Coal gasification is the process of producing syngas–a mixture consisting primarily of carbon monoxide (CO), hydrogen (H2), carbon dioxide (CO2), natural gas (CH4) , and water vapour (H2O)–from coal and water, air and/or oxygen.
Historically, coal was gasified to produce coal gas, also known as "town gas". Coal gas is combustible and was used for municipal lighting, and heating, before the advent of large scale production of natural gas from oil wells.
In current practice, large-scale coal gasification installations are primarily for electricity generation, or for production of chemical feedstocks. The hydrogen obtained from coal gasification can be used for various purposes such as making ammonia, powering a hydrogen economy, or upgrading fossil fuels.
Alternatively, coal-derived syngas can be converted into transportation fuels such as gasoline and diesel through additional treatment, or into methanol which itself can be used as transportation fuel or fuel additive, or which can be converted into gasoline.
Natural gas from coal gasification can be cooled until it liquifies for use as a fuel in the transport sector.
Historically, coal was gasified to produce coal gas, also known as "town gas". Coal gas is combustible and was used for municipal lighting, and heating, before the advent of large scale production of natural gas from oil wells.
In current practice, large-scale coal gasification installations are primarily for electricity generation, or for production of chemical feedstocks. The hydrogen obtained from coal gasification can be used for various purposes such as making ammonia, powering a hydrogen economy, or upgrading fossil fuels.
Alternatively, coal-derived syngas can be converted into transportation fuels such as gasoline and diesel through additional treatment, or into methanol which itself can be used as transportation fuel or fuel additive, or which can be converted into gasoline.
Natural gas from coal gasification can be cooled until it liquifies for use as a fuel in the transport sector.
History
In
the past, coal was converted to make coal gas, which was piped to
customers to burn for illumination, heating, and cooking. High prices of
oil and natural gas are leading to increased interest in "BTU
Conversion" technologies such as gasification, methanation and liquefaction. The Synthetic Fuels Corporation
was a U.S. government-funded corporation established in 1980 to create a
market for alternatives to imported fossil fuels (such as coal
gasification). The corporation was discontinued in 1985.
Early history of coal gas production by carbonization
The Flemish scientist Jan Baptista van Helmont used the name "gas" in his Origins of Medicine (c. 1609) to describe his discovery of a "wild spirit" which escaped from heated wood and coal, and which "differed little from the chaos of the ancients". Similar experiments were carried out in 1681 by Johann Becker of Munich and in 1684 by John Clayton of Wigan, England. The latter called it "Spirit of the Coal". William Murdoch (later known as Murdock) discovered new ways of making, purifying and storing gas. Among others, he illuminated his house at Redruth and his cottage at Soho, Birmingham in 1792, the entrance to the Manchester Police Commissioners premises in 1797, the exterior of the factory of Boulton and Watt in Birmingham, and a large cotton mill in Salford, Lancashire in 1805.
Professor Jan Pieter Minckeleers lit his lecture room at the University of Louvain in 1783 and Lord Dundonald lit his house at Culross, Scotland, in 1787, the gas being carried in sealed vessels from the local tar works. In France, Philippe le Bon
patented a gas fire in 1799 and demonstrated street lighting in 1801.
Other demonstrations followed in France and in the United States, but,
it is generally recognized that the first commercial gas works was built
by the London and Westminster Gas Light and Coke Company in Great Peter Street in 1812 laying wooden pipes to illuminate Westminster Bridge with gas lights on New Year's Eve in 1813. In 1816, Rembrandt Peale and four others established the Gas Light Company of Baltimore, the first manufactured gas company in America. In 1821, natural gas was being used commercially in Fredonia, New York.
The first German gas works was built in Hannover in 1825 and by 1870
there were 340 gas works in Germany making town gas from coal, wood,
peat and other materials.
Working conditions in the Gas Light and Coke Company's Horseferry Road Works, London, in the 1830s were described by a French visitor, Flora Tristan, in her Promenades Dans Londres:
Two rows of furnaces on each side were fired up; the effect was not unlike the description of Vulcan's forge, except that the Cyclops were animated with a divine spark, whereas the dusky servants of the English furnaces were joyless, silent and benumbed.... The foreman told me that stokers were selected from among the strongest, but that nevertheless they all became consumptive after seven or eight years of toil and died of pulmonary consumption. That explained the sadness and apathy in the faces and every movement of the hapless men.
