Clean Coal


Introduction

Coal is the most widely used fossil fuel in the United States, providing more than 50% of America’s electrical power [12]. The highly combustible black-brown sedimentary rock is made mostly of carbons and hydrocarbons. [13] Not only is it one of the most used fossil fuels, but it is also the dirtiest when burned. Coal produces nearly two times more carbon dioxide than natural gas, making it the biggest contributor to global warming. There are a few benefits of coal - it is cheap, reliable and is readily available in the United States, limiting dependence of foreign fuel. [12] However, with the recent focus on climate change, people have begun to challenge the use of coal as the primary energy source because of the toxic release of carbon dioxide into the atmosphere. To help combat coal’s bad imagine, coal companies have touted the idea of “clean” coal. Even U.S. politicians, including President Barack Obama, have claimed that “clean” coal will drastically help improve the climate change crisis. But, the question is, can coal truly be “clean”?

Background

Since clean coal technologies focus mainly on carbon capture and storage (CCS), there is no difference between “clean” coal and the coal that has been used for hundreds of years. The entire life cycle of coal needs to be considered when examining the environmental effects. It takes millions of years to create coal, making it a nonrenewable resource. Millions of years ago, the earth was partly covered by swamps. Plants that died in these swaps were eventually buried under layers of water and dirt. Over millions of years, the heat and pressure from the layers turned the dead plants into coal. [13]

how_coal_was_formed.JPG
http://www.eia.doe.gov/kids/energyfacts/sources/non-renewable/coal.html#coalformation


To retrieve the coal, there are a two main categories of mining - surface and
area_surface_mining.JPG
http://www.umwa.org/index.php?q=content/surface-coal-mining
underground mining. When the coal is near the surface, it is more economical to use surface mining techniques. This includes strip mining and mountain top removal mining. The process of strip mining exposes seams of coal by first removing the overburden then drilling holes into the coal where explosives can be set. After the explosives have blasted the coal apart, trucks remove the coal and the overburden that was removed is used to fill in the mine. The coal is then transported by truck to a coal preparation plant or directly to where it will be used. Area surface mines are used on flat terrain and mining strips can be 100 to 200 feet wide. Contour mining is used in mountainous areas and follows the coal seam along the ridges and valleys of the mountain. Highwall mining is used when contour mining becomes too expensive. In highwall mining, augers are used in the side of the mountain to collect the coal. Open pit mines are often used where the coal seam is very thick; open pit mines can reach depths of several hundred feet. [14]



Mountain top removal mining (MTR) has been used for nearly 40 years. [15] This process involves striping the land of trees and most other wildlife, then using strong explosives to expose the coal seam in the middle of the mountain. The overburden is often put into adjacent valleys or hollows, sometimes filling in streams and blocking water flow. This method of mining also results in plateaus where there used to be mountains. [16]

mt_top.JPG
http://www.mountainjusticesummer.org/facts/steps.php

Underground mining is used in places where the coal is deep underground. The two main types of underground mining are room & pillar mining and longwall mining. Room and pillar mining is when workers create a series of rooms in the coal, leaving behind pillars of solid coal to support the roof of the mine. This is the safer of the two methods, but results in a lot of wasted coal which is left behind for support.
room_mining.JPG
http://www.umwa.org/index.php?q=content/room-and-pillar-mining
Longwall mining uses a cutter which moves back and forth over an exposed strip of coal about 800 feet in width and 7,000 feet in length. As the cutter removes the coal, it falls onto a conveyor belt that moves it out of the mine. There are no natural roof supports in this method. Instead, hydraulic supports are used and moved along with the cutter. The roof of the mining area caves in as the cutter and supports move down the mine. [14]

There are numerous environmental effects from mining coal. Large areas of land are greatly disturbed by mining. Surface mining requires that the entire habitat be disrupted, completely removing all vegetation and displacing any animals that live there. This normally leads to erosion, loss of habitat and creates dust pollution. For each ton of coal mined, approximately 25 tons of overburden must be removed. [17]

Mountaintop mining has devastating effects on the environment. (For more pictures, visit http://www.ohvec.org/galleries/mountaintop_removal/index.html) Not only is there great waste from mountaintop mining, but often streams and rivers become grossly contaminated and sometimes blocked completely, cutting off water from downstream habitats. Flooding from the blocked streams often force communities to relocate and the landscape is permanently changed.

