GB2362847A

GB2362847A - Fuel burner nozzle

Info

Publication number: GB2362847A
Application number: GB0013318A
Authority: GB
Inventors: Roger Shaun Lanary; George Leonard Gubbins
Original assignee: Hamworthy Combustion Engineering Ltd
Current assignee: Hamworthy Combustion Engineering Ltd
Priority date: 2000-06-02
Filing date: 2000-06-02
Publication date: 2001-12-05
Also published as: GB0013318D0; WO2001094845A1

Abstract

The atomiser assembly 22 comprises at least one fuel duct 24 connected to a supply of fuel 4, at least one atomising medium duct 2 connected to a supply of atomising medium 1, an atomising and mixing chamber 14, and a main outlet 20. The fuel duct 24 and atomising medium duct 2 are connected to supply fuel and atomising medium to the atomising and mixing chamber 14. The atomised and mixed fuel and atomising medium are ejected from the main outlet 20. The atomiser assembly 22 further comprises a second atomising medium duct 16 which has an outlet 17 in the region of the main outlet 20. The second duct 16 is arranged to provide a flow of atomising medium in the region of the main outlet 20.

Description

2362847 1 ATOMISER ASSEMBLY FOR A BURNER The present invention relates to

combustion equipment and, in particular burners and specifically atomising type burners and atomiser assemblies for the combustion of a liquid type fuel.

In order for liquid fuels, for example fuel oil, to be combusted in stationary sources such as boilers, furnaces or heaters, they must be mixed with air or some other suitable oxidant and ignited in a device generally referred to as a burner. The efficient combustion of a liquid fuel requires that it is broken down from a bulk liquid into tiny droplets and this process is known as atomisation.

There are several different known methods and burner types for achieving atomisation. One particular method is known as twin fluid atomisation. In this method of atomisation an atomising fluid (or other medium) is used and directed at, so as to impinge on, a stream or f low of the liquid fuel in order to break up the fuel liquid flow into small droplets. Typically the atomising fluid or medium is steam, compressed air, fuel gas, or another gas. Although generally used with liquid fuels this method may also be used with other fuels which have liquid properties, for example emulsions.

Some liquid fuels and in particular heavy residual fuel oils, have a high viscosity. In order to achieve good atomisation and hence good combustion this viscosity of the liquid fuel must be lowered by, for example, preheating the liquid fuel to temperatures substantially above ambient prior to being fed into the atomiser and burner and/or being combusted.

To provide the atomisation of the fuel with the atomising medium the burner of the combustion equipment is provided with an atomiser, and for twin fluid atomisation as described above a twin fluid atomiser is used. Such an 2 atomiser within a burner is generally part of a larger assembly, known as an oil gun or an oil sprayer, which provides a means to feed the atomising fluid and liquid fuel separately to the atomiser.

A significant problem associated with oil burners and atomisation devices is the formation and growth of hardened carbon deposits, referred to as coke and coking, on and around the faces and passage ways of the atomiser. Over a period of time this coking can increase in size to a level where it affects the atomised spray issuing from the atomiser and has a detrimental effect on the resultant flame. This effect can be in the form of increased particulate emission levels (i.e. non combusted fuel) due to poor atomisation, mixing and fuel spray distribution. This is leads to poor performance and efficiency of the combustion equipment. It can also result in increased emissions and pollution with the emission levels possibly exceeding the statutory requirements. In the extreme, coking of the atomiser can result in extinction of the flame which not only results in severe inconvenience but can give rise to a hazardous situation with unburnt fuel being sprayed into the combustion chamber.

To accommodate this problem it is industry practice to regularly shut down individual burners to remove and clean the burner, oil gun and atomiser assemblies. This results in loss of use of the combustion equipment, possible disruption, inconvenience and also results in a significant and expensive ongoing maintenance cost.

A further general problem with combustion equipment and with the combustion of liquid fuels is the formation and emission of oxides of nitrogen, commonly referred to as NOx. Such oxides of nitrogen (NOx)are a pollutant and emissions to the atmosphere are considered to be harmful. There is strict legislation restricting the maximum permitted levels and with any combustion equipment NOx emissions should be 3 minimised as far as possible.

It is therefore desirable to provide an improved burner and atomiser assembly and method of operation which minimises coke formation and growth and/or reduces NOx emissions so addressing the above described problems and/or which offers improvements generally.

According to an aspect of the present invention there is provided a burner and atomiser assembly as described in the accompanying claims 1 to 16.

