Burner comprising a pilot

0001The present invention relates to a burner comprising a pilot for igniting a combustible fluid stream.

0002Burners are known for the combustive destruction of noxious substances in a fluid stream. The combustion can be performed in a combustion chamber by an open flame arrangement or a radiant burning arrangement.

0003In an open flame burner 50 as shown partially in <figref idref="f0005">Figure 6</figref>, a fluid stream 52 is introduced through an inlet 54 of a combustion chamber 56 as a mixture with a fuel gas 58. The mixture is ignited by a pilot flame 60 of a pilot 62 and burns as an open flame 64 combusting the noxious substances. The pilot 62 comprises an injection tube 66 having an end proximate the inlet of the combustion chamber for introducing a fuel gas which is ignited by igniter 68 and which in turn ignites the mixture as it is introduced to the combustion chamber through the inlet 54.

0004A number of problems exist with the burner 50. The pilot flame 60 is located at only one position relative to the flame 64 and therefore burning of the flame is more readily achieved in the region close to the pilot flame and may not be achieved, or not fully achieved, at a region remote from the pilot flame. Consequently, mixture entering the combustion chamber may not be completely combusted resulting in noxious substances being exhausted from the burner. Further, the injection tube of the pilot is typically relatively narrow and therefore the fuel gas is injected through the tube in a narrow concentrated jet which has a propensity to disrupt the flame 64 causing incomplete combustion of the mixture. If more than one inlet is provided (not shown in <figref idref="f0005">Figure 6</figref>), for instance for introducing a fluid stream from more than one source of noxious substances, a pilot is required for each of the inlets causing additional expense in addition to the problems identified above.

0005In a radiant burner 70 as shown in <figref idref="f0006">Figure 7</figref>, a combustion chamber 72 is formed by a surrounding porous generally cylindrical wall 74. Fuel gas 76 is introduced to an outer chamber 78 through inlet 80 and passes through the porous wall 74. A pilot 82 produces a pilot flame 84 for igniting fuel gas at the inner surface of the wall producing a high temperature reaction zone 86. A weir arrangement 88 produces a weir of cold liquid 90 (typically water) for dissolving constituents of the combusted fluid stream and for washing away particulate matter. The cold liquid also cools fluid exhausted from the burner so that it can be conveniently disposed. A fluid stream 92 containing at least one noxious substance is introduced into the combustion chamber 72 through inlet 94 and is combusted by contact with the gases from the high temperature reaction zone 86 near the surface of the wall 74. Combustion is also achieved by the heat which is generated and reflected from an opposing surface of the wall 74. A radiant burner is shown in more detail in <patcit id="pcit0001" dnum="EP0694735A"><text>EP 0694735</text></patcit>.

0006The burner 70 suffers from a number of problems. First, the flame 86 when fully ignited is cylindrical and as the pilot flame 84 is located at only one position the high temperature reaction zone 86 ignition is assured in this region. The flame 86 may not burn or may not burn fully at areas of the wall 74 which are remote from the pilot. Areas of the wall surface at which a high temperature reaction zone is not maintained may be susceptible to depositing of particulates which causes damage to the porous wall. Further, the proximity of the cooling column cools the surface of the wall 74 particularly at the lower part of the wall. Also, heat is not reflected from the cooling water which further acts to cool the combustion chamber 72. Consequently, complete combustion of the fluid stream 92 may not be achieved along with incomplete combustion of the fuel / air mix 76. Additionally, the provision of a single relatively high energy concentrated flame can damage the delicate porous wall of the burner requiring expensive replacement.

0007It is know to provide discontinuous annular burner arrangements from <patcit id="pcit0002" dnum="GB2136557A"><text>GB2136557</text></patcit>; <patcit id="pcit0003" dnum="US3501255A"><text>US3501255</text></patcit>; and <patcit id="pcit0004" dnum="FR2546273"><text>FR2546273</text></patcit>.

0008The present invention provides a burner according to Claim 1.

0009Other preferred and/or optional aspects of the invention are defined in the accompanying claims.

