The Experimental and Numerical Approach of Catalytic Combustion on Noble Metals Disc Burners of the...

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The Experimental and NumericalApproach of Catalytic Combustionon Noble Metals Disc Burners of theTurbulent Gaseous Fuel Jet DiffusionFlamesS. F. Deriase a , S. A. Ghoneim a , A. S. Zakhary a & A. K. Aboul-Gheita

a Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo,Egypt

Available online: 31 Jan 2012

To cite this article: S. F. Deriase, S. A. Ghoneim, A. S. Zakhary & A. K. Aboul-Gheit (2012): TheExperimental and Numerical Approach of Catalytic Combustion on Noble Metals Disc Burners of theTurbulent Gaseous Fuel Jet Diffusion Flames, Energy Sources, Part A: Recovery, Utilization, andEnvironmental Effects, 34:6, 492-507

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Energy Sources, Part A, 34:492–507, 2012

Copyright © Taylor & Francis Group, LLC

ISSN: 1556-7036 print/1556-7230 online

DOI: 10.1080/15567030903551208

The Experimental and Numerical Approach of

Catalytic Combustion on Noble Metals Disc

Burners of the Turbulent Gaseous Fuel JetDiffusion Flames

S. F. DERIASE,1 S. A. GHONEIM,1 A. S. ZAKHARY,1 and

A. K. ABOUL-GHEIT1

1Egyptian Petroleum Research Institute (EPRI), Nasr City, Cairo, Egypt

Abstract Catalytic combustion is proposed and developed as an efficient method

of promoting stability and oxidation of gaseous fuel with minimum pollutants. Theeffect of catalytic combustion of gaseous turbulent diffusion flames over catalytic

discs containing Pt, Pd, and (Pt C Pd) supported on -Al2O3 were experimentallyand mathematically studied. These flames have proved to be highly stable over the

three catalytic burners and their catalytic enhancement is found to be in the order(Pt C Pd) > Pt > Pd. The axi-symmetric thermal distribution of flames developing

over these burners record higher values due to enhancing the fuel oxidizability onthe noble metal sites in the reaction zone of flames via improving homogeneous-

heterogeneous chemical reactions. Lower values of CO and NO are measured at theaxial flames direction in the presence of catalytic burners. A numerical approach has

been investigated for the catalytic combustion process on the three noble metals discburners showing high numerical evaluation of different predicted functions. Stability

limits are analyzed following a 1st-degree polynomial. The model of temperaturesdistribution is described by Gaussian function. For CO distribution, a non-linear

four parameters model has been predicted. A differential equation for NOx indicatesperfectly the location of the peak values. These models have been strictly confirmed

with the experimental data.

Keywords catalytic combustion, mathematical modeling, noble metal burners, pol-lutants, stability, thermal distribution

1. Introduction

Designers of industrial burners and gas turbine combustors use various methods for en-

hancing flame stabilization and improving the thermal structure of turbulent jet diffusion

flames (El-Banhawy et al., 1993; Zakhary, 1996, 2005; Mansour, 2003).The most effective

technique for flame stabilization uses a catalytic surface in the combustion domain. This

implies complete oxidation of the fuel over a solid catalyst. Zakhary and Aboul-Gheit

(2006a) reveal that catalytic combustion has a strong effect on the stability of gaseous

fuel jet diffusion flames. High stability limits are attained using perforated alumina disc

burners with two different diameters at a specified position over the fuel jet nozzle at

Address correspondence to Salwa Ghoneim, Egyptian Petroleum Research Institute, 1 AhmedEl Zomour Street, Hay El Zohoor, Nasr City, Cairo 11727, Egypt. E-mail: [email protected]

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Catalytic Combustion on Noble Metals Disc Burners 493

different levels of combustion air flow. Zakhary and Aboul-Gheit (2006b) clarify the

effectiveness of catalytic combustion on increasing the thermal distribution of turbulent

confined lifted flames and show that the catalytic combustion process has significantly

accelerated the fuel burning rate and released transport, both upstream by diffusion and

downstream through flames, proportional to the active surface of the catalytic disc burners.

