Physical pretreatment of lignocelluloses typically involves size comminution by grinding, milling, or chipping. The aim is to reduce the crystallinity of the cellulose fibers in the biomass. Size reduction is also necessary to eliminate mass and heat transfer limitation during the required reaction. The size of the resulting materials is typically 10–30 mm after chipping and 0.2–2 mm after milling or grinding. Ball milling is used for particle sizes smaller than 90 μm, with cellulose content lower than for larger particle sizes. For hammer mill, the energy requirements for size reduction increase linearly as the particle size is reduced and the moisture content increases, where the energy requirements tend to level off for particle sizes less than 2 mm. Generally, the higher the moisture content of the biomass, the more energy is required for size reduction.

There are three types of physicochemical pretreatment, which are steam explosion, ammonia fiber explosion (AFEX), and CO₂ explosion. In steam explosion, the reduced size of biomass is subjected to high-pressure saturated steam for a short time before a sudden drop in pressure causes an explosive decompression of biomass. The process causes transformation of lignin and degradation of hemicelluloses. Steam explosion is known to be a cost-effective pretreatment for hardwood and agricultural residues. AFEX and CO₂ explosion are similar to steam explosion, where the biomass is exposed to liquid ammonia or CO₂ at high temperatures and pressures for a short period of time, followed by a sudden drop in pressure. AFEX does not solubilize hemicelluloses but does require recovery of ammonia for cost and environmental reasons.

Chemical pretreatment of biomass includes the use of ozone, acids, alkali, organic solvents, and peroxides. Ozonolysis is carried out at room temperature and is effective for lignin removal without the formation of toxic by-products. Mild acid pretreatment with sulfuric acid is efficient for the removal of hemicelluloses but fails to effectively remove lignin. It also helps in the removal of ash in biomass. Dilute alkali pretreatment using sodium hydroxide devastates the intermolecular bonds between lignin and hemicelluloses and improves the porosity of the biomass. Other studies on dilute alkali pretreatments have examined the use of ammonia water and hydrated lime. The use of methanol, ethanol, acetone, and ethylene glycol along with inorganic and organic acids as catalyst has also been studied but the pretreatment cost is relatively high as compared with physicochemical pretreatments.

This treatment involves the use of microorganisms that selectively degrade lignin and hemicellulose. Biological pretreatments are less energy intensive compared to chemical and physicochemical processes and only require mild reaction conditions. However, the process is very slow, making it unattractive for commercial use.

Awareness of the potential of recovering, utilization of energy, and upgraded products from agricultural wastes must be made known to researchers. There is a huge amount of energy locked in these solid wastes. By processing them in mills, tremendous savings can be realized through the potential of converting them into energy and value-added products. The dependence of industries on conventional energy and fuels can be reduced to a great extent. The potential offered by biomass energy for reducing a country’s energy problem seems viable to a certain degree. Figure 8.2 shows the main processes of thermal energy conversion. There are several technologies for conversion of biomass into energy and higher value products. These are mainly classified as biochemical, thermochemical, physical, and liquefaction.

Presently, direct combustion of biomass is used in medium and large industries to produce electrical power and process heat, which is the simplest route for energy recovery from these wastes. There are various combustion technologies available in the market but their suitability depends on the characteristics of the biomass itself. Generally, biomass burns efficiently in an inclined moving bed combustion unit. Staged combustion techniques are used to improve the emissions standards. The combustion of biomass generally consists of volatile and char combustion. Therefore, the heat transfer and residence time for the two kinds of combustion will be different for different types of biomass. Thus, the design for the combustion process will be different for different types of biomass. The heat from the combustion process can then be used for drying, and steam can be used for thermal heating and steam electrical power generation.

Biomass pyrolysis is a thermochemical decomposition of organic material at medium temperatures in the absence of oxygen. It is one of the oldest processes of making charcoal or biochar, which is often called carbonization that produces solid residues high in carbon content. The aim is to remove moisture and in the process convert the volatiles in the biomass material into higher carbon content. Long residence time and medium heating with an inert environment are the key requirements for the process. There are various techniques for the conversion of biomass to charcoal ranging from burying heated biomass, to beehive stoves, to modern carbonization plants. Table 8.3 shows the various pyrolysis processes with their respective residence time and terminal temperatures.

Biomass gasification is used to provide clean gaseous fuel for combustion in furnaces, boilers, and internal combustion engines for power generation and process heating. Biomass in the form of char is usually used rather than in its dried form because the producer gases are relatively free of tar, water, and corroding components. Downdraft gasifiers are of popular design, which specifically eliminates the tars and oils from the gas for gas engine application. In the fixed bed gasifier, the moisture is usually driven off at the top drying zone before entering the pyrolysis zone. The tars and oils pass through the bed of hot char where they are synthesized into simpler gases. The gas velocity is low in the downdraft gasifier and the ash settles through the bottom grate so that very little ash is carried over with the gases. Prior to the utilization of the gas in engines, the gas passes through the dry cleaning system, which usually consists of cyclone, filter bags, and gas coolers. Presently, small-scale biomass gasification is not popular for power generation due to the messy maintenance issue of the system, but commercial scale would be the biomass integrated plasma gasification combined cycle where it was initially designed and operated for coal gasification.

The torrefaction process can be described as a mild form of pyrolysis at temperatures ranging from 200°C to 320°C. During the process, the water contained in the biomass as well as superfluous volatiles are removed. The biomass loses about 20% of its mass and about 10% of its heating value, without any change in its volume, thus decreasing its energy density. The purpose is to obtain a much better fuel quality for combustion and gasification applications as well as allow the material to be easily pelletized or briquetted.

Biomass like coal can be converted into higher value hydrocarbons: liquid fuels, methane, and petrochemicals. Biomass to liquid fuels or “BTL” mimics “coal to liquid fuels” or “CTL.” Biomass liquefaction is the production of liquid fuels from biomass using high pressure and temperature with the presence of solvents or catalysts. Specific liquefaction technologies generally fall into two categories: direct liquefaction and indirect liquefaction processes. Indirect liquefaction processes generally involve gasification of coal or biomass to a mixture of carbon monoxide and hydrogen (syngas) and then use a process such as the Fischer–Tropsch process to convert the syngas mixture into liquid hydrocarbon. The direct liquefaction processes converts coal or biomass into liquids directly, without the intermediate step of gasification, by breaking down its organic structure with the application of solvents or catalysts in a high pressure and temperature environment. Since liquid hydrocarbons generally have a higher hydrogen–carbon molar ratio than coals or biomass, either hydrogenation or carbon-rejection processes must be used in both direct and indirect liquefaction technologies. Biomass or coal liquefaction generally is a high-temperature/high-pressure process requires significant energy consumption at an industrial scale (thousands of barrels/day), and needs multibillion dollar capital investments. Thus, biomass/coal liquefaction is only economically viable at historically high oil prices, and therefore presents a high investment risk.