Maximizing the energy recovery from rice straw through two-step conversion using eggshell-catalytic pyrolysis followed by enhanced anaerobic digestion using calcium-rich biochar.

Anaerobic digestion of lignocelluloses for biogas production is greatly restricted by the poor biomass degradability. Herein, a novel approach is suggested to enhance the energy recovery from rice straw through a two-step conversion using eggshell-based catalytic pyrolysis followed by biochar-based anaerobic co-digestion. Pyrolysis with eggshell significantly enhanced the crude bio-oil yield by 4.6 %. Anaerobic digestion of rice straw using 4 g L-1 of rice straw biochar (RB) showed the highest recorded biogas yield of 503.7 L kg-1 VS, with 268.6 L kg-1 VS biomethane yield. However, 4 g L-1 of calcium-enriched eggshell rice straw biochar (ERB) enhanced the biomethane yield to 281.8 L kg-1 VS, which represented 95.6 % higher than the control. It was attributed to enhancement of biomethanation, which resulted in 74.5 % maximum recorded biomethane content at the 7th day of anaerobic digestion. Microbial analysis confirmed that Methanosarciniales was the most dominant Archael group in the control (14.84 %), which increased sharply to 73.91 % and 91.66 % after addition of 4 g L-1 RB and ERB, respectively. The suggested route enhanced the energy recovery in the form of bio-oil and biomethane by 41.6 %.