GB1570180A - Combustion of fuels - Google Patents

Description

(54) IMPROVEMENTS IN OR RELATING TO THE COMBUSTION OF FUELS (71) We, EXXON RESEARCH AND ENGINEERING COMPANY, a Corporation duly organised and existing under the laws of the State of Delaware, United States of America, of Linden, New Jersey. United States of America, do hereby declare the invention for which we pray that a patent may be granted to us, and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention concerns improvements in or relating to the combustion of fuels, particularly gaseous, liquid and/or solid hydrocarbon and carbonaceous fuels.

The growing concern about both energy conservation and atmospheric pollution demands that combustion systems be operated with maximum combustion efficiency (with minimum excess air) and without any emission of pollutants. The two goals, however, are not readily compatible, and indeed have proved difficult to realise. This is because any reduction in excess air to promote combustion efficiency increases smoke, carbon monoxide and unburned hydrocarbons, and often also oxides of nitrogen (NOx).

We have now developed a system and method in which both objectives can be achieved, not just with relatively clean burning gaseous and light distillate fuels but also with fuel oils. The system basically involves staged combusiton, as described below, in which improvements have been made in the design of the combustion system, and catalysts used in different stages. conventional staged combustion involves firing of all the fuel in a first stage with a sub-stoichiometric quantity of air and the provision of sufficient air to complete combustion in a second stage. Such systems have been claimed to reduce NOX by c.50, but they usually render the control of other pollutants, notably carbon, more difficult.

According to the invention, in one aspect, fuel is partly burned in a first stage flame to yield gaseous state, hot substantially carbon-(or smoke)-free partly-combusted fuel or first stage product, the latter being contacted at a temperature in the range of from 800 to 1600do with a catalyst which is active for promoting the conversion of nitrogen, the partly-combusted fuel (or first stage product), after contact with the catalyst, then being at least partially burned in a second stage flame to yield hot gaseous products having a low nitrogen oxide (and other pollutants) content.The temperature at which the partly-combusted fuel or firststage product contacts the catalyst may be in the range of from 900 to 1400"C, preferably 10000C to 13000C, although the actual temperature will depend upon the fuel and on the amount of oxygen supplied for combustion in the first stage flame, and on the amount of nitrogen introduced with the oxygen.

In order to reduce still further the nitrogen oxide content of the products, there may be employed, in addition, a catalyst to reduce or inhibit the formation of nitrogen oxide, located within the flame of the first stage and/or within the flame of the second stage. Preferably, a catalyst for reducing or inhibiting NOX formation is located within the flames of both stages.

The catalyst for the, or each, stage, is preferably so disposed within the flame as to contact or be contacted by the hottest region of the flame usually at between 30V and 45( of the - flame length from its upstream end.

The catalyst employed between the stages, and the catalyst(s) used in the first and/or second stages may be the same or different. They are preferably selected from substances consisting of or containing the elements or compounds of chromium, iron, cobalt, nickel, molybdenum, tungsten, silicon, aluminium, magnesium, calcium, manganese, barium, strontium, neodymium vanadium, alkali metals or any mixture thereof. A preferred catalyst, for cost effectiveness, is Fe/Cr, although cobalt may alternatively be favoured.The total amount of oxygen supplied in relation to the fuel may be a substoichiometric amount so that the hot gaseous products contain combustible material, or a substantially stoichiometric amount so that the hot gaseous products substantially comprise a flue gas containing no free oxygen or combustible material, or a superstoichiometric amount so that a flue gas containing free excess oxygen is produced.

The part-combustion in the first stage may be performed in any way which produces partly combusted fuel (or first stage products) which is free of, or substantially free of, carbon or smoke. A preferred expedient for part combustion is described in the description and drawings of the Appendix forming part of this patent specification.

The second stage combustion may also be performed in the manner described in the Appendix. The second stage combustion may be effected by mixing an oxygencontaining gas (such as air) into the hot partly-combusted fuel or first-stage products.

