2.3. Classification of biofuel generations
2.3.2. Second-generation biofuel
Inventing second-generation biofuels was to solve the issues linked to first-generation biofuels. It is essential to underline that the feedstock used to produce first-generation biofuels is mainly agricultural, forest residues and non-edible food crops. The production of second-generation biofuels saw the light of life to overcome the limitations of the first-generation. Second-generation biofuels are produced from plants, waste material, or non-edible plants [25].
The consideration of second-generation biofuels is down to the inability of first-generation biofuels to solve the issues linked to fossil-based fuels. Nevertheless, the first-generation biofuels added problems and did not provide enough solutions to environmental concerns; instead, they impacted food security, they increased competitiveness with the food industry and prices [26, 27].
Second-generation biofuels use a different source of biomass in comparison to first-generation. Second- generation biomass consists mainly of lignocellulosic biomass, woody biomass, and inedible seeds such as Jatropha curcas and waste cooking oil. The feedstock used in the manufacturing process of second- generation biofuels is available and cheaper [26, 27]. According to Naik et al. and Mohammadi et al. [26, 27], the source of second-generation is either food industry remnants or those produced during the manufacturing process of the first-generation biofuels.
Second-generation biofuels are different from one another. The difference is in the conversion platforms used to produce fuels. There are two sets of second-generation biofuels, which are biochemical and Thermochemical second-generation biofuels.
A thermochemical conversion platform is a novel technology that produces biofuels. The conversion platform that makes second-generation biofuels has no first-generation analogue [28].
The mechanisms used in producing second-generation biofuels via the thermochemical conversion platform and those used in producing fossil-based fuels are similar. Figure 2.1. illustrates the similarity between fossil-based fuels and thermochemical second-generation biofuel production.
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Figure 2. 1: Production pathways to liquid fuels from biomass and, for comparison, from fossil fuels [28]
2.3.2.1. Second-generation biochemical biofuels
The feedstock used to produce first-generation biofuels (FGBs) presents no difference in properties from the one used to produce second-generation biofuels (SGBs). However, the methods or techniques used are different. Second-generation biofuels produced using lignocellulosic biomass are generally called cellulosic biofuels [21].
The second-generation biofuels (SGBs) production via the biochemical conversion platforms uses the following steps: pretreated, saccharified, fermented, and distilled. The feeds are pretreated to separate three main components of biomass: cellulose, hemicellulose, and lignin, into simple sugars to allow the enzyme to catalyze through the addition of water which decomposes carbohydrate molecules parts "cellulose and hemicellulose" [22]. Cellulose is a crystalline lattice composed of long chains of glucose C6 sugar molecules. The decomposition of cellulose into simple has never been easy, resulting in difficulty fermenting cellulose. The disintegration of biomass enables a quick fermentation process using microorganisms. However, the fermentation of hemicellulose is challenging. It isn't easy to convert biomass consisting of C5 sugar molecules to biofuels. It is a complex process despite the hemicellulose being less tenace and easy to decompose. The newly developed microorganisms have helped to ease the fermentation process of hemicellulose [22].
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Figure 2. 2: Simplified depiction of process steps for the production of second-generation fuel ethanol [28]
The investigation of an alternative energy source has been ongoing for an extended period resulting in the production of biodiesel and biogas at the industrial level. However, bio-gasoline production is still staggering [29].
Several process designs have been elaborated and proposed regarding the production of second-generation biofuels via a biochemical conversion platform, including the production of bioethanol. One of the innovations is the combination of saccharification and fermentation to produce bioethanol. The well- conceived conversion platform makes second-generation bioethanol presented in figure 2.2 [29]. Besides the above-modified technologies, the consolidated bioprocessing technology has also seen the light of the day in producing second-generation biofuels. The latest technology merges enzyme production from biomass with saccharification and fermentation [30].
It is worth highlighting that the technologies developed have been solely used to produce bioethanol. There is a possibility to extend the use of these technologies to process other second-generation biofuels.
2.3.2.2. Second-generation thermochemical biofuels
A thermochemical conversion platform produces second-generation biofuels at extremely high pressure and temperature [28].
The thermochemical conversion platform is an essential technique. It can accommodate several feedstocks and yield a variety of finished biofuels compared to the biochemical conversion platform. Another advantage of the thermochemical conversion platform is the quality of its end products. The thermodynamical conversion platform yields clean-finished fuel ready for engines. Gasification or pyrolysis is the first thermochemical process used; this process is not cost-effective. Therefore, it needs a considerable scale for the best economy [28].
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The gasification process is an exciting step through which different biofuels are produced, including Fisher- Tropsch Liquids (FTL), dimethyl ether (DME), and various alcohols. Gasification uses combustion to convert biomass to gas; the transformation of biomass to gas results in combustible and non-combustible gases. The impurity in the gas is purged, followed by specific processes by adjusting, which performs using the "water-gas shift" reaction of the synthesis gas, also known as syngas, to prepare it for additional downstream treatment (figure 2.3).
Figure 2. 3: Simplified depiction of process steps for thermochemical biofuel production [28].
The downstream inlet stream consists of a mixture of a solution of syngas and a selected solvent. Mixing the syngas and the solvent is to sequestrate carbon dioxide through its dilution. The sequestration of carbon dioxide in the syngas facilitates a series of reactions downstream. After cleaning the mixture of syngas and solvent, the resulting syngas consists mainly of carbon monoxide (CO), hydrogen, and a small quantity of methane (CH4) [28].
The outlet stream comprising CO, H2, and a small quantity of methane undergoes a catalytic process to produce biofuel. Methane is inert in this process. The catalyst is the main element on which biofuel production depends. This process has the disadvantage of incomplete syngas conversion to biofuel in most plants; therefore, some syngas does not convert to biofuel. The unreacted portion of syngas is used as an energy supplier to run the facility and sometimes exports electricity to the grid. Syngas can be converted to liquid through fermentation using microorganisms (figure 2.3), even though this technology is not yet at an advanced stage commercially compared to the catalytic cracking process [28]. The equipment required to convert biomass to fuel through catalytic synthesis is readily obtainable. However, progress is necessary
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for feeding biomass into a large-scale pressurized gasifier and cleaning the raw gas produced by the gasifier [28].
This study investigates biogasoline production using second-generation biomass. The conversion platform of interest is the thermochemical and its variety.