DEVICE FOR STABILIZING DILUTE COMBUSTION IN A COLD-WALL
COMBUSTION CHAMBER
The present invention concerns a device for stabilizing dilute combustion in a
combustion chamber: it applies, in particular, to high environmental performance,
energy-efficient combustion technologies for combustion chambers known as "coldwall"
chambers, wherein the skin temperature of the walls containing the load is of
5 the order of or less than 1,000 K, such as refining furnaces or industrial furnaces.
Most combustion methods are confronted by unwanted emissions of nitrogen
oxides (NOx) in the combustion fumes. Nitrogen oxides have adverse effects on
human beings and on the environment. For example, they are responsible for acid
rain and play a significant role in the formation of atmospheric ozone.
10 European regulations are moving towards a substantial reduction in nitrogen
oxide emissions. Because of this, manufacturers of combustion equipment, in
particular burners, are constantly striving to limit nitrogen oxide emissions as much
as possible. In 201 1, according to the European LCP Directive (no. 2001- 80-EC),
the nitrogen oxide emission limit values ("ELV") for large combustion plants were
15 fixed at 200 m g l ~ mat 3~% O2 for existing industrial furnaces and 100 m g l ~ mat 3~%
O2 for new furnaces. It is very likely that these ELVs will be reduced in the years to
come.
There are many chemical pathways for the formation of nitrogen oxides. For
natural gas combustion in furnaces, the two main contributors are thermal nitrogen
20 oxides (Zeldovich mechanism) and prompt nitrogen oxides (Fenimore mechanism).
The rate of thermal nitrogen-oxide production is very dependent on the temperature.
Formation increases significantly when the temperature in the reaction zone exceeds
1,500 K. As well as being dependent on the temperature, thermal nitrogen oxide
formation is also dependent on the residence time in the hot zones.
25 Generally, given the relative contributions of the two types of nitrogen oxide
formation, the initial focus is on reducing thermal nitrogen oxides, then the effect of
the modifications on the prompt nitrogen oxides is observed.
Nitrogen oxide reductions can be achieved by means of two principles,
referred to as "primary methods" and "secondary methods". The primary methods
30 consist of preventing the formation of nitrogen oxides, while the secondary methods
are aimed at destroying the nitrogen oxides formed. The secondary methods have
the disadvantages of high implementation costs for the reduction method, high levels
of ammonia releases and decreased robustness for the plant.
Thus, the primary methods seem preferable. Most low nitrogen oxide emission
furnace burners are based on non-premixing of the air andlor fuel (e.g. as described
5 in patent 6 485 289). In this architecture, two reaction zones are created: a first rich
zone, supplying the energy required to stabilize a second, lean, zone, diluted by the
internal circulation of combustion products. A technology to further improve the
performance of low nitrogen oxide emission burners consists of using an external
recirculation of the combustion products. The combustion air is then pre-diluted with
10 fumes (approximately 30% of the flow of fumes in the flue). This makes it possible to
reduce the oxygen content in the oxidant flow and thus reduce the temperature peak
and the thermal nitrogen oxides. Patent US 6 869 277 can be cited. However, these
systems are complex, expensive and require frequent maintenance. For these
reasons they are not installed very often.
15 Flameless combustion, also called "dilute combustion", is a type of combustion
making it possible to limit the temperature peaks, and thus to substantially reduce
nitrogen oxide emissions. This combustion is based on an intense dilution of the
oxidant and fuel jets by means of internal recirculations of products of combustion,
generated by the jets of oxidant and fuel which are injected separately using high
20 velocities. The dilution makes it possible to switch from an intense localized
combustion to a more moderate combustion intensity. The high temperature of the
products used as diluent allows the stability of this mode of combustion to be
ensured. This type of combustion is characterized by a large-size heat emission
zone, uniform temperature at the flame front, much lower temperature peaks and
25 much smaller temperature fluctuations than in traditional combustion, reduced
nitrogen oxide emissions and a much weaker link between acoustic waves and heat
emission.
Patent US 5 154 599 describes an example of a flameless combustion burner.
