To uncouple the complex behavior of sulfur transformation during thermal processing of coal and to elucidate the main mechanism, typical organic and inorganic sulfur compounds impregnated on or mixed with a low-ash char are studied through temperature-programmed decomposition coupled with online mass spectrum analysis (TPDMS) and followed by temperature-programmed oxidation coupled also with online mass spectrum analysis (TPOMS) in a temperature range of up to 800 degrees C. It is evident that the cleavages of Cal-S and Car-S bonds, where the subscripts al and ar stand for aliphatic and aromatic carbon, respectively, in the organic compounds result in the formation of SH radicals, which then undergo secondary reactions with the char to form various sulfur compounds such as H2S, SO2, COS, and elemental sulfur, as well as sulfur structures in the char. H2 has the ability to stabilize the -* SH radicals and weaken the interactions between the -* SH radicals and the char. For the sulfur compounds, which do not generate the *SH radical, the only sulfur products detected are those formed directly from the decomposition of the starting sulfur compounds, H2S from FeS2 in H2 or SO2 from Fe2(SO4)3 in He, for example, and no sulfur structure is formed in the char. Minerals have significant effects on the bond cleavage temperature and the reactions of the *SH radicals with the char. It is clear that the *SH radical is a key species interacting with the char to form secondary sulfur compounds, while H2S and SO2 play no role in the sulfur transformation to the carbon structure.
The gasification reactivity of 13 carbonaceous materials in CO 2 or in steam was studied in the temperature range 1000−1600 °C. The gasification reaction was carried out in a drop-in-fixed-bed reactor under atmospheric pressure. The gasifying agent fed into the reactor either as pure gas or as 36% volumetric concentration in argon with a total gas flow rate of 500 mL/min. The test samples included different rank coals, petcokes, and graphites. The raw materials were used to eliminate the problem related to char prepreparation. The dynamic profiles of gasification rate were used to compare the gasification behaviors for different samples. The physicochemical characteristics of chars were evaluated by scanning electron microscopy and N 2 adsorption method. The experimental results reveal that the difference in gasification reactivity among samples decreases as the temperature increases and is not distinguishable for most coals at 1600 °C. However, the temperature is still critical for gasification of petcokes and some high-rank coals at high temperature. The gasification reactivity of petcokes is 2−9 times lower than that of coals at 1600°C. The kinetic analysis reveals that the temperature dependence of reactivity varies with the type of materials. It is interested to find that, in the temperature range 1400−1600 °C, the gasification reactivity in CO 2 is higher than that in steam for coals but not for petcokes. From the views of the reaction thermodynamics, the gas diffusion difficulty, and the catalytic effect, the high temperature is favorable to the CO 2 -gasification. The effect of AAEMs (alkali and alkaline earth metals) should be a key factor. The content of AAEMs is apparent in coals but limited in petcokes. The Arrhenius plots reveal that the gasification mechanism may be altered around 1200 °C for most of coals. The petcokes are appeared with the most compact physical structure and the least gasification reactivity. Either the shrinking core model (SCM) or the volume reaction model (VRM) is suitable for most of the samples and conditions but not suitable for the petcokes. A diffusion term associated with the carbon structure may be needed for modelling the gasification behaviors of the petcoke-like materials.
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