Catalytic pre-mixed fibre burners

Chemical Engineering Science 54 (1999) 3599}3608

Catalytic pre-mixed "bre burners

Guido Saracco!,*, Isotta Cerri!, Vito Specchia!, Romano Accornero"

!Dipartimento di Scienza dei Materiali e Ingegneria Chimica, Politecnico di Torino, Corso Duca degli Abruzzi, 24-10129 Torino, Italy"Societa% Italiana per il Gas, Via XX Settembre, 41-10121 Torino, Italy

Abstract

The performances of three pre-mixed "bre burners (maximum rated power: 30 kW), based on the same FeCrAlloy porous support,were tested in a speci"c pilot plant. The "rst one was a commercial FeCrAlloy panel (the reference burner); the other two werecatalytically activated by deposition onto the "bres of the LaMnO

3perovskite, according to two di!erent procedures: &direct' and

&indirect route'. The latter, in which an LaAlO3layer was placed between the catalyst and the "bres to prevent deactivation, should be

preferred for the presumably major stability and constant performance in the long term. The #ue gas temperature, the NOx, CO and

HC #ue gas concentrations and the emission intensity of the panel surface, were measured as a function of Ea

(excess of air) andQ (speci"c heat power). Besides, the operating combustion regimes (radiant, transition and blue-#ame) were identi"ed by directobservation of the burner surface. As compared to the non-catalytic burner, the two catalytic ones enabled, with nearly unchangedNO

xproduction, up to about 5 times lower CO and HC emissions, particularly in the radiant combustion regime. As a result, a wider

rangeability of the burner (down to about 10% of the maximum speci"c operating power, where non-catalytic burner failed), withenvironmentally acceptable #ue gas composition, was achieved. ( 1999 Elsevier Science Ltd. All rights reserved.

Keywords: Premixed combustion; Methane; Fibre burner; FeCrAlloy; LaMnO3

perovskite

1. Introduction

The growing concern of the environmental problemsrelated to the emissions of natural gas boilers has forcedseveral European governments to "x lower and lower#ue gas concentration limits for CO, unburned hydrocar-bons (HC) and NO

x. The best strategy to reduce NO

xemissions strictly depends on the control of the #ue gastemperature, which, together with the residence time inthe combustion chamber and the square root of oxygenresidual concentration (Shari" et al., 1993), a!ects NO

xproduction during a combustion process (particularly,any increase of the above parameters results in a higherNO

xproduction).

In this context, various di!usive-type radiative heatersbased on catalytic "bre burners were developed forhousehold applications since the early 1970s (Radcli!e& Hickman, 1975; Trimm & Lam, 1980; Specchia et al.,1981); the panel (generally based on noble metals), host-ing the combustion between reactants di!using from

*Corresponding author. Tel.: #39-11-5644654; fax: #39-11-5644699; e-mail: [email protected].

opposite sides, emits the heat by radiation to the sur-rounding environment leading to a mat temperature ofabout 4003C.

As opposed to di!usive-type burners, fully pre-mixed"bre burners have gained in the last decade an increasinginterest, thanks to some properties which make themparticularly attractive compared with conventional non-pre-mixed ones, aerated or not aerated (Hargreaves et al.,1986). Particularly, they allow: (i) lower combustion tem-peratures and NO

xemissions, due to quick heat removal

by means of either radiative or convective transfer mech-anisms; (ii) a more compact design of the combustionchamber and the heat exchanger; (iii) a very large #exibil-ity in siting the burner inside the appliance, since the#ame is controlled by fuel gas momentum rather thannatural convection (burner can "re upwards, downwards,sideways); (iv) lower air excess levels, allowing higherthermal e$ciency; and (v) capability of being shaped soas to properly "t almost any required geometry.

When operating at low speci"c heat power Q andexcess of air E

a, a radiant combustion regime takes place,

where the oxidation occurs mostly in a thin layer insidethe panel, at the opposite side to the inlet #ow, so thatthe surface glows #amelessly. In these conditions, a

0009-2509/99/$} see front matter ( 1999 Elsevier Science Ltd. All rights reserved.PII: S 0 0 0 9 - 2 5 0 9 ( 9 8 ) 0 0 4 6 3 - 1

signi"cant fraction of thermal energy is transferred by aradiative mechanism to the heat exchanger, with lowtemperatures of the #ue gas and low NO

xemissions.

