Coal pollution mitigation
Coal pollution mitigation is a series of systems and technologies that seek to mitigate health and environmental impact of burning coal for energy. Burning coal releases harmful substances that contribute to air pollution, acid rain, and greenhouse gas emissions. Mitigation includes precombustion approaches, such as cleaning coal, and post combustion approaches, include flue-gas desulfurization, selective catalytic reduction, electrostatic precipitators, and fly ash reduction. These measures aim to reduce coal's impact on human health and the environment.
The combustion of coal releases diverse chemicals into the air. The main products are water and carbon dioxide, just like the combustion of petroleum. Also released are sulfur dioxide and nitrogen oxides, as well as some mercury. The residue remaining after combustion, coal ash often contains arsenic, mercury, and lead. Finally, the burning of coal, especially anthracite, can release radioactive materials.
Mitigation technologies
Mitigation of coal-based pollution can be divided into several distinct approaches. Coal pollution mitigation seek to minimize negative impacts of coal combustion.Precombustion
Prior to its combustion, coal can be cleaned by physical and by chemical means.Physical cleaning of coal usually involves gravimetric processes, often in conjunction with froth flotation Such processes remove minerals and other noncombustible components of coal, exploiting their greater density vs that of coal. This technology is widely practiced.
Coal can also be cleaned in part by chemical treatments. The concept is to use chemicals to remove deleterious components of coal, leaving the combustible material behind. Typically, coal cleaning entails treatment of crushed coal with acids or bases. This technology is expensive and has rarely moved beyond the demonstration phase. During World War II, German industry removed ash from coal by treatments with hydrofluoric acid and related reagents.
Post-combustion
The wastes produced by the combustion of coal can be classified into three categories: gases, particulates, and solids. The gaseous products can be filtered and scrubbed to miminize the release of SOx, NOx, mercury:- can be removed by flue-gas desulfurization
- can be removed by selective catalytic reduction.
- Mercury emissions can be reduced by up to 95%.
Ash
The solid residue, coal ash, requires separate set of technologies but usually involves landfilling or some immobilization approaches. Reducing fly ash reduces emissions of radioactive materials.Carbon capture
Several different technological methods are available for carbon capture:- Pre-combustion capture – This involves the gasification of a feedstock to form synthesis gas, which may be shifted to produce an and -rich gas mixture, from which the can be efficiently captured and separated, transported, and ultimately sequestered, This technology is usually associated with Integrated Gasification Combined Cycle process configurations.
- Post-combustion capture – This refers to capture of from exhaust gases of combustion processes.
- Oxy-fuel combustion – Fossil fuels such as coal are burned in a mixture of recirculated flue gas and oxygen, rather than in air, which largely eliminates nitrogen from the flue gas enabling efficient, low-cost capture.
Satellite monitoring
Combined cycle power plants
A few Integrated gasification combined cycle coal-fired power plants have been built with coal gasification. Although they burn coal more efficiently and therefore emit less pollution, the technology has not generally proved economically viable for coal, except possibly in Japan although this is controversial.Case studies
In conjunction with enhanced oil recovery and other applications, commercial-scale CCS is currently being tested in several countries. Proposed CCS sites are subjected to extensive investigation and monitoring to avoid potential hazards, which could include leakage of sequestered CO2 to the atmosphere, induced geological instability, or contamination of water sources such as oceans and aquifers used for drinking water supplies. As of 2021, the only demonstrator for CCS on a coal plant that stores the gas underground is part of the Boundary Dam Power Station.The Great Plains Synfuels plant supports the technical feasibility of carbon dioxide sequestration. Carbon dioxide from the coal gasification is shipped to Canada, where it is injected into the ground to aid in oil recovery. A drawback of the carbon sequestration process is that it is expensive compared to traditional processes.
The Kemper County IGCC Project, a proposed 582 MW coal gasification-based power plant, was expected to use pre-combustion capture of to capture 65% of the the plant produces, which would have been utilized and geologically sequestered in enhanced oil recovery operations. However, after many delays and a cost runup to $7.5 billion, the coal gasification project was abandoned and as of late 2017, Kemper is under construction as a cheaper natural gas power plant.
The Saskatchewan Government's Boundary Dam Integrated Carbon Capture and Sequestration Demonstration Project will use post-combustion, amine-based scrubber technology to capture 90% of the emitted by Unit 3 of the power plant; this will be pipelined to and utilized for enhanced oil recovery in the Weyburn oil fields.
An early example of a coal-based plant using carbon-capture technology is Swedish company Vattenfall’s Schwarze Pumpe power station located in Spremberg, Germany, built by German firm Siemens, which went on-line in September 2008. The facility captures and acid rain producing pollutants, separates them, and compresses the into a liquid. Plans are to inject the into depleted natural gas fields or other geological formations. Vattenfall opines that this technology is considered not to be a final solution for reduction in the atmosphere, but provides an achievable solution in the near term while more desirable alternative solutions to power generation can be made economically practical.
Other examples of oxy-combustion carbon capture are in progress. Callide Power Station has retrofitted a 30-MWth existing PC-fired power plant to operate in oxy-fuel mode; in Ciuden, Spain, Endesa has a newly built 30-MWth oxy-fuel plant using circulating fluidized bed combustion technology. Babcock-ThermoEnergy's Zero Emission Boiler System is oxy-combustion-based; this system features near 100% carbon-capture and according to company information virtually no air-emissions.
Other carbon capture and storage technologies include those that dewater low-rank coals. Low-rank coals often contain a higher level of moisture content which contains a lower energy content per tonne. This causes a reduced burning efficiency and an increased emissions output. Reduction of moisture from the coal prior to combustion can reduce emissions by up to 50 percent.
In the late 1980s and early 1990s, the U.S. Department of Energy conducted projects called the Clean Coal Technology & Clean Coal Power Initiative.
Financial impact
Whether carbon capture and storage technology is adopted worldwide will "...depend less on science than on economics. Cleaning coal is very expensive."Cost of converting a single coal-fired power plant
Conversion of a conventional coal-fired power plant is done by injecting the into ammonium carbonate after which it is then transported and deposited underground. This injection process however is by far the most expensive. Besides the cost of the equipment and the ammonium carbonate, the coal-fired power plant also needs to use 30% of its generated heat to do the injection. A test-setup has been done in the American Electric Power Mountaineer coal-burning power plant.One solution to reduce this thermal loss/parasitic load is to burn the pulverised load with pure oxygen instead of air.
Cost implications for new coal-fired power plants
Newly built coal-fired power plants can be made to immediately use gasification of the coal prior to combustion. This makes it much easier to separate off the from the exhaust fumes, making the process cheaper. This gasification process is done in new coal-burning power plants such as the coal-burning power plant at Tianjin, called "GreenGen".Country by country experiences
Local pollution standards include GB13223-2011, India, the Industrial Emissions Directive and the Clean Air Act.China
Since 2006, China releases more than any other country. Researchers in China are focusing on increasing efficiency of burning coal so they can get more power out of less coal. It is estimated that new high efficiency power plants could reduce emission by 7% because they won't have to burn as much coal to get the same amount of power.costs of retrofitting CCS are unclear and the economics depends partly on how the Chinese national carbon trading scheme progresses.