Aqueous Acid Pretreatment
Acid pretreatment is a well known process to obtain cellulosic materials suitable for enzymatic hydrolysis. Concentrated acids and dilute aqueous acids have been used for wood, agricultural wastes, and soft leafy biomass forms. The most commonly used acid is dilute aqueous sulfuric acid, but other acids have also been studied such as hydrochloric acid, phosphoric acid, nitric acid [130, 151,152], organic acids (oxalic, citric, tartaric, acetic acid) [153], and formic acid [154, 155].
The acid pretreatment usually consists of the addition of concentrated or diluted aqueous acid solution (usually between 0.2% to 2.5% w/w) to the powdered or chopped lignocellulosic biomass, followed by constant mixing at temperatures between 130°C and 210°C. A number of reactor configurations are known, including, liquid percolation through a bed, spraying on to the residue after which the residue is heated, or agitation with the biomass in a reactor. The mixture of acid and biomass can be heated indirectly through the vessel walls or by direct steam injection. Depending on the type of biomass and concentration of the acid, pretreatment can take from a few minutes to hours. A wide variety of biomass types have been tested using acid pretreatment technique including wheat stover [156, 157], corn stover [158, 159], rice straw [159, 160], rice hull [161], sorghum bagasse [162-164], sugarcane bagasse [165, 166], purple guinea grass [167], paulownia [168], waste paper [169], Eucalyptus globulus [170], and hardwoods (red maple, sweet gum, trembling aspen and red alder) [171].
Usually, heating high hemicellulose-containing biomass with dilute aqueous sulfuric acid has been used to manufacture furfural by hydrolyzing the hemicellulose to simple sugars such as xylose, which continue to convert into furfural. Acid pretreatments have been used as a part of overall processes in fractionating the components of lignocellulosic biomass due to its ability to hydrolyze hemicellulose. In this process, acid pretreatment followed by alkali pretreatment is used for the removal of lignin, producing relatively pure cellulose.
The effect of dilute sulfuric acid pretreatment on Bermuda grass and rye straw was studied by Sun et al. [172]. Bermuda grass and rye straw were pretreated with a solid loading of 10% at 121°C with different sulfuric acid concentrations (0.6, 0.9, 1.2 and 1.5%, w/w) and residence times (30, 60, and 90 min). Then the pretreated biomass samples were subjected to a 48-hour enzymatic hydrolysis, and the maximum total reducing sugars were found to be 197.1 mg/g and 229.3 mg/g of dry biomass for Bermuda grass and rye straw, respectively [172]. In another study, 0.75% sulfuric acid pretreated (120-190°C) wheat straw and rice hull produced maximum sugar yield of 565 ± 10 mg/g for wheat straw (76% yield based on total carbohydrate content) and 287 ± 3 mg/g for rice hull (60% yield based on total carbohydrate content) [173, 174].
Chen et al. recently investigated [158] the dilute sulfuric acid pretreatment of corn stover by varying the acid concentration (0.5%-1.25%(w/w)) and the temperature (130-160°C). Given the overall sugar yield, the most favorable pretreatment was performed with 0.75% sulfuric acid at 150°C for 30 min, and then with a cellulase enzyme loading of 15 FPU per gram of cellulose, which resulted in a total of 49.74g glucose and xylose from 100g dry corn stover [158]. Then, fiber physical features, structure and properties of pretreated corn stover samples were studied using scanning electron microscope (SEM) and Fourier transform infrared spectroscopy (FTIR). Furthermore, The SEM pictures indicated that the biomass structure was deformed and its fibers were exposed by the pretreatment. FTIR study showed that lignin and hemicellulose were partially removed during the diluted sulfuric acid pretreatment [158].
The conventional dilute acid pretreatment processes use relatively large quantities of sulfuric acid and require alkali for neutralization of the sulfuric acid used, before hydrolyzing with cellulases. For this step, lime is used in equivalent amounts, and significant amounts of sulfate salts are generated as byproducts. The calcium sulfate produced can be sold as a byproduct from the cellulosic ethanol plant to recover some of the costs. However, a large quantity of wastewater is generated in the whole process. Wastewater treatment is an expensive, yet indispensable part of commercial level biomass-to-ethanol plants. Therefore, reducing acid use to the minimum level possible would be of great interest to large-scale implementation of this technology. Therefore a number of researchers have investigated the possibility of lowering the acid concentration to a minimum, without compromising the pretreatment efficiency.
Chen and coworkers at Novozymes, USA, recently reported [175] their results on reducing acid in dilute acid pretreatment of corn stover and the impact on enzymatic saccharification. The pretreatment was conducted at lower acid levels than the conventional process reported in the literature, while using longer residence times. The study indicates that a 50% reduction in acid consumption can be achieved without compromising pretreatment efficiency when the pretreatment time was extended from 1-5 min to 15-20 min. To avoid undesirable sugar degradation and inhibitor generation, temperatures should be controlled below 170°C. When the sulfuric acid to lignocellulosic biomass ratio was kept at 0.025 g acid/g dry biomass, a cellulose to glucose conversion of 72.7% can be achieved at an enzyme loading of 0.016 g/g corn stover [175]. In addition to this, they found that acid loading based on total solids (g acid/g dry biomass) governs the pretreatment efficiency rather than the acid concentration (g acid/g pretreatment liquid).
In some cases, extremely low acid (ELA) conditions have been employed with encouraging results. Lee and coworkers have investigated the enzyme digestibility of brown macro-algae, Laminaria japonica, treated with 0.02-0.14% sulfuric acid at 150-180°C for 5-20 min [176, 177]. A sample of their results is shown in Figure 5.11. Under optimal conditions, a fourfold improvement of digestibility
could be achieved utilizing 0.06% sulfuric acid at a temperature of 170°C for 15 min.
A two-stage dilute acid pretreatment method to enhance biomass digestibility and maximize sugar recovery was reported by Nguyen and coworkers [178]. In this technique, first a low temperature, low acid concentration acid pretreatment was used to promote hemicel — lulose hydrolysis/recovery, and in the high-severity second stage a higher temperature, with higher acid concentration was used to hydrolyze a portion of the remaining cellulose to glucose [178].
In comparison with other methods, there are some advantages in acid pretreatment technique. Acid pretreatment can significantly improve hemicellulose and cellulose hydrolysis by varying the severity of the pretreatment, so the concept of combined severity can be conveniently applied. The C5 sugars separated in the hydrolysis of hemicellulose can be fermented separately, or can be combined with the enzyme hydrolyzate [178]. The major disadvantage in the acid pretreatment is the formation of fermentation inhibitors such as 5-hydroxymethylfurfural, furfural, and levulinic acid. Therefore, it is necessary to remove these fermentation inhibitors by applying a washing step, which adds to the overall cost of the process.
Another disadvantage is that expensive pretreatment vessels constructed with acid-resistant material are required in this process [1]. Generally, acid pretreatment costs more than most other physicochemical pretreatment methods such as steam explosion and ammonia fiber/freeze explosion (AFEX), especially the two — stage acid pretreatment techniques, due to the inability to recycle the acid; hence, the cost of acid and lime required to neutralize the acid.