JPS58173302A - Improved gas combustion with low nox discharge - Google Patents

Abstract

(57) [Summary] This bulletin contains application data before electronic filing, so abstract data is not recorded.

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to a method and apparatus for combusting gaseous fuels utilizing preheating for increased thermal efficiency and reduced production of NOx. It is related to.

[Prior art and its problems]

Although the use of preheating in gas combustion increases thermal efficiency, it is known that *,t-like combustion generally produces higher amounts of NOx in the combustion products or flue gases. Over time, with the increasing interest in and need to limit the release of NOx into the atmosphere, the use of preheating in gas-fired (J4) systems is becoming limited, resulting in reduced thermal efficiency. However, rising fuel prices have emphasized the desire to achieve even greater gas combustion thermal efficiencies.

[Purpose 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 all the above considerations, 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 Ia woven burner, and the fuel gas is passed through the porous fiber layer of a porous Ia woven burner. Burning occurs on the external surface of the layer to cause said external surface to become incandescent without any flame damage, thereby producing a low NO
characterized in that it produces a hot flue gas of x content;
A method of combustion of fuel gas is provided that achieves improved thermal efficiency and low NOx production.

[Embodiments of the invention]

Preheating can be applied to the fuel gas or to the combustion medium (usually air, or both), but is most often applied to the combustion medium. It is clear that there is little benefit in thermal efficiency from preheating the methane, since it requires at least about 10 times the volume of air.Furthermore, #1 requires only about 10 times the volume to achieve complete combustion. Combustion air is often used, generally in excess of about 10 to 15 liters over the stoichiometrically required amount.

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 only aqueous, or - together with carbon oxides. The mixture can also be used as fuel gas. Refined gas and landfill gas are examples of gas mixtures, where fuel 11tt
111M and can be fired 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 only about 25 times the volume of air is required to combust one volume of hydrogen and/or carbon monoxide.

Porous fiber burners provide flameless combustion at the external surface of the porous fiber layer, thereby making the external surface incandescent and generating a high proportion of radiant heat. Flameless radiant burners that can be used to carry out the present invention include US 14
No. 4,275,497 and No. 85,159. A porous fiber burner containing powdered aluminum is disclosed in U.S. Pat. It is something.

Unlike conventional flame burners, which often provide primary and secondary air to effect combustion of the 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 more
The increase in thermal efficiency is also large, p, which for the purposes of the present invention is defined as P or the percentage of heat input that is input into the combustion zone and converted into useful heat.

For example, if a furnace requires a thermal efficiency of 80 sacks, this means all that 20+ of the heat input is lost as heat, primarily into the flue gases exhausted to the open air.

Another reason for the loss in thermal efficiency is the incomplete combustion of the fuel gas as evidenced by monovalent carbon, hydrocarbons, water volume and soot (carbon) K in the four-way gas.

Rounding to make it simple and easy to measure, the amount of preheating added to the combustion reactants is - in the specification the temperature of the preheating reactants (°
F). Residential furnaces generally use flue gas t-
Industrial and industrial F often have the combustion reactants preheated by passing necessary air to tangentially exchange heat with this hot flue gas before exhausting.

With other available preheating sources for the combustion reactants, i.e. the waste heat of the operation may or may not be combined with the furnace, this type of waste heat can be used to supply the furnace flue gas. The preheat resulting from the heating can be increased or all of the desired preheat can be provided.

In most cases, the amount of preheat that can be practically applied to the combustion reactants will raise the temperature of the total combustion air to a value of about 200 to 1,0 OOTO yoke. A preheat source below 200° F. does not increase the thermal efficiency of the furnace by a fraction of a second and therefore does not increase the cost of the heat exchanger. For fuel gases of the type L000°F000 heat source &
#May cause pre-ignition or flash fire.

