Before flue gas desulfurization was installed, the emissions from this power plant in New Mexico contained excessive amounts of sulfur dioxide. New Mexico ( is a state located in the southwestern region of the United States of America.

Flue gas desulfurization (FGD) is the current state-of-the art technology used for removing sulfur dioxide (SO₂) from the exhaust flue gases in power plants that burn coal or oil to produce steam for the steam turbines that drive their electricity generators. Flue gas emissions from fossil fuel combustion refers to the Combustion product gas resulting from the burning of Fossil fuels. A fossil fuel power plant burns Fossil fuels such as Coal, Natural gas or Petroleum (oil to produce Electricity. A steam turbine is a mechanical device that extracts Thermal energy from pressurized Steam, and converts it into useful mechanical work In Electricity generation, an electrical generator is a device that converts Mechanical energy to Electrical energy, generally using Electromagnetic

Sulfur dioxide is responsible for acid rain formation. Acid rain is Rain or any other form of precipitation that is unusually Acidic It has harmful effects on plants aquatic animals and infastructure Tall flue gas stacks disperse the emissions by diluting the pollutants in ambient air and transporting them to other regions. A 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

In 1988 Prince Charles of Britain said, "I would have thought that the Central Electricity Generating Board was doing too little and too late. The Central Electricity Generating Board (CEGB was the cornerstone of the British electricity industry for almost 40 years from 1957 to Privatisation in the 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 SO₂ emissions that have been enacted in a great many countries, SO₂ is now being removed from flue gases by a variety of methods:

• Wet scrubbing using a slurry of sorbent, usually limestone or lime, to scrub the gases. Wet scrubber is a form of Pollution control technology The term describes a variety of devices that use Pollutants from a Furnace Flue gas A sorbent is a material used to Adsorb either liquids or gases Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3 Lime is a general term for various naturally occurring Minerals and materials derived from them in which Carbonates Oxides and Hydroxides of
• Spray-dry scrubbing using similar sorbent slurries. Spray drying is a commonly used method of drying a liquid feed through a hot gas
• Dry sorbent injection systems.

For a typical coal-fired power station, FGD will remove 95 percent or more of the SO₂ 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 SO₂ from a single site began to concern the public. The SO₂ 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 SO₂ 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 SO₂ controls on all such power plants. ^[1]

During this period, major FGD installations went into operation in the United Kingdom at the Battersea Power Station, the Swansea Power Station, and the Fulham Power Station. Battersea Power Station is a defunct coal-fired power station in Battersea, London, that was the first in a series of large coal-fired electrical generating Often confused with Lots Road Power Station. Fulham Power Station was a coal-fired power station on the north bank of the 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. ^[1]

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. ^[2] 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. ^[3][4][5]

Sulfuric acid mist formation

Fossil fuels such as coal and oil contain significant amounts of sulfur. Fossil fuels or mineral fuels are fossil source Fuels that is Hydrocarbons found within the top layer of the Earth’s crust. When burned, about 95 percent or more of the sulfur is generally converted to sulfur dioxide (SO₂). 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 SO₂ is further oxidized to sulfur trioxide (SO₃). Sulfur trioxide (also spelled sulphur trioxide) is the chemical compound with the formula SO3 Actually, too much oxygen is only one of the ways that SO₃ is formed. Gas temperature is also an important factor. At about 800 °C, formation of SO₃ is favoured. Another way that SO₃ can be formed is through catalysis by metals in the fuel. This is particularly true for heavy fuel oil, where significant amounts of vanadium are present. Vanadium (vəˈneɪdiəm is a Chemical element that has the symbol V and Atomic number 23 In whatever way that SO₃ is formed, it does not behave like SO₂ in that it forms a liquid aerosol known as sulfuric acid (H₂SO₄) mist that is very difficult to remove. Technically an aerosol is a suspension of fine solid particles or liquid droplets in a gas Sulfuric (or sulphuric acid, H 2 S[[oxygen O]]4 is a strong Mineral acid. Generally, about 1% of the sulfur dioxide will be converted to SO₃. 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 precipitators. An electrostatic precipitator (ESP or electrostatic air cleaner is a Particulate collection device that removes particles from a flowing gas (such as air using

FGD chemistry

Scrubbing with a basic solid or solution

SO₂ is an acid gas and thus the typical sorbent slurries or other materials used to remove the SO₂ from the flue gases are alkaline. The reaction taking place in wet scrubbing using a CaCO₃ (limestone) slurry produces CaSO₃ (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 CaSO₃ (calcium sulfite) and can be expressed as:

Ca(OH)₂ (solid) + SO₂ (gas) → CaSO₃ (solid) + H₂O (liquid)

When wet scrubbing with a Mg(OH)₂ (magnesium hydroxide) slurry, the reaction produces MgSO₃ (magnesium sulfite) and can be expressed as:

Mg(OH)₂ (solid) + SO₂ (gas) → MgSO₃ (solid) + H₂O (liquid)

Some FGD systems go a step further and oxidize the CaSO₃ (calcium sulfite) to produce marketable CaSO₄ · 2H₂O (gypsum):

CaSO₃ (solid) + ½O₂ (gas) + 2H₂O (liquid) → CaSO₄ · 2H₂O (solid)

Types of wet scrubbers used in FGD

To promote maximum gas-liquid surface area and contact time, a number of wet scrubber designs have been used. Gypsum is a very soft Mineral composed of Calcium sulfate dihydrate with the Chemical formula Ca[[sulfur S]] O 4·2 An important parameter in wet scrubbing systems is the rate of liquid flow Common ones are:

Mobile-bed scrubbers

Venturi-rod scrubbers

Main article: venturi scrubber

A venturi scrubber is a converging/diverging section of duct. A 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 A 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 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.

