论文部分内容阅读
Cement production process is the predominant anthropogenic source of atmospheric mercury emissions worldwide and in China.The Minamata Convention requires to reduce the mercury emissions from cement plants in the near future.The mercury behavior in cement production process is more complex than that in other industries,such as power plants.The recycling of collected dust and preheat of raw materials and coal cause several mercury cycling in the cement production process.Mercury is accumulated to a high level in the production process,and over 90%of mercury input from raw materials and fuel is emitted into the atmosphere because of the mercury cycling.Therefore a comprehensive understanding of the characteristics of the mercury cycling is crucial for the mercury control through the disposal of collected dust in cement plants.In this study,field tests were conducted in three typical cement plants in China.The flue gas emitted from the kiln system,before and after each facility including raw mill,fabric filter(FF)of raw mill and coal mill,electrostatic precipitator(ESP)at the kiln head were sampled with Ontario Hydro Method(OHM).Simultaneously the solid samples from the whole manufacturing process were collected,including raw materials,coal and dust from dust collectors.The mercury concentrations in the flue gas and solid samples were determined.It is found that 64.7-97.7%of the mercury input for the kiln system was emitted from the kiln tail,and there was only 0.3-2.6%of the mercury input remaining in the clinker.The mercury emitted from the kiln head accounted for 2.0-32.8%.The mercury cycling between kiln system and raw mill+FF,coal mill+FF,and ESP at kiln head is based on the high removal efficiencies of the above facilities.The mercury from the kiln tail was removed by the raw mill and FF with efficiencies of 73.1%and 78.3%,and recycled to the kiln system.Only 9.9%of the mercury from the kiln tail was emitted into the atmosphere.The removal efficiencies of coal mill and FF were 96.5%and 42.5%,indicating that 95.9%of the mercury in the flue gas was cycled to the kiln system.The removal efficiencies of ESP at kiln head ranged from 22.0%-82.3%,and 52.3%of the mercury from the kiln head was cycled to the kiln system.The effects of dust disposal are finally assessed based on the characteristics of the mercury cycling in the cement manufacturing process.