Gas burner

A gas burner particularly intended to produce diffusion flames of good emissivity and luminosity in an industrial furnace, for example a reverberation furnace such as may be used in glass making. The burner has a plate forming a burner nozzle having a central through first bore surrounded by a circular array of six smaller diameter second bores. One of the second bores is parallel with the axis of the central first bore. Whereas the other five second bores are each respectively disposed at an acute angle of substantially 22.5.degree. to the axis of the central first bore. Fuel gas is supplied to the central first bore through a central tube and to the second bores through an annular passage between the central tube and a surrounding tube.

BACKGROUND OF THE INVENTION 
This invention concerns a gas burner and also a reverberatory furnace 
provided with at least one gas burner. 
Herein a reverberatory furnace means a furnace in which heating flames are 
directed into a combustion chamber above a tank containing a load to be 
melted or maintained molten and wherein hot products of combustion 
circulate above the tank and heat radiates down onto the surface of the 
tank load from said flames and from surfaces of the chamber. 
High temperature, for example 1000.degree. C. and above, melting of metals 
and non-metal materials can be achieved in a reverberatory furnace by the 
above surface firing of the process material or load with gas flames. Heat 
transfer to the process material or load occurs through convection from 
the hot combustion product flows and from radiant heat transfer either 
directly from the flame and/or from adjacent furnace surfaces. The thermal 
efficiency and rate of melting of the process normally depends on the 
flame radiant heating properties since this heat transfer mechanism is 
more effective than convection at higher process temperatures (i.e. 
greater than 1200.degree. C.). 
High process temperatures also require high flame temperatures and this is 
often achieved by using pre-heated combustion air or mixing the fuel gas 
with oxygen. Both techniques result in increased levels of combustion 
generated pollutants, in particular NOx. Increasing the radiant emissivity 
of gas flames would result in greater heat transfer to the process, lower 
flame temperatures, and hence lower NOx emission levels. 
An object of the invention is to provide a gas burner intended to produce 
flames having greater flame luminosity and emissivity, a higher heat 
transfer rate and lower NOx emission levels than gas burners (not having 
the increased luminosity and emissivity) currently available for high 
temperature processes, for example processes performed at substantially 
1000.degree. C. or above. 
SUMMARY OF THE INVENTION 
According to the invention a gas burner comprises a nozzle formed with a 
plurality of first and second through apertures for the passage of fuel 
gas therethrough, there being a first aperture surrounded by a plurality 
of second apertures disposed in an array about the first aperture, means 
whereby fuel gas can be supplied to the nozzle to emerge from the first 
and second apertures, the cross-sectional area of the first aperture being 
greater than the cross-sectional area of any said second aperture, and at 
least one said second aperture facing along a direction which is at an 
acute angle to a direction along which the first aperture faces. 
Preferably fuel gas, for example natural gas, is injected through the 
burner nozzle into a cold or pre-heated combustion air flow to produce 
diffusion flames. This may occur in a reverbatory furnace in which the 
resultant diffusion flame has a higher flame emissivity and luminosity 
than is currently possible with conventional gas burner designs used in 
such furnaces. The increased emissivity produces higher radiant heat flux 
to the tank load and adjacent surfaces of the combustion chamber and 
enhances the load melting capacity. Also the increased emissivity can 
result in a lower flame temperature being possible and thus less NOx 
production. 
It is believed that the increased or higher flame emissivity is caused by 
the partial thermal cracking of the fuel gas flow. The thermal cracking is 
due to high heat transfer rates between the individual jets of gas 
emerging from the first and second apertures of the nozzle. It is thought 
that the jet of gas from the first aperture is pre-heated by flamelets 
from the surrounding second apertures to induce thermal cracking. Thermal 
cracking is the heat induced conversion of the fuel gas chemical species 
into free carbon and/or other hydrocarbons of higher carbon to hydrogen 
ratio than are otherwise contained in the fuel. The presence of these 
produces an increased flame emissivity and luminosity.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 1 shows a reverberatory furnace 2 comprising a combustion chamber 4 
over a tank 6 extending substantially for the length of chamber 4 and 
containing a load 8 to be melted or kept molten. A plurality of spaced 
openings 10 or 11 is provided in each opposite longitudinal wall of the 
combustion chamber along the length thereof (only one opening 10 or 11 
being shown in each longitudinal wall). Each opening 10 or 11 contains one 
or more gas burners 12a or 12b each of which may protrude into the 
combustion chamber 4 and is supplied with fuel gas, for example natural 
gas, under pressure from a suitable supply including supply piping 14 
coupled at 16 with the burner. 