By effecting the part-combustion in the first stage using the technique described in the Appendix, the NOX production in the absence of the catalyst which is employed in the practice of this invention is up to c.60 /n less than the NOX production by single stage combustion. Moreover, the technique of the Appendix markedly reduced carbon formation even at very high fuel:oxygen ratios, and facilitates complete combustion (if desired) in the second stage with substantially no excess air. By employing catalysts between the stages, and also within the flames of both stages, a reduction in the NOX concentration of over 90% to an innocuously low concentration may be realised.

When the fuel contains suphur, operation with no, or very little, excess air reduces the formation of S03, most of the sulphur appearing in the flue gases as SO2 with a reduction in the acid dew point and potential improvements in heat recovery.

The invention will now be further described with reference to the accompanying drawings in which the graphs of Figures 1 to 9 each show on the abscissa the ratio of total actual air supplied to a burner to the stoichiometric air requirement for the burning of the specified fuel, and on the ordinate, the amount of the specified product contained in the burned fuel products. In Figures 1 to 3 where the specified product is nitric oxide (NO), the amount of NO has been "corrected" to the well-accepted standard in the art of a notional amount of excess or diluting air providing 3% oxygen in the gas products. The influence of different factors on the efficiency of combustion is now described.

Control of Nitrogen Oxides (NOx) NOX are reduced most effectively when catalysts are used in the interstage zone.

Less, but significant further reductions are achieved by placing catalysts within the first and second stage flames.

The data below are quoted for a system where either light fuel oil or gas oil, doped with pyridine to increase the NOX forming potential to the highest likely to be encountered from any petroleum fuel, was fired with a burner of the type which recirculates a part of the burnt gas into the fresh charge. The combustion chamber was lined with silica/alumina refractory. It was 90 in. and 8 in. I.D.

Catalyst in Interstate Zone The effect of an iron/chromium oxide catalyst, coated on a perforated alumina/silica refractory block (3 in. thick with numerous 1/2 in. diameter channels through it), when placed at different positions in the interstage zone, is shown in Figure 1 and Table 1. The catalyst markedly reduced NOx. Its effect increased the further downstream it was placed, and also as the proportion of fuel-to-air was increased. Table 1 also shows that cobalt oxide, while an effective catalyst, is less effective catalyst than iron/chromium oxide.

Catalyst in the Second Stage Flame An iron/chromium oxide treated refractory block similar to that in the interstage zone, but only half its thickness (1 in.), was placed in 3 in. downstream of the air injector. As shown in Figure 2, this brought about a further reduction in NOx.

For instance, with the primary air 73% of the stoichiometric and the overall air input stoichiometric, the second stage catalyst reduced NO from 140 ppm to 120 ppm-a 14% reduction.

TABLE I Influence of interstage catalyst at different distances from the primary burner and at different air consumption rates (Fuel: Light fuel oil Firing rate, 2.5 gph) Position of Air: NO catalyst from Actual/ Reduction Catalyst the primary burner Stoichiometric Fe/Cr 66" 0.75 62 0.80 41 0.85 25 52" 0.75 53 0.80 34 0.85 21 40" 0.75 44 0.80 30 0.85 19 Co 52" 0.75 45 0.80 30 0.85 19 A different design of catalyst support-as an elongated ellipsoid with ca 6 in. and 2 in.

axes, supported ca 2 in. From the burner (air injector) along the central axisproduced virtually the same effect as the perforated block catalyst.

Catalyst in the First Stage Flame The use of iron/chromium catalyst in the first stage flame, in a manner similar to that in the second stage flame, substantially reduced NOX (Figure 3). for instance, whilst the inter-stage catalyst, with primary air 74 of stoichiometric reduced NO from 275 ppm to 115 ppm (58% reduction) the additional first stage catalyst reduced NO 70 ppm (75% reduction). At this primary air level, second stage combustion with a catalyst therein, produced 100 ppm of NO at an overall stoichiometry of 1.0. A conventional combustion system, with the high nitrogen content fuel used, would generate well over 1000 ppm of NO.

Control of Sulphur Trioxide Formation of sulphur trioxide from any sulphur impurity in the flame is highly undesirable because of its corrosive nature.

