Technical field of the invention
[0001] The present invention relates to the combustion of granular solid fuels or liquid
fuels on a granular solid carrier, generally sedimentary materials which are often
moist, and in particular to a method of burning such materials and a combustion device.
The combustion device is, for example, particularly well suited for the combustion
of oily sludge.
Background of the invention
[0002] A technique generally used for the combustion of the kind of solid materials mentioned
above, is combustion on a base, with feeding of the materials at the bottom by means
of a screw. For certain products, this technique can cause problems with the agglomeration
of ashes, as well as a poor combustion (formation of CO).
[0003] Fluidised beds may be used, as described in EP-0 092 622. In such a fluidised bed,
the temperatures may not exceed 800°C, otherwise the ashes will agglomerate into slag
and block further combustion.
[0004] In order to overcome this problem, cyclonic burners have been tried. A cyclone of
air is created along the wall, which should carry along the injected combustibles.
However, due to centrifugal forces, the combustibles will be thrown against the wall,
and will simply slide down, thus shortening the time of residence of the combustibles
in the combustion process and creating a blocking layer at the bottom of the cyclonic
burner.
[0005] US-4,351,251 describes a combustion apparatus for burning pelletised organic material
and/or liquid fuel. A primary air supply supplies air to the burner combustion chamber
and the fuel is fed with the primary air. A swirling pattern of air in combination
with the fuel feed is created. The swirling action creates an area of negative pressure
located in the middle of the swirling pattern, and a flow of air under pressure is
directed into this area of negative pressure. Furthermore, air entering towards the
centre rear of the combustion chamber generates a second swirling pattern. The air
being directed in a reverse direction to the flow of air serves to entrap heavier
particles of fuel in the combustion chamber to come to a more complete combustion,
while the second swirling pattern serves to avoid a dead zone at the rear end of the
combustion chamber, and to avoid that heavier particles leave the combustion chamber
at that rear end.
[0006] The combustion apparatus described above will present, when combusting solid fuel,
the disadvantage that ashes will agglomerate and possibly adhere to the bottom of
the combustion apparatus. In order to be able to evacuate the ashes, the combustion
chamber would have to be cleaned which would almost certainly necessitate stopping
the combustion. Generally, the known combustion apparatus appears to be better designed
for liquid fuel.
[0007] A cyclonic combustion apparatus in vertical position with evacuation of particles
is described in US-4,002,25. Two fluid streams enter the combustion chamber in two
distinct fluid paths moving is vortex flows about the axis in the same direction of
rotation, and the respective energy flow rates of fluid in these paths are comparable.
The vortex flows progress axially towards one another before combining in the middle
where they meet, to create a localised inward radial flow. Subsequently, the fluid
leaves the radial flow and moves axially towards and out through a discharge tube.
Fuel is tangentially introduced at the upper side of the combustion apparatus and
moves downwards along the inside wall of the combustion chamber. Lighter parts of
the fuel are moving radially where both vortex flow paths meet each other, and are
then taken up by the axially upward flow. Heavy particles go down and will be evacuated
from the bottom of the combustion apparatus without being totally burnt. There is
no circulatory flow of the fuel in the chamber.
[0008] WO 92/14969 describes a vertical cyclone furnace. Primary air together with the fuel
is blown in through tangential injection nozzles, thus creating a vortex. Combustion
takes place in a downwardly-directed spiral movement. All particles collect at the
bottom of the furnace, where they are further combusted and thus will agglomerate.
A rotating cooled ash scraper is provided for removing ash from the bottom area of
the furnace. The ashes thus will not stick to the wall of the furnace, but they will
agglomerate to each other, and complete combustion may be impaired.
[0009] It is an object of the present invention to provide a method and a device for burning
granular solid fuel or liquid fuel on a granular solid carrier, whereby the residence
time of the solid material in the combustion process is long enough to obtain a complete
and good combustion.
[0010] It s a further object of the present invention to provide a method and a device for
burning granular solid fuel or liquid fuel on a granular solid carrier, whereby the
combustion is not blocked by agglomeration of solid materials.
Summary of the invention
[0011] The above objectives are accomplished by a device and a method for the combustion
of granular solid fuel or liquid fuel on a granular solid carrier according to the
present invention.
