TECHNICAL FIELD

The present invention relates to a nozzle for feeding a particulate material into a fluidized bed, wherein the parti¬ culate material preferably consists of a fuel which is fed in mixed with a liquid into a paste. The invention further relates to a device and methods by which plugs, formed or initiated in the nozzle, in the particulate material may be cleaned away and, in addition, prevent material from pene¬ trating from outside into the nozzle, that is to say that the nozzle is provided with a plug cleaning and blocking function.

BACKGROUND ART

When feeding, for example, particulate fuel into a fluidized bed in a power plant, there is utilized, for reasons of trans¬ port engineering, among other things, the possibility of supplying the particulate material to the bed mixed with water, another liquid, or an emulsifier to impart to the mixture a paste-like consistence. When being fed into the bed, the paste is distributed by means of nozzles especially designed for paste feeding. One such known nozzle for feeding fuel in the form of a paste is described in the patent specification SE-B-469145. According to this publication, paste is fed to the nozzle in a pipe of even thickness, at the outermost part of which there being arranged a conical section which forces the paste to pass through a constriction. At the narrowest part of the conical section, the outlet of the nozzle is arranged in the form of a circular outlet opening through which the paste leaves the nozzle. To obtain a good distribution inside the bed of the paste which leaves the nozzle, a pipe for splitting air is disposed inside the nozzle pipe, along the axis thereof, which splitting air pipe termi¬ nates in a jet immediately inside the outlet of the nozzle. By

supplying splitting air at a definite distance inside the out¬ let opening of the nozzle, air leaving the air nozzle will occur as a cone-shaped air jet which brings along paste on its way out through the outlet opening and thus split the paste into droplets which are spread in a conical region out from the outlet opening. By this arrangement, paste and hence the particulate material, for example a fuel, will be well distributed in the bed.

When a plant according to the above is adjusted, for example, to low-load operation, problems may arise with regard to the fuel feed at a nozzle of the above-described kind, in that the paste, which usually consists of pulverized coal mixed with water, dries out or segregates. As a consequence of this, a plug may occur in the conical section of the fuel nozzle. This may entail expensive downtime for the entire plant, since the plug, which consists of a hardened coal mass, must be removed from inside, for example by mechanical intervention.

Attempts have been made to achieve cleaning of the plug while feeding fuel in a power plant by utilizing the splitting air flow. If the splitting air flow is intensified, a hole through the plug may be achieved; however, no paste may penetrate through the hole when resuming the feeding of paste. With this solution, however, no undisturbed feeding is achieved, since it has proved that the splitting air only opens a central hole through the plug, whereas the surrounding residual plug remains intact. Consequently, mechanical cleaning is necessary also when using this method.

When starting a plant according to the above, there is a need to keep the nozzle pipe completely clean from material pene¬ trating therein before the paste feed starts, that is, a blocking function is desirable. In this connection, known fuel nozzles suffer from the drawback that this blocking function is missing completely or partially. This problem with the

above known fuel nozzle manifests itself also in case of completed fuel feed through the nozzle in that the bed material may penetrate through the outlet opening into the nozzle and clog the nozzle.

SUMMARY OF THE INVENTION

The present invention relates to a nozzle for feeding a parti¬ culate material in the form of paste into a fluidized bed in a power plant, wherein the nozzle comprises an air pipe which is disposed inside the nozzle and which directs a jet of splitt¬ ing air against the orifice of the nozzle and a cleaning medium pipe outside the splitting air pipe and arranged coaxially therewith which comprises a vortex generator to achieve a cone of a flowing medium, preferably air or water, wherein the cone has such a top angle that it hits the inside of the nozzle at that conical section of the nozzle which tapers towards the orifice, in order thus to make possible dissolution of a plug of hardened paste, formed or initiated in the nozzle, or prevent bed material from penetrating into the nozzle.

