Prospects of Underground Coal Gasification in view of the Specific Geological formation of Thar Coal Deposits.
All fossil fuels will eventually run out and it is essential that we use them as efficiently as possible. Significant improvements continue to be made in industrially developed countries, in how efficiently coal is used so that more energy can be generated from each ton of coal produced.
The most economical method of coal extraction from coal seams depends on the depth, the quality of the seams, the geology, the hydrology and environmental factors.
World Coal Deposits Vs Thar Coal Deposits.
Coal is called a fossil fuel because it was formed from the remains of ancient plants and vegetation that have undergone sedimentary and metamorphic transformation over millions of years.
Worldwide most of the coal deposits were formed 300 million years ago, from the compaction and indurations of accumulated remains of plants and trees that once grew in extensive swamp and coastal marsh areas.
As the water level of sea or lake dropped due to any change in geological or climatic conditions, the forest in shallow areas became dried, the plants and trees died, their remains sank to the bottom of the swampy and marsh areas due to fluctuating sea level, which had little biological activity at its bottom, led to preservation of the plant leaves, stems, pollen and other structures.
Over long periods of time, the makeup of the earth's surface changed, and seas and great rivers caused deposits of sand, clay and other mineral matter to accumulate layer upon layer. Several hundred feet of sediments layers continued to accumulate and added weight, eventually forming a soggy material called peat. Sandstone and other sedimentary rocks were formed, and the pressure caused by their weight squeezed water from the peat. Increasingly deeper burial and the heat caused chemical changes associated with it gradually changed the peat to coal of different grades worldwide.
Process of Coalification.
Coal formation began during the Carboniferous Period (known as the first coal age), which spanned 360 - 290 million years before present. Coals formed during the first coal age are older, so they are generally located deeper in the earth's crust. The greater heat and pressures at these depths produces higher-grade coals such as anthracite and bituminous coals in Europe and North America.
Coal formation continued throughout the Permian, Triassic, Jurassic, Cretaceous, and Tertiary Periods (known collectively as the second coal age), which spanned 290 - 1.6 million years before present. Conversely, coals formed during the second coal age under less intense heat and pressure is generally located at shallower depths. Consequently, these coals tend to be lower-gradesub bituminous and lignite coals (like Thar Coal deposits Sindh Pakistan).
Geological Time Scale.
The path of peat to coal transformation could be shown as a function ofpressure, temperature and time as under.
Plants debris -> peat -> lignite -> sub-bituminous coal -> bituminous coal -> anthracite coal -> graphite (a pure carbon mineral) -> Diamond.
Credit: Kentucky Geological Survey, University of Kentucky USA.
Peat is an unconsolidated accumulation/ deposits of plant remains from a water-saturated environment such as a bog (marsh) or mire (swamp).The process of peat accumulation continues until terminated by an event like an invasion of a nearby river channel, a marine transgression, or unfavorable climate. Each time shorelines retreated coal swamps migrated with them, along vast deltas which received seemingly limitless supplies of sediment sank to the bottom of the coastal areas.
During deep burial the peat undergoes Coalification process which squeezes out up to 98% of the water and some of the volatile hydrocarbons. The older and more deeply-buried a coal seam is, the less water and volatile matter it contains.
Bedded and compacted coal layers are geologically considered to be rocks. Because peat is not consolidated or compacted, it is classed as organicsediment. Lignite and bituminous ranks are classed as organic sedimentaryrocks.
Metamorphic rocks are comprised aggregates of minerals formed by the recrystallization of pre-existing rocks in response to changes of pressure, temperature, or volatile content. Anthracite Coal, formed when bituminous beds of coal are subjected to the folding and regional deformation affiliated with drifting and tectonic mountain building processes, is listed as a metamorphicrock.
Paleontology of Thar Coal Deposits.
Paleontology seeks information about several aspects of past organisms and tells us Earth's organic and inorganic past. On the basis of paleontological information the Thar coals are Paleocene to Eocene in age; Paleocene-Eocene eras 66- 44 million years ago while Indo-Pakistan landmass was drifting towards Eurasian plate shown below. Available evidence indicates that the Thar coals may have been deposited in a raised-bog environment landward of a west trending coastline of Tethys sea on Indo-Pakistan land mass, forming a low rank coal deposits (one of the youngest coal deposit in terms of formation).
