Effect Of Metals And Chlorine On The Oxidation Of Mercury In Flue Gases During Coal Combustion

The wide use of coal has led to serious air pollution in China. In addition to common pollutants such as SOx, NOx and CO2, mercury has received increasing concern around the world in recent years. The oxidization of Hg0 to Hg2+ during or after coal combustion is believed to be an effective way for controlling mercury emissions. However, the underlying mechanisms of mercury transformation during combustion are not very clear up to now. The present thesis investigates the effects of alkali and alkaline metals, such as Na, K, and Ca, and Cl on Hg oxidation through experiments and thermodynamic simulations. These results are very helpful to both prediction of Hg transformation and control of Hg emissions from coal combustion. The work involved in this thesis is as follows:(1) Various models of chemical and thermal equilibrium are used to predict chemical species of Hg in the coal combustion flue gas. The software used is HSC chemistry 5.1, the temperature range is from 473K to 2000K, and the pressure is 0.1MPa. Calculation models are based on different flue gas compositions, and different Cl concentrations. Possible reactions in different models are predicted. The emphasis is the combined effects of trace and alkali metals on Hg oxidation. The results show that only a small amount of HgCl2 is generated in the oxygen-rich environment.(2) The experimental study of oxidation of mercury in real flue gas has been carried out. By changing the contents of Cl and metals in the coal before combustion, the concentrations of HCl and SO2 in the flue gasare measured. The results show that after the addition of metal-carbonates, the proportion of oxidized Hg decreases from 86% to about 22%. At the same time, the detected HCl has a significant decrease in concentration. The oxidized Hg in the flue gas from combustion of the washed coal samples undergoes a reduction to only 7% and 10%, respectively. Therefore, Cl is considered to play a dominant role in the oxidation of mercury, while the addition of metals could greatly suppress the oxidation of mercury.