Heating – Work chamber having heating means – Combustion products heat work by contact
Reexamination Certificate
1999-10-15
2001-09-11
Wilson, Gregory (Department: 3749)
Heating
Work chamber having heating means
Combustion products heat work by contact
C432S072000, C431S005000
Reexamination Certificate
active
06287111
ABSTRACT:
BACKGROUND OF THE INVENTION
The present invention relates to residential and commercial heat release processes, which requires the burning of fuel with air. Specifically, the invention relates to processes regulated for achieving low nitrogen oxide (NOx), carbon monoxide (CO) and volatile organic chemicals (VOC) emissions.
This invention provides more economical means for achieving low emission levels, particularly oxides of nitrogen, while maintaining high efficiency. High efficiency also reduces greenhouse gases.
Residential units may include, among others, forced hot air heating and water heaters. Commercial units may include boilers and water heaters. Industrial units may include indirect heating such as radiant tubes.
The fuels combusted might be natural gas, refinery or process gas, and liquid distillate fuels like diesel fuel or #6 (residual) fuel oil.
NOx is an abbreviation for the collective species of nitrogen oxide and nitrogen dioxide, which are responsible for health problems, and the creation of smog. NOx is created in the intense heat of a combustion flame where nitrogen and oxygen are present. It has been long known that the suppression of flame temperature can reduce NOx significantly. NOx can also be created from fuel bound nitrogen and ammonia or other nitrogen bearing compounds that find its way into the fuel or combustion air. In this case, NOx is not reduced by a suppression of flame temperature, but NOx can be “chemically reduced” to N
2
and carbon dioxide (CO
2
) in a hot fuel-rich flame that is carbon monoxide (CO) rich.
There are a number of methods for reducing NOx, as listed below. However, all come with some economic penalty or degradation in process performance. It is an object of this invention to provide a novel method and structure that will minimize the economic penalties associated with these NOx reducing methods.
FIELD OF THE INVENTION
There are a number of techniques for reducing NOx that are well known in the prior art. Some definition of terms employed in the art and some explanation of the nature of processes known in the prior art are described below.
Ballast is a mass of gas added to a region of combustion that reduces flame temperature.
A nozzle mix burner is one in which air and gas enter the combustion zone unmixed. Such a burner has the advantage that there is essentially no chance of flashback. However, it has the disadvantage that combustion is slower than with premixing so that more CO and VOC are produced, and usually the NOx is higher.
A premix burner brings air and gas together before entering the combustion zone. This results in lower NOx and cleaner combustion. However, it allows the possibility that flashback can happen.
Diluents are gases added to a region of combustion which reduce speed of reaction and heat liberation by reducing molecular collisions between reactive molecules.
Flue gases are gas mixtures which are the results of combustion, gases which normally flow out the stack.
Rich mix has more fuel than air in the burn mixture and results in insufficient air to complete combustion.
Lean mix has more air than required for stoichiometric combustion of the fuel. Normal natural gas combustion produces about 2-3% oxygen in the flue gases. The flame extinguishes at about 12-15% oxygen in the flue gases..
Fuel bound nitrogen is nitrogen that is chemically bound in the fuel molecules.
Oxygen trim means reducing the excess oxygen too close to stoichiometric levels while maintaining a watch on the carbon monoxide regulated limits. This produces a small decrease in NOx from oil and gas nozzle-mix burners but an increase in the NOx from premix burners. Operating closer to stoichiometric improves thermal efficiency but increases pollutants such as CO and VOC.
Lean premix is a lean mix mixed before it is introduced into the combustion space. One can achieve very low-NOx by diluting the stoichiometric gas mixture with large amounts of air ballast. This moderates the flame temperature. However, the air ballast carries large amounts of heat out the stack, so that the process is thermally inefficient. This method is acceptable for air dryers where the moderated temperatures are required. The air ballast increases NOx for a nozzle-mix burner.
Quick Mix is another burner design technology that uses a nozzle mix type burner and rapidly mixes the air and the fuel in the combustion space before it can ignite. This simulates premix and is reported to obtain low-NOx.
Rich burn quench introduces air over the fire of a burner. It is mainly used to reduce oxides of nitrogen emission for nozzle-mix burners using fuel oils or gases with high levels of fuel bound nitrogen. Most of the fuel bound nitrogen is converted to NOx in the flame. This is in excess of the thermal NOx created in the hot flame. However the CO in the very rich flame reduces some of the nitrogen oxide back to nitrogen and carbon dioxide. Therefore, the remainder of the fuel requires burning before venting to atmosphere, usually with natural gas and overfire air. The proplem is matching the heat release to the heat absorbing capabilities (e.g. steam/water tubes in a boiler) and using natural gas in place of less expensive fuel oil and the additional piping and costs of natural gas.
Water or steam injection employed as ballast (as used in lean premix) and diluent injected to cool the flame temperature and reduce the combustion reaction rate thus reducing thermal NOx. The ballast carries heat out the stack. Thus it is inefficient compared to stoichiometric combustion. Water and steam may be expensive in arid areas like California.
Staged air combustion is similar to “rich burn quench” except that there is no quench so that the introduction of additional air combusts the remainder of the fuel. Usually the final combustion is close to stiochiometric; thus the process is thermally efficient. Often staged air combustion is used in combination with other techniques for lowering NOx.
Flue gas recirculation (FGR) is a technique using an extra fan to suck flue gases from the stack and usually to force them into the air stream. This is a very popular method of reducing NOx on existing and new boilers and works well with both liquid and gaseous fuels. Usually 10-30% flue gas is recirculated. There is a loss of efficiency due to recirculation fan power and cooling and condensing of the recirculated flue gas in the piping. With high sulfur fuels, condensing must be avoided to prevent corrosion; thus recirculation usually requires clean fuels. Flue gas recirculation does not reduce NOx from fuel bound nitrogen.
In situ FGR are methods of FGR whereby flue gas from the furnace is inspirated into the burner body and mixed with the air or fuel without an external loop or very short external loop. This technique is only moderately successful in reducing NOx since the inspiratation of hot flue gases requires large amounts of energy.
Zone combustion or bias combustion for long or tall furnaces allows the combustion to be split up into zones. One patent has a lean premix burner firing at one end of the furnace and fuel injectors before the end of the furnace (zone
2
) where combustion and heat transfer can be completed before exiting. Thus, there are two types of burners. However, the flue products of one burner does not enter into the combustion of the other burner as in the present invention.
Co-generation is normally thought of as method for improving heat recovery from turbine-generator set or diesel-generator set. However, it does reveal a method of reducing NOx. The waste gas stream from the generator set contains a lot of oxygen and heat. To reclaim some of the waste-heat the waste gas can be sent (15% in the case of a gas-fired turbine at about 900 F.)directly to a waste heat recovery unit (boiler). Still, thermal efficiency is low due to the large amounts of excess air (ballast). Some waste recovery unit's (boiler) have burners, which uses the oxygen in the exhaust gas to complete combustion close to stoichiometric. Thus, the overall heat balance of the plant is improved and th
Dann Dorfman Herrell and Skillman, P.C.
Wilson Gregory
LandOfFree
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