Reduction of mercury concentration from flue gases

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Reduction of mercury concentration from flue gases

We are engaged in studies and consultancy focusing on technologies for reducing and removing mercury concentrations from flue gases generated during the combustion of brown coal in power plants and heating plants. We process studies for small local sources up to large industrial units in the range of flue gas flows from 10,000 – 3,000,000 m3/h.

The study is prepared by our engineering division, see the contact.

Combustion of brown coal in coal-fired boilers of thermal power plants produces flue gases containing various harmful substances. Among other things, combustion produces also elements of heavy metals, which are very harmful to the surrounding environment. The most common heavy metals in flue gases include Hg (mercury) and Se (selenium). Mercury (hereinafter referred to as Hg) is released during combustion in three forms. Mercury can be released in gaseous form as elemental (metallic) Hg0, part of this mercury is oxidized in the flue gas by the presence of halogens and non-combustion to the so-called oxide (divalent) form of mercury Hg2 +. A small amount of mercury is absorbed on the surface of the solid particles of fly ash and occurs in a form bound to solid particles called HgP. This mercury is discharged together with the fly ash by the lower hopper of the boiler as ash and it is further captured in the subsequent filtration stage in the installed electrostatic precipitator or fabric filter. The normal concentration of mercury in the flue gases generated by the combustion of brown coal is in the range of 2 - 80 μg / Nm3 with the considered amount of mercury in the fuel in the range of 20 - 900 μg / kg. As mentioned above, mercury occurs in flue gases in various forms and the capture of individual forms of mercury requires specific procedures and technologies based on mechanical and physicochemical principles. The elemental form of mercury (Hg0) and its capture from flue gases is a very complicated process. The elemental form of mercury passes through the entire flue gas cleaning system and is released by the flue gas into the surrounding environment. In practice, many methods are used to remove this form of mercury. Most methods are based on the principle of oxidation of elemental mercury to the so-called divalent oxide form of mercury (Hg2 +), which is very soluble in water. The process of oxidation of elemental mercury to the divalent form of mercury can occur, for example, in the installed technology to reduce the concentration of NOx, so-called SCR, directly on the surface of the installed catalyst. Another method is the dosing of halogen-based compounds, either directly into the combusted fuel or as an additive to the activated carbon sorbent. Activated carbon is the most commonly used additive to reduce the concentration of mercury in flue gases based on the principle of adsorption. The divalent form of mercury can be captured very well, thanks to its solubility, in the installed wet desulphurisation technology or in the fabric filter device in combination with the dosing of activated carbon. In practice, the installation of a fabric filter plays an important role in capturing mercury, much larger than the installation of an electrostatic precipitator. As mentioned above, there are many methods for removing mercury from flue gases, including combinations thereof. The most commonly used methods are described below.

Dosing of activated carbon into flue gases

In practice, the most common method is to reduce the mercury content by dosing powdered activated carbon into the flue gas. Thanks to its large specific surface area, activated carbon is able to bind mercury to the surface on the basis of the absorption principle. Dosing coal into the system is a complex process, as it depends on the very thorough distribution of activated carbon in the flue gas stream. Most often, activated carbon is injected into the system at a point before the flue gas enters the air heater, namely the flue gas temperature range 140 – 180 ° C. The activated carbon is then carried by the flue gas stream to the subsequently installed hose filter, where the mercury bound to the surface of the activated carbon on the filter fabric is separated. The separated coal is then taken together with the separated ash from the hose filter hoppers to the storage system by an installed pneumatic transport or a part of the separated material is led back to the entrance to the fabric filter for reuse.

The system using activated carbon dosing in combination with the installation of a fabric filter can separate up to 99% of the mercury contained in the flue gas. In addition to the oxidic forms of mercury, the elemental form of mercury can be trapped to a certain extent on the surface of activated carbon. In order to increase the capture efficiency of the elemental form of mercury, halogen-based compounds (eg bromides) can be added to the activated carbon additive due to the oxidation to the divalent form of mercury described above. The amount of activated carbon fed into the process must be optimized based on the actual operating parameters of the boiler and the connected system. Because when burning brown coal with high ash content, a higher concentration of fly ash can be expected in the flue gas. The ash produced by burning coal has similar properties as injected activated carbon, ie a large specific surface area. With a sufficient amount in the flue gas, the fly ash is able to bind mercury to the surface. This process can save on the consumption of expensive activated carbon.

Modifications of desulphurisation absorbers

This method is based on a suitable modification and modification of droplet separators of existing absorbers of wet desulphurisation technologies installed at coal-fired power plants.

Engineering

Reduction of mercury concentration from flue gases

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Dosing of activated carbon into flue gases

Modifications of desulphurisation absorbers