Note : Les descriptions sont présentées dans la langue officielle dans laquelle elles ont été soumises.
WO92/16792 PCT/US92/020~
7 2 l1 1
APPARATUS AND METHOD FOR COMBUSTION
WITHIN POROUS MATRIX ELEMENTS
FIELD OF THE INVENTION
This invention relates generally to combustion apparatus
and methodology, and more specifically relates to an
improved combustion apparatus and method which provides
increased flame control and stability, and which is
especially effective in the reduction of NO~ emissions.
BACKGROUND OF THE INVENTION .
Environmental pollution caused by combustion-generated NO~
emissions, is a matter of great concern to the public,
and as well to industrial fuel users. Beginning in the
1960's, governmental agencies, indeed prompted by public
concern with increasing levels of smog and air
pollutants, imposed NO~ reduction requirements upon exist-
ing power plants in major metropolitan areas. Theserestrictions were expanded in the 1970's and 1980's to
include virtually all industries with combustion
equipment. Industry, accepting the challenge, has
already developed a large variety of technologies to meet
the new needs. Modifying the combustion process has
become the most widely used technology for reducing
combustion generated NO~ . In addition, a number of flue
gas treatment technologies have been developed and are
emerging as the primary method of control for certain
applications, but have seen limited use where natural gas
is the fuel of choice.
Oxides of nitrogen (NOI) are formed in combustion
processes as a result of thermal fixation of nitrogen in
the combustion air (~thermal NO~"), by the conversion of
chemically bound nitrogen in the fuel, or through
"prompt-NO~" formation. Thus, in addition to generating
"thermal NOx'', i.e., by high temperature combination of
free nitrogen and oxygen, where the fuels employed by
WO92/167s2 PCT/US92/020
~ ~ 2
such users (e.g. coal gas) contain substantial quantities
of chemically bound nitrogen, certain combustion
conditions will favor the formation of undesirable NO-
type compounds from the fuel-bound nitrogen. "Prompt NO~"
refers to oxides of nitrogen that are formed early in the
flame and do not result wholly from the Zeldovich
mechanism. Prompt-N0~ formation is caused by l)
interaction between certain hydrocarbon components and
nitrogen components and/or, 2) an overabundance of oxygen
atoms that leads to early NO~ formation. For natural gas
firing, virtually all of the N01 emissions result from
~; thermal fixation, i.e. "thermal N0~", or from prompt NO~.
The formation rate is strongly temperature dependent and
generally occurs at temperatures in excess of 1800K
(2800F) and generally is more favored in the presence of
excess oxygen. At these temperatures, the usually stable
nitrogen molecule dissociates to form nitrogen atoms
which then react with oxygen atoms and hydroxyl radicals
to form, primarily, NO.
In general, NO~ formation can be retarded by reducing the
concentrations of nitrogen and oxygen atoms at the peak
combustion temperature or by reducing the peak combustion
temperature and residence time in the combustion zone.
i 25 This can be accomplished by using combustion modification
techniques such as changing the operating conditions,
modifying the burner design, or modifying the combustion
system.
Of the combustion modifications noted above, burner
design modification is most widely used. Low NO~ burners
are generally of the diffusion burning type, designed to
reduce flame turbulence, delay the mixing of fuel and
air, and establish fuel-rich zones where combustion is
initiated. Manufacturers have claimed 40 to 50 percent
nominal reductions, but significant differences in the
predicted NOz emissions and those actually achieved have
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WO 92/16792 ~ 2 ~ L PCT/US92/02084
i
been noted. The underlying cause for these discrepancies
is due to the complexity in trying to control the
simultaneous heat and mass transfer phenomena along with
the reaction kinetics for diffusion burning.
Illustrative of the foregoing and related techniques for
` NO~ reduction, are the disclosures of the following United
States patents:
DeCorso, U.S. patent 4,787,208 discloses a low-NOI
combustor which is provided with a rich, primary burn
zone and a lean secondary burn zone. NO~ formation is
inhibited in the rich burn zone by an oxygen deficiency,
and in the lean burn zone by a low combustion reaction
temperature. Ceramic cylinders are used at certain parts
of the combustion chambers.
Furuya et al, U.S. patent 4,731,989 describes a
combustion method for reducing NO~ emissions, wherein
catalytic combustion is followed by non-catalytic thermal
combustion.
Davis Jr. et al, U.S. patent 4,534,165 seeks to minimize
NO~ emissions by providing operation with a plurality of
catalytic combustion zones and a downstream single
"pilot" zone to which fuel is fed, and controlling the
flow of fuel so as to stage the fuel supply.
~eCorso, U.S. patent 4,112,676 shows a combustor
generally of the diffusion burning type for a gas turbine
engine.
Pil~s~ury, U.S. patent 4,726,181 provides combustion in
two catalytic stages in an effort to reduce NO~ levels.