mt_top_after.JPG
Effects of Mountain Top Removal Mining


When coal is mined from underground, mine subsidence is a problem when the ground sinks because it is no longer supported by the coal. Mine subsidence usually presents itself as sinkholes or troughs. This can lead to the damage of homes and habitats that have been built on top of or near abandoned mines. [18] Another problem with underground mining is acid mine drainage (AMD). When pyrite, which is sulfur rich, is exposed to water and air, it reacts and makes sulfuric acid. Heavy metals such as lead, mercury and copper can be dissolved by sulfuric acid which can lead to ground and surface water contamination. [17]

After the coal is mined, it is brought to a processing plant by truck or train. At the processing plants, the coal is crushed, sized and dried, then packaged and shipped off again to the final destination. The processing plants create air emissions of particulate matter, carbon monoxide, hydrogen and volatile organic compounds. [19] There are ways to help curb the emissions from the processing plant. Baghouse filters, cyclones and water spray systems are currently being used to capture most particulate matter. Water can also become contaminated from coal processing plants. Pollutants include hydrocarbons, ammonia and amines, oxygenated compounds, acids, inorganic salts, and traces of heavy metal ions. [19]

The coal that leaves the processing plant is most likely headed for a coal power plant. Most of the time, coal is transported by a train. A good animation of how a coal power plant works can be found at http://www.eas.asu.edu/~holbert/eee463/coal.html. The United States, in 2006, had 616 coal-fired power plants. These plants generate nearly 50% of the power consumed in the USA. [20]
coal_plant.JPG
Example of coal-fired power plant http://www.tva.gov/power/coalart.htm

Coal-fired power plants are the leading cause of acid rain, smog, global warming and air toxics because of the high amounts of carbon dioxide, sulfur dioxide, and nitrogen dioxide that are released when coal is burned. Other pollutants that are released by coal-fired power plants include particulates, carbon monoxide, hydrocarbons, lead, arsenic and mercury. [10] Because the coal cannot be completely burned, there is waste. There is nearly 300,000 tons of waste from a single coal-fired power plant each year; 75% of that waste ends up in landfills across the country. Arsenic, mercury, chromium and cadmium are all found in that waste, which can eventually find a way into the ground water below the landfill sites. [21] The coal-ash that is left behind after the coal has been burned is often considered more hazardous than the coal itself. The EPA does not regulate the amount of coal-ash that is stored around the country, but when there is a coal-ash spill, it can be very dangerous to the surrounding habitats.

Unfortunately, coal-fired power plants are also not very efficient. Only about 33-35% of the heat generated from the burning coal is used to make electricity, the rest is wasted and absorbed by the atmosphere or into the cooling water. [21] Each power plant uses 2.2 billion gallons of water each year, which is enough to last a for a year in a city of 250,000 people.[21] One of the key factors in coal-fired power plants is the cooling water which condenses the steam back into water so that it can be used again, or dischar ged back into the source. The cooling water is drawn from a local source such as a lake, or river. However, when this water is discharged back into the river or lake it is taken from, it can be 20 – 25 degrees (F) warmer than the surrounding water. Power plants also add chlorine to the water to reduce algae growth. This changes the local habitat for the fish and plants, causing health and habitat problems. [21]

It is clear that currently coal pollutes when it is mined, transported, and burned. So, if there was a possibility of having clean coal, how could we do it?

Pre-Combustion Cleaning Alternatives
To help reduce sulfur dioxide emissions from coal, pre-combustion measures can be taken to remove pyritic sulfur (FeS2) from coal before it is burned [1]. These practices include physical processes for washing coal of minerals or impurities, chemical, and biological processes. It should be noted that most processes create new waste pollution streams.





Coal Cleaning by Washing


In order to make coal clean and help it burn more efficiently, the mineral content in the coal needs to be removed before it is burnt. Coal washing involves grinding the coal into smaller pieces and passing it through a process called gravity separation. One technique involves feeding the coal into barrels containing a liquid that has an intermediate density which causes the coal to float, while unwanted material sink and can be removed from the fuel mix. The coal is then pulverized and prepared for burning. Coal cleaning by washing has been standard practice in developed countries for some time. It has been demonstrated that it reduces emissions of ash and sulfur dioxide when the coal is burned. Standard methods of cleaning fine coal (< 28 mesh) are less efficient than processes for larger sizes because enhanced separation process between clean coal and minerals is required, reducing ash and sulfur to lower levels. New commercial processes are being developed that include column froth flotation and oil agglomeration. These processes are still in various stages of development. They include fine heavy-media cycloning, enhanced-gravity separators, true heavy-liquid cycloning, agglomeration, electrostatic separations, and magnetic separations. (“Coal”).

