GB2048456A - Reducing NOx emission from burners - Google Patents

Abstract

A method of reducing the NOx emission during the combustion of nitrogenous fuels in burners in closed combustion chambers, in which the combustion air is divided into two streams, the first stream 2 being supplied directly to the burner to effect partial combustion of the fuel in a primary zone 6, and the second stream 4 being supplied separately from the burner to its flame in a secondary zone 7 to effect complete combustion of the fuel. The air in the first stream is insufficient to effect complete combustion of the fuel, and the air in the second stream is in excess of that required for complete combustion. <IMAGE>

Description

SPECIFICATION Method of reducing the NOx emission during the combustion of nitrogenous fuels in burners in closed combustion chambers The invention relates to a method of reducing the NOx emission during the combustion of nitrogenous fuels in burners in closed combustion chambers, wherein the addition of the combustion air is effected in two stages, a first which is under-stoichiometric and a second which is over-stoichiometric.

The reaction mechanisms which cause the formation of nitric oxides in industrial furnaces are largely known. A distinction is made today essentially berween two different formation reactions: the thermal NOX formation which is based on the oxidation of molecular nitrogen which occurs abundantly, for example, in the combustion air. Since.the oxidation of molecular nitrogen requires atomic oxygen or aggressive radicals (for example OH, 02, etc.), it is greatly dependent on temperature, hence thermal NOx; the formation of fuel NOx which is effected through the oxidation of nitrogen compounds bonded in the fuel.During the pyrolysis, nitrogen-carbon and nitrogen-hydrogen radicals (CH, HCN, CH etc.) are formed from these nitrogen compounds and, because of their ability to react with molecular oxygen, oxidize to NOx already at relatively low temperatures, in the presence of oxygen.

Therefore a reduction in the thermal NOx formation is achieved above all by lowering the combustion temperature and the dwell times at high temperatures. Since a large proportion of the total NOX formation results from the fuel NOx reaction, however, during the combustion of fuels with bonded nitrogen, the above-mentioned means are not sufficient to achieve the standard emission values existing in some countries, with such fuels. For this it is necessary to reduce the nitrogen compounds to molecular nitrogen (N2) during the pyrolysis in the absence of oxygen.Experiments have shown that these reduction reactions to molecular nitrogen take place, for example, if the fuels are burnt under understoichiometric conditions, that is to say with the supply of less oxygen or air than is necessary for complete combustion. In order to achieve an optimum result, an air ratio between 0.9 and 0.5 should be selected for the primary combustion zone, depending on marginal conditions (for example wall temperature of the combustion chamber). Then, in order to achieve a complete burning of the hydrocarbon compounds of the fuel, the reaction products formed in the under-stoichiometric primary region must be after-burned.

Experiments have shown that with such a two-stage combustion, not only the fuel NOx formation with simultaneous extraction of heat from the under-stoichiometric region, but also the thermal NOX formation can be considerably reduced. In experiments, by the use of two-stage combustion, the reduction of the NOx emission values was achieved up to about 70% in comparison with unstaged combustion.

It was proved by experiment that during operation of the burners in the near-or understoichiometric range, the formation of fuel NOx could be reduced appreciably. In order to avoid losses through incomplete combustion and an increase in the emission of other harmful substances (CO, hydrocarbons and particles), with under-stoichiometric operation, additional air must be blown into the burners, above these in the combustion chamber. The disadvantage of this mode of operation is that slagging and corrosion of the tube walls may occur in the lower part of the combustion chamber which is operated under-stoichiometrically. Thus the operational reliability of the installation is endangered.

It has further been found that a considerable reduction in the NOx emission can be achieved by slowing down the mixing between the streams of air and fuel.

For this, jet burners, for example, are suitable, in which both the jets of air and fuel are blown parallel into the combustion chamber.

In order to achieve a satisfactory ignition, however, the burner jets must support one another mutually, for example in a corner firing.

If the burners are arranged in a front or counter firing, the mixing of air and fuel can be slowed down, for example, by the fact that the secondary air surrounding the jet of dust is blown in at substantially the same speed.

In a known burner, the stream of secondary air is supplied separately in two tubes disposed annularly in relation to one another, for example to allow the inner stream of secondary air, which is thus immediately adjacent to the jet of dust, to emerge at a lower speed and the outer stream of secondary air at a higher speed. It is a disadvantage of this arrangement that a lengthening of the flame occurs, which leads to larger combustion chambers, and that when the secondary air is reduced because of the load, the speed of the secondary air is reduced below the speed of the dust air, as a result of which the character and the shape of the flame alter. In some circumstances, the ignition can be disadvantageously influenced in this case.

