Kazuo Asakura

Mercury emissions from a coal-fired power plant in Japan

The emissions study for mercury was conducted at a 700 MW coal-fired plant for the combustion of three types of coal with mercury concentrations of 0.0063, 0.0367 and 0.065 mg/kg. The power plant is equipped with a cold-side electrostatic precipitator and wet type flue gas desulfurization system. During full load operation of the boilers, samples of the input and output streams such as coal, coal ash, ESP ash and post-ESP particulates and flue gas were collected. The Hg concentrations in solid were measured by cold-vapor atomic absorption spectrometry (AAS) after appropriate preparation and acid digestion. Gaseous Hg was collected using a mixed solution of potassium permanganate and sulfuric acid and the Hg concentrations in the samples were measured using cold-vapor AAS. The results were used to examine: (1) overall mass balances; (2) relative distribution in the power plant; (3) equilibrium of Hg species using MALT-2 calculation program; and (4) Hg concentrations in stack emissions. The mass balances estimated in this study were 100, 138 and 89%, respectively, for the coals. Total Hg concentrations in stack gas were 1.113, 0.422 and 0.712 microg(m3N), respectively, for the coals. More than 99.5% of the Hg in the stack emissions were in gaseous form and the proportion in particulate form was extremely low. The relative distribution of Hg in ESP, FGD and Stack ranged from 8.3 to 55.2%, 13.3 to 69.2% and 12.2% to 44.4%, respectively. The results indicated that factors other than the Hg concentration of coals and efficiency of pollution control devices might affect Hg emissions from coal-fired plant. The calculated equilibrium of the distribution of Hg species using the MALT2 program suggest that it is necessary to consider condensation mechanism to interpret the affect of Hg species on the variations of the removal efficiencies of Hg in the ESP.