Can microwave treat biomass tar? A comprehensive study based on experimental and net energy analysis

Tar pollution is the most serious problem hampering the application of biomass gasification. Microwave exhibits unique effects for treating biomass tar and thus have attracted considerable attention. However, as an energy-intensive technology, the optimal operating parameters of microwave have not been developed, and the energy feasibility remains unknown. In this study, a bench-scale microwave catalytic reforming (MCR) was built and tested for the thorough conversion of biomass model tar under ex-situ hot gas conditions. The effects of tar concentration levels and reaction temperatures on product distributions and reaction routes were carefully investigated, and the optimal MCR parameters were determined based on the net energy analyses of three corresponding gasification scenarios. The results showed that net energy efficiencies of 29.02%, 76.01%, and 93.04% can be obtained in scenarios with 100 g Nm(-3), 10 g Nm(-3), and 1 g Nm(-3) of inlet tar, respectively. The results clearly proved that the MCR can be self-powered in the gasification processes regardless of the tar level. Moreover, this conclusion was tested in an operational biomass gasification power plant, and a comprehensive comparison was conducted between the MCR and conventional tar treatment. It was demonstrated that net energy efficiencies of 85.60% and 73.03% can be achieved by coupling the MCR with a water-cooling unit and with a single MCR, respectively. Additionally, the catalyst lifetime was tested in successive eight-hour experiments, and the carbon consumption of the catalyst was thoroughly investigated using a thermogravimetric analysis with multiple atmospheres. This study provided a foundation and new insights for treating biomass tar with microwaves, and it may be a milestone for its commercialisation.