Coal was the first of the fossil fuels to be exploited for stored energy; it fueled the industrial revolution in Europe that inaugurated the modern era of that supplanted horses and sweat with machines and electricity. Wood burning was the predominant global energy source until about 1880 when the use of coal was necessitated by wood depletion engendered by rising population pressures coupled with an increased demand for high energy density sources for nascent manufacturing enterprises. By 1900 at the turn of the twentieth century, coal comprised about 95 percent of all fossil fuel energy production and in 2008 coal is still used for about 35 percent of global energy production. While the use of coal must eventually be curtailed as is the case for all fossil fuels, its continued use in the near future to bridge the gap to renewable energy technologies will almost certainly be a necessity barring any heretofore undiscovered emerging energy technologies. It is therefore best to consider how to make the transition – to explore strategies for coal combustion that will minimize the impact that its consumption will inevitably entail. The return to renewable resources will necessarily have a different trajectory, as wood phytomass will no longer suffice and petroleum will be a distant memory. The indomitable human inventiveness and spirit will hopefully prevail in the coming struggle with the countervailing realities of population, energy, and climate.
The provenance of coal is well understood, as its constituent components attest to its origins. Coal can be considered an organic rock composed of carbon mixed with a variety of petrological components known as macerals; it is deposited as a sedimentary rock that may be transformed by temperature and pressure into a metamorphic rock. Coal was formed from peat bogs, which are essentially wetland areas that contained plant matter (mostly ferns, horsetails and club mosses) that were decomposed by anaerobic (without air) bacteria. As geologic time progressed, layers of peat accumulated as the land subsided into a depression known as a geosyncline; inorganic sedimentary deposits such as shale and sandstone eventually covered the inchoate coal seams. Subsequent tectonic plate movements and their attendant orogenies (mountain building events) subjected the plant layers to temperature and pressure that increased the carbon content of the material in a process known as coalification. The hardest coals are called anthracite, with 90 percent carbon, the soft coats are called bituminous and have about 80 to 85 percent carbon and the low grade brown coals are called lignite and have less that 75 percent carbon.
The vast coal beds of the northern hemisphere were deposited during the Carboniferous Period of the Paleozoic Era between 345 and 225 million years ago (mya). The “carbon” period was aptly named, as the earth was dominated by the evolving land plants that had emerged from the oceans during the prior Devonian Period and thrived in a world devoid of nettlesome consumption by an established faunal population. The vast reaches of swampy plant matter endured for millennia; it is estimated that the peat deposits were up to three miles thick. The supercontinent Pangaea was formed in the Carboniferous Period from the combination of the northern hemisphere supercontinent Laurasia which was comprised of North America and Eurasia and it’s southern hemisphere counterpart Gondwana which consisted of South America, Africa, India and Australia. The coal formation of the Carboniferous occurred across two great bands, the global concentration of coal in the 21st Century is a reflection of this distribution. The largest concentrations of coal are in five nations, which together hold about three quarters of the total proved recoverable coal reserves of about 900 gigatons (Gt). The first three are the United States with 247 Gt, Russia with 157 Gt and China with 115 Gt and are all of the northern Laurasian Carboniferous Period wetlands. The other two are India with 92 Gt and Australia with 79 Gt of southern Gondwanan provenance when these two areas were part of the larger protocontinent landmass; India was affixed to Africa at about the location of Madagascar and Australia was contiguous with the eastern edge of India.
A second period of coal formation occurred from the late Cretaceous Period during the Mesozoic Era about 70 mya through the Tertiary Period of the Cenozoic Era until about 2.5 mya. As the process continues at a geologic-time glacial pace, today’s peat bogs are tomorrows coal seams; poor quality brown coal lignite was produced as late at the Quaternary Period 1.5 mya. These later, less important coal deposits were not subject to the metamorphosing transformations of pressure and temperature as they formed in consonance with the breakup of Pangaea, a period of reduced orogenic activity; they consequently consist of lower carbon lignite coals. These deposits are the brown coals of western North America from Utah to Alaska, and more importantly from the historical perspective, many of the coal fields of Western Europe.