0010In order that the present invention may be well understood, embodiments thereof, which are given by way of example only, will now be described with reference to the accompanying drawings, in which: <ul id="ul0001" list-style="none" compact="compact"><li><figref idref="f0001">Figure 1a</figref> shows a section through a pilot taken along line I-I in <figref idref="f0002">Figure 2</figref>;</li><li><figref idref="f0001">Figure 1b</figref> shows a section through a pilot taken along line J_J in <figref idref="f0002">Figure 2</figref>;</li><li><figref idref="f0002">Figure 2</figref> shows an elevation of the pilot indicating the lines of section I-I and J-J;</li><li><figref idref="f0003">Figure 3</figref> shows an open flame burner comprising the pilot shown in <figref idref="f0001">Figure 1</figref>;</li><li><figref idref="f0004">Figure 4</figref> shows a radial burner comprising the pilot shown in <figref idref="f0001">Figure 1</figref><figref idref="f0005">Figure 5</figref> shows a further open flame burner comprising the pilot shown in <figref idref="f0001">Figure 1</figref>;</li><li><figref idref="f0005">Figure 6</figref> shows a prior art open flame burner;</li><li><figref idref="f0006">Figure 7</figref> shows a prior art radial burner;</li><li><figref idref="f0007">Figures 8 and 9</figref> show a further pilot made integrally with a weir arrangement of a burner.</li></ul>

0011Referring to <figref idref="f0001">Figures 1a, 1b</figref> and <figref idref="f0002">2</figref>, a pilot 10 is shown for igniting a combustible fluid stream. The pilot comprises an inlet 12, which may comprise a venturi arrangement, for receiving a fuel gas 14, including a fuel and air/oxygen mixture, into the pilot. Ignition means 16, for example a spark igniter or an ignition electrode, are provided for igniting the fuel gas 14. A generally annular conduit 18 is open along an annular extent forming a circumferential aperture 20. The conduit 18 conveys fuel gas from the inlet to the circumferential aperture. When the fuel gas flowing out of the circumferential aperture 20 is ignited a continuous annular pilot flame is generated radially of the conduit.

0012The annular conduit as shown is circular, but may be formed of other shapes such as a square, rectangle or hexagonal. In this regard, the annular conduit conveys fuel gas from the inlet through 360° and allows the gas to flow from the inlet so that when it is ignited an annular flame is generated. Likewise, the annular flame generated need not be ring-shaped, but instead corresponds generally to the shape of the conduit. For example, if the conduit is rectangular then a generally rectangular flame is generated.

0013The conduit as shown in <figref idref="f0001">Figures 1a, 1b</figref> and <figref idref="f0002">2</figref> is open along its annular extent by the circumferential aperture 20. The aperture extends through 360° to allow fuel gas to flow from the conduit approximately equally about its circumference. The aperture is determined in size to ensure that an equal flow of fuel gas is distributed around the circumference generating a consistent flame.

0014The fuel gas typically contains a mixture of fuel such as a hydrocarbon and oxygen or air.

0015As shown in <figref idref="f0001">Figures 1</figref> and <figref idref="f0002">2</figref>, the conduit 18 is open along a radially inner annular extent so that the circumferential aperture 20 faces radially inwardly. When the fuel gas 14 is ignited by the igniting means 16 the annular pilot flame is generated radially inwardly of the conduit for surrounding a fluid stream to be ignited.

0016Although the pilot flame generated is annular only a single igniter 16 is required since once the flame is ignited in the region of the igniter it readily spreads around the circumference of the pilot. Therefore, the ignition means 16 as shown is disposed at a single location in the circumference of the pilot and in use ignites the fuel gas to form the continuous annular pilot flame. The pilot flame may not be fully annular at all times after ignition as the flow of fuel gas may not be distributed completely equally about the circumference or the fluid stream may occasionally extinguish part of the annular flame. However, generally in use the pilot is capable of generating a continuous annular pilot flame.

0017Monitoring or sensing means may be placed at intervals around the pilot for monitoring or sensing characteristics of the pilot, such as the presence of a flame, or the flow or pressure of fuel gas. Such sensing means may feed back to a control for the pilot which may be connected to a fuel gas supply valve or the igniting means.

