Combustion characteristics of hydrogenâ€“air premixed gas in a sub

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Combustion characteristics of hydrogen–air premixed gasin a sub-millimeter scale catalytic combustor

Wonyoung Choi�, Sejin Kwon, Hyun Dong Shin

School of Mechanical, Aerospace & Systems Engineering, KAIST, 335 Gwahangno, Yuseong-gu, Daejeon, Republic of Korea

a r t i c l e i n f o

Article history:

Received 26 December 2007

Received in revised form

20 February 2008

Accepted 21 February 2008

Available online 10 April 2008

Keywords:

Micro Combustion

Catalytic Combustion

Hydrogen

Platinum

nt matter & 2008 Internae.2008.02.070

hor. Tel.: +82 42 869 5017;: [email protected], bl

a b s t r a c t

A sub-millimeter scale catalytic combustor was fabricated. Platinum catalyst coated on a

porous ceramics support was placed in the combustion chamber. The chamber, sized 10�

10� 1:5 mm was covered with a GaAs window which is transparent to infrared radiation.

The conversion rate was measured using gas chromatography. The temperature distribu-

tion in the combustion chamber was measured with an infrared thermal imager. The

activated region where the catalytic reaction was violent was formed in the vicinity of the

inlet. More than 95% of supplied gas reacted in spite of a small chamber space. The area of

activated region increased with increased volume flow rate and decreased equivalence

ratio. The concentration of platinum catalyst did not affect the reaction. A large content of

heat generation and a broad activated region prevented reaction termination by soaked

liquid water. The results of this work are important design factors for the development of

an effective micro-catalytic combustor.

& 2008 International Association for Hydrogen Energy. Published by Elsevier Ltd. All rights

reserved.

1. Introduction

Batteries are typically used as a small-sized power source for

micro-scale systems. Conventional batteries are not effective

as a micro power source because their energy density is small

[1–4]. This is a significant problem for micro reformers and

reactors which require a high temperature for operation [5].

Micro power sources using micro combustors are suitable for

such systems because such power sources have more energy

densities than conventional batteries as shown in Fig. 1 [2].

A flame is difficult to sustain in a small space below millimeter

scale. Heat loss becomes more significant as the combustion

chamber shrinks because the surface to volume ratio increases

[3,6]. Conventional flame combustion is not sustainable for a

micro-combustor because of problems like flame instability,

flame quenching and low combustor efficiency [7].

Catalytic combustion was applied as an alternative method.

Catalytic combustion is a sustained reaction by catalyst

tional Association for Hy

fax: +82 42 869 [email protected] (W. Ch

without a flame. The reaction is sustained by surface reaction

of fuel and oxidizer which are adsorbed on the catalyst

surface. The rate of surface reaction is lower than the volume

reaction. The reaction rate of catalytic combustion is lower

than for conventional combustion. The reaction temperature

in the combustion chamber also decreases. The amount of

heat loss becomes smaller than for conventional combustion.

Flame quenching does not occur because there is no flame

[7–10]. For these reasons, catalytic combustion is suitable for

micro-combustor.

Hydrogen is easy to be applied as the fuel of micro-combustor

because it can catalytically react with air in the absence of

ignition source. An application of micro-catalytic combustor

using hydrogen is a micro-scale fuel reformer-fuel cell system

by Ryi et al. [11]. Heat energy for the reforming reaction is

generated by a micro-catalytic combustor using anode-off

hydrogen. The storage problem of hydrogen is also avoided

because reforming fuel is easier to store than hydrogen.

drogen Energy. Published by Elsevier Ltd. All rights reserved.oi).

dx.doi.org/10.1016/j.ijhydene.2008.02.070

mailto:[email protected],

mailto:[email protected]

