Staged Oxyfuel Combustion Method Using Pre-Heated Reagents

Abstract

The invention relates to a staged oxyfuel combustion method in which a combustion chamber is injected with a jet of fuel inside a jet of primary oxygen gas which is introduced through a hole having a diameter D and a secondary oxygen gas which is introduced through a hole having a diameter d, which is positioned at a distance l from the hole for the primary oxygen gas. According to the invention, the jet of fuel emerges at a distance r set back from the wall of the combustion chamber. The oxygen-rich oxygen gas is pre-heated to a temperature of at least 300° C. The r/D ratio is between 5 and 20 or between 0.75 and 3 and the l/d ratio is at least 2.

Description

The present invention relates to a process for burning a fuel using an oxygen-rich oxygenated gas in which the oxygen-rich oxygenated gas is preheated to a temperature of at least 300° C.

At the present time, the two main preoccupations when designing burners intended for industrial furnaces are the efficiency of heat transfer to the charge and the reduction in polluting emissions, particularly nitrogen oxides. One of the most promising methods for meeting these two preoccupations is staged oxycombustion using preheated reactants. This is because it is known that by staging the combustion it is possible to reduce the NOx emissions and that by preheating the reactants it is possible to make energy savings.

Oxycombustion is combustion employing an oxygenated gas having an oxygen concentration greater than air. The process of staged combustion of fuels consists in dividing the amount of oxidant needed for complete combustion of the fuel into at least two oxidant streams introduced at different distances from the fuel stream. Thus, a first oxidant stream is introduced at a very short distance from the fuel stream. This first oxidant stream closest to the fuel stream is called the primary stream—it allows partial combustion of the fuel at a controlled temperature that limits the formation of NOx. The other oxidant streams are introduced at a greater distance from the fuel than the primary oxidant stream. They are used to complete the combustion with the fuel that has not reacted with the primary oxidant. These streams are called secondary streams.

Preheating the reactants (oxygenated gas and fuel) has already been proposed in various schemes for recovering heat from the flue gases (U.S. Pat. No. 6,071,116). Also disclosed, in WO 00/79182, is a burner in which both the fuel and the oxidant are preheated. However, these documents do not mention the existence of problems associated with the use of hot reactants. However, the use of hot reactants may result in the following disadvantages:

• • increase in the flame velocity of the reactant mixture—the flame front then stabilizes close to the outlets of the various burner orifices, increasing the risk of overheating and premature destruction of the injectors;
  • increase in the flame temperature and thereby increasing the radiative transfer to the charge and to the surface of the burner; and
  • increase in the level of thermal NOx, as a result of the increase in flame temperature.

The object of the present invention is to propose design rules for a burner employing staged oxycombustion and preheated reactants so as to retain the advantages of staged oxycombustion without incurring the risks associated with the use of preheated reactants.

For this purpose, the invention relates to a process for burning a fuel using an oxygen-rich oxygenated gas in which the following are injected into a combustion chamber:

• • a jet of the fuel; and
  • at least two jets of the oxygenated gas:
    • the first jet of the oxygenated gas, called the primary jet, being introduced via an orifice having a diameter D and being injected around the jet of fuel and in an amount such that it causes a first incomplete combustion, the gases resulting from this first combustion still comprising at least some of the fuel and
    • the second jet of the oxygenated gas, being introduced via an orifice having a diameter d, placed at a distance l from the orifice for introducing the first, primary jet of oxygenated gas, so as to enter into combustion with that part of the fuel present in the gases resulting from the first combustion;and in which:
  • the jet of the fuel emerges within the primary jet of oxygenated gas at a point set back from the wall of the combustion chamber, said point being located at a distance r therefrom;
  • the oxygen-rich oxygenated gas is preheated to a temperature of at least 300° C.;
  • the r/D ratio is:
    • either between 5 and 20, preferably between 7 and 15;
    • or between 0.75 and 3; and
    • the l/d ratio is at least 2, preferably at least 10.

The process according to the invention employs combustion in which the oxidant is an oxygen-rich oxygenated gas, that is to say one having a higher oxygen concentration than air. This may be oxidant-enriched air or pure oxygen. Preferably, the oxygen concentration of this oxygenated gas is at least 70% by volume.

The process according to the invention employs combustion in which at least the oxygenated gas is preheated to a temperature of at least 300° C. This preheating may be carried out by any known method of the prior art. Preferably, this is preheating using a recuperator, for recovering the heat of the combustion flue gases. The fuel may also be preheated, preferably to at least 300° C.