This document presents a regenerative burner architecture, i.e. the fumes are
30 aspirated by the burner, their energy is stored in a heat reservoir and then transferred
to the combustion air so as to heat it up to 1,200 K. Dilution of the reagents before
combustion is ensured by having a considerable distance between the air and fuel
injection points (at least twice the diameter of the central injector) and a high flow rate
for fumes that recirculate (recirculation ratio greater than two for natural gas).
This technology is implemented industrially in methods known as "hot", i.e.
methods where the average temperature of the chamber is much higher than the
self-ignition temperature of the fuel in question. Flameless oxidation is self-sustained
by means of the self-ignition of an oxidanfffuel/burnt gasses ternary mixture. For self-
5 ignition to occur, the temperature in the mixing zone must be higher than the selfignition
temperature. There are two possible ways of fulfilling this condition. In the
first, at least one of the reagents (typically the oxidant) is preheated by means of
energy recovered from the fumes or by means of an external energy source. In the
second, internal recirculations of the hot combustion products are used to exceed the
10 self-ignition temperature in the reagents mixing zone. These two stabilization
methods are widespread in high-temperature applications (chamber temperature
higher than the self-ignition temperature). in effect, in this case the fumes have
sufficient energy to enable the fuel and/or the oxidant to be preheated to a high
temperature and thus the self-ignition temperature to be exceeded in the mixing
15 zone.
The stability of the dilute combustion, and therefore its sustainability, is
jeopardized in "cool-wall" type methods. As the combustion products cool on contact
with the walls, they do not let the self-ignition temperature be exceeded in the
recirculating oxidanfffuellcombustion products mixing zone. Flameless oxidation, as
20 utilized in high-temperature applications, cannot therefore be extended to cold-wall
type chambers.
However, several technologies have been developed, but not yet deployed on
an industrial scale, for furnace-type applications, in order to remove this barrier. One
can cite stabilizations by means of: . , ." . .~ . .
25 - a pilot flame,
- a catalytic element for lowering the self-ignition temperature, or
- preheating the combustion air to a high temperature.
Each of these technologies has drawbacks, in terms of cost, performance,
complexity andlor reliability.
30 The problem of flame stabilization in industrial environments is not specific to
dilute combustion. For furnace types of chambers, the flame stabilization of "low
nitrogen oxide" burners is often based on a primary intense rich combustion zone
that helps to stabilize the lean combustion zone, whose characteristics are close (in
terms of dilution of the air by combustion products) to those of a flarneless
combustion. Patent US 5 407 347 can be cited as a modern low nitrogen oxide
burner technology. As a dilute combustion application, patent EP 1 850 067, which
envisages stabilizing a highly dilute combustion by means of a pilot burner, can be
cited. However, this type of stabilization has the inconvenience of creating hot zones
5 that are potentially high nitrogen oxide emitters.
For internal combustion applications, "HCCi" (acronym for Homogeneous
Charge Compression Ignition) combustion in gasoline engines, whose properties, in
terms of mixing, are those of a dilute combustion, is performed by spark plugs. As an
industrial burner operates continuously, spark ignition technology cannot be applied
10 here.
This invention aims to remedy all or part of these drawbacks.
To this end the present invention envisages, according to a first aspect, a
device for stabilizing dilute combustion in a cold-wall type of combustion chamber,
equipped with a burner comprising at least one oxidant inlet and at least one fuel
15 inlet, the oxidant and fuel inlets opening separately into the chamber at a distance
suitable for establishing combustion which is highly diluted by internal recirculations
of the combustion products towards the burner zone. Said device comprises a
heating element designed to heat, during steady operating conditions, the
combustion products in order to sustain self-ignition conditions, said heating element
20 being positioned in the dilution zone and surrounding the set of oxidant and fuel jets.
Thanks to these provisions, stabilization of a dilute combustion is enabled. The
present device combines many interesting properties for industrial applications, such
as very low nitrogen oxide and carbon monoxide emissions, high uniformity of the
- .
