EP0479378B1

EP0479378B1 - Fluidized bed furnace with internal gas combustion

Info

Publication number: EP0479378B1
Application number: EP19910202512
Authority: EP
Inventors: Jozef Weedaeghe
Original assignee: Bekaert NV SA
Current assignee: Bekaert NV SA
Priority date: 1990-10-02
Filing date: 1991-09-27
Publication date: 1995-07-12
Anticipated expiration: 2011-09-27
Also published as: DE69111173T2; EP0479378A1; DE69111173D1

Description

The invention relates to a fluidized bed furnace for internal gas combustion. As is known, a fluidized bed furnace comprises a container that is filled to a certain height with granules that form the fluidized bed. The granules are inert to high temperatures of 1500°C and more. At the bottom of the granule bed, there is an inlet adapted for blowing a carrying gas upwards into the bed, with an input flow that is as equally as possible distributed over the bottom surface of the bed. Between a minimum and maximum blowing speed, the granules come to whirl up and down and the bed swells up so as to behave like a fluid in which a body can easily be immersed or which can easily be traversed continuously by a body such as by metal wires or wire mesh. This fluid then has a high heat exchange coefficient with such bodies, which already comes near to the coefficient for liquids, as in lead or salt baths, and owing to the great mobility of the granules, the heat is very rapidly distributed over the bed. A fluidized bed is consequently very adapted for heat treatment of such bodies. Typical granule materials are silica-, alumina- or zirconiasand, silicon carbide or ferrosilicon, and typical granule dimensions lie in the range between 0.03 and 0.5 millimeter, preferably between 0.1 and 0.3 millimeter. The blowing speed into the bed depends on the chosen type of granule, and typical speeds lie in the range between 0.06 and 0.15 meter per second. At a too low blowing speed, the fluidized bed collapses, and gas bubbles, bubbling up from the bed are only obtained. At a too high blowing speed, the granules are blown out of the bed, so that in both cases there is no fluidization.
Because of this behaviour as a fluid, a fluidized bed is often used for the warming up of bodies in the fluidized bed, in cases where the heat delivery has to be distributed as equally as possible over the surface of the body that has to be warmed up. The heat source can be located outside said container. In that case, the heat has first to pass, by conduction, through the container wall before it can be delivered to the bed (external heating). But the heat source can also be located inside the container, in which case the heat is directly delivered to the whirling granules (internal heating). In the latter case, the heat source can be an electrical resistance, but it can also be a gas flame, when a fuel gas is blown in the bed together with oxygen, and burns in the bed. It is also possible to burn a gas outside the container and blow the flue gases thereof as a carrying gas into the bed. The drawback of this latter possibility, as compared to combustion inside the fluidized bed itself, is that a separate fireproof combustion room must be provided with refractory connection ducts towards the fluidized bed. When using a gas flame inside the fluidized bed itself, the heat is very rapidly and equally distributed over the granules of the bed. Such a fluidized bed with internal gas flame is what is called hereinafter a fluidized bed with internal gas combustion.
An application for such fluidized bed furnace lies in the field of heat treatment of metallic products, especially where a fluidized bed temperature is necessary above the self ignition temperature of conventional fuel gas (i.e. above about 750°C), so that accidentally extinguished flame parts immediately come to inflame again, whereby a stable flame can be maintained.
