WO2001022004A1

WO2001022004A1 - Combustion furnace

Info

Publication number: WO2001022004A1
Application number: PCT/SE2000/001821
Authority: WO
Inventors: Anders Granudde
Original assignee: KUBI AB
Current assignee: KUBI AB
Priority date: 1999-09-24
Filing date: 2000-09-20
Publication date: 2001-03-29
Anticipated expiration: 2002-03-24
Also published as: SE9903468L; AU7695800A; SE9903468D0; SE521216C2

Abstract

The invention relates to a combustion furnace specially intended for burning solid, finely separated fuel, for example in the formof pellets, including a furnace hearth (1) connected to a gas combustion space (2) so that gases can be transmitted and an outlet (11) positioned after these when seen in the direction of flow ofthe fumes for the exiting fumes, a grating arranged at the furnace hearth that can be supplied with fuel form a fuel store, plus inlets (3, 4) for the regulated supply of air that comprises at least primary and secondary combustion air, whereby the secondary combustion air is supplied in such a way that it rotates in a screw-like manner in the gas combustion space. To improve the controland efficiency of the combustion, the jacket that demarcates the outside of the gas combustion space (2) is mounted to rotate and arranged to turn around a preferably but not necessarily perpendicular axis (15) where the direction of rotation of the jacket is chosen so that it coincides with the direction of rotation of the vortex formation that is generated in the centre of the gas combustion space through the action of the Coriolis force.

Description

Combustion furnace

The present invention relates to a combustion furnace of the type stated in claim 1.

A combustion furnace for incinerating fuel that preferably comprises solid fuel, e.g. fossil fuels, is considered to be an arrangement of very simple design that essentially includes two separate zones for vaporising the fuel, i.e. transforming the chemical energy bound up in the fuel to heat energy. These sections or zones in principle include a furnace and a gas combustion zone. The air that is supplied to the furnace and that participates in the formation of gas is named primary air. The gases that are formed in the furnace still include the major part of the energy of the fuel and need a high temperature to be completely combusted. This complete combustion takes place in the so-called gas combustion chamber where extra oxygen is provided by introducing so-called secondary combustion air.

During all combustion, and especially with fossil fuels such as peat and wood, the time of combustion and the temperature of combustion in the gas combustion section located between the furnace and the convection section are essentially the most important parameters to control. It should be realised that combustion time refers to the time it takes for the fumes to make their way through the gas combustion chamber.

It has been previously known to prolong the combustion time by making the gases rotate or, more specifically, move along a spiral line in the combustion space. In other words, this means that the path taken by the gases as they move through the combustion section is extended without the plant needing to be made larger. A significant advantage of prolonged combustion times and high temperatures is that the formation of poisonous gases such as nitrogen monoxide (NO) as well as the formation of tar and soot particles can be avoided or reduced. For example, it can be mentioned that if a fume temperature of 850°C - 950°C can be maintained for at least half a second, ammonia (NH₃) is formed in the gases, which in turn reacts with the nitrogen monoxide (NO) that arises during the combustion and that itself is poisonous in such a manner that it is transformed into harmless nitrogen gas (N₂O).

Prolonging the time spent in the oven normally takes place by leading the secondary air into the gas combustion space at a tangent or by the fumes emitted from the furnace being led by guides in the gas combustion space so that the fumes are brought to move in a vortex or screw-like manner through the gas combustion space. Through the centrifugal force that arises as a result of the formation of the vortex, the gases rotating in the chamber are separated according to their temperature and specific weight, whereby the supplied secondary combustion air, which is colder than the hot combustion fumes, distributes itself along the insides of the gas combustion space while the significantly hotter fumes can be found concentrated in the centre of the space from which they can be led out. In this way, over-heating of the walls of the gas combustion chamber can also be avoided.

The secondary combustion air that is introduced at a tangent supports or amplifies the natural Coriolis effect that, due to the thermal and dynamic forces that arise in the oven, creates vortex formations of very hot gases in the centre of the combustion space. As a result of this extra force or acceleration acting on the rotating gases in the space, it is possible to achieve a very efficient separation of the warm respective cold masses of gas from one another in the gas combustion space. The vortex formation in the middle or centre of the space arises due to the effect obtained from the Coriolis force, which is that gases that move towards an area with lower pressure always defect to the right as viewed by the observer. The condition named above applies to movements of gas that take place in the Northern Hemisphere, whereby the opposite condition applies in the Southern Hemisphere. As the gases deflect to the right, vortexes are formed around the defined low pressure areas that in the Northern Hemisphere rotate anti-clockwise and thus in a direction that coincides with the direction of rotation of the earth.

Even if the combustion furnaces that utilise the vortex-forming technique described above have shown that they function well and are clearly efficient, there is nevertheless a wish to as far as possible further improve and make more efficient such devices, and the aim of this invention is also to achieve such an improved combustion furnace.

This aim is achieved though a combustion furnace that has the features and characteristics stated in claim 1. For carrying out the invention, an essentially perpendicular axis of rotation is preferable, but it should be realised that the invention should even give a good result when executed even if the axis of rotation is arranged with a certain degree of inclination towards the horizontal plane. Despite the jacket that demarcates the extent of the gas combustion space in the embodiment described here including a stationary and a rotating part, it can naturally be considered from a pure construction point of view to design the furnace so that the jacket includes one single part mounted so that it can rotate.

