- Email: [email protected]
Experimental investigations of hydrogen catalytic combustion plates at KACST
Ini.
Pergamon PII: SO360-3199(96)000894
EXPERIMENTAL
J tfj cl,r~qen Ewy>.
( I997
Vol. 22. No. 4. pp. 3X3 387. 1997
IntixnatmnalAsx~~atwn for Hydrogen Energy
Elsevier Scmm Ltd Prmted in Great Britain. All rights reserved 036OG3I99 :97 s I7 no + 0.00
INVESTIGATIONS OF HYDROGE:N COMBUSTION PLATES AT KACST
CATALYTIC
M. AL-GARNI Solar Programs. Energy Research Institute, King Abdulaziz City for Science and Technology (KACST). Riyadh 11441. Saudi Arabia
P.0. Box 6086.
Abstract--Catalylic combustion of hydrogen is considered to be one of the safest, cleanest and most efficient forms of utilizing hydrogen for heating purposes. At KACST. in order to find a general purpose catalytic combustion module. thorough application-oriented investigations were carried out upon ditferent hydrogen catalytic combustion plates. In this paper. experimental setup, investigation procedures and study results are discussed in detail. (” 1997 International Association for Hydrogen Energy. All rights reserved
INTRODUCTION Previous investigations have shown that hydrogen catalytic combustion can satisfy the most stringent safety, pollution-free and combustion efficiency requirements [I-IO]. In addition. several investigations have been carried out to show the numerous possibilities of utilizing the catalytic combustion of hydrogen in different domestic uses. In this regard. Brewer investigated hot catalytic combustion plates designed for food heating and similar household purposes [2]. Mercea et al. investigated the possibility of heating rooms using hydrogen catalytic combustion [3]. They further investigated the performance of hydrogen catalytic combustors in the form of platinum-supported plates of different sizes. By defining the combustion efficiency as the ratio between reacted and inlet hydrogen flow rates, they showed that it could reach up to 100% especially at relatively low flow rates [4]. In a series of interesting papers. Haruta r/ (11.thoroughly investigated the catalytic combustion of hydrogen [5-S], and several inexpensive transition metal oxides as well as platinum-group metals were also investigated. They concluded that, in diffusive combustion. Pd-powder coated Ni foam with relatively large pores could offer the highest combustion efficiency. However. they showed that it suffered front considerable non-uniformity in surface temperature distribution. They also showed that premixing of air and hydrogen could significantly improve the combustion efficiency. Most of their investigations were carried out on vertically positioned plates. This caused a large non-uniformity in the reaction, and hence, in the temperature distribution both inside and at the plate surface. Ledjeff introduced and discussed the activity and features of catalytic hydrogen combustion
plates fabricated by using highly hydrophobic substrates coated with catalytically active materials like palladium [9]. The substrate, he pointed out, consisted of PTFE and active charcoal. He indicated that the hydrophobicity significantly helped in reducing the vapor pressure inside the catalyst substrate by avoiding capillary condensation. Ledjeff and Gieshoff designed a hydrogen catalytic combustion-based heating system for divers [IO]. They proposed a combustion-rate limiting arrangement so that temperature can not exceed a maximum safe limit of 250 C. In this study. application-oriented investigations were carried out upon several different samples of hydrogen catalytic combustion plates which are planned to be used for different domestic heating purposes. The influences and interrelations of the investigated physical parameters are presented. Drawbacks and features of different conditions and setups are introduced in detail. The study is carried out under the cover of the Saudi-German research and development program. HYSOLAR. This program was initiated in 1986 to produce hydro,gen using solar energy and investigate its utilization possibilities. EXPERIMENTAL
SETUP
Four different catalytic sample plates were selected for investigation at our laboratory. All sample plates shared the same physical dimensions of 5 x 5 x 1 cm. According to previous studies [5--S], it has been realized that vertical positioning of catalytic plates caused considerable temperature-distribution non-uniformity. Consequently, we preferred to discard this option and tix sample plates horizontally. In addition, it has been shown that putting
M. AL-GARNI
384 r
Catalytic Plate
Gas Dstribution Plate NOZZk
diffusion into its internal pores, the Johnson Matthey (JM) sample was drilled from all sides using a drill bit of 0.8 mm diameter. This was because it had a honeycomblike structure and, hence, had no interconnected pores as was the case with the other samples.
