Pretreatment of Lignocellulosic Wastes to Improve Ethanol and Biogas Production: A Review

Abstract

Lignocelluloses are often a major or sometimes the sole components of different waste streams from various industries, forestry, agriculture and municipalities. Hydrolysis of these materials is the first step for either digestion to biogas (methane) or fermentation to ethanol. However, enzymatic hydrolysis of lignocelluloses with no pretreatment is usually not so effective because of high stability of the materials to enzymatic or bacterial attacks. The present work is dedicated to reviewing the methods that have been studied for pretreatment of lignocellulosic wastes for conversion to ethanol or biogas. Effective parameters in pretreatment of lignocelluloses, such as crystallinity, accessible surface area, and protection by lignin and hemicellulose are described first. Then, several pretreatment methods are discussed and their effects on improvement in ethanol and/or biogas production are described. They include milling, irradiation, microwave, steam explosion, ammonia fiber explosion (AFEX), supercritical CO 2 and its explosion, alkaline hydrolysis, liquid hot-water pretreatment, organosolv processes, wet oxidation, ozonolysis, dilute-and concentrated-acid hydrolyses, and biological pretreatments.

Sodium Hydroxide Pretreatment and Enzymatic Hydrolysis of Oil Palm Mesocarp Fiber

“…Previous works done by [21] and [9] also agree that, alkaline pretreatment could cause cellulose fibers to swell rather than directly degrading it. They concluded that this pretreatment is the best method to avoid further fragmentation of hemicelluloses polymers as well as break up the ester bonds between lignin, hemicelluloses and cellulose [22].…”

“…Numbers of research and development activities were previously done in order to exploit biomass content potential and transform it into higher valued compounds [5]- [8]. In order to achieve these targets, most biomass conversion process requires pretreatment to increase the accessibility of enzyme during hydrolysis process [9]. The process to remove lignin compound from lignocellulosic biomass prior to be used for enzymatic hydrolysis and fermentation process become an important factor in order to produce suitable source fermentable substrate in the next process.…”

Biological Pretreatment of Lignocellulosic Substrates for Enhanced Delignification and Enzymatic Digestibility

“…The effect of this area may correlate with crystallinity or lignin protection or hemicellulose presentation or all of them [10].…”

“…An effective and economical pretreatment should meet the following requirements according to Taherzadeh and Karimi [10]. [6] describes these processes in detail, a list of which is given below.…”

“…Dilute acid pretreatment is probably the most commonly applied method among the chemical pretreatment methods. It can be used either as a pretreatment of lignocellulose for enzymatic hydrolysis, or as the actual method of hydrolysing to fermentable sugars [10]. High rate of hydrolysis is obtained with dilute acids, but they generate toxic degradation products.…”