Catalytic Reaction Mode Of Organic-bound Ca And The Interaction Between Ca And Other Minerals During Coal Gasification Process

Gasification is a promising option for the clean and efficient utilization of coals.Low-rank coals usually contain high contents of alkali and alkaline earth metals(AAEMs),which have obvious catalysis during gasfication process.Among all the AAEMs,the organic-bound calcium(Ca)has large content,low volatility and high dispersion.Thus,it causes great concerns.There were lots of works involved the Ca catalysis and much progress have been achieved.Nevertheless,the Ca catalysis mechanism,especially the Ca catalytic reaction mode,is still a controversial topic.And the studies about the interactions between Ca and other minerals in coals are not sufficient.This work is aimed to solve these issues.By doing a seires of reasearches,the transformation and catalysis of Ca and the interaction rules of the minerals in coals are known.This is significant for the development of gasification technology for the low-rank coals.Since the content and occurance of minerals are complexed in the raw coals,the demineralizing and loading method are used.Then,the isothermal gasification experiments were conducted in a thermogravimetric reactor.With different atmospheres,temperatures and pyrolysis conditions,the transformation and catalysis of minerals were adequately studied.The related contents are as follows.Fisrt,to solve the controversial problem about the Ca catlalytic mechanism,the catalytic actions of Ca in the steam and CO₂ gasification are studied.The results show that there are different catalytic reaction modes in steam and CO₂.In steam gasification,the highly dispersed Ca O particles catalyze the reation,which lead to the formation of quantities of channels in the char.This is described as a channeling reation mode.In CO₂,Ca transforms into Ca O and Ca CO₃ mixtures.A Ca layer is formed on the char surface owing to the sintering of Ca.It follows a surface recession reaction mode.These two different reaction modes have close relationships with the gasification reactivity and pore structures of char.In addition,the effects of pyrolysis conditions on the transformation and catlalysis of Ca during gasification process are studied.The results show that with the contents of pyrolysis condition increasing,the dispersion of Ca in the chars declines obviously.The catlalytic reaction modes of Ca in steam and CO₂ are unchanged with the the different pyrolysis conditions.For the fast pyrolyzed chars,a much dense Ca layer is formed on the partially-gasified char in CO₂ gasification.Increasing temperatures can greatly promote the gasification reactivity.In steam gasification,the partially-gasified chars have developed char structures.High temperature can cause the increasing particle size of Ca O and decline the Ca dispersion.This leads to a phenomenon that the gasfication reactivity stays nearly unchanged when the temperature increases.The above results indicate that for the samples with high content of Ca,the gasification reactivity characteristic of long-time pyrolyzed chars can not be used to represent the chars with fast heating rate.According to the obtained Ca catalytic mechanism and the experiment method(pyrolysis with fast heating rate following by isothermal in-situ gasification),a series of works were conducted.These involve the interactions between Ca and other minerals in coals and their effect on the gasfication reactivity.By using the methods of single-element loading and double-elements loading,the synergistic effect of Mg and Ca is studied.In CO₂gasification,the existence of Mg does not change the Ca catalytic reaction mode.There is a positive synergistic effect,which is much obvious at 800?.It is because Mg O can inhibit the sintering of Ca CO₃.In steam,there is a negative synergistic effect on gasification rate when the two elements exist.Mg decreases the Ca-C interface and suppresses the channeling process.Besides,the synergistic effect of Na and Ca is studied.It promotes the gasification reactivity,especially in CO₂ gasification.In CO₂,Na inhibits the formation of Ca CO₃,which makes calcium exsit as Ca O or low-temperature eutectic minerals.The reaction follows the surface recession mode.In steam,at 800?,the existence of Na leads to the change of wetting characteristics of Ca O.There is no obvious channeling process.At900?,the volatilization of Na becomes higher and the particle size of Ca O becomes larger.The partially-gasified chars have more mesopores and marcopores.The whole gasifcaiton process follows the channeling reaction mode.In order to study the interactions between Si-Al containing minerals and Ca-Na on the catalytic gasification process,kaolin and silica are added to the loaded samples.The results show that the direct interaction between kaolin(or silica)and Ca is very poor.Only some amorphous minerals are formed.Owing to the low contact area and reaction content between kaolin(or silica)and Ca,the two minerals have no obvious effect on the Ca catalysis.When the coal simultaneously contains Ca and Na,the kaolin and silica can both capture the volatilized Na.The Na-fixed effect of kaolin is slightly better than silica.After Na is captured,there are quantities of amorphous mineral particles formed.The melting content of particles formed by kaolin and Na is relatively lower,which is not good for its contact and reaction with Ca particles.Thus,the inhibited effect of kaolin on Ca-Na catalysis is not very obvious.When silica is added,it can react with Na and forms amorphous particles with high melting content and mobility.These particles can easily react with Ca and Na particles on the char surface.The interaction consumes Ca and Na.Thus,it obviously inhibits the gasification rate.