JPS6319764B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION Field of the Invention The present invention relates to a method for combusting gaseous fuels using preheating for increased thermal efficiency and reduced production of nitrogen oxides (NO _x ). It is.

[Prior art and its problems]

The use of preheating in gas combustion increases thermal efficiency, but unfortunately such combustion generally produces higher amounts of _NOx in the combustion products, i.e., the flue gas.
is known to produce. Therefore, there is an increasing interest in, and a growing need for, limiting the emissions of _NOx into the atmosphere.
The use of preheating in gas combustion is becoming limited, resulting in a loss of thermal efficiency. However, rising fuel prices have emphasized the desire to achieve greater gas combustion thermal efficiencies.

OBJECTS OF THE INVENTION It is an object of the present invention to provide a method and apparatus for combustion of gaseous fuels that overcomes the above-mentioned drawbacks associated with conventional gas combustion.

[Key points of the invention]

In view of the above, according to the present invention, a preheated mixture of fuel gas and a combustion medium is formed, the preheated mixture is passed through a porous fiber layer of a porous fiber burner, and the fuel gas is passed through the porous fiber layer of a porous fiber burner. improved thermal efficiency and characterized in that the method is characterized in that it is combusted on the external surface of said external surface to cause said external surface to incandescent substantially without flame, thereby producing a hot flue gas with a low _NOx content. A method of combustion of fuel gas is provided that achieves low _NOx production.

Preheating can be applied to the fuel gas and/or the combustion medium (usually air), but is most often applied to the combustion medium. For example, it is clear that there is little benefit in thermal efficiency from preheating methane, since even stoichiometrically it requires at least about 10 times the volume of air to combust one volume of methane. It is. Additionally, combustion air is often used in excess of the stoichiometric amount, generally about 10-15%, to achieve nearly complete combustion. Therefore, the benefit of preheating the fuel gas is reduced.

Natural gas, primarily methane, is abundant and widely used as a fuel gas, but other hydrocarbon gases, such as propane, can also be used, or hydrogen alone or with or without carbon monoxide. The mixture can also be used as fuel gas. Refined gas and landfill gas are examples of gas mixtures that have fuel value and can be combusted according to the present invention. If the fuel gas is primarily hydrogen and/or carbon monoxide, preheating of the fuel gas and combustion air is recommended. This is because it takes only about
This is because only 2.5 times the volume of air is required.

Porous fiber burners provide flameless combustion at the external surface of the porous fiber layer, thereby producing incandescence of the external surface and a high proportion of oblique heating. Flameless radiant burners that can be used to practice the present invention are disclosed in US Pat. Nos. 3,275,497 and 3,383,159. Porous fiber burner containing powdered aluminum receives U.S. patent no.
No. 3,383,159, which suppresses the formation of carbon monoxide and is therefore more preferred for the purposes of the present invention. Unlike conventional flame burners, which often provide primary and secondary air for combustion of fuel gas, all the air desired for combustion is passed through the porous fiber burner mixed with the fuel gas.

Generally, the greater the amount of preheat transferred to the combustion reactants, the greater the increase in thermal efficiency, which for the purposes of this invention is the amount of heat input into the furnace or combustion zone that is converted into useful heat. Defined as a percentage (%) of For example, if the furnace is 80
% thermal efficiency, this is 20% of the heat input
is lost primarily as heat into the flue gas exhausted outdoors. Another reason for losses in thermal efficiency is incomplete combustion of the fuel gas as evidenced by carbon monoxide, hydrocarbons, hydrogen and soot (carbon) in the flue gas.

For simplicity and ease of measurement, the amount of preheat added to the combustion reactant is expressed herein as the temperature of the preheat reactant (〓). While residential furnaces generally have the combustion reactants preheated by passing the required air to indirectly exchange heat with this hot flue gas before exhausting the flue gas, industrial and Furnaces often have other available sources of preheating for the combustion reactants, ie, waste heat of operation may or may not be combined with the furnace. This type of waste heat can be used to increase the preheat generated from the furnace flue gas, or to provide all the desired preheat.

