Boiler for fuel combustion and method for conducting gases therein

Abstract

 The invention relates to a boiler (1) for burning a fuel, such as wood, and a method for conducting gases in this. The boiler (1) comprises a primary combustion space (4), where fuel is gasified to form primary gases (A), and a secondary combustion space (6), where unburned primary gases (A) are burnt into flue gases (D). In the method, the air (B) required for combustion is taken to the secondary combustion space (6), from where a circulating gas mixture (E) is taken near the area to which the air (B) required for combustion is introduced, and which mixture is subsequently taken to the primary combustion space (4).

Description

[0001] The invention relates to a boiler for burning a fuel, such as wood, and a method for conducting gases in such a boiler, as defined in the preambles of the independent claims below.

[0002] The combustion process of solid fuels such as wood comprises many stages. With a rough division of the process, the heated fuel first dries, i.e. any water present evaporates, and then part of the fuel is gasified forming burning gases, which subsequently burn with a flame after having been brought into contact with oxygen. The fuel leaves carbon as a solid residue, which, when further heated, is also gasified and burns, and non-combustible components, i.e. ashes. In conventional combustion, for instance when wood is burnt in a fireplace, the gasified wood constituents, called volatile constituents, will contact oxygen and burn almost immediately after their gasification. The idea of combustion based on gasification of fuel is that the fuel is heated in an oxygen-free space, or at least with such a small amount of oxygen that the combustion remains incomplete. In other words, the fuel is transformed into unburned flue gases, which are subsequently burnt in a space separate from the gasification location.

[0003] US patent specification 5,564,348 discloses a fireplace, in which partly unburned gases produced in the fuel bed in the primary combustion space are circulated upwards in a duct formed in the wall of the primary fire box, further across the firebox cover, then downwards in a duct formed on the second wall, and finally back to the fuel bed. In this publication, primary air can be added to the circulated gas in order to maintain fuel combustion, on the one hand, and further heating the fuel bed to maintain gasification, on the other hand. The partly unburned gases are mixed with secondary air, and the gas mixture thus produced is conducted to the secondary combustion space, where secondary air and primary gases react by burning into flue gases, which are subsequently evacuated from the fireplace. In such a fireplace, the structure becomes complex and thus difficult to clean due to the ducts of the primary combustion space. In addition, cool air is mixed with the circulated gas, thus retarding the fuel gasification.

[0004] The object of the present invention is to provide a boiler and a method that minimise the prior art problems mentioned above.

[0005] A special object of the invention is to provide an efficient low-emission method and boiler suitable for burning solid fuels.

[0006] The drawbacks mentioned above are eliminated or reduced and the objects mentioned above are achieved with the boiler and method of the invention, which are characterised by the features defined in the characterising clause of the independent claims below.

[0007] The boiler of the invention for burning a fuel such as wood is characterised by comprising a jacket and a primary combustion space defined by first walls and a secondary combustion space defined by second walls, both formed within the jacket. The primary and secondary combustion spaces are connected by a primary gas connection, through which gases are conducted from the primary combustion space to the secondary combustion space. At least the following connections penetrate through the boiler jacket:

• an air inlet connection for introducing air required for combustion into the boiler,
• a fuel connection for conducting fuel through the first wall into the primary combustion space, for instance in order to form a fuel bed at the bottom of the primary combustion space, and
• an exhaust connection for evacuating flue gases from the secondary combustion space.

[0008] The boiler also comprises a gas circulating connection for conducting gas to the primary combustion space. In accordance with the invention, said air inlet connection opens in the secondary combustion space, in the vicinity of the wall of this space, and the gas circulating connection extends from the vicinity of the mouth of said air inlet connection to one or more circulating gas inlet connections extending through the first wall, in order to circulate gas from the secondary combustion space to the primary combustion space.

