Abstract:
Coal field fire can release massive amounts of heat, affecting the surrounding coal. When the fire area is effectively controlled or extended to other areas, a large amount of oxidized coal is formed at the edge of the fire area or adjacent coal seam. Compared with raw coal, oxidized coal has low calorific value and high ash content, which makes it difficult to be used as conventional coal for combustion. Currently, it is mostly mixed with high-quality coal, resulting in a decrease in the overall calorific value and an increase in the amount of slag, which seriously affects the operation efficiency of power plant boilers. To make a better use of oxidized coal and maximize the utilization of resources, the idea of staged catalytic gasification of oxidized coal is put forward. It is proposed to adopt low-cost bentonite as the catalytic core material in the high-temperature stage. It can realize the high-efficiency gasification utilization of oxidized coal by the initial activation and the persistent catalytic gasification in the later stage. The X-Ray diffraction(XRD),Scanning electron microscope(SEM),and pore analysis are adopted to characterize the evolution of coal. The TG-MS analyzer is used to carry out gasification experiments. The results demonstrate that the staged catalyst is composed of an outer layer of K
2CO
3 and an inner layer of bentonite. The surface layer of K
2CO
3 can significantly promote the formation of pore structures in the initial pyrolysis of oxidized coal, and a large number of microporous structures are formed in the initial stage of gasification, increasing the amount of contact area of oxidized coal and gasifying agent CO
2. Bentonite containing Na
+ can play a stable catalytic role at high temperatures, making up for the defects of K
2CO
3 being volatile and deactivated. The growth rate of the carbon lattice unit is slower than that of raw coal char with the action of the staged catalyst. This indicates that the staged catalyst can be inserted into the edge of the carbon aromatic ring, resulting in defects in the internal lattice, thereby effectively inhibiting the graphitization process of oxidized coal char and ensuring the continuous catalytic gasification process. When the weight ratio of K
2CO
3 and Na-based bentonite is 1∶1,the overall catalytic gasification effect is the best. The staged catalyst plays a catalytic role in the pyrolysis stage as well, which can promote the release of hydrogen from the pyrolysis of oxidized coal. During the gasification stage, the staged catalyst can effectively reduce the gasification reaction temperature of oxidized coal and the peak temperature of the synthesis gas yield, and increase the gasification reaction rate as well. The temperature values corresponding to the peak concentrations of CO and H
2 decrease by 97.4 ℃ and 129.1 ℃,respectively, showing an obvious catalytic effect. The high-efficiency production of syngas by the staged catalytic gasification proposed in this work provides a new way for the efficient utilization of oxidized coal in the fire area, which can avoid the waste of a large number of coal resources in the coal field fire area.