Clean Coal

Clean coal is the name attributed to coal chemically washed of minerals and impurities, sometimes gasified, burned and the resulting flue gases treated with steam, with the purpose of almost completely eradicating sulfur dioxide and re-burned so as to make the carbon dioxide in the flue gas economically recoverable. The carbon dioxide can then be captured and stored instead of being released into the atmosphere (see carbon capture and storage).

Byproducts

The byproducts of clean coal are very hazardous to the environment if not properly contained. This is seen to be the technology's largest challenge, both from the practical and public relations perspectives.

While it is possible to remove most of the sulfur dioxide (SO2), nitrogen oxides (NOx) and particulate (PM) emissions from the coal burning process, carbon dioxide (CO2) emissions will be more difficult to address. Technology does exist to capture and store CO2 but they have not been made available on a large-scale commercial basis due to high economic costs^[1].

Potential uses of clean coal

The primary example of clean coal is the proposed US FutureGen plant — a zero-emissions coal-fired power plant.

It is also believed that some process similar to the natural gas fuel cell or microbial fuel cell (charged from biomass or sewage) may be practical using coal as fuel. Those technologies are used mostly for stationary fuel cells as charging is slow. A large power plant in a coal mine might be the most energy efficient approach and require the least transport of coal to the users, though the return of the coal chute and use in homes may be possible in some places, especially if home sewage or natural gas lines can be tapped as well by an improved fuel reformer technology such as that used already to convert methanol, gasoline to the natural gas form.

Support and opposition

Clean Coal has been mentioned by United States President George W. Bush on several occasions, including his latest State of the Union Address. Bush's position is that clean coal technologies should be encouraged as one means to reduce the country's dependence on foreign oil. Senator Hillary Clinton has also recently said that "we should strive to have new electricity generation come from other sources, such as clean coal and renewables."^[2].

Despite the supportive comments from U.S. President Bush about clean coal, the White House has only granted $18 million (USD) to develop zero-emission coal-fired power plants over the next decade out of a $388 billion omnibus spending bill. ^[3]

In addition, some prominent environmentalists (such as Dan Becker, director of the Sierra Club's Global Warming and Energy Program) believe that the term clean coal is misleading: "There is no such thing as 'clean coal' and there never will be. It's an oxymoron". ^[4] Complaints focus on the environmental impacts of coal extraction, the prohibitively high costs to sequester carbon, and uncertain of how to store end result pollutants.

Acid mine drainage

Acid mine drainage (AMD), or acid rock drainage (ARD), refers to the outflow of acidic water from (usually) abandoned metal mines or coal mines. However, other areas where the earth has been disturbed (e.g. construction sites, subdivisions, transportation corridors, etc.) may also contribute acid rock drainage to the environment. In many localities the liquid that drains from coal stocks, coal handling facilities, coal washeries, and even coal waste tips can be highly acidic, and in such cases it is treated as acid rock drainage. Acid rock drainage occurs naturally within some environments as part of the rock weathering process but is exacerbated by large-scale earth disturbances characteristic of mining and other large construction activities, usually within rocks containing an abundance of sulfide minerals.

Occurrence

Sub-surface mining often progresses below the water table, so water must be constantly pumped out of the mine in order to prevent flooding. When a mine is abandoned, the pumping ceases, and water floods the mine. This introduction of water is the initial step in most acid rock drainage situations. Tailings piles or ponds may also be a source of acid rock drainage.

After being exposed to air and water, oxidation of metal sulfides (often pyrite, which is iron-sulfide) within the surrounding rock and overburden generates acidity. Colonies of bacteria and archaea greatly accelerate the decomposition of metal ions, although the reactions also occur in an abiotic environment. These microbes, called extremophiles for their ability to survive in harsh conditions, occur naturally in the rock, but limited water and oxygen supplies usually keep their numbers low. Special extremophiles known as acidophiles especially favor the low pH levels of abandoned mines. In particular, Acidithiobacillus ferrooxidans is a key contributor to pyrite oxidation [1].

Metal mines may generate highly acidic discharges where the ore is a sulfide or is associated with pyrites. In these cases the predominant metal ion may not be iron but rather zinc, copper, or nickel. The most commonly-mined ore of copper, chalcopyrite, is itself a copper-iron-sulfide and occurs with a range of other sulfides. Thus, copper mines are often major culprits of ARD.

Chemistry

The chemistry of oxidation of pyrites, the production of ferrous ions and subsequently ferric ions, is very complex, and this complexity has considerably inhibited the design of effective treatment options.

Although a host of chemical processes contribute to ARD, pyrite oxidation is by far the greatest contributor. A general equation for this process is:

2FeS₂(s) + 7O₂(g) + 2H₂O(l) → 2Fe²⁺(aq) + 4SO₄^2-(aq) + 4H⁺(aq)

The solid pyrite, when introduced to oxygen and water, is catalyzed to form Iron(II) ions, sulfate ions, and hydrogen ions. The hydrogen ions bind to the sulfate ions to produce sulfuric acid.

Effects on pH

In some ARD systems temperatures reach 120 degrees Fahrenheit (50 °C), and the pH can be as low as -3.6[2]. ARD-causing organisms can thrive in waters with pH very close to zero. Negative pH occurs when water evaporates from already acidic pools thereby increasing the concentration of hydrogen ions.

About half of the coal mine discharges in Pennsylvania have pH under 5 standard units. However, a significant portion of mine drainage in both the bituminous and anthracite regions of Pennsylvania is alkaline, because limestone in the overburden neutralizes acid before the drainage emanates.

ARD has recently been a hindrance to the completion of the construction of Interstate 99 near State College, but this ARD didn't come from a mine: pyritic rock was unearthed during a road cut and then used as filler material in the I-99 construction.