[0009] In this context, a boiler implies the device of the invention for burning a fuel. The primary combustion space is the part of the boiler where fuel is gasified. Such gases are also burnt to some extent in the primary combustion space of the invention. This primary combustion reaction is controlled i.a. by the amount of oxygen entering the primary combustion space. The secondary combustion space is the part of the boiler where the unburned gases derived from the primary combustion space are burnt in a reaction called the secondary combustion reaction. The primary gas connection connecting the primary and secondary combustion spaces is any pipe, opening or other tube, through which gases are conducted from the primary combustion space to the secondary combustion space. The boiler preferably also comprises a separate connection for the removal of ashes from the primary combustion space. The secondary combustion space may have any shape, provided that it allows for efficient and pure gas combustion. Owing to the collection of circulating gas in the vicinity of said air supply location adjacent to the wall of the secondary combustion space, part of the air introduced into the secondary combustion space passes at least occasionally to the gas circulating connection, and through this to the primary combustion space to work as primary air. In normal use, the gas flows in the secondary combustion chamber are very turbulent and strong, and hence the circulating gas must be taken in the vicinity of the mouth of the air inlet connection, as mentioned above. Otherwise the gas flows in the secondary combustion space may capture all of the introduced air, and then only unburned primary gas or already burnt flue gases would pass to the gas circulating connection. Collection of circulating gas in the vicinity of said air supply location means that circulating gas is collected in the same part of the secondary combustion space to which air is introduced. If the boiler is large-sized, the term "in the vicinity" may imply even a relatively long distance. The gas circulating connection may be any duct or pipe. Thus, for instance, the circulating connection may consist of a masonry duct encircling the primary combustion space on one or more sides and having a width equal to the entire primary combustion space.

[0010] In a preferred boiler of the invention, the gas circulating connection is disposed within the jacket in its totality. This allows pipes or similar running outside the boiler to be avoided, and heat insulation and transfer of heat energy between the circulating connection and the remaining structure is easier to control. A substantial section of the gas circulating connection is preferably located between the outer surface of the first wall and the inner surface of the boiler jacket.

[0011] In a specially preferred embodiment of the boiler of the invention, a substantial section of the gas circulating connection communicates with the outer surface of the first wall. This design allows maximally efficient heat exchange between the circulating gas mixture passing in the gas circulating connection and the primary combustion space.

[0012] In a preferred boiler of the invention, the gas circulating connection extends directly from under the mouth of said air inlet connection, from the vicinity of the wall of the secondary combustion space, downwards at least to the bottom level of the secondary combustion space. Such a design allows air to be efficiently guided to the circulating connection and the circulating gas mixture to be conducted to the level of the fuel bed at the bottom of the primary combustion space. A preferred solution further comprises one or more gas circulating inlet connections opening in the lower part of the primary combustion space, preferably at the bottom of the primary combustion space, where for instance a fuel bed of solid fuel is located.

[0013] In an especially preferred embodiment of the invention, the boiler comprises one or more circulating gas inlet connections, which open in the upper part of the primary combustion space. Thus the primary gases will mix with the circulating gases sufficiently well to ensure proper combustion. There may be a plurality of circulating gas inlet connections, and they may conduct gas to different locations in the primary combustion space.

[0014] In an especially preferred embodiment of the invention, the secondary combustion space of the invention is located substantially above the primary combustion space and the primary gas connection extends from the upper part of the primary combustion space to the secondary combustion space. Such a solution makes benefit of the natural gas flows. It is well known that hot gases tend to rise upwards.

[0015] In a further preferred embodiment of the invention, at least part of the second walls defining the secondary combustion space are formed substantially hood-like or dome-like above the primary gas connection. Then the walls of the secondary combustion space will efficiently collect the naturally ascending hot gases, conducting them towards the exhaust connection. In such a boiler, part of the second walls may consist of any other part than the hood-like part mentioned above. The boiler jacket, for instance, may act as the wall of the secondary combustion space, for instance over the section between the primary combustion space and the hood-like wall portion. The air required for combustion, which is led from the air inlet connection to the secondary combustion space, may then be conducted for instance adjacent to said wall portion formed by the jacket, between the jacket and the hood-like wall portion.