- ..~ transfer to theload and reduced intensity of the temperature peaks, a reaction zone
25 well distributed in volume, more uniform heat generation and a flame less sensitive to
thermo-acoustic instabilities. Inserting the heating element in the furnace makes it
possible, by heating recirculating combustion products, to sustain the self-ignition
conditions required for stabilizing the highly dilute combustion locally in the mixing
zone.
30 According to particular features, the heating element comprises an electrical
resistance.
According to particular features, the heating element comprises a heating tube
supplied with heated products via an auxiliary combustion, products of combustion or
intermediary fluid.
According to particular features, the heating element comprises a multiperforated
tube supplied with combustion products.
According to particular features, the heating element is located at the base of
the burner.
5 This heating element -heats the combustion products, before they reach the
burner zone where they are mixed with, firstly, the oxidant and, secondly, the fuel.
The oxidant and fuel are injected separately. Thanks to the fumes being heated by
the heating element, the temperature in the fuelloxidant mixing zone thus exceeds
the mixture's self-ignition temperature. The heating element, having heated the
10 combustion products before mixing with the reagents, produces a hot zone
(temperature higher than the self-ignition-temperature) at the point where the jets of
oxidant and fuel meet.
According to particular features, the heating element is designed to maintain
the temperature of the heated combustion products in recirculation below the
15 temperature limit for the exponential rise in the rate of thermal nitrogen oxide
production.
According to particular features, the device comprises a means of preheating
air by recovering the radiative flow emitted by the heating element.
According to particular features, the air preheating means is a heat exchanger
20 located inside the combustion chamber opposite the heating element and designed
to recover a portion of the power supplied by the heating element transferred in the
form of a radiative flow to the cold walls of the combustion chamber and to transfer,
to the oxidant and/or to the fuel, at least a portion of this recovered power.
. .. According toparticular features, theair preheating:means:is.aheat exchanger
25 located inside the combustion chamber opposite the heating element and designed
to recover a portion of the power supplied by the heating element transferred in the
form of a radiative flow to the cold walls of the combustion chamber and to transfer,
to a fluid load to be heated, at least a portion of this recovered power.
The stability of the flameless combustion is thus further increased by
30 preheating the oxidant or the fuel, and performance is improved. Finally, the power
thus generated by the heating element is recovered in two ways: firstly, by the
oxidant via the exchanger and secondly by the fumes in recirculation via a
conductive-convective exchange. In addition, the heat exchanger makes it possible
to reduce the thermal stress on the cold walls located opposite the heating element.
Good uniformity of the heat transfer over the whole of the combustion chamber's
heating walls is thus retained.
According to particular features, the air preheating means is a radiative wall
located inside the combustion chamber between the heating element and the cold
5 walls.
This radiative wall, or screen, confines the heat to the inside of the heating
unit. In this way the radiative heat transfer from the heating element to the opposite
walls is limited. The length of this wall must be equal to the height of the heating
element. The two key points are the material forming this wall and its diameter.
10 According to particular features, the oxidant and fuel inlets have injection holes
'' with suitable diameters such that the oxidant an& fuel speeds are favorable for
utilizing a highly dilute combustion.
According to a second aspect, the present invention envisages a cold-wall
type of combustion chamber, which comprises a combustion stabilization device that
15 is the subject of the present invention.
According to a third aspect, the present invention envisages a furnace, which
comprises a combustion stabilization device that is the subject of the.present
invention.
Other advantages, aims and characteristics of the present invention will
20 become apparent from the description that will follow, made, as an example that is in
. no way limiting, with reference to the drawings included in an appendix, in which:
- figure 1 is a functional diagram of a first embodiment of the device that is
the subject of the present invention for utilizing flameless combustion in a
.: ~ . . . ..- - .. .-. .. . . . Y - - cold-wall combustion chamber, with no preheatingof theoxidantor fuel,
25 - figure 2 is a functional diagram of a second embodiment of the device that
is the subject of the present invention for utilizing flameless combustion in
a cold-wall combustion chamber coupled with a system for recovering the
radiative flow emitted by the heating element for the purposes of
preheating the oxidant,
- figure 3 is a diagram of a longitudinal view of a combustion chamber
equipped with the heating element for stabilizing flameless combustion and
- figure 4 represents a variant of the device illustrated in figure 3 comprising
a means of preheating air by recovering the radiative flow emitted by the
heating element.
