Highlights
Rice straw was pretreated using acidified glycerol for bioethanol production.
Optimal condition was determined to at 190 °C for 10 h with 5% solid loading.
Acidified glycerol pretreatment improved digestibility of sugars by 4–5 times.
Fermentation yielded ethanol yield of 0.44 g/g with high productivity.
SHF of acidified glycerol hydrolysates showed better performance than SSF.
Abstract
Rice straw was pretreated using an industrial grade glycerol for ethanol production. The pretreatment was conducted at 130–210 °C for 1–24 h with 5% solid loading. The glucan content in the regenerated rice straw increased with increasing pretreatment temperature and time. The production of fermentable sugars initially increased as the pretreatment temperature and reaction time increased, but then decreased somewhat at the higher temperatures and with longer reaction duration. The highest amount of reducing sugar produced by the enzymatic hydrolysis was achieved at 190 °C for 10 h with 5% solid loading, optimal condition for the glycerol pretreatment of rice straw. Furthermore, it was observed that glycerol pretreatment with the addition of HCl improved the digestibility of fermentable sugars by 4–5 times that of untreated samples. Fermentation of hydrolysates resulted in an ethanol yield of 0.44 g/g sugar, corresponding to a theoretical yield of 84.3%. It was concluded that acidified glycerol is one of the good candidates of the organic solvent for the pretreatment of lignocellulosic biomass.
Introduction
Ethanol, which can be biologically produced from a variety of biomass, is accepted as an alternative to fossil fuels. Recently, there has been increasing interest over the utilization of non-starch feedstock such as lignocellulosic biomass for the production of ethanol [1], [2]. Lignocellulosic biomass is an abundant, inexpensive, and renewable source of sugars, which is considered as a desirable feedstock for the sustainable production of liquid fuels and chemical products through the biorefinery process [3].
Rice straw is one of the largest lignocellulosic biomass sources in the world, and is a common as well as abundant agricultural waste in Asian countries. It contains cellulose (24–34%), hemicellulose (19–29%), lignin (5–11%), and crude ash (10–22%). Rice straw has drawn attention for its potential use in the production of biofuel, since its high content of cellulose and hemicellulose can be readily hydrolyzed into fermentable sugars [4], [5], [6].
In general, lignocellulosic biomass is composed mainly of cellulose, hemicellulose, and lignin. The cellulose and hemicellulose are densely packed by layers of lignin, resulting in a crystalline structure that is highly recalcitrant to chemicals or enzymes [7]. This recalcitrance is largely responsible for the high cost of the bioconversion of lignocellulosic biomass. Cellulose can be enzymatically hydrolyzed to the glucose molecules that are then subsequently fermented to ethanol. However, the high crystalline structure of rice straw hinders the bioconversion process in the enzymatic saccharification step. Therefore, rice straw first needs to be pretreated in order to decrease the crystallinity of cellulose, increase the biomass surface area, remove hemicellulose, and break the lignin structures [8]. Through pretreatment, cellulose can be more accessible to enzymes, allowing the conversion of carbohydrate polymers into fermentable sugars to achieve more rapidly and with more yields. Thus, the pretreatment of lignocellulosic biomass is one of the most important processes in ethanol production [9], [10].
Many pretreatment technologies have been proposed and developed to overcome the recalcitrance of the lignocellulosic biomass and to enhance the rate and yield of enzymatic saccharification of the lignocellulosic biomass [11], [12], [13]. These methods are classified as physical, chemical, biological, and multiple or combinatorial ones. Among the chemical methods, organosolv pretreatment using organic solvents has gained increased attention because it produces less fermentation inhibitors and is more environmentally friendly than acid pretreatment. In addition, organic solvent pretreatment primarily removes hemicellulose and lignin, thus increasing the rate of enzymatic hydrolysis of cellulose [14]. The organic solvents used include solvents with low boiling point (ethanol, methanol) and high boiling point (ethylene glycol, glycerol, tetrahydrofurfuryl alcohol) solvents, and other organic compounds such as dimethyl sulfonics, ethers, and ketones for solubilizing the lignin [10], [15].
Of organic solvents, glycerol, which produces 10% of the products in biodiesel production, is widely available at low cost. Being a high boiling point and non-toxic organic solvent, glycerol increases the pretreatment reaction rate, allowing high temperature processing, and its reaction can occur at atmospheric conditions [16], [17], [18]. In addition, organosolv pretreatment is usually carried out using strong inorganic acid catalyst, such as HCl and H2SO4, in order to hydrolyze the lignin bonds in the biomass [15]. As far as we know, acidified glycerol pretreatment of rice straw was has not been widely studied, even though it is one of the most abundant agricultural residues.
In the present study, the influence of operation conditions on the bioconversion efficiency of glycerol-pretreated rice straw was investigated. Glycerol pretreatment was examined in the temperature range of 130–210 °C for reaction times over 1–24 h, with 5% solid loading. Furthermore, the addition of HCl under the optimum pretreatment conditions was considered to improve the bioconversion efficiency in the acidified organosolv pretreatment. Using optimal enzymatic hydrolysis condition, rice straw was pretreatment and fermentation of regenerated biomass was carried out.
Comments