Plate towers

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 SO₂ 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

Main article: spray tower

A spray tower is the simplest type of scrubber. Spray towers or spray chambers are a form of Pollution control technology Spray towers or spray chambers are a form of Pollution control technology It consists of a tower with spray nozzles, which generate the droplets for surface contact. Spray towers are typically used when circulating a slurry (see below). Spray towers or spray chambers are a form of Pollution control technology 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 lime or limestone scrubbing slurries are used. Lime is a general term for various naturally occurring Minerals and materials derived from them in which Carbonates Oxides and Hydroxides of Limestone is a Sedimentary rock composed largely of the Mineral Calcite ( Calcium carbonate: CaCO3

Scrubbing reagent

As explained above, alkaline sorbents are used for scrubbing flue gases to remove SO₂. Depending on the application, the two most important are lime and sodium hydroxide (also known as caustic soda). Lime is a general term for various naturally occurring Minerals and materials derived from them in which Carbonates Oxides and Hydroxides of Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature Sodium hydroxide ( Na[[hydroxide OH]]) also known as Lye, caustic soda and (incorrectly according to IUPAC nomenclature 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 (CaSO₃) that must be disposed of. Fortunately, calcium sulfite can be oxidized to produce by-product gypsum (CaSO₄ · 2H₂O) 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. Kraft paper is Paper produced by the Kraft process from Wood pulp.

Scrubbing with sodium sulfite solution

It is possible to scrub 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 extraction of an inert gas such as xenon or radon (or some other solute which does not undergo a chemical change during the extraction) from water to another phase. Liquid-liquid extraction, also known as solvent extraction and partitioning, is a method to separate compounds based on their relative solubilities in two "Inert gases" is also used in a narrower sense for Noble gases An inert gas is any Gas that is not reactive with elements Xenon (ˈzɛnɒn or) is a Chemical element represented by the symbol Xe. Radon (ˈreɪdɒn is the Chemical element that has the symbol Rn and Atomic number 86 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 international organization that seeks to promote the peaceful use of nuclear energy and to inhibit its ^[6] It is a radiation technology where an intense beam of electrons is fired into the flue gas at the same time as ammonia is added to the gas. Radiation, as in Physics, is Energy in the form of waves or moving Subatomic particles emitted by an atom or other body as it changes from a higher energy The electron is a fundamental Subatomic particle that was identified and assigned the negative charge in 1897 by J Ammonia is a compound with the formula N[[hydrogen H3]] It is normally encountered as a Gas with a characteristic pungent Odor 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. ^[7][8] However, the accelerator design principles and manufacturing quality need further improvement for continuous operation in industrial conditions. ^[9]

No radioactivity is required or created in the process. Radioactive decay is the process in which an unstable Atomic nucleus loses energy by emitting ionizing particles and Radiation. The electron beam is generated by a device similar to the electron gun in a TV set. An electron gun (also called electron emitter) is an electrical component that produces an Electron beam that has a precise Kinetic energy and is most often This device is called an accelerator. This is an example of a radiation chemistry process^[8] 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 which can be used as a fertilizer according to the IAEA. Ammonium sulfate, (NH42SO4 is an inorganic chemical compound commonly used as a fertilizer In addition, it can be used to lower the nitrogen oxide content of the flue gas. This method has attained industrial plant scale. ^[7][10][11]

Facts and statistics

The information in this section was obtained from a US EPA published fact sheet. ^[12]

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 SO₂ 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 to avoid wet solids deposition on downstream equipment and plugging of baghouses.

The capital, operating and maintenance costs per short ton of SO₂ 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 from the flue gases after burning is to remove the sulfur from the fuel before or during combustion. The short ton ( S/T) is a unit of mass equal to 2000 lb (around 907 Sulfur or sulphur (ˈsʌlfɚ see spelling below) is the Chemical element that has the Atomic number 16 Hydrodesulfurization of fuel has been used for treating fuel oils before use. Hydrodesulfurization (HDS is a Catalytic chemical process widely used to remove Sulfur (S from Natural gas and from refined petroleum products Fuel oil is a fraction obtained from Petroleum Distillation, either as a distillate or a residue Fluidized bed combustion adds lime to the fuel during combustion. Fluidized bed combustion (FBC is a combustion technology used in power plants The lime reacts with the SO₂ to form sulfates which become part of the ash. Coal assay techniques are specific analytical methods designed to measure the particular physical and chemical properties of coals

See also

• Incineration
• Scrubber
• Flue gas emissions
• Flue gas stacks

References

External links

• Schematic of desulfurization plant
• 5000 MW FGD Plant (includes a detailed process flow diagram)
• New innovative design using Gas Suspension Absorption
• Flue Gas Desulfurization Fact Sheet