Preface

Biochar is the solid residue recovered from the thermal cracking of biomasses in an oxygen-poor atmosphere. Recently, biochar has been increasingly explored as a sustainable, inexpensive and viable alternative to traditional carbonaceous materials for the development of many cutting-edge applications. Biochar exhibits high thermal stability, high surface area and electrical conductivity, and its principal properties can be tuned appropriately by controlling the conditions of the pyrolysis process. Due to its intriguing characteristics, biochar is currently in competition with high-performing fillers in multifunctional approaches to environmental remediation, electrochemistry, energy application and materials science, and represents a promising aspect of the movement toward a biomass-based circular bioeconomy.

For this book, we selected crucial topics ranging from pollutants removal to the electrochemical and energetic applications of biochar, focusing both on contributions that could provide an introduction to the biochar sector and those that provide enlightening new perspectives on this subject and its future applications. The book promotes the spread of innovative rethinking of old technologies and problems that can merge the great potential of biochar-based technologies with respect for our world.

This book will be highly accessible to any reader with a strong scientific and technological background, from scientific advisors in private companies to academics. Students enrolled in graduate science programs may also find this text useful for deeper insights into the very complex field proposed by the authors. In view of its very strong scientific content, we believe that this book may come to be the reference text for any future study and application of biochar-based technologies. We hope that *Biochar - Productive Technologies, Properties and Applications* will contribute to focusing the attention of the scientific community on this emerging sector.

> **Mattia Bartoli** CSFT@POLITO, Fondazione Istituto Italiano di Tecnologia, Turin, Italy

**Mauro Giorcelli** Department of Applied Science and Technology, Polytechnic University of Turin, Turin, Italy

#### **Alberto Tagliaferro**

Department of Applied Science and Technology, Polytechnic University of Turin, Turin, Italy

Section 1

Perspectives on the

Biochar Future

**1**

Section 1

## Perspectives on the Biochar Future

#### **Chapter 1**

### Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive on Ruminant' s Methanogenesis

*Ana R.F. Rodrigues, Margarida R.G. Maia, Ana R.J. Cabrita, Hugo M. Oliveira, Inês M. Valente, José L. Pereira, Henrique Trindade and António J.M. Fonseca*

#### **Abstract**

The use of biochar has been suggested as a promising strategy in bio-waste management and greenhouse gases mitigation. Additionally, its use, as a feed additive, in ruminants has been reported to have contrasting effects on enteric methane production. Hence, this chapter intends to overview the most relevant literature that exploited the use of biochar as a mitigation strategy for methane. This includes the reported effects of biochar on methane production and rumen fermentation observed in *in vitro* and *in vivo* assays, as well as manure's methane emission. The information available about the biochar and the experimental conditions used in the different studies is still limited, which created additional challenges in identifying the biological mechanisms that potentially drive the contrasting results obtained. Nevertheless, it is clear from the current state-of-the-art that biochar may be a key player in the modulation of gut fermentation and in the reduction of greenhouse gases produced by ruminants that need to be consolidated by further research.

**Keywords:** biomass, biochar, enteric methane, *in vitro*, *in vivo*, ruminants

#### **1. Introduction**

The livestock sector was estimated to emit 14.5% of global anthropogenic greenhouse gases (GHG), mainly methane (CH4), nitrous oxide (N2O), and carbon dioxide (CO2) [1], with enteric CH4 corresponding to 40% of total livestock sector emissions, 77% of which emitted by cattle [1].

Ruminants are herbivorous animals that host a complex symbiotic microbial population composed of bacteria, protozoa, archaea, fungi, and bacteriophages in the two forestomach (reticulum and rumen) where feeds undergo fermentation, before entering the true stomach, the abomasum. Microbial population ferments structural

and non-structural polysaccharides, and proteins originating volatile fatty acids (VFA) (mainly acetate, propionate, and butyrate), ammonia-N (NH3-N), CO2, and hydrogen (H2) [2]. Volatile fatty acids are absorbed through the rumen wall and comprise the major energy source of the host animal. Hydrogen is mainly eliminated by the reduction of CO2 by methanogenic archaea [3]. Enteric CH4 represents a loss from 2 to 12% of total gross energy intake [4] and it is the second GHG contributor to climate change, with a global warming potential 28 times larger than CO2, in a time horizon of 100 years. Mitigation of enteric CH4 emissions is thus important not only to minimize the environmental impact of ruminant production but also to improve feed efficiency.

Several strategies have been evaluated to reduce enteric CH4 production, including feeding management (e.g., ingredient selection, feed supplements, rate of passage, and better-quality ingredients), rumen modifiers (e.g., defaunation, bacteriocins, and immunization), and improvement of animal production through genetics (e.g., nutrient utilization, feed efficiency, and CH4 production) [5], but effects are often transient [6] or conflicting [7]. Greenhouse gases (CH4, N2O) and ammonia (NH3) are also produced during cattle manure decomposition in housing, storage, and treatment, and ultimately during land spreading [8]. Different strategies have been proposed to reduce gaseous emissions in each stage of manure management, from dietary manipulation to chemical application in slurry [9, 10]. One emerging strategy to cope with the mitigation of both enteric CH4 and GHG from ruminants' manure is the use of biochar. Biochar is a stable porous carbon-rich material (between 65 and 90%), mainly produced by the pyrolysis method under oxygen-limited conditions, containing mineral elements whose physical and chemical characteristics are determined by feedstocks and technologies involved in the production process [11, 12]. Due to its characteristics, biochar has been studied for multiple uses, such as soil amending [13–15], mitigating GHG emissions from soil [16–19], recovering nutrients from wastewaters [20], and reducing GHG emissions from cattle manure during storage [21, 22]. Its porous structure promotes soil moisture retention, reduces bulk density, enhances the organic matter content, and can positively affect soil cation exchange capacity [23, 24]. Due to these properties, interest has emerged in biochar as a feed supplement to mitigate enteric and fecal CH4, and manure gaseous emissions [25, 26], in a cascade approach, thus enhancing its effect along the cattle production system [27]. In this context, the European biochar foundation has developed guidelines for biochar production to be used as a feed additive [28] under the requirements of the European Food Safety Authority (EFSA) and respecting the commission regulation (EC) 178/2002 [29] and 834/2007 [30].

#### **2. The role of biomass and production conditions on biochar characteristics**

The biomass source and the type and conditions of production are key factors in biochar physicochemical properties resulting in different functional characteristics and applications [31], being pyrolysis the most common process for the production of biochar. The characteristics of biochar can be highly variable, especially in terms of elemental composition, surface chemical composition, structure, and stability. Each component's decomposition and depolymerization occurs through several reactions at different temperatures, contributing to the structural differences among biochars [32, 33].

*Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive… DOI: http://dx.doi.org/10.5772/intechopen.108952*

In the works reviewed here, biochars were mainly produced by the pyrolysis of agriculture and forestry lignocellulosic biomasses, which are primarily composed of cellulose (4045%), hemicellulose (2535%) and lignin (2030%), although their distribution varies among biomasses [34]. In terms of gaseous capture, the most relevant characteristics of biochar are the organic matter content (given by polarity and aromaticity), mineral content, cation exchange capacity, surface charge, and textural properties (surface area and pore size) [35].

The adsorption capacity of biochar related to the polarity and aromaticity is highly modulated by the pyrolysis conditions [35]. Due to the high carbon content and porous structure, adsorption is a valuable property of biochar, which has been used for environmental purposes, such as the reduction of GHG levels [35]. Therefore, the physical-chemical characteristics of the biochar have a strong influence on the capabilities of the materials for a particular application (**Figure 1**).

For example, the ash content that results from the decomposition of the inorganic matter of biomass [23] is expected to be low in wood-based biomass when compared to mineral-rich biomass, such as grass, manure, litter, and solid waste [36]. Wood, bamboo, corncob, corn stover, pellets (miscanthus, softwood, wheat straw, and oilseed rape straw), rice straw, and potato peel biochar reported less than 25% of ash content, while rice husk presented higher than 40% [37–44]. The ash content has been demonstrated to be relevant for the surface polarity and distribution of pores, thus influencing the sorption capacity of the material. The mineral content in biochar (such as carbonates, oxides, phosphates, alkali, or alkaline earth metals) has been shown to

*Biochar post-production functionalization and potential applications. Reprinted with permission from Ghodake et al. [33].*

increase the sorption capacity for acidic gases, such as sulfur dioxide, hydrogen sulfide, and CO2 [12].

The surface area and pore size can also be modified by chemical and physical activation following the carbonization process [45]. The modification of biochar with a CO2-NH3 mixture resulted in a surface area increase besides improving the chemical properties of the surface by a nitrogen modification [46]. The microporous structure has a key role in CO2 capture at low temperatures [47].

Using lignocellulosic biomasses (the main raw material present in the application herein described), a microporous structure is expected with higher cellulose and hemicellulose content, whereas mesoporous structures are expected with higher lignin content [27]. The increase in pyrolysis temperatures also increases the porosity, surface area, pH, ash, and carbon content of biochar due to the release of volatile components, while reducing biochar exchange capacity and yield [20, 48]. In the study of Calvelo Pereira *et al.* [38], an increase in surface area, carbon, nitrogen, and ash contents of biochar produced from the pyrolysis of pine chips and corn stover was observed. Also, biomass has been shown to highly influence the surface area, as demonstrated by other authors [40, 44].

#### **3. Effects of biochar on** *in vitro* **rumen fermentation**

There is a paucity of data on the effects of biochar on CH4 production by shortand long-term *in vitro* studies. Therefore, these will be addressed separately.

#### **3.1** *In vitro* **short-term studies**

**Table 1** presents the results obtained in 14 studies evaluating the effects of biochar addition up to 16% on rumen fermentation and CH4 production through *in vitro* short-term incubations (up to 48 h). No clear association is evident between effects on CH4 production and biochar characteristics (e.g., biomass, temperature of pyrolysis) and level of inclusion. Increasing pyrolysis temperature increases surface area, which has the potential to improve biofilm formation and promote the adsorption capacity of microorganisms, nutrients, and gases, thus reducing CH4 production [26, 49]. Indeed, some studies [44, 50–54] reported a decrease in CH4 production with the addition of biochar produced at very high temperatures (7001000°C), whereas in the studies using biochar produced at lower temperatures (350700°C) no effect [38, 55] or an increased [42] CH4 production was observed. However, Saenab *et al.* [56] reported a decrease in CH4 production when biochar from cashew nutshell was produced at 300°C and Cabeza *et al.* [40] found higher CH4 production with biochar produced at 700°C than 550°C. It must be realized that *in vitro* systems do not effectively reproduce the *in vivo* situation, particularly the adaptation of rumen microbiome to novel materials, and for this reason, effects *in vitro* might not be observed *in vivo* [5].

The information about biochar characteristics (besides pyrolysis temperature), is absent in the majority of the studies, making impossible any association between the results and the biochar characteristics and their respective effects on CH4 production. Despite not having evaluated the effect on CH4 mitigation, McFarlane *et al.* [39] found biochar particle size to affect rumen fermentation, being inhibited with large particles (>178 μm *vs.* <178 μm). Although without impact on gas production and


*Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive… DOI: http://dx.doi.org/10.5772/intechopen.108952*

#### **Table 1.**

*Biochar biomass, temperature of pyrolysis (°C), and inclusion level (% dry matter basis) effects on methane (CH4) production in short-term in vitro studies.*

VFA proportions, these authors reported *in vitro* true digestibility of orchard grass hay to be increased by the inclusion of fine biochar particle size [39].

