GB2134242A - Fuel burners - Google Patents

Abstract

A gas burner has one or more gas nozzles 3 emitting gas into a primary air swirler consisting of curved blades 9 which may have an axial entry region which merges through an arcuate transition region into an oblique outlet region. The swirl blades 9 are mounted on the end of a central air pipe 6 which accommodates a liquid fuel supply pipe 4 and atomiser 5 secondary swirl blades are positioned between a shroud 8 and outer casing 2. <IMAGE>

Description

SPECIFICATION Improvements in and/or relating to fuel burners The present invention relates to fuel burning apparatus, more generally known as a burner.

According to the present invention there is provided a novel combination of air stabilising swirler, also known as a primary air swirler, and design of fuel gas nozzles to enable improvements to be made to the combustion performance and safety of the burner on gaseous fuels and combined firing of liquid and gaseous fuels. The invention is suitable for burners for use on all types of boilders, process heaters and other combustion systems requiring the burning of gaseous and liquid fuels.

The invention provides burner apparatus for improving combustion performance and safety, comprising a curved blade primary air high efficiency swirler with at least one gas nozzle for introducing gaseous fuels behind the front face (combustion side) of the said swirler.

The invention is applicable to burners having a main liquid fuel atomiser along the central axis of an air register through which the combustion air flows. Within this air register a stabilising swirler is normally fitted around the liquid fuel atomiser to swirl a proportion of the combustion air so that an air flow reversal pattern is set up downstream of the swirler into which atomised liquid fuel and gaseous fuels are injected for flame stability.

By this means products of combustion at a temperature sufficient for ignition of new fuel are forced to recirculate back to or near to the front of the burner to enable continuous combustion to take place.

Fuel gas nozzles on burners designed for firing both liquid and gaseous fuels normally consist of a number of discrete tubes passed through the burner from a common gas manifold at the outside of the burner frontplate.

These tubes end in gas nozzles which direct the fuel gas into the reversal zones set up by the primary air swirler to ensure flame stability. It has been found that to obtain improvements in the combustion performance of the burner the design of the primary air swirler and gas nozzles are related and that the relative positioning of the gas nozzles to the primary air swirler is important.

The following performance criteria (not listed in order of merit) is quoted to explain the merits of the invention and to explain the terminology used.

(a) That when operating on gaseous fuels only, the burner has an adequate operating range when the combustion air flow rate is correctly matched to the fuel flow rate. The ratio of maximum to minimum fuel flow rate is more normally termed the turn-down ratio such that the higher this ratio the greater is the operating range.

(b) That when operating over the turndown range at correctly matched fuel gas and air flow rates the combustion efficiency is as high as possible, with maximum burn-out of fuel and air flow rates approaching the theoretical value of stoichiometric.

(c) At all points in the operating range the flame to be stable on the burner.

(d) The burner to be able to operate with a stable flame over as wide a range of excess air rates as possible without the flame "blowing out". For example, with air at maximum the minimum gas flow before the flame blows out should be as small as possible to allow safe operation of the burner. This out-of-ratio condition should ideally be as high as 6 or 7 to one (out-of-ratio turndown) based on an air flow rate sufficient to burn the maximum flow rate of gas with zero excess air, the gas flow should be able to be reduced to one sixth or one seventh of its maximum flow rate without the flame blowing out.

(e) The gas nozzles and gas supply tubes to these nozzles should be able to be removed from the burner whilst it is firing an alternative fuel.

(f) The gas nozzle should be of simple design, with adequate strength to enable long life, ease of cleaning and to maintain their original calibration to prevent over-firing etc.

(g) The above performance noted in (a) to (f) to be maintained over a wide range of different gaseous fuels, combustion air temperatures, combustion air velocities through the air register, the central liquid fuel atomiser in position or withdrawn, or with the liquid fuel atomiser in position and say with atomising steam on.

(h) Performance of the burner on gaseous fuels to be relatively unaffected by the gas velocity from the nozzles or the number of gas nozzles used.

(i) On liquid fuels as per notes (a)-(d) above.

(j) The burner should operate without producing any carbonaceous deposits on the liquid fuel atomiser, primary air swirler etc under all conditions.

(k) The burner should be able to operate with various combinations of liquid and gaseous fuels firing together, with stable flame fronts, and comparable combustion performance as on liquid or gaseous fuels separately.

An embodiment of the invention will be described with reference to the accompanying drawings in which: Figure 1 is a sectional elevation through the end of a burner, Figure 2 is a section on line A-A in Figure 1, and Figure 3 shows details of the design of the primary air swirler and gas nozzles and posi- tioning of these components.

Fig. 1 shows the downstream end of a gas/liquid fuel burner mounted in a combustion chamber wall 1. The burner comprises a tubular metal casing 2 in which are mounted four converging gas spuds 3 mounted at equal angular intervals around a central axial liquid fuel supply pipe 4 and atomiser 5 mounted in a central air supply tube 6. Air is also supplied along the annular space between the inner and outer tubes 6, 2. The gas spuds 3 and an igniter assembly 7 are also mounted in this annular space. At its outlet end is an internal shroud 8. Secondary air swirl blades 10 are mounted in the annular space between the shroud 8 and the outer casing or tube 2.

The invention is also applicable to a burner without liquid fuel firing means.

