GB2140910A - Heating of enclosures - Google Patents

Abstract

In the operation of an enclosure such as a glass tank, heat is supplied by firing one or more burners of a kind in which air is initially employed to atomise the fuel. Oxygen-rich gas, typically substantially pure oxygen, is subsequently substituted for the air. This substitution is carried out without decreasing the flame length. Typically, on making the substitution, the rates of supply of atomising agent, and secondary air are reduced. A burner head with a central passage 32 for oil, and outer passages 30 for atomising gas, projects atomized fuel from an orifice 48. The cross section of a passage 44, and the orifice size, may be reduced when oxygen-rich gas is used. <IMAGE>

Description

SPECIFICATION Heating of enclosures This invention relates to a burner and a method of heating an enclosure. It is particularly concerned with the operation of burners to heat, for example, tanks in which molten glass is formed by melting selected minerals (hereinafter referred to as 'glass tanks'), furnaces and kilns, collectively referred to herein as 'enclosures', and with the kind of burner that employs a gas to atomise a liquid fuel, typically a heavy fuel oil.

Various techniques are known for improving the efficiency of combustion of liquid fuel by supplying oxygen-rich gas (ie commercially pure or oxygen enriched air) to the flame. A commonly practised technique is that of directing a jet of oxygen rich gas at the flame from underneath the flame. This and other techniques of supplying oxygen rich gas to the flame are known to offer a number of advantages which are described in the literature (For example, see 'The use of oxygen in glass making furnaces' H R Miller and K Royds, Glass Technology, Vol.14, No.6 December 1973, pp 171-181). Briefly, these advantages include an increased flame temperature and an improvement in the heat transfer from the flame to its surroundings by both radiation and convection. These advantages derive at least in part from an increase in the rate at which free radicals are formed in the flame.This latter phenomenon results in there being an increased rate of reassociation of the radicals in lower temperature zones of the flame or in parts of the enclosure outside the flame with concommitant increase in the rate of release of their latent heat of dissociation causing an improvement in convective heat transfer. The use of oxygen-rich gas also assists in achieving complete combustion of the fuel and in making possible a reduction in the requirements of the flame for secondary air.

In the aforesaid paper by Miller and Royds there is discussion of using oxygen-rich gas towards the end of the lifetime of the plant to achieve an increase in the working lifetime of regenerators or recuperators associated with glass tanks for the purpose of pre-heating secondary air. More recently, interest has focused on the use of oxygen-rich gas to increase output of glass (or other material being fired or melted in the enclosure) per unit energy of fuel, generally with a view to achieving fuel savings rather than increasing production.

Another phenomenon associated with the introduction of oxygen-rich gas into a flame is a shortening of the flame. Such shortening is particularly pronounced when oxygen-rich gas is used as the atomising agent, as is illustrated in Figure 1 of the paper of Miller and Royds who recommend other techniques of adding oxygen-rich gas to a flame in preference to using it as an atomising agent.

More recently, there has been renewed interest in the use of oxygen-rich gas as an atomising agent when heavy fuel oil is burned. There are two overt advantages associated with the use of oxygen-rich gas as atomising agent. First, the need for lances and the like, as required in under-flame injection, or for oxy-fuel burners of relatively complex construction and hence high capital cost is avoided. Second, there is an immediate cost saving in that the need to compress the air that is conventionally used, as the atomising agent is removed.

Contrary to the teaching of Miller and Royds, experiments have successfully been performed using commercially pure oxygen as the atomising agent and significant fuel savings have been achieved. However, these experiments have been performed in an enclosure in which there was no need to avoid a shortening of flame length when the oxygen was substituted for compressed air as the atomising agent, and accordingly, the shorter flame length that was produced had no adverse affect.