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Nomenclature

f equivalence ratio_V volume flow rate

CPt concentration of platinum catalyst in the support

N number of moles

x conversion rate

H2 hydrogen

0 inlet condition

MFC mass flow controller

XRD X-ray diffraction

GC gas chromatography

I N T E R N A T I O N A L J O U R N A L O F H Y D R O G E N E N E R G Y 3 3 ( 2 0 0 8 ) 2 4 0 0 – 2 4 0 8 2401

A millimeter scale catalytic combustor was fabricated by

Choi et al. [7]. The basic combustion characteristics of

hydrogen–air premixture and the potentiality of scale down

to sub-millimeter scale were investigated. Platinum catalyst

loaded on cordierite monolith support was used. The support

installed in the combustion chamber. Then hydrogen–air

premixed gas was supplied to the combustor. The reaction

started at room temperature without external ignition source

and was sustained stably. Above 95% of supplied gas reacted

on 5% of area in the combustion chamber. This means that a

catalytic combustor will properly work if the size of the

combustion chamber is reduced below millimeter scale.

This research investigates the catalytic combustion char-

acteristics of hydrogen–air premixture in a sub-millimeter

scale combustor in which platinum catalyst on porous media

support was installed.

2. Methods

2.1. Experimental apparatus

A schematic of the experimental system is presented in Fig. 2.

Hydrogen and air were supplied by mass flow controllers and

mixed in a buffer tank. The mixture was supplied to the

catalytic combustor. The temperature distribution in the

combustion chamber was measured by an infrared thermal

Fig. 1 – Energy density of va

imager outside of the combustor and recorded by personal

computer. The conversion rate of hydrogen was measured by

gas chromatography which was connected to an exhaust line.

The burnt gas was passed through silica gel to remove water

vapor generated by the reaction.

An exploded view of the catalytic combustor is seen in

Fig. 3. The combustor consisted of a body, window and cap.

The combustion chamber, the inlet and the outlet are in the

body part which is made of ceramics to minimize conduction

heat loss and the fracture of the window caused by thermal

expansion. The structure of the combustion chamber is seen

in Fig. 4. The shape of the chamber is like a thin square plate

with a size of 10� 10� 1:5 mm. The gas inlet and the outlet

are on the side of the chamber. A porous support for the

coated catalyst was installed in the combustion chamber

which was covered by the window. The window was prepared

with various materials with a diameter and thickness of 25.4

and 5 mm, respectively. Magnesium fluoride ðMgF2Þ and

Gallium Arsenide (GaAs) were used as the window material

in this work. The body and the window were jointed by the

cap part, giving an airtight combustor.

2.2. Catalyst preparation

Platinum (Pt) was used as the combustion catalyst. A porous

firebrick material (Isolite B5TM) was used as the support for

rious power sources [2].

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Fig. 2 – Schematics of whole experiment system.

Fig. 3 – Exploded view of the sub-millimeter scale catalytic

combustor.

Fig. 4 – Structure of the combustion chamber.


the coated catalyst. Isolite B5 is a porous ceramic with a

porosity of 70% and an average pore diameter of 50mm. Porous

media is widely used as the support of the precious metal

catalyst because of the large surface area [12,13]. The support

was sliced to a dimension of 10� 10� 1:5 mm, the same as

the size of the combustion chamber. The precursor was

Hexachloro platinic acid ðH2PtCl6Þ, and acetone was used as

the solvent for the precursor.

The catalyst was prepared by the incipient wetness method

[12,14]. The pore volume of the support was measured by

absorbing water into a well-dried piece of the support.

A solution of the precursor and the solvent whose volume is

the same as the pore volume of the support was prepared,

and was absorbed into the support. The support was dried at

70 �C oven for 6 h. The specimen was reduced and calcined in

a quartz reactor from 20 to 300 1C for 3 h and for 6 h at 600 1C

under hydrogen flow diluted by nitrogen [14]. The procedure

for catalyst preparation is shown in Fig. 5. The support

appeared dark gray due to coated platinum.

X-ray diffraction (XRD) analysis was carried out to deter-

mine if other chemicals exist on the support except platinum.

Platinum was coated for all specimens and the average

particle diameters ranged from 5 to 13mm, according to the

concentration of the platinum.

2.3. Temperature measurement with infrared thermalimager

The temperature distribution on the catalyst surface is

an important measure of the surface reactivity inside

Fig. 5 – Procedure of catalyst preparation.

