Fluidized bed combustion boiler

Abstract

 Fluidized bed combustion boiler comprising a combustion chamber (2) with in its lower part a bottom plate (3) with openings. The bottom plate is destined for carrying a fluid bed, such a sand bed (4), thereon. A burner (5) is present for providing combustion air, which is transported through a combustion air pipe (7). The combustion air pipe comprises whirl pipes (8), which extend through the openings in the bottom plate. A cooling element (9) is placed at a distance over the bottom plate and comprises a number of cooling pipes for containing a cooling agent, such as water. A dosage system (10) doses the fuel, preferably wood, to the combustion chamber. The fluidized bed combustion boiler is further provided with temperature measuring means for measuring the temperature of the fluid bed and for providing measuring signals. Movement means change the distance between the cooling element and the bottom plate by moving the cooling element. Control means receive the measuring signals and control the movement means in reaction to the measuring signals.

Description

[0001] The present invention relates to a fluidized bed combustion boiler comprising a combustion chamber with in its lower part a bottom plate with openings, which bottom plate is destined for carrying a fluid bed, such a sand bed, thereon, a burner for providing combustion air, a combustion air pipe for transporting the combustion air, which combustion air pipe comprises whirl pipes, which extend through the openings in the bottom plate, a cooling element placed at a distance over the bottom plate, which cooling element comprises a number of cooling pipes for the bottom plate of a cooling agent, a dosage system for dosing the fuel to the combustion chamber.

[0002] Such a fluidized bed combustion boiler has been used for quite some time for combusting coals for for instance supplying steam or hot water. Such a boiler can also be used as pre-heater or as main boiler. Exactly because of the small emissions of furnace gasses such as NO_x and SO₂ a combustion boiler is highly suitable from an environmental point of view. A broader use of a combustion boiler, for instance by also combusting wood, however, has not been possible up until now. In particular the small weight of wood in relation to coal plays a part here, because of which the combustion of the wood mainly takes place in the upper part of the fluid bed and even above it, and the fact that wood supplied to the boiler may have strongly varying humidity contents. As a result of this a constant and protracted combustion of wood in a known fluidized bed appeared not to be possible.

[0003] It is an object of the present invention to provide a fluidized bed combustion boiler with which it is possible among others to combust wood in a correct manner.

[0004] For that purpose a combustion boiler of the kind described above according to the present invention is characterized in that the fluidized bed combustion boiler is provided with temperature measuring means for measuring the temperature of the fluid bed and for providing measuring signals, with movement means for changing the distance between the cooling element and the bottom plate by moving the cooling element, and with control means for receiving the measuring signals and for controlling the movement means in reaction to the measuring signals. The invention is based on the insight that as a result of the small weight and the inconstant degree of humidity of wood, the temperature of the fluid bed, for instance a sand bed, is not constant enough to have the known fluidized bed combustion boiler function well. By changing the place of the cooling element, which is located in the sand bed during use of the boiler, in reaction to the temperature of the sand bed, according to the invention a sufficiently constant temperature of the sand bed can be obtained for efficiently and correctly combusting wood, in particular waste wood, and other fuels.

[0005] Some embodiments of a fluidized bed combustion boiler according to the present invention will, by way of example, be described on the basis of the drawing, in which:

Figure 1 schematically shows a known fluidized bed combustion boiler for combusting coal;

Figure 2 schematically shows a fluidized bed combustion boiler according to the invention for combusting in particular waste wood;

Figure 3 schematically shows a part of a movement means of the fluidized bed combustion boiler of figure 2.

[0006] In figure 1 a known fluidized bed combustion boiler for combusting coal is schematically shown. As such a coal boiler is generally known, it will only be discussed in short hereafter.

[0007] The combustion of coal in the known fluidized bed combustion boiler 1 takes place in a combustion chamber 2. In the lower part of this combustion chamber 2 a bottom plate 3 has been arranged, which has been provided with openings. At the start of the combustion process a fluid bed, such as a sand bed 4 lies on the bottom plate 3. A burner 5, for instance a natural gas or light oil burner, possibly provided with a ventilator 6, provides combustion air, which is transported to the combustion chamber 2 through a combustion air pipe. The combustion air pipe 7 comprises whirl pipes 8, which extend through the openings in the bottom plate 3. A cooling element 9 is placed at a distance over the bottom plate 3, and contains a number of cooling pipes for containing a cooling agent, such as for instance water. A dosage system 10 doses coal to the combustion chamber 3.

[0008] During use of the coal boiler the hot flue gasses are blown through the pipes 8, so that the sand is brought in whirling turbulence and heated. The sand here reaches beyond the cooling element. At a temperature of about 550°C coal is introduced in doses into the sand bed 4. The coal reach ignition so that the temperature rises further. When the sand bed 4 has reached a temperature of about 750°C the burner 5 is switched off and cold air is blown through the sand bed 4. Because of the combustion of the coal the temperature further rises up to about 850°C and this temperature is maintained in the sand bed 4. This is obtained by directly discharging heat via the cooling pipes, of which the heat transfer from the hot sand to the boiler water is very efficient.

[0009] According to the present invention this known fluidized bed combustion boiler for coal is improved such that it is also suitable for combusting wood, in particular waste wood. Other fuels can also be combusted in the inventive fluidized bed combustion boiler.

[0010] For that purpose the fluidized bed combustion boiler 1' (see figure 2) of the present invention is provided with temperature measuring means 11, for instance a thermistor, for measuring the temperature of the sand bed 4' carried by the bottom plate 3'. The temperature measuring means 11 provide measuring signals that are representative of the temperature, and transfer these measuring signals to the control means 13 via a line 12. Movement means 14 and 15 are present for changing the distance between the cooling element 9' and the bottom plate 3', by moving the cooling element 9'. By means of lines 16 and 17 the control means 13 are connected to the movement means 14 and 15, respectively. The control means 13 receive the measuring signals from the temperature measuring means 11 and control the movement means 14 and 15 in reaction to the measuring signals. Should for instance the temperature of the sand bed 4' drop below a pre-determined level, the movement means 14, 15 are controlled such that the cooling element 9' is moved upwards step by step until the wanted temperature value is reached again. By moving the cooling element 9' a flexible connection of the supply and discharge end of the cooling element with the rest of the boiler is desirable. This flexible connection can be realized by tubes, and is schematically shown in figure 2 by the lines 18 and 19.

[0011] The movement means 14, 15 preferably each contain a piston rod 20 (see figure 3) which is connected with one end to the cooling element and with the other end to the so-called solid world of the boiler 1'. The piston rods 20 may be driven hydraulically, pneumatically or electrically, for instance by means of spindle engines.

Claims

1. Fluidized bed combustion boiler comprising a combustion chamber with in its lower part a bottom plate with openings, which bottom plate is destined for carrying a fluid bed, such a sand bed, thereon, a burner for providing combustion air, a combustion air pipe for transporting the combustion air, which combustion air pipe comprises whirl pipes, which extend through the openings in the bottom plate, a cooling element placed at a distance over the bottom plate, which cooling element comprises a number of cooling pipes for containing a cooling agent, a dosage system for dosing the fuel to the combustion chamber, characterized in that, the fluidized bed combustion boiler is provided with temperature measuring means for measuring the temperature of the fluid bed and for providing measuring signals, with movement means for changing the distance between the cooling element and the bottom plate by moving the cooling element, and with control means for receiving the measuring signals and for controlling the movement means in reaction to the measuring signals.

2. Fluidized bed combustion boiler according to claim 1,
characterized in that, the dosage system for dosing the fuel to the combustion chamber is a dosage system for dosing wood, in particular waste wood.

Drawing