Flue gas desulfurization (FGD) is the existing state-of-the art an innovation used for removing sulfur dioxide (SO2) indigenous the exhaust flue gases in power plants that burn coal or oil to produce steam for the steam turbines that drive their electrical energy generators.

Sulfur dioxide is responsible for acid rain formation. Tall flue gas stacks disperse the emissions through diluting the pollutants in ambient air and transporting lock to various other regions.

In 1988 Prince Charles of britain said, "Iwould have actually thought the the central Electricity Generating board was doing too small and also late. Our responsibilities execute lie in no exporting our difficulties abroad." His words highlighted the fact that sulfur dioxide exhaust need to be removed, quite than emitted high into the setting where that affects many much more people. A variety of countries now have actually regulations limiting the height of flue gas stacks.

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As a result of stringent eco-friendly protection regulations concerning SO2 emissions that have actually been spread in a great many countries, SO2 is currently being removed from flue gases by a selection of methods:

Dry sorbent injection systems.

For a common coal-fired power station, FGD will eliminate 95 percent or an ext of the SO2 in the flue gases.


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3 FGD chemistry3.1 Scrubbing with a simple solid or solution3.1.1 varieties of wet scrubbers used in FGD

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History

Methods for removing sulfur dioxide from boiler and also furnace exhaust gases have actually been studied because that over 150 years. Early on concepts beneficial for flue gas desulfurization show up to have actually germinated in England circa 1850.

With the building of big scale power plants in England in the 1920s, the problems linked with huge volumes the SO2 from a solitary site started to worry the public. The SO2 emissions trouble did no receive much attention till 1929, when the home of Lords upheld the insurance claim of a landowner against the Barton power Works that the Manchester coporation, group for damages to his land resulting from SO2 emissions. Quickly thereafter a press project was launched versus the erection of power plants in ~ the boundaries of London. This outcry led to the imposition that SO2 controls on every such strength plants.<1>

During this period, significant FGD installations got in operation in England in ~ the Battersea power Station, the Swansea strength Station, and the Fulham power Station. The very first major FGD installation on a energy was put right into operation at the Battersea Station, owned by the London power Company, in 1931. In 1935, one FGD system comparable to that installed at Battersea go into business at the Swansea power Station. The third major FGD system was set up in 1938 at the Fulham strength Station. These three at an early stage large-scale FGD installations were exit during people War II. Large FGD units did not reappear in commercial procedure at utilities until the 1970s, where most of the task occurred in the unified States and also Japan.<1>

As the June 1973, there were 42 FGD units, ranging in dimension from 5 MW come 250 MW, in operation: 36 in Japan and 6 in the joined States.<2> together of about 1999-2000, FGD systems were being used in 27 countries and there to be 678 FGD devices operating on a full power plant volume of about 229 gigawatts. About 45% of the FGD capacity was in the U.S., 24% in Germany, 11% in Japan and also 20% in assorted other countries. About 79% of the units, representing around 199 gigawatts of capacity were making use of lime or limestone wet scrubbing. Around 18% (or 25 gigawatts) made use of spray-dry scrubbers or sorbent injection systems.<3><4><5>

Sulfuric acid mist formation

Fossil fuel such together coal and also oil contain far-ranging amounts the sulfur. When burned, around 95 percent or an ext of the sulfur is typically converted come sulfur dioxide (SO2). This wake up under normal conditions of temperature and also of oxygen existing in the flue gas. However, there are scenarios under which this may not it is in the case.

For example, as soon as the flue gas has actually too lot oxygen and also the SO2 is further oxidized come sulfur trioxide (SO3). Actually, too much oxygen is only one of the methods that SO3 is formed. Gas temperature is also vital factor. At around 800 °C, development of SO3 is favoured. Another means that SO3 have the right to be created is with catalysis by metals in the fuel. This is an especially true for hefty fuel oil, where far-reaching amounts that vanadium room present. In whatever way that SO3 is formed, the does no behave favor SO2 in the it develops a fluid aerosol known as sulfuric acid (H2SO4) mist that is very complicated to remove. Generally, about 1% the the sulfur dioxide will be convert to SO3. Sulfuric mountain mist is regularly the cause of the blue haze the often shows up as the flue gas plume dissipates. Increasingly, this problem is being addressed through the use of wet electrostatic precipitators.

FGD chemistry

Scrubbing through a simple solid or equipment

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

CaCO3 (solid) + SO2 (gas) → CaSO3 (solid) + CO2 (gas)

When wet scrubbing v a Ca(OH)2 (lime) slurry, the reaction also produces CaSO3 (calcium sulphite) and also can it is in expressed as:

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

When wet scrubbing v a Mg(OH)2 (magnesium hydroxide) slurry, the reaction produces MgSO3 (magnesium sulphite) and also can it is in expressed as:

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

Some FGD systems go a action further and oxidize the CaSO3 (calcium sulphite) to create marketable CaSO4 · 2H2O (gypsum):

CaSO3 (solid) + ½O2 (gas) + 2H2O (liquid) → CaSO4 · 2H2O (solid) types of wet scrubbers offered in FGD

To promote maximum gas-liquid surface ar area and also contact time, a number of wet scrubber designs have been used. Common ones are:

Mobile-bed scrubbers Venturi-rod scrubbers
Main article: venturi scrubber

A venturi scrubber is a converging/diverging ar of duct. The converging section speeds up the gas stream come high velocity. When the fluid stream is injected at the throat, which is the allude of best velocity, the turbulence led to by the high gas velocity atomizes the fluid into little droplets, which creates the surface area crucial for mass transport to take place. The higher the press drop in the venturi, the smaller the droplets and the higher the surface area. The penalty is in power consumption.

key towers pack bed scrubbers

A packed scrubber consists of a tower through packing material inside. This pack material can be in the form of saddles, rings or part highly dedicated shapes designed to maximize contact area in between the dirty gas and also liquid. Pack towers typically operate at much reduced pressure drops than venturi scrubbers and are thus cheaper come operate. They also typically offer higher SO2 remove efficiency. The border is that they have actually a greater tendency to plug up if corpuscle are existing in overfill in the exhaust wait stream.