Geological Background of Thar Coal Deposits.
Present shape of Thar coal deposits is the product of all the geological changes have occurred in the past in Thar area, discussed in the above link.
RANKS OF COAL.
Peat, an unconsolidated accumulation of partly decomposed plant material, has an approximate carbon content of 20 percent. In many classification schemes, peat is listed as the initial stage of coal formation. Moisture content is quite high, at least at the 75 percent level.
When dry, peat has an oxygen content of about 30 percent, is flammable, freely but inefficiently burns slowly and steadily for months at a low heat-content value of 5,000- 5,400 Btu’s per pound.
A method of classifying coal based on the amounts of carbon and volatile matter it contains, is known as Rank of coal.
The ratio of fixed carbon to volatile matter is used to determine acoal's rank. The higher the ratio, the higher its rank. It signifies the degree of coalification of organic material.
Forty seven percent (47%) of World Reserves are Low Rank Coals and fifty three percent (53%) are Hard Coals in terms of moisture and carbon contents.As the amount of moisture increases, the coal has a lower heating value.
The use of coal depends on its chemical composition and how much moisture it contains. The above sketch shows the types of coal and their specific uses.
The amount of energy produced by a ton or a pound of coal is largely dependent upon the type of coal being burned. The above four major categories of coal, each differing in composition and age. Lignite, also known as brown coal, is the least valuable of the four, in terms of energy production.
Organic part of the coal primarily contains carbon, hydrogen and oxygen with some sulfur and nitrogen.
The inorganic part of the coal is much less than the organic part and usually contains a large number of ash forming compounds.
Discovery of Thar Coal Deposits.
Thar coal deposits were discovered accidentally in the year 1989, during an exploration project of defunct SAZDA (Sindh Arid Zone Development Authority) to find drinkable water in the desert. The size and the quality of Thar Coal reserves were known to a great extent since1992- 93 after a study by John T Boyd Company, US Mining consultants. Thar Lignite Coal resource was initiallyestimated to be around 135 billion tons which was subsequently enhanced to175 billion tons after review of data by USGS (United States Geological Survey) and GSP (Geological Survey of Pakistan). This estimate was based on distancedbore holes over an area of 9,000 sq kms. Subsequent a constricted explorations carried over an area of 1,200 sq have provided even higher figures of provenreserves of being around 200 to 350 million tons of lignite per sq km.
As per reports, the Thar Lignite Coal resources are equivalent to around 50billion tons of oil, which is more than the combined oil resources of Saudi Arabia and Iran. In terms of gas reserves, these are around 68 times the present resources of natural gas in Pakistan (resource estimation will be discussed later).
Configuration Of Thar Coal Deposits.
Thar Desert is a desolate region where sand is piled up into huge wind blown dunes. The soils of the desert are generally sandy to sandy-loam in texture which is a personal observation during Thar Coal visit. As per reports the low-lying loams have a hard pan of clay, calcium carbonate, silica and gypsum. Several minerals and petroleum also found below the ground level of the Thar Desert.
The dominant clay stones represent low-energy shallow-water conditions, whereas occasional thin sandstone beds and siltstone within clay stone show that the area was periodically affected by flood events.
Thar Coal Overburden.
Overburden is the waste rock material overlying the Coal seams (layers of coal), also called spoil. It is removed to reach the coal seams and either dumped or used in reclaiming mined areas. As per reports, Thar Coal overburden consists of three kinds of material; dune sand, alluvium and sedimentary sequence.
Dune sand is an accumulation, mound or ridge of sand formed by wind. Winds are the driving force behind sand dunes that have so significantly shaped and modified the surface of land. Generally, active dunes (unvegetated dunes) have a gentle slope on their windward side. During wet periods, rate of dune migration commonly decreases, enables vegetation to encroach onto the margins of the sand mass, stabilizing the margins and inhibiting further movement along them.
Alluvium soil deposits are formed when the rivers slowly lose their carrying capacity due to decrease in velocity. While slowing down, a river loses its potential to hold the large soil particles in a suspended state and these particles thus settled dawn on the river bed or flood plains.
