NL1023570C2 - Homogeneous oxidation. - Google Patents

Description

-1 - I · * «* HOMOGENE OXIDATION 5

The invention relates to a method for homogeneously oxidizing from a main burner of a fuel, wherein combustion air and hot flue gases are supplied to the fuel in a combustion space, wherein a mixture is formed with a temperature above the self-ignition temperature of the fuel. This homogeneous oxidation takes place virtually invisible to the naked eye and is therefore also called flameless combustion.

Such a device is for example known from US 6,206,686 B1.

In this document a burner device is described for homogeneously oxidizing fuel, the hot flue gases coming entirely from the oxidation process of the fuel itself. These hot flue gases flow outside the zone I where the actual homogeneous oxidation takes place through the combustion space back I to the place where fuel and combustion air are supplied. The hot flue gases are entrained by the supplied fuel and combustion air and a combustion mixture forms with a temperature above the self-ignition temperature of the fuel. The combustion takes place with an I low resulting ΝΟχ-content in the flue gases.

A drawback of the known method is that the reducing flue gases must always have a sufficiently high temperature to bring the described three-component mixture to the required temperature. This requires in practice that the entire combustion chamber must remain at a high temperature, with the homogeneous oxidation of natural gas above about 800 ° C, in order to prevent the reducing flue gases from cooling down too much on the way back to the fuel supply, for example by contact with the wall of the combustion chamber. If the flue gases cool too much, the mixture will no longer rise above the auto-ignition temperature of the fuel, as a result of which a dangerous situation in the combustion space can arise. An additional problem here is that the disappearance of the homogeneous oxidation can only be easily detected after some time, so that the dangerous situation may not be immediately recognized.

The invention has for its object to prevent this disadvantage wholly or to a considerable extent.

This object is achieved according to the invention in that the hot flue gases come at least in part from a permanently operated auxiliary burner.

By using the hot flue gasses of an auxiliary burner instead of just that of the combustion of the fuel itself, one is no longer bound to keep the entire combustion space at a high enough temperature for self-ignition because by controlling the auxiliary burner can supply sufficient heat to the three-component mixture. It also appears that the gehalte-content in the final combustion gases is still very low, even when conventional combustion takes place in the auxiliary burner and no homogeneous oxidation.

An additional advantage is that flame monitoring on the conventional auxiliary burner is easily possible so that a sufficient supply of sufficiently hot flue gases can be better monitored directly. In US 6,206,686 B1 the use of an auxiliary burner is mentioned.

However, this is an auxiliary burner that is used to heat the combustion chamber at the start of the combustion to bring the fuel to self-ignition temperature. The auxiliary burner is switched off again after the start-up phase.

In the method according to the invention, a mixture of the fuel to be oxidized, combustion air and hot flue gases is formed.

Suitable fuels are the known gaseous fuels, in particular natural gas, but also liquid fuels, of which petroleum and domestic fuel oil are suitable examples.

Suitable combustion air are gas mixtures in which at least 10% by volume of oxygen is present. The best-known example of this is ambient air. The combustion air can be preheated, for example with the off-gases discharged from the combustion space. The temperature of the combustion air is thus between ambient temperature and approximately 1100 °

The hot flue gases come from at least partly from an auxiliary burner in stationary operation. For the rest, the heat required to heat the mixture of fuel to be oxidized and the combustion air to above the auto-ignition temperature can come from the combustion of said fuel itself, as in the known burner. In the auxiliary burner, the same fuel is preferably burned as the fuel to be oxidized, because this is technically the simplest. If desired, both mentioned fuels can also be selected differently. The auxiliary burner is preferably of a type that generates flue gases with a low NOx content. The temperature of these flue gases when entering the combustion chamber is preferably at least 900 ° C and is preferably between 800 ° C and 1400 ° C. The velocity at which these flue gases exit can be considerably lower than that described below for the fuel to be oxidized and the combustion air and preferably lie between 5 and 15 m / s. The said speed provides a stable "flow shell" which can shield the main burner from the low-oxygen combustion chamber atmosphere and thus delay a further dilution of the oxygen in the combustion air.

The auxiliary burner is designed in such a way that the formed flue gases I enter the combustion space in the vicinity of the place where the fuel I and the combustion air are supplied to that space. The person skilled in the art is able to devise many suitable embodiments for this purpose, which should have the feature in common that the hot flue gases are entrained to the desired extent by the fuel and combustion air supplied. In a suitable embodiment the combustion air and the fuel I to be oxidized is supplied through two coaxial feeds which open into the combustion space. The flue gases from the auxiliary burner are then preferably supplied by coaxial around the aforementioned coaxial feeds. The flue gases can be supplied through a number of separate openings, but it is very suitable to supply them through an annular opening, coaxial with said feeds.

