Process for combustion of a fuel with an oxygen-rich oxidant

Details Process for combustion of a fuel with an oxygen-rich oxidant Process for combustion of a fuel with an oxygen-rich oxidant

1999-07-02

2001-02-20

Lazarus, Ira S. (Department: 3743)

Combustion

Process of combustion or burner operation

Controlling or proportioning feed

C431S010000, C431S037000, C431S076000, C431S090000, C110S188000, C110S186000

Reexamination Certificate

active

06190160

ABSTRACT:

This application claims priority under 35 U.S.C. §§119 and/or 365 to 98 08742 filed in France on Jul. 8. 1998; the entire content of which is hereby incorporated by reference.
BACKGROUND OF THE INVENTION
1. Field of the Invention
The invention relates to a process for combustion of a fuel with an oxidant whose oxygen level is more than 25 mol %, in a combustion chamber, in which, cyclically:
at least one principal variable representing the combustion in the said combustion chamber is measured,
as a function of the result of the measurement of the said at least one principal variable, a control instruction for regulating the flowrates of fuel and oxidant to be injected into the said combustion chamber is determined, and
the said regulating control instruction is applied in order to make the fuel burn with the oxidant in the said combustion chamber.
2. Description of the Related Art
A large number of industrial processes are based on the combustion of various fuels such as natural gas, propane, fuel oil, etc. Among these processes, mention may particularly be made of processes for incinerating waste or for melting glass in a furnace.
Conventionally, for carrying out the combustion processes in these furnaces, air is used as the oxidant. The situation is then referred to as “air combustion”.
For glass furnaces, rotary furnaces or incineration furnaces operating with air as the oxidant, various processes for regulating the combustion are known. These are generally processes in which one or two variables connected directly with the combustion are measured, such as for example a roof temperature and/or the oxygen level of the outlet fumes of the furnace, and an empirical control taking the measurement results into account is applied in order to regulate the amounts of fuel and air injected into the furnace in order to sustain the combustion.
Recently, in order on the one hand to be able to comply with the constraints relating to new standards governing the emission, for example, of nitrogen oxides or carbon monoxide at the outlet of the furnace and, on the other hand, to increase the power of such furnaces, furnace operators are making increasing use, as the oxidant, of oxygen-enriched air or substantially pure oxygen (that is to say having an oxygen level higher than 90 mol %) instead of air. This situation is referred to as “oxy-combustion”.
However, the inventors have has become aware that the conventional processes mentioned above cannot provide satisfactory regulation of oxy-combustion. Indeed, the more the oxygen level in the oxidant moves away from a 25 mol % oxygen level, that is to say the closer the situation comes to so-called “all oxygen” combustion (that is to say combustion using substantially pure oxygen as the oxidant), the more conventional regulation becomes unsatisfactory.
By way of explanators, the invention have observed that the regulation of oxy-combustion is more difficult than that of air combustion because of the appearance of new parameters which have a non negligible effect on the combustion.
Thus, for example, they have found that in the case of oxy-combustion, fluctuations of the oxygen level in the oxidant have a non negligible impact on the power of the combustion, that is to say the amount of heat released by the flame.
Furthermore, given that in the case of combustion with substantially pure oxygen, the volume of fumes is reduced by approximately a factor of five compared with the volume of fumes from air combustion, the inventiors have found that inadvertent intake of air into a furnace can greatly interfere with the oxy-combustion and increase the emission of nitrogen oxides.
In addition, the fact that the temperature of the flames of oxy-combustion is generally higher than those of the flames of air combustion can accelerate wear of the refractory lining material of the inner walls of the furnace, depending on the nature of the molten material.
The object of the invention is therefore to take these observations by the inventiors into account in order to obtain more satisfactory regulation of the oxy-combustion in a furnace.
SUMMARY OF THE INVENTION
To that end, the invention relates to a process for combustion of a fuel with an oxidant whose oxygen level is more than 25 mol %, in a combustion chamber, in which, cyclically:
at least one principal variable representing the combustion in the said combustion chamber is measured,
as a function of the result of the measurement of the said at least one principal variable, a control instruction for regulating the flowrates of fuel and oxidant to be injected into the said combustion chamber is determined, and
the said regulating control instruction is applied in order to make the fuel burn with the oxidant in the said chamber, characterized in that
at least one secondary variable associated with an operational constraint of the said combustion chamber or with a perturbation in the operation of the latter is additionally measured, and
for the determination of said regulating control instruction, the measurement result of the said at least one secondary variable is also taken into account.
The process according to the invention may also have one or more of the following characteristics:
the said at least one variable associated with an operational constraint of the said chamber is a variable in the following group: the level of nitrogen oxides in the outlet fumes of the combustion chamber, the level of carbon monoxide in the outlet fumes of the combustion chamber and a wall temperature of the said combustion chamber,
the said at least one variable associated with an operational perturbation of the said combustion chamber is a variable in the following group: the nitrogen level of the fuel, the oxygen level of the oxidant and the operational state of at least one burner of the said combustion chamber,
a plurality of secondary variables associated with operational constraints of the said combustion chamber and/or with perturbations in the operation of the latter are measured and, for the determination of the said regulating control instruction, the measurement result of each secondary variable is taken into account,
the regulating control instruction is determined by a fuzzy-logic controller employing the steps consisting in:
converting each principal or secondary variable with a view to establishing its degree of membership in at least one state of a fuzzy variable,
applying preestablished rules between at least one state of a first fuzzy variable and a state of a second fuzzy variable in order to determine a control instruction for regulating the flowrates of fuel and oxidant to be injected into the said combustion chamber, and
applying the said regulating control instruction to make the fuel burn with the oxidant in the said chamber,
the input fuzzy variables are established on the basis of curves defined throughout the universal set of the principal and secondary variables.


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