Literature Review
Several studies have been conducted to evaluate how lignocelluloses can e used to generate bioethanol. According to Tran et al. (2019), there are two main processes involved in producing bioethanol from lignocelluloses. The pretreatment process is carried out before fermentation and hydrolysis. It increases the amorphous regions, which can easily be hydrolyzed. Pretreatment also liberates cellulose from lignin and hemicelluloses (Tran et al., 2019). The pretreatment methods used can be physical, chemical, and biological. Mechanical processes decrease biomass to size and improve the contact surface. Thermophysical methods such as steam exploding reduce biomass crystallinity and cellulose polymerization degree. The chemical methods involve HCL and H2SO4 in the pretreatment of lignocellulose (Tran et al., 2019). However, highly concentrated acids are not recommended because they would result in toxicity and hydrolysis of hemicellulose and cellulose during the pretreatment process (Tran et al., 2019). Equally, diluted acids are not the best as they can generate furfural compounds, which may prevent microorganisms’ growth during the fermentation process. The biological process involves using microorganisms, such as Phlebia and Pleurotus, in the pretreatment process. However, Limayem & Ricke (2012) also include distillation or separation as crucial processes in generating ethanol from lignocellulose sources. It involves the separation of ethanol from water using their different volatilities.
Similarly, Gonçalves et al. (2016) carried out research to compare Zymomonas mobilis, Pichia stipitis, and Saccharomyces cerevisiae performance in producing bioethanol from coconut fibre mature (CFM). Semi Simultaneous saccharification and fermentation and Simultaneous saccharification and fermentation (SSF) methods were used. The catalyst was sodium hydroxide (HPCSH). The SSF is considered an optimal lignocellulose conversion method because it results in a high amount of bioethanol (Tran et al., 2019). Quantitative acid hydrolysis with 5ml, or 72%, was used for chemical characterization (Goncalves et al., 2016). The delignified pretreated CF solids were used in SSF. The study revealed that the most efficient microorganism for ethanol generation is determined by the generation methods used (Goncalves et al., 2016). Equally, according to Puttaswamy et al. (2016), chemically defined media also improves overall cellulolytic activity. The pretreated lignocelluloses can also be converted to bioethanol through hydrolysis, fermentation, and direct microbial conversion (Tran et al., 2019). However, Saccharomyces cerevisiae is commonly used in first-generation bioethanol (Kang et al., 2014). Besides, the fermentation rate is higher when Zymomonas mobilis is used. Correspondingly, Vaithanomsat et al. (2011) carried out a study to determine coconut husk efficiency in generating ethanol. The authors noted a high generation of ethanol through SSF and separate hydrolysis and fermentation (SHF) from the cellulose of coconut husk. However, Scott et al. (2013) suggest that glucose and xylose-produced saccharification can reduce the saccharification rate.
De Santana et al. (2019) also conducted to determine the amount of ethanol generated from a green coconut. The study reveals that untreated, regardless of its high content in lignin, untreated coconut is a potential source of sugars that can be fermented. The authors suggest that it is necessary to check the potential amount of sugar released from treated and untreated materials before using green coconut as a source of ethanol. The pretreatment process has huge impacts on the final quality and cost of ethanol. Contrary to using sodium hydroxide for pretreatment, De Santana et al. (2019) revealed that pressurized liquid might be a good option in the pretreatment process because it reduces the catalyst used also eliminates the need to use filtration.
Kang et al. (2014) reveal that “first generation” bioethanol results from sugar-based raw materials’ fermentation. However, “second generation” bioethanol results from the fermentation of lignocellulose raw materials. The development of strategies that help in generating bioethanol from sugar released from cellulose helps to deal with agricultural byproducts such as sawdust, wood trimmings, and straw (Kang et al., 2014). Puttaswamy et al. (2016) also agree that agricultural residues such as grass, sawdust, and wood chips can be used as bioethanol sources. However, Limayem & Ricke (2012) group the sources of lignocelluloses into forest woody feedstock, marine algae, and agricultural residues. Kang et al. (2014) categorize hydrolysis into the first stage and second stage hydrolysis. First stage hydrolysis involves the use of acids and enzymes to hydrolyze the pretreated lignocellulose. The second stage, hydrolysis, is the conversion of the released cellulose biomass into glucose.
Puttaswamy et al. (2016) conducted a study to determine the amount of bioethanol produced from water hyacinth, wheat straw, sugarcane bagasse, ragi straw, and rice straw. The sugarcane bagasse generated the highest amount of bioethanol, and the least amount was produced from water hyacinth. On the contrary, Johnson et al. (2020) focused on various benefits of ethanol generation in Jamaica. It will help the country to reduce the amount of money spent to get ethanol from international markets. It will also help them to reduce greenhouse gases by producing ethanol from sugarcane.
Therefore, the studies agree that pretreatment and hydrolysis are the major processes involved in generating bioethanol from lignocelluloses sources. The sources can include sugarcane, coconut, sawdust, and marine algae. The major gap noted is the lack of a sustainable process of producing bioethanol (Scott et al., 2013). There is also limited research on genetically modified microorganisms