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Flue-gas desulfurization (FGD) is a set of technologies used to remove
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
() from
exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste
incineration
Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. Industrial plants for waste incineration are commonly referred to as waste-to-energy facilities. Incineration and other high ...
.
Methods
Since stringent environmental regulations limiting emissions have been enacted in many countries, is being removed from flue gases by a variety of methods. Common methods used:
*
Wet scrubbing using a slurry of alkaline
sorbent
A sorbent is a material used to absorb or adsorb liquids or gases. Examples include:
*A material similar to molecular sieve material, which acts by adsorption (attracting molecules to its surface). It has a large internal surface area and good t ...
, usually
limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
or
lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
, or seawater to scrub gases;
*
Spray-dry scrubbing using similar sorbent slurries;
*
Wet sulfuric acid process
The wet sulfuric acid process (WSA process) is a gas desulfurization process. After Danish company Haldor Topsoe introduced this technology in 1987, it has been recognized as a process for recovering sulfur from various process gases in the form ...
recovering
sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
in the form of commercial quality
sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
;
*
SNOX Flue gas desulfurization removes sulfur dioxide, nitrogen oxides and particulates from flue gases;
*Dry sorbent injection systems that introduce powdered hydrated lime (or other sorbent material) into exhaust ducts to eliminate and from process emissions.
For a typical coal-fired power station, flue-gas desulfurization (FGD) may remove 90 percent or more of the in the flue gases.
History
Methods of removing
sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
from boiler and furnace exhaust gases have been studied for over 150 years. Early ideas for flue gas desulfurization were established in
England
England is a country that is part of the United Kingdom. It shares land borders with Wales to its west and Scotland to its north. The Irish Sea lies northwest and the Celtic Sea to the southwest. It is separated from continental Europe b ...
around 1850.
With the construction of large-scale power plants in England in the 1920s, the problems associated with large volumes of from a single site began to concern the public. The emissions problem did not receive much attention until 1929, when the
House of Lords
The House of Lords, also known as the House of Peers, is the Bicameralism, upper house of the Parliament of the United Kingdom. Membership is by Life peer, appointment, Hereditary peer, heredity or Lords Spiritual, official function. Like the ...
upheld the claim of a landowner against the Barton Electricity Works of the
Manchester Corporation
Manchester City Council is the local authority for Manchester, a city and metropolitan borough in Greater Manchester, England. Manchester is the sixth largest city in England by population. Its city council is composed of 96 councillors, three f ...
for damages to his land resulting from 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 controls on all such power plants.
The first major FGD unit at a utility was installed in 1931 at
Battersea Power Station
Battersea Power Station is a decommissioned Grade II* listed coal-fired power station, located on the south bank of the River Thames, in Nine Elms, Battersea, in the London Borough of Wandsworth. It was built by the London Power Company (LPC) ...
, owned by
London Power Company
The London Power Company was an electricity generating and bulk supply company in London, England, formed in 1925 by the merger of ten small electricity companies. In 1948 Britain's electricity supply industry was nationalised under the Electri ...
. In 1935, an FGD system similar to that installed at Battersea went into service at Swansea Power Station. The third major FGD system was installed in 1938 at
Fulham Power Station
Fulham Power Station was a coal-fired power station on the north bank of the River Thames at Battersea Reach in Fulham, London
Station A ran from 1901, with station B opening in 1936, until their decommissioning in 1978.
History Fulham A
Th ...
. These three early large-scale FGD installations were suspended during
World War II
World War II or the Second World War, often abbreviated as WWII or WW2, was a world war that lasted from 1939 to 1945. It involved the vast majority of the world's countries—including all of the great powers—forming two opposin ...
, because the characteristic white vapour plumes would have aided location by enemy aircraft. The FGD plant at Battersea was recommissioned after the war and, together with FGD plant at the new
Bankside B power station opposite the City of London, operated until the stations closed in 1983 and 1981 respectively. Large-scale FGD units did not reappear at utilities until the 1970s, where most of the installations occurred in the
United States
The United States of America (U.S.A. or USA), commonly known as the United States (U.S. or US) or America, is a country primarily located in North America. It consists of 50 states, a federal district, five major unincorporated territorie ...
and
Japan
Japan ( ja, 日本, or , and formally , ''Nihonkoku'') is an island country in East Asia. It is situated in the northwest Pacific Ocean, and is bordered on the west by the Sea of Japan, while extending from the Sea of Okhotsk in the north ...