A plurality of spaced openings 18 or 19 is also provided in each opposite 
longitudinal wall of the combustion chamber along the length thereof (only 
one opening 18 or 19 being shown in each longitudinal wall). Each opening 
18 on one side is associated with a respective burner opening 10 and is in 
communication with a passage arrangement 20 through which combustion air 
under pressure can be supplied to the combustion chamber 4 when the 
burners 12a are supplying fuel gas. Each opening 11 in the other side is 
associated with a respective burner opening 11 and is in communication 
with a passage arrangement 21 which acts as a flue through which 
combustion products can leave the chamber 4 when the burners 12a are 
supplying the fuel gas and the burners 12b are not because their fuel 
supply is turned off. On the other hand when the burners 12b are supplying 
fuel gas and the supply to the burners 12a is turned off combustion air is 
supplied under pressure to the chamber 4 through the openings 19 from the 
passage arrangement 21 whereas the openings 18 and passage arrangement 20 
act as a flue. A control is provided to operate gas supply valve means, 
combustion air and flue valve means, and air blower mean to ensure that 
(i) when gas is supplied to the burner 12a the gas supply to the burner 12b 
is off and combustion air is supplied to the openings 18 from passage 
arrangement 20 whereas passage arrangement 21 is connected to a flue 
system; and 
(ii) when gas is supplied to the burners 12b the gas supply to the burners 
12a is off and combustion air is supplied to the openings 19 from passage 
arrangement 21 whereas passage arrangement 20 is connected to the flue 
system. The control may operate to automatically switch the burners 12a or 
12b on or off as it switches the passage arrangement 20 or 21 to air 
supply mode or to flueing mode in a timed cycle of operation, and/or 
according to temperature measurement. 
Although the combustion air supplied may be cold, it is preferably 
pre-heated by including in each passage arrangement 20 or 21 a respective 
regenerator 22 or 23 comprising an air permeable arrangement of heat 
storage material which gives up to combustion air passing therethrough 
heat which the material absorbed from a previous flow of flue gases 
therethrough from the combustion chamber 4. 
The reverberbatory furnace 2 may be used to perform heating processes 
requiring high temperatures, for example substantially 1000.degree. C. and 
above. Such processes include metal melting and glass making, the load 8 
in FIG. 1 indicating the molten metallic or non-metallic load. The furnace 
2 in FIG. 1 is particularly suited to producing molten glass. 
Each burner 12a or 12b from which heating flames issue over the surface of 
the load 8 is of substantially similar construction to the burner 12 in 
FIG. 2. Burner 12 comprises a central tube 24 of circular section 
concentrically surrounded by an outer tube 25 of circular section. Both 
tubes 24,25 are welded in a gas tight manner to a circular end plate or 
nozzle 26 having a central cylindrical through bore 28 opening into 
interior passage 30 of the tube 24. The bore 28 is surrounded by an array 
of cylindrical through bores 32 and 34 in the plate 26. At the exposed or 
outer face of the plate 26 (see FIG. 3), the openings into the bores 32,34 
are substantially equally spaced circumferentially from one another and 
are substantially equally spaced from the central bore 28. The axis of the 
bore 28 coincides with axis B of the burner and the axis of the bore 34 is 
substantially parallel with the axis B. Each bore 32 has an axis C which 
is inclined at an acute angle D, for example substantially 22.5.degree., 
to the axis B. This means that when inner face of the plate 26 is 
considered as in FIG. 4 the opening into the bore 34 no longer lies on the 
same circle as the openings into the bores 32. All the bores 32,34 open 
into the annular passage 36 between the inner and outer tube 24,25. All 
the bores 32,34 have substantially the same diameter and each has a 
cross-sectional area less than that of the bore 26. 
The burner 12 is made of heat withstanding material, for example a heat 
resistant metal and may be surrounded by a cooling water jacket 38 having 
an inlet 40 and an outlet 42. 
A fuel gas supply pipe 14 (FIG. 1) can be attached to the burner 12 using a 
coupling 16 (FIG. 1) screwed to the outer tube 25 using screw thread 44 
thereon. A desired relative distribution of gas flows between the nozzle 
aperture or bore 28 and the nozzle apertures or bores 32,34 can be 
achieved using a restriction formed by a plug 46 screw fitted into the 
tube 24 and having a central bore 48. The plug 46 has a head 50. The head 
50 can be of a size which has little or no effect on the supply of fuel 
gas to the annular passage 36. 
Considering the direction gas or flame flow from the burner, the direction 
of gas or flame flows from the apertures or bores 32 diverge from the 
direction of flow from the aperture or bore 28, whereas that from the bore 
34 is substantially parallel to that from the bore 28. Thus when the 
burner is mounted in the furnace 2 as at 12a or 12b, the bore 34 is 
located nearer to the surface of the load 8 (in other words lowermost) 
than the bores 32. Preferably each burner is mounted so that the axis B of 
the burner and aperture 28 is at an acute angle E to the direction F of 
the flow of combustion air through the associated opening 18 or 19. Angle 
E is preferably in the range 5.degree. to 50.degree.. Angle E may be in a 
vertical plane but need not be so. The resultant diffusion flames from the 
burners may be at an angle to the surface of the load 8 or horizontal 
thereto. The burners may point transversely to the direction F of the 
combustion air flow, in which case the openings 10,11 may be in the walls 
of the openings 18,19. 
It is believed that the burner 12 used in the furnace 2 gives an increased 
radiant emissivity and luminosity of natural gas flames resulting greater 
flame heat flux and heat transfer to the load 8. 
As an example, the plate 26 may be substantially 13 mm thick and 
substantially 42 mm in diameter. The diameter of central aperture 28 may 
be substantially 5.0 mm and the diameter of each aperture 32,34 may be 
substantially 3.0 mm. The internal diameter of tube 25 may be 
substantially 34 mm and that of the tube 24 may be substantially 9 mm. The 
external diameter of the tube 24 may be substantially 14 mm. The length of 
each tube 24, 25 may be substantially 89 mm. The diameter of plug bore 48 
may be substantially 3.0 mm. The length of the plug bore 48 may be 
substantially 34 mm and the length of the male threaded part of the plug 
46 may be substantially 25 mm.