**1. Introduction**

Biochar is a porous carbonaceous solid material produced by the thermal decomposition of biomass from plant or animal waste under oxygen-free or limited oxygen conditions [1–3]. The International Biochar Initiative [4] defines biochar as "a solid material obtained from the thermochemical conversion of biomass in an oxygen-limited environment". Biochars have a wide range of physicochemical properties [5], which greatly affect their potential applications in various agronomic and industrial sectors. The feedstock and the method of biochar production have a significant impact on the biochar characteristics, such as concentrations of elemental constituents, density, porosity, and pH, which collectively impact the suitability of the biochar for various applications [3]. Biochar is used to upgrade the soil quality [6, 7] in agricultural areas. It slows down the rate of decomposition and release of nutrients from the soil and hence, enhances the soil quality. In various industries, biochar is used in waste treatment [8–10] to remove organic contaminants and

heavy metals. Biochar can be used as a fuel in power generation because it contains a high carbon percentage in it.

Biomass is a very potential source of renewable energy [11] materials and chemicals [12, 13]. Agricultural residues, algal biomass, forest residues, manures, activated sludge, energy crops, digestate, etc. are the main sources of biomass [14] to be used as a raw material. Biomass can be converted into high-value products using various physical, thermochemical and biochemical processes. Thermochemical conversion processes, such as pyrolysis, gasification, torrefaction, and hydrothermal carbonization of carbonaceous biomass are used for biochar is production, at high temperatures ranging from 300–900°C and under O2-free conditions [15]. The physicochemical and mechanical properties of biochars depend on the pyrolysis operating conditions and feedstock used [16]. The selection of a suitable kind of feedstock is usually determined by the availability of that material in areas where the biochar is likely to be produced, as this reduces the cost of transport while decreasing the carbon footprint of the biochar technology. Biochar production from the biomass depends upon the thermochemical process used and process parameters considered. The literature on the biomass pyrolysis revealed that the production of the biochar depends upon several factors such as type of biomass, moisture content, and particle size, reaction conditions (reaction temperature, reaction time, heating rate) and surrounding environment (carrier gas type, flow-rate of carrier gas) and other factors (catalyst, reactor type) [3, 6, 8, 11, 16].

The main objective of this chapter is to show the potential use of waste biomass for the production of biochar which is an important material with numerous industrial and environmental applications. The biochar characterization is presented to understand the physical and chemical properties of biochar, including variations of the biochar properties as a function of production conditions and feedstocks, and to evaluate the applicability of biochar in desired fields. This chapter exhaustively describes the possible feedstocks for biochar production, biochar production processes particulary slow pyrolysis process, and pyrolysis process conditions, the properties of biochar such as biochar characterization, proximate and ultimate analysis of biochars. Some important industrial and environmental biochar applications are discussed and finally conclusion is presented.

### **2. Raw materials for biochar production**

The main sources of raw material for biochar production include: municipal waste, agricultural and forest residues, energy crops, and animal waste, which are grouped as lignocellulosic and non-lignocellulosic biomasses. Lignocellulosic biomass [17–19] are abundant fibrous plant parts, non-food 'second generation' feedstock, including agro-industrial residues, forest-industrial residues, energy crops, municipal solid waste, etc. Chemically, biomass is a complex composition of carbon, hydrogen, oxygen, sulfur, nitrogen, and small quantities of few other elements which include alkali metals, alkaline earth metals, and heavy metals, depending upon the species or type of biomass. The proportion of these elements in the biomass is a function of species of biomass, growing condition, and geographical situation of the region [20]. Lignocellulosic biomass is mainly composed of cellulose (38–50%), hemicelluloses (23–32%), lignin (15–25%) and small amounts of extractives [3, 16, 21, 22]. Among these components, cellulose and hemicelluloses are linear and chain polysaccharides respectively, while lignin is a cross-linked phenolic polymer. Biomass with varying contents of hemicellulose, cellulose, and lignin may yield biochars with distinctive physicochemical properties. The abundant biomass reserves and its renewability have been the main driving forces for research and

*Recent Perspectives in Biochar Production, Characterization and Applications DOI: http://dx.doi.org/10.5772/intechopen.99788*

utilization of biomass. Thus, such agricultural and animal waste disposal can be reduced and converted into value-added products such as biochars using pyrolysis processes. A review by Li and Jiang presented on non-lignocellulosic biomass characteristics, thermochemical behaviors of main components (e.g., C, O, N, P, and metals), characterization methods, conversion process, and the main applications of non-lignocellulosic biochar [23]. Song and Guo studied the quality variations of poultry litter biochar generated at different pyrolysis temperatures [24]. Unlike lignocellulosic biomass, the non-lignocellulosic biomass has a greater threat to the ecological environment because of its higher contents of heavy metals and *heteroatom* like nitrogen, phosphorus, sulfur [25], which may dissolve in a water systems, leading to water pollution and accumulation in the food chains [26].

Poultry litter (PL), a solid waste resulting from chicken rearing, is being explored as a feedstock for biochar production and examined the effect of pyrolysis temperature on the quality PL biochar and identify the optimal pyrolysis temperature for converting PL to agricultural-use biochar [24]. Physically, PL is a mixture of bedding materials (e.g., wood shavings, sawdust, and peanut hull), bird excreta, feather, feed spills, and chemical treatments like alum and sodium bisulphate. Through pyrolysis, PL can be readily transformed into biochar [24].

