Coal is by far the most abundant fossil fuel on earth. It is essentially carbon and is mainly used as a combustion fuel. The large-scale use of coal began with the Industrial Revolution in the 19th century. As the number of industries increased, demand for more sources of energy grew.
Coal is the product of plants, mainly trees, that died tens or hundreds of millions of years ago. Due to water logging in low-lying swampy areas or in slowly sinking lagoons, dead trees and plants did not decompose as they normally would. The dead plant matter was covered with water and protected from the oxidizing effect of air. The action of certain bacteria released the oxygen and hydrogen, making the residue richer and richer in carbon. Thick layers of this carbon-rich substance, called peat, built up over thousands of years. As more material accumulated above the peat, the water was squeezed out leaving just carbon-rich plant remains. Pressure and temperature further compressed the material. This aided the process of producing coal as more gases were forced out and the proportion of carbon continued to increase. The carbon slowly metamorphosed into coal over millions of years.
There are three main types of coal: lignite, bituminous, and anthracite. Lignite and bituminous have a lesser percentage of carbon in them and therefore burn faster. They release a great deal of pollutants into the atmosphere. Anthracite has about 98% carbon and therefore burns slowly and releases much less smoke. Coal of all types contains sulphur to some degree. Sulphur is the worst of the pollutants and causes damage to human health and to vegetation.
Though petroleum gained importance over the 20th century and continues to do so, coal remains essential for the industrial sector. It is the principal heat source for electricity generation in most countries and is used directly in such heavy industries as iron and steel making.
Coal utilization in India
Coal is used primarily as an energy source, either for heat or electricity. It was once heavily used to heat homes and power locomotives and factories. Bituminous coal is also used to produce coke for making steel and other industrial process heating. Coal gasification and coal liquefaction (coal-to-liquids) are also possible uses of coal for producing synthetic fuel.
Highly dependent on imports for this crucial raw material needed for steel and power generation, India has decided to tackle its coking coal deficit by acquiring a foreign coking coal asset, and washing certain grades of coal to make it fuel-ready.
State-run CIL, the country’s near-monopoly coal producer, is said to be looking at coking coal assets overseas as the country is faced with constraints of commercially viable domestic metallurgical coal reserves,
Ironically, India sits on a massive natural stockpile of coking-grade coal, but these resources have not been fully exploited, and much of it is not suitable for power plants, forcing India to rely on imports for electricity. India generates about 60% of its total energy from coal and about 10% using natural gas and even diesel fuel.
In view of the climate commitments made by India at the Paris Climate Conference, it needs to bring non-fossil fuels up to 40% of its energy mix and the carbon intensity of its GDP down to 33% by 2020. Washing of the low-grade coal would mean it can then be used as coking coal for steel plants, and cheaper imports of coal for power plants can then be used.
Non coking coal
Non- coking coal comprises the lion’s share of Indian coal. Based on Useful Heat Value (UHV), it is classified into grades A to G for commercial use. A to C grades are considered as Superior and are used in cement, fertilizer and sponge iron industries. D to G grade, available in almost in all the coalfields, is considered as Inferior and is mostly used in power sector.
These are coals without coking properties and nainly used as thermal grade coal for power generation. Also used for cement, fertilizer, glass, ceramic, paper, chemical and brick manufacturing, and for other heating purposes.
Environmental issues associated with coal
There are numerous damaging environmental impacts of coal that occur through its mining, preparation, combustion, waste storage, and transport.
Acid mine drainage (AMD) refers to the outflow of acidic water from coal mines or metal mines, often abandoned mines where ore- or coal mining activities have exposed rocks containing the sulphur-bearing mineral pyrite. Pyrite reacts with air and water to form sulphuric acid and dissolved iron, and as water washes through mines, this compound forms a dilute acid, which can wash into nearby rivers and streams.
Air pollution from coal-fired power plants includes sulfur dioxide, nitrogen oxides, particulate matter (PM), and heavy metals, leading to smog, acid rain, toxins in the environment, and numerous respiratory, cardiovascular, and cerebrovascular effects. Air pollution from coal mines is mainly due to emissions of particulate matter and gases including methane(CH4), sulfur dioxide (SO2), and nitrogen oxides (NOx), as well as carbon monoxide (CO).
Coal-fired power plants are responsible for one-third of America’s carbon dioxide (CO2) emissions, making coal a huge contributor to global warming. Black carbon resulting from incomplete combustion is an additional contributor to climate change. Coal dust stirred up during the mining process, as well as released during coal transport, which can cause severe and potentially deadly respiratory problems.
Coal fires occur in both abandoned coal mines and coal waste piles. Internationally, thousands of underground coal fires are burning now. Global coal fire emissions are estimated to include 40 tons of mercury going into the atmosphere annually, and three percent of the world’s annual carbon dioxide emissions.
Coal sludge, also known as slurry, is the liquid coal waste generated by washing coal. It is typically disposed of at impoundments located near coal mines, but in some cases it is directly injected into abandoned underground mines. Since coal sludge contains toxins, leaks or spills can endanger underground and surface waters.
According to a 2010 study, mountaintop removal mining releases large amounts of carbon through clearcutting and burning of trees and through releases of carbon in soil brought to the surface by mining operations. These greenhouse gas emissions amount to at least 7% of conventional power plant emissions. Since coal seams are often serve as underground aquifers, removal of coal beds may result in drastic changes in hydrology after mining has been completed.