Combustion according to the invention greatly reduces SO3 formation. For instance, whereas in a conventional system, operating at 2.1At excess oxygen, light fuel oil with 2.5 ' sulphur gave rise to 44 ppm of SO,; combustion employing the technique described hereinafter, in the appendix forming part of the specification, without staging or the use of catalysts, produced 34 ppm of sulphur trioxide (23 ," reduction) at the same stoichiometry. Staging reduced sulphur trioxide from 34 ppm to 26 ppm. The use of an interstage catalyst reduce it to 20 ppm, and that of the first and second stage catalysts to 18 ppm.Since, unlike the conventional system, staged combustion can be operated smoke-free under stoichiometric conditions, this further greatly reduces sulphur trioxide to only 5 ppm-that is ca 90% overall reduction.

Control of Smoke, Carbon Monoxide, Hydrogen and Unburned Hydrocarbons in the Second Stage For complete combustion of smoke, carbon monoxide, hydrogen and unburned hydrocarbons in the second stage, without any excess air, and in a limited space, it is essential that the system design promotes thorough mixing of the products of the first stage with the added air. The necessary design features are essentially the same as those developed for the first stage. Using the same narrow combustion chamber with baffles as in the first stage, we were successful in effecting satisfactory combustion with stoichiometric air overall.

Smoke emission in the flue gas from the second stage was not greater than 3 Bacharach Number-that is, no smoke was visible in the stack gas. Carbon monoxide, hydrogen and unburned hydrocarbons were absent. And, as previously stated, both NOX and sulphur trioxide had been reduced by about 90%.

The foregoing description is given to illustrate the invention and is not limitative of its scope.

There now follows an Appendix, including drawings (figure 4, 5, 6, 7, 8 and 9) describing a technique which is a preferred mode of performing the combustion in the first stage, and which mode may also be employed in the second combustion stage.

The Appendix, as previously stated, forms part of this patent specification.

Appendix This appendix relates to a technique for improving combustion of hydrocarbon and/or carbonaceous fuels.

According to one aspect of the technique, combustion of such fuels in flames is improved by laterally confining at least part of the length of a flame produced by a burner in a combustion chamber having cross-sectional dimensions smaller than the natural or unconfined cross-sectional dimensions (measured in the same crosssectional planes) of the flame.

In another aspect, the technique comprises laterally confining a flame produced by a burner consuming such fuels for at least part of its natural or unconfined length in a combustion chamber such that the length of the flame is increased.

It is preferred that the natural or unconfined cross-sectional dimensions (e.g.

diameter) of the flame be reduced by containment in the combustion chamber by from 0.5 inch (c. 1.25 cms) or thereabouts to 2.5 inches (c. 6.3 cms) or thereabouts, more preferably from 0.75 inch (c. 1.8 cms) or thereabouts to 2 inches (c. 5.1 cms) or thereabouts. In many cases, the reduction of the flame's cross-sectional dimensions may suitably be from 1.0 inch (c. 2,5 cms) or thereabouts to 1.5 inches (c. 3.8 cms) or thereabouts.

It is preferred that the cross-sectional dimension should not be reduced by more than 25%.

At least 50% of the natural length of the flame is preferably contained or confined by the combustion chamber and more preferably 60% or more (e.g. 70%). Better improvements in combustion may be realized when the combustion chamber laterally confines the upstream part (towards the burner) rather than the downstream part of the flame. The combustion chamber may confine the flame starting from a position either at the exit from the burner or spaced downstream therefrom. It may be convenient to attach the combustion chamber to the burner or burner support.

The combustion chamber may contain at least one internal fixed baffle for promoting recirculation of reactive species in the flame. The, or each, baffle may have any convenient form such as a refractory ring or annulus (in the case of cylindrical combustion chambers) extending inwardly from the periphery at the internal wall.

The baffle(s) should cause the smallest pressure drop which is economically acceptable for the realized improvement in combustion. For most cases, a pressure drop of up to 2 inches (c. 5.0 cms) water will be acceptable. With one baffle, the pressure drop will usually tend to be about 1 inch (c.