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Table 1 – Volume flow rate supplied to the combustorðf ¼ 1:0Þ

Volume flow rate (ml/min)

Mean velocity (cm/s) Re

Total H2 air

84 25 59 9.33 19.6

169 50 119 18.78 39.5


the combustor. An infrared thermal imager, NEC Sanei

TH9100, that can detect the bandwidth of 8214mm of infrared

ray was used to measure the temperature in the combustion

chamber.

A Gallium arsenide (GaAs) window was used to allow

infrared transmission from the catalyst bed to measure its

temperature in an optical manner. GaAs is opaque to visible

light but transmits 55% of 8214mm infrared ray as shown in

Fig. 6 [15].

Table 3 – Change in equivalence ratio ðCPt ¼ 0:75 wt%Þ

253 75 178 28.11 59.2

338 100 238 37.56 79.0

676 200 476 75.12 158.0

1014 300 714 112.68 237.0

Table 2 – Variation in catalyst concentration (f ¼ 1:0,V ¼ 253 ml=min)

Mass of the precursor per unit massof the support ðmg=mgÞ

Platinumconcentration

(wt%)

8.06 0.40

20.16 0.75

50.41 1.87

100.82 3.73

2.4. Test conditions

Volume flow rate, equivalence ratio of premixed gas and mass

of platinum on supports influence combustion characteristics

and were selected as experimental parameters. Volume flow

rate of the gas mixture was varied under fixed equivalence

ratio and platinum concentrations. The volume flow rate for

each condition is shown in Table 1. All flows were laminar by

calculation of Reynolds number. Results of change in

platinum concentration under fixed equivalence ratio and

volume flow rate are presented in Table 2. Changing mass of

precursor per unit pore volume during catalyst preparation

varied with the platinum concentration. The effect of change

in equivalence ratio under the same heat generation is shown

in Table 3.

The temperature in the combustion chamber was recorded

every 6 s for 80 min. Analysis of burnt gas was performed by

gas chromatography along with temperature measurements.

The results were recorded every 2.5 min for 60 min.

Equivalence ratio Volume flow rate (ml/min) Re

H2 Air Total

0.2 50 600 650 147.2

1.0 50 119 169 39.5

Fig. 6 – Transmittance of gallium arsenide.

3. Results and discussion

3.1. Direct observation in the combustion chamber andmeasurement of conversion rate of hydrogen

The result of direct observation in the combustion chamber

by time is shown in Fig. 7. The equivalence ratio was 1.0, and

the volume flow rate was 253 ml/min. Platinum concentration

was 0.75 wt%. The hydrogen–air premixture entered the

chamber at the left side and exhausted at the right side.

The reaction started promptly when the premixed gas was

supplied to the combustor. Liquid water condensation

was visible around the inlet. After 15 s, a small red spot was

formed around the inlet, and the water drop started to move

to the exhaust. After a minute, the reaction became active, so

that a red bright region formed in the vicinity of the inlet. The

water drop pooled in the combustor, soaking into the support.

The water formed by the reaction did not drain causing the

activated region to be reduced. Later, all of the water was

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Fig. 7 – Result of direct observation in the combustion

chamber where t is the elapsed time from the start of the

reaction (s).

Fig. 8 – Conversion rate of supplied hydrogen (/ ¼ 1:0,

V ¼ 338 ml=min, CPt ¼ 0:75 wt%).


vaporized due to continuous heat generation. The reaction

became steady state without further changes after 20 min.

The conversion rate of hydrogen with respect to time is

shown in Fig. 8. The conversion rate of a species means the

ratio of the amount of the species consumed in the reactor

and that supplied through the inlet. The conversion rate of

hydrogen is defined like

xH2¼ 1�

NH2

NH2 ;0. (1)

The conversion rate of hydrogen increased immediately

after the start of reaction. Thus, catalytic light-off occurred

without any time delay from the supply of premixed gas. This

is a distinct property of catalytic combustion of hydrogen

with platinum catalyst in macro-scale experiments [16,17]. It

is also seen in the sub-millimeter scale reactor. This means

that the structure and the handling of a micro-catalytic

combustor can be simple because it needs no external

ignition equipment or preheating source.

