Thermal pretreatment of microalgae for biomethane production: experimental studies, kinetics and energy analysis

BACKGROUND: Microalgae grown on wastewater are promising feedstocks for biomethane production by anaerobic digestion. However, the hemicellulose composition of the microalgae cell wall inhibits hydrolysis of intracellular substances and limits their anaerobic digestibility. This study investigated enhancement of biomethane production rates during anaerobic digestion of Chlorella sp. using thermal pretreatment at varying temperatures. Experimental data were fitted to three simplified kinetic models and an energy analysis was performed to gain insights into this potential application of thermal pretreatment. RESULTS: Methane yields from untreated algae were 155 mL g(-1) VSadd, while thermal pretreatment at 70 degrees C and 90 degrees C for 0.5 h increased the methane yield by 37% and 48%, respectively. Thermal pretreatment at 121 degrees C for 0.3 h resulted in the highest methane yield ( 322 mL g(-1) VSadd), which is 108% higher than the untreated algae. Data from digestion of thermally pretreated microalgae were best described by a first-order kinetic model. However, for untreated microalgae the Gompertz model, which includes a lag phase, provided the best fit to the methane production data. CONCLUSIONS: Thermal pretreatment improved the maximum methane production rate and shortened the lag period during anaerobic digestion. However, the energy balance indicated that pretreatment of microalgae could not achieve a positive energy balance compared with anaerobic digestion of untreated microalgae. Co-digestion with other biomass or increasing the solids concentration of anaerobic digestion could be used to increase the overall energy efficiency. (C) 2016 Society of Chemical Industry