Method for igniting a burner

The present invention relates to a method for igniting a burner, especially burners used in industrial furnaces.

There are burners of various kinds, having in common that a gaseous fuel and a gaseous oxidant are injected near, or at a distance from each other into the combustion zone. The gases are usually injected through lances in a burner head.

For security reasons, a burner needs to be monitored, with respect to the fact that there is a flame present during operation. Such a monitoring is usually carried out by the use of a UV sensor, which is sensitive to ultraviolet radiation. The sensor is usually mounted in the burner in such a way that the UV sensor can see a part of an existing flame.

Ignition of the fuel/gas mixture normally takes place via a spark plug at the orifice or orifices of the burner, which emits a spark, igniting the mixture. An electrode is commonly mounted inside a ceramic tube that runs up to the surface of the burner head facing the furnace space. This tube has a relatively low strength, and is therefore easily broken. The tube can also crackle due to thermal stress. Moreover, it has turned out to be relatively difficult to obtain a correct electric arc.

During operation, the spark plug often gets clogged up, resulting in that it cannot give off a sufficient spark for igniting the mixture.

Also, it is usual practice that the spark ignites a pilot flame, which in turn ignites the main flame. This arrangement increases the cost.

The present invention solves these problems by providing another way of igniting a burner.

Thus, the present invention relates to a method for use when burning a fuel together with an oxidant in an industrial furnace, where the fuel and the oxidant are supplied to a burner head, and where the flame is monitored by a detector for ultra violet light, or other light, where at least one duct, for the supply of fuel and at least one duct for the supply of oxidant are provided, opening out on the side of the burner head facing the furnace space, characterized in that a laser is made to shine on a point on the burner head at said ducts, and is made to heat up the said point to a temperature which is above the ignition temperature of the fuel and the oxidant, and in that when the burner has ignited, the said detector is made to emit a signal to a control circuit, and in that the control circuit is made to turn off the laser.

Below, the invention is described more in detail, partly in conjunction with an embodiment of the invention shown in the attached drawing, wherein

• Fig. 1 schematically shows a longitudinal section of a burner head according to the invention.

In Fig. 1, a burner for combustion of a fuel together with an oxidant in an industrial furnace is shown. The burner is arranged in such a way that fuel and oxidant are supplied to the burner head 1. A detector 2 for the detection of ultraviolet light or other light, is provided in order to monitor a flame outside of the burner head 1.

According to the invention, there is provided at least one duct 3 for the supply of fuel and at least one duct 4 for the supply of oxidant that open out on the surface 5 of the burner head facing the furnace space.

It is evident that the ducts can be formed in other ways, and can number more than two.

Said detector 2 is provided at the duct for fuel 3, or in the other duct for oxidant 4. Conveniently, the detector is provided at the furnace distal end of the duct, and is arranged so that UV light from the flame, led into the duct, hits the detector. The detector is connected to a control circuit 6, comprising a detector circuit by which is assessed whether a flame exists or not. In the case a flame cannot be detected, the supply of fuel and oxidant is interrupted.

For use with the present invention, oxidants with an O₂ content of over 85 percent are preferred. The fuel can be natural gas, propane, butane, liquefied petroleum gas, light fuel oil, etc.

The oxidant is injected into the combustion space via one or several nozzles, formed as straight tubes, or laval-, or venturi nozzles.

According to the invention, a laser 7 is made to shine on a point on the burner head near said ducts, and is made to heat said point to a temperature exceeding the ignition temperature of the fuel and the oxidant. When the burner 1 has been ignited, said detector 2 emits a signal to the control circuit 6, whereas the control circuit turns the laser 7 off.

The laser 7 is a suitable, known laser with a sufficient power to heat the said point on the burner head to more than 700 - 800°C within a short period of time, such as within a few seconds. This will result in a very reliable ignition. As the said detector 2 detects, when the burner ignites, the detector will emit a signal to the control circuit 6 at the moment of ignition, at which time the control circuit immediately turns off the laser. Even if the laser 7 has a sufficient power to quickly heat the point on the burner head to the above mentioned temperatures, it will not melt the material in any point on the burner head, because of the rapid extinction of the laser 7.

Instead of shining on a point on the burner head, the laser 7 can shine on, for example, a protrusion of suitable metal that is not shown, projecting from the burner head. The important thing is that the point that the laser shines upon is located along the path of transportation of the fuel/oxidant mixture.

According to a preferred embodiment, the laser 7 shines on an area of the side of the burner head facing the furnace space, between said ducts 3, 4.

According to another preferred embodiment, the laser beam 8 is directed at an angle slightly less than a right angle with respect to the longitudinal axis of the burner 1, as is illustrated by a laser 9 in Fig. 1.

According to an alternative embodiment, the laser beam 10 is directed towards the side of the burner head facing the furnace space in a direction essentially coinciding with the longitudinal axis of the burner 1, as is illustrated by a laser 11 in Fig. 1.

According to an alternative embodiment the laser beam 12 is directed in such a way that it runs in one of the said ducts 4, in a direction towards the side 5 of the burner head 1 facing the furnace space, so that the duct 4 is being illuminated on its inner side proximate to its orifice, as is also illustrated in Fig. 1.

Above, different embodiments have been described. The laser can be of various known models, and it can be directed towards any suitable point on the burner head. The laser can be integrated in the burner or separate.

The present invention shall therefore not be considered to be limited to the above indicated embodiments but can be varied within the scope of the attached claims.