Conventional flamethrower burners are known to rapidly increase the amount of M4 in the flue gas as the preheat temperature increases. For example, 660? At a preheat source t, the NOx content of the flue gas is about 220 ppm of the flue gas on a dry basis and corrected for zero oxygen), and at a preheat temperature of 900 ff, the NOx content increases at about 370 prints. do. With these extremely high NOx contents and strikingly symmetrical K, the use of the porous fiber burner of US Pat. And the combustion air is 664? Preheat to a high temperature of 4 NO
, the content shall not increase beyond 1a7m). Obviously, preheating with a flame burner is environmentally unobtrusive, but with porous fiber burners this becomes both acceptable and desirable. Because,
BRIEF DESCRIPTION OF THE DRAWINGS The present invention will now 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.
f: Shown. The combustion reactants flow via line 12 to burner 11, which line 12 is supplied with fuel gas via line 16 and preheated nitrogen gas 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 birth canal gas is passed from the furnace 1o through a pipe 18 to a heat exchanger S 19.
released into the body. All of the combustion air supplied to the burner 11 also enters the coil 2o in the heat exchanger 619 via the inlet 21, flowing countercurrently to the flue gas, and the a-way gas passes through the line 22 to the exchanger 19. flows out from The combustion air preheated in this way is passed from the coil 2o to the pipe 1.
It reaches the pipe 12 through 4t and is supplied to the burner 11.

To demonstrate the benefits gained from preheating the combustion air, a standard 11 cubic feet)! fN) 1000BTU
Natural gas with a calorific value of (English ait weight unit) and 701
A residential furnace operating with a 15% excess of air with a temperature of 0 S has a thermal efficiency of 84 and releases flue gas at a temperature of 7501. If the flue gas is used to preheat the same amount of combustion air to a temperature of 400°F as shown in Figure @1, the thermal efficiency of the furnace increases to 91 bags. at the same time,
The NOx content of the flue gas is virtually unchanged by preheating; in both cases the NOx content is less than 18 ppm.

Of note is that in both cases the flue gas from the porous fiber burner contains no carbon monoxide wood and produces a flue gas containing less than 5 ppm of unburned carbon water.

Figure 2 shows the 15-
1 is a graph showing how the thermal efficiency of a furnace operating with excess air (shown as waste circle T0 on the horizontal axis) is increased by preheating the combustion air up to four different times; The straight line in the graph marked 200°500.750 and 1000 indicates the preheating temperature f ('F) t of the combustion air flowing into the portal fiber burner, and the increase in thermal efficiency obtained is the preheating temperature shown on the vertical axis of the graph. # with T, E, s! For example, a furnace with a circumference of -70*r,i, will yield a preheat of 76 degrees T,E, when preheating the combustion air to 500"F.Another furnace with a circumference of 1155 degrees T, , preheating 15% excess air to a temperature of 750°F shows a preheat of 64%. With other furnaces, preheating the combustion air to a temperature of 750°F gives a temperature of 8Q%T. ,lii.

increases after preheating 94'r, n, and t.

Tests carried out in accordance with the present invention further showed several advantageous features. For example, a porous fibrous layer may be fabricated from a stainless steel screen according to the teachings of U.S. Pat. No. 4,384,159.
- open a burner designed to provide a heat input capacity of 1000 BTUL per cubic foot in standard conditions and supplied with natural gas deposited on top of the cylinder and having a calorific value of 1000 BTUL per cubic foot in standard conditions; It was used in several combustion tests using a burner in air (not in a furnace). All of these tests were conducted with a 10% excess of air and 1% combined with 800”F for air and natural gas.
With preheating mft reaching m, the temperature of the radiant surface of the burner m
#i, if natural gas is supplied vertically to the burner at a rate of 3QOOOj3TU per hour, approximately 60° from the average temperature of 17307
2QOO per hour of natural gas flow without changing more than F
When reduced with OBTU1N, the temperature of the burner's radiant surface was reduced to 1650°F when operating at 960°F preheat am. Therefore, a porous fiber burner can be used successfully if the heat input is significantly reduced below its design capacity.
Even with a 1 degree preheat extending at about t000° Ffi and with these changes in operating conditions, the NOx content of the flue gas typically remains about 20°C.
The lower value WC@maru is less than p-.

