Thermogravimetric analysis and kinetics of co-pyrolysis of raw/torrefied wood and coal blends

The properties of biomass can be improved via torrefaction, and torrefied wood is a fuel with the potential to partially replace coal. In this study, raw Cryptomeria japonica (WRaw) is torrefied at 250 (TW250) and 300 °C (TW300) for 1 h, and then mixed with an anthracite coal to undergo co-pyrolysis. A thermogravimetric analyzer is used to examine the co-pyrolysis characteristics of fuel blends and five different biomass blending ratios (BBRs) of 100, 75, 50, 25, and 0 wt.% are taken into consideration. When WRaw, TW250, and the coal are tested, the pyrolysis is characterized by a three-stage reaction, whereas four-stage thermal degradation is found for TW300 and fuel blends. The predictions from the linear superposition of the thermal decomposition of individual fuels fit the experimental data of the fuel blends, suggesting that the interaction or synergistic effect of co-pyrolysis between the raw/torrefied C. japonica and the coal is slight. The co-pyrolysis kinetics of the fuel blends is also analyzed. The variation of chemical kinetics with decreasing BBR in the second stage is different from that in the third stage. That is, an increase in BBR leads to an increase in the activation energy in the second stage, whereas it causes a decrease in the third stage. This is attributed to that the reactivities of cellulose and lignin in biomass are different from that of coal in the two stages.