A comparison between studies is further complicated by the diversity of biomass sources used (e.g., rice husk, pine wood, corn stover, cashew nutshell, tree pruning, rice straw, corncob, bamboo) that might affect VFA profile, thus introducing a confounding effect on the mechanism of CH4 reduction. Most studies that compared the impact of biomass sources on enteric CH4 production [37, 38, 41, 42] observed no differences among biochar sources. Conversely, Van Dung *et al.* [44] found rice straw and bamboo biomass to reduce CH4 production compared to corncob, at 4 and 48 h of incubation, but not at 24 h. Moreover, these authors observed an interaction effect between biomass source and pyrolysis temperature [44], supporting the need for a multi-aspect analysis of biochar's chemical and physical properties. The effects on VFA profile were further assessed [38, 40, 42, 55, 56]. In the study of Calvelo Pereira *et al.* [38], despite a decrease in propionate proportion found with some mixtures, which might indicate an increase in H2 produced, the effects were insufficient to affect CH4 production. In the study by Cabeza *et al.* [40], the addition of biochar slightly reduced CH4 production, but it kept unchanged the amounts of total VFA or acetate produced and reduced those of propionate and butyrate. Saenab *et al.* [56] observed a reduction of CH4 production by 11.5% with 3% [dry matter (DM) basis] cashew nutshell biochar supplementation, although total and individual VFA produced were unaffected. Rodrigues *et al.* [42] attributed the reduction of VFA production through biochar addition to a reduced energy supply for microbial growth. Supplementation of tree pruning biochar up to 4% (DM basis) did not affect CH4 or VFA content and profile [55].

The study by Leng *et al.* [57] was the only one that evaluated the effect of rumen fluid adapted to biochar. The authors attributed the reduction in CH4 production with rumen-adapted inoculum to a larger ruminal population that oxidizes CH4. Indeed, adapted rumen inoculum is expected to present a higher density of methanotrophs [58], possible the effect of biochar on rumen CH4 is solely due to the increase in potential habitat for this consortium. However, in the study by Leng *et al.* [57], CH4 reduction was higher with biochar addition to unadapted rumen inoculum than without biochar addition to adapted rumen inoculum. Biochar addition promotes either the association of microorganisms that more efficiently ferment feed materials or facilitates CH4 oxidation by bringing together methanogenic archaea and methanotrophic consortia [59].

However, from the available studies, the mechanism of CH4 reduction through biochar is unclear. Although biochar favors methanotrophism in the soil [60], the anaerobic rumen precludes the growth of aerobic methanotrophs, thus the action of biochar is most possibly through the promotion of micro-environments by the large surface area of biochar [40].

#### **3.2** *In vitro* **long-term studies**

The long-term effects of biochar supplementation on rumen fermentation and CH4 production were further assessed *in vitro* using the rumen simulation technique system (**Table 2**). Despite differences among biochar biomass, pyrolysis temperature, and chemical and physical characteristics, only one study observed a CH4 mitigation effect of biochar when compared to control [41]; supplementation levels (0.5, 1, and 2%, DM basis) having a quadratic effect, greatest with 0.5% inclusion. Jackpine biochar also improved most fermentation parameters (e.g., NH3-N, total VFA,

*Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive… DOI: http://dx.doi.org/10.5772/intechopen.108952*


*NH3-N- ammonia-N, DMD- dry matter digestibility, CPD- crude protein digestibility, NDFD- neutral detergent fiber digestibility, ADFD- acid detergent fiber, LAB- liquid associated bacteria, and VFA- volatile fatty acids.*

#### **Table 2.**

*Biochar biomass, temperature of pyrolysis (°C), and inclusion levels (% dry matter basis) effects on rumen fermentation and methane (CH4) production in long-term in vitro studies.*

acetate, propionate, butyrate, and branched-chain VFA yield), nutrient digestibility (DM, crude protein, neutral detergent fiber, acid detergent fiber), and microbial N of total and liquid associated bacteria while decreased that of loosely associated bacteria [41]. Conversely, mineral-activated blackbutt [61], jack/yellow pine [43] and spruce stem [62] biochar supplementation kept unaffected gas production, fermentation parameters (pH, NH3-N, total and individual VFA yield), nutrient digestibility, microbial N produced, protozoa count, and bacterial diversity, richness, and relative abundance. Inconsistency of biochar effects has been attributed to variations in biochar chemical and physical properties, including particle size, adsorptive potential, electrical conductivity, and electron-mediation in redox reactions [37, 39]. Several modification methods have been used to improve biochar properties, such as acidification of surface area, to increase biochar adsorption [23]. Teoh *et al.* [61] further suggested that biochar pH could be of particular importance in enteric CH4 reduction, based on the notable CH4 reduction (25%, as mg/g DM incubated) of the acidic (pH 4.8) jack pine biochar used in Saleem *et al.* [41] study. Acidic biochar has been associated with improved carbon sequestrum and higher redox potential in soils, whereas neutral mineral-rich biochar lacked this ability [63]. However, acidic (pH 4.9) pine biochar failed to reduce enteric CH4 production [43] similarly to observed with basic (pH 8.2) biochar supplementation [37, 38, 61].

Acidic biochar has also been suggested to improve the redox potential and thus increase biofilm development by the mediation of electrons among the microbial population [61, 64]. However, more developed biofilms were observed on readily digestible

substrates than on biochar surfaces [62, 65]. Even though microbial diversity, richness, and relative abundance were not affected by long-term biochar supplementation, discriminant analysis unveiled biochar-type specific changes in rumen bacterial families [43, 61, 62]. Of particular interest, Teoh *et al.* [61] found a 19.8-fold reduction in the abundance of *Methanomethylophilaceae* with the supplementation of mineral-activated biochar. Members of *Methanomethylophilaceae* family are methanogenic archaea that use sources of hydrogen to reduce methylated compounds and produce CH4 [66, 67], thus suggesting the potential mitigation effect of hardwood biochar [61].

#### **4. Effects of biochar** *in vivo*

The porous structure of biochar can adsorb gases and provide habitat for microbial biofilms [37, 68], which in addition to electron-mediation properties in biological redox reactions [69] suggest its potential to reduce enteric CH4 production and promote rumen fermentation. As previously stated, *in vitro* studies present several advantages, but do not fully simulate the *in vivo* animal. Few studies have evaluated, *in vivo*, the effects of dietary biochar inclusion on ruminant performance and CH4 production (**Table 3**). Globally, dietary supplementation with biochar from different sources increased or not affected ruminant performance and reduced or kept


*Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive… DOI: http://dx.doi.org/10.5772/intechopen.108952*


#### **Table 3.**

*Effect of biochar biomass and inclusion level (% dry matter basis) on ruminant performance and methane (CH4) production.*

unaffected CH4 production. Leng *et al.* [52] pointed out the need for CH4 mitigation strategies to include alternative electron sinks rather than just focused on methanogens inhibition, due to the need for symbiotic associations in biofilm microbial colonies on feed particles for successful ruminal fermentation to occur. Rumen microbial biofilms are of particular importance for fiber fermentation, with microbial attachment to feed particles allowing pit formation as well as glycocalyx emission to fibrous amorphous material [75].

In Angus Hereford heifers, Terry *et al.* [65] found that, although total tract digestibility, nitrogen balance, and CH4 production were not affected by dietary biochar inclusion, the relative abundance of *Fibrobacter* and *Tenericutes* were reduced and that of *Spirochaetaes*, *Verrucomicrobia*, and *Elusimicrobia* increased. Modulation of the manure microbial population was also found to be affected by dietary biochar supplementation. Al-Azzawi *et al.* [74] reported decreased methanogenic population by 30% with a corresponding increase in the non-methanogenic archaeal species in manure, suggesting that formed CH4 could be reduced by further utilization by methanotrophic species. Moreover, biochar was shown to affect nitrification by increasing ammonia-oxidizing organisms and reducing ammonooxygenase activity [76].

Although dietary biochar supplementation had variable effects on ruminant performance, these were overall promising and suggest potential benefits beyond methanogenesis. Indeed, 0.6% biochar increased the live weight gain of yellow cattle and DM feed conversion by 25% [52]. Terry *et al.* [73] found no effect on body weight gain or DM intake in beef steers up to 2% biochar, but lean meat yield increased with the highest biochar level tested (2%). In lambs, 2% biochar kept feed intake and average daily gain unaffected, and improved DM intake [71], while up to 1.5% biochar was found to maintain DM intake and increase average daily gain and feed conversion ratio [72]. In addition, milk production of cows fed 0.5% (DM basis) activated carbon was improved [74]. Furthermore, in an innovative solution for biochar utilization reported by Joseph *et al.* [77], biochar was mixed with molasses and fed directly to cows, the dung-biochar mixture being incorporated into the soil profile by dung beetles and the costs and benefits of integrating biochar with animal husbandry and improvement of pastures were assessed. These authors found that dung-biochar had an outer coating of mineral elements (P, K, Mg, Ca, Al, Si, and Fe) and nitrogen, adsorbed in the cow gut, that were available for soil, thus being an effective strategy to improve soil properties. In addition, increasing returns to farmers were calculated, suggesting the profitability of dietary biochar supplementation in ruminant production systems [77].

Notwithstanding, the inconsistent results in the literature on the effect of biochar on reducing CH4 emissions, rumen *in vitro* fermentation, and *in vivo* rumen function limits the mechanistic understanding of the underlying mode of action. This is particularly difficult due to the use of different sources of biomass and production conditions, such as duration and temperature, of pyrolysis as well as post-treatment modifications, which alter the composition, porosity, and chemistry of biochar [65], but also to the poorly characterized biochar used in ruminant studies. These challenges make comparisons between studies difficult, and in addition to the lack of knowledge of the long-term effects of dietary biochar supplementation, could have limited its use in ruminant feeding practices on-farm.

#### **5. Effects of biochar on manure CH4 production**

Ruminant production generates high amounts of manure that need to be stored until the land application. Manure is a rich source of nutrients, and its application is shown to improve soil quality, to reduce the use of mineral fertilizers and costs of production [21]. However, during manure storage and land application, malodorous compounds and GHG, such as CH4, CO2, and N2O, as well as NH3, are formed and emitted [78], with a detrimental impact on ecosystems [22]. Biochar application to manure can be an effective strategy to improve its environmental impact, as it can absorb and retain GHG, NH3, and nutrients [79, 80]. Moreover, when applied to soils, biochar-enriched manure may provide nutrients, sequester carbon, and improve soil's structure [22, 79]. Although the already identified biochar potential in manure, differences have been reported among biochar biomass, production conditions, pH, hydrophobicity, and particle size [22, 68, 81]. Moreover, a life cycle assessment of the environmental implications of stored cattle slurry (a mixture of manure, split feed, and water) treatments revealed biochar to be one of the less effective approaches to suppress GHG emissions from liquid slurry, except for N2O [21]. The inconsistent results from biochar application to manure pinpoint the need for more research in this field.

#### **6. Conclusions**

Biochar is undoubtedly a material with high potential to deal with ruminant methanogenesis due to its availability, stability, and large surface areas. Nevertheless, there is a significant knowledge gap about the mechanisms that govern the interactions between biochar and the plethora of microorganisms that are present in the ruminant's gut and manure. In this chapter, we addressed the most relevant literature on the topic, seeking additional clarification about the potential role of biochar in methanogenesis. The absence of detailed characterization of biochar used, and the diversity of the experimental conditions applied in the different studies, create additional challenges for a critical comparison of the past findings. Therefore, for future studies, some level of standardization and the detailed characterization of the biochar(s) used will have a significant impact on the clarification of its role in the mitigation of GHG emissions from ruminants.

*Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive… DOI: http://dx.doi.org/10.5772/intechopen.108952*

#### **Acknowledgements**

The work received financial support from the Portuguese Foundation for Science and Technology (FCT) through projects UIDB/04033/2020, UIDB/50006/2020, UIDP/50006/2020, and OPTIMA: Optical monitoring of environmental emissions of ammonia by an integrated and autonomous membrane-based fluorescence platform (PTDC/CTA-AMB/31559/2017), and from project R&W Clean: new solutions for sensing environmental and biological parameters to help demedicalize the agricultural sector (POCI- 01-0247-FEDER-70109) supported by PORTUGAL 2020 program through the European Regional Development Fund. Ana R.F. Rodrigues thanks FCT and European Social Fund through Programa Operacional Capital Humano (POCH) and SANFEED Doctoral Programme, for funding her Ph.D. grant (PDE/BDE/114434/ 2016). Margarida R.G. Maia and Inês M. Valente thank FCT for funding through program DL 57/2016 – Norma transitória (Ref. SFRH/BPD/70176/2010 and SFRH/ BPD/111181/2015, respectively).

