The variety and transition of key intermediate liquid products during the process of coal‐to‐biohydrogen fermentation

Abstract

Organic liquid intermediates were generated during the coal-to-biohydrogen fermentation process. The study on the generation of these organic liquid intermediates can provide information on the biogenic gas formation mechanism. In this paper, high volatile bituminous coal from Yima colliery was collected as an effective carbon source. The indigenous microflora extracted from a coal mine water was used as the bacterial source to set up simulation experiment to get biogenic gas. Then, gas chromatography and ultraviolet spectrophotometry, combined with liquid-liquid extraction gas chromatography–mass spectrometry, were used to analyze the gas composition, as well as the production of saccharide, pyruvic acid, and volatile intermediates in the bioreactor fluids. The results show that the gases generated from the fermentation experiment contained hydrogen, carbon dioxide, and nitrogen. Twelve volatile component types were detected in the bioreactor fluids: straight alkanes, branched alkanes, alkenes, alcohols, aldehydes, ketones, acids, lipids, amines, quinolones, phenols, and epoxy alkanes. The key components of organic intermediates produced were saccharides, pyruvic acid, organic acids and alcohol, straight alkanes, and amide. Saccharides, pyruvic acid, alkanes, and organic acids were closely related to the generation of hydrogen and carbon dioxide, whereas amide stimulated nitrogen generation. Three stages are divided in the fermentation process. The first stage is hydrolysis, where the total, reducing, and polysaccharides production reached a maximum, with generation of long-chain fatty acids as well as other liquid organics. The second stage is hydrogen production stage, which includes peak hydrogen production and hydrogen production stabilization stages. In the peak hydrogen production stage, saccharide production rapidly decreases, with an initial increase and subsequent decrease in pyruvic acid, straight alkanes, and branched alkanes. With the production of acetic acid and hydrogen, carbon dioxide production was initially slow but then rapidly increases. In stabilization of hydrogen generation stage, the saccharides were mostly consumed, pyruvic acid and branched alkanes' production increased slowly, but straight alkanes' production continued to decrease. In this stage, the production of acetic acid, hydrogen, and carbon dioxide was relatively stable, and the production of amide and nitrogen increased slowly. The third stage is the homoacetogenic stage, where pyruvic acid and branched alkane production continued to increase and straight alkanes' production continued to decrease. The pyruvic acid production increased abnormally, while amide and nitrogen production began to decrease and gas production nearly terminates. The liquid phase intermediate plays an important role in the coal-fermenting hydrogen generation process and in revealing the hydrogen generation mechanism.