Combustion – Process of combustion or burner operation – In a porous body or bed – e.g. – surface combustion – etc.
Patent
1987-05-14
1989-04-25
Yeung, James C.
Combustion
Process of combustion or burner operation
In a porous body or bed, e.g., surface combustion, etc.
431 12, 431170, 110347, 110345, 423239, F23B 700, F23C 900
Patent
active
048243604
DESCRIPTION:
BRIEF SUMMARY
The present invention relates to a method for decreasing emissions of nitrogen oxides and sulfur oxides when burning a fuel which contains nitrogen and sulfur. The method is based on the control of the combustion process so as to decrease the formation of nitrogen oxides and/or on the reduction of nitrogen oxides present in the flue gases and on the binding of the sulfur oxides present in the flue gases to a pulverous material. The method is especially suitable for the treatment of gases produced from a solid fuel such as pulverized coal.
The burning of fossil fuels produces sulfur oxides and nitrogen oxides, which are deleterious to the environment; the environmental hazards due to them became central problems in energy technology in the late 1970s. In Japan, the United States and certain Western European countries, statutory norms have been set regarding the maximum allowed emissions of sulfur dioxides (SO.sub.x) and nitrogen oxides (NO.sub.2), and most likely there will be corresponding developments in all industrialized countries in the near future.
In the known solutions to the problem, emissions of nitrogen oxides are limited primarily by affecting the combustion process so that oxides of nitrogen are formed at a minimal rate. A method known to be effective is to introduce the combustion air in steps so that the pyrolysis and preoxidation of the fuel occur in substoichiometric conditions. Thereby the nitrogen bound in the organic part of the fuel is at least in part rendered to the form of a stable N.sub.2 molecule, whereupon its oxidation remains low. In general, the maximum temperature in the combustion chamber can also be limited by introducing the air in steps, and this has a decreasing effect on the so-called thermal NO.sub.x. The returning of cold flue gases into the combustion chamber has a similar effect.
It is also known to reduce NO.sub.x by means of special catalyst reactors, in which the reduction of nitrogen oxide to molecular nitrogen is accomplished, usually by means of ammonia (NH.sub.3).
By pyrotechnical means alone it is not possible to comply with the strictest NO.sub.x norms in solid-fuel boilers, but in these cases it is also necessary to treat in the above-mentioned catalyst reactors at least a proportion of the flue gases.
The known catalyst reactors for NO.sub.x are solid-bed or cell structures coated with a catalyst material; these structures typically operate at temperatures of 300.degree.-400.degree. C., and the reductant most commonly used is ammonia gas (NH.sub.3). In order to accomplish a good mass transfer contact, the gas conduits formed by the catalyst sheets must be oblong and the hydraulic diameter of the conduits must be small. Since a large amount of catalyst surface is needed, the catalyst reactor must be constructed so as to be uncooled. From this it follows that it is not advantageous to raise the operating temperature of catalyst reactors above 400.degree. C. Present-day catalyst reactors are not suitable for fuels containing sulfur. In terms of the NO.sub.x reactors, fuels which contain both ashes and sulfur, such as coal, are very problematic.
The disadvantages of catalyst reactors include high investment costs and considerable operating costs. Furthermore, practical experience has shown that during operation the catalyst sheets lose some of their catalytic effect owing to soiling and poisoning. One important cause of such poisoning is SO.sub.3, which in general sulfates an oxidic catalyst material. In the form of sulfate the catalyst material loses its effect. Another central problem encountered in the reduction of nitrogen oxides present in sulfur-containing gases by means of ammonia gas in catalyst reactors is the corroding and soiling of the air heaters of the steam boilers, caused by the forming ammonium sulfates. In the known reduction reactors for nitrogen oxides it has not been possible to solve these problems. The known catalyst reactors have had a further disadvantage in the compounds, detrimental to the environment, resulting from the unreacted am
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Janka Pentti
Ruottu Seppo
OY Tampella AB
Price Carl D.
Yeung James C.
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