Furthermore, it is known to effect a primary combustion under under-stoichiometric conditions in a precombustion chamber, and to admix the air necessary for complete burning with the waste gases which leave the precombustion chamber. The disadvantage of this arrangement consists in the risk of tube wall corrosion in the precombustion chamber oper ated under-stoichiometrically.

It is therefore the object of the present invention to provide a method of reducing the NOx emission during the combustion of nitrogenous fuels in burners in closed combustion chambers, with which assurance is provided that a lower formation of NOx is achieved by influencing the stream of secondary air, and at the same time the security of the operation against slagging and corrosion of the tube walls remains assured while at the same time adhering to an intensive ignition and a satisfactory burning.

In order to solve this problem, it is proposed according to the invention, that the stream of secondary air should be divided into two component streams of which the first component stream is supplied directly to the burner to form an under-stoichiometric primary zone and the second component stream is supplied separately from the burner to its flame in the region of the over-stoichiometric zone in the combustion chamber.

The second component stream of secondary air supplied separately from the burner to its flame as stage air is preferably regulated depending on the load.

It has proved particularly advantageous that the stream of secondary air supplied directly to the burner is so dimensioned that an air number between n = 0.9 and n = 0.5 is achieved in the primary zone.

In addition, the second component stream of secondary air (stage air) may in turn be divided into at least two component streams and these component streams fed into the combustion chamber over a divided circle surrounding the burner concentrically, and flue gases drawn out of the combustion chamber into the primary zone through the impulse of the flame via the free space present between the streams of stage air.

The advantages which are achieved by the method according to the invention, namely low NOx emmission, no slagging and corrosion at the tube walls of the combustion chamber and a reliable ignition and satisfactory burning over a wide working range, are achieved by the division of the stream of secondary air.

Whereas the one part of the secondary air which is supplied directly to the burner can be influenced with regard to twist and speed so that an intensive ignition is ensured over the whole load range, the second part of the stream of secondary air which is added to the flame as a stream of stage air outside the burner, is so formed that after ignition and primary combustion have been effected by the mixed energy of the stream of stage air, the burning in the secondary zone is realized with the object described.

The method according to the invention is described in more detail with reference to the basic sketch of a burner and the flame produced thereby.

With the burner consisting of a core air tube 2, fuel and supporting air part 1 and jacket-air part 3, a partial combustion zone (primary zone) 6 is produced, the air number of which is between 0.9 and 0.5 times the stoichiometry.

The burner is so constructed that a zone of intensive flow back 5 is produced from a region of advanced combustion in the interior of the flame by specific means (twist of the jacket air, burner mouth widened out in a taper, closed core air). As a result, the mixture of fuel and air is rapidly heated and ignited.

The heating and ignition can be influenced by the addition of core air. Thus the best ignition is ensured if the core air is closed.

The air necessary for the residual burning is blown in as stage air 4 through some nozzles at the periphery so that it only supplies the secondary flame or after-burning zone 7 with oxygen after development of the primary flame. The flow of stage air 4 is disposed in a divided circle which corresponds to double the diameter of the jacket-air tube, thus ensuring that the stage air 4 only reaches the actual flame downstream of the burner mouth after a distance of about one to two diameters of the jacket-air tube.

At the sections of the peripheral area of the flame which are not adjacent to the flow of stage air 4, flue gases are drawn out of the combustion chamber by impulse exchange. By this means, the flame temperature is reduced.

Claims (5)

1. A method of reducing the NOx emmission during the combustion of nitrogenous fuels in burners in closed combustion chambers, wherein the addition of combustion air is effected in two stages, a first which is understoichiometric and a second which is overstoichiometric, characterised in that the stream of secondary air is divided into two component streams of which the first component stream is supplied directly to the burner to form an under-stoichiometric primary zone and the second component stream is supplied separately from the burner to its flame in the region of the over-stoichiometric secondary zone in the combustion chamber.

2. A method as claimed in claim 1, characterised in that the second component stream of the secondary air which is supplied separately from the burner to its flame as stage air is regulated depending on the load.

3. A method as claimed in claim 1 or 2, characterised in that the stream of secondary air supplied directly to the burner is so dimensioned that an air number between n = 0.9 and n = 0.5 is reached in the primary zone.

4. A method as claimed in any of claims 1 to 3, characterised in that the second component stream of secondary air (stage air) is in turn divided into at least two component streams and these component streams are fed into the combustion chamber over a divided circle surrounding the burner concentrically, and that flue gases are drawn out of the combustion chamber into the primary zone through the impulse of the flame via the free space between the streams of stage air.

5. A method of reducing the NOx emission during the combustion of nitrogenous fuels in burners in closed combustion chambers, substantially as hereinbefore described with reference to the accompanying drawing.