Diffuser plate for boiler burner feed assembly

Abstract

A diffuser or orifice plate is positioned across the bore of a feed pipe supplying a mixture of pressurized air and recycled flue gas to the combustion chamber of a gas burner associated with a boiler. The diffuser plate comprises inner and outer, radially spaced apart, annular baffles. The baffles define a central opening and an outer series of openings forming an annular array. Part of the gas mixture is accelerated as it passes through the restrictive outer openings and forms an annular stream. Part of the gas mixture passes through the central opening in the form of a lance-like stream. Fuel gas is injected into the annular stream immediately downstream of the diffuser plate. The resulting air/flue gas/fuel gas mixture mixes and adopts an elongated form in the combustion chamber. The burner feed assembly described contributes to maintaining the Ox content in the burner flue gas at a low level such as less than 15 ppm.

Description

FIELD OF THE INVENTION

The present invention relates to a burner feed assembly incorporating a diffuser plate. The burner feed assembly is used in connection with the burner of a boiler, such as a boiler used in an oilfield operation for generating steam. The diffuser plate assists in mixing air, recycled flue gas and fuel gas being fed to the burner combustion chamber.

BACKGROUND OF THE INVENTION

Boilers are commonly used in oilfield operations. These boilers are usually provided to generate steam.

The burners in these boilers produce flue gases, which are commonly emitted into the atmosphere. The flue gases comprise NOx compounds. The production of these NOx compounds is environmentally undesirable. They can react to form ground-level ozone and nitrates, which can trigger respiratory problems in humans. They also contribute to the formation of acid rain. And nitrous oxide contributes to the formation of ‘greenhouse’ gases in the atmosphere.

Governments are now issuing increasingly stringent guidelines with respect to acceptable levels of NOx emissions, such as those produced by oilfield boiler burners.

It is well understood in the art that thorough mixing of the air and fuel being fed to a burner combustion chamber can lead to a reduction in NOx emissions.

It is also known that reducing combustion temperature in the burner chamber can ameliorate NOx emissions.

One prior art embodiment of an oil burner feed assembly, commercially supplied for steam boilers, is illustrated schematically in FIGS. 1 and 2. The burner feed assembly (a) comprises a diffuser plate (b) mounted within the downstream end of an air feed pipe (c). In use, the feed pipe (c) is connected to the combustion chamber of a boiler burner. The diffuser plate (b) extends across the throat or bore of the feed pipe (c). The feed pipe (c) is a conduit for a stream of pressurized air issuing from a blower or fan (d). Some flue gas is recycled from a burner stack and is fed into the feed pipe bore, upstream of the diffuser plate (b). As shown, the diffuser plate (b) comprises an annular baffle (e). An oil nozzle (f) is mounted in the central opening (g) of the baffle (e), for injecting atomized oil. The baffle (e) forms a plurality of outer openings (h). The mixture of air and recycled flue gas flows through the openings (h) and mixes with the oil injected by the nozzle (f) in the burner chamber, to produce the combustion mixture.

The flue gas is added to produce a “lean” air/flue gas/fuel gas mixture which burns at a moderate temperature.

One state government has recently regulated that oilfield steam boiler burner NOx emissions should be less than 15 ppm. The state of the art burners described above produce NOx emissions of about 25 ppm.

There is therefore a need for a burner feed assembly adapted to better enable the burner to produce reduced NOx emissions, for example at a concentration less than 15 ppm.

It is the purpose of the present invention to provide such an assembly.

SUMMARY OF THE INVENTION

The present invention relates to a modified burner feed assembly for use in the context of a fuel gas-fired boiler.

The objective of the invention is to reduce NOx emissions in flue gases leaving the boiler burner. This objective is achieved by the combination of:

• • arranging for a central air/flue gas flow that causes elongation of the flame in the burner combustion chamber;
  • arranging for an outwardly spaced, generally annular air/flue gas flow that surrounds the central flow; and
  • introducing fuel gas inwardly from the side into the outer gas flow; whereby a desirable degree of mixing of the air, flue gas and fuel gas is obtained to produce a generally homogeneous combustion mixture and the resulting flame is elongated and is characterized by a moderate flame temperature.

These results are achieved by the combination of using a diffuser plate of specific configuration and introducing the fuel gas inwardly, immediately downstream of the plate.

More particularly, the diffuser plate comprises:

• • inner and outer, radially spaced apart, annular baffles forming a central opening and one or more arcuate openings spaced outwardly from the central opening; and
  • means, such as radial connectors, for interconnecting the baffles.

It is believed that the upstream surfaces of the annular baffles deflect part of the air/flue gas mixture flow moving through the feed pipe and cause it to move through the arcuate openings, which constrict and accelerate it, thereby inducing a venturi effect which assists in entraining the fuel gas being injected inwardly, immediately downstream of the plate. In addition, it is believed that the lance-like flow through the central opening has the effect of causing the flame produced in the burner chamber to elongate, which contributes to reducing flame temperature.

In another aspect, the invention is concerned with a method for controlling NOx emission from a burner having a feed pipe for supplying a flow of a mixture of air and flue gas under pressure into a burner combustion chamber, comprising:

• • providing an apertured diffusion plate extending across the bore of the feed pipe, said plate having a central opening and one or more outwardly spaced, arcuate openings;
  • forming part of the mixture flow into an inner stream having an elongated configuration, by passing it through the central opening;
  • forming another part of the mixture flow into an outer stream having a generally annular configuration by passing it through the outer openings;
  • the area of the outer openings being selected to be restrictive so that the mixture flow is accelerated as it passes therethrough; and
  • injecting fuel gas inwardly into the annular stream immediately downstream of the plate.