The first public piped gas supply was to 13 gas lamps, each with three glass globes along the length of Pall Mall, London in 1807. The credit for this goes to the inventor and entrepreneur Fredrick Winsor and the plumber Thomas Sugg, who made and laid the pipes. Digging up streets to lay pipes required legislation and this delayed the development of street lighting and gas for domestic use. Meanwhile, William Murdoch and his pupil Samuel Clegg were installing gas lighting in factories and work places, encountering no such impediments.
Early history of coal gas production by gasification
In the 1850s every small to medium-sized town and city had a gas
plant to provide for street lighting. Subscribing customers could also
have piped lines to their houses. By this era, gas lighting became
accepted. Gaslight trickled down to the middle class and later came gas
cookers and stoves.
The 1860s were the golden age of coal gas development. Scientists like Kekulé and Perkin
cracked the secrets of organic chemistry to reveal how gas is made and
its composition. From this came better gas plants and Perkin's purple
dyes, such as Mauveine. In the 1850s, processes for making Producer gas and Water gas from coke were developed. Unenriched water gas may be described as Blue water gas (BWG).
Mond gas, developed in the 1850s by Ludwig Mond,
was producer gas made from coal instead of coke. It contained ammonia
and coal tar and was processed to recover these valuable compounds.
Blue water gas (BWG) burns with a non-luminous flame which makes
it unsuitable for lighting purposes. Carburetted Water Gas (CWG),
developed in the 1860s, is BWG enriched with gases obtained by spraying
oil into a hot retort. It has a higher calorific value and burns with a
luminous flame.
The carburetted water gas process was improved by Thaddeus S. C. Lowe
in 1875. The gas oil was fixed into the BWG via thermocracking in the
carburettor and superheater of the CWG generating set. CWG was the
dominant technology in the US from the 1880s until the 1950s, replacing
coal gasification. CWG has a CV of 20 MJ/m3 i.e. slightly more than half that of natural gas.
Development of the coal gas industry in the UK
The advent of incandescent gas lighting in factories, homes and in the streets, replacing oil lamps and candles with steady clear light, almost matching daylight in its colour, turned night into day for many—making night shift work possible in industries where light was all important—in spinning, weaving
and making up garments etc. The social significance of this change is
difficult for generations brought up with lighting after dark available
at the touch of a switch to appreciate. Not only was industrial
production accelerated, but streets were made safe, social intercourse
facilitated and reading and writing made more widespread. Gas works were
built in almost every town, main streets were brightly illuminated and
gas was piped in the streets to the majority of urban households. The
invention of the gas meter and the pre-payment meter in the late 1880s played an important role in selling town gas to domestic and commercial customers.
1934 gas cooker in England
The education and training of the large workforce, the attempts to
standardise manufacturing and commercial practices and the moderating of
commercial rivalry between supply companies prompted the founding of
associations of gas managers, first in Scotland in 1861. A British Association of Gas Managers was formed in 1863 in Manchester and this, after a turbulent history, became the foundation of the Institute of Gas Engineers (IGE). In 1903, the reconstructed Institution of Civil Engineers (ICE) initiated courses for students of gas manufacture in the City and Guilds of London Institute. The IGE was granted the Royal Charter
in 1929. Universities were slow to respond to the needs of the industry
and it was not until 1908 that the first Professorship of Coal Gas and
Fuel Industries was founded at the University of Leeds. In 1926, the Gas Light and Coke Company opened Watson House adjacent to Nine Elms Gas Works. At first, this was a scientific laboratory. Later it included a centre for training apprentices
but its major contribution to the industry was its gas appliance
testing facilities, which were made available to the whole industry,
including gas appliance manufacturers.
Using this facility, the industry established not only safety but also
performance standards for both the manufacture of gas appliances and
their servicing in customers' homes and commercial premises.
During World War I, the gas industry's by-products, phenol, toluene and ammonia and sulphurous compounds were valuable ingredients for explosives. Much coal
for the gas works was shipped by sea and was vulnerable to enemy
attack. The gas industry was a large employer of clerks, mainly male
before the war. But the advent of the typewriter and the female typist
made another important social change that was, unlike the employment of
women in war-time industry, to have long-lasting effects.
The inter-war years were marked by the development of the continuous vertical retort which replaced many of the batch fed horizontal retorts. There were improvements in storage, especially the waterless gas holder,
and distribution with the advent of 2–4 inch steel pipes to convey gas
at up to 50 psi (340 kPa) as feeder mains compared to the traditional cast iron pipes working at an average of 2–3 inches water gauge (500–750 Pa). Benzole as a vehicle fuel and coal tar as the main feedstock for the emerging organic chemical industry provided the gas industry with substantial revenues. Petroleum supplanted coal tar as the primary feedstock of the organic chemical industry after World War II and the loss of this market contributed to the economic problems of the gas industry after the war.
