Dark fermentative biohydrogen production from lignocellulosic biomass: Technological challenges and future prospects

Biohydrogen is a promising low-carbon energy vector because its high energetic density, and emerging technologies has been studied aiming achieving higher efficiency and competitive H-2 production, as is the case of dark fermentation. The objective of this paper is to review dark fermentative biohydrogen production from lignocellulosic biomass, presenting insights of biomass pretreatment methods, influential factors in dark fermentation, and environmental and economic aspects. Rice, corn, and wheat residues have been the main lignocellulosic sources studied, and biohydrogen production ranged from 12 to 7019 mL H-2/L. This wide variation is due to the source of lignocellulosic and its pretreatment method, the source and treatment conditions of the inoculum, and the operational conditions of dark fermentation. Acid hydrolysis has been the most applied method to breakdown the complex structure of lignocellulosic biomass, and enzymatic hydrolysis has been used in sequence to improve the process. Moreover, additives (mainly metal materials) have been studied to enhance dark fermentation and lignocellulosic biomass pretreatment. Heat-treated mixed culture is the main used source of inoculum -100 degrees C for 30 min is the most usual condition. Temperature, pH, and hydraulic retention time (HRT) have also high influence in the biohydrogen production and yield. Mesophilic temperatures (around 37 degrees C), pH near 7.0, and HRT of 72 h, are recurrent parameters of dark biohydrogen fermentation. Finally, most studies focused on laboratory scale, which suggest advanced studies on a large scale, and alternatives to improve lignocellulosic biomass pretreatment and biohydrogen production is necessary to make this technology efficient, economical and sustainable.