The vortex generator, located inside the orifice of the cleaning medium pipe between the splitting air pipe and the outer cleaning pipe, consists of oblique slots which set medium flowing through the pipe in rotation and hence force the medium, when the medium has left the orifice of the cleaning pipe, to be thrown outwards because of the centri¬ fugal force acting on the medium, thus forming a cone-shaped jet of the medium which thus fills up the cone-shaped jet. The air which simultaneously flows through the splitting air pipe will thus be set in the same rotary motion as the cleaning medium when the two flows, the cleaning medium and the splitting air, are brought together.

If, for example, water is used as medium through the cleaning pipe when the nozzle is plugged by paste in the conical section of the nozzle, the cone of water which is supplied to the plug from inside the nozzle will cause water to force its way through the nozzle wall and the plug, the plug thus being dissolved from outside and inwards, which results in the plug after awhile being dissolved and flushed out through the orifice and being completely dissolved. This makes it possible to dissolve a paste plug during operation without downtime in the plant by supplying, for example, water through the clean¬ ing pipe of the nozzle and, by means of the vortex generator, to set the water in rotation. Depending on the hardness of the paste plug, differently hard pressures may be applied to the cleaning water. A hard plug is dissolved more rapidly if the water pressure is higher.

By the supply of water as medium in the cleaning pipe, a good blocking function is obtained with respect to bed material penetrating into the nozzle. The air cone which arises when the blocking medium (in this case air) flows from the cleaning pipe fills up the whole conical section of the nozzle from the point where the air cone hits the conical section of the nozzle and up to the orifice and prevents bed material from penetrating in since the whole area of the orifice is filled with the outflowing cleaning air. This function of the nozzle is particularly advantageous when fuel is fed in the form of a paste in a power plant, during periods of downtime in the paste feed, for example upon start-up or trip of the plant, when the supply of air and the creation of the above-described air vortex and the ensuing cone of air prevent penetration of bed material into the nozzle. As mentioned above, only using splitting air is not sufficient in these contexts, since the almost ejector-like splitting air jet creates a sub-atmos¬ pheric pressure in the nozzle pipe which rather contributes to suck bed material into the nozzle pipe.

The time for dissolution of a plug by means of water in the cleaning pipe is dependent on the distance between the orifice of the cleaning pipe and the orifice of the nozzle and on the water pressure. Further, the pitch of the slots arranged in the vortex generator is also formed such that the top angle in the cone-like flow of flowing water leaving the nozzle of the cleaning pipe is adapted to give the water cone such a cross section that the outlet of the nozzle is filled up to a satis¬ factory extent by the vortex of flowing medium.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 schematically shows a representation of a PFBC power plant, wherein a nozzle according to the invention is used.

Figure 2 shows a schematized cross section through a nozzle according to the invention.

Figure 3 illustrates an enlarged section of the front part of the splitting air pipe with the surrounding ςleaning pipe, the nozzle of the cleaning pipe, and the position of the vortex generator between these two pipes.

Figure 4 illustrates the vortex generator seen in a plane view.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention will be described in the following with reference to the accompanying drawings and exemplified for use in a PFBC power plant wherein fuel in the form of a paste comprising pulverized coal is fed into a fluidized bed which is under a high pressure.

The nozzle for feeding of paste according to the invention may advantageously be used with other types of bed as well, for

example circulating fluidized beds, MBC beds etc., where the bed is not subjected to a higher pressure.

In an overall figure (Figure 1), the central units of a PFBC power plant are represented, wherein a combustor 1 is housed in a pressure vessel 2. Air from a compressor (not shown) is supplied to the pressure vessel 2 via the air inlet 3 for pressurization of the pressure vessel 2 and hence also the combustor 1. The compressed air 4 is supplied to the combustor 1 via fluidization nozzles 5 at the bottom of the combustor for fluidization of a bed 6 enclosed within the combustor. The bed consists of bed material, a particulate absorbent and a particulate fuel which is burnt in the fluidizing air 4 supplied to the bed 6. Combustion gases from the bed 6 pass through a freeboard 7 above the surface 8 of the bed and are forwarded via the outlet 9 for cleaning in dust separators, whereafter the combustion gases are expanded in a gas turbine (not shown) , where the energy contents in the gases are transformed into useful energy.

