April 25, 2011 (Coal Geology): Coal has played significant part in the history of human civilization. Coal is still one of the major sources of energy today. The use of coal is predicted to increase for at least two more decades irrespective of various environmental concerns for coal burning. This article tries to capture some of the most notable problems and their existing solutions.
Effect of coal usage on human health and environment:
Effect on Land: In case of surface mining, overburden material has to be removed to extract the coal seam. Many believe coal mining is the major cause of destruction of landscape and degradation of visual environment. In extensively surface mined states such as West Virginia, this also caused disturbance of surface and groundwater resources. Recent mining cultures in the US have proven that land restoration is possible. For example, many of surface mine sites restored by Alpha Natural Resources have won environmental awards. However, there is no doubt that extensive coal mining during the pre-regulation period destroyed large areas of agricultural and forest lands. Removal of soil and rock overburden covering the coal resource could causes burial and loss of top soil and creates vast infertile wastelands, exposes parent material. Pit and spoil areas are not capable of providing food and cover for most species of wildlife. However recent mining practices typically save the topsoil separately for land restoration purposes.
The reclamation process should be taken seriously throughout the mining industry. Proper planning for reclamation is required which should address the availability of material to fill up the final pit area; possibility of erosion at the steep slopes due to the unconsolidated nature of the material and other issues related to the chemistry of the topsoil such as sodium adsorption ratio, major elements, size fraction; cation exchange capacity etc. For example, presence of soluble salts could lead to saline condition in the topsoil.
Acid Mine Drainage (AMD): The exposure of the broken overburden material to oxygen and water (rain) could potentially lead to acid mine drainage (AMD) and contaminate surface and ground water. AMD is produced as the sulfide materials (most commonly pyrite) in the coal seams and the overburden material is exposed to air and water. As coal forms in a reducing environment, sulfide minerals are common in coal and surrounding rock strata. In the US, Appalachian coal fields are notorious for AMD issues. There are still many closed underground mines in the states of Pennsylvania, Ohio, West Virginia and Maryland that produce acidic drainage. AMD is well studied and documented as a series of complex geochemical reactions. Microbes often play an important role in AMD generation. Pyrite could be present in the coal seam, in the coal refuse or in the overburden.
AMD could be treated using both chemical and passive techniques. Lime application is commonly used to balance acid generation. Sodium hydroxide, sodium bicarbonate and anhydrous ammonia are also used to treat AMD. Those chemicals raise the pH of the solution and help most of the dissolved constituents to precipitate (such as iron). However, chemical treatment is expensive and the cost of treatment must be addressed during the feasibility study of the coal reserve.
Passive treatment of AMD is widely used by the mining industry. If is most common to see use of series of settling ponds before the effluent enters the main creek. There are many different kinds of passive treatments available such as a) anoxic limestone drains; b) diversion wells; c) open limestone channels; d) aerobic and anaerobic wetlands; e) vertical flow reactors etc.
It is also common to find more than one passive AMD treatment at a mine site such as use of aerobic wetlands along with open limestone channels.
Subsidence: Subsidence is a common problem in the deep mines. Subsidence could adversely affect the topography. The roof materials may cave following the removal of coal in a deep mine. This leads to the collapse of overlying strata, commonly noted as “roof falls” and marked as important features in most of the mine maps. Collapse of roof underground lead to subsidence at the surface. For room and pillar mining, subsidence occurs when a pillar support fails. In some mining technique, such as longwall mining, roof is allowed to collapse as mining progress. It is important to evaluate subsidence before start of coal mining. Longwall mining typically produce a shallow trough due to subsidence which could change the local drainage pattern. Many subsidence modeling program are available and used in the industry to address the issue. The mount of subsidence depends on a) thickness of coal seam; b) mining height; c) mining technique, size, shape and distribution of pillars; d) amount of coal extraction; e) thickness of overburden; f) strength and characteristics of the roof strata; g) degree of fracturing in the overburden strata and fracture orientation and density; h) type of rock strata (shale, fireclay, sandstone); i) presence or absence of rider seams in the roof.
Methane Gas (CH4): Coal seam contains various amount of methane gas. Coal Bed Methane (CBM) is produced during the coalification process. Only a small fraction of the methane gas is trapped within the coal bed. Some of the coal seams emit very high amount of methane gas (erg some of the Pocahontas coals) while some others emit negligible amount. The rate of release of the coal gas is not constant. If released at high amount at the working face of the mine, it could be highly hazardous. Many of the coal mine disasters are caused by the methane gas. The amount of methane gas emitted is directly related to a) the rank of coal seam; b) method of mining; c) depth of seam.
The amount of methane gas increases as the rank of the coal increases. Most of the shallow coal seams do not have much methane content. Shallow horizon is subject to intense weathering and fracturing. Methane gas in shallow coals emits to the atmosphere through natural cracks. The adsorption capacity of coal seams improves with depth. It is common not to test coal seams for methane gas above 150 feet-200 feet. The methane released during underground mining is called the “coal mine methane (CMM)”.
In many cases, local gas companies built degasification system such as vertical and horizontal wells into the coal seam to recover the coal bed methane (CBM) before the start of mining, during mining and during post mining conditions. In some cases, if undermining has occurred and the underlying mine is closed, methane trapped inside the closed mine could travel to the mine in the upper seams through fissures and fractures or even by exploratory drill holes.