Roderick et al. (2003) clarify the effectiveness of catalytic combustion of premixed

methane in air over a heated hexa-aluminate catalytic surface and clarify its suitability

for the high temperature zones.

High temperature catalytic combustion is regarded as a highly efficient and clean

energy system for industrial applications of furnaces and gas turbines. Noble metals

possess the highest catalytic activities that initiate the catalytic oxidation of fuels at

relatively lower reaction temperatures. Burch and Loader (1994), Forsth et al. (1999),

Davis et al. (2000), and Reinke et al. (2004) investigated catalytic combustion over

different noble metals and revealed that platinum is more effective than palladium for

methane combustion, and they clarify the contribution of homogeneous and heterogeneous

reactions at the oxidation process. Appel et al. (2005) investigated the catalytically

stabilized turbulent combustion of fuel-lean hydrogen/air pre-mixtures over platinum and

found that nearly half of the fuel is converted heterogeneously and the remaining part is

combusted in the post-catalyst.

Zakhary and Aboul-Gheit (2005) show the effectiveness of Pt/ -Al2O3 catalytic disc

burner in the combustion domain of a confined turbulent flame. The authors indicate

that the progress of the combustion process over the platinum sites has controlled the

combustion emission products minimizing the environmental pollution.

Catalytic combustion over noble metal burners can be numerically examined in some

experimental investigations. Reinke et al. (2004) try to correlate the data of the catalytic

combustion of the lean mixture of methane over platinum in the pressure range 4 to

16 bar. The numerical predictions were carried out with a two-dimensional elliptic code

that included elementary heterogeneous and homogeneous chemical reaction schemes.

Also, the modeling of surface kinetics of heterogeneous/homogeneous chemistry and its

coupling to transport in catalytic combustion have been reviewed by Mantzaras (2006).

In the present work, we compare the effectiveness of using three catalytic noble metal

disc burners of Pt/ -Al2O3, Pd/ -Al2O3, and (Pt C Pd)/ -Al2O3 situated in the combus-

tion domain of confined turbulent gaseous diffusion flame. The stability behaviors of these

flames and the axi-symmetric thermal distribution and pollution emissions are monitored

in the catalytic combustion domain. Numerical approach and statistical analyses are

performed using software packages, POLYMATH 6.0 (The MathWorks, Inc., Natick, MA)

(Shacham et al., 2004) and MATLAB (The MathWorks, Inc., Natick, MA) (Cutlip and

Schacham, 1999; Constantinides and Mastoufi, 1999), in order to investigate the catalytic

combustion process and explain the interaction effects of different process variables.

2. Experimental Procedures

2.1. Combustion Operation

The experimental set up (Figure 1) comprises a vertical cylindrical combustion chamber

filled with an arrangement supplying the fuel and combustion air. The combustion

chamber (Figure 2) is 150 mm in diameter, 5 mm thick, and 1.0 m long. The combustor

is equipped with a thermal resistant glass window. The fuel jet was discharged vertically

through a nozzle with a diameter of 2.5 mm connected at the center of the fuel supply

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494 S. F. Deriase et al.

Figure 1. Experimental set up.

Figure 2. Combustor and burner arrangements.

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line in the axial direction at the base of the combustor. Commercial liquified petroleum

gas fuel having an average composition of 76% butane, 23% propane, and 1% pentane

is used in all experiments.

Three catalytic disc burners of Pt, Pd, and (Pt C Pd), all possessing a diameter of

40 mm, 4 mm thick, and perforated with 25 holes, are connected at the base of the

combustor at the specified supporting distance of 40 mm over the fuel jet nozzle. They

have been separately used as a catalytic flame burner. These discs are made of -Al2O3,

with a surface area of 60 m2 g�1. Each Al2O3 disc support was made by mixing -Al2O3

powder with a suitable binder, pasted, and formed. After drying at 110ıC overnight, the

disc was perforated by drilling to acquire suitable perforation (3-mm diameter holes).

The disc was then calcined at 400ıC for 4 h in a muffle furnace. This heat treatment

gives the highest crushing strength while retaining the catalytic activity.

2.2. Catalytic Disc Burners

The first alumina disc was impregnated with H2PtCl6 solution, such that the Pt content is

0.0001 wt% of the disc weight. The disc is again dried at 110ıC overnight and calcined

at 550ıC for 4 h.