Rendall et al, U.S. patent 4,730,599 discloses a gas-fire
radiant tube heating system which employs heterogeneous
W092/16792 ~1 ~ 7 ~ ~i PCT/US92tO2084
catalytic combustion and claims low-NO~ catalytic combus-
tion.
Shaw et al, U.S. patent 4,285,193 describes a gas turbine
; 5 combustor which seeks to minimize NO~ formation by use of
multiple catalysts in series or by use of a combination
of non-catalytic and catalytic combustion.
Pfefferle, U.S. patent 3,846,979 describes low NO~
emissions in a two-stage combustion process wherein
combustion takes place above 3300F., the effluent is
quenched, and the effluent is subjected to catalytic
oxidation.
Beremand et al, U.S. Patent No. 4,087,962, discloses a
combustor which utilizes a non-adiabatic flame to provide
a low emission combustion for gas turbines. The fuel-air
mixture is directed through a porous wall, the other side
of which serves as a combustion surface. A radiant heat
sink is disposed adjacent to the second surface of the
burner so as to remove radiant energy produced by the
combustion of the fuel-air mixture, and thereby enable
operation below the adiabatic temperature. The inventors
state that the combustor operates near the stoichiometric
mixture ratio, but at a temperature low enough to avoid
excessive NO~ emissions. In one embodiment the radiant
heat sink comprises a further porous plate.
In U.S. patent 4,811,555, of which Ronald D. Bell, one of
the applicants of the present application, is patentee,
there is described a cogeneration system in which NO~ is
controlled by the treatment of the turbine exhaust by a
combination of combustion in a reducing atmosphere and
catalytic oxidation.
In McGill et al, U.S. patent 4,405,587, for which Ronald
D. Bell is a co-patentee, the NO~ content of a waste
WO92/16792 PCT/US9~/020
stream is controlled by treating ~ and subjecting it to
high-temperature combustion in combined reducing and
oxidation zones.
i
Recent work by several of the present co-inventors and
others, has resulted in a combustion device which
utilizes a highly porous inert media matrix to provide
for containment of the combustion reaction within the
porous matrix ("PM") -- which may comprise fibers, beads,
or other material which has a high porosity and a high
melting temperature. Preferably, a ceramic foam is used.
This ceramic, sponge-like material has a porosity
(typically about 90%) which provides a flow path for the
t~ combustible mixture. The energy release by the gas phase
reactions raises the temperature of the gases flowing
through the porous matrix in the postflame zone. In
turn, this convectively heats the porous matrix in the
postflame zone. Because of the high emissivity of the
solid in comparison to a gas, radiation from the high
temperature postflame zone serves to heat the preflame
zone of the porous material which, in turn, convectively
heats the incoming reactants. This heat feedback
mechanism results in several interesting characteristics
relative to a free-burning flame. These include higher
burning rates, higher volumetric energy release rates,
and increased flame stability resulting in extension of
both the lean and rich flammability limits. In addition
to the ability to achieve very high radiant output from a
very compact combustor, flame temperature increases are
negligible. This is an important consideration with
respect to NOI control purposes.
A one-dimensional mathematical model was constructed that
included both radiation and accurate multi-step chemical
kinetics. This model was used to predict the flame
structure and burning velocity of a premixed flame within
an inert, highly porous medium. The various predictions
W092/~6792 PCT/US92/020~
7~
of this model have been discussed by Chen et al. See
"The Effect of Radiation on the Structure of Premixed
Flames Within a Hiahly Porous Inert Medium", Y-K Chen,
R.D. Matthews, and J.R. Howell, Radiation Phase Chanae
Heat Transfer and Thermal Systems, ed. by Y. Jaluria,
V.P. carey, W.A. Fiveland, and W. Yuen (eds.), ASME
Publication HTD-Vol. 81, 1987. "Premixed Combustion in
Porous Inert Media"; Y-K Chen, R.D. Matthews, J.R.
Howell, Z-H Lu, and P.L. Varghese, Proceedings of the
Joint Meetinq of the Japanese and Western States Sections
of the Combustion Institute, pp. 266-268, 1987; and
"Experimental and Theoretical Investigation of Combustion
in Porous Inert Media", Y-K Chen, R . D . Matthews, I -G Lim,
Z. Lu, J.R. Howell, and S.P. Nichols, Paper PS-201,
Twenty-Second Symposium (International) on Combustion,
1988. These papers demonstrate that a porous matrix (PM)
combustor can provide a number of advantages over
diffusion burners. However, these papers are focused on
the development of this new concept, but are not
concerned with the problem of N0~ emissions, much less
with the effective reduction of same.
