Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
2001-03-07
2002-12-31
Clarke, Sara (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S009000, C431S115000, C431S174000, C431S178000
Reexamination Certificate
active
06499990
ABSTRACT:
FIELD OF THE INVENTION
In one aspect, the present invention relates to methods and apparatuses for reducing NO
x
emissions from burners of the type used in process heater, boilers, and other fired heating systems. More particularly, but not by way of limitation, the present invention relates to achieving low NO
x
emissions in and with a burner having only one combustion stage.
BACKGROUND OF THE INVENTION
Many industrial applications require large scale generation of heat from burners for process heaters, boilers, or other fired heating systems. If the burner fuel is thoroughly mixed with air and combustion occurs under ideal conditions, the resulting combustion products are primarily carbon dioxide and water vapor. However, when the fuel is burned under less than ideal conditions, such as in a high temperature environment, nitrogen present in the combustion air reacts with oxygen to produce nitrogen oxides (NO
x
). It is well known that, other conditions being equal, NO
x
production increases as the temperature of the combustion process increases. NO
x
emissions are generally considered to contribute to ozone depletion and other environmental problems.
Prior to today's increasing concern over the environmental effects of NO
x
, emissions, single staged burners were in common use in generally all types of fired heater applications. Prior art single stage burners typically comprise one or more fuel nozzles or distributors positioned inside the burner wall. As compared to typical low NO
x
, burners now in use, the prior art single stage burners are less expensive, less complex, safer, more stable, and simpler to operate, control, and maintain. Prior art single stage burners also typically provide much broader acceptable operating ranges (turndown ratios). Unfortunately, however, the simpler single stage burners heretofore used in the art produce very high levels of NO
x
, emissions and are not capable of meeting today's demanding environmental standards and regulations.
Burners designed for combusting fuel with air in a manner resulting in less NO
x
emissions are commonly referred to as “low NO
x
” burners. One type of apparatus now used for reducing NO
x
emissions is a “staged air” burner. Staged air burners operate by dividing the flow of combustion air to create a first combustion zone (wherein the fuel is introduced) having a deficiency of air so as to create a reducing environment that suppresses NO
x
formation and a second combustion zone wherein the remaining portion of air is introduced and the combustion process is completed.
Another type of low NO
x
apparatus is a “staged fuel” burner wherein all of the combustion air, but only a portion of the fuel to be burned, are introduced in a first combustion zone. The remaining fuel is introduced into a second combustion zone utilizing the oxygen-rich effluent of the first zone. In such a burner, the excess air in the first zone serves to dilute the fuel, which lowers the temperature of the burning gases and thereby reduces the formation of NO
x
.
Other low NO
X
methods and apparatuses recirculate and mix furnace flue gases with fuel/air mixtures to dilute the mixtures and to thereby lower the combustion temperature so that NO
x
formation is reduced. Flue gases are captured from the furnace space and conducted via pipes, ducts, or passageways to a mixer assembly, typically within the burner housing, where the flue gases are mixed with fuel or with fuel and air. The resulting mixture is then burned.
It will be appreciated that to accomplish the foregoing, each of the conventional types of low NO
x
burners must be rather complex in structure and operation. As compared to high NO
x
prior art single stage burners, conventional low NO
x
systems must provide for and include additional hardware, conduits, passageways and other structures to achieve staged introduction of fuel or air, to allow for the burning of fuel/air mixtures in multiple combustion zones, and/or to accommodate the recirculation of furnace gases. This increased level of complexity does not lend itself to low-cost manufacture, reliability, or ease of maintenance. Moreover, in staged burner systems, the necessity of splitting flows to and balancing the performance of multiple combustion zones/stages increases the difficulty of achieving and maintaining operational stability and greatly reduces the available operating range (turndown ratio) of the burner.
One type of low NO
x
staged fuel burner heretofore known in the art is described in U.S. Pat. No. 5,195,884. In the '884 burner, a primary portion (preferably about 75%) of the fuel gas used in the burner must be burned in a first (“primary”) combustion zone within and surrounded by the burner wall. The primary fuel is mixed with air and discharged into the primary combustion zone using one or more mixing and discharge assemblies which project into and are at least partially contained within the throat of the burner. Each of these assemblies comprises a Venturi aspirating tube having a primary fuel gas nozzle positioned at the lower end thereof.
The remaining (secondary) fuel gas used in the '884 burner is delivered to a secondary combustion zone by four secondary fuel gas nozzles outside of the burner wall. Each of the secondary fuel nozzles has an array of multiple flow ports provided therein which must spread the secondary flue gas in a fan-type pattern covering essentially one-quarter of the exterior of the burner wall. The burner wall has an exterior frusto-conical surface which is contacted by the secondary fuel gas as it spreads outwardly and moves upwardly to the secondary combustion zone. Such contact is said to promote the mixing of internal flue gases with the secondary fuel gas.
Unfortunately, the primary combustion stage alone of the burner described in the '884 patent produces more NO
x
emissions than are allowable, for example, under Texas Gulf Coast restrictions and other regulatory requirements. Moreover, without using the primary combustion stage, it has not been possible heretofore to obtain adequate heating or to achieve and maintain stable burner operation.
Like the stage fuel burner described in the '884 patent, few, if any, of the other low NO
x
burners presently available are capable of meeting Texas Gulf Coast requirements and other increasingly stringent air quality standards. If suitable new burner technologies capable of satisfying these requirements are not found, the industry will be required to use more expensive and elaborate techniques, such as catalytic reduction, to reduce NO
x
emissions.
Thus, a need exists for a new burner technology which produces even less NO
x
emissions than the low NO
x
burner systems currently available in the art. The new, extremely low NO
x
burner would preferably also be less complex, less expensive, more stable, and much simpler to operate, maintain, and control than current low NO
x
burner systems. Further, the new, extremely low NO
x
burner would preferably provide a much larger available turndown ratio than is provided by current low NO
x
burners.
SUMMARY OF THE INVENTION
The present invention provides a low NO
x
burner apparatus and method which satisfy the needs and alleviate the problems discussed above. The inventive burner and method are capable of providing NO
x
emission levels of 10 parts per million by volume (ppmv) or less (preferably 7 ppmv or less) based on the total volume of combustion gas product produced by the burner. The inventive low NO
x
burner also provides much stabler operation and is less complex and less costly than the low NO
x
burner systems currently available. Moreover, the inventive burner is much simpler to maintain and control and provides a desirably broad available operating range. In this regard, the inventive burner has a turndown ration in the range of from about 5:1 to about 10:1, comparable to the broad operating ranges provided by prior high NO
x
burners.
The inventive burner and method also greatly reduce the need for metal components within the throat of t
Isaacs Rex K.
Kirk Tim
McDonald John
Zink Darton J.
Clarke Sara
Fellers Snider Blankenship Bailey & Tippens, P.C.
Zeeco Inc.
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