However, the methane in this regime may not burn outcompletely, especially at very low E

avalues, which leads

to high CO and HC emissions.Conversely, at high Q and/or E

aa blue-#ame combus-

tion regime takes place, with the presence, on the panelsurface, of numerous short blue #ames, whose lengthincreases with Q and/or E

a, until #ame lift-o!; the tem-

perature and NOxlevels within the combustion chamber

are very high, especially at low Ea

values, whereas COand HC emissions are very low. A transition regime canbe "nally pointed out, for intermediate Q and/orEa

values, characterised by the simultaneous presenceof red- and blue-#ames covered parts over the burnersurface.

Such pre-mixed burners have found application asretro"t for "re-tube boilers (Krill & Kesserling, 1985), asheating elements for process heaters in the petroleumindustry (Gotterba et al., 1985; Kessler et al., 1987), forre"nery storage tank heaters, for commercial warm-airfurnaces (Gotterba et al., 1985). More recent applicationslay in the drying of paper, in annealing furnaces for theglass industry, in baking furnaces in the food industry,and in kitchen stoves (Nakamachi et al., 1994).

If a catalyst is deposited onto the "bres of thepre-mixed burners, lower CO and HC emissions shouldbe expected (especially at low thermal load, where thepre-mixed non-catalytic burners generally fail), lowerNO

x#ue gas concentrations (due to the increased

emissivity of the burner), and wider range of environ-mentally friendly steady-state operation in terms either ofspeci"c heat power or excess of air (a wide power rangea-bility is needed to save energy, especially in householdheating applications). For this purpose, the catalystshould have good catalytic activity, high thermal stabil-ity (up to 10003C), good resistance to deactivation by thesulphurised odorising compounds and chemical}phys-ical stability.

Deposition of noble metal catalysts, as Pt and Pd, wasattempted on either ceramic (Sullivan, 1991; Bos et al.,1992) or metallic (van Wingerden et al., 1990; van Looij etal., 1992) burners. The use of these catalysts for methanecombustion led to satisfactory results in short-time tests,but the relatively quick catalyst deactivation either bypoisoning, sintering or volatilisation compromised theirfunctionality in the long term (Sullivan, 1991).

The catalytic materials belonging to the perovskiteclass (A

1~xA@

xB1~y

B@yO

3Bd where A, A@" La, Sr, Ba,2while B"Co, Mn, Cr,2 ) seem to be the more promis-ing candidates, by generally assuring a good catalyticactivity, a reasonable speci"c surface area, a good resist-ance to the deactivation at high temperatures(800}12003C), where traditional catalysts based on sup-ported noble metals notoriously fail (Seyama, 1992;

Zwinkels et al., 1993; Zwinkels, 1996). In this context,ECN (Petten, Holland) has recently undertaken a studyabout advanced burners based on alumina foam ac-tivated by the deposition of the La

0.25Sr

0.75MnO

3per-

ovskite compound (Bos & Doesburg, 1993). On thegrounds of this pionieristic attempt and of earlier studieson non-catalytic "bre burner (Saracco et al., 1996), thispaper enlightens the improvements that can be obtainedvia the deposition of perovskite catalysts onto the "bresof a metallic pre-mixed burner, by studying methanecombustion over three di!erent "bre burners: the refer-ence FeCrAlloy burner, the same burner activated withLaMnO

3catalyst and the last one characterised by the

interposition of an LaAlO3

layer between the "bres andLaMnO

3, to avoid interactions between these two last

counterparts.

2. Experimental

2.1. Materials

The basic "bre burner used in this study is a metalpanel (NIT 100S by ACOTECH bv, a joint venturebetween Shell and Bekaert), made of FeCrAlloy (Cr 20%,Al 4.75%, Y 0.2%, trace elements about 1%, Fe balance).It has been characterised by SEM observation (Fig. 1):the "bres, having rectangular cross-section (size: about10]20 lm), are arranged in a well-ordered knitted con-"guration; the mattress is about 2 mm thick. At hightemperature and in oxidising enviroments, aluminiumdevelops a super"cial layer of oxide (alumina). Thealumina layer should protect the core of the "bre againstfurther oxidation (Pint et al., 1995). In fact, after a pre-treatment in air at high temperature (800}12003C), the"bre surface gets covered by a uniform and compactalumina layer.

Fig. 1. Scanning electron micrograph of the "bre arrangement of thecommercial FeCrAlloy burner.