In most cases, the amount of preheat that can be practically applied to the combustion reactants is approximately 200 to 1000
〓(93-538°C). Preheating temperatures below 200°C (93°C) do not sufficiently increase the thermal efficiency of the furnace and therefore do not rationalize the cost of the heat exchanger. Preheating temperatures above 1000°C (538°C) are rarely achieved economically and, for some fuel gases, cause pre-ignition or flashback of these combustion reactants before passing them through the porous fiber burner. Sometimes.

Conventional flame burners are known to rapidly increase the amount of _NOx in the flue gas as the preheat temperature increases. For example, at a preheat temperature of 660㎓ (349℃), the _NO
220 ppm, and at a preheating temperature of 900㎓ (482°C), the NO _x content rises to about 370 ppm. In striking contrast to these extremely high _NOx contents, the use of the porous fiber burner of U.S. Pat.
_{The NOx} content did not increase beyond 18.7 ppm even when the combustion air was preheated to temperatures as high as 664°C (351°C). Obviously, preheating with flame burners is environmentally unacceptable, but with porous fiber burners this is not only acceptable but desirable. This is because the resulting increase in thermal efficiency is consistent with the national desire to conserve fuel.

〔Example〕

Hereinafter, the present invention will be described in detail with reference to the accompanying drawings.

FIG. 1 shows a furnace 10 with a porous fiber burner 11 at the bottom. The combustion reactants flow via line 12 to burner 11 , which line 12 is supplied with fuel gas via line 13 and with preheated air via line 14 . Flameless combustion occurs on the external surface of burner 11, producing radiant heat and hot flue gases. Water or other desired fluid enters the heat exchanger coil 15 in the furnace 10 through an inlet 16 and flows down the coil 15 to extract heat from the rising hot flue gases and absorb infrared radiation from the burner 11 . The thus heated fluid exits the coil 15 via its outlet 17 and is utilized for its intended use. The cooled but still hot flue gases are discharged from the furnace 10 via piping 18 into a heat exchanger 19.
All of the combustion air supplied to the burner 11 is transferred to the coil 20 in the heat exchanger 19 via the inlet 21.
flows countercurrently to the flue gas, which exits the exchanger 19 via line 22.
The combustion air preheated in this way is sent to the coil 2.
0 to the pipe 12 via the pipe 14 to the burner 1
1.

To demonstrate the benefits derived from preheating combustion air, standard conditions per cubic foot
A residential furnace operating with natural gas having a heat value of 1000 BTU (British Thermal Units) and a 15% excess of air with an ambient temperature of 70〓 (21°C) has a thermal efficiency of 84% and a thermal efficiency of 750〓 (399 The flue gases are released at a temperature of (°C). If the flue gas is used to preheat the same amount of combustion air to a temperature of 400°C (204°C), as shown in Figure 1, the thermal efficiency of the furnace increases to 91%.
At the same time, the NO _X content of the flue gas is practically unchanged by preheating, and in both cases the _NO
Less than 18ppm. Notably, in both cases the flue gas from the porous fiber burner contains no carbon monoxide and produces a flue gas containing less than 5 ppm of unburned hydrocarbons.

Figure 2 shows the thermal efficiency of a furnace operating with natural gas and 15% excess air at an ambient temperature of 70㎓ (ambient T on the horizontal axis).
FIG. 2 is a graph showing how the combustion air (expressed as %) is increased by preheating the combustion air to four different temperatures. The straight lines in the graph marked 200, 500, 750 and 1000 indicate the preheating temperature (〓) of the combustion air flowing into the porous fiber burner, and the increase in thermal efficiency obtained is due to the preheating temperature shown on the vertical axis of the graph.
Read at TE%. For example, a furnace with an ambient 70% TE results in a preheated 76% TE when preheating the combustion air to 500°C (260°C). Perimeter 55%
Other furnaces with TE have 15% excess air at a temperature of 750
When preheated to 〓(399℃), it shows 64% of preheating. Furthermore, other furnaces allow the combustion air to be heated to
By preheating to 750〓, the ambient temperature can be reduced to 80% T.
E. rises to preheating 94% TE.