Sedimentary rocks are those rocks made up of pieces of other rocks. The sedimentary rocks we see today were once gravel, sand, silt, mud,or living things (Sea shales,Trees, Plants etc), carried by water or wind , characteristically laid down in strata which are initially horizontal or nearly so and deposited on the surface of the land.Sedimentary structures and mineralogy of a sandstone may distinguish whether it is of Aeolian (wind- blown dunes), fluvial (river- born) or marine origin.
As per reports, the Thar coal total overburden is around 150 to 230 meters. The various Coal seams start at around 150 meter depth and occur up to 215meters. The thickness of the lignite seams varies greatly from place to place in Bara formation and discontinuous, separated by silt or clay parting.
The roof above Coal seams and the floor rocks under Coal seams are clay stone and loose sandstone beds, making an envelope ( 50- 65 meter thick) for a deposit of discontinuous Coal seams. Creation of Ground waters/ Aquifers.
As discussed in the above link, the river systems of the past have several flow regimes associated with them. The most common is the surface flow. River channels have sometimes hundreds of feet of sand and sediment deposits. There is groundwater in these sands and this groundwater flows through this sand or even through subsurface bedrocks.
Another type of underground flow is when there is a subterranean cave system. Surface waters can disappear underground and flow through such caves. So the underground flow of the Ghaggar-Saraswati river is really groundwater flowing along the present and abandoned channels replenished by monsoon water from the channels and the surrounding plains. Some places this groundwater appears at the surface as springs.
Rates of groundwater flow vary depending on permeability of the aquifer and hydraulic gradient. So the groundwater system associated with the above rivers drainage is likely to be a series of disconnected aquifers being recharged primarily through monsoon discharge of the old rivers channels. Also the monsoon floods and repeated Sea inundation in the area affects the water table at various depths. Hypothetical Sketch of Thar Coal Aquifers
The generalized stratigraphic sequence in Thar Coalfield Blocks is shown in the above sketch. Drilling data has indicated three aquifers (water-bearing Zones) at an average
depth of 50 m, 120 m and more than 200 meters:
Ø One aquifer above the coal zone:
Ranges between 52.70 and 93.27 meters depth.
Ø Second aquifer with the coal zone at 120 meters depth:
Varying thickness up to 68.74 meters.
Ø Third aquifer below the coal zone at 200 metes depth:
Varying thickness up to 47 meters. Ø Water quality is brackish to salin
Chemical Composition of Thar coal: The weighted average chemical analysis of the coal samples of the four blocks
show variation and are as given below:
Moisture (%) 43.24 to 49.01
Ash (%) 5.18 to 6.56
Volatile Matter (%) 26.50 to 33.04
Fixed Carbon (%) 19.35 to 22.00
Sulphur(%) 0.92 to 1.34
Heating value (Btu/lb) As Received 5780 to 6398
Dry
10723 to 11353
DAF 11605 to 12613
MMM Free 6101 to 6841
The knowledge of the geology, geometry, Hydrology and the quality characteristics of the specific Coal deposit are of the utmost importance in designing it’s efficient exploitation.
Underground Coal Gasification:
History.
The prospect of recovering the energy of coal from deep or unmineable deposits through Underground Coal Gasification was first suggested by two German engineers, brothers Carl Werner and Carl Wilhelm Siemens, in 1868 over 144(hundred forty four) years ago.
The first experimental work on UCG was planned to start in 1912 in Durham, the U K, under the leadership of Nobel Prize winner Sir William Ramsay. However, he was unable to commence the UCG field work before the beginning of the World War I, and the project was abandoned.
In 1913, Ramsay's work, attracted the attention of Viladimir Lenin of Soviet Union who recommended the experimentation of underground coal gasification. Ultimately an extensive program of work carried out in various places of Soviet Union between a period of 1920-1955, but wasn't developed experimentally until the first half of the Twentieth Century in Soviet Russia. These activities resulted in several pilot plants and five industrial sized UCG plants in the 1960s, but efforts were abandoned as large natural gas discoveries made the process uneconomical. Today of these only the Yerostigaz plant Angren, owned by the Australian Linc
Energy in Uzbekistan remains.
