CaSO4与气固产物的竞争反应对煤热解特性的影响

Effect of competing reactions between CaSO_(4)and gas/solid products on characteristics of coal pyrolysis

基于灰热载体的煤热解过程中,煤灰对煤热解特性的影响与其所含的CaSO_(4)密切相关,但由于CaSO_(4)与还原气和半焦之间存在复杂的竞争反应,导致CaSO_(4)对热解的影响规律和作用机制尚不明确。在固定床反应器中考察了500℃~850℃条件下CaSO_(4)对煤热解特性的影响规律,同时开展了CaSO_(4)-模拟热解气的气-固反应实验和CaSO_(4)-半焦的固-固反应实验,用于探究CaSO_(4)与还原气和半焦之间的竞争反应。结果表明:添加CaSO_(4)在低温时对H2收率无显著影响,但850℃时添加CaSO_(4)导致H2收率由1.11%降到0.96%,模拟热解气与CaSO_(4)的气-固反应验证了2H_(2)+CaSO_(4)→CaS+2H2O是H2收率降低的主要原因。高于700℃时,添加CaSO_(4)导致CO和CO_(2)的收率显著增加,如850℃时CO和CO_(2)收率分别由原煤热解时的11.57%和15.2%增加到添加CaSO_(4)热解时的31.60%和20.3%,CaSO_(4)与半焦的固-固反应证实了2C+CaSO_(4)→CaS+2CO_(2)和4C+CaSO_(4)→CaS+4CO是CO和CO_(2)收率增加的主要原因。结合热解过程中CaSO_(4)的分解率及质量衡算,固-固反应导致的CaSO_(4)的分解量约占总分解量的91.95%,表明相较于CaSO_(4)与还原气的气-固反应,热解过程中CaSO_(4)的分解主要归因于CaSO_(4)与半焦的固-固反应,这是CO和CO_(2)的变化量远高于H_(2)变化量的本质原因。

The impacts of coal ash are closely linked with CaSO_(4)during coal pyrolysis with ash as heat carriers.However,due to the existence of complicated competing reactions among CaSO_(4)-pyrolytic gas and CaSO_(4)-char,the influence of CaSO_(4)on the characteristics of coal pyrolysis remains elusive.The impacts of CaSO_(4)in a fixed-bed reactor at temperatures of 500℃-850℃were investigated,and the experiments on gas-solid reactions and solid-solid reactions were also performed to clarify the competing reactions.The results show that H2 yield is less effected by CaSO_(4)at low temperatures,however,a decrease from 1.11%to 0.96%at 850℃is observed when CaSO_(4)is added.The gas-solid reactions between simulated pyrolytic gas and CaSO_(4)show that the reaction 2H_(2)+CaSO_(4)→CaS+2H_(2)O is mainly responsible for the varie-ties in H2.CaSO_(4)is found to promote CO and CO_(2)yields at temperatures above 700℃.For ex-ample,CO and CO_(2)yields are significantly increased from 11.57%and 15.2%to 31.60%and 20.3%,respectively,at 850℃with CaSO_(4)addition,which is ascribed to the reactions 2C+CaSO_(4)→CaS+2CO_(2)and 4C+CaSO_(4)→CaS+4CO according to investigations on the solid-solid reactions between char and CaSO_(4).Based on the analysis of mass balance and decomposition ratios,the decomposition amount of CaSO_(4)due to solid-solid reactions accounts about 91.95%of the overall decomposition amount.This indicates that compared to gas-solid reactions,the solid-solid reactions play a predominant role in how CaSO_(4)influences coal pyrolysis,elucidating the substantial increase in CO and CO_(2)compared to that of H_(2).