Figures 1 and 2 show a cold-wall combustion chamber 245 type of installation,
for example an industrial gas furnace equipped with a burner and a flue. Generally,
the burner permits a plane of symmetry. The chamber permits an axis of symmetry in
the case of a fire-tube furnace.
Only the upper half of the installation is shown in figures 1 and 2. The burner
proposed here has a simple architecture. The air inlet 105 is located in the center
and two fuel injection points 110 are positioned on the periphery. In variants, there
are more than two fuel injection points. In variants, the fuel injection point is
positioned in the center and oxidant injection points are placed on the periphery. Two
important parameters characterize the burner: the distance between the injection
holes and the velocities of the fuel and oxidant jets. The distance between the jets of
fuel and oxidant must allow these to be mixed with the combustion products that
recirculate, cooled on contact with the walls, before the jets of oxidant and fuel meet.
Intense recirculations of combustion products are fed by the high velocities on output
from the oxidant injector. For instance, for flameless combustion of natural gas, the
recirculation ratio, defined as the ratio of the recirculating flow rate to the sum of the
injected flow rates, is greater than or equal to four.
Figure 1 shows that the air enters at a temperature of 300 K (the temperatures
of the fluids are indicated, in figures 1 and 2, in italic figures, beside the arrows
representing their movements). The air is mixed with the combustion products at a
temperature of at least 1,100 K in 115 before being mixed with the fuel; in 125. The
fuel enters at a temperature of 300 K and it, also, is mixed with the combustion
products in 120. The mixture obtained in 125 has a temperature of 850 K.
Combustion occurs in 130 and the-combustion products exit atL1,700 K. The heat..
exchange between the combustion products and the load occurs in 135. A portion of - ~
the combustion products performs a recirculation in 140 to return to be mixed with
the fuel and the incoming air.
In the cold-wall combustion chambers considered, the temperature of the
fumes at the base of the burner is less than 700 K. In this case, the average
temperature of the mixture, fuel/oxidant diluted by the fumes, is less than the selfignition
temperature of all the usual fuels. Dilute combustion, sustained by the selfignition
of the mixture, cannot therefore be correctly established.
In the embodiments of the device that is the subject of the invention
represented in the figures, a heating element 145 that heats the portion of the fumes
that recirculate is positioned to take part in the dilution of the reagents. This heating
by the heating element 145 aims to reach, in the fuelloxidant mixing zone, firstly, a
temperature higher than the self-ignition temperature of the fuel considered and,
secondly, an oxygen concentration of the order of 5% in the oxidant jet. A stable
5 dilute combustion, sustained by the self-ignition of the mixture of reagents, is
therefore established in 130.
The maximum temperature in the reactive zone thus obtained is of the order of
1,700 K. Compared to conventional combustion, this moderate temperature produces
a significant reduction in nitrogen oxide emissions. The combustion occurs over a
10 large area and no longer in a concentrated flame. The load to be heated can be
'.-water (fire-tube furnace or water-tube furnace) or another liquid product (a refinery's
atmospheric distillation furnace, for example). All the fumes that recirculate transfer a
significant portion of their energy to the load. The fumes that recirculate up to the
level of the mixing zone are therefore heated from 850 K to at least 1,100 K by the
15 heating element.
The skin temperature of the heating element must enable the combustion
products that recirculate to be heated sufficiently while not exceeding the
temperature limit for thermal nitrogen oxide production. The inventors have
determined that a value of 1,200 K represents a good compromise for respecting
20 these two conflicting constraints.
. . . . Low temperatures are also obtained for the walls containing the load (water or
fluid). In addition, the temperature of the heating element is preferably at least 1,100
K. Thus, the net incident radiative flow at the walls containing the load to be heated,
received bythe heating element, relative to thedifference in heating element/walls:- - ..~ .