In many situations a shortening of the flame length and other associated changes to the shape of the flame is intolerable. One of these situations is in a cross-fired glass tank (ie a glass tank in which the burners fire in a direction generally transverse to the flow of molten glass). In such a tank a shortening of the flame length would lead to non-uniform and incomplete heating of the glass, the material most remote from the burners in operation remaining at a lower temperature than the rest of the glass. Such uneven heating could lead to some mineral remaining unmelted or unrefined or to faults occuring in the glass during the forming process.

Contrary to the teaching in the art that the addition of oxygen rich gas to a flame, particularly if the oxygen-rich gas is used as an atomising agent shortens the flame length we have now found it possible to use oxygen-rich gas in this way without any shortening of flame length while obtaining a fully combusted flame and such that an increase in thermal efficiency is achieved. We obtain such results by appropriate selection of the rates and velocities at which atomising oxygen and the liquid fuel issue from the burner and of the rate at which secondary air is supplied to the burner flame.

According to one aspect of the present invention there is provided a method of operating a liquid fuel burner of a kind that conventionally employs air as a medium to atomise the liquid fuel and to help support combustion of the fuel and which fires into an enclosure, excess secondary air being supplied to the enclosure to create therein an atmosphere including oxygen, whereby a stable substantially fully combusted flame of not less than a given length is produced, wherein oxygen-rich gas (as hereinafter defined) is substituted for air as the atomising medium and the rates of supply of oxygen-rich gas and fuel, their velocities and combined atomising momentum and the rate of supply of secondary air are selected so as to give a stable fully combusted flame of substantially the given length, and a flame temperature higher than that achievable with air as the atomising medium.

The method according to the present invention are particularly suited for use with burners employing medium pressure atomisers (e.g. the atomising gas is supplied at 20 to 50 psig, and the fuel is consequently supplied at such pressures) and to burn heavy fuel oils. Conventionally air has been used as the atomising gas in such burners but in accordance with the invention oxygen-rich gas is substituted for air.

With such a burner and such a fuel the increase in flame temperature is typically produced by increasing the rate which oxygen molecules are supplied to the flame from the atomising gas. Typically, the said rate is increased by an amount in the range 200 to 300%. Consequently, in the event that substantially pure oxygen (i.e. oxygen having a purity greater than 99%) is for example selected as the oxygen-rich gas, this will entail an actual reduction in the rate at which the atomising agent is supplied upon the substitution of the oxygen-rich gas for the air. We prefer to supply substantially pure oxygen at a rate from 2 to 3% of that volume of air required for the complete theoretical combustion of the heavy fuel oil.

By the term 'oxygen-rich gas' as used herein is meant pure oxygen or a gas mixture containing at least 22% and typically at least 25% by volume of oxygen, typically oxygen-enriched air.

It is to be appreciated that rates of supply of oxygen molecules from the atomising gas to the flame substantially in excess of those discussed above are likely to yield excessive flame temperatures and a concommitanttendancyto create 'hot spots' in the enclosure.

With burners of heavy fuel oil having medium pressure atomisers we recommend typically aiming at fuel savings in the order of 10 to 15%, ie a reduction in the rate of supply of fuel oil of 10 to 15%.

With such a reduction, we recommend typically reducing the rate of supply of atomising gas by from 40 to 50% upon the substitution of substantially pure oxygen for air as the atomising gas and typically to make a reduction in secondary air flow rate sufficient to reduce the oxygen concentration in the atmosphere in the enclosure to not more than 3% by volume.

