GB2098314A - Controlled combustion of gases - Google Patents

Abstract

A gas burner apparatus and method comprises a combustion chamber 16 and first and second stage burners for totally premixing fuel gas and air supplied to the chamber 16. The first stage burner includes a mixer section 31 and burner orifices 18 in the rear end wall 141 of the combustion chamber 16. The second stage burner comprises a mixer means 32 and burner orifices 18 in rear end wall 142 of the combustion chamber which has a greater forward projection than the wall 141 and defines a cylindrical first stage combustion space 161 forwardly of the end wall 141. Fuel gas is supplied separately through ducts 101 and 102 to the first and second stage mixer sections wherein the gas is mixed in venturi mixers 81 and 82 with air from a common supply 2. The air/gas premixes remain separate throughout each burner stage. To obtain a reduced heat output, both stages of the burner are progressively shut down until the second stage burner is completely shut off, further reduction being obtained by regulating the first burner stage alone. <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 suitability 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 and flame temperature, the availability of oxygen and nitrogen and the time during which the combustion gases are held at a high temperature.

In our GB Patent Application No. 8019218 we describe a method of burning a fuel gas, and a gas burner apparatus, in which 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 limits of the flammability of the gas.

The gas burner apparatus disclosed in the aforesaid application 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, the mixer means being configured to produce a total premix 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 when a mixture marginally weaker than the accepted limits of flammability of the fuel gas concerned, and a flame trap between the mixer means and the burner orifices.

To obtain the intimacy of mixing required, the preferred gas burner apparatus preferably employs one or a plurality of air/gas venturi mixers. 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 gas in the expanding sections of the venturi nozzles ensures that good mixing is obtained.

The air/gas ratio is maintained at a level which is as high as possible, for example in excess of 15:1 for natural gas. At the high levels of excess air employed, the flame speed is low 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 burner disclosed in the above application is a single stage burner and with any single stage burner there is a problem on turndown. A burner is not always required to operate at maximum heat.

In the majority of environments there are periods of operation when a burner is required to deliver less than its maximum heat output. The turndown (i.e. the ratio of maximum/minimum heat liberation) of a burner is a factor which will affect the choice of a burner for many applications. A turndown of 5/1 is achievable with the aforesaid burner, but this can be restrictive in certain applications.

It is accordingly an object of the present invention to improve a single stage burner to achieve a greatly increased turndown ratio while continuing to provide combustion conditions which minimise the amounts of oxides of nitrogen in the products of combustion.

The present invention proposes accordingly an apparatus for burning a fuel gas with air which comprises a combustion chamber having an outlet for products of combustion and first and second burner orifices, a first mixer means for producing a total premix as hereinafter defined of the fuel gas and air and for supplying said premix to the first burner orifices, second mixer means for producing a total premix as hereinbefore defined of the fuel gas and air and for supplying said total premix to the second burner orifices, first means for supplying fuel gas to said first mixer means, second means for supplying fuel gas to said second mixer means, and common means for supplying air to both said first and second mixer means, said second burner orifices surrounding said first burner orifices and defining a combustion space for the mixture issuing from the first burner orifices.

A "total premix" is defined as an intimate mixture of the fuel gas and air in which the air/gas 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 crosssection of flow of the mixture, and at all longitudinal positions. By ensuring that such a constant 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%.

Preferably the first and second burner orifices are disposed respectively in a first and a second rear wall of the combustion chamber, the second wall being annular and surrounding the first wall and having a greater projection into the combustion chamber Turndown is achieved initially by progressively reducing the gas supply to the second mixer means and thus to the second burner orifices. The air supply to both mixer means and to both sets of burner orifices is a common supply. Thus, even when the second stage is not alight, air will pass through the second stage at about the same rate as when it is alight. This allows cooling of the apparatus when not firing and also prevents any pull back of hot combustion product into the second stage.

The first mixer means and first burner orifices together comprise what is herein referred to as a first stage burner, and similarly the second mixer means and the second burner orifices comprise what is herein referred to as a second stage burner.

The fuel gas is supplied separately to the two burner stages, and the air/gas premixes remain separate throughout each burner stage. The first stage burner, which is disposed centrally in the apparatus and is surrounded by the second stage burner, remains on all the time the apparatus is functioning, only the second stage burner ever being completely shut off.

