Investigation into the fate of mercury in bituminous coal during mild pyrolysis

Two high-volatile bituminous coal (Lower Freeport #6A and Pittsburgh #8), used primarily for electricity production, were tested to determine the fate of their mercury content during mild pyrolysis. Mono-sized samples of the well characterized coals were tested under nitrogen in a horizontal tube furnace at different residence times at different temperatures throughout the range 275–600°C. The resulting char was analyzed for mercury, and compared to the original parent coal concentration. The percent Hg removal was found to be a function of both residence time and temperature. The data for both coals have shown two distinct regimes; a low temperature chemical evolution mechanism which follows an Arrhenius form (apparent activation energies for the Lower Freeport #6A and the Pittsburgh #8 coals are 25.6±1.5 and 21.7±1.9 kcal/mol respectively), and a higher temperature regime where the Hg evolution dramatically decreases. This can be attributed to the changing structure of the coal at these higher temperatures. The results of bomb calorimetry analysis performed on the Lower Freeport #6A coal samples verify that the overall heating value of the coal is essentially unaffected by mild pyrolysis at temperatures lower than 400°C.