Pilot-scale study of volatilization behavior of Hg, Se, As, Cl, S during decoupled conversion of coal

The main aim of this paper is to determine the potential capability of removing volatile hazardous elements from coal before its further utilization by decoupled conversion of coal. The so called decoupled conversion isolates the combustion/gasification into two consecutive stages: pyrolysis followed by char combustion/gasification. Previous studies indicated that pyrolysis could transfer a part of the volatile hazardous elements of coal into gas form. The concentrated pollutants in the pyrolysis gas could be removed easier than those diluted ones in the gas product of direct combustion/gasification. However, the experiments were carried out in very small reactors, mostly in fixed beds, with batch feeding. The operating conditions, especially the reaction atmosphere and residence time, were quite different from the industry production process. Therefore, a 50 kg/h pilot scale facility combining a fluidized bed coal pyrolyzer and a fluidized bed char gasifier has been installed. A bituminous coal, a lignite or the binary mixture were continuously pyrolyzed together with circulating silica sands at different temperatures in the fluidized bed pyrolyzer and then gasified in the fluidized bed char gasifier. The volatilities of selected five hazardous volatile elements (mercury, arsenic, selenium, chlorine and sulfur) have been determined by analyzing each element concentration in the pyrolysis products. Pyrolysis in the large scaled fluidized bed has been proved to be a potential efficient method to remove volatile hazardous elements from coal before gasification. 600 °C is an optimal temperature for removing volatile hazardous elements from the two selected coals by flash pyrolysis, which also has been verified as a suitable temperature to get a satisfied tar yield.