Besides physical cleaning, biological or chemical methods are also being used. Chemical coal cleaning processes include the following groups:

a) Use of elevated temperatures and pressures to oxidize pyritic sulfur to water-soluble sulfur compounds.

b) Use of caustic chemicals to leach pyritic and/or organic sulfur species from the coal.

c) Use a chemically-induced pyritic sulfur alteration to improve later physical separations.

It has been reported that other chemical processes developed have the potential to remove 90 - 95 % of the pyritic sulfur and 40 - 85 % of the organic sulfur associated with coal. These processes include an iron sulfate oxidation leach known as TRW Inc.-Meyers; a molten caustic leach process (TRW-DOE); a carbonyl pyritic sulfur alteration process (Hazen Research, Inc.-Magnex); and an elevated temperature and pressure leach oxydesulphurization process). The fact that these processes involve high processing costs, unsafe processing conditions, safety and environmental concerns is making the commercial success of these processes difficult. [7]

The biological cleaning methods include microorganisms that have been found to oxidize the insoluble Fe+2 (pyrite) in pulverized coal to the soluble Fe+3 form. There are two families of microorganisms that can be used to remove pyritic sulfur from coal: thiobacillus ferrooxidans and sulfolobus acidocaldarius. It should be noted that the time of the reaction is an important factor when using microorganisms. In some cases, weeks will be required to complete the reaction. Also, bacteria culture can be employed to consume the organic sulfur in coal. [5]



Combustion Cleaning Alternatives

The combustion conditions of coal can be modified to reduce the creation of pollutants, and/or substances can be injected into the coal to capture pollutants as they form. These practices include gasification, the integrated gasification combined cycle, and fluidized bed combustion.

Gasification of Coal

Gasification converts coal in to a burnable gas with the maximum amount of potential energy from the coal being in the gas. [2] Coal gasification plants are seen as a primary component of a zero-emissions system and effective way to reduce emissions of sulfur dioxide, particulates and mercury. Coal gasification plants are preferred because they are flexible and have high levels of efficiency and offer one of the most versatile and cleanest ways to convert the energy content of coal into electricity, hydrogen, and other energy forms. Gasification breaks down coal into its basic chemical constituents. In a modern gasifier, coal is typically exposed to hot steam and carefully controlled amounts of air or oxygen under high temperatures and pressures. Under these conditions, carbon molecules in coal break apart, setting into motion chemical reactions that typically produce a mixture of carbon monoxide, hydrogen and other gaseous compounds. The hydrogen has a heat value of 121 MJ/kg[3] about five times that of the coal, so it is a very energy-dense fuel.
http://en.wikipedia.org/wiki/File:Gasification.gif
http://en.wikipedia.org/wiki/File:Gasification.gif

The gas can be used to power electricity generators, or it can be used in transportation or the chemical industry. Coal gasification may offer a further environmental advantage in addressing concerns over the atmospheric buildup of greenhouse gases, such as carbon dioxide. In gasification, oxygen is used in a coal gasifier instead of air. The O2 supply is much less than required for full combustion to yield CO and H2. Carbon dioxide is emitted as a concentrated gas stream. In this form, it can be captured more easily and at lower costs for ultimate disposition. In contrast, when coal is burned with excess air to give complete combustion, 80% of which is nitrogen, the resulting carbon dioxide is much diluted and more costly to separate from the larger mass of gases flowing from the gasifier. The capability to produce electricity, hydrogen, chemicals, or various combinations while virtually eliminating air pollutants and potentially greenhouse gas emissions makes coal gasification one of the most promising technologies for the energy plants of tomorrow. However, the technology remains unproven on a widespread commercial scale. (“Gasification”)


The Integrated Gasification Combined Cycle (IGCC)


