The growing demand for power is being largely met through coal-based power generation. However, the steady increase in coal-based generation has led to a rise in particulate emissions, which necessitates the adoption of emission control technologies. Particulate emissions generally constitute the solid and liquid matter of organic or inorganic composition suspended in flue gas, which are released into the atmosphere. Since Indian coal is of low grade and has a high ash content of 30-45 per cent as well as low heat value, it is a major source of particulate emissions. These emissions have adverse effects such as impaired visibility due to smog and haze, soiling of surrounding areas, impaired plant growth, and respiratory problems in humans.
In order to minimise the harmful impact of emissions, the industry is now increasingly using particulate control equipment and technologies, which are designed to remove particulates from the flue gas stream, prevent them from re-entering the flue gas and release the collected residue. There are various types of emission control devices that can be used in power plants. These include electrostatic precipitators (ESPs), fabric filters, cyclone separators and the high temperature high pressure (HTHP) particulate control solution. These help thermal units to meet the high efficiency and high reliability requirements.
Current situation
As per the Central Electricity Authority’s latest data, around 62 per cent of the total installed power generation is based on coal. As of March 2015, the demand for coal by the power sector stood at 602 million tonnes, having increased at a compound annual growth rate of 5.5 per cent since 2010. With the growing dependence on coal for power generation, particulate emissions have been rising steadily.
In February 2015, the Centre for Science and Environment published the results of a two-year-long green rating project. This move was aimed at monitoring the environmental performance and compliance of the industrial sector. Under the project, 47 power plants accounting for 55 per cent of the country’s capacity were rated. As per the results, 60 per cent of the coal-based power plants under the study were violating the air pollution standards, their fly ash discharge was very high with improper utilisation and they were unable to meet the mandatory target of utilising 90 per cent of solid waste set by the Ministry of Environment, Forest and Climate Change (MoEF&CC). The project highlighted that the national norms for particulate matter were not stringent and not in line with global standards. National norms for SOx and NOx were absent and the ash handling policy was not proper with limited stress on ash utilisation.
Subsequently, the MoEF&CC released new emission standards for coal-based power plants in December 2015. As per the new standards, power plants are expected to cut down particulate emissions by 25 per cent. Earlier, the particulate emission norm for an average power plant was 150 milligram per normal cubic metre (mg per Nm3). However, according to the new standards, a plant installed in 2017 will have to meet a particulate standard of 30 mg per Nm3.
Particulate emission control equipment
Various equipment and technologies are being deployed commercially to control particulate emissions from coal combustion in thermal power plants (TPPs). Power Line takes a look at some of these …
Electrostatic precipitators
ESPs are the most commonly used particulate emission control equipment in coal-based power generation plants. An ESP electrically charges the ash particles in the flue gas stream to collect and remove the particulate matter. It comprises a series of parallel vertical plates that contain electrodes to create the electric field through which the flue gas passes and particles are separated. 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. Its collection efficiency is very high at 99-99.99 per cent for a particle range of 0.01-100 micrometre (µm); however, it does not work well for fly ash with high electrical resistivity. 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 the flue gas is an established technique that is used to restore the performance of an ESP in coal-based power plants, which produce high-resistivity fly ash resulting from burning low-sulphur fuels.
Fabric filter
A fabric filter collects the dry particulate matter as the flue gas passes through the filter material. The fabric filter comprises a multiple compartment enclosure, with each compartment consisting of small fabric bags. The flue gas passes from the porous bag material, which separates the particulate matter from the gas. Fabric filters operate by passing flue gas through a felted or woven fabric. The particulate matter in the flue gas gets deposited on the fabric surface, thereby improving the collection efficiency.
There are two types of fabric filters that are used by utilities: reverse air fabric filters and pulse jet fabric filters. In the reverse air fabric filter, fabric bags are fastened on to the cell plate at the bottom of the filter whereas in a pulse jet fabric filter, this is done at the top of the filter. The pulse-jet-type filter is chosen over the reverse air filter due to its smaller size and lower cost. 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. The commonly used conditioning agents are elemental sulphur, ammonia and sulphur trioxide.
Cyclone separator
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 applied centrifugal force. Various industries use these devices because of inherent advantages 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 particulate 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 reduces. Most wet particulate scrubbers are designed to control both sulphur dioxide and particulates by utilising alkaline fly ash as a sorbent.
HTHP particulate control
An HTHP particulate control device is an important component of combined cycle power systems. HTHP devices enable efficient hot gas particulate filtration to protect against the fouling and erosion of downstream heat exchanger and gas turbine components.
Best practices
The collection efficiency of particulate emission control equipment depends upon the flue gas temperature, moisture content, particle size distribution and the chemical composition of particles. System efficiency reduces as the particle size decreases and resistivity increases.
In order to improve collection efficiency, several best practices can be adopted. Flue gas conditioning is widely used to increase the size of particulate matter and improve collection efficiency. It involves the injection of chemical agents such as sulphur trioxide, sulphuric acid, ammonium sulphate, ammonium bisulphate and ammonia along with water or steam into the flue gas to alter the physico-electrical properties of fly ash.
The way forward
Among all particulate control devices, ESPs are the most widely deployed in thermal power stations. However, the fly ash generated by Indian coal comprises sulphur and alkali, resulting in high resistivity, which hampers ash collection by ESPs. Going forward, new techniques to collect high-resistivity fly ash can be adopted. These include gas temperature control, intermittent energisation, wide-pitch electrodes, pulsed power supply or semi-pulsed operation, conditioning of flue gas, and changing coal properties by blending it with sulphur and high-heat-value coal.
Given the increasingly stringent environmental regulations, developers need to explore appropriate particulate emission control devices and technologies for their TPPs.