.
[
In 1970, the ]U.S. Congress
The United States Congress is the legislature of the federal government of the United States. It is Bicameralism, bicameral, composed of a lower body, the United States House of Representatives, House of Representatives, and an upper body, ...
passed the Clean Air Act of 1970
The Clean Air Act (CAA) is the United States' primary federal air quality law, intended to reduce and control air pollution nationwide. Initially enacted in 1963 and amended many times since, it is one of the United States' first and most inf ...
(CAA). The law authorized development of federal regulations in the United States covering emissions from both stationary (industrial) and mobile sources, which were subsequently published by the U.S. Environmental Protection Agency
The Environmental Protection Agency (EPA) is an Independent agencies of the United States government, independent executive agency of the United States federal government tasked with environmental protection matters. President Richard Nixon pro ...
(EPA). In 1977, Congress amended the law to require more stringent controls on air emissions. In response to the CAA requirements, the American Society of Mechanical Engineers
The American Society of Mechanical Engineers (ASME) is an American professional association that, in its own words, "promotes the art, science, and practice of multidisciplinary engineering and allied sciences around the globe" via "continuing ...
(ASME) authorized the formation of the PTC 40 Standards Committee in 1978. This committee first convened in 1979 with the purpose of developing a standardized "procedure for conducting and reporting performance tests of FGD systems and reporting the results in terms of the following categories: (a) emissions reduction, (b) consumable and utilities, (c) waste and by-product characterization and amount." The first code draft was approved by ASME in 1990 and adopted by the American National Standards Institute
The American National Standards Institute (ANSI ) is a private non-profit organization that oversees the development of voluntary consensus standards for products, services, processes, systems, and personnel in the United States. The organi ...
(ANSI) in 1991. The PTC 40-1991 Standard was available for public use for those units affected by the 1990 Clean Air Act Amendments. In 2006, the PTC 40 Committee reconvened following EPA publication of the Clean Air Interstate Rule (CAIR) in 2005. In 2017, the revised PTC 40 Standard was published. This revised standard (PTC 40-2017) covers Dry and Regenerable FGD systems and provides a more detailed Uncertainty Analysis section. This standard is currently in use today by companies around the world.
As of June 1973, there were 42 FGD units in operation, 36 in Japan and 6 in the United States, ranging in capacity from 5 MW to 250 MW.[Beychok, Milton R., ''Coping With SO2'', Chemical Engineering/Deskbook Issue, 21 October 1974] As of around 1999 and 2000, FGD units were being used in 27 countries, and there were 678 FGD units operating at a total power plant capacity of about 229 gigawatts
The watt (symbol: W) is the unit of power or radiant flux in the International System of Units (SI), equal to 1 joule per second or 1 kg⋅m2⋅s−3. It is used to quantify the rate of energy transfer. The watt is named after James Watt ...
. About 45% of the FGD capacity was in the U.S., 24% in Germany
Germany,, officially the Federal Republic of Germany, is a country in Central Europe. It is the second most populous country in Europe after Russia, and the most populous member state of the European Union. Germany is situated betwe ...
, 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.[Nolan, Paul S., ''Flue Gas Desulfurization Technologies for Coal-Fired Power Plants'', The Babcock & Wilcox Company, U.S., presented by Michael X. Jiang at the Coal-Tech 2000 International Conference, November 2000, Jakarta, Indonesia]
FGD on ships
The International Maritime Organization ( IMO) has adopted guidelines on the approval, installation and use of exhaust gas scrubbers (exhaust gas cleaning systems) onboard ships to ensure compliance with the sulphur regulation of MARPOL Annex VI. Flag States must approve such systems and port States can (as part of their port state control
Port state control (PSC) is an inspection regime for countries to inspect foreign-registered ships in port other than those of the flag state and take action against ships that are not in compliance. Inspectors for PSC are called PSC officers (PS ...
