Abstract
For the Polish power industry, the basic fuel is hard coal and lignite, which contains significant amounts of mercury. Current emission standards in the European Union (IED directive and in the near future BAT conclusions) create conditions for investment in flue gas cleaning installations for coal-fired power plants. During the combustion of coal, mainly metallic mercury (Hg0) is produced, which is difficult to remove from the flue gas in dedusting installations (electrostatic precipitators and fabric filters) and wet flue gas desulphurization plants. In these installations, oxidized mercury (Hg2+) and ash bound mercury (Hg(p)) are removed. In order to reduce the mercury concentration in flue gases, the DEMERTEC technology was created, which is based on the oxidation of metallic mercury to an oxidized form and its removal from flue gas in an existing flue gas de-dusting or de-sulphurization installations. The article presents the results of field tests of the DEMERTEC technology for flue gas from hard coal and lignite. This technology was tested on a ~ 400 MWe unit fed with lignite and on an FGD absorber in which flue gas was purified from two units with a capacity of 195 and 220 MWe fed with hard coal. In both cases, mercury emissions were reduced below the levels required by the BAT conclusions.
Highlights
The amount of mercury present in the exhaust gases after the boiler is proportional to the mercury content in coal, for Polish deposits, the average content is as follows: hard coal - from 50 to 150 ppb (1 ppb = 1 μg / kg); lignite - from 120 to 370 ppb [1]
The mercury contained in the coal in the combustion process at the temperature > 600 °C [2] passes into the gas phase as elemental mercury Hg0
The efficiency of mercury removal in the flue gas desulphurization (FGD) absorber increased with the amount of oxidant injected into the flue gas, which was related to the increased concentration of Hg2 + in the flue gas before the absorber
Summary
The mercury contained in the coal in the combustion process at the temperature > 600 °C [2] passes into the gas phase as elemental mercury Hg0. Lowering the flue gas temperature in heat exchangers causes that part of Hg0 is oxidized to the Hg2+ form (called ionic mercury or oxidized mercury) [3]. Hg2 + is the name of all water-soluble mercury compounds, the main of such compounds in the process of combustion of hard coal is HgCl2 (there is HgBr2, HgI2, HgF2, HgO, HgSO4 and Hg (NO3)2) [4]. The permissible mercury concentrations in the flue gas resulting from the BAT conclusions [6] refer to the total mercury HgT defined as: HgT = Hg0 + Hg2 + + Hg (p)
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