Tests carried out in accordance with the present invention further showed several advantageous features. For example, U.S. Pat.
per hour when formed by depositing a porous fibrous layer onto a stainless steel screen according to the teachings of No.
A burner designed to provide a heat input capacity of 30,000 BTU was used in several combustion tests using the burner in open air (not in a furnace). All of these tests were conducted with a 10% excess of air. 800〓 (427℃) for mixed air and natural gas
With a preheating temperature that reaches I didn't.
When the natural gas flow is reduced to 20000 BTU/hour, the temperature of the radiating surface of the burner is 930〓 (499℃)
When operating at a preheat temperature of 1650㎓ (899℃). Therefore, a porous fiber burner can be operated successfully if the heat input is significantly reduced below its design capacity, and even approximately 1000 〓
Even with these changes in operating conditions, the flue gas
_NOx content generally remains surprisingly low, below about 20 ppm. If the fuel gas is substantially pure hydrogen, combustion of the hydrogen-air mixture at the external surface of the porous fiber burner tends to migrate through the porous fiber layer and cause flashback of the combustion reactants. It turned out to be. A flashback in a porous fiber burner is achieved by installing a perforated ceramic plate (generally a metal screen) near the internal surface of the porous fiber burner and attaching a gold film to the side of the ceramic plate facing the internal surface of the burner. It can be prevented. Perforated ceramic plates should be used whenever the fuel gas exhibits a tendency to flashback when carrying out the combustion process of the present invention.

The above description establishes flexibility in operating conditions and in all cases produces flue gases with extremely low _NO Establish the amount of preheat provided to the combustion reactants that is allowed. Modifications to the invention will be apparent to those skilled in the art without departing from the spirit of the invention. For example, coil 2 in heat exchanger 19 in FIG.
Combustion air passing through zero to obtain its preheating can also be combined with air at ambient temperature or with air preheated with waste heat from a separate source. Similarly, an air heating coil similar to coil 20 is connected to outlet piping 18 for obtaining flue gas into furnace 10.
By installing a separate heat exchanger 19 near the
can also be omitted.

〔Effect of the invention〕

Combustion of fuel gases to achieve improved thermal efficiency and suppress _NOx production is accomplished by preheating the combustion reactants and then flameless combustion on the external surface of the porous fiber burner. Preferably, all of the desired combustion air is preheated by indirect heat exchange with the flue gas resulting from flameless combustion, and the preheated air is then mixed with the fuel gas and fed to the porous fiber burner. . Waste heat from another available source can also be used to preheat the combustion reactants.

Thus, according to the invention, in the flue gas
Thermal efficiency can be increased while keeping NO _x content low.

[Brief explanation of the drawing]

FIG. 1 is a schematic diagram of a combustion apparatus illustrating a preferred embodiment of the present invention, and FIG. 2 is a graph illustrating the increased thermal efficiency of a furnace utilizing preheating according to the present invention compared to the thermal efficiency of a furnace without preheating. 10... Furnace, 11... Fiber burner, 12... Piping, 1
3... Piping, 14... Pipe, 15... Heat exchanger coil, 16... Inlet, 17... Outlet, 18... Piping, 19
...Heat exchanger, 20...Coil, 21...Inlet, 22...
Piping.

Claims (1)

[Claims] 1. Forming a preheated mixture of fuel gas and combustion air in excess of 15% or less over the stoichiometrically required amount, and passing this preheated mixture through a porous fiber layer of a porous fiber burner. , combusting the fuel gas on the outer surface of the fibrous layer to cause the outer surface to become incandescent substantially without flame, thereby producing hot flue gas with a low _NOx content of not more than about 20 ppm. A method of combustion of fuel gas achieving improved thermal efficiency and low _NOx production. 2. The method of claim 1, wherein hot flue gases are used to preheat a mixture of fuel gas and combustion air. 3 heating the combustion air to a temperature of at least 200 °C (93 °C) by indirect heat exchange with hot flue gases;
A method of burning fuel gas according to claim 1, wherein the heated air is mixed with the fuel gas to form a preheated mixture, which is passed through the porous fiber layer. 4. The fuel gas combustion method according to any one of claims 1 to 3, wherein the fuel gas is natural gas. 5 The combustion air is heated by indirect heat exchange with hot flue gases to a temperature of at least about 400°C (204°C), and the porous fiber layer contains a small amount of finely divided aluminum powder evenly distributed. Range 1
The method for burning fuel gas according to any one of items 1 to 4.