The latter issue is, however, addressed in our parent
Serial No. 554,748 application in which low N0~ combustion
is effected by a method wherein a fuel, e.g., natural
gas, and a source of oxygen, e.g., air, are mixed and the
mixture is combusted in at least two successive
combustion zones filled with a porous matrix, the void
spaces of which provide sites at which substantially all
of the said combustion occurs. Preferably, the method
utilizes three such combustion zones. The first or most
upstream zone is filled with a said porous matrix, and
the mixture provided thereto is fuel-lean. In the second
succ~ssive zone the mixture is fuel-rich; and in the
third zone the mixture is fuel-lean.
Serial No. 670,286, of which this application is a
W092/16792 PCT/US92/020~
7~
continuation-in-part, addresses a serious problem that
has been experienced with PM burners, i.e. flame
flashback from the postflame to preflame zones. The
latter may include ceramic foam and/or flow mixing and
distributing means such as ceramic honeycomb, glass beads
or other media, or simply media void mixing space.
Flashback of the flame from the postflame zone where
combustion is desired, aside from creating potential or
actual danger, by definition is uncontrolled burning --
which is precisely the condition sought to be avoided inorder to preclude or limit NOX formation. It might be
thought that by providing a sufficient rate of fuel/air
flow through the PM combustion zone, the problem could be
eliminated, i.e. by using a flow rate exceeding the
possible rate of back propagation of the flame. It
develops, however, that in the real system present in the
PM burner, the porous media, as for example where same is
in the general shape of a solid cylinder, acts with
respect to the normally axial flow of the fuel-air
mixture through such cylinder, to cause an uneven rate of
flow across a plane transverse to the cylinder.
Specifically, there will tend to be flow stagnation at
the peripheral walls of the cylinder, as opposed to the
generally maximum flow rate occurring at the axis.
Accordingly, merely increasing the rate of flow of the
fuel-air mixture is not generally sufficient to assure
the absence of u~desired flame flashback to the preflame
zone.
The problem presented by the foregoing is recognized in
Fleming, U.S. Patent No. 4,643,667. In this, Flemin~
discloses a noncatalytic porous phase combustor
comprising a porous plate having at least two discrete
and contiguous layers, a first preheat layer comprising a
material having a low inherent thermal conductivity, and
a second combustion layer comprising a material having a
high inherent thermal conductivity and also providing a
, . ~ .
W092/16792 PCT/US92/020~
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radiating surface. The presence of the low conductivity
material tends to limit the heating in that initial zone,
thereby discouraging flashback. The construction
recommended by Fleming is, however, a very complex and
difficult one to achieve. Furthermore, the presence of
; the contiguous low conductivity material, while affording
? advantages as aforementioned, also introduces a pressure
~ drop into the flow, with no commensurate benefits.
., .
' 10 In the apparatus of the S.N. 670,286 invention, mixing
and flow directing means are provided for receiving and
mixing a fuel, e.g. natural gas, and a source of oxygen,
} e.g. air, and forming a flow of the combustible mixture.
The combustible mixture is flowed downstream to a combus-
tion zone defined by a porous high temperature-resistant
matrix, the void spaces of which provide sites at which
substantially all of the combustion occurs, which zone
includes an input end for receiving the combustible flow
from the mixing and flow directing means. Cooling means
are mounted in proximity to the input end of the
combustion zone for maintaining the temperature of the
combustible mixture at the input end below ignition
temperature, to thereby limit the flame produced by
combustion in the porous matrix to the downstream or
postflame side of the cooling means. The cooling means
typically comprises a generally toroidal metal body which
is provided with one or more internal cooling channels.
This body surrounds, and is in thermal contact with the
input end of the combustion zone. Means are provided for
¦ 30 circulating a coolant through the body, which coolant can
typically be water but may be other liquid media or a
gas, including air. The cooling body is so mounted as to
be nonintrusive with respect to the porous matrix in the
combustion zone, so as to introduce no impedance to the
flowing fuel and oxygen source mixture.
The S.N. 670,286 invention is applicable to a single
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~092/16792 PCT/US92/020~
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- 9 C~ 72~l
stage porous matrix burner, as well as to the multiple
stage devices which are disclosed in parent application
Serial No. 554,748. In any of these instances, the
cooling means is positioned as to be at the input end
(i.e. in advance) of the first (or single) stage whereat
combustion is to be effected. The cooling stage in each
instance acts to produce a sharp discontinuity in
temperature so that even where the flow stagnation effect
aforementioned (which tends to occur at the periphery of
the porous matrices) is present, there is substantially
no danger of flashback from the flame of combustion which
exists in the postflame PM zone(s). By eliminating the
flashback potential, it is found that extremely stable,
well-formed flames result, which in turn provide the
highly controlled combustion conditions which are one of
the objectives sought after in porous media burners, for
the special objective of reducing generation of NOX.
A combustion process is thus provided enabling controlled
low NOX combustionO Fuel and an oxygen source such as
~ air are mixed and formed into a combustible flow stream.
