GB2126704A - Igniters - Google Patents

Abstract

A gas-electric igniter 1 comprises a tube 6 which encloses concentric tubes 7 and 8 defining an annular passage 13 connected to a gas line for issuance of gas to a flame holder 14 at the front end of the igniter 1. Flame holder 14 has six axial tubes (15 and 16) spaced about the axis of the igniter 1 and communicating with the annular passage 13. Four tubes 15 pass, through an ignition zone 21 to the front end of the flame holder 14 to issue as main gas jets. The two tubes 16 have their front ends closed so that raw gas only issues through radial holes 20 as pilot gas jets which are spark ignited in the ignition zone 21 to provide a pilot flame which then ignites the main gas jets. Support tube 6 carries a deflector F which causes combustion air C, which is blown by a fan forwardly along the outside of the support tube 6, to reversely flow into the ignition zone to form a combustible mixture with the pilot and main gas jets. <IMAGE>

Description

SPECIFICATION Ignites This invention relates to igniters.

It is known for a gas-electric igniter to be incorporated in an oil fuel burner of which a multiple number are arranged in the furnace walls of a fossil fired boiler for firing the boiler's fuel. Oil fuel is used as the prime fuel for firing burners, or as a secondary fuel for igniting pulverised coal when that is the primary fuel, or in combination with gas as an alternative primary fuel. The boiler would generate steam, and have land, marine or other industrial applications. Such a gas-electric igniter is used as a stable source of ignition to provide lighting up of such oil or pulverised coal burners, and it achieves this by first igniting with an electric spark a stream of inflammable gas mixed with air. The resulting gas flame is then used to light the oil or pulverised coal burner.

Two kinds of such a gas-electric igniter exist.

The first utilises a supply of gas and air which is pre-mixed. The second kind utilises a supply of raw gas which entrains free air in the region of the ignition source. These two kinds of igniter may be described as "premixed" or "raw gas" types respectively.

The required attributes of any gas-electric igniter can be summarized as follows: (a) ignition of the gas by the electric spark must be reliable.

(b) the gas flame must be stable and of a predictable shape.

(c) gas combustion must be compiete so as to achieve a hot sootless flame.

These attributes must be maintained in the hostile environment of an oil and/or coal burner, where air velocities parailel to the igniter may exceed 200 ft/sec and where it may be expected that the igniter flameholder will be contaminated by fuel oil spray or pulverised coal dust.

Most currently available gas-electric igniters are designed to remain in their forward position at all times and are therefore susceptible to contamination, particularly when the burner itself is out of use and there is no airflow to help in keeping it clear. As a result, the attributes (a), (b) and (c) suffer considerably, mainly due to blockage of the gas jets and/or spark gap by pulverised fuel dust or cracked fuel oil deposits. In general it is not desirable to withdraw the igniter after use, since automatic withdrawal and insertion equipment has proved unreliable in practice.

An object of the present invention is to provide a gas-electric igniter which is of the "raw gas" kind mentioned above and which minimises the enumerated disadvantages of prior igniters to provide a more reliable igniter system.

According to the invention there is provided an electric igniter of the raw gas type wherein a pilot flame generated from one or more pilot gas jets serves to ignite axially issuing main gas jets with combustion air being caused to flow reversely thereto to form a combustible mixture with said pilot and main gas jets.

In order that the invention may be well understood there will now be described some preferred embodiments thereof, given by way of example, with reference to the accompanying drawings, in which: Figure 1 is an axial sectional view taken along line I-I of Figure 2 of a gas-electric igniter operatively positioned in relation to a fluid injector shown in chain dot outline; Figure 2 is an end elevation of the same igniter; Figure 3 is a cross-section to a larger scale of the same igniter taken along line Ill-Ill of Figure 4; Figure 4 is a scrap axial section of the front end of the same igniter taken along line IV--IV of Figure 3; Figure 5 is a scrap isometric view of the front end of the same igniter; Figure 6 is a scrap view similar to Figure 1 showing the front end of a modified gas-electric igniter;; Figure 7 is a view similar to Figure 4 of the same modified igniter; and Figure 8 is a scrap side elevation, partly sectioned of the front end of yet another modified gas-electric igniter.

In the various figures, like references denote like parts.

Referring first to Figure 1, a gas-electric igniter 1 is shown operatively positioned relative to a fluid injector 2 of the burner whose combustion it is to initiate and which may be constructed in accordance with GB application 2080703A or anyofGBpatentNos.1233317,1231631 or 1497271. Attention is directed to those patents and application for a full description of the construction and operation of thè fluid injector which will not be described here save to say that a carrier tube within which the fluid injector is mounted is denoted by reference numeral 3 and its flame stabilizer is referenced 4.It should also be mentioned that the igniter 1 and the fluid injector 2 (with its carrier tube 3 and flame stabilizer 4) are mounted within a channel (not shown) to which is connected a combustion air fan (also not shown) for providing a flow of combustion air to both the igniter and fluid injector. Finally, in this instance the fuel which is to be burned by the flame issuing from the fuel injector 2 is pulverized coal which is supplied to a further channel adjacent or coaxially surrounding that along which flows the combustion air.The general arrangement of the aforementioned system is conventional and will be well understood by those versed in the art as will the means by which the igniter 1 and the fluid injector carrier 3 are supported in and relative to the boiler wall which, accordingly, also will not be described apart from mentioning the provision of a conventional centre carrier plate 5 which supports the rear end of the igniter.

