GB2098721A

GB2098721A - A method of igniting a pulverised fuel main burner and a burner arrangement for carrying out the method

Info

Publication number: GB2098721A
Application number: GB8203056A
Authority: GB
Original assignee: Steag GmbH
Current assignee: Steag GmbH
Priority date: 1981-02-16
Filing date: 1982-02-03
Publication date: 1982-11-24
Also published as: ZA82897B; AU8041182A; JPS57161419A; DE3105540A1; GB2098721B

Abstract

A method of igniting a pulverised fuel main burner comprising an annular burner 4', and a pulverised fuel pilot burner 5', also in the form of an annular burner, arranged centrally in the main burner and provided with a supporting air flow 5 and a secondary air flow 6. The ignition energy for the main burner flame is provided by the powdered fuel igniting flame ignited centrally in the internal return flow area of the main burner, and the powdered fuel igniting flame is stabilised by imparting a strong axial rotation and a high speed to the secondary air flow of the powdered fuel pilot burner. The secondary air flow of the powdered fuel pilot burner and the supporting air of the igniting powdered fuel only supply part of the air necessary for complete combustion. A burner arrangement for carrying out the method is also described. <IMAGE>

Description

SPECIFICATION A method of igniting a pulverised fuel main burner and a burner arrangement for carrying out the method This invention relates to a method for igniting a pulverised fuel main burner in the form of an annular burner, in particular a coal dust main burner, with a pulverised fuel pilot burner, also in the form of an annular burner, arranged centrally in the main burner, in particular a coal dust pilot burner, in which the ignition energyforthe main burner flame is provided by the pulverised fuel igniting flame ignited centrally in the internal return flow area of the main burner, and in which the pulverised fuel igniting flame is stabilised.

The object of the pulverised fuel pilot burner is to initiate, and if necessary to assist, reliable ignition of the main burner, if the conditions necessary for self-stabilisation are not yet fulfilled or if the fuel of the main burner is not such as to guarantee reliable ignition also during continuous operation. This can be necessary if the ignition quality of the fuel is not adequate, if its calorific value does not result in a sufficiently hot flame due to a high proportion of ballast, and/or if the fuel is not sufficiently refined for reliable combustion.

It is known that, for igniting pulverised coal flames, pilot burners can be used which require oil or gas as ignition fuel. Gas and oil are fuels which have a relatively high ignition quality, so that the only things necessary for stabilising the ignition flame are an adequate mixture with the quantity of air fed to the burner and the exceeding of the ignition temperature at the base of the flame. The ignition energy necessary for achieving the ignition temperature usually produces a back eddy downstream from the mouth of the burner, which conducts hot gases from the ignition flame back along the jet axis. Combustion of the oil and gas occurs very quickly. A relatively small area is sufficient for the back eddy, as it is forced by so-called flame supports or impellers.

Usually in annular burners, the pilot burner is arranged in a central pipe, which also provides the air supply. The pilot burner itself usually consists of an oil or gas lance, an electric igniter or an igniter supplied with gas, and a flame control. This pilot burner is surrounded by the coal dust pipe of the main burner.

The secondary air, conducted along an outer pipe, flows into the area of the burner. This outer pipe for the secondary air opens out conically into a burner tube in the direction of the combustion chamber.

This burner tube conducts the flow of secondary air which, due to an axial rotation imparted to it, flows along the wall of the tube in a funnel-shaped manner into the combustion chamber, and on leaving the tube mixes with the gases in the combustion chamber. Within the jet of secondary air there is a vacuum area which leads to the formation of the back eddy necessary for stabilising the flame.

Moreover, this back eddy forces the mixture of coal dust and primary air emerging from the coal dust tube of the main burner into the jet of secondary air flowing in a funnel-like manner. However, before this occurs, ignition of the coal dust by the hot flue gases drawn back from the burning flame must take place. After entry into the flow of secondary air the burning coal particles find the oxygen necessary for combustion.

A method of igniting a coal dust annular burner flame is known from German Auslegeschrift No.

2933040, in which the ignition energy is introduced centrally into the internal backflow area of the main burner flame by an ignited pulverised fuel ignition flame, whereby the ignition flame is operated by a fuel, in particular coal dust, which differs from the coal dust used as the main fuel in its grain size and/or consistency.

A corresponding burner arrangement with the pilot burner arranged in the main burner is known from German Auslegeschrift No. 2933060.

The construction of a pulverised fuel pilot burner in the central area of a pulverised fuel main burner, and in particular in the inner tube of a main burner, can naturally start from the same principle of construction as is used in main burners. In the known burner arrangement, the pilot burner has an individual burner tube widening conically inside the inner tube.

