EP0050105A2

EP0050105A2 - A method and a device for the combustion of solid fuels

Info

Publication number: EP0050105A2
Application number: EP81850184A
Authority: EP
Inventors: Kurt Sjöstrand; Karl-Eric Johnsson
Original assignee: Individual
Current assignee: Individual
Priority date: 1980-10-09
Filing date: 1981-10-09
Publication date: 1982-04-21
Also published as: EP0050105A3

Abstract

The invention relates to a method and a device for the combustion of preferably solid fuels. The fuel is combusted in a combustion chamber, from which the exhaust gases produced by the combustion are discharged.

By the fact that the temperature of the exhaust gases is elevated and/or the exhaust gases are supplied with oxygen after the gases have left the combustion chamber, it is possible to ignit the exhaust gases for combusting combustible substances present in the exhaust gases.

Description

The present invention relates to a method and a device for the combustion of fuels, preferably solid fuels.
The increasing oil prices.have made the use of solid fuels, such as wood, peat and waste products for heating purposes-more and more desirable. However, the drawbacks of previously'known methods and devices for the combustion of solid fuels are considerable. Bad efficiency, impure exhaust gases including high percentage of carbon and carbon monoxide and unsatisfying control possibilities are problems . which have proved to be difficult to solve in this connection.
Therefore, the object of the present invention is to provide an improved method and an improved device for the combustion of solid fuels.
This object is satisfied by means of the method according to the invention in which the solid fuel is combusted in a combustion chamber, from which the exhaust gases created by the combustion are discharged and which is characterized in that the exhaust gases after having left the combustion chamber by means of an elevation of the temperature and/or an addition of oxygen are brought to a combustible condition, whereupon the inflammable substances present in the exhaust gases are combusted
In accordance with the invention it- is-thereby preferred that the exhaust gases are supplied with oxygen by the addition of air and that the temperature of the exhaust gases is elevated by conducting the exhaust gases along a surface which is heated by the supply of heat from-the combustion of the fuel in the combustion chamber by radiation and/or convection. Thereby, it is preferred that the air is heated before it is supplied to the exhaust gases.
Suitably the exhaust gases are conducted from the combustion chamber through a channel, the exhaust gases while flowing through the channel being supplied with oxygen as well as being heated and the exhaust gases being combusted in substantially the same way as in a burner for gaseous or liquid.fuels when the exhaust gases flow out from the channel. Because of the fact that the addition of oxygen to the exhaust gases takes place at a controlled positive pressure it is possible to control the flame created by the combustion of the exhaust gases dependent on the temperature of a medium which is heated by said coinbustion.
The method according to the invention makes it possible to combust solid fuels with high efficiency, pure exhaust gases. and substantially as good control possibilities as in the combustion of.gas and oil.
According to the invention there is also provided a device for the combustion of solid fuels in the method defined above, said device comprising a combustion chamber, in which the combustion of the fuel is intended to take place, and a device for discharging the exhaust gases created by the combustion from the combustion chamber, said device being characterized by. a device for elevating the temperature of the exhaust gases after the exhaust gases have left the . combustion chamber,.and/or by a device for supplying oxygen to the exhaust gases, whereby the exhaust gases are provided with such properties so as to be combustible.
In an advantageous embodiment the device according to the invention is designed so that the device for discharging the exhaust gases comprises a channel system which is at least partially positioned between partitions adjoining the combustion chamber and the outer walls of the combustion device.
In an embodiment of the device according to the invention the channel system comprises two at opposite sides of the device positioned, in parallel with the adjoining side of the device extending, substantially U-shaped channels, each channel having an exhaust gas inlet at the upper end of one leg of the U-shaped channel and an exhaust gas outlet at the upper end of the other leg of the U-shaped channel. Thereby, it is suitable that the exhaust gas outlets of the U-shaped channels open into a collection chamber, which is connected with a heat exchanger system in order to conduct the exhaust gases into this system, Because of the fact that the heat exchanger system is not positioned in direct connection with the combustion chamber the combustion therein will take place at a high temperature which leads to a high efficiency and-pure exhaust gases.
The invention is described in the following with reference to the accompanying drawings.

