Flue gas desulfurization is commonly known as FGD and is the technology used for removing
sulfur dioxideSulfur dioxide is the chemical compound with the formula SO
2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...
(SO
2) from the exhaust
flue gasesFlue gas emissions from fossil fuel combustion refers to the combustion product gas resulting from the burning of fossil fuels . Most fossil fuels are combusted with ambient air...
in
power plantsA fossil-fuel power plant is a power plant that burns fossil fuels such as coal, natural gas or petroleum to produce electricity.Fossil-fuel power plants are designed on a large scale for continuous operation...
that burn coal or oil to produce steam for the steam turbines that drive their electricity generators.
Sulfur dioxide is responsible for
acid rainAcid rain is rain or any other form of precipitation that is unusually acidic, i.e. elevated levels of hydrogen ions . It has harmful effects on plants, aquatic animals, and infrastructure. Acid rain is mostly caused by emissions of compounds of sulfur, nitrogen, and carbon which react with the...
formation. Tall
flue gas stackA flue gas stack is a type of chimney, a vertical pipe, channel or similar structure through which combustion product gases called flue gases are exhausted to the outside air. Flue gases are produced when coal, oil, natural gas, wood or any other fuel is combusted in an industrial furnace, a power...
s disperse the emissions by diluting the pollutants in ambient air and transporting them to other regions.
In 1988 Prince Charles of Britain said, "
I would have thought that the Central Electricity Generating Board was doing too little and too late. Our responsibilities do lie in not exporting our problems abroad." His words highlighted the fact that sulfur dioxide exhaust should be removed, rather than emitted high into the atmosphere where it affects many more people. A number of countries now have regulations limiting the height of flue gas stacks.
As a result of stringent environmental protection regulations regarding SO
2 emissions that have been enacted in a great many countries, SO
2 is now being removed from flue gases by a variety of methods, with the following being the most common:
- Wet scrubbing
Wet scrubber is a form of pollution control technology. The term describes a variety of devices that remove pollutants from a furnace flue gas or from other gas streams...
using a slurry of alkaline sorbentA sorbent is a material used to absorb liquids or gases. Examples include:*A material similar to molecular sieve material. It has a large internal surface area and good thermal conductivity. It is typically supplied in pellets of 1 mm to 2 mm diameter and roughly 5 mm length or as grains of the...
, usually limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
or limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
, or seawaterSeawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5%. This means that every of seawater has approximately of dissolved salts...
to scrub the gases.
- Spray-dry
Spray drying is a method of producing a dry powder from a liquid or slurry by rapidly drying with a hot gas. This is the preferred method of drying of many thermally-sensitive materials such as foods and pharmaceuticals. A consistent particle size distribution is a reason for spray drying some...
scrubbing using similar sorbent slurries.
- Wet sulfuric acid process
The wet sulfuric acid process is one out of many gas desulfurization processes on the market today. Since its introduction in the 1980s, where it was patented by the Danish catalyst company Haldor Topsøe A/S, it has been recognised as an efficient process for recovering sulfur from various process...
recovering sulfur in the form of commercial quality sulfuric acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
- Dry sorbent injection systems.
For a typical coal-fired power station, FGD will remove 95 percent or more of the SO
2 in the flue gases.
History
Methods for removing sulfur dioxide from boiler and furnace exhaust gases have been studied for over 150 years. Early concepts useful for flue gas desulfurization appear to have germinated in England circa 1850.
With the construction of large scale power plants in England in the 1920s, the problems associated with large volumes of SO
2 from a single site began to concern the public. The SO
2 emissions problem did not receive much attention until 1929, when the House of Lords upheld the claim of a landowner against the Barton Electricity Works of the Manchester Corporation for damages to his land resulting from SO
2 emissions. Shortly thereafter a press campaign was launched against the erection of power plants within the confines of London. This outcry led to the imposition of SO
2 controls on all such power plants.