A fluidized bed furnace with internal gas combustion comprises a gas supply arrangement that comprises the necessary means to send a given flow of carrying gas, with a given composition, towards the inlet for the carrying gas at the bottom of the fluidized bed. The outlet of that gas supply arrangement is consequently connected with the inlet for the carrying gas. For the carrying gas, in general, a mixture is used of the fuel gas that has to be burnt, and of an insufficient quantity of primary air, i.e. a quantity below the necessary proportion for ignition. At a certain height above the bottom of the fluidized bed container, an additional quantity of secondary air is blown in, so that the flame begins at the height at which the mixture reaches said necessary proportion for ignition. In this way it is avoided that, at a low flow rate of carrying gas, the flame would enter into the gas supply arrangement. This gas supply arrangement for making up the gas mixture consequently comprises a first inlet that has to be connected with a source of fuel gas, e.g. a gas pipe under pressure, or a chamber in which the fuel gas is produced. That supply arrangement also comprises a second inlet that has to be connected with an air source, e.g. an air duct where the air is blown, by means of a ventilator, towards the gas supply arrangement. The temperature in the fluidized bed is regulated from a sensor of the temperature, and correction is given by means of a control device which, in dependence on the deviation of the measured temperature from the desired reference temperature, makes that more or less fuel gas is supplied.
In general terms, the invention consequently relates to a fluidized bed furnace for internal gas combustion, having a fluidized bed container of which the inlet for the carrying gas is connected with a gas supply arrangement that comprises a first and second inlet for fuel gas, respectively air, and that comprises a sensor of the temperature in the fluidized bed, said sensor being connected with the input of a control device.
With such fluidized bed furnaces, a difficulty exists in keeping the fluidized bed operation parameters at a constant level, which have to guarantee a continuously equal treatment, and consequenty continuously equal properties of the treated products. This is especially important in applications where the furnace is arranged in a continuous production line, and the product is continuously passed through the furnace, such as in the case of steel wire.
A first important parameter of operation is the temperature T in the fluidized bed. This temperature has to be regulated in order to be maintained at a desired reference value. Otherwise, and without any control, the temperature can deviate too far from the desired temperature, for instance when the production line speeds up or slows down, or when there is a switch-over towards another product, so that the heat that has to be taken up by the product is considerably changed, or when there is a switch-over towards another fuel gas with more or less combustion heat, or when fuel gas is received with largely fluctuating combustion heat. For that reason, it is necessary that the control device would be able to vary the supply of the fuel gas between very large limits, and even towards zero, in order to be able to react very strongly on the temperature variations.
A second important parameter of operation is the degree of oxydation/reduction of the combustion gas athmosphere above the flame. Sometimes a slight oxydation is desired at the surface of the steel wire, but in that case always to the same degree, or a slight reduction (e.g. decarbonization of steel), but then also always to the same degree. In most cases, a neutral athmosphere is desired, where the combustion gas athmosphere is the product of a complete combustion without excess of oxygen, and has to be maintained as such.
A third important parameter of operation is the condition of fluidization of the fluidized bed. It is not sufficient that the upward speed of the blown-in carrying gas would not come below the fluidization limit. It is also necessary that the pattern of the gas stream in het fluidized bed should be stable, so that a constant flame pattern can be maintained. It must also be avoided as much as possible that the flame would go up and down in dependence on the content of fuel gas in the carrying gas, or on the inlet speed of the latter. And it must especially be avoided that the flame would come down to the bottom of the fluidized bed container, which would then be damaged by heat, and further penetrate upstream into the supply arrangement.
In order to meet these conditions for these three parameters, it is known to control the furnace arrangement as described hereabove by means of the flow rates Q of fuel gas, of the primary and of the secondary air. When, for instance, the temperature raises, then a constrol system makes that less fuel gas is sent to the supply arrangement for the carrying gas. As the speed of the blown-in carrying gas has to remain constant, the control system will compensate the reduction of volume flow rate of fuel gas, by increasing the volume flow rate of primary air by a same quantity. But, as the degree of oxydation/reduction of the flue gas has to remain constant, the flow rate of secundary air shall have to be strongly reduced by the control system, in order to keep the total quantity of air, primary and secundary, in the same proportion with respect to the reduced quantity of fuel gas. The inverse occurs when the temperature falls down.