When the fuel gas is substantially pure hydrogen, the combustion of the water-air mixture at the external surface of the porous fiber burner is
It has been found that there is a tendency to migrate through the porous fiber layer and cause a 7 lash pack of combustion reactants. Flashback in porous fiber burners is achieved by installing a perforated Cerac board (generally a metal screen) near the internal surface of the porous fiber burner and applying a gold film to the side of the Ceric board facing the internal surface of the burner. This can be prevented by The perforated ceramic plate is used when carrying out the combustion method of the present invention.

It should be used whenever the fuel gas exhibits a 7 rushback tendency.

The above description establishes flexibility in the operating conditions and in all cases produces flue gases with extremely low NOx contents and by the use of porous fiber burners containing virtually no carbon monoxide lA. Establish the preheat provided to the combustion reactants allowed. Modifications to the invention will be apparent to those skilled in the art without departing from the spirit of the invention. For example, the combustion air that passes through the coil 20 in the heat exchanger 19 of FIG. You can also Similarly, an air heating coil similar to coil 20 may be provided in the furnace 10 in the vicinity of the outlet pipe 18 for obtaining flue gas.
The separate O heat exchanger 19 can also be omitted.

〔Effect of the invention〕

Combustion of the fuel gas 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 flue gas living from flameless combustion, and the preheated air is mixed with the fuel gas in the primary layer to form a porous Supply to fiber burner. Waste heat from another available source can also be used to preheat the combustion reactants.

Thus, according to one aspect of the present invention, thermal efficiency can be increased while keeping the NOx content in the exhaust gas 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 diagram illustrating a comparison of the increased thermal efficiency of a furnace utilizing preheating according to the present invention with the thermal efficiency of a furnace without preheating. 10... Furnace 11... Fiber burner 12...
・Piping 13・-Piping 14...Pipe 15...Heat exchanger coil 16...In
Port 17...Outlet 18...Piping 1
9...Heat exchanger 20...Coil 21...Inlet 22...Piping patent applicant Alzeta Corporation 51,
=D 2==D □Yo2=D 2D FIG, 2 f [”y'zn1 6e10

Claims (1)

[Scope of Claims] (1) Forming a preheated mixture of fuel gas and combustion medium, passing the preheated mixture*1 through a porous fiber layer of a porous fiber burner, and passing the fuel gas outside of the fiber layer. Improved thermal efficiency and lower N41 mold by burning on a surface to cause said external surface to glow incandescent substantially without flame, thereby producing hot flue gas with a low NOx content.
2. The method of claim 1, wherein: (2) hot flue gas is used to preheat a heated mixture of fuel gas and combustion medium. β) air heating the combustion medium to at least 200° F (Q temperature f) by indirect heat exchange with MI11 road gas, and mixing the so heated air with the fuel gas to form a preheated mixture; The method according to claim 1, in which the fuel gas is passed through a porous fiber layer. (4) The fuel gas is natural gas, and the desired combustion air is in excess of the theoretically required amount by 15 bags or less. 6. A method according to claim 8, wherein the porous fibers are heated to a temperature of at least about 400° F. by indirect heat exchange with hot flue gases over a combustion medium. Claims 1 and 2, 1. in which the layer contains a small amount of evenly distributed fine aluminum powder. The fifth term is Ij
Method 0 according to IIF4 (The method according to any one of claims j11 to 5, wherein the production of 61NOx is about 209 pm or less in the flue gas. a heat exchanger (19) connected to this furnace to receive a flow of flue gases from the furnace, and a connection (20) in said heat exchanger for passing a flow of flue gas from said furnace. ), a pipe (14) connected to this passage and the outlet of the burner for flowing heated air from the casing into the burner, and a means for supplying fuel gas into the burner. An improved combustion device for fuel gas for producing flue gases with low NOx content and increasing good thermal efficiency. 8. The improved combustion device of claim 7 containing a moldy aluminum powder.