Methane detectors are a must in the deep mines of the United States under current regulations. Methane gas is extremely explosive in air with concentration range between 5-15%. Proper and adequate ventilation is extremely important for safe operation in any underground coal mine. Current ventilation process emits the coal bed methane into the atmosphere. Methane is a greenhouse gas and China and United states together produce nearly 50% of all Coal Mine Methane (CMM) produced in the world (US EPA, 2006, Global Migration of Non-CO2 Greenhouse Gases).
US EPA has established a Coalbed Methne Outreach Program to reduce CMM emissions in the US and internationally. CBM is commonly used to generate electricity or sold to natural gas pipeline systems. US coal mines recovers nearly 90% of the coal bed methane gas.
Coal slurry: Coal is hardly delivered to the end user right after mining. Different end user of coal has different specifications based on size, heat value, and sulfur content and so on. Beneficiation is generally combination of processes that typically include – a) sizing, b) increasing heat value by removing ash content (using gravity separation), c) removing undesirable mineral content such as sulfur, trace elements etc. Preparation plants that perform specific beneficiation processes are constructed close to the location where the coal is mined. The coal beneficiation process generates “coal slurry” (waste streams) that has to be controlled. The coal slurry typically includes “fine materials” and discharged to a tailing impoundment. The coarse reject material is transported away as a solid waste.
Hydrologic impacts: Subsidence of underlying strata could cause diversion of surface drainage and even change local water table to change direction of ground water flow. Mining operation and Coal Handling and Processing Plant (CHPP) requires extensive use of water not existed before the beginning of the mining operations. Miners and other workers need fresh water most commonly drawn from the deep aquifer. Water is also required at the coal processing stages and for dust suppression. Pre-mining evaluation of water supply is of utmost importance and must be included as part of the feasibility study of the coal reserve. In many countries, such as in Australia, is common to prepare flow models using advanced modeling software such as MODFLOW to determine water demand during mining in different years. If the coal seam dips below the water table, the over lying aquifer is also drained during the underground mining operation. It is important to assess the users of the aquifers to be impacted by the mining. Coal mining operations in the United States are highly regulated. Every mine permit requires at least 6 months of baseline sampling along with monthly and bi-monthly sampling at various surface and groundwater stations during and after mining conditions. All mine permits also require a section on Probable Hydrologic Considerations (PHC) to assess any impact on water supply. Both quality and quantity are addressed in such sections.
Contamination from coal utilization: Coal combustion produces different kind of emissions that could have adverse environmental effects if not handled properly. The by-products of coal combustions must be assessed for environmental concerns. Sulfur dioxide (SO2), nitrogen oxides (NOx), particulate matter (PM) and carbon dioxide (CO2) are of greatest concern from emission.
Coal Fires: Coal seams sometimes catch fire underground. Weathered coal near the outcrop can also increase ground temperatures. Almost all fires in coal are caused by surface fires started by people, or by lightning. Spontaneous combustion is caused when coal oxidizes and air flow is insufficient to dissipate heat, but this more commonly occurs in stockpiles and waste piles, rarely in bedded coals underground. Coal seam fires may burn underground for decades.
Health Effects and Coal Dust: Coal dust is a very common problem for both the surface and undergrounds mine. The terrible “Black Lung” disease has its origin in the coal mine. Apart from the coal dust itself, the transportation, hauling and storage of coal could produce dust. Mining is a dangerous occupation irrespective of all modern technologies and improvements. The recent 2010 explosion at the Massey operated Big Branch Mine, West Virginia, proves the fact again. More than a million miners were killed in the United States between 1885-1985 (Coal: the energy sources of the Past and Future, American Chemical Society, 1987). Current mining conditions have remarkably improved in parallel with the growing number of regulations in the mining industry. Mine Safety and Health Administration (MSHA) has inspectors who visit various mine sites to check how well the rules are implemented. Mines are penalized and even closed if violations are cited. MSHA website www.msha.org has enormous amount of information on mine safety. Everybody working in a mine site is required to have 40 hr. MSHA training. Apart from deaths in the mine due to explosion, black lung disease (pneumoconiosis) has taken many lives. In present practice, pulverized lime dust is spread over the walls of coal seams to minimize and settle coal dust. Water sprayers are also used at the working face of the mine to reduce dust level. Miners also wear MSHA approved air purifiers to fight dust problems.
Fly Ash Spills: The burning of coal leads to substantial fly ash sludge storage ponds which could lead to contamination of surface or ground water.
Apart from some of the common problems due to coal mining, complaints about noise pollution, increased truck traffic, vibration from blasting have also been noted in many cases.
Today we have a highly regulated coal mining industry in the US that overcame unsafe working conditions, lack of training, child labor, land destruction and poor living condition of the miners.
Should we then stop coal mining?
No, absolutely not. Coal mining poses problems. But if properly managed, all of problems related to coal mining can be eliminated. The coal mining regulations in the United States have become stringent over time and significant amount of background study is currently required for getting any mine permits. Each permit addresses the post mining effects and presents a plan to address any adverse post mining issues. Quality of water in the nearby creeks and ponds are tested periodically (monthly or bi monthly) based on agreed surface and groundwater monitoring stations during the permit process. Various geochemical tests are also performed on the overburden material and on the coal seams such as acid-base-accounting (ABA), elemental analysis. The chemical understanding of the mine site gives insight about any potential problem, and feasible solution. Stopping coal mining means elimination of thousands of high paying jobs. Coal currently produces nearly half of all electricity generated in the US. No other alternate energy could replace the need of coal based on the current technological advancement.