Study On The Distribution Of Mercury In Various Combustion Modes

Mercury is another pollutant which has caused concern after the SOx and NOx. Mercury is the only metal which is liquid at room temperature and highly volatile. Mercury is able to enrich in the environment, which causes soil and water pollution of the natural environment, then invades the human body through the respiratory tract, skin or digestive tract and other ways. Mercury contamination from coal combustion is the main source of anthropogenic artificial mercury contamination, which accounts for more than one-third. Mercury distribution from coal combustion is influenced by a variety of factors, such as types of coal, combustion modes and air pollution control equipments.In this thesis, different burning ways of coal-fired boilers are taken as the object of study. By using EPA Method30B and Lumex Mercury Analyzer, when types of coal and loads are changed, solid and gas samples are collected. Solid samples include coal, limestone, bottom ash, fly ash and gypsum; gas sample is collected at the flue which is before stack and after desulfurization. The emission characteristic of mercury can be obtained in various combustion modes by measuring the content of mercury. The result shows that the majority of mercury entering the pulverized coal boiler is exhausted to the atmosphere with the flue gas, which accounts for56%～85%; the majority of mercury entering circulating fluidized bed is enriched in fly ash and captured by precipitator, which accounts for50%～85%; the majority of mercury entering layer combustion boiler is enriched in gypsum, which accounts for56%. By calculating the enrichment factors and emission factors of mercury in fly ash in various combustion modes, it is found that the enrichment factors of mercury in the circulating fluidized bed are higher than that in the pulverized coal boiler and layer combustion boiler, while the emission factors of mercury in the pulverized coal boiler are the highest.In this thesis, in order to study the influence of operating conditions on the ability of capturing the mercury by fly ash, the content of mercury in fly ash is measured from the different boiler loads. The content of mercury first increases and then decreases with the increase of the boiler-load. The content of mercury is the highest at the load of90%. Through the micro-analysis of fly ash from different loads, it is found that the most probable pore size and pore size distribution of the fly ash at the load of90%is the biggest, which is beneficial to adsorbing the mercury by fly ash.By analyzing the content of mercury in different particle size of fly ash, the result shows that the particle size influences the ability of accumulation of mercury in fly ash. The content of mercury first increases and then decreases with the increase of the particle size of fly ash. The content of mercury is the highest among the particle size of90～106μm. Through the micro-analysis of fly ash under the different particle size, it is found that the most probable pore size of the fly ash among the particle size of90～106μm is the biggest, which is beneficial to adsorbing the mercury by fly ash.