GB2043871A

GB2043871A - Burner

Info

Publication number: GB2043871A
Application number: GB8006860A
Authority: GB
Original assignee: L&C Steinmueller GmbH
Current assignee: Hitachi Zosen Inova Steinmueller GmbH
Priority date: 1979-03-05
Filing date: 1980-02-28
Publication date: 1980-10-08
Also published as: FR2450999B1; SE8001481L; FI65853B; DK148928B; AU536420B2; GB2043871B; IT1135969B; ZA801258B; NO800297L; NO149710C; FR2450999A1; NL8000995A; IT8004811A0; JPS6115962B2; FI800680A7; JPS55121308A; DK148928C; DE2908448C2; DK84180A; NO149710B

Abstract

A burner for the combustion of nitrogenous fuels in a closed combustion chamber, comprises a core air tube 1 fitted with a centrally disposed oil atomiser lance, a coal dust tube 6 surrounding the core air tube, a jacket air tube 9 surrounding the dust tube with an axially displaceable ring of swirl vanes 10 disposed at the air inlet, and a burner chalice 14 which widens out towards the combustion chamber. Air nozzles for supply of additional air are arranged in a concentric arrangement around the burner chalice. The air supplied to the burner chalice is less than that required for complete combustion, and the reaction products from the partial combustion of the fuel at the burner outlet are subsequently burnt in a secondary zone by air supplied through the nozzles in excess of that required for complete combustion. <IMAGE>

Description

SPECIFICATION Burner for the Combustion of Nitrogenous Fuels The invention relates to a burner for the combustion of nitrogenous fuels, consisting of a core air tube with a centrally disposed oil atomizer lance, a dust tube surrounding the core air tube, a jacket air tube surrounding the dust tube with an axially displaceable ring of swirl vanes disposed at the air inlet and a burner chalice which widens out in a taper towards the combustion chamber.

Burners with the above-mentioned construction features produce flames which produce a considerable concentration of NOx in the flue gases.

The reaction mechanisms which cause the formation of nitric oxides in industrial furnaces are largely known. A distinction is made today essentially between two different formation reactions:- the thermal NOx formation; which Is based on the oxidation of molecular nitrogen which occurs abundantly in the combustion air for example.

Since the oxidation of molecular nitrogen requires atomic oxygen or aggressive radicals (for example OH, 03, 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.) form 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.

A reduction in the thermal NOx formation is therefore achieved above all by lowering the combustion temperature and reducing the dwell times at high temperatures. Since a high proportion of the total NOx formation results from the fuel NOx reaction during the combustion of fuels with bonded nitrogen, however, the abovementioned means are not sufficient to achieve the standard values of emission 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 under-stoichiometric conditions, that is to say with the addition of less oxygen or air than is necessary for complete combustion. In order to achieve the optimum results, an air ratio between 0,9 and 0.5 should be selected for the primary combustion zone depending on the marginal conditions (for example wall temperature of the combustion chamber). Then, however, the reaction products formed in the understoichiometric primary region must be after-burnt in order to achieve a complete burning of the hydrocarbon compounds of the fuel.

Experiments have shown that with such a twostage 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, the reduction of the NOx emission values to about 70% in comparison with unstaged combustion was achieved through the use of two-stage combustion.

It was proved by experiments that when the burners were operated in the near-stoichiometric or under-stoichiometric 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 can occur in the lower part of the combustion chamber which is operated understoichiometrically. Thus the operational reliability of the installation is endangered.

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

For this jet burners, for example, are suitable wherein both the air jet and the fuel jet are blown into the combustion chamber in parallel. In order to achieve a satisfactory ignition, however, the burner jets must support one another mutually, for example in a corner firing.

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

In a known burner, the secondary stream of air is added separately in two tubes disposed annularly in relation to one another, for example in order to allow the inner stream of secondary air, which is thus immediately adjacent to the dust jet, to emerge with a lower speed and the outer stream of secondary air with 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 on a reduction of the secondary air 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 may 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 corrosion of the tube walls of the precombustion chamber operated understoichiometrically.

