Laboratory Study on Gasification Reactivity of Coals and Petcokes in CO_2/Steam at High Temperatures

摘要

The gasification reactivity of 13 carbonaceous materials in CO_2 or in steam was studied in the temperature range 1000-1600 ℃. 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 ℃. 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 ℃. 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 ℃, 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 ℃ 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.