#### **Conflict of interest**

The authors declare no conflict of interest.

### **Author details**

Ana R.F. Rodrigues1†, Margarida R.G. Maia<sup>1</sup> , Ana R.J. Cabrita<sup>1</sup> , Hugo M. Oliveira<sup>2</sup> , Inês M. Valente1,3, José L. Pereira4,5, Henrique Trindade5 and António J.M. Fonseca<sup>1</sup> \*

1 REQUIMTE, LAQV, ICBAS, School of Medicine and Biomedical Sciences, University of Porto, Porto, Portugal

2 INL, International Iberian Nanotechnology Laboratory, Braga, Portugal

3 REQUIMTE, LAQV, Chemistry and Biochemistry Department, Faculty of Sciences University of Porto, Porto, Portugal

4 Centre for the Research and Technology of Agro-Environmental and Biological Sciences (CITAB), Inov4Agro, University of Trás-os-Montes and Alto Douro, Vila Real, Portugal

5 Agrarian School of Viseu, Polytechnic Institute of Viseu, Viseu, Portugal

\*Address all correspondence to: ajfonseca@icbas.up.pt

† Present Address: FeedInov CoLab, Estação Zootécnica Nacional, R. Professor Doutor Vaz Portugal, 2005-424, Vale de Santarém.

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

*Review: Heads or Tails? Toward a Clear Role of Biochar as a Feed Additive… DOI: http://dx.doi.org/10.5772/intechopen.108952*

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#### **Chapter 2**

## Biochar: Production, Application and the Future

*Edward Kwaku Armah, Maggie Chetty, Jeremiah Adebisi Adedeji, Denzil Erwin Estrice, Boldwin Mutsvene, Nikita Singh and Zikhona Tshemese*

#### **Abstract**

Biochar, or carbon obtained from biomass, is a particularly rich source of carbon created by thermal burning of biomass. There is a rise of interest in using biochar made from waste biomass in a variety of disciplines to address the most pressing environmental challenges. This chapter will provide an overview on the methods employed for the production of biochar. Biochar has been considered by a number of analysts as a means of improving their ability to remediate pollutants. Process factors with regards to biochar properties are mostly responsible for determining biomass production which is discussed in this present chapter. Several characterization techniques which have been employed in previous studies have received increasing recognition. These includes the use of the Fourier transform infrared spectroscopy and the Scanning electron microscope which duly presented in this chapter. This chapter also discusses the knowledge gaps and future perspectives in adopting biochar to remediate harmful contaminants, which can inform governmental bodies and law-makers to make informed decisions on adopting this residue.

**Keywords:** biochar, biomass, characterization, future perspective, pyrolysis, pretreatment

#### **1. Introduction**

The word char, is a common terminology used for the solid product of the combustion of carbonaceous material [1]. Generally, char product is rich in carbon content; an example is charcoal, which is almost the earliest invention of humans from fire or heat creation. Another vivid example of char is biochar. In this case, the study, is made from organic compounds such as forest, agricultural or animal products but in the absence/ limited supply of oxygen compared to charcoal. Therefore, biochar is derived from biomass combustion in the presence of a limited oxygen supply and at relatively low temperatures below 700°C. The earliest known purpose for creating biochar was specifically for soil application such as carbon storage or sequestration in soil; improvement of soil performance such as increase in nutrient availability, reduction of compactness in soil, soil pH improvement; soil water filtration. Recent applications involve energy production, biochemical process stability and improvement, climate change mitigation, and construction additive [1–3]. The raw material determines carbonized organic matter properties and the operational parameters used during it production. Pyrolysis (slow or fast) and gasification are the main methods for the production of biochar. The physical nature of the biochar produced is directly affected by the chemical composition of the biomass feedstock. Most organic matter begin to thermally decompose at temperatures above 120°C. Hemicelluloses degrade between 200 and 260°C, cellulose between 240 and 350°C, and lignin between 280 and 500°C. As a result, the proportions of these components will affect the degree of reactivity and, as a result, the extent to which the physical structure is modified during processing [4]. Biochar is characterized with high porosity with pores ranging in size from micro to macropores. Large holes, which originate from the raw biomass's vascular bundles, are critical for increasing soil quality because they can serve as habitats for symbiotic microbes. Biochar major components are carbon, volatile matter, mineral matter (ash), and moisture. The percentage composition of each components varies based on the feedstock material and the operating parameters [1]. Biochar from plant-based materials have higher carbon composition which range from as low as 51% to as high. The understanding of the key mechanisms for changes in physicochemical properties of biochar during processing for various feedstock types and operating parameters is required to determine biochar's potential for application both now and in future. Therefore, this chapter explains biochar production techniques, factors affecting its properties and compositions and its application.

#### **2. Biochar production techniques**

An ever-growing appetency for using biochar for various applications has orchestrated an increase in converting it into biochar. Thermochemical conversion is a common technology for making biochar. Thermochemical conversion techniques are pyrolysis, hydrothermal carbonization (HTC), gasification, torrefaction, and hydrothermal liquefaction [5, 6].

#### **2.1 Pyrolysis**

Pyrolysis is a thermochemical technique that produces biochar, bio-oil, and syngas derived from biomass [7]. The process involves heating and thermally decomposing biomass under anaerobic conditions or limited oxygen supply (low stoichiometric oxygen atmosphere) with temperatures ranging between 400°C and 1200°C [2]. The absence of oxygen enables biomass heating beyond its thermal stability limit, causing the creation of more robust products, including solid residues. By creating an anaerobic atmosphere, it is also ensured that combustion will not occur when the biomass is heated. It is a highly complex process involving many distinct reactions in the reacting zone [8]. In another study, a low-temperature range for pyrolysis was recorded between 250°C and 900°C. Biomass from Agriculture comprises lignin, cellulose, hemicelluloses, and silica. Typically, cellulose pyrolyzes at 350°C, whereas the melting point of lignin is well above 350°C [6]. Although the product yield depends on various operating variables, char formation is generally favored by low temperatures and long residence times [9]. Therefore, it can be decoded that the effective temperature range for pyrolysis was between 300 and 700°C. The cracking of heavy chemicals happens in secondary pyrolysis and converts biomass into biochar or gases. **Figure 1** is a summary of the pyrolysis technique and the operating variables affecting pyrolysis.

*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

**Figure 1.** *Schematic representation of pyrolysis process [3].*

In essence, this is an alternative way to valorize biomass into various products such as bio-oil, syngas and biochar. Depolymerization, fragmentation, and cross-linking are chemical mechanisms that occur during the process at specific temperature points, resulting in a different product state for lignocellulosic components, including cellulose and hemicellulose (solid, liquid and gas). Biochar and bio-oil are the solid and liquid products, whereas CO2, CO, H2, (collectively known as syngas) are evolved as the gaseous by-products (C1-C2 hydrocarbons) [3]. Biochar is made in a different type of reactors, such as paddle kiln, bubbling fluidized bed, wagon reactor, and agitated sand rotating kiln. The biomass nature and employed type determine the biochar yield during the pyrolysis route. The major operating parameter that impacts product efficiency is the temperature [10, 11]. When the pyrolysis temperature is increased, biochar's yield decreases and the generation of syngas increases. The gas yield is represented by the initial section of the product side (as shown in Eq. (1)), with various gases created during the process.

$$\left(\mathbf{C}\_{6}\mathbf{H}\_{6}\mathbf{O}\_{6}\right)\_{\mathrm{n}} \rightarrow \left(\mathbf{H}\_{2} + \mathbf{CO} + \mathbf{CH}\_{4} + \dots + \mathbf{C}\_{5}\mathbf{H}\_{12}\right) + \left(\mathbf{H}\_{2}\mathbf{O} + \mathbf{CH}\_{3}\mathbf{OH} + \mathbf{CH}\_{3}\mathbf{COOH} + \dots\right) + \mathbf{C} \tag{1}$$

The mixture of multiple sorts of liquid outputs is shown in the second part of the products' side, and the solid yield is represented in the last component [12]. One of the most significant masteries of this technology is that it may be optimized to achieve the desired outcomes. Slow pyrolysis, for example, can be utilized to produce a considerable amount of biochar, whereas fast pyrolysis is better for dominantly producing bio-oil [13].

#### *2.1.1 Types of pyrolysis*

Pyrolysis is strongly dependent on the operating parameters, namely temperature, heating rate, and residence time [14]. These operating conditions further help to categorize pyrolysis into other six subclasses. These subclasses are slow pyrolysis, fast pyrolysis, flash pyrolysis, vacuum pyrolysis, intermediate pyrolysis, and hydropyrolysis [15]. Each classification of pyrolysis has its own documented benefits and drawbacks. The subclasses in question foster an environment for different reaction conditions and mechanisms to have various products. The pyrolysis technology mechanism is shown in **Figure 2**.

#### *2.1.1.1 Slow pyrolysis*

As indicated by the name, to complete the process, slow pyrolysis has a long residence time (more than 1 hour), and biochar is produced as a major product [16]. Slow pyrolysis is dubbed conventional pyrolysis, where biomass is heated at temperatures ranging between 300 and 600°C accompanied by a heating rate of 5–7°C/ min [12, 17]. A lower heating rate and longer vapor residence time provide a suitable environment and adequate time for the secondary reactions to proceed. Furthermore, a prolonged residence period permits vapors created during the secondary reaction to be evacuated [15, 18]. This leads to the creation of solid carbonaceous biochar in the end. Slow pyrolysis favors char development, but liquid and gaseous products are also created in modest quantities. Biochar is formed as a primary product (35–45%) together with other products such as bio-oil (25–35%) and syngas (20–30%), as indicated in Eq. (1) [6, 19].

*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

#### *2.1.1.2 Fast pyrolysis*

Fast pyrolysis is a direct thermochemical process for converting solid biomass into high-energy liquid bio-oil. A high-efficiency thermochemical technique to produce biomass-derived biofuels, with reduced amounts of solids and gases produced [20, 21]. Fast pyrolysis is carried out without oxygen at temperatures above 500°C and a heating rate of over 300°C/min. Fast pyrolysis is a rapid biochar generation technique that takes only a few seconds. Fast pyrolysis produces 60% bio-oil, 20% biochar, and 20% syngas, as reported in other studies [21, 22]. Even higher temperatures in the range of 850–1250°C with a heating rate of 10–200°C for a short residence time ranging from 1 to 10 s have been reported in several experiments. 60%-75% of liquid products, 15%-25% of biochar and 10–20% of non-condensable gaseous products are produced by a typical pyrolysis process [23]. Fast pyrolysis takes biomass to temperatures in which thermal cracking can occur and minimizes the exposure time, which supports biochar production [24].

#### *2.1.1.3 Flash pyrolysis*

This is dubbed to be an enhanced and modified version of fast pyrolysis. Biomass decomposes quickly, usually in less than a minute, at 1000°C and even higher temperatures. Heating rates of above 1000°C/sec have been recorded on occasion. Flash pyrolysis is carried out at temperatures between 900 and 1200°C, which can be reached in less than one second (usually between 0.1 and 1 s) [25, 26]. A high bio-oil yield combines a high heating rate with a high temperature and a short vapor residence time. However, the yield of biochar is reduced because of the process [27, 28]. In flash pyrolysis, heat and mass transfer processes, reaction chemical kinetics and biomass phase transition behavior all play a role in product distribution. Although flash pyrolysis is performed in a fluidised bed reactor and a twin-screw mixing reactor, it has limited industrial applicability because of the reactor's architecture, which requires it to run at a high temperature with a very high heating rate [12].