In the fluidized bed 6, a tube bundle 10, which is completely immersed into the bed at full-load operation, is also shown. Water is supplied to the tube bundle 10 at 11 for cooling the bed 10 and further for generating steam in the tubes in the tube bundle. The steam is forwarded at 12 to a steam turbine (not shown) in a steam cycle in the plant.

At the lower part of the bed 6, a number of fuel lances 14 for the supply of particulate fuel in the form of paste are arranged. The fuel lances 14 terminate inside the bed 6 in a fuel nozzle 15 for distribution of fuel paste.

The nozzle 15 for feeding paste 15a according to the invention is shown in Figure 2. The nozzle consists of an extension of the fuel lance 14 for paste added to the fuel lance. Further, the front part of the nozzle 15 is formed with a constriction

in the form of a conical section 17, which at its narrowest part changes into the orifice 18 of the nozzle 15, through which orifice paste 15a flows out into the surrounding bed 6. Inside and along the axis of the nozzle 15, a pipe 19 for splitting air 19a is disposed in accordance with known technique. Air or another gas which is supplied as splitting air 19a through the pipe 19 flows out of the pipe in a cone- shaped jet which splits up paste in the region surrounded by the conical section 17 into droplets which are thrown out of the orifice 18 of the nozzle 15 in an evenly distributed beam.

According to the invention, a cleaning pipe 20 is arranged coaxially with the splitting air pipe 19 and outside thereof, the terminating end of the cleaning pipe inside the nozzle 15 being extended and somewhat longer than the end of the splitt¬ ing air pipe. That part of the end of the cleaning pipe 20, which is located in front of and outside the end of the splitting air pipe 19, is formed as a nozzle 21.

Between the two coaxially placed splitting air pipes 19 and the cleaning pipe 20, a vortex generator 22 which will be described below is arranged in the front part of these two pipes. When a medium 20a, for example air or water is supplied from outside to the cleaning pipe 20, this medium 20a is forced to traverse the vortex generator 22, which causes the medium to be set in a helical motion. The vortex which is imparted to the medium 20a propagates out through the nozzle 21, where the speed of the medium increases, whereafter the vortex leaves the nozzle and expands while forming a medium cone under the influence of the centrifugal force until the medium cone 23 formed encounters the inner walls of the coni¬ cal section 17 at the nozzle 15 and follows the inner contour of this conical section before exiting from the orifice 18 of the nozzle 15. The medium flow out of the nozzle 21 is given the shape of a cone with a top angle of the cone determined by

the pitch of oblique slots arranged in the vortex generator 22.

As medium 20a in the cone 23 water is utilized to dissolve plugs which have formed in the conical part 17 of the nozzle and the orifice 18. Air or other gas may be supplied as alter¬ native medium in the cone 23 since the intention is to achieve a barrier to material penetrating into the orifice 18 from outside. By a suitable choice of the distance between the jet 21 and the orifice 18 of the nozzle, it is possible to build up an air or gas barrier which completely fills up the orifice 18 and hence prevents material from penetrating from the bed 6 which is under a high pressure.

Splitting air through the pipe 19 is distributed and utilized according to known technique.

Figure 3 shows in cross section a detailed enlargement of the end of the splitting air pipe 19, the surrounding cleaning pipe 20 with the end of this tube 20 formed into a nozzle 21, and the position of the vortex generator 22 placed in the annular space between the end of the splitting air pipe and the cleaning pipe 20.

Finally, Figure 4 illustrates the vortex generator 22 in side view, wherein oblique slots 24 for the flowing medium 20a are shown with a pitch angle α according to the figure, which according to a preferred embodiment has the magnitude 45°. Under certain circumstances, the angle α may be varied within the interval ±20°. Between the slots 24 for the medium there are walls 25, the function of which is to direct the flow of the medium.