Power station flue gases

The reaction chambers used to remove nitrogen oxides from power station flue gases constitute the largest type of fixed-bed reactors as regards reactor volume and throughput, while automobile exhaust purification... 

Figure 11. Reaction chamber for removal of nitrogen oxides from power station flue gases [23].

Purely adiabatic fixed-bed reactors are used mainly for reactions with a small heat of reaction. Such reactions are primarily involved in gas purification, in which small amounts of noxious components are converted. The chambers used to remove NO, from power station flue gases, with a catalyst volume of more than 1000 m3, are the largest industrial adiabatic reactors, and the exhaust catalyst for internal combustion engines, with a catalyst volume of ca. 1 L, the smallest. Typical applications in the chemical industry include the methanation of traces of CO and CO2 in NH3 synthesis gas, as well as the hydrogenation of small amounts of unsaturated compounds in hydrocarbon streams. The latter case requires accurate monitoring and regulation when hydrogen is in excess, in order to prevent complete methanation due to an uncontrolled temperature runaway. 

There are, of course, several assumptions behind this apparently simple description. One is that the 834S signal of all fossil fuel is in the above range. At the moment only a rather small number of samples of power-station flue gases from limited locations have been analysed. A second assumption is that the 534S signal of seawater SO4 is not altered significantly when DMS crosses the air-sea interface and is oxidized to S02 and SO4 in the atmosphere. The evidence to date indicates that neither of these assumptions introduces much error, but more work is required to prove this approach. 

SCR process power station flue gases mordenite NOj -free off-gas... 

An investigation into potential catalytic cycles for the removal of oxides of nitrogen from power station flue gases has resulted in the observation that in aqueous solution NO, HONO, and NOJ react with Fe(II)(edta) to produce Fe(II)(edta)NO. Although the reaction with NO is too fast for SF and TJ techniques, the reactions with HONO/NO2 are within the SF timescale. This reaction follows two parallel paths which are pseudo-first and pseudo-zero order in Fe(II)(edta) concentration, respectively. The contribution of each path depends on the pH and the total nitrite concentration, and the overall rate law is given by Eq. (29), where K and = 62 and 90 dm mol... 

Untreated power station flue gases generally contain between 3% and 16% CO2 (Holloway et al. 1996) with much of the rest being nitrogen. 

CO2 is ubiquitous - it can be either extracted pure from natural weUs or recovered from various industrial sources. For instance, quite pure CO2 is recovered from urea synthesis. Several other industries, such as those using fermentation or similar methods, also provide a convenient source of pure CO2 (above 99%) at low recovery cost, but their seasonality prevents fuU exploitation so that several million tons per year (Mt year ) of pure CO2 are vented. Today, there is a growing interest in recovering CO2 from power station flue gases that contain around 14% of CO2, but the separation techniques are quite expensive [4] and are seldom applied on a large scale [5]. 

The milk of lime-sulfite process was widely used for early power station flue gas desulfurisation projects. It had a lower capital cost than the limestone-sulfite process and gave high absorption and reagent efficiencies. Subsequently, the problem of disposal of the calcium sulfite sludge led to three variants based on lime to be adopted — the gypsum process, the dual alkali process and the maglime process. 

Nitrogen in the coal itself also contributes significantly to the formation of gases. Of the nitrogen oxides emitted in power station flue gas, 95% is nitric oxide (NO), which oxidizes rapidly in the atmosphere to form nitrogen dioxide (NO2) (Commission on Energy, 1981). 

Flue gas desulfurization (FGD) (Figure 8.30) removes sulfur dioxide from the exhaust gases before they leave the power station flue (chimney). 

Other Uses. Other uses include intermediate chemical products. Overall, these uses account for 15—20% of sulfur consumption, largely in the form of sulfuric acid but also some elemental sulfur that is used directly, as in mbber vulcanization. Sulfur is also converted to sulfur trioxide and thiosulfate for use in improving the efficiency of electrostatic precipitators and limestone/lime wet flue-gas desulfurization systems at power stations (68). These miscellaneous uses, especially those involving sulfuric acid, are intimately associated with practically all elements of the industrial and chemical complexes worldwide. 

Environmental considerations in recent years have dictated that sulphur bearing compounds are removed from the exhaust gases of coal burning power stations in order to reduce the incidence of acid rain . The flue gas... 

This process consists of the separation of C02 from flue gas produced during the combustion of fossil fuels and can be applied to large flue gas stationary sources as thermal power stations and industrial processes. 

Battersea A pioneering flue-gas desulfurization process, operated at Battersea power station, London, from 1931 until the station was closed. The flue-gases were washed with water from the River Thames whose natural alkalinity was augmented by chalk slurry. One of the problems of this process was cooling of the stack gases, which caused the plume to descend on the neighborhood. 

Bergbau-Forschung/Uhde A flue-gas desulfurization process that uses a movable bed of hot coke. Operated in a power station in Arzberg, Germany, since 1987. 

Holter A flue-gas desulfurization process in which the sulfur dioxide is absorbed in an aqueous suspension of calcium hydroxide and calcium chloride, yielding gypsum. Operated in an experimental plant at the Weiherr III power station in Quierscheid, Germany, in 1988. 

