ACID HYDROLYSIS

During acid hydrolysis, concentrated acids like HCl and H2SO4 have been used to pretreat lignocellulosic biomass. Although acids are influential agents for cellu­lose hydrolysis, intense acids are poisonous, corrosive, and require chemical reactors that are resistant to corro­sion. In addition, concentrated acid must be removed af­ter hydrolysis of celluloses into simple sugars, which simultaneously enter into alcoholic fermentation (Potumarthi et al., 2013; Ravichandra et al., 2013). Hydro­lysis using dilute acid has been industriously developed for pretreatment of lignocellulosic biomass (O’Donovan et al., 2013). The dilute sulfuric acid pretreatment can attain high reaction rates and drastically improve cellu­lose hydrolysis. Dilute acids at lower temperatures, saccharification suffered from low yields because of sugar decomposition (Chen et al., 2009). High tempera­tures, with dilute acids are favorable for cellulose hydro­lysis. In recent times, dilute acid hydrolysis processes use less harsh environment and achieve high xylan to xylose conversion rates. Achieving high xylan to xylose conver­sion yields is required to achieve favorable economics, because xylan is the third most promising carbohydrate in many lignocellulosic feedstocks (Sun and Cheng,

2002) . Primarily two types of dilute acid pretreatment processes are well studied: high-temperature

(T > 160 °C), continuous flow process for low solids loading (5—15% (weight of biomass/weight of reaction mixture)) (Converse et al., 1989), and low-temperature (T < 160 °C), batch process for high solids loading (10—40%) (Esteghlalian et al., 1997). Although dilute acid hydrolysis can significantly improve the cellulose
breakdown, overall cost is typically higher when compared with few other physicochemical pretreatment processes such as steam explosion.

ALKALINE HYDROLYSIS

Usually alkaline hydrolysis was carried out at low temperature and pressure and it may be completed even at ambient conditions. The only drawback of this process is time; it might be hours or even days to com­plete the hydrolysis. During lime pretreatment, some cal­cium is tainted into nonrecoverable salts or included in the biomass (Chang et al., 2001). Other alkaline pretreat­ment methods include calcium, potassium, sodium and ammonium hydroxides as reactants based on biomass category. Among these reactants, sodium hydroxide re­ceives the most attention followed by lime, due to its advantage of being low cost and secure to use, as well as it is easily recoverable from water as insoluble CaCO3 by reaction with CO2. Further delignification of feedstocks can be enhanced by supplying surplus air/ oxygen (Hu et al., 2008). We can compare alkaline pre­treatment of feedstocks to Kraft pulping, where lignin was removed efficiently, thus improving the reactivity of polysaccharides. Alkaline hydrolysis also effectively removes acetyl groups and uronic substitutions from hemicellulose; thus, the surface of hemicellulose becomes more accessible to the hydrolytic enzymes.

AMMONIA HYDROLYSIS

Ammonia has abundant desirable characteristics as a pretreatment reagent. It is a valuable swelling reagent for lignocellulosic biomass, and ammonia has high selectivity for reactions with lignin over those with car­bohydrates (Kim et al., 2003). It is one of the most exten­sively used commodity chemicals with about one-fourth the cost of sulfuric acid on molar basis. Its high volatility makes it easy to recover and recycle. It is a nonpolluting and noncorrosive chemical. One of the known reactions of aqueous ammonia with lignin is cleavage of ether (CeOeC) bonds in lignin as well as ester bonds in the
ligninecarbohydrate complex (Lewin and Roldan, 1971). This above reaction indicates that ammonia pre­treatment selectively cuts the lignin content in biomass. Lignin is believed to be a major hindrance in enzymatic hydrolysis and there are several advantages by removing lignin early in the conversion process before it faces the biological treatment.

OZONOLYSIS

Ozone is a leading oxidant that demonstrates high delignification efficiency. This ozonolysis is done at room temperature and at normal pressure. In this case we do not locate any inhibitory by-products, which affect the simultaneous fermentation steps (Saritha et al., 2012). An important drawback is ozone require­ment in large quantities, which can make the process economically unapproachable (Sun and Cheng, 2002).

Bioethanol Fermentation

Once the lignocelluloses were hydrolyzed into simple sugars, they have to be fermented to ethanol. The hydro — lyzate now contains various hexoses and pentoses, mainly glucose and xylose, depending on the substrate and the pretreatment method applied. Currently, fermentation of simple sugars is mostly done using yeast cultures (Saccharomyces cerevisiae), because of its well-known char­acteristics, toughness and high ethanol yield. However, S. cerevisiae can only ferment hexoses and not the pentoses. The pentose sugars can be fermented in an additional step by another microorganism or by S. cerevisiae itself through genetic engineering approaches, so that it is able to ferment pentoses as well (Van Zyl et al., 2007). List of most popular yeast strains used for ethanol fermen­tation are mentioned in Table 1.2. Besides a high yield, an important aspect with fermentation is alcohol tolerance in the fermenting organisms. A strategy to defeat this crisis is to have a system where the ethanol is recovered at reg­ular intervals to keep the alcohol concentrations under control. Another problem is inhibitory compounds that

are produced during the pretreatment. As mentioned above they can be reduced by an additional detoxification step, but this is an expensive operation (Van Maris et al.,

2006) .