Ultra low NOx burner for process heating

Details Ultra low NOx burner for process heating Ultra low NOx burner for process heating

2002-02-05

2004-08-10

Clarke, Sara (Department: 3749)

Combustion

Process of combustion or burner operation

Flame shaping, or distributing components in combustion zone

C431S008000, C431S115000, C431S187000, C431S284000

Reexamination Certificate

active

06773256

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention is directed to a gaseous fuel burner for process heating. In particular, the present invention is directed to a burner for process heating which yields ultra low nitrogen oxides (NOx) emissions.
Energy intensive industries are facing increased challenges in meeting NOx emissions compliance solely with burner equipment. These burners commonly use natural gas as a fuel due to its clean combustion and low overall emissions. Industrial burner manufacturers have improved burner equipment design to produce ultra low NOx emissions and call them by the generic name of “Low NOx Burners” (LNBs) or various trade names. Table I (Source: North American Air Pollution Control Equipment Market, Frost & Sullivan) gives the LNB market share based on industry for the year 2000. An objective for new burners is to target the industrial sectors that have the largest need for LNBs based on geographic region and local air emission regulations.
TABLE I
Low NOx Burner Market
Paper,
Food,
Public
Refinery
Power
Rubber,
Year
Utilities
Incineration
or
Generation
Other
Generation
(%)
(%)
CPI (%)
(%)
(%)
2000
46.5
15
21.3
6.4
10.8
As shown in Table I, public utilities and refineries (Chemical and Petroleum Industries) utilize the largest share of low NOx burners. These burners are used in industrial boilers, crude and process heaters (atmospheric and vacuum furnaces) and hydrogen reformers (steam methane reformers).
Nitrogen oxides (NOx) are among the primary air pollutants emitted from combustion processes. NOx emissions have been identified as contributing to the degradation of environment, particularly degradation of air quality, formation of smog (poor visibility) and acid rain. As a result, air quality standards are being imposed by various governmental agencies, which limit the amount of NOx gases that may be emitted into the atmosphere.
Primary goals in combustion processes related to the above are to (1) decrease the NOx emissions levels to <9 parts per million by volume (ppmv) and (2) improve the overall heat transfer uniformity and combustion efficiency of process heaters, boilers and industrial furnaces. For example, in southern California, for process heaters with a firing capacity greater than 20 MM Btu/hr, it is required that the NOx emissions be less than 7 ppmv and that the exhaust gas stream from the process heaters must be vented to a Selective Catalytic Reduction (SCR) unit. At present, this is only possible using best available control technology such as an SCR system. The SCR systems use post treatment of flue gas by reaction of ammonia in the presence of a catalyst to destruct NOx into nitrogen. In addition, California law also requires a fixed temperature window (600° F. to 800° F.) for >90% NOx removal efficiency as well as the avoidance of ammonia slip below 5 ppmv. A typical SCR unit for a 100 million Btu/hr process heater would cost approximately $700,000 in capital costs with annual operating costs of $200,000. See, for example, Table 2 of R. K. Agrawal and S.C. Wood, “Cost-Effective NOx Reduction”,
Chemical Engineering
, February 2001.
The above compliance costs create a higher cost burden on furnace/process plant operators or utility providers. Generally, emission control costs are transferred to the public in the form of higher overall product costs, local taxes and/or user fees. Thus, power utilities and process plants are looking for more cost effective NOx reduction technologies that would control NOx emissions from the source and do not require post treatment of flue gases after NOx is already formed.
In order to comply cost-effectively for NOx emissions, many combustion equipment manufacturers have developed LNBs. See, e.g., D. Keith Patrick, “Reduction and Control of NOx Emissions from High Temperature Industrial Processes”,
Industrial Heating
, March 1998. The cost effectiveness of an LNB compared to the SCR system would generally depend on the type of burner, consistent NOx emissions from burner, burner costs and local compliance levels. In many ozone attainment areas, the LNBs (for >40 MM Btu/hr) have not been capable of producing low enough NOx emissions to comply with regulations or provide an alternative to SCR units. Therefore, SCR remains today as the only best available control technology for large process heaters and utility boilers.
The greatest challenge in designing a low NOx burner is keeping NOx emissions consistently at sub 9 ppmv level or comparable to NOx emissions at the outlet of the SCR system. The prior art includes low NOx or ultra low NOx burners that produce low NOx emissions using various fuel/oxidant mixing techniques, fuel/oxidant staging techniques, flue gas recirculation, stoichiometry variations, fluid oscillations, gas rebuming and various combustion process modifications. However, most burners are unable to produce NOx emissions at less than 9 ppmv and those that do so in a lab, cannot reproduce such NOx levels in an industrial setting. The technical reasons or challenges in designing a sub 9 ppmv low NOx burner will become evident as described below.
Most large capacity gaseous fuel fired industrial burners used for process heating applications are nozzle mixing type burners. As the name implies, the gaseous fuel and combustion air do not mix until they leave various fuel/oxidant ports of this type of burner. The principal advantages of nozzle mix burners over premix burners are: (1) the flames cannot flash back, (2) a wider range of operating stoichiometry; and (3) a greater flexibility in burner/flame design. However, most nozzle mix air-fuel burners require some kind of flame holder/arrester for maintaining flame stability. One prior art generic nozzle mix burner is shown in
FIG. 1
, where a metallic flame holder disk is used for providing flame stability. Here, combustion air is induced surrounding the main fuel pipe with flame holder in a large box type burner shell.
The example burner of
FIG. 1
also uses staging fuel for secondary combustion to reduce overall NOx formation. However, for successful staged combustion processes, it is very important to have a stable primary flame attached to the flame holder.
FIG. 2
shows a typical flame holder geometry in which a multiple-hole fuel nozzle is located in the center and several perforated slots are used on the flame holder conical disk outside for passing through a small amount of combustion air for mixing with the injected fuel. The bluff body shape flame holder creates an air stream reversal as shown in FIG.
2
. The opposite direction air stream creates almost stagnant condition (zero axial velocity) for air fuel mixing at the inside cavity of the flame holder cone. This stagnant air-fuel mixture with almost no positive firing axis velocity component is used for attaching the main flame to the flame holder base.
Flame holders of various hole patterns and external shapes (conical, perforated disk, ring, etc.) are used for anchoring flames. For example, U.S. Pat. No. 5,073,105 (Martin, et al.) and U.S. Pat. No. 5,275,552 (Schwartz et al.) describe low NOx burner devices where such flame holders are used to anchor the flame. In U.S. Pat. No. 5,073,105, a primary fuel (30-50% of total fuel) is injected radially inwardly over the flame holder disk with flue gas entrainment (through a hole in the burner tile) for anchoring the primary flame. The remaining, secondary fuel is injected surrounding and impacting the external burner block (tile) surface for fuel staging and furnace gas recirculation. Combustion air mixing with the primary fuel takes place inside the burner block over the flame holder and some NOx is formed due to limited heat dissipation volume inside the burner block cavity and due to creation of locally fuel rich regions.
A very similar approach involving flame holder, primary fuel and secondary fuel injection is used in U.S. Pat. No. 5,275,552. Here, the primary gas, with entrained furnace gas through holes in the burner tile, is swirled in the burner block cavity for better mixing. The swirling primary fuel/f

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