Spray towers
Main article: spray tower

A spray tower is the simplest kind of scrubber. It consists of a tower with spray nozzles, which create the droplets for surface contact. Spray towers are typically used once circulating a slurry (see below). The high rate of a venturi would reason erosion problems, if a pack tower would certainly plug increase if the tried come circulate a slurry.

Countercurrent packed towers space infrequently used due to the fact that they have a tendency to become plugged by accumulated particles or to scale as soon as lime or limestone scrubbing slurries are used.

Scrubbing reagent

As explained above, alkaline sorbents are used for scrubbing flue gases to remove SO2. Relying on the application, the 2 most crucial are lime and sodium hydroxide (also recognized as caustic soda). Lime is frequently used on huge coal or oil fired boilers as discovered in power plants, as it is very much less expensive than caustic soda. The difficulty is that it results in a slurry gift circulated through the scrubber rather of a solution. This makes it more tough on the equipment. A spray tower is frequently used because that this application. The usage of lime results in a slurry that calcium sulfite (CaSO3) that must be disposed of. Fortunately, calcium sulfite have the right to be oxidized to create by-product gypsum (CaSO4 · 2H2O) i beg your pardon is marketable for use in the building products industry.

Caustic soda is limited to smaller burning units because it is much more expensive 보다 lime, but it has actually the advantage that it creates a solution fairly than a slurry. This makes it less complicated to operate. It produces a systems of salt sulfite/bisulfite (depending ~ above the pH), or salt sulfate that should be disposed of. This is no a trouble in a kraft pulp mill for example, where this can be a resource of makeup chemistry to the restore cycle.

Scrubbing with sodium sulfite equipment

It is possible to scrub sulfur dioxide by utilizing a cold equipment of salt sulfite, this creates a sodium hydrogen sulfite solution. By heater this equipment it is feasible to reverse the reaction to type sulfur dioxide and the sodium sulfite solution.

In some methods this have the right to be thought of together being comparable to the reversible liquid-liquid extraction of an inert gas such as xenon or radon (or some other solute i m sorry does no undergo a chemical change during the extraction) native water to one more phase. When a chemical change does occur throughout the extraction of the sulfur dioxide indigenous the gas mixture, the is the case that the exploit equilibrium is shifted by transforming the temperature rather than through the use of a chemical reagent.

Gas step oxidation adhered to by reaction with ammonia

A new, emerging flue gas desulfurization technology has been explained by the IAEA.<6> the is a radiation an innovation where an intense beam of electron is fired into the flue gas in ~ the exact same time together ammonia is added to the gas. The Chendu strength plant in China began up together a flue gas desulfurization unit top top a 100 MW range in 1998. The Pomorzany power plant in Poland additionally started increase a similar sized unit in 2003 and that plant removes both sulfur and also nitrogen oxides. Both plants are reported to be operating successfully.<7><8> However, the accelerator architecture principles and also manufacturing quality require further advancement for consistent operation in commercial conditions.<9>

No radiation is required or produced in the process. The electron beam is produced by a maker similar to the electron pistol in a TV set. This an equipment is referred to as an accelerator. This is an instance of a radiation chemistry process<8> where the physical results of radiation are supplied to procedure a substance.

The action of the electron beam is to promote the oxidation of sulfur dioxide come sulfur(VI) compounds. The ammonia reacts with the sulfur compounds thus developed to develop ammonium sulfate which deserve to be used as a fertilizer follow to the IAEA. In addition, it can be used to lower the nitrogen oxide content of the flue gas. This technique has attained industrial plant scale.<7><10><11>

Facts and also statistics

The information in this section was obtained from a united state EPA published reality sheet.<12>

Flue gas desulfurization scrubbers have been used to burning units shoot coal and also oil that selection in size from 5 MW to 1500 MW. Scottish Power are spending £400 million installation FGD in ~ Longannet power terminal which has a capacity of over 2 GW. Dried scrubbers and spray scrubbers have normally been used to units smaller sized than 300 MW.

Approximately 85% of the flue gas desulfurization units mounted in the US space wet scrubbers, 12% are spray dry systems and 3% are dry injection systems.

The highest SO2 removed efficiencies (greater than 90%) are accomplished by wet scrubbers and the lowest (less 보다 80%) by dry scrubbers. However, the more recent designs for dry scrubbers are qualified of achieving efficiencies in the stimulate of 90%.

In spray drying and dry injection systems, the flue gas must an initial be cooled to about 10-20 °C above adiabatic saturation to prevent wet solids deposition ~ above downstream equipment and also plugging of baghouses.

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The capital, operating and maintenance prices per short ton that SO2 gotten rid of (in 2001 us dollars) are:

For wet scrubbers larger than 400 MW, the cost is $200 come $500 every ton because that wet scrubbers smaller sized than 400 MW, the price is $500 come $5,000 every tonFor spray dried scrubbers larger than 200 MW, the cost is $150 come $300 per tonFor spray dried scrubbers smaller sized than 200 MW, the expense is $500 to $4,000 per ton

different methods of reducing sulfur dioxide emissions

An alternate to removing sulfur from the flue gases after burning is to eliminate the sulfur indigenous the fuel prior to or during combustion. Hydrodesulfurization of fuel has actually been supplied for dealing with fuel oils before use. Fluidized bed combustion adds lime to the fuel throughout combustion. The lime reacts with the SO2 to type sulfates which become component of the ash.