U.S. patent application number 11/976430 was filed with the patent office on 2008-05-29 for porous burner as well as a method for operating a porous burner.
Invention is credited to August Lackner, Frank Stocker.
Application Number | 20080124666 11/976430 |
Document ID | / |
Family ID | 38805076 |
Filed Date | 2008-05-29 |
United States Patent
Application |
20080124666 |
Kind Code |
A1 |
Stocker; Frank ; et
al. |
May 29, 2008 |
Porous burner as well as a method for operating a porous burner
Abstract
The invention relates to a porous burner with a housing, which
has an inlet for a fuel-air mixture and an outlet for the exhaust
gas mixture generated in the burner, where in flow-direction of the
fuel-air mixture the housing contains an ignition space with an
ignition device and adjacent to this space a porous burner medium.
On the inlet side the ignition space is provided with a stabilizing
element, which reduces the inlet cross-section and directs the flow
of the fuel-air mixture essentially perpendicular to the inlet
cross-section of the porous medium. The porous burner is provided
with a device for controlling the mass flow of the fuel-air
mixture, which serves to shift the combustion zone from the
ignition space into the porous medium.
Inventors: |
Stocker; Frank; (Zeltweg,
AT) ; Lackner; August; (Salzburg, AT) |
Correspondence
Address: |
DYKEMA GOSSETT PLLC
FRANKLIN SQUARE, THIRD FLOOR WEST, 1300 I STREET, NW
WASHINGTON
DC
20005
US
|
Family ID: |
38805076 |
Appl. No.: |
11/976430 |
Filed: |
October 24, 2007 |
Current U.S.
Class: |
431/7 ;
431/326 |
Current CPC
Class: |
F23D 14/02 20130101;
F23C 99/006 20130101; F23D 11/448 20130101; F23D 14/82 20130101;
F23D 17/002 20130101 |
Class at
Publication: |
431/7 ;
431/326 |
International
Class: |
F23D 3/40 20060101
F23D003/40 |
Foreign Application Data
Date |
Code |
Application Number |
Oct 24, 2006 |
AT |
A 1794/2006 |
Claims
1. A porous burner comprising a housing, which housing has an inlet
for a fuel-air mixture and an outlet for the exhaust gas mixture
generated in the burner, an ignition space with an ignition device
and, following said ignition space, a porous medium being provided
in the housing in flow direction of the fuel-air mixture, wherein
the ignition space contains a stabilizing element at its inlet
side, which reduces the inlet cross-section and directs the flow of
the fuel-air mixture essentially perpendicular to the inlet
cross-section of the porous medium, and wherein the porous burner
is provided with a device for controlling the mass flow of the
fuel-air mixture.
2. A porous burner according to claim 1, wherein in front of the
stabilizing element in flow direction of the fuel-air mixture,
there is located a mixing chamber for pretreatment of combustion
air and liquid fuel
3. A porous burner according to claim 2, wherein the mixing chamber
is heatable.
4. A porous burner according to claim 1, wherein the stabilizing
element is a perforated ceramic plate.
5. A porous burner according to claim 4, wherein the perforated
ceramic plate has a flow cross-section which amounts to 10% to 30%
of the free flow cross-section of the porous burner.
6. A porous burner according to claim 4, wherein the perforated
ceramic plate has a flow cross-section which amounts to 15% to 20%
of the free flow cross-section of the porous burner.
7. A porous burner according to claim 4, wherein the diameters of
the bores in the perforated plate are dimensioned such that the
flames are quenched in the case of flame blowback, with the
diameters of the bores amounting to 8% to 15% of the thickness of
the perforated plate.
8. A porous burner according to claim 4, wherein the diameters of
the bores in the perforated plate are dimensioned such that the
flames are quenched in the case of flame blowback, with the
diameters of the bores amounting to approximately 10%, of the
thickness of the perforated plate.
9. A porous burner according to claim 4, wherein the perforated
ceramic plate is attached to the inlet of the porous burner by
means of a conical clamping ring.
10. Method for operating a porous burner, which has an ignition
space with an ignition device in flow direction of the fuel-air
mixture in front of a porous medium, comprising: a) directing and
accelerating the flow of the fuel-air mixture essentially
perpendicular to the inlet cross-section of the porous medium by
means of a perforated plate at the inlet of the ignition space; b)
ignition of the fuel-air mixture in the ignition space and
stabilization of a flame front in a combustion zone on the side of
the perforated plate facing the porous medium; c) increasing the
mass-flow of the fuel-air mixture and shifting the combustion zone
into the porous medium.
11. Method according to claim 10, wherein in the case of liquid
fuels the fuel-air mixture is prepared in a heatable mixing chamber
located in front of the perforated plate.
12. Method according to claim 10, wherein the air ratio .lamda. is
increased for the purpose of increasing mass flow of the fuel-air
mixture at the transition from start-up phase to steady-state
operation.