The activated region was stably formed around the inlet.

The conversion rate of hydrogen was sustained above 95% in

the steady state after 20 min. This means that the premixture

reacted violently but stably in the activated region even

though there was no flame [18].

3.2. Effect of volume flow rate

The results of temperature distribution in the combustion

chamber are shown in Figs. 9–11. Each square means the

combustion chamber. The temperature distribution for var-

ious volume flow rates is shown in Fig. 9. The activated

region, which is not black in the temperature distribution,

was small and biased to inlet after 10220 min. As time went

on, water started to vaporize due to heat generation in the

combustor. After 30 min, the reaction went into steady state

and no appreciable change appeared in the temperature

distribution for all volume flow rate. This is the same as the

result shown in Fig. 7.

The maximum and average temperatures in the combus-

tion chamber at 60 min are shown in Table 4. The more the

volume flow rate increased, the higher the average tempera-

ture increased. The maximum temperature was much higher

than the average temperature in the combustion chamber for

all volume flow rate because the region where the maximum

temperature was measured was very small like a spot in the

vicinity of the inlet.

The area where the temperature was above 300 �C is shown

in Fig. 10. The area broadened as the volume flow rate

increased because the amount of fuel and oxidizer and

the flow velocity in the combustion chamber increased.

The time to obtain steady state was reduced with the increase

of the volume flow rate. This means that a large heat

generation is helpful in the initial stage of the combustor

operation.

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Fig. 9 – Temperature distributions in the combustion chamber under various volume flow rate (/ ¼ 1:0, CPt ¼ 0:75 wt%).

Fig. 10 – Comparison of the area whose temperature is above 300 �C (/ ¼ 1:0, CPt ¼ 0:75 wt%).


The average conversion rate of hydrogen in steady state

was shown in Table 5. The measurement of the conversion

rate was not reliable when the volume flow rate was higher

than 1014 ml/min because the excess temperature gradient

caused by the high volume flow rate tended to damage

the window part. However, the limit volume flow rate that

exceeds the catalyst capability might be higher than 1014 ml/

min because the conversion rate of hydrogen sustained above

97% as the volume flow rate increased.

3.3. Effect of lean premixed gas

The temperature distributions when the equivalence ratios

were 1.0 (stoichimetric premixture) or 0.2 (lean premixture)

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Fig. 11 – Temperature distributions in the combustion chamber under stoichiometric and lean case ðCPt ¼ 0:75 wt%Þ.

Table 4 – Maximum and average temperature in thecombustion chamber under various volume flow rate

Volume flow rate(ml/min)

Maximumtemperature (�CÞ

Averagetemperature

(�CÞ

84 353.3 40.8

169 778.3 209.0

253 745.0 262.4

338 804.3 321.4

Table 5 – The average conversion rate of hydrogen in thesteady state under various volume flow rate higher than338 ml/min (f ¼ 1:0, CPt ¼ 0:75 wt%)

Volume flow rate (ml/min)

Average conversion rate ofhydrogen (%)

338 97.63

676 97.34

1014 98.33


are shown in Fig. 11. The area of activated region was larger

for the case of 0.2 at the same time before 60 min. The

temperature distribution between the two cases was differ-

ent. When the lean premixture was supplied, the temperature

distribution of the left half of the combustion chamber did

not change until steady state, in contrast to the stoichio-

metric case. This is due to the difference of in volume flow

rate for the two cases. The volume flow rate, when the

equivalence ratio was 0.2, was about 4 times larger than for

the stoichiometric case. The water generated by the reaction

evaporated and drained better because the convection heat

transfer was greater. The area of the activated region became

broad, and the temperature distribution sustained stably in

the initial stage of the operation for the case of lean

premixture.

The burning velocity of lean premixed gas is lower than for

the stoichiometric premixture in conventional flame combus-

tion [6,19,20]. This means that the reaction rate of the lean

premixture is lower. The larger activated region when the

equivalence ratio was 0.2, was also affected by the lower

reaction rate.