3600 G. Saracco et al. /Chemical Engineering Science 54 (1999) 3599}3608

2.2. Catalyst preparation and characterisation

On the basis of previous studies about LaCr1~x

MgxO

3and LaMg

1~xMn

xO

3systems, performed at Pol-

itecnico di Torino (Saracco and Specchia, 1996), a seriesof perovskite catalysts were studied for their "nal depos-ition on the FeCrAlloy "bre panel. Di!erent perovskitecompounds (e.g.: LaMnO

3, LaMg

0.2Mn

0.8O

3, LaMg

0.5Mn

0.5O

3, LaCrO

3, LaCr

0.5Mg

0.5O

3,2 ) were pre-

pared in a supported form on either alumina powders(size: 38}63 lm) or alumina powders coated with LaAlO

3(30% b.w.) as inert phase; the so-called &citrate method',suitably modi"ed (Abbattista & Vallino, 1983), was em-ployed. The LaAlO

3layer should in fact either support

the perovskite catalysts or avoid the deactivating reac-tions between alumina and the catalyst. The physico-chemical catalyst structure was characterised (SEM,EDAX, X-ray di!raction analysis) and the catalytic activ-ity towards methane combustion was measured. Aftercharacterisation and testing, LaMnO

3appeared as the

best available catalyst towards methane combustion.

2.3. Catalyst deposition technique

First, the best pre-treatment operating conditions todevelop the alumina protective layer on the burner "breswere applied: a calcination temperature of 11003C for 3 h.In order to optimise the catalyst loading and adhesion onthe knitted "bres of the burner two di!erent techniqueswere then followed (Saracco & Specchia, 1997a):f &direct route': LaMnO

3perovskite was directly depos-

ited on the pre-calcined "bres;f 0indirect route': the pre-calcined "bres were "rst coated

with the LaAlO3

inert phase, then the LaMnO3

cata-lyst was added.While the "rst procedure is interesting for its simplicity

and the relatively low costs involved, the more complexsecond one is attractive for the major catalytic stabilityand activity that would result. The following steps werecarried out for the catalyst deposition:f pre-heating of the metallic panel in an oven up to

9003C;f quick immersion of the hot panel in a solution contain-

ing LaMnO3

(1 M) or LaAlO3

(0.3 M) precursor salts,respectively: due to the heat released by the panel, theinterfacial water rapidly evaporates causing a localprecipitation of catalyst-precursor crystals on the "bresurface;

f panel drying by a compressed air #ow for removing thesolution droplets, which remain over the "bre surfaceafter the previous treatment step;

f "nal calcination at 9003C for 8 h for the LaMnO3

catalyst, or at 11003C for 2 h for the intermediateLaAlO

3layer.

The above procedure was repeated several times inorder to obtain the suitable amount of catalyst (about

Fig. 2. Scanning electron micrograph of a FeCrAlloy "bre catalysed bythe &indirect route'.

26% b.w.) and/or LaAlO3

layer (about 2% b.w.). AnSEM microphotograph of the "bres of the indirect routecatalytic burner is shown in Fig. 2; the small cracks onthe catalyst surface are probably due to the quick coolingduring the deposition procedure.

2.4. The pilot plant

The two catalytic burners and the reference non-cata-lytic one (shape: disk; diameter: 140 mm; thickness: about2 mm; maximum burning rate: 30 kW) were tested in thepilot plant schematically drawn in Fig. 3. The mostimportant items of the apparatus are:f methane and air supply system;f a Venturi-type fuel}air mixing system;f a plenum chamber and a mixture distributor;f a #at-mattress "bre burner, vertically placed, sealed at

its perimeter and equipped with both an ignition deviceand a #ame-extinction controller;

f gas temperature measuring thermocouples, placed justupstream the burner in order to prevent any #ash-backby acting on an on}o! electrovalve on the methanefeed line;

f a cylindrical jacketed combustion chamber, watercooled, horizontally mounted, carrying a glass peep-hole (for direct observation of the combustion surfaceand for the insertion of a radiometer) and a samplingdevice for a suction pyrometer to measure the temper-ature of the #ue gases in the combustion chamber;

f an ellipsoidal radiometer probe (IFRF, Holland), watercooled and nitrogen purged;

f a shell-and-tube heat exchanger, with one-pass "retube and two-pass water shell;

f a sampling device for #ue gas analysis;f an analysis section: NO

xchemiluminescence analyser,

CO and CO2infrared analysers, O

2paramagnetic ana-

lyser and HC #ame-ionisation detector;f a chimney.