According to a second aspect of the present invention there is provided a method of operating a burner and atomiser assembly as described in the accompanying claims 17 and 18.

In aspects of embodiments of the invention there is provided an atomising type burner and atomiser assembly for the combustion of a liquid type fuel. The burner and atomiser assembly comprises at least one fuel duct connected in use to a supply of fuel, at least one atomising medium duct connected in use to a supply of atomising medium, an atomising and mixing means, and a main outlet. The fuel duct and atomising medium duct are connected to in use supply fuel and atomising medium to the atomising and mixing means which is arranged in use to atomise and mix the fuel with the atomising medium. In use the atomised and mixed fuel and atomising medium are ejected from the main outlet and from the atomising means. The atomiser type burner and atomiser assembly further comprises a second atomising medium duct which has an outlet in the region of the main outlet. In use the second duct is arranged to provide a flow of atomising medium in the region of the main outlet.

By providing a flow of atomising medium in the region 4 of the main outlet using the second duct, the formation of coke on the atomiser burner and/or atomiser assembly, and in particular in the region of the main outlet is reduced. Consequently the problems associated with such coke formation are alleviated and at least in part addressed. In addition a flow of atomising medium from the second duct in the region of the main outlet will in use be at the root of the combustion flame from the atomiser assembly and atomiser burner. Such a flow has been found to have an effect on Nox 10 emissions.

The atomiser assembly and atomising type burner assembly is typically generally annular and has a central axis. The main outlet comprises a plurality of outlet ducts which are preferably disposed around the central axis at one end of the atomiser assembly and atomising type burner. The outlet of the second atomising medium duct is preferably located on the central axis at one end of the atomiser assembly.

The atomising and mixing means preferably comprise an internal chamber within the atomiser assembly.

Preferably the atomising and mixing means includes at least one fuel injection duct and at least one atomising injection duct which are arranged to in use provide impingement of the fuel and atomising medium injected into the atomising and mixing means.

In further aspects of embodiments of the invention there is provided a method of operating an atomiser assembly and atomising type burner comprising the steps of:- a) providing a supply of fuel to the atomiser assembly b) providing a supply of atomising medium to the atomiser assembly, c) atomising and mixing the fuel supplied to the atomiser assembly with the atomising medium supplied to the atomiser assembly within an is atomiser section of the atomiser assembly to produce an atomised and mixed flow of fuel and atomising medium, d) ejecting the atomised and mixed flow of fuel and atomising medium from a main outlet of the atomiser assembly and burner, characterised in that the method further comprises e) providing a separate flow of atomising medium in the region of the main outlet of the atomiser assembly and burner.

The present invention will now be described by way of example only with reference to the following figure in which:

Figure 1 is a cross section through an atomiser assembly of a burner according to the present invention.

Combustion equipment for burning in particular liquid fuels typically comprises a number of individual burners and atomiser assemblies forming a complete burner assembly. The atomiser assemblies of the burner(s) are arranged to atomise the liquid fuel and produce a spray of the atomised liquid fuel into the combustion equipment where it is combusted with air or some other suitable oxidant. Each of the burners and atomiser assemblies typically include an atomiser section within which the liquid fuel is atomised with a suitable atomising medium which is supplied to the burner and atomiser assembly. The atomising medium is used within the atomising section of the atomiser assembly and burner to break up a stream of the liquid fuel, supplied through orifices into the atomising section of the atomiser assembly and burner, into tiny droplets. The tiny droplets accordingly form a spray which is more easily and more completely combustible and which is discharged with the atomising medium in a mixed flow from the atomiser assembly.

6 Such combustion equipment and atomiser assemblies are generally known as such and will not therefore be described further.

The liquid fuels typically used are residual fuel oils. However other fuels, including for example emulsions, which exhibit liquid type properties can be used with the burners and atomiser assemblies. Typically steam is used as the atomisation medium. The use of steam has the advantage that due to its temperature it will heat the liquid fuel as it is used to atomise the liquid fuel. This will reduce the viscosity of the liquid fuel which aids atomisation of the fuel. Alternatively compressed air, fuel gas or another gas can and are used as the atomising medium.

A single atomiser assembly 22 of a twin fluid atomiser is type with an internal mixing atomising chamber 14 providing the atomising means, and incorporating an embodiment of the present invention is shown in figure 1. In this embodiment steam is used as the atomising medium and the liquid fuel is oil.

