Combustion – Process of combustion or burner operation – In a porous body or bed – e.g. – surface combustion – etc.
Patent
1992-07-15
1995-04-18
Langel, Wayne
Combustion
Process of combustion or burner operation
In a porous body or bed, e.g., surface combustion, etc.
431 4, 4232391, B01D 5334
Patent
active
054076498
DESCRIPTION:
BRIEF SUMMARY
The invention relates to a method for reducing emission of NO.sub.x in a combustion process, wherein ammonia or another substance containing nitrogen, or a substance producing ammonia or nitrogen is supplied to the flue gas, and the obtained mixture is supplied to a reaction vessel.
U.S. Pat. No. 4,793,981 describes a method for controlling the emission of SO.sub.x, NO.sub.x and particles from a coal fired boiler system, wherein a powder which is a reagent/catalyst is supplied to the flue gas between the boiler and an economizer and is made to react with SO.sub.x and NO.sub.x, while the flue gas is passing through the economizer. An additional reaction is obtained when the flue gas is then led through filter bags and the accumulated filter cake thereof.
The publication KOLBLADET 89-3, 1989-03-20, describes purifying of flue gas at temperatures above 400.degree. C. by means of suitable catalysts, by which NO.sub.x can be removed up to 90%. Ammonia and alkali are injected into the flue gas, and the mixture is then led through a ceramic filter bag on which ashes rich in alkali are deposited, through which the flue gas passes then to be brought into contact with a catalyst inside the filter bag.
In order to obtain a high NO.sub.x -reduction, the ammonia is injected in a larger quantity than that corresponding to the stochiometric ratio. This over-dosage, which is made in order to overcome incomplete mixing and to obtain satisfactory NO.sub.x reduction, causes the unreacted ammonia to escape with the flue gases, so called "slip". U.S. Pat. No. 4,423,017 describes a way to reduce "slip" in a non-catalytic combustion procedure for reducing NO-emission, wherein a reducing gas comprising ammonia only or ammonia in combination with one or more reducing gases, is injected into flue gas, containing NO and oxygen, in a reaction zone. At the end of this zone a metallic material is placed so as to substantially reduce the discharge of ammonia into the atmosphere.
The method according to the invention is based on the conditions existing at combustion in a pressurized fluidized bed with addition of absorbant, for example limestone or dolomite. The dust that occurs in the flue gas from this combustion system has a special mineralogical composition and is of amorphous character. Compared to atmospheric combustion systems in which SO.sub.2 is reduced by the addition of for example limestone (CaCO.sub.3), a calcination to CaO occurs, which is the active component against SO.sub.2. The product is anhydrite, CaSO.sub.4. Since the limestone due to inertia in the chemical reactions, always has to be supplied in surplus, there will always be CaO in the flue gas dust in an atmospheric system. It is known that CaO catalyses the formation of NO.sub.x, i.e. the net yield for reduction of NO.sub.x with ammonia will potentially be lower when CaO is present.
At combustion in a pressurized fluidized bed with supply of limestone (or dolomite), it has been found that CaO is present neither in the flue gas dust nor in the discharged bed material. This condition is explained by the partial pressure of CO.sub.2 being higher in a pressurized fluidized bed than in, for example, an atmospheric fluidized bed. The requirements for calcination at the current levels of temperature up to 1000.degree. C. are not satisfied, which can be shown also thermodynamically. The sulphur absorbance in the limestone has been postulated to occur by direct substitution, CaCO.sub.3 +SO.sub.2 +1/2 O.sub.2 .fwdarw.CaSO.sub.4 +CO.sub.2, in a pressurized system. Thus, the net yield for reduction of NO.sub.x by ammonia in the flue gas from a pressurized fluidized bed can become higher than in an atmospheric system. Furthermore, the dust and the sulphate surfaces, which are formed in sulphating the limestone, have a favourable effect by catalysing the NO.sub.x -reduction.
With knowledge of these special conditions at combustion in a pressurized fluidized bed, the method according to the invention has obtained the characterizing features according to claim 1.
For further exp
REFERENCES:
patent: 4154803 (1979-05-01), Uckikawa et al.
patent: 4423017 (1983-12-01), Dean
patent: 4782771 (1988-11-01), Bergkvist
patent: 4793981 (1988-12-01), Doyle et al.
patent: 4847054 (1989-07-01), Weisweiler
Andersson Mats
Damsgaard Hans-Jorgen
Greis Ingemar
Lindgren Lotta
Nystrom Olle
ABB Carbon AB
Dunn Jr. Thomas G.
Langel Wayne
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