[0012] The granular solid fuel or the liquid fuel on a granular solid carrier, hereinafter
called fuel, is burnt in suspension in a vertical combustion device comprising a combustion
chamber with a top and a bottom and a vertical peripheral wall disposed around a longitudinal
axis. The solid materials have for example, granular dimensions between 0.1 and 6
mm in diameter. Examples of such granular solid materials are grape-stones, almond
shells, waste products from forest industries, waste materials after squeezing olives.
Some materials may have to be forced through pelletising mills in order to obtain
the right dimensions.
[0013] A first opening is provided in the top of the chamber for removal of hot combustion
products such as gas, fly ashes and smoke. Typically the combustion device will be
cylindrical, but this is not necessarily needed. It should be symmetrical around a
longitudinal axis, and any deviation from cylindrical must be within the practical
confines of operability. Any shape which prevents the efficient creation of a first
and a second vortex flow as explained hereinafter, would be unsatisfactory.
[0014] The combustion device comprises a fuel inlet device for inputting through the peripheral
wall granular solid fuel or liquid fuel on a granular solid carrier. It comprises
a first gas inlet device for inputting a gas to generate a first vortex flow adjacent
to the peripheral wall, and a second gas inlet device for inputting a gas to form
a second vortex flow within the first vortex flow. The second gas inlet device is
located at or adjacent the bottom of the chamber. The second vortex flow is directed
upwards towards the first opening in the top of the chamber.
[0015] A second opening is provided in the bottom of the chamber for continuous removal
of dense particles.
[0016] Furthermore, a deflector is provided, which co-operates with the second vortex flow
to propel the fuel upwards towards the first opening and which allows dense particles
to exit continuously from the second opening.
[0017] Preferably the fuel inlet device is located at the upper half of the combustion chamber,
and the first gas inlet device is located at the lower half of the combustion chamber.
[0018] By means of the creation of the first and second vortex flow, the fuel materials
are separated and circulated inside the combustion chamber up to complete combustion.
Preferably the second vortex flow has a velocity which is higher than the velocity
of the first vortex flow.
[0019] The method provided in accordance with the present invention comprises the steps
of inputting fuel at the peripheral wall, inputting a primary gas at the peripheral
wall to generate a first vortex flow adjacent to said peripheral wall, inputting a
secondary gas at or adjacent the bottom of the chamber to form a second vortex flow
within the first vortex flow, the second vortex flow being directed upwards towards
the first opening and having sufficient energy to propel fuel particles towards the
first opening, thus forming a fuel circulation path with the first vortex flow, and
continuously preventing fuel to leave the combustion chamber, allowing dense particles
to leave the combustion chamber through the second opening, and removing combustion
products via the first opening.
Typically the temperatures in the combustion chamber will be between 700 and 1200
°C, preferably between 900 and 1000°C. Higher temperatures could cause the ashes to
melt.
[0020] For burning oily sludge or polymeric resins, chalk may be added in the combustion
chamber, in order to avoid the formation of dioxins.
[0021] Other features and advantages of the present invention will become apparent from
the following detailed description, taken in conjunction with the accompanying drawings,
which illustrate, by way of example, the principles of the invention.
Brief description of the drawings
[0022]
Fig. 1 is a schematic vertical cross-section of a combustion device according to an
embodiment of the present invention;
Fig. 2 is a schematic vertical cross-section of a further embodiment of a combustion
device with double walls;
Fig. 3 represents a top view of the position of trapezoidal blades generating the
second vortex flow;
Fig. 4 illustrates the circulation action in the combustion chamber; and
Fig. 5 is a schematic overview of the system with controlling devices.
[0023] Similar objects in the different drawings bear the same reference numerals.
Description of the preferred embodiments
[0024] The present invention will be described with reference to certain drawings and certain
embodiments but the present invention is not limited thereto but only by the claims.
In the text and claims reference is made to a vertically mounted combustion chamber,
and words such as "top", "bottom", "side", "falling" relate to this vertical orientation
of the combustion chamber.
[0025] Fig. 1 represents a vertically positioned, cylindrical combustion device 1. The combustion
chamber 2 is formed by a vertical peripheral wall 3, a top 4 and a bottom 5. The top
4 is provided with a first opening 6 for removal of hot combustion products such as
gas, smoke and fly ashes. The bottom 5 is provided with a second opening 7 for continuous
removal of dense particles.