Fig. 1 is a vertical section of a combustion device according to the invention.
Fig. 2 is a horizontal section on line II-II in Fig. 1.
Fig, 3 is a vertical section of a modified embodiment of a combustion device according to the invention.
Fig. 4 shows a modification of the combustion device according to Fig. 1.
Fig. 5 is a vertical section of a further embodiment of a combustion device according to the invention on line V-V in Fig. 7.
Fig. 6 is a vertical section of the combustion device according to Fig. 5 on line VI-VI in Fig. 7.
Fig. 7 is a horizontal section of the combustion device shown in Figs 5 and 6 on line VII-VII in Fig. 5.

The combustion device for solid fuels shown in Figs 1 and 2 comprises a cover 2 having an upper door 4 through which the solid fuel can be supplied to the combustion device. The combustion-device is. divided into a combustion chamber 6, . which is defined by the cover 2, a partition 8 and a grate 10, an exhaust gas channel 12, which is defined by the partition 8 and the cover 2, and a space 14 posi.tioned below the grate 10 and i.a. being used for the collection of ash and can have some type of device (not shown) for this purpose. The exhaust gas channel 12 which as appears from Fig.'2 is of less width than the combustion chamber 6 is connected with the combustion chamber through an upper opening 16 and a lower opening 18. At the lower end of the exhaust gas channel 12 the channel is connected with a burner pipe 20 which preferably consists of a ceramic fibre material in order to resist high temperatures and to-be efficiently heat conductive.
An air supply pipe 22 opens into the upper portion of the combustion chamber 6
and is provided with a fan 24 for supplying air to the upper portion of the combustion chamber 6 through the pipe 22. An air supply pipe 26 is connected with the space 14 positioned below the grate 10, and a fan 28 is connected with the supply pipe 26 for supplying air to the space 14. The air which is supplied to the space 14 through the pipe 26 by means of the fan 28 is discharged in part to the lower portion of the combustion chamber 6 through-the grate 10 in order to promote the combustion in the combustion chamber 6 and in part to the lower portion of the exhaust gas channel 12 through a duct 30. The air supply pipes 22 and 26 as well as the duct 30 are provided with throttles 32, 34 and 36, respectively, in order to make it possible to control the supply of air to the combustion chamber and the exhaust gas channel in any desired way. The supply of air is effected in such a way that the air is heated in the combustion device before it is introduced into the exhaust gas channel 12.
Preferably the burner pipe 20 opens into a secondary combustion chamber which for example can be constituted by the combustion chamber of a conventional heating boiler 38 shown by means of broken lines, which is adapted to heat water for radiators and a water heater. As shown by means of the broken line 40 the fans 24 and 28 can be connected with the heating boiler in order to be controlled dependent on the temperature of the water of the heating boiler 38.
The combustion'device is started up by setting fire to a solid fuel, for example wood, which has been introduced through the door 40 and is supported by the grate 10, in a conventional way. Thereby the supply of the air necessary for the combustion is provided either by means of self-draught through the air supply pipe 26 and optionally also the air supply pipe 22 or by the fact that the fan 28 and optionally also the fan 24 compulsory supplies air to the space 14 and the combustion chamber 6, respectively. The exhaust gases created by the combustion are discharged to the exhaust gas channel 12 through the openings 16 and 18 and arrives through the burner pipe 20 into the heating boiler 38 which in a conventional way is connected with a chimney. During the initial course of the combustion the throttles 32 and 34 are somewhat closed in order to throttle the. supply of air through the supply pipes 22 and 26.
When the combustion device has reached a suitable temperature the throttles 32 and 34 are opened and the fans 24 and 28 are started, if the starting thereof has so far not been provided. Thus, there is provided a supply of air to the lower portion of the combustion chamber through the grate 10, a supply of air to the lower portion of the channel 12 through the duct 30 and a supply of air to the upper portion of the combustion chamber, which provides that exhaust gases mixed with air are supplied to the channel-12 through the openings 16 and 18 while a further supply of air to the exhaust gases is concurrently taking place before the exhaust gases arrive into the burner pipe 22. The air supplied to the exhaust gas channel 12 has been conducted through the heating device before it is supplied to the channel 12, so that the temperature thereof has been elevated and the air does not provide any cooling of the exhaust gases in the channel 12. In the lower portion of the exhaust gas channel 12 and in the burner pipe 20 a further heating of the exhaust gases takes place'by the fact that the exhaust gases flow along surfaces which have been heated to a high temperature by the conduction of heat from the lower portion of the combustion chamber 6. By means of the supply of air to the exhaust gases and the heating thereof the exhaust gases are provided with- such properties, that the gases are ignited by self-ignition when leaving the burner pipe 20. The flame created thereby will act in the same way as the flame of an oil burner. In order to provide a complete combustion the properties of the flame can be controlled by positioning the throttles 32, 34 and 36 in the desired way.
