Volatility and Speciation of Mercury during Pyrolysis and Gasification of Five Chinese Coals

The fate of mercury in coal gasification systems is closely relevant to its transformation in fuel gases and subsequent release into the environment. This paper presents a study of the volatility and speciation of mercury during pyrolysis and gasification of five typical Chinese coals commonly used in gasification stations. Experiments were conducted on a bench-scale fixed bed under atmospheric pressure at final temperatures from 400 to 1200 degrees C, in 200 degrees C increments. The mercury concentration in coals ranged from 0.259 to 8.339 mu g/g. The gas-phase mercury was analyzed by an atomic fluorescence mercury analyzer according to the Ontario Hydro Method. It was observed that mercury volatility increased monotonically with temperature over 30 min. Hg-0(g) was the dominant species at most of the temperatures and holding times examined. It was also observed that high temperatures and long times enhanced mercury oxidation. With the temperature increasing from 800 to 1200 degrees C, Hg2+(g) increased from 5% to 35% during pyrolysis, and from 20% to 60% during gasification. The ratio of Hg2+/Hg-T reached its minimum at 800 degrees C for one coal (GZA) during pyrolysis, whereas for two other coals (BS and SF) the maximum value of that ratio was higher than the other coals and occurred during high temperature steam gasification at 1200 degrees C, respectively. The maximum value of the ratio of Hg2+/Hg-T was achieved during the high temperature steam gasification. Comparative studies on mercury emission under different conditions indicated that the volatility and speciation of mercury may correlate with the halogen concentration in the coals. However, no obvious correlation between Hg2+/Hg-T and basic oxides or acidic oxides in the ash was observed.