In an apparatus aspect, the invention is concerned with a burner feed assembly for supplying a combustion mixture to the combustion chamber of a burner associated with a boiler, comprising: means for supplying a stream of pressurized air; a feed pipe, having a bore, for connecting the air supply means with the combustion chamber, said bore having upstream and downstream ends; a diffuser plate, comprising inner and outer, generally annular baffles forming a central opening and at least one outwardly spaced, arcuate opening, said diffuser plate being positioned transversely across the downstream end of the bore of the feed pipe; means for recycling flue gases, produced in the combustion chamber, to the feed pipe bore upstream of the diffuser plate, for mixing with the stream of air to form an air/flue gas mixture; and means for feeding fuel gas into the air/flue gas mixture immediately downstream of the diffuser plate.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a simplified side view showing a burner feed assembly in accordance with the prior art, said assembly being connectable with the burner of a boiler;

FIG. 2 is a simplified end view of the prior art burner feed assembly of FIG. 1, showing the diffuser plate;

FIG. 3 is a simplified schematic side view showing a burner feed assembly in accordance with the invention, connected with the burner of a boiler, with a fanciful burner flame shown;

FIG. 4 is a front view showing a diffuser plate in accordance with the invention; and

FIG. 5 is a view similar to that of FIG. 4, labelled to indicate the dimensions of one embodiment, as set forth in Table 1.

DESCRIPTION OF THE PREFERRED EMBODIMENT

The burner feed assembly 1 is adapted for operation with a burner 2 of a steam boiler 3.

The burner feed assembly 1 comprises means, such as a blower 4, for supplying a stream of pressurized air 5 into the axial bore 6 of a feed pipe 7. The feed pipe 7 connects the blower 4 with the combustion chamber 19 of the burner 2.

A recycle line 8 connects the stack 9 of the burner 2 with the blower 4, for recycling some of the produced flue gas 10 into the bore 6. The recycled flue gas 10 and air 5 mix in the bore 6 to form an air/flue gas mixture 11.

A diffuser plate 12 is mounted in the feed pipe 7 at its downstream end, so as to extend across the bore 6. The diffuser plate 12 comprises inner and outer annular baffles 13, 14. The baffles 13, 14 are interconnected by connectors 15 and are fixed in a common plane. The baffles 13, 14 and connectors 15 combine to define a central opening 16 and a plurality of outwardly spaced, arched or arcuate openings 17. The arcuate openings 17 combine to form a substantially annular passageway 18.

As a result of experimentation, applicant has identified a preferred combination of dimensions for the diffuser plate, which yield particularly desirable results. These dimensions are identified in FIG. 5 and are set forth in the following tabulation:

TABLE 1
a 23 1/16″
b 18½″
c 11 2/4″
d 5½″

A fuel line 20 connects a source 2a of fuel gas 21 with the bore 6 of the feed pipe 7, immediately downstream of the diffuser plate 12. The fuel line 20 feeds fuel gas 21 inwardly into the annular stream 22 of air/flue gas mixture 11 which issues from the arcuate passageway 18.

In operation, the blower 4 and recycle line 8 supply pressurized air 5 and recycled flue gas 10 into the feed pipe bore 6, to form a cylindrical stream 24 of mixture 11. Part of the mixture stream 24 flows through the central opening 16 to produce a lance-like stream. The remainder of the mixture stream 24 is accelerated as it passes through the restrictive arcuate openings 17 and produces a generally annular stream which entrains fuel gas 21 being fed in from the side and mixes it with the air and flue gas.

Example

A diffuser plate dimensioned in accordance with Table 1 was installed in the bore of the feed pipe of a North American Manufacturing Company Model G burner connected to a steam boiler. Air was supplied to the feed pipe at 10,000 C.F.M. and 1 psig. Flue gas was recycled to the feed pipe, upstream 30 inches from the diffuser plate, at a rate of 1,000 C.F.M. Fuel gas was injected from the side, 2 inches downstream of the plate, at a rate of 833 C.F.M.

The flue gas emitted by the burner was tested using a Teco™ Chemiluminescent Analyzer and was found to contain 12 ppm NOx.

Claims

1. A burner feed assembly for supplying a combustion mixture to the combustion chamber of a burner associated with a boiler, comprising:

means for supplying a stream of pressurized air;

a feed pipe, having a bore, for connecting the air supply means with the combustion chamber, said bore having upstream and downstream ends;

a diffuser plate, comprising inner and outer, generally annular baffles forming a central opening and at least one outwardly spaced, arcuate opening, said diffuser plate being positioned transversely across the downstream end of the bore of the feed pipe;

means for recycling flue gases, produced in the combustion chamber, to the feed pipe bore upstream of the diffuser plate, for mixing with the stream of air to form an air/flue gas mixture; and

means for feeding fuel gas into the air/flue gas mixture immediately downstream of the diffuser plate.

2. The burner feed assembly as set forth in claim 1 comprising radial connectors interconnecting the inner and outer annular baffles, said baffles and connectors combining to define a plurality of arcuate, outwardly spaced openings, collectively forming a substantially annular passageway.

3. A method for controlling NOx emission from a burner having a feed pipe for supplying a flow of a mixture of air and flue gas under pressure into a burner combustion chamber, comprising:

providing an apertured diffusion plate extending across the bore of the feed pipe, said plate having a central opening and one or more outwardly spaced, arcuate openings;

forming part of the mixture flow into an inner stream having an elongated configuration, by passing it through the central opening;

forming another part of the mixture flow into an outer stream having a generally annular configuration by passing it through the outer openings;

the area of the outer openings being selected to be restrictive so that the mixture flow is accelerated as it passes therethrough; and

injecting fuel gas inwardly into the annular stream immediately downstream of the plate.