In IGCC the first gasification step is pyrolysis, from 400°C and up, where the coal, in the absence of oxygen, rapidly gives off carbon-rich char and hydrogen-rich volatiles. The coal is not combusted directly but reacts with oxygen and steam to form a "syngas" (primarily hydrogen). After being cleaned, it is burned in a gas turbine to generate electricity and to produce steam to drive a steam turbine-generator. The main problem for IGCC is its extremely high capital cost, up to $3,593/kW[8].






http://en.wikipedia.org/wiki/File:IGCC_diagram.svg
http://en.wikipedia.org/wiki/File:IGCC_diagram.svg



In the second step the char is gasified from 700°C up to yield gas, leaving ash. Clean coal technology field is moving in the direction of coal gasification with a second stage so as to produce a concentrated and pressurized carbon dioxide stream followed by its separation and geological storage. To achieve futher clean coal technology in the future, the water-shift reaction will become a key part of the process as follows:


(i) C + O2 → CO
(ii) C + H2O → CO + H2
(iii) CO + H2O → CO2 + H2 (the water-shift reaction)
The products are then concentrated CO2 which can be captured, and hydrogen, which is the final fuel for the gas turbine. An overall thermal efficiency for oxygen-blown coal gasification, including carbon dioxide capture and sequestration, has been reported at about 73% and using the hydrogen in a gas turbine for electricity generation has the potential to achieve overall system efficiency up to 60%. [10]

Fluidized Bed Combustion (FBC)

FBC is commonly used by power plants. In Fluidized beds , pulverized coal and Limestone or dolomite are mixed and then suspended on jets or air (fluidized) in the combustion chamber, increasing heat transfer and chemical reaction. Almost all sulfur dioxide is captured in solid form as calcium sulfite before the gas can escape, thus reducing the emissions. This procedure allows for much-reduced combustion temperatures and therefore also highly decreases the amount of nitrogen oxide that is formed and released.

File:Combustion systems for solid fuels.gif
File:Combustion systems for solid fuels.gif


By burning at low temperatures, polycyclic aromatic hydrocarbon emissions are higher. It has been suggested that at least 95% of the sulfur pollutants present in coal can be confined inside the boiler by the sorbent. Commercial FBC units operate at viable efficiencies and have NO2 and SO2 emissions below federal standards levels.[9]

Post-combustion


After combustion a variety of flue gas treatment methods can be utilized to reduce the environmental impact of the emissions. In the case of carbon capture technology the emissions are reduced more directly.


Particulate Control

Coal combustion exhaust contains an array of contaminants, such as NOX, SO2, Hg, and particulate matter. There are a variety of pollution control systems to remove these, such as settling chamber, cyclones, bag houses, scrubbers, and electrostatic precipitators. Scrubbers are the most versatile with the ability to remove both wet and dry particulates of all size ranges along with contaminates like Hg. Scrubbers are not typically used to remove CO2 from flue gas due the large amounts of calcium carbonate that would need to be used and the difficulties it would create for disposing of such large amount of wet byproduct. These processes all add to the energy needs leading to a net increase in the amount of CO2 produced, leaving us further unable to meet Kyoto targets. For more information on the Kyoto Protocol please see the Kyoto Protocol wiki. [23]

Carbon Capture and Storage (CCS)

Carbon dioxide can be captured from the various combustion processes described above. Even though operating a capture system would result in a 25%-40% increase in fuel needs, current technology can capture 85%-95% of CO2. This results in an 80%-90% reduction in CO2 emissions. This captured CO2 must then be stored, the two promising means of storing CO2 are under geological features and in the ocean. [11]



In oxy-fuel combustion the nitrogen is removed from the air to produce an oxygen rich input for combustion. As a result the exhaust for the combustion is reduced in volume with leads to greater efficiency through less heat loss and less exhaust to treat. The resulting flue gas now consist primarily of CO2 circumventing the trouble of flue gas separation, and can be used for sequestration. The energy and economic costs of oxygen separation are great, making oxy-fuel combustion only practical as an alternative to separating CO2 from flue gas. [22]

  • Geological Storage

Carbon dioxide can be stored within geological features, when properly characterized and chosen these sites could retain carbon dioxide for thousands of years. A variety of locations and methods are already being used to store carbon underground. Figure 10 illustrates the following storage options: 1) Depleted oil and gas reservoirs 2) The use of CO2 in enhanced oil and gas recovery 3) Deep saline formations (a) offshore (b) onshore 4) Use of CO2 in enhanced coal bed methane recovery 5) Deep unmineable coal seams 6) Other suggested options (basalts, oil shales, cavities).[11]






Figure 10 Options for storing carbon dioxide in geologic formations.
Figure 10 Options for storing carbon dioxide in geologic formations.