The atomiser assembly 22 is generally cylindrical and has a central axis A. The atomiser assembly 22 comprises an inner steam tube 2 which is connected to a steam supply 1 via for example a manifold assembly and pipework (not shown) which feeds steam to the, and/or each, atomiser assembly 22 and atomiser assembly as a whole. Concentrically with and outside of the inner steam tube 2 there is an outer oil tube 5 such that an annular volume and passageway or duct 24 is defined between the outer oil tube 5 and the inner steam tube 2. A separate oil supply 4 provided via a suitable manifold and pipework (not shown) is connected to and supplies oil to the annular volume 24 defined between the outer oil tube 5 and the inner steam tube 2. The end of the outer oil tube 5 is engaged within a cylindrical, tubular, housing 10 which is coaxially arranged and welded to the end of the oil tube 5. The housing 10 generally continues the 7 oil tube 5.

A generally cylindrical plug member 7 is fitted to and closes off the end of the inner steam tube 2 and the housing 10. The radially outer portion 26 of an end of the plug 7 abuts and seats against the end 28 of the housing 10 to close off the annular volume 24 between the inner steam tube 2 and outer oil tube 5 / housing 10. The plug 7 also has an inner cylindrical bore 30 which is coaxially arranged with the inner steam tube 2 and is fitted over the end of the steam tube 2. The end of the plug bore 30 is closed off by a blanking face 32 such that a steam chamber 3 is defined between the inner surface of the plug bore 32 and the end of the steam tube 3. The plug 7, and in particular the plug bore 32 diameter is arranged so that the plug 7 is a close sliding fit over the end of the steam tube 2. This allows for thermal differential expansion of the plug 7 and inner steam tube 2 whilst preventing steam leakage. Similarly the outer portion of the plug end face 26 and the end 28 of the housing 10 have a conical angled cooperating profile to aid seating of the plug 7 and sealing.

A cylindrical atomiser assembly tip 9 at the end of and coaxial with the atomiser assembly 22 abuts against a mating end face 34 of the plug 7. The outside of the housing 10 is threaded 40 and a cap nut 11 fits over the tip 9 and screws onto the housing 10. The cap nut 11 is arranged to engage with shoulders 42 on the tip 9 such that when the cap nut 11 is tightened the tip 9 is pulled up against the plug 7 and the plug 7 is pulled against the housing 10 and inner steam tube 2.

An annular groove 44 in the mating end face 34 of the plug 7 defines with a mating face 46 of the tip 9 an annular oil chamber 6. A number of axially extending small holes 8 forming passages are drilled within the plug 7 to interconnect the oil chamber 6 with the annular volume 24 defined between the inner steam tube 2 and housing 10 8 outer oil tube 5.

An atomising and mixing chamber 14 is def ined by a blind cylindrical bore 48 within the atomiser assembly tip 9 and the mating end face 34 of the plug 7. This mixing and 5 atomising chamber 14 is internal to the atomiser assembly 22. A number of radially directed oil transfer slots or holes 12 forming passages are disposed at one end of the atomising and mixing chamber 14 to interconnect the oil chamber 6 with the atomising and mixing chamber 14. A number of axially directed steam transfer holes 13 forming passages similarly extend through the end face 32 of the plug 7 and interconnect the steam chamber 3 with the atomising and mixing chamber 14.

At the end of the tip 9 there are a plurality of spray outlet holes 15 forming outlet passages. Theses outlet holes 15 are disposed about and around the end of the tip 9 and atomiser assembly axis A. The outlet holes 15 are connected to the atomising and mixing chamber 14 and define an outlet 20 from the atomising and mixing chamber 14 and from the atomiser assembly 22 overall. The outlet holes 15 are generally aligned at a divergent angle to the atomiser assembly axis A.

Within the tip 9 there is a central steam transfer tube 16 which is coaxial with the tip 9 and atomiser assembly 22. one end (the inlet end) of the steam transfer tube 9 is fitted and slid into a close fitting hole 50 within the plug 7 so that this end of the steam transfer tube 16 protrudes into and makes a connection with the steam chamber 3. The other end (outlet end) of the steam transfer tube 16 is located at the end of the tip 9 providing a steam transfer tube outlet 17 at the centre of the end of the tip 9. The steam transfer tube 16 thereby provides a separate passageway and duct for a separate flow of steam from the steam chamber 3 to an outlet 17 at the end of, and within an end face of, the tip 9. The outlet 17 of the steam transfer 9 tube 16 can particularly be of a divergent profile forming a divergent atomiser assembly outlet 17 which will disperse the outlet flow from the steam transfer tube 16 to produce a divergent outlet steam flow 18. The steam transfer tube 16 may be integral with and part of the tip 9 or may comprise a separate tube which is suitably attached to the tip 9. The internal bore of the steam transfer tube may be of a constant diameter along its length. Alternatively it may, as shown, incorporate a reduced diameter bore section 52 which is sized to meter the steam flow rate through the steam transfer tube 16.