With this method of control, the three fluidized bed operation parameters cannot simultaneously be regulated in order to be maintained each at a desired reference value. When the temperature is regulated by means of steering the fuel gas flow rate, then the total flow rate of the primary and secondary air has to be adjusted in proportion, and then no constant gas stream pattern can be maintained. Moreover, in this method, the proportion varies between the primary air and the fuel gas, so that the flame goes up and down under such variations, and can penetrate inside the supply arrangement when the proportion comes below the ignition limit. As a consequence, this method does not allow very broad limits between which the fuel gas flow rate can be steered. Always a minimum flow rate of fuel gas is needed in order to deliver, together with the primary air, the necessary constant flow rate of carrying gas. And, at any rate, the carrying gas has to be blown in at a sufficient speed and in a sufficient concentration of fuel gas in order not to be inflammable and not to penetrate upstream into the gas supply arrangement. The fuel gas has also a maximum flow rate, above which so much secondairy air has to be supplied, which is then insufficiently mixed with the carrying gas, in such a way that locally varying oxydation problems arise. In this way, it is not possible to simultaneously regulate the three fluidized bed operation parameters to a desired level each, without having to abandon the control of one of these, or to concede something with respect to two or all three of them.
It is a purpose of the present invention to provide a fluidized bed furnace for internal gas combustion, having a control system allowing to regulate, i.e. to maintain at a constant reference value or to make them independently follow the course of a varying reference value, the said three fluidized bed parameters: fluidized bed temperature T, the proportion air/fuel gas (L/G), and the flow rate Qc of the carrying gas.
According to the invention, the gas supply arrangement comprises, in addition to the two inlets for air and fuel gas, a further third inlet for flue gas, and each of said three inlets comprises a supply device adapted for steering the corresponding inlet flow rate and having a steering signal input, connected with the output of said control device, said control device being arranged for steering the flow rate of said three inlets in a way as to maintain the carrying gas flow rate, the ratio air flow rate to fuel gas flow rate, and the temperature of the fluidized bed each at a respective reference value.
In the systems above with fuel gas and air, without flue gas, it is in principle not possible to regulate three variables (temperature, proportion air/fuel gas, flow rate of carrying gas) towards a reference value, because only two independent steering variables are available: fuel gas flow rate and air flow rate. A third steerable variable is necessary. Dividing the air flow in two steerable flow rates of primary and secondairy air is a deficient solution, because both variables are not independent from each other. But when, according to a first step towards the invention, the flow rate of flue gas is added as a really independently steerable variable, then it becomes possible to design a control system in which three variables can be regulated, each to its own independent reference value. As a consequence, in a second step towards the invention, the opportunity is taken of this possibility offered by said first step, and a control device is added to the system, in which these three flow rates are used as independently steerable variables in order to control three other variables that depend thereon: temperature of the bed, air to fuel gas proportion, and flow rate of carrying gas, by which the operating conditions of the fluidized bed are the determined.
In such control device, the three inlets are consequently provided each with a supply device of which the flow rate can be steered. In general, these will be supply valves with steerable opening and having a steering signal input for receiving the signal for steering the opening. The control device shall have to take into account that the flow rate is determined not only by the opening, but also by the pressure drop over the valve. This can be arranged, either by the control device keeping constant the pressure drops in the gas supply system (so that a steering signal that corresponds to a desired opening also univocally corresponds to a desired flow rate), or by the fact that each valve is provided by its own flow rate control system that adapts the actual flow rate to a steering signal that is representative of the desired flow rate), or by a combination of both methods, depending of the design of the control device. It consequently also depends on the design of the control device whether the steering signal, that is transmitted from the output of the control device towards the input of the supply arrangement, will be representative, either of the desired opening, or of the desired flow rate, or of a combination of both.
The manner in which the flow rate Qc of the carrying gas will be regulated to a reference value, will similarly also depend on the design of the control device, as well the way for measuring the actual value that has to be regulated, as the way for readjusting that value. When the total flow rate deviates from the reference value, then this can be observed by the device, either via addition of the three measured flow rates, or by measurement of the total flow. The latter measurement can be made, either by measuring the pressure drop over a narrowing of the flow duct, or by measuring the overpressure, above athmospheric, of the gas on its transition from the supply arrangement towards the inlet for the carrying gas in the fluidized bed, because this overpressure is a function of the flow rate of the carrying gas. Further, when the flow rate deviates from the desired reference value, then the correction can be given, either to the flow rate of one of the components, or to two of them, or to all three in the desired proportion.
