US20120291679A1

US20120291679A1 - Method for correcting the combustion settings of a set of combustion chambers and apparatus implementing the method

Info

Publication number: US20120291679A1
Application number: US13/512,207
Authority: US
Inventors: Patrick Giraud
Original assignee: Five Stein
Current assignee: Five Stein; Fives Stein SA
Priority date: 2009-11-30
Filing date: 2010-11-26
Publication date: 2012-11-22
Also published as: EP2507551A1; BR112012013060A2; CN102686946B; KR20120106965A; CN102686946A; FR2953280B1; EP2507551B1; WO2011064752A1; FR2953280A1

Abstract

The invention relates to a method for real-time, continuous adjustment of a set of thermal combustion chambers (2, 2 i) having similar characteristics supplied by the same combustive agent (8) and fuel (9) networks, according to which one of the chambers (2 i) is used as a reference chamber, combustion parameters are measured (7) in the fumes output from the reference chamber, and the control parameters of all the chambers (2) are adjusted according to the combustion parameters measured at the output of the reference chamber (2 i), in particular according to the ratio of combustive agent to fuel and/or the composition of the combustive agent and/or fuel.

Description

The present invention relates to a method for the adjustment of devices for supplying a set of combustion chambers supplied with a combustion agent and a fuel, the properties thereof being variable. The invention enables possible fluctuations of said properties to be taken into account so as to maintain the desired quality of combustion.
Numerous industrial heating devices consist of a set of separate combustion chambers supplied by a common device for supplying combustion agent and fuel. For example, in the iron and steel industry the devices are furnaces for continuous processing lines for metal strips, provided with radiant tubes.
In many cases, the combustion agents or fuels used have properties which may vary over time. For example, when using enriched air or depleted air, or even when using fuels co-produced on site such as coke oven gas, steel furnace gas, a mixture of gases or even alternative fuels such as biogas or generator gas.
Numerous methods exist for correcting the regulation of a furnace according to the properties of the fuel. Said methods make it possible to correct the fluctuation of one of the principal parameters of the fuel, for example the calorific power PCI, the combustive power, the combustion index or Wobbe index.
Said methods do not permit the properties of the fuel and the combustion agent to be taken properly into account as, in the majority of cases, the sampling process modifies the properties of the fuel and the combustion agent. Said methods also have the drawback of having a relatively long response time and require devices for measuring in protected areas to be put in place.
A specific method is proposed by FR2712961. According to this method, a burner is controlled by removing a given quantity of fuel, said fuel being combusted in a dedicated chamber with a quantity of air resulting in combustion in excess air, and a variable reflecting the deviation from stoechiometry is measured in the fumes from the complete combustion of the fuel. This variable is then used for determining the representative value of the coefficient for regulating the quantity of combustion air from the burner. Said device for measuring the combustion consists of a combustion tunnel, a burner and appropriate measuring means.
Said system has the drawback of using a specifically constructed mini-furnace in which an air/fuel mixture is combusted in proportions which are controlled in order to provide an excess level of air. Said mini-furnace consumes fuel and has to operate with an excess level of air. The combustion is carried out in said mini-furnace at an operating point which may be very different from that of the apparatus to be regulated, for example in terms of the power of the burner, the air/gas ratio, the temperature of the furnace and the containment of the flame. The processing of the results thus requires the use of mathematical correction formulae.
Said system also has the drawback of being exclusively dedicated to taking into account variations in the properties of the fuel. It does not take into account variations in the properties of the combustion agent nor the combustion parameters appropriate for the monitored apparatus, namely the proportion of unburnt residues or even the proportion of nitrogen oxide NOx emissions.
The object of the invention is primarily to improve the control of the combustion of a set of combustion chambers involved in a manufacturing process; in particular, the invention aims to improve the control of a set of radiant tubes for a continuous processing line for metal strips.
Said combustion chambers are of the same type. The power of each of said combustion chambers may be variable but the technologies implemented are similar. The potential differences between said combustion chambers do not influence the combustion parameters which are desired to be monitored.
In order to ensure the control of the combustion of the set of radiant tubes, one of said tubes installed in the furnace shell is used as a reference tube. Said reference tube takes part in the heating of the strip, as do all the other tubes. Said radiant tube has a similar construction to that of the other tubes and is acted upon in a manner representative of the operation of the other tubes which take part in the heating process.
The combustion products at the outlet of the radiant tube are cooled by taking part in the process of heating the furnace. A measurement device is installed at the outlet of said reference tube, said measurement device providing information about the status of the combustion products downstream of the tube. Generally, said device is a gas analyzer which permits information, for example about the excess air and/or unburnt residues, to be obtained, in particular.
Thus, according to the invention, the method for real-time and continuous control of a set of thermal combustion chambers having similar characteristics, supplied by the same combustion agent and fuel networks, is characterized in that:

