Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1992-07-16
1994-04-19
Heller, Gregory A.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
423235, B01J 800, C01B 1700
Patent
active
053043626
DESCRIPTION:
BRIEF SUMMARY
The invention refers to a method in cleaning flue gas in a PFBC plant including a gas turbine driven thereby, the term PFBC plant referring to a boiler with a pressurized fluidized bed or several such beds and, if necessary, a cyclone separator and/or filter for cleaning the flue gas from the boiler before such gas being supplied to the gas turbine.
In order to reduce emission of NO.sub.x in the combustion, it is known to apply a process known as thermal reduction, wherein ammonia or another nitrogen containing substance, or a nitrogen generating substance is supplied to the flue gas within the boiler itself. When henceforth ammonia is mentioned, it is intended to include therein also such substances that contain or generate ammonia or nitrogen when supplied to the hot flue gas. The supply of ammonia can be obtained in a differentiated manner by supplying the ammonia to a number of different sites in the boiler and adjusting the amount of ammonia at each site to the amount of NO.sub.x which occurs in different zones in the boiler. It also happens that the ammonia is injected into the flue gas downstream of the boiler in a separate reaction vessel, in which flue gas and ammonia are mixed intimately in order to interreact. In order to obtain a high NO.sub.x reduction, ammonia is supplied in a greater amount than that corresponding to the stoichiometric ratio, which may cause escape of unreacted ammonia with the flue gas, what is called "slip", specific measures having to be taken in order to render the surplus ammonia harmless before the flue gas is released to the atmosphere, which, of course, is a disadvantage and involves enhanced capital costs. Another and more obvious disadvantage in connection with a PFBC plant is, however, that by supplying ammonia the thermal reduction of NO.sub.x in the boiler becomes unacceptably low or even non-existent at too low partial loads due to the low temperature of the gas from the boiler.
Another known way of reducing NO.sub.x in the flue gas is the catalytic reduction, ammonia being supplied downstream of the boiler and the mixture of flue gas and ammonia being contacted with a catalyst e.g. vanadin. The catalyst does not participate in the reaction between flue gas and ammonia but must, nevertheless, from time to time be replaced or exchanged, which adds to the operating costs of the plant.
In order to obtain a reduction of NO.sub.x in the flue gas, which is advantageous as to capital and operating economy, to an acceptable value in a plant of the type referred to above over a large range of partial load of the PFBC plant without unacceptable "slip" being obtained, the method proposed according to the invention has obtained the characteristics specified in claim 1. Then a smaller amount of catalyst can be used than if the total reduction of NO.sub.x would be effected by catalytic reduction since the catalyst needs to be dimensioned only for catalytic reduction to the smaller extent that is required in order to obtain a satisfactory NO.sub.x reduction also at low partial loads, the catalyst at the same time providing the supplementary reduction that may be necessary for consumption of possible surplus of ammonia leaving the boiler, because at that site it has not had the opportunity to react with present NO.sub.x. Due to the unique combination of thermal and catalytical reduction in a plant of the kind referred to herein it is possible to achieve totally a NO.sub.x reduction without release of unpermitted amounts of unreacted ammonia, which are lower than the demands presently stipulated.
BRIEF DESCRIPTION OF THE DRAWINGS
In order to further explain the invention, the accompanying drawing is referred to, on which
FIG. 1 diagrammatically shows a PFBC plant with a turbine driven thereby, and
FIG. 2 is a diagram illustrating the relationship between the NO.sub.x content in the flue gas and the load of the PFBC plant.
The plant according to FIG. 1 comprises a PFBC plant with a pressure vessel 10, in which a boiler 11 is located, said boiler having one or several fluidize
REFERENCES:
patent: 4393031 (1985-07-01), Henke
patent: 4473536 (1984-12-01), Carberg et al.
patent: 4473537 (1984-12-01), Ford et al.
patent: 4737345 (1988-04-01), Henke
patent: 5132103 (1992-07-01), Schoubye
ABB Carbon AB
Heller Gregory A.
Mason, Jr. Joseph C.
Smith Ronald E.
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