Combustion – Process of combustion or burner operation – Flame shaping – or distributing components in combustion zone
Reexamination Certificate
1999-09-02
2001-03-06
Yeung, James C. (Department: 3743)
Combustion
Process of combustion or burner operation
Flame shaping, or distributing components in combustion zone
C431S010000, C431S187000, C431S190000, C239S424000, C239S424500
Reexamination Certificate
active
06196831
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for burning a fuel, in which at least one fuel jet and, some distance therefrom, at least one main jet of an oxdizer are injected into a combustion zone.
2. Description of the Related Art
A combustion process is known from U.S. Pat. No. 4,988,285, which makes it possible to reduce the formation of nitrogen oxides of the type NO
x
, in which a jet of fuel, for example natural gas, and a main jet of an oxidizer, for example air or oxygen-enriched air, arranged a short distance from the fuel jet, preferably between 4 to 20 times the diameter of the main oxidizer jet, are injected into a combustion zone.
The Applicant has however found that such a known combustion process leads to the production of too great a quantity of nitrogen oxides when the fuel and main oxidizer jets are arranged a short distance apart.
When the oxidizer and fuel jets are moved further apart in order to reduce the emission of nitrogen oxides, one is then confronted with problems regarding the stability of sustained combustion (the flame may at times go out) and with the presence of unburnt fuel in the fumes, this also being harmful to the environment.
The invention aims to alleviate these drawbacks by proposing a combustion process making it possible to obtain stable combustion, with low emission of nitrogen oxides, despite the distance between the oxidizer and fuel jets being much greater than that described in the prior art such as U.S. Pat. No. 4,988,285.
SUMMARY OF THE INVENTION
To this end, the subject of the invention is a combustion process for burning a fuel, in which at least one fuel jet and some distance therefrom at least one main jet of an oxidizer are simultaneously injected into a main combustion zone, characterized in that the point of injection of each main oxidizer jet with respect to the point of injection of the fuel jet closest to it is arranged a distance D away satisfying at least one of the following relations:
D
A
>
5
(and preferably >10) and/or
D
B
>
5
(and preferably >10)
D being defined as the minimum distance between the outer edge of the relevant oxidizer jet and the outer edge of the fuel jet closest to it, at their respective points of injection, and A and B being respectively the cross section of the main jet of the oxidizer and the cross section of the fuel jet, the cross sections being considered at the point of injection of the jets, in such a way as to keep the fuel and main oxidizer jets separated until the said at least one main oxidizer jet and/or the fuel jet has entrained a quantity of a substantially inert surrounding fluid. The quantity of surrounding fluid entrained is preferably greater than five, even more preferably than ten times its own flow rate.
According to a preferred variant, the invention is characterized in that at least one auxiliary jet of an oxidizer is injected into an auxiliary combustion zone situated upstream of the said main combustion zone so as to stabilize the combustion in the said main combustion zone, the point of injection of the said auxiliary oxidizer jet being arranged a distance D
s
away from the associated fuel jet, D
s
satisfying the following relation:
D
s
A
s
<
5
D
s
being the minimum distance between the outer edge of the relevant auxiliary oxidizer jet and the outer edge of the associated fuel jet, at their respective points of injection, and A
s
being the cross section of the relevant auxiliary oxidizer jet at its point of injection, so as to obtain substantially uniform combustion.
The use of a distance D which satisfies at least one of the above two relations enables the main oxidizer jet and the fuel jet to entrain a quantity of surrounding fluid, in particular a substantially inert one, before they react with one another. By taking as reference as the beginning of their interaction (and at the start of the main combustion zone) the point at which the edges of the main oxidizer jet and the fuel jet meet, for substantially parallel jets, each of the relations implies that the total flow rate contained in the jet is at least 1.8 times the initial flow rate of the entraining jet. The ratio (jet flow rate/initial flow rate) increases as the ratio (density of entraining fluid/density of entrained fluid) decreases. By satisfying each of the two inequalities it is possible to obtain a dilution of each of the fuel and main oxidizer jets. This invention will be implemented with a distance D satisfying at least one of the above relations, preferably satisfying D/A
0.5
>10 and/or D/B
0.5
>10, so that the flow rate of at least one of the jets and preferably of each jet (initial flow rate plus substantially inert surrounding fluid) is at least 3.6 times the initial flow rate of the entraining jet.
According to a preferred embodiment, the process is characterized in that the total flow rate of oxidizer injected by the main and auxiliary oxidizer jets is adjusted to a value above the stoichiometric flow rate of oxidizer required to burn all the fuel injected into the combustion zone by the at least one fuel jet. Likewise preferably, the flow rate of oxidizer injected by the at least one auxiliary jet is adjusted to a value below 30%, preferably between 2% and 15% of the total flow rate of oxidizer injected into the combustion zone.
The process according to the invention can moreover include one or more of the following characteristics:
several main oxidizer jets are injected symmetrically about the at least one fuel jet,
two main oxidizer jets arranged diametrically opposite with respect to at least one central fuel jet are injected into the combustion zone,
three central fuel jets which are coplanar with the two main oxidizer jets arranged diametrically opposite with respect to the three central fuel jets are injected into the combustion zone,
at least one jet of a first fuel, in particular natural gas, and at least one jet of a first fuel, in particular natural gas, and at least one jet of a second fuel, in particular fuel oil, are injected into the said combustion zone (the fuel may in all cases be solid, liquid and/or gaseous).
The term “substantially uniform combustion” signifies that a zone of substantially uniform combustion is obtained characterized by a combustion zone volume which is at least doubled with respect to a flame where the fuel and oxidizer jets mix rapidly without prior dilution with combustion products, and a temperature field with low gradients within the volume of the flame, such that, for an oxidizer composed of pure oxygen, the maximum mean temperature is at least 500° C. below the theoretical adiabatic temperature of the fuel/oxidizer mixture.
The total momentum (fuel+combustible)of the fluid jets, referred to as a unit of power (and which will therefore be expressed in Newtons/Megawatts), will preferably be greater than around 3 N/MW, as to obtain satisfactory mixing of the gases (the momentum is defined here as the product of a mass flow rate (kg/s) times a velocity (m/s)).
The table below (referred to a burner power of 1 MW) summarizes the various results obtained with an oxygen
atural gas flame (of 1 MW):
OXYGEN
NATURAL GAS
TOTAL
Momentum
Velocity
Momentum
Momentum
Case
Velocity
(N)
(m/s)
(N)
(N)
1
10
0.9
50
1.1
2.0
2
10
0.9
100
2.2
3.1
3
60
5.1
5
0.1
5.2
4
100
8.5
100
2.2
10.7
5
300
25.5
400
8.8
34.3
Case 1 corresponds to injection velocities which are very small for the oxidizer and small for the natural gas. Practice shows that the flames produced are sensitive to buoyancy forces and may create hotspots on the roof of an oven, owing to the raising of the rear part of the flame. Cases 2 to 5 show various examples where the mixing of the gases is ensured by momentum supplied either by the oxidizer jets, or by the fuel jets, or by both.
The term substantially inert surrounding fluid signifies the fluid (in general a gas) situated in proximity to the main oxidizer jet. In general, it consists of the combustion gases which recirculate throughout the combustion zone as well as in the vic
Charon Olivier
Dugue Jacques
Labegorre Bernard
Samaniego Jean-Michel
Burns Doane , Swecker, Mathis LLP
L'Air Liquide, Societe Anonyme pour l'Etude et l'
Yeung James C.
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