[0026] A first vortex flow 8, is created adjacent to the peripheral wall 3 by tangentially
injecting primary gas 9. Typically primary gas 9 is injected at a speed between 30
and 35 m/s. The flow of this first vortex flow 8 is very intense at the level of the
peripheral wall 3. A deflector 28 is provided at the inside of the combustion chamber
2 at or adjacent the bottom 5. Preferably the deflector 28 is formed by a wall which
is slanted in a vertical sectional view. The slope of the deflector 28 depends on
the material used, but generally is between 45° and 50°. A groove 15 is provided between
the bottom 5 and the deflector 28. The width of the groove 15 preferably is about
3 mm.
[0027] At or adjacent the bottom 5, a second vortex flow 10 is created by means of injection
of secondary gas 11, typically with a velocity of about 60 or 70 m/s. The second vortex
flow 10 is preferably created by means of a ring 12 of trapezoidal blades 13, which
in their prolongation are tangential to a fictive central circle 14 of small diameter,
as can be seen on the top view represented in Fig. 3. The flow of injected secondary
gas 11 is thus more radial than tangential. The secondary gas 11 moves past the groove
15 between the bottom 5 and the deflector 28. Therefore, a vortex with high speed
is created, the second vortex flow 10. The speed of the second vortex flow 10 is controlled
by controlling the speed of the injected secondary gas 11. The lighter the particles
to be burnt are, the lower the speed of the second vortex flow 10 and the bigger its
diameter has to be.
[0028] The fuel inlet device for inputting granular solid fuel or liquid fuel on a granular
solid carrier may be mechanical or pneumatic devices, for example a screw 17, at the
upper part of the combustion chamber 2. The fuel goes down following a helical line
18 along the peripheral wall 3 of the combustion chamber 2, and then falls down and
slides along the deflector 28 (represented by the line 19) towards the second opening
7 in the bottom 5, to be presented to a horizontal flow of gas creating the second
vortex flow 10. Particles which fall into the almost radially injected flow of gas,
are carried to the second vortex flow 10 and are then transported pneumatically and
vertically upwards (represented by the line 20), but under centrifugal force fall
move or fall into the first vortex flow 8 again (represented by the line 21), and
come down following a helical line 33 (see Fig. 4). In this way, fuel is circulated
in the combustion chamber until completely burnt. Some solid matter, when completely
burnt may form light ash. These light ashes are evacuated through the first opening
6 in the top 4 of the combustion chamber 2 in the form of fly ashes following path
35. Heavy elements (such as stones for example, which may be included in the fuel,
but also dense glassy or stony material formed by agglomeration of ashes) are not
carried along with the first and second vortex flows 8 and 10, but are eliminated
through the second opening 7 at the bottom 5 of the combustion device 1 following
line 34, as explained with reference to Fig. 2. The circulation action is illustrated
in Fig. 4.
[0029] For the combustion of humid products, the secondary gas 11 may be preheated in a
device with double walls, thus allowing the ignition of the products. Fig. 2 represents
a practical realisation of this embodiment.
[0030] The combustion device 1 comprises an internal peripheral wall 3 and an external wall
22. The combustion device 1 is divided in a pre-combustion chamber 23 and a post-combustion
chamber 24. Again, the top 4 of the pre-combustion chamber 23 is provided with a first
opening 6, and the bottom 5 is provided with a second opening 7. Fuel is fed in by
means of a screw 17 or pneumatically.
[0031] Secondary gas 11 is introduced between the internal peripheral wall 3 and the external
wall 22, at the upper side of the combustion device 1. The gas 11 is warmed up while
travelling along the internal peripheral wall 3, and it is then introduced in the
pre-combustion chamber 23 through the groove 15 between the bottom 5 and the deflector
28, thus creating an second vortex flow 10 as explained hereinabove. The temperature
of the injected secondary gas 11 typically is about 400°C.
[0032] Primary gas 9 for creating the first vortex flow 8, is injected tangentially through
different channels 25, preferably situated in the peripheral wall 3 just above the
bottom 5 (at the junction between the wall 3 and the bottom 5) and in any case situated
in the lower half of the peripheral wall 3. This tangentially injected primary gas
9 creates a vortex of large diameter, the first vortex flow 8, lying adjacent to the
peripheral wall 3 of the combustion chamber 23 and around the second vortex flow 10.