The second disc (Pd/ -Al2O3) has been prepared via wet impregnation of an aqueous

solution of Pd (NO3)2 containing 10�4 g of the Pd metal. The impregnation is adjusted

to incorporate the Pd-containing solution on the external surface (1 mm in depth).

The third disc (Pt C Pd/ -Al2O3) has been prepared via two successive impregna-

tions: the first using Pd (NO3)2 solutions and the second used H2PtCl6. These impregnated

solutions contained the requisite quantity of the metal precursor, such that 10�4 g of Pt

and 10�4 g of Pd are absorbed and cover the surface of the finished catalyst. The catalysts

were dried in an oven at 110ıC overnight followed by calcinations at 550ıC for 4 h.

Every catalytic disc was then held at the combustor position for each experiment.

2.3. Combustion Procedure

The experiments were carried out using the three catalytic disc burners of Pt, Pd, and

(Pt C Pd) connected at a specified supporting distance of 40 mm, for ensuring the high

flame stability. The stability limits of jet diffusion flames were determined via recording

the fuel jet velocity at the stability conditions by visual observation. This procedure was

repeated several times at each set of different values of combustion air velocity ranging

between 1.2–4.2 ms�1 .

Detailed local measurements of the axial mean temperature were examined along

the stable flames operating in the presence of each catalytic disc burner at the supporting

distance of 40 mm over the fuel jet nozzle. Temperature measurements have been

obtained by using thermocouple probes made of platinum/platinum-13% rhodium, 0.1-

mm diameter wire. The thermocouple output voltage was integrated over a period of 20

sec using a microprocessor integrator. The present gaseous diffusion flames operate at

the same fuel mass flow rate (m0

f ) of 2.6 kg/h, combustion flow rate (ma) of 40 kg/h,

air fuel ratio (A/F) D 15.34 at the stoichiometric condition and overall equivalence ratio

(') D 1.0.

The axial distribution of CO and NOx concentrations has been achieved along the

flames developed over the catalytic disc burners at a fixed supporting distance of 40

mm. The concentrations were obtained through the use of a stainless steel water-cooled

sampling probe with a 1.0 mm inner diameter and 6.0 mm outer diameter. Samples of the

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combustion products were conditioned and measured using an on line Anapol EU-200/4

gas analyzer.

3. Results and Discussion

The stability of gaseous fuel jet flames is a topic of critical consideration in turbulent

combustion. Stability conditions of diffusion flames are to be considered in developing

rational designs of gas burners in industrial furnaces and gas turbines and in ensuring their

safe and efficient operation. The main function of catalytic combustion is stabilizing the

jet flame and enhancing efficient oxidative exothermic reactions in contact with the active

catalytic disc burners. The high generative heat energy evolved after catalytic ignition

will anchor the upstream region of the reaction zone of the jet flame, such that improved

stabilization tendency is achieved to a great extent.

In the present article, the combustion of turbulent jet flames over three noble metals

catalytic disc burners composed of Pt/ -Al2O3, Pd/ -Al2O3, and (Pt C Pd)/ -Al2O3 is

investigated at a specified supporting distance over the fuel jet nozzle. The jet flame

operates at high Reynolds’s numbers exceeding 3,000, to be satisfactory for turbulent

conditions.

3.1. Experimental Results of the Stability Limits

The experimental results of the stability limits of turbulent flames using catalytic disc

burners of Pt (Zakhary and Aboul-Gheit, 2005) as well as Pd and mixed (Pt C Pd)

are depicted in Figure 3. In this figure, the variation of the fuel jet velocity versus air

velocity is evident using these catalytic disc burners. The fuel and air velocities plotted

are the mean values calculated based on both flow rates and the passage area. The stability

limits of the gaseous flames have been investigated throughout air flow velocities ranging

between �1.2–4.2 m/sec. For each operating catalytic burner, all trials to increase the

fuel jet velocity at each air velocity during the range mentioned above, stable flames

prevail without fluctuations. However, extra trials for increasing the fuel jet velocities

to the limit of the experimental system, flames do not approach the blowout limit. This

success in improving the stability behavior of turbulent flames operates in the catalytic

Figure 3. Experimental data for velocity.