2.5 cms) water in most cases.

When there are two or more baffles, they should be separated by a distance equal to at least the cross-sectional dimension (e.g.

diameter or equivalent) of the combustion chamber.

The location of the baffle(s) in the flame tends to influence the improvement in combustion. With one baffle, the baffle should be located preferably less than halfway down the total length of the flame from the burner, e.g. about 33% of the total flame length from the burner.

When two baffles are employed, it is preferred that the upstream baffle is located from 25% to 33 /" of the length of the flame downstream of the base of the flame, e.g. at the burner, and the downstream baffle from 50% to 67% of the flame length from the flame base.

With three baffles, the location of the upstream and middle baffles is preferably in the same range as for two baffles, the downstream baffle being located within the flame at any distance downstream of the middle baffle but separated therefrom by a distance no smaller than the internal diameter (or its equivalent) of the combustion chamber.

While the combustion chamber tends to increase the length of the flame, each baffle reduces the flame length so that a shorter combustion chamber can be used to effect the same improvement in combustion. One baffle alone can reduce the flame length by up to 25%, e.g. 15 to 20%, while three baffles can reduce the flame length by up to 50%.

The technique is particularly useful in reducing the amount of air (or other oxygen-containing gas) required to eliminate, or reduce to an acceptable level, smoke or carbon, so that it is possible to burn relatively heavy fuel oils and solid fuels substoichiometrically to produce hot, clean reducing gas relatively efficiently. The techinque may also be employed with lighter fuels, e.g. naphthas up to LPG containing more than 2% of butane and may also be employed in the combustion of any of the foregoing fuels to produce a substantially smoke-and carbon-free hot neutral (i.e. neither oxidizing nor reducing) gas useful in processes requiring inert gas blanketing and in power generation. With regard to the latter, the absence of excess air tends to reduce the formation of SO3 from sulphur in the fuel whereby greater heat recovery may be effected without the risk of sulphuric acid corrosion. Moreover, the production of nitrogen oxides also tend to be reduced in the substantial absence of excess air.

The technique will now be described in connection with the production of hot, clean, reducing gases.

Hot reducing atmospheres are extensively generated for heat treatment of metals.

Their far wider use in future has been forecast for injection into the bosh zone of blast furnaces and eventually for the production of raw steel by direct reduction of iron ore.

Currently reducing atmospheres are generated mostly by partial combustion of gaseous fuels-natural gas, town gas, propane/butane-in the presence of a catalyst. The operation requires a careful control of fuel and catalyst quality and maintenance of the optimum operating conditions to ensure prevention of carbon formation and deposition on the catalyst.

Much work has already been carried out on improved burner designs for minimizing carbon formation. What we have now discovered, in accordance with the invention, is that far greater reduction in carbon-forming tendency can be achieved by careful design of combustion chamber.

With our combustion chamber, in conjunction with a suitable burner, we can now generate a highly reducing atmosphere with, e.g., liquid fuels of wide compositional range, without the aid of a catalyst.

Whilst the burners modification have been chiefly based on better mixing of the fuel and air feed and/or on the injection into the fuelair feed of water/steam or product of combustion, the combustion chamber modifications are based on our observation that increased mixing and recirculation of the flame reactants and products in the flame itself can greatly reduce carbon formation. We have systematically investigated the combustion chamber design parameters which promote mixing and recirculation and find that these entail: optimum choice of combustion chamber diameter optimum choice of chamber length provision of suitable baffles in the chamber These features are easy to incorporate and unlike burner modifications, do not require any complex and expensive subsidiary control devices.

The effect of combustion chamber modifications was investigated with a burner which recirculates a part of the combustion products into the fuel-air feed.

This burner produces considerably less carbon than a typical medium pressure air atomizing burner. Comparative results with this burner firing into a refractory lined combustion chamber of conventional size (24 in. diameter and 50 in. long) are shown in Figure 4.

The influence of different parameters on the efficiency of combustion was investigated, as described below.