) ensure that such systems are functioning correctly. If a scrubber system is not functioning probably (and the IMO procedures for such malfunctions are not adhered to), port States can sanction the ship. The United Nations Convention on the Law Of the Sea also bestows port States with a right to regulate (and even ban) the use of open loop scrubber systems within ports and internal waters.
Sulfuric acid mist formation
Fossil fuel
A fossil fuel is a hydrocarbon-containing material formed naturally in the Earth's crust from the remains of dead plants and animals that is extracted and burned as a fuel. The main fossil fuels are coal, oil, and natural gas. Fossil fuels m ...
s such as coal and oil can contain a significant amount of sulfur. When fossil fuels are burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
(). Such conversion happens under normal conditions of temperature and of oxygen present in the flue gas
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced a ...
. However, there are circumstances under which such reaction may not occur.
can further oxidize into sulfur trioxide
Sulfur trioxide (alternative spelling sulphur trioxide, also known as ''nisso sulfan'') is the chemical compound with the formula SO3. It has been described as "unquestionably the most important economically" sulfur oxide. It is prepared on an ind ...
() when excess oxygen is present and gas temperatures are sufficiently high. At about 800 °C, formation of is favored. Another way that can be formed is through catalysis by metals in the fuel. Such reaction is particularly true for heavy fuel oil, where a significant amount of vanadium
Vanadium is a chemical element with the symbol V and atomic number 23. It is a hard, silvery-grey, malleable transition metal. The elemental metal is rarely found in nature, but once isolated artificially, the formation of an oxide layer ( pas ...
is present. In whatever way is formed, it does not behave like in that it forms a liquid aerosol
An aerosol is a suspension (chemistry), suspension of fine solid particles or liquid Drop (liquid), droplets in air or another gas. Aerosols can be natural or Human impact on the environment, anthropogenic. Examples of natural aerosols are fog o ...
known as sulfuric acid
Sulfuric acid (American spelling and the preferred IUPAC name) or sulphuric acid ( Commonwealth spelling), known in antiquity as oil of vitriol, is a mineral acid composed of the elements sulfur, oxygen and hydrogen, with the molecular formu ...
() mist that is very difficult to remove. Generally, about 1% of the sulfur dioxide will be converted to . 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 precipitator
An electrostatic precipitator (ESP) is a filterless device that removes fine particles, like dust and smoke, from a flowing gas using the force of an induced electrostatic charge minimally impeding the flow of gases through the unit.
In con ...
s.
FGD chemistry
Basic principles
Most FGD systems employ two stages: one for fly ash
Fly ash, flue ash, coal ash, or pulverised fuel ash (in the UK) plurale tantum: coal combustion residuals (CCRs)is a coal combustion product that is composed of the particulates (fine particles of burned fuel) that are driven out of coal-fired ...
removal and the other for removal. Attempts have been made to remove both the fly ash and in one scrubbing vessel. However, these systems experienced severe maintenance problems and low removal efficiency. In wet scrubbing systems, the flue gas normally passes first through a fly ash removal device, either an electrostatic precipitator or a baghouse, and then into the -absorber. However, in dry injection or spray drying operations, the is first reacted with the lime, 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 . These gases are highly corrosive to any downstream equipment such as fans, ducts, and stacks. Two methods that may minimize corrosion are: (1) reheating the gases to above their dew point
The dew point is the temperature to which air must be cooled to become saturated with water vapor, assuming constant air pressure and water content. When cooled below the dew point, moisture capacity is reduced and airborne water vapor will cond ...
, or (2) using materials of construction and designs that allow equipment to withstand the corrosive conditions. Both alternatives are expensive. Engineers determine which method to use on a site-by-site basis.
Scrubbing with an alkali solid or solution
is an acid gas Acid gas is a particular typology of natural gas or any other gas mixture containing significant quantities of hydrogen sulfide (H2S), carbon dioxide (CO2), or similar acidic gases. A gas is determined to be acidic or not after it is mixed with wate ...