Flow Regulator
RESULTS
Hydrogen
-
Flow Meter
-
Pressure Regulator
Air
Fig. 1. Schematic of the test setup. a catalyst-free porous plate in front of the catalytic plate significantly improved the gas distribution [4, 6-81. Having these two points in mind and taking into account the hydrogen-air premixing option, the experimental setup schematically illustrated in Fig. 1 was built at our laboratory. It mainly consisted of hydrogen supply cylinders connected to the test rig through a pressure regulator and a flow meter. In addition, a compressed air supply line was also connected through a separate pressure regulator and flow meter. Both lines come to a mixing chamber at the base of the test rig as shown in the figure. The test rig itself comprises of the mixing chamber for fuel-air premixing, a nozzle, a compartment for the gas distribution porous plate and the fixture for the catalytic plate. The purpose of the nozzle was to act as a flame trap in the case of backfire. This happens frequently when hydrogen is premixed with air or oxygen and will be discussed later. Fast response thermocouple probes were fixed at different locations on the top surface and through the depth of the catalytic plate. In addition, a hand-held fast response surface thermocouple probe was frequently used for cross checking throughout the experimental work. A sensitive hand-held hydrogen detector was exposed to the combustion products in order to check for any unburned traces of hydrogen during the experiment. However, in nearly all of our experiments, no unburned hydrogen was detected. The investigated samples were procured from different suppliers. Their physical and catalytic specifications are shown in Table 1. All were cut from bigger samples in order to suit the test rig configuration. No chemical or basic physical modifications were introduced to the test samples. However, in order to improve the chances of air
AND DISCUSSION
Extensive experimentation was carried out with the aim of completely characterizing the sample plates. From the beginning, it was clear that premixing of hydrogen and air would have to be excluded for two reasons. Firstly, premixing allows an explosive mixture to form which can very easily ignite and cause a backfire. This occurred in our test on several occasions. Secondly, it was noticed that premixing caused the combustion to take place mainly at the bottom surface of the catalytic plate. This in turn caused the bottom surface to overheat at certain flow rates, and to have some spots on it which exceeded the hydrogen self-ignition temperature. Both phenomena caused the combustion to be unstable and unsafe under most conditions. Therefore, premixing was excluded as an option for practical domestic use. However, it was noticed that putting a nozzle before the hydrogen-air distribution plate acted as a flame trap, which increased the possibilities of eliminating the backfire phenomenon. After excluding the premixing option, external mixing was selected for the characterization. With this option, hydrogen first diffused into the catalytic-free distribution porous plate and then into the catalytic plate. By having the surrounding air at the top surface, combustion started at that surface and immediately its main core shifted to the inside of the plate where diffusion of air took place as well. In the beginning, the four sample plates selected for investigation were tested at a constant flow rate of about 600 l/h and a constant pressure of about 0.1 bar. This helped to narrow the investigation range further by excluding those samples that did not show a relatively high combustion efficiency, defined as the ratio between reacted and inlet hydrogen flow rates, and concentrating upon the remaining ones. This screening process resulted in the activity curves shown in Fig. 2. Temperature readings of all the surface thermocouples indicated a relatively good combustion uniformity, and hence, the average temperature value was considered. It was noticed that the top surface temperature of the JM plate was the lowest, and regardless of whether the temperature probe was fixed at the surface or inside the plate, it did not give
Table 1. Physical characteristics of tested samples Sample
Catalytic material
Substrate
EK 10 (Schumacher & Heraeus, Germany) EK 20 (Schumacher & Heraeus, Germany) UOP MM90F (UOP Limited, UK) JM (Johnson Matthey, USA)
0.5% Pd 0.5% Pd l-3% mixture of Pd and Pt Undisclosed by supplier
Ceramic, 30 nrn pore size Ceramic, 40 nm pore size Metallic foam, 950 pm pore size Honeycomb ceramic, 1 mm2 cell area
HYDROGEN Average.Temperature
CATALYTIC
( aC )
COMBUSTION Average ml
350)
300
0’
0
10
20
30
40
I 50
50
___
Time ( rmn )
Fig. 2. Catalytic
combustion
activity plates.
