GB2054822A - Controlled combustion of gases - Google Patents

Abstract

A gas burner apparatus and method comprises a combustion chamber 16 and means 3 for totally premixing fuel gas and air supplied to the chamber 16 through a refractory orifice plate 14. <IMAGE>

Description

SPECIFICATION Controlled combustion of gases This invention relates to the burning of gases under conditions in which the combustion of a fuel gas with air is controlled to minimise the amount of oxides of nitrogen (NOx) contained in the products of combustion.

When a mixture of fuel gas and air is burnt in a combustion chamber, the combustion gases leaving the chamber contain NOx in an amount depending on the efficiency of the combustion process. The NOx leaving the chamber consists substantially of nitric oxide (NO). Prior art gas burner apparatus typically produces combustion gases containing between 50 and 1000 parts per million of NOx.

NO by itself is relatively harmless, but in the atmosphere it oxidises to NO2, and NO and NO2 together form Nitrous Acid with water which is capable of producing very objectionable contaminants.

Requirements have arisen for burners, particularly for air heating applications, in which the presence of oxides of nitrogen must be maintained at very much lower levels than previously thought acceptable.

In such air heating applications substantial quantities of dilution air are mixed with the hot combustion gases to produce the heated "air" used for drying, so if nitrogen oxides in the combustion gases are kept to the order of one part per million then, depending on the dilution, a proportion of the order of 0.04 part per million (above ambient) can be achieved and is regarded as acceptable.

The factors which influence the formation of oxides of nitrogen are flame temperature, the availability of oxygen and nitrogen and the time during which the combustion gases are held at a high temperature.

According to the present invention, the combustion reaction is extremely closely controlled in order to mitigate against the formation of oxides of nitrogen, by burning to completion a total premix of fuel gas and air close to the lower limit of flammability of the gas. The invention resides both in a method and a gas burner apparatus for operating in this manner, and in one aspect provides a method of burning a fuel gas with air in a combustion chamber wherein the fuel gas is burnt to not less than 99.5% completion in the chamber, which comprises introducing the fuel gas and air into the chamber as a total premix as hereinbefore defined.

A "total premix" is defined as an intimate mixture of the fuel gas and air in which the fuel gas/air ratio is near the limit of flammability of the fuel gas with air such that the products of combustion at the mouth of the chamber, before any dilution with extraneous air, contain not less than 8.5% of free oxygen, and is constant at all points within the volume occupied by the mixture and at all throughputs over the range of operation of the burner. Such ratio is constant both over the cross-section of flow of the mixture, and at all longitudinal positions. By ensuring that such a constant fuel gas/air ratio is maintained at all times the flame temperature is controlled at the lowest practicable level and does not rise and fall due to variations in the mixture and develop flame conditions which are opposed to those required to minimise the formation of oxides of nitrogen.

While it might appear that similar conditions are obtained in known types of burners like post mix or nozzle mix patterns, in fact no such burner is designed to work near the limits of flammability.

Moreover, due to imperfections in mixture proportions at the point of combustion, variable burning conditions apply and the levels of oxides of nitrogen produced are unacceptably high for the purpose.

By means of the invention fuel gas and air can be burnt in a way such that the total concentration of nitrogen oxides in the combustion gases at the burner exit, before addition of dilution air, is reproducibly as low as 1 part per million or better.

In particular, the invention is capable of attaining a concentration of not more than 1 part per million of oxides of nitrogen in the gases at the burner exit.

This represents a very great improvement over known types of burner claimed to produce one part per million of nitrogen peroxide (NO2) at the burner mouth, because at the burner mouth NO2 represents only a very small proportion of the total nitrogen oxides. Such a burner would also be producing 50 or more parts of nitric oxide which, later, would be converted to NO2 by reaction with oxygen.

The preferred amount of free oxygen in the products of combustion before dilution will be between 9% and 12%.

Another aspect of the present invention also provides apparatus for burning a fuel gas with air which comprises a combustion chamber having a refractory rear wall and an outlet at the end opposite the rear wall, mixer means upstream of the combustion chamber, means for feeding fuel gas and air to said mixer means in a ratio close to the lower limit of flammability of the gas, said mixer means being configured to produce a total pre-mix as hereinbefore defined of the fuel gas and air, a plurality of burner orifices in the rear wall of the chamber designed and disposed so as to maintain a stable flame even with a mixture marginally weaker than the accepted limits of flammability of the fuel gas concerned, and a flame trap between said mixer means and the burner orifices.