¦ The flow stream is passed to an input end of a combustion
I zone defined by a porous high temperature-resistant
I matrix. The mixture is combusted at the matrix, the void
1 25 spaces of which provide sites at which substantially all
¦ of the said combustion occurs, and the combustion
i products are flowed from an output end of the matrix.
The input end of the combustion zone is cooled, to
maintain the temperature of the combustible mixture at
the said input end below ignition temperature, thereby
limiting the flame produced by combustion in the porous
matrix to the downstream side of the cooling means.
SUMMARY OF INVENTION
In our above-cited prior applications the advantagas of a
two-stage PM burner were considered to be best achieved
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~ WO92/16792 PCT/US92/020~f
~ 2~ 10 fr~
by maintaining fuel-rich conditions in the first, i.e.
upstream zone, and fuel-lean conditions in the second,
: i.e. downstream zone. Unexpectedly, it has now been
. found that outstanding reductions of NOs and Co are
5, 5 achieved by utilizing a fuel-lean first stage and a fuel-
rich second stage. Although not required for NO~
' reduction, preferably such an arrangement is used in
.~ conjunction with a first-stage cooling means as above
described, i.e. which are mounted in proximity to the
~? 10 input end of the first combustion zone. Incorporation of
the cooling means is preferred in order to achieve the
previously discussed advantage of same, including to best
achieve a broad range of equivalence ratios in operation
of the invention.
` 15
Apparatus for low NOI combustion in accordance with the
i invention, may thus comprise first and second combustion
zones, each filled with a said porous matrix, and said
second zone being downstream of said first zone. Means
are provided for mixing fuel and oxygen and providing
same to said first combustion zone to establish fuel-lean
conditions therein; and means for providing the
combustion products from said first zone to said second
zone and aug~enting same with sufficient fuel and addi-
tional oxygen to create fuel-rich burning conditions
therein to complete the oxidation of the products from
the first zone.
.'
Heat transfer by convection and radiation within the
porous matrix element of the first zone preheats the
! incoming fuel/air mixture to yield a flame temperature
which is higher than the theoretical adiabatic flame
temperature for said mixture, thus allowing a broader
range of fuel/air mixtures to be combusted under fuel
lean conditions, and in which heat transfer by radiation
from the non-porous walls of the second stage result in
an overall lower-flame temperature for the second 7One ~ :
. .
- . . ~ .. . .
W~) 9~/~6'7g2 Pcr~JS92~02084
operating in a rich f~el/air ratio condition, and thu
minimizing the formation of thermal and prompt NOI-
he porous matriX can comprise a porous ceramic foam,
i~ 5 e.g. a reticulated silica-alumina or zirconia foam~ in
which case the voids are defi~ed by the pores Of the
~oam. Similarly the said matrix can comprise a pacXed
bed -- e.g. o~ ceramic balls, rods~ fibers or other media
which can withstand the high temperature of the
combustion processes. In these instances the voids are
defined by the interspaces among the media. It is
important to point out here, that in the present
invention, unlike certain prior art methodology,
substantially all of the process combustion occurs in the
void spaces of the matrix -- not at surfaces of a ceramic
or porous tube or the like. Also to be noted is that
~, differing matrices can be used at the successive zones --
and indeed the matrix at a given zone can comprise
combinations of one or more contiguous sections, one of
Which may e.g. CompriSe a poroUS Ceramic foam and another
a packed bed, or so forth.
BRIEF DESCRIPTION OF THE DRAWINGS
The invention will be more readily apparent from the
following detailed description, which should be read in
¦ conjunction with the appended drawings, in which:
I
FIG. 1 is a longitudinal sectional view, schematic in
nature, of a preferred e~bodi~ent of two stage combustion
apparatus in accordance with the present invention;
FIG. 2 is a graph of NO~ concentration as a function of
equivalence ratios for the apparatus of FIG. 1 where same
is operated in a single stage configuration;
FIG. 3 is a graph showing equilibrium NO~ formation for a
W092/16792 PCT/US92/020~
.
2 ~ g3 7 ~ 12 `~
mixture of methane and air at various temperatures and
equivalence ratios; and
FIG. 4 is a graph, showing axial temperature dis-
tributions in combustion apparatus of the type shown in
Figure 1, for a fixed flow rate and specified equivalence
ratios in the two stages.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Referring to the drawings and particularly to Figure 1,
combustor or burner apparatus embodying features of the
invention is designated generally by the reference
numeral 50. The combustor or burner 50 is oriented with
its axis vertical such that the flow of gases is upward
along the vertical axis. Burner 50 conveniently has a
base 12 which may be of metal such as steel. Attached to
base 12 is a hollow vertical column 14, the interior of
j which defines a conduit 15. Column 14 extends upwardly
to a flange 17. Threaded rods 19 extend between flange
17 and the outer portion of a toroidally shaped body or
ring 36 between which is secured an encapsulating sleeve
42 which may comprise quartz. Premixed reactants (i.e.