During this period, major FGD installations went into operation in the United Kingdom at the
Battersea Power StationBattersea Power Station is a now unused coal-fired power station located on the south bank of the River Thames, near Battersea in London. The station comprises two individual power stations, built in two stages in the form of a single building. Battersea A Power Station was built first in the...
, the Swansea Power Station, and the
Fulham Power StationFulham Power Station was a coal-fired power station on the north bank of the River Thames at Battersea Reach in Fulham, London, not to be confused with Lots Road Power Station, a mile or so downstream.- History :...
. The first major FGD installation on a utility was put into operation at the Battersea Station, owned by the London Power Company, in 1931. In 1935, an FGD system similar to that installed at Battersea went into service at the Swansea Power Station. The third major FGD system was installed in 1938 at the Fulham Power Station. These three early large-scale FGD installations were abandoned during World War II. Large-scale FGD units did not reappear in commercial operation at utilities until the 1970s, where most of the activity occurred in the United States and Japan.
As of June 1973, there were 42 FGD units, ranging in size from 5 MW to 250 MW, in operation: 36 in Japan and 6 in the United States. As of about 1999-2000, FGD units were being used in 27 countries and there were 678 FGD units operating on a total power plant capacity of about 229 gigawatts. About 45% of that FGD capacity was in the U.S., 24% in Germany, 11% in Japan and 20% in various other countries. Approximately 79% of the units, representing about 199 gigawatts of capacity were using lime or limestone wet scrubbing. About 18% (or 25 gigawatts) utilized spray-dry scrubbers or sorbent injection systems.
Sulfuric acid mist formation
Fossil fuelFossil fuels or mineral fuels are fuels formed by natural resources such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
s such as coal and oil contain significant amounts of sulfur. When burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (SO
2). This happens under normal conditions of temperature and of oxygen present in the flue gas. However, there are circumstances under which this may not be the case.
For example, when the flue gas has too much oxygen and the SO
2 is further oxidized to
sulfur trioxideSulfur trioxide is the chemical compound with the formula SO3. In the gaseous form, this species is a significant pollutant, being the primary agent in acid rain...
(SO
3). Actually, too much oxygen is only one of the ways that SO
3 is formed. Gas temperature is also an important factor. At about 800 °C, formation of SO
3 is favoured. Another way that SO
3 can be formed is through catalysis by metals in the fuel. This is particularly true for heavy fuel oil, where significant amounts of
vanadiumVanadium is the chemical element with the symbol V and atomic number 23. It is a soft, silvery grey, ductile transition metal. The formation of an oxide layer stabilizes the metal against oxidation. Andrés Manuel del Río discovered vanadium in 1801 by analyzing the mineral vanadinite, and named it...
are present. In whatever way that SO
3 is formed, it does not behave like SO
2 in that it forms a liquid
aerosolTechnically, an aerosol is a suspension of fine solid particles or liquid droplets in a gas. Examples are smoke, oceanic haze, air pollution, smog and CS gas. In general conversation, aerosol usually refers to an aerosol spray can or the output of such a can...
known as
sulfuric acidSulfuric acid, , is a strong mineral acid. It is soluble in water at all concentrations. Sulfuric acid has many applications, and is one of the top products of the chemical industry. World production in 2001 was 165 million tonnes, with an approximate value of US$8 billion...
(H
2SO
4) mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to SO
3. Sulfuric acid mist is often the cause of the blue haze that often appears as the flue gas plume dissipates. Increasingly, this problem is being addressed by the use of wet
electrostatic precipitatorAn electrostatic precipitator , or electrostatic air cleaner is a particulate collection device that removes particles from a flowing gas using the force of an induced electrostatic charge...
s.