To obtain the intimacy of mixing required, in a preferred gas burner apparatus according to the invention a plurality of air/gas venturi mixers are employed. At the throat of each venturi, a plurality of holes is provided to allow gas which surrounds the venturi mixers to penetrate as a number of jets into the air streams flowing through the venturis, and diffusion of gas into the air in the expanding sections of the venturi nozzles ensures that good mixing is obtained. The use of a plurality of venturis ensures that the gas penetration into the air mass need only be small, whereby stratification of mixing is eliminated by diffusion during travel through the diffuser-sections of the venturis.

Nevertheiess, the invention does not exclude the use of a single venturi mixer suitably designed in relation to the capacity of the burner to achieve the desired "total premix". Preferably the venturis are mounted longitudinally in a cylindrical vessel between partition walls and fuel gas is fed into the interior of the vessel in a manner ensuring equal distribution to all the gas holes in the venturis.

The gas/air ratio is maintained at a level which is as high as possible, for example in excess of 1:15 for natural gas. At the high levels of excess air employed, the flame speed is iow and this can result in problems of flame stability. With this in mind various design features may be employed to prevent the flame being extinguished.

As time and temperature are also important factors influencing the formation of nitric oxide, it is preferred for the actual combustion to operate with a plurality of burner orifices to secure more rapid completion of burning, these being designed to produce early recirculation of partly burnt products and mutual support by sharing early heat, whereby completion of combustion in an otherwise unhelpful situation is secured, before allowing the products to be quenched by ambient air which, in the case of the preferred air heating application, may be taken around the burner itself in sufficient quantity to provide up to a 40 :1 dilution. The provision of a forwardly convergent outlet to the combustion chamber, which reflects heat back onto the burning mixture has been found to improve the stability of combustion.

Flame-hold devices can also be incorporated in some at least of the burner orifices.

The invention will now be further described, by way of example, with reference to the accompanying drawing in which: Figure 1 is a longitudinal section through a gas burner apparatus according to one embodiment of the invention; Figure 2 is a detail cross-sectional view of an alternative flame hold feature, and Figures 3 and 4 are views similar to Figure 2 showing further alternative flame hold features.

Referring to Figure 1, combustion air is supplied through an air inlet duct 2 to a mixer module or section generally indicated by 3 and comprising a cylindrical vessel 4 with partition walls 5 and 6 between which a plurality of venturis 8 extend. In the embodiment being described, six venturis 8 are provided, and are arranged in a ring about the axis of the mixer section 2. Fuel gas is supplied through a duct 10 to the interior of the mixer section 2, and enters the venturis 8 through holes 9 provided in the throat of each venturi. By virtue of this arrangement a total premix, as hereinbefore referred to, of the gas and air is obtained without stratification. It is of great importance that the venturis should be accurately made without internal ridges and have a highly smooth inner surface.

The thus intimately mixed gas and air issuing from the venturis passes through a flame arrester or trap 12 to the rear side of an orifice plate 14 which forms the end wall of a refractory combustion chamber 1 6. The orifice plate 14 contains a large number of orifices 18 through which the mixture of air and gas passes. The emerging jets are ignited and burn to not less than 99.5% completion within the combustion chamber 1 6 before leaving the combustion chamber through an outlet 28 at the end of chamber 1 6 opposite the end wall 14 and being diluted and quenched by surrounding air. The outlet 28 is preferably forwardly convergent, suitably conical, and reflects heat back onto the burner jets to improve combustion stability.The flame arrester or trap 12 is provided to stop any possibility of flash back into the venturis 8.

The supply of the fuel gas and air for combustion through the ducts 2 and 10 is regulated so that the ratio of gas/air is close to the lower limit of flammability of the gas concerned and the products of combustion at the outlet 28 of the combustion chamber, before any dilution with extraneous air, contain not less than 8.5% of free oxygen.

The burner orifices 1 8 are designed and disposed so as to maintain a stable flame even when the mixture is marginally weaker than the accepted limits of flammability of the fuel gas concerned.

Incorporated within at least some of the orifices 18 are flame hold or flame stabilising devices 20.

The metal devices 20 shown in Figure 1 allow the major portion of the mixture to pass through the centre but a small amount is fed out onto the wall of the tubular orifice 1 8 where by the loss of velocity it stabilises and in itself provides conditions which permit the main flame alsq to stabilise. The refractory material defining the combustion chamber 1 6 and specifically the orifice plate/end wall 14 rapidly becomes heated and as such radiates back on the small flames encouraging stability and rapid combustion.

Alternative flame hold devices are shown in Figures 2 to 4 and consist of ceramic tubes inset into the orifice plate 14 to provide an even greater number of flame jets. Sufficient area should be left between neighbouring jets to ensure that there is space for the recirculation of partly burnt products which both encourage flame stability and also due to temperature at the flame root enhance rapid ignition and burn out.