Similarly, the ratio air flow rate to fuel gas flow rate (L/G) can be regulated in different ways, depending on the design of the control device. For instance, the device may measure the actual proportion (e.g. by division of the two measured inlet flow rates), and compare this to a reference value, in order to produce a signal for readjustment. But the device may for instance also multiply one measured inlet flow rate with the reference-ratio, and use the result as a reference value for steering the other inlet flow rate.
The reference values for the three magnitudes (temperature, flow rate of the carrying gas and air to fuel gas ratio), are desired values towards which the actual values are regulated by the control device. Each reference value can be a constant value, determined by the internal parameters of the device, but shall preferably be an externally adjustable value, that is adjusted when switching over from one product or production manner to another. It can however also be a steerable value, that is steered by an input signal that is transmitted towards the control device from another control device, that continuously adjusts the reference value in function of, for instance, continuous measurements of the exit properties of the product. The regulation towards the reference value can be done, as is known in control engineering, either by the proportional, or the proportional-integral, or the proportional-integral-differential principle.
The control device can be carried out either in an analog or in a digital manner, for instance, by a computer, in which the functions are not performed by separate components, but by programmes. The measurements, signals and corrections can be continuous or intermitting.
A great many of mathematical control concepts are possible, an many ways of carrying them out. The important thing herein is however, that they regulate the temperature, the flow rate of the carrying gas, and the air/fuel gas ratio towards a respective reference value, by means of the steering of three flow rates: of the fuel gas, of the air, and of the flue gas.
According to another aspect, the invention also relates to a process for steering a fluidized bed furnace with internal gas combustion, in which a carrying gas is blown into the fluidized bed and has, as a first and second component, a quantity of fuel gas and of air respectively, and in which the temperature of the fluidized bed is regulated on a reference value.
The process according to the invention is here characterized by the fact that said carrying gas comprises, as a third component, a quantity of flue gas, and that said carrying gas is supplied from three sources of said three components respectively, the flow rate of each of which being steered so as to regulate, simultaneously with the temperature, also the carrying flow ratio and the ratio of air flow rate to fuel gas flow rate, on a respective reference value.
Preferably, the combustion gases of the fluidized bed itself are used as a source for said third component.
The flue gas that is blown in must not necessarily be neutral as to its oxydation/reduction decree, i.e. to be the procuct of a complete combustion without excess of oxygen. As a function of the desired oxydation/reduction degree of the atmosphere created by the combustion in the fluidized bed, and of the content of oxygen and CO of the flue gas, the reference value for the air to fuel gas ratio can be adapted and continuously regulated, so that the desired oxydation/reduction degree is obtained after combustion.
The regulation is however very simple when the combustion athmosphere of the fluidized bed has to be neutral, and the flue gas is also derived from above the same fluidized bed. In that case, the reference value for the air to fuel gas ratio is set at the stoechiometric ratio air/fuel gas, and must not further be adapted in function of the oxydation/reduction degree of the flue gas. This reference value shall however possibly have to be set in function of the fuel gas in use, and have to be readapted when the composition of the fuel gas fluctuates.
The invention will here further be explained with reference to two figures, given by way of example, and of which:

Figure 1 is a schematic view of an embodiment, showing the principle of the invention;
Figure 2 is a particularly simple embodiment of the control device.