#### *2.1.1.4 Vacuum pyrolysis*

This is the thermal decomposition of biomass under vacuum or relatively low pressure in an isolated oxygen environment [15, 29]. Pressure is usually regulated in the region between 0.5 and 2 bar, and temperature is maintained at 450–600°C [30]. Like slow pyrolysis, vacuum pyrolysis has comparably low heating rates. However, these two techniques, in comparison, yield significantly different products. This owes to the constant and effective discharge of the vapor produced during vacuum pyrolysis through condensation train. The rapid evacuation of organic vapors created during the primary pyrolysis also considerably minimizes the vapor residence time, which in turn minimizes the occurrence of secondary reactions and assures a high liquid product yield during the secondary pyrolysis [31]. As a result, only vacuum or lowpressure extraction is utilized to remove vapor evolved during pyrolysis, which substantially affects product quality and yield by preventing inorganic devolatilisation.

#### *2.1.1.5 Intermediate pyrolysis*

As the name suggests, this is a combination of slow and fast pyrolysis processes, and it is crucial when there is a need to balance solid and liquid products. This means that

slow pyrolysis is more efficient at producing large amounts of char, but it also results in lower amounts of liquid products, while it is vice versa with fast pyrolysis. Generally, pressure is kept at 1 bar during the process. Intermediate pyrolysis has temperatures ranging between 500 and 650°C, with heating rates between 0.1 and 10°C/min and residence time between 5 and 17 mins [32]. 40–60% liquid, 20–30% non-condensable gases, and 15–25% biochar are typical constituents of finished products [33, 34]. Using intermediate pyrolysis conditions prevents the synthesis of high molecular reactive tars and results in dry biochar, which can be utilized for agricultural purposes or directly in boilers and engines in conjunction with high-quality bio-oil [2].

#### *2.1.1.6 Hydropyrolysis*

It relatively a novel technique that is used for the conversion of biomass into high quality products by injection of hydrogen or hydrogen based material into the reactor under high pressure, typically above the atmospheric pressure, stretching from 50 bar to 200 bar [15, 35]. The heating rate (10–300°C/s), residence time (over 15 sec) and temperature (350–600°C) are not highly deviated from fast pyrolysis [36]. In essence, hydropyrolysis can be considered a special type of fast pyrolysis subjected to high pressure in an atmosphere infused with hydrogen or hydrogen-based material. This method is not ideal for the production of biochar as the introduction of hydrogen under high temperature and pressure acts as a reducing agent, hence reducing oxygen content in the bio-oils produced and synchronously inhibiting the production of biochar [37, 38]. The employment of a catalyst to eradicate oxygen, water, and COx from the liquid product is typically linked with hydropyrolysis. Catalysts also reduce depolymerisation and coking reactions [39]. However, developing the catalyst for this intention remains a notable example of the difficult aspects of catalytic hydropyrolysis.

#### **2.2 Carbohydrate decomposition**

The majority of the material used in biochar production via pyrolysis contain carbohydrates in various forms (cellulose, hemicellulose and lignin), and these react differently based on the operating conditions they are subjected to, thus influencing the product yield of pyrolysis [15]. More specifically, lignin and cellulose are the major parts of biomass, making up its bulk [40]. On pyrolysis, cellulose mostly creates tar, a mixture of discrete ketones, aldehydes, organic liquids, and char, whereas lignin essentially produces char and a minimal amount of water. As the cellulose content grows but the char and tar content decreases, the yield of gaseous content increases. It's also been discovered that structural differences in biomass cause changes in the pyrolysis product's composition [41].

#### *2.2.1 Cellulose decomposition*

By lowering the extent of polymerization, the process of cellulose degradation is determined, which consists of two principal reactions:


*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

In addition to producing the solid product biochar, levoglucosan is dehydrated to generate hydroxymethylfurfural, which can break down to produce liquid and gaseous products such as bio-oil and syngas, respectively. Furthermore, the hydroxymethylfurfural can undergo several processes, including aromatization, condensation, and polymerization, to generate solid biochar [42, 43]. At low temperatures, cellulose degrades to a reasonably stable anhydrocellulose that produces a lot of char, but it decomposes into volatiles [25, 44].

#### *2.2.2 Hemicellulose decomposition*

The hemicellulose degradation mechanism is like that of cellulose. Depolymerisation of hemicellulose leads to oligosaccharides production [45]. Decarboxylation, intramolecular rearrangement, depolymerisation, and aromatisation reactions can be used to synthesize biochar or the compound can degrade into syngas and bio-oil [46]. The volatile products and lignin are responsible for the char yield of the cellulose and hemicellulose components in biomass [40].

#### *2.2.3 Lignin decomposition*

Unlike the degradation of cellulose and hemicellulose, lignin decomposition is more complicated [47]. The creation of a more condensed solid structure and the shattering of relatively weak bonds result in the formation of char from lignin [48]. The β-O-4 lignin bond is broken and causes free radicals to be released. The protons emanating from other particles are captured by these free radicals, causing the production of degraded substances or compounds. Chain propagation is accomplished by free radicals moving to other molecules. Different amounts of lignin related to variable wood types bring about different breakdown rates. Coniferous lignin has been discovered to be more stable than deciduous lignin, and the former creates more char [49, 50].

#### **2.3 Gasification**

This is a thermochemical process that decomposes carbon-rich materials into gaseous products, including CO, CO2, CH4, H2, and traces of hydrocarbons; these gases are referred to as syngas [51, 52]. Gasification happens at high temperatures between 700 and 900°C in an environment with restricted oxidizing agents such as oxygen, air, nitrogen, steam, carbon dioxide, or a mixture of these gases. It was discovered that when the temperature rose, carbon monoxide and hydrogen production increased, while other components such as methane, carbon dioxide, and hydrocarbons declined [53]. The main product of this process is syngas (mostly hydrogen), while char is referred to as a by-product (or waste) with a lower yield, along with ash, tar, and some oil [51]. Partial oxidation of biomass, unlike combustion, takes the energy available in the biomass and bundles it into chemical bonds in the form of gaseous products. The intrinsic chemical energy of carbon in biomass is transformed into combustible fuel gases, which are more efficient and convenient to utilize than raw biomass [54]. Commercial use of the gasification technique has also been documented. Because of its lower Levelised emissions and higher volume of syngas, gasification outperforms other traditional techniques including pyrolysis, combustion, and fermentation. The O/C ratio is critical to achieving high gasification efficiency. High gasification efficiency is achieved by using biomass with a low O/C

ratio during gasification. Biomass can be reduced in its O/C ratio by the process of torrefaction. Before conventional gasification, torrefaction might be regarded as a pretreatment for better product quality. It is a low-temperature process between 200 and 300°C with a heating rate of roughly 50°C/min depending on the biomass composition and type [55, 56]. Pyrolysis and gasification are closely related processes. When gasification and pyrolysis are combined, there is no apparent separation between the two approaches [57, 58]. The little composition of oxygen used in gasification causes the biomass to undergo partial oxidation, changing the final product's characteristics. The product type is one of the most significant variations between pyrolysis and gasification. Gasification produces around 85% gaseous products, 10% solid char, and 5% liquid products [15, 58]. The schematic of the gasification process is shown in **Figure 3**.

The gasification mechanism can be sub-divided into many steps as follows [5]:

#### *2.3.1 Drying*

Biomass moisture is entirely removed from the material, and no energy is recovered in the process. Different types of biomass have varying moisture contents. When the biomass has a high moisture content, drying is used as a distinct step during gasification.

**Figure 3.** *Process diagram for gasification [54].*

#### *2.3.2 Pyrolysis*

The biomass is heated from 200 to 700°C with restricted oxygen or air during the pyrolysis process. The volatile components of the biomass are evaporated under these circumstances. The volatile vapor contains CO, CO2, CH4, H2, tar (heavier hydrocarbon) gases, and water vapor [59]. Tar and char are also formed [60].

#### *2.3.3 Oxidation/combustion*

The oxidation and combustion reactions of the gasification agents are the primary energy sources for the gasification process. These gasification agents react with the gasifier's combustible species to create CO2, CO, and water.

#### *2.3.4 Reduction*

The CO2 and H2O are produced when the oxygen provided to the gasifier combines with the combustible elements. Upon contact with the char formed by pyrolysis, some of this CO2 and H2O are converted to CO and H2 [60, 61]. Furthermore, the hydrogen in the biomass can be oxidized, resulting in the production of water. The reduction reactions that take place inside the gasifier are endothermic, and the energy necessary for them comes from the combustion of char and volatiles. Through a series of reactions, biomass reduction produces combustible gases such as hydrogen, carbon monoxide, and methane [62, 63].

#### *2.3.5 Cracking*

Furthermore, during the gasification process, the tar gases formed during the pyrolysis step are cracked, resulting in non-condensable gasses, light hydrocarbons, and unconverted tar [64]. The cracking stage follows more or less Eq. (2).

$$a\mathbf{C}\_{n}H\_{x} \to b\mathbf{C}\_{m}H\_{y} + CH\_{2} \tag{2}$$

Where *C*n*H*x is tar and *C*m*H*y is dehydrogenated hydrocarbons; a, b and c are mole ratios.

#### **3. Factors affecting the properties of biochar**

#### **3.1 Feedstock**

Biomass is a composite solid substance made up of organic, inorganic and biological material produced from living or non-living creatures/organisms. There are two main categories of biomass, namely Woody and Non-woody biomass. Woody biomass is mainly forestry and tree residue [1]. It is characterized by low moisture and ash content, high calorific and bulk density values, and low voidage; in contrast, Nonwoody biomass is made up of agricultural crop residue, animal waste, and municipal and industrial solid waste [1]. Non-woody biomass is characterized by high moisture and ash content, decreased calorific value, low bulk density, and increased voidage compared to woody biomass [1]. The moisture content of the biomass has been shown to have a significant effect on the physicochemical characteristics of the derived biochar [2]. A study conducted by [3] comparing the pyrolytic charcoals produced from hard and softwood bark samples reported a direct correlation between initial sample moisture content and the surface chemistry derived charcoal; the study found that a decrease in the moisture content of maple bark resulted in charcoal surface becoming more graphite-like and polyaromatic attributed to prolonged pyrolysis time. The effect of feedstock lignin and cellulose content on biochar formation is a well-researched area [4]. Lignin is an amorphous, high molecular weight polymer that is hydrophobic in nature and has several aromatic functional groups in comparison; cellulose and hemicelluloses are made up of simple sugar monomers that disintegrate at temperatures below 450 degrees Celsius [5]. Studies conducted by Tripathi et al. 2016 and Yu et al. 2014 [2, 6] showed that the cellulose content of feedstock aided the formation of tar (which comprises aldehydes, organic liquids, ketones, and char); while a high lignin concentration is beneficial to the formation of char during pyrolysis. According to Demirba (2004) [7], high feedstocks lignin content will increase char formation. It has been shown that increased lignin content in plant biomass promotes carbonization and increases biochar carbon and ash content [8, 9].

#### **3.2 Residence time**

Residence (pyrolysis time) has been shown to affect the degree of carbonization and biochar yield of feedstock; this effect is particularly pronounced at low temperatures [18]. According to Zornoza et al. (2016), increased residence time during pyrolysis results in a higher degree of carbonization, reducing the liable organic matter mitigation the vulnerability of the biochar to microbial attack [19]. Residence time has also been shown to influence the specific surface area of biochar produced. A study conducted by Wang et al. (2019) found that the surface area of biochar's derived from the co-pyrolysis of sewage sludge and cotton stalks increased as residence time increased from 30 minutes to 90 minutes [20]. This was attributed to the formation and extension of pore structures of the biochar caused by the increased thermal decomposition of organic matter and volatiles released from etching pores during the increased residence time [21]. The same study noted a decrease in the surface area of the biochar's as the residence time was increased from 90 minutes to 150 minutes; this reduction was accounted for by the collapse of the pore structure of the biochar during the extended residence time [20]. Residence time has also been shown to affect the calorific value of the biochar produced; a study conducted by Ahmad et al. (2020) on coconut shell derived biochar showed an increase in calorific value from 25.99 MJ/kg to 29.54 MJ/kg as residence time increased for 45 minutes to 75 minutes [22].

