Energy Recovery from Rice Straw through Hydrothermal Pretreatment and Subsequent Biomethane Production

Rice straw is an abundant agricultural waste in Asia. Anaerobic digestion (AD) as an environmentally friendly process for bioenergy recovery is expected to solve the environmental issues brought about by open burning of rice straw. In order to test the feasibility and scalability of hydrothermal treatment (HTT) on rice straw for subsequent methane production from pretreated straw, this study attempted two peak HTT temperatures (150 and 210 degrees C, i.e. HTT150 and HTT210, respectively) for holding 0-30 min to pretreat rice straw which was then used for mesophilic methane fermentation. Thereafter energy balance and energy recovery were analyzed on HTT and subsequent AD of rice straw. Results show that HTT150 exhibited a positive effect on subsequent methane production, achieving the highest methane yield of 134 mL (STP) for per gram of added volatile solid (VSadded) of rice straw after being hydrothermally pretreated at 150 degrees C for 20 min. The maximum specific methane production rate (mu), around 17-40% higher than the control (without pretreatment), was achieved from HTT150 pretreated rice straw. HTT210 was found to have a negative effect on subsequent AD. Considering disposal of rice straw by HTT coupling with subsequent methane production, the highest net energy gain (Delta E = E-out - E-in), energy ratio (E-out/E-in), and energy recovery (in comparison to direct combustion) were obtained at HTT150 for 20 min, about 2741 MJ/t, 2.7, and 30.7%, respectively. Results from this work imply that HTT temperature is critically important when subsequent AD for enhanced methane production and energy balance of the whole disposal system are targeted.