Figure 1 shows a fluidized bed furnace according to the invention, adapted for continuously austenitizing steel wires in the patenting heat treatment operation, although the invention is not limited to this application. In the patenting operation, the steel wires are firstly heated up to a temperature in the range from about 900 to about 1050°C, and directly thereafter quenched to a temperature in the range around about 550°C, e.g. in a quenching bath or in a fluidized bed. This is in general a continuous operation, in which the wires that have to be treated are continuously guided side by side and in parallel in axial direction of the wire through the heating furnace and the quenching device. In this example, the invention is applied to a heating furnace that is carried out as a fluidized bed furnace, that is consequently provided with the necessary access openings an wire guiding means (not shown), in order to guide the wire 33 in and out the fluidized bed. A typical bed height ranges then around 40 to 60 centimeter.
The furnace comprises a container 1, made of refractory material. At the bottom of the container, there is a distribution chamber 2 that extends over the whole bottom surface of the container. The distribution chamber has a horizontal upper wall. This wall is traversed by a multitude of small vertical gas pipes 3, that project for about 6 centimeter above that wall. They serve to blow the carrying gas vertically upward. They are equally distributed over the upper surface of the distribution chamber, so that the flow of carrying gas is about equally distributed over the horizontal cross-section of the container. These pipes are carried out as explained in European patent N° 181 653.
The function of the distribution chamber is to take care that the carrying gas would be blown in under the same pressure over the whole bottom surface of the fluidized bed, and this is obtained by the fact that the carrying gas comes from a common room 2 at the same pressure. In most cases, the carrying gas is not blown in through gas pipes, as in this example, but through a ceramic floor that separates the distribution chamber from the fluidized bed above, and that is porous or has a multitude of openings that are equally distributed over its surface. By using a pipe system however, of which the exit orifices are located at a given level above the bottom of the bed, there remains a layer of non-fluidized granules at the bottom, that protects the latter from overheating when the flame would sink too much, and this layer may, if desired, comprise cooling pipes. This provides the possibility to use, instead of a ceramic floor, an easily replaceable steel plate box 2, as the carrying gas in the distribution chamber is still cold. Although this way of carrying out the distribution chamber and the pipe system for blowing in the carrying gas can very advantageously be applied in an installallation according to the invention, the invention is not to be limited thereto. Each sort of inlet to the fluidized bed is usable, even without distribution chamber, but where the carrying gas is blown in upwardly and with a flow that is equally distributed over the horizontal cross section of the bed. With a pipe system, the pressure for blowing in can approximately be made equal over the pipes, by supplying them from ducts that do not lie in series, but are connected in parallel.
In the container 1 above the distribution chamber 2, there is a bed (4) of whirling granules of alumina sand (Al₂O₃) of a size of about 250 micron. And due to a carrying gas stream having an upward speed of about 13 m/sec, the bed is kept in fluidized state.
In addition to the distribution chamber 2, the supply arrangement for the carrying gas also comprises a mixing chamber (5) with three entries 6, 7 and 8, respectively for air, fuel gas and flue gas. A supply valve, respectively 9, 10 and 11, is associated to each of these entries, the inlet flow rate of each valve being steerable in response to a steering signal that is entered at a steering signal input 12, 13 and 14 respectively. This signal can be a pneumatic, mechanical or electrical signal, either analog or digitally coded. The steering signal input shall consequently be adapted to the sort of signal that it has to receive.
The supply valve for the air is connected at its inlet 22 to an air source 15 that is at sufficient overpressure with respect to the mixing chamber 5, e.g. 1500 mm above athmospheric pressure, so that the air can be blown towards that mixing chamber with the desired flow rate. This pressure is obtained by sucking the air through ventilator 15, possibly preceded by a filter. The same applies to the supply of the flue gas towards the inlet 24 of supply valve 11 via ventilator 17. This ventilator is connected with an outlet 18 for the combustion gases of the fluidized bed itself, via a cyclone 19, a heat exchanger 20 for cooling the combustion gases towards a temperature that is no longer detrimental to the construction of the gas supply arrangement (e.g. 200°C to 250°C), and a filter 21 that, together with cyclone 19, separates the sand, that is dragged along with the combustion gases, from those gases. The supply valve for the fuel gas is connected at its inlet 23 to a fuel gas source under sufficient pressure (not shown), e.g. a gas pipe 16 supplying propane or natural gas.
