Enhanced biogas production from lignocellulosic materials using NMMO and organosolv pretreatments

Biogas production via anaerobic digestion is a well-established technology for renewable energy production and a steadily growing bioprocess worldwide. It promotes a sustainable development of the energy supply, while offering the opportunity to deal with the reduction of the amount of organic wastes, decreasing the environmental impact (Jeihanipour et al., 2011). Lignocellulosic materials (LMs) present several features that make them particularly attractive among the organic substrates commonly employed in anaerobic bioreactors. In particular, LMs under the form of agricultural residues have been acknowledged as the most suitable biomass for biomethane production due to their high availability, low cost, sustainability and no direct competition with food and feed production (Kabir et al., 2015). However, their recalcitrance to biological conversion still hinders their application for commercial production of biogas and requires a pretreatment step to improve their microbial degradability. Thus, research is focusing to implement pretreatment strategies that are able to overcome the challenges due to the LMs structure, increasing their microbial degradability. Among the several methods applied to pretreat LMs, solvent pretreatments and organosolv are rising as the most promising (Karimi, 2013). This study investigates the effects of two different chemical pretreatments on the methane yields of three LMs, namely hazelnut skins and cocoa beans, generated during the industrial processing of the raw agricultural products, and rice straw, which is one of the most abundant agricultural wastes worldwide. Initially, the three LMs were separately pretreated with an organic solvent, N-Methylmorpholine-N-oxide (NMMO) at 120°C for both 1 and 3 h. Consequently, batch bio-methane production (BMP) tests were performed under mesophilic (i.e. 37 ± 2°C) conditions for approximately 45 d, as described by Esposito et al. (2011). Improvements on the cumulative methane yields were observed for all the substrates, compared to the untreated ones. In a similar experimental run, the effects of organosolv pretreatment on the three LMs were investigated. Ethanol was used as organic solvent and the pretreatment was carried out at 150 and 170°C for 30 min. The organosolv pretreatment was able to improve the bioconversion of the LMs, resulting in higher biomethane yields than the untreated feedstocks. Compositional analyses and Fourier transform infrared spectroscopy (FTIR) were performed in both experiments. Changes in chemical composition and crystallinity after the pretreatment were considered the most important factors affecting the biogas production enhancement from all of the investigated LMs.