**5.4 Thermo-chemical processes**

Thermochemical conversion techniques involve the use of heat or chemicals for the conversion of biomass into fuel and heat (as in the case of combustion and gasification). The thermochemical means is sometimes preferred since it requires limited time, little or no pre-treatment, and generation of variable end-products as compared to biochemical means. The overview of the main thermochemical conversion techniques is summarized in the following section.

## *5.4.1 Pyrolysis*

An irreversible chemical reaction impacted by heat in the absence of oxygen is known as pyrolysis. This method generally converts lignocellulosic biomass into solid, liquid, and gaseous fractions which are further processed into another product spectrum [59]. Pyrolysis is commonly adopted, since its end-product ranges from gaseous to solid fuels in varying percentages, when compared to other thermochemical biomass conversion methods. Pyrolysis can be a precursor to some other conversion processes such as gasification and combustion, as well as a succeeding step to some pre-treatment methods, such as torrefaction and degradative solvent extraction (DSE) [60]. Pyrolysis is a temperature, heating rate and time dependent process, and varying these conditions with addition of selected catalysts give a specified, desired end-product. Based on the specific conditions, pyrolysis is classified into slow, intermediate, and fast mode; and from which different percentages of solid, liquid, and gaseous product are derived. The classification of pyrolysis base on temperature and residence time is shown in **Table 3**, and it ranges from the low temperatures (~ 300°C) to high temperatures (approximately 900°C).

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and transported [19, 61].

*5.4.2 Gasification*

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

**rate**

Slow 250–400°C Long hours

**Mode of pyrolysis Heating** 

*Mode of pyrolysis and product distribution [61, 62].*

Torrefaction Intermediate

*Approximate values.*

*\**

**Table 3.**

Slow pyrolysis involves lower heating rates between 0.1 and 2°C, and the products are mainly solids. Carbonization is a form of slow pyrolysis which is the old technique used in the production of charcoal (biochar) and the vapor produced during the process is usually not condensed, but rather used for heating. Torrefaction mainly aims at improving the energy density and biomass fuel properties, such as reduced weight and volume, which renders the fuel easy to transport and be crushed when needed. The intermediate pyrolysis process produces less solid and liquid with low viscosities. Fast pyrolysis is one of the biomass conversion technologies currently gaining prominence because of its ability to generate more liquid fuel (bio-oil) which could be easily upgraded to diverse valuable products,

**Temperature Residence** 

Carbonization Low ~400°C\* 35 30 35

Fast High ~500°C < 2 s 12 75 13

**time**

to days

Medium 300–500°C 5–30 s 25 50 25

~280°C 10–60 mins 80 3 20

**Product percent by weight (% wt) Solid Liquid Gas**

Gasification is the conversion of biomass or carbonaceous feedstock into gaseous

Boudouard reaction : C CO 2CO + =<sup>2</sup> (7)

Carbon hydrogen reaction : C H CH − + =2 4 (8)

components at temperatures higher than 650°C. The gaseous component of the gasification process may vary depending on the operating temperature. At a lower temperatures below 1200°C, gas constituents may vary from CO, CO2, CH4 and H2, which are collectively known as producer gas while at a higher temperatures, the constituents are CO and H2, which are collectively known as synthesis gas or syngas [62]. Coal gasification is a well-known process in the generation of electricity which can also be used for biomass conversion into gas. The four steps involved in the gasification process are drying or heating, pyrolysis, gas–solid reaction, and gas phase reaction. These steps occur successively and may take a fraction of a minute depending on the reactor (gasifier) design [52]. Drying/heating is mainly adopted to remove the moisture content of biomass, thereby converting the biomass into dry mass. This is done to attain the required temperature for gasification and the desired products. This process is followed by pyrolysis. The gas–solid reaction step involves the reaction of the gas and solid (char) produced from the pyrolysis phase. The char which is carbon reacts with carbon dioxide, hydrogen, oxygen, and water (vapor) to form gaseous compounds as shown in the conversion Eqs. (7)–(10).

**Figure 8.** *Schematic of a microbial fuel cell.*


#### **Table 3.**

*Biotechnological Applications of Biomass*

**5.4 Thermo-chemical processes**

*5.4.1 Pyrolysis*

differ based on the application of the fuel cell.

conversion techniques is summarized in the following section.

reduction of various compounds to other forms (such as CO2 to acetate, nitrate to N2 and O2 to H2O). It is an electrochemical reaction that utilizes microorganisms for catalysis; therefore, it is referred to as a bioelectrochemical process. The anode in an MFC is usually carbon based such as carbon cloth, felt, fiber, rod, and paper, while the cathode is either of the latter coated with platinum [57, 58]. **Figure 8** shows a schematic of the working principles of a microbial fuel cell. The catalyst used may

