The particulate matter (PM) emissions in a thermal power plant (TPP) can be reduced and controlled by the installation of emission controlling devices such as electrostatic precipitators (ESPs), fabric filters and cyclone separators. Empirical data shows that TPPs prefer installing ESPs over fabric filters, since ESPs concur with emission standards while also meeting cost considerations. According to the environment standards issued in 2015, TPPs need to meet the prescribed PM emission standards based on their installation dates – 100 mg per Nm3 for plants commissioned till December 31, 2003, 50 mg per Nm3 for plants commissioned between January 1, 2004 and December 31, 2016, and 30 mg per Nm3 for plants commissioned January 1, 2017 onwards.
ESPs are the most commonly used PM control equipment in coal-based power generation plants. ESPs collect dust in the flue gas emitted from the boiler of the power plant, which is then safely disposed. A well-designed ESP is characterised by high collection efficiency, high reliability, low flue gas pressure loss, resistance to moisture and temperature upsets, and low maintenance. Since Indian coal produces fly ash, it is very difficult to precipitate, and this often limits the collection efficiency of ESPs. In this context, the conditioning of fly ash in flue gas is an established technique that is used to restore the performance of an ESP.
As per the phasing plan prepared by the Central Electricity Authority, out of a total plant capacity of 197 GW as of December 2017, PM non-compliant capacity stood at 73 GW (37 per cent). ESP implementation/upgradation has already taken place for 65 GW of capacity (out of the 66 GW considered for implementation/upgradation).
The country’s largest power generator, NTPC Limited, has equipped its power stations with more than 99.8 per cent efficient ESPs. Performance enhancement is being carried out to achieve the desired emission level by augmenting the size of the ESPs (increased height, additional fields, charging of dummy fields, retrofitting of advanced ESP controllers) and deploying new technology – moving electrode electrostatic precipitators. The company has planned for renovation and modernisation of ESPs in 64 units, out of which work has been completed in 48 units. Work is in progress as per schedule in the remaining 16 units (aggregating to 5 GW of capacity).
A fabric filter consists of a casing with a filter medium and operates like a vacuum cleaner. Flue gas emitted by a boiler is blown or sucked through a fabric filter bag. The fabric bag then collects the dust and removes ash and particulate matter by periodically shaking the bag. Fabric filters can be very efficient collectors for even sub-micrometre-sized particles. The removal efficiency of fabric filters is in the range of 99-99.99 per cent for a particle size range of 0.01-100 µm. Flue gas conditioning is also implemented in fabric filters, enabling lower emissions. Jhajjar Power Limited, at its 1,320 MW coal-based power plant, has implemented a hybrid technology comprising ESPs and fabric filters for efficient PM control.
A cyclone separator is a device that applies centrifugal force to separate suspended particles from the flue gas stream. The coarser particles are moved towards the cyclone separator wall due to the ap-plied centrifugal force. Various industries use these devices because of inherent ad-vantages such as simple structures, low cost, less space requirement and capability to withstand high temperature and high-pressure conditions.
Wet particulate scrubbers
Wet scrubbers for PM control capture fly ash in addition to sulphur dioxide. The most widely used wet scrubber is the venturi scrubber, in which 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. The venturi scrubber has a removal efficiency of 90-99.9 per cent for a particle size range of 0.5-100 µm. However, if the particle size is less than the specified range, the system efficiency drops. Most wet particulate scrubbers are designed to control both sulphur dioxide and particulates by utilising alkaline fly ash as a sorbent.
As per the environmental norm, mercury emissions need to be less than 0.03 mg per Nm3. These standards are based on the characteristics of Indian coal, meteorological conditions and the best available control technology. The most common technology involves the injection of activated carbon into the plant’s exhaust stream. Mercury can be removed through chemical adsorption on powdered activated carbon (PAC). Activated carbon is injected upstream of an ESP or FF and is removed along with fly ash. The main drawback of the PAC system is its potentially adverse impact on ESP performance at the time of particulate collection. A new market entrant, the injection of amended silicates, can potentially negate both the increased SO3 concentration in flue gas and the adverse effect on ESP performance. Another widely used technology is halogen injection into coal using calcium bromide or sodium iodide. It is an inexpensive process and results in very high mercury oxidation.
To conclude, ESPs are widely preferred to alternative technologies for PM control; however, they are incapable of controlling the fly ash generated by Indian coal comprising sulphur and alkali. Going forward, new techniques for collecting high-resistivity fly ash can be adopted.