25 temperatures raised to the fourth power, is high. This additional radiative flow can
thus degrade the uniformity of the transfer to the load.
One solution, described here with respect to figure 2, consists of installing an
exchanger 150 in the chamber. The oxidant circulates in this exchanger. The
preheating obtained in this way allows the stability of the flameless combustion to be
30 further improved, while re-homogenizing the transfer to the load, for an equal level of
performance. It has been estimated that a preheating of the air of the orderof 700 K
can be obtained with such an exchanger. As the combustion is highly diluted,
preheating the combustion air has only a minor effect on nitrogen oxide emission
levels.
Figure 3 is a diagram of a longitudinal view of a combustion chamber
equipped with the heating element for stabilizing the highly dilute flameless
combustion, coming from a burner with separate jets of liquid or gaseous fuel, in a
cold-wall combustion chamber.
. . The combustion chamber 245, supplied with fuel 1, and air.2, is a confined
area having an axis of symmetry 250 in the case of a fire-tube furnace, for example.
The burner is a burner type with separate injection points and a large distance
between the jets. The air injection point 205 is located in the center and the fuel is
transported on the periphery by two ports 210. As can be seen in figure 3, the
distance between the central air injection point 205 and the two gas injectors 210 is
between -1:5 and 3 times the diameter of the oxidant injector 205 for a combustion
with natural gas.
Thus, before the fuel and the oxidant are in contact in 235, they are diluted by
recirculated combustion products 230, for the oxidant, and 225, for the fuel. These
recirculated combustion products are heated beforehand by the heating element 21 5,
here in the form of a coil surrounding the fuel and oxidant jets 295.
The heating element 215 can be of various types. For example, it can consist
of an electrical resistance, a hollow heating element supplied with hot products from
an auxiliary combustion, or possibly a combination of the two, depending on the
resources available on site.
The-sizing and positioning of the heating element 215 allow the exchange
surface to be maximized while not introducing any confinement effect for the mixing
zone with respect to internal recirculations. As shown in figure 3, the flow of products
passes between the coils withammoderatelo ss of load. The spiral pitch is sufficientto., . .
limit the loss of load and allow dilution. Conversely, this distance is sufficiently small
and the diameter of the coils is sufficiently large as to maximize the heat exchange
surface. The heating element 215 is positioned as close as possible to the base of
the burner, the wall where the fuel and oxidant emerge. The length, along axis 250,
of the heating element 215 allows the heating element to go beyond the position of
the pointwhere the fuel and oxidant jets merge. At this level, the diameter of the
heating element 215 is such that the heating element 215 is not impacted by the
lateral jets. A safety distance of several centimeters is preferable, so as to limit the
thermal stresses on the heating element 215 and thus increase its lifespan.
Regardless of the energy supply (products of an ancillary combustion or electric
power), the material of the heating element 215 allows the convective heat transfer to
be maximized and the emitted radiative flow to be minimized. A significant roughness
of the order of one millimeter is preferred, so as to increase the convective transfer
coefficient. In contrast, a low emissivity, of the order of 0.1, is preferable so as to
reduce the loss by radiative transfer.
The velocities of the jets (mainly the oxidant jet) are the source of intense
advective movements 220. It has been shown that a recirculation ratio of at least 4 is
required in a combustion with natural gas to ensure sufficient dilution in 235. With
respect to the sizing of the injection point holes of the burner, these correspond to a
speed of the oxidant on output from the injector, preferably equal to at least 30 mls.
During their.recirculation; the combustion products lose a large part of their- ~ '
energy by convection and radiation to the benefit of the load to be heated. This load
can be contained in tubes (water-tube furnace or refining furnace) or in a space in
which the combustion chamber 245 is immersed (fire-tube furnace).
The heating element 215 is then used to partially compensate for this heat
loss by heating the combustion products that recirculate and are involved in diluting
the reagents. These combustion products being heated, the temperature in 235
exceeds the self-ignition temperature of the fuel, and the dilute combustion is
therefore stable. The dilute combustion develops in area 240 and produces few
nitrogen oxides. Finally, the fumes are evacuated from the combustion chamber by
the flue 260. The supplying of the heating element 215 is shown in 255. . ... ~ . .
