Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Patent
1989-05-23
1991-05-21
Heller, Gregory A.
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
423239, C01B 2100, B01J 800
Patent
active
050173472
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
The present invention relates to a process for the reduction of nitrogen oxides (NO.sub.x) in the effluent, especially the oxygen-rich effluent, from the combustion of a carbonaceous fuel while minimizing the production of other pollutants, such as ammonia (NH.sub.3) and/or carbon monoxide (CO).
Carbonaceous fuels can be made to burn more completely, and with reduced emissions of carbon monoxide and unburned hydrocarbons, when the oxygen concentrations and air/fuel ratios employed are those which permit high flame temperatures. When fossil fuels are used in suspension fired boilers such as large utility boilers, temperatures above about 2000.degree. F. and typically about 2200.degree. F. to about 3000.degree. F. are generated. Unfortunately, such high temperatures, as well as hot spots of higher temperatures, tend to cause the production of thermal NO.sub.x, the temperatures being so high that free radicals of oxygen and nitrogen are formed and chemically combine as nitrogen oxides. Nitrogen oxides can form even in circulating fluidized bed boilers which operate at temperatures which typically range from 1300.degree. F. to 1700.degree. F., as well as gas turbines and diesel engines.
Nitrogen oxides are troublesome pollutants which are found in the combustion effluent streams of boilers when fired as described above, and comprise a major irritant in smog. It is further believed that nitrogen oxides can undergo a process known as photo-chemical smog formation, through a series of reactions in the presence of sunlight and hydrocarbons. Moreover, nitrogen oxides comprise a significant contributor to acid rain.
Unfortunately, the temperatures within a suspension fired or circulating fluidized bed boiler render most common methods of reducing NO.sub.x concentrations, such as effluent scrubbing or catalyst grids, either uneconomical, infeasible, or both, particularly when combined with the need to minimize the production of other pollutants such as ammonia (NH.sub.3) and/or carbon monoxide (CO).
BACKGROUND ART
Processes and compositions for the reduction of nitrogen oxides in an effluent from the combustion of a carbonaceous fuel have been developed extensively over recent years. With the increased attention to the health risks and environmental damage caused by agents such as smog and acid rain, it is expected that NO.sub.x reduction research will continue to be pursued.
In the past, most processes for the reduction of nitrogen oxides levels have concentrated on achieving maximum NO.sub.x reductions without addressing the problems raised by the production of other pollutants, such as ammonia and carbon monoxide. More recently, in a unique application of NO.sub.x reducing principles, Epperly, Peter-Hoblyn, Shulof and Sullivan, in U.S. patent application entitled "Multi-Stage Process for Reducing the Concentration of Pollutants in an Effluent" having Ser. No. 022,716, filed Mar. 6, 1987, disclose a method of achieving substantial NO.sub.x reductions without the production of a major amount of other pollutants through a multiple stage injection process. The disclosed process, though, is more concerned with maintaining a low level of other pollutants than with achieving a maximum amount of nitrogen oxides reduction because each injection is designed not to maximize NO.sub.x reduction but to minimize the production of other pollutants.
Although effective for reducing the level of nitrogen oxides in an effluent, the fact that the NO.sub.x reduction is not maximized at each injection indicates that further reductions are possible. Furthermore, even in the prior art disclosures which teach methods which may lead to a lessened production of other pollutants, most do so under relatively static conditions and do not account or compensate for changes in effluent conditions, such as effluent temperature variations as load varies, which are often encountered. There exists a present need, therefore, for a process which can achieve maximum nitrogen oxides reductions without the production of substantial amou
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Epperly William R.
O'Leary John H.
Sprague Barry N.
Sullivan James C.
Fuel Tech Inc.
Heller Gregory A.
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