fuel and air) may enter the burner 50 through a two-stage
mixing system (not shown) consisting of a primary mixing
section into which fuel and air are introduced before
being provided through the inlet 16 for the first stage,
and inlet 18 for the second stage. The premixed fuel and
air proceed from inlet 16 into a secondary mixing chamber
effectively defined within conduit 15. Premixed air and
fuel for the secondary stage proceeds via inlet 18
through the conduit 20 to a distributor 22, which can be
a porous ceramic cylinder or comprised of other
refractory material which includes multiple flow paths
for rendering the flow of reactants uniform. In any
event, the objective is to provide a well mixed fuel with
air or other oxidant combustible mixture at two
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WO92/16792 PCT/US92/020~
.
13 2~ ~ l'.'t~
equivalence ratios, one for the first stage, and the
other for the second stage.
''
A void space 28 is located above this mixing section (at
~` 5 conduit 15) and below the preheat section 30 of the
burner core. The burner core in Figure 1 comprises the
preheat or preflame section 30 and a combustion or
postflame section 32, each being a porous ceramic cylin-
;der constituted of partially stabilized zirconia (PSZ)
having the general appearance of a sponge. Other ceramic
?~foams such as reticulated silica alumina foam are
suitable as are packed beds such as beds of saddles,
balls, rods and the like; or other formulations with low
pressure drop and capable of withstanding the
`15 temperatures typically present in combustion apparatus
;may be used. Foams utilizable in the invention include
the silica alumina partially stabilized zirconia as
mentioned, silicon nitride and silicon carbide foams of
High Tech ceramics, characterized as having from about 5
to 65 pores per inch (ppi). Typically the ceramic foam
of section 30 has about 65 ppi; that of section 32 about
lO ppi. The average porosity of the ceramic media varies
from 84 to 87% while the thermal conductivity, for
example for the 10 ppi ceramic, is approximately 1 W/m-K.
A cooling means comprising a nonintrusive flame holder
¦ 34, is utilized to stabilize the first reaction or
combustion zone 44 defined within the porous ceramic
section 32. The cooling means 34 is seen to be a
generally toroidally shaped body 36 comprised, for
- example, of brass, which is water cooled by a channel 38
extending internally around the entire toroidal body.
Cooling water is pumped through the channel 38 by an
inlet and an outlet (not shown) which project from
channel 38 to outside body 36. Other cooling media can
also be furnished to the interior channel 38 and cooling
can also be accomplished by a gas, including air. Water,
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WO92/167s2 PCT/US92/~20
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however, is readily available and is a preferred medium
for the cooling purposes. It is noted that the generally
toroidal body 36 includes an inwardly extending lip
portion 40, which reaches the inner diameter of the flow
5 encapsulation sleeve 42. Hence, it is seen that the
innermost lip 40 of body 36 is in virtual contact with
the outer periphery of the ceramic core, i.e. with
~ sections 30 and 32. Typically in construction of the
t ceramic core, several adjacent ceramic sections such as
10 at 30 and 32 are utilized, which may have differing
porosity; i.e. as mentioned, in Figure 1, the core
~' section 30 being actually in the preflame area, may have
a porosity of 65 ppi, whereas the main core section 32
whereat the actual flame combustion exists, may have a
15 porosity of lo ppi. Where separate sections are used as
indicated, the cooling means or flame holder 34 is thus
inserted between the two sections of the porous ceramic.
i However, noteworthy is that the said cooling means is
thus positioned proximate to the combustible flow input
' 20 end of core section 32, and is in thermal contact with
the flow input end 43 of the first combustion zone 44.
¦ Ignition of the fuel-air mixture flowing through burner
lO can be enabled by any conventional means, including by
25 igniting the flow at the final output 35 or at a
convenient intermediate flow point.
. .
Use of flame holder 34 is found to allow a broad range of
equivalence ratio and flow rate combinations to be
30 utilized in the apparatus 10, while maintaining a stable
reaction zone. (By "equivalence ratio" is meant the
ratio of fuel to oxygen on a stoichiometric basis.)
I't is found that in apparatus as shown in Figure 1, the
35 flame stability limits for different equivalence ratios
is very substantially increased in comparison to what may
be achieved where apparatus similar to Figure 1 but
WO92/16792 PCT/US92/020~
. ,, ~
,~ 7 ~
without the flame holder 34 is operated. Without the
flame holder the only effective flame stabilization
mechanism is heat loss from the entrance and exit regions
; of the burner. With the flame holder 34 present, lower
flow rates can be used while maintaining the reaction
zone at a relatively constant position. Such use also
allows for rapid transition between such stable operating
condi~ions. These are important characteristics in
practical applications due to the common need to have a
turndown ratio between 2:1 and 3:1.
