A review on the role of pretreatment technologies in the hydrolysis of lignocellulosic biomass of corn stover

Highlights

• Corn stover, a lignocellulosic biomass has a great potential for biofuel production.

• Physical pretreatment does not produce chemical pollutants.

• Chemical pretreatment is efficient in enhanced biodegradation.

• The biological pretreatment energy efficient and eco-friendly.

• A suitable pretreatment method can be used for sugar production.

Abstract

Corn is one of the major crops in most of the agricultural countries. In the US and China, the corn production shows increasing trends with the annual production of corn stover of 250 and 220 million tons respectively. Corn stover as an abundant renewable lignocellulosic biomass, has an excellent potential to be used for biofuel production. As it is resistant to physical and biological attacks due to the presence of lignin, it requires suitable pretreatment methods that can increase the yield of fermentable sugars after enzymatic saccharification. However, all types of processes used for pretreatment affect the total sugar yield before fermentation and result in the formation of inhibitory products. Thus, it is challenging to select the best pretreatment process. Therefore, researchers are more focused on identifying the best pretreatment method by combining two or more pretreatment techniques, which is convenient rather than a single pretreatment method to enhance the digestibility of maximum available sugars. In this review, detailed insight into the prospective and challenges of commonly used pretreatment techniques, particularly for corn stover as biomass, is presented to understand pretreatment's role in biofuel production. However, still there is a need for research to develop novel and more efficient pretreatment processes for the utilization of lignocellulosic biomass to produce useful products for conversion into biofuel.

Introduction

Lignocellulosic feedstocks are readily available bioresource materials found globally as agricultural waste, renewable, sustainable, clean, and inexpensive biomass energy sources. This abundant lignocellulosic biomass can facilitate the replacement of conventional fossil fuel resources [1,2]. China is the biggest agricultural country and produces massive crop residues such as corn stover, wheat straw, rice straw, sugarcane bagasse, cotton stalk, and bamboo. Corn stover is obtained from corn crop in a significant amount worldwide, which is a typical harvest in agricultural countries, such as the US, China, Brazil, India, and Pakistan (Fig. 1). As a large agricultural country, China produces a colossal amount of crops, providing 0.7 billion tons of crop straw [[3], [4], [5], [6], [7]]. This tremendous amount of corps straw is a significant source of biomass for the production of biofuel. The annual corn stover production is 250 million tons in the US and 220 million tons in China [8]. According to China's national bureau of statistic, corn production was reported as 257 million tons in 2018. Among the produced corn stover more than 81.48% could be used in China for production of biofuel [9].

However, half of the corn stover remains unutilized due to a lack of proper disposal methods, whereas some of it is used for animal feeding, and some of it is burned in the open air [10,11]. This informal disposal of corn stover tends to release pollutants into the atmosphere and negatively impacts the local and regional environment [12].

Corn stover is lignocellulose and has a complex structure mainly composed of cellulose, hemicellulose, and lignin. Cellulose is a highly crystalline polymer surrounded by hemicellulose as a matrix, while lignin is a tough covering layer [13]. This recalcitrant nature of corn stover is the main obstacle to its effective utilization. Many other structural and compositional factors are the bottleneck in the hydrolysis of cellulose to sugars and generation of value-added products [14]. Therefore, various pretreatment processes have been developed to enhance cellulose accessibility to enzymatic hydrolysis for conversion into sugar. In general, pretreatment processes aim to change the physical and chemical structure of the lignocellulosic biomass and improve hydrolysis rates and high sugar yields [15]. The initial step of the pretreatment process is to disrupt the cell wall and uncover the cellulose and hemicellulose fractions for enzyme accessibility. During the pretreatment, the effect of consecutive hydrolysis and fermentation of biomass is a significant challenge [16]. The careful selection of the most appropriate pretreatment method is essential, which should be low cost, energy-efficient with minimal formation of degradation products. In recent years researchers are focusing on development of pretreatment technologies which are time efficient, cost efficient and higher enzymatic hydrolysis. An ideal pretreatment approach required to produce highest fermentable sugars, with minimum inhibitors release, and fastest hydrolysis with overall efficiency of the system [17].

A limited number of reviews have been presented about different pretreatment processes suitable and apt for efficient use of corn stover in future growth and development. The present study evaluates and analyzed selected references on the pretreatment process used for corn stover for the sugar recovery and subsequent enzymatic hydrolysis [16]. The objective is to find the merits and demerits of various pretreatment methods used for corn stover and to identify the most suitable pretreatment methods for industrial-scale acceptance. This review gives information about the corn stover pretreatment process and discusses the high yield of valuable products from corn stover by different pretreatment conditions.

The present review is divided into four main sections besides the introduction part. In section-2, the corn stover to biofuel is described. Whereas the section-3 explains the methods of pretreatment of corn stover, including physical, chemical, physiochemical and biological pretreatments. Finally, in section-4, the conclusion has been made for overall review.