A wide variety of appliances and uses for gas developed over the years. Gas fires, gas cookers, refrigerators, washing machines, hand irons, pokers for lighting coal fires, gas-heated baths, remotely controlled clusters of gas lights, gas engines of various types and, in later years, gas warm air and hot water central heating and air conditioning,
all of which made immense contributions to the improvement of the
quality of life in cities and towns worldwide. The evolution of electric lighting made available from public supply extinguished the gas light, except where colour matching was practised as in haberdashery shops.
Process
Scheme of a Lurgi gasifier
During gasification, the coal is blown through with oxygen
and steam (water vapor) while also being heated (and in some cases
pressurized). If the coal is heated by external heat sources the process
is called "allothermal", while "autothermal" process assumes heating of
the coal via exothermal chemical reactions occurring inside the
gasifier itself. It is essential that the oxidizer supplied is
insufficient for complete oxidizing (combustion) of the fuel. During the
reactions mentioned, oxygen and water molecules oxidize the coal and produce a gaseous mixture of carbon dioxide (CO2), carbon monoxide (CO), water vapour (H2O), and molecular hydrogen (H2).
(Some by-products like tar, phenols, etc. are also possible end
products, depending on the specific gasification technology utilized.)
This process has been conducted in-situ within natural coal seams
(referred to as underground coal gasification) and in coal refineries. The desired end product is usually syngas (i.e., a combination of H2 + CO), but the produced coal gas may also be further refined to produce additional quantities of H2:
- 3C (i.e., coal) + O2 + H2O → H2 + 3CO
If the refiner wants to produce alkanes (i.e., hydrocarbons present in natural gas, gasoline, and diesel fuel),
the coal gas is collected at this state and routed to a Fischer-Tropsch
reactor. If, however, hydrogen is the desired end-product, the coal gas
(primarily the CO product) undergoes the water gas shift reaction where more hydrogen is produced by additional reaction with water vapor:
- CO + H2O → CO2 + H2
Although other technologies for coal gasification currently exist,
all employ, in general, the same chemical processes. For low-grade coals
(i.e., "brown coals") which contain significant amounts of water, there
are technologies in which no steam is required during the reaction,
with coal (carbon) and oxygen being the only reactants. As well, some
coal gasification technologies do not require high pressures. Some
utilize pulverized coal as fuel while others work with relatively large
fractions of coal. Gasification technologies also vary in the way the
blowing is supplied.
"Direct blowing" assumes the coal and the oxidizer being
supplied towards each other from the opposite sides of the reactor
channel. In this case the oxidizer passes through coke and (more likely)
ashes to the reaction zone where it interacts with coal. The hot gas
produced then passes fresh fuel and heats it while absorbing some
products of thermal destruction of the fuel, such as tars and phenols.
Thus, the gas requires significant refining before being used in the
Fischer-Tropsch reaction. Products of the refinement are highly toxic
and require special facilities for their utilization. As a result, the
plant utilizing the described technologies has to be very large to be
economically efficient. One of such plants called SASOL is situated in
the Republic of South Africa (RSA). It was built due to embargo applied
to the country preventing it from importing oil and natural gas. RSA is
rich in Bituminous coal and Anthracite and was able to arrange the use
of the well known high pressure "Lurgi" gasification process developed
in Germany in the first half of 20th century.
"Reversed blowing" (as compared to the previous type
described which was invented first) assumes the coal and the oxidizer
being supplied from the same side of the reactor. In this case there is
no chemical interaction between coal and oxidizer before the reaction
zone. The gas produced in the reaction zone passes solid products of
gasification (coke and ashes), and CO2 and H2O contained in the gas are additionally chemically restored to CO and H2.
As compared to the "direct blowing" technology, no toxic by-products
are present in the gas: those are disabled in the reaction zone. This
type of gasification has been developed in the first half of 20th
century, along with the "direct blowing", but the rate of gas production
in it is significantly lower than that in "direct blowing" and there
were no further efforts of developing the "reversed blowing" processes
until 1980-s when a Soviet research facility KATEKNIIUgol' (R&D
Institute for developing Kansk-Achinsk coal field) began R&D
activities to produce the technology now known as "TERMOKOKS-S" process.
