Application of Sorbent Polymer Catalyst (SPC) modules for mercury emission control

摘要

Power plants producing electricity and/or heat which fire lignite or bituminous coal are deemed to be major anthropogenic sources of mercury emissions. High toxicity of Hg contributes to significant damage to the environment and constitutes a direct threat to human health. The content of Hg in coal deposits is at trace levels but taking into account the amount of fuel burned for energy production (36,616 thousand tons of hard coal and 62,371 thousand tons of lignite were used by thermal power plants in 2015 in Poland - Fuel and energy economy) the average emissions of mercury from a typical Polish coal-fired thermal power plant ranges from 52 to 57 kg of Hg/year [1]. In 2013, the total amount of mercury emitted to the environment in Poland reached 10.4 tonnes, including 5.7 tons from combustion processes of which 4.9 tons were emitted from thermal power plants and CHP plants. Coming in 2017, the European Commission Integrated Pollution Prevention and Control (IPPC) / Industrial Emissions Directive (IED) Best Available Techniques (BAT) make the emission regulation outlined in 2010/75/EU more strict for NO_x, SO_x and heavy metals emission like mercury. Updated BREF LCP standards for mercury emission will require the use of new integrated and/or flue gas purification technologies. Current primary mercury control techniques using wet Flue Gas Desulphurization (FGD), electrostatic precipitator are not sufficient to ensure required mercury reduction levels. Accordingly, there is a need to research and develop new and efficient mercury removal techniques. The proposed solution attempts to fill the emerging market technological gap. Such solution is particularly important for east European countries as the energy production is based on bituminous and lignite coals. The burning of lignite coal cause serious problem for mercury control due to high content of this heavy species. This work presents the possible application of the mercury control techniques by installation of polymer adsorption modules. Due to the mercury content in the fuel, power stations will not be able to operate in the market without use of highly efficient systems reducing mercury emissions. Currently on the east European market, there is no experience in the area of reduction of Hg and the choice of technology is essential to optimize costs in achieving environmental results. There is also a high risk that the draft standards in IED for Hg emissions can be further reduced below 3 ug/Nm3 for bituminous and 7 ug/Nm3 for lignite coals. During the coal combustion process which occurs at temperatures higher than 1200°C the liberation of nearly all mercury from coal to flue gas in the elemental form Hg° occur. Above 677 °C the portion of Hg° may react with chlorine Cl_2 created fallowing the Deacon reaction [2] from species like, HC1, HOC1 to form inorganic oxidized mercury Hg2+ like HgCl_2 [3,4], Cooling the flue gases, as it passes the steam superheaters and other convective heat transfer surfaces, the flue gas temperature is decreased and the Deacon reaction is inhibited and fallowing the Grifina reaction SO_2 bounds with chlorine Cl_2 and water to form hydrochloric (HC1) acid and SO_3. This process inhabit the possible conversion of elemental mercury to inorganic oxidized form Hg~(2+). The mercury which is carried by the flue gas can be partially reduced within air pollutant control (APC) devices. In order to remove Hg° or Hg~(2+) from downsteam cold-end flue gases, it is expected that the use of passive or active Hg control techniques, which complicate and increase of costs of the whole process, need to be applied. The most frequently used passive technique is an injection of powder activated carbon (PAC), where the effectiveness of mercury removal depends on: the surface area of the sorbent, the pore size, vapor concentrations and types of mercury (elemental mercury Hg or oxidized Hg~(2+)) and also the exhaust gas temperature and composition. The drawbacks of injecting PAC are the continued operational costs of injected AC and increasing of the carbon content within ashes. Currently investigated possibility is to utilize some relatively new techniques based on installing Hg adsorption modules Sorbent Polymer Catalyst (SPC) Composite within wet FGD which should ensure high mercury reduction with low running cost in contrary to PAC injection. To ensure appropriate working condition for polymer modules they should be installed within the upper portion of the wet FGD where saturation temperature is maintained. Taking the advantages of using polymer modules it is always questionable how the reduction efficiency can change depending on conditions at which modules are operated i.e., different flue gas composition, mercury content in flue gases, fraction of mercury before modules. To give answer for that question a numerical model developed using computational fluid dynamic (CFD) can be utilized for that purpose. To model adsorption rete of mercury at the modules surface it is necessary to define certain model parameters. For that purpose the pilot installation can be used for determining mathematical model constant for modeling adsorption rate. Validated model can be finally used for modeling large scale system. Such simulation can provide answer how many modules are required or how and where they should be placed to reach desired reduction level.