3.3.2 Biomass Conversion Process
Worldwide biomass assets and the latest state-of-the-art in the biomass conversion process were reviewed by Xu et al. [7] viz., thermochemical and biological conversion process, including torrefaction and carbonization, pyrolysis, gasification, combustion, and anaerobic digestion and also reported on their technological and large-scale challenges.
3.3.2.1 Thermochemical Conversion
Thermochemical processes of transformation were carried out at various temperatures by multiple types; torrefaction and carbonization, rapid pyrolysis, gasification, and combustion [5]. For thermochemical conversion, proper temperature, pressure, and heating rate are essential. Nitrogen oxides (NOx), particulate matter (PM), and tar are released from biomass by thermochemical conversion [80]. Existing methods have virtually eliminated NOx and PM emissions, while tar has been minimized by definitive treatment and in situ process management.
3.3.2.1.1 Torrefaction and Carbonization
Torrefaction is a low-temperature (under 400 °C) process of biomass and bio-solid pyrolysis that results in bio-coal, charcoal, and torrefied production biomass. This method creates energy carriers with hydrophobic properties and high calorific content. When heated to high temperatures in an inert environment, Biomass converts carbon-based energy-rich in carbon by carbonization. However, an outdated method is still in usage as it paves the way for commercialization and scientific implementation [81]. Hydrothermal carbonization is a promising technology for complete wastewater treatment without the dryness process and any discharge of waste. Often it is carried out at high pressure or moderate temperatures in the presence of water [5]. This conversion process yields carbon-rich hydrochar, solid fuel with high-density material, and process water with high organic content [75]. Even at low temperatures, Torrefaction extracts chlorine from biomass that induces metal corrosion when treated by combustion or gasification processes [82].
This process increases the biofuel properties of biomass in the energy field. Due to less awareness about its method and technology, Torrefaction is still progressing in biomass conversion. Nunes, 2020 [83] was the first to record large-scale industrialization of torrefied biomass. They investigated the criteria for biomass torrefaction processing in large-scale industry: large unit size, reaction kinetics, optimized residence time and temperature, unit technical activity management [83]. However, studies on the relationship between different biomass and torrefaction processes and integrated tasks with biomass pre-processing systems are still required. Agroforest residues, such as almond shells, kiwifruit pruning, vine pruning, olive pomace, pinewood chips, and eucalyptus wood chips, all have a high potential for a large-scale torrefaction process at 300 AC for energy synthesis [84].
3.3.2.1.2 Pyrolysis
To turn biomass into energy products (bio-oil, stable biochar, and pyrolytic gas), employed pyrolysis as a dynamic thermochemical method carried between 400 °C and 700 °C [85]. The properties of pyrolysis products are significantly evaluated by temperature, whereas the heating rate specifies the type of biomass pyrolysis that is either fast or slow. In the absence of O2, slow pyrolysis carried out at an extended period and optimized temperature results in maximum biochar yield compared to rapid pyrolysis [86]. Biochar can be used in boilers, as a catalyst, as an adsorbent, and in other manufacturing processes as solid fuel. For internal combustion engines and other techniques, pyrolytic gas may be a sustainable alternative fuel [85, 86]. Currently, the Life Cycle Assessment (LCA) approach evaluates the environmental effects of fuel combustion, electricity production, eutrophication, and N fertilizer synthesis from agricultural biomass in Shandong’s northern area in China [87]. They claimed in the study that the distributed-centralized agricultural straw pyrolysis (DCP) system was both socially and economically more advantageous for managing the disposal of crop residues compared to the traditional straw incorporation biprocess.
3.3.2.1.3 Gasification
Biomass gasification is accomplished by thermochemical reactions between 700 °C and 900 °C with high potential applications. Low-cost biomass and urban solid waste to gasify biomass for electricity generation [88]. Guran [89] showed that syngas production through gasification could be a sustainable resource of alternative fuel or converted as liquid fuels and chemicals.
3.3.2.1.4 Combustion
Combustion is a process of thermochemical reaction carried above 900 °C that releases energy in the form of heat and bioenergy products. Direct combustion produces carbon-rich solid fuels, liquid hydrocarbon materials, gaseous fuels, and the immediate release of all energy through the thermochemical conversion process [90].
3.3.2.2 Biological Conversion
Biological conversion is the biomass conversion method by the mechanism of anaerobic digestion, fermentation, or composting using enzymes from bacteria or other microorganisms [9].
3.3.2.2.1 Anaerobic Digestion (AD)
Anaerobic digestion (AD) is defined as biomass’s bioconversion into different biofuel products, biogas, and biohydrogen. Inappropriate treatment and accumulation of AD digestate, energy consumption costs, digestate preservation costs, and biogas as a significant product are a substantial concern behind the use of AD. Through generating many bioproducts concurrently in addition to the biogas, the operation can be made feasible. It may also provide an ideal approach for the processing of bio-fuels and waste treatment [91]. In the AD phase, alkali pre-treatment demonstrates high methane production [92]. Co-digestion with sludge results in the highest methane yield will resist the inhibition of acetogens in the AD phase transforming propionic acid into acetic acid phase [92]. Volatile organic compounds (VOCs), sulfur-containing gases (H2S), wastewater, and biogas slurry generated by biomass bioconversion [80]. The slurry of biogas used as a valuable resource and physical-chemical