The flow of the combustion products from first combustion
zone 44, is seen to be provided to a second combustion
zone 52. Zone 52 is also constituted by a porous ceramic
matrix 54, which can be the same or different from the
matrix 32 in zone 44.
In operation of the two-stage embodiment of Figure 1, the
fuel and oxygen-containing gas to be fed are mixed by
conventional mixing means to provide a mixture to chamber
15 containing oxygen which is present in the mixture in
150 to 250~, typically 200% of the stoichiometric amount
for the fuel, so that the mixture is a fuel "lean"
mixture. The mixture typically has a temperature of 40
to 80F. if no air preheat is employed. In first
combustion zone 44 the mixture of fuel and oxygen-con-
taining gas is ignited, and combustion takes place at a
temperature of 2000 to 2800F, typically 2400F.
After the fuel-lean mixture has been combusted in zone
44, additional fuel and oxygen-containing gas are added
to the product gases from zone 44 via inlet 18 and
conduit 20, to produce a fuel "rich" mixture wherein the
oxygen present is 60 to 95~, typically 80% of the
stoichiometric quantity, and the augmented rich mixture
is combusted in the second combustion zone 52 at a
temperature of 1800 to 2600F., typically about 2200F.
~ . ,,. . :, . . ... , . .. :, ... . .
~ WO92/16792 PCT/US92/020~
- ~iO7~
16
This temperature range is low enough to prevent the
formation of oxides of nitrogen either by "thermal" or
"prompt" reaction mechanisms. Control of this
temperature range is accomplished by the combined effects
of fuel-air staging and of radiant heat transfer from the
surface of the porous media.
,
,"
In this operation, a portion of the combustion air and/or
fuel bypasses the initial premix of fuel and air in the
lo interior of the PM first combustion zone 44. Ignition
and combustion of the initial mixture occurs under fuel
.lean conditions as a result of preheat generated by
radiant feedback. Peak flame temperature occurs in this
zone as a result of radiant and convective preheat with
! 15 minimum N0x formation. The air and/or fuel which is
bypassed is then mixed with the products formed in the
first combustion zone 44 to oxidize the excess combus-
tibles, prior to exiting the PM burner at 35. The
cooling effect of the radiant heat transfer from the PM
burner results in a lower temperature than the theo-
retical flame temperature for the total combined fuel/air
mixture in the second zone which is overall reducing.
This combined effect results in lower NO~ levels being
achieved than would be possible for either a single
staged or multiple staged burner employing diffusion
burning.
¦In consequence, significant improvement in terms of NOX
reduction is achieved vis-a-vis passage of all of the
fuel and all of the oxygen through a single combustion
zone, such as zone 44. Typically, e.g., a reduction of
from 50 to 80% is achieved compared to a standard
diffusion flame burner or a single stage pre-mix burner
wherein combustion occurs either in the matrix or on the
surface.
Thus in the process and apparatus depicted in Figure 1,
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W092/16792 PCT/US921020~
l7 ~ 7 ~ l71
sufficient fuel mixes with the air in the first (lean)
stage of apparatus 50 to provide for a combustion
temperature in zone 44 below 1500K (2500F.), to
minimize thermal NOI. In this stage, the residence time
is minimized to convert fuel to co but not totally to co2.
In the second stage, i.e., at zone 52, the remainder of
the fuel is added to obtain additional heat release, but
` again at a temperature below 1500K. (2500F. ) . Prompt
N0~ formation will be retarded because radicals from the
first stage will attack the fresh fuel and energy will be
rapidly released from the oxidation of C0. At the same
time, the presence of cooling means 34 precludes flame
back to the preflame section, assuring that the
downstream combustion in zone 44 is completely stable and
- lS controlled to minimize N0x as aforementioned.
I
~ EXAMPLE
.~
In operation of apparatus 50, burner start-up was
effected by delivering a low flow rate, stoichiometric
reactant mixture from the first-stage inlet section. The
burner was then ignited at the second-stage exit 35. The
low flow rate, stoichiometric mixture allowed the
reaction zone to propagate upstream through the second-
stage burner core. This process was monitored visually
i through the burner walls 42, which were comprised of
quartz. As the flame traveled down into the first-stage
burner core, the fuel and air flow rates were gradually
increased until the desired first-stage equivalence ratio
and flow rate was achieved. If a single-stage experiment
was to be performed, the start-up sequence was complete.
For two-stage experiments, the burner was allowed to
reach steady-state operation in the first stage before
the second-stage reactants were introduced through inlet
18.
