*3.1.2. Thermochemical conversion processes*

extensively in the production of bioenergy for both domestic and industrial applications [41]. The process involves the utilization of microorganism for conversion of moist organic sub-

Fat and oils Protein Sugar and starch Lignocellulosic

as hydrogen sulfide [21]. Along the product a waste stream digestate is generated which are usually utilized as manure of the farmland. The generated biogas is characterized with highenergy content of one-third of the lower heating magnitude of the feedstock from which it is produced [42]. In the quest for renewable energy production in the form of biogas, this method has been studied succinctly. Moreover, there is inherent advantage of carbon capture

algae stand prominent as an agricultural residue producing significant amount of biogas in

Besides, another vital approach of biomass conversion is an enzymatic controlled anaerobic process [43], which is employed in the synthesis of bioethanol from lignocellulosic biomass. In this process, the first action is the pretreatment of the raw biomass and subsequent hydrolysis prior to fermentation process. The cellulosic component of the biomass is transformed into glucose via enzymatic hydrolysis converts the cellulose component of the biomass into glucose while the hemicellulose part affords pentose and hexoses. Microorganism then converts the glucose into ethanol. This is affected by the action of biological catalysts to turn fermentable sugars to important chemicals (usually alcohols or organic acids). The most essential product of fermentation has been ethanol; however, there are some other useful substances such as hydrogen, methanol, and succinic acid that are generated. The major fermentation substrates are hexoses, which are mostly glucose, while modified fermentation organisms are

Furthermore, fermentation process is a conventional and extensively considered method in the treatment of waste streams, as well as for ethanol synthesis from agricultural residues, such as corn cobs and sugar beets [43]. Using fermentation sugars in sugarcane as feedstock, Brazil established a successful bioethanol plant. In 2011, about 5.57 billion gallons of ethanol is generated as fuel from this program, an equivalent of about 24.9% of the world's total

used to convert pentose, glycerol, and other hydrocarbons to ethanol [44].

mitigation [39, 41]. Among the various biomass resources that has been investigated,

, biogas and some other impurities such

stance in an anaerobic environment to generate CO2

**Table 2.** Primary biomass conversion process and processed biomolecules [21].

**Conversion processes Biomass Components**

Direct combustion ✓ ✓

Pyrolysis ✓ ✓ ✓ ✓ Gasification ✓ ✓ ✓ ✓

Anaerobic digestion ✓ ✓ ✓ Cellulose only Fermentation ✓ ✓ Cellulose only

for CO2

many locations of the world [39].

Transesterification ✓

78 Agricultural Waste and Residues

ethanol utilization in form of fuel [21].

Various other methods of thermochemical conversion processes for biomass conversion abounds, which are carried out at supercritical temperature and pressure and are usually at higher reaction condition compared to biochemical processes [46]. This process has been employed to generate a number of important bio-based products. These methods include direct combustion, pyrolysis, gasification and hydrothermal liquefaction (**Table 2**).

An important method for biomass conversion via thermochemical route is direct combustion methods is employed to produce the major bioenergy resource of the world accounting well above 97% of world bioenergy index [43]. It is the most common way of extracting energy from biomass. Direct combustion methods produce energy only in the form of heat and electric power as such it is not employed for biofuel production [47] and it considered several feedstocks such as energy crops, agriculture residues, forest residues, industrial and other wastes [48].

Another production process is pyrolysis, which is an important biomass conversion method that heralds the combustion or gasification of solid fuels. It comprises of thermal degradation of biomass feedstock at temperatures of about 350–550°C, under pressure, in air tight compartment [21]. This approach affords three fractions: liquid fraction (bio-oil), solid (largely ash), and gaseous fractions. Pyrolysis has been useful over time in charcoal production, however, it is only been recently considered due to the mild temperature and short residence time [49]. The product generated from the fast pyrolysis technic is known to be made up of more than two-third of the feedstock in liquid content and is suitable for use in engines, machinery and myriads of other applications [49]. An integrated approach where fast-pyrolysis can be co-processed with fossil fuel in conventional refinery is the current trends in research in which refined hydrogen can be utilized for blend to upgrade the oil into locomotive fuels and, in turn, some gases of pyrolysis are employed in the refinery [42, 50]. The feasibility of this approach is a measure of the comparable cost of natural gas, biomass feedstock, and incremental capital costs. Co-processing of petroleum with renewable agricultural residues offers advantages from both technological and economic considerations.