#### **3.3 Biomass pretreatment**

The pre-treatment of biomass before the pyrolysis has been shown to influence biochar characteristics. Pre-treatment is primarily divided into four categories: physical, physiochemical/thermal, chemical, and biological. Physical pre-treatment describes methods (milling, grinding etc.) that use mechanical energy to alter biomass properties. The most common form of physical pre-treatment is particle size reduction via mechanical comminutions. The effect of particle size reduction and fractionation of ash content is well researched. A study conducted by Liu et al. showed that the ash content of switchgrass and pine bark varied considerably

#### *Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

with particle size fractions [22]. The study also reported the potential 20% removal of inorganic constituents from switchgrass and a 30% removal of inorganic constituents from raw pine bark. A similar study conducted by Bridgeman et al. found that the ash content of switchgrass and reed canary greatly increased in fines with particle sizes smaller than 90 micrometers, increasing to 3.62 wt. % to 6.0 wt. % for reed canary grass and 3.12 wt. % to 6.88 wt. % (dry basis) for switchgrass [23]. Besides the ash content, feedstock particle size is also correlated to biochar particle size, with finer feedstocks producing finer biochar particle sizes [18]. Studies have found that biochar's derived from finer feedstocks exhibit lower nitrogen content as well as increased surface area, electrical conductivity, and pH [24, 25]. A study conducted by Sun et al. (2012) evaluating the properties of fine apple wood and corn stover-derived biochar (feedstock = 0.25 mm) reported a higher surface area when compared to applewood or corn stover-derived biochar stover-derived biochar of feedstock particle size = 1.5 mm [27]. Thermal pre-treatment describes methods that make use of thermal energy to produce changes in biomass properties; the most common forms of thermal pre-treatment are steam explosion, HTC and hot water extraction. Steam explosion involves the subjection of biomass to high temperatures and pressures between (160-260°C) and (0.69–4.83 MPa); the biomass subsequently undergoes sudden decompression scattering the fiber material and breaking the covalent bonds between the hemicellulose and lignin [28, 29]. Steam explosion increases the lignin content of the biomass by facilitating the depolymerisation of lignin into lower molecular weight molecules, which then condense with other degradation products [30]. A study conducted by Chen et al. 2017 [46] evaluating the effect of the steam explosion of crop straws before pyrolysis reported a change in the surface structure of the derived biochar; exhibiting a rougher surface when compared to the smoother, clearer and distinct pore structure of the untreated crop straw [31]. The same study also showed an approximate increase in the specific surface area of oil-rape straw-derived biochar 16 times greater than that on the untreated sample.

#### **4. Biochar characterization**

Properties of biochar produced depend on the composition, type of biomass and the conditions under which it is carbonized. Both physical and chemical characterizations are necessary when identifying the basic properties of biochar and predicting the various application uses. Biochar serves as a promising alternative to its surface area, charged surfaces and functional groups. **Figure 4** below displays the different physical and chemical methods used for biochar characterization, focusing on BET and FTIR, belonging to the chemical characterization and SEM as physical characterization.

The main aim of quantification to distinguish biochar from organic matter and other forms of black carbon produced. Majority of the potential technology is dependent on spectroscopic characteristics rather than physical separation or isolation.

Biochar being produced from a range of biomass that has different chemical and physical properties results in materials of different properties. Properties of each biomass are important during thermal conversion processes, proximate analysis (ash and moisture content); calorific value; fractions of fixed carbon; volatile components; fractions of lignin, cellulose and hemicellulose; inorganic substances; true density; particle size and moisture content.

**Figure 4.** *Overview of a proposed characterization techniques for biochar [65].*

#### **4.1 Porosity and surface area**

Chemical composition of biomass feedstock and biomass is subjected to a range of analyses to achieve the basic physicochemical characteristics of each raw material. **Figure 5** displays the physiochemical characteristics of biochar. Biochar production is often assessed through changes in the elemental concentrations of C, H, O, S and N and the associated ratios. The fixed carbon is the solid residue that remains after the particle size is carbonized and the volatile matter is expelled. The H/C and O/C ratios are used to determine the degree of aromaticity and maturation. Elemental ratios of O/C, O/H and C/H have been used to provide a reliable measure of the extent of pyrolysis and the level of oxidative adjustment of the biochar. Irrespective of the pyrolytic temperature, the BET areas increased with an increase in carbon burn off, indicating that the carbon burns off had a significant role in increasing pore volume and surface area while the average pore size increased with residence time and pyrolytic temperature. The BET surface area of biochar value of (1057 m2 .g − 1) has been reported, which appears slightly higher than that of activated carbon (970m2 .g−1). Biochar micropore volume of (0.24 mL .g−1) also appeared smaller than that of activated carbon, having a value of (0.32 mL .g−1), however having an average pore diameter of (5.2 nm).

#### **4.2 Scanning electron microscope (SEM)**

Scanning electron microscopy is categorized as a physical characterization technique used to determine the samples macroporosity and the physical morphology of solid substance (**Figure 6**). A study by Amin 2016 [1] approximated that the biochar produced from cellulose plant materials had a pore diameter of 1 m. This characteristic is highly dependable in the intrinsic architecture of the feedstock use.

SEM micrographs displayed that the biochar produced at different pyrolytic temperatures has a distinguishable and clear honeycomb structural appearance due to the original tubular structures present in plant cell materials (**Figure 6**). The welldeveloped pores have a direct impact on the high surface area. According to Cantrell et al. (2012), biochar produced at lower temperatures is appropriate for regulating

*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

#### **Figure 5.**

*Fourier-transform infrared spectra (FTIR) of the biochar samples [66].*

**Figure 6.** *SEM micrograph of biochar with magnification of 500x [67].*

fertilizer nutrients and absorbing pollutants from the soil. Higher temperatures lead to material analogous to activated carbon and environmental remediation. SEM micrographs of biochar displayed a clean surface as the pyrolysis process had stabilized the volatile hydrocarbons, therefore smoothening the surface of the biochar. Pyrolysis at lower temperatures displays molded structures with small pores and uneven surface structure. In general, it is safe to say that since the biomass wastes contain lignin and high volatile matter content, the pore creation in biochar is directly affected.

#### **4.3 Fourier transform infrared spectroscopy (FTIR)**

FTIR spectroscopy serves as a great tool to observe the shift change of chemical compositions. The commonly used technique for biochar characterization using the FTIR is the pellet technique, which mixes 1 mg of dried biochar with 300 mg of pre-dried and pulverized spectroscopic grade KBr. Novak ae al. (2012) used the pellet technique to conclude 3400to 3410 cm−1, H-bonded O–H stretching vibrations of hydroxyl groups from alcohols, phenols, and organic acids, 2850 to 2950 cm−1, C–H stretching of alkyl structures; 1620–1650 cm−1, aromatic and olefinic CDC vibrations, CDO in amide (I), ketone, and quinone groups; 1580 to 1590 cm−1, COO- asymmetric stretching; 1460 cm−1, C–H deformation of CH3 group; 1280–1270 cm−1, O–H stretching of phenolic compounds; and three bands around 460, 800, and 1000–1100 cm−1, bending of Si–O stretching [68]. **Figure 5** illustrates the FTIR spectra of biochar collected during different stages of the production, i.e. (Biochar: Original, −1: pre-incubation, −2: jointing, −3: Heading; −4: Mature).

#### **5. Applications of biochar and future perspective**

Biochar is a product (together with bio-oil and gases) resulting from biomass pyrolysis. Biochar usage has increased because it reduces the negative impacts of biomass on the environment [69]. The physicochemical properties of biochar are what govern the applications of this material. Depending on the feedstock type, production technology and process conditions [70]; the quality, yield and toxicity of the resulting biochar differs (as shown in **Table 1**) [72, 73]. These applications (including potential applications) range from adsorption for water and air pollutants [74], activated carbon [75], anaerobic digestion promoter/catalyst [76], construction material [77], agriculture and horticulture use such as soil conditioning, compost additive [78], carbon sequestration, etc. [73]. **Figure 7** demonstrates these applications and how biochar contributes to the circular economy through its uses in agriculture and horticulture. Also, these numerous biochar benefits show a great potential to contribute to the economic sustainability of emerging cellulosic bioenergy production systems [79, 80]. It is worth noting that as the number of applications of biochar increases, so does the number of manufacturers, leading to a need for regulated standards and guidelines for the production of this material (see **Table 2**) [81, 82].

#### **5.1 Biochar in agriculture and horticulture**

Biochar application in agriculture and horticulture has been explored both on a laboratory scale and in the field. These applications include being used as a component of chemical fertilizer [83], soil microbial activity, soil amendment for crop productivity improvement through nutrient availability [84, 85] as well as water holding capacity [86]. Biochar has also been reported to alleviate heavy metals release in the soil while having a limiting effect that aids in increasing the pH of highly acidic soils [87, 88]. Though biochar is another soil conditioner type, it differs from compost by production pathways. Biochar is produced by thermal decomposition of food, horticultural and municipal solid waste in the absence of oxygen, while natural biodegradation of organic substrates produces compost by the microbial community under aerobic conditions. Another difference is that; compost degrades fast, making its benefits relatively shortlived compared to biochar which persists in the soil for more prolonged periods [78, 89].

*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*


**Table 1.** *List of notable chemical characterisations of biochar.*

**Figure 7.**

*Biochar uses in agriculture and horticulture and its contribution to the circular economy [78].*

#### *5.1.1 Biochar as a compost additive*

Low soil organic carbon and fertility are challenges faced by many agricultural farmers around the globe. Biochar offers a solution to this challenge because it gives two options, i.e. returning nutrients and carbon to the soil while producing energy [90]. Also, the compositing rate can be increased by using biochar as an additive.



*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

Zhang and Sun [91] have examined spent mushroom compost and biochar cocomposting. Their results showed a great increase in nutrients content of the resultant compost product and an improved composed quality while reducing the composting time from 90 to 270 days to only 24 days. Also, the large porosity of biochar enables it to facilitate microbial growth in the compost pile, leading to accelerated nutrient recycling [92]. The addition of biochar to poultry manure has been found to increase the maximum temperature reached and shorten the thermophilic phase [93].

#### *5.1.2 Biochar as an adsorbent*

An issue of heavy metals/metalloids (HMS) and polycyclic aromatic hydrocarbons (PAHs) in soil and water poses detrimental environmental problems and poor quality of agriculture, affecting all forms of life [94, 95]. These pollutants are toxic, persistent, non-biodegradable and potentially bioaccumulate [96]. Among other bioremediation technologies used to solve the HMS and PAHs issue, biochar is one of the best solutions due to its advantages [97]. These advantages include sustainability, low costs, sequestration of carbon, etc. [94]. Various physical and chemical characteristics of biochar, such as pore structure, specific surface area and functional groups, have been used to adsorption different pollutants [98]. For instance, Mahmoud, et al. [99] have used modified Switchgrass biochar for efficient decolorization of reactive red 195 A dye from aqueous and wastewater samples. Other biomass materials such as rice husks and dairy manure have also been used for biochar production with varying adsorption capacities according to the biomass used upon other factors [100].

#### **5.2 Biochar in construction**

Biochar has been used in road construction and as a concrete admixture. Wang, et al. [77] assessed this where a novel production of fill material and pedestrian/vehicle paving blocks were done. In this study, biochar addition was found to be beneficial to cement hydration even though it was noticed that the studied particle sizes could incur microcracks and strength degradation. Also, biochar's incorporation resulted in enhanced immobilization of potentially organic contaminants and toxic elements in the sediment product, which is significant for moderately to heavily contaminated products. Therefore, biochar from wood can be used as a green combination for cement-based recycling procedures for highly contaminated waste. The use of biochar in construction material to trap atmospheric carbon dioxide in buildings also offers the potential to reduce greenhouse gasses by 25%. High pH and high water retention rate of biochar enable it to absorb some of the mixing water used in concrete mixing, thereby reducing the amount of free water in the concrete [101].