for four different
study
Temperature
Temperature
385
( aC )
-EK20
0
2
4 Distance
6 from sample face ( mm )
a
10
lower
Fig. 4. Temperature profile across the depth of the test sample.
more than 230°C. Because we were only interested in plates which operate in the range 350-7OO”C, this sample plate was excluded from any further investigation. Furthermore, because the samples EK 10 and EK 20 were nearly similar when compared with the other two samples, one of them (EK 10) was also excluded. With the remaining two samples, UOP and EK 20, A thorough characterization investigation was carried out. First, the temperature was measured at different depths through the plates. It is interesting to notice that combustion activity about 4 mm inside showed an opposite trend to that of the surface. This is probably because of the difference in the average pore diameter. For UOP, with its bigger bore diameter (average of 950 pm), it was much easier for air to diffuse into the plate and meet hydrogen very early so that the peak combustion activity took place much nearer the bottom surface. On the other hand, this was not the case for the EK 20 sample where the relatively narrow pores (40 pm) did not allow for a similar activity. These results are illustrated by Figs 3-5. In the test condition represented by Fig. 3, the temperature probe was fixed at a depth of about 4 mm inside the sample. This figure also shows that the combustion temperatures of Average
PLATES
metallic substrates tended to be relatively non-uniform, especially when external air flows existed. It was observed that with the same room conditions, when a windowtype air conditioning unit was running, the blown air flow affected the temperature uniformity of the UOP metallic-based substrate much more than the EK 20 ceramic-based substrate. Figure 3 represents the temperature profiles inside both samples. It is clear that, with its larger bore diameter, the UOP sample gave a higher combustion temperature near to the bottom surface, while the EK 20 gave a better activity in the middle and towards the top surface, and had a better temperature uniformity. In Fig. 5, it is clear that with larger bore size (UOP), the instantaneous temperature distribution was much more uniform. This confirmed the obvious fact that TernDenature
( “C 1
Temperature
( “C )
aoJ-----Tw.
’
( a C )
500,
0 0
-
UOP
-
EK20
Probe 0
10
20
30 Time
Fig. 3. Combustion
40 (min
4 mm 50
mslde so
70
)
activity at a depth of 4 mm inside the test samples.
Fig. 5. Temperature distribution at the plate surface; UOP above and EK 20 below.
386
M. AL-GAKNI
Average
Temperature
(‘C
)
Average 550 I
Temperature
( ’ C )
500
-EK
450
20
400 350 300 250 200 150 100 50 “” 01
02
03
Pressure
04
05
0
( bar )
0
5
10
15
Fig. 6. Influence of varying supply pressure on the activity.