The invention is described in more detail below with reference to the enclosed drawings where Fig. 1 shows a side view of a longitudinal cross-section of a furnace according to the invention for burning solid fuels, and Fig. 2 shows a cross-section through the combustion space along the line II-II in Fig. 1.

Fig. 1 shows an example of a furnace according to the present invention. It includes a furnace hearth with associated grating, generally designated with the reference number 1 , that via the gas combustion area 2 located above it is connected so that gas can be transmitted to a section for a heated medium for heat exchange with a medium such as water or steam. In a known manner, the furnace hearth 1 as well as the gas combustion space 2 are arranged with inlets generally designated with the reference numbers 3, 4 for introducing primary respectively secondary combustion air, plus an inlet duct 5 arranged for supplying fuel whose one end is connected to a fuel store (not shown in the figures) and whose other end is arranged to open downwards towards the grating device of the furnace hearth 1. As is evident from Fig. 1 , the first and second rotation surfaces 6 and 7 respectively that outwardly demarcate the furnace hearth 1 and the gas combustion space 2 are themselves accommodated in an outer space 8 that is demarcated by an enveloping jacket 9. At its upper end, this jacket 9 is connected to a flange 10 that forms part of a pipe-shaped outlet 11 exiting from the gas combustion space 2, and at its lower end it is demarcated by the bottom 12. The enclosure demarcated by the said rotating surfaces 6 and 7 and that defines the furnace hearth 1 and the gas combustion space 2 has a rotationally symmetric design and primarily the shape of half a globe. The first rotation surface 6 includes a stationary upper jacket formed from sheet metal that can most closely be described as a lid that in a collar-like manner is connected to the lower end of the outlet 11. The second rotating surface 7 that forms the lower part of the enclosure is demarcated by a lower jacket formed from sheet metal. This lower jacket 7, which has a shape that most closely resembles a bowl, is supported in an overhung manner by an axle 13 that is mounted to rotate via a bearing 14 in the bottom 12 for turning around a centre axis 15 that extends perpendicularly through the furnace hearth and the gas combustion space as illustrated by the loop 16 with an arrow attached. At 17, the upper edge section running around the top of the rotatable lower jacket 7 joins with the lower edge section running around the stationary upper jacket 6 in an essentially gas-tight manner so that the said jacket sections 6, 7 can rotate between themselves within a plane that is oriented at right angles to the centre axis 15 while keeping the hot gases within the enclosure that forms the furnace hearth 1 and the gas combustion space 2. The insides of the stationary upper jacket 6 and the rotatable lower jacket 7 are provided with a lining 18 of a suitable material that tolerates high temperatures, e.g. a ceramic.

The primary and secondary combustion air is led in a common way into the outer space 8 via a number of pipe projections 19 that extend through the jacket 9 and that for this purpose are connected to a source (not shown in the figure) that generates forced air flow. Axle 13 is provided with an impeller 20 that, for driving the said axle and thereby also the lower rotatable jacket 7 in a rotary manner, is arranged to be affected by the flow of air that flows in to the outer space 8 via the pipe projections 19. As the inflowing primary and secondary combustion air sweeps over the axle 13, the bearings 14 and the impeller 20, an effective and continuous cooling of these parts is obtained, at the same time as the supplied air is pre-heated to a certain degree. The said flow of air is appropriately and in a. per se known manner arranged to be controlled by a means of regulating the flow not shown in the figure. A glow body 21 that is rotatable and fed by electric current is positioned in the bottom of the rotatable jacket 7 using slip rings or other known means and is arranged to project out somewhat from the inside of the rotatable jacket 7 in a direction towards the centre of the jacket. The primary task of this glow body 21 is to facilitate the start-up of the furnace, but also to a certain extent to support its operation.

The detailed enlargement in Fig. 1 shows a channel 22 that is found in the form of drilled holes adjacent to the glow body 21 and that is arranged to extend through a part of the axle 13 and the bottom of the rotatable jacket 7. This channel 22 is connected with the outer space 8 and thus forms an air inlet 3 that is used for introducing the primary combustion air to the furnace hearth 1. In the stationary jacket 6 that demarcates the upper section of the gas combustion space 2, a number of channels 23 are arranged that, on closer inspection of Fig. 2, will be seen to be connected to the outer space and that thus form the air inlets 4 that are used for introducing secondary combustion air into the gas combustion space 2.

According to the principles of the invention, the secondary combustion air introduced through the channels 23 is brought to circulate in a screw-like manner along the insides of the rotatable jacket 7 down towards the furnace hearth 1 situated in the bottom of the this with a direction of movement that is opposite to the direction of movement of the rotatable jacket 7, which is illustrated with the arrow 24 in Fig. 2. In this section, the lower jacket 7 thus rotates anti-clockwise at the same time as the introduced combustion air is brought to circulate clockwise along the insides of the said jacket 7, which is illustrated by the arrows 25 in Fig. 2.