#### **5.3 Future perspective**

Since biochar's applications depend greatly on its properties, future research must elucidate the production process effects on biochar's properties. Biochar used in water treatment would differ from the one used in energy/agriculture. Likewise, there are diverse literature findings on the effects of biochar on agriculture, particularly on crop production caused by soils being different. For instance, crop yields may be increased or decreased by adding biochar depending on the soil type and fertilizer management [90, 102]. Also, the chemical behavior of biochar with heavy metal ions has been found to be inconsistent [103]. It is apparent that the interaction mechanisms between biochar, soil and plants are critical and yet not thoroughly known. Therefore, more efforts are still needed concerning biochar properties to soil and crop responses equally in the field and climate-controlled environment.

#### **6. Conclusion**

Biochar has been applied to remediate contaminated agricultural soil and improve soil fertility by reducing acidity and increasing the availability of nutrients. Thus, the addition of biochar to soils can be one of the best practices to overcome any biotic stress in soil and increase crop productivity, mainly in the agricultural sector. The properties of biochar have significantly been influenced by processes such as pyrolyscould, which have been discussed in this chapter. Thus, biochar appears as a highly promising option for pollutant removal. Economic impacts and recyclability should be considered in developing recoverable biochar for wide environmental applications. The relationship between various solutions for waste management and energy production differs in parameters and multiple techniques for its production and economic, social and ecological constraints. This review paper detailed the state-of-art information that would be helpful to find new opportunities in scientific innovation in the field of biochar research.

#### **Acknowledgements**

The authors are thankful to the Green Engineering and Sustainability research group in the Department of Chemical engineering at the Durban University of Technology, South Africa.

### **Conflict of interest**

The authors declare no conflict of interest.

*Biochar: Production, Application and the Future DOI: http://dx.doi.org/10.5772/intechopen.105070*

#### **Author details**

Edward Kwaku Armah1,2\*, Maggie Chetty1 , Jeremiah Adebisi Adedeji3 , Denzil Erwin Estrice1 , Boldwin Mutsvene1 , Nikita Singh1 and Zikhona Tshemese4

1 Faculty of Engineering and the Built Environment, Department of Chemical Engineering, Durban University of Technology, Steve Biko Campus, Green Engineering and Sustainability Research Group, Durban, South Africa

2 Department of Applied Chemistry, C.K. Tedam University of Technology and Applied Sciences, School of Chemical and Biochemical Sciences, Navrongo, Ghana

3 University of KwaZulu-Natal, School of Engineering, Discipline of Chemical Engineering, Howard College Campus, Durban, South Africa

4 Faculty of Applied Sciences, Department of Chemistry, Durban University of Technology, Steve Biko Campus, Durban, South Africa

\*Address all correspondence to: ekarmah@cktutas.edu.gh

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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#### **Chapter 3**

## Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth

*Minister Obonukut, Sunday Alabi and Alexander Jock*

#### **Abstract**

Waste is unwanted material left after useful parts have been removed and found to affect our environment and health adversely. Waste from agro-allied industries is massive and claims most land, which would have been used for agricultural purposes when used as a landfill, including other environmental and health issues. This chapter will assess wastes generated during the processing of cassava for variety of products and review their properties when characterized. In the course of characterizing the wastes, which emerged during processing, pre-processing, and post-processing depending on the products, various reports on the physical, chemical, and biological properties of cassava wastes will be presented. The properties of cassava waste when subjected to biochemical and thermochemical processes will be compared with those of conventional raw materials for biochar production. This chapter will showcase the potential of cassava wastes for efficient valorization, especially as adsorbents *via* biochar. It will be of great significance to engineers, farmers, and manufacturers in their quest to manage cassava wastes for the betterment of our environment and health.

**Keywords:** cassava waste, biochar, characterization, biochemical, thermochemical

#### **1. Introduction**

Manihot esculenta Crants (Manihot utilissima phol), commonly known as cassava, tapioca, mandioca, and manioc, is regarded as the bread of the tropics as it is mainly grown and consumed in the tropical world [1, 2]. Its primary attraction is its tuberous root, which serves as one of the highest yielding starchy staples [3]. The root has been processed into varieties of food, including garri and fufu, among others. Specifically, Abiagom [4] reported that 15% of cassava was consumed as fresh roots; 5% as garri; 10% as starch; and 10% as flour and others. Similarly, other parts of the crop are equally valuable. The stem of the cassava plant is mainly exploited for propagation, while the leaves—found to be nutritious—are equally consumed.

Besides domestic consumption, cassava has recently been processed into many products of high demand, including ethanol, glucose, starch, animal feed, baking flour, pulp, and paper [5, 6]. Its application beyond consumption has increased as cassava is presently regarded as an industrial/cash crop [7]. The economic importance of cassava transcends the tropics as a staple food to a global industrial raw material for the production of myriad products [8]. Despite being exploited in a variety of ways for domestic consumption and industrial products, waste generation is inevitable and this occurs at almost every stage of the production process [9]. However, the benefits of cassava wastes are yet to be assessed as they are generally discarded and disposed of due to their toxicity [6].

The adverse impact of cassava wastes on the environment and health has been one of the challenges confronting the tropics. Acknowledging cassava waste as a major source of pollution in the cassava processing areas, Okunade and Adakalu [10] reported that cassava waste has deleterious effects on the receiving soil and water source as well as the adjourning environment. Research shows that as far as cassava processing is concerned, waste would be inevitably generated [9]. Since human life generally revolves around the activities that result in the production of cassava wastes, it is necessary to examine previous research works on the characterization of cassava waste and they have been chronicled in one piece for accessibility and posterity. Due to the toxicity of the waste [11] and its adverse impact on the environment, wastes generated from cassava processing need to be handled bereft of levity.

This chapter presents the prospect and challenges confronting cassava processing, especially in Nigeria: the world's largest producer of cassava (Section 2). Waste is inevitable in most industrial processes and cassava processing is not an exemption. The estimated quantity of wastes generated during cassava processing as well as the nature of these wastes is considered next (Section 3). This review is necessary for better management, as the availability of the waste for valorization is paramount. Furthermore, a review on the characteristics of the wastes constituting Section 4 of this chapter showcased the biochemical and physicochemical properties of cassava processing wastes. The concluding section (Section 5) will be on biochar production using cassava waste and will focus on process parameters and the choice of feedstock. This information is a useful guide in our quest to manage cassava wastes for the betterment of our environment and health as well as reveals the potential of these wastes for efficient valorization.

#### **2. Prospect and challenges of cassava processing in Nigeria**

This section presents the prospect as well as challenges of processing cassava in Nigeria. Nigeria is the largest producer of cassava in the world for decades, and has a robust cassava industry with prospects and challenges. It is expected that other cassava-producing countries, such as Brazil and Thailand, among others, may have similar challenges. The progress in cassava production in Nigeria is reported next.

#### **2.1 Progress in cassava production in Nigeria**

Cassava has been identified as one of the most cost-effective and nutritionally vital native African tubers [12–14]. Its origin is traced back to South America, and the crop made its trans-Atlantics journey close to the beginning of the slave trade in the sixteenth century into Nigeria [15]. Cassava is a recurrent, vegetative bred shrub, and is cultivated through the plain tropics [16]. It is a dearth resilient crop grown mostly in temperate areas and adds appreciably to the nourishment and livelihood of many countries, including Nigeria. Cassava is one of the major staple foods in Nigeria, and its cultivation is a priority in almost every household, especially in the Southern part

#### *Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*

of the country [8, 15]. Hence, it is the most extensively farmed crop in Nigeria and it is largely cultivated by small-scale farmers that depend on seasonal rainfall [17].

Presently, cassava has been changed from a low-yielding dearth spare crop for consumption to a high-producing cash crop, with its many different uses in livestock feeds, source of raw materials for agro-industry, and beyond [18, 19]. Over half a billion people around the world depend on cassava as a major food source. It is the third largest source of calories after rice and corn [5, 6, 20]. The crop is known to thrive well on any soil and its ability to grow well in poor soils and withstand drought makes it an ideal crop to cultivate in places where other crops struggle [21, 22].

Fortunately, Nigerian soil is fertile and the crop thrives very well making Nigeria the world's largest producer of cassava for about a decade. The country went from harvesting 36 million tons of cassava in 2003 to 53 million in 2013, a 47% increase [15]. Growth was driven by a substantial increase in yields—which jumped by 44% over this period as Nigeria overtakes Brazil as the top producer of cassava. This increase is attributed to several interventions and initiatives, including the Institute of Tropical Agriculture (IITA), the presidential initiative on cassava, etc., by the Nigerian governments at all levels. It was reported that through these programs large hectares of land were dedicated to cassava cultivation in order to boost the production and exports of processed cassava products [23, 24]. With 59.5 million tons of cassava produced in 2019 (**Figure 1**), the country maintained its top spot in global cassava production since it outpaced Brazil in total production output in 1991 [26, 27].

The plant as a whole is useful as its roots and leaves are consumed, while the stems are mainly exploited for propagation. About 70% of Nigeria's cassava is processed into garri—a granular flour that is used mainly to make porridges and fufu—a type of mash [22]. Specifically, the roots are processed by several methods to form products that are used in diverse ways according to local preferences [28, 29]. However, when left unprocessed, the cassava roots perish quickly, often spoiling within 48 hours [30]. Processing offers not just the ability to produce higher-value, exportable cassavaderived items like garri and essential items, such as glucose, starch, and flour, but also to preserve the root (**Figure 2**).

Meanwhile, Thailand, the second-largest producer of cassava, considered the crop more of a cash crop than a staple food with the vast majority of the root being processed and exported [31–33]. Currently, Thailand is the world's leading country in the exportation of cassava-based products (**Figure 3**). It is reported that in Thailand, about 90% of all cassava is processed and exported [22]. Thailand accounted for approximately 76% of global trade and 84% of its cassava exports were sent to China,

**Figure 1.**

*Top cassava-producing countries (in million tons). Data are adapted from PwC [25].*

### **Figure 2.**

*Potentials of cassava root. Data are adapted from PwC [25].*

#### **Figure 3.**

*Top cassava exporting countries (in million tons). Data are adapted from PwC [25].*

where the products are largely used for ethanol production [34]. Specifically, over 6.4 million tons of cassava-based products were exported from Thailand accounting for about 50% of the total volume of exported products worldwide generating huge revenue for the country [22, 27].

#### **2.2 Challenges of cassava processing in Nigeria**

In Nigeria, about 80% of the cassava is processed into garri for local consumption and exportation [22]. Specifically, processing cassava into garri is relatively simple—it requires only that cassava roots are peeled, grated, and sieved, and then placed in a porous bag from which excess water can be squeezed out (**Figure 4**). The resulting

*Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*

**Figure 4.** *Cassava processing facility in Nigeria.*

dry flour can be stored for several months [29]. Approximately 70% of cassava processing occurs at small- and medium-size centers near villages.

In 2012, it was reported that there were 75,000 total small- and mediumprocessing centers that employed roughly 3 million people—most of which were small-scale farmers, and generated less than 5 tons of high-quality cassava flour per day [25]. However, medium- and large-scale processors struggle to stay afloat due to high transportation costs, mainly due to the poor condition of rural Nigerian roads [16]. The challenge of limited access to cassava processing facilities has not only hindered efficient large-scale processors, but also extends to storage facilities as unreliable transportation compounds the problem presented by the crop's perishability. Consequently, post-harvest losses for cassava are high as estimated by Deutsche Gesellschaft für Internationale Zusammenarbeit (GIZ) and found to be more than \$600 million annually in Nigeria alone [22, 35].