The substantial reductions in the ratio of the rate of supplying of atomising agent to the rate of supplying fuel (notwithstanding any 10 to 15% decrease in the rate of supplying fuel) entailed in carrying out our recommendations above will result in a substantial reduction in the atomising momentum (i.e. momentum available for atomisation). This will result in a less thorough atomisation of the fuel (ie a production of a larger average droplet size) than in the conventional practice of using air as the atomising agent.Although this tends somewhat to counteract the tendency for the substitution of oxygen-rich gas for the air to lead to a shorter flame length, reduction in flame length cannot be avoided by mere adjustment of flow rate of the atomising agent or fuel when substantially pure oxygen is used as the atomising agent, or in this instance by reducing the rate of flow of secondary air. When substantially pure oxygen is to be used as the atomising agent we have typically found it necessary to restrict the passage through which the atomising agent flows into the atomising chamber or zone of the burner and we have also typically found it necessary to restrict the passage or orifice through which the mixture of atomised fuel and atomising agent leaves the burner when the burner is converted as aforesaid.This restriction increases the velocity at which the said mixture of atomised fuel and oxygen-rich gas leaves the burner.

Typically burners with medium pressure atomisers each include a body portion, sometimes known as a spud, in which there are passages for the fuel and atomising agent and a cap which is adapted to engage the spud and which has an orifice through which in use of the burner the mixture of atomising agent and fuel leaves the burner. The cap and spud are so dimensioned as to define an atomising chamber or zone therebetween and a restricted passageforthe atomising medium leading to such chamber or zone. We have, on converting such burner for use with substantially pure oxygen, adjusted the fitting of the cap on the spud so as to restrict further said passage.

Typically, when using substantially pure oxygen as the atomising agent instead of air, we reduce the cross sectional area of the passage by about 2/3rds.

This has the effect of increasing the velocity and hence momentum at which the atomising agent enters the atomising chamber or zone and thereby helps to compensate for the reduction in momentum occasioned by substituting oxygen-rich gas at a lower mass flow rate for the atomising agent. In practice, we prefer this compensation not to be complete when converting the burner from operation with air as the atomising medium to operation with substantially pure oxygen as the atomising medium.In other words if m and v are respectively the mass flow rate and velocity of the atomising agent entering the atomising chamber or zone when air is employed as the atomising agent and m2 and v2 are respectively the mass flow rate and velocity of the atomising agent entering the atomising chamber or zone when oxygen rich gas is employed as the atomising agent, we prefer m to be greater than m2 and v2 to be greater than v, and the product of m and v to be greater than the product of m2 and v2.

Typically, when converting a burner of the aforesaid medium pressure atomising kind to operating with substantially pure oxygen as the atomising agent instead of air, we reduce the cross sectional area of the outlet orifice in the cap by about 25%.

One advantage we attribute to the increase in velocity of the mixture of oxygen-rich gas and atomised fuel as it leaves the burner is that it makes it possible to prevent any reduction in the flame 'lift' (i.e. the gap between the start of the flame and the burner tip) when using the burner after conversion from the use of air as the atomising agent to the use of oxygen-rich gas in the atomisation. A reduction in the lift associated with a higher flame temperature would tend to increase the role of erosion or other wear of any refractory block or the like in which the burner is mounted. Moreover, molten refractory is typically caused to run down into the molten glass and subsequently solidify, causing 'stones' or other flaws to be created. Such refractory blocks are commonly employed in mounting burners to fire into cross-fired regenerative glass tanks and the like.

Normally, in conventional operation of a crossfired regenerative glass tank each burner is adapted (with air as the atomising medium) to produce uniform heating of the contents of the tank from one side to the other of the tank. This requires careful 'tailoring' of the shape of the flame produced by the burner. In particular, if the flame is too short the molten glass is inadequately heated at the opposite side ofthetankto said burner, while if the flame is too long the wall of the tank opposite said burner is likely to be damaged.By using oxygen rich gas as the atomising medium in the method according to this invention, it is possible to avoid any deterimental change to the shape of the flame, particularly its length, and therefore we prefer to maintain the flame length substantially unaltered, i.e. the flame extends across at least 4/5ths of the width of the tank without impinging against the opposite wall.