3.4. Effect of platinum concentration

The region where the temperature was above 300 �C in the

steady state of the combustor under various catalyst con-

centrations is shown in Fig. 12. The shapes of the region were

different, but the area of the region for each case was similar

regardless of platinum concentration.

This means that the platinum concentration had no effect

on the sub-millimeter scale catalytic combustion character-

istics and the performance of the combustor. Cost-effective-

ness is guaranteed because the output of the combustor does

not change, although the amount of platinum catalyst is

small, below 0.5 wt%.

3.5. Termination of reaction by soaked water and itsrecovery

Initially, the reaction was vigorous around the inlet when the

premixed gas was supplied. Then the liquid water produced

by the reaction soaked into the porous support (Fig. 7). The

soaked water evaporated and exhausted as time passed by

the heat generation for most of the experiment condition.

However, for the cases of small volume flow rate and small

catalyst concentration, the soaked water encroached on the

activated region and covered the active sites. The reaction

stopped when if all of the active sites were covered by

liquid water; no activated region was detected by both of

direct observation and thermal imager. The temperature

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distribution when the reaction stopped after 100 s elapsed

from the start of the reaction is shown in Fig. 13.

The reaction started again after the reaction stopped if the

premixture was continuously supplied. Immediately after the

reaction stopped, no water was formed. As time passed, some

active sites which had been covered by liquid water became

exposed because small amounts of water were evaporated by

Fig. 12 – Comparison of the area whose temperature is

above 300 �C for the case of the various catalyst

concentrations in the steady state of the combustor (/ ¼ 1:0,

v ¼ 253 ml=min).

Fig. 13 – Temperature distribution with time when the catalytic r

CPt ¼ 0:40 wt%Þ.

supplied gas. At the active sites, the premixture started to

react, and the water around there became evaporated by heat

generation. Then the activated region was formed again in a

short time. This is the process of recovery from the stop of

reaction. After that, the reaction went to steady state or

stopped again according to the temperature of the combus-

tion chamber and the amount of heat generation. However,

the recovery condition could not be identified because it was

not predictable.

Liquid water in the combustion chamber must be removed

to decrease the delay time before steady state. This may be

accomplished by larger volume flow rate and lean premixed

gas. Heat generation increases as the volume flow rate

increases accelerating the evaporation of water. The area of

the activated region broadens as the equivalence ratio

decreases, preventing water from coming into the support.

3.6. Effect of temperature gradient by activated region

The activated region was biased toward the inlet of the

combustion chamber for all of the experiment condition. It

causes a great temperature gradient in the catalyst bed

material inside the combustion chamber because the tem-

perature in the vicinity of inlet is higher than the temperature

around the exhaust. Thus, a thermal stress was added to the

structure of the combustor. The thermal stress may result in

significant damage if the size of a combustor is micro-scale.

4. Conclusion

A sub-millimeter scale catalytic combustor with platinum

catalyst on a porous ceramic support was fabricated. The

supplied premixed gas reacted stably, and the activated

eaction terminated by liquid water (/ ¼ 1:0, V ¼ 253 ml=min,

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region was formed around the inlet. Above 95% of supplied

gas stably reacted in spite of the small activated region. Thus,

catalytic combustion reaction is applicable in sub-millimeter

scale.

The area of activated region in the combustion chamber

increased as the equivalence decreased. The concentration of

platinum catalyst had no effect on the reaction character-

istics, making it cost effective to produce a micro combustor.

Termination of the reaction by water initially may decrease

the combustor performance. A larger volume flow rate and

less equivalence ratio were effective ways to suppress liquid

water formation. The results of this work are important

design factors for the development of an effective micro-

catalytic combustor.

However, the activated region was formed around inlet for

all experiment condition. Thermal stress to the structure of

combustor by the temperature gradient in the micro-scale

combustion chamber may result in significant damage. More

research is required to address this problem.

Acknowledgment

This research has been done with the support of Korea

Science and Engineering Foundation (KOSEF) through the

Combustion Engineering Research Center (CERC).

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