Chemistry of inorganic compounds – Modifying or removing component of normally gaseous mixture – Nitrogen or nitrogenous component
Reexamination Certificate
1999-03-26
2001-07-10
Griffin, Steven P. (Department: 1754)
Chemistry of inorganic compounds
Modifying or removing component of normally gaseous mixture
Nitrogen or nitrogenous component
C110S203000, C110S210000, C110S212000, C423S235000
Reexamination Certificate
active
06258336
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a method and apparatus for reduction of nitrogen oxide emissions by industrial heating furnaces, such as utility boilers, fired by carbonaceous fuels, in particular, fuels with fixed nitrogen, such as coal.
2. Description of Prior Art
During the combustion of fuels having fixed nitrogen, such as coal, oxygen from the air combines with the nitrogen to produce nitrogen oxide (NO). At sufficiently high temperatures, oxygen also reacts directly with atmospheric nitrogen to form nitrogen oxide. A small fraction of the nitrogen oxide formed in the flame is oxidized to nitrogen dioxide (NO
2
) downstream of the flame. The total emission of nitrogen oxides, NO+NO
2
, is denoted by NO
x
. The generation and emission of nitrogen oxides are undesirable because they are toxic. In addition, nitrogen oxides, along with oxides of sulfur (SO
2
, SO
3
) contribute to acid rain precipitation, and in the presence of sunlight, react with hydrocarbons to produce photochemical smog and ozone.
The 1990 Clean Air Act amendments require industrial furnace operators to reduce nitrogen oxide emissions from fossil fuel-fired furnaces under Title IV of the Act. In addition, measures for ozone attainment under Title I of the 1990 Clean Air Act amendments are also being established. Because nitrogen oxides contribute to tropospheric ozone formation, limitations on NO
x
emissions are more stringent during the summer “ozone season.” Consequently, many industrial furnace operators that have installed low NO
x
burner/overfire air systems or post-combustion control systems will require additional NO
x
controls, particularly during the summer months. Thus, there is a need for methods and apparatuses which reduce the nitrogen oxide emissions from such industrial furnace facilities.
Commercially available techniques for reducing nitrogen oxide emissions in furnace flue gases include low-NO
x
burners, overfire air, selective non-catalytic NO
x
reduction (SNCR), selective catalytic reduction (SNCR), and reburning.
Reburning, that is, in-furnace nitrogen oxide reduction or fuel staging, has been described in several patents and publications. See for example, “Enhancing the Use of Coals by Gas Reburning Sorbent Injection,” presented at the Energy and Environmental Research Corporation (EER), First Industry Panel Meeting, Pittsburgh, Pa., Mar. 15, 1988; “GR-SI Process Design Studies for Hennepin Unit No. 1-Project Review,” Energy and Environmental Research Corporation (EER), presented at the Project Review Meeting on Jun. 15-16, 1988; “Reduction of Sulfur Trioxide and Nitrogen Oxides By Secondary Fuel Injection,” Wendt, et al., published at the Symposium of the Combustion Institute, 1972; “Mitsubishi” MACT In-Furnace NO
x
Removal Process For Steam Generator,” Sakai et al., published at the U.S.-Japan NO
x
Information Exchange, Tokyo, Japan, May 25-30, 1981.
