Characterstics Of Calcium Microstructure Evolution During Catalytic Gasification Of Shengli Lignite

There are abundant low rank coal reserves in Inner Mongolia.Lignite is a typical kind of low rank coals with low calorific value due to high moisture and high volatility.Direct combustion of lignite not only produces low thermal efficiency,but also releases harmful gas polluting the environment seriously.However,efficient and clean utilization of lignite can be realized through gasification technology of lignite originated high reactivity.Alkali/alkaline earth metals compents present signanfantly catalytic activity on lignite gasification process,of which the catalytic effect of potassium,sodium and calcium is remarkably.Calcium is the preferred catalytic material for lignite gasification due to its cheap and easy availability and environmental friendliness.The Shengli lignites from Inner Mongolia were used as the materials in this dissertation.The prepared coal/chars samples were characterized by FTIR,XPS,XRD and Raman.The catalytic effect of calcium on Shengli lignite gasification process was investigated under different reaction atmosphere.The evolution process of calicium microstructure is analyzed to further reveal the catalytic mechanism of calcium component during lignite catalytic gasification.The ruslts of this dissertation can provide theory and technical support for the clean and efficient utilization of lignite and other low-rank coal resources in Inner Mongolia.The main results are as follows:（1）The thermal evoltion characteristics of Ca（OH）₂ and calcium carbonate under different reaction atmospheres were investigated.Under the atmosphere with CO₂,Ca（OH）₂ reacts with carbon dioxide to form calcium carbonate at low temperature,and then decomposes into calcium oxide and carbon dioxide with the increase of temperature.With the increase of CO₂ concentration,the decomposition temperature of calcium carbonate into calcium oxide gradually increases,indicating that the gradually enhanced inhibition effect of increasing CO₂ concentration on the decomposition process of calcium oxide.（2）The gasification performace of demineralized Shengli lignite(SL⁺⁺,minerals were removed by hydrochloric acid and hydrofluoric acid)under pure steam,mixed gas（50%CO₂+50%H₂O）and pure carbon dioxide atmosphere was tested.The results show that the gasification temperature windows increases gradually with the increase of CO₂ concentration,and the gasification temperature windows uhder pure CO₂reached the highest.The presences of carbon dioxide result in poor gasification performance and the rising of initial gasification temperature.The greater the inhibition of the decomposition and transformation of calcium components into Ca O,the greater the adverse impact on calcium-catalyzed lignite gasification will be.（3）The gasification char samples prepared by the gasification of calcium-added lignite under the pure steam,mixed gas（50%CO₂+50%H₂O）and pure CO₂atmosphere at 700℃were characterized by XRD and XPS.The characterization shows that the calcium components transform to metastable vaterite beside the formation of the calcite structure during the thermal evolution under pure steam gasification condition,while only the calcite crystal phase is formed under the carbon dioxide containing gasification condition（50%CO₂+50%H₂O）.These indicate clearly that the presence of carbon dioxide is not conducive to the formation and stability of metastable vaterite.Three occurrence structures of Ca CO₃,COOCa and Ca O are formed during the steam gasification of calcium-added lignite samples.With the increase of gasification conversion,the relative content of COOCa structure in gasified char continues to decrease until finally disappears,and the content of Ca O generally increases.