- one of the chambers is used as reference chamber,
- combustion parameters are measured in the fumes at the outlet of the reference chamber,
- the control parameters of all the chambers are adjusted according to the combustion parameters measured at the outlet of the reference chamber, in particular according to the ratio of combustion agent to fuel and/or the composition of the combustion agent and/or fuel.

The combustion parameters measured in the fumes at the outlet of the reference chamber may consist at least of the concentration of oxygen and/or the concentration of carbon dioxide and/or the concentration of hydrogen and/or the concentration of nitrogen oxides and/or the rate of unburnt residues.
The level of excess air may be determined according to the concentration of oxygen or carbon dioxide in the fumes at the outlet of the reference chamber. The lack of air may be determined by the concentration of carbon monoxide or hydrogen or unburnt residues in the fumes at the outlet of the reference chamber.
Advantageously, the control parameters for all the chambers are adjusted so as to obtain the desired level of excess air and/or rate of unburnt residues. It is possible to adjust the control parameters for all the chambers so as to obtain the desired lack of air and/or the rate of unburnt residues.
The concentration of nitrogen oxides may be regulated by adjusting the composition of the combustion agent and/or fuel, in particular by diluting with combustion products.
The thermal chambers may consist of radiant tubes supplied with proportions of gas which are different from those of the reference tube.
The invention also relates to an apparatus for implementing a method as defined above, comprising a set of thermal combustion chambers having similar characteristics, supplied by the same combustion agent and fuel networks, characterized in that:

- one of the chambers is used as a reference chamber,
- means for measuring the combustion parameters are installed at the outlet of the reference chamber to measure said combustion parameters in the fumes at the outlet,
- control means are provided to adjust the control parameters of the set of chambers according to the measures implemented at the outlet of the reference chamber, in particular according to the ratio of combustion agent to fuel and/or the composition of combustion agent and/or fuel.

The thermal chambers may consist of radiant tubes and a reference tube may be supplied together with the other tubes, the control members being placed on supply circuits common to all the tubes.
According to a further possibility, the radiant tubes are supplied separately from the reference tube and have different control members from those of the reference tube, on different supply circuits, the control members of the tubes being controlled such that said tubes are supplied with the same proportions of gas as the reference tube. According to a further possibility, the control members of the radiant tubes may be controlled such that said tubes are supplied with proportions of gas which are different from those of the reference tube, in particular with a different level of excess air.
Relative to the equipment used in the prior art, according to the invention a standard device is used as the combustion chamber, said device being similar to the others and being used in the method, and the control thereof being entirely representative of the operation of all the devices. In particular, it permits the parameters which depend on the mode of operation of the entire apparatus to be controlled.
Generally, the main parameter is the level of excess air measured by the concentration of oxygen. However, it may be necessary to monitor other parameters, for example the unburnt residues when the furnace is cold or even the nitrogen oxide emissions which may be regulated, for example according to variations in the properties of the combustion agent and/or fuel.
The method according to the invention also enables devices functioning in the absence of air to be regulated. In this case, the monitored parameter may be a component representing one of the gases representative of this combustion mode, present in a high proportion in the fumes. This gas may, for example, be carbon monoxide CO.
The reference tube is supplied by circuits supplying combustion agent and fuel provided with control members enabling the proportion thereof to be adjusted. Said reference tube may also be supplied by additional circuits, for example for combustion fumes or oxygen.
The analyses carried out on the combustion products at the outlet of the reference tube are taken into account to control the control members of the supply circuits so as to obtain the desired quality of combustion.
When the reference tube is supplied together with the other tubes, i.e. when the control members are placed on supply circuits common to all the tubes, including the reference tube, the other tubes benefit from this same control.
When the radiant tubes are supplied separately from the reference tube, i.e. they have control members on the different supply circuits, said control members are controlled such that said tubes are supplied with the same proportions of gas as the reference tube, or with a different proportion.
If the two sets of radiant tubes are designed to have identical controlled parameters, for example an identical supply pressure for a different operating power, whilst maintaining the same proportion of gas, the controlled parameters of the reference tube are reproduced for the members of the supply circuits of the second set of tubes.
If the two sets of radiant tubes are designed to have different controlled parameters, for example a different supply pressure, whilst maintaining the same proportion of gas, the controlled parameters of the reference tube are corrected for the members of the supply circuits of the second set of tubes. Said correction depends on the law of physics of the measured value, for example the variation in the flow rate according to the variation using the square of the differential pressure.
According to the invention, when the radiant tubes are supplied separately from the reference tube, their control members may be controlled such that said tubes are supplied with different proportions of gas from those of the reference tube, for example with a different level of excess air.
The control of the calorific requirement of the reference tube may be implemented by altering the flow rate or altering the duration. The control may be associated with that of other radiant tubes or it may be separate.

