Furnaces – Process – Treating fuel constituent or combustion product
Reexamination Certificate
1999-09-13
2001-11-20
Bennett, Henry (Department: 3749)
Furnaces
Process
Treating fuel constituent or combustion product
C110S214000, C110S343000, C110S344000, C110S297000, C110S210000
Reexamination Certificate
active
06318277
ABSTRACT:
FIELD AND BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a technique and apparatus for reducing NOx emissions, and in particular to a new and useful method and apparatus for reducing nitrogen oxide emissions without increasing the presence of unburned carbon and without causing conditions in the furnace which increase corrosion.
2. Description of the Related Art
There are many papers and patents that describe the use of staged combustion for controlling NOx emissions during the combustion of pulverized coal.
In a tangentially fired furnace, pulverized coal and the required air for combustion are introduced at the furnace corners tangent to an imaginary circle in the middle of the furnace. Controlling the emissions of nitric oxides (NOx) from these furnaces is accomplished through the use of staging, that is, the introduction of some of the combustion air downstream of the fuel for the purpose of allowing nitrogenous compounds from the fuel to convert to molecular nitrogen rather than to nitric oxide (NOx). The combustion air that has been used to accomplish this is called overfire air since it is introduced above the main combustion zone. This method of controlling NOx emissions has been very effective. However, it results in increased levels of unburned carbon due to the inefficiency of the combustion process and corrosion of the furnace's heat absorption surfaces due to the reducing (lack of oxygen) atmospheres required for the control of NOx emissions.
SUMMARY OF THE INVENTION
The present invention includes a system for tangentially-fired units where a portion of the combustion air is injected above the main combustion zone causing the stoichiometry of the combustion zone to be less than 1.0. The air injected above the main combustion zone, called overfire air (OFA), is introduced into the furnace at two different locations (elevations). The first elevation is located as close to the main combustion zone as possible. This air is also introduced through multiple locations at the same horizontal plane. Air injectors of the invention are designed such that they can be yawed horizontally and vertically to allow for adequate mixing with main combustion product gases. The amount of air injected through this lower OFA location represents 15 to 40% of the total amount of OFA, with the actual amount depending on the overall stoichiometry required for NOx emissions reduction and the chemical properties of the coal.
The upper OFA injection ports are located above the lower ports and allow the combustion gases a residence time of 0.1 to 0.2 seconds prior to mixing with the air injected by the upper OFA ports. The upper OFA ports can be multiple locations in the same horizontal plane. The amount of air introduced through these ports is enough to complete the combustion process.
Controlling NOx emissions in tangentially fired boilers when combusting pulverized coal through the use of air staging results, as noted, in operating the main combustion zone under substoichiometric conditions achieved by the invention. These conditions result in the creation of a reducing atmosphere creating gases that promote corrosion of the furnace's heat absorption surfaces. The corrosion of the furnace heat absorption surfaces is most severe in the area of the highest heat release rates and lowest stoichiometry, which is immediately above the main burner zone (i.e., the main area where the combustion air and fuel are introduced). It is this area that the lower level of OFA is located. The introduction of this air at several locations at this elevation allows for a protective layer of oxidizing atmosphere to be formed, preventing gases, such as H
2
S and CO, that are major contributors to corrosion.
The introduction of this lower OFA at those locations under the conditions described according to this invention also reduces corrosion created through the direct deposition of corrosion inducing solids. Under this mechanism, ash particles containing corrosion promoting constituents will deposit and directly attack the heating surfaces. The oxygen contained in the air introduced through these lower OFA ports oxidizes these compounds resulting in reduced corrosion rates.
The use of substoichiometric conditions in the main combustion zone also produces an increase in combustible losses due to the inefficient mixing of fuel and combustion air. The use of multiple ports at the lower and upper OFA locations allows for more complete mixing, thereby reducing levels of unburned combustibles and improving boiler efficiency. The use of the multiple elevations of OFA also allows for maintaining significant reduction of NOx emissions.
Accordingly, an object of the present invention is to provide a method of reducing NOx emissions in a furnace having a main combustion zone with a waterwall and means for supplying main combustion air and fuel to the main combustion zone, the method reducing unburned carbon and waterwall corrosion in the furnace, the method comprising providing at least one lower overfire air injector at a first level over the main combustion zone of the furnace for supplying overfire air to create a lower overfire air zone in the furnace over the main combustion zone and providing at least one upper overfire air injector at a second level over the lower overfire air zone for supplying overfire air to create an upper overfire air zone in the furnace over the lower overfire air zone. The overfire air in the lower and upper overfire air zones are supplied at a rate for reducing the stoichiometry in the main combustion zone which reduces unburned carbon and a corrosive reducing atmosphere in the furnace.
A further object of the present invention is to provide an apparatus for achieving the same effect.
The various features of novelty which characterize the invention are pointed out with particularity in the claims annexed to and forming a part of this disclosure. For a better understanding of the invention, its operating advantages and specific objects attained by its uses, reference is made to the accompanying drawings and descriptive matter in which a preferred embodiment of the invention is illustrated.
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Bennett Henry
Marich Eric
Rinehart K. B.
The Babcock & Wilcox Company
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