Combustion – Flame holder having protective flame enclosing or flame... – Including means feeding air axially spaced points of the flame
Reexamination Certificate
1997-12-17
2002-07-16
Bennett, Henry (Department: 3743)
Combustion
Flame holder having protective flame enclosing or flame...
Including means feeding air axially spaced points of the flame
C431S177000, C431S179000, C431S186000, C431S010000, C431S012000, C431S165000, C431S011000, C431S185000
Reexamination Certificate
active
06419480
ABSTRACT:
CROSS-REFERENCE TO RELATED APPLICATIONS
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a combustion device as well as to a method of providing low NO
x
and low CO combustion with substantially separate inputs for fuel and combustion air into a combustion chamber, wherein the entire or most of the combustion air is fed to the combustion chamber in continuous steps at several points in the chamber.
As used herein, the term fuel is intended to connote substances which react exothermally with oxygen and which are in a gaseous or vaporous state at ambient temperature and/or when fed into the combustion chamber. The term fuel further includes liquid or pulverized substances suspended in air, vapor and/or waste gas as a carrier gas. In this context, the term combustion air includes gas and/or vapors having an oxygen content sufficient to ensure a stable combustion of the selected fuel. The combustion air may contain waste gases. The term combustion zone as used herein is intended to include the spatial area in which the combustion takes place.
2. The Prior Art
In burners of the kind known from German published patent specifications DE-OS 4,419,345 and DE-OS 4,231,788 with separate feeding of fuel and combustion air into a combustion chamber, the combustion air is usually fed coaxially of the fuel injection. For this purpose, a fuel jet is generated near the mouth of the combustion device. The combustion air is fed peripherally of the fuel jet and outside of the flame region, through a substantially annular distributor, wherein the distributor is placed in the proximity of the fuel nozzle and substantially coaxially with respect to the fuel nozzle. Because of the significant space between the combustion air distributor and the flame region and, more particularly, the core of the flame, a uniform mixture of fuel and combustion air or a mixture composed of predetermined shares cannot be achieved in practice when operating this type of combustion device. In an attempt to reduce this disadvantage, the combustion air is segregated into primary air and secondary air, whereby locally limited peak values of oxygen concentration are lowered and the automatic control of stoichiometric ratios during combustion may thus be improved to some extent. The principal or main disadvantage of this type of combustion device, namely the unsatisfactory automatic control of the stoichiometric ratios of fuel and combustion air, is that the resultant formation of harmful and noxious substances of contaminants and pollutants, such as nitric oxides and carbon monoxide, can only be reduced at relatively high expense. A cause of this disadvantage is that the input of combustion air extends only over a relatively small spatial area of the combustion zone. Therefore, the stoichiometric ratios during combustion are essentially determined solely by the difficulty of controlling convection in the combustion zone. An attempt to reduce this disadvantage by special installations which provide for more intensive turbulence of fuel and combustion air does, however, entail a larger consumption of energy due to increased pressure losses.
Another method of reducing the formation of NO
x
in the combustion chamber of combustion devices not providing for partial premixing comprises injecting the combustion air and fuel at a high velocity into a combustion zone preheated to about 950° C. This is, however, an energetically and structurally complex solution, and pyrotechnically it is of little interest since it leads to long flames and does not result in an optimum mixture.
Frequently, a multi-stage input of combustion air is carried out to improve combustion and to lower the emission of harmful substances, such as pollutants and contaminants, in combustion devices not calling for premixing. Such a proposal is, for example, the subject of the combustion device according to German laid-open patent specification DE OS 4,041,360. This combustion device with a horizontal burner pipe provided at its upper surface with a plurality of gas exit openings comprises, within so-called jet-flow rods, additional openings above the primary air input for feeding of secondary air. The flow rods are intended to cool the flames. The thermal load and stress on such jet-flow rods is, however, very high so that only high-temperature resistant materials can be employed for these jet-flow rods. Moreover, an optimum automatic control of the combustion device as to harmful substances is rendered substantially more difficult at different thermal load levels because the ratio of primary air to secondary air quantities can be changed only within narrow limits. In particular, the input of secondary air into the upper flame zone is insufficient in the region of the full thermal load.
Attempts have been made to reduce these drawbacks by separate automatic controls of the secondary air input or by partial premixing, which may require expanding the two-step air input to a three-step or four-step air input. A combustion device of the kind operating in this manner has been described in German laid-open patent specification DE OS 4,142,401. The combustion device utilizes premixing and is operated substantially below stoichiometric levels of oxygen. The oxygen which is lacking for combustion is supplied only at a noticeable distance from the combustion mouth at one or several sites, whereby the direction in which the oxygen is injected must not be the same as, and parallel to, the main flow direction of the combustion gases. This method undoubtedly improves the operation of large size industrial furnaces, such as cylindrical rotary kilns, drum-type furnaces and the like, even though it is relatively difficult to control because of the complicated flow guidance of the combustion air which must be tuned to, or adjusted in accordance with, the geometry of the furnace walls. This method is, however, too expensive for the operation of compact combustion devices with lower thermal output rates. Moreover, this method suffers from the general disadvantage that in essence the combustion air is supplied to the area of the combustion zone in which the flame temperature is relatively high.
The advantages of a multi-step air supply are also applied in a special variant of burners without premixing and provided with a combustion chamber which expands conically in the direction of the flame, wherein the combustion chamber forms a diffuser and provides more intensive mixing of fuel and combustion air. Fuel and primary combustion air are fed into the combustion devices or burners of this type (see German laid-open patent specification DE OS 3,600,784) by the diffuser and are burned in said combustion devices. Secondary combustion air is additionally fed through wall openings in the diffuser in a direction radially of the flame. However, the length of the diffuser cannot be arbitrarily extended because the diffuser would then excessively shield the flame which would impair the transfer of heat to the furnace wall. Since the length of the flames can substantially surpass the length of the diffuser in case of higher thermal output rates, this means that insufficient secondary combustion air is fed to the region of the tip of the flames particularly at higher heat output rates. The effect as regards the emission of pollutants from the combustion device or burner is unfavorable.
In combustion processes of this method, the hottest flame zones are always in the interior of the diffuser and cause the walls of the diffuser to glow red hot. This is disadvantageous for two reasons: First, the glowing leads to an increased formation of environmentally hazardous nitric oxide due to the increased temperature and, second, special temperature resistant materials are required for the walls of the diffuser. Overall, the input of combustion air is restricted to relatively small spatial areas of the combustion zone in this kind of combustion device and, moreover, the relatively small spatial areas exhibit very high flame temperatures.
Another kind of air distribution as d
Bennett Henry
Cocks Josiah C.
Hormann Karl
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