Apart from the arrangements set forth above, the invention consists of a certain number of other arrangements which will be referred to in more detail hereinafter with reference to embodiments, disclosed with reference to the accompanying drawings, but which are in no way limiting. In the drawings:

FIG. 1 is a schematic view of a continuous treatment line for metal strips,

FIG. 2 is a schematic view of a radiant tube,

FIG. 3 shows an example for supplying a pair of radiant tubes,

and FIG. 4 is a second example for supplying a set of radiant tubes.

With reference to FIG. 1 of the drawings it is possible to see a furnace 1, shown schematically, provided with radiant tubes 2 providing the heating of a metal strip 3, passing along rollers, not shown. The heating is carried out in a protective atmosphere, generally composed of a mixture of nitrogen oxide and hydrogen. In each radiant tube, the combustion is ensured in a confined combustion chamber, separated from the furnace shell by the tube. Thus indirect heating takes place.
With reference to FIG. 2 of the drawings, it is possible to see a U-shaped radiant tube 2, shown schematically. The radiant tubes may have other shapes, for example they may be I-shaped, P-shaped, double P-shaped or W-shaped. A burner 4 is located at one end of the tube 2. The flame 5 is propagated in the tube and releases its energy toward the walls of the tube which heats the shell and the strip. As they pass into the tube, the fumes thermally dissipate on the walls of the tube. A heat recovery system, not shown, enabling the combustion air to be preheated is generally positioned at the end of the tube opposing the burner. At the outlet of the tube, the partially dissipated fumes are evacuated toward a collector 6 connected to a flue. An analyzer 7 positioned downstream of the outlet of the tube, permits the analysis of the combustion products.
With reference to FIG. 3 of the drawings, it is possible to see an embodiment of the circuit for supplying combustion agent and fuel to a set of U-shaped radiant tubes, including a reference tube 2 r, shown schematically. The burners 4 are supplied by a system for supplying combustion agent 8, with a control valve FCVc and a system for supplying fuel 9 with a control valve FCVf. The automatic control of the valves may be ensured by an automatic system (not shown) which receives at the inlet information about the combustion parameters measured by the analyzer 7 at the outlet of the reference tube 2 r. The automated system provides, at the outlets connected to the controls of the valves, specific instructions for control as a function of the input data.
In this embodiment, the furnace zone is provided with radiant tubes 2, the burners 4 thereof being supplied in parallel by common combustion agent and fuel networks. The variation in the calorific requirement is controlled by the operating times of the burners, i.e. for each burner the proportion of the duration of opening of its combustion agent valve FCVc and its fuel valve FCVf during a given time.
Each of the burners has been previously controlled during commissioning for a known operating point by means of manual flow limiters 10. For a given operating point, for example at 100% power, each of the burners has been previously controlled by an individual flow limiter device. In these conditions, for a given general operation of the furnace zone, the operation of a tube selected as a reference tube generally illustrates the operation of the set of burners connected to the same supply systems. By modifying the settings of the system for supplying the reference tube, i.e. the valves FCVc and FCVf, to take into account the change in the characteristics of the fuel or combustion agent, the control of all the radiant tubes is modified.
The control of the valves FCVc and FCVf causes a variation in the pressure of the combustion agent Pc or the pressure of the fuel Pf, enabling the proportion of the gases to be adjusted to be identical with all the tubes.
The measurements carried out by the analyzer 7 placed at the outlet of the reference tube may, for example, be limited to two parameters: the oxygen content which represents the level of excess combustion agent and the proportion of unburnt residues, the quantity thereof resulting in corrections to the level of excess air, in particular at low temperatures.
With reference to FIG. 4 of the drawings, it is possible to see a furnace provided with U-shaped radiant tubes, shown schematically.