Time
20
25
30
( min )
Fig. 8. Cooling rate profile of the test samples
air can mix with hydrogen at a depth inside the plate directly proportional to the pore size. In order to investigation the influence of varying hydrogen supply pressure, data was collected for the range O.llO.5 bar. Figure 6 shows the result of the pressure-varying test. It is clear that varying the pressure did not significantly affect the combustion activity. Indeed, it shows that temperatures remained almost constant along the selected range. Varying the hydrogen flow rate had some influence on temperature. Figure 7 represents flow rate vs temperature for the range 1000-3000 1/h at 0.1 bar. The figure shows that the UOP experienced a slow decrease in the surface temperature. Again, this could be because of its larger bore size and also because of its metallic substrate. When hydrogen blew in larger quantities, it prevented air from diffusing as easily as it did with lower flow rates, and hence, the combustion was shifted towards the upper surface where insufficient mixing took place within the plate. In addition, the hydrogen supply was at room temperature and this caused the plate to relatively cool down. With the other sample (EK 20) bore size and nature of substrate caused the opposite trend. With increased hydrogen supply and smaller pore size, air
diffused into the plate and more combustion activity took place. In practice, stability of operation could be one of the most important factors in the daily use of catalytic combustion heating plates. In case of intermittent discontinuities of the fuel supply, it would be favorable to have a better heat-storage capacity that could help to avoid severe fluctuations in performance. With this in view, a test was carried out to see the cooling-down behavior of the sample plates. As could be seen in Fig. 8, the ceramic-based substrate plate (EK 20) showed a slower cooling rate than the metallic-based one (UOP). For example, if the fuel discontinuity time period is 5 min, the figure shows that UOP would cool down to about 80°C while the EK 20 plate would not go below about 200°C. This gives, in general, an important advantage for the ceramic-based plates over the metallic-based ones. Finally, stability with time was also investigated for about 500 h. Figure 9 represents the performance stability of both the test plates at a constant pressure and flow rate of about 0.1 bar and 600 1/h, respectively and with Average Temperature
Average 400
Temperature
( “C )
...”.._. .,,._.
( aC ) 320
350 I,
0
300
280
0 300
260
---tEKZO
250
240
220
Probe at surface 200 1000
1500
2000
2500
0.1 bar; 600 I / h.
200
3000
Flow rate ( I / hr )
Fig. 7. Influence of varying supply flow rate on the activity.
100
200
300
400
500
Operational Time ( hr )
Fig. 9. Average surface temperature vs time
600
HYDROGEN
CATALYTIC
temperature probe fixed at a depth of about 4 mm from the bottom surface. In addition, the figure confirms the previous findings about the non-uniformity of the metallic-based plate (UOP).
COMBLJSTION PLATES
l
decrease and to shift towards the upper surface of the plate. With hydrogen, in the absence of products causing catalyst poisoning, degradation of the catalytic combustion activity was shown to be relatively unlikely for long operation cycles,
CONCLUSIONS With the investigations carried out in this study, one can summarize the conclusions with the following points. l
l
Catalytic combustion with ceramic-based porous substrates proved to be probably the most suitable option for stable heating purposes with hydrogen. They exhibited good temperature uniformity and satisfactorily resisted surrounding air fluctuations. Furthermore, their cooling-down rate was relatively slow. It seemed. however, that plates with larger average pore diameters (X0-120 pm) could give better combustion activity and a more uniform surface temperature distribution. In general, varying the fuel supply pressure proved to have a negligible effect upon combustion activity. However, varying the supply flow rate showed some influence. especially with metallic plates with larger pore sizes. With such plates. combustion activity tended to
3x7
REFERENCES I. J. Pangborn, M. Scott and J. Sharer, I/r/. J. Hj&qc,n &rr~/r 2,431 (1977).
4. J. Mercea, E. Grecu, T. Fo’dor and S. Kreibik. fnt. .I. H&oyew Ener-y?,6, 482 ( 19X2). 5. M. Haruta and H. Sane. Inr. J. Hj,tlro,qr,r .!+rc,~,q!~ 6. 601 (1981). 6. M. Haruta, Y. Souma and H. Sano. I/?/. J. //j.&oq(~r Enprq:yl. 9, 729 (1982). 7. M. Haruta and H. Sane. Int. J. H~/ro~,qenE/rer,qj,9. 737 (1982).
9. K. Ledjeff. 1f1/.J. Hrrlrcjqm 0re~qy 5. 361 (1987). 10. K. L.edjeff and J. dieshoff. Hydrogen heating systems for divers, Proccwiin,q.r o/‘the Irrto.r~utionul H\droqen C‘o~fi~t~c~c~
IX. pp. 289 293. Paris (1992).
Pergamon PII: SO360-3199(96)000894
EXPERIMENTAL
J tfj cl,r~qen Ewy>.