Basic principles
Most FGD systems employ two stages: one for
fly ashFly ash is one of the residues generated in the combustion of coal. Fly ash is generally captured from the chimneys of coal-fired power plants, and is one of two types of ash that jointly are known as coal ash; the other, bottom ash, is removed from the bottom of coal furnaces...
removal and the other for SO
2 removal. Attempts have been made to remove both the fly ash and SO
2 in one scrubbing vessel. However, these systems experienced severe maintenance problems and low simultaneous removal efficiencies. In wet scrubbing systems the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a wet scrubber, and then into the SO
2 absorber. However, in dry injection or spray drying operations, the SO
2 is first reacted with the sorbent and then the flue gas passes through a particulate control device.
Another important design consideration associated with wet FGD systems is that the flue gas
exiting the absorber is saturated with water and still contains some SO
2. (No system is 100%
efficient.) Therefore, these gases are highly corrosive to any downstream equipment - i.e.,
fans, ducts, and stacks. Two methods that minimize corrosion are: (1) reheating the gases to
above their
dew pointThe dew point is the temperature to which a given parcel of air must be cooled, at constant barometric pressure, for water vapor to condense into water. The condensed water is called dew. The dew point is a saturation point...
and (2) choosing construction materials and design conditions that
allow equipment to withstand the corrosive conditions. The selection of a reheating method
or the decision not to reheat (thereby requiring the use of special construction materials) are
very controversial topics connected with FGD design. Both alternatives are
expensive and must be considered on a by-site basis.
Scrubbing with a basic solid or solution
SO
2 is an acid gas and thus the typical sorbent slurries or other materials used to remove the SO
2 from the flue gases are alkaline. The reaction taking place in wet scrubbing using
a CaCO
3 (
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
) slurry produces CaSO
3 (calcium sulfite) and can be expressed as:
- CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)
When wet scrubbing with a Ca(OH)
2 (lime) slurry, the reaction also produces CaSO
3 (
calcium sulfiteCalcium sulfite, or calcium sulphite, is a chemical compound which is the salt of calcium cation and sulfite anion with the molecular formula CaSO
3. As a food additive it is used as a preservative under the E number E226...
) and can be expressed as:
- Ca(OH)2 (solid) + SO2 (gas) → CaSO3 (solid) + H2O (liquid)
When wet scrubbing with a Mg(OH)
2 (
magnesium hydroxideMagnesium hydroxide is an inorganic compound with the chemical formula Mg2. As a suspension in water, it is often called milk of magnesia because of its milk-like appearance. The solid mineral form of magnesium hydroxide is known as brucite.Magnesium hydroxide is common component of...
) slurry, the reaction produces MgSO
3 (
magnesium sulfiteMagnesium sulfite is a chemical compound containing magnesium and sulfite, with the formula . Its most common hydrated form has 6 water molecules making it a hexahydrate, ·6. When heated above , it is dehydrated to magnesium sulfite trihydrate, or ·3. The anhydrous form is hygroscopic, meaning that...
) and can be expressed as:
- Mg(OH)2 (solid) + SO2 (gas) → MgSO3 (solid) + H2O (liquid)
To partially offset the cost of the FGD installation, in some designs, the CaSO
3 (calcium sulfite) is further oxidized to produce marketable CaSO
4 · 2H
2O (
gypsumGypsum is a very soft mineral composed of calcium sulfate dihydrate, with the chemical formula CaSO
4·2H
2O.-Crystal varieties:...
). This technique is also known as
forced oxidation:
- CaSO3 (solid) + H2O (liquid) + ½O2 (gas) → CaSO4 (solid) + H2O
A natural alkaline usable to absorb SO
2 is seawater. The SO
2 is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO
4- and free H
+. The surplus of H
+ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO
2 gas:
- SO2 (gas) + H2O + ½O2 (gas)→ SO42- (solid) + 2H+
- HCO3- + H+ → H2O + CO2 (gas)
Types of wet scrubbers used in FGD
To promote maximum
gas-liquid surface areaAn important parameter in wet scrubbing systems is the rate of liquid flow. It is common in wet scrubber terminology to express the liquid flow as a function of the gas flow rate that is being treated...
