Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1990-10-10
1992-07-21
Heller, Gregory A.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
B01J 800, C01B 2100
Patent
active
051321033
DESCRIPTION:
BRIEF SUMMARY
FIELD OF THE INVENTION
The present invention relates to a process for the removal of nitrogen oxides (NO.sub.x) from offgases from turbines by a selective catalytic reduction (SCR) with ammonia. It is possible to obtain up to 98-99% removal (deNO.sub.x) of the nitrogen oxides in the offgases (exhaust gas), e.g. the offgases from the turbo-charger of a Diesel engine, by the SCR with ammonia and at the same time obtain after purification, gas containing less than a few ppm of ammonia.
BRIEF DESCRIPTION OF THE DRAWINGS
In the following the prior art and the invention will be described with reference to the drawings, in which
FIG. 1 schematically shows a boiler according to prior art, provided with combustion and SCR catalyst sections,
FIG. 2 shows the relationship between the NH.sub.3 /No.sub.x proportion in the gas before the SCR catalyst, the obtained degree of deNO.sub.x and the NH.sub.3 slip from the SCR catalyst layer, and
FIG. 3 schematically illustrates the principle of the process according to the invention.
BACKGROUND OF THE INVENTION
Known processes for the removal of NO.sub.x from offgases from turbines, gas engines and Diesel engines are described in, i.a., the periodical "Power", October 1988, in the article "Reducing NO.sub.x Emissions from Boilers, Gas Turbines end Engines". The most commonly used of these processes is explained with reference to FIG. 1. Here the gas is cooled from a turbine or exhaust turbine 11 in a boiler 12 in which the gas is passed first through a layer of a combustion catalyst 13 for combusting CO and hydrocarbons in the gas, after which ammonia is added to the gas by means of a system consisting of a large number of nozzles 14 distributed over the cross section of the boiler. Subsequently the gas is cooled in a section 15 of the boiler, typically at 340.degree.-380.degree. C., before it is passed through one or more layers or panels of a catalyst 16 catalyzing the SCR reaction: are removed selectively, i.e. without the ammonia being oxidized by the oxygen present, which is typically between 5% and 16% O.sub.2. Thereafter the gas is further cooled down to typically 100.degree. C. in a subsequent boiler section 17 before the gas is passed to a stack 18. Because of an inhomogenous admixture of the ammonia into the gas it is difficult by this process to obtain a degree of purification for NO.sub.x above about 90% if the gas contains more than about 500 ppm of NO.sub.x and if the residual content of NH.sub.3, the so-called NH.sub.3 -slip, must not exceed, e.g., 10 ppm of NH.sub.3 after the layer of SCR catalyst.
SCR catalysts are well-known. Frequently they contain as the catalystically active material oxides of vanadium, molybdenum, tungsten, iron (Fe.sub.2 O.sub.3) or the lanthanides on a carrier of, notably, TiO.sub.2 and/or Al.sub.2 O.sub.3.
An improved admixture of the ammonia may be obtained if the addition takes place in the pipe leading from the turbine to the boiler, which prevents the application of a combustion catalyst before the SCR catalyst; or if the cross sectional area of the flow is narrowed strongly at the point of adding the ammonia according to FIG. 1, which, however, will entail other disadvantages in the form of increased initial expenditure and bulk as well as an increased loss of pressure. Another drawback inherent in this prior art is that the use of combustion catalysts based on platinum metals known for the purification of exhaust gases cause the residual content of ammonia to be predominantly or almost completely oxidized to NO.sub.x.
It is the object of the present invention to provide a process for the removal of nitrogen oxides from the exhaust gas from turbines while at the same time obtaining a minimum residual content of ammonia in the purified gas.
DISCLOSURE OF THE INVENTION
The process of the invention is characterized in that the stream of gaseous ammonia needed for the catalytic reduction of the nitrogen oxides is added before the turbine at a temperature of the offgases of 400.degree. C. to 1200.degree. C. and a pressure of
REFERENCES:
patent: 4115516 (1978-09-01), Takami et al.
patent: 4393031 (1983-07-01), Henke
patent: 4473536 (1984-09-01), Carberg et al.
patent: 4473537 (1984-09-01), Ford et al.
Haldor Topsoe A/S
Heller Gregory A.
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