It has been assumed in the aforegoing description that upon converting a burner from use with air as the atomising medium to use with oxygen-rich gas as the atomising medium, no change is made to the fuel or its viscosity. It is the viscosity of the fuel that determines the amount of energy that is required to atomise it. In commercial practice, heavy fuel oils are heated so as to reduce their viscosity prior to entering burners in which they are burned. Even so, it has been generally found necessary (at least when melting glass) to use medium pressure atomisers (i.e. employing pressures in the order of 20 to 80 psig) to atomise the heavy fuel oil. We now believe it is possible by using oxygen rich gas as the atomising agent to burn successfully a heavy fuel oil in a burner with a low pressure atomiser (ie one that employs a gaseous atomising agent at a pressure of 10 psig or less).

According to yet another aspect of the present invention there is provided a method of burning a heavy fuel oil with a burner having a low pressure gas atomiser, wherein oxygen-rich gas is used as the atomising medium. The oxygen-rich gas is preferably substantially pure and typically supplies in the order of 20 to 30% of the oxygen required for complete combustion of the fuel. By the term 'heavy fuel oil' as used herein we mean a fuel oil having a viscosity of at least 3500 SR, (Seconds Redwood no 1).

The invention also includes within its scope a burner adapted to perform any one of the above defined methods.

Methods and burners according to the present invention will now be described by way of example with reference to the accompanying drawings, in which: Figure 1 is a schematic perspective view partly cut away, of a cross-fired regenerative glass furnace; Figure 2 is a schematic sectional side elevation of a burner with a medium pressure atomiser; Figure 3 is a schematic side view partly in section of a burner port shown in Figure 1.

Referring to Figure 1 of the accompanying drawings, a furnace 2 for melting glass has a hopper 4 for feeding the minerals from which the glass is made into one end of a tank 6. The minerals are melted by means of a plurality of oil-fired burners 8 located below ports 10. (Only one of the burners 8 is shown in Figure 1). Molten glass flows out of the tank 6 through a throat 12.

As can be seen in Figure 1,fourports 10 are located on one side of the tank 6, and another four ports 16 are located on the opposite side. The furnace 2 is provided with a pair of regenerators 14, one regenerator being in communication with the four ports 10 along one side of the furnace, and the other regenerntor 14 being in communication with the other four ports 10 along the other side of the tank 6. The regenerators 14 have common inlet 15 for incoming ambient air and flues 17 for incoming air or exiting hot gases. The flues 17 communicate with a stack 18 through a reversal value (not shown).

One of the burners 8 is illustrated in Figure 2 of the accompanying drawings. It includes a body or spud 20 and a complementary cap 22. Ths spud 20 is in the form of a solid cylindrical body 24 having an integral head 26 projecting therefrom. A central passage 28 extends from end-to-end and through the body 24 and head 26 and is coaxial with the longitudinal axis of the spud 20. The passage 28 is surrounded by a circumferential arrangement of twelve passages 30 (of which only two are shown in Figure 2) formed through the body 24. A pipe 32 for fuel oil is received fluid tight in the passage 28 at the body and thereof. The pipe 32 is coaxial with a pipe 34 for atomising agent inside of which the pipe 32 extends. The cap 22 engages the body 24 of the spud 20 and the pipe 34 in a fluid-tight manner.The head 26 of the spud 20 has a face 36 with a central planar portion 38 and a peripheral portion 40 that slopes away from said portion 38 at a shallow angle. The inner surface 42 of the cap 20 complements the portion 40 of the face 26 of the spud 20 and defines therewith a restricted passage 44 leading to an atomising zone or chamber 46 intermediate the restricted outlet of the central passage 28 and an orifice 48 in the cap 22 coaxial with the passage 28.

A swirl plate 50 is engaged between the end of the oil pipe 32 and the inner wall of the passage 28 so as, in operation, to impart a swirling motion to the fuel oil entering the atomising zone or chamber 46.