The challenges confronting Nigeria's cassava processing capacity are responsible for the country's poor role in international cassava trade as large-quantity cassava needs to be processed in order to be exported. Thailand's position as the global leader in cassava processing and exportation is a clear indication that Nigeria—as the largest producer—is not doing well in the area of processing. Nigeria's emergence in the international cassava market coupled with achieving self-sufficiency in cassava-based products, such as starch, glucose, and flour production, is not out of reach. The country has already proven success in improving raw cassava production through its increase in yields and needs to extend these successes throughout the value chain. The economic potential of cassava is huge as seen in **Figure 5**.

Specifically, the country has the huge economic potential to generate USD 427.3 million from domestic value addition and derive an income of USD 2.98 billion in the exportation of cassava-based products [25, 36]. According to PwC [25], the local addition to cassava *via* local manufacturing and processing could potentially unlock about USD 16 million as revenue for Nigeria.

Realizing the incredible amount of untapped potential that lies waiting in this sector, Nigerian government has waded in to address the challenges confronting cassava processing. In an attempt to support the cassava processing industry, the government launched the Cassava Transformation Agenda in 2011 [22]. The initiative is working to expand the cassava value chain and Nigeria is making millions from cassava production and export. It was reported that Nigeria exported 509 tons of cassava products, half of which went to China, the world's top importer of the product in 2019 [8, 36]. Several medium- and large-scale cassava processing facilities are set up

#### *Biochar - Productive Technologies, Properties and Applications*

#### **Figure 5.**

*Current demand for cassava-based products in Nigeria. Data are adapted from PwC [25].*

#### **Figure 6.**

*Cassava processing waste (cassava peel).*

on a daily basis and processing clusters are domiciled in every community as seen in the Ojapata processing cluster, Kogi state, and Nigeria [8, 37].

Although the commercialization of cassava processing and subsequent exportation and expansion of its value chain generate huge revenue and job opportunities for Nigerians, this development is welcomed with mixed feelings due to poor waste management (**Figure 6**) and its adverse impact on the environment [38]. The cassava processing environment (**Figure 4**) is heavily polluted in all ramifications (air, water, and land) and the impact of environmental pollution attributed to cassava wastes is significant, especially with commercial processing of the cassava.

#### **3. Cassava processing and quantity of wastes generated**

Processing cassava into varieties of products inevitably result in the generation of wastes. Basically, according to Ubalua [37] and Zhang et al. [38], these wastes are categorized into (i) peels prior to crushing, (ii) sieved fibrous residue after crushing, (iii) bagasse and settling starch, and (iv) wastewater effluent. In view of this, cassava

#### *Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*

processing facilities continuously generate waste as the demands for cassava-based products soar. Chunk amount of cassava processing wastes and residues generated has been reported to be one of the major environmental threats, especially in rural regions of developing countries [39, 40]. These regions are mainly dominated by small-scale processing facilities managed mainly by rural dwellers bereft of standardized waste treatments and disposal strategies. The informed operators of these facilities considered the cost of treatment and disposal of these wastes a huge financial burden.

Zhang et al. [38] estimated that the processing of fresh cassava roots generates liquid waste between 8.85 and 10.62 MT per MT of fresh cassava processed, containing approximately 1% total solids (TS). In the case of dry cassava processed, the authors reported that between 0.93 and 1.12 MT of wet cassava bagasse and peels are produced per metric ton of dry cassava processed.

A breakdown of wastes generated from fresh cassava root during the production of high-quality cassava flour (HQCF), starch, garri, and fufu are presented in **Figures 7**–**10** respectively. Specifically, during the production of 150–200 kg HQCF from 1 MT of fresh cassava roots, 550–700 kg of wastes was generated constituting peels, fibrous waste, sifting juice, and wastewater (**Figure 7**).

A recent report stated that for every ton of cassava processed, 10–15% constituting 125 kg/tons are lost in form of wet peels, which are poorly utilized, dumped as waste, or burnt [42, 43]. These methods of disposing cassava wastes though easy and cheap

**Figure 7.** *Flow sheet of high-quality cassava flour production process. Data are adapted from Sanni and Jaji [7]; FAO [41].*

**Figure 8.**

*Flow sheet of the starch production process and waste generated. Data are adapted from FAO [41].*

**Figure 9.**

*Flow sheet of garri production process and waste generated. Data are adapted from Sanni and Jaji [7]; FAO [41].*

*Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*

#### **Figure 10.**

*Flow sheet of fufu production process and waste generated. Data are adapted from Sanni and Jaji [7]; FAO [41].*

are not economically viable in terms of lands claimed and environmental-friendly due to pollution.

Starch is one of the cassava-based products with high demand in several industries, including laundry, and the flow sheet for its production process is presented in **Figure 8**. FAO [41] reported that 1 MT of fresh cassava roots when processed can produce between 180 and 200 kg starch with about 680 kg of waste generated.

Currently, garri (cassava flake) is the most preferred cassava-based product widely recognized as a staple food in the tropics. **Figure 9** presents the flow sheet of garri production process. However, it is reported that of the 200–240 kg of garri produced from 1 MT of fresh cassava roots, and 500–600 kg of waste was generated [7, 41].

Fufu is another cassava-based product known mostly in southern Nigeria. In terms of consumer preference, fufu was the most preferred in the early 1970s with more than 60% of cassava exploited for its production [21]. However, by the early 1980s, the consumption of fufu had declined to 14% of all cassava eaten, while consumption of garri rose to 65% according to a national consumption survey by the Federal Office of Statistics (FOS) [44]. It is considered that the consumer preference for fufu had reduced due to its inherent undesirable characteristics of poor odor, short shelf life,

and tedious preparation [27, 29]. The flow sheet for the production of fufu is presented in **Figure 10**. It has been found that 1 MT of fresh cassava roots produced between 280 and 300 kg of fufu generating between 80 and 130 kg of waste [41].

#### **4. Characteristics of cassava wastes**

The properties of wastes generated from cassava processing constituting the peels prior to crushing, the sieved fibrous residue after crushing, the bagasse and settling starch, and the wastewater effluent are presented in this section. The physicochemical properties of the wastes are critical as they reveal the way in which the wastes interact with other substances physically and chemically when discharged. Section 4.1 presented the physicochemical characteristics of these wastes. The thermochemical properties of the wastes are presented in Section 4.2. The biochemical properties of the wastes are equally useful as the wastes when discharged are expected to interact with the fauna and flora content of the medium as well as the environment. This is discussed in Section 4.3.

#### **4.1 Physicochemical characteristics of cassava wastes**

These are the intrinsic physical and chemical characteristics of cassava wastes. These include appearance, boiling point, density, toxicity, volatility, water solubility, and flammability,. In the case of cassava wastes, several studies have been conducted by various researchers, and the outcome of some of the works is presented in this section. Zhang et al. [38] reported that cassava starch wastes (wastewater and solid waste) are weakly acidic liquids with high nitrogen and phosphorus contents of about 1300 and 780 mg/L, respectively. In addition, this category of cassava waste contains between 9.6 and 37.5 g/L of total carbohydrates and 2.3 total proteins. **Table 1**


#### **Table 1.**

*The physiochemical characteristics of cassava wastes.*

#### *Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*

presents the physicochemical properties of cassava starch wastes as well as their composition.

In the case of cassava bagasse, Zhang et al. [38] further reported that a typical solid residue of cassava processing contains between 40.1 and 75.1% starch (dry weight) and between 14.9 and 50.6% fiber. **Table 2** shows the composition of the cassava residue.

Wastewater is inevitably generated during cassava processing either as a byproduct of the initial production process or arises when the cassava tubers are indiscriminately discharged to a nearby water body. Okunade and Adekalu [10] reported on the organic components of cassava wastewater (**Table 3**) as they


#### **Table 2.**

*Cassava residue composition % by dry weight.*


#### **Table 3.**

*Concentration of organic compounds in cassava wastewater.*

found that cassava wastewater, which is five times denser than water contains alcohols, acids, and others (3-penten-2-ol, 1-butanol, 3-hexanol, octadecanoic acid, oleic acid, n-hexadecanoic acid, acetoin, dibutyl phthalate, squalene, and bis (2-ethylhexyl) phthalate). The rust-removing properties of the wastewater from metallic substances, such as nails, are attributed to the presence of these organic compounds [10].

In the related development, Aripin et al. [45] have recognized that without proper waste management, and the organic wastes like cassava peels could result in an increased amount of solid waste dumped into landfills. This eventually leads to less soil available for agricultural purposes as most of these soils are rendered infertile, in an attempt to utilize these organic wastes as pulp for paper-making industries and to promote the concept of "from waste to wealth and recyclable material." The authors exploited Kurscher-Hoffner and Chlorite methods to determine the chemical properties of the wastes in accordance with the relevant Technical Association of the Pulp and Paper Industry (TAPPI) test. It was found that the cassava waste was rich in holocellulose, cellulose, hemicellulose, lignin, and ash content with 1% of sodium hydroxide and hot water solubility (**Table 4**).

In order to determine the suitability of cassava peel as an alternative fiber resource in pulp and paper making, its properties were compared to other published literature, especially from wood sources. Aripin et al. [45] reported that the amount of holocellulose contents in cassava peels (66%) is within the limit suitability to produce paper although it is the least when compared with that of the wood (70–80.5%) and canola straw (77.5%). Similarly, the lignin content (7.52%) is the lowest than those of all wood species (19.9–26.22%). However, the morphological properties of the cassava peel are promising as the authors went further to subject the peels to scanning electron microscopy (SEM) under different magnifications (**Figures 11** and **12**).

Aripin et al. [45] observed that under different levels of magnifications (27 and 300), there exist differences in fiber morphology of the peels (**Figures 11** and **12**). The surface morphology of cassava peels depicted in **Figure 11** shows that it is dominated by a low (micro) pore size structure, while that of **Figure 12** differs. The differences in fiber morphology of the peel indicate variation in the major character of the fiber's physical structure. This variation had been reported to be attributed to differences in the physical properties of the cassava peels [46]. The porous structure of the peel can be exploited for several industrial applications.


#### **Table 4.**

*Chemical composition of oven-dried cassava peels.*

*Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*

#### **Figure 11.**

*Surface morphology (SEM) of cassava peel at 27. Data are adapted from Aripin et al. [45].*

**Figure 12.**

*Surface morphology (SEM) of cassava peel 300. Data are adapted from Aripin et al. [45].*

#### **4.2 Thermochemical characteristics of cassava wastes**

The thermochemical properties of agricultural wastes have been a subject of research interest recently. One of these was reported by Pattiya [47] on cassava wastes. The study includes proximate, ultimate, structural, inorganic matter, heating value, and thermogravimetric analyses. Cassava waste was found as shown in **Table 5** to have high volatile contents (78–80%, dry basis) and contains 51% carbon, 7% hydrogen, 41% oxygen, 0.7–1.3% nitrogen, and <0.1% sulfur. Structural analysis reveals that cassava residues are composed of about 36% cellulose, 44% hemicellulose, and 24% lignin. The main inorganic elements found are potassium, phosphorus, and calcium. The lower heating values (LHV) of the biomass are approximately 18 MJ kg<sup>1</sup> .

Similarly, Aro et al. [48] carried out a similar study on cassava tuber wastes (CTW) produced by a cassava starch-processing factory in the Ondo State of Nigeria. They investigated the properties of five different types of CTW wastes: cassava starch


#### **Table 5.**

*Proximate, ultimate, structural, inorganic matter, heating value, and thermo-gravimetric analyses of cassava wastes.*


*\*N.F.E. = nitrogen free extractives, CAP = cassava peels, CAE = cassava effluent, CAW = cassava whey, CSR = cassava starch residues, CAS = cassava stumps, ND = not detected, and D.M. = dry matter. Data are adapted from Aro et al. [48].*

#### **Table 6.**

*Proximate composition (g/100g D.M.) of different types of fresh cassava tuber wastes (CTW) collected from the factory.*

residues (CSR) or pomace cassava peels (CAP), cassava effluent (CAE), cassava stumps (CAS), and cassava whey (CAW). The proximate composition of samples collected in respect of these five types of wastes (**Tables 6**–**8**) showed that moisture was the highest in CAW (96.7%) and the lowest in CAS (64.1%). Crude fiber was highest in CAP (29.6%) but was not detected in the whey (CAW). The CAS had the highest content of fat (5.35%), while it was not detected in CAW. Protein was the highest in CAP (4.20%) and lowest in CSR (1.12%). Ash content was the highest in CAP (7.47%) and lowest in CAW (1.88%). The nitrogen-free extractives (NFE) were the highest in CAW (95.7%) and lowest in CAP (55.5%).

