Gasification characteristics of different rank coals at H2O and CO2 atmospheres

- Effects of gasification temperature, pyrolysis rate and atmosphere in the coexistence of H2O and CO2 were evaluated.
- Mixed reaction model was put forward and kinetic parameters were calculated.
- Fast pyrolysis rate attained a better reactivity.
- Synergistic effect under the CO2/H2O gasification atmosphere was approved.

Gasification characteristics of Shenhua bituminous (SH) and Baorixile lignite coal (BRXL) were investigated using both a thermogravimetric analyzer and tubular furnace. Coal samples were pyrolized in a tubular furnace at five different temperatures (800-1000 °C in 50° increments), at two heating rates, 10 °C/min and 1000 °C/min. The resultant chars were gasified isothermally at (i) each of these five temperatures under a constant atmosphere (40% H2O(g) + 60% CO2) and (ii) at 1000 °C while varying the atmosphere from pure H2O(g) to pure CO2 by increasing the CO2 component stepwise in 20 vol.% increments. Carbon conversion, reactivity index, and average specific gasification rate were calculated and fit to a mixed reaction model. It was found that both the gasification temperature and atmosphere composition greatly impacted gasification characteristics, and the temperature effect was more pronounced in a heterogeneous atmosphere. The char produced with a high heating rate pyrolysis process promoted gasification reactivity likely a result of the more open pore structure. The gasification reactivity was lowest under a pure CO2 gasification atmosphere and increased continuously with the H2O partial pressure. SH and BRXL char samples achieved peak gasification reactivity under atmospheres with 60% and 80% H2O(g), respectively. Also, a synergistic effect was observed for heterogeneous CO2/H2O gasification atmosphere and not all the gasification reactions obeyed a first-order reaction model. The results of this work indicate that there is an optimal gasification atmosphere and temperature combination, specific to the coal sample, for efficiently converting char to gasification products.