The steering signal inputs 12, 13 and 14 for the supply valves are connected with the output 26 of a control device 25. As the nature of the steering signals can be mechanical as well as pneumatic or electrical, analog or digitally coded, said output has to be adapted to the nature of the signal that it has to send out.
In this schematic embodiment showing the principles of the invention, the mixing of the three gases is carried out in one single mixing chamber 5. But it is also possible to mix two gases first, e.g. fuel gas with air, at a first location of confluence that must not necessarily be a chamber, and to mix then the third gas, at a second location of confluence, together with the mixture of the first two gases. It must be taken into account that, at such a location of confluence of two or three gases, the flow rate of each gas is not only determined by the opening of the supply valves, but also by the pressure drop between the inlet of each valve and the mixing chamber. The steering of each of said flow rates shall consequently have to take this pressure drop into account. For that reason, in the embodiment according to Figure 1, the inlet pressure of each gas is measured at the inlet locations 22, 23 and 24, and also the pressure in the mixing chamber 5, and the measurement signals are transmitted towards an input 27 of the control device. The nature of these signals can be mechanical, as well as pneumatic or electrical. In this control device, everything is calculated and steered from a central control device 25 that treats these pressure signals together with the other input signals so as to create output signals that are then to be transmitted towards the steering signal inputs of the supply valves. But, as will appear from a further example, it is clear that a more decentralized supply device can be designed, in which these pressure drops are taken into account in another way.
The control device further comprises a sensor 31 of the flow rate Qc of the carrying gas that is sent to the fluidized bed chamber, and the output of that sensor is connected with the input 28 of the control device. This sensor can for instance be a meter of the pressure difference between both sides of a diaphragm, where this voltage difference is representative of the volumetric flow rate. As the pressure losses of the gas stream, between the location of sensor 31 and the exit from the fluidized bed into the athmosphere, are constant when a constant stream pattern is maintained, the overpressure above atmospheric can also be representative of this volumetric flow rate, and consequently, a sensor of this overpressure can also be used. In that case, the control device shall keep this overpressure at a constant value, e.g. at 800 mm.
The control device further comprises another sensor 32 of the temperature T in the fluidized bed, and of which the output is connected with the input 29 of the control device. Such sensor can, for instance, be a thermocouple. The input of the control device is further provided with an adjustment button 30, where the desired ratio of air to fuel gas (L/G) can be preset. In this case, the desired ratio is determined by the position of the button. But this desired ratio can also be introduced as a signal entering via a signal input, when this ratio has to vary frequently without any human intervention. When the system only works at a fixed value, e.g. the stoechiometric ratio, then such preset button can be left out, and the desired ratio can then be determined by the internal parameters of the control circuit.
The fluidized bed is traversed by steel wires 33, perpendicular to the plane of the figure, that continuously travel through the bed in their longitudinal direction.
The control device 25 is presented in this example as a black-box, because it can be designed in various ways, pneumatically, electrically, digital or analog, with continuous or intermitting signals, steered by a computer programme or not. The important thing in this invention is not how the regulation is realized, but what the variables are that are regulated, and by means of what independently steerable variables. The function of the control device lies herein, that the flow rate Qc (volumetric) of carrying gas, the fluidized bed temperature T, and the ratio air flow rate to fuel gas flow rate (L/G) are each regulated on a respective reference value, and this by means of three steering variables that have to achieve the reduction of the deviations from the reference values: the flow rates of fuel gas, of air, and of flue gas. Different concepts are possible for the control device, where some types may aim at simplicity, others at accuracy or reaction speed, and still others at safety. In general, such design lies in the normal activity of the man, skilled in control engineering, although particularly ingenuous concepts may exist.
