Thermal power plants (TPPs) are well known for releasing emissions such as particulate matter (PM) and mercury. These emissions can be controlled and even reduced by installing suitable emission controlling devices. Numerous technologies are being deployed for emission control by TPPs in India in order to comply with emission standards. As per 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 till 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 January 1, 2017, onwards.
PM emission control
Electrostatic precipitators (ESPs), the most commonly used particulate emission control equipment in coal-based power generation plants, specialise in the removal of PM by electrically charging the ash particles in a flue gas stream to collect PM. An ESP comprises a series of parallel vertical plates containing electrodes, which create an electric field through which the flue gas passes 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, the collection efficiency of ESPs is often limited as it does not work well for fly ash with high electrical resistivity.
Meanwhile, wet particulate scrubbers for PM control 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. Most wet scrubbers are designed for the purpose of controlling both sulphur dioxide and particulates by utilising alkaline fly ash as a sorbent.
Another technology solution namely fabric filters can be extremely efficient collectors for even sub-micrometre-sized particles. They consist of a casing with a filter medium, wherein 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 with 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 that comprises of ESPs and fabric filters in order to facilitate efficient PM control.
Another device that specialises in the application of a centrifugal force for separating suspended particles from the flue gas stream is the cyclone separator. Offering 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.
For particulate emission reduction, most NTPC units are complying with SPM norms. Furthermore, to control PM emissions, the majority of NTPC stations are equipped with over 99.8 per cent efficient ESPs. With ESP augmentation being taken up through renovation and modernisation projects, work is in progress for 16 units with a total cumulative capacity of 5 GW.
Mercury emission control
Mercury emissions from power plants not only cause human mercury exposure but are also known to bioaccumulate in fish and animal tissue in their most toxic form, called methylmercury. The adverse effects of mercury include reduction in reproductive success, impaired growth, behavioural abnormalities and sometimes death among fish, mammals and birds. Mercury can be controlled as a co-benefit of the NOx, SOx, and PM control technologies that exist, and via sorbent injection technology. 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 delsulphurisation systems. In addition, the injection of amended silicates can potentially negate both the increased SO3 concentration in flue gas as well as any adverse effect on ESP performance. Halogen injection into coal is another widely used technology, and is done using calcium bromide or sodium iodide. At NTPC, mercury analysers for emission and air monitoring have been installed. In addition, the necessary environmental monitoring data has been made available to the central and state pollution control boards, as per their requirements. To further minimise efficiency losses at stations, plant information system-based applications for real-time efficiency and loss calculations have been put in place.
Conclusion
Net, net, with technologies such as ESPs, fabric filters, cyclone separators, wet particulate scrubbers and mercury emission controls in place, there is tremendous potential in the power sector for collecting high-resistivity fly ash so as 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., to achieve emission control targets as per the norms.