[0016] The method of the invention for leading gases in a boiler for burning a fuel such as wood is characterised by

• feeding fuel into the primary combustion space,
• heating the fuel in the primary combustion space so as to gasify fuel into primary gas,
• leading primary gases from the primary combustion space into the secondary combustion space,
• introducing the air required for combustion to the secondary combustion space,
• bringing at least part of said air as secondary air to react with the unburned secondary gases in the secondary combustion space,
• leading the secondary gases produced in the secondary combustion space to the exhaust connection to be evacuated from the boiler,
• leading circulating gas mixture from the secondary combustion space near the area to which air required for combustion is introduced, to the primary combustion space.

[0017] In a preferred method of the invention, the circulating gas mixture is conducted to the fuel bed in the primary combustion space along the outer surface of the wall of the primary combustion space, so that indirect heat transfer occurs between the circulating gas mixture and the primary combustion space by the intermediary of the wall.

[0018] In a preferred embodiment of the invention, the air required for combustion is introduced into the secondary combustion space in the vicinity of the wall, and the circulating gas mixture is collected substantially directly below said air supply location.

[0019] In an especially preferred method of the invention, solid fuel is fed into the primary combustion space, so that a fuel bed is formed at the bottom of the primary combustion space.

[0020] A chief advantage of the boiler and the method of the invention is that the heat required for fuel gasification is supplied directly from the gas mixture derived from the hot secondary combustion space.

[0021] A second major advantage of the boiler and the method of the invention is that fuel combustion in the boiler is easy to control by controlling the amounts of fuel, primary and secondary air, and circulating gas mixture, respectively.

[0022] A further vital advantage of the boiler and method of the invention is that the combustion process easily achieves the high temperatures required for pure combustion, because in the boiler of the invention, the combustion spaces are not necessarily cooled directly, but the energy produced in combustion is recovered only from the flue gases, after the secondary combustion process.

[0023] An advantage of the boiler and method of the invention is that the gas flow in the primary combustion space can be reduced by control to such an extent that the flue gases entrain hardly any ashes or sooth at all. Thus emissions are reduced, the need for cleaning chimneys is reduced and no hot material will escape through the chimney.

[0024] The invention is described in further detail below with reference to the accompanying schematic drawings, in which

figure 1 is a schematic view of an exemplifying boiler of the invention in a vertical cross-sectional projection,

figure 2 is a simplified schematic view of the gas flows in the boiler of figure 1, and

figure 3 is a schematic view of a second exemplifying boiler of the invention in a vertical cross-sectional projection.

