Impact Mechanism Of Calcium-Based Compounds On The Synergistic Effect For The Reactivity Of Coal Char Gasification With H₂O/CO₂ Mixtures

Partial replacement of H₂O by CO₂ to realize coal char gasification with H₂O/CO₂ mixtures,i.e.co-gasification,in the pressurized fixed-bed dry bottom gasifier can not only reduce the amount of phenolic wastewater but also limit the emission of CO₂ and make full use of the carbon source.However,the introducing of CO₂ into the gasifier may inhibit the H₂O gasification and the gasification reaction rate will be reduced.The calcium-containing mineral matters inherent in coal,which can show excellent catalytic activity during CO₂gasification,solve this problem effectively.The reactivity of coal char gasification with H₂O/CO₂ mixtures was better than that with H₂O or CO₂ alone due to the synergistic effect between H₂O and CO₂ on the gasification reactivity caused by the catalytic action of calcium.Taking full advantage of synergistic effect,gasification at relatively lower temperature can be developed,which can both reduce the energy consumption and improve the energy conversion.Moreover,this method is favorable to the water-gas shift reaction as well as the methanation reaction.Up to now,the synergistic effect during calcium-catalyzed gasification in H₂O/CO₂ mixtures and the influence of calcium on the composition and distribution of product gases are still lack of thorough discussion.Clearly understanding the catalytic effect of calcium on co-gasification,shedding light on the synergistic effect mechanism during co-gasification,and revealing the influence mechanism of calcium on the gas production behavior at different co-gasification conditions have the important theoretical and practical significance.It can not only provide the theoretical basis for the industrial application of partial replacement of H₂O by CO₂,including the selection of coal and determination of gasification parameters,but also offer the guidance for the use of coals that are rich in alkali and alkaline earth metals（AAEMs）in the pressurized fixed-bed dry bottom gasification.Therefore,two calcium-enriched coals,including WCW sub-bituminous coal from Zhundong coal mine in Xingjiang province and YN sub-bituminous coal used in pressurized fixed-bed dry bottom gasifier in Xingjiang province,were selected as the raw coal in this study.A thermal gravimetric analyzer（TGA）and a self-made pressurized fixed-bed reactor were used to conduct the gasification experiments to investigate the reaction characteristics and gas production behavior for coal char gasification with H₂O/CO₂ mixtures,respectively.XRD,N₂ adsorption,SEM-EDS,and Raman spectrum were adopted to reveal the synergistic effect mechanism of co-gasification combining with the kinetic analysis.The following are the main conclusions:（1）The synergistic effect during calcium-enriched coal char gasification with H₂O/CO₂ mixtures was studied,and the results show that:There is a synergistic effect during the co-gasification of WCW coal and YN coal,respectively,and the intensity of synergistic effect is affected by the loading precursor and amount of calcium,as well as the ratio of H₂O to CO₂ in the gasifying agents and gasification temperature.The most obvious synergistic effect is observed when 1wt.%Ca catalyzing coal char gasification in 66.7%H₂O+33.3%CO₂ with Ca（OAc）₂ as the loading precursor of calcium.The synergistic effect is relatively stronger at lower temperature but becomes weaker with increasing temperature and no longer exists above 900oC.（2）The impact of calcium on the synergistic effect during coal char gasification with H₂O/CO₂ mixtures was analyzed,and the results show that:The main active form of calcium,CaO,plays a remarkable catalytic role during coal char gasification with H₂O/CO₂ mixtures and promotes the synergistic effect.H₂O can not only inhibit the sintering of CaO particles and increase their dispersity on the char surface,but also facilitate the adsorption of CO₂ on CaO,so that CO₂ can be more easily dissociated and react with coal char.Moreover,the pores of coal char are enlarged by the interaction between H₂O and CO₂ with calcium participation,thus facilitating the diffusion of the gasifying agents and accelerating the gasification reaction rate.However,the adsorption capacity of CaO for CO₂ decreases with temperature up due to the sintering of CaO.It is the inactivation of CaO that leads to the weak synergistic effect during co-gasification at higher temperature.（3）The influence of sodium addition on the synergistic effect during calcium-catalyzed co-gasification was discussed,and the results show that:The excellent mobility of sodium at high temperature and the formation of eutectic system Na₂Ca（CO₃）₂ can inhibit the sintering of CaO effectively.Thus,the weaker synergistic effect during co-gasification caused by the sintering of CaO at higher temperature can be improved by the addition of sodium.During Ca/Na-catalyzed coal char gasification with H₂O/CO₂ mixtures,sodium plays a most obvious synergic part with calcium in catalytic gasification.And the remarkable synergy between sodium and calcium is closely related to the excellent dispersity of Ca/Na bearing particles on the char particle surface and the most obvious improvement of CaO dispersion.（4）The kinetic study of synergistic effect during calcium-catalyzed and calcium/sodium-catalyzed gasification with H₂O/CO₂ mixtures was performed,and the results show that:The kinetic model（eICM-II）,which is established by taking acount of the pore structure evolution of chars and the mobility of sodium on the char surface,can be used to simulate the calcium-catalyzed and calcium/sodium-catalyzed gasification with H₂O,CO₂ and their mixtures.The catalytic activity of calcium is higher at the beginning of gasification process but decreases gradually,whereas sodium shows better catalytic activity at the later stage.The catalytic effect of calcium on coal char gasification is different from that of sodium and the difference varies with the gasifying agents.Calcium together with sodium shows a cooperation effect towards the gasification reactivity,which is the most obvious on the co-gasification process and reduces the activation energy significantly.Within the carbon conversion of 50%,calcium has the excellent catalytic activity,and when carbon conversion is higher than 50%,the obvious mobility of sodium and its ability to crack the large rings in coal char promote the catalytic activity of calcium.Besides,the priority consumption order of different structures in char is different between H₂O gasification and CO₂ gasification.In contrast to the H₂O gasification and CO₂ gasification,active sites of the char increases significantly during calcium-catalyzed co-gasification process.Furthermore,the synergistic effect mechanism during coal char gasification with H₂O/CO₂ mixtures is revealed by combining the char structure evolution during gasification process with the kinetic parameters.（5）The gas production behavior of calcium-catalyzed coal char gasification with H₂O/CO₂ mixtures was investigated,and the results show that:The gas release rate during co-gasification is much faster than that during H₂O gasification because of the high catalytic activity of calcium,and the production of CO and H₂ during gasification process is affected by the introducing of CO₂.Water-gas shift reaction may be inhibited at high temperature,and the addition of CO₂ can also influence the water-gas shift reaction to a certain extent.Thus,the ratio of H₂ to CO in the product gas can be adjusted effectively.In addition,the catalytic effect of calcium can accelerate the methanation reaction and further increase the production of CH₄.This effect can be strengthened by the cooperation between calcium and sodium,whereas the clay mineral matters inherent in coal may have a negative impact.