Description
BACKGROUND OF THE INVENTION
[0001] The invention relates to a porous burner with a housing,
which is provided with an inlet for a fuel-air mixture and an
outlet for the exhaust gas mixture generated in the burner, and
contains in flow direction of the fuel-air mixture an ignition
space with an ignition device and adjacent thereto a porous body.
The invention further relates to a method of operating a porous
burner of this kind.
[0002] In contrast to conventional combustion using a free flame,
the combustion reactions of combustion in porous inert media do not
take place in a free flow of gas but within a coherent structure of
cavities of an inert porous body. Stabilization in the porous
matrix is made possible by its significantly better heat transport
properties as compared to the pure gas phase. Gas-phase and solid
state matrix are approximately in thermal equilibrium and thus no
free flames will occur.
[0003] The operation of conventional porous burners generally shows
the following operational phases. [0004] Start-up: during the
start-up phase the burner is brought to operating temperature by
the hot exhaust gases of free combustion taking place in front of
the porous body or medium. To generate the start-up flames
pressurized oil atomizer nozzles may be employed. [0005]
Intermediate phase: the start-up phase is terminated by a short
interruption of fuel feed. [0006] Steady-state operation: with
renewed fuel feed the fuel-air mixture is brought to combustion by
self-ignition in the hot porous medium. The combustion zone is thus
stabilized in the porous medium.
DESCRIPTION OF PRIOR ART
[0007] DE 102 28 411 C1 describes a porous burner with reduced
start-up emissions, in which after start-up no intermediary phase
is required during which fuel feed would have to be interrupted.
The burner has a housing with an inlet for the fuel-air mixture and
an outlet for the hot smoke gases, with finely porous material
being provided in a zone on the inlet side and coarsely porous
material being provided in a zone on the outlet side. The burner is
furthermore provided with a shifting device by means of which a
free intermediary space can be created between the finely porous
and the coarsely porous zone during the start-up process. In the
start-up phase the fuel-air mixture is fed through the finely
porous zone into the free intermediary space where it is burnt. The
smoke gases generated by combustion in the free intermediary space
will heat the coarsely porous region to operating temperature,
which region does not take part in the combustion process during
start-up. After the start-up phase, when optimum operating
temperatures have been reached in the second zone, the coarsely
porous material is moved back towards the finely porous material of
the first zone. The presence of movable parts in the burner and
their actuating means is of disadvantage, since they will lead to
increased maintenance expense in addition to greater manufacturing
and operating costs of the porous burner.
[0008] In this context there is known from DE 197 29 718 A1 a
combustor body for a burner for gaseous fuels, where at the inlet
of the burner in flow direction of the process gases a first zone
of porous material is provided in the form of "spaghetti-ceramics".
This is followed by a free ignition chamber with an ignition
device, and behind the ignition chamber there is located an element
consisting of a plurality of corrugated sheet metal platelets
placed side by side. The faces of the platelets are parallel to the
flow direction of the gas-air mixture. During operation of the
burner the gas-air mixture ignites in the open ignition chamber,
and the flame front propagates downstream, heating the element
consisting of corrugated platelets and stabilizing itself in this
material.
[0009] Furthermore DE 43 22 109 A1 describes a burner in which the
combustion chamber is filled with porous material whose porosity
changes along the length of the combustion chamber in such a way
that porous cavity size increases in flow direction of the process
gases, resulting in optimum parameters for porous cavity size and
thus flame development at a boundary surface or in a certain zone
of the porous material, which will permit a flame to develop. Flame
stabilization thus occurs at the transition from a finely porous to
a coarsely porous medium.
[0010] Finally, DE 199 60 093 A1 describes a gas burner and a
method for the flameless combustion of a gas-air mixture. The
burner consists of a hollow cylindrical perforated plate and a
porous body placed concentrically and at a certain distance above
the plate. The gas is fed into the cavity between the hollow
cylindrical perforated plate and the porous medium, and the
oxidizing air is directly fed into the hollow cylindrical
perforated plate. The reaction zone of this gas burner is
permanently located in the cavity.
SUMMARY OF THE INVENTION
[0011] It is the object of the present invention to improve a
porous burner of the initially described kind and a method of
operating this burner in such a way that the burner has no movable
parts and that noxious emissions are minimized in the start-up
phase as well as during steady-state operation. Furthermore, this
burner design should be applicable for gaseous fuels as well as
oil, and the transition from start-up to steady-state operation
should be controllable without problems.
[0012] According to the invention this object is achieved by
providing that the ignition space has a stabilizing element at the
inlet side, which reduces the inlet cross-section and directs the
flow of the fuel-air mixture essentially at a right angle to the
inlet cross-section of the porous medium, and that the porous
burner is furnished with a device for controlling the mass-flow of
the fuel-air mixture, which serves to shift the combustion zone
from the ignition space into the porous medium.
