Study On Mercury Speciation And Emission During Coal Pyrolysis And Gasification

As the largest source of atmospheric mercury emission, mercury emitted from coal utilization has a pollution effect on the environment and does great harms to the human health. Control of mercury and its compounds has been an emerging and frontier area in the research fields of pollutants emission. These reviews, however, do not consider in-depth the speciation of mercury in pyrolysis and gasification flue gases.This paper particularly studied the speciation and emisstion behavior of mercury during coal pyrolysis and gasification with experimental and computational methods.Calculation models of chemical thermal dynamical equilibrium are used to predict the chemical speciation of Hg in the coal pyrolysis and gasification flue gas. The software used is F*A*C*T, the temperature range is from 500K to 2000K, and the pressure range is from 1atm to 80atm. Calculation results showed that mercury species at equilibrium are all distributed in gas phase. Hg⁰ is the main species at temperatures higher than 500K.Two bituminous coals were used to determine the mercury behavior during coal pyrolysis. The experiments were carried out in a tube furnace with a nitrogen blanket at deferent resident times and different temperatures. The experimental data indicated that the percentage of mercury removed from coal was increased with increasing temperature and resident time. Elemental mercury(Hg⁰) is the dominant species of mercury presents in gas, the content of elemental mercury(Hg⁰) becomes lower and the content of divalent mercury becomes higher with incerasing temperature and resident time during coal pyrolysis.The gasification experiments were also carried out in a tube furnace at deferent resident times and different temperatures. We use carbon dioxide(CO₂) as the reaction gas. It is clear that most of the original mercury is removed from these coals at these conditions. Elemental mercury(Hg⁰) is the main species at temperatures lower than 600℃, and divalent mercury(Hg²⁺) becomes the main species at temperature higher than 600℃.