G. Saracco et al. /Chemical Engineering Science 54 (1999) 3599}3608 3601

Fig. 3. Scheme of the pilot plant. (1) air "lter; (2) air-#ow control valve; (3) blower; (4) Venturi mixer; (5) "bre burner; (6) combustion chamber; (7) glasspeep hole/radiometer entry hole; (8) shell-and-tube heat exchanger; (9) sampling port to analytical system; (10) chimney; (P) suction pyrometer; (T

1)

thermocouple.

Several tests were carried out by varying either thespeci"c heat power (Q"200}2000 kW/m2) or theexcess of air (E

a"0}50%). During the tests the com-

position of the #ue gases (CO2, CO, HC, NO

xand

O2

referred to dry gases at 03C, 1013 mbar and for 0%O

2equivalent concentration * except the last one), the

temperature of the combustion chamber ¹cc

and theburner surface emissivity, measured by the radiometer,were monitored. Particular care was taken on the obser-vation of the panel surface through the peep-hole inorder to recognise the actual combustion regime of theburner.

3. Results and discussion

Fig. 4 shows the di!erent combustion regimes (radiant,transition and blue-#ame), for all the studied panels, asa function of E

afor various Q values.

For each panel, the lines limiting the various combus-tion regimes are drawn; E

abeing constant, the blue-#ame

regime prevails at high heat powers while the #amelessone at low Q values.

The two catalytic panels have practically the samebehaviour; their #ameless zone are smaller than that ofthe non-catalytic burner. In fact, for the formers, the#ameless regime establishes only at Q values lower than850 kW/m2 and at low E

alevels ((10}15%), while, for

the non-catalytic one, the Q upper limit, for low excess ofair, is about 1400}1500 kW/m2. For each panel, by in-creasing E

a, the Q limit value decreases and, from

Ea

equal to 30%, all the tested burners have the samebehaviour as concerns the radiant regime. The blue-#amemode seems to set up, for the same E

avalue, at similar

speci"c heat power values, for all the three burners (theblue-#ame regime establishes only at Q higher than

1900}2000 kW/m2 for Ealower than 15%). However, the

Q limit values of the non-catalyitic burner are a slightlyhigher than those of the catalytic ones, at least forEalower than about 50%. Therefore, the presence of the

catalyst on the "bre burner enlarges the transition zone,mainly by reducing the radiant regime, and leads toa slight increase of the blue-#ame one. Porosity, per-meability and emissivity of the burner should be respon-sible for the above behaviour. In principle, the presenceof the catalyst might lead to an enlargement of theradiant regime since the LaMnO

3catalyst is a dark

material with an emissivity higher than that of the FeC-rAlloy "bres. However, the catalyst deposition onto the"bre burner reduces the burner porosity (a decrease ofpanel porosity from about 85%, for the non-catalyticburner, to 79%, for the catalytic ones, was measured) andtherefore increases the local #ue gas momentum, whichplays against the radiant combustion regime (the #ametends to be blown out of the burner). This latter phe-nomena could also be responsible for the earlier start ofthe blue-#ame regime.

Since there are no signi"cant di!erences between thetwo catalytic panels, freshly prepared, as regards the CO,HC and NO

x#ue gas concentrations, the temperatures of

the combustion chamber (¹cc) and the emission intensity

of the burner surface as well, the catalytic panel activatedby &direct route' will not be considered in the following.Notwithstanding the appreciable simplicity in its prep-aration procedure, X-ray di!raction analysis on aged&direct route' panels detected a partial deactivation of theLaMnO

3catalyst by a chemical reaction with the

alumina of the support to form lanthanum aluminates(LaAlO

3, LaAl

11O

18) (Saracco & Specchia, 1996). On

the contrary, the interposition of the LaAlO3

phase,between the catalyst and the alumina, hindered thisdeactivation.


Fig. 4. Comparison of the operating combustion regimes for the three "bre burners tested.

The ¹cc

values depend on the fraction of the heatdirectly transferred to the exchange surface by radiationand that retained by #ue gases, mainly transferred byconvective mechanisms in the following shell-and-tubeexchanger. The presence of the radiant regime, ratherthan the blue-#ame one, induces relatively lower temper-atures within the combustion chamber.