0018The generally annular conduit 18 in <figref idref="f0001">Figures 1a, 1b</figref> and <figref idref="f0002">2</figref> comprises a plenum chamber 22 for receiving fuel gas from the inlet 14 and a circumferential channel 24 for channelling fuel gas from the plenum chamber to the circumferential aperture 20 for forming the continuous annular pilot flame. In use the plenum chamber 22 distributes the fuel gas generally equally around the annular extent of the conduit 18 at an equal pressure so that when ignited the continuous annular pilot flame burns generally evenly at approximately the same intensity and size about the annular extent of the conduit. This arrangement promotes the generation of a pilot flame which is annular and unbroken.

0019The pressure of gas in the conduit 18 may be greatest in the region proximate the inlet 12. Accordingly, the amount of gas exiting the circumferential aperture 20 may be greatest near the inlet and hence the annular flame may be largest near the inlet 12. The plenum chamber arrangement as described above serves to distribute the gas equally around the conduit. Additionally it will be seen that inlet 12 is not aligned with channel portion 24. Instead gas entering through the inlet must flow along a tortuous path prior to exiting the pilot through the circumferential aperture. In this way, the surface of the plenum chamber opposed to the inlet and the upper surface of the baffle as shown in <figref idref="f0001">Figures 1a and 1b</figref> act as baffles to the flow of fuel gas.

0020The pilot may be moulded from a metallic material in a single piece or two or more pieces which are subsequently fixed together. The channel portion 24, or other portion of the pilot which is in contact with or closely adjacent to the pilot flame, may be formed from a heat or flame resistant material, such as a ceramic material or a metal such as stainless steel. It is preferable to avoid a metal surface in contact with the pilot flame since this may generate undesirable nitrogen oxide compounds.

0021In use, fuel gas is introduced to the conduit 22 and conveyed around the conduit so that it can flow out of the circumferential aperture 20. When the flow through the aperture is established the ignition means 16 ignites the fuel gas generating a pilot flame which extends radially inwardly from the aperture and in a generally annular configuration. The rate at which fuel gas is introduced to the conduit and the size of the aperture are selected to reduce the possibility of flash back into the plenum chamber 22. In this regard, the flame speed should be less than forward velocity of the fuel gas. The flame speed is the speed a flame passes back towards its source fuel and oxidant. The forward velocity is the speed at which gas is conveyed through the aperture in a generally radially inwards direction and should be greater in velocity than the flame speed. As the gas mixture leaves the aperture, it will spread out (fan like), thereby reducing the forward velocity. The anchor region at which the forward velocity equals flame speed is consequently just beyond the exit of the aperture. If the material from which the main portions of the pilot is made is metallic, flame resistant portions, made for instance of ceramic, may be disposed in the region of the aperture 20.

0022The pilot is formed in this example by a first, or lower, pilot plate 116, which together with a second, or upper, pilot plate 117 forms the plenum chamber 22, the channel portion 22 and circumferential aperture 20. The first and second pilot plates may be fixed together by any suitable means such as by bolts. Spacers 121 are placed at intervals around the pilot plate for spacing the first pilot plate from the second pilot plate. The spacers ensure that the channel portion 24 and circumferential aperture 20 are sized correctly and uniformly around the pilot. Ignition means 16 which in this example is an ignition electrode extends through the wall of the first pilot plate and is made gas tight by means of a swageing system. The ignition electrode extends into the circumferential aperture to provide an ignition source when the fuel gas is flowing through the aperture. Inlet 12 is formed in the first pilot plate 116 which may be sealed with a Swagelok component.

0023An open flame burner 27 comprising pilot 10 is shown partially in <figref idref="f0003">Figure 3</figref>. The pilot 10 is not shown in detail in <figref idref="f0003">Figure 3</figref> for simplicity.