In this apparatus, a set of four radiant tubes 2 is supplied by combustion agent 8 and fuel 9 networks common to the set of four tubes by means of the control valves FCVc and FCVf. In this apparatus, a reference radiant tube 2 i is supplied separately by its own system for supplying combustion agent 8 i regulated by a valve FCVci and fuel 9 i regulated by a valve FCVfi.
According to a preferred embodiment of the invention, the transmitters used on the reference tube 2 i and the set of tubes 2 to be regulated, make use of the same laws of physics. For example, it is possible to use a differential pressure transmitter to measure the flow rate.
At a given operating point, for example at 100% power, each of the burners, including that of the reference tube, has been previously controlled during commissioning by a separate flow limiter device 10, in particular by manual control, so as to obtain the same ratio of combustion agent/fuel at each burner. This control is carried out in the same conditions for all the tubes, i.e. with the same quality of combustion agent and fuel.
It should be noted that each of the tubes may have a nominal power and/or different dimensions. The tubes may also be of different shapes, for example they may be U-shaped or W-shaped.
In control mode, the flows of combustion agent and fuel from the reference tube are adjusted so as to have the correct rate of oxygen in the fumes according to variations in the properties of the fuel and/or the combustion agent. The analyzer 7 enables the valves FCVci and FCVfi to be controlled so as to obtain the desired combustion quality in the reference tube. The corresponding operating point measured by the pressures Pci and Pfi defines the control variables Pc and Pf used to control the valves FCVc and FCVf.
As the settings of the reference tube 2 i are representative of the desired combustion at all the tubes, the values measured at its supply system are used to govern the regulation of the set of tubes 2.
For example, the fuel valve FCVf will be controlled according to the calorific requirement and the combustion agent valve FCVc will be controlled so that the value Iva of the measurement signal which is representative of the flow of combustion agent at the set of tubes to be regulated is defined according to the following relation:
Iva/Ivf=K×Ivai/Ivfi
In this relation, K is a constant value, Ivai expresses the measurement signal which is representative of the combustion agent at the reference tube 2 i, Ivfi expresses the signal of the measurement which is representative of the fuel at the reference tube 2 i and Ivf expresses the measurement signal which is representative of the fuel at the set of tubes 2 to be regulated.
The advantage of this system is that it enables corrections to be easily made to the regulation of a heating system. The response time is thus very short and the control parameters are entirely representative of the desired control.
An extension of this application is to control an apparatus of which the composition of the combustion agent is variable. This variation may be unintentional, for example the composition of the air is dependent on the humidity content.
This variation may be intentional, for example by modifying the rate of oxygen of the combustion agent. Thus, oxygen enrichment may be carried out to increase the output of a furnace, reduce the consumption of fuel or reduce CO2 emissions. Oxygen depletion may be implemented in order to modify the thermal transfer, for example by extending the flame, or to reduce the NOx emissions. In this application, the measurement of NOx in the fumes of the reference tube serves to regulate the rate of dilution of the combustion agent.
The invention makes it possible to adjust the settings which may be different from those of the reference burner. According to the invention, the flame develops in a reference chamber which is similar to the other chambers, but not in the open air.
The combustion in a chamber is significantly influenced by the geometry thereof. Said geometry dictates the containment of the flame, the nature of the flow of gas, the recirculation of part of the fumes and the temperature cartography in the chamber. All these parameters influence the combustion, in particular the temperature of the flame.
The results of the combustion measured at the outlet of the reference chamber are thus directly representative of the combustion as produced in the other chambers.

