Clean Coal Technology: Myth or Reality?
Clean Coal? Is This for Real?
Chances are you've heard of something called "clean coal". If you're like me you're skeptical. It sounds more than a little like typical corporate greenwash. How can you make coal clean? Have you seen the stuff? Perhaps, like me, you've lived in close proximity to a coal-fired power plant. In college my dorm was right across the street from the campus power plant. Everyone in the building had to sign waivers stating that we wouldn't hold the school responsible if the waste steam falling on the parking lot stripped the paint off of our cars. It's difficult for me to believe that calling something like coal 'clean' is anything other than a cynical marketing device. This impression is only intensified by blatant propaganda websites such a www.coal-is-clean.com.
However, we live in a time of wonders. Technologies I don't understand do things I wouldn't have thought possible all the time these days. Perhaps there is something to it. This is the result of my attempts to sort it out.
Coal and the Modern World
More than half of the electricity generated in the United States comes from the burning of coal. The US accounts for about one quarter of worldwide coal use. Across the developing world, the pace of coal use is accelerating. As coal is abundant, widely distributed, and cheap, the fuel offers the best hope of these countries to obtain the energy necessary to modernize their societies. In addition, the technology of using coal is mature and the infrastructure for its use is already in place. Barring some unforeseen technological breakthrough, or a very unlikely acceptance of a lower standard of living, coal is going to be a major part of the world economy for quite some time.
The abundant power generated by the burning of coal has largely made modern society and technology possible. However, the use of coal has some very detrimental effects on the environment.
Coal burning is considered to be the largest source of air pollution in the world. According to the Union of Concerned Scientists, an average coal-fired power plant creates:
- 3.7 million tons of carbon dioxide, the major contributor to global warming. For comparison the Empire State Building weighs 365,000 tons.
- 10,000 tons of sulfur dioxide, which combines with water and falls as acid rain
- 10,200 tons of nitrogen oxide, which creates smog and damages lung tissue
- 720 tons of carbon monoxide
- 220 of hydrocarbons, which also contribute to smog
- 170 pounds of mercury, an element extremely toxic in very small amounts
- 114 pounds of lead, 4 pounds of cadmium, and small amounts of other toxic heavy metals.
These pollutants are released into the atmosphere in these quantities each year by one such plant. There are 594 such plants in the United States alone.
Solid Waste Products and Water Pollution
In addition to these airborne pollutants, in a year the typical power station produces 125,000 tons of ash and 193,000 tons of sludge. More than three quarters of this waste is deposited in unmonitored landfills or simply piled up on the surface. Toxins such as arsenic and heavy metals in these wastes find thier way into water supplies, where they damage natural ecosystems and cause disease in humans.
Environmental Impact of Coal Mining
The process of coal mining has some ugly consequences as well. Over 60% of US coal is mined by stripping away the surface of the ground, drastically altering the landscape. In Appalachia, mining operations often remove the entire tops of mountains to get at the coal below. Mining wastes are usually deposited in valleys and streams. This practice also destroys hundreds of thousands of acres of forest. Add to this the polltuion caused by the many trains and trucks needed to transport the coal, and the toxic runoff caused by rainfall on the piles of stored coal, and you get a pretty depressing assessment of the environmental consequences of this kind of power generation.
So What Is Clean Coal, Then?
The term 'clean coal' refers to a group of technologies and process designed to minimize the harmful effects of coal burning. Broadly speaking, the techniques focus on management of the wastes produced by coal combustion and keeping them from being released into the atmosphere. Others aim to increase the efficiency of coal-fired power generation, enabling less fuel to be used per unit of power generated, meaning fewer emissions.
Purifying the Coal and Cleaning the Emissions
Chemical washing of raw coal to remove impurities has been standard in the industry for years. Advanced techniques of purification remove the components of many kinds of pollution from the coal before it is burned. Some of these elements, such as sulfur, can then be recovered from the process and used.
Coal burning produces an exhaust known as flue gas. Flue gas is what you see billowing out of the smokestacks of powerplants. Various technologies are used to remove pollutants from these gases before they leave the plant. Most of these methods have been in use for years. These include:
- Electrostatic precipitators: These electrically charge particulate matter and remove them from the flue gas.
- Fluidized-bed combustion: This process uses limestone and dolomite to reduce the formation of sulfur dioxide
- "Scrubbers" : These are fitted on smokestacks and remove sulfur and other impurities from emissions
- Low Nitrogen Oxide Burners: These burners control the combustion process to reduce the generation of nitrogen oxide
Carbon Capture and Sequestration
Perhaps the most promising of the clean-coal technologies involve the capture of the carbon dioxide resulting from the combustion. The CO2 is then stored permanently away in underground geological formations. This directly addresses the primary component of coal-combustion pollution, the prolific generation and release of this greenhouse gas. However, this carries the risk of unforeseen seismic disturbances. Also, precautions would have to be taken to prevent the gas escaping into the atmosphere.
Most schemes for doing this involve separating the CO2 from the combustion products, compressing it into a near-liquid supercritical state, and injecting it into geological formations such as depleted natural gas fields. It can also be pumped into oil fields to assist the process of extraction, a practice which is already used in some older fields. One advantage of using gas or oilfields for carbon sequestration is that the geology of these fields is thoroughly explored, minimizing the risks associated with permanent storage.
Another possible storage strategy is the combination of the gas with existing minerals to form stable carbonates. This would eliminate the possibility of leakage entirely, as the carbon would be bound into the mineral structure of the rock. This can be done at the plant, but must be done at high temperatures, entailing significant energy costs. Another approach would be to utilize underground formations of suitable minerals. Sufficient minerals of these kinds have been identified to safely bind over 500 years worth of carbon emissions. However, the rate at which the binding takes place at ambient temperatures is very slow.
Advanced Techniques of Combustion
New methods of burning coal can also be used to mitigate the harmful effects of the process. Especially when used in conjunction with sequestration techniques, these could offer a far less destructive way of using coal.
Rather than burning coal in air, pure oxygen can be used. This higher-temperature process results in pure CO2 and water. The water can then be condensed out of the gas, and the pure CO2 captured and stored.
Another process which seems very promising is the technique of gasification. In this process coal is converted into syngas, which is a mixture of hydrogen and carbon monoxide gas. This can take place either in the plant or underground before the coal is extracted.
The gases are then separated. The pure hydrogen is burned for fuel, while the carbon monoxide gas is converted into carbon dioxide. This CO2 is captured as a pure stream and sequestered. This promises to be a way to use coal in a way which produces no emissions.
Good Ideas, but Enough to Make a Difference?
The various clean-coal techniques seem to have a good deal of promise to reduce the hazards of coal use. However, much more investment and implementation than is currently seen will be needed to make a major impact on the environmentally damaging aspects of the coal industry. The contribution of the combustion of coal to climate change, in particular, may cause drastic changes to the circumstances of human civilization long before the current halfhearted efforts make much difference. This is all the more likely given the continuing coal-based industrialization of the developing world.