From the perspective of human civilization, the use of coal as an energy source transcends millennia; the Chinese employed coal locally for heating and pottery making during the Han Dynasty more that 2,000 years ago and there is evidence that its use in the Shanxi Province since the emergence of homo sapiens at the beginning of the last interglacial period 10,000 years ago. There was also some early use of coal for heat and light in Britain under the Roman occupation. As outcrops of coal were relatively common, it is indubitable that many localities discovered its energy potential, the verisimilitude to charcoal derived from wood providing the impetus for experimentation. But coal was difficult to obtain in quantity due to its inherent geologic associations with deep and practically intractable telluric seams; wood was readily available and provided adequate heat and light. A small but growing coal trade economy began in Belgium and England in the 12th Century and spread to France over the next several hundred years. However, it was not until wood began to be seriously depleted by larger populations clearing more land for agriculture and using more heat for more habitations that coal came into widespread use as an alternative energy source.
The Industrial Revolution that transformed human civilization from a phytomass based energy culture that used about 30 Gigajoules (Gj) of wood and charcoal in a year to a fossil fuel based mega culture consuming over ten times that amount (360 Gj per capita in the United States) began in England. Although the conversion to the modern energy coal economy is traditionally consigned in the late 18th Century, in actuality it was a very gradual process that took several centuries. By 1600, coal was in widespread use in households and for a number of major industries including glass, bricks, soap and salt; all major English coalfields were in operation before 1650 and by 1700 the production rate reached 3 million tonnes (a tonne or metric ton is 1,000 kg which is equivalent to 2200 pounds). The use of coal expanded asymptotically in consonance with the scientific discovery of coke for iron smelting and the engineering invention of the steam engine to convert energy to power, the rate of doing work.
The use of iron as the primary metal for manufacture was the inevitable result of its relative abundance, its malleability, and the simplicity of the extraction chemistry. The production of pure iron from iron ore requires only one relatively challenging requirement, a very high temperature to reduce the iron from its oxide. The basic process consists of the production of carbon monoxide (CO) that reduces iron oxide (Fe2O3) to pure iron (the Fe++ of the oxide is reduced by two negative electrons so that its oxidation number goes from +2 to 0, the oxidation number of pure iron, Fe0). Charcoal made from wood had been used to provide the necessary heat energy for this endothermic reaction. The combination of the depletion of the forest resources of England, the expansion of the iron industry, and the large amount of wood needed to make charcoal contributed to the demand of coal for coke.
The improvements made to Thomas Newcomen’s steam engine by James Watt in 1769 by the addition of a condenser and vacuum expansion arguably defined the beginning of the modern energy era. It was the first practical machine that transformed the chemical energy of fuel into the mechanical energy of motion. The steam engine used water heated by wood or coal to add the energy of heat to produce steam to drive a piston; levers, pawls and ratchets were added to convert reversing reciprocation to continuous rotation for industrial applications. Ironically, one of the first uses of the steam engine was to pump water out of mine shafts to ameliorate the coal extraction process.
During the 18th Century British (England became Great Britain with the passage of the Act of Union by the Scottish Parliament and Westminster in 1707) coal production grew from 3 million tonnes to 10 million tonnes to provide for the expanded use of coal that definitized the fossil fuel industrial revolution. Coal-fired steam engines powered factories that manufactured products for consumption and transport to the ports by steam engine railroads to be sent throughout the world on steam engine ships. The sun never set on the 19th Century British Empire that the dominance of coal production engendered. At apogee, the coal industry of the United Kingdom (Ireland voted to join Great Britain in 1801 and became a part of the United Kingdom of Great Britain and Ireland – later Northern Ireland when the southern counties seceded in 1922) accounted for 80 percent of the world’s total. Other countries followed suite; the United States surpassed the UK as the leading coal producing country in the 1870’s. By 1900, 95 percent of the world’s commercial energy was derived from coal.
Coal was the dominant global fuel until the 1960’s when it was supplanted by petroleum with its higher energy density, its fungibility for ease of transport and its chemical versatility. The diminution of coal as an energy resource was also precipitated by problems associated with its combustion, it is an inherently dirty fuel. The natural grayness of London was not enhanced by the soot effluent of its coal-fired furnaces. In December, 1952, a week of coal smoke mixed with fog during a temperature inversion resulted in smog conditions that contributed to the death of 12,000 people in London. The introduction of other alternative energy sources such as nuclear power plants in the latter half of the 20th Century resulted in a further erosion of the global coal industry. It was once considered an ignis fatuus (a deceptive hope or foolish fancy) to “sell coal to Newcastle” because Newcastle-upon-Tyne in northeastern England was a traditional entrepot for coal production and it would have therefore been nearly impossible to sell coal there. The United Kingdom of Great Britain and Northern Ireland is now a net importer of coal, some of which has even been sold to Newcastle.