As mentioned above, the fans 24 and 28 can be controlled dependent on the temperature of the water in the heating boiler 38. Thus, when the temperature
of the water in the heating boiler 38 reaches the desired value, there is generated a signal for stopping the fans 24 and 28, the supply of'air to the combustion device being thereby reduced to such a degree that the.flame of the burner pipe 20 goes out. The combustion of the solid fuel in the combustion chamber 6 is maintained by means.of a small supply of air by self-draught also when the fans-24 and 28 are not working.
When the temperature of the water.in the heating boiler 38 has decreased to a predetermined temperature, there is produced a signal for starting the fans 24 and 28, whereby the supply of air to the combustion device is again started as described above and the exhaust gases are again self-ignited outside the burner pipe 20 while producing a flame for heating the heating boiler 38.
In a combustion device-according to the invention it has proved to be possible to provide such a complete combustion of solid fuels, for example wood, that the exhaust gases leaving the combustion device contain 0 % carbon and 0 % Co. Thus., the combustion device according to the invention provides possiblities not previously known for obtaining a complete combustion of solid fuels and quite unique possibilities of controlling the combustion of solid fuels with regard to the desired effect.
In Fig. 3 there is shown a somewhat modified embodiment of a combustion device according to the invention. The combustion device according to Fig. 3 is with regard to essential portions designed in the same way as the combustion device according to Figs 1 and 2, and the portions of the combustion device according to Fig. 3 which correspond to the portions of the combustion device according to Figs 1 and 2 have the same reference numerals as in the last mentioned figures with the addition "a".. The most importantdifference between the combustion device according to Fig. 3 and the combustion device according to Figs 1 and 2 is that the combustion device according to Fig. 3 has its burner pipe 42 positioned at the upper portion of the exhaust gas channel 12a instead of at-the lower portion thereof. Thereby the discharging of the exhaust gases from the combustion chamber 6a is facilitated, which also entails that there is not required any upper air supply pipe with a fan belonging thereto. When the fan 28a is activated, the air is supplied through the air supply pipe 26a into the space 14a from which the air is discharged to the combustion chamber 6a through the grate 10a as well as to the exhaust gas channel 12a through the duct 30a. Because of the positive pressure created in the combustion chamber 6a the exhaust gases are forced through the opening 16a to the burner pipe 42, whereby the exhaust gases before arriving into the.burner pipe 42 is supplied with excess air from the channel 12a. When the exhaust gases leave the burner pipe 42 there is provided a self-ignition, and the exhaust gases burn with a flame in the same way as described with reference to Figs 1 and 2. .
Also the combustion device according to Fig. 3 can be connected with a heating boiler in order to be utilized for heating water for radiators and a hot water boiler. Preferably there is in the embodiment according to Fig. 3 also provided a control of the supply of air to the combustion device by means of the fan 28a dependent on the temperature of the water in the heating boiler, so that the flame from the burner pipe 42 is ignited and extinguished in order to maintain the temperature of the water within predetermined limits.
In a modification of the device according to the invention it is advantageous to connect a duct between the exhaust gas channel opposite to the upper opening 16 in the partition 8 and the burner pipe 20. Thereby the discharge of exhaust gases from the upper portion of the combustion chamber 6 is facilitated. By providing said duct with a control throttle there is provided a further possibility of controlling the flame from the burner pipe 20. This discharge of exhaust gases from the upper portion of the combustion chamber is provided in an especially advantageous way in the modification.of the combustion device according to Fig. 1 which is shown in Fig. 4. In accordance therewith a separate duct 44 is connected between the upper portion of the combustion chamber 6 and an annular space between the outer surface of the burner pipe-20 and a pipe section 46 concentrically enclosing the burner pipe. The exhaust gases which are supplied to said annular space flow out around the outlet of the burner pipe 20 and take part in the combustion.
