Coal-based plants emit dust of about 10 microns size on burning pulverised coal. Given its adverse environmental impact, particulate control equipment and technologies have been designed to remove particulates from the flue gas stream, preventing them from re-entering the flue gas. Some popular technology solutions are electrostatic precipitators (ESPs), fabric filters, cyclone separators and the high temperature high pressure (HTHP) particulate control solution.
In order to contain rampant particulate and gaseous emissions from thermal power plants (TPPs), the Ministry of Environment, Forest and Climate Change released new emission standards for coal-based power plants in December 2015. The revised particulate matter (PM) standard for thermal power units installed before March 31, 2003 is 100 mg per normal cubic metre (Nm3), for units installed after March 31, 2003 is 50 mg per Nm3 and for units installed after January 2017 is 30 mg per Nm3.
ESP is the most widely used technology for managing particulate emissions at coal-based plants. Broadly, an ESP comprises collecting electrodes, discharge electrodes and dust dislodging systems. These components are enclosed within a structure called field. Passing current through discharge electrodes charges dust particles in the flue gas stream, thereby drifting them towards the collecting electrodes.
Ideally, the collection efficiency of ESPs is as high as 99-99.99 per cent for the particle range of 0.01-100 micrometre. Its performance depends on a number of factors such as the specific collection area and voltage used to create electric fields, as well as the volume of the flue gas and the duration for which it comes in contact with the electrode. Some basic measures for proper functioning of ESPs include ensuring that the hopper (dust collection container) switches are not bypassed, ash is being effectively dislodged, flue gas temperature matches with the design flue gas temperature and seal air systems are maintained.
Solutions to upgrade ESPs
Currently, the most widely adopted solution for fine tuning and improving PM management is upgrading the existing ESPs. This helps improve the performance of the system and ensures compliance with tighter norms. Established ESP upgrade technology available at low investment makes upgrade convenient. As per the estimates of the Centre for Science and Environment, renovation of ESPs would cost Rs 0.5 million per MW-Rs 1.5 million per MW, depending upon the extent of upgradation. The shutdown time for retrofitting could be a maximum of 30 days, subject to the technique chosen for refurbishment.
A popular method for upgrading ESPs is augmenting the collection area of ESPs. In the case of limited space, the collection area could be increased by installing additional ESPs in series to the existing one. In case of sufficient space, placing additional ESPs in parallel to the existing ones is possible. However, as parallel ESPs would have multiple inlets, it requires redesigning the flue gas flow and dust distribution. It also leads to excess pressure drops, resulting in greater electricity consumption. An example of ESP upgrade with additional ESPs in parallel is the Maharashtra State Power Generation Company’s (Mahagenco) Parli TPP.
In those cases when neither parallel nor series addition is possible for increasing the collection area, adding new internals by increasing the casing height is useful. Also, filling dummy fields of ESPs, which are casings without internals, installed at either the flue gas inlet or outlet improves ESP performance. This has been carried out at Tamil Nadu Generation and Distribution Corporation’s Tuticorin 1–2 (2×210 MW) TPP and Mahagenco’s 5 (1×200 MW) TPP, among others.
Another important measure to increase the efficiency of ESPs is by manipulating ESP electrodes. An option is to increase the space between the electrodes from 250–300 mm to 400 mm. Besides improving efficiency, it reduces the weight of ESP internals. Also, electrodes can be made movable and use brushes can be fitted in the hopper to scrape off the dust. NTPC Limited’s Rihand TPP is the country’s first to have deployed this technique. With moving electrode precipitators, dust emission at the plant is expected to be reduced from 500 mg per cubic metre to 50 mg per cubic metre.
In addition to the mechanical methods for improving ESP performance, flue gas conditioning, which involves injection of chemical additives, and/or water or steam (water fogging) into the flue gas to alter the physical and electrical properties of dust particles, too increases the collection efficiency. It is necessary to ensure optimal injection and mixing of chemicals or water for desired results as over injection may lead to fouling, corrosion, clogging and unsalable ash generation.
Another technique to collect dry PM is through the use of fabric filters. Long fibre or cloth bags are used to filter dust, which is then periodically shaken off. The collection efficiency of fabric filters depends on parameters such as the air-to-cloth ratio, and volume and temperature of flue gas. Bag filters are recommended for flue gas volumes in the range of 100,000–500,000 Nm3 per hour, which is the flow rate in units smaller than 150 MW. Efficiency of bag filters may be up to 99 per cent, but it could drop to 90 per cent even if one of the thousands of bags in the filter gets damaged.
In contrast with ESPs, bag filters occupy less space. However, they consume more auxiliary power. Unlike in an ESP, the flue gas passing through bag filters encounters high resistance to flow as the difference of pressure between the inlet and outlet of the bag filter is much higher than in an ESP. This pressure drop necessitates a bigger fan size in bag filters than in an ESP to either push or pull the exhaust gas, resulting in higher auxiliary energy consumption. Often, bag filters are installed in combination with an ESP.
However, the use of bag filters for particulate emission management has failed to see adequate uptake in the country. A pilot project for the installation of bag filters at Mahagenco’s Koradi TPP had proved unsuccessful owing to concerns relating to the reliability of the spray cooling system to cool the flue gas before entering bag filters, shrinkage and wear and tear of the bags due to high velocity and particulate concentration of flue gases. However, the use of bag filters is popular globally — 35 per cent of coal-based power plants in the US and 10 per cent in China have installed bag filters or ESPs and bag filter hybrids.
Cyclone separators can also be used for particulate control. It operates on centrifugal force and separates suspended particles from the flue gas stream. These have simple structures, entailing low cost and less space requirement. However, they have low collection efficiency and are therefore usually used in combination with ESPs. Besides, HTHP particulate control device is an important component of combined cycle power systems. HTHP devices enable efficient hot gas particulate filtration as a protection against fouling and erosion of downstream heat exchanger and gas turbine components.
To conclude, there are a number of technology solutions available for particulate emission management and it is necessary to ensure the adoption of the best solution depending upon space availability and age of the plant, among others.