Thermochemical conversion techniques involve the use of heat or chemicals for the conversion of biomass into fuel and heat (as in the case of combustion and gasification). The thermochemical means is sometimes preferred since it requires limited time, little or no pre-treatment, and generation of variable end-products as compared to biochemical means. The overview of the main thermochemical

An irreversible chemical reaction impacted by heat in the absence of oxygen is known as pyrolysis. This method generally converts lignocellulosic biomass into solid, liquid, and gaseous fractions which are further processed into another product spectrum [59]. Pyrolysis is commonly adopted, since its end-product ranges from gaseous to solid fuels in varying percentages, when compared to other thermochemical biomass conversion methods. Pyrolysis can be a precursor to some other conversion processes such as gasification and combustion, as well as a succeeding step to some pre-treatment methods, such as torrefaction and degradative solvent extraction (DSE) [60]. Pyrolysis is a temperature, heating rate and time dependent process, and varying these conditions with addition of selected catalysts give a specified, desired end-product. Based on the specific conditions, pyrolysis is classified into slow, intermediate, and fast mode; and from which different percentages of solid, liquid, and gaseous product are derived. The classification of pyrolysis base on temperature and residence time is shown in **Table 3**, and it ranges from the

low temperatures (~ 300°C) to high temperatures (approximately 900°C).

**442**

**Figure 8.**

*Schematic of a microbial fuel cell.*

*Mode of pyrolysis and product distribution [61, 62].*

Slow pyrolysis involves lower heating rates between 0.1 and 2°C, and the products are mainly solids. Carbonization is a form of slow pyrolysis which is the old technique used in the production of charcoal (biochar) and the vapor produced during the process is usually not condensed, but rather used for heating. Torrefaction mainly aims at improving the energy density and biomass fuel properties, such as reduced weight and volume, which renders the fuel easy to transport and be crushed when needed. The intermediate pyrolysis process produces less solid and liquid with low viscosities. Fast pyrolysis is one of the biomass conversion technologies currently gaining prominence because of its ability to generate more liquid fuel (bio-oil) which could be easily upgraded to diverse valuable products, and transported [19, 61].

### *5.4.2 Gasification*

Gasification is the conversion of biomass or carbonaceous feedstock into gaseous components at temperatures higher than 650°C. The gaseous component of the gasification process may vary depending on the operating temperature. At a lower temperatures below 1200°C, gas constituents may vary from CO, CO2, CH4 and H2, which are collectively known as producer gas while at a higher temperatures, the constituents are CO and H2, which are collectively known as synthesis gas or syngas [62]. Coal gasification is a well-known process in the generation of electricity which can also be used for biomass conversion into gas. The four steps involved in the gasification process are drying or heating, pyrolysis, gas–solid reaction, and gas phase reaction. These steps occur successively and may take a fraction of a minute depending on the reactor (gasifier) design [52]. Drying/heating is mainly adopted to remove the moisture content of biomass, thereby converting the biomass into dry mass. This is done to attain the required temperature for gasification and the desired products. This process is followed by pyrolysis. The gas–solid reaction step involves the reaction of the gas and solid (char) produced from the pyrolysis phase. The char which is carbon reacts with carbon dioxide, hydrogen, oxygen, and water (vapor) to form gaseous compounds as shown in the conversion Eqs. (7)–(10).

Boudouard reaction : C CO 2CO + =<sup>2</sup> (7)

$$\text{Carbon} - \text{hydrogen reaction} : \text{C} + \text{H}\_2 = \text{CH}\_4 \tag{8}$$

$$\text{Carbon} - \text{oxygen reaction} : \text{C} + \text{O}\_2 = \text{CO}\_2 \tag{9}$$

$$\text{Carbon} - \text{water reaction} : \text{C} + \text{H}\_2\text{O} = \text{CO} + \text{H}\_2 \tag{10}$$

Thus, the gas phase reaction is shown in Eqs. (11)–(12).

$$\rm{CO} + \rm{H}\_2\rm{O} = \rm{CO}\_2 + \rm{H}\_2\tag{11}$$

$$\rm{CH}\_4 + \rm{H}\_2\rm{O} = \rm{CO} + \rm{3H}\_2\tag{12}$$

#### *5.4.3 Solvent liquefaction*

This is the conversion of biomass into liquid or solubilized products at moderate temperatures (105–400°C) and pressure (2–20 MPa) with the aid of solvents. Solvents such as water, ethanol, phenol, tetralin, sulfuric acid, phosphoric acid, nitric acid, and other ionic solvents have been utilized. When water is used as the solvent in the liquefaction process, it is known as hydrothermal liquefaction [63]. For optimal efficiency of the process, the main parameter is the choice of solvent as the process requires the solvent to be in the liquid phase during the reaction. Solvents such as creosote have been utilized to achieve a bio-oil yield of 74 wt% as compared to water of 35 wt% yield or acetone of 10 to 60 wt% yield, for lignocellulosic biomass. This process has been mainly used for the processing of lignocellulosic biomass, algae, and other biomass feedstocks. Unlike other thermochemical conversion processes, this process does not need much residence time for intermediate drying/heating of the biomass as it could be used to process biomass with 15 to 80% moisture content.

