To minimize the NOX emission to the surroundings, it is crucial to be able to control the stoichiometric proportions during the combustion process. This is achieved by controlling the air supply to the flame by means of a suitable mixture of primary, secondary, and, if relevant, tertiary air, the air flows further being given different velocities and turbulences in guide vanes arranged in the individual air passages.

A condition for the efficient course of the combustion process is that a reliable and stable ignition zone is established for the flame. In conventional burners, known, for example, from EP patent application No. 0571704, and disclosing a burner of the type mentioned in the introduction, this is provided by a flame-stabilizing member positioned on the outer primary air pipe near the burner tip and between the outer primary and secondary air pipes of the burner.

The result of this arrangement is that a ring of recirculating zones are formed, contributing to a rise in temperature which gasifies the fossil fuel and increases its ignitionability, and that the secondary

combustion air is mixed with the primary air in an annular shape. This, in turn, means that changes in the secondary air flow affect the ignition conditions. As it is desirable to reduce the secondary air flow to increase the air-staging, such reduction will mean that it is no longer possible to obtain low emission values due to the combustion conditions.

International publication No. WO 98/21524 discloses a burner in which the primary and secondary air flows are kept separate by a transition zone provided by an annular wall surrounding the primary air pipe. The annular wall results in an increased diameter of the entire burner, and a relatively complicated structure.

Against this background, the object of the invention is to improve a burner of the type mentioned in the introduction ensuring stable and controlled combustion conditions and accompanying low emission of Nô ;,, while being of a simple structure.

This object is achieved with a burner of the type mentioned in the introduction, characterized in that the flame-stabilizing member is positioned on the inner primary air pipe.

With this design, it turns out that it is possible to provide an ignition zone in which the ignition occurs from the core towards the surface of the flame rather than from the outer rim and towards the centre of the flame, as is the case in conventional burners.

This results in a more complete combustion, as the coal particles near the core ignite first and achieve a better combustion compared with conventional burners and a stable ignition substantially independent of secondary air, which in its turn permits the air- staging intended. Furthermore, the burner according to the invention is of a simple structure, and any

replacement of the flame-stabilizing member can be carried out in a simple manner by pulling the inner primary air pipe out and mounting another member during operation of the plant instead of stopping the plant to gain access to the burner from the boiler room side. In addition, with the burner according to the invention it is no longer necessary to dismount large parts of the burner as was previously the case.

In a preferred embodiment of the invention, the said element is positioned at a small distance from the burner tip. In this way, a longer life will be obtained for the member, which will not be exposed to the high temperatures at the burner tip, as the temperature decreases with the distance from the burner tip while the output is maintained at a substantially unchanged level.

The invention will now be described in more detail below with reference to the schematic drawing, in which Fig. 1 is a longitudinal sectional view through a burner according to an embodiment of the invention.

The burner shown in Fig. 1 comprises a number of coaxially arranged pipe members, viz. from the inside and outwards, a pipe 1 for transportation of a first fuel B, such as oil or gas, an inner primary air pipe or core air pipe 2, an outer primary air pipe 3 with an air flow P for transportation of a second, pulverized fuel, such as coal, a secondary air pipe 4 and a tertiary air pipe 5. The oil and gas combustion takes place mainly through the insertion of an oil lance arrangement 11, alternatively a corresponding gas lance arrangement in the pipe 1. At oil combustion, the air flow K in the inner primary air pipe 2 acts as combustion air, in other cases as cooling air.

In the secondary and tertiary air pipes 4 and 5, vanes 6 and 7, respectively, are arranged to create

rotation in the secondary and tertiary air flows S and T, respectively.

The burner further comprises an ignition device 8 and a flame-stabilizing member 9 or a flame holder, which is positioned on the inner primary air pipe 2, and whose function will be described in detail below.

During operation, pulverized coal is transported with the air flow P in the outer primary air pipe in a direction towards the burner tip generally designated 10. In the ignition zone established by the flame- stabilizing member 9, the coal is combusted from the inside and outwards, as the coal particles in the middle, that is, near the inner primary air pipe 1, will ignite first.

As the coal located at the middle of the burner has the lowest stoichiometry and thus the largest uncombusted potential while the coal located radially outside has a higher stoichiometry, the combustion will be more complete than what is the case in conventional burners, where the coal located in the middle will ignite last. Furthermore, it will also be possible to use coal with a lower reactivity than in conventional burners.

The position of the member 9 on the inner primary air pipe 2 makes it affect mainly the primary air flow P, which in its turn means that contrary to conventional burners the ignition becomes largely independent of the volume of secondary air flow S.

Adjustment of the velocity and the formation of rotation in the secondary and tertiary air flows S and T makes it possible to obtain the stoichiometric proportions that provide the desired low emission values for NOX. This embodiment particularly permits air-staging, as the secondary air flow can be reduced substantially without affecting the ignition and thus

the combustion.

The position of the member 9 at a small distance from the burner tip 10 means that it is exposed to lower temperatures than what is the case in burners in which the flame holder is located near the burner tip.

In the embodiment shown, the member 9 is formed as a ring mounted on the inner primary air pipe 2, and in case of wear it can easily be replaced by pulling the inner primary air pipe 2 out and mounting another member 9.