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Injection quenching and the high temperature water-splitting reactor
Solar Energy Vol. 35, No. 6, pp. 535-537, 1985 Printed in the U.S.A.
0038-092X/85 $3.00 + .00 © 1985 Pergamon Press Ltd.
INJECTION QUENCHING AND THE HIGH TEMPERATURE WATER-SPLITTING REACTOR J . W . W A R N E R a n d R . STEPHEN BERRY
Department of Chemistry and the James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (Received 16 January 1985; revision received 9 May 1985; accepted 13 May 1985)
Abstractmlt is shown that injection quenching may be an effective method for recovering hydrogen gas generated by the direct, uncatalyzed thermal splitting of water. The method is simple and efficient enough to offer encouragement for further study of direct splitting as a realistic possibility for the generation of H2 from water. Several research groups have built small scale solar reactors for hydrogen production by direct high temperature water splitting. J. Ledr, et al.,[1] quenched the hot effluent with streams of argon gas and steam and verified hydrogen production with gas chromatography. R. B. Diver, et al.,[2] verified hydrogen production by igniting the effluent. Another reactor was that of Bilgen[3], who employed mass spectrometry to show the presence of hydrogen in the effluent. The work of these groups demonstrates that a solar water-splitting reactor, operating in the range 2000 K to 2500 K, can be built with currently available materials. The largest barrier to a practical system is the problem of separating the hydrogen. Bilgen[4] has shown that high temperature separation would be efficient, but no presently known means are available for such a process. Ihara[5] and Jellinek[6] have discussed the difficulty of membrane separation; membranes decompose rapidly at high temperature in the presence of water. Another approach is to cool the effluent so rapidly that the back reaction is stopped, and then separate the hydrogen at low temperature, with known materials. Led~[l] employed a cooling rate of 106 to 107 degrees per second, and recovered hydrogen from the mixture. At 2500 K, hydrogen comprises about 4% (mole fraction) of the reaction mixture. If the effluent is quenched most of its heat is lost. Since the degree of splitting is small, the heat loss by quenching per unit of hydrogen recovered is large, and the process efficiency is necessarily low. We have investigated models for a number of quenching strategies and report here our findings, with emphasis on injection quenching, which seems to be the most favorable. A more detailed report, including analysis of gas quenching, will be published elsewhere[7]. Besides the heat loss just mentioned, there are two other major losses involved with quenching. First, the work of separation may be large. To bring the effluent from 2500 K to 500 K, a large quantity of coolant is required, on the order of 500 moles of
coolant per mole of hydrogen. The separative work, depending on the device, can be as large as 50 to 80 kJ/moleH2. A second major loss is the work required for handling and recycling the coolant. In order to obtain the high cooling rate, the coolant must be mixed rapidly with the reaction effluent. This in turn requires that the coolant be pressurized and the work consumption, again strongly dependent on the device, can be as large as 1.7 × 103 kJ/ moleH:. It is important to distinguish the heat loss during quenching from the two sources of work consumption. In most models, the reactor is heated with solar energy, which can be considered almost free. On the other hand, pressurizing the coolant and performing the separation would most likely be done with electric pumps and compressors. If the work consumption exceeds the 247 kJ/moleH2 enthalpy of combustion of hydrogen, the device will not break even in the sense of requiring more energy in nonsolar form that it yields. Gas quenching may be achieved without large work consumption, with, for example, an unrecycled steam coolant. The additional heat needed to produce the steam, though, again greatly lowers the efficiency. Injection quenching is free from these limitations. In this process, the effluent is injected directly into the base of a column of water and the sparingly soluble hydrogen and oxygen bubble out at the top. Most of the water vapor in each bubble condenses, and only the hydrogen and oxygen need to be separated, with a work requirement of about 3 kJ/moleH2. The work of handling the coolant consists of lifting water into the column, is less than 1 kJ/moleH2. These values assume that the work is delivered at 50% efficiency. We have investigated injection quenching with a coupled kinetic, heat and mass transfer model for bubble collapse, to be described in a future report[7]. Heat transfer coefficients for bubbles are enormous[8], due to the absence of a resistant thermal boundary layer for the new, clean surface, and we expected to find nearly complete hydrogen re-
535
536
J. W. WARNER and R. S. BERRY
Table 1. Percentage of hydrogen recovery and cooling times for the injection quenched directsplitting water reactor, as a function of bubble radius at injection. The recovery percentage is the amount of hydrogen in the cooled bubble divided by the amount on injection, x 100. A(OH) is the accomodation coefficient for OH depletion at the bubble wall Initial Bubble Radius (cm)