The velocity of the first vortex flow 8 is much smaller than that of the second vortex
flow 10.
[0033] A particular aspect of the present invention is that the first vortex flow 8 stabilises
the second vortex flow 10, which would otherwise broaden, slow down and loose its
ability of upwardly carrying fuel.
[0034] An adjustment device 26 is provided for performing an adjustment of the proportion
of the quantity of secondary gas 11 injected for generation of the second vortex flow
10, related to the quantity of primary gas 9 injected for generation of the first
vortex flow 8. This is done in order to have the fuel circulated inside the combustion
chamber 2 or pre-combustion chamber 23 up to complete combustion.
[0035] As described above in relation to Fig. 1, the fuel goes down following a helical
line along the peripheral wall 3 of the pre-combustion chamber 23, and then falls
down and slides along the bottom 5 to be presented to a horizontal flow of gas creating
the second vortex flow 10. At or adjacent the bottom 5 a deflector 28 is provided
which co-operates with the second vortex flow 10 to propel the fuel towards the first
opening, and which allows dense particles to exit continuously from the second opening
7. The fuel is transported pneumatically and vertically upwards, but under centrifugal
force moves or falls in the first vortex flow 8 again. As for the previous embodiment,
the fuel is circulated. Light, completely burnt ashes are evacuated out of the pre-combustion
chamber 23 and into the post-combustion chamber 24 by means of the second vortex flow
10.
[0036] Heavy particles are evacuated out of the combustion chamber 23 into a collecting
room 27 through a second opening 7 in the bottom 5, located under the level of injection
of secondary gas 11.
[0037] The extraction of heavy particles out of the collecting room 27 takes place by means
of an extraction device, e.g. a screw 29 or an ash-pan.
[0038] In view of the high amount of fine dust contained in certain products to be burnt,
a central tube 30 may be provided, preferably made of ceramic. It is mounted through
the top 4 and extends at least partially inside the pre-combustion chamber 23. At
the exit of the pre-combustion chamber 23 it creates a reverse flow of the first vortex
flow 8, thus augmenting the residence time of the fine fuel particles in the combustion
chamber 23. The tube 30 forms a second deflector at the top of the chamber for enhancing
circulation of fuel. This tube 30 also stabilises the second vortex flow 10.
[0039] As for the combustion, the fuel is taken at the exit of the screw 17 by the first
vortex flow 8, which is poor in oxygen, and undergoes a drying and a pyrolytic flash.
This gives a liberation of gas towards the axis 31 of the combustion device 1, where
it will be mixed with mounting gas.
[0040] The speed of combustion may controlled by introducing tertiary gas 32 in the exit
throat of the pre-combustion chamber 23. In the post-combustion chamber 24, a flame,
still in vortex, is obtained.
[0041] As represented schematically on Fig. 5, preferably the temperature inside the combustion
chamber 2 is measured by means of a pyrometric stick 36, preferably provided with
a thermocouple, which generates a temperature signal. The fuel inlet device can, for
example, be a screw driven with variable speed by fuel inlet driving device 37. The
driving speed of the fuel inlet driving device 37, and thus the speed of the fuel
inlet device 17, depends on the temperature measured by the pyrometric stick 36. On
the basis of this temperature signal, the fuel inlet driving device 37 derives an
optimal driving speed for the fuel inlet device 17, and that way the flow of fuel
introduced by the fuel inlet device 17 is affected.
[0042] The combustion may be controlled using a parameter relating the combustion of the
fuel. For instance, control may be effected by adjusting, by means of first and second
O
2 adjustment devices 38 and 39, the amount of O
2 injected into the combustion chamber 2. The efficiency of the combustion can be determined
using a CO measuring device 40 for measuring the amount of CO generated. The amount
of O
2 injected depends on the measured value of the amount of CO.
[0043] The minimal value for the CO concentration is obtained for values of O
2 of 5% of the primary and secondary gas 9 and 11 injected. For these values, the gas
which leaves the combustion chamber 2 through the first opening 6 at the top 4, contains
dust particles between 100 and 200 ppm, according to the degree of dust contained
in the fuel. This explains the importance of the central tube 30 which keeps the dust
in the combustion chamber 2. By heightening the upper part 41 (free board) of the
central tube 30 the combustion will further continue in the central tube 30, which
implies that less CO will be generated (as the combustion will be more complete),
and that gasses leaving the combustion chamber 2 may contain dust particles less than
100 ppm.
