GB1570179A

GB1570179A - Combustion in flames

Info

Publication number: GB1570179A
Application number: GB4565175A
Authority: GB
Original assignee: Exxon Research and Engineering Co
Current assignee: ExxonMobil Technology and Engineering Co
Priority date: 1976-11-03
Filing date: 1976-11-03
Publication date: 1980-06-25

Description

(54) IMPROVING COMBUSTION IN FLAMES (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 relates to improving combustion of hydrocarbon and/or carbonaceous fuels in flames.

The present invention provides 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 crosssectional dimensions (measured in the same cross-sectional planes) of the flame, but not more than 25% smaller than the said natural or unconfined cross-sectional dimensions of the flame.

The lateral confinement of the flame in accordance with the invention tends to increase the length of the flame relative to its length when not so confined.

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 substantially 0.5 inch (c. 1.25 cms) to substantially 2.5 inches (c. 6.3 cms), more preferably from substantially 0.75 inch (c. 1.8 cms) to substantially 2 inches (c. 5.1 cms). In many cases, the reduction of the flame crosssectional dimensions may suitably be from substantially 1.0 inch (c. 2.5 cms) to substantially 1.5 inches (c. 3.8 cms).

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 half-way 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 1flame, 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 invention is particularly useful in reducing the amount of air (or other oxygencontaining 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 invention 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 blanketting and in power generation.With regard to the latter, the absence of excess air tends to reduce the formation of SO, 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 tends to be reduced in the substantial absence of excess air.

The invention 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, pro pane/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 bumer modifications 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 products 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 cham ber 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 invention is now further described with reference to the accompanying drawings in which: Figures 1 to 5 are graphs showing, on the abscissae, the ratio of actual air supplied to the stoichiometric air requirement for combustion of the specified fuel, and on the ordinates, the Bacharach smoke number, each graph illustrating the influence on smoke formation of a particular parameter or factor; and Figure 6 is a graph showing, on the abscissa, the ratio of actual air supplied to the stoichiometric air requirement for fuel combustion, and on the ordinate, the volume % composition of some components of the products of fuel combustion.

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 1.

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

Influence of Combustion Chamber Diameter Using the exhaust gas recirculation burner mentioned in connection with Figure 1, its smoke emission performance in the 24 in. I.D.

combustion chamber of Figure 1 was compared with at in an 8 in. I.D. chamber of the same length. The narrower diameter chosen was about 1 in. smaller than the flame diameter at its widest. The results (Figure 2) 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 was 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 3 for light fuel oil and in Figure 4 for gas oil. It is seen that an increase in length reduces the carbon-forming 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 5, 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 processes 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% 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% 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 6 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 H, and correspondingly less of CO, and H2O.

This can be readily achieved by passing the product gas through a bed of incandescent coal in accordance with well known practice.

WHAT WE CLAIM IS: 1. 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 cross-sectional planes) of the flame, but not more than 25% smaller than the said natural or unconfined crosssectional dimensions of the flame.

2. A method according to claim 1 in which the confined flame has a cross-sectional dimension reduced by from substantially 0.5 to substantially 2.5 inches relative to the crosssectional dimension of the unconfined flame in the same cross-sectional plane.

3. A method according to claim 2 in which the reduction in cross-sectional dimension is from substantially 0.75 to 2.0 inches.

4. A method according to claim 2 or claim 3 in which the reduction in cross-sectional dimension is from substantially 1.0 to substantially 1.5 inches.

5. A method according to any one of claims 1 to 4 in which at least 50% of the natural or unconfined length of the flame is laterally confined.

6. A method according to claim 5 in which at least 60% of the natural or unconfined length of the flame is laterally confined.

7. A method according to any one of claims 1 to 6 in which the flame is laterally confined towards its upstream end.

8. A method according to claim 7 in which the flame is laterally confined from a position which is in the plane of the exit of a burner, and downstream therefrom.

9. A method according to any of claims 1 to 8 comprising laterally confining the flame by at least part of a combustion chamber.

10. A method according to claim 9 in which the combustion chamber contains at least one internal fixed baffle for promoting recirculation of reactive species in the flame.

11. A method according to claim 10 in which the or each baffle extends inwardly from the combustion chamber wall.

12. A method according to claim 10 or claim 11 in which the baffle(s) cause a pressure drop of up to 2 inches of water.

13. A method according to any one of claims 10 to 12 comprising two or more baffles, the baffles being separated by a distance at least equal to the cross-sectional width of the chamber.

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

Claims

**WARNING** start of CLMS field may overlap end of DESC **. 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 was 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 3 for light fuel oil and in Figure 4 for gas oil. It is seen that an increase in length reduces the carbon-forming 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 5, 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 processes 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% 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% 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 6 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 H, and correspondingly less of CO, and H2O. This can be readily achieved by passing the product gas through a bed of incandescent coal in accordance with well known practice. WHAT WE CLAIM IS:

1. 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 cross-sectional planes) of the flame, but not more than 25% smaller than the said natural or unconfined crosssectional dimensions of the flame.

14. A method according to claim 13 in

which there are at least two baffles, the upstream baffle being from 25% to 33% of the length of the flame downstream from the base of the flame, and the adjacent downstream baffle being from 50% to 67% of the flame length from the flame base.

15. A method according to claim 13 or claim 14 comprising three baffles, the most downstream of the baffles being within the flame and separated from the second of the baffles by a distance no smaller than the crosssectional width of the chamber.

16. A method according to claim 10 or claim 11 in which there is a single baffle which is located less than half-way down the total length of the flame.

17. A method according to any one of claims 1 to 16 comprising recovering from the flame a substantially smoke- and carbon-free gas.

18. A method according to any one of claims 1 to 17 wherein the flame is produced by burning the fuel with no more than a stoichiometric amount of combustion-supporting gas.

19. A method according to claim 18 in which the flame is produced by burning the fuel with a stoichiometric amount, relative to the fuel, of the combustion-supporting gas.

20. A method according to claim 19 comprising recovering from the flame a hot, substantially clean reducing gas.

21. A method according to any one of claims 1 to 20 wherein the fuel is a gas oil, a fuel oil and/or a solid fuel.

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

23. A hot, substantially smoke- and soot-free gas obtained by burning a hydrocarbon and/ or carbonaceous fuel in accordance with the method of any one of claims 1 to 22.