In operation of the burner shown in Figure 2, fuel oil is supplied at a pressure of, say, 30 psig to the pipe 32 and atomising agent at a similar pressure to pipe 34. The fuel oil passes into the passage 28 and flows through the swirl plate 50 which imparts to the fuel a swirling motion in a manner well known in the art. It then leaves the passage 28 and passes into the atomising chamber 46 where it is impacted by the atomising agent that enters said chamber or zone 46 via the pipe 34, passages 30 and restricted passage 44. The supply pressure of the atomising agent and the radial cross-sectional width of the restricted passage 44 determine the flow rate and velocity at which the atomising agent enters the chamber or zone 46.Its momentum is chosen to be sufficient adequately to atomise the fuel and droplets of atomised fuel dispersed in the atomising agent leave the burner through the orifice 48, and, upon ignition a flame. It is to be appreciated that not all the oxygen molecules necessary for combustion are supplied by the atomising agent. Indeed, referring again to Figure 1, the majority of this air is supplied through the ports 10 from one of the regenerators 14.

Referring now to Figure 3 of the drawings, below each port 10, a plurality of burners 8 is housed. The burners 8 are supported in part by a refractory block 60 having convergent-divergent passages 62, into.

each one of which the tip of an associated burner extends, terminating in the convergent portion of the associated passage near to the throat 64 thereof. The position of each block 60 relative to the exit of its port 10 is such that in operation the 'lift' of the flame is sufficient for there to be no damage to the block 60.

Referring again to Figure 1 of the accompanying drawings, the operation of the regnerators 14 and burners 8 to effect the melting of the glass will now be described. The burners 8 and regenerators 14 thus operate on a continuously repeating two-step cycle, the burners 8 and regenerator 14 on one side of the tank 6 operating on the cycle out-of-phase with the burners 8 and regenerators 14 on the other side of the tank 6. In the first step of the cycle, the burners on one side of the tank are fired, the tank being supplied with secondary air from the regenerator 14 associated with the burners being fired, this air being heated by the regenerator as it passes tberethrough. The hot combustion gases heat the regenerator 14 opposite the burners being fired.In the second step of the cycle the previously firing burners are cooled internally by air, while hot combustion gases from the tank pass through the ports 10 associated with the burners being fired and then through the regenerator, heating said regenerator. Since, the burners on one side of the tank operate out-of-phase with the burners on the other side, the glass is continuously heated. Moreover, it is necessary for each burner flame to reach across the tank from one side to the other so as to avoid any detremental underheating ofthe glass on the one side of the tank remote from the burners being fired.

The burner flames transfer heat to the glass both by convention and radiation.

As well as it being necessary to avoid any local underheating of the material in the tank it is also necessary to ensure that no combustion of the combustion products takes place in the regenerators 14 or ports 10 and it is moreover desirable to keep to a minimum deposition of solid carbon in regenerators 14. There are accordingly a number of constraints on the supply of the atomising agent.

In conventional operation, it is desirable to keep to a minimum the amount of atomising air that is required since energy is consumed in pressurising this air. Moreover, being supplied directly to the burners 8 without passing through one of the regenerators 14the primary air unlike the secondary air, is approximately at ambient temperature and therefore has a cooling effect on the flame. However, if too little atomising air is supplied the resulting droplets of oil are too large to be fully combusted in the tank and there would therefore be a risk of carry over of carbon or products of incomplete combustion into the regenerators 14.

Moreover, there is a limit to the velocity at which the air-atomised fuel oil mixture can be allowed to leave the orifices 48 (see Figure 2) of the burners 8. If this velocity is excessive, there will be inadequate time available for complete combustion to take place before the velocity of the gases has taken them into those ports opposite the burners being fired. Thus, there is a limitation upon the degree to which the necessary momentum for adequate atomisation can be contributed to by the velocity component of the atomising agent.