Hydrogen Recovery P~rcentage w i t h A(0H)=l with A ( O H ) = O
Cooling Time 2500 to 370K
(~ec)
-7 0.01
97
82
6.0"10
0.05
81
68
2.3"10
0.i
76
66
4.2"10
0.5
76
73
1.2"10
1.0
77
76
2.8"10
3.0
78
78
8.5"10
5.0
76
75
1.3"10
-6 -6 -5 -5 -5 -4
covery, even for large bubbles. We were surprised, however, to uncover an anomaly in the kinetics, making the rates of some hydrogen depletion actually increase during cooling. We develop the explanation for this as follows. The rate of a hydrogen depletion half-reaction, for bimolecular collisions may be written R = Ae°/Tclc2
(1)
where 0 = E a / R is the activation temperature, and Cl and C2 are the concentrations of the two species involved. In a constant pressure model, the volume of the bubble is directly proportional to the temperature so that c, and c2 vary inversely with T. Then the temperature dependence of the rate is given by k e - OlT
R -
Tz
(2)
where k incorporates all numbers that do not depend on T. Equation (2) has a maximum at T = 0/ 2. If the activation temperature is large, the rate decreases during cooling. If however the activation temperature is less than 2T, the rate increases during cooling. It is the free radical reactions that have low activation temperatures and increase in rate during injection quenching. The principal hydrogen depletion reaction is H2 + OH--> H20 + H
(3)
Reaction (3) has an activation temperature of 2600 K, so the rate does not begin to decrease until the temperature is below 1300 K. At 700 K, the temperature factor of eqn (2) is still greater than at 2500 K. The equilibrium ratio of OH to Hz is 0.56 at 2500 K, so that if all the atomic hydrogen produced by reation (3) goes to molecular hydrogen, about 28% of the H2 will be lost. Additionally, the equilibrium ratio of H to H2 at 2500 K is 0.12, which can supply another 6% molecular hydrogen. Considering only reaction (3) and recombination of atomic hydrogen, one expects to lose about 22% of the molecular hydrogen during injection quenching. The results of simulations of the bubble collapse are given in Table 1. The quantity A(OH) is the accommodation coefficient for OH. When A(OH) = 1, all OH that strikes the bubble wall is depleted, and removed from the bubble. When A(OH) is zero, OH rebounds from the bubble wall. It is not possible to calculate A(OH), so we carried out the calculations for both extremes. It is seen that, usually, about 24% of the hydrogen is lost, so that the explanation given above accounts for most of the depletion. In order to verify our picture, we repeated the calculations with a fixed bubble volume, and in all cases found nearly complete hydrogen recovery. The minimum recovery ratio, for bubble radius about 0.1 cm, seems to be due to recombination of atomic hydrogen with atomic oxygen, to produce more OH, and increase the degree of depletion of H2. In larger bubbles, the atomic oxygen usually combines to give molecular oxygen. To heat the water from room temperature to 2500
Injection quenching and the high temperature water-splitting reactor K, and react, 3.82 x 103 kJ/moleH2 is required. Of this, about 134 kJ/moleH2 can be recovered from the coolant, assuming it is raised to 80°C by the quenching. With 76% hydrogen recovery and an enthalpy of combustion of hydrogen of 247 kJ/ moleH2, the efficiency will be about 5%. Additionally, Bilgen[4] has suggested a 30% loss in the solar collectors and reactor, so that the overall efficiency will be about 3.5%. Although this is an encouraging value for a fuel-producing solar device, a more appealing feature is the technological simplicity of the injection quenching process. Acknowledgements--This work was supported by Contract No. 5083-260-0834 with the Gas Research Institute, Chicago, Illinois. REFERENCES
1. J. Led6, F. Lapieque and J. Villermaux, Production of hydrogen by direct thermal decomposition of water. Int. J. Hyd. Energy 8, 675 (1983).
537
2. Richard B. Diver, Stephen Pederson, Todd Kappauf and Edward A. Fletcher, Hydrogen and oxygen from water-VI. Quenching the effluent from a solar furnace. Energy 8, 947 (1983). 3. E. Bilgen, M. Ducarrioer, M. Foex, F. Sibleude and F. Trombe, use of solar energy for direct and two-step water decomposition cycles. Int. J. Hyd. Energy 2, 251 (1977). 4. E. Bilgen, Solar hydrogen production by direct water decomposition process: a preliminary engineering assessment. Int, J. Hyd. Energy 9, 53 (1984). 5. S. Ihara, Int. J. Hyd. Energy 3, 287 (1978) and Direct thermal decomposition of water. In Solar-Hydrogen Systems (T. Ohta, Ed.), Chapter 4. Pergamon Press, Oxford (1979). 6. H. H. G. Jellinek and H. Kachi, The catalytic thermal decomposition of water and the production of hydrogen. Int. J. Hyd. Energy 9, 667 (1984). 7. J. W. Warner and R. Stephen Berry, The hydrogen separation problem in the direct high-temperature splitting of water. Int. J. Hyd. Energy (in press). 8. Theodore T. Robin and Nathan W. Snyder, Theoretical analysis of bubble dynamics for an artificiallyproduced vapor bubble in a turbulent stream. Int. J. Heat and Mass Transfer 13, 523 (1970).
