Pollution Control For Coal-Fired Plants
While industry likes to use the term “clean coal,” nothing can actually make a coal plant clean. Even the most efficient plants with the best pollution control technology still suck up local water supplies and discharge toxic materials. However, operators and governments can adopt employee practices and technologies to reduce some of the deadly effects associated with coal-fired power. Any new coal plant should, at a minimum, include the equipment and processes outlined below, which will help to:
· Make the project more efficient by burning less coal to produce the same amount of electricity, thereby releasing less harmful pollution.
· Reduce the impact on the water supply by using less water and employing best practices to store toxic byproducts.
· Limit air pollution by installing and using the best available pollution control technology and operating practices.
· Ensure projects operate efficiently and utilize available pollution control technology through continuous monitoring.
Types of coal-fired power plants
Most coal plants use pulverized coal combustion technology that grinds coal into powder and then burns it to boil water, using the steam to drive an electrical generator. There are four classifications for pulverized coal plants, which indicate their efficiency: subcritical, supercritical (SCPC), ultra-supercritical (USCPC), and advanced ultra-supercritical (AUSCPC).
Sixty percent of the heat energy in coal is wasted in the process of making electricity. The excess heat is discharged to the air and to the cooling water used by the plant. The most efficient plants reduce this loss, generating the same amount of electricity with less coal.
· Subcritical units have an efficiency 33% and 37% (i.e. between 33% and 37% of the energy in the coal is converted into electricity).
· Supercritical technology was commercialized in the 1960s, and the efficiency of these plants ranges from 37% to 40%.
· Ultra-supercritical units began coming online in the 1990s, and their efficiency is between 44% and 46%.
Circulating fluidized bed (CFB) power plants burn coal with air in a circulating bed of limestone. CFB is advantageous because it can burn a variety of fuels, but they are less efficient than PC plants.
The final type of coal plant is integrated gasification combined cycle (IGCC), which converts coal into synthetic gas and then use the gas to power a turbine, recovering wasted heat for additional power. In theory, IGCC plants emit fewer pollutants and separate out carbon dioxide. These plants can be up to 20 percent more efficient than a PC plant.
Overall efficiency can also be improved by using waste heat for other purposes, such as district heating in cities or to support integrated industrial uses. This technique is called combined heat and power (CHP). The efficiency of plants that burn lignite and other forms of coal that contain large amounts of moisture can be improved by using waste heat or solar power to dry the fuel before burning it. Importantly, regular and significant maintenance of the unit is required throughout the life of the unit to maintain as much of the initial performance of the unit as possible. Poor maintenance programs will allow the efficiency of the unit to degrade by one-third or more over time.
Water
Coal plants millions of gallons of water for their cooling systems, sometimes reducing the amount available for drinking, agriculture, and ecosystems. Large numbers of fish are killed when this water is sucked out of local sources, and when water is eventually discharged from coal plants it contains toxic metals and other harmful chemicals that pose a threat to human health and wildlife.
Cooling Systems
Once-through cooling is an antiquated cooling system that sucks millions of gallons of water the closest lake, river or ocean into the plant’s cooling system, and indiscriminately sucks in whatever aquatic life is near the intake pipe. In this process, fish and other aquatic life are smashed and mutilated against crude screens or are sucked into the cooling system itself. The water is then discharged at an elevated temperature, causing severe ecosystem destruction. Each of these plants withdraws between 20,000 and 50,000 gallons for each megawatt-hour (MWh).
More modern closed-cycle cooling or recirculating systems recirculate cooling water, reducing withdrawals and fish kills. These plants still consume between 480-1,100 gallons per MWh. These projects require more energy to operate the cooling system, making them less efficient and potentially generating more heat-trapping emissions overall.
Coal Ash
Coal ash is the leftover material after burning coal, and it also contains high levels of toxic heavy metals such as arsenic, cadmium, and hexavalent chromium. Exposure to toxics in coal ash can cause increased risk of cancer, learning disabilities, neurological disorders, birth defects, reproductive failure, asthma and other sickness, as well as poison and kill fish and wildlife. When coal ash comes in contact with water, a soup of hazardous pollutants can leach out of the waste and poison our water.