On the basis of the measured ¹cc

values, the fractionf of the heat power Q received by the #ue gases andtransferrable by convection to the heat sink of the com-bustion chamber, can be determined by the followingequation:

f"=c

p(¹

cc!¹

a)

QS, (1)

where= (kg/s) is #ue gas mass #ow rate, cpspeci"c heat

of the #ue gases (assumed equal to 0.96 kJ/kg/K at alloperating conditions), ¹

a(203C) the room temperature,

and S (0.0154 m2) the burner surface area. Fig. 5a andb shows, for the non-catalytic and the indirect catalyticburner respectively, the fraction f as a function of E

afor

di!erent Q values.It is worth noticing that, in the radiant regime, the

catalytic burner is characterised by f values lower thanthose typical of the non-catalytic one. This is likely en-tailed by the higher emissivity of the black LaMnO

3perovskite compared with the alumina covering and pro-tecting the "bres of the non-catalytic burner. As a result,lower combustion temperatures and NO

xemissions

should be expected in this operating regime for the cata-lytic burner. This point will be addressed in the following.

For both burners f increases gradually at constantEavalue by increasing Q due to the progressive transition

from the #ameless regime to the blue-#ame one.The f increase for the non-catalytic panel is more

gradual than that for the catalytic one (Fig. 5b), for whicha quicker increase with E

ais evident in the Q range of

about 600}1300 kW/m2. The same trends have beenmeasured for ¹

cctoo: Q being constant, ¹

ccalways in-

creases with Eaindependently of the combustion regime.

In fact, in these conditions the warming up of the #uegases due to the progressive reduction of the radiationmechanism prevails, as compared to the convective one,notwithstanding the increasing mass #ow rate with E

a.

This happens more slowly in the #ameless regime andquickly in the transition from this latter to the blue-#ameone, where for the catalytic burner a ¹

ccvariation from

about 7003C to about 10003C has been observed. Eitherthe increased local gas momentum in the radiant regimeor the presence of the blue #ames in the transition regime,reduce the fraction of heat transferred by radiation andwarm the #ue gases, thus enhancing the heat exchange byconvection to the sink.

The emission intensity of the burner surface measuredduring the tests with the radiometer probe gave onlyqualitative informations, due to the practical impossibil-ity to calibrate the instrument. However, it allowed tocon"rm the data obtained by direct observation of theburner surface concerning the existance of the di!erentcombustion regimes (more details are given in Saracco etal., 1997). Any attempt to derive estimations of the burnersurface temperature, a useful parameter for data inter-pretation, got unfortunately frustrated.


Fig. 5. Variation of f with Ea

for di!erent Q values for both the non-catalytic (a) and the &indirect route' catalytic burner (b).

Fig. 6a and b shows the experimental NOxconcentra-

tions in the #ue gas for the catalytic and non-catalyticburners versus E

aat di!erent Q values. For these two

"gures and the following ones, only three lines concern-ing the trend of the experimental data points have beendrawn for sake of clearness: at a low, an intermediate anda high Q value. As a general behaviour of each panel, atconstant Q values, NO

xconcentration decreases by in-

creasing Ea; besides, the higher is Q, the quicker is the

NOxdecrease. Notwithstanding the #ue gas temperature

increase with Ea, the residence time decrease, due to the

increase of either the #ue gas mass or the volumetric #owrate, is prevailing. Besides, the highest NO

xlevels at low

Ea

values occur at the highest Q values where the blue-#ame mode, characterised by high #ame temperature, isestablished.

Comparing the three burners, the NOx

emissions donot vary signi"cantly, despite the di!erences in combus-tion regimes and ¹

ccvalues. Particularly, in the radiant

regime, where the burner reaches high temperatures, thepossibly increased nitrogen oxidation due to the catalytice!ect seems to be balanced by the decreased #ue gastemperature due to larger emissivity. In the blue-#ameregime, where the panel remains relatively cold, loweringthe catalytic activity towards NO

xformation, there are

no evident reasons for a di!erent behaviour between thecatalytic and the non-catalytic burner. However, forEa'15%, independently of the Q value, the NO

xemis-

sions remain lower than about 70 ppmv and thereforequite acceptable.

As concerns CO (Fig. 7a and b) and HC (Fig. 8a and b)concentrations in the #ue gases, the catalytic activation,as expected, plays an important role: the catalytic panel,in fact, burns better (lower HC emissions) and morecompletely (lower CO formation), if compared to thenon-catalytic one. This e!ect appears as more evident aslower are the Q values, especially for those typical of theradiant regime (where the panel temperature is high).