, and, therefore, the typical sorbent slurries or other materials used to remove the from the flue gases are alkaline. The reaction taking place in wet scrubbing using a (limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
) slurry produces calcium sulfite
Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfite with the formula CaSO3·x(H2O). Two crystalline forms are known, the hemihydrate and the tetrahydrate, respectively CaSO3·½(H2O) and CaSO3·4(H2O). All fo ...
() and may be expressed in the simplified dry form as:
: + → +
When wet scrubbing with a (hydrated lime
Calcium hydroxide (traditionally called slaked lime) is an inorganic compound with the chemical formula Ca( OH)2. It is a colorless crystal or white powder and is produced when quicklime (calcium oxide) is mixed or slaked with water. It has ma ...
) slurry, the reaction also produces (calcium sulfite
Calcium sulfite, or calcium sulphite, is a chemical compound, the calcium salt of sulfite with the formula CaSO3·x(H2O). Two crystalline forms are known, the hemihydrate and the tetrahydrate, respectively CaSO3·½(H2O) and CaSO3·4(H2O). All fo ...
) and may be expressed in the simplified dry form as:
: + → +
When wet scrubbing with a (magnesium hydroxide
Magnesium hydroxide is the inorganic compound with the chemical formula Mg(OH)2. It occurs in nature as the mineral brucite. It is a white solid with low solubility in water (). Magnesium hydroxide is a common component of antacids, such as milk ...
) slurry, the reaction produces (magnesium sulfite
Magnesium sulfite is the magnesium salt of sulfurous acid 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 anhydr ...
) and may be expressed in the simplified dry form as:
: + → +
To partially offset the cost of the FGD installation, some designs, particularly dry sorbent injection systems, further oxidize the (calcium sulfite) to produce marketable (gypsum
Gypsum is a soft sulfate mineral composed of calcium sulfate dihydrate, with the chemical formula . It is widely mined and is used as a fertilizer and as the main constituent in many forms of plaster, blackboard or sidewalk chalk, and drywall. ...
) that can be of high enough quality to use in wallboard
Drywall (also called plasterboard, dry lining, wallboard, sheet rock, gypsum board, buster board, custard board, and gypsum panel) is a panel made of calcium sulfate dihydrate (gypsum), with or without additives, typically extruded between thick ...
and other products. The process by which this synthetic gypsum is created is also known as forced oxidation:
: + 2 + →
A natural alkaline usable to absorb is seawater. The is absorbed in the water, and when oxygen is added reacts to form sulfate ions and free . The surplus of is offset by the carbonates in seawater pushing the carbonate equilibrium to release gas:
: + + → + 2
: + → +
In industry caustic
Caustic most commonly refers to:
* Causticity, a property of various corrosive substances
** Sodium hydroxide, sometimes called ''caustic soda''
** Potassium hydroxide, sometimes called ''caustic potash''
** Calcium oxide, sometimes called ''caus ...
() is often used to scrub , producing sodium sulfite
Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2 SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of ...
:
:2 + → +
Types of wet scrubbers used in FGD
To promote maximum gas–liquid surface area and residence time, a number of wet scrubber designs have been used, including spray towers, venturis, plate towers, and mobile packed bed
In chemical processing, a packed bed is a hollow tube, pipe, or other vessel that is filled with a packing material. The packing can be randomly filled with small objects like Raschig rings or else it can be a specifically designed structured ...
s. 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 concurrently, 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 removal than other absorber designs.
FGD scrubbers produce a scaling wastewater that requires treatment to meet U.S. federal discharge regulations. However, technological advancements in ion-exchange membrane An ion-exchange membrane is a semi-permeable membrane that transports certain dissolved ions, while blocking other ions or neutral molecules.
Ion-exchange membranes are therefore electrically conductive. They are often used in desalination and c ...
s and electrodialysis
Electrodialysis (ED) is used to transport salt ions from one solution through ion-exchange membranes to another solution under the influence of an applied electric potential difference. This is done in a configuration called an electrodialysis ...
systems has enabled high-efficiency treatment of FGD wastewater to meet recent EPA discharge limits. The treatment approach is similar for other highly scaling industrial wastewaters.
=Venturi-rod scrubbers
=
A venturi scrubber
A venturi scrubber is designed to effectively use the energy from a high-velocity inlet gas stream to atomize the liquid being used to scrub the gas stream. This type of technology is a part of the group of air pollution controls collectively refe ...