In this case a very uniform mixing of the three supplied components takes place. This annular opening can be separated from the combustion space by a mesh for achieving a uniform inflow of the flue gases from the auxiliary burner into the combustion space. By not premixing fuel and combustion air but supplying it separately, combustion is prevented from occurring before the combustion air is sufficiently diluted with the hot flue gases and avoidable NOx formation is avoided. The mouth of the I feeds for fuel and combustion air in the combustion space and of the opening through which the flue gasses from the auxiliary burner are supplied to that space are as a rule substantially in one plane, said mouths preferably being some distance beyond the protrude the opening for the flue gases into the combustion space. More preferably, the mouth of the fuel supply protrudes over some distance with respect to the supply for the combustion air. Said distances depend on the total size of the I 'burner and lie between 20 and 200% of its largest cross-section perpendicular to the feeds. A more homogeneous mixing of the three components appears to be achieved with this embodiment.

The fuel and the combustion air are preferably supplied in such a way that a turbulent flow is created. This can be achieved, for example, by choosing the inflow speed sufficiently high. The inflow rate is preferably at least 25 m / s and is preferably between 50 and 100 m / s. The inflow rates of the fuel and the combustion air are preferably approximately equal to each other to achieve a controlled slow mixing of both. As a result, the conditions for homogeneous oxidation are only formed at some distance from the lances. There, the oxidizing mixture is already strongly diluted with (hot) flue gases from the auxiliary burner and, optionally additionally, from the furnace atmosphere, which greatly limits NOx formation.

Due to this turbulent inflow the hot flue gases from the auxiliary burner are sucked in and a mixture of the three components is formed. In a cold combustion chamber, the flow rate of the combustion gases coming from the auxiliary burner must be at least 20% of that of the combined fuel and combustion air flow from the main burner.

This mixture has a low oxygen content due to the dilution of the combustion air with the hot flue gases, preferably of at most 10% by volume, which leads to a low NOx production in the homogeneous oxidation of the fuel.

I By mixing with the hot flue gases, the temperature of the mixture rises above the auto-ignition temperature of the fuel so that it oxidizes homogeneously flamelessly at a certain distance and separate from the inflow openings. I The auxiliary burner remains in operation as long as fuel I and via the main burner combustion air is supplied. At the start of the combustion process, when the entire device in which the combustion takes place is still at a temperature far below the operating temperature, the auxiliary burner is operated at a higher power output than when the operating temperature has been reached. During the heating process, the capacity of the auxiliary burner can be gradually reduced. The part of the total capacity of the auxiliary burner and the main burner supplied by the auxiliary burner is between 20 and 50% at start-up and is reduced to a share I under stationary operating conditions. 35 between 5 and 15%.

-5-

The invention also relates to a device for homogeneously oxidizing a fuel, provided with a main burner for supplying combustion air and the fuel to be oxidized and provided with an auxiliary burner for generating hot flue gases, wherein the burner and the auxiliary burner have been designed and positioned relative to each other in operation to form a mixture of the supplied combustion air, the fuel to be oxidized and the hot flue gases with a temperature above the auto-ignition temperature of the fuel.

In the usual devices for homogeneous oxidation of fuel as, for example, known from US 6,206,686, instead of the flue gasses of an auxiliary burner during continuous operation, only the flue gasses of the fuel are used to bring the mixture to the desired temperature.

The drawbacks of this have been described above and are substantially eliminated by the device according to the invention. The additional advantages of the various preferred embodiments have also already been mentioned above.

For example, the main burner in the device according to the invention is preferably a non-premixed burner. Furthermore, the combustion air and the fuel to be oxidized are preferably supplied by coaxially placed feeds which open into the combustion space. Also preferably, the auxiliary burner is arranged annularly around the main burner. For supplying the flue gases from the auxiliary burner to the combustion space, the device can be provided with a number of separate supply openings, but preferably a continuous annular supply opening is present.

The device according to the invention makes it possible, upon starting up the process for which the device is designed, to achieve very quickly the regime in which homogeneous oxidation of the fuel takes place because the entire combustion space does not have to be at the high temperature required for this purpose. but only the auxiliary burner needs to come to a stationary state in which it supplies sufficient heat to bring the fuel-combustion air mixture above the fuel's auto-ignition temperature. The latter generally takes place within a considerably shorter time than is necessary for bringing the entire combustion space to temperature.

The invention therefore also relates to a method for effecting combustion of a fuel from a main burner by homogeneous oxidation, wherein an apparatus according to the invention is used and wherein the auxiliary burner is operated at substantially full power until homogeneous I oxidation occurs and is then reduced to a level where at least the combustion is maintained by homogeneous oxidation.

With this method the condition under which homogeneous oxidation occurs is achieved in a considerably shorter time than in the known applications of auxiliary burners in which the entire combustion space must be brought to the required temperature. As the auxiliary burner is operated with a higher capacity, the homogeneous oxidation regime will be achieved more rapidly, in particular when the auxiliary burner is operated at substantially full power, for example at at least 75 or even 90% of its full capacity. In a cold combustion chamber, the flow rate of the combustion gas from the auxiliary burner must be at least 20% of that of the combined fuel and combustion air flow from the main burner.