In practice, it is generally found necessary using air as the atomising medium to supply the air at a rate such that it contributes from about 2 to 8% of the total air needed for stoichiometric combustion of the fuel oil. Further it is also necessary to supply the secondary air at a rate in excess of that needed for the purposes of combustion. In a typical operation, a flame temperature in the order of 1725"C may be attained while the hot combustion gases typically enter those of the ports 10 opposite the burners that being fired (for the time being) at a temperature in the range 1300 to 1600"C.These gases are effective to raise the temperature of the regenerator associated with said ports sufficiently to preheat air in the next step of the operating cycle to a temperature typically in the range 600 to 10000C. The duration of each step of the operating cycle is arranged to be sufficient for adequate preheating of the secondary air to take place.

It is to be understood that the mere substitution of substantially pure oxygen or other oxygen-rich gas for the air used as the atomising medium will cause a substantial shortening and flattening of the flame produced by the burner. This will lead to inadequate heating of the material in that region of the tank 6 that is the most remote from the burners being fired.

Moreover, there will be an appreciable reduction in the flame lift which will tend rapidly to erode the burner blocks and the intensification of the flame may be so great as to cause local 'hot spots' which will result in damage being caused to the tank 6 and regenerators 14. In addition, it is not possible to overcome these difficulties merely by adjusting the rates at which oxygen, secondary air and fuel oil are supplied without running the risk of obtaining inadequate combustion of the fuel.

In accordance with the invention, the velocity at which the atomising agent is supplied to the atomising chamber 46 of each burner 8 is increased as is the velocity at which the mixture of atomised fuel oil and atomising agent leaves the burner orifice 48.

The necessary adjustments to the burners 8 can be determined empirically. Typically in order to be able to substitute substantially pure oxygen for air as the atomising agent, the rate of supply of the oxygen is selected to be in the order of from 40 to 60% of the rate at which the atomising air was supplied. This enables a reduction to be made in the amount of fuel oil is burned per unit weight of glass produced.

Typically, the fuel consumption can be reduced by from 10 to 15% and thus the rate of supply of fuel oil to each burner may be reduced by 10 to 15%. We have found that substantially the same flame length and shape may be retained upon the substitution of substantially oxygen for air as the atomising agent with such rates of supply of the fuel oil and oxygen provided that the cap 22 is relocated nearer to the spud 20 by a distance sufficient to make a reduction of about 2/3rds in the diameter of the passage 44 defined between the face 36 and the surface 42 and the cross-sectional area of the orifice 48 is reduced by about 25%.

The resulting adjustments to the atomising momentum of the oxygen and the velocity at which the mixture of fuel oil and oxygen leaves the burner enables the flame length, shape and lift to be maintained and a stable flame to be produced. Even through the velocity of the atomising agent is increased by virtue of the reduction in the diameter of the passage 44, the overall atomising momentum of the oxygen is reduced as a result of the greater reduction in the rate of supply of the atomising agent that is made. This results in a less complete or coarser atomisation of the fuel oil in the chamber 46 and this helps to prevent the flame length from being reduced. In addition a reduction is made in the rate at which secondary air is supplied (see below).

The reduction in the cross-sectional area of the orifice 48 result in an increase in the velocity at which the mixture of oxygen and fuel oil leaves the burner 8. This increase also contributes to maintaining the flame length constant, notwithstanding the coarser atomisation as it compensates for the increase in the rate of combustion caused by the substitution of oxygen for air as the atomising medium.

Although the flame length, shape and lift are maintained substantially unaltered, the substitution of oxygen for air as the atomising medium makes possible the attainment of a number of beneficial changes in other parameters of the process. Thus, the flame temperature is increased typically by from 150 to 250"C. Moreover, the efficiency of convective heat-transfer is improved as a result of the heat released when the free radicals created as a result of the higher flame temperature recombine. It is moreover possible substantially to reduce the excess secondary air requirement (i.e. the excess over that needed for stoichiometric combustion). Typically, the tank 6 may be operated with oxygen levels in its atmosphere of from 2 to 3% instead of the typical conventional levels of from 4 to 6%.In addition, the efficiency of the regenerators is improved in that higher air preheat temperatures can be attained. In addition, a reduction in the amount of carbon monoxide and hydrogen in the combustion products can be obtained.

