396

aTg = teragram = 1 x 109 kg; 1 Tg CH4 = 52 billion cubic feet (Bcf).

Source: EPA, International Anthropogenic Methane Emissions in the United States (Office of Policy, Planning and Evaluation, U.S. Environmental Protection Agency, U.S. Government Printing Office, Washington, D.C., 1994).

aTg = teragram = 1 x 109 kg; 1 Tg CH4 = 52 billion cubic feet (Bcf).

Source: EPA, International Anthropogenic Methane Emissions in the United States (Office of Policy, Planning and Evaluation, U.S. Environmental Protection Agency, U.S. Government Printing Office, Washington, D.C., 1994).

The total volume of CMM liberated in the United States in 2000 was 196 billion cubic feet (Bcf); of that, underground mining activities liberated 142 Bcf [8]. CMM emissions account for approximately 10% of total U.S. methane emissions. Globally, coal mines account for 8% of all methane emissions. Underground mines are the largest source of CMM and account for 72% of the total CMM liberated.

The U.S. coal industry has made substantial progress in recovering and using CMM through drainage systems. Of the 142 Bcf of CMM liberated from underground mines in 2000, about 42 Bcf were emitted through drainage systems, with the remainder being emitted through ventilation systems [8]. Coal mines in the United States recovered 86%, or 36 Bcf, of the gas liberated through the drainage systems, which is nearly a threefold increase over 1990, when only 14 Bcf of gas were recovered. Approximately 100 Bcf of CMM are emitted through ventilation systems each year. Ventilation air represents a considerable source of greenhouse gas emissions and is a major focus area for recovery and use.

Some coal seams, particularly in Australia but also in France and Poland, contain high carbon dioxide concentrations, which can comprise as much as 100% of the gases in the coal seam [6]. This gas is thought to have a magmatic origin. Although carbon dioxide is not toxic, it can cause asphyxiation by displacing breathable oxygen.

Liquid Effluents/Acid Mine Drainage

The generation of liquid effluents from major mining techniques tends to be higher for underground mining than for surface mining. For underground mining, liquid effluent rates of 1.0 and 1.6 tons per 1000 short tons of coal produced have been documented for conventional and longwall mining, respectively [9]. If groundwater systems are disturbed, the possibility then exists for serious pollution from highly saline or highly acidic water.

The highly acidic water, commonly known as acid mine drainage, is produced by the exposure of sulfide minerals (most commonly pyrite) to air and water, resulting in the oxidation of sulfur and the production of acidity and elevated concentrations of iron, sulfate, and other metals [1]. Pyrite and other sulfide minerals are generally contained in the coal, overburden, and coal processing wastes.

Historically, coal extraction in the northern Appalachian coalfields has resulted in serious problems related to contaminated mine drainage [10]. Acid drainage from closed and abandoned mines (both underground and surface) has far-ranging effects on water quality and, therefore, on fish and wildlife. Drainage from closed mines is particularly acidic in Pennsylvania, Ohio, northern West Virginia, and Maryland.

The formation of AMD is primarily a function of the geology, hydrology, and mining technology employed for the mine site [11,12]. AMD is formed by a series of complex geochemical and microbial reactions that occur when water comes in contact with pyrite (iron disulfide minerals) in coal, refuse, or the overburden of a mine operation. The resulting water is usually high in acidity and dissolved metals. The metals stay dissolved in solution until the pH raises to a level where precipitation occurs.

Four commonly accepted chemical reactions represent the chemistry of pyrite weathering to form AMD. An overall summary is:

Pyrite + Oxygen + Water —> "Yellowboy" + Sulfuric Acid

The first reaction in the weathering of pyrite includes the oxidation of pyrite by oxygen. Sulfur is oxidized to sulfate and ferrous iron is released. This reaction generates 2 moles of acidity for each mole of pyrite oxidized:

Pyrite + Oxygen + Water —> Ferrous Iron + Sulfate + Acidity

The second reaction involves the conversion of ferrous iron to ferric iron. The conversion of ferrous iron to ferric iron consumes one mole of acidity. Certain bacteria increase the rate of oxidation from ferrous iron to ferric iron. This reaction rate is pH dependent, with the reaction proceeding slowly under acidic conditions (pH of 2 to 3) with no bacteria present and several orders of magnitude faster at pH values near 5. This reaction is often referred to as the rate-determining step in the overall acid-generating sequence:

Ferrous Iron + Oxygen + Acidity —> Ferric Iron + Water

The third reaction which may occur is the hydrolysis of iron. Hydrolysis is a reaction which splits the water molecule. Three moles of acidity are generated as a by-product. Many metals are capable of undergoing hydrolysis. The formation of ferric hydroxide precipitate (i.e., a solid product) is pH dependent. Solids form if the pH is above about 3.5, but below pH 2.5 few or no solids will precipitate. The third reaction is:

Ferric Iron + Water —> Ferric Hydroxide ("Yellowboy") + Acidity

The fourth reaction is the oxidation of the additional pyrite by ferric iron. The ferric iron is generated in Reactions (3-2) and (3-3). This is the cyclic and self-propagating part of the overall reaction that takes place very rapidly and continues until either ferric iron or pyrite is depleted. Note that in this reaction ferric iron is the oxidizing agent, not oxygen:

FeS2 + 14Fe3+ + 8H2O 15Fe2+ + 2SO2" + 16H+ (3-5)

Pyrite + Ferric Iron + Water —> Ferrous Iron + Sulfate + Acidity

Treatment of AMD includes both chemical and passive techniques. In Pennsylvania, for example, strict effluent discharge limitations were placed on mine operations in 1968 [11]. Many companies used chemical treatment methods to meet these new effluent limits. In these systems, the acidity is buffered by the addition of alkaline chemicals such as calcium carbonate, sodium hydroxide, sodium bicarbonate, or anhydrous ammonia. These chemicals raise the pH to acceptable levels and decrease the solubility of dissolved metals. Precipitates form from the solution. These chemicals are expensive, however, and the treatment system requires additional costs associated with operation and maintenance as well as the disposal of metal-laden sludges.

Many variations of AMD passive treatment systems were studied as early as 1978 [11]. During the last 15 years, passive treatment systems have been implemented on full-scale sites throughout the United States with promising results. The concept behind passive treatment is to allow the naturally occurring chemical and biological reactions that aid in AMD treatment to take place in the controlled environment of the treatment system and not in the receiving water body. Passive systems do not require the expensive chemicals necessary for chemical treatment systems, and operation and maintenance requirements are considerably less. Passive AMD treatment technologies being implemented include:

• Aerobic wetland;

• Compost or anaerobic wetland;

• Open limestone channels;

• Diversion wells;

• Anoxic limestone drains;

• Vertical flow reactors;

• Pyrolusite process.

Hydrologic Impact

With underground mining, subsidence and fracturing of overlying strata may cause surface runoff to be diverted underground and may disrupt aquifers, causing local water level declines and changing the direction of groundwater flow near the mine [1]. Dewatering required by mining operations affects groundwater quantity by depleting aquifers when mine features extend below the water table and become a drain [5]. Although groundwater depletion is the most obvious and immediate effect of mining on groundwater, longer term effects on the environment may be equally important. Redistribution and/or change in groundwater recharge rates may affect the time and degree to which aquifers will recover to a static condition. Conscientious management practices minimize water-related environmental impacts [13]. Coal mining activities are highly regulated, requiring extensive surface and groundwater sampling and monitoring to ensure compliance with federal, state, and local statutes. Also, hydrological impacts must be taken into consideration as part of the permitting process; therefore, coal company hydrologists study and monitor the quality of surface and underground water resources before, during, and after mining activity to ensure minimal hydrological impacts.

Health Effects/Miner Safety

There is no argument that mining is a dangerous occupation and that historically there has been a tremendous social cost associated with coal mining. Mine accidents and diseases have taken their toll on miners. In the United States, more than 100,000 miners died over the period of about 1885 to 1985 [4]. In 1900, the annual death rate among underground miners was 3.5 deaths per thousand miners [4]; however, a commitment to enhancing safety training and the development of new technologies have yielded remarkable improvements in the mine as a workplace. In addition, coal mines are subject to regular, comprehensive inspections by the federal Mine Safety and Health Administration (MSHA), and safety and health reporting requirements are much more stringent than those required by the Occupational Safety and Health Administration (OSHA), which regulates most other U.S. industries. According to MSHA, the twentieth century saw remarkable improvements in safety and health for U.S. miners, and the rate of fatal injuries for underground miners declined by 92% over the period from 1960 to 1999 [13]. Data from the U.S. Department of Labor show that in 2002 there were 12 fatalities in underground mining, 6 in surface mining, and 2 in preparation plants [14]. This is an annual death rate of 0.2 per thousand mining personnel, nearly a 20-fold decrease since 1900. The annual death rate for underground miners, which experienced the largest incidences of fatalities with 12, was 0.3 per thousand miners. According to the U.S. Department of labor, mining has a lower rate of injuries and illness per 100 employees than the agricultural, construction, or retail trades [13]. The accident and injury rates for miners today are comparable to those of grocery store workers.

Dust issues in mines, particularly underground mines, have had a negative impact on the coal industry. Many miners, particularly underground miners, have contracted respiratory diseases. These diseases— pneumoconiosis (black lung) and silicosis—have taken years to manifest themselves, and the industry has been working to minimize dust in mines. In an underground mine, the walls of the tunnels or shafts are covered with pulverized limestone to help settle coal dust [13]. Water sprayers on the mechanical equipment, such as in continuous and longwall mines, help reduce dust concentrations in the mines. The ventilation fans that remove methane also remove the lingering dust, and a continuous supply of fresh air is brought into the mine. Plus, miners now wear air-purifying systems. Dust production in underground mines is 0.0006 and 0.01 short tons per 1000 short tons of coal produced for conventional and longwall mining, respectively [9].

0 0

Post a comment