To ensure sufficient heat transfer, two parameters have to be considered in
addition to the exchange surface: the temperature differential between the skin of the
heating element 215 and the fumeslin. contact with it, andthe convective transfer- ~- :...:C?~L
coefficient at the wall of the heating element 215. The temperature of the fumes is of
the order of 850 K for an industrial fire-tube furnace. It has therefore been estimated
that a temperature.of at least 1,200 K on the surface of the heating element 215 is
preferable, for heating the fumes sufficiently and ensuring a temperature in 235
higher than the self-ignition temperature of the usual fuels. The second parameter
influencing the value of the convective transfer coefficient between the heating -
element 215 and the fumes is the convective transfer coefficient h. This coefficient
mainly depends on the speed of the fluid, its viscosity and the roughness of the walls.
Among these parameters, one can play mainly on the roughness of the walls. To
maximize the energy transmitted to the fluid by the heating element 215, the transfer
coefficient and the temperature differential are maximized.
Nevertheless, with a very high temperature at the wall of the heating element
215, a significant portion of the energy dissipated by the heating element 215 will be
recovered by the walls of the chamber 245 located opposite the heating element 215, .- -
in the form of incident radiative heat flow. In figure 4, the radiative flow emitted by the
heating element 215 mainly depends on the emissivity coefficient of the material that
the heating element 215 is made of and on the temperature differential between the
walls of the heating element and the walls containing the load to be heated, raised to
the fourth power. It is therefore preferable to use a material with a low coefficient of
emission for the heating element 215. ..~, .~
However, with current materials, as the temperature differential is large, it is
difficult to reduce the incident radiative flow on the walls containing the load to a
negligible amount. Thus, a loss of the uniformity of the heat transfer, obtained by
means of the flameless combustion, can appear.
A variant of the device illustrated in figure 3 can comprise a means of
preheating air 280 by recovering the radiative flow emitted by the heating element
215. This preheating means 280 can thus be associated with the heating element
215 so as to recover the radiative flow emitted by the heating element 215. Therefore
either a radiative screen (not shown), which confines the heat to the inside of the
heating unit, or, as in figure 4; a heat-exchanger 280, which recovers the incident
radiative flow coming from the heating element 215 to preheat the combustion air
before it enters the chamber 245, is provided. The heat exchanger 280 is installed
facing the heating element 215,:preferably.all around-thewalls of the furnace.ln 290, ~
-
the fuel enters, cold, into the exchanger, and exits in 285, before being injected into
the combustion chamber by inlet 205. Typically, the heating temperature can reach
700 K for combustion with air. This heat exchanger 280 allows further improvements
in the combustion stability, while maintaining a uniform transfer to the load and low
nitrogen oxide emissions, and all for an equal level of performance. One of these
variants is illustrated in figure 4.
The stability of the flameless combustion is thus further increased by this
preheating of the oxidant, and performance is improved. Finally, the power thus
generated by the heating element is recovered in two ways: firstly, by the oxidant via
the exchanger and secondly by the recirculated fumes via a conductive-convective
exchange.
In variants, a process fluid to be heated is circulated, rather than the
combustion air, and the available energy is thus recovered for a use other than
5 preheating the oxidantor fuel. . .. .. ~. .
In the case where the exchanger 280 is replaced by a radiative wall located
between the heating element and the walls, the length of this wall is preferably equal
to or greater than the length of the heating element.
Simulations have shown a sizing that allows a flameless combustion to be
10 stabilized in a 20 kW pilot furnace. The temperature of the walls of the combustion
chamber is of the order of 350 K: The-jets of fuel and oxidant are not preheated to ~~ .
begin with. The burner is a burner type with separate jets having a central air injector
and two lateral methane injectors. The distance between the jets is twice the
diameter of the air jet. The parameters are given in the following table.