It is therefore the object of the present invention to develop a burner wherein by influencing the stream of secondary air and introducing the same at various points of the combustion chamber, but associated with the burner, the combustion is influenced in such a manner that in a partial combustion zone (primary zone) following directly on the burner outlet, a stable ignition is achieved over the whole load range under under-stoichiometric conditions and the residual burning is effected in an after-burning zone (secondary zone) following on the primary zone, under over-stoichiometric conditions.

In order to solve this problem, there is provided according to the invention a burner for the combustion of nitrogenous fuels, consisting of a core air tube with a centrally disposed oil atomizer lance, a dust tube surrounding the core air tube, a jacket-air tube surrounding the dust tube with an axially displaceable ring of swirl vanes disposed at the air inlet and a flared burner chalice which widens out towards the combustion chamber, characterised in that air nozzles which are connected through additional passages to the main air passage are provided in concentric arrangement round the burner chalice.

The air nozzles may be constructed in the form of hole-type nozzles or of slit-type nozzles, the slit-like openings being produced, for example, by removing the fins between the tubes.

The burner of the invention may have at least two, but preferably a maximum of six air nozzles disposed on a divided circle extending concentrically to the jacket air tube, the diameter of the divided circle being at least 1.5 times and a maximum of 3 times the diameter of the jacket air tube.

The stream of air emerging from the air nozzles can be regulated through a flap.

The advantages which are achieved by the invention consist in that through the addition of some of the secondary air through air nozzles outside the jacket air tube of the burner, to the combustion chamber, the course of combustion of the nitrogenous fuel being burnt is effected so that the NOx values are reduced to a minimum without the ignition of the burner being endangered over the whole load range, slagging and corrosion occurring at the combustionchamber tubes and the burning being adversely affected.

The invention is described by way of example with reference to the Figures illustrated in the drawing.

Figure 1 shows, in principle, the burner according to the invention in longitudinal section, Figure 2 shows a view of the burner in the direction of the arrow F.

The burner consists of a central core air tube 1 which is adapted to receive an oil atomizer lance for ignition firing or alternative power firing for oil.

The core air tube is connected to the main air passage 4 by the passage 2 through the flap 3.

Disposed coaxially to the core air tube is the dust air tube 6 which is connected to the dust conduit 8 by the dust distributor chamber 7. The dust conduit is supplied with the mixture of dust and air provided for burning from a coal dust tube.

Disposed coaxially round the dust air tube is a jacket air tube 9 which is connected to the main air passage 4 through the flaps 13. A ring of swirl vanes 10, through which the jacket air flows axially, can be displaced axially through a plurality of spindles 11 and the hand-wheel 12. The jacket air passage is connected to the combustion chamber through the burner chalice 14 widening out in a taper. Stage air nozzles 16, which are uniformly distributed over an imaginary divided circle of the burner periphery, are supplied with air from the main air passage 4 through a plurality of passages 1 5. The burner chalice 14 is made of ceramic material, for example. It is installed in a tube basket 1 8 which is formed from the tubes of the wall tubing of the combustion chamber.

The stage air nozzles 1 6 may be constructed either as holetype nozzles 1 6 or as slit-type nozzles. The slit-type nozzles are formed by removing the tubing of the combustion-chamber wall formed from tube-web-tube.

The stream of stage air which enters the combustion chamber through the passage 1 5 with the nozzles 16, can be regulated by a flap 17.

Claims

1. A burner for the combustion of nitrogenous fuels, consisting of a core air tube with a centrally disposed oil atomizer lance, a dust tube surrounding the core air tube, a jacket-air tube surrounding the dust tube with an axially displaceable ring of swirl vanes disposed at the air inlet and a flared burner chalice which widens out towards the combustion chamber, characterised in that air nozzles, which are connected through additional passages to the main air passage are provided in concentric arrangement round the burner chalice.

2. A burner as claimed in claim 1, characterised in that the air nozzles are constructed in the form of hole-type nozzles or slit-type nozzles.

3. A burner as claimed in claim 1 or 2, characterised in that at least two and a maximum of six air nozzles are disposed on a divided circle concentric with the jacket air tube and the diameter of the divided circle is at least 1.5 times and a maximum of 3 times the diameter of the jacket air tube.

4. A burner as claimed in any of claims 1 to 3, including a flap for regulating the stream of air emerging from the nozzles.

5. A burner for the combustion of nitrogenous fuels, substantially as hereinbefore described with reference to the accompanying drawings.