0024A fluid stream 29 is introduced through an inlet 31 of the combustion chamber 33 as a mixture with a fuel gas 35. The mixture is ignited by a pilot flame 37 of the pilot 10 and burns as an open flame 39 combusting the noxious substances. Although only a semi-circular portion of the flame 37 is shown in <figref idref="f0003">Figure 3</figref>, the pilot flame 37 is annular and therefore surrounds the fluid stream and fuel gas mixture as it enters the combustion chamber 33 through the inlet 31. The term "horizontal pilot" as used herein is used to describe the arrangement shown in that the pilot extends generally perpendicularly to the direction of flow of gases entering the burner. Usually a burner is upright as shown and in this case the pilot is generally horizontal. Accordingly, the mixture is exposed to the pilot flame laterally from all sides ensuring relatively complete combustion of the mixture. This arrangement constitutes an improvement over the prior art described above in which only the portion of the fluid stream proximate the pilot flame is consistently burnt. Additionally, as the circumferential aperture 20 is relatively large and the flame is therefore a distributed low energy flame the fuel gas flowing through the aperture does not significantly disrupt the main flame 39. Referring to <figref idref="f0004">Figure 4</figref>, a radial burner 26, comprising pilot 10, is shown for removing noxious substances from a fluid stream 28. The burner 26 comprises a combustion region in which a fuel gas can be burnt for combusting the fluid stream. In this arrangement the combustion region is formed by a chamber 30 surrounded by a generally cylindrical wall 32. The wall 32 is porous to allow passage of fuel gas through it into the combustion chamber for burning on the inner surface of the wall. Fuel gas 34 is introduced to an outer chamber 36 through inlet 38 and passes through the wall 32. The wall may form a right circular cylinder, elliptic cylinder, parabolic cylinder, or hyperbolic cylinder such that the wall forms a surface on which fuel gas 34 can burn radiating hear radially inwardly and combusting the fluid stream. It will also be understood that the pilot 10 is located away from the top, or head, of the burner where space is limited.

0025A weir arrangement 44 produces a weir of cold liquid 46 (typically water) for dissolving constituents of the combusted fluid stream and for washing away particulate matter. The cold liquid also cools fluid exhausted from the burner so that it can be conveniently disposed.

0026The pilot 10 is shown in simplified form in <figref idref="f0004">Figure 4</figref> and is located below the wall 32 and between the combustion chamber 30 and the weir arrangement 44. In use the pilot 10 generates an annular flame 40 although only a semi-circular portion of which is shown in <figref idref="f0004">Figure 4</figref>. The pilot flame 40 ignites fuel gas at the inner surface of the wall producing a flame 42. The fluid stream 28 containing at least one noxious substance is introduced into the combustion chamber 30 through inlet 48 and is combusted by contact with the hot reaction zone 42 near the surface of the wall 74.

0027The pilot 10 is located so that when ignited the continuous annular pilot flame 40 and the generally cylindrical wall 32 are adjacent along their respective annular extents such that the fuel gas 34 passing through the generally cylindrical wall can be efficiently ignited and a flame at the surface maintained. As the annular pilot flame 40 ensures that the full circumferential extent of the lower portion of the flame 42 is ignited and maintained alight, combustion of the fuel gas 34 over the full inner surface of the wall 32 is increased.

0028Additionally, the pilot 10 is located so that when ignited the continuous annular pilot flame 40 thermally insulates the base of the main combustor reaction zone 42 from chilling effect generated by the relatively cold liquid 46 passing over the weir arrangement. Accordingly, heat is more efficiently generated at the base of the reaction zone 42, thereby improving the emissions from the combustor at the base nearest the weir, for example carbon monoxide and hydrocarbon emissions (CxHy). Table 1 shows the improvement observed in testing the embodiment of the invention. <tables id="tabl0001" num="0001"><table frame="all"><title>Table 1</title><tgroup cols="4"><colspec colnum="1" colname="col1" colwidth="74mm" /><colspec colnum="2" colname="col2" colwidth="29mm" /><colspec colnum="3" colname="col3" colwidth="31mm" /><colspec colnum="4" colname="col4" colwidth="15mm" /><thead><row><entry valign="top">Conditions</entry><entry valign="top">Residual oxygen</entry><entry valign="top">Carbon monoxide</entry><entry valign="top">CxHy</entry></row></thead><tbody><row><entry>Prior Art shown in <figref idref="f0006">Figure 7</figref></entry><entry>5.2%</entry><entry>196 ppm</entry><entry>0.34%</entry></row><row><entry>Embodiment shown in <figref idref="f0004">Figure 4</figref> with horizontal pilot</entry><entry>5.0%</entry><entry>100 ppm</entry><entry>0.17%</entry></row></tbody></tgroup></table></tables>

0029The improvement is due to a numbers of aspects of the embodiment. For example, heating the base of the radiant burner pad improves the combustion at the base.