On the basis of Soviet research work ,later the efforts for underground Coal gasification began in countries like USA, Britain, Poland etc., the criteria for the process was based on the following factors.
i) The minimum cumulative thickness of combustible coal shall be 10 feet and the individual combustible coal beds shall be separated by no more than 5 feet of non-combustibles( overburdens).
ii) The depth of overburden shall be more than 500 feet but less than 2000 feet, selected as the maximum depth of Soviet experiments.
iii) Overburden permeability should be substantially less than the natural permeability of the coal. iv) There shall be no fertile acquifers in the roof rock directly above the coals and the coal seam should not be a desolate acquifer.
v) Roof and floor rock material preferably shall be composed of shale or clay or relatively plastic rocks.vi) The topography at the project site should be suitable for drilling holes and for the installation of surface facilities. vii) The proposed site should be sufficiently remote to avoid interference with neighboring operators.
The Past and present Underground Coal Gasification Pilot Projects activities.
· Underground gasification was trialed in the UK during the 1950s at near Bayton, near Cleobury Mortimer in Worcestershire, but was later abandoned – and at the time questions were raised in British Parliament about the environmental impacts on communities up to 10 miles away.
· The United States of America had studied the concept of UCG during1970’s because of energy crises. In USA Livermore’s 1970s test site at Hoe Creek, Wyoming, unfortunately resulted in contaminated groundwater, as did one pilot in Carbon County, Wyoming. At Hoe Creek, operation of the burn cavity at pressures higher than that in the surrounding rock strata pushed contaminants away from the cavity, which introduced benzene, a carcinogen, in potable groundwater. Since these problematic tests in the 1970s,most of the projects have served to identify various risks and problems: groundwater contamination, leaking byproducts like benzene, seismic instability, and other issues
· The costs of electricity produced with UCG based syngas were estimated in the study of the University of Indiana USA. The average cost of electricity production in 2010 in the state of Indiana according to the report was 5.7 cents per kWh, which at present makes exploitation of UCG in Indiana economically difficult since only seams up to 3.5 meters thick are available at depths greater than 200 meters. The number of holes drilled and depth and thickness of coal seams ( thin coal seams exhaust quickly thereby terminating the gasification process after short intervals and considered uneconomical for UCG like most of European deposits) also a key cost effecting factor. So the 3-3.5 meter thin coal seams are left unused in UCG process.
· Syn gas can be burned as it is, but is a relatively dirty fuel in its raw state due to presence of excessive CO2 that is why expensive carbon captured and storage ( CCS) techniques are being proposed with UCG.Carbon Capture and Storage (animation).
Leakage of CO2 out of reservoir is a major safety concern. Even stable rock formations shift in earthquakes. Seismic activity presents a danger of undoing all that sequestration. Because CO2 is denser than air, when it leaks out of the ground it forms an invisible, undetectable cloud that pools near the ground and displaces the oxygen, suffocating any life nearby.
As per report, this has happened naturally and given us a vision of what could occur:
in 1986, Lake Nyos in Cameroon released a large amount of CO2, silently killing nearly two thousand people and a large number of livestock as shown above.
· It is alleged that once extracted, Syn gas can also be liquefied, allowing it to be used as feedstock for gas-to-liquid processes like diesel etc. China’s Coal to Liquids Program have been prohibited in the United States. Producing oil from coal is a technology that has been around for a long time. Germany used it to fuel its tanks and aircraft during World War II and South Africa is using it today to provide about 30 percent of its gasoline and diesel supply. But as reported (June 29, 2011) Institute for Energy Research, United States have stymied coal to liquid plants and shut out of that market for military use because it is argued that Coal to Liquids process life cycle greenhouse gas emissions would be higher than that of conventional oil and has proven a lucrative & expensive business.
· As per report Sydney (Platts)--8Jul 2011,the Queensland state government has permanently shut down Cougar Energy's underground coal gasification pilot plant at Kingaroy in eastern Australia.
The Department of Environment and Resource Management said it had advised Cougar that only "rehabilitation and monitoring" could be conducted at the project. "No further underground coal gasification will be permitted at the site," the DERM added.