( I997
Vol. 22. No. 4. pp. 3X3 387. 1997
IntixnatmnalAsx~~atwn for Hydrogen Energy
Elsevier Scmm Ltd Prmted in Great Britain. All rights reserved 036OG3I99 :97 s I7 no + 0.00
INVESTIGATIONS OF HYDROGE:N COMBUSTION PLATES AT KACST
CATALYTIC
M. AL-GARNI Solar Programs. Energy Research Institute, King Abdulaziz City for Science and Technology (KACST). Riyadh 11441. Saudi Arabia
P.0. Box 6086.
Abstract--Catalylic combustion of hydrogen is considered to be one of the safest, cleanest and most efficient forms of utilizing hydrogen for heating purposes. At KACST. in order to find a general purpose catalytic combustion module. thorough application-oriented investigations were carried out upon ditferent hydrogen catalytic combustion plates. In this paper. experimental setup, investigation procedures and study results are discussed in detail. (” 1997 International Association for Hydrogen Energy. All rights reserved
INTRODUCTION Previous investigations have shown that hydrogen catalytic combustion can satisfy the most stringent safety, pollution-free and combustion efficiency requirements [I-IO]. In addition. several investigations have been carried out to show the numerous possibilities of utilizing the catalytic combustion of hydrogen in different domestic uses. In this regard. Brewer investigated hot catalytic combustion plates designed for food heating and similar household purposes [2]. Mercea et al. investigated the possibility of heating rooms using hydrogen catalytic combustion [3]. They further investigated the performance of hydrogen catalytic combustors in the form of platinum-supported plates of different sizes. By defining the combustion efficiency as the ratio between reacted and inlet hydrogen flow rates, they showed that it could reach up to 100% especially at relatively low flow rates [4]. In a series of interesting papers. Haruta r/ (11.thoroughly investigated the catalytic combustion of hydrogen [5-S], and several inexpensive transition metal oxides as well as platinum-group metals were also investigated. They concluded that, in diffusive combustion. Pd-powder coated Ni foam with relatively large pores could offer the highest combustion efficiency. However. they showed that it suffered front considerable non-uniformity in surface temperature distribution. They also showed that premixing of air and hydrogen could significantly improve the combustion efficiency. Most of their investigations were carried out on vertically positioned plates. This caused a large non-uniformity in the reaction, and hence, in the temperature distribution both inside and at the plate surface. Ledjeff introduced and discussed the activity and features of catalytic hydrogen combustion
plates fabricated by using highly hydrophobic substrates coated with catalytically active materials like palladium [9]. The substrate, he pointed out, consisted of PTFE and active charcoal. He indicated that the hydrophobicity significantly helped in reducing the vapor pressure inside the catalyst substrate by avoiding capillary condensation. Ledjeff and Gieshoff designed a hydrogen catalytic combustion-based heating system for divers [IO]. They proposed a combustion-rate limiting arrangement so that temperature can not exceed a maximum safe limit of 250 C. In this study. application-oriented investigations were carried out upon several different samples of hydrogen catalytic combustion plates which are planned to be used for different domestic heating purposes. The influences and interrelations of the investigated physical parameters are presented. Drawbacks and features of different conditions and setups are introduced in detail. The study is carried out under the cover of the Saudi-German research and development program. HYSOLAR. This program was initiated in 1986 to produce hydro,gen using solar energy and investigate its utilization possibilities. EXPERIMENTAL
SETUP
Four different catalytic sample plates were selected for investigation at our laboratory. All sample plates shared the same physical dimensions of 5 x 5 x 1 cm. According to previous studies [5--S], it has been realized that vertical positioning of catalytic plates caused considerable temperature-distribution non-uniformity. Consequently, we preferred to discard this option and tix sample plates horizontally. In addition, it has been shown that putting
M. AL-GARNI
384 r
Catalytic Plate
Gas Dstribution Plate NOZZk
diffusion into its internal pores, the Johnson Matthey (JM) sample was drilled from all sides using a drill bit of 0.8 mm diameter. This was because it had a honeycomblike structure and, hence, had no interconnected pores as was the case with the other samples.
