Biomass hydrogen donor assisted microwave pyrolysis of low-rank pulverized coal: Optimization, product upgrade and synergistic mechanism

Wheat straw (WS) has been used as a hydrogen donor to provide sufficient hydrogen for hydrogenated microwave pyrolysis of low-rank pulverized coal (PC). In this work, the effects of pyrolysis time, microwave power, the particle size of PC, and the ratio of PC to WS on microwave co-pyrolysis of PC and WS were investigated to optimize the experimental conditions. The pyrolysis products generated under the optimal conditions for the maximum tar yield were contrastively discussed, and the relevant synergistic mechanism was proposed. Results showed the temperature-rising rate of other conditions was positively correlated with the tar yield but not for the condition of the ratio of PC to WS. The tar yield reached the maximum value of 17.20% during microwave co pyrolysis under the conditions of 0.68-1.00 mm of particle size of PC, 700 W of microwave power, 50% of WS, and 20 min of pyrolysis time. The microwave pyrolysis of PC was significantly improved when adding WS, resulting in increased yields of tar and pyrolysis gas by 13.21% and 12.40%, respectively, compared with PC alone. The aliphatic and aromatic hydrocarbons in the coal tar sharply decreased, but the phenols, alcohols, and others increased compared with those in the microwave pyrolysis of PC. There was a positive synergistic effect between PC and WS in microwave co-pyrolysis caused by volatiles and biochar generated from WS.