Influence of Combustion Chamber Using the exhaust gas recirculation burner, its smoke emission performance in the 24 in. I.D. combustion chamber was compared with that in a 8 in. I.D. chamber of the same length. The narrower diameter chosen was ca 1 in. smaller than the flame diameter at its widest. The result (Figure 5) show the markedly less carbon forming tendency in the narrower chamber.

Studies were next carried out in a still narrower (5 in. I.D.) chamber. But now the flame length has become far too long to effect complete combustion within the 50 in. long chamber.

Influence of Combustion Chamber Length With the 8 in. I.D. chamber, the influence of increasing the length of the combustion chamber from 50 in. to 75 in. and then to 90 in. is shown in Figure 6 for light fuel oil and in figure 7 for gas oil. It is seen that an increase in length reduces the carbonforming tendency, although the effect is less marked than that of reducing the diameter.

Influence of Baffles In the 8 in. diameter 50 in. long combustion chamber, three refractory baffles, each with a 2.5 in. hole in the centre, were spaced in the combustion chamber at 16 in., 30 in. and 48 in. from the burner.

These baffles, as shown in Figure 8, considerably reduced carbon formation.

The effect, in fact, is greater than of increasing the length and thus these provide an inexpensive way of reducing carbon forming tendency even with short combustion chambers.

In general, therefore, carbon formation can be markedly reduced by reducing the chamber diameter, increasing the chamber length and by inserting baffles. Of the three factors, the effect of diameter is most marked. However, care has to be taken that the diameter is not reduced excessively- preferably by not more than one-to-inches, or by more than 25 /,, of that of the unrestricted/unconfined flame, since otherwise the resulting excessive inhibition of flame reactions and aerodynamic factors can necessitate the use of an impracticably long chamber for completion of the flame processes. For a given length constraint, carbon formation can be considerably reduced by the insertion of baffles in the chamber.Again care is necessary that these are not placed so closely as to inhibit the combustion process to any significant extent. In the system described here by way of example, the position and size of baffles and the diameter of the chamber are particularly suited for maintaining a stable flame even with barely 50 Mn of the stoichiometric air. In conventional systems stable flames are usually hard to maintain with air less than 70% of the stoichiometric.

Successful maintenance of flame process with air barely 50 /n of the stoichiometric amount produces a highly reducing atmosphere. The amounts of CO and H2 that are formed, along with CO2, are shown in Figure 9 for gas oil fired into the 8 in. diameter, 50 in. long chamber fitted with 3 baffles.

For some applications it is desirable to generate even greater amounts of CO and H2 and correspondingly less of CO2 and H2O. This can be readily achieved by passing the product gas through a bed of incandescent coal in accordance with well known practice.

Attention is direction to our co-pending UK patent application No. 45651/75 (Serial No. 1570179) which describes and claims a method of burning a hydrocarbon and/or carbonaceous fuel, comprising burning the said fuel to produce a flame and laterally confining at least part of the length of the flame in a chamber having cross-sectional dimensions which are smaller than the natural or unconfined cross-sectional dimensions (measured in the same crosssectional planes) of the flame, but not more than 25 /,, smaller than the said natural or unconfined cross-sectional demensions of the flame.

WHAT WE CLAIM IS: 1. A method of burning a hydrocarbon and/or carbonaceous fuel comprising the following steps: (a) partially burning the fuel in a first stage flame to produce a hot, substantially carbon-or smoke-free partly-combusted fuel in the gaseous state; (b) contacting the said partlycombusted fuel at a temperature in the range of from 800 to 16000C with a catalyst which is active for promoting the conversion of nitrogen oxide(s) in the partly-combusted fuel to nitrogen; and then (c) at least partly burning the partlycombusted fuel in a second stage flame to yield hot gaseous products of relatively low pollutant content.

2. A method according to claim 1 in which step (b) is performed at a temperature in the range of from 900 to 14000C.

3. A method according to claim 2 in which step (b) is performed at a temperature in the range of from 1000 to 13000C.

4. A method according to any one of claims I to 3 comprising, in addition, in a step (d), contacting the first and/or second stage flame(s) with a catalyst or a respective catalyst for reducing or inhibiting the formation of nitrogen oxide.