Claims

1-12. (canceled)

13. A method for real-time and continuous control of a set of thermal combustion chambers (2, 2 r, 2 i) having similar characteristics, supplied by the same combustion agent (8) and fuel (9) networks, the method comprising:

using one of the chambers (2, 2 r, 2 i) as a reference chamber;

measuring combustion parameters (7) in the fumes at an outlet of the reference chamber; and

adjusting control parameters of all the chambers according to the combustion parameters measured at the outlet of the reference chamber, in particular according to the ratio of combustion agent to fuel and/or the composition of combustion agent and/or fuel.

14. The method as claimed in claim 13, wherein the combustion parameters measured in the fumes at the outlet of the reference chamber include at least a concentration of oxygen and/or a concentration of carbon dioxide and/or a concentration of carbon monoxide and/or a concentration of hydrogen and/or a rate of unburnt residues and/or a concentration of nitrogen oxides.

15. The method as claimed in claim 14, wherein the level of excess air is determined according to the concentration of oxygen or carbon dioxide in the fumes at the outlet of the reference chamber.

16. The method as claimed in claim 14, wherein the lack of air is determined by the concentration of carbon monoxide or hydrogen or unburnt residues in the fumes at the outlet of the reference chamber.

17. The method as claimed in claim 13, wherein the control parameters of all the chambers are adjusted so as to obtain the desired level of excess air and/or rate of unburnt residues.

18. The method as claimed in claim 16, wherein the control parameters of all the chambers are adjusted so as to obtain a desired lack of air.

19. The method as claimed in claim 14, wherein the concentration of nitrogen oxides is regulated by adjusting the composition of the combustion agent and/or fuel, in particular by diluting with combustion products.

20. The method as claimed in claim 13, wherein the thermal chambers comprise radiant tubes, one of the radiant tubes being a reference tube, and the other radiant tubes being supplied with proportions of gas which are different from those of the reference tube.

21. An apparatus for implementing a method as claimed in claim 13, comprising a set of thermal combustion chambers (2, 2 r, 2 i) having similar characteristics, supplied by the same combustion agent (8) and fuel (9) networks, wherein:

one of the chambers (2, 2 r, 2 i) is used as a reference chamber,

means for measuring (7) the combustion parameters are installed at the outlet of the reference chamber to measure said combustion parameters in the fumes at the outlet,

control means (FCVc, FCVf) are provided to adjust the control parameters of the set of chambers according to the measures implemented at the outlet of the reference chamber, in particular according to the ratio of combustion agent to fuel and/or the composition of combustion agent and/or fuel.

22. The apparatus as claimed in claim 21, wherein the thermal chambers comprise radiant tubes, wherein a reference tube (2 r) is supplied together with the other tubes, the control members (FCVc, FCVf) being placed on supply circuits (8, 9) common to all the tubes.

23. The apparatus as claimed in claim 21, wherein the thermal chambers comprise radiant tubes, wherein one of the radiant tubes is a reference tube (2 i), and wherein the other radiant tubes are supplied separately from the reference tube (2 i) and have different control members (FCVc, FCVf) from those (FCVci, FCVfi) of the reference tube (2 i), on different supply circuits (8, 9, 8 i, 9 i), the control members (FCVc, FCVf) of the tubes (2) being controlled such that said tubes (2) are supplied with the same proportions of gas as the reference tube (2 i).

24. The apparatus as claimed in claim 23, wherein the control members (FCVc, FCVf) of the radiant tubes (2) are controlled such that said tubes are supplied with proportions of gas which are different from those of the reference tube (2 i), in particular with a different level of excess air.