Reburning is a technique whereby a fraction of the total thermal input to the furnace is injected above the primary combustion zone to create a fuel rich zone. Hydrocarbon fuels such as coal, oil, or gas are more effective NO
x
reducers than non-carbon containing fuels such as hydrogen or non-hydrogen containing fuels such as carbon monoxide. Stoichiometry of about 0.90 (10% excess fuel) in the reburn zone is considered optimum for NO
x
control. Thus, it is apparent that the amount of reburn fuel required for effective NO
x
control is directly related to the stoichiometry of the primary combustion zone and, in particular, the amount of excess air therein. Under typical furnace conditions, a reburn fuel input of over 10% of the total fuel input to the furnace is usually sufficient to form a fuel-rich reburn zone. The reburn fuel is injected at high temperatures in order to promote reactions under the overall fuel-rich stoichiometry. Typical flue gas temperatures at the injection point are above 2600° F. Overfire air is introduced into the flue gases downstream of the fuel-rich reburn zone in order to complete combustion of any unburned hydrocarbons and carbon monoxide (CO) remaining in the flue gases leaving the fuel-rich reburn zone. In addition, it is also known that rapid and complete dispersion of the reburn fuel in the flue gases is beneficial. Thus, the injection of reburn fuel is frequently accompanied by the injection of a carrier fluid, such as recirculated flue gases, for the purpose of promoting mixing. To the extent that the recirculated flue gas contains oxygen, the amount of reburn fuel must be increased. Furthermore, due to the requirement of at least three combustion zones within the furnace, a primary combustion zone, a fuel-rich reburn zone downstream of the primary combustion zone, and a fuel-lean completion zone downstream of the fuel-rich reburn zone, implementation of conventional reburn technology requires a relatively tall furnace. Unfortunately, many furnaces in the United States do not have the internal volume required for retrofitting this technology.
Full scale demonstrations of conventional natural gas reburning with flue gas recirculation for mixing and overfire air for completion of combustion of unburned hydrocarbons and carbon monoxide have shown up to 65% NO reduction under the high temperature fuel-rich conditions in several cyclone, wall, and tangentially fired boilers. However, conventional natural gas reburn is expensive due to the capital and operating expenses required for recirculating flue gases and providing overfire air (also known as burn-out air). In addition, the requirement of a fuel-rich zone suggests the use of greater than 10% natural gas, making conventional gas reburn uneconomical for most coal-fired furnaces.
Because it is cheaper than natural gas, coal has also been used as a reburn fuel. However, coal reburn requires a finer grind of coal than the typical utility grind in order to improve coal devolatilization and char burnout in the upper furnace which is diminished by the lack of O
2
inherent in the fuel-rich zone requirement. Because coal has inherent bound nitrogen which can be oxidized to NO, the use of coal as a reburn fuel is limited to initial NO concentrations greater than 200 ppm. This effectively precludes the use of coal reburn in many furnaces equipped with low-NO
x
burners.
Selective non-catalytic reduction (SNCR) processes for NO
x
reduction are based on the injection of chemical reagents into the combustion flue gases. In these processes, NO
x
, substantially all of which is NO, is reduced to nitrogen, N
2
, by injection into the flue gases of a nitrogenous compound such as ammonia (NH
3
), urea (NH
2
)
2
CO, or cyanuric acid (HNCO)
3
. All of these compounds either directly or indirectly form NH
i
radicals which subsequently react with NO in the flue gases to produce N
2
. These processes are called “selective” because the chemical reagents selectively react with NO. Thus, only small amounts of the ammonia, urea, or cyanuric acid are required.
A further process for NO
x
reduction in combustion flue gases involves the injection of a reburn fuel, such as natural gas, into the flue gases downstream of the primary combustion zone so as to maintain an overall fuel lean stoichiometry in the upper furnace. The chemical kinetic mechanisms of this fuel lean gas reburn process and the selective non-catalytic reduction process have many of the same selective reactions. The injection of natural gas in hot, low oxygen furnace gases results in the formation of hydrocarbon radicals (CH
i
), and the injection of urea results in the formation of amine radicals (NH
i
). Both of these radicals reduce NO to molecular nitrogen through a series of very similar selective reactions.
The selective non-catalytic reduction process reactions are highly efficient in reducing NO
x
in a narrow temperature window of about 1700° F. to about 1900° F. At higher temperatures, the process performance drops off due to oxidation of the amine additive to NO. At lower temperatures, the kinetics are too slow and result in high reagent le
Breen Bernard P.
Hura Harjit S.
Gas Research Institute
Griffin Steven P.
Pauley Petersen Kinne & Fejer
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