and residence time, a number of wet scrubber designs have been used in wet FGD systems, including spray towers, venturis, plate towers, and mobile packed beds. Because of scale buildup, plugging,
or erosion, which affect FGD dependability and absorber efficiency, the trend is to use simple scrubbers such as spray towers instead of more complicated ones. The configuration of the tower may be vertical or horizontal, and flue gas can flow cocurrently, countercurrently, or crosscurrently with respect to the liquid. The chief drawback of spray towers is that they require a higher liquid-to-gas ratio requirement for equivalent SO2 removal than other absorber designs.
Venturi-rod scrubbers
A
venturi scrubberA venturi scrubber is designed to effectively use the energy from the inlet gas stream to atomize the liquid being used to scrub the gas stream...
is a converging/diverging section of duct. The converging section accelerates the gas stream to high velocity. When the liquid stream is injected at the throat, which is the point of maximum velocity, the turbulence caused by the high gas velocity atomizes the liquid into small droplets, which creates the surface area necessary for mass transfer to take place. The higher the pressure drop in the venturi, the smaller the droplets and the higher the surface area. The penalty is in power consumption.
For simultaneous SO
2 and fly ash removal, venturi scrubbers can be used. In fact, many of the industrial sodium-based throwaway systems are venturi scrubbers originally designed to remove particulate matter. These units were slightly modified to inject a sodium-based scrubbing liquor. Although removal of both particles and SO
2 in one vessel can be economically attractive, the problems of high pressure drops and finding a scrubbing medium to remove heavy loadings of fly ash must be considered. However, in cases where the particle concentration is low, such as from oil-fired units, simultaneous particulate and SO
2 emission reduction can be effective.
Packed bed scrubbers
A packed scrubber consists of a tower with packing material inside. This packing material can be in the shape of saddles, rings or some highly specialized shapes designed to maximize contact area between the dirty gas and liquid. Packed towers typically operate at much lower pressure drops than venturi scrubbers and are therefore cheaper to operate. They also typically offer higher SO
2 removal efficiency. The drawback is that they have a greater tendency to plug up if particles are present in excess in the exhaust air stream.
Spray towers
A
spray towerSpray towers or spray chambers are a form of pollution control technology. They consist of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed...
is the simplest type of scrubber. It consists of a tower with spray nozzles, which generate the droplets for surface contact.
Spray towerSpray towers or spray chambers are a form of pollution control technology. They consist of empty cylindrical vessels made of steel or plastic and nozzles that spray liquid into the vessels. The inlet gas stream usually enters the bottom of the tower and moves upward, while liquid is sprayed...
s are typically used when circulating a slurry (see below). The high speed of a venturi would cause erosion problems, while a packed tower would plug up if it tried to circulate a slurry.
Countercurrent packed towers are infrequently used because they have a tendency to become plugged by collected particles or to scale when
limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
or
limestoneLimestone is a sedimentary rock composed largely of the mineral calcite . The deposition of limestone strata is often a by-product and indicator of biological activity in the geologic record...
scrubbing slurries are used.
Scrubbing reagent
As explained above, alkaline sorbents are used for scrubbing flue gases to remove SO
2. Depending on the application, the two most important are
limeLime is a general term for calcium-containing inorganic materials, in which carbonates, oxides and hydroxides predominate. Strictly speaking, lime is calcium oxide or calcium hydroxide. It is also the name for a single mineral of the CaO composition, occurring very rarely...
and
sodium hydroxideSodium hydroxide , also known as lye and caustic soda, is a caustic metallic base. It is used in many industries, mostly as a strong chemical base in the manufacture of pulp and paper, textiles, drinking water, soaps and detergents and as a drain cleaner. Worldwide production in 1998 was around...