Fig. 6. NOx

emissions as a function of Ea

for di!erent Q values and for both the non-catalytic (a) and the &indirect route' catalytic burner (b).

Besides, at the highest Q values, CO emissions enhance,especially at low E

avalues (notwithstanding some data

points belong to the transition regime, they fall down inthe area of the radiant regime * see the points sur-rounded by a shaded circle in Fig. 7a* due to their highCO level). This result presumably depends on di!erentfactors typical of high Q operations: a possible #ameinstability; the lower retention time in the combustionchamber; some temperature decrease for those #ames incontact with cooled surfaces of the combustion chamberjacket.

Particularly, in the Ea

operating range generally ad-opted (15}25%) in combustion processes, CO concentra-tions for the catalytic burners are lower than30}40 ppmv, whereas those for the non-catalytic one arelower than 120 ppmv. Similarly for the HC emissions inthe usual E

arange, the values obtained for the catalytic

burners are 50}60 ppmv, whereas those for the non-catalytic one are lower than 300}400 ppmv.

4. Conclusions

Three di!erent "bre burners made of the same FeCrAl-loy mattress (one non-catalytic and two catalysed bydeposition of LaMnO

3with or without interposition of

inert LaAlO3) were tested in view of their "nal applica-

tion in domestic or industrial boilers.The performances of these burners were compared and

discussed:f both catalytic burners enable to work with a wide

steady-state operating regime (200}2000 kW/m2;rangeability 10 : 1; 15%(E

a(25%) with emission

levels acceptable from an environmental point of view(CO(40 ppmv; HC(60 ppmv; NO

x(70 ppmv);

conversely, at the same emission levels, the rangeabilityfor the non-catalytic burner is lower: 5 : 1;

f as concerns the operating regimes (#ameless, transi-tion and blue-#ame), the only remarkable di!erenceis that for both the catalytic burners the #ameless


Fig. 7. CO emissions as a function of Ea


regime establishes, for Ea(30%, at Q values

(600}900 kW/m2) lower than those of the non-catalyticone (600}1500 kW/m2). This result could be primarilylinked to the porosity reduction of the mattress asa consequence of the catalyst deposition onto the"bres.

f NOx

emissions (always lower than 70}80 ppmv forEa'15%) are almost una!ected for each burner what-

ever the working conditions (Q, Ea) are; this could be

due to the balance of the opposite e!ects of eitherthe catalytic activity (favouring NO

xformation) or the

increased emissivity of the catalyst-coated "bres (allow-ing higher heat fraction transferred by radiation to theheat exchanger and consequently lower ¹

ccvalues).

f The major goal obtained with the catalytic activation isthat, in the E

arange generally used for a combustion

process (15}25%), CO and HC emissions are respec-tively 3}4 and 6}8 fold lower than those of the non-catalytic burner. Particularly, the reduction of CO and

HC emissions at low Q values ((800 kW/m2) is due tothe activity of the oxidising catalyst. Therefore, HC andCO concentrations respectively lower than 30}40 and50}60 ppmv are guaranteed.Summarising, the catalyst promotes a more complete

methane combustion especially when operating in thelow-Q (300}600 kW/m2) radiant regime: since the oxida-tion reaction takes place inside the burner, when thepanel surface temperature is at 500}6003C the supportedcatalyst should markedly show its promoting activity.Moreover, it remains stable also when the transitionregime is approached after as high temperatures as800}9003C are reached (high-Q radiant regime). In theseconditions, the system behaves like a catalyst-supported#ame combustor, which is possible only if the catalystpossesses a very high thermal stability, till at least10003C.

Further, the deposition of the catalyst on the FeCrAl-loy mattress, by reducing the porosity of the panel,


Fig. 8. HC emissions as a function of Ea


increases the #ue gas momentum; as a consequence,the transition between the radiant and the blue-#ameregime occurs at Q and/or E

avalues lower than those of

the non-catalytic burner, while the radiant zone becomessmaller. Therefore, in the pure blue-#ame regime, wherethe temperature of the burner surface is relatively lowbecause the mattress is not hot any more, the e!ect of thecatalyst is only a physical one, since the catalytic activitycannot be exploited.

Thanks to these promising results, some tests of cata-lyst "bre burners are in progress in a larger scale pilotplant (maximum burning rate: 150 kW) at Centro Studie Ricerche ITALGAS, Asti.

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