The invention relates also to an apparatus for facilitating the activation of a main burner in two-stage combustion, wherein the main burner to be activated functions as an afterburner for the hot combustion gases of a gas generator, which has its furnace in contact with an appliance or material to be heated, the main burner including a fuel manifold for supplying fuel into a hot flow of combustion gas coming to the main burner from the gas generator.

In one embodiment of the invention, the appliance or material to be heated comprises a convection compartment in a boiler.

By dividing the combustion in two main stages and by managing the process carefully, numerous benefits can be achieved, regarding e. g. emission release and improvement of a performance/volume ratio. In terms of a boiler, for example, the latter objective requires that a large number of small-diameter convection pipes or other heat exchange surfaces be fitted in a small space. However, this results in a counterpressure for the combustion gas flow and an overpressure existing in the main burner's furnace. The overpressure can be within the range of 30-1000 mbars, depending on a parttcu application. This overpressure existing in the furnace causes

nevertheless a problem at the moment of igniting the main burner. The flame is difficult to set alight and/or it strives to withdraw from the furnace into the actual burner, nor does the gas generator reach its normal operating range.

It is an object of the invention to provide a method and apparatus of the foregoing type for facilitating and/or enabling the activation particularly of a main combustion in an overpressure furnace without the occurrence of the above problem.

This object is accomplished with a method of the invention, having its characterizing features presented in the appended claim 1. The object is also accomplished with an apparatus of the invention, having its characterizing features presented in the appended claim 4. Preferred embodiments of the invention are disclosed in the dependent claims.

One exemplary embodiment of the invention will now be described with reference to the accompanying drawings, in which Fig. 1 shows in a schematic sectional view an apparatus implementing a method of the invention, in connection with a boiler 3 provided with a furnace 12 for a burner 2, and Fig. 2 shows in a schematic view an entire combustion facility, involving a method and apparatus of the invention.

The description deals first with a two-stage burner assembly comprehensively in reference to fig. 2.

The firing chamber 1 of a gas generator is connected by way of a pipe 8 to an afterburner 2, which functions as a main burner and has its flame in the furnace of a bd 3. The gas generator lncludes a turblne 7, which is driven

by a combustion gas flow discharging from the firing chamber 1 into the pipe 8. The turbine 7 in turn drives a supercharger 6, which charges combustion air into the firing chamber 1 through a conduit 9. As the gas generator is activated, the required combustion air is supplied by a fan unit 10. The firing chamber 1 is provided with an igniter 11 and a fuel manifold 4, whereby one or a variety of fuels can be supplied into pipes extending to the chamber 1 and having fuel nozzles at the ends thereof. Thus, combustion occurs first in the gas generator's firing chamber 1 between the supercharger 6 and the turbine 7 and then, by means of the afterburner or main burner 2, in an afterburning chamber or actual furnace 12 (fig. 1). About 15-35% of the total combustion output is released in the gas generator's firing chamber 1.

The main combustion, i. e. the flame of the burner 2, is producing 85-65% of the total combustion output. The gas generator can be operated autonomously and it is always first to be activated. In order to operate, the burner 2 always requires operation of the gas generator. The gas generator 1,6, 7 has a combustion gas which typically contains about 14-18% oxygen and has a temperature typically within the range of 300-900°C. The firing chamber 1 has a pressure of 0,2-3 bars and downstream of the turbine 7 the pressure is within the range of 30-1000 mbars, typically within the range of 50-150 mbars.

The pipe 8 connects tangentially to the burner's 2 cylindrical section, the burner 2 developing a powerful turbulent flow supplied with fuel from a fuel manifold 5, one or more fuel lines, with a fuel nozzle at the end thereof, extending therefrom to the interior of the burner 2. By virtue of a high flow rate of combustion gases, the mixing of fuel is effective and a flame emerging from the mouth of the burner 2 is typically intensive and compact or small in its aerodynamics. If necessary, the burner's 2 flame can be reshaped by means of regulating devices constructed for the burner.

Fig. 1 depicts a furnace 12 for the boiler 3, in which the burner's 2 flame is burning. In order to keep the boiler 3 in a compact size, a mutiplicty of

small-diameter convection pipes 13 are fitted at a dense pitch in the boiler's 3 water compartment. As a result of this, the convection pipes 13 develop a considerable counterpressure for the flow of flue gas. In order to facilitating the activation or enabling the activation of the burner 2 despite this counterpressure, the invention involves arranging on the centre axis of the boiler 3, i. e. on the extension of the burner's 2 centre axis, a sizable conduit 14 connecting the furnace 12 to a discharge duct 17 of flue gases. The conduit 14 is equipped with a flow regulating and/or flow shutting device 15, provided with an external operating mechanism 16 which is controlled to open the conduit 14 for the duration of a boiler activation procedure. Thus, during the boiler activation procedure, the conduit 14 functions as a pressure reduction conduit, by way of which the flue gas flow by-passes the convection pipes 13. When the burner 2 has been activated, the combustion gas flow passing through the conduit 14 is shut off or adjusted to a lesser rate by means of the regulating and/or shutting device 15 for gradually forcing the combustion gas flow into passageways constituted by the convection pipes 13. After the burner 2 has been ignited and the process stabilized, the problem caused by counterpressure does not exist any more or it is otherwise manageable. After the burner 2 has ignited and its flame has stabilized during a by-pass flow through the conduit 14, the inventive regulating and/or shutting device 15,16 sets an appropriate counterpressure and/or shuts off the entire by-pass conduit 14 and sustains a proper counterpressure in a normal running condition. If necessary, of course the device 15,16 can also be used in the process of normal operation.

In the present invention, the activation of the burner includes ignition and starting-up of the burner 2 until its flame has stabilized to bear up rather high counter pressure.

It is obvious that the invention is not limited to the above exemplary embodiment. For example, the position of the conduit 14 and the number of conduits can ba subjected to a muMtude of variations. One or more conduits

can be positioned in the peripheral regions of the boiler 3 or extended from the boiler's 3 furnace 12 directly outside can be a conduit or conduits for delivering fumes in a controlled manner into the discharge duct 17 of flue gases. It should be appreciated that the inventive apparatus is not a conventional blast gate and/or safety device for a possible furnace explosion.

Such safety devices function at substantially higher pressures, nor are able to eliminate said counterpressure problem by providing a reduction of counterpressure for the duration of a firing or ignition process. Instead, the inventive apparatus enables a more or less unimpeded flue gas flow from the furnace 12 into the discharge duct 17, such that the pressure loss caused by the convection compartment 13 does not raise a counterpressure in the furnace 12 at the moment of ignition. Hence, the inventive apparatus is capable of opening in a controlled and supervised manner a flue gas passage to the discharge duct 17 until the by-pass conduit 14 can be closed for thus raising and adjusting a pressure level in the furnace 12, whereby the flue gases can be delivered, to the full extent, if necessary, exclusively into the convection pipes 13. Instead of the boiler 3 fitted with the convection compartment 13, the furnace 12 can be associated with any appliance or material to be heated. This can be e. g. a process vessel, containing a material to be processed by heating or firing.