**3. Pyrolysis**

Pyrolysis is the thermochemical decomposition of biomass or other feedstock into biochar, bio-tar, bio-oil, bio-gas and other related products in absence or very limited supply of oxidizing agents [10, 11, 21]. This process can be broadly classified as fast, slow and flash, depending on degradation temperature, heating rate and vapor retention time as stated earlier. Other factors that affect pyrolysis are nature and moisture content of the chosen biomass [22]. **Figure 2** presents possible products of pyrolysis of biomass, while **Figure 3** illustrates pyrolysis mechanism. Incipient pyrolysis products are solid bio-char and condensable vapor. The later breaks down into char, liquid and non-condensable gases through gas-solid and

*Pyrolysis: A Convenient Route for Production of Eco-Friendly Fuels and Precursors for Chemical… DOI: http://dx.doi.org/10.5772/intechopen.101068*

**Figure 2.** *Possible products of biomass pyrolysis and their applications.*

### **Figure 3.**

*Pyrolysis of biomass [11].*

gas phase heterogeneous and homogeneous reaction respectively at high temperature. In homogeneous gas phase reaction, condensable vapor cracks into small molecules of non condensable gases (CO, CO2, H2O, H2, N2 and CH4). In gas-solid phase heterogeneous reaction, high molecular weight hydrocarbons are broken into low molecular weight hydrocarbons (such as CH4, C2H4, C2H6, C6H6 etc.). Several reactors called pyrolyzers have been used by many researchers to obtain two or more of these products from biomass. Some of these pyrolyzers are bed, vacuum, microwave and solar reactor [11].

Pyrolysis of lignocellulosic biomass involves several stages of thermal decomposition. Evaporation of moisture occurs between 40 and 115°C, at this stage, biomass dries up as a result of heat absorption that evaporates free moisture and eradicates loosely bonded water [7]. The heat transfers into biomass interior through convention to wipe out all internally bonded moisture. Biomass incipient degradation occurs through decomposition of pectin and other extractives between temperatures of 115 and 180°C, generating syn-gas and organic liquids (such as carbon(ii) oxide, carbon(iv) oxide, methane, ethane, ethane, benzene, methanol, pyrogallol and ethanoic acid) [22].

Hemicellulose degrades between temperatures of 180–250°C, giving off considerable amount of carbon(ii) oxide, carbon(iv) oxide, water, hydrocarbon of low carbon content, organic liquids and minor bio-oil [23]. Temperature from 250–350°C decomposes cellulose to yield mixture of non-condensable vapor (syn-gas), condensable vapor (bio-oil and tar) and minor bio-char [24]. Proper formation of solid fraction (primary bio-char) commences from thermal decomposition of lignin from 400°C, this is the stage at which primary pyrolysis occurs. As temperature increases from 500°C upward, minor tar and bio-oil contained in the bio-char get separated and high quality bio-char obtained. Although, a low bio-char yield is obtainable

from temperature above 700°C, but a high grade bio-char formed [25]. This is the final stage of thermochemical degradation called secondary pyrolysis. It involves cracking of condensable vapor (bio-tar and bio-oil) into non condensable gases (CO, CO2, CH4 etc.) and on cooling; part of the products solidifies to form char (secondary bio-char). Summary of the entire pyrolysis process is presented in **Table 3**.