15 The same modeling method as that for an earlier study into flameless
combustion in a hot chamber was used. The cold chamber simulations made it
possible to show that the chosen geometry - coiled heating element with the
dimensions given below - allowed the same operating mode to be kept as that for the
burner alone (identical mixture and aerodynamic fields).
20 Thus the flameless combustion dilution conditions are respected with the
presence of the heating element 215;-In addition; with a skin temperature of the
heating element 215 of the order of 1,200 K, a roughness of 1 mm and a low
emissivity of 0.1, heating the fumes that recirculate in the burner zone produced by
... .. .:., . . I . i . .. . . the heating element 215 makes- it - possible ito;:findt~.the ..thermal conditions for - ~~ ~ . .
25 stabilizing the combustion In the Airlmethane mixing zone.
I I
I Volume 1 0.12 mJ
Burner
Combustion
Temperature of the
chamber
internal walls
I I
Heating I HE diameter 1 8.6 cm
Burner capacity 20 kW
Aeration rate I 1.1
element
I
HE height 16 cm
This invention extends to applications for combustion in a cold-wall
combustion chamber equipped with a burner comprising an inlet for oxidant and an
inlet for fuel (natural gas, process gas, coke oven gas, synthesis gas, etc.). The
~ . ~~ 5 oxidant and fuel inlets open separately into a furnace at a relative distance allowing
the establishment of combustion which is highly diluted (distance between the jets
between 1.5 and 3 times the diameter of the oxidant injector and injection speeds
between 20 and 100 mls for a natural gaslair combustion) by internal recirculations of
the combustion products towards the burner zone.
10 Stabilization of the flameless combustion in a cold-wall type of combustion
chamber (temperature below 1,000 K) is obtained by using a solid heating element
(electrical resistance or heating tube supplied with products heated by combustion or
an auxiliary system) located at the base of the burner and surrounding the
oxidant/fuel jets. This spiral-shaped element heats the combustion products, before
~ .~ 15 they reach the burner zone where they are mixed with; firstly, the oxidant and,
secondly, the fuel. The oxidant and fuel are injected separately. Thanks to the fumes
being heated by the heating element, the temperature in the fuelloxidant mixing zone
- . . . . . ... -ttius exceeds the mixture's self-ignition temperature!.' . ' . , . ~ , .
The velocities of the jets are high in order to generate intense recirculations of
20 the combustion products towards the burner. Typically, for an airlnatural gas
combustion, the speed of the air entering the chamber is more than 30 mls. With this
type of burner architecture, the combustion is self-sustained subject to having a
temperature in the diluted fuelldiluted air mixing zone that is higher than the selfignition
temperature of the mixture.
25 The heating element, having heated the combustion products before mixing
with the reagents, produces, at the point where the jets of oxidant and fuel meet, a
hot zone (temperature higher than the self-ignition temperature) with a dilution such
HElburner base distance
HE temperature
Diameter of the coils
Spiral pitch
Emissivity
Roughness
2.5 cm
1200 K
4 mm
6 mm
0.1
I mm
that the oxygen content' is of the order of 5 to 8%, instead of 20% for non-diluted air.
Combustion, highly diluted in this way, is then generated and self-sustained by selfignition
according to the flameless combustion principle.
In order to ensure sufficient dilution, the recirculation ratio (defined as the ratio
5 of the recirculated flow rate to the sum of the injected flow rates) is greater than fourfor
flameless combustion with natural gas, for example. This condition is ensured by
the high speeds of the reagents coming out of the injectors.
In addition to purely technical constraints (ease of installation, ease of
maintenance, cost), the two constraints concerning the heating element are: firstly,
10 for the flow not to be confined to the mixing zone, thus allowing the reagents to be
sufficiently diluted.by the recirculated products, while being sufficiently close to the
mixing zone. And, secondly, to maximize the convective heat transfer between the
heating element and the recirculated fumes taking part in the dilution, while avoiding
hot points (temperature below the temperature limit for the exponential rise in the rate
15 of thermal nitrogen oxide production).
