MICROSTRUCTURAL AND REACTIVITY CHANGES OF COAL CHAR DURING STEAM GASIFICATION AT LOW-MEDIUM TEMPERATURE

摘要

The purpose of this study is to investigate the relationship between the steam gasification reactivity of aChinese lignite char (raw-char) and char generated from raw coal loaded with potassium carbonate (K-char) andmicrostructural changes. Steam gasification of the raw-char and the K-char was performed in a fixed bed reactor. Aftergasification, the gasified chars at different carbon conversions were examined using X-ray diffraction (XRD), and theXRD patterns of raw-char at various conversions showed that as gasification proceeded, the graphitization degree ofraw-char increased with an increase in carbon conversion. On the other hand, the microstructure of carbon of K-charcould not be detected by XRD due to the surface of K-char was covered with dense K2CO3 dispersing uniformly on it. Itwas revealed that laser Raman spectroscope (RS) and a scanning electron microscope combined with an energydispersive X-ray spectrometer (SEM/EDX) were very effective for the comprehensive evaluation of carbonaceousstructure evolution during gasification. The SEM images of the raw-char showed that porous structure with roughsurface and structure of well organized, longitudinal tubes coexisted, but in the case of K-char, only porous structurewith rough surface was observed during steam gasification. The Raman spectra of raw char at different carbonconversions showed that the disordered carbon having higher reactivity than the graphitic carbon was gasifiedpreferentially and as a result the ratio of graphitic carbon increased. Graphitic carbon and disordered carbon of K-charwere consumed simultaneously at the same rate during K-catalyzed steam gasification due to uniformly dispersion ofK2CO3 in the char matrix and, redistribution and penetration of potassium compounds through the pores of the K-charmatrix during gasification.