Furthermore, one frequently proceeds from the principle that as much air should reach the coal dust pilot burner as is necessary for complete combustion ofthe coal being fed in. The cross-sectional area required for the mixture of primary air and coal dust of the pilot burner, which is greater than that of a gas-oil lance, the cross-sectional area for the secondary air necessary for the complete combustion of the ignition coal dust, and the burner tube, considered to be absolutely necessary, in the extension of the secondary airtube, all lead to a large outer diameter ofthe pilot burner. This is of disadvantage, particularly when converting existing main burners, since then greater structural alterations have to be undertaken.Also in boiler plants being newly constructed, the known geometry of the burner can be greatly altered by the great increase in the central cross-sectional area required for receiving the pilot burner, so that difficulties can occur in the combustion chamber which have not previously been encountered.

An object of the present invention is to provide a method of stabilising the igniting flame without any special structural elements.

This object is achieved in that the pulverised fuel igniting flame is stabilised by imposing a strong axial rotation and a high velocity on the flow of secondary air of the pulverised fuel pilot burner, whereby the secondary air of the pulverised fuel pilot burner and the supporting air of the igniting pulverised fuel only supply part of the air necessary for complete combustion of the igniting flame.

As has been known until now, the mixture of primary air and igniting pulverised fuel is fed to the pilot burner in a central tube, in the middle of which the igniter is arranged, for example in the form of a small propane gas burner to be ignited electrically, or an electrical igniter in the form of an electric element, or an electric arc device. The mixture of primary air and igniting pulverised fuel emerges from the primary air tube at the lowest possible speed.

The secondary air duct is arranged around the primary air tube, also called the igniting pulverised fuel tube. The air emerges from a narrow transverse cross-sectional area at high speed. It only constitutes a fraction of the amount of air which is necessary for complete combustion of the igniting pulverised fuel.

Moreover, an adjustable strong axial rotation is imparted to this flow of secondary air.

During orafterthereleaseofthisannularflowin the duct, the high momentum leads to an intensive mixing with the surroundings, that is, with the primary air emerging from the primary air duct of the main burner at relatively low speed during ignition, and with the air emerging from the secondary air duct of the main burner, the amount and speed of which can be suited to the requirements for ignition. By this means it is guaranteed that the igniting pulverised fuel, in particular igniting coal dust, encounters the air necessary for combustion after ignition.

The strong axial rotation imposed on the secon daryairflowofthe pilot burner causes a funnel-like widening of the annular jet even without a fixed burner tube, which is considered absolutely necessary in German Auslegeschrift No.2933040 and German Auslegeschrift No. 2933060, and a back flow along the jet axis. Thus, with a relatively small pilot burner cross-section, the ignition energy is fed back from the burning flame along the jet axis to the base of the flame.

The axial rotation and velocity of the secondary air flow of the pilot burner must have such a high momentum that the funnel-shaped "compressed air burner tube" must be retained, even when, on putting the main burner into operation, its primary air duct is loaded with a mixture of pulverised fuel and air, in particular a mixture of coal dust and air.

The high mixing power of the pilot burner secondary airflow moreover improves the mixing of the ignition flame with the flame of the main burner, if, in continuous operation, the pilot burner assists with the ignition of the main burner when poor burning conditions prevail.

This invention is not only aimed at a method for igniting a pulverised fuel main burner in the form of an annular burner, with a pulverised fuel pilot burner arranged centrally in the main burner, but also at a burner arrangement for carrying out this method.

Thereby the invention proceeds from the burner arrangement according to German Auslegeschrift No. 2933060. According to the invention, with this burner arrangement it is provided that the secondary air tube opens out cylindrically to the combustion chamber, and the angled vane ring arranged with it is placed close to the air outlet. Otherwise, in the narrow transverse cross-section of the secondary air duct, a large proportion of the rotating energy would be lost by friction with the air flowing at high speed.

The method according to the invention is approp riately carried out so that the igniting flame is driven by a quantity of air in the region of 0.2 to 0.8.

It is also of advantage if the axial velocity compo nentofthecombustion airforthe pulverised fuel igniting flame is greater than that for the main burner flame.

It is also desirable if the rotation and/orthe velocity can be adusted.

The following is a detailed description of a preferred-embodiment of the burner arrangement, reference being made to the accompanying drawing.