So is Figure 2 showing a very simple concept for the control device. In this latter figure, not all elements of the fluidized bed, as they appeared in Figure 1, are shown again, but the elements that also appeared in Figure 1 are referred to by means of the same reference numbers. In this system, there is no mixing chamber, but the fuel gas, as delivered from gas pipe 16 and via duct 37, flows together first with the air, the latter being delivered via ventilator 15, and supplied via duct 36 to a first location of confluence. The obtained mixture of fuel gas with air is then led to a second location of confluence, where the mixture flows together with the combustion gases of the fluidized bed chamber, as delivered by ventilator 17 and supplied via duct 38 to said second location of confluence.
The flue gas duct runs from ventilator 17 via a steerable control valve 51 towards duct 38. This control valve 51 is steered from a sensor 61 of the gas pressure (overpressure above athmospheric) in the supply duct 40 that goes from said second location of confluence towards the distribution chamber 2. The measuring signal of this gas pressure is supplied towards a control circuit 62, that generates a correction signal, according to the proportional, integral or differential principle or a mixture thereof, as known in control engineering. This correction sinal is the led as a steering signal towards control valve 51, and the operation is such, that the gas pressure (overpressure) in supply duct 40 is maintained at a constant value.
The duct for the fuel gas runs from gas pipe 16, via differential control valve 48 and, in series and downstream therewith, via steerable control valve 50 towards duct 37. The differential valve 48 serves to make the steering of the opening of control valve 50 independent from the overpressure at which the fuel gas is supplied from gas pipe 16. This differential valve is in the form of a membrane valve, where at one side of the membrane the pressure is brought that exists in the supply duct 40 towards the distribution chamber, and at the other side, the pressure is brought that exists just before control valve 50. The operation is such, that the valve, in function of the difference between both pressures, is set more or less open, so that this pressure difference is kept at a constant value. As the pressure in supply duct 40 is regulated in order to keep a constant value, the pressure just before control valve 50 is also constant, and then, the pressure existing after that latter valve is then a value that is representative of the flow rate. The control valve 50 is steered from a sensor 32 of the fluidized bed temperature, in a way that it closes more or less, according as the temperature comes above, respectively below, the reference value.
The air duct from ventilator 15 similarly runs via a differential control valve 47 and, in series and dowstream therewith, via steerable control valve 49 towards duct 36. The differential control valve 47 serves to make the steering of the opening of control valve 49 independent from the pressure drop over that valve, so that a given opening corresponds to a given flow rate. The operation is analog to the operation of differential control valve 48, and such that the pressure drop over control valve 49 remains constant. This control valve 49 is steered from a sensor 46 of the pressure in the fuel gas duct 37 (this is a measure for the fuel gas flow rate als explained above), and in a manner that control valve closes more or less according as the pressure drops more or less, and in a way that the ratio air to fuel gas flow rate is maintained at a constant value.
The operation of this control device is as follows: when the temperature T in the fluidized bed exceeds the reference value, then this is announced by sensor 32 towards the control valve 50, that will close somewhat more, in order to reduce the combustion. But then the air to fuel gas ratio is too high. By the fact however that the control valve 50 closes somewhat more, the pressure, as measured by sensor 46, will drop, and this makes control valve 49 to close more, to that extent as to bring the ratio air to fuel gas back to the reference value. But, as to the flow rates of air and fuel gas have dropped, the total flow rate Qc of carrying gas has then dropped below the reference value. This is felt by sensor 61 in the supply duct 40, and this makes control valve 51 to open somewhat more for admitting more flue gases, whereby the total flow rate is brought back towards its reference value.
When the air to fuel gas ratio L/G would exceed the reference value, e.g. by an increased pressure as delivered by ventilator 15 or a reduced pressure in fuel gas pipe 16, then these variations of the pressures are directly observed by the differential control valves 47 and 48 and corrected, and the ratio is brought back to the reference value by control valve 49.
Variants of this control device are possible. The regulation of the fuel gas supply can for instance be switched with the regulation of the air supply, where, upstream of differential control valves 47 and 48, are then not located respectively the air and fuel gas supply, but inversely, the fuel gas and air supply respectively.