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combustion domain and the enhancement order of the discs is found as follows:

.Pt C Pd/= -Al2O3 > Pt= -Al2O3 > Pd= -Al2O3

The very high catalytic activities of these noble metals exhibit how important the

catalytic oxidation process is, where the final combustion products of the burned gaseous

fuel are CO2 C H2O. Obviously, the most effective catalyst is the bimetallic one (Pt C

Pd), which indicates that Pt and Pd metals combined together on the alumina surface

give a synergistic effect that accelerates the oxidative efficiencies of both metals and,

hence, better flame stabilization.

3.1.1. Stability Limit Analysis. Linear regression analysis is performed to estimate a

model equation that can correlate the maximum fuel jet velocity as a function of air

velocity. The experimental data of stability limits of the turbulent flame operating at the

three catalytic burners, shown in Figure 3, are statistically analyzed and found to predict

a model equation applicable for the three catalytic flames following a linear first-degree

polynomial:

Uf max D Ao C A1Ua; (1)

where Uf max is the predicted response, which stands for maximum fuel jet velocity

(m/sec); Ua is the value of air velocity (m/sec); Ao and A1 are the regression coefficients.

The regression coefficient (Ao) denotes the value of Uf max at Ua D 0. The slope of

the straight line�Uf

�Uadepicts the value of (A1). A comparison between experimental data

and fitting results of the predicted models for each catalytic flame are shown in Figure 4.

Table 1 concludes the regression coefficients and the corresponding values of R2 for the

three predicted model equations.

To assess the accuracy of the model, the value of determination coefficient, R2,

measures the reliability of model fitting. In the present work, the values of R2 for the

three predicted model equations using the corresponding three catalysts, Pt, Pd, and (Pt C

Pd), are 0.9964, 0.997, and 0.9969, respectively, which ensures the good adjustment of

the predicted results to experimental data. Also, the validity of the empirical models was

tested with the analysis of variance (ANOVA), the used a 95% confidence level. The

Figure 4. Experimental and calculated data for velocity.

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Table 1

Model coefficients with 95% confidence interval

Regression coefficients

Catalyst A0 A1 R2

Pt/Al2O3 40.757 ˙ 1.738 11.997 ˙ 0.641 0.996

Pd/Al2O3 21.562 ˙ 0.973 8.011 ˙ 0.359 0.997

(Pt C Pd)/Al2O3 54.772 ˙ 1.214 12.009 ˙ 0.448 0.997

results of ANOVA are given in Table 2. The Fischers “F” value with a low probability,

prob (F), value indicates high significance of the regression model.

It is noticed from Table 1 that the slope of linear relation (Uf max � Uair ) using the

three catalytic disc burners, increases according to the following order: (Pt C Pd) >

Pt > Pd.

This increase in the slope indicates that air velocity has a large influence on flame

stability. In other words, it can be assumed that the stable flames prevail at high fuel jet

velocities with the increase in air velocity.

3.2. Experimental Results of the Axi-symmetric Mean

Temperature Distribution

The axi-symmetric mean temperature distribution of the stable flames developed in the

presence of the Pt/ -Al2O3 catalytic burner (Zakhary and Aboul-Gheit, 2005), Pd/ -

Al2O3, and (Pt C Pd)/ -Al2O3 are shown in Figure 5. The axial mean temperature of

the flames operating at the three catalytic disc burners exhibits similar trends; a gradual

increase of temperature as a function of the axial flame distance reach a maximum due to

the progress of the combustion process followed by a gradual decay with almost parallel

rates at the downstream. It is evident that the temperature at the upstream distances

over the three noble metals catalytic disc burners are high compared to non-catalytic

combustions. The maximum axial temperature recorded was 1850ıC for a flame operating

over the (Pt C Pd) catalytic burner at the corresponding axial distance x of 80 mm. For

a flame operating over Pt the temperature is 1750ıC at x of 90 mm, while for the flame

operating over Pd the temperature is 1500ıC at x of 110 mm.