It is recognized that the air supply pipes 22 and 26 can be provided with a common fan instead of having separate fans.
The combustion device shown in Figs 5-7 has a cover 102 of sheet metal material and an insulating mat 104 positioned inside the cover and having such properties that a heat admission from the side walls of the combustion device is prevented in an effective way. The cover 102 and the insulating mat 104 form a front wall 106, a back wall 108, side walls 110 and 112 and a bottom 114 of the combustion device. Inside the insulating mat 104 the walls 106-112 and the bottom 114 have walls 116 manufactured from a refractory cast compound having the capacity of resisting temperatures up to about 1.500°C. At the side walls 110 and 112 there are inside the-walls 116 positioned partitions 118 and 120 which together with transverse walls 122 and _'124 define U-shaped channels 126 and 128,-respectively, at the side walls 110 and 112. Thus, the channel 126 has a vertical portion 130, a horizontal, lower portion 132 and a vertical portion 134.. The U-shaped.channel 128 has a vertical portion 136, a horizontal portion 138 and a vertical portion 140. Openings 142 and 144 extend from the environment up to the horizontal portions 132 and 138, respectively, of the U-shaped channels 126 and 128, respectively, in order to supply secondary air to the channels 126 and 128 at these points. The openings 142 and 144 are controllable by means of throttles (not shown).
The combustion chamber 146 is positioned between the partitions 118 and 120 an_'d is at its lower part defined by a grate 148 'below which a space 150 for the collection of ash is positioned. The combustion chamber 146 has a combustion chamber door 152 and the space.150 has an ash door 154. The combustion chamber 146 is connected with the U-shaped channel 126 through an opening 156. Thus exhaust gases from the combustion chamber are supplied to the U-shaped channel 126 through said opening 156, whereupon the exhaust gases flow downwards through the vertical portion 130, the horizontal portion-132 and the vertical portion 134 of the U-shaped channel 126, the vertical portion 134 ending in an opening 158 which is connected with a collection chamber 160 for the exhaust gases. The.U-shaped channel 128 is connected with the combustion-chamber 146 through an opening 162, and exhaust gases from the combustion chamber will flow into the opening 162 downwards through the vertical portion 136, the horizontal portion 138 and upwards through the vertical portion 140 of the U-shaped channel 128 to an opening 164 which conducts the exhaust gases to the collection chamber 160. As appears from the drawings the inlet opening 156 to the U-shaped channel 126-is positioned diagonally in relation to the inlet opening 162 to the U-shaped channel 128 and the outlet openings 158 and 164 are in a corresponding way positioned diagonally in relation to each other. From the collection chamber 160 the exhaust gases are conducted upwards to a heat exchanger 166 which in accordance therewith is positioned entirely above the combustion device.
The embodiment of the combustion device shown in Figs 5-7 is adapted to be used for the combustion of solid fuels but it is recognized that it is possible to modify the combustion device so that it can be used also for oil firing.
When firing with wood the wood is thrown into the combustion device through the combustion chamber door 152 to the grate 148 where the wood is ignited. The exhaust gases are conducted through the U-shaped channels 126 and 128 and is supplied with secondary air at the horizontal portions 132 and 138, respectively, of said channels, which'air together with the fact that the temperature.of the exhaust gases is elevated because of the flowing of the gases outside the partitions 128 and 120 adjoining the combustion chamber 146.provides that.the exhaust gases are self-ignited when leaving the collection chamber 160. Thereby there is obtained.a combustion.of the combustible substances of the exhaust gases so that the exhaust gases leaving the combustion device contain 0 % carbon and 0 % CO.
In a way not shown the supply of air to the combustion chamber and to the channels 126 and 128 can be provided by means of fans which makes it possible to control the combustion in a more exact and effective way.
Because of the fact that the combustion chamber is separated from cold surfaces the combustion in the combustion device takes place at a high temperature providing for a very advantageous efficiency. Also the sub-combustion of the combustible substances of the exhaust gases contributes to the high efficiency.
In order to provide a lower temperature of the outer surface of the combustion device according to Figs 5-7 it is also possible to provide the combustion device with a space between the cover 102 and the insulating mat 104, said space being connected with the heat exchanger so.that the water flows through said space. Because of the fact that said space is positioned outside the insulating mat 104 it will not have any influence on the combustion temperature in the combustion chamber 46 or the temperature of the-exhaust gases in the channel systems 26 and 28.