Nothing can make coal ash disposal 100% safe, but surface impoundments, or wet dumps are the most dangerous. These dumps are subject to catastrophic failure if dams holding back toxic sludge fail, flooding nearby communities and releasing pollutants into the water, as well as leaching, whereby heavy metals and other pollutants seep into groundwater. Dry storage is the better alternative. Thea risk of toxic dust blowing off the site and water contamination from rainwater mixing with the material must be managed by covering all but a small working area of the storage site and by lining the base of the site with impervious materials. Properly designed dry storage sites are less susceptible to, sudden, devastating failures and should be designed to completely prevent toxic materials from leaching into the groundwater or blowing offsite.
Air pollution
Air pollution from coal causes serious health effects and contributes to cancer, stroke, heart disease, and upper respiratory disease. While it is impossible to build a coal plant that does not emit deadly material, there are technology and practices that can reduce harm to local communities.
Fine Particulate Matter (PMf or soot).
PMf or soot pollution is the release of airborne particles composed of a mixture of metals, chemicals, and acid droplets – or gases that later become particles. It is one of the deadliest and most dangerous air pollutants. This fine particulate matter can travel deep into our lungs and even enter the bloodstream. Exposure to soot pollution is linked to premature death, heart attacks, irregular heartbeats, chronic bronchitis, lung damage, irritation of the airways, and a variety of other significant health problems
Some soot is formed directly when coal is burned, and more soot is formed after the exhaust gases that leave the stack as SO2 and NOx react to form small sulfate and nitrate particles. Soot pollution may travel for hundreds of miles.
Coal fired power plants emit larger sized particulate matter as well as fine PM. There are two types of technology available that can remove 99.5% of the mix of fine and coarse PM materials emitted as particles. The gases that will later form PMf can be controlled by the devices described below.
A fabric filter, also known as a bag house, uses filters to collect dangerous materials. The second method is an electrostatic precipitator (ESP), which uses an electrostatic charge to remove the particles that lead to soot pollution. To maintain their effectiveness, the plant has to operate within set parameters and the device must be cleaned and repaired over time. With these devices any small tear in a bag or failure of a section of an ESP can lead to a very large increase in emissions
Ozone
Ozone is beneficial when in the stratosphere but is highly reactive and is harmful at lower elevations. Coal plants emit nitrogen oxides, which react in the presence of volatile organic compounds (VOCs) and sunlight to create ozone. At times ozone combines with other particles, including carbon monoxide, volatile organic compounds (VOC), and oxides of nitrogen (NOx), to produce smog, a toxic compound and a dangerous irritant. Doctors liken inhaling smog or ozone to getting a sunburn on your lungs. It can cause chest pain, coughing, and breathing difficulties. It triggers asthma attacks, and it can lead to irreversible lung damage or even death. Smog exacerbates conditions like bronchitis, emphysema, and asthma—sometimes fatally. Children are at the greatest health risk from air pollution because they are more likely to be active outdoors and their lungs are still developing.
To limit ozone pollution, power plant operators should reduce NOx (see below) emissions, one of the compounds critical to its formation.
Sulfur Dioxide (SO2)
SO2 is created by the combustion of fuels containing elemental sulfur and sulfur compounds. Is a pollutant in its own right, but more importantly, it contributes to the formation of fine PM and acid rain, which leads to the acidification of streams and lakes, destroying local ecosystems. It can cause severe damage to forests and agriculture, as well as damage to buildings and monuments
There are three methods that, when used together, can help reduce the amount of SO2 emitted from a coal plant:
- Use coal with low sulfur content.
- Clean the coal before burning to further reduce sulfur.
- Use scrubbers (flue Gas desulfurization or FGD) to remove SO2 after the coal is burned and before the emissions are released from the smokestack. This technology uses sprays or slurries to bind with the sulfur before it is released into the atmosphere. The leftover material then must be disposed of with the coal ash and other toxic byproducts.