The gas-electric igniter 1 comprises a support tube 6 carrying a flared gutter F at its forward end and which is supported by the carrier plate 5.

Within the support tube 6 is slidably mounted a rigid assembly of inner and outer concentric tubes 7 and 8 spaced apart by studs 9 and which are arranged concentrically within the support tube by spacing studs 9a integral with the outer tube 8. A cap head screw 9b carried by a clamp ring 9c integral with the support tube 6 secures the rigid tube assembly within the support tube by clamping against the outer tube 8. The rear ends of the inner and outer tubes 7 and 8 are fast with an end cap 10 with which they make an integral assembly and with respect to which they are sealed respectively by O-rings 11. A male coupling 12 is secured to the end cap 10 and is connected to a gas line (not shown) for passing raw gas into an annular passage 13 between the inner and outer tubes 7 and 8 for issuance to a flame holder 14 at the front end of the igniter 1.

The flame holder 14 comprises six separate axial tubes 1 5 and 16 equiangularly spaced in concentric formation about the axis of the igniter 1 and having their inner ends fast with an inner mounting plate 1 7 secured between the inner and outer tubes 7 and 8, the outer ends of the tubes being fast with an end plate 18. The tubes 1 5 pass through an ignition zone 1 9 to the front end of the flame holder 14 to convey raw gas thereto through their open ends which provide generally axially issuing main gas jets G.The other two tubes 16 are diametrically opposed and whilst they also pass through the ignition zone 1 9 their front ends are closed to allow the raw gas only to issue laterally therefrom through radial holes 20 provided in the side wall of each tube.Such radial holes provide generally radially issuing pilot gas jets P and are positioned on opposite sides of a spark gap 21 in the ignition zone 19. Reference is also directed to Figures 2 to 5 for showing of the above-described structure.

The inner tube 7 of the igniter 1 carries an electrode rod 22 supported by suitable insulation 23 so as to run along the central axis of the tube to make an electrical connection at its rear end with an electrode 24 screwed into the end cap 10. The forward end of the electrode rod 22 penetrates into the flame holder 14 where it "meets" an electrode screw 25 threaded into the end plate 1 8 from the opposite direction.

Adjustment of the electrode screw 25 determines the length of the spark gap 21.

The function of the flared gutter F is to provide stabilization for the gas flame and is a term used in gas turbine parlance to denote an aerodynamic device for creating a reverse gaseous flow. The relative positions of the flame holder 1 4 and flared gutter F are such that the flame holder is protected from contamination, since it is almost completely inset into the gutter support tube 6.

This has not been possible with previous designs.

The flame holder components and the gutter F may be of stainless steel with the inner and outer tubes 7 and 8, support tube 6 and clamp ring 9c of mild steel, and the end cap 10 of aluminium.

In operation of the gas-electric igniter 1, gas is passed from the coupling 12 along the annular passage 13 to issue radially as pilot gas jets P into the region of the spark gap 21 through the two pilot gas holes 20 in the tubes 1 6. The raw gas also issues axially as main gas jets G into the region immediately downstream of the flame holder 14 through the four main gas holes in the open front ends of the tubes 1 5. At the same time, combustion air to the fluid injector 2 passes along the outer surface of the fluid injector 2 passes along the outer surface of the igniter support tube 6 as indicated by arrows C. At the end of the support tube 6 the gutter F intercepts this air stream, creating a low pressure zone Z immediately ahead of it.This low pressure causes a backward flow of air into the flame holder 14 where it mixes with the pilot gas jets P from the pilot gas holes 20 to form an ignitable mixture.

When a high voltage is applied to the electrode rod 22 (between 5 and 10 thousand volts) a spark jumps across the spark gas 21 to ignite the pilot gas jets P which then ignites the main gas jets G.

Because of the backward flow caused by the gutter F, a proportion of the ignited gas is continually fed back towards the flame holder 14 and so maintains a stable flame when the spark is removed.

The design of the gas-electric igniter 1 is such as to permit the flame holder 14 to be almost totally withdrawn inside the gutter F thus protecting it from contamination, provides a stable gas flame in the presence of high velocity air flows, and ensures complete cobmustion of the gas, providing a high temperature, sootless flame. Since the main gas jets G issue from the open ends of the tubes 1 5 the diameter thereof is governed by that of the internal axial passage within those tubes which can be relatively large compared with that of the radial jets P issuing from the lateral holes 20 in the closed ended tubes 1 6 since radial holes weaken the wall of the tube and hence their diameter is restricted.Thus, by providing axially issuing main gas jets, a longer flame is possible than if only radial gas jets were utilized, enabling the igniter 1 to be retracted further away from the forward end of the fluid injector (whose issuing fuel is ignited by the flame igniter) than has previously been possible thus further protecting the igniter from contamination by the pulverized coal dust (or oil spray which carbonises at high temperature).