5. A method according to claim 4 in which in step (d) the or each catalyst contacts the hottest region of the flame.

6. A method according to claim 4 or claim 5 in which in step (d) the or each catalyst contacts the or the respective flame at a region from 30 to 45% of the length of the flame from its. upstream end.

7. A method according to any one of claims 4 to 6 in which in step (d) the or each flame is contacted by a catalyst selected from substances consisting of or containing the elements or compounds of chromium, iron, cobalt, nickel, molybdenum, tungsten, silicon, aluminium, magnesium, calcium, manganese, barium, strontium, neodymium, vanadium, alkali metals and any mixture of two or more of the foregoing.

8. A method according to claim 7 in which in step (d) both flames are contacted by catalysts which are the same or different.

9. A method according to any one of claims 1 to 8 in which the catalyst in step (b) is selected from substances consisting of or containing the elements or compounds of chromium, iron, cobalt, nickel, molybdenum, tungsten, silicon, aluminium, magnesium, calcium, manganese, barium, strontium, neodymium, vanadium, alkali metals and any mixture of two or more of the foregoing.

10. A method according to any one of claims 1 to 9 in which the or each catalyst comprises either iron and chromium or cobalt.

11. A method according to any one of claims 1 to 10 in which either the first stage flame or the flame in the first stage and the flame in the second stage is or are laterally confined for at least part of its or their length(s) by a chamber or a respective chamber which reduces the cross-sectional width of zone flame(s) relative to its or their natural or unconfined cross-sectional width(s).

12. A method according to claim 11 in which the cross-sectional width(s) is/are reduced by not more than 25%.

13. A method according to claim 11 or claim 12 in which the cross-sectional width of the or each flame is reduced by from 0.5 to 2.5 inches.

14. A method according to claim 13 in which the cross-sectional width of the or each flame is reduced by from 0.75 to 2.0 inches.

15. A method according to any one of claims 11 to 14 in which at least 50 MO of the natural or unconfined length of the or each flame is laterally confined.

16. A method according to any one of

**WARNING** end of DESC field may overlap start of CLMS **.

Claims (26)

**WARNING** start of CLMS field may overlap end of DESC **. combustion process to any significant extent. In the system described here by way of example, the position and size of baffles and the diameter of the chamber are particularly suited for maintaining a stable flame even with barely 50 Mn of the stoichiometric air. In conventional systems stable flames are usually hard to maintain with air less than 70% of the stoichiometric. Successful maintenance of flame process with air barely 50 /n of the stoichiometric amount produces a highly reducing atmosphere. The amounts of CO and H2 that are formed, along with CO2, are shown in Figure 9 for gas oil fired into the 8 in. diameter, 50 in. long chamber fitted with 3 baffles. For some applications it is desirable to generate even greater amounts of CO and H2 and correspondingly less of CO2 and H2O. This can be readily achieved by passing the product gas through a bed of incandescent coal in accordance with well known practice. Attention is direction to our co-pending UK patent application No. 45651/75 (Serial No. 1570179) which describes and claims a method of burning a hydrocarbon and/or carbonaceous fuel, comprising burning the said fuel to produce a flame and laterally confining at least part of the length of the flame in a chamber having cross-sectional dimensions which are smaller than the natural or unconfined cross-sectional dimensions (measured in the same crosssectional planes) of the flame, but not more than 25 /,, smaller than the said natural or unconfined cross-sectional demensions of the flame. WHAT WE CLAIM IS:

1. A method of burning a hydrocarbon and/or carbonaceous fuel comprising the following steps: (a) partially burning the fuel in a first stage flame to produce a hot, substantially carbon-or smoke-free partly-combusted fuel in the gaseous state; (b) contacting the said partlycombusted fuel at a temperature in the range of from 800 to 16000C with a catalyst which is active for promoting the conversion of nitrogen oxide(s) in the partly-combusted fuel to nitrogen; and then (c) at least partly burning the partlycombusted fuel in a second stage flame to yield hot gaseous products of relatively low pollutant content.