Table 2

Analysis of variance for 1st degree polynomial models

Catalyst DF SS MS V F value Prob(F)

Pt/Al2O3 1 1,704.237 1,704.237 0.7684 2,217.89 0.00001

Pd/Al2O3 1 637.7 637.7 0.24096 2,646.41 0.00001

(Pt C Pd)/Al2O3 1 984.0232 984.0232 0.3752 2,622.48 0.00001

DF: degrees of freedom; SS: sum of squares; MS: mean squares; V: variance; F: Fischer’s value;Prob(F): probability value.

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Figure 5. Experimental data for axial mean temperature distribution.

It is obviously shown that the three flames operate using the three noble metal

catalytic burners where the main reaction zone encloses the flame core and merges to the

flame axis. The hot portion of the flame core is generated fast, reaching near downstream

location rapidly, hence, enhancing the oxidation process in the following order:

.Pt C Pd/ > Pt > Pd:

This increase of thermal distribution of the catalytic flames confirms their increase in the

stability limits. The catalytic combustion with high heat release rate anchors the base of

the flame to be in stable condition.

It can be assumed that the combination of Pd with Pt in a catalytic disc may have

formed some sort of alloy of the two metals, which activate catalytic combustion. This can

be most probably attributed to enhancing the spill-over for oxygen in the combustion air

and, hence, increasing the oxidizability of the fuel molecules. Moreover, it is observed

that the dispersion of Pt is improved when mixed with Pd in the supporting alumina

disc. Furthermore, it deserves mentioning that the addition of the Pd to Pt on the Al2O3

supporting disc has improved the life time of Pt crystallites (Zakhary and Aboul-Gheit,

2005).

3.2.1. Mathematical Model for Temperature Distribution. Numerical investigation is

performed using POLYMATH and MATLAB software packages, to find the relationship

between the axial mean temperature distribution (T ) along the flame and the axial distance

(x) over the above-mentioned three catalytic disc burners. The relationship is found to

be described by the Gaussian model for fitting peaks (Giraud, 2008), which is given by

the equation:

T D

nX

iD1

ai eŒ�. x�bi

ci /2�; (2)

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Table 3

Converged values of the parameters with corresponding

R2 in Gaussian models

Catalyst/

parameter Pt/Al2O3 Pd/Al2O3 (Pt C Pd)/Al2O3

a1 1,338.350 1,011 1,462.930

b1 164.248 170.95 161.357

c1 129.032 134.18 133.302

a2 950 700 1,000

b2 98.229 105.16 89.784

c2 37.702 42.63 37.489

R2 0.983 0.988 0.967

where:

Parameter “a” is the height of the curve’s peak. The T values of the points making the

graph are multiplied by the value of “a.”

Parameter “b” is the position of the center of the peak.

Parameter “c” controls the width of the curve and is related to the full width at half

maximum (FWHM) of the peak and n is the number of the peaks to fit 1 � n � 8,

in our investigation (n) equals 2.

The numerical evaluation for axial mean temperature distribution is conducted for

the three used catalysts, and the converged values of ai , bi , and ci for flames operating

over Pt, Pd, and (Pt C Pd) together with the corresponding values of R2 are represented

in Table 3.

Figures 6a–c indicate the predicted results in comparison with corresponding exper-

imental data. It is observed that the experimental data are in close agreement with the

model prediction based on higher values of R2.

3.3. Experimental Results of Axial Distribution of the

Volumetric CO%

The axial distribution of the volumetric percentage of carbon monoxide (CO%) along

the flames developing over Pt (Zakhary and Aboul-Gheit, 2005), Pd, and (Pt C Pd)

catalytic burners is shown in Figure 7. This figure indicates the effectiveness of the noble

metals on the surface of catalytic burners in the combustion domain. The reaction of

the heterogeneous and homogeneous processes simultaneously co-operate leading to the

combustion that proceeds to completion. The percentage of carbon monoxide values all

over the axial distance of the flame in the presence of (Pt C Pd) disc burner is lower

than Pt and Pd in consequence. The comparatively lower CO values at upstream locations

among the three developing catalytic flames operate in the catalytic domain confirming

that decomposition and partial oxidation of the fuel goes rapidly towards CO2 formation.