(also known as caustic soda). Lime is typically used on large coal or oil fired boilers as found in power plants, as it is very much less expensive than caustic soda. The problem is that it results in a slurry being circulated through the scrubber instead of a solution. This makes it harder on the equipment. A spray tower is typically used for this application. The use of lime results in a slurry of calcium sulfite (CaSO
3) that must be disposed of. Fortunately, calcium sulfite can be oxidized to produce by-product gypsum (CaSO
4 · 2H
2O) which is marketable for use in the building products industry.
Caustic soda is limited to smaller combustion units because it is more expensive than lime, but it has the advantage that it forms a solution rather than a slurry. This makes it easier to operate. It produces a solution of sodium sulfite/bisulfite (depending on the pH), or sodium sulfate that must be disposed of. This is not a problem in a kraft pulp mill for example, where this can be a source of makeup chemicals to the recovery cycle.
Scrubbing with sodium sulfite solution
It is possible to scrub
sulfur dioxideSulfur dioxide is the chemical compound with the formula SO
2. It is produced by volcanoes and in various industrial processes. Since coal and petroleum often contain sulfur compounds, their combustion generates sulfur dioxide...
by using a cold solution of sodium sulfite, this forms a sodium hydrogen sulfite solution. By heating this solution it is possible to reverse the reaction to form sulfur dioxide and the sodium sulfite solution.
In some ways this can be thought of as being similar to the reversible
liquid-liquid extractionLiquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two different immiscible liquids, usually water and an organic solvent. It is an extraction of a substance from one liquid phase into another liquid...
of an
inert gasAn inert gas is any gas that is not reactive with elements.Unlike noble gases, inert gas is not necessarily elemental and is often a compound gas. Like the noble gases the tendency for non-reactivity is due to the valence, the outermost electron shell, being complete in all the inert gases...
such as
xenonXenon is a chemical element represented by the symbol Xe. Its atomic number is 54. A colorless, heavy, odorless noble gas, xenon occurs in the Earth's atmosphere in trace amounts...
or
radonRadon is a chemical element with symbol Rn and atomic number 86. It is a radioactive, colorless, odorless, tasteless noble gas, occurring naturally as the decay product of radium. It is one of the heaviest substances that remains a gas under normal conditions and is considered to be a health...
(or some other solute which does not undergo a chemical change during the extraction) from water to another phase. While a chemical change does occur during the extraction of the sulfur dioxide from the gas mixture, it is the case that the extraction equilibrium is shifted by changing the temperature rather than by the use of a chemical reagent.
Gas phase oxidation followed by reaction with ammonia
A new, emerging flue gas desulfurization technology has been described by the IAEA. It is a
radiationIn physics, radiation describes any process in which energy emitted by one body travels through a medium or through space, ultimately to be absorbed by another body...
technology where an intense beam of electrons is fired into the flue gas at the same time as
ammoniaAmmonia is a compound of nitrogen and hydrogen with the formula NH
3. It is normally encountered as a gas with a characteristic pungent odor. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to foodstuffs and fertilizers...
is added to the gas. The Chendu power plant in China started up such a flue gas desulfurization unit on a 100 MW scale in 1998. The Pomorzany power plant in Poland also started up a similar sized unit in 2003 and that plant removes both sulfur and nitrogen oxides. Both plants are reported to be operating successfully. However, the accelerator design principles and manufacturing quality need further improvement for continuous operation in industrial conditions.
No radioactivity is required or created in the process. The electron beam is generated by a device similar to the
electron gunAn electron gun is an electrical component that produces an electron beam that has a precise kinetic energy and is most often used in televisions and monitors which use cathode ray tube technology, as well as in other instruments, such as electron microscopes and particle accelerators.Electron...
in a TV set. This device is called an accelerator. This is an example of a radiation chemistry process where the physical effects of radiation are used to process a substance.
The action of the electron beam is to promote the oxidation of sulfur dioxide to sulfur(VI) compounds. The ammonia reacts with the sulfur compounds thus formed to produce
ammonium sulfateAmmonium sulfate , 2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen as ammonium cations, and 24% sulfur as sulfate anions...
which can be used as a fertilizer according to the IAEA. In addition, it can be used to lower the nitrogen oxide content of the flue gas. This method has attained industrial plant scale.