Burner operating conditions were chosen to allow
WO92J16792 PCT/US92~020~
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',3~ 7 2 ;.t 1 18
; comparison of emissions from a single-stage versus a two- stage burner at comparable energy release rates and
, overall equivalence ratios. Single-stage burner
emissions were obtained using the two-stage burner
apparatus with no additional fuel or air added to the
second stage. For the two-stage experiments, both
lean/rich and rich/lean staging configurations were
investigated. The fuel and air flow rate in the first
stage were calculated from,
~` 10
air = V~ (1)
V~ V (pr ")(~F~c) l~)
where the stoichiometric fuel air ratio is 17.2 for a
methane air mixture and the density ratio of air to
methane is 1.805. In Equations 1-4, the equivalence
ratio (~) is defined as the stoichiometric air/fuel ratio
divided by the actual air/fuel ratio. Thus, equivalence
ratios less than one represent lean operating conditions
while equivalence ratios greater than one represent rich
operating conditions. The second stage air flow rate was
derived as a function of the overall equivalence ratio,
the first- and second-stage equivalence ratios (~l and
~2)~ and the first-stage air flow rate.
V2 Vl (~~0~)
where ~ represents the overall eguivalence ratio of the
first and second stage combined. The second-stage fuel
flow rate was derived as a function of second-stage air
flow rate and equivalence ratio.
V fuel ' V air ( P~
~ WO92/16792 PCT/US92/02084
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~ The overall equivalence ratio was maintained in the
; rich/lean two-stage configuration by setting a desired
rich operating condition for the first stage (equivalence
ratio and total flow rate of reactants), a lean
equivalence ratio for the second stage and calculating
the necessary fuel and air flow needed in the second
stage to produce the desired overall equivalence ratio.
The lean/rich configuration used to make the comparison
0 was achieved by inverting the operating conditions
obtained by the above analysis.
; The porous media burner S0 was operated at 50 slpm in a
single-stage configuration to determine the baseline NO~
formation at various equivalence ratios which exhibited
stable burning within the matrix. As shown in Figure 2,
stable burning was achieved at equivalence ratios from
0.6 (67~ excess air) to 1.5 (50% excess fuel). NO~ levels
at equivalence ratios of 0.6 to 0.8 were quite low, in
the range of 5 to 15 ppmv, dry corrected to 3% 2- At
high equivalence ratios, 1.0 to 1.5, NO~ levels ranged
from 25 to 50 ppmv, dry corrected to 3% 2-
The reason for the higher NO~ levels being formed under
operating conditions having an excess of fuel compared toconditions having an excess of oxygen is not readily
understood, but may be the results of two reaction paths
that are taken. Under oxidizing conditions, most of the
NO~ is formed by Zeldovich reactions, consisting of the
following:
O + N2 ~ NO + N
N + 2 - NO + o
The first step is rate-limiting and occurs at elevated
temperatures (>2799F) (5). At equilibrium, very high
WO 92~16792 PCI/US92/02084
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levels of N0~ can be formed under oxidizing conditions.
Figure 3 shows equilibrium N0;~ formation for a mixture of
methane and air at various temperatures and equivalence
ratios. At an equivalence ratio of 0.87 (approximately
3% 2) ~ N0~ levels in the range of 1000 to 4000 ppmv are
possible at temperatures above 2400F. However, due to -
the high activation energy and long residence times
required for 2eldovich r~actions to go to completion,
only a small fraction of the equilibrium levels of N0;~ are
realized. Figure 2 shows that at an equivalence ratio of
0.87, only 30 to 35 ppmv of N0~ was formed in the PM
~urner due to the low residence time in the matrix and
;- the cooling effect of radiant heat transfer.
!~ .
- 15 In fuel-rich flames, equivalence ratios of 1.0 to 1.5, NOj~
is formed from HCN which is produced by a reaction
between the excess hydrocarbon radicals and elemental
nitrogen. Under most conditions, the dominant path from
HCN to NO is the sequence initiated by the reaction of
HCN with atomic oxygen:
. "'~
CHz + N2 ~ HCN + NH
HCN + 0 - N0 + HC (5)
Equilibrium N0;~ formation, under fuel-rich conditions, is
in the range of 10 to 200 ppm, dry at temperatures above
2800F. The single-stage data presented in Figure 2
indicates that, under actual firing conditions, the PM
burner will generate 25 to 50% of the equilibrium N0~
levels. The conclusion which can be drawn from these
data is that, under oxidizing conditions, N0;~ formation is
rate limited. Whereas, at conditions of excess fuel, N0
formation may approach equilibrium conversions, which is
the limiting factor for levels of N0;~ that are formed.
It will be noted that at temperatures below 2400F,
; WO92/167s2 PCT~US92/020
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21 ~iJ.~ ~
equilibrium NO~ formation for fuel-rich combustion
conditions approaches zero. This points out the need for
maintaining reduced temperatures in the PM burner for
operation under reducing as well as oxidizing conditions.