#### *5.4.4 Combustion*

Combustion is the complete oxidation of carbon containing materials to CO2 and H2O in the presence of air (oxygen). For a complete combustion process, the four stages involved are heating or drying, pyrolysis, volatiles combustion (known as flaming) and char combustion (smoldering). These stages are similar to the stages in gasification and the only difference is that combustion requires excess air [64]. Combustion depends on the operating temperature, feedstock type, the particle size of the feedstock, the design of the reactor and atmospheric conditions. Other by-products of this process are nitrogen oxides, sulfur, ash, and particulate matter which are environmentally unfriendly [63].

#### **6. Conclusion**

The conversion of LB into value-added products is vital to meet the global demand for lignocellulosic products. The concept of biorefining arose since the potential of lignocellulosic and microalgae-based products were substituted for fossil fuel derived products, which accounted for increased usage of non-renewable fuels. The reduction in the demand on fossils, creation of opportunities in the job market and the provision of sustainable forms of energy has highlighted the role of

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**Author details**

South Africa

and Donald Tyoker Kukwa

Edward Kwaku Armah\*, Maggie Chetty, Jeremiah Adebisi Adedeji

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

provided the original work is properly cited.

Department of Chemical Engineering, Durban University of Technology, Durban,

© 2020 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,

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

biorefineries in tackling climate change. This chapter presented the insights into the various components of lignocellulose and microalgae, the pretreatment techniques adopted in the past decades for delignification and conversion into useful products, and the applications coupled with future prospects for valorization of biomass.

*Valorization of Lignocellulosic and Microalgae Biomass DOI: http://dx.doi.org/10.5772/intechopen.93654*

*Biotechnological Applications of Biomass*

*5.4.3 Solvent liquefaction*

80% moisture content.

which are environmentally unfriendly [63].

*5.4.4 Combustion*

**6. Conclusion**

Thus, the gas phase reaction is shown in Eqs. (11)–(12).

Carbon – oxygen reaction : C O CO + =2 2 (9)

Carbon water reaction : C H O CO H − + =+ 2 2 (10)

This is the conversion of biomass into liquid or solubilized products at moderate temperatures (105–400°C) and pressure (2–20 MPa) with the aid of solvents. Solvents such as water, ethanol, phenol, tetralin, sulfuric acid, phosphoric acid, nitric acid, and other ionic solvents have been utilized. When water is used as the solvent in the liquefaction process, it is known as hydrothermal liquefaction [63]. For optimal efficiency of the process, the main parameter is the choice of solvent as the process requires the solvent to be in the liquid phase during the reaction. Solvents such as creosote have been utilized to achieve a bio-oil yield of 74 wt% as compared to water of 35 wt% yield or acetone of 10 to 60 wt% yield, for lignocellulosic biomass. This process has been mainly used for the processing of lignocellulosic biomass, algae, and other biomass feedstocks. Unlike other thermochemical conversion processes, this process does not need much residence time for intermediate drying/heating of the biomass as it could be used to process biomass with 15 to

Combustion is the complete oxidation of carbon containing materials to CO2 and H2O in the presence of air (oxygen). For a complete combustion process, the four stages involved are heating or drying, pyrolysis, volatiles combustion (known as flaming) and char combustion (smoldering). These stages are similar to the stages in gasification and the only difference is that combustion requires excess air [64]. Combustion depends on the operating temperature, feedstock type, the particle size of the feedstock, the design of the reactor and atmospheric conditions. Other by-products of this process are nitrogen oxides, sulfur, ash, and particulate matter

The conversion of LB into value-added products is vital to meet the global demand for lignocellulosic products. The concept of biorefining arose since the potential of lignocellulosic and microalgae-based products were substituted for fossil fuel derived products, which accounted for increased usage of non-renewable fuels. The reduction in the demand on fossils, creation of opportunities in the job market and the provision of sustainable forms of energy has highlighted the role of

CO H O CO H +=+ 2 22 (11)

CH H O CO 3H 4 2 + =+ <sup>2</sup> (12)

**444**

biorefineries in tackling climate change. This chapter presented the insights into the various components of lignocellulose and microalgae, the pretreatment techniques adopted in the past decades for delignification and conversion into useful products, and the applications coupled with future prospects for valorization of biomass.