The Kingaroy underground coal gasification pilot plant was ordered closed in July 2010 after water quality tests found two isolated readings of 2 parts per billion of benzene in a nearby groundwater monitoring bore.
Another environmental concern is that the void created by gasification may cause the land surface to subside. Subsidence is likely to be more of a problem if gasification occurs in a shallow coal seam, closer to the surface.
Brief About UCG Process & Problems Encountering in View of Thar Coal Deposits.
Gasifying coal underground involves drilling two wells into a suitable coal seam, igniting the coal at one well and pumping in air or oxygen enriched air. Because the amount of oxygen available in air is limited, the coal only partially burns to provide heat and carbon monoxide will be the main combustion product which can then be piped to the surface and burned there.
Obviously this is not going to be a particularly efficient process since much of the energy liberated will go into heating up rocks underground. In order to try to make the process a bit more efficient,controlled amount of water is usually injected along with the air/oxygen. When it comes in contact with the superheated rock some of that heat can be used to split the water (H20) into hydrogen and oxygen which react with more of the coal and additional reactions such as below.
CO + 3H2 < = > CH4 + H2O + Q( HEAT)
CO2 + 4H2 < = > CH4 + 2H2O + Q(HEAT)
Controlled amount of water injection keeps the reactions to produce a mixture of combustible gases, including hydrogen, butane, methane, carbon monoxide and carbon dioxide.
This mixture of gases is known as syn gas which is removed from the second well and is quite similar to the town gas that was made from coal during the 19thand much of the 20th century in America and Europe. The above method of introducing steam, somewhat increases the efficiency of the process but as per reports even so around 25-40 percent of the energy embodied in the coal is lost in heating surrounding rocks during the Underground coal gasification process.
The underground coal gasification is a complex physico- chemical process and as per reports, water is unavoidably added into the reaction zone by leakage from surrounding formations. The rate of ground water influx into the gasification zone has an important influence on the reactions and the product gas. With higher level of natural water specially aquifer’s water intrusion, theheating value of the product decreases and becomes inconsistent (by increasing the CO2 concentration at the expense of CO and H2) and making ituneconomical to utilize even at source/site for power generation.
Energy that enters into process (coal, oxidizer) is equal to output energy (syngas) and residual energy (unburned coal, ash, tar). Control of expenditure and energy consumption (compliance with the law of conservation of energy) is based on material balance and heat balance of the process which is used for the purpose of tracking temperature changes. Heat balance allows us to track the impact of temperature changes on the gasification process and the proportion of obtained energy in the form of the gas from the process. Specifically the presence of such pressurized aquifers surrounding Coal Seams, does not permit Material and Heat (Energy) balance calculations, essential to run the process economically.
Conclusion and Suggestions.
· In 1961 Soviet era, an experimental UCG facility near Angren Uzbekistan, commenced gasifying a sub-bituminous coal seam at a depth of between 100 and 250 meters ranging 3-24 meter thick. It is the only longest operating UCG facility. The results obtained from specific Underground Coal Gassification Angren field Uzbekistan and Escom South Africa, can not be duplicated in Thar lignite field because of the adverse geological and hydrological conditions at Thar Coal, discussed above.
· As the UCG process progresses, the chamber fills with rubble, ash and char. When the process reaches the roof of the coal seam, more of the barren overburden will be exposed, Clearly this is not going to be a particularly efficient process since much of the energy liberated will go into heating up rocks.
· One of the most serious problems in Underground Coal Gasification specifically like Thar Coal deposit, is the presence of pressurized Aquifers. If there is too much water, underground fires won’t be able to burn for long as these will be doused by the underground water lead to failure of the project.
· For any combustion process, the material and heat balance calculations are carried out in order to carry out the thermal efficiency audit of the process essential to run the process economically which is not possible under the prevailing hydrological conditions at Thar coal deposits.
· Under prevailing geological and hydrological conditions of Thar Coal deposits, discussed above, sustainable & efficient production of Syn gas with required amount of heating value for power generation is a question mark and need to be reviewed.
· In view of the above discussion, Open-cast or Open pit mining is an appropriate option which allows maximum and efficient utilization of such kind of lignite deposits as Thar Coal.