Desirably, all the burners are converted to use with oxygen-rich gas as the atomising agent. However, if desired, one or only some of the burners may be so converted, but the resultant advantages thereby obtained in terms of overall fuel consumption per unit weight glass produced and in the efficiency of operation of the furnace 2 will be diminished.

Moreover, although oxygen-enriched air containing, say, from 50 to 95% oxygen may be used instead of substantially pure oxygen, the greater the purity of the oxygen the larger the benefits than can be attained from its use as an atomising agent.

It is desirable to construct the burners from materials which are known to be safe for use with oxygen. Thus, if the burners used with air are found to include other materials, in converting the burners from use with airto use with oxygen those ports made of the other materials will be charged for ports made of the materials known to be suitable.

Claims (14)

1. A method of operating a liquid fuel burner of a kind that conventionally employs air as a medium to atomise the liquid fuel and to help support combustion of the fuel and which fires into an enclosure, excess secondary air being supplied to the enclosure to create therein an atmosphere including oxygen, whereby a stable substantially fully combusted flame of not less than a given length is produced, wherein oxygen-rich gas (as hereinbefore defined) is substituted for air as the atomising medium and the rates of supply of oxygen-rich gas and fuel, their velocities and combined atomising momentum and the rate of supply of secondary air are selected so as to give a stable fully combusted flame of the given length, or not substantially less, and a flame temperature higher than that achievable with air as the atomising medium.

2. A method as claimed in claim 1, in which the enclosure is a cross-fired glass tank and said flame extends across at least 4/5ths of the width of the tank without impinging against the opposite wall.

3. A method as claimed in claim 1 or claim 2, in which the fuel is a heavy fuel oil and the burner has a medium pressure atomiser.

4. A method as claimed in any one of the preceding claims in which the atmosphere in the enclosure contains not more than 3% by volume of oxygen.

5. A method as claimed in any one of the preceding claims, in which the burner has a body portion in which there are passages for the fuel and atomising agent, and a cap engaging the body portion and having an orifice through which a mixture of atomising agent and fuel leaves the burner, the cap and body portion being so dimensioned as to define an atomising chamber or zone therebetween and a restricted passageforthe atomising agent leading to such chamber or zone, and in which prior to substituting oxygen-rich gas for air as the atomising agent, the position of the cap relative to the body is adjusted to reduce the cross-sectional area of the restricted passage, andthe area of said orifice is reduced.

6. A method as claimed in claim 5, wherein m is greater than m2, v2 is greater than v and the produce of m and v greater than the product of m2 and v2, where m and v are respectively the mass flow rate and velocity of the atomising agent entering the atomising chamber prior to the substitution of oxygen-rich gas for air as the atomising agent and m2 and v2 are respectively the mass flow rate and velocity of the atomising agent entering the atomising chamber after said substitution.

7. A method as claimed in claim 5 or claim 6, in which the cross-sectional area of said restricted passage is reduced by about two-thirds.

8. A method as claimed in any one of claims 5 to 7, in which the area of said orifice is reduced by about 25%.

9. A method as claimed in any one of claims 5 to 8, in which the rate at which secondary air is supplied to the enclosure is reduced upon making said substitution.

10. A method as claimed in any one of the preceding claims, in which the flame lift is not reduced on making said substitution.

11. A method of operating a burner, substantially as herein described with reference to the accompanying drawings.

12. A method of burning a heavy fuel oil with a burner having a low pressure gas atomiser, wherein oxygen-rich gas (as herein defined) is used as the atomising medium.

13. A method as claimed in claim 12, in which the oxygen-rich gas is substantially pure oxygen.

14. A method as claimed in claim 12 or claim 13, in which the atomising medium supplies 20 to 30% of the oxygen required for complete combustion of the fuel oil.