### **3. Biochar production processes**

Biomass-derived biochar production is formed via a complex process, but the reaction mechanism of biomass pyrolysis can be described as occurring mainly through three general steps, as depicted in Eq. (1):

$$\begin{aligned} \text{Biomass} & \xrightarrow{heat} \text{Moisture} + \text{Dryresidues} \\ \text{Dryresidues} & \xrightarrow{heat} \text{Volatile} \& \text{Gases} + \text{Pre} - \text{biochar} \\ \text{Pre} - \text{biochar} & \xrightarrow{heat} \text{Volatile} \& \text{Gases} + \text{Biochar} \end{aligned} \tag{1}$$

The first step is the removal of available moisture from the biomass, which becomes dry feedstock by heating. Then pre-biochar and volatile compounds are formed. In the last step, chemical compounds in the pre-biochar rearrange and form a carbon-rich solid product known as biochar. Major thermochemical technologies for biochar production include pyrolysis, gasification, torrefaction, and hydrothermal carbonization. Pyrolysis is one of the thermochemical technologies for converting biomass into energy and chemical products consisting of liquid bio-oil, solid biochar, and pyrolytic gas [3, 11, 20, 27–29]. Depending on the heating rate, pyrolysis temperature, and residence time, biomass pyrolysis can be divided into slow, intermediate, fast and flash pyrolysis mainly aiming at maximizing either the bio-oil or biochar yields. Operating conditions of various pyrolysis processes and product distribution (biochar, bio-oil, and gas). Thus, biochar yield greatly depends on the type of pyrolysis used. Slow pyrolysis conducted at longer residence time and at a moderate temperature (350–550°C) in the absence of O2 results in a higher yield of biochar (�30%) than the fast pyrolysis (�12%) or gasification (�10%) [3, 11, 27, 28]. Pyrolysis requires relatively dry feedstock (usually moisture content <30 wt %, but moisture contents of ≤ 10 wt % are preferred), and grinded to different particle sizes based on the type of pyrolysis. Feedstock with high moisture content consumes more energy accounting for increasing heat of vaporization during the heating of biomass towards the pyrolysis temperature. Additionally, the gases and vapors produced in pyrolysis using a high moisture feedstock are diluted with steam


### *Recent Perspectives in Pyrolysis Research*


## *Recent Perspectives in Biochar Production, Characterization and Applications DOI: http://dx.doi.org/10.5772/intechopen.99788*


### *Recent Perspectives in Pyrolysis Research*


### *Recent Perspectives in Biochar Production, Characterization and Applications DOI: http://dx.doi.org/10.5772/intechopen.99788*


### **Table 1.**

*Characterization of raw biomass samples and biochar produced: Values for proximate and ultimate analysis.*

### *Recent Perspectives in Biochar Production, Characterization and Applications DOI: http://dx.doi.org/10.5772/intechopen.99788*

and have a lower calorific value. Wet biomass, typically with 70 wt% or more water can be converted using hydrothermal carbonization processes. The common processes [11, 27, 28] include slow and fast pyrolysis, and the most successful approach for high-yield biochar production is via slow pyrolysis [3].

Slow pyrolysis is a conventional type of pyrolysis which is operated at moderate temperatures ranging from 300–550°C, slow heating rates of 0*:*1 °C <sup>s</sup> up to 0*:*8 °C <sup>s</sup> , and longer residence time of 5 to 30 min or even 25 to 35 h [3, 30] conducted at atmospheric pressure. Slow pyrolysis is commonly used to produce biochar, with bio-oil and syngas as co-products. The typical yields of biochar, bio-oil, and syngas are 35%, 30%, and 35% of the dry biomass feedstock, respectively [3, 24] by slow pyrolysis. The main purpose conducting of slow pyrolysis is to maximize the biochar yield. The longer vapor residence times in slow pyrolsis favors the secondary reactions. Biochar produced in slow pyrolysis consists of both primary and secondary chars. The slow heating rate with moderate pyrolysis temperatures also promotes the production of biochar. Biochar yield usually depends on the raw material type & properties, and pyrolysis conditions such as processing temperature, heating rate, and pyrolysis environment [30]. The final biochar yields are decreased by increasing the process temperature because more volatiles are produced from tars at higher temperatures, leading to the production of more gases and bio-oils. Biomass containing more minerals yields less biochar [3]. The overall slow pyrolysis process can generally be exothermic due to the extensive occurrence of secondary reactions. Slow pyrolysis can accept a wide range of particle sizes (5–50 mm).

Pyrolysis of biomass also produces syngas and bio-oil as co-products together with biochar. The fraction of each that is produced depends on the pyrolysis process, but slow heating rates are recommended when biochar is the main product desired. Furthermore, pyrolysis temperatures above 250°C are recommended for the conversion of lignocellulosic biomass because decomposition of hemicellulose and cellulose begins at 250°C and is maximal at around 400°C, whereas changes in lignin structure only start to occur after heating for long durations or higher temperature pyrolysis reactors. The pyrolysis conditions used for biochar productions are related to the type of biomass and biochar quality required. Variation of these reaction parameters finally results in a variety of physicochemical properties of the biochars and affects their final application types and performances. Thus several studies have been conducted to determine the suitable raw material and optimal pyrolysis condition. The challenge is to be able to predict the quality and performance of biochars produced from given biomass and a given pyrolysis process via analysis of its physicochemical properties. Thus, to produce the right type of biochar for specific applications from certain lignocellulosic biomass, elemental composition of different biomass resources and produced biochars need to be measured. A summary of proximate analysis and ultimate analysis along with elemental composition of different raw materials for biochar production are shown in **Table 1**. Conversion of raw biomass to biochars resulted in higher contents of fixed carbon and ash, and lower contents of moisture and volatiles. Fixed Carbon (FC) of biochar was calculated as the sum of moisture, ash, and volatile matter subtracted from 100, (FC %ð Þ¼ 100 � moisture %ð Þ� ash %ð Þ� VM %ð Þ) [41].
