Flue-gas desulfurization
Flue-gas desulfurization (FGD) is a set of technologies used to remove sulfur dioxide (SO2) from exhaust flue gases of fossil-fuel power plants, and from the emissions of other sulfur oxide emitting processes such as waste incineration, petroleum refineries, cement and lime kilns.
Methods
Since stringent environmental regulations limiting SO2 emissions have been enacted in many countries, SO2 is being removed from flue gases by a variety of methods. Common methods used:
- lime, or seawater to scrub gases;
- Spray-dry scrubbing using similar sorbent slurries;
- Wet sulfuric acid process recovering sulfur in the form of commercial quality sulfuric acid;
- 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 SO2 and SO3 from process emissions.[1]
For a typical coal-fired power station, flue-gas desulfurization (FGD) may remove 90 per cent or more of the SO2 in the flue gases.[2]
History
Methods of removing sulfur dioxide from boiler and furnace exhaust gases have been studied for over 150 years. Early ideas for flue gas desulfurization were established in England around 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 SO
2 emissions problem did not receive much attention until 1929, when the
2 controls on all such power plants.[3]
The first major FGD unit at a utility was installed in 1931 at Battersea Power Station, owned by London Power Company. 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. These three early large-scale FGD installations were suspended during World War II, because the characteristic white vapour plumes would have aided location finding by enemy aircraft.[4] 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.[5] Large-scale FGD units did not reappear at utilities until the 1970s, where most of the installations occurred in the United States and Japan.[3]
In 1970, the
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
FGD on ships
The International Maritime Organization (
Sulfuric acid mist formation
SO2 can further oxidize into sulfur trioxide (SO3) when excess oxygen is present and gas temperatures are sufficiently high. At about 800 °C, formation of SO3 is favored. Another way that SO3 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 is present. In whatever way 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. 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.
FGD chemistry
Basic principles
Most FGD systems employ two stages: one for
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 SO2. 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, 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
SO2 is an acid gas, and, therefore, 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 calcium sulfite (CaSO3) and may be expressed in the simplified dry form as:
- CaCO3(s) + SO2(g) → CaSO3(s) + CO2(g)
When wet scrubbing with a Ca(OH)2 (
- Ca(OH)2(s) + SO2(g) → CaSO3(s) + H2O(l)
When wet scrubbing with a Mg(OH)2 (magnesium hydroxide) slurry, the reaction produces MgSO3 (magnesium sulfite) and may be expressed in the simplified dry form as:
- Mg(OH)2(s) + SO2(g) → MgSO3(s) + H2O(l)
To partially offset the cost of the FGD installation, some designs, particularly dry sorbent injection systems, further oxidize the CaSO3 (calcium sulfite) to produce marketable CaSO4·2H2O (
- 2 CaSO3(aq) + 4 H2O(l) + O2(g) → 2 (CaSO4·2H2O(s))
A natural alkaline usable to absorb SO2 is seawater. The SO2 is absorbed in the water, and when oxygen is added reacts to form sulfate ions SO2−4 and free H+. The surplus of H+ is offset by the carbonates in seawater pushing the carbonate equilibrium to release CO2 gas:
- SO2(g) + H2O(l) + 1/2 O2(g) → SO2−4(aq) + 2 H+
- HCO−3 + H+ → H2O(l) + CO2(g)
In industry
:- 2 NaOH(aq) + SO2(g) → Na2SO3(aq) + H2O(l)[15]
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 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 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 SO2 removal than other absorber designs.
FGD scrubbers produce a scaling wastewater that requires treatment to meet U.S. federal discharge regulations.[16] However, technological advancements in ion-exchange membranes and electrodialysis systems has enabled high-efficiency treatment of FGD wastewater to meet recent EPA discharge limits.[17] The treatment approach is similar for other highly scaling industrial wastewaters.
Venturi-rod scrubbers
A venturi scrubber 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 SO2 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 SO2 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 SO2 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 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
A spray tower is the simplest type of scrubber. 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). 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
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
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 "
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. 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.
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. 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
No
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 nitrogenous fertilizer. In addition, it can be used to lower the nitrogen oxide content of the flue gas. This method has attained industrial plant scale.[19][22]
Facts and statistics
- The information in this section was obtained from a US EPA published fact sheet.[23]
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 are spending £400 million installing FGD at Longannet power station, 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
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
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
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
See also
- Incineration
- Scrubber
- Flue-gas emissions from fossil-fuel combustion
- Flue-gas stacks
- Wellman–Lord process
References
- ^ "Dry Sorbent Injection Technology | Nox Control Systems".
- ^ Compositech Products Manufacturing Inc. "Flue Gas Desulfurization – FGD Wastewater Treatment | Compositech Filters Manufacturer". www.compositech-filters.com. Retrieved 30 March 2018.
- ^ .
- ISBN 0-19-854673-4.
- S2CID 159395306.
- ^ "Evolution of the Clean Air Act". Washington, D.C.: U.S. Environmental Protection Agency (EPA). 3 January 2017.
- ^ ASME, 2017, "Flue Gas Desulfurization Units", ASME PTC 40-2017
- ^ "Clean Air Interstate Rule". EPA. 2016.
- ^ Beychok, Milton R., Coping With SO2, Chemical Engineering/Deskbook Issue, 21 October 1974
- ^ 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
- S2CID 28265636. Archived from the originalon 9 October 2014.
- ^ Beychok, Milton R., Comparative economics of advanced regenerable flue gas desulfurization processes, EPRI CS-1381, Electric Power Research Institute, March 1980
- ^ "Index of MEPC Resolutions and Guidelines related to MARPOL Annex VI". Archived from the original on 18 November 2015.
- ^ Jesper Jarl Fanø (2019). Enforcing International Maritime Legislation on Air Pollution through UNCLOS. Hart Publishing.
- ISSN 0976-0083.
- ^ "Steam Electric Power Generating Effluent Guidelines – 2015 Final Rule". EPA. 30 November 2018.
- ^ "Lowering Cost and Waste in Flue Gas Desulfurization Wastewater Treatment". Power Mag. Electric Power. March 2017. Retrieved 6 April 2017.
- ^ IAEA Factsheet about pilot plant in Poland.
- ^ a b Haifeng, Wu. "Electron beam application in gas waste treatment in China" (PDF). Proceedings of the FNCA 2002 workshop on application of electron accelerator. Beijing, China: INET Tsinghua University.
- ^ a b Section of IAEA 2003 Annual Report Archived 21 February 2007 at the Wayback Machine
- ISSN 0029-5922.
- ^ Industrial Plant for Flue Gas Treatment with High Power Electron Accelerator by A.G. Chmielewski, Warsaw University of Technology, Poland.
- ^ "Air Pollution Control Technology Fact Sheet: Flue Gas Desulfurization" (PDF). Clean Air Technology Center. EPA. 2003. EPA 452/F-03-034.
- ^ "HIOPAQ Oil & Gas Process Description". Utrecht, The Netherlands: Paqell BV. Retrieved 10 June 2019.
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