This fast chemical reaction takes place early in the fuel jet more favorably over the (Pt C

Pd) and least effectively over the Pd disc burner. A significant decay of carbon monoxide

is observed at the axial location of maximum temperature for each flame operating in the

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Figure 6. Experimental and calculated data of temperature using (a) Pt/Al2O3, (b) Pd/Al2O3, and

(c) (Pt C Pd)/Al2O3.

Figure 7. Experimental data for CO%.

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combustion domain. Obviously, during the downstream location using the three current

catalytic burners, CO values with environmentally friendly concentrations are obtained.

3.3.1. The Mathematical Model for Carbon Monoxide Distribution. The experimental

data of the axial distribution of the volumetric percentage of CO at different axial distance

x over the noble metal catalytic disc burners Pt/Al2O3, Pd/Al2O3, and (Pt C Pd)/Al2O3

along the flames have been mathematically modeled. Several equations modeled are

investigated to match the data and assess the relationship. A non-linear four-parameter

model equation provides the actual relationship. The predicted model is:

y D bo C

�

b1

1 C e.b2x�b3/

�

; (3)

where y is the predicted response that stands for CO% and x represents the axial distance

over the catalytic disc burner.

Figures 8a–c demonstrate a comparison between the experimental and the predicted

exponential function, which indicates the evaluation of CO% along the flames with respect

Figure 8. Experimental and calculated data of CO% using (a) Pt/Al2O3, (b) Pd/Al2O3, and (c) Pt C

Pd/Al2O3.

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Table 4

Converged values and statistical parameters of the

non-linear model equation

CatalystRegression

parameters Pt/Al2O3 Pd/Al2O3 Pt C Pd/Al2O3

b0 0.218 0.328 0.155

b1 4.652 4.752 4.404

b2 0.049 0.044 0.050

b3 3.388 3.565 3.260

SS 35.4 44.2 29.2

V 0.004 0.007 0.006

R2 0.998 0.998 0.997

F value 2,862.49 2,212.13 1,581.8

Prob(F) 0.00001 0.00001 0.00001

to x. In the proposed model, the exponential function [an algebraic term exists in the

dominator of Eq. (3)] indicates that:

� In Figures 8a and 8b, the predicted model of flames over Pt and (Pt C Pd)

catalytic disc burners indicate an exponential decay in the range 30 � x � 60 and

exponential growth in the range 60 < x � 280.

� In Figure 8c, the predicted model of flame over the Pd catalytic disc burner

indicates an exponential decay in the range 30 � x � 80 and exponential growth

in the range 80 < x � 280.

� The predicted model yields in the three operated flames on the catalytic combustion

domain reflecting a good adequacy of the process fitting with the experimental

data due to the higher R2 values achieved.

Table 4 indicates the regression coefficients with 95% confidence interval.

3.4. Experimental Results of NOx Formation

Figure 9 shows the NOx formation as the combustion process proceeds over the noble

metal catalytic burners Pt (Zakhary and Aboul-Gheit, 2005), Pd, and (Pt C Pd) at the same

operating conditions. The formation of NOx records a comparatively lower percentage

along the flames operating over the three current burners. For the corresponding three

curves of such flames, the peak values appear at different axial distances located at the

position of the maximum mean temperature for each flame as shown in Figure 5. The

formation of thermal NOx is resulting from N2 and O2 in the combustion air mixture.

The mechanism is strongly favored by the high temperature due to the high activation

energy required for the chemical reaction (Svensson, 2008).

Finally, catalytic combustion over noble metal disc burners has been shown to be a

valuable technology for a lower formation of NO while maintaining stable combustion

with high efficiency.

3.4.1. Mathematical Formulation of NOx Distribution. The axial distribution of the

volumetric percentage of NOx at different axial distances along the flames, which operates

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Figure 9. Experimental data for NOx using the current catalysts.

over the noble catalytic burners of Pt/Al2O3, Pd/Al2O3, and (Pt C Pd)/Al2O3, has been

mathematically studied. Mathematical correlations reveal the instantaneous rate of change

of y, which stands for NOx with respect to the axial distance x, and it can elucidate the

rate at which the relations of y versus x ascend or descend per unit change of x.