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 removal of and fly ash, 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 in one vessel can be economic, 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, it can be more effective to remove particulate and simultaneously.
=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 the 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 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 tower
A spray tower (or spray column or spray chamber) is gas-liquid contactor used to achieve mass and heat transfer between a continuous gas phase (that can contain dispersed solid particles) and a dispersed liquid phase. It consists of an empty c ...
is the simplest type of scrubber. It consists of a tower with spray nozzles, which generate the droplets for surface contact. Spray tower
A spray tower (or spray column or spray chamber) is gas-liquid contactor used to achieve mass and heat transfer between a continuous gas phase (that can contain dispersed solid particles) and a dispersed liquid phase. It consists of an empty c ...
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.
Counter-current packed towers are infrequently used because they have a tendency to become plugged by collected particles or to scale when lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
or limestone
Limestone ( calcium carbonate ) is a type of carbonate sedimentary rock which is the main source of the material lime. It is composed mostly of the minerals calcite and aragonite, which are different crystal forms of . Limestone forms whe ...
scrubbing slurries are used.
Scrubbing reagent
As explained above, alkaline sorbents are used for scrubbing flue gases to remove . Depending on the application, the two most important are lime
Lime commonly refers to:
* Lime (fruit), a green citrus fruit
* Lime (material), inorganic materials containing calcium, usually calcium oxide or calcium hydroxide
* Lime (color), a color between yellow and green
Lime may also refer to:
Botany ...
and sodium hydroxide
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alkali ...
(also known as caustic soda
Sodium hydroxide, also known as lye and caustic soda, is an inorganic compound with the formula NaOH. It is a white solid ionic compound consisting of sodium cations and hydroxide anions .
Sodium hydroxide is a highly caustic base and alka ...
). 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 () that must be disposed of. Fortunately, calcium sulfite can be oxidized to produce by-product gypsum () 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 "spent caustic Spent caustic is a waste industrial caustic solution that has become exhausted and is no longer useful (or spent). Spent caustics are made of sodium hydroxide or potassium hydroxide, water, and contaminants. The contaminants have consumed the majo ...
" solution of sodium sulfite
Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2 SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of ...
/bisulfite (depending on the pH), or sodium sulfate that must be disposed of. This is not a problem in a kraft pulp
Kraft paper or kraft is paper or paperboard (cardboard) produced from chemical pulp produced in the kraft process.
Sack kraft paper (or just sack paper) is a porous kraft paper with high elasticity and high tear resistance, designed for packag ...
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 dioxide
Sulfur dioxide (IUPAC-recommended spelling) or sulphur dioxide (traditional Commonwealth English) is the chemical compound with the formula . It is a toxic gas responsible for the odor of burnt matches. It is released naturally by volcanic activ ...
by using a cold solution of sodium sulfite
Sodium sulfite (sodium sulphite) is the inorganic compound with the chemical formula Na2 SO3. A white, water-soluble solid, it is used commercially as an antioxidant and preservative. A heptahydrate is also known but it is less useful because of ...
; 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. Since the sodium sulfite solution is not consumed, it is called a regenerative treatment. The application of this reaction is also known as the Wellman–Lord process The Wellman–Lord process is a regenerable process to remove sulfur dioxide from flue gas (flue-gas desulfurization) without creating a throwaway sludge product.
In this process, sulfur dioxide from flue gas is absorbed in a sodium sulfite solutio ...
.
In some ways this can be thought of as being similar to the reversible liquid–liquid extraction
Liquid–liquid extraction (LLE), also known as solvent extraction and partitioning, is a method to separate compounds or metal complexes, based on their relative solubilities in two different immiscible liquids, usually water (polar) and an org ...
of an inert gas
An inert gas is a gas that does not readily undergo chemical reactions with other chemical substances and therefore does not readily form chemical compounds. The noble gases often do not react with many substances and were historically referred to ...
such as xenon
Xenon is a chemical element with the symbol Xe and atomic number 54. It is a dense, colorless, odorless noble gas found in Earth's atmosphere in trace amounts. Although generally unreactive, it can undergo a few chemical reactions such as the ...
or radon
Radon is a chemical element with the symbol Rn and atomic number 86. It is a radioactive, colourless, odourless, tasteless noble gas. It occurs naturally in minute quantities as an intermediate step in the normal radioactive decay chains through ...