In operation fuel oil (or any other suitable liquid fuel) is supplied to the atomiser assembly 22 and to the annular volume 24 between the outer oil tube 5 and inner is steam tube 2. This supply 4 of oil flows through the annular volume 24 to the oil chamber 6 within the atomiser assembly 22. Similarly steam (or any other suitable atomising medium) is supplied to the atomiser assembly 22 and to the inner steam tube 2. This supply 1 of steam flows through the inner steam tube 2 to the steam chamber 3 within the atomiser assembly 22.

A flow or stream of oil passes from the oil chamber 6 via the oil transfer slots 12 and is injected in a generally radially inward direction into, and at one end of, the atomising and mixing chamber 14. At this end of the atomising and mixing chamber 14, and adjacent to the oil transfer slots 12, steam is injected in a generally axial direction into the atomising and mixing chamber 14 via the steam transfer holes 13. Accordingly the steam transfer holes 13 and oil transfer slots 12 are arranged so that this injected flow of steam from the individual steam transfer holes 13 impinges upon the radially injected flow of oil. In particular the steam transfer holes 13 are aligned with, and directed towards, the oil transfer slots 12 and the flow of oil therefrom. This impingement of the steam onto the oil flow causes the continuous stream of oil from the oil transfer slots 12 to be broken down into small droplets thereby atomising the oil. Further atomisation, mixing, and heating of the oil by the steam occurs within the atomising and mixing chamber 14. The injected flow of steam also directs the injected oil in a generally axial direction through the atomising and mixing chamber 14 with a turbulent flow being generated by the injected steam and oil which further atomises and mixes the oil and steam. Accordingly the oil is atomised by the steam within the atomiser assembly 22 and in particular within the atomising and mixing chamber 14 into which the steam and oil are injected. The atomised oil and steam is then ejected from the atomising and mixing chamber 14 through the outlet holes 15 in the end of the tip 9 in the form of a very fine spray 19. This spray 19 is directed slightly radially outwardly in the form of a cone diverging from the end of the tip 9 by virtue of the angling of the outlet holes 15. This provides further dispersal of the atomised oil and steam ejected from the outlet holes 15 and atomiser assembly nozzle 22 as a whole. The spray 19 is subsequently ignited and combusts within the combustion equipment in the form of a consolidated flame or several flamelets.

A separate flow of steam also flows from the steam chamber 3 through the steam transfer tube 16. This flow of steam is ejected from the end of the steam transfer tube 16 at the outlet 17 at the centre of the end of the tip 9. The ejected steam is dispersed by the outlet 17 profile of the steam transfer tube 16 and will flow over the outer end of the tip 9 providing a steam flow over the outer surface of the tip 9 of the atomiser assembly 22. Such a separate flow 18 of steam, distinct from the atomised and mixed flow 19 of oil and steam, has a beneficial effect in reducing coke formation on the end of the tip 9 and in particular in the region of the outlet holes 15 of the atomiser assembly 22.

11 Consequently the problems associated with coke formation on the atomiser assembly 22 are at least in part reduced and alleviated.

In addition since the flow 18 of steam ejected from the outlet 17 of the steam transfer tube 16 is located at the centre of the atomiser assembly 22, and in particular at the centre of the outlet holes 15, the ejected flow 18 of steam is at the root of the oil flame associated with the atomiser assembly 22. Such a f low 18 of steam at the root of the flame can restrict the formation of NOx.

The flow rate of steam through the steam transfer tube 16 is metered by the size of the bore of the tube and/or of any reduced bore section 52. Accordingly the flowrate is adjusted and selected by choosing an appropriate bore diameter so as to provide the optimum reduction in coke formation and/or to optimise the restriction of NOx formation.

Although in this embodiment a central steam tube 16 provides a central separate flow 18 of steam from the atomiser assembly 22, and this is the preferred arrangement, it will be understood that in other embodiments the separate flow of steam over the atomiser assembly tip 9 in the region of the outlet 15 of the atomised mixed flow does not have to be provided f rom a central outlet 17. Indeed a series of steam outlets providing separate flows of steam over the tip 9 could be provided from steam transfer passageways and outlets located around the circumference of the tip 9 of the atomiser assembly 22. Such an arrangement could be used in addition to or in place of the central steam transfer tube 16.