2. A method according to claim 1 in which step (b) is performed at a temperature in the range of from 900 to 14000C.

3. A method according to claim 2 in which step (b) is performed at a temperature in the range of from 1000 to 13000C.

4. A method according to any one of claims I to 3 comprising, in addition, in a step (d), contacting the first and/or second stage flame(s) with a catalyst or a respective catalyst for reducing or inhibiting the formation of nitrogen oxide.

5. A method according to claim 4 in which in step (d) the or each catalyst contacts the hottest region of the flame.

6. A method according to claim 4 or claim 5 in which in step (d) the or each catalyst contacts the or the respective flame at a region from 30 to 45% of the length of the flame from its. upstream end.

7. A method according to any one of claims 4 to 6 in which in step (d) the or each flame is contacted by a catalyst selected from substances consisting of or containing the elements or compounds of chromium, iron, cobalt, nickel, molybdenum, tungsten, silicon, aluminium, magnesium, calcium, manganese, barium, strontium, neodymium, vanadium, alkali metals and any mixture of two or more of the foregoing.

8. A method according to claim 7 in which in step (d) both flames are contacted by catalysts which are the same or different.

9. A method according to any one of claims 1 to 8 in which the catalyst in step (b) is selected from substances consisting of or containing the elements or compounds of chromium, iron, cobalt, nickel, molybdenum, tungsten, silicon, aluminium, magnesium, calcium, manganese, barium, strontium, neodymium, vanadium, alkali metals and any mixture of two or more of the foregoing.

10. A method according to any one of claims 1 to 9 in which the or each catalyst comprises either iron and chromium or cobalt.

11. A method according to any one of claims 1 to 10 in which either the first stage flame or the flame in the first stage and the flame in the second stage is or are laterally confined for at least part of its or their length(s) by a chamber or a respective chamber which reduces the cross-sectional width of zone flame(s) relative to its or their natural or unconfined cross-sectional width(s).

12. A method according to claim 11 in which the cross-sectional width(s) is/are reduced by not more than 25%.

13. A method according to claim 11 or claim 12 in which the cross-sectional width of the or each flame is reduced by from 0.5 to 2.5 inches.

14. A method according to claim 13 in which the cross-sectional width of the or each flame is reduced by from 0.75 to 2.0 inches.

15. A method according to any one of claims 11 to 14 in which at least 50 MO of the natural or unconfined length of the or each flame is laterally confined.

16. A method according to any one of

claims 11 to 15 in which the upstream part of the or each flame is laterally confined.

17. A method according to any one of claims 11 to 16 in which the or each chamber comprises at least one internal baffle for promoting recirculation of reactive species in the flame or a respective flame which contacts the baffle.

18. A method according to claim 17 in which there is a single internal baffle positioned so as to be contacted by the flame or a respective flame less than halfway down the total length of the flame from its upstream end.

19. A method according to claim 17 in which there are two internal baffles positioned so as to be contacted by the flame or a respective flame, the upstream baffle being located from 25% to 35% of the length of the flame downstream from its upstream end, and the second baffle being located from 50 on to 67 , of the flame length downstream from the upstream end of the flame.

20. A method according to claim 19 comprising three baffles, the third baffle being positioned to contact the flame or a respective flame and being separated from the second baffle by a distance, measured in the downstream direction, which is no smaller than the cross-sectional width of the chamber.

21. A method according to any one of claims 1 to 20 in which the fuel comprises hydrocarbons boiling in the gas oil, and/or heavy fuel oil range, and/or carbonaceous solids.

22. A method according to any one of claims 1 to 21 in which the total amount of oxygen supplied in relation to the fuel is substoichiometric.

23. A method according to any one of claims 1 to 21 in which the total amount of oxygen supplied is substantially stoichiometric in relation to the fuel.

24. A method according to any one of claims 1 to 21 in which the total amount of oxygen supplied is superstoichiometric in relation to the fuel.

25. A method of burning a hydrocarbon and/or carbonaceous fuel substantially as hereinbefore described.

26. A hot gas product substantially free of pollutants produced by the method of any one of claims 1 to 25.