The process according to the invention employs staged combustion in which the oxygenated gas is divided into two streams, which are introduced into the combustion chamber at various distances from the fuel jet. The primary jet is introduced in contact with the fuel jet and surrounds it. The orifice of the injector introducing the primary jet into the combustion chamber has a diameter D. The end of this primary jet injector emerges directly in the combustion chamber. However, the end of the fuel injector does not emerge directly in the combustion chamber but is set back from the wall of the combustion chamber: the end of the fuel injector emerges in the middle of the primary jet injector at a point set back from the wall of the combustion chamber, situated at a distance r from said wall. The diameter of the fuel injector is generally adjusted so as to produce a stable flame. In general, 2 to 20% of the total amount of oxygenated gas needed to burn the fuel is introduced by means of the primary jet. The complement of oxygenated gas is introduced by means of the secondary jet via an injector whose end opens directly in the combustion chamber and whose orifice has a diameter d. This injector of the secondary jet is placed at a distance l from the primary oxygenated gas orifice, the distance l being measured between the edges closest to the orifices of the primary oxygenated gas injector and of the secondary oxygenated gas injector.

One of the essential features of the invention is that it is necessary to prevent the staged oxycombustion process being used with the fuel injector set back within the primary oxygenated gas injector with dimensions such that r/D is between 3 and 5. The respective dimensions r and D are critical as they influence the volume of the cavity created by the fuel injector being set back within the primary oxygenated gas injector. By choosing the dimensions correctly, the invention makes it possible, on the one hand, to prevent the premixing flame developing in this cavity from damaging the ends of the fuel and primary oxygenated gas injectors and, on the other hand, to prevent the radiation emanating from the combustion chamber from damaging the end of the combustion injector.

Another essential feature of the invention is that the l/d ratio must be at least 2 and preferably at least 10.

Usually, the orifices via which the reactants are injected have a circular cross section. However, the invention also covers the cases in which the cross sections of these orifices are not of circular shape. In the latter cases, the diameters d and D mentioned above correspond to the hydraulic diameters of noncircular cross sections, the hydraulic diameter being defined as the ratio of 4 times the area of the cross section of the orifice divided by the perimeter of the cross section of the orifice.

FIG. 1 is a diagram for illustrating the various elements of the dimensions (r, d, D, l) for the process according to the invention. The fuel l is introduced by means of the injector 7 placed in the injector 6 for the primary oxygenated gas 2. The end of the fuel injector is set back by a distance r from the wall 4 of the combustion chamber. The end of the primary oxygenated gas injector 2 has a diameter D. The secondary oxygenated gas 3 is introduced via the injector 5, the end of which has a diameter d. The distance between the edges of the ends of the oxygenated gas injectors 5 and 6 is l.

By implementing the process as described above, it is possible to preheat the combustion reactants in a staged oxycombustion process without prematurely damaging the reactant injectors and without increasing the NOx emission.

Claims

1-7. (canceled)

8. A process for burning a fuel using an oxygen-rich oxygenated gas in which the following are injected into a combustion chamber: a jet of the fuel; andat least two jets of the oxygenated gas: the first jet of the oxygenated gas, called the primary jet, being introduced via an orifice having a diameter D and being injected around the jet of fuel and in an amount such that it causes a first incomplete combustion, the gases resulting from this first combustion still comprising at least some of the fuel andthe second jet of the oxygenated gas, being introduced via an orifice having a diameter d, placed at a distance l from the orifice for introducing the first, primary jet of oxygenated gas, so as to enter into combustion with that part of the fuel present in the gases resulting from the first combustion;

9. The process of claim 8, wherein the r/D ratio is between 7 and 15.

10. The process of claim 8, wherein the l/d ratio is at least 10.

11. The process of claim 8, wherein the oxygen-rich oxygenated gas has an oxygen concentration of at least 70% by volume.

12. The process of claim 8, wherein the fuel is preheated to at least 300° C.

13. The process of claim 8, wherein said process is employed in a glass melting furnace.

14. The process of claim 8, wherein said process is employed in a reheating furnace.

15. The process of claim 9, wherein the l/d ratio is at least 10.

16. The process of claim 15, wherein the oxygen-rich oxygenated gas has an oxygen concentration of at least 70% by volume.

17. The process of claim 16, wherein the fuel is preheated to at least 300° C.

18. The process of claim 17, wherein said process is employed in a glass melting furnace.

19. The process of claim 17, wherein said process is employed in a reheating furnace.

20. The process of claim 15, wherein said process is employed in a glass melting furnace.

21. The process of claim 15, wherein said process is employed in a reheating furnace.

22. The process of claim 16, wherein said process is employed in a glass melting furnace.

23. The process of claim 16, wherein said process is employed in a reheating furnace.