Claims

1. A method for the combustion of preferably solid fuels, in which the fuel is cdmbusted in a combustion chamber, from which the exhaust gases produced by the combustion'are discharged, characterized in that the exhaust gases after having left the combustion chamber by elevation of the temperature and/or the.supply of oxygen are caused to be ignited for the combustion of combustible substances present in the exhaust gases.

2. A method as claimed in claim 1, characterized in that the exhaust gases are supplied with oxygen by the supply of-air.

3. A method as claimed in claim 2, characterized in that the air is heated before it is supplied to the exhaust gases.

4. A method as claimed in any of claims 1-3, characterized in that the temperature of the exhaust gases is elevated by conducting the exhaust'gases along a surface which is heated by the supply of heat from the combustion of the fuel in the combustion chamber by radiation and/or conduction.

5. A method as claimed in any of the preceding claims, characterized in that the exhaust gases are discharged from the combustion chamber through a channel, the combustion of the exhaust gases taking place at the outlet of said channel.

6. A method as claimed in claim 5, characterized in that the supply of oxygen to the exhaust gases is provided by the supply of air to the upper portion of the combustion chamber as well as to the channel somewhat upstream from the outlet thereof.

7. A method as claimed in any of the preceding claims, characterized in that the supply of oxygen to the exhaust gases is controlled dependent on the temperature of a medium heated by means of the combustion of the exhaust gases.

8. A device for the combustion of preferably solid fuels according to the method as claimed in claim 1, comprising a combustion chamber, in which the combustion of the fuel is intended to take place, and a device for discharging the exhaust gases produced by the combustion from the combustion chamber, characterized by a device for elevating the temperature of the exhaust gases after the exhaust gases have left the combustion chamber and/or a device for supplying oxygen to the exhaust gases, whereby the exhaust gases are provided with such properties that they are combustible.

9. A device as claimed in claim 8, characterized by an exhaust gas channel which is connected with the combustion chamber, said channel adjoining the combustion chamber and having an outlet at which the exhaust gases are adapted to be combusted.

10. A device as claimed in claim 9, characterized in that a device for the supply of air is connected with the channel adjacent the outlet thereof-.

11. A device as claimed in any of claims 8-10, characterized in that a duct for supplying air is connected with.the upper portion of the combustion - chamber.

12. A device as claimed in any of claims 9-11, characterized in that the channel is connected with the combustion chamber at at least two points positioned at different heigh'ts of the combustion chamber.

13. A device as claimed in any of claims 9-12, characterized in that the channel constitutes a burner pipe at the portion adjacent the outlet.

14. A device as claimed in any of claims 8-13, characterized in that the bottom of the combustion chamber is constituted by a grate and that an air supply device is connected with a space positioned below the grate.

15. A device as claimed in claim 14, characterized in that the space positioned below the grate is connected with the channel for allowing the supply of air thereto from the air supply device through said space.

16. A device as claimed in any of claims 8-15, characterized by a device for controlling the supply of oxygen to the exhaust gases dependent on the temperature of a medium heated by the combustion of the exhaust gases.

17. A device as claimed in claim 8, characterized in that the device for discharging the exhaust gases comprises a channel system which at least partially is positioned between partitions adjoining the combustion chamber and the outer walls of the combustion device.

18. A device as claimed in claim 17, characterized in that the channel system comprises two at opposite sides of the combustion device positioned, in parallel with said sides extending., substantially U-shaped channels, each having an exhaust gas inlet at the upper end of one leg of the U-shaped channel and an exhaust gas outlet at the upper end of the other leg of the U-shaped channel.

19.. A device as claimed in claim 18, characterized in that the exhaust gas outlets of the U-shaped channels end in a collection chamber which is connected with a heat exchanger system for conducting the exhaust gases into said system.

20. A device as claimed in claim 18 or 19, characterized in that the exhaust gas inlets are positioned diagonally in relation to each other in the upper portion of the combustion chamber.

21. A device as claimed in any of claims 17-20, characterized by a device for the supply of air to the exhaust gases while the exhaust gases flow through the channel system.

22. A device as claimed in-any of claims 17-21, comprising a heat exchanger system, characterized in that the combustion chamber as well as the channel system are separated from the environment by means of heat insulated surfaces and that the heat exchanger system is positioned separate from the combustion chamber and the channel system in such a way that the temperature of the combustion chamber and the channel system is substantially uninfluenced of the heat exchanger system.

23. A device as claimed in any of claims 17-22, characterized in that the combustion chamber and the channel system are defined by walls consisting of refractory cast compound.