Scrubbers must receive regular maintenance to ensure they continue operating correctly. Additionally, scrubbers use a large amount of the power generated by the power plant. For this reason, it is not uncommon for unscrupulous operators to turn the technology off in the evening or on the weekends, putting local communities at risk, as well as crops and animals. Continuous monitoring is critical to ensuring compliance with any laws or promises governing sulfur emissions. The best SO2 controls can remove 96-98 percent of the SO2 produced by the plant. Industry and government representatives often attempt to substitute less effective (but less expensive) SO2 controls.
Nitrogen oxides (NOx)
During the combustion process nitrogen in the atmosphere and in the fuel can be converted to several oxides of nitrogen (NO2, NO3, etc). NOx are dangerous pollutants with their own issues, and they can also play a role in climate disruption. Most critical for nearby communities, NOx interact with other particles and sunlight to form ozone and soot.
Power plant designers can take steps to limit NOx pollution through the combustion chamber design. They can also reduce NOx by adding technology to control emissions after the coal is burned. Selective catalytic reduction (SCR) involves passing the exhaust from the power plant through a catalyst, removing 90-95% of NOx. Industry and government representatives often attempt to substitute less effective (but less expensive) NOx control levels. These include Selective non-catalytic reduction (SNCR) involves using ammonia or urea in the combustion process to react with the nitrogen, but without a catalyst to accelerate the chemical reaction. This reduces NOx by only 60-80%. Other controls that are often employed include staged air combustion, over fire air and low NOx burners. These techniques should be part of a comprehensive NOx control system, but should not be considered a substitute for the best technology.
Mercury
Burning coal releases toxic mercury into the air that then is transported, either by wet (during rainfall) or dry (particulate) deposition to surface waters. Under certain conditions, the deposited mercury will transform into methylmercury. This poison accumulates in the food chain, eventually making its way into our bodies. Mercury is a powerful neurotoxin that can damage the brain and nervous system. Mercury is of special concern to women who are pregnant or thinking of becoming pregnant, since exposure to mercury can cause developmental problems, learning disabilities, and delayed onset of walking and talking in babies and infants.
Mercury can be emitted as elemental mercury, oxidized mercury or reactive mercury, and the form of the mercury determines what happens to it. Elemental mercury can remain in the atmosphere for very long times and can be transported globally. Reactive forms of mercury generally will be deposited within 125 km of the source. If conditions are right, it can then enter the food chain.
To limit the amount of mercury, a power plant operator should implement coal washing before the fuel is burned, as well as maintain SO2 and NOx controls, which will also help reduce mercury. Finally, activated carbon injection (ACI) involves injecting activated carbon particles into the flue gas to bond with the mercury particles. It is critical that activated carbon injection is paired with good PM controls, as the PM controls are necessary to collect the activated carbon particles before they are released.
Continuous Emission Monitoring
Much of the pollution control technology available is expensive to run, creating a strong incentive for operators to turn it off at night or on weekends, even though the technology is installed on the power plant. Moreover, without maintenance, coal plants become less efficient over time. Regular inspections of power plants and continuous emission monitoring of generation and smokestack emissions are critical to ensuring that projects continue to make use of available tools and technology to protect public health. Continuous monitoring is a common practice in countries such as the United States, where emissions data by hour, day, month and year for every plant in the country is available to the public on a government website. [1]
“Clean Coal”
For over 40 years, proponents of coal-fired power plants have used the phrase “Clean Coal” to describe new projects, even as they resisted development and implementation of better air pollution controls, water use policies and management of coal ash. The phrase is defined by no commonly accepted level of control. References to “Best Available Techniques” (BAT) are also vague. For this reason, when a new plant is proposed it is important to understand the specific controls that will be installed, the level of performance of those controls that will be required by the plant permit and the monitoring scheme that will allow the public to know that these levels are being maintained.
[1] See, www.ampd.epa.gov