**2.2 Biochemical methods**

*Biotechnological Applications of Biomass*

*2.1.3 Torrefaction*

pulverized [20, 33].

*2.1.4 Combustion*

15–20% were reported and it affects the properties of the solid fuels produced during

Torrefaction is a slow and mild pyrolysis process that is usually carried out at low temperatures between 225°C-300°C. The process is aimed at increasing the biomass energy density and as well its fuel properties [33]. This is achieved by removal of biomass moisture content and other superfluous volatiles. During the process, the biopolymeric substances such as cellulose, hemicellulose and lignin were partly decomposed to release organic volatiles. The product obtained at the end of the process is a dry and black residual solid regarded as torrified biomass. The torrified biomass is hydrophobic and soft which can easily be crush, grind or

The process of combustion is a widely applied biomass conversion technology that was functional to a sizeable portion of human race since the advent of human civilization. It is widely applied even today for burning of wood and agricultural residues to make pot fires and stoves in order to provide heat and light energy for cooking and heating. Combustion process is frequently used for the conversion of lignin-rich biomass. The process could be applied in two broad ways, that is either by direct conversion of the whole biomass feedstock or by biochemical conversion in which some portions of the biomass remained. Compared with the other biomass conversion technologies, the process is largely non-selective in terms of the biomass feedstock. During the process, biomass feedstock is converted to CO2 and water including smaller amount of other species which depends on the composition of the biomass and the process parameters. However, combustion of biomass largely depends on energy content of the feedstock. The amount of heat energy released during the process depends on feedstock energy content and as well as the conversion efficiency of the reaction. The fact that biomass feedstock composition plays a vital role in the combustion process was well established by many researchers worldwide in various reports [34–36]. The major share of energy in the biomass is formed by the assembly of organic matter during photosynthesis and respiration in plants. However, the inorganic fractions in the biomass are important in design and operation of the combustion system, especially when using the fluidized bed reactor. The amount of volatile matter in biomass feedstock is higher when compared with its fossil counterpart in which it is around 70–80%. The presence of this high volatile matter, greatly influence the thermal decomposition of the biomass feedstock and as well as the combustion performance of the solid fuels. This is because, large portion of the biomass feedstock has to be vapourized before the homogeneous combustion reaction takes place and the remaining char will then undergo hetero-

The main elements that constitutes the biomass feedstock are C, H, and O, while herbaceous feedstock such as agricultural waste and grasses contain higher amounts of ash forming minerals [37, 38]. Biomass is more oxygenated compared to the conventional fossil fuel. This is due to the biomass carbohydrate structure and its dry mass usually contains about 30–40% oxygen [37]. During the combustion process, part of the oxygen required is supplied by the organically bonded oxygen from the biomass, while the rest is supplied through air injection into the system. The primary constituent of a biomass is carbon which made up about 30–60% by weight of dry matter depending on its ash content. The carbon present in biomass

the process [20]. The biomass feed sizes can vary from ground to a whole log.

**10**

geneous combustion reaction.

Biochemical biomass conversion technologies refer to conversion of biomass through biological pre-treatments. These pre-treatments were aimed to turn the biomass into a number of products and intermediates through selection of different microorganisms or enzymes. The process provides a platform to obtain fuels and chemicals such as biogas, hydrogen, ethanol, butanol, acetone and a wide range of organic acids [42]. However, this process was aimed at producing products that could replace petroleum-based products and as well as those obtained from the grains. Biomass biochemical conversion technologies are clean, pure, and efficient when compared with the other conversion technologies [43].

#### *2.2.1 Digestion*

Anaerobic digestion (AD) is one of the most sustainable and cost-effective technology for lignocellulosic and other form of waste treatment for energy recovery in form of biofuels. This process does not only minimize the amount of waste, but also transforms such waste into bioenergy. Also, the digestates produced during the process are rich in nutrients, which can serve as fertilizer for agricultural purposes [44].

**Figure 7.** *Various reactors for combustion process [41].*

The digestion of lignocellulosic biomass anaerobically produces energy rich methane (CH4). The CH4 yield per unit area is usually employed for the determination of energy output of an individual feedstock which significantly varies between species and as well with maturity, location and inputs (such as fertilizer, water etc.) within the same variety (Yang et al., 2013). The Biochemical methane potential (BMP) test is commonly used to evaluate the anaerobic digestibility of a biomass substrate. The biomass yield and CH4 production potentials of some selected feedstocks were presented in **Table 1** [45].

Anaerobic digestion is a process used to produce biogas through biological treatment of biomass. It is performed at temperature ranges between 30 and 35°C, or 50 and 55°C using two stages. The first stage is the breaking down of the complex organics in the biomass by acid-forming bacteria into simpler compounds such as acetic and propionic acids along with volatiles. The second stage is conversion of such acids into CO2 and CH4 commonly called biogas through the use of methane producing bacteria. Usually, both stages of biogas production are performed in a single tank. The produced biogas contains about 60% CH4, 35% CO2, and a mixture of other gases such as H2, NH3, CO, and H2S which account for about 5%. The biogas has a heating value of about 22,350 KJ/m3 for a mixture that contains a ratio (CH4:CO2:inerts) of 60: 35: 5 (**Figure 8**) [46].
