Sequential production of biomethane and bioethanol through the whole biorefining of rice straw: Analysis of structural properties and mass balance

This study demonstrates a new biorefinery approach for efficient conversion of rice straw into biomethane and bioethanol. The main objective of this study was to investigate the production of these two bioenergy products from rice straw through sequential anaerobic digestion (AD) and fermentation. The mesophilic AD and thermophilic AD of the rice straw were investigated for biogas production and digestate characterization. Mesophilic and thermophilic AD generated biogas yield of 236.8 and 275.8 L/kgVS, respectively. The mesophilic digestate fiber is better for sequential production of biomethane and bioethanol. Different acid and alkali pretreatments were investigated for the bioethanol production. Both alkali and acidic pretreatments degraded the lignocellulose; the alkali pretreatment was more effective than the acidic pretreatment. The changes in the lignocellulose biomass of the pretreated digestate were investigated by Fourier transform infrared (FTIR) spectroscopy and X-ray diffraction (XRD). The pretreatment with 3% NaOH for 6 h significantly increased the glucose conversion and achieved the highest ethanol production of 87.13 g/kg dry fiber. The sequential route of AD and fermentation of rice straw enhanced the total gross energy output rate to 1.468 GJ ton⁻¹ day⁻¹. The current research illustrates a revolutionary rice straw-to-energy strategy that will have a massive effect on the biofuel industry progress. Graphical abstract: [Figure not available: see fulltext.]