Effects Of CO₂on Structure And Gasification Reactivity Of Char

The pressurized fixed-bed dry bottom gasifier provides the mostcompetitive coal gasification technology for SNG because of the matureindustry experience and relative high CH4concentration in crude gas. To ensurethe ash discharge in solid form, excessive steam is injected into the gasifier tocontrol the temperature, thus resulting in the emissions of large amounts ofphenolic wastewater and high concentration CO2. Utilization of CO2as partialgasification agent in the gasifier has been proposed to save excess steam andreduce CO2emission. The introduced CO2would inevitably affect the thermalchemical process of beds. Especially, taking CO2as gasifying agent could adjustthe ratios of H2/CO. As a heat carrier gas in the carbonization layer, CO2wouldaffect the coal pyrolysis process. Based on above-mentioned backgrounds, it isnecessary to study the influences of CO2on structure and gasification reactivityof chars. Meanwhile, to find out the gas distribution during coal chargasification with mixed CO2and H2O is of great significance.Yining bituminous coal (YN) and Ximeng lignite (XM) with the particlesize of5~8mm were used as coal samples in this study. The samples werepyrolyzed in CO2and N2in the temperature range of500~800°C at differentpressures in a fixed-bed reactor. The structural features of chars werecharacterized using nitrogen adsorption, FT-Raman spectroscopy, and FT-IRspectroscopy, the reactivity of chars under CO2was analyzed in the TGA.Therefore, the influence of CO2on coal chars structure and gasification reactivity under different conditions were obtained. Moreover, the fixed-bedreactor was employed to investigate coal char gasification characteristics withmixture of CO2and H2O under different pressures, and the gas composition wasanalyzed on-line. The main conclusions are as follows:(1) The char yields obtained under CO2are higher than that under N2at thepyrolysis temperature of500°C and600°C, which is caused by the inhibition ofCO2on the release of volatiles. Due to the occurrence of char-CO2gasification,the char yields under CO2are lower than that under N2at the pyrolysistemperature of700°C and800°C. The inertinite-rich YN has a higher charyield compared with vitrinite-rich XM under same pyrolysis conditions.(2) Compared with N2atmosphere, CO2can accelerate the fracture of–OHand–CH2–in coal. CO2has a positive effect on CH4release, while it has littleinfluence on the maximum release temperature. In additional, CO2can promotethe release of H2in advance and react with chars to increase CO contentobviously. The chars obtained under CO2have higher specific surface area,higher aromaticity, and lower ratios of small to large rings. With the pressureincreasing, the ratios of small to large rings will decrease.(3) The influence of the specific surface area on gasification reactivity isrelatively weak when compared to the ratios of small to large rings in coal char.The XM chars prepared under CO2, which have lower ratios of small to largerings, have lower reactivity when compared with that prepared under N2. Thereactivity of YN chars obtained under CO2at700°C and800°C are higherthan that obtained under N2, which may be attributed to the catalytic effectcaused by the minerals in the coal chars.(4) The synergistic effect of co-gasification can improve gas yield. Atatmospheric pressure, when the substitute amount of CO2is75%(by volume),the highest gas yield is obtained. At elevated pressures, when the substituteamount of CO2is25%(by volume), the highest gas yield is obtained. The syngas with different ratios of H2/CO can be obtained by changing the substituteamount of CO2. With increasing gasification pressures, the ratios of H2/CO areobviously increased.