Because of the pressure differences that prevail in the furnace hearth 1 and the gas combustion space 2, a continuous pressure equilibration takes place whereby, due to the action of the Coriolis force, vortex formation occurs around the centre axis 15 that extends through the enclosure. The said vortexes generated by the Coriolis force are illustrated by the arrows 26 in Fig. 2, and as gases that move towards the area with lower pressure deflect to the right, the deflection of these said gases is as illustrated by the arrows 27 in Fig. 2. The vortexes 26 generated by this deflection of the gases rotate anti-clockwise and consequently also in agreement with the direction of rotation given to the lower rotatable jacket 7.

The directions of rotation chosen achieve an interaction between the rotatable jacket 7 and the vortexes 26 that arise in the centre of this due to the Coriolis force. In this way, the formation of vortexes in the centre is amplified or rather initiated, which is advantageous, not least because the Coriolis forces that arise are comparatively small. It can also be mentioned that the said vortex formation is also supported by the section of the glow body 21 that has a certain projection in towards the furnace hearth 1.

As mentioned above, the gas masses rotating in the enclosure have different temperatures, whereby the differences in the temperature gradient vary in such a way that the relatively hotter gases can be found concentrated at the area of the centre axis 15 of the gas combustion space 2, while the colder and therefore heavier gases are concentrated at the inside of the enclosure or its inner periphery. In other words, the gases have a density that increases radially outwards from the centre axis 15.

Due to the thermodynamics and vortex formation that occurs, the lighter heated gasses in the centre of the enclosure flow upwards outlet 11 while executing a screw-like motion, which is illustrated by the lines of flow 26 in Fig. 1. As the furnace hearth 1 and the gas combustion space 2 are arranged co-axially after one another, one on top of the other, the heated gases in the centre of the furnace hearth 1 as well as in the gas combustion space 2 can freely move upwards towards the outlet duct 11. In practice, it has been shown that the amplification of the vortex movements that are obtained according to the invention are to a large extent dependent on the flow of the air that is introduced into the gas combustion space. In other words, a large flow gives a significantly higher effect and therefore also a higher efficiency in the combustion furnace according to the invention. The duct 5 for supplying fuel down towards the furnace hearth 1 also extends through the stationary upper jacket 6. As the lower jacket 7 rotates, the fuel fed in will be distributed evenly and finely on the bottom of the rotatable jacket 7 and thus also in the area of the grate of the furnace hearth 1. Even if it is not shown in the figure, it can be mentioned that the duct 5 for supplying fuel is connected to a fuel store and that a device for regulating the amount of fuel supplied is arranged at this, whereby the parts that are included in the fuel feeding unit are designed so that there is a break, for example in the form of a fall shaft or similar, between the fuel store and the furnace hearth to prevent fire progressing backwards.

The present invention is not limited to that described above and shown in the drawings but can be changed and modified in a number of ways within the scope of the invention as stated in the following claims.

Claims

1. Combustion furnace specially intended for burning solid, finely separated fuel, for example in the form of pellets, including a furnace hearth (1) connected to a gas combustion space (2) so that gases can be transmitted and an outlet (11) positioned after these when seen in the direction of flow of the fumes for the exiting fumes, a grating arranged at the furnace hearth that can be supplied with fuel from a fuel store, plus inlets (3, 4) for the regulated supply of air that comprises at least primary and secondary combustion air, whereby the secondary combustion air is supplied in such a way that it rotates in a screw-like manner in the gas combustion space c h a r a c t e r i s e d in that the jacket that demarcates the outside of the gas combustion space (2) is mounted to rotate and arranged to turn around a preferably but not necessarily perpendicular axis (15) where the direction of rotation of the jacket is chosen so that it coincides with the direction of rotation of the vortex formation that is generated in the centre of the gas combustion space due to the action of the Coriolis force.

2. Combustion furnace according to claim l c h a r a c t e r i s e d in that the jacket that demarcates the gas combustion space (2) includes an upper jacket (6) that is stationary in relation to the axis (15) and a lower jacket (7) that is supported by an axle (13) in an overhung manner and mounted so that it can rotate, and that is connected to the upper stationary jacket (6) in a way that is gas-tight and that allows rotation between the two.

3. Combustion furnace according to claim 2 c h a r a c t e r i s e d in that the furnace hearth (1) and the associated grating are arranged in the bottom of the rotatable jacket

(7) and that the upper stationary jacket (6) includes an outlet (11) for leading out fumes from the gas combustion space, the inlet (4) for introducing secondary combustion air into the gas combustion space (2), plus an inlet duct (5) for supplying fuel down towards the grating of the furnace hearth.

4. Combustion furnace according to claim 3 c h a r a c t e r i s e d in that it includes an outer jacket (9) that encloses the furnace hearth (1) and the upper stationary jacket (6) and the lower rotatable jacket (7) that define the gas combustion space (2) so that a space

(8) is defined between the said jackets (6, 7, 9) to which space (8) the inlets (3, 4) for introducing the primary and secondary combustion air are connected, whereby forced air in introduced into the said space (8) via inlets (19) arranged in the outer jacket (9) to obtain the primary and secondary combustion air.