; 5
Figure 4 shows the axial two-stage temperature profile
for staged combustion having a first stage equivalence
S ratio of 1.2 and a second stage of 0.4 for an overall
ratio of 0.87 (3% excess 0~). The average temperature
under staged conditions was 1324C (2416F). The average
, axial temperature for single-stage burning at the same
conditions was 1420C (2588F) (4).
!.
., .
The lower temperature profile for combustion under staged
15 conditions is due to the combined effects of distributing
fuel and air along the axis of the porous matrix burner
and the heat losses in the second stage due to radiant
heat transfer.
¦ 20 Table 1 summarizes the results of emissions measurements
obtained at various burner operating conditions. case 1
is single-stage combustion, Case 2 is two-stage
combustion with a rich first stage and a lean second
stage, and Case 3 is a two-stage combustion with a lean
25 first stage and a rich second stage. The heat release
rate (Q) is the mass flow rate of fuel multiplied by the
lower heating value of the fuel. These results indicate
that NO formation may be reduced in a two-stage burner in
which the first and second stages are fuel-lean and fuel-
30 rich, respectively (Case 3). A similar trend in NO
emission was observed for an overall equivalence ratio of
1. 0.
WO92/16792 PCT/US92/02084
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22
5~ ~ ti~
::
;`.
Table 1. Two-Stage vs. Single-Stage Burning in a PM Burner
Q ¦ NO ¦ CO
,~ Case I ~O- _ 1 2 (kw) (ppm) (%) _
,. 1 87 _ 87 . ~ 4.5 23 o.01
. _ .87 1.4 0.6 4.7 35 1.6
,. .; .. .
. 3 87 0.6 1.4 4.7 lO ~
, 4 1.0 l.O ~ . 5.1 36 0.8
'. 5 l.O 1.4 0.6 4.9 38 ~2.5
, _ _ _
6 1.00.6 1.4 4.9 20
._
~ ~ No CO deteoted.
W092/l6792 PCT~US92/020
23 ~i v.~
As expected, the best results were achieved under staged
conditions with a very lean mixture in the first stage,
~=0.6, and a fuel-rich mixture, ~=1.4, in the second
stage (see Cases 3 and 6). Relative flow rates in each
stage were varied between Cases 3 and 6 to get the
desired overall equivalence ra~ios.
.
Cases 1 and 4 are single-stage burning conditions
operating at ~=0.87 and 1.0, respectively. Note that NO~
lo levels of 23 and 36 ppmv, dry, were obtained compared to
10 and 20 ppmv, dry, respectively, for staged burning at
the same overall equivalence ratios.
Cases 2 and 5 are two-stage burning conditions with fuel-
rich combustion in the first stage and fuel-lean in the
second, resulting in overall equivalence ratios of 0.87
and 1.0, respectively. The formation of NO~ was in the 35
to 38 ppmv dry range even with excessive CO emissions
(1.6 to >2,5%). Note that cases 3 and 6 not only had the
lowest NO~ emissions but also the lowest co levels.
These results indicate that staged burning with fuel-lean
equivalence ratios in the first stage and fuel-rich
equivalence ratios in the second stage provides a
significant advantage over single-stage combustion at
equivalent overall equivalence ratios. The reason for
these results is the fact that thermal NOI formation
resulting from Zeldovich mechanisms is retarded in the
first stage due to the low flame temperature achieved
with high excess air conditions. In the second stage, a
fuel-rich mixture is added, but formation of N0l from the
cyano mechanism is retarded due to the combined effects
of the unreacted oxygen (at a reduced concentration) from
the first stage and the effect of radiant heat transfer
lowering the flame temperature at the exit end of the
ceramic porous matrix tube. Nondetectable levels of CO
for Cases 3 and 6 indicate good combustion
, : . ; - :. ~ :
WO92~16792 PCT/US92/020
;J ~ i 2 4
..
. characteristics even at a stoichiometric fuel air ratios
(~=l.o).
The following conclusions may be drawn:
1. Two-stage burning in a porous media burner
~; results in lower average axial temperature compared
to single-stage combustion;
..;
~!
,; 10 2. Two-stage burning, in which the first stage is
lean and the second stage is fuel-rich, results in
lower NO~ and cO emissions than single-stage burning
, at the same overall equivalence ratio;
3. Two-stage burning, in which the first stage is
fuel-rich and the second stage is lean, does not
offer a significant advantage over single-stage
combustion at the same equivalence ratios; and
..
4. Two-stage burning, in which the first stage is
lean and the second stage is fuel-rich, results in
very low NOI and CO emissions even at overall
stoichiometric fuel:air ratios and, as such, affords
maximum fuel efficiency with minimum emissions.
It will be understood that various changes and
modifications may be made in the embodiments described
and illustrated without departing from the invention as
defined in the appended claims. It is intended,
therefore, that all matter contained in the foregoing
description and in the drawings shall be interpreted as
I illustrative only, and not in a limiting sense.
I
