| dc.description.abstract | Emission of trace metals from coal-fired power plants has become an important environment issue worldwide. Studies conducted by the US EPA, EPRI, and DOE indicate that coal contains mercury in the range of 0.02?0.25 ?g/g. It is significantly higher than heavy oil (0.002?0.008 ?g-Hg/g). In Taiwan, more than 60% of electricity is supplied by coal-fired power plants. This fact may result in mercury pollution in Taiwan and needs to be investigated in detail and controlled as soon as possible. Coal-fired utility boilers are the largest anthropogenic emission sources of mercury in the world. Concentration and speciation of mercury emitted depend on coal type and air pollution control devices (APCD) applied. Mercury with a relatively high vapor pressure easily escapes from traditional APCDs (i.e., FGD, EP, and wet scrubbing). Several methods have been proposed for capturing trace mercury from the flue gases, including adsorption by activated carbon or fly ash, or removal by wet scrubbers. It is reported that only 30-40% Hg can be captured by FGD and more than 50% Hg is emitted into the atmosphere. In addition to mercury, many kinds of trace metals also exist in coals. Notably, parts of them are toxic. Therefore, understanding the current emissions of trace metals from coal-fired power plants is deemed necessary.
In this study three coal-fired power plants were selected for conducting flue gas and ash sampling for determination of the metals emission including mercury, cadmium, lead, chromium, arsenic, and nickel and the removal efficiency achieved by the APCDs. Plant-A was equipped with fabric filter and flue gas desulfurization, and plant-B was equipped with selective catalytic reduction, electrostatic precipitators and flue gas desulfurization, while plant-C was equipped with electrostatic precipitators and flue gas desulfurization. The results indicate that removal efficiency of heavy metal could reach 90% for Plant-B (ESP+FGD). The removal efficiency of heavy metals reaches more than 60% (except for Cr and Cd). The overall removal efficiencies of mercury were 75% and 82% and 59% for plant-A and plant-B and Plant-C, respectively. Because SCR is installed in Plant-B for NOx control, lower Hg emission from Plant-B compared to Plant-A is measured. The results of stack sampling show that metals with high vapor pressure and low boiling point, such as Hg, gas-phase portion were about 80~90% of the total concentration and dominated the partitioning. The results indicate that metals with low boiling points tend to penetrate through APCD and effective control device is needed to collect them. Preliminary results indicate that removal efficiency of particulate was about 95% for Plant-A, 99% for Plant-B. At the stack of the plant-A, plant-B and plant-C total mercury emission concentration were 0.36 ug/Nm3, 0.45 ug/Nm3 and, 1.11ug/Nm3, respectively. Moreover, Hg, Cd, Pb, Cr, As and Ni were mainly retained in the fly ash for Plant-A. As far as Hg was concerned, 85% of total Hg were retained in fly ash, and 10% of total Hg would be emitted into the atmosphere. The concentrations of stack in the Plant-A were Pb＞Cr＞Ni＞Cd＞As＞Hg；Plant-B were Ni＞Cr＞Pb＞Cd＞As＞Hg in order. | en_US |