[0044] Tests have been carried out for burning chloride containing products. These have
been burnt with chalk added. The amounts of PCB contained in the ashes were less than
0.001 mg/kg, which makes the device according to the present invention very suitable
for burning chloride containing products.
[0045] While the invention has been shown and described with reference to preferred embodiments,
it will be understood by those skilled in the art that various changes or modifications
in form and detail may be made without departing from the scope and spirit of this
invention.
1. A combustion device (1) for burning solid fuel or liquid fuel on a granular solid
carrier, comprising:
a combustion chamber (2) with a top (4) and a bottom (126) and a vertical peripheral
wall (3) enclosing the chamber (2) , a first opening (6) being provided in the top
(4) of the chamber (2) for removal of hot combustion products;
a fuel inlet device (17) for inputting fuel through the peripheral wall (3);
a first gas inlet device (25) for inputting a gas to generate a first vortex flow
(8) adjacent to the peripheral wall (3);
a second gas inlet device (12) for inputting a gas to form a second vortex flow (10)
within the first vortex flow (8), the second gas inlet device (12) being located at
or adjacent the bottom (5) of the chamber (2) and the second vortex flow (10) being
directed upwards towards the first opening (6) in the top (4) of the chamber (2);
a second opening (7) in the bottom (5) of the chamber (2) for continuous removal of
dense particles; and
a deflector (28), the second vortex flow (10) and the deflector (28) cooperating to
propel the fuel upwards towards the first opening (6) and to allow dense particles
to exit continuously from the second opening (7).
2. A combustion device according to claim 1, characterised in that the fuel inlet device (17) is located at the upper half of the combustion chamber
(2).
3. A combustion device according to any of claims 1 or 2, characterised in that the first gas inlet device (25) is located at the lower half of the combustion chamber
(2).
4. A combustion device according to any or the preceding claims, characterised in that the second gas inlet device comprises a ring (12) of trapezoidal blades (13) which
in their prolongation are tangent to a fictive central circle (14) of small diameter.
5. A combustion device according to any of the preceding claims characterised in that the first gas inlet device is formed by channels (25) for tangentially injecting
gas into the combustion chamber through the peripheral wall (3).
6. A combustion device according to any of the preceding claims, characterised in that it furthermore comprises an adjustment device (26) for performing an adjustment of
the proportion of the quantity of secondary gas (11) injected for generation of the
second vortex (10) related to the quantity of primary gas (9) injected for generation
of the first vortex (8).
7. A combustion device according to any of the preceding claims, characterised in that it furthermore comprises a pyrometric stick (36) for generating a temperature signal,
a fuel inlet driving device (37) deriving the optimal driving speed for driving the
fuel inlet (17) from said temperature signal.
8. A combustion device according to any of the preceding claims, characterised in that it furthermore comprises a CO measuring device (40) which generates a signal reflecting
the amount of CO generated, this signal being fed to first and second O2 adjustment devices (38, 39) which adjust the amount of O2 in the primary and secondary gas (9, 11) injected in the combustion chamber(2).
9. A combustion device according to any of the preceding claims, characterised in that it furthermore comprises a central tube (30) mounted through the top (4) and extending
at least partially inside the combustion chamber (2).
10. A method of burning granular solid fuel or liquid fuel on a granular solid carrier
in a combustion chamber (2) having a top (4) and a bottom (5) and a vertical peripheral
wall (3) defining the chamber (2), there being a first opening (6) in the top (4)
and a second opening (7) in the bottom (5) of the chamber (2), comprising the steps
of:
inputting fuel at the peripheral wall (3);
inputting a primary gas (9) at the peripheral wall (3) to generate a first vortex
flow (8) adjacent to the peripheral wall (3);
inputting a secondary gas (11) at or adjacent the bottom (5) of the chamber (2) to
form a second vortex flow (10) within the first vortex flow (8), the second vortex
flow (10) being directed upwards towards the first opening (6) and having sufficient
energy to propel fuel particles towards the first opening (6), thus forming a fuel
circulation path with the first vortex flow (8); and
allowing dense particles to leave the combustion chamber (2) through the second opening
(7) and removing combustion products via the first opening (6).