The device shown in Figure 2 comprises ceramic tubes 22 which are flush with both end faces of the orifice plate 14. In Figure 3, the ceramic tubes 24 are recessed relative to the front end face of plate 14, whereas in Figure 4 the ceramic tubes 26 project from the front end face of the plate 14. In addition, although not shown, some at least of the tubes can be provided with a reduced diameter portion at the upstream end to give an abrupt reduction in velocity and consequent added stability.

A mixture of the different types of flame hold devices described may, if desired, be incorporated in a single apparatus.

The basic burner apparatus described above can be assembled in various configurations of modules and sizes to suit applications for various types of air heater. In addition, the apparatus may be arranged for controlled operation such that parts of the burner can be extinguished leaving the remainder operating and still capable of providing the same controlled combustion conditions at a reduced heat output.

Claims (1)

1. A method of burning a fuel gas with air in a combustion chamber wherein the fuel gas is burnt to not less than 99.5% completion in the chamber, which comprises introducing the fuel gas and air into the chamber as a total premix as hereinbefore defined.

2. A method according to Claim 1 , wherein the total premix is obtained by mixing fuel gas with air in at least one venturi mixer.

3. A method according to Claim 2, wherein the air is fed longitudinally through the at least one venturi mixer and the fuel gas is admitted through one or more apertures in the venturi throat or throats.

4. A method according to any one of the preceding claims, wherein the amount of nitric oxide (NO) in the burner gases issuing from the combustion chamber is of the order of one part per million.

5. A method according to any one of the preceding claims, wherein the premixed gases enter the combustion chamber through a refractory orifice plate and flame stabilising means are incorporated in at least some of the orifices thereof.

6. Apparatus for burning a fuel gas with air which comprises a combustion chamber having a refractory rear wall and an outlet at the end opposite the rear wall, mixer means upstream of the combustion chamber, means forfeeding fuel gas and air to said mixer means in a ratio close to the lower limit of flammabiiity of the gas, said mixer means being configured to produce a total premix as hereinbefore defined of the fuel gas and air, a plurality of burner orifices in the rear wall of the chamber designed and disposed so as to maintain a stable flame even with a mixture marginally weaker than the accepted limits of flammability of the fuel gas concerned, and a flame trap between said mixer means and the burner orifices.

7. Apparatus according to Claim 6, wherein said mixer means comprise at least one venturi mixer and the means for feeding the fuel gas and air comprise means for feeding the gas and air separately to said venturi mixer to form a total premix therein.

8. Apparatus according to Claim 7, wherein the at least one venturi mixer comprises at least one venturi, the air is fed longitudinally of the at least one venturi and the venturi throat or throats comprise one or more apertures for admitting the fuel gas.

9. Apparatus according to Claim 8, wherein the mixer means comprises a cylindrical mixing vessel, a plurality of venturis extending longitudinally of the vessel between transverse partition walls and one or more apertures in each venturi throat, said means for feeding fuel gas being connected to supply such gas to the space within the vessel between the partition walls.

10. Apparatus according to any one of Claims 6 to 9, wherein flame stabilising means are incorporated in at'least some of the burner orifices.

1 Apparatus according to Claim 10, wherein at least some of the flame stabilising means comprise metal devices arranged to allow the major portion of the mixture to pass through the centre of the orifice and to feed out a minor amount thereof onto the wall of the orifice whereat it stabilises by the loss of velocity.

1 2. Apparatus according to Claim 10, wherein at least some of the flame stabilisation means comprise ceramic tubes in the burner orifices.

1 3. Apparatus according to Claim 12, wherein at least some of the tubes are flush with both faces of the end wall.

14. Apparatus according to Claim 12, wherein at least some of the tubes are recessed relative to the downstream face of the rear wall.

1 5. Apparatus according to Claim 12, wherein at least some of the tubes project from the downstream face of the rear wall.

1 6. Apparatus according to any one of Claims 13 to 1 6, wherein at least some of the ceramic tubes have a reduced diameter portion at the upstream end.

1 7. Apparatus according to any one of Claims 11 to 17, wherein the outlet of the combustion chamber is a forwardly convergent refractory member for reflecting heat back onto the burning mixture within the combustion chamber to improve the stability of combustion.

18. A method of burning a fuel gas with air, according to any one of Claims 1 to 6 and substantially as hereinbefore described.

19. Apparatus for burning a fuel gas with air, substantially as hereinbefore described with reference to the accompanying drawings.