There is a great deal that is not yet understood about the potential consequences of geo-sequestration. If carbon dioxide were to leak from such storage the results would be deadly for plants and subterranean animals. Some of the costs of CO2 geo-sequestration could be offset by enhanced harvest of methane, but this methane would then be used to generate more CO2, negating the original benefit. Geo-sequestration does appear promising as a temporary fix for the CO2 that is currently being produced.
[11]

  • Oceanic Storage

The idea behind ocean storage is that by pumping liquid CO2 deep into the ocean it would remain there, isolated from the atmosphere, for hundreds of years, providing more solutions to controlling emissions of greenhouse gases as posed in the Kyoto Protocol. Figure 11 shows a few of the methods that could be used to deliver the carbon to the ocean. The deeper the injection the longer the carbon would be potentially retained. Other strategies to increase CO2 retention include forming solid CO2 hydrates and liquid CO2 lakes on the sea floor. As illustrated in Figure 11 liquid carbon dioxide is denser than sea water at depths greater than 3 km and thus sinks. Liquid carbon dioxide is less dense than sea water at depths shallower than 2.5 km and will generally float. The behavior in the transition zone between these two depths varies by location depending on temperature. Liquid CO2 is generally stable at depths greater than 0.4 km (depending on temperature), at these depths CO2 reacts with sea water to form a solid ice-like hydrate CO2-6H2O. Solid CO2 hydrate sinks and may provide an easier way to transport carbon to the sea floor. Solid CO2 hydrate will dissolve into aqueous phase CO2 when exposed to seawater that is not saturated, the dynamics of such processes are not perfectly understood. Another method to store carbon in the ocean is by converting carbon dioxide to bicarbonate using limestone; this method has the advantage of not needing to be pumped deep into the ocean.[11]
Figure 11 Ocean storage strategies.
Figure 11 Ocean storage strategies.
These methods have the potential to remove carbon from the atmosphere for hundreds or thousands of years; the ocean and atmosphere will eventually equilibrate. An increase in the ocean's CO2 concentration does have the potential to negatively impact marine life.[11]

Carbon dioxide sequestration is not a sustainable solution due to the net removal and sequestration of oxygen from the atmosphere. This point reflects the underlying problem with fossil fuels, they are not sustainable. For more information on sustainable energy sources see the Green Power wiki page. The clean coal dilemma is that clean coal technologies are another sad example of treating the symptoms and not the causes of a problem. The more energy we spend on clean coal technology, in the futile attempt to make something dirty cleaner, the more we undermine only reason we use coal, convenient energy.






Conclusion


There are emerging technologies that suggest that coal can be a clean source of energy. Unfortunately, none of these technologies have been proven and there are currently no coal-fired power plants in the United States that are using any of these technologies because of the uncertainty of the technologies and expensive that is required to implement them. The fact is that coal is dirty from beginning to end. Coal mining is destroying entire ecosystems beyond repair and coal burning pollutes the atmosphere with toxic greenhouse gases. There are some benefits of coal - it's cheap, reliable, and readily available in the United States. Nearly 50% of the electricity in the United States comes from coal-fired power plants, so it will be a difficult transition to renewable power sources. But instead of focusing the green effort on trying to fix coal, resources should be allocated toward truly renewable sources of energy such as wind and solar.

Many sources claim that clean coal is a solution to climate change and global warming. However, environmental groups believe it is an marketing scheme for coal companies and that emissions and wastes are just transferred from one waste stream to another, instead of being eliminated. In thier opinion, coal will never be clean. Critics of clean coal believe that coal-fired power plants which burn clean coal will still release significantly more pollution than truly renewable energy sources such as wind power and solar power.

The processes that make coal "clean" create their own waste streams. Instead of decreasing emissions and waste, the processes end up creating their own environmental problems. Carbon Capture and Storage also has some major concerns associated with it. No one is sure about what will happen to the carbon dioxide that is buried under the ocean or in land masses. It may have an immediate, short lived positive effect on the amount of carbon dioxide that is being released into the atmosphere, but it is not a sustainable method of improving the condition of the environment. While it is tempting to believe that coal can be clean, it is more appropriate to say it will be "slightly less polluting" coal instead of "clean" coal.