Once the homogeneous oxidation has been started, the temperature of the combustion chamber will rise and the reducing combustion gases from the main burner will also be able to contribute to heating up the fuel-combustion air mixture for the main burner. The additional amount of heat that must still be supplied by the auxiliary burner will consequently decrease. As a result, the power of the auxiliary burner can also be reduced. This reduction may not take place beyond at least a level at which the homogeneous oxidation is maintained. Further reducing the power would otherwise lead to the supply of insufficient heat to keep the fuel-combustion air mixture above the fuel's auto-ignition temperature and thereby the homogeneous oxidation would extinguish. This can lead to dangerous situations which, although it can be prevented by installing monitoring equipment known per se, but it is also undesirable for reasons of undisturbed operation.

Depending on the capacity of the main burner, the size of the combustion chamber and other parameters that influence the heat consumption in and of the combustion chamber, the situation may arise that the additional heat from the auxiliary burner is no longer needed. to maintain homogeneous oxidation I. In such a, mostly stationary, situation, the auxiliary burner I can even be switched off completely. However, the advantage of the rapid entry of the homogeneous oxidation regime has already been achieved.

V * -7-

The invention will be elucidated with reference to the following drawings.

FIG. 1. a front view of a burner with main and auxiliary burner, suitable for use in the method and the device according to the invention; and

FIG. 2. a cross section of the burner of FIG. 1 along the AA line.

In FIG. 1. 2 is a burner with a rectangular housing 4, within which 6 is the annular outflow opening for the combustion gases from an auxiliary burner. This outflow opening is concentric with the also annular outflow opening 8 for the combustion air for the main burner, which in turn is concentric with the inside circular outflow opening 10 for the fuel, in particular the gas, for the main burner. These outflow openings can also take other forms or consist of different individual openings, provided that the outflow openings for the combustion gases are situated around the other outflow openings. The shape of the housing is also not critical and can be adapted to the combustion space and to the requirements that the supply, the discharge, the further nature and construction of the burner and the nature of the various gases possibly make.

In FIG. 2, 204 is the housing of burner 202 and 206,208 and 210 are the outflow openings of resp. the combustion gases from the auxiliary burner, the combustion air for the main burner and the fuel for the auxiliary burner.

In the illustrated embodiment, the auxiliary burner is a premixed burner. Through annular channel 212, a premixed gas-air mixture is introduced, supplied from a conventional mixing system (not shown here), which is burned at the bottom of the burner bowl 214. This bottom may be, for example, a metal fiber or ceramic burner.

Combustion air is supplied through supply channels 216 and 218 respectively. gas supplied to the main burner from conventional plena not shown here. The main burner is thus of the non-premixed type. Channels 216 and 218 both protrude slightly beyond the outlet opening 206, the mouth of channel 218 again slightly protruding beyond the mouth of channel 216. Gas and combustion air from the main burner mix and flow into an imaginary conical profile 220. The hot combustion gases coming from the auxiliary burner are sucked in and mix with the said gases for the main burner, the entire mixture being at a certain distance from the outflow openings in a

I

Η The imaginary area 222 rises above the auto-ignition temperature of the gas and I oxidizes there homogeneously.

The head and auxiliary burner assembly surrounded by housing 204 opens into a combustion space not shown here.

H

Claims (9)

A method for homogeneously oxidizing from a main burner of a fuel, wherein combustion air and hot flue gases are supplied to the fuel in a combustion space, wherein a mixture is formed with a temperature above the self-ignition temperature of the fuel, with the characterized in that the hot flue gases come at least in part from a permanently operated auxiliary burner. 2. Method according to claim 1, wherein the main burner is a non-premixed burner. 3. Method as claimed in claim 2, wherein the combustion air and the fuel to be oxidized are supplied through two coaxial feeds which open into the combustion space. 4. Method as claimed in claim 2 or 3, wherein the flue gases from the auxiliary burner are supplied from openings which surround the supplied fuel and combustion air. 5. Apparatus for homogeneously oxidizing a fuel, provided with a main burner for supplying combustion air and the fuel to be oxidized and provided with an auxiliary burner for generating hot flue gases, the burner and the auxiliary burner being designed in this way and I are positioned relative to each other during operation to form a mixture of the supplied combustion air, the fuel to be oxidized and the hot flue gases with a temperature above the fuel's auto-ignition temperature. I 25 6. Device as claimed in claim 5, wherein the main burner is a non-premixed burner. 7. Device as claimed in claim 6, wherein the combustion air and the fuel to be oxidized are supplied by coaxially placed feeds which open into the combustion space. I 30 Device as claimed in any of the claims 5-7, wherein the auxiliary burner is arranged annularly around the main burner. 9. Method for effecting combustion of a fuel I from a main burner by homogeneous oxidation, wherein a device I according to any one of claims 5-8 is used and wherein the auxiliary burner is operated at substantially full power until homogeneous oxidation occurs and I 'Η H is then adjusted back to a level where at least the homogeneous oxidation is maintained. ·· ->