Transformation Of Aaems And Its Influence On Reactivity During Char Gasification With H₂O/CO₂

The endeavor to develop coal-based synthetic natural gas can not only alleviate the contradiction between supply and demand of natural gas, advance the optimization of energy structure in our country and have a strategic significance on domestic energy security, but promote the energy-saving and emission reduction. The technology meets the requirement that coal chemical industry should develop a way of clean and efficient utilization. Because of the mature technology and relatively high CH4 concentration in crude gas, pressurized fixed bed dry bottom gasifier has been the most widely used and competitive coal gasification technology for SNG. In order to solve the problem of phenolic waste water, replacing partial steam with high purity CO₂ as gasifying agents has been proposed. In our previous studies, gasification of Yining coal and Wucaiwan coal in mixture of H₂ O and CO₂ show a synergistic effect, the coals feature high content of alkali and alkaline earth metallic species（AAEMs）, and the synergistic effect proved to be related to catalysis of alkali and alkaline earth metallic mineral. To reveal synergistic mechanism, the transformation rules of AAEMs during coal char gasification with mixture of H₂ O and CO₂ should be studied, which is in favor of further exploration for the influence of AAEMs transformation on catalysis of char gasification.Wucaiwan coal, which is rich in AAEMs, was used as coal samples in this study. Sequential chemical extraction method was applied to investigate the different forms of AAEMs in WCW coal char. A series of partial gasified coal chars in different carbon conversion were prepared in 800 °C and 900 °C. Variation trends of AAEMs content during char gasification with CO₂,H₂ O and H₂O/CO₂ was studied, the morphology of AAEMs minerals was studied by SEM. By using sequential demineralized method, transformation rules of AAEMs and its influence on catalysis during gasification were associated. The main conclusions are as follows:（1） Most of the sodium and potassium in WCW char are water-soluble, and exchangeable ion is the main form of calcium. Changing atmospheres has little effect on the release of Na and K, whereas calcium is more likely to release under CO₂ atmosphere than that under H₂ O atmosphere. Temperature is an important factor that affects the release of AAEMs during WCW char gasification, the release amount in 900 °C is obviously higher than that in 800 °C.（2） The reactivity of WCW char would improve during isothermal gasification gasification process in 800 °C by comparing the reactivities of WCW char and gasified-chars. The order of gasified-chars is as following: H₂O-gasified-char>H₂O/CO₂-gasified-char>CO₂-gasified-char.（3） By contrasting the reactivities of sequential demineralized chars, the catalysis effect of AAEMs is the main factor which influences the gasification reactivity of WCW coal. The contents of water-soluble sodium and potassium in char will have an enhancement and present an increasingly better catalytic ability during gasification. Therefore, while the Aromatic ring condensation improves as a function of carbon conversion, the gasification reactivity still increases.（4） It is the difference in transformation of calcium under different atmospheres rather than the pore structure and chemical structure which makes the different reactivities of gasified-chars. The calcium species would have a large release and severe sintering under CO₂ atmosphere, resulting in a higher catalytic ability of calcium in H₂ O atmosphere than that in CO₂ atmosphere.