Scaled-up microwave pyrolysis of sludge for hydrogen-rich biogas and life cycle assessment: Parameters synergistic optimization, carbon footprint analysis and technology upgrade

Abstract

Microwave pyrolysis is one of the emerging technologies for biomass conversion into clean energy. To promote the commercial application of microwave pyrolysis, the scale-up technology and the life cycle assessment (LCA) were used to optimize the quality of hydrogen-rich biogas and to provide ideas for process upgrading. Compared to the batch equipment, the horizontal microwave continuous pyrolysis equipment could effectively improve the biogas yield (50.15 wt%) while increasing the sludge treatment amount. The instantaneous high temperatures and the control of sludge moisture content were more favorable to increase the H2 concentration (45.71 vol%) via promoting the steam reforming and the secondary cracking of bio-oil. The energy consumption and carbon emission of each scenario were evaluated using the LCA. The drying and microwave pyrolysis scenarios were the main energy consuming stages, emitting 503 and 587 kg CO2 eq., respectively. For this reason, controlling the sludge moisture content not only contributed to the H2 generation, but also reduced the CO2 emissions. Meanwhile, the global warming could be reduced by 45.60 % when the energy consumption was reduced by 35 %. To improve the utilization efficiency of microwave energy, the combination of different frequency magnetrons and the design of biochar microwave absorption bed can be optimized to reduce equipment manufacturing costs and CO2 emissions. This work provides a new insight into the development of biomass energy and the innovation of microwave pyrolysis technology.