NOx reduction using coal based reburning

Details NOx reduction using coal based reburning NOx reduction using coal based reburning

2000-05-19

2001-03-27

Kastler, Scott (Department: 1742)

Specialized metallurgical processes, compositions for use therei

Processes

Producing or treating free metal

C075S466000, C431S005000

Reexamination Certificate

active

06206949

ABSTRACT:

TECHNICAL FIELD
This invention relates generally to combustion and, more particularly, the reduction of nitrogen oxides (NOx) emissions stemming from the combustion.
BACKGROUND ART
The combustion of fuel and oxidant to provide heat is used in a large number of commercial furnaces in such industrial applications as the generation of electricity, the production of steel and other metals, the production of glass, and the treatment of wastes. Typically the oxidant employed in the combustion process is air which contains a substantial amount of nitrogen. In the course of the combustion reaction, nitrogenous species in the fuel and nitrogen in the air are oxidized to form NOx. Other combustion gases, which typically include carbon dioxide and water vapor are also formed.
The emission of NOx from industrial and other furnaces has been shown to have a serious impact on both environmental quality and human health. The compounds cause acid rain, are a factor in the formation of ground level ozone, and have been implicated in the formation of fine particulate.
One approach to reducing NOx emissions from a combustion reaction is reburning wherein fuel is provided to the NOx-containing combustion gases to react with the NOx and form nitrogen gas. Typically the fuel employed for the reburning is methane or natural gas.
It is desirable to use a less expensive fuel such as coal or other granular carbonaceous fuel for reburning to reduce NOx emissions. However, the use of such fuels can reduce the efficiency of the reburning because longer contact times are necessary, and can result in the ash having a high carbon content causing the need for further processing steps for the ash.
Accordingly, it is an object of this invention to provide an improved method for reducing NOx emissions from a combustion reaction using reburning with a granular carbonaceous fuel.
SUMMARY OF THE INVENTION
The above and other objects, which will become apparent to those skilled in the art upon a reading of this disclosure, are attained by the present invention which is:
A method for reducing the NOx content of combustion gases from a combustion reaction comprising:
(A) combusting fuel and oxidant to generate heat and to form combustion gases comprising NOx;
(B) heating oxygen to a temperature within the range of from 1000° F. to 3000° F. to form hot oxygen;
(C) mixing the hot oxygen with granular carbonaceous fuel containing volatiles to form a mixture having a stoichiometric ratio less than 1.0, devolatizing the granular carbonaceous fuel within the mixture, and reacting hot oxygen with volatiles and granular carbonaceous fuel within the mixture to form a fuel-rich gas phase mixture containing hydrocarbon radicals; and
(D) contacting the combustion gases with the fuel-rich gas phase mixture and reacting hydrocarbon radicals of the fuel-rich gas phase mixture with NOx of the combustion gases to form nitrogen gas.
As used herein the term “stoichiometric” means the ratio of oxygen molecules to fuel for combustion purposes. A stoichiometric ratio less than 1.0 means there is less oxygen present than the amount necessary to completely combust the fuel present, i.e. fuel-rich conditions.
As used herein the term “oxygen” means a gaseous fluid having an oxygen concentration of at least 30 mole percent. It may have an oxygen concentration exceeding 70 mole percent or may be commercially pure oxygen having an oxygen concentration of 99.5 mole percent or more.
As used herein the terms “nitrogen oxides” and “NOx” mean one or more of nitrous oxide (N
2
O), nitric oxide (NO), dinitrogen trioxide (N
2
O
3
) nitrogen tetroxide (N
2
O
4
), nitrogen dioxide (NO
2
), trinitrogen tetraoxide (N
3
O
4
) and nitrogen trioxide (NO
3
).
As used herein the term “volatiles” means organic compounds that are evolved from solid or liquid fuels when heated. The compounds include tars, methane and other organic compounds with moderate boiling points.
As used herein the term “devolatize” means to cause volatiles to gasify from a solid or liquid.
As used herein the term “hydrocarbon radicals” means highly reactive species that are formed by dissociation of, or reaction with, hydrogen or carbon containing species. Examples of hydrocarbon radicals include NH, CH and CH
2
.
As used herein, the term “granular” means finely divided material such as crushed or pulverized solid material or atomized liquid material.


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