Above the self ignition temperature of about 750°C, it also appears that with a system according to the invention, it is possible to go to surprisingly low flow rates of fuel gas, and even to zero. When the carrying gas indeed is very poor in fuel gas and air, and very rich in flue gases, the it will still come to burn with the very small quantity of air as present, also after the flame was extinguished either locally or in total, because the hot granules initiate a rapid self ignition. Also, owing to the constant flow rate of the carrying gas, the speed at the blowing orifices can be so designed, that the flame cannot penetrate inside the gas supply arrangement. In this way, it is no longer necessary to blow in the carrying gas with an air quantity above the ignition ratio, but all freedom is here kept.

Claims

Fluidized bed furnace for internal gas combustion, having a fluidized bed container (1) of which the inlet for the carrying gas is connected with a gas supply arrangement that comprises a first (23) and second (22) inlet for fuel gas, respectively air, and that comprises a sensor (32) of the temperature T in the fluidized bed, said sensor being connected with the input (29) of a control device (25), characterized in that the gas supply arrangement further comprises a third (24) inlet for flue gas, and that each of said three inlets (22-23-24) comprises a supply device (9-10-11) adapted for steering the corresponding inlet flow rate and having a steering signal input (12-13-14), connected with the output (26) of said control device, said control device being arranged for steering the flow rate of said three inlets in a way as to maintain the carrying gas flow rate Qc, the ratio of air flow to fuel gas flow (L/G), and the temperature T of the fluidized bed at a respective reference value.
Fluidized bed furnace according to claim 1, characterized in that said third inlet (24) is connected with the combustion gas outlet (18) of the fluidized bed itself.
Fluidized bed furnace according to one of claims 1 or 2, characterized in that at least one of said reference values can be preset or steered.
Fluidized bed furnace according to one of claims 1 to 3, characterized in that it is provided with wire access and guiding means, for continuously guiding a number of metal wires side by side in parallel through said fluidized bed.
Fluidized bed furnace according to any one of claims 1 to 4, characterized in that said control device is provided with a first device part (48-50), adapted to steer the flow rate of the supply device for the fuel gas, or of the air, in a sense as to reduce the deviation of the measured temperature from its corresponding reference value, and is further provided with a second device part (47-49) having a sensor (46) of the flow rate of said supply device and adapted to steer the flow rate of the supply device of the air, respectively the fuel gas, in a sense as to maintain the ratio air flow rate to fuel gas flow rate at its corresponding reference value, and is further provided with a third device part (51) having a sensor (61) of the carrying gas flow rate, and arranged to steer the flow rate of the supply device of the flue gas in a sense as to reduce the deviation of the measured carrying gas flow rate from its corresponding reference value.
A process for steering a fluidized bed furnace for internal gas combustion, in which a carrying gas is blown in the fluidized bed and has, as a first and second component, a quantity of fuel gas and of air respectively, and in which the temperature T of the fluidized bed is regulated on a reference value, characterized in that said carrying gas comprises, as a third component, a quantity of flue gas, and that said carrying gas is supplied from three sources of said three components respectively, the flow rate of each of which being steered so as to regulate, simultaneously with the temperature T, also the carrying gas flow rate Qc and the ratio of air flow rate to fuel gas flow rate (L/G), each on a respective reference value.
A process according to claim 6, characterized in that the combustion gases of the fluidized bed itself are used as a source for said third component.
A process according to one of claims 6 or 7, characterized in that the stoechiometric ratio is taken as the reference value for the ratio airflow rate to fuel gas flow rate.
A process according to one of claims 6 to 8, characterized in that a temperature above 750°C is chosen as a reference value for the fluidized bed temperature.
A process for continuously austenitizing a row of steel wires that are continuously guided side by side through a fluidized bed furnace, characterized in that said fluidized bed furnace is steered according to claims 7 and 8, in which a temperature between 900°C and 1050°C is chosen for the reference value for the fluidized bed temperature.