Zhiping Zhu

Thermogravimetric analysis of gasification reactivity of coal chars with steam and CO2 at moderate temperatures

Comparative study on the gasification reactivity of the three types of Chinese coal chars with steam and CO2 at 850–1050xa0°C was conducted by isothermal thermogravimetric analysis. The effects of coal rank, pore structure, ash behavior, and gasification temperature on the gasification reactivity of coal chars were investigated. It is found that the gasification reactivity difference between different coal chars changes with reaction degree and gasification temperature, and has no immediate connection with coal rank and initial pore structure. Ash behavior plays an important role in the char reactivity, and changes with gasification temperature and reaction degree due to the variation in the compositions and relative amount. The influence of pore structure is more noticeable during a relatively moderate reaction process. The relative reactivity ratio of steam to CO2 gasification generally decreases with the increasing temperature, and is related with the catalytic effect of inherent minerals. The characteristic parameters of the chars were analyzed, finding that the value of half reaction specific rate is approximate to the average specific rate under the same conditions. The nth-order distributed activation energy model is proposed to describe the coal char gasification process, and the results show that the activation energy increases with the increasing carbon conversion.

Regasification properties of industrial CFB-gasified semi-char

Fluidized bed (FB) gasification provides a promising way for the clean and effective utilization of low-rank coal. However, massive amounts of high-carbon-containing gasified semi-chars are produced, which greatly reduces its total carbon conversion. The disposal of the semi-char has become an intractable problem. Regasification of the semi-char is a possible way to realize further utilization of the residual carbon and achieve higher carbon conversion of coal gasification. In this paper, the regasification properties of the ultrafine semi-char, obtained from an industrial circulating fluidized bed gasifier, were investigated via thermogravimetric analysis and bench-scale study in a FB reactor. The kinetic parameters are derived by four different reaction models, namely homogeneous model (HM), shrinking core model, random pore model and modified random pore model (MRPM). Both non-isothermal and isothermal tests show that the chemical reactivity of the semi-char is poorer than the coal char. A higher temperature greatly favors regasification of the semi-char. For the studied reaction models, MRPM is the most favorable one for both coal char and semi-char. The activation energy of the semi-char is slightly higher than that of the coal char. Stable regasification of the semi-char in a FB reactor is achieved, and the residual carbon is further utilized. Increasing oxygen concentration could largely improve the heat value of the fuel gas, but slightly decrease the carbon conversion. Higher temperature operation greatly benefits the conversion of the residual carbon in the semi-char. Consequently, it will be considered feasible that the semi-chars are reclaimed and reused as feedstocks for gasification.