The reason for reviving the interest in this type of gasification
process is that it is ecologically clean and able to produce two types
of useful products (simultaneously or separately): gas (either
combustible or syngas) and middle-temperature coke. The former may be
used as a fuel for gas boilers and diesel-generators or as syngas for
producing gasoline, etc., the latter - as a technological fuel in
metallurgy, as a chemical absorbent or as raw material for household
fuel briquettes. Combustion of the product gas in gas boilers is
ecologically cleaner than combustion of initial coal. Thus, a plant
utilizing gasification technology with the "reversed blowing" is able to
produce two valuable products of which one has relatively zero
production cost since the latter is covered by competitive market price
of the other. As the Soviet Union and its KATEKNIIUgol' ceased to exist,
the technology was adopted by the individual scientists who originally
developed it and is now being further researched in Russia and
commercially distributed worldwide. Industrial plants utilizing it are
now known to function in Ulaan-Baatar (Mongolia) and Krasnoyarsk
(Russia).
Pressurized airflow bed gasification technology created through the joint development between Wison Group
and Shell (Hybrid). For example: Hybrid is an advanced pulverized coal
gasification technology, this technology combined with the existing
advantages of Shell SCGP waste heat boiler, includes more than just a
conveying system, pulverized coal pressurized gasification burner
arrangement, lateral jet burner membrane type water wall, and the
intermittent discharge has been fully validated in the existing SCGP
plant such as mature and reliable technology, at the same time, it
removed the existing process complications and in the syngas cooler
(waste pan) and [fly ash] filters which easily failed, and combined the
current existing gasification technology that is widely used in
synthetic gas quench process. It not only retains the original Shell
SCGP waste heat boiler of coal characteristics of strong adaptability,
and ability to scale up easily, but also absorb the advantages of the
existing quench technology.
Underground coal gasification
Underground coal gasification (UCG) is an industrial gasification
process, which is carried out in non-mined coal seams. It involves
injection of a gaseous oxidizing agent,
usually oxygen or air, and bringing the resulting product gas to the
surface through production wells drilled from the surface. The product
gas can be used as a chemical feedstock or as fuel for power generation.
The technique can be applied to resources that are otherwise not
economical to extract. It also offers an alternative to conventional coal mining
methods. Compared to traditional coal mining and gasification, UCG has
less environmental and social impact, though environmental concerns
exist, including the potential for aquifer contamination.
Carbon capture technology
Carbon
capture, utilization, and sequestration (or storage) is increasingly
being utilized in modern coal gasification projects to address the
greenhouse gas emissions concern associated with the use of coal and
carbonaceous fuels. In this respect, gasification has a significant
advantage over conventional combustion of mined coal, in which CO2
resulting from combustion is considerably diluted by nitrogen and
residual oxygen in the near-ambient pressure combustion exhaust, making
it relatively difficult, energy-intensive, and expensive to capture the
CO2 (this is known as “post-combustion” CO2 capture).
In gasification,
on the other hand, oxygen is normally supplied to the gasifiers and
just enough fuel is combusted to provide the heat to gasify the rest;
moreover, gasification is often performed at elevated pressure. The
resulting syngas is typically at higher pressure and not diluted by
nitrogen, allowing for much easier, efficient, and less costly removal
of CO2. Gasification and integrated gasification combined cycle’s unique ability to easily remove CO2 from the syngas prior to its combustion in a gas turbine (called "pre-combustion" CO2
capture) or its use in fuels or chemicals synthesis is one of its
significant advantages over conventional coal utilization systems.
CO2 capture technology options
All coal gasification-based conversion processes require removal of hydrogen sulfide (H2S;
an acid gas) from the syngas as part of the overall plant
configuration. Typical acid gas removal (AGR) processes employed for
gasification design are either a chemical solvent system (e.g., amine gas treating systems based on MDEA, for example) or a physical solvent system (e.g., Rectisol or Selexol).
Process selection is mostly dependent on the syngas cleanup requirement
and costs. Conventional chemical/physical AGR processes using MDEA,
Rectisol or Selexol are commercially proven technologies and can be
designed for selective removal of CO2 in addition to H2S from a syngas stream. For significant capture of CO2 from a gasification plant (e.g. > 80%) the CO in the syngas must first be converted to CO2 and hydrogen (H2) via a water-gas-shift (WGS) step upstream of the AGR plant.
