Combustion Plant

This invention relates to an apparatus for combustion or a fuel while in suspension, and in particular to a steam boiler plant including a whirl chamber in which the fuel is suspended and burned.

Steam boiler plants in which the combustion of the fuel necessary for the generation of steam takes place in a fluidized-bed are known. In the fluidized-bed, the fuel is mixed with an inert, grain-shaped material, which does not itself participate in the combustion. The inert material serves to maintain an even distribution of the fuel on the fluidized-bed and to transfer the heat generated to heat absorbers, e.g. built-in heat absorbing boiler tubes. Also, at least part of the oxygen necessary for the combustion is taken from the fluidizing air between the inert material.

However, in fluidized-beds the bottom on which the fluidized material rests is disadvantageously rather complex, inasmuch as it is composed of a number of nozzles through which the fluidizing air is blown in. Such nozzles are likely to block up, and it calls for a considerable pressure drop over the nozzles in order to maintain an even air flow over the entire cross-section of the bed. This in turn results in a substantial loss of power. I have invented an improved whirl chamber type boiler plant which avoids the above-noted limitations of the prior art.

The invention relates to a whirl chamber boiler plant which comprises at least one combustion chamber having boiler tubes therein. The bottom portion of the chamber slopes generally downwardly from the walls thereof toward a generally centrally positioned combustion air inlet means. The invention further comprises means for directing fuel and at least one inert material to the bottom portion of the chamber, and outlet means at the upper portion of the chamber for exit gases and particle products of combustion. Preferably the inert material is in the form of generally grain-shaped particles.

According to the invention the inert material is preferably an ash which is similar to the kind produced by the burning of the fuel. Upon introduction into the chamber, the fuel and hot, inert material are mixed intimately into a mixture which slides downwardly along the sloping bottom towards the air intake. The inert material preheats the fuel to its ignition temperature. Upon coming into contact with the flowing-in combustion air the fuel starts to burn. The exit gases and part of the ash particles leave the combustion chamber at its top though an outlet. The remaining ash particles slide down along the walls of the chamber, give off their heat to the built-in boiler tubes, and at the bottom mix with fresh fuel before again meeting with blown-in combustion air. Thus a whirl is formed in the combustion chamber, having an upwardly directed flow centrally in the chamber and a downwardly directed flow along the chamber walls. The advantage over the fluid-bed is achieved by the combustion air inlet having a large cross-section which avoids any blockage and provides only a small pressure drop.

As the exit gases constantly entrain part of the inert material, it is necessary continuously to supply fresh inert material. According to the present invention this may be achieved by providing the combustion chamber at its outlet with a separator, which separates part of the hot ash particles and returns at least part of these to the combustion chamber.

If insufficient material is separated in the separator for maintaining the necessary amount of inert material in the combustion chamber, it may according to the invention be expedient to provide means for returning cold separated ash to the combustion chamber. The cold ash may be ash continuously being separated off before the exit gases are passed out through a chimney, or ash stored for this use.

According to the invention the plant may be equipped with measuring devices for determining the size of the charge in the whirl chamber, so as to decide on the need to return inert material to the combustion chamber and the amount of such material.

The heat released by the combustion is removed and utilized by means of the built-in boiler tubes, which according to the invention may preferably be located along the walls f the combustion chamber, preferably oriented in a vertical direction. Such an arrangement of the boiler tubes ensures that they do not interfere with the whirling movement of the inert material, which movement consists of a central, upwardly directed stream in which the inert material is heated while the added fuel is burned, and a downwardly directed stream along the walls of the combustion chamber, during which movement the inert material gives off its heat to the boiler tubes. Eventually, the inert material is mixed with fresh fuel at the bottom of the chamber.

According to a preferred embodiment, lime or dolomite may be fed along with the fuel in order to remove sulphur oxides. The lime supplied is either fed separately or mixed with the fuel and in doses proportioned to the sulphur content of the fuel.