[0025] Figure 1 shows the boiler 1 of the invention and figure 2 gas flows within this boiler. The primary combustion space 4 of the boiler 1 is supplied with fuel from a fuel connection, which is not shown in the figure. The fuel connection may be for instance an openable hatch formed in the first walls 3 defining the boiler jacket 2 and the primary combustion space 4, through which split billets, wood chips or any other suitable fuel is fed to the bottom 9 of the primary combustion space 4. The fuel is heated so as to be gasified into primary gases A, however, due to the small amount of primary air, i.e. oxygen, the gases will burn only partly in the primary combustion space 4. Even this partial combustion heats the fuel and thus maintains gasification. The partly unburned primary gases A are led through the primary gas connection 7 located in the upper part 16 of the primary gas space 4 into the secondary combustion space 6, where the primary gases A are contacted with the secondary air C, whereby the unburned primary gases A burn. The hot flue gases D thus formed are led to the exhaust connection 10. The energy contained in the flue gases D can be recovered for instance by heating the water in a hot water tank. In the boiler 1 of figure 1, the air B required for combustion is taken from air inlet connection 8 in-between the jacket 2 and the hood-like wall 5 of the secondary combustion space 6, adjacent to the wall 5a of the secondary combustion space 6 formed by the jacket 2, from where the turbulent gas flow captures the secondary air C required for the secondary combustion reaction. Unless the turbulent gas flow captures all of the air B conducted adjacent to the wall 5a of the secondary combustion space 6, the remaining air will flow down along the wall 5a of the secondary combustion space to the gas circulating connection 11, which conducts the air to form the primary air in the primary combustion space 4. Unburned primary gases A and flue gases D having already reacted with the air will also pass to the circulating connection 11. The circulating gas mixture E thus formed is conducted along circulating connection 11 on the outer surface 13 of the wall 3 of the primary combustion space 4 below the primary combustion space 4, and through the circulating gas connection 12 into the primary combustion space 4, to the bottom 9 of this. As the circulating gas mixture E is flowing down on the wall 3 of the primary combustion space 4, heat energy is transferred between the circulating gas mixture E and the primary combustion space 4. If the circulating gas mixture E contains a large amount of relatively cool air B, it is probably cooler than the outer surface 13, whereby the gas mixture E naturally tends to flow down in the gas circulating connection 11, while being preheated before being fed into the primary combustion space. If on the other hand, the circulating gas mixture E contains a large amount of flue gases D, the gas is probably warmer than the outer surface 13 of the primary combustion space 4, whereby the circulating gas mixture E heats the fuel to be gasified indirectly. The primary air contained in the circulating gas mixture E feeds the combustion in the primary combustion space 4, and if the circulating gas mixture E is sufficiently warm, it will transfer its inherent heat directly to the fuel. One crucial idea of the invention is that, with the dimensions of boiler 1, the fuel, its supply and the other conditions correctly designed, the amounts of primary and secondary air, respectively, will be balanced according to the cycle described below. If there is an increase in gasification, the secondary combustion reaction accelerates, a greater portion of air B is used for secondary combustion, the amount of primary air decreases, the primary reaction decelerates and the gasification decreases. By adopting various control systems, the combustion process can be brought to the desired output level. Thus, for instance, gas flows, temperatures, pressures and concentrations can be measured at different locations of the system, and these values will then allow e.g. control of the fuel and air supply. A fan may be fitted e.g. in the gas circulating connection 11 in order to increase the flow of circulating gas mixture E if desired.

[0026] Figure 3 shows a second exemplifying boiler 1 of the invention. The boiler is otherwise similar to those of figures 1 and 2, with the exception that the upper part 16 of the primary combustion space extends further up towards the exhaust connection 10. In addition, the circulating gas connections 12 extend from the circulating gas connection 11 through the walls 3 to the upper part 16 of the primary combustion space. In figure 3, arrow E illustrates the flow of circulating gas mixture in the boiler of the figure. In other respects, the gas flows are identical to those shown in figure 2 in principle. By introducing the circulating gas mixture E containing unburned oxygen directly into the upper part 16 of the primary combustion space, the combustion of primary gases is increased already in the primary combustion space 4. In the boiler exemplified in figure 3, a major portion of the primary gases are in fact burnt already in primary combustion space 4, and the primary gas A flowing through the primary gas connection 7 to the secondary combustion space 6 contains a smaller amount of unburned gases than in the example of figures 1 and 2.

[0027] It is obvious to those skilled in the art that the invention is not restricted to the examples given above, but may vary within the scope of the following claims. The boiler of the invention may have other dimensions than those exemplified in the figures. It may vary in size, from a fireplace for heating e.g. a single-family house to a boiler of a large power plant. The boiler may be made of various materials, such as tiles or metal. Any gasified fuel, such as wood pellets, wood chips or split billets, is suitable as the fuel of the boiler. An automatic device for feeding pellets, wood chips, split billets or any other fuel has preferably been connected to the boiler. The boiler of the invention can be combined with a conventional oil burner, allowing the benefits of both the methods of combustion. The primary and secondary combustion spaces may be in any relationship other than on top of each other, and they may be spaced from each other. The fuel may be supplied to the boiler of the invention in batches or continuously. The primary combustion space may be equipped with an additional separate primary air feed connection. In order to enhance combustion and gas mixing, secondary air may be introduced into the secondary combustion space at different locations of the secondary combustion space, for instance at the primary gas connection, or only at the trailing end of the secondary combustion space.