Flow Regulator
RESULTS
Hydrogen
-
Flow Meter
-
Pressure Regulator
Air
Fig. 1. Schematic of the test setup. a catalyst-free porous plate in front of the catalytic plate significantly improved the gas distribution [4, 6-81. Having these two points in mind and taking into account the hydrogen-air premixing option, the experimental setup schematically illustrated in Fig. 1 was built at our laboratory. It mainly consisted of hydrogen supply cylinders connected to the test rig through a pressure regulator and a flow meter. In addition, a compressed air supply line was also connected through a separate pressure regulator and flow meter. Both lines come to a mixing chamber at the base of the test rig as shown in the figure. The test rig itself comprises of the mixing chamber for fuel-air premixing, a nozzle, a compartment for the gas distribution porous plate and the fixture for the catalytic plate. The purpose of the nozzle was to act as a flame trap in the case of backfire. This happens frequently when hydrogen is premixed with air or oxygen and will be discussed later. Fast response thermocouple probes were fixed at different locations on the top surface and through the depth of the catalytic plate. In addition, a hand-held fast response surface thermocouple probe was frequently used for cross checking throughout the experimental work. A sensitive hand-held hydrogen detector was exposed to the combustion products in order to check for any unburned traces of hydrogen during the experiment. However, in nearly all of our experiments, no unburned hydrogen was detected. The investigated samples were procured from different suppliers. Their physical and catalytic specifications are shown in Table 1. All were cut from bigger samples in order to suit the test rig configuration. No chemical or basic physical modifications were introduced to the test samples. However, in order to improve the chances of air
AND DISCUSSION
Extensive experimentation was carried out with the aim of completely characterizing the sample plates. From the beginning, it was clear that premixing of hydrogen and air would have to be excluded for two reasons. Firstly, premixing allows an explosive mixture to form which can very easily ignite and cause a backfire. This occurred in our test on several occasions. Secondly, it was noticed that premixing caused the combustion to take place mainly at the bottom surface of the catalytic plate. This in turn caused the bottom surface to overheat at certain flow rates, and to have some spots on it which exceeded the hydrogen self-ignition temperature. Both phenomena caused the combustion to be unstable and unsafe under most conditions. Therefore, premixing was excluded as an option for practical domestic use. However, it was noticed that putting a nozzle before the hydrogen-air distribution plate acted as a flame trap, which increased the possibilities of eliminating the backfire phenomenon. After excluding the premixing option, external mixing was selected for the characterization. With this option, hydrogen first diffused into the catalytic-free distribution porous plate and then into the catalytic plate. By having the surrounding air at the top surface, combustion started at that surface and immediately its main core shifted to the inside of the plate where diffusion of air took place as well. In the beginning, the four sample plates selected for investigation were tested at a constant flow rate of about 600 l/h and a constant pressure of about 0.1 bar. This helped to narrow the investigation range further by excluding those samples that did not show a relatively high combustion efficiency, defined as the ratio between reacted and inlet hydrogen flow rates, and concentrating upon the remaining ones. This screening process resulted in the activity curves shown in Fig. 2. Temperature readings of all the surface thermocouples indicated a relatively good combustion uniformity, and hence, the average temperature value was considered. It was noticed that the top surface temperature of the JM plate was the lowest, and regardless of whether the temperature probe was fixed at the surface or inside the plate, it did not give
Table 1. Physical characteristics of tested samples Sample
Catalytic material
Substrate
EK 10 (Schumacher & Heraeus, Germany) EK 20 (Schumacher & Heraeus, Germany) UOP MM90F (UOP Limited, UK) JM (Johnson Matthey, USA)
0.5% Pd 0.5% Pd l-3% mixture of Pd and Pt Undisclosed by supplier
Ceramic, 30 nrn pore size Ceramic, 40 nm pore size Metallic foam, 950 pm pore size Honeycomb ceramic, 1 mm2 cell area
HYDROGEN Average.Temperature
CATALYTIC
( aC )
COMBUSTION Average ml
350)
300
0’
0
10
20
30
40
I 50
50
___
Time ( rmn )
Fig. 2. Catalytic
combustion
activity plates.