The present invention is described in detail below with reference to the drawing which illustrates a boiler plant having a whirl combustion chamber constructed according to the present invention.

Referring to the drawing, a boiler or combustion plant including a whirl combustion chamber 1 is shown having an inlet, an outlet, and built-in boiler tubes 2 which serve to conduct away the thermal energy generated during the combustion. Preferably, the boiler tubes are arranged along the walls of the chamber and in a vertical direction. Fuel and an inert, grain-shaped material are led to the bottom 3 of the combustion chamber 1. The bottom 3 slopes downwardly towards a central inlet 4 for passage of combustion air which is indicated by arrow 15. The drawing shows the input of fuel by fuel supply line and arrow 5, and of inert material by inert supply line and arrow 6. A supply line and arrow 7 provides lime for removal of sulphur oxides. The supply lines and arrows 5, 6, and 7 signify only that the respective materials are supplied to the combustion chamber and are not indicative of the exact location in the combustion chamber 1 where the supply lines end. Thus, in the case of coal firing, the addition of lime may occur by adding lime or dolomite in the coal mill so as to feed coal and lime through the same supply tube.

The added inert material, in one embodiment of the present invention is an ash similar to the kind produced by the burning of the fuel. Preferably the inert material has a temperature of combustion above the ignition temperature of the fuel. In this manner the fuel, which is intimately mixed with the inert material during its entry into and passage further downwardly along the inclined bottom of the combustion chamber 1, is evenly distributed and preheated to such a temperature as to start burning upon contact with the blown-in combustion air. The combustion takes place in a whirl which moves upwardly and centrally in the combustion chamber 1 and also downwardly along the walls of the combustion chamber 1. The combustion is preferably controlled in such a way that the hot exiting gases can be conducted away at the top of the combustion chamber 1 through an outlet at a temperature of approximately 700 degrees C. Part of the exiting gases conducted away from the chamber is led to a cyclone 8 through its tangential inlet in which entrained ash particles are separted and are returned through the material outlet 9 of the cyclone to the inert material supply line 6. The exiting gas from the cyclone 8 is, together with any bypassed exiting gas from the outlet, passed in a known manner, as shown in the figure, from the combustion chamber 1 past an economizer 10, and a preheater 11 for combustion air indicated by arrow 15. The preheater 11 is in thermal communication with the combustion air. As shown in the figure, a conduit in communication with the inlet of the combustion chamber 1 passes integrally through the preheater 11 without any mixing of the respective contents therein. The exiting gases further pass to an ash separator 12 through its inlet, in which the remaining ash is separated in a known manner by means of cyclones and electrofilters. From the ash separator 12 and dedusted exiting air is passed to a chimney 13. Also, part of the separated dust may, if desired, be returned through an outlet and through a tube 14 in communication with supply line 6 to the combustion chamber 1 as inert material. Thus it is possible by the addition of cold ash to maintain the required amount of inert material in the combustion chamber 1, should the amount of ash separated in the cyclone 8 prove insufficient. By measuring the pressure drop over the chamber 1 by suitable means (not shown), it may be determined whether the combustion chamber contains the required amount of inert material. Thus the pressure drop measured can be used for controlling the amount of ash to be returned to the combustion chamber 1.

A vane 16 is provided in the conduit coupling the outlet of combustion chamber 1 with the inlet of economizer 10 so as to regulate the flow of exiting gases through the conduit. If desired, the vane 16 can be coupled to and regulated by the pressure measuring means so as to control the flow of ash laden gases passing through the conduit. This in turn, serves to regulate the quantity of both cold and hot ash material which is returned to the combustion chamber 1.

In the drawing, the boiler plant comprises a single combustion chamber. Since the useful gas distribution to be maintained in the chamber may place an upper limit on the size of such a chamber, larger plants can be made of several combustion chambers working in parallel to each other with the combustion being controlled separately in each chamber.