Coal contains many heavy metals, as it is created through compressed organic matter containing virtually every element in the periodic table – mainly carbon, but also heavy metals. The heavy metal content of coal varies by coal seam and geographic region. Small amounts of heavy metals can be necessary for health, but too much may cause acute or chronic toxicity (poisoning). Many of the heavy metals released in the mining and burning of coal are environmentally and biologically toxic elements, such as lead, mercury, nickel, tin, cadmium, antimony, and arsenic, as well as radio isotopes of thorium and strontium.
R&D in Coal Sector in India
The Government of India through its Coal Science & Technology (S&T) scheme and Coal India Limited (CIL) through its Research & Development (R&D) scheme undertake R&D activities for improvement in production, productivity and safety in coal mines, coal beneficiation and utilization, protection of environment and ecology and other related activities.
Since introduction of research funding of Ministry of Coal [MoC] and Coal India Limited, a large number of research projects have been carried out by various academic and research institutes related to the coal and allied industries with active participation of coal and lignite mining companies. Many of these projects have yielded considerable benefits, resulting in operational improvement, safer working conditions, better resource recovery and protection of the achievement.
At present, there are 34 on-going research projects with total outlay of more than Rs. 200 Cr at various academic and research institutions in association with coal and lignite producing companies. Under the above R&D scheme, different Government academic/research institutions are involved in the research work in coal sector.
For enhancing the research work needed to address the complexity of operations of the coal industry and for wider involvement of research institutes including private organizations with adequate infrastructure and expertise in India and abroad, requests are made from time to time, inviting research proposals for undertaking research work beneficial to the coal industry,
Important Coal Related Technologies
In situ coal gasification
In-situ means “in place” and refers to recovery techniques at the location of the energy source. In-situ coal gasification is also known as Underground Coal Gasification (UCG). It is the chemical conversion of deeply-buried coal in its original coal seam into a mixture of hydrogen, carbon monoxide, carbon dioxide and methane by creating the right process conditions in the coal seam to cause a series of chemical reactions to occur.
The mixture of hydrogen and carbon monoxide is known as Syngas or Synthetic gas that can be used as fuel or feedstock for further chemical processes such as ammonia production or liquid fuels. The carbon dioxide generated during the process can be captured to be piped back into the seam or pumped into oil wells to boost recovery rates.
The process works by injecting an oxidant (usually air, oxygen, or steam) into a coal seam. The oxidant reacts with the coal and water present in the seam to produce syngas that is extracted through a production well. Coal resources that are deeper than 60 meters and not suitable for conventional miningare ideally suited for UCG.
The process of coal liquefaction creates synthetic liquid fuels from solid coal as substitutes for various petroleum products. There are two types of liquefaction – direct and indirect.
Direct liquefaction converts solid coal directly into liquid form with no intermediate step, which results in only the partial dismantling of the coal structure. Indirect liquefaction requires an intermediate gasification of the solid coal to form a synthesis gas, which is then converted to the liquid product. This process results in the complete dismantling of the coal structure. In direct liquefaction, coal is exposed directly to hydrogen at high temperatures (450C) and high pressures (14000-20000kPa) for approximately one hour in the presence of a solvent that breaks down the hydrocarbon structure. Catalysts are used to improve rates of conversion of coal from solid to liquid form. The resulting liquid coals have molecular structures that require further upgrading to produce usable fuels like gasoline and fuel oil.
Indirect coal liquefaction takes solid coal through a gas phase before being converted into a raw liquid form. The synthesis gas (syngas) is made up of hydrogen and carbon monoxide, which is then reacted over an F-T catalyst to form a liquid hydrocarbon. The resulting liquid forms a range of hydrocarbon fuels and products including gasoline, diesel, methanol, and other chemicals. This method was also used to produce motor fuel during World War II, and has been used in South Africa since the 1960s to produce motor fuels and petrochemical feedstocks. Though this indirect process yields a larger number of byproducts and has a lower overall thermal efficiency, it results in more clean fuels.
Clean coal technology
Coal is an extremely important fuel and will remain so. Some 23% of primary energy needs are met by coal and 39% of electricity is generated from coal. About 70% of world steel production depends on coal feedstock. Coal is the world’s most abundant and widely distributed fossil fuel source. The International Energy Agency (IEA) expects a 43% increase in its use from 2000 to 2020.
However, burning coal produces almost 14 billion tonnes of carbon dioxide each year which is released to the atmosphere, most of this being from power generation.
Development of new ‘clean coal’ technologies is addressing this problem so that the world’s enormous resources of coal can be utilised for future generations without contributing to global warming. Much of the challenge is in commercialising the technology so that coal use remains economically competitive despite the cost of achieving low, and eventually ‘near-zero’, emissions. The technologies are both costly and energy-intensive.
As many coal-fired power stations approach retirement, their replacement gives much scope for ‘cleaner’ electricity. Alongside nuclear power and harnessing renewable energy sources, one hope for this is via ‘clean coal’ technologies, such as carbon capture and sequestration, also called carbon capture and storage (both abbreviated as CCS) or carbon capture, use and storage (CCUS). It involves the geological storage of CO2, typically 2-3 km deep, as a permanent solution.
Consequently the term ‘clean coal’ is increasingly being used for supercritical and ultra-supercritical coal-fired plants without CCS, running at 42-48% thermal efficiency. These are also known as high-efficiency low-emission (HELE) plants. The capital cost of ultra-supercritical (USC) HELE technology is 20-30% greater than a subcritical unit, but the higher efficiency reduces emissions and fuel costs to about 75% of subcritical plants. A supercritical steam generator operates at very high temperature (about 600°C) and pressures (above 22 MPa), where liquid and gas phases of water are no longer distinct. In Japan and South Korea about 70% of coal-fired power comes from supercritical and ultra-supercritical plants