0030Powders tend to form at the base of the combustor due to reduced temperature of the combustor pad 32 / reaction zone at the base. The provision of a horizontal pilot thermally insulates the base of the pad increasing efficiency of the pad allowing it to sustain hotter temperatures. Additionally, the horizontal pilot decreases the propensity for powder to adhere to the delicate porous structure of the pad 32. The problem of powder deposition still occurs but in the embodiment it occurs downstream of the pad 32. However, the pilot plates are relatively robust and the deposition of powder on the pilot is not considered a significant problem. The solids may include silica which is easily removed by light agitation of the surface of the pilot by compressed air or water flows. Further, the pilot can be cleaned by washing with water in an in situ cleaning method (described in more detail below with reference to <figref idref="f0007">Figures 8 and 9</figref>)

0031Advantageously, the pilot 10 provides a low energy distributed flame surface which has less propensity to damage the delicate porous wall 32.

0032Although not depicted, the radial burner shown in <figref idref="f0004">Figure 4</figref> can be used in combination with the open flame burner shown in <figref idref="f0003">Figure 3</figref> for combusting certain types of noxious substances in the fluid stream. In this arrangement, the inlet upstream of the combustion chamber 30 shown in <figref idref="f0004">Figure 4</figref> is adapted for introducing the fluid stream and a fuel gas as a mixture into the combustion region as shown in <patcit id="pcit0005" dnum="WO2006013355A"><text>WO2006/013355</text></patcit>. The mixture at the inlet can be ignited by the flame 42 formed at the surface of the generally cylindrical wall which is itself ignited by pilot 10 at the base of the cylindrical wall. Alternatively, the pilot 10 is located between the wall 32 and the inlet (at the top of the wall as shown in <figref idref="f0004">Figure 4</figref>) for igniting both the mixture at the inlet and the fuel gas at the surface of the wall.

0033<figref idref="f0005">Figure 5</figref> shows a modification of the open flame burner shown in <figref idref="f0003">Figure 3</figref>. The burner 45 is suitable for removing noxious substances from a plurality of fluid streams 47, for instance, from a respective plurality of semi-conductor wafer processing chambers. Burner 45 comprises an inlet 41, 43 for each fluid stream 47. Although only two inlets are shown the burner may comprise more than two inlets. The inlets 41, 43 introduce the plurality of fluid streams 47 and fuel gas 49 as respective mixtures into the combustion chamber 33. The pilot 10 is located so that when ignited the continuous annular pilot flame 37 surrounds mixtures to be burned so that all of the mixtures can be ignited by a single pilot.

0034Typically, cleaning of the pilot 10 requires removal of the burner arrangement 26 and this removal is labour intensive and results in significant tool downtime.

0035However, in-situ cleaning of the pilot plate can be performed Cleaning is performed by pumping pressurized fluid, such as water or air, through inlet 12 of the pilot, through the plenum chamber and out of the circumferential aperture 20. Advantageously, tthe fluid is a liquid, as a liquid falls under gravity removing particulates from the outer wall of the lower pilot plate 116.

0036Further, the pilot plate 10 may be integrated with the weir 44. As shown in <figref idref="f0007">Figures 8 and 9</figref>, a passage 112 is machined between the pilot plate plenum 105 and the weir volume 107. A bung 106 is moveable from a first position in which it blocks the passage 112 and a second position in which the passage is open. When the passage is open, water injected through inlet 12 of the pilot can flow into the pilot plenum 105 and out through the circumferential aperture 109, running down the entirety of the outer wall of the lower pilot plate 102, removing solid residue by physical and chemical (dissolving) action. After cleaning the water is switched off and the plenum chamber allowed to drain. Once drained, the bung 106 is then reengaged. The pilot is dried by blowing air through the inlet. Once dry, the pilot can be ignited. It will be appreciated that the process described for in-situ cleaning is more efficient and less time intensive than disassembling the burner, disconnecting various pipes, cleaning the pilot and subsequently re-assembling the burner and reconnecting the pipes. Further, in-situ cleaning reduces the possibility of the operator coming into contact with potentially dangerous combustion byproducts (ie oxides of Arsenic on AsH3 processes).

0037Accordingly, a method of cleaning pilot 10 in situ in a burner comprises: disconnecting a source of fuel from an inlet to the pilot; connecting a source of cleaning fluid to the inlet; and cleaning the pilot with said fluid.