If a greater turndown is required than can be achieved by shutting off the second burner stage, then the fuel gas and combustion air supply to the first stage burner can be progressively reduced.

The distribution of heat liberation between the first and second stages is chosen so that there is an overlap, namely the low heat output of both stages together is lower than the high heat output of the first stage alone.

At all times during operation of the apparatus according to the invention the air/gas ratio is maintained at a level which is as high as possible.

At the high levels of excess air employed, the flame. speed is low and this can result in problems of flame stability. It is therefore preferred to incorporate flame hold or flame stabilising devices in at least some of the burner orifices By appropriate use of the two stages of the gas burner apparatus of the invention, a turndown of 10/1 can readily be achieved. This is twice the turndown attainabie with a single stage burner and is achieved without duplication of controls.

Thus, a single burner/fan/air damper control system can be used with the apparatus of the present invention.

A comparable turndown could be obtained by using two individual burners but these can have disadvantages where space restrictions prevent siting in the best position for optimum dilution with extraneous air. Also the use of two individual and physically separate burners would require duplication of many of the controls leading to a substantially more complex and expensive system.

The invention will now be further described, by way of example, with reference to the accompanying drawing the single Figure of which is a longitudinal section through a gas burner apparatus according to one embodiment of the invention.

Referring to the drawing, combustion air is supplied through an air inlet duct 2 to a mixer module generally indicated by 3 and comprising a first stage mixer section 31 and a second stage mixer section 32. The first stage mixer section 31 comprises a cylindrical vessel 41 having transverse partition walls 51 and 61 between which a plurality of venturis 81 extend. The venturis can be arranged in a ring about the axis of the mixture module 3. The second stage mixer section 32 comprises an outer cylindrical vessel 42 coaxial with and surrounding the first stage vessel 41 and having transverse annular partition walls 52 and 62 between which a plurality of venturis 82 extend. As in the case of the venturis 81, the venturis 82 can be arranged in a ring about the axis of the mixer module 3.

Fuel gas is supplied separately through ducts 101 and 1 02 to the respective interiors of the mixer sections 31 and 32, and enters the venturis 81 and 82 through holes 91 and 92 provided in the throats of the venturis. Air is supplied to the mixer sections 31 and 32 through the duct 2 from a common fan and damper system (not shown).

Within the venturis a total premix as hereinbefore referred to of the gas and air is obtained without stratification. It is important 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 81 of the first stage mixer section 31 passes to the rear side of an orifice plate 141 which forms the first stage rear end wall of a refractory combustion chamber 1 6. Similarly, the intimately mixed gas and air issuing from the venturis 82 of the second stage mixer section 32 passes to the rear side of an annular orifice plate 142 which forms the second staae rear end wall of the combustion chamber 1 6. Each refractory orifice plate 141 and 142 contains a large number of orifices 1 8 through which the respective mixtures of gas and air pass.The emerging jets are ignited and burn to not less than 99.5% completion within the combustion chamber 1 6 before leaving the same through an outlet 28 at the further end thereof and being diluted and quenched by surrounding air. The outlet 28 is preferably forwardly convergent, suitably conical, and reflects heat back into the burner jets to improve combustion stability. Fusible links 121 and 1 22 are provided in the passages between the venturis 81 and the orifice plate 141 and the venturis 82 and the orifice plate 142 respectively.

The fusible links 121 and 122 comprise a low melting point wire. In the event of flash back of the flame from the burner jets in one or both of the passages then the sudden increase in temperature will immediately melt the link or links concerned.

Meeting of the links trips the electrical control circuit for the burner (not shown) and the gas supply to the burner is thus cut off before any possibility of overheating and damage occurs.

The forward faces of the end walls 141 and 142 i.e. the faces adjacent the combustion chamber 1 6 are staggered whereby the wall 142 has a greater forward projection than the wall 141 and defines a cylindrical first stage combustion space 161 forwardly of the end wall 141.

The supply of fuel gas through the ducts 101 and 102 for combustion with the air supplied through duct 2 is regulated so that within each burner stage 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 the fuel gas concerned.

Incorporated within at least some of the orifices 18 in each plate 141 and 142 are flame hold or flame stabilising devices comprising ceramic tubes 22. The flame hold devices 22 operate as disclosed in the aforesaid Application No.

8019218 to provide flame stability and encourage rapid combustion. The other types of flame hold device described in the aforesaid application may also be used in the present apparatus.

