Experimental Study On The Kinetic Characteristics Of In Situ Demineralized Coal Char Under O₂/H₂O Atmosphtere

Oxy-Coal Combustion Steam System of Near-zero Emissions（OCCSS）has obvious advantages in high efficiency and low cost of CO₂ emission reduction.In order to realize the combustion reliability and high burnout rate of the demineralized coal under O₂/H₂ O atmosphere,we used the demineralized coal leached by HCl-HF-HCl as raw material,which ash content is 0.2%.Then we systematically investigated the gasification and combustion characteristics of in-situ char under 925-1075℃ 、15%-35%H₂O,625-750℃ 、 10%-30%O₂,respectively.The experiments were conducted on the micro fluidized bed system,which can separated pyrolysis from gasification and combustion by switching experimental atmosphere.At last,we analyzed the effects of gasification on combustion process in order to understand the reaction mechanism of demineralized coal char in O₂/H₂ O atmosphere.During the gasification process,the absolute carbon conversion rate can reach 100% when temperature is higher than 975℃.At the same temperature,the concentration of vapor has no obvious influence on carbon conversion.The influence of water vapor concentration on carbon conversion rate is related to the gasification temperature.When the temperature is 1025℃ and 1075℃,the carbon conversion rate does not increase obviously if the water vapor concentration is higher than 25%.The activation energy E calculated from shrinking core and random pore model is located between 186 and 194kJ/mol.The poor gasification reactivity of demineralized coal char under low temperature（925、975℃）and low concentration of water vapor（15%）has negative effects on the calculation accuracy of pore structure parameter ψ of random pore model,resulting in its lower fitting accuracy compared with the shrinking core model.The gasification reaction order is 0.49-0.61.Compared with raw coal,the process of the water-gas shift reaction（CO + H₂ O ? CO₂ + H₂）is hindered during the gasfication of demineralized coal char,inhibiting the conversion from CO to CO₂ and improving the CO content in gas product.The sensitivity of each product to temperature is poor and almost keeps constant with the increase of temperature.The steam gasification of demineralized coal char is a stable process,the relative content of each product has no obvious changes under different stages of the reaction（X=30%-70%）.Compared with the deminerized coal char,the release rate of H₂、carbon conversion rate and absolute carbon concersion were faster for raw coal char.The activation energy is 160.2kJ/mol.The combustion kinetic parameters fitted by homogeneous and shrinking core model are close: when the oxygen concentration is 21% and 30%,the combustion process of deminerized coal char was controlled by chemical reaction under 650-715℃,the activation energy is 122 kJ/mol and 128kJ/mol,respectively;when the oxygen concentration is decreased to 10%,due to the insufficient oxygen,the combustion reaction is shifted to “diffusion control” region when temperature is higher than 690℃,the activation energy decreases from 150kJ/mol to 88kJ/mol.With the process of gasification at 1025℃,the combustion reactivity of gasified char decreases gradually,indicating positive change of enlarging the pore by water vapor is weaker than the accumulation of inert carbon structure.It witnesses an obvious decrease of combustion reactivity when the concentration of the water vapor increases from 15% to 25%.As the concentration continues increase to 35%,the reactivity is close to 25% limited by the number of active sites on the surface of carbon.