Harnessing biomethane through thermally enhanced co-digestion of Rice straw and Napier grass

Abstract

Rice straw (RS), an abundant agricultural residue in India, is often subjected to open-field burning, contributing to severe environmental pollution. However, its potential for energy through biogas production remains underutilized due to its high carbon to nitrogen (C/N) ratio and seasonal availability. This study proposes a green pathway through anaerobic co-digestion (AnCoD) with perennial energy grass, Napier grass (NG). The maximum methane yield was achieved at a substrate-to-inoculum (S/I) ratio of 0.5. Among various substrate mixing ratios (SMR) tested, SMR 20:80 (RS: NG) yielded 200.6 mL/g-VS_added. To further enhance the energy recovery, thermal pretreatment techniques including autoclave, hot air oven, hot water bath and microwave were applied. At respective optimum temperatures, these pretreatment techniques boosted the soluble chemical oxygen demand (sCOD) by 2.3, 1.83, 1.57 and 1.56 folds, respectively, compared to the untreated sample. Further, the thermally enhanced co-digestion study augmented biomethane by 1.45 times (291.86 mL/g-VS_added) compared to untreated co-digestion (200.6 mL/g-VSadded), highlighting improved biodegradability of lignocellulosic biomass. The structural disruption in lignocellulosic structure upon autoclave pretreatment was confirmed through Fourier Transform Infrared Spectroscopy (FTIR) and Field Emission Scanning Electron Microscopy (FESEM) instrumental study. Although the net energy balance of pretreatment appeared slightly negative compared to total energy, a pilot scale techno-economic-environmental assessment is necessary to evaluate the feasibility and potential benefits of this co-digestion strategy.