The rate of change of NOx with respect to x for flames operates at Pt/Al2O3 and

(Pt C Pd)/Al2O3 catalytic burners and are given by the equation:

Dx Ddy

dxD

�2a1

c1

.x � b1/e�.

x�b1c1

/2� a2b2e

�b2x; (4)

while the rate of change of NOx with respect to x for flame operating over Pd/Al2O3

catalytic burner is given by the equation:

Dx Ddy

dxD

�2a1

c1

.x � b1/e�.

x�b1c1

/2�

2a3

c3

.x � b3/e�.

x�b3c3

/2: (5)

The values of the parameters [constants .a1; a2; a3/, .b1; b2; b3/, and .c1; c3)] for each

catalyst used are given in Table 5.

By integrating each of the above two equations, the resulting concentration profiles

of NOx versus axial distance x for each catalyst are represented in Figures 10a–c. It

is evident that these profiles are strictly confirmed with the experimental data and the

following remarks are concluded:

1. The slope of the tangent line is positive in the intervals: 30 � x < 105:5,

30 � x < 101, and 30 � x < 105:04 for Pt/Al2O3, Pd/Al2O3, and (Pt C

Pd)/Al2O3 catalytic disc burners, respectively. It is shown that NOx increases as

x increases at the above-mentioned intervals.

2. The slope of the tangent line is negative in the intervals: 105:5 < x � 280,

101 < x � 280, and 105:04 < x � 280 for Pt/Al2O3, Pd/Al2O3, and (Pt C

Pd)/Al2O3 catalytic disc burners, respectively. It is clearly indicated that NOx

decreases as x increases at these intervals.

It is evident from the mathematical formulation of the above differential equations that

the peak values of NOx distribution are located perfectly in the above mentioned intervals.

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Table 5

Values of the parameters of the differential equations

Catalysts/

parameters Pt/Al2O3 Pd/Al2O3 (Pt C Pd)/Al2O3

a1 0.053 0.033 0.0604

b1 105.5 101.6 105.04

c1 27.2 20.3 33.6

a2 0.006 — 0.007

b2 0.0004 — 0.0002

a3 — 0.0098 —

b3 — 130.5 —

c3 — 89.6 —

Figure 10. Experimental and calculated data for NOx% using (a) Pt/Al2O3, (b) Pd/Al2O3, and

(c) (Pt C Pd)/Al2O3.

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4. Conclusions

The results of the present investigation verify that:

1. Gaseous turbulent diffusion flames behave in stable condition over noble metal

catalytic disc burners of Pt/ -Al2O3, Pd/ -Al2O3, and (Pt C Pd)/ -Al2O3.

2. The highly efficient oxidative exothermic reactions in contact with an active (Pt C

Pd)/ -Al2O3 surface greatly enhance the heat evolved after catalytic ignition that

improves stabilization tendency to a greater extent.

3. The axi-symmetric temperature distribution of flames developing over Pt, Pd, and

(Pt C Pd) noble metals in the catalytic burners result in higher values through

the catalytic combustion domain, in particular, using the bimetallic catalytic

combination (Pt C Pd) due to the progress of combustion process resulting from

the rapid oxidizability of the resulting noble metals alloy.

4. Comparatively lower axial CO and NO values measured through flames operating

in the presence of Pt, Pd, and (Pt C Pd) catalytic burners potentially minimize

the environmental pollution.

5. Stability limits of the three catalytic flames are found to be compatible with

a linear regression fitting. The predicted model following a linear 1st-degree

polynomial strongly confirms the experimental data.

6. Mathematical modeling of temperature distribution for the above-mentioned three

catalytic flames is performed by Gaussian model function indicating high numer-

ical compatibility with the experimental data.

7. For carbon-monoxide distribution along the three catalytic flames, a non-linear

four-parameter model equation has been predicted with good adequacy in the

three operating catalytic flames on fitting relative to the experimental data.

8. For NOx distribution along the three catalytic flames, the mathematical formula-

tion using a differential equation for each NOx distribution relative to the axial

distance along the flame indicate perfectly the location of the peak values at

accurate mentioned intervals. The mathematical formulation has been strictly

confirmed with the experimental data.

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