Even though the relative importance of coal has declined for the last 50 years, its production in absolute terms has not, as the rising world population and concomitant energy demand have risen more that any single source could provide. Coal production has increased by about a factor of 10 since 1900 and global production now stands at about 4 billion tonnes a year – and that coal accounts for about 35 percent of global electricity generation. China is the leading producer with 1.6 billion tonnes followed by the United States (900 million tonnes [mt]), India (370 mt), Australia and South Africa (200 mt each). It is now widely recognized that renewable energy sources will ultimately need to be developed to replace the waning fossil fuel sources. This conversion will only be possible if new alternative energy sources are discovered and those that are currently available are improved and expanded. However, even with a concerted international effort, it will take decades. During the transition period, fossil fuels will be necessary to bridge the gap. The proven reserves of coal will last for about another 165 years at the current rate of consumption. So if it must be used, it is best to consider how to do this with minimal environmental impact and maximum energy production. This is the genesis of the notion for “clean coal.”
Clean coal is a term that has entered the lexicon of the energy debate as a means of promoting coal as a viable energy alternative, not the dirty coal of yesterday, but the clean, modern coal of tomorrow. It is not a singular technology, but rather a combination of technologies that reduce and in some cases nearly eliminate the impact of coal combustion on the environment. There is not a universal consensus on what constitutes clean coal technology, but it is generally considered to mean any or all of the following:
• Pre-combustion – Removal of impurities before combustion by pulverizing, washing and filtering. Other pre-combustion processes include upgrading and treatment
• Purification – Removing sulfur, nitrogen oxide, and particulate contaminants from the waste stream
• Conversion – Chemically changing coal into a gas or a liquid to improve combustion properties
• Carbon Capture and Sequestration (CCS) – Capturing and storing the carbon dioxide that is produced to prevent its contribution to global warming.
If all of these things were actually accomplished to a measurable and verifiable extent, then coal would indeed be relatively clean from an environmental standpoint. And even if they are not all done, it is certainly a viable argument that it is better to improve coal combustion somewhat when ending it altogether is not a feasible alternative at this time.
Coal contains many contaminants when it is removed from the ground. The origin of these contaminants has not been conclusively resolved; they were either integral to the primordial plants when they first decayed in peat bogs, transported to the coal bed by the sediments that were deposited on top of the decomposing plants, or absorbed by the coal beds during or after the coalification process. The latter provenance is considered the most likely as the carbonaceous constituency of a coal bed provides an electrochemically reducing environment that would favor precipitation of the relevant elements. In other words, the carbon in coal provides the electrons to reduce the ionic state of an element in a compound as in the case of iron oxide(Fe2O3) discussed above. The diversity of coal contaminants is extensive, a reflection of the complexity of the coalification process. Coal can contain everything from the rare earths germanium and gallium to the common metals iron and aluminum. But it is the contaminants with consequences to human health that are of most concern.
Coal-fired power plants are the largest singly source of mercury emissions, accounting for about 40 percent of the total. Atmospheric mercury is not of great concern except when it precipitates with rainwater. Once waterborne, mercury is transformed into methylmercury – subsequent ingestion by successive organisms results in sequentially increasing mercury concentrations according to the relative position in the food chain. The human consumption of fish and shellfish at the top of the food chain is consequently subject to the higher levels, and excessive mercury can result in fatal damage to the brain and the kidneys which is of particular concern to developing embryos and thus to pregnant women. Coal also contains measurable amounts ( between 1 and 4 parts per million or ppm) of uranium and thorium and their radioactive decay products such as radium and radon. Populations living in close proximity to coal-fired power plants experience some increase in their radiation dosage. However, the amount represents an increase of between 1 and 5 percent over the radiation absorbed due to natural background sources and is not considered to be a significant health hazard. The environmentally damaging effluents that are of most concern, however, are particulate matter (fly ash and soot), sulfur dioxide (SO2) and nitrogen oxides (NOx). The atmospheric oxidation of the latter two compounds into sulfates and nitrates is largely responsible for the light-scattering haze that beleaguers most large industrial urban areas. They also contribute to the formation of sulfuric and nitric acids, the harbingers of acid rain.