The air/gas premixes formed in the respective mixer sections 41 and 42 remain separate through the flame traps 121 and 122 and are burnt separately at the respective burner orifices.

Thereafter the combustion products come together in the c-ombustion chamber 1 6 and leave through the outlet 28.

Maximum heat liberation is obtained when both burner stages are operating at full heat output. To obtain a reduced heat output, both stages of the burner are progressively shut down by reducing the gas and combustion air supplies through the ducts. If a greater reduction in heat output is required than can be achieved in this manner, then the gas supply through the duct 102 is completely shut off and the gas and combustion air supplies through the ducts 101 and 2 to the first stage burner are regulated to obtain the desired further progressive reduction. The distribution of heat liberation between the first and second stages is chosen so that there is an overlap, namely the low output of both stages together is less than the full output of the first stage alone.An automatic control system (not shown) can be associated with the burner and controls valves in the ducts 101 and 102 to regulate the heat output of the burner apparatus in accordance with the heat requirements.

The first stage burner thus remains on all the time whilst the burner is firing, only the second stage burner being completely shut down when required. Because there is a common air supply to both stages through the duct 2, even when the supply of fuel gas through the duct 102 to the second stage burner is completely shut off air will continue to pass through the second stage venturis 82 and the second stage burner orifices 18 in the plate 142. This allows cooling of the outer section of the burner apparatus when not firing and also prevents any pull back of hot combustion products of the first stage burner from the combustion chamber 1 6 into the second stage.

At the same time, due to the staggering of the orifice plates 141 and 142 the first stage burner jets are ignited within the combustion space 161 where the flame benefits from radiation and conduction from the refractory wall surrounding the space. The space 161 also affords time to establish the first stage flame before it can be affected by either cold air passing through the second stage or the combustion products of the second stage.

The gas burner apparatus disclosed is capable of providing the same controlled combustion conditions throughout a wide range of reduced heat outputs and a turndown of 10/1 can be achieved.

Claims (9)

1. Apparatus for burning a fuel gas with air, which comprises a combustion chamber having an outlet for products of combustion and first and second burner orifices, a first mixer means for producing a total premix as hereinbefore defined of the fuel gas and air and for supplying said premix to the first burner orifices, second mixer means for producing a total premix as hereinbefore defined of the fuel gas and air and for supplying said total premix to the second burner orifices, first means for supplying fuel gas to said first mixer means, second means for supplying fuel gas to said second mixer means, and common means for supplying air to both said first and second mixer means, said second burner orifices surrounding said first burner orifices and defining a combustion space for the mixture issuing from the first burner orifices.

2. Apparatus according to Claim 1, wherein the first and second burner orifices are disposed respectively in a first and a second rear wall of the combustion chamber, the second rear wall being annular and surrounding the first rear wall and having a greater projection into the combustion chamber whereby to define said combustion space.

3. Apparatus according to Claim 1 or Claim 2, wherein the first mixer means and first burner orifices together comprise a first stage burner, the second mixer means and the second burner orifices together comprise a second stage burner, the fuel gas is supplied separately to the two burner stages and the air/gas premixes remain separate throughout each burner stage.

4. Apparatus according to Claim 3, including fusible links in said burners intermediate the respective mixer means and burner orifices thereof for interrupting the supply of fuel gas to the respective burner or burners in the event of flashback of the flame from a burner orifice.

5. Apparatus according to Claim 4, wherein the fusible links comprise a low melting point wire connected in an electrical control circuit for the respective burner.

6. Apparatus according to Claim 3, wherein the first and second stage burners are dimensioned so that the low heat output of both stages together is lower than the high heat output of the first stage alone.

7. Apparatus according to any one of the preceding Claims, wherein the first mixer means and the second mixer means each comprises at least one venturi, the air being fed longitudinally of the at least one venturi and the venturi throat or throats comprising one or more apertures for admitting the fuel gas.

8. A method of operating the apparatus claimed in any one of Claims 3 to 6, to obtain a reduced heat output, which comprises progressively shutting down both burner stages by reducing the supplies of fuel gas and combustion air thereto, whereafter a further reduction in heat output is achieved by completely shutting off the supply of fuel gas to the second stage burner and regulating the supply of fuel gas and combustion air to the first stage burner alone.

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