Effect of atmosphere and temperature on syngas production during gasification of Zhundong lignite and water-washed Zhundong lignite in a fixed-bed reactor

Abstract

The effect of temperature and atmosphere on the syngas yields of CO and H2 during gasification of Zhundong lignite (ZD) and water-washed Zhundong lignite (WWZD) in a fixed-bed reactor was investigated. The experiments were run over temperatures from 750 to 950 °C, atmospheres of pure steam or with CO2 addition ratios of 25–75%. The syngas yield rate, overall yield, carbon conversion rate and the activation energy for gasification of carbon in coal were obtained. Results showed that the yield rates of CO and H2 during pure steam gasification of ZD at 750 °C increased from 0 to a maximum of 5.4 and 19.4 ml/min and decreased afterwards, when H2 was found as the main syngas. Moreover, CO was not detected while H2 was identified at a later stage of gasification, indicating that the char-steam gasification and water–gas shift reaction were dominated at this stage. As the temperature increased to 950 °C, the maximum yield rates of CO and H2 increased, confirming that the increase in temperature promoted gasification. Likewise, the increase in CO2 ratio from 25 to 75% resulted in a maximum CO yield rate of 163.87 ml/min at 950 °C, promoting CO yield when CO2 was the gasifying agent. The maximum yield of H2 was, however, decreased while the Boudouard reaction was promoted. Water washing was found to significantly influence gasification. The maximum yield rates of CO and H2 were decreased and the gasification time was prolonged, indicating that the reactivity of char-steam gasification was decreased due to the removal of Na contents by water washing. In addition, the overall yield of CO was increased as temperature and the CO2 percentage increased, whereas that of H2 decreased. The activation energy for steam gasification of ZD as determined by the momogeneous model (HM), shrinking core model (SCM) and isoconversional method was in the range of 53.36–56.14 kJ/mol, all of which were applicable for determining the activation energy of ZD gasification under steam.