| China is the largest anthropogenic source atmospheric mercury emitter worldwide and coal-fired flue gas is one of the main sources of atmospheric mercury emission. Mercury in coal-fired flue gas is present in three basic forms:oxidized mercury (Hg2+), parti culate-bound mercury (Hgp), and elemental mercury (Hg0). The oxidized mercury and particulate-bound mercury can be easily removed by the existing electrostatic precipitators or fabric filters, while elemental mercury is highly volatile and insoluble in water and is not easily removed by existing air pollution control devices, so Hg0 is an important factor in mercury removal of coal-fired flue gas. However, if the elemental mercury can be converted to oxidized mercury by using appropriate oxidizing chemicals or catalyzer, the emission of mercury will be controlled effectively with fully utilization of downstream dust-removing equipment and desulfurization and denitration units. And this method will be the most efficient way to control the mercury emission in coal-fired flue gas in China.Aiming at domestic typical coals, the impact of halogen addition on mercury emission and speciation transformation in coal-fired flue gas were investigated by using mercury oxidation rate as evaluating indicator and one-dimensional drop tube furnace system as experiment device. Firstly, mass balance of mercury was checked and oxidation rates of mercury in coals without halogen addition were ascertained. Secondly, researches about the impact of adding chlorine, bromine on mercury transformation were conducted through experiments, and the effects mechanism of additive’s concentration, type, adding mode and coal quality on mercury speciation transformation in coal-fired flue gas were also investigated.According to the experiment results, the following conclusions can be obtained: (1) When halogen (Cl, Br) was added in a specific concentration range, the oxidation rate of mercury in coal-fired flue gas could be promoted remarkably. While mercury oxidation rate could not be improved anymore when the addition concentration exceeded the range. What’s worse, there would be a certain degree of inhibition in mercury oxidation rate. (2) The mercury oxidation rates were different by halogen type and coal quality. The elemental mercury oxidation rates sequence of lignite flue gas under same chlorine addition were as follows:HC1 injection after the furnace< HCl injection before the furnace< CaCl2 addition in coal< NaCl addition in coal. The elemental mercury oxidation rates sequence of sub-bituminous flue gas under same chlorine addition were as follows:HCl injection after the furnace< HCl injection before the furnace< NaCl addition in coal< CaCl2 addition in coal. The elemental mercury oxidation rates sequence of lignite and sub-bituminous flue gas under same bromine addition were as follows:HBr injection after the furnace< HBr injection before the furnace< CaBr2 addition in coal< NaBr addition in coal. (3) The mercury oxidation obtained by solid immersion was much better than it obtained in flue gas whether adding halogen (Cl, Br) in lignite or sub-bituminous, and the mercury oxidation effect of lignite was better than it of sub-bituminous. Besides, the mercury oxidation rate of bromine addition was better than it of chlorine addition. (4) The optimal addition concentration and its mercury oxidation rates acquired by this experiments as follows:the highest oxidation rate of mercury of 72% was obtained with addition NaBr in lignite at 200 mg/kg; the highest oxidation rate of mercury of 83% was obtained with addition NaBr in sub-bituminous at 1000 mg/kg. (5) Build-up a database which contains data of different power plants using halogen addition method to control the emission of mercury was proposed. In the design stage, based on the practical cases with similar working condition in database, the optimization of operating conditions will be conducted using the theory basis of halogen addition obtained in the research of laboratory as the design guideline, consequently the effective control measures of mercury emission in flue gas of coal-fired power plant can be ascertained eventually. |
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