Reaction characteristics investigation of CeO_(2)-enhanced CaSO_(4) oxygen carrier with lignite

Calcium sulfate(CaSO_(4))has been verified as a promising oxygen carrier(OC)in the chemical looping combustion(CLC)for its high oxygen capacity,abundant reserve and low cost,but its low reactivity and deleterious sulfur species emission from the side reactions of CaSO_(4) should be well considered for its wide application in CLC.In order to promote the reactivity of CaSO_(4) and increase its potential to inhibit the gaseous sulfur emission,a CeO_(2)-enhanced CaSO_(4) OC mixed OC of core–shell structure was prepared using the combined template synthesis method.Reaction characteristics of the prepared CaSO_(4)-CeO_(2) mixed OC with a typical lignite was first conducted and systematically investigated,and an improved reactivity of the prepared CaSO_(4)-CeO_(2) mixed OC was demonstrated than its single component CaSO_(4) or CeO_(2) due to the fast transfer and exchange of oxygen from the CaSO_(4) substrate to coal via the doped CeO_(2).Furthermore,the solid products formed from the mixed CaSO_(4)-CeO_(2) OC with the selected coal were collected and analyzed.Especially,evolution and redistribution of the sulfur species of different forms were focused.At the latter reaction stage of YN reaction with the CaSO_(4)-CeO_(2) mixed OC,the SO_(2) emitted from the side reactions of CaSO_(4) was greatly diminished and the doped CeO_(2) was proven effective to directionally fix the SO_(2) released to turn into different solid sulfur compounds,which were determined as Ce_(2)O_(2)S,Ce_(2)S_(3) and Ce_(2)(SO_(4))_(3)·5H_(2)O and formed through the different pathways.In addition,good regeneration of the reduced CaSO_(4)-CeO_(2) mixed OC could be reached in spite of the unavoidable interaction between the included minerals in coal and the reduced mixed OC.Overall,the combined template method-made CaSO_(4)-CeO_(2) mixed OC reported herein was not only endowed with enhanced reactivity for coal conversion,but also owned the potential to directionally fix the gaseous sulfur emission,which is quite applicable as OC for simultaneous decarbonatization and desulfurization in the real CLC process.

基于新型循环载氧体的天然气化学链燃烧热力学研究

以CH4为燃料对基于CaSO4载氧体的化学链燃烧的热力学性能进行了分析研究,计算了CaSO4在CH4氛围中的还原-氧化的热力学参数与温度的关系,分析结果显示,在一定的温度范围内,以CaSO4为载氧体实现化学链燃烧具有可行性,是一种理想的载氧体。基于Gibbs能最小化方法建立了化学链燃烧技术模型,模拟了温度,CH4与CaSO4摩尔比对燃料反应器和空气反应器的影响。结果表明,燃料反应器最佳反应温度850℃～900℃,空气反应器最佳反应温度为1000℃～1050℃,CH4与CaSO4的摩尔比最佳摩尔比为1。研究结果对燃煤化学链燃烧具有参考价值。

Investigation into Syngas Generation from Solid Fuel Using CaSO4-based Chemical Looping Gasification Process

Chemical-looping gasification （CLG） is a novel process for syngas generation from solid fuels, sharing the same basic principles as chemical-looping combustion （CLC）. It also uses oxygen carriers （mainly metal oxide and calcium sulfate） to transfer heat and oxygen to the fuel. In this paper, the primary investigation into the CLG process with CaSO4 as oxygen carrier was carried out by thermodynamic analysis and experiments in the tube reactor. Sulfur-contained gas emission was mainly H2S rather than SO2 in the CLG process, showing some different features from the CLC. The mass and heat balance of CLG processes were calculated thermodynamically to determinate the auto-thermal operating conditions with different CaSO4/C and steam/C molar ratios. It was found that the CaSO4/C molar ratio should be higher than 0.2 to reach auto-thermal balance. The effect of temperature on the reactions between oxygen carrier and coal was investigated based on Gibbs free energy minimum method and ex- perimental results. It indicated that high temperature favored the CLG process in the fuel reactor and part of syngas was consumed to compensate for auto-thermal system.

基于钙基载氧体燃烧分离CO_2还原反应动力学研究

根据化学链燃烧原理,进行了以CaSO4为载氧体,以CO为燃料的化学链燃烧还原反应动力学试验研究,并基于标准Gibbs自由能最小化原理,研究了以煤为燃料,CaSO4为载氧体化学链燃烧周期性的还原-氧化反应过程.结果表明:CO还原CaSO4的气相产物主要为CO2,固相产物主要为CaS而不是CaO;通过CO浓度为20％～70％,温度为880～950℃条件下的还原反应验证了还原反应动力学模型,求出相关的动力学参数;试验得到的结果和拟合结果非常吻合.

Thermodynamics on Sulfur Migration in CaSO4 Oxygen Carrier Reduction by CO

CaSO4 is an attractive oxygen carrier for chemical looping combustion（CLC） because of its high oxygen capacity and low price. The utilization of a CaSO4 oxygen cartier suffers the problems of sulfur release, and deacti- vation caused by sulfur loss. With respect to the fact that partial sulfur release could be recaptured and then recycled to CaSO4 by CaO sorbent, the mixture of CaSO4-CaO can be treated as an oxygen carrier. Thermodynamics of CaSO4 and CaSO4-CaO reduction by CO have been investigated in this study. The sulfur migrations, including the sulfur migration from CaSO4 to gas phase, mutual transformation of sulfur-derived gases and sulfur migration from gas phase to solid phase, were focused and elucidated. The results show that the releases of S2, S8, COS and CS2 from CaSO4 oxygen carrier are spontaneous, while SO2 can be released at high reaction temperatures above 884 ℃. SO2 is the major emission source of sulfur at low CO/CaSO4 molar ratios, and COS is the major part of the byproducts as soon as the ratio exceeds 4 at 900℃. Under CO atmosphere, all the sulfur-derived gases, SO2, S2, S8 and CS2, involved are thermodynamically favored to be converted into COS substance, and are spontaneously absorbed and solidified by CaO additive just into CaS species, which may be recycled to CaSO4 as oxygen carrier in the air reactor. But high reaction temperatures and high CO2 concentrations are adverse to sulfur capture.