Pre-combustion is perhaps the most obvious of the clean coal technologies, the incoming coal is cleaned in addition to being ground and pulverized in advance of the combustion process. This removes most of the incident debris that was mined with the coal and at least some of the heavy metal contaminants. Washing the pulverized coal takes advantage of the fact that coal has a lower density compared to other material (the specific gravity of coal ranges from 1.3 to 1.6 compared to 1.8 to 2.4 for other material); the upward velocity of the water lifts the coal for separation. Other pretreatments may be combined with the sorting and cleaning process to remove moisture by heating or to employ additives to chemically scavenge harmful constituents. However, many of the contaminants cannot be removed by pre-combustion treatments and are consequently burned with the coal, forming a waste material known as fly ash. The principle constituents of fly ash are silicon dioxide (SiO2), the metal oxides of aluminum and iron (Al2O3 and Fe2O3) and the alkaline compounds commonly found in sedimentary rock formations (CaO, MgO, and K2O). Fly ash combined with unburned black carbon soot make up the visible black smoke that emanates from coal fired power plants, euphemistically known as particulate matter.
Purification is the removal of the particulate matter, sulfur dioxide (SO2) and nitrogen oxides (NOx) from the exhaust gas waste stream. Electrostatic precipitators and mechanical filters are used in clean coal plants to remove 99 percent of the particulate matter. Electrostatic precipitators apply a charge to the particles in the waste stream and use charged plates to extract them; filters remove particulate mechanically according to size. A characteristic formula for bituminous coal is C153H115N3O13S2, accounting for the various elemental constituents in the appropriate ratios. The sulfur (S) is oxidized, or burned, producing sulfur dioxide:
S(s) + O2(g) —– > SO2(g)
The SO2 undergoes additional oxidation to SO3 which reacts with water vapor to produce sulfuric acid, H2SO4, one of the primary contributors to acid rain. Sulfur dioxide is removed from the stack effluent by Flue Gas Desulphurization (FGD) with about 90 percent efficiency. This process involves the chemical reaction of SO2 with a variety of basic compounds such as CaO, MgO and CaCO3 (limestone) which are mixed with water and sprayed into the flue gas waste stream. The limestone reaction proceed according to:
CaCO3(s) + SO2(g) — > CaSO3(s) + CO2(g)
which has the rather unfortunate chemical consequence of producing more carbon dioxide. In some FGD processes, the calcium sulfate is oxidized with water to produce gypsum that is sold for manufacture into wallboard for interior wall construction
CaSO3(s) + ½ O2(g) + H2O(l) — > CaSO4 · H2O(s)
The United States has the largest percentage of coal fired plants equipped with FGD (over 30% of all plants), followed by Germany and Japan.
Nitrogen oxides (NOx) are produced during the combustion process from the nitrogen present in the coal , called fuel NOx and from the nitrogen gas that makes up 80 percent of air, called thermal NOx. The latter increases exponentially when combustion temperatures exceed 2800ºF. The nitrogen content of U. S. coals ranges between 0.5% and 2.0%; burning coals that have low nitrogen levels is one obvious improvement. Reducing the fuel NOx level is accomplished by several technologies that either lower the combustion temperature, reduce the time that the amount of time that the nitrogen gas is in the high temperature region, or reduce the amount of oxygen in the combustion area. The most prevalent is the use of low NOx burners that control the mix of fuel and air, achieving about a 50 percent reduction in nitrogen oxide; there are several other related methods that include gas recirculation and staged combustion. However, to remove the nitrogen oxide from the waste stream almost entirely (up to 90 percent), it is necessary to treat the gaseous effluent directly, which is generally called flue gas treatment (FGT). The most developed technology is called selective catalytic reduction (SCR), in which the nitrogen oxide is reacted with ammonia (NH3) in the presence of a catalyst (a mixture of titanium dioxide, vanadium pentoxide, and tungsten trioxide) to produce nitrogen gas (N2), in a manner similar to that used for automobile catalytic converters. Though successful, it is expensive and its widespread use is expected to increase the price of coal generated electricity by about $60 per kilowatt (KW).
Conversion is the most compelling of the clean coal technologies, as it chemically converts the coal into either a gas or a liquid fuel with purportedly reduced environmental impact. The use of coal to make gas is not a new technology; In 1812, coal was to produce low energy town gas to illuminate the streets of London, and up until the 1940’s and the availability of cheap natural gas, it was widely used in the United States as producer gas or town gas. The process consisted of passing steam through heated coal to generate methane according to:
C(s) + H2O(g) —- > CO(g) + H2(g)
C(s) + H2(g) —- > CH4(g)
This process was very inefficient with a heating value of about 10 percent of that of natural gas; the output consisted of only about 1 percent methane (CH4) with most of the remainder being carbon monoxide (CO) and nitrogen (N2), from the air used in the process.