[0013] A method according to the invention for operating such a
burner, which is provided with an ignition space with an ignition
device in front of a porous medium in flow direction of the
fuel-air mixture, is characterized by the following steps: [0014]
Essentially perpendicular direction and acceleration of the
fuel-air mixture by means of a perforated plate at the inlet of the
ignition space; [0015] Ignition of the fuel-air mixture in the
ignition space and stabilization of a flame front in a combustion
zone on the side of the perforated plate facing the porous medium;
[0016] Increasing the mass-flow of the fuel-air mixture and
shifting the combustion zone into the porous medium.
[0017] In the case of an oil-burner the fuel-air mixture is
prepared in a preferably heatable mixing chamber preceding the
perforated plate.
[0018] According to the invention the stabilizing element consists
of a perforated plate made of ceramics, with a flow cross-section
amounting to 10% to 30%, and preferably 15% to 20%, of the free
flow cross-section of the porous burner.
[0019] The working principle of the porous burner according to the
invention will now be described for the case of an oil burner.
[0020] Pretreatment of the combustion air and the fuel, i.e.,
heating of both components to target temperature and evaporation of
the oil, takes place in a mixing chamber preceding the porous
burner itself (see AT 408 904 B, for example). The generated
turbulent oil-vapor air mixture then flows through the perforated
ceramic plate, which has a significantly smaller flow cross-section
than the porous burner and will thus accelerate the fuel-air
mixture and give it an axis-parallel flow direction in the case of
a cylindrical burner.
[0021] During start-up the burner is operated at low power and with
a low excess air ratio (e.g., .lamda.=1.1), resulting in low
mass-flow of the fuel-air mixture and in a low exit velocity from
the perforated ceramic plate. With increasing process temperature
the flame velocities also increase and the combustion zone shifts
upstream from the ignition site towards the perforated plate. A
flame front consisting of a plurality of single flames is
established, which is stabilized by the perforated plate. The hot
smoke gases generated by combustion will flow through the porous
medium and heat it. Subsequently, the excess air ratio .lamda. is
increased while power is kept constant, which causes the mass flow
and thus the exit velocity of the fuel-air mixture to increase. The
combustion zone now shifts downstream into the porous medium and is
stabilized there. Steady-state operation of the burner has set in.
The desired steady-state power output may now be set with a
corresponding limiting excess air ratio. The limiting excess air
ratio is given by the amount of combustion air necessary for
shifting the combustion zone completely into the porous medium and
stabilizing it there, keeping all noxious emissions below their
prescribed threshold values.
[0022] As an example the operating sequence for an oil burner (for
instance a burner for domestic fuel oil) with a power range from 2
to 16 KW will now be described. [0023] Preheating of the mixing
chamber: the mixing chamber is first preheated by an electric
heating element to a temperature of 350.degree. C. After a
temperature of 230.degree. C. has been reached combustion air is
fed into the mixing chamber, which flushes combustion residues from
the porous medium. At a mixing chamber temperature of 345.degree.
C. high-voltage spark ignition is activated. [0024] Start-up phase
(ignition and heating of the porous medium): at 350.degree. C. the
fuel-air mixture with an excess air ratio of .lamda.=1.1 (at a
power output of 5.5 KW) is fed into the ignition space and ignited
by the high-voltage spark ignition device. The combustion zone will
then move upstream with increasing flame temperature towards the
perforated ceramic plate and develop a flame front on the side of
the perforated plate facing the porous medium. The porous medium is
heated by the hot smoke gases. [0025] Shifting of the combustion
zone: the excess air ratio is now increased to .lamda.>1.1. Due
to the increased mass flow of the mixture the exit velocity also
increases. The flame front stabilized by the perforated plate
becomes unstable and the combustion zone finally shifts into the
porous medium. [0026] Steady-state operation: the burner is now
adjusted to deliver the desired steady-state power output. The
necessary excess air ratios are to be chosen depending on the given
power value.
[0027] In addition to the tasks of stabilizing, respectively
destabilizing, the starting flame as described above, the
perforated ceramic plate has the following functions: [0028]
Avoiding flame blowback; [0029] Redirecting the originally circular
flow of the fuel-air mixture in such a way that it moves
essentially perpendicular to the inlet cross-section of the porous
medium; [0030] Protecting the mixing chamber against overheating
due to radiation from the porous medium.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] The invention will now be explained in more detail with
reference to the enclosed drawings, wherein
[0032] FIG. 1 shows a porous burner according to the invention in a
longitudinal section;
[0033] FIG. 2 a variant of the porous burner of FIG. 1 with a
mixing chamber placed in front, in a sectional view;
[0034] FIG. 3 a diagram showing the relationship between the
limiting excess-air ratio .lamda..sub.k and the heating power of
the porous burner; and
[0035] FIG. 4 a variant of the porous burner according to FIG.