A gas-electric igniter 1 a shown in Figures 6 and 7 is a modified version of the igniter 1 in that the closed-ended tubes 1 6 are extended through the end plate 18 and are made fast at their projecting front ends with a circular deflector plate 26. At high gas pressures the velocity of the gas stream formed by the main gas jets G issuing axially from the four tubes 1 5 can exceed the velocity of the flame front, causing the gas flame to move out from the ignition zone 1 9 and extinguish. By disposing the deflector plate 26 in the path of the main gas jets G the gas stream is slowed down thereby preventing that happening.

The modified gas-electric igniter 1 b shown in Figure 8 is similar to the igniter 1 a but incorporates an alternative gutter Fb which is flared at a less acute angle than the gutter F to provide a flow reversal of the air stream, as before, but not such a violent one. This is of particular relevance with very high air stream velocities because if the flow reversal is too violent the resulting eddy currents can propagate downstream and cause oil entrainment from the burner. it will be appreciated that the igniter 1 could also incorporate a less acute angied gutter.

Thus, in all the described embodiments the igniter provides first a pilot flame by ignition of the radially issuing gas jets which then ignites the axially issuing gas jets, in conjunction with the provision of the gutter which creates a low pressure zone at the exit of the air flow so that combustion air will reversely flow and be entrained with the gas jets to form an ignitable mixture.

Claims (11)

Claims

1. An electric igniter of the raw gas type wherein a pilot flame generated from one or more pilot gas jets serves to ignite axially issuing main gas jets with combustion air being caused to flow reversely thereto to form a combustible mixture with said pilot and main gas jets.

2. An electric igniter of the raw gas type comprising an ignition zone, means for passing one or more pilot gas jets laterally from axially disposed first gas passage means into said ignition zone, axially disposed second gas passage means extending through said ignition zone for passing axially issuing main gas jets downstream of said ignition zone, a gutter for causing a reverse flow of combustion air to form a combustible mixture with said pilot and main gas jets, and ignition means in said ignition zone for igniting said or each pilot gas jet to create a pilot flame which serves to ignite said main gas jets.

3. An electric igniter as claimed in claim 2, including an annular gas supply duct for supplying gas to said first and second gas passage means which themselves comprise tubes open at one end to said supply duct, the opposite ends of the tubes comprising the second gas passage means also being open to issue said main gas jets, and the opposite end of the or each tube comprising the first passage means being closed and having a radial hole formed in its peripheral wall to issue said pilot gas jet.

4. An electric igniter as claimed in claim 3, wherein there are two tubes comprising said first gas passage means and four tubes comprising said second gas passage means, said tubes being equiangularly spaced in concentric formation about the axis of the igniter with said two tubes being diameterically opposed with their radial holes disposed to issue the pilot gas jets towards said ignition means.

5. An electric igniter as claimed in claim 3 or claim 4, including a rigid assembly of inner and outer concentric tubes between which is defined said annular gas supply duct, the inner ends of said tubes comprising said first and second gas passage means being fast with a mounting plate secured between said inner and outer tubes, and the outer ends of said tubes comprising said first and second gas passage means being fast with an end plate.

6. An electric igniter as claimed in claim 5, including a deflector plate disposed in the path of the main gas jets issuing from said tubes comprising said second gas passage means to slow down the gas stream formed by said jets, said deflector plate being fast with the closed ends of said tubes comprising said first gas passage means which tubes are extended through said end plate.

7. An electric igniter as claimed in claim 5 or claim 6, wherein the rigid assembly of inner and outer concentric tubes is slidably mounted within a carrier tube having said gutter at its forward end.

8. An electric igniter as claimed in claim 7, including a clamping means carried by said carrier tube for securing the rigid assembly of inner and outer concentric tubes within said carrier tube by clamping against said outer tube.

9. An electric igniter as claimed in any of claims 5 to 8, including an electrode rod supported by insulation within said inner tube and an electrode screw mounted in said end plate to provide an adjustable spark gap between itself and said electrode rod, said electrode rod and said electrode screw comprising said ignition means.

10. An electric igniter as claimed in claim 9, wherein the rear ends of said inner and outer concentric tubes are fast with an end cap, a coupling being carried by said end cap for connection to a gas line by which to pass gas into said annular gas supply duct, and an electrode also being carried by said end cap and making an electrical connection with said electrode rod.

11. An electric igniter as claimed in any of claims 2 to 10, wherein the gutter is flared at an angle to suit the required degree of reversal of the air flow.

1 2. A gas-electric igniter substantially as herein described with reference to Figures 1 to 5, Figures 6 and 7, or Figure 8 of the accompanying drawings.