Whereas conventional air swirl blades are flat, set obliquely relative to the incoming air flow, in the present burner the primary air swirl blades 9 are curved as seen in the radial direction, as is shown clearly in Fig. 3. Each blade comprises a longitudinal entry region 11 which merges through an arcuate translation region 1 2 into an oblique outlet region 1 3 set, for example, at 55 to the longitudinal direction of the burner. Selected blades are cut away to accommodate gas nozzles 1 4 which terminate upstream of the combustion face of the primary air swirler. A shroud ring 1 5 encircles the outer edges of the swirl blades in their downstream region. The inner edges of the swirl blades are mounted on an extension 1 6 of the tube 6.

It has been found that the radial positioning of the gas nozzles affects the out-of-ratio turn down (see note d above) and the flame stability pattern when firing gaseous and liquid fuels together. When the gas nozzles are positioned further away from the central axis of the burner less fuel is entrained within the recirculation zone induced by the primary air swirler, which reduces the out-of-ratio turndown. Positioning the gas nozzles too close to the axis of the burner normally results in a fuel rich zone close the burner axis particularly when firing gaseous fuels in combination with liquid fuels and the flame front may lift off from the zone adjacent to the front of the burner.The present invention provides a primary air swirler designed to allow maximum air to pass through it and hence to eliminate these fuel rich zones and with gas nozzles positioned close to the axis of the burner for high out-of-ratio turn downs satisfactory control of flame front conditions when gaseous and liquid fuels are fired together can be achieved.

In consideration of notes (a) to (k) above the gas nozzles do not pass completely through the primary air swirler. With this design, the gas nozzles are protected from the high temperatures in the combustion zone and with the primary air swirler of this invention, they interfere less with this primary air flow. It has been further found that when the gas nozzles pass completely through the primary air swirler overheating of this swirler can occur on high flame speed gases, such as hydrogen, even though other burner performance criteria may be satisfied.Also, when the gas spuds are placed around the outside of the primary air swirler not all of the burner performance criteria can be met. Other important advantages found has been that, by introducing the gaseous fuel back from the front (combustion side) face of the swirler, the primary air swirler aids the distribution o the gseous fuel around the burner such that variations of gas pressure/velocity have minimal effect on burner performance, also the design of the gas nozzle discharge port or ports can be quite simple to aid cleaning, improve life and maintain correct geometry over long periods. A single forward facing hole in the gas nozzle about parallel to the burner axis has been found to be satisfactory.Because the primary air swirler assists the distribution of the gaseous fuel the gas nozzle and gas supply pipe may be at an angle to the axis of the burner and more than one gas discharge hole may be used.

Referring to Fig. No. 3 the present invention relates to the design of the primary air swirler and gas nozzles. The primary air swirler consists of a number of blades which are curved to gradually increase the swirl of the air. In addition the blades preferably extend in a direction parallel to the axis of the burner upstream into the approaching air stream to control and maximise the air flow into the primary air swirler. This blade extension is optional. The angle through which the air is swirled from the burner axis can be varied, as can the number of swirler blades. This part of the present invention is designed to minimise the variations in air flow rate around the circumference between blades that occurs for example with flat blade swirlers as a result of air flow separations. Almost uniform air flow rates are possible with the present invention circumferentially around the primary air swirler and at all radii from near the central sleeve or hub out to the outer rim. The use of an extended outer rim upstream into the air register is optional. By all these means the air volume through the primary air swirler is maximisd to ensure good stabilising air flow swirl and adequate air for complete combustion of all fuel(s) at the flame root for the reasons stated earlier.

The maximised air flow through the primary air swirler allows high flame speed gases for example Hydrogen to be burnt with the gas nozzles in the position quoted above behbind the front face of the swirler without excessive overheating of the swirler. Positioning of the gas nozzles close to the axis of the burner for high out-of-ratio turn downs allows the the combustion of liquid and gaseous fuels to be fired together without forming fuel rich zones near to the burner front for good flame stability.

Claims (9)

1. Burner apparatus comprising a highefficiency primary air swirler with curved blades, and at least one gas nozzle arranged to introduce gaseous fuel upstream of the downstream face of the swirler.

2. Burner apparatus comprising a primary air passage, at least one fuel gas nozzle in the primary air passage, and a primary air swirler in the primary air passage, and in which the primary air swirler comprises a plurality of generally radial swirler blades with respective downstream edges defining a downstream face of the swirler adjacent to the combination zone of the burner apparatus, the gas nozzle is or nozzles are upstream of the said face of the swirler, and each swirler blade, seen radially, is curved thereby presenting to the primary air flow a progressively increasing angle of deflection.

3. Burner apparatus as claimed in claim 1 or 2 in which each swirler blade has a curved region and a substantially straight region extending generally axially upstream from the curved region.

4. Burner apparatus as claimed in claim 1, 2 or 3 in which each swirler blade has a curved region and a substantially straight oblique region extending downstream from the curved region.

5. Burner apparatus as claimed in any preceding claim in which a shroud ring encircles the downstream region of the primary air swirler blades.

6. Burner apparatus as claimed in any preceding claim in which the gas outlets of the gas nozzles are downstream of the upstream side of the primary air swirler.

7. Burner apparatus as claimed in any preceding claim in which there is a plurality of the said gas nozzles spaced angularly.

8. Burner apparatus as claimed in any preceding claim further including a central axial liquid fuel burner the primary air swirler being disposed about the liquid fuel burner.

9. Burner apparatus substantially as herein described with reference to the accompanying drawings.