2.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0036] The porous burner shown in FIG. 1 (for instance, a gas
burner) has a housing 2 with an inlet 3 for a fuel-air mixture 4
and an outlet 5 for the exhaust gas mixture 6 generated in the
burner, where in flow direction of the process gases there is
provided in the housing 2 an ignition space 7 with an ignition
device 8 followed by a ceramic porous medium 9. The ignition space
7 is bounded on the inlet side by a stabilizing element 10, a
perforated ceramic plate, which reduces the inlet cross-section and
redirects the flow of the fuel-air mixture 4 in such a way that it
is perpendicular to the inlet cross-section of the porous medium 9.
In the example shown, which is a cylindrical porous burner, the
fuel-air mixture is directed parallel to the axis 1' of the porous
burner 1. The porous burner 1 furthermore is provided with a device
11 for controlling the mass flow of the fuel-air mixture 4, for
instance a pressure blower in front of the burner or a suction fan
behind the burner. Preferably, control is effected by increasing
the excess air ratio .lamda. when the mass flow of the fuel-air
mixture 4 is to be increased at the transition from the start-up
phase to steady-state operation.
[0037] The housing 2 of the porous burner 1 essentially comprises a
water-cooled combustion pipe 12, which is provided with a
heat-insulating lining 13. The perforated ceramic plate 10 is held
in a conical opening at the inlet 3 of the porous burner 1 by means
of a conical clamping ring 14.
[0038] In the porous oil-burner shown in FIG. 2 a preferably
heatable mixing chamber 15 for pretreating the combustion air and a
liquid fuel is placed in flow direction of the process gases in
front of the stabilizing element 10. The heater element 16 of the
mixing chamber 15 is positioned in the area of the inlet opening
18, the mixing chamber itself being attached to the water-cooled
combustion pipe 12 via an intermediate ring 17.
[0039] The material data and numerical values cited in the
following pertain to a concrete embodiment and are not to be
interpreted as restrictions for the present invention.
[0040] Regarding design and material, the porous medium 9, in which
process temperatures between 1500.degree. C. and 1800.degree. C.
can be attained, corresponds to a standard model of porous burner
technology. For example, corrugated Al.sub.2O.sub.3 ceramics could
be used. The porous medium could also consist of ZrO.sub.2,
SiO.sub.2 or other high-temperature ceramics. The heat-insulating
lining 13 has a thickness of 5 mm, for instance, and is made of a
material with low heat conductivity (e.g., 0.85 W/mK at 745.degree.
C.), with a small expansion coefficient (e.g. 0.9*10-61/K) and a
porosity of 20%. The stabilizing element 10 is a circular,
perforated ceramic plate with an outer diameter of 67 mm, a
thickness of 22 mm and a reduced free cross-section area of 689
mm.sup.2, the individual bores 10' of the plate having a diameter
of 2.19 mm. Axial directing of the fuel-air mixture is achieved by
means of the relatively long axial bores 10', the diameter of the
bores 10' in the perforated plate 10 being such that in case of a
flame blowback the flames are quenched. The diameter of the bores
10' amounts to 80% to 15%, and preferably 100%, of the thickness of
the perforated plate 10, for instance.
[0041] In the example shown the circular perforated ceramic plate
10 is bedded in a conical brass clamping ring and via the ring
attached to the inlet 3 of the combustion pipe 12. The conical
clamping ring 14 is pressed into the conical opening of the
combustion pipe 12 by the intermediate ring 17 and is thereby held
in place. It is also possible to manufacture a perforated plate
with a conical rim which can be directly inserted into the conical
opening of the combustion pipe 12. The whole attaching system of
the perforated plate is kept almost free of heat-expansion due to
the watercooling of the combustion pipe 12, and thus the
stabilizing element 10 can withstand even highest temperature
loads. At the same time the watercooling prevents overheating of
the stabilizing element 10 and an undesirable, premature
self-ignition of the fuel-air mixture.
[0042] The shifting of the combustion zone from the side of the
perforated plate 10 facing the ignition space 7 into the porous
medium 9 is caused by destabilization of the starting flames. This
is achieved when the flow velocity exceeds the flame velocity at
all points of the flame front. While sufficiently high total mass
flow can be attained with low excess air ratios in the high power
range of the burner, a higher proportion of combustion air in the
mixture is required at low power, which is associated with
decreased fuel mass flow. This defines a power-dependent limiting
excess air ratio .lamda..sub.k as shown in FIG. 3.
[0043] In the embodiment shown in FIG. 4 the perforated plate 10 is
configured in two parts and is held by two clamping flanges 19, 19'
and clamping brackets 20. The front clamping flange 19 is also used
to attach the mixing chamber 15.
* * * * *