Comparative study of continuous and batch microwave-assisted pyrolysis of waste printed circuit boards: product characteristic, bromine transformation, and energy efficiency assessment

To realize the efficient processing and resource utilization of waste printed circuit boards (WPCBs), products characteristics, pollutants control, and energy conversion between batch microwave-assisted pyrolysis (BMAP) and continuous microwave-assisted pyrolysis (CMAP) were compared and analyzed. CMAP promoted decomposition of oxygenated and Br/N-containing compounds in char, producing char with stronger aromatization and graphitization. CMAP facilitated bromine fixation in char, with higher bromine yield in solid product and lower bromine yield in condensate (71.69 and 18.80 wt %, respectively) than BMAP (67.51 and 29.11 wt %, respectively). For catalytic CMAP, continuous mixing promoted heat exchange between WPCBs and hot K2CO3 particles thus providing a higher heating rate, which led to a higher condensate yield than BMAP. K2CO3 promoted >95% of bromine retained in char during both processes. The higher KBr content in char, and lower concentrations of Br-containing compounds in pyrolysis oil and lower HBr yield were achieved from catalytic CMAP compared to BMAP, mainly due to more uniformly mixing of solid and volatiles in CMAP process. CMAP had more supremacy than BMAP in significantly enhancing energy efficiency. The results provide theoretical basis for the green utilization of WPCBs and development of microwave pyrolysis from batch to continuous modes.