for four different
study
Temperature
Temperature
385
( aC )
-EK20
0
2
4 Distance
6 from sample face ( mm )
a
10
lower
Fig. 4. Temperature profile across the depth of the test sample.
more than 230°C. Because we were only interested in plates which operate in the range 350-7OO”C, this sample plate was excluded from any further investigation. Furthermore, because the samples EK 10 and EK 20 were nearly similar when compared with the other two samples, one of them (EK 10) was also excluded. With the remaining two samples, UOP and EK 20, A thorough characterization investigation was carried out. First, the temperature was measured at different depths through the plates. It is interesting to notice that combustion activity about 4 mm inside showed an opposite trend to that of the surface. This is probably because of the difference in the average pore diameter. For UOP, with its bigger bore diameter (average of 950 pm), it was much easier for air to diffuse into the plate and meet hydrogen very early so that the peak combustion activity took place much nearer the bottom surface. On the other hand, this was not the case for the EK 20 sample where the relatively narrow pores (40 pm) did not allow for a similar activity. These results are illustrated by Figs 3-5. In the test condition represented by Fig. 3, the temperature probe was fixed at a depth of about 4 mm inside the sample. This figure also shows that the combustion temperatures of Average
PLATES
metallic substrates tended to be relatively non-uniform, especially when external air flows existed. It was observed that with the same room conditions, when a windowtype air conditioning unit was running, the blown air flow affected the temperature uniformity of the UOP metallic-based substrate much more than the EK 20 ceramic-based substrate. Figure 3 represents the temperature profiles inside both samples. It is clear that, with its larger bore diameter, the UOP sample gave a higher combustion temperature near to the bottom surface, while the EK 20 gave a better activity in the middle and towards the top surface, and had a better temperature uniformity. In Fig. 5, it is clear that with larger bore size (UOP), the instantaneous temperature distribution was much more uniform. This confirmed the obvious fact that TernDenature
( “C 1
Temperature
( “C )
aoJ-----Tw.
’
( a C )
500,
0 0
-
UOP
-
EK20
Probe 0
10
20
30 Time
Fig. 3. Combustion
40 (min
4 mm 50
mslde so
70
)
activity at a depth of 4 mm inside the test samples.
Fig. 5. Temperature distribution at the plate surface; UOP above and EK 20 below.
386
M. AL-GAKNI
Average
Temperature
(‘C
)
Average 550 I
Temperature
( ’ C )
500
-EK
450
20
400 350 300 250 200 150 100 50 “” 01
02
03
Pressure
04
05
0
( bar )
0
5
10
15
Fig. 6. Influence of varying supply pressure on the activity.