For gasification applications, or Integrated Gasification
Combined Cycle (IGCC), the plant modifications required to add the
ability to capture CO2 are minimal. The syngas produced by
the gasifiers needs to be treated through various processes for the
removal of impurities already in the gas stream, so all that is required
to remove CO2 is to add the necessary equipment, an absorber and regenerator, to this process train.
In combustion applications, modifications must be made to the exhaust stack and because of the lower concentrations of CO2 present in the exhaust, much larger volumes of total gas require processing, necessitating larger and more expensive equipment.
IGCC (Integrated Gasification Combined Cycle) based projects in the United States with CO2 capture and use/storage
Mississippi Power’s Kemper Project
is in late stages of construction. It will be a lignite-fuel IGCC
plant, generating a net 524 MW of power from syngas, while capturing
over 65% of CO2 generated using the Selexol
process. The technology at the Kemper facility, Transport-Integrated
Gasification (TRIG), was developed and is licensed by KBR. The CO2 will be sent by pipeline to depleted oil fields in Mississippi for enhanced oil recovery operations.
Hydrogen Energy California
(HECA) will be a 300MW net, coal and petroleum coke-fueled IGCC
polygeneration plant (producing hydrogen for both power generation and
fertilizer manufacture). Ninety percent of the CO2 produced will be captured (using Rectisol) and transported to Elk Hills Oil Field for EOR, enabling recovery of 5 million additional barrels of domestic oil per year.
Summit’s Texas Clean Energy Project
(TCEP) will be a coal-fueled, IGCC-based 400MW power/polygeneration
project (also producing urea fertilizer), which will capture 90% of its
CO2 in pre-combustion using the Rectisol process. The CO2 not used in fertilizer manufacture will be used for enhanced oil recovery in the West Texas Permian Basin.
Plants such as the Texas Clean Energy Project which employ carbon capture and storage
have been touted as a partial, or interim, solution to climate change
issues if they can be made economically viable by improved design and
mass production. There has been opposition from utility regulators and
ratepayers due to increased cost; and from environmentalists such as Bill McKibben, who view any continued use of fossil fuels as counterproductive.
By-products
The by-products of coal gas manufacture included coke, coal tar, sulfur and ammonia; all useful products. Dyes, medicines, including sulfa drugs, saccharin and many organic compounds are therefore derived from coal gas.
Coke is used as a smokeless fuel and for the manufacture of water gas and producer gas. Coal tar is subjected to fractional distillation to recover various products, including
- tar, for road surfacing
- benzole, a motor fuel
- creosote, a wood preservative
- phenol, used in the manufacture of plastics
- cresols, disinfectants
Sulfur is used in the manufacture of sulfuric acid and ammonia is used in the manufacture of fertilisers.
Commercialization
According to the Gasification and Syngas Technologies Council, a
trade association, there are globally 272 operating gasification plants
with 686 gasifiers and 74 plants with 238 gasifiers under construction.
Most of them use coal as feedstock.
As of 2017 large scale expansion of the coal gasification
industry was occurring only in China where local governments and energy
companies promote the industry to provide jobs and a market for coal.
For the most part, the plants are located in remote, coal-rich areas.
The central government is aware of the conflicts with
environmental goals: in addition to producing a great deal of carbon
dioxide, the plants use a great deal of water in areas where water is
scarce.
Environmental impact
Environmental impact of manufactured coal gas industry
From its original development until the wide-scale adoption of
natural gas, more than 50,000 manufactured gas plants were in existence
in the United States alone. The process of manufacturing gas usually produced a number of by-products that contaminated the soil and groundwater in and around the manufacturing plant, so many former town gas plants are a serious environmental
concern, and cleanup and remediation costs are often high. Manufactured
gas plants (MGPs) were typically sited near or adjacent to waterways
that were used to transport in coal and for the discharge of wastewater
contaminated with tar, ammonia and/or drip oils, as well as outright
waste tars and tar-water emulsions.
In the earliest days of MGP operations, coal tar was considered a
waste and often disposed into the environment in and around the plant
locations. While uses for coal tar developed by the late-19th century,
the market for tar varied and plants that could not sell tar at a given
time could store tar for future use, attempt to burn it as boiler fuel,
or dump the tar as waste. Commonly, waste tars were disposed of in old
gas holders, adits or even mine shafts (if present). Over time, the
waste tars degrade with phenols, benzene (and other mono-aromatics – BTEX) and polycyclic aromatic hydrocarbons released as pollutant plumes that can escape into the surrounding environment. Other wastes included "blue billy", which is a ferroferricyanide compound—the blue colour is from Prussian blue, which was commercially used as a dye.