It is noted that several forms of heating element can be used: coil, grids,
burner tube with multiple perforations, radiant burner, etc. For each case, sizing is
done so as to allow a maximum exchange surface and a minimum loss of load for the
crossing of the heating element. In the case of the coiled heating element, the
20 important dimensions are the diameter of the heating element, its length, its spacing
-.- --relative to the burner base, the diameter of the coils and the spiral pitch. The heating
element thus makes it possible to overcome the conflict between high temperature
and high dilution in the recirculated productsloxidantlfuel mixing zone. As the
, - ~.apparatus.doesn ot change the burner's aerodynamic properties,.the~mainp roperties
25 of highly dilute combustion are obtained: the nitrogen oxide emissions are reduced,
the reaction zone is distributed in volume, heat generation is more uniform, the flame
is less sensitive to thermo-acoustic instabilities and the temperature peaks are less
intense.
CLAIMS
1. Device for stabilizing dilute combustion which is intended to be used in a cold-wall
type of combustion chamber (245), equipped with a burner comprising at least one
oxidant inlet (105, 205) and at least one fuel inlet (110, 210), the oxidant and fuel
inlets opening separately into the chamber at a distance suitable for establishing
5 combustion (130, 240) which is highly diluted by internal recirculations (140, 220) of
the products of combustion towards the burner zone,
characterized in that it comprises a heating element (145, 215) designed to heat,
- during steady operating conditions, the combustion products in order to sustain selfignition
conditions, said heating element being positioned in the dilution zone (120)
10 and surrounding the set of oxidant and fuel jets (295).
2. Device according to claim 1, wherein the heating element (145, 215) comprises an
electrical resistance.
15 3. Device according to one of claims 1 to 2, wherein the heating element (145, 215)
comprises a heating tube supplied with heated products via an auxiliary combustion,
products of combustion or intermediary fluid.
4. Device according to one of claims 1 to 3, wherein the heating element (145, 215)
20 comprises a multi-perforated tube supplied with combustion products.
. ~ ~.. . . ~~
.. ~.~- - = -
~.,..L ~ . . - ~ ~ . ~ ..* .: . - ... . ~ . : . . ,.,
5. Device according to one of claims 1 to 4, wherein the heating element (145, 215)
is located at the base of the burner.
25 6. Device according to one of claims 1 to 5, wherein the heating element (145, 215)
is designed to maintain the temperature of the heated combustion products in
recirculation below the temperature limit for the exponential rise in the rate of thermal
nitrogen oxide production.
30 7. Device according to one of claims 1 to 6, that comprises a means of preheating air
(280) by recovering the radiative flow emitted by the heating element (215).
8. Device according to one of claims 1 to 7, wherein the air preheating means (280)
is a heat exchanger (150, 280) located inside the combustion chamber opposite the
heating element (145, 215) and designed to recover a portion of the power supplied
5 by the heating element transferred in the form of a radiative flow to the cold walls of
.the combustion chamber (245) and to transfer, to the oxidant and/or to the fuel, at
least a portion of this recovered power.
, 9. Device according to one of claims 1 to 7, wherein the air preheating means (280)
I
10 is a heat exchanger (150, 280) located inside the combustion chamber opposite the
. . heating element (145, 215) and designed to- recover a portion of the power~supplied
by the heating element transferred in the form of a radiative flow to the cold walls of
the combustion chamber (245) and to transfer, to a fluid load to be heated, at least a
portion of this recovered power.
15
10. Device according to one of claims 1 to 7, wherein the air preheating means is a
radiative wall located inside the combustion chamber (245) between the heating
element (145, 215) and the cold walls.
20 11. Device according to one of claims 1 to 10, wherein the oxidant and fuel inlets
have injection holes with suitable diameters such that the oxidant and fuel speeds .
are favorable for utilizing a highly dilute combustion.
~ . . ~ 12. Cold-wall type of combustion chamber(245),:characterized in that it comprises a . .:!,:-.:-
25 device for stabilizing dilute combustion according to claims 1 to 11.
13. Furnace, that comprises a device according to one of claims 1 to 11