Yellow boy

Yellow boy in a stream receiving acid drainage from surface coal mining.

When the pH of ARD is raised past 3, either through contact with fresh water or neutralizing minerals, soluble Iron(II) ions hydrolize to form Iron(III) hydroxide, a yellow-orange solid colloquially known as Yellow boy. Yellow boy discolors water and smothers plant and animal life on the streambed, disrupting stream ecosystems. The process also produces additional hydrogen ions, which can further decrease pH. Research is currently being conducted as to the feasibility of using Yellow boy as a commercial pigment.

Heavy metal contamination

Many acid rock discharges also contain elevated levels of toxic metals, especially nickel and copper with lower levels of a range of other heavy metal ions such as lead, arsenic, aluminium, and manganese. In the coal belt around the south Wales valleys in the UK highly acidic nickel-rich discharges from coal stocking sites have proved to be particularly troublesome.

Oversight

In the United Kingdom, many discharges from abandoned mines are exempt from regulatory control. In such cases the Environment Agency working with partners has provided some innovative solutions, including constructed wetland solutions such as on the River Pelena in the valley of the River Afan near Port Talbot.

Although abandoned underground mines produce most of the ARD, some recently mined and reclaimed surface mines have produced ARD and have degraded local ground-water and surface-water resources. Acidic water produced at active mines must be neutralized to achieve pH 6-9 before discharge from a mine site to a stream is permitted.

In Canada, work to reduce the effects of ARD is concentrated under the Mine Environment Neutral Drainage (MEND) program. Total liability from acid rock drainage is estimated to be between $2 billion and $5 billion CAD [3]. Over a period of eight years, MEND claims to have reduced ARD liability by up to $400 million CAD, from an investment of $17.5 million CAD [4].

Methods

Carbonate neutralization

Generally, limestone or other calcareous strata that could neutralize acid are lacking or deficient at sites that produce acidic rock drainage. Limestone chips may be introduced into sites to create a neutralizing effect. Where limestone has been used, such as at Cwm Rheidol in mid Wales, the positive impact has been much less than anticipated because of the creation of an insoluble calcium sulfate layer on the limestone chips, blinding the material and preventing further neutralization.

Ion exchange

Cation exchange processes were investigated as a potential treatment for ARD. Not only would ion exchangers remove potentially toxic heavy metals from mine runoff, there was also the possibility of turning a profit off of the recovered metals. However, the cost of ion exchange materials compared to the relatively small returns, as well as the inability of current technology to efficiently deal with the vast amounts of mine discharge, renders this solution unrealistic at present.

Constructed wetlands

Constructed wetlands systems have shown promise as a more cost-effective treatment alternative to artificial treatment plants. A spectrum of bacteria and archaea, in consortium with wetland plants, may be used to filter out heavy metals and raise pH. Anaerobic bacteria in particular are known to be capable of reverting sulfate ions into sulfide ions. These sulfide ions can then bind with heavy metal ions, precipitating heavy metals out of solution and effectively reversing the entire process.

Interestingly enough, T. ferrooxidans - the very bacteria which appears to be the problem - has also been shown to be effective in treating heavy metals in constructed wetland treatment systems.

The attractiveness of a constructed wetlands solution lies in its passivity - building an artificial wetlands is a relatively cheap one-time investment which continuously works to reduce acidity and heavy metal concentration. Although promising, constructed wetlands take much time to completely cleanse an area, and are simply not enough to deal with extensively polluted discharge. Constructed wetland effluent often requires additional treatment to completely stabilize pH. Also, the products of bacterial processes are unstable when exposed to oxygen, and require special disposal to ensure no further contamination. Other issues include seasonal variation in the activity of cleansing organisms, as well as the lack of a practical passive means of moving mine discharge through the most efficient regions of purification.

Active Treatment with Aeration

In some discharges, HCO3-, a base, enters into the runoff from the breakdown of organic matter in the mine, such as mine timbers, or from the groundwater interaction with limestone. The base then neutralizes the acid in the runoff, forming carbonic acid.

H⁺ + HCO₃^- = H₂CO₃. (1)

When this solution reaches the ground surface, the water is exposed to the air and the dissolved CO2 will degas into the atmosphere. This lowers the concentration of CO2, allowing more H2CO3 to decompose, which in turn allows the neutralization of more acid.

H₂CO₃ = H₂O + CO₂. (2)

The rise in pH promotes the oxidation of the iron and the formation of iron hydroxide, which will precipitate out of the solution, leaving little iron left in the water. Large air pumps and diffuser tubes can be used to allow more CO2 to outgas, and thus precipitate more iron out of the solution. This explained method can only work, however, for runoff which is naturally basic.

List of acid mine drainage sites worldwide

This list includes both mines producing ARD and river systems significantly affected by such drainage.

North America

• Britannia Beach, British Columbia, Canada
• Iron Mountain Mine, Shasta County, California, USA
• Clinch-Powell River system, Virginia and Tennessee, USA
• Berkeley Pit superfund site, covering the Clark Fork River and 50,000 acres (200 km²) in and around Butte, Montana, USA
• Various Coal Mines in the anthracite and bituminous coal regions of Pennsylvania, USA
• Pronto mine tailings site, Elliot Lake area, Ontario, Canada
• North Fork of Kentucky River, Kentucky, USA
• Cheat River Watershed, West Virginia, USA
• Copperas Brook Watershed, from the Elizabeth Mine in S. Strafford, Vermont, impacting the Ompompanoosuc River

Europe

• Cwm Rheidol, Wales
• Aznalcollar mine on the Agrio River, Spain

Oceania

• Buller coalfield in the north-west of the South Island, New Zealand

South America

• Potosi, Bolivia, metal mines in and around Cerro Rico