The effect of coal volatility on mercury removal from bituminous coal during mild pyrolysis

Two high volatile and one low volatile bituminous coals (Lower Freeport #6A, Pittsburgh #8 and Lower Kittanning, respectively) used primarily for electricity production were tested to determine the percentage of mercury content removed during mild pyrolysis. Size-segregated samples of the well-characterized coals were tested in a tube furnace with a nitrogen blanket at different residence times for different processing temperatures through the range 275–600°C. The resulting char was analyzed for mercury and compared to the original parent coal concentration to determine the percentage of removal. Experiments have shown that as much as 80% of the original mercury is removed from these coals at these conditions. The percentage mercury removal was found to be a function of residence time and temperature. The high volatile bituminous coals show a near-constant mercury removal rate as the temperature increases until the temperature reaches a level where mercury removal is inhibited and the rate decreases with increasing temperature. For the low volatile coal, the rate of mercury does not show a change in mechanism as temperature increases and follows the Arrhenius form throughout the temperature range studied. The results were modeled as a homogeneous reaction with distinct maximum percent mercury available for conversion (Xmax) for a specified temperature. Data analysis indicates the following: at 500°C, mild pyrolysis of the Lower Kittanning low volatile sample resulted in 75% maximum mercury removal and the corresponding reaction rate coefficient is 1.56 min−1; mild pyrolysis of Lower Freeport #6A coal sample at 500°C resulted in 74% maximum mercury removal and the corresponding reaction rate coefficient is 0.42 min−1; the Pittsburgh #8 coal sample had a maximum mercury removal amount of 80% at a furnace temperature of 400°C and a reaction rate coefficient of 0.44 min−1.