Facts and statistics
- The information in this section was obtained from a US EPA published fact sheet.
Flue gas desulfurization scrubbers have been applied to combustion units firing coal and oil that range in size from 5 MW to 1500 MW. Scottish Power are spending £400 million installing FGD at Longannet power station which has a capacity of over 2 GW. Dry scrubbers and spray scrubbers have generally been applied to units smaller than 300 MW.
Approximately 85% of the flue gas desulfurization units installed in the US are wet scrubbers, 12% are spray dry systems and 3% are dry injection systems.
The highest SO
2 removal efficiencies (greater than 90%) are achieved by wet scrubbers and the lowest (less than 80%) by dry scrubbers. However, the newer designs for dry scrubbers are capable of achieving efficiencies in the order of 90%.
In spray drying and dry injection systems, the flue gas must first be cooled to about 10-20 °C above adiabatic
saturationSaturation or saturated may mean:* Dew point, which is a temperature that occurs when atmospheric humidity reaches 100% and the air is saturated with moisture...
to avoid wet solids deposition on downstream equipment and plugging of baghouses.
The capital, operating and maintenance costs per
short tonThe short ton is a unit of weight equal to . In the United States it is often called simply ton without distinguishing it from the metric ton or the long ton ; rather, the other two are specifically noted. There are, however, some U.S...
of SO
2 removed (in 2001 US dollars) are:
- For wet scrubbers larger than 400 MW, the cost is $200 to $500 per ton
- For wet scrubbers smaller than 400 MW, the cost is $500 to $5,000 per ton
- For spray dry scrubbers larger than 200 MW, the cost is $150 to $300 per ton
- For spray dry scrubbers smaller than 200 MW, the cost is $500 to $4,000 per ton
Alternative methods of reducing sulfur dioxide emissions
An alternative to removing
sulfurSulfur or sulphur is the chemical element that has the atomic number 16. It is denoted with the symbol S. It is an abundant, multivalent non-metal. Sulfur, in its native form, is a yellow crystalline solid. In nature, it can be found as the pure element and as sulfide and sulfate minerals...
from the flue gases after burning is to remove the sulfur from the fuel before or during combustion.
HydrodesulfurizationHydrodesulfurization is a catalytic chemical process widely used to remove sulfur from natural gas and from refined petroleum products such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils...
of fuel has been used for treating
fuel oilFuel oil is a fraction obtained from petroleum distillation, either as a distillate or a residue. Broadly speaking, fuel oil is any liquid petroleum product that is burned in a furnace or boiler for the generation of heat or used in an engine for the generation of power, except oils having a flash...
s before use.
Fluidized bed combustionFluidized bed combustion is a combustion technology used in power plants. Fluidized beds suspend solid fuels on upward-blowing jets of air during the combustion process. The result is a turbulent mixing of gas and solids. The tumbling action, much like a bubbling fluid, provides more effective...
adds lime to the fuel during combustion. The lime reacts with the SO
2 to form
sulfateIn inorganic chemistry, a sulfate is a salt of sulfuric acid.-Chemical properties:...
s which become part of the ash.
See also
- Incineration
Incineration is a waste treatment technology that involves the combustion of organic materials and/or substances. Incineration and other high temperature waste treatment systems are described as "thermal treatment". Incineration of waste materials converts the waste into incinerator bottom ash,...
- Scrubber
Scrubber systems are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. Traditionally, the term "scrubber" has referred to pollution control devices that use liquid to wash unwanted pollutants from a gas stream...
- Flue gas emissions
Flue gas emissions from fossil fuel combustion refers to the combustion product gas resulting from the burning of fossil fuels . Most fossil fuels are combusted with ambient air...
- Flue gas stacks
External links