Application of pretreatments to enhance biohydrogen and/or methane from lignocellulosic residues: linking performances to compositional and structural features

In the future, various forms of renewable energy, such as second generation biofuels from lignocellulosic residues, will be required to replace fossil fuels. Among these, biohydrogen and methane produced through fermentative processes appear as interesting candidates. However, biohydrogen and/or methane production of lignocellulosic residues is often limited by the recalcitrant structure and a pretreatment step prior to fermentative processes is often required. Up to date, information on lignocellulosic characteristics limiting both hydrogen and methane production are limited. Therefore, this work aims to investigate the effect of compositional and structural features of lignocellulosic residues on biohydrogen and methane performances for further developing appropriate pretreatments strategies. Firstly, a panel of twenty lignocellulosic residues was used to correlate both hydrogen and methane potentials with the compositional and structural characteristics. The results showed that hydrogen potential positively correlated with soluble carbohydrates only. Secondly, methane potential correlated negatively with lignin content and, in a lesser extent, with crystalline cellulose, but positively with the soluble carbohydrates, amorphous holocelluloses and protein contents. Pretreatments strategies were further developed to enhance both hydrogen and methane production of sunflower stalks. Dilute-acid and combined alkaline-enzymatic pretreatments, which were found efficient in solubilizing holocelluloses into soluble carbohydrates, were applied prior to biohydrogen potential tests. By combined alkaline-enzymatic pretreatment, hydrogen potential was fifteen times more than that of untreated samples. On the contrary, hydrogen production was inhibited after dilute-acid pretreatments due to the release of byproducts (furfural, 5-HMF and phenolic compounds) that led to microbial communities shift toward no hydrogen producing bacteria. Similarly, methane production, five thermo-chemical pretreatments (NaOH, H₂O₂, Ca(OH)₂, HCl and FeCl₃) found efficient in delignification or solubilization of holocelluloses, were considered. Among these pretreatments, the best conditions were 55 deg. C with 4% NaOH for 24 h and led to an increase of 29-44 % in methane potential of sunflower stalks. This pretreatment condition was validated in one stage anaerobic mesophilic continuous digester for methane production and was found efficient to enhance from 26.5% the total energy produced compared to one stage-CH₄ alone. Two-stage H₂ (batch) / CH₄ (continuous) process was also investigated. Nevertheless, in term of energy produced, no significant differences were observed between one-stage CH₄ and two-stage H₂ /CH₄. (author)