(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
The International Atomic Energy Agency (IAEA) is an intergovernmental organization that seeks to promote the peaceful use of nuclear energy and to inhibit its use for any military purpose, including nuclear weapons. It was established in 1957 ...
. It is a radiation
In physics, radiation is the emission or transmission of energy in the form of waves or particles through space or through a material medium. This includes:
* ''electromagnetic radiation'', such as radio waves, microwaves, infrared, visi ...
technology where an intense beam of electrons
The electron ( or ) is a subatomic particle with a negative one elementary electric charge. Electrons belong to the first generation of the lepton particle family,
and are generally thought to be elementary particles because they have no ...
is fired into the flue gas at the same time as ammonia
Ammonia is an inorganic compound of nitrogen and hydrogen with the formula . A stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct pungent smell. Biologically, it is a common nitrogenous was ...
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.[Section of IAEA 2003 Annual Report]
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 gun
An electron gun (also called electron emitter) is an electrical component in some vacuum tubes that produces a narrow, collimated electron beam that has a precise kinetic energy. The largest use is in cathode-ray tubes (CRTs), used in nearly ...
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 sulfate
Ammonium sulfate (American English and international scientific usage; ammonium sulphate in British English); (NH4)2SO4, is an inorganic salt with a number of commercial uses. The most common use is as a soil fertilizer. It contains 21% nitrogen a ...
, which can be used as a nitrogenous fertilizer
A fertilizer (American English) or fertiliser (British English; see spelling differences) is any material of natural or synthetic origin that is applied to soil or to plant tissues to supply plant nutrients. Fertilizers may be distinct from ...
. In addition, it can be used to lower the nitrogen oxide content of the flue gas. This method has attained industrial plant scale.[''Industrial Plant for Flue Gas Treatment with High Power Electron Accelerator'']
by A.G. Chmielewski, Warsaw University of Technology, Poland.
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 1,500 MW. Scottish Power
Scottish Power is a vertically integrated energy company based in Glasgow, Scotland. It is a subsidiary of Spanish utility firm Iberdrola.
ScottishPower is the distribution network operator for Central and Southern Scotland, Merseyside, North ...
are spending £400 million installing FGD at Longannet power station
Longannet power station was a large coal-fired power station in Fife, and the last coal-fired power station in Scotland. It was capable of co-firing biomass, natural gas and sludge. The station stood on the north bank of the Firth of Forth, nea ...
, which has a capacity of over 2,000 GW. Dry scrubbers and spray scrubbers have generally been applied to units smaller than 300 MW.
FGD has been fitted by RWE npower
Npower Limited is a British supplier of gas and electricity to businesses. It has been a subsidiary of E.ON UK since January 2019. The company was formerly known as Innogy plc and was listed on the London Stock Exchange and was a constituent ...
at Aberthaw Power Station
Aberthaw Power Station refers to two decommissioned coal-fired and co-fired biomass power stations on the coast of South Wales, near Barry in the Vale of Glamorgan. They were located at Limpert Bay, near the villages of Gileston and West Aberth ...
in south Wales using the seawater process and works successfully on the 1,580 MW plant.
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 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 saturation
Saturation, saturated, unsaturation or unsaturated may refer to:
Chemistry
* Saturation, a property of organic compounds referring to carbon-carbon bonds
**Saturated and unsaturated compounds
** Degree of unsaturation
**Saturated fat or fatty aci ...
to avoid wet solids deposition on downstream equipment and plugging of baghouses.
The capital, operating and maintenance costs per short ton
The short ton (symbol tn) is a measurement unit equal to . It is commonly used in the United States, where it is known simply as a ton,
although the term is ambiguous, the single word being variously used for short, long, and metric ton.