In the secondary air supply 1 of a main burner an angled vane ring 2 is provided, the vanes of which are adjustable. The angled vane ring 2 is arranged in the secondary air tube of the secondary air supply, this tube not being shown in detail. A burner tube 3 is connected to the secondary air tube. The means for supplying fuel and air mixture to the main burner are given the reference numeral 4, and the means for supplying fuel and air mixture to the pilot burner are given the reference numeral 5. Said means comprise a pulverised fuel tube for the power burner 4' and a pulverised fuel tube for the pilot burner 5'. Secondary air is supplied to the pilot burner via a connecting pipe 6, and angled vanes 8 are arranged in an annular manner in the secondary air duct 7 just in front of the outlet.An igniter 9 is arranged centrally inside the primary air duct of the pilot burner, this igniter having an air supply 10 and a gas supply 11. The burner arrangement is arranged in the wall 12 of a combustion chamber - as is shown diagrammatically. In the embodiment shown, the inner wall of the annular main burner duct is defined by a secondary air tube 7' defining the outer wall of the secondary air duct 7 of the pilot burner.

However, it is also possible for the inner wall of the duct for supplying fuel and air mixture to the main burner to be formed by a central tube into which the secondary air tube 7' is inserted; this is the case in particular in the assembly of burner arrangements with an inner air tube. Basically, the main thing is that the corresponding ducts are provided for the mixture of secondary air and primary air and fuel.

The pulverised fuel tube 5' for the pilot burner is centred by means of the angled vane ring 8 in the secondary air tube 7'. The angled vanes of the vane ring 8 are appropriately arranged so that the rotating effect on the flow can be varied so as to be able to adapt the burner to the varying operating conditions.

When using coal dust as the fuel it can be appropriate to also impart axial rotation to the mixture of primary air and igniting coal dust, depending on the quality of the coal dust available (features of quality are: grain size, ballast content and gas content). This would result in faster mixing of the pulverised fuel with the secondary air. When using igniting coal dust with poor ignition properties, it can be particularly appropriate to make the rotation of the secondary air of the pilot burner very strong.

Claims

1. A method of igniting a pulverised fuel main burner in the form of an annular burner, in particular a coal dust burner, with a pulverised fuel pilot burner, also in the form of an annular burner, arranged centrally in the main burner, in particular a coal dust pilot burner, in which the ignition energy for the main burner flame is provided by the powdered fuel igniting flame ignited centrally in the internal return flow area of the main burner, and in which the powdered fuel igniting flame is stabilised, wherein the stabilisation of the powdered fuel igniting flame is achieved by imparting a strong axial rotation and a high speed to the secondary air flow of the powdered fuel pilot burner, and wherein the secondary air flow of the powdered fuel pilot burner and the supporting air of the igniting powdered fuel only supply part of the air necessary for complete combustion.

2. A method according to claim 1, wherein the igniting flame is operated by an amount of air in the region of 0.2 to 0.8.

3. A method according to claim 1 or claim 2, wherein the axial velocity of the combustion air for the powdered fuel igniting flame is greater than that of the main burner.

4. A method according to any one of claims 1 to 3, wherein the axial rotation and/or the velocity are adjustable.

5. A burner arrangement, consisting of a pulverised fuel main burner with a central duct, a pipe for the pulverised fuel encircling the central duct and a secondary air pipe encircling the pipe for the pulverised fuel with an angled vane ring arranged at the air inlet, and also consisting of a pulverised fuel pilot burner arranged concentrically in the central duct of the main burner with an igniter arranged centrally, a pipe for the igniting pulverised fuel arranged concentrically to the igniter and a pipe for secondary air with an angled vane ring encircling the pipe for the igniting pulverised fuel, wherein the opening of the pipe for the secondary air for the pilot burner is in line with the combustion chamber, and its angled vane ring is arranged close to the air outlet.

6. A burner arrangement according to claim 5, wherein the central duct of the main burner is formed by a central pipe and the pipe for the secondary air for the pilot burner is inserted into the central pipe.

7. A burner arrangement according to claim 5, wherein the pipe for the secondary air for the pilot burner itself defines the central duct of the main burner.

8. A burner arrangement according to any one of claims 5 or 7, wherein the angled vane ring of the pilot burner is adjustable in the pipe for the secondary air for the pilot burner.

9. A burner arrangement according to any one of claims 5 to 8, wherein an angled vane ring is also arranged in the pipe for the igniting pulverised fuel for the pilot burner.

10. A burner arrangement according to any one of claims 5 to 9, wherein the angled vane ring of the pipe for the secondary air for the pilot burner serves for centring the pipe for the igniting pulverised fuel.

11. A method of igniting a pulverised fuel main burner substantially as hereinbefore described with reference to the accompanying drawing.

12. A burner arrangement substantially as hereinbefore described with reference to the accompanying drawing.