It will be appreciated that a significant aspect and feature of the invention is that a separate flow of steam (or other atomising medium) is provided over the outlet end of the atomiser assembly in order to alleviate coke formation. This separate flow is separate and distinct from 12 the outlet flow of atomised and mixed flow of oil and atomising medium produced within and f lowing out of the atomiser assembly.

13

Claims

CLAIMS is An atomiser assembly for combustion of a liquid type fuel, the

atomiser assembly comprising -at least one fuel duct connected in use to a supply of fuel, -at least one atomising medium duct connected in use to a supply of atomising medium, -an atomising and mixing means to which the fuel duct and atomising medium duct are connected to in use supply fuel and atomising medium to the atomising and mixing means which is arranged in use to atomise and mix the fuel with the atomising medium, and -a main outlet from which in use the atomised and mixed fuel and atomising medium from the atomising and mixing means are ejected; characterised in that the atomiser assembly further comprises a second atomising medium duct which has an outlet in the region of the main outlet and which in use is arranged to provide a flow of atomising medium in the region of the main outlet.

An atomiser assembly as claimed in claim 1 in which the second atomising duct in use is arranged to provide a flow of atomising medium which bypasses the atomising and mixing means, said flow arranged to be separate and distinct to the atomised and mixed fuel and atomising medium flow ejected from the main outlet.

3.

An atomiser assembly as claimed in claim 1 or 2 in which the atomising medium is supplied to a internal chamber within the atomiser assembly, the second atomising medium duct is connected at an inlet end to the internal chamber and at least one further 4.

is 6.

14 interconnects the internal chamber with the atomising means and in use supply a flow of atomising medium to the atomising and mixing means.

An atomiser assembly as claimed in any preceding claim in which the atomiser assembly is generally annular and has a central axis.

An atomiser assembly as claimed in any preceding claim in which the atomising and mixing means comprises an internal chamber within the atomiser assembly.

An atomiser assembly as claimed in any preceding claim in which the atomising and mixing means includes at least one fuel injection duct and at least one atomising injection duct which are arranged to in use provide impingement of the fuel and atomising medium injected into the atomising and mixing means.

7. An atomiser assembly as claimed in any one of claims 4 to 6 in which the main outlet comprises a plurality of outlet ducts disposed around the central axis at one end of the atomiser assembly.

8. An atomiser assembly as claimed in any one of claims 4 to 7 in which the outlet of the second atomising medium duct is located on the central axis at one end of the atomiser assembly.

9. An atomiser assembly as claimed in any preceding claim in which the second atomising medium duct incorporates a reduced bore section which is sized to in use meter the flow of atomising medium through the second atomising medium duct.

is 10. An atomiser assembly as claimed in any preceding claim in which the atomising medium is a gas.

An atomiser assembly as claimed in claim 10 in which the atomising medium is a fuel gas.

12. An atomiser burner as claimed in claim 10 in which the atomising medium is compressed air.

13. An atomiser assembly as claimed in any preceding claim in which the atomising medium is steam.

14. An atomiser assembly as claimed in any preceding claim in which the liquid fuel is oil.

A burner incorporating an atomiser assembly as claimed in any one of claims 1 to 14.

16. Combustion equipment incorporating an atomiser assembly as claimed in any one of claims 1 to 14.

17. A method of operating an atomiser assembly and/or burner comprising the steps of:a) providing a supply of fuel to the atomiser assembly, b) providing a supply of atomising medium to the atomiser assembly, c) atomising and mixing the fuel supplied to the atomiser assembly with the atomising medium supplied to the atomiser assembly to produce an atomised and mixed flow of fuel and atomising medium, d) ejecting the atomised and mixed flow of fuel and atomising medium from an main outlet of the atomiser assembly, 16 characterised in that the method further comprises e) providing a separate flow of atomising medium in the region of the main outlet of the atomiser assembly.

18. A method of operating an atomising assembly and/or burner as claimed in claim 15 in which the atomiser assembly comprises an atomiser assembly as claimed in any one of claims 1 to 14.

19. An atomiser assembly as hereinbefore described with reference to the accompanying figure 1.

20.

A method of operating hereinbefore described accompanying figure 1.

an atomiser assembly as with reference to the is