Hydrothermal gasification of corn-cob hemicellulose to produce hydrogen and methane

Abstract

Hydrogen and methane are drawn attention due to their potential of using as clean energy sources recently. Production of these valuable gases by hydrothermal gasification (supercritical water gasification) from the biomasses is of interest and novel technology. In this technique, organic part of the lignocellulosic biomasses is converted into gaseous products substantially above the critical temperature and pressure of water (374^oC, 221 atm). The most significant advantage of the method is to be able to gasify the wet biomass materials without the need for drying.

Lignocellulosic biomasses mainly consist of cellulose, lignin, hemicelluloses and extractive parts. They show different attitude in hydrothermal gasification. Significant varieties are observed in the gasification yields and product distributions of these parts. In recent years, intensive studies have being done using cellulose, lignin and model compounds forming them. There is not enough work carried out with the hemicelluloses which are being approximately 1/3 of a plant, and this is a significant deficiency in the literature.

The experiments were carried out in the set up of hydrothermal gasification with reactor inner volume of 100 cm³. The corn-cob hemicellulose was used as feedstock. The effect of the reaction temperature (300, 400, 500 and 600°C) and the catalysts (K₂CO₃ and KOH) were investigated to reach maximum mol of H₂ and CH₄. Carbon gasification efficiency (CGE; g C in gaseous /g C in biomass), carbon liquefaction efficiency, (CLE; g C in aqueous /g C in biomass), and residue efficiency, (RE; g C in residue /g C in biomass) were given in Fig.1, the composition and amount of the products were identified with the analyses by GC, GC-MS, HPLC, TOC, SSM and the colorimeter. Gas product compositions were shown in Fig. 2.

Main gaseous products were CO₂, H₂, CH4, small amount of CO and C₂-C₄ hydrocarbons. In aqueous product, there were lots of organic substances such as carboxylic acids, alcohols, aldehydes, ketones, furfurals and phenols. The highest yields were obtained in hydroxy acetic acid, formic acid, acetic acid, 5MF and phenol mostly. Maximum gas conversion, highest H₂ and CH₄ yields were reached at 600°C and in the presence of catalyst. As the reaction temperature increased, CGE, H₂ and CH₄ yields increased while CLE and RE were decreased. K₂CO₃ and KOH improvement ratios on the gasification efficiency were found as 11.3% and 11.6%, respectively. H₂, CH₄ yields were increased 30.8% and 34.4% in the presence of K₂CO₃; 32.3% and 33.3% in the presence of KOH, respectively.