The proximate analysis of cassava waste conducted by Janz and Uluwaduge [49] is presented in **Table 9**, while that of Obadina et al. [50] on cassava peel is presented in **Table 10**.

#### *Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*


#### **Table 7.**

*Mineral composition (mg/kg D.M.) of fresh cassava tuber wastes (CTWs) collected from the factory site.*


*Data are adapted from Aro et al. [48].*

#### **Table 8.**

*Antinutrient composition of cassava wastes (dry matter basis).*


#### **Table 9.**

*Proximate analysis of cassava waste on the basis of 100 g.*


#### **Table 10.**

*Proximate analysis of cassava peel.*

#### **4.3 Biochemical characteristics of cassava wastes**

Apart from the nutritional value of cassava, the chemical oxygen demand (COD), biochemical oxygen demand (BOD), and cyanide content are the parameters of interest because of their effect on the flora and fauna as well as the environment. Based on cyanide contents, cassava can be classified as sweet and bitter and it wastes are often laden with suspended solids, high COD, and BOD making them toxic. Some of the works carried out in this direction show that one liter of cassava wastewater has 23.9 g of COD, 23.1 g of volatile solids (VS), and 22.9 g of total solids (TS) [51, 52].

In the case of nutritional value, some of the works presented earlier (vide supra) indicated the presence of carbohydrates, proteins, and other nutritional components that support life. Dresden [11] conducted research on the nutritional profile of cassava waste and reported its nutritional composition as presented in **Table 11**.

Glanpracha et al. [53] reported on the cyanide content of cassava waste as presented in **Table 12**. The cyanide content includes hydrocyanic acid (HCN) and cyanogenic glucoside (linamarin).


#### **Table 11.**

*The nutritional profile of cassava waste.*


#### **Table 12.**

*Biochemical composition of cassava waste.*


#### **Table 13.**

*Physicochemical and biochemical characteristics of nine cassava varieties.*

Mégnanou et al. [54] conducted a study on the physicochemical and biochemical characteristics of nine varieties of cassava roots (V4, V23, V60, V61, V62, V63, V64, V65, and V66). The mean values of their physicochemical and biochemical characteristics are presented in **Table 13**.

In a similar development, Izah et al. [55] investigated the heavy metal content of cassava mill effluents (cassava processing wastes) collected from a cassava processing mill at Ndemili in Ndokwa west local government area of delta state, Nigeria. It was found that the effluent contains 1.46 mg/l of copper, which is comparable to the value 1.83 mg/l reported by Orhue et al. [56], 1.91 mg/l reported by Adejumo and Ola [57], and lower than the value of 2.50 mg/l as reported by Patrick et al. [58] as well as 2.60 mg/l by Olorunfemi and Lolodi [59] and higher than the value of 0.00 mg/l reported by Omomowo et al. [60]. Another heavy metal present was zinc with a concentration of 4.35 mg/l. This is comparable to the value of 4.1 mg/l reported by Patrick et al. [58] and lower than the value of 5.90 mg/l reported by Olorunfemi and Lolodi [59] and higher than the value of 1.07 mg/l reported by Orhue et al. [56] as well as 0.00 mg/l as reported by Adejumo and Ola [57].

Manganese was equally found with a concentration of 4.64 mg/l, which is lower than the value of 0.71 mg/l reported by Adejumo and Ola [57], as well as 0.00 mg/l by Omomowo et al. [60] and lower than the value of 7.10 mg/l reported by Olorunfemi and Lolodi [59]. About 28.27 mg/l of iron was reported to be found in the effluent, which is far higher than the value of 2.35 mg/l reported by Adejumo and Ola [57], as well as 2.30 mg/l reported by Omomowo et al. [61] and 2.00 mg/l by Orhue et al. [56] and lower than the value of 30.9 mg/l reported by Olorunfemi and Lolodi [59]. The study further revealed the presence of 0.18 mg/l of chromium, which was lower than the value of 1.14 mg/l reported by Olorunfemi and Lolodi [59].

Generally, the arbitrary variation (with no trend) in the heavy metal concentration (**Table 14**) could be attributed to the age of the cassava prior to processing, activities leading to individual heavy metals disposition in the plantation where the cassava was cultivated, and possible leaching of metals from the processing equipment.

From the foregoing, it is obvious that a huge quantity of waste from cassava processing would unavoidably be generated irrespective of the cassava-based products of interest. The waste constituents are not all toxic as researchers found that in it (the waste) contains 11% of the crop energy coupled with valuable mineral nutrients [61–64]. These valuable contents of the waste can be exploited to boost the economic potential of cassava processing. Thus, the characteristics of cassava wastes as reviewed are within the range, presented in **Table 15**.


#### **Table 14.**

*Heavy metal contents of various effluents from cassava processing mill.*

*Biochar from Cassava Waste: A Paradigm Shift from Waste to Wealth DOI: http://dx.doi.org/10.5772/intechopen.105965*


**Table 15.** *Parameters of interest.*

#### **5. Biochar from cassava wastes: parameters and choice of feedstock**

Characterizations of cassava wastes are necessary for efficient valorization. Through characterization, it is obvious that embedded in the wastes are fractions of the crop energy and minerals. The huge quantity of waste generated during cassava processing translates to a huge quantity of energy and minerals that need to be recovered. The cassava wastes can be converted into biogas (energy recovery) as well as digestate filtrate and residue for biofertilizer, bio-oil, and biochar (mineral recovery) [65]. Several studies have been carried out on bio-oil production using cassava wastes but much has not been reported on biochar from cassava wastes [66, 67]. A brief description of biochar properties would be beneficial to identify its applications.

Biochar is a carbon-rich product obtained when biomass (cassava waste) is heated in a closed system with restricted oxygen. Structurally, it is similar to charcoal but with different properties. However, biochar has a high surface area (highly porous) and negative surface charge, and charge density [68]. Due to its superlative adsorption properties, biochar has been extensively used as an adsorbent [69]. Biochar as adsorbent finds application in removing "emerging contaminants" from flue gas and wastewater. It is equally applied to soil to improve soil properties as biochar can hold nutrients and become more stable than most fertilizer or other organic matter in soil [69].

Biochar (pyrochar) is a solid product from the pyrolysis process. Of all the thermochemical conversion processes (combustion, incineration, etc.), pyrolysis offers a great opportunity of transforming wastes into wealth. Varieties of biomass, including cassava wastes, are exploited as feedstock to produce valuable gas, liquid, or solid products, including biochar. Research has shown that pyrolysis is relatively environmentally friendly when compared with its counterparts as it produces low emissions [70]. During the production process, it was reported that biochar is able to scrub carbon dioxide, nitrous oxides, and sulfur dioxide from the flue gas constituting greenhouse gases (GHG) [68]. These gases (GHG) contribute immensely to global warming leading to climate change with heat waves, flooding, and typhoons [71–73].

Scrubbing GHG during biochar production can be harnessed as a potential tool to slow global warming [74].

Pyrolysis process is versatile—fast or slow depending on residence time—and can be optimized to enhance the production of desired product. Fast pyrolysis with residence time in seconds generates more liquid products (bio-oil), while slow pyrolysis with residence time in hours favors more solid products (biochar) [75–77]. In addition, the properties of biochar produced are varied by the pyrolysis parameters and choice of feedstock.

Besides residence time, temperature and heating rates equally influence the yield and composition of the pyrolysis products. The temperature and heating rates are two of the pyrolysis parameters that affect the yield and composition of the pyrolysis [70, 78]. Noor et al. [79] reported that temperature has a more significant influence than the heating rate during biochar production. Although the nature of feedstock determines the fixed carbon content in the biochar produced, it was found that a higher pyrolysis temperature increased more fixed carbon in the biochar than a higher heating rate [79, 80]. Thus, the effect of production parameters is significant. However, the choice of feedstock on the quality of biochar is equally important.

We extensively reviewed the complementary role of thermochemical conversion process after subjecting the wastes to a biochemical (anaerobic) process [81]. Anaerobic digestate of cassava waste is a valuable pyrolysis feedstock for biochar production due to its high volatile matter content, low ash, and sulfur content [81]. Consequently, thermally treated digestate is more suitable than any other materials subjected to pyrolysis for biochar production. Meanwhile, cassava plantation residues: cassava stem and cassava rhizome have been exploited for biochar production [79, 80]. However, the biochar produced from cassava wastes contains a high percentage of fixed carbon, which is about five to eight times higher than that from cassava plantation residues.

Cassava irrespective of its components can be exploited for biochar production *via* pyrolysis. Three categories of cassava waste can be exploited as pyrolysis feedstock for biochar production (cassava plantation residues, cassava processing waste, and digestate from anaerobic digestion of cassava processing waste). The quality of biochar produced depends on the process parameters and choice of feedstock. Slow pyrolysis when optimized produces high-quality biochar suitable for several applications [79, 80]. Nevertheless, digestate from anaerobic digestion of cassava waste is the most preferred pyrolysis feedstock. This is followed by cassava processing wastes and the least is the cassava plantation residues, especially the stem, this is subject to further investigations. Besides the production of high-quality biochar, the digestate from cassava processing waste has been effectively exploited for biogas and biofertilizer production.

#### **6. Conclusion**

This review has established that waste generation during cassava processing is inevitable irrespective of the cassava-based products. In the course of characterizing the wastes, which emerged during processing, pre-processing, and post-processing depending on the products, various researchers reported that the physical, chemical, and biological properties are within the range as paired: carbon/nitrogen (17.64–30.0), total solid (4.5–38.2 mg), volatile solid (3.4–33.0 mg), pH (3.6–6.2), total chemical oxygen demand (8.0–66.2), soluble chemical oxygen demand (14.2–34.5), total

carbohydrate (9.6–37.5 mg), cellulose (36.0–43.2 mg), hemicellulose (44.0–64.4 mg), lignin (24.0–46.2 mg), and cyanide (45–154 mg).

It can be confirmed that the cassava waste through toxicity contains a valuable component of interest. This was proven as its energy content is about 11% of the crop energy. This can be harnessed and added to our energy mix. The microporous structure of cassava residue, especially the peel is equally promising for adsorbent formulation. This review pinpoints the potential of these wastes for biochar production. The quality of biochar produced depends on the process parameters and choice of feedstock.

Three categories of cassava waste can be exploited as pyrolysis feedstock for biochar production (cassava plantation residues, cassava processing waste, and digestate from anaerobic digestion of cassava processing waste). Digestate from anaerobic digestion of cassava waste is the most preferred pyrolysis feedstock. Slow pyrolysis when optimized produces high-quality biochar suitable for several applications. Anaerobic digestion of cassava processing wastes generates much more than high-quality biochar. It is an effective waste to wealth strategy.

#### **Conflicts of interest**

The authors declare that they have no conflicts of interest regarding the publication of this paper.

#### **Data availability**

No data were used to support this study.

#### **Author details**

Minister Obonukut<sup>1</sup> \*, Sunday Alabi<sup>1</sup> and Alexander Jock<sup>2</sup>

1 Department of Chemical Engineering, Topfaith University, Mkpatak, Nigeria

2 Department of Chemical and Petroleum Engineering, University of Uyo, Uyo, Nigeria

\*Address all correspondence to: obonukutminister@gmail.com

© 2022 The Author(s). Licensee IntechOpen. This chapter is distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/3.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Section 2
