Fine particulates (PM2.5 – those that are 2.5 µm or smaller) are coming under tighter control as they cause smog, other air pollution and detrimental health effects. Coal-fired power plants are a major source of PM2.5. Fine particulates are made up of primary and secondary PM2.5. Primary PM2.5 are emitted directly, and secondary PM2.5 form from chemical reactions in the air of other pollutants including SOx, NOx, VOC and ammonia. So legislation and controls on these pollutants will also effect PM2.5 in the air.
During the last decade, regulations for PM2.5 emissions have been implemented around the world and are discussed by me in my latest report* for the IEA Clean Coal Centre. However, PM2.5 emitted from coal-fired power plant are still regulated as PM which includes PM10 and other particles. It is difficult to compare emission standards across countries, but based on a rough comparison, although China’s air quality standards for PM2.5 and its precursors are less strict than the World Health Organisation’s (WHO) 2005 guidelines and the European Union standards, it has stricter PM emission limits for new coal-fired power plants.
The most commonly used PM2.5 measuring methods were developed by the US EPA. The ISO has also issued three testing and measuring standards in recent years, and Canada developed their own. Most of these methods concentrate on determining the total mass of PM2.5. High quality and comprehensive measurement methods to determine the chemical components of PM2.5 still need to be developed.
PM2.5 emissions from coal-fired power plants can be controlled before, during and after combustion. Pre-combustion control includes choosing a suitable coal type and pulverising the coal to the correct size. Optimising combustion temperature, burning time, and boiler load can reduce the formation and emission of fine PM. Injecting high temperature sorbents leads to higher emissions of coarse PM, while the emissions of fine PM are reduced. However, post-combustion control systems are needed to meet emission limit regulations.
Electrostatic precipitators (ESP) and fabric filters (FF) are the two most commonly used conventional particulate emission control devices. An ESP can collect 98% of PM2.5 when combined with flue gas desulphurisation (FGD) and/or other pollutant control systems, while FFs have a higher collection efficiency – up to ~99.7% for PM2.5.
Several innovations have been made to improve the removal efficiency of ESPs. Of these, flue gas conditioning (FGC) and wet ESPs (WESP) are the most successful. There are over 600 FGC installations worldwide. Low temperature ESPs have drawn attention in recent years, especially ultra-low temperature precipitation technology, which has the co-benefit of SOx control. FGC involves injecting chemical agents and/or water or steam into the flue gas stream to alter the physico-electrical properties of fly ash, consequently reducing fly ash resistivity. The most common conditioning agents are sulphur trioxide, ammonia, and sodium compounds. The SO3 FGC system is relatively easy to retrofit as it has a small footprint and a low capital cost, and only a short outage period is needed for its installation. But use of SO3 for PM2.5 control is not recommended if an amine-based carbon capture system may be added later.
The removal efficiency for fine particles can be improved by employing a WESP after the FGD system. Additional benefits such as keeping SO3 at a low level and capturing mercury, NH3, and HCl can make WESPs a preferred option. WESPs could have favourable economics for smaller coal-fired power plants or act as a final polishing stage for larger plants. There are numerous WESP installations, especially in China.
Hybrid systems combine the advantages of ESP and FF. This technology has improved PM collection efficiency to 99.99%. The US EPRI’s Compact Hybrid Particulate Collector (COHPAC), China Fujian Longking’s Electrostatic Fabric Integrated Collector (EFIC), and China Feida’s ESP-FF hybrid system (EFF) are all commercially available. To date, there are over 1,700 MW of COHPAC and 25,000 MW of EFIC installed. The Advanced Hybrid Collector (AHPC), Electrostatically Stimulated Fabric Filter (ESFF) – Max-9TM, and Multi-Stage Collector (MSC) all have promise but need further investment and demonstration trials before they can be brought to the market.
Multi-pollutant control systems that include a FF or ESP element are commercially available. As well as achieving a high PM2.5 collection efficiency, they can capture additional pollutants and can have lower capital and operating costs than a series of traditional systems to remove the same number of pollutants. Commercially available technologies with the most benefit for fine particulate control are the TOXECON™ and ECO® systems.
There are no miracle technologies for PM2.5 emission control. Individual coal-fired power plants vary in many aspects, such as type of coal used, location, water resources, space availability, funding and local labour cost. So, performance from one particulate control technology on a specific plant may not be achieved at another. However, providing correct assessments and management are undertaken, the emission standards set are achievable with currently available pollution control technologies.
*The report Emission standards and control of PM2.5 from coal-fired power plants CCC/267, by Xing Zhang, ISBN 978-92-9029-590-7, 80 pp, July 2016 is available for download from the IEA Clean Coal Centre Bookshop http://bookshop.iea-coal.org.uk/site/uk/clean-coal-technology-research-reports. Residents of member countries and employees of sponsoring organisations can download the report at no charge after a one-off registration.