1.5.2. Thermochemical Conversion

This conversion process consists of pyrolysis, gasification, and combustion. In this process, the treatment of agro-waste into valuable and important products such as biochar, bio-oil, biofuels, etc., usually requires a high temperature. Pyrolysis refers to the thermal depolymerization of agro-waste in an atmosphere with a constant supply of heat. Among the sources of feedstock used for pyrolysis is agro-waste such as rice husk, corn stover, wheat straw, etc., woody biomass (redwood, teak, etc.), and energy crops like bamboo, sorghum, etc., and municipal solid wastes [35]. As a result of the constant and rapid heating of such agro-waste leads to the production of vapor made up of various hydrocarbons coupled with condensation to yield an organic liquid called bio-oil [36]. Moreover, the product obtained from pyrolysis depends primarily upon the composition of the agrowaste used and the interaction between the produced liquefaction products influenced by different parameters such as temperature, heating rate, inert flow rate, and particle size, and conversion time. Due to the influence of these parameters, pyrolysis can be classified as fast, slow, or flash. Pyrolysis is a flash when it operates at a lower temperature, with a lower heating rate and longer vapor formation time. In comparison, fast pyrolysis tends to work at a higher temperature, higher heating rate, and short vapor formation time. Hence, the primary end-products of slow pyrolysis are biochar, bio-oil, and pyrolyzed gas with varying percentages of 35–40% for biochar and 30–35% for pyrolyzed gas. Similarly, flash and fast pyrolysis produce an end-product of biochar (12%) and pyrolyzed gas (13%), while bio-oil is about 75% of the end-product (Equation (2)) [37].