Claims

1. A boiler (1) for burning a fuel such as wood, the boiler (1) comprising a jacket (2) and a primary combustion space (4) defined by first walls (3) and a secondary combustion space (6) defined by second walls (5, 5a), both formed within the jacket, and a primary gas connection (7) connecting the primary and secondary combustion spaces for leading gases from the primary combustion space (4) to the secondary combustion space (6), and through the jacket (2) of the boiler (1) are disposed

- an air inlet connection (8) for introducing air required for combustion into the boiler (1),

- a fuel connection for conducting fuel through the first wall (3) to the primary combustion space (4), and

- an exhaust connection (10) for evacuating flue gases from the secondary combustion space (6),

the boiler (1) further comprising a gas circulating connection (11) for conducting gas to the primary combustion space (4),
characterised in that the air inlet connection (8) opens in the secondary combustion space (6) in the vicinity of the wall (5, 5a) and that the gas circulating connection (11) extends from the vicinity of said opening area of the air inlet connection (8) to the circulating gas inlet connection (12) leading through the first wall (3).

2. A boiler (1) as defined in claim 1, characterised in that the gas circulating connection (11) is disposed within the jacket (2) in its totality.

3. A boiler (1) as defined in claim 1 or 2, characterised in that a substantial section of the gas circulating connection (11) is located between the outer surface (13) of the first wall (3) and the inner surface (14) of the jacket (2) of the boiler (1).

4. A boiler (1) as defined in any of the preceding claims, characterised in that a substantial section of the gas circulating connection (11) is disposed in connection with the outer surface (13) of the first wall (3).

5. A boiler (1) as defined in any of the preceding claims, characterised in that the gas circulating connection (11) extends directly from below the opening area of said air inlet connection (8), from the vicinity of the wall (5, 5a) of the secondary combustion space (6), downwards, at least to the level of the bottom (9) of the primary combustion space (4).

6. A boiler (1) as defined in any of the preceding claims, characterised in that one or more gas circulating inlet connections (12) open in the lower part (15) of the primary combustion space (4), preferably at the bottom (9) of the primary combustion space.

7. A boiler (1) as defined in any of the preceding claims, characterised in that one or more gas circulating inlet connections (12) open in the upper part (16) of the primary combustion space (4).

8. A boiler (1) as defined in any of the preceding claims, characterised in that the secondary combustion space (6) is substantially directly above the primary combustion space (4) and the primary combustion connection (7) extends from the upper part (16) of the primary combustion space (4) to the secondary combustion space (6).

9. A boiler (1) as defined in claim 8, characterised in that the second walls (5) defining the secondary combustion space (6) have been formed substantially hoodlike or dome-like above the primary gas connection (7).

10. A method for conducting gases in a boiler (1) for burning a fuel, such as wood, a primary combustion space (4) and a secondary combustion space (6) having been provided within the boiler (1), the method comprising

- feeding fuel into the primary combustion space (4),

- heating the fuel in the primary combustion space (4) so as to gasify fuel into primary gas (A),

- leading primary gases (A) from the primary combustion space (4) to the secondary combustion space (6),

- introducing air (B) required for combustion into the secondary combustion space (6),

- bringing at least part of said air (B) as secondary air (C) to react with the unburned primary gases (A) in the secondary combustion space (6),

- leading the secondary gases (D) produced in the secondary combustion space (6) to the exhaust connection (10) to be evacuated from the boiler (1),

- leading circulating gas mixture (E) to the primary combustion space (4),

characterised in that the circulating gas mixture (E) is taken from the secondary combustion space (6) near the area to which air required for combustion is introduced.

11. A method as defined in claim 10, characterised in that the circulating gas mixture (E) is conducted to the primary combustion space (4) along the outer surface (13) of the wall (3) of the primary combustion space (4), resulting in indirect heat transfer between the circulating gas mixture (E) and the primary combustion space (4) through the wall (3).

12. A method as defined in claim 10 or 11, characterised in that the air (B) required for combustion is introduced into the secondary combustion space (6) in the vicinity of the wall (5, 5a) and the circulating gas mixture (E) is taken from substantially directly below said air (B) supply location.

13. A method as defined in claim 10, 11 or 12, characterised in that solid fuel is fed into the primary combustion space (4), so that a fuel bed is formed at the bottom (9) of the primary combustion space (4).

Drawing