The vari ...
of 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 sulfur
Sulfur (or sulphur in British English) is a chemical element with the symbol S and atomic number 16. It is abundant, multivalent and nonmetallic. Under normal conditions, sulfur atoms form cyclic octatomic molecules with a chemical formula ...
from the flue gases after burning is to remove the sulfur from the fuel before or during combustion. Hydrodesulfurization
Hydrodesulfurization (HDS) is a catalytic chemical process widely used to remove sulfur (S) from natural gas and from refined petroleum products, such as gasoline or petrol, jet fuel, kerosene, diesel fuel, and fuel oils. The purpose of remov ...
of fuel has been used for treating fuel oil
Fuel oil is any of various fractions obtained from the distillation of petroleum (crude oil). Such oils include distillates (the lighter fractions) and residues (the heavier fractions). Fuel oils include heavy fuel oil, marine fuel oil (MFO), bun ...
s before use. Fluidized bed combustion
Fluidized bed combustion (FBC) is a combustion technology used to burn solid fuels.
In its most basic form, fuel particles are suspended in a hot, bubbling fluidity bed of ash and other particulate materials (sand, limestone etc.) through which ...
adds lime to the fuel during combustion. The lime reacts with the to form sulfate
The sulfate or sulphate ion is a polyatomic anion with the empirical formula . Salts, acid derivatives, and peroxides of sulfate are widely used in industry. Sulfates occur widely in everyday life. Sulfates are salts of sulfuric acid and many ar ...
s which become part of the ash
Ash or ashes are the solid remnants of fires. Specifically, ''ash'' refers to all non-aqueous, non- gaseous residues that remain after something burns. In analytical chemistry, to analyse the mineral and metal content of chemical samples, ash ...
.
This elemental sulfur is then separated and finally recovered at the end of the process for further usage in, for example, agricultural products. Safety is one of the greatest benefits of this method, as the whole process takes place at atmospheric pressure
Atmospheric pressure, also known as barometric pressure (after the barometer), is the pressure within the atmosphere of Earth. The standard atmosphere (symbol: atm) is a unit of pressure defined as , which is equivalent to 1013.25 millibars, 7 ...
and ambient temperature. This method has been developed by Paqell, a joint venture between Shell Global Solutions
Shell plc is a British multinational oil and gas company headquartered in London, England. Shell is a public limited company with a primary listing on the London Stock Exchange (LSE) and secondary listings on Euronext Amsterdam and the New Yo ...
and Paques.
See also
*Incineration
Incineration is a waste treatment process that involves the combustion of substances contained in waste materials. Industrial plants for waste incineration are commonly referred to as waste-to-energy facilities. Incineration and other high ...
*Scrubber
Scrubber systems (e.g. chemical scrubbers, gas scrubbers) are a diverse group of air pollution control devices that can be used to remove some particulates and/or gases from industrial exhaust streams. An early application of a carbon dioxide scr ...
*Flue-gas emissions from fossil-fuel combustion
Flue gas is the gas exiting to the atmosphere via a flue, which is a pipe or channel for conveying exhaust gases from a fireplace, oven, furnace, boiler or steam generator. Quite often, the flue gas refers to the combustion exhaust gas produced ...
*Flue-gas stack
A flue-gas stack, also known as a smoke stack, chimney stack or simply as a 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 g ...
s
*Wellman–Lord process The Wellman–Lord process is a regenerable process to remove sulfur dioxide from flue gas (flue-gas desulfurization) without creating a throwaway sludge product.
In this process, sulfur dioxide from flue gas is absorbed in a sodium sulfite solutio ...
References
External links
Schematic process flow of FGD plant
5000 MW FGD Plant
(includes a detailed process flow diagram)
Alstom presentation to UN-ECE on air pollution control
(includes process flow diagram for dry, wet and seawater FGD)
Flue Gas Treatment
article including the removal of hydrogen chloride, sulfur trioxide, and other heavy metal particles such as mercury.
Institute of Clean Air Companies
– national trade association representing emissions control manufacturers
{{DEFAULTSORT:Flue-Gas Desulfurization
Air pollution
Pollution control technologies
Air pollution control systems
Acid gas control
Incineration
Chemical processes
Environmental engineering
Environmental impact of the energy industry
Gas technologies
Sulfur