References


1. "Coal vs. Wind". Union of Concerned Scientists. http://www.ucsusa.org/clean_energy/coalvswind/c01.html. Retrieved on 2009-03-04.
2. "Fossil Energy: DOE's Clean Coal Technology Program". U.S. Department of Energy. http://www.fossil.energy.gov/programs/powersystems/cleancoal/. Retrieved on 2009-03-08.
3. “Heat of combustion”. http://en.wikipedia.org/wiki/Heating_valueRetrieve on 2009-03-17
4. “Integrated Gasification Combined Cycle”. http://en.wikipedia.org/wiki/Integrated_Gasification_Combined_Cycle. Retrieve on 2009-03-15
5. “Coal cleaning primer”.http://www.cq-inc.com/Coal_Primer.pdf. Retrieve on 2009-03-16.
6. “Gasification”. http://en.wikipedia.org/wiki/Gasification. Retrieve on 2009-03-17.
7. Toole, B., 1982. Coal Beneficiation Process Survey. Babcock and Wilcox Research and Development Division: Alliance, OH. January 20, 1982.
8. Excelsior's Mesaba Project . http://www.eia.doe.gov/oiaf/aeo/assumption/pdf/electricity.pdf#page=3.
9.“Fluidized Bed Combustion”. http://en.wikipedia.org/wiki/Fluidized_bed_combustion
10. "Current and Future IGCC Technologies: Bituminous Coal to Power".
http://www.netl.doe.gov/technologies/coalpower/turbines/refshelf/igcc-h2-sygas/CurrentFutureIGCC2Revisionfinal.pdf
11. IPCC special report on Carbon Dioxide Capture and Storage . Prepared by working group III of the Intergovernmental Panel on Climate Change. Metz, B., O.Davidson, H. C. de Coninck, M. Loos, and L.A. Meyer (eds.). Retrieved on March 24, 2009.

12. The illusion of Clean Coal. The Economist. [Online] March 5, 2009. [Cited: March 23, 2009.] http://www.economist.com/opinion/displaystory.cfm?story_id=13235041.
13. Coal Basics 101. Energy Information Administration. [Online] [Cited: March 23, 2009.] http://www.eia.doe.gov/basics/coal_basics.html.
14. Who we are, where we work. United Mine Workers of America. [Online] [Cited: March 23, 2009.] http://www.umwa.org/index.php?q=content/who-we-are-where-we-work.
15. Mountain Top Mining. [Online] [Cited: March 23, 2009.] http://www.mountaintopmining.com/.
16. Mountaintop Removal Mining Fact Sheet. Ohio Valley Environmental Coalition. [Online] [Cited: March 23, 2009.] http://www.ohvec.org/issues/mountaintop_removal/articles/mtr_fact_sheet.pdf.
17. Coal Mining. The Environmental Literacy Council. [Online] [Cited: March 23, 2009.] http://www.enviroliteracy.org/article.php/1122.html.
18. What Is Mine Subsidence? Pennsylvania DEP. [Online] [Cited: March 23, 2009.] http://www.dep.state.pa.us/MSIHomeowners/WhatIsMS.html#1pics.
19. Environmental, Health and Safety Guidelines for Coal Processing. IFC: International Finance Corporation. [Online] April 30, 2007. [Cited: March 23, 2009.] http://www.ifc.org/ifcext/sustainability.nsf/AttachmentsByTitle/gui_EHSGuidelines2007_CoalProcessing/$FILE/Final+-+Coal+Processing.pdf.
20. FQA - Electricity. Energy Information Administration. [Online] [Cited: March 23, 2009.] http://tonto.eia.doe.gov/ask/electricity_faqs.asp#Top.
21. Coal vs. Wind. Union of Concerned Scientists. [Online] 2008. [Cited: March 23, 2009.] http://www.ucsusa.org/clean_energy/coalvswind/c01.html.
22. Oxy-fuel combustion process. Wikipedia. Retrieved on March 25, 2009.
23.Clean coal technology. Wikipedia. Retrieved on March 25, 2009.