Subject to sustainable practices and advocacy as well as the availability of feedstock, the utilization of biomass feedstock in biofuel and bioenergy production promise to be prominent approach and the generated biofuel products are known to be comparative in characteristic feature with petroleum products. The first large-scale plant facility employing fast pyrolysis and bio-crude refining method in the United States amounting to about \$215 million projects is the KiOR Inc. plant situated in Columbus, Mississippi [50].

Pyrolysis of biomass and their direct liquefaction method with water are often used mistaking to mean the same thing; however, there exist a striking difference between the two processes. Although they are both thermochemical conversion methods that involve the alteration of various components of biomass into liquid products. Whence liquefaction involves decomposition of macro-molecule feedstock into smaller fragments of light molecules where an appropriate catalyst is employed in the conversion. Subsequently, the unstable smaller fragments are re-polymerized into oily constituent with comparable molecular weights with fossil equivalent. Whereas in pyrolysis, the generated fragments are instantaneously merged to an oily compound and the use of catalyst is predominantly may be subject to necessity [43].

Furthermore, numerous works have assessed the technical feasibility of crop residue production in China. Jiang et al. [63] used a GIS-based approach to examine the availability of crop residues in China. A number of cereal crop were considered and the findings demostrated China potential to provide about 506 million dry biomass metric tons of the residues annually. In another study, Qiu et al. [64] adopted remote sensing data and reported about 729 million MT crop residues in 2010, of which about 20–45% of this amount could substitute coal subject to regional utilization and customary needs of crop residues. Liu et al. [65] discovered that about 630 million MT of crop residues was harvested annually over a decade between 1995 and 2005. The observable dicotomy is as a result of the several factors such as considered crops, assumptions relative to crop-to-residue ration, and residue collection methodology, which is evidence in the estimated technical availability of crop residues available in the

Significance of Agricultural Residues in Sustainable Biofuel Development

http://dx.doi.org/10.5772/intechopen.78374

81

In estimation of the technical potential of crop residues production, production cost of the residues and the cost of feedstock were never considered in past reports. In certainty, farmers' preparedness to collect crop residues rely significantly on the yields and production costs of crop residues as well as on the biomass prices provided in the market. Specifically, the biomass prices offered must cover the costs of collecting crop residues. In this regard, Chen [66] examined the potential yield of each type of crop residue in China at various prices and subsequently, estimated the collective supply of crop residues at these prices. As regards the crop residues, different residues were considered as potential residues and due to the inherent yield and cost uncertainty, they derived the supply curves of the crop residues using alternative assumptions about the production costs of crop residues and residue collection

In Tanzania, the major commercially sourced after agricultural crops include sugar, cotton, tea, cashew nut, tobacco, coffee, and sisal. Significant amounts of residues from these crops have been utilized for the cogeneration of electricity in the sugar sector. Convesely, only a small amount of sisal residues had been utilized as substrate in a pilot biogas plant to generate electricity since 2008. Moreover, almost all biomass can be converted into energy; crop residues are not an exception. The types of residues available for energy generation in the commercial crop sector in Tanzania were bagasse, coffee husks, cashew nut shells, tobacco

The energetically available share of these residues was determined by the termed non-energy applications, whence the energy content of residues is influenced by the plant structure and the moisture content of the residue. Considering the account of these different parameters, the heating value for every tonne of dry matter had been reported. Although they submitted to probability of the estimation due to expedient losses during collection and transportation, the upper bound demonstrated that all residue types contain a incredible energy propensities. The combined potential of 6053 TJ is equivalent to 1680 Gigawatt hours (GWh). This estimated maximum potential is equivalent to over 37% of the country's electricity generation of 4553

results.

technology.

stems and sisal pulp [67].

GWh in 2008 [68].