The modern processes of coal gasification employ a variety of techniques that improve on the coal gas methods of the past; the most prevalent are Lurgi, Winkler, and Koppers-Totzek which differ according to how the coal and steam are mixed. Fundamentally, these processes use oxygen and steam (for H2O) at high temperatures to produce a gas mixture now frequently called simply syngas that is about 40 percent hydrogen, 16 (Lurgi) to 51 (Koppers-Totzek) percent carbon monoxide and only about 1 percent nitrogen. The combination of the carbon monoxide and hydrogen can be used directly as a source of energy or converted to methane in the presence of a catalyst:
CO(g) + 3H2(g) —- > CH4(g) + H2O(g)
The gasification process extracts some of the contaminants that must be removed from the waste flue gas from conventional coal-fired plants. However, this amounts to only about 50 percent of the sulfur and nitrogen oxides so that some form of flue gat treatment would be necessary to minimize environmental damage.
In what are called Integrated Gasification Combined Cycle (IGCC) plants, syngas is burned in a combustor whose output energy is used directly to turn a gas turbine and indirectly to heat water to generate steam to turn a second steam turbine. Both turbines are connected to electrical generators so that the output power of the two turbines is combined; the output efficiency of the process is about 40 percent. The Department of Energy has invested billions of dollars in this technology over the past several decades and there are a number of commercial plants in operation. It is the best technology that is ready to be deployed for new power plants that would otherwise be fired by conventional pulverized coal facilities.
Another conversion process that has some potential is hydrogenation, the conversion of coal to a liquid fuel. The process was developed in Germany in the 1930’s and basically consists of using the hydrogen (H2) and carbon monoxide (CO) produced from coal in the gasification processes described above and using a tungsten based catalyst to form hydrocarbons:
nCO + (2n + 1)H2 —- > CnH(2n + 2) + nH2O
Germany produced about 75 percent of its oil using this process from 1941 to 1944. At peak production in 1942, 32 million gallons of aviation gas were manufactured. The process is very inefficient, however, and it is not likely that it would become economically viable without significant improvements in the chemistry. Arguably, Germany would not have initiated World War II without the potential for converting German coal to fuel the Panzer tanks on which the Blitzkrieg depended.
Carbon Capture and Sequestration (CCS) is the bane of all fossil fuel power plants as carbon dioxide (CO2) is an essential product of the combustion of carbon in air. Since carbon dioxide is the predominant greenhouse gas whose production must be curbed to prevent global warming, the use of any energy source that produces it is problematic at best and catastrophic at worst. The general idea of capturing carbon dioxide is proffered as a panacea to this conundrum. It is not entirely clear at this juncture whether that CCS will be either physically possible or economically feasible. There is some scientifically supported credibility to the idea of chemically removing carbon dioxide from coal during the gasification process, which would at least take it out of the direct output to the atmosphere. There is still the problem of where to put it. Among the ideas that may be feasible are storage in geological formations like depleted oil fields and saline formations, storage in the ocean possibly by augmenting the solid clathrate hydrates that are already there, or mineral storage by reacting carbon dioxide with metal oxides to produce stable carbonates. It will be the challenge of the 21st Century to solve this problem or, conversely, to find adequate energy sources to replace coal.
In summary, it is fair to say that coal has come a long way from the sooty smokestacks of the 19th Century. The reality of global warming coupled with the insatiable demand for energy is, in the view of many, an irresistible force engaged against an immovable object. Globally, every year of fossil fuel energy consumption accounts for 400 years of geologic history that it took for that amount to be deposited – we are living on the earth’s energy conversion process that took place hundreds of millions of years ago. It is not sustainable. Coal has greater known reserves than any other fossil fuel. In 2005, the global reserve to production (R/P) ratio for coal was 165 years; if xenophobia were possible in the interconnected economic and environmental world, the United States in the absence of any foreign trade has enough coal to last for 245 years. By contrast the R/P ratio for petroleum in 2005 was 44 years. It is therefore abundantly clear that any semblance of human civilization in the 22nd Century and beyond must find alternative energy sources. The use of clean coal to provide electrical power will only buy time to find solutions to the seemingly intractable problems of global warming and the exhaustion of the resources of relatively cheap energy – it is the great hope of mankind that the solution will be found. The Revelation of John would likely be the alternative result – famine, pestilence, war and death culminating with the apocalypse on the plains of Megiddo.