Time
20
25
30
( min )
Fig. 8. Cooling rate profile of the test samples
air can mix with hydrogen at a depth inside the plate directly proportional to the pore size. In order to investigation the influence of varying hydrogen supply pressure, data was collected for the range O.llO.5 bar. Figure 6 shows the result of the pressure-varying test. It is clear that varying the pressure did not significantly affect the combustion activity. Indeed, it shows that temperatures remained almost constant along the selected range. Varying the hydrogen flow rate had some influence on temperature. Figure 7 represents flow rate vs temperature for the range 1000-3000 1/h at 0.1 bar. The figure shows that the UOP experienced a slow decrease in the surface temperature. Again, this could be because of its larger bore size and also because of its metallic substrate. When hydrogen blew in larger quantities, it prevented air from diffusing as easily as it did with lower flow rates, and hence, the combustion was shifted towards the upper surface where insufficient mixing took place within the plate. In addition, the hydrogen supply was at room temperature and this caused the plate to relatively cool down. With the other sample (EK 20) bore size and nature of substrate caused the opposite trend. With increased hydrogen supply and smaller pore size, air
diffused into the plate and more combustion activity took place. In practice, stability of operation could be one of the most important factors in the daily use of catalytic combustion heating plates. In case of intermittent discontinuities of the fuel supply, it would be favorable to have a better heat-storage capacity that could help to avoid severe fluctuations in performance. With this in view, a test was carried out to see the cooling-down behavior of the sample plates. As could be seen in Fig. 8, the ceramic-based substrate plate (EK 20) showed a slower cooling rate than the metallic-based one (UOP). For example, if the fuel discontinuity time period is 5 min, the figure shows that UOP would cool down to about 80°C while the EK 20 plate would not go below about 200°C. This gives, in general, an important advantage for the ceramic-based plates over the metallic-based ones. Finally, stability with time was also investigated for about 500 h. Figure 9 represents the performance stability of both the test plates at a constant pressure and flow rate of about 0.1 bar and 600 1/h, respectively and with Average Temperature
Average 400
Temperature
( “C )
...”.._. .,,._.
( aC ) 320
350 I,
0
300
280
0 300
260
---tEKZO
250
240
220
Probe at surface 200 1000
1500
2000
2500
0.1 bar; 600 I / h.
200
3000
Flow rate ( I / hr )
Fig. 7. Influence of varying supply flow rate on the activity.
100
200
300
400
500
Operational Time ( hr )
Fig. 9. Average surface temperature vs time
600
HYDROGEN
CATALYTIC
temperature probe fixed at a depth of about 4 mm from the bottom surface. In addition, the figure confirms the previous findings about the non-uniformity of the metallic-based plate (UOP).
COMBLJSTION PLATES
l
decrease and to shift towards the upper surface of the plate. With hydrogen, in the absence of products causing catalyst poisoning, degradation of the catalytic combustion activity was shown to be relatively unlikely for long operation cycles,
CONCLUSIONS With the investigations carried out in this study, one can summarize the conclusions with the following points. l
l
Catalytic combustion with ceramic-based porous substrates proved to be probably the most suitable option for stable heating purposes with hydrogen. They exhibited good temperature uniformity and satisfactorily resisted surrounding air fluctuations. Furthermore, their cooling-down rate was relatively slow. It seemed. however, that plates with larger average pore diameters (X0-120 pm) could give better combustion activity and a more uniform surface temperature distribution. In general, varying the fuel supply pressure proved to have a negligible effect upon combustion activity. However, varying the supply flow rate showed some influence. especially with metallic plates with larger pore sizes. With such plates. combustion activity tended to
3x7
REFERENCES I. J. Pangborn, M. Scott and J. Sharer, I/r/. J. Hj&qc,n &rr~/r 2,431 (1977).
4. J. Mercea, E. Grecu, T. Fo’dor and S. Kreibik. fnt. .I. H&oyew Ener-y?,6, 482 ( 19X2). 5. M. Haruta and H. Sane. Inr. J. Hj,tlro,qr,r .!+rc,~,q!~ 6. 601 (1981). 6. M. Haruta, Y. Souma and H. Sano. I/?/. J. //j.&oq(~r Enprq:yl. 9, 729 (1982). 7. M. Haruta and H. Sane. Int. J. H~/ro~,qenE/rer,qj,9. 737 (1982).
9. K. Ledjeff. 1f1/.J. Hrrlrcjqm 0re~qy 5. 361 (1987). 10. K. L.edjeff and J. dieshoff. Hydrogen heating systems for divers, Proccwiin,q.r o/‘the Irrto.r~utionul H\droqen C‘o~fi~t~c~c~
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