Howden An early flue-gas desulfurization process using a lime or chalk slurry in wooden grid-packed towers. The calcium sulfate/sulfite waste product was intended for use in cement manufacture, but this was never commercialized. The key to the process was the use of a large excess of calcium sulfate in suspension in the scrubbing circuit, which minimized the deposition of scale on the equipment. The process was developed by Imperial Chemical Industries and James Howden Company in the 1930s and operated for several years at power stations at Fulham, London, and Tir John, South Wales, being finally abandoned during World War II. British Patents 420,539 433,039. 

LIFAC [Limestone in-fumace and Added Calcium] A dry flue-gas desulfurization process in which limestone is injected into the lumace and calcium hydroxide is injected after it. Developed by Tampella in 1984 and used in a power station in Finland. A demonstration plant was built for Saskatchewan Power, Canada, in 1990. 

Powerclaus A flue-gas desulfurization system which applies the Aquaclaus process to power station effluent gases. 

SHU [Saarberg-Holter-Lurgi] A flue-gas desulfurization process using wet limestone as the sembbing medium, assisted by the addition of dilute formic acid. Developed by the companies named, and used in 11 power stations in Germany and Turkey in 1987. 

SNOX A combined flue-gas desulfurization and denitrification process. The NOx is first removed by the SCR process, and then the S02 is catalytically oxidized to S03 and converted to sulfuric acid by the WSA process. Developed by Haldor Topsoe and first operated at a power station in Denmark in the 1990s. 

The nse of low-snUur coals and the introduction of more efficient flue gas desulfurization facilities in conventional power stations have contributed to a significant lowering of emissions, but further improvement by these means is limited. 

When coal is combusted a number of ash products are produced. In a conventional coal-fired power station the ash that enters the flue gas stream is referred to as the fly ash or pulverized fuel ash. This is volumetrically the most important fraction and although considerable progress has been made in utilizing this material, nevertheless there is an excess production, much of which ends in lagoons or, on a longer term basis, in landfill sites. The potential impacts of fly ash on surface water and groundwater therefore have to be considered both in the short and long term. The annual European production of fly ash in 2000 was 38.96 x 106 t, of which... 

Bottom ash from power stations is less of a problem compared with fly ash for the contamination of natural waters firstly because the proportions of fly ash to bottom ash are approximately three to one and a greater proportion of the bottom ash is used (ECOBA 2003). Secondly, the volatile elements are depleted compared with fly ash (Clarke Sloss 1992). Other combustion residues include fluidized-bed boiler ashes and the products from flue gas desulphurization (FGD). The non-regenerable FGD systems commonly use limestone, slaked lime, or a mixture of slaked lime and alkaline fly ash that are sprayed into the flue gases to remove SO2 (Clarke Sloss 1992). Although 90 wt% of the product is used to replace natural gypsum in plasters and wallboards, there is currently a small excess production in Europe of that is disposed of in landfill and equivalent sites (ECOBA 2003). Because the FGD plant treats the cooled flue gases volatile elements are concentrated and there will be similarities with fly ash. 

Units called flue gas desulfurisation (FGD) units are being fitted to some power stations throughout the world to prevent the emission of sulfur dioxide gas. Here, the sulfur dioxide gas is removed from the waste gases by passing them through calcium hydroxide slurry. This not only removes the sulfur dioxide but also creates calcium sulfate, which can be sold to produce plasterboard (Figure 11.25). The FGD units are very expensive and therefore the sale of the calcium sulfate is an important economic part of the process. 

Flue gas desulfurisation (FGD) The process by which sulfur dioxide gas is removed from the waste gases of power stations by passing them through calcium hydroxide slurry. 

According to the data for volumetric mass transfer coefficient measured in the device on a small pilot plant scale, for a certain load of flue gas to be processed, the required total volume of the reactor under consideration would be very small, only about 1/3 that of existing wet FGD equipment. In addition, the arrangement of the internal wet cyclone shown in Fig. 7.23 enables the reactor to have simultaneously high ash-removal efficiency. The reactor is especially suitable for the wet desulfurization of flue gas with hydrated lime or dilute ammonia solution as the absorbent. The design of the large-scale reactor suitable for a power station has now been accomplished and is expected to be applied industrially in the very near future. 

Many criteria must be considered when choosing or developing absorbents for C02 removal, including selectivity, cost, and stability. There are mainly two C02 separation issues (1) For post combustion capture from flue gas (e.g., coal-fired power station), a major obstacle is the low pressure of the flue gas (1 atm), with just 15 % C02 concentration together with other component gasses, predominantly N2. 

Satriana (2) provides a summary of the development of flue gas treatment technology. The first commercial application of flue gas scrubbing for sulfur dioxide control was at the Battersea-A Power Station [228 MW(e)] in London, England, in 1933. The process used a packed spray tower with a tail-end alkaline wash to remove 90 percent of the sulfur dioxide and particulates. Alkaline water from the Thames River provided most of the alkali for absorption. The scrubber effluent was discharged back into the Thames River after oxidation and settling. A similar process was also operated at the Battersea-B Power Station [245 MW(e)] beginning in 1949. The Battersea-B system operated successfully until 1969, when desulfurization efforts were suspended due to adverse effects on Thames River water quality. The Battersea-A system continued until 1975, when the station was closed. 

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