Besides sulphur oxides (SOx) and nitrogen oxides (NOx), thermal power plants (TPPs) also emit particulate matter (PM) and mercury. These emissions can be reduced by installing suitable emission controlling devices. Various technologies such as electrostatic precipitators (ESPs), wet particulate scrubbers, fabric filters and cyclone separators are deployed for PM control by TPPs. Meanwhile, mercury emissions can be controlled as a co-benefit of the NOx, SOx and PM control technologies. In the environment standards issued in 2015, the PM emission standards prescribed for TPPs vary on the basis of their installation dates. To keep emissions under control, a limit of 100 mg per Nm3 has been set for plants commissioned by December 31, 2003, while for plants commissioned between January 1, 2004, and December 31, 2016, it is 50 mg per Nm3, followed by 30 mg per Nm3 for plants commissioned from January 1, 2017 onwards. Further, mercury emissions need to be lower than 0.03 mg per Nm3.
PM emission control
ESPs: ESPs are the most commonly used PM emission control equipment in coal-based TPPs. They specialise in the removal of PM by electrically charging the ash particles in a flue gas stream. An ESP comprises a series of parallel vertical plates containing electrodes, which create an electric field through which the flue gas is passed in order to separate the particles. Given their high collection efficiency of over 99.99 per cent for a particle range of 0.01-100 micrometres, most power plants in the country have already installed ESPs. However, ESPs do not work well for fly ash with high electrical resistivity, limiting their collection efficiency. The country’s largest power generator, NTPC Limited has awarded packages for the renovation and retrofitting of 52 ESP units in 10 stations. As of March 2022, work has been completed on 39 units in 8 stations. Work is currently under way for around 4 GW of capacity across 13 units.
Wet particulate scrubbers: These aim to capture fly ash in addition to sulphur dioxide. In the most widely used wet scrubber, called the venturi scrubber, water is injected into the flue gas stream at the venturi throat to form droplets. Fly ash particles react with the droplets to form a wet by-product, which is then disposed of. Most wet scrubbers are designed for the purpose of controlling both sulphur dioxide and particulates by utilising alkaline fly ash as a sorbent.
Fabric filters: These can be extremely efficient collectors for even submicrometre-sized particles. They consist of a casing with a filter medium. The flue gas emitted by a boiler is blown or sucked through a fabric filter bag. The fabric bag collects the dust, subsequently removing ash and particulate matter via periodic shaking of the bag. With a removal efficiency of 99-99.99 per cent for a particle size range of 0.01-100 µm, fabric filters can lower emissions by implementing flue gas conditioning. At its 1,320 MW coal-based power plant, Jhajjar Power Limited has implemented a hybrid technology comprising ESPs and fabric filters for efficient PM control.
Cyclone separator: This is another device that specialises in the application of a centrifugal force to separate suspended particles from the flue gas stream. With advantages such as low costs, smaller space requirement benchmarks, simple structures, and the capability to withstand high temperatures and pressures, these devices are used extensively by multiple industries.
Mercury emission control
Control systems for NOx and SOx, as well as ESP and sorbent injection technology, offer the co-benefit of mercury emissions control. The level of control is strongly affected by the type of mercury emitted, the type of air pollution controls deployed, coal type and chlorine levels. Mercury control involves the oxidisation of all metallic mercury to ionic mercury to facilitate its removal from flue gas desulphurisation systems. The most common technology involves the injection of activated carbon into a plant’s exhaust stream. Mercury can be removed through chemical adsorption on powdered activated carbon (PAC). The main drawback of the PAC system is its potentially adverse impact on ESP performance at the time of particulate collection. However, the injection of amended silicates can potentially negate both the increased SO3 concentration in flue gas as well as any adverse effects on ESP performance. Halogen injection into coal is another widely used technology, and is done using calcium bromide or sodium iodide. It is an inexpensive process and results in very high mercury oxidation. At NTPC, mercury analysers for emission and air monitoring have been installed and the necessary environmental monitoring data has been made available to the central and state pollution control boards, as per their requirements. At many of its stations, mercury emissions are below the minimum detectable limit.
Conclusion
Net, net, with these technologies in place, the power sector will be able to keep PM and mercury emissions under control. Numerous other technologies are available and are being worked upon, depending on the type of coal being used in plants, the age of the plants, etc., in order to achieve emission control targets as per the norms.