Methane production from the anaerobic digestion of substrates from corn stover: Differences between the stem bark, stem pith, and leaves

Highlights

• The methane yields and hydrolysis kinetic constants of SB, SP, and LV were evaluated.

• The chemical and physical changes of the substrates during digestion were inspected.

• Correlation was established between chemical changes and methane production.

• The crystalline structure of cellulose had limit effect to anaerobic digestion of SB.

• The abundance of Bacteroidetes and Firmicutes was positively related to methane yields.

Abstract

This work evaluated the methane potential and methane production rate of the stem bark (SB), stem pith (SP), and leaves (LV) of corn stover from batch anaerobic digestion. The obtained cumulative methane potential and the hydrolysis kinetics constant were 0.201, 0.214, and 0.199 L g⁻¹ VS (volatile solids) and 0.090, 0.149, and 0.227 d⁻¹ for SB, SP, and LV, respectively. The chemical composition and the crystalline structure of the substrates as well as their changes during the anaerobic digestion were inspected, and their impacts on the characteristics of methane production were assessed. The methane production rate correlated positively with the hemicellulose and soluble compounds content and negatively with the cellulose and lignin content, but the degradation rates of hemicellulose and cellulose in the specific substrate were complex and comparable. The methane production has limit correlation with the crystalline structure of the substrates. Microbial community structure was analyzed to elucidate functional microorganism contributing to methane production of different substrate. The abundance of Bacteroidetes and Firmicutes was most affected by the substrate, and positively related to methane yields.

Introduction

China is one of the largest agricultural countries in the world and has abundant biomass resources, and thus has a significant amount of crop residues awaiting utilization. In 2017, 884 million tons of agricultural straw was produced in China, about 32.5% of which was corn stover (National Burean of Statistics of China, 2018). Although many methods have been developed to reutilize corn stover as energy source, livestock feeding stuff, pulp, biofertilizer, etc., still a large amount of it is burnt in the open field, which causes serious problems including air pollution, surface water pollution, fire disaster, and potential safety concerns for air craft and ground traffic (Wang et al., 2007). Research works about the anaerobic digestion of corn stover to produce of methane has attracted significant attention in the past decade because they cannot only solve the environmental problems but also help in the energy structure adjustment and ecological agriculture development (Dang et al., 2014).

The production of biogas from lignocellulosic biomass by anaerobic digestion has great potential for bioenergy but has not been widely adopted because lignocellulose has a complicated structure that readily resists microbial attack (Chandra et al., 2012), which prolongs the reaction time of the anaerobic digestion and undermines the economic feasibility of the biogas production due to the slow degradation of lignocellulose.

Treating the different substrates having different anaerobic fermentation characteristics with different processes can improve the project operation efficiency. Corn stover is an inhomogeneous material composed of stem bark (SB), stem pith (SP), and leaves (LV). These parts differ significantly from each other in compositional and structural properties, and their properties in biotransformation are thus very different. Hence, the different parts should be handled with different technology or at least with different parameters of anaerobic digestion for the sake of effective utilization and optimal operation performance. Before using corn stover as feedstock at an industrial scale, it is necessary to evaluate the effect of the chemical and structural properties of the different parts on the bioconversion.

Pretreatment of the recalcitrant lignocellulosic biomass is essential for attaining high biogas yield in the anaerobic digestion. Various techniques of physical, chemical, and biological pretreatments have been developed over the past decades (Yang and Wyman, 2008), but they were most based on the pretreatment of the whole corn stover (Chandra et al., 2012; Ward et al., 2008). In view of the big differences in the chemical composition of different parts of corn stover, the uniform pretreatment and anaerobic digestion would unsurprisingly give rise to different performance for different parts. Furthermore, this may be a very important reason as for why comparable results are hard to achieve when it comes biogas production from corn stover.

On the other hand, the different parts of corn stover are good model substrates to study the anaerobic digestion process of lignocellulosic material. Lignocellulose, which consists of cellulose, hemicellulose, and lignin, is the main component of corn stover, and the hydrolysis of lignocellulose is the main limiting step in the anaerobic degradation of corn stover (Divya et al., 2015). Some works used a single component (e.g., microcrystalline cellulose, xylan, etc.) as the substrate to study the anaerobic digestion of lignocellulosic biomass (Wang et al., 1994; Yu et al., 2012). However, since the primary hurdle in the degradation of lignocellulose is the bonding and wrapping between the structures of cellulose, hemicellulose, and lignin, it makes much better sense to use realistic lignocellulose materials with different chemical compositions as the substrates to study the anaerobic digestion process.

With these considerations in mind, this paper evaluated the methane potential and studied experimentally the anaerobic degradation kinetics of different parts of corn stover. We firstly estimated the methane potential and methane production rate of the SB, SP, and LV of corn stover, and we then examined the effects of their chemical composition and physical structure on the anaerobic digestion properties. Finally, we evaluated the correlation between methane production and the chemical composition changes of these parts of corn stover.