Blue billy is typically a granular material and was sometimes sold
locally with the strap line "guaranteed weed free drives". The presence
of blue billy can give gas works waste a characteristic musty/bitter almonds or marzipan smell which is associated with cyanide gas.
The shift to the Carburetted Water Gas process initially resulted
in a reduced output of water gas tar as compared to the volume of coal
tars. The advent of automobiles reduced the availability of naphtha for
carburetion oil, as that fraction was desirable as motor fuel. MGPs that
shifted to heavier grades of oil often experienced problems with the
production of tar-water emulsions, which were difficult, time consuming,
and costly to break. (The cause of tar change
water emulsions is complex and was related to several factors, including
free carbon in the carburetion oil and the substitution of bituminous
coal as a feedstock instead of coke.) The production of large volumes of
tar-water emulsions quickly filled up available storage capacity at
MGPs and plant management often dumped the emulsions in pits, from which
they may or may not have been later reclaimed. Even if the emulsions
were reclaimed, the environmental damage from placing tars in unlined
pits remained. The dumping of emulsions (and other tarry residues such
as tar sludges, tank bottoms, and off-spec tars) into the soil and
waters around MGPs is a significant factor in the pollution found at
former manufactured gas plants (known as "FMGPs" in environmental remediation) today.
Contaminants commonly associated with FMGPs include:
- BTEX
- Diffused out from deposits of coal/gas tars
- Leaks of carburetting oil/light oil
- Leaks from drip pots, that collected condensible hydrocarbons from the gas
- Coal tar waste/sludge
- Typically found in sumps of gas holders and decanting ponds.
- Coal tar sludge has no resale value and so was always dumped.
- Volatile organic compounds
- Polycyclic aromatic hydrocarbons (PAHs)
- Present in coal tar, gas tar, and pitch at significant concentrations.
- Heavy metals
- Leaded solder for gas mains, lead piping, coal ashes.
- Cyanide
- Purifier waste has large amounts of complex ferrocyanides in it.
- Lampblack
- Only found where crude oil was used as gasification feedstock.
- Tar emulsions
Coal tar and coal tar sludges are frequently denser than water and are present in the environment as a dense non-aqueous phase liquid.
In the UK, a number of former gasworks sites have been redeveloped for residential and other uses (including the Millennium Dome),
being seen as prime developable land within the confines of city
boundaries. Such development opportunities are now leading to problems
associated with planning and the Contaminated Land Regime and have
recently been debated in the House of Commons.
Environmental impact of modern coal gasification
Coal gasification processes require controls and pollution prevention measures to mitigate pollutant emissions. Pollutants or emissions of concern in the context of coal gasification include primarily:
- Ash & slag
Non-slagging gasifiers produce dry ash similar to that produced by
conventional coal combustion, which can be an environmental liability if
the ash (typically containing heavy metals) is leachable or caustic,
and if the ash must be stored in ash ponds. Slagging gasifiers, which
are utilized at many of the major coal gasification applications
worldwide, have considerable advantage in that ash components are fused
into a glassy slag, capturing trace heavy metals in the non-leachable
glassy matrix, rendering the material non-toxic. This non-hazardous slag
has multiple beneficial uses such as aggregate in concrete, aggregate
in asphalt for road construction, grit in abrasive blasting, roofing
granules, etc.
- Carbon dioxide (CO2)
CO2 is of paramount importance in global climate change.
- Mercury
- Arsenic
- Particulate matter (PM)
Ash is formed in gasification from inorganic impurities in the coal.
Some of these impurities react to form microscopic solids which can be
suspended in the syngas produced by gasification.
- Sulfur dioxide (SO2)
Typically coal contains anywhere from 0.2 to 5 percent sulfur by dry weight, which converts to H2S
and COS in the gasifiers due to the high temperatures and low oxygen
levels. These "acid gases" are removed from the syngas produced by the
gasifiers by acid gas removal equipment prior to the syngas being burned
in the gas turbine to produce electricity, or prior to its use in fuels
synthesis.
- Nitrogen oxides (NOx)
(NOx) refers to nitric oxide (NO) and nitrogen dioxide (NO2).
Coal usually contains between 0.5 and 3 percent nitrogen on a dry
weight basis, most of which converts to harmless nitrogen gas. Small
levels of ammonia and hydrogen cyanide are produced, and must be removed
during the syngas cooling process. In the case of power generation, NOx also can be formed downstream by the combustion of syngas in turbines.
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