0038-092X/85 $3.00 + .00 © 1985 Pergamon Press Ltd.
INJECTION QUENCHING AND THE HIGH TEMPERATURE WATER-SPLITTING REACTOR J . W . W A R N E R a n d R . STEPHEN BERRY
Department of Chemistry and the James Franck Institute, The University of Chicago, Chicago, Illinois 60637 (Received 16 January 1985; revision received 9 May 1985; accepted 13 May 1985)
Abstractmlt is shown that injection quenching may be an effective method for recovering hydrogen gas generated by the direct, uncatalyzed thermal splitting of water. The method is simple and efficient enough to offer encouragement for further study of direct splitting as a realistic possibility for the generation of H2 from water. Several research groups have built small scale solar reactors for hydrogen production by direct high temperature water splitting. J. Ledr, et al.,[1] quenched the hot effluent with streams of argon gas and steam and verified hydrogen production with gas chromatography. R. B. Diver, et al.,[2] verified hydrogen production by igniting the effluent. Another reactor was that of Bilgen[3], who employed mass spectrometry to show the presence of hydrogen in the effluent. The work of these groups demonstrates that a solar water-splitting reactor, operating in the range 2000 K to 2500 K, can be built with currently available materials. The largest barrier to a practical system is the problem of separating the hydrogen. Bilgen[4] has shown that high temperature separation would be efficient, but no presently known means are available for such a process. Ihara[5] and Jellinek[6] have discussed the difficulty of membrane separation; membranes decompose rapidly at high temperature in the presence of water. Another approach is to cool the effluent so rapidly that the back reaction is stopped, and then separate the hydrogen at low temperature, with known materials. Led~[l] employed a cooling rate of 106 to 107 degrees per second, and recovered hydrogen from the mixture. At 2500 K, hydrogen comprises about 4% (mole fraction) of the reaction mixture. If the effluent is quenched most of its heat is lost. Since the degree of splitting is small, the heat loss by quenching per unit of hydrogen recovered is large, and the process efficiency is necessarily low. We have investigated models for a number of quenching strategies and report here our findings, with emphasis on injection quenching, which seems to be the most favorable. A more detailed report, including analysis of gas quenching, will be published elsewhere[7]. Besides the heat loss just mentioned, there are two other major losses involved with quenching. First, the work of separation may be large. To bring the effluent from 2500 K to 500 K, a large quantity of coolant is required, on the order of 500 moles of
coolant per mole of hydrogen. The separative work, depending on the device, can be as large as 50 to 80 kJ/moleH2. A second major loss is the work required for handling and recycling the coolant. In order to obtain the high cooling rate, the coolant must be mixed rapidly with the reaction effluent. This in turn requires that the coolant be pressurized and the work consumption, again strongly dependent on the device, can be as large as 1.7 × 103 kJ/ moleH:. It is important to distinguish the heat loss during quenching from the two sources of work consumption. In most models, the reactor is heated with solar energy, which can be considered almost free. On the other hand, pressurizing the coolant and performing the separation would most likely be done with electric pumps and compressors. If the work consumption exceeds the 247 kJ/moleH2 enthalpy of combustion of hydrogen, the device will not break even in the sense of requiring more energy in nonsolar form that it yields. Gas quenching may be achieved without large work consumption, with, for example, an unrecycled steam coolant. The additional heat needed to produce the steam, though, again greatly lowers the efficiency. Injection quenching is free from these limitations. In this process, the effluent is injected directly into the base of a column of water and the sparingly soluble hydrogen and oxygen bubble out at the top. Most of the water vapor in each bubble condenses, and only the hydrogen and oxygen need to be separated, with a work requirement of about 3 kJ/moleH2. The work of handling the coolant consists of lifting water into the column, is less than 1 kJ/moleH2. These values assume that the work is delivered at 50% efficiency. We have investigated injection quenching with a coupled kinetic, heat and mass transfer model for bubble collapse, to be described in a future report[7]. Heat transfer coefficients for bubbles are enormous[8], due to the absence of a resistant thermal boundary layer for the new, clean surface, and we expected to find nearly complete hydrogen re-
535
536
J. W. WARNER and R. S. BERRY
Table 1. Percentage of hydrogen recovery and cooling times for the injection quenched directsplitting water reactor, as a function of bubble radius at injection. The recovery percentage is the amount of hydrogen in the cooled bubble divided by the amount on injection, x 100. A(OH) is the accomodation coefficient for OH depletion at the bubble wall Initial Bubble Radius (cm)
Hydrogen Recovery P~rcentage w i t h A(0H)=l with A ( O H ) = O
Cooling Time 2500 to 370K
(~ec)
-7 0.01
97
82
6.0"10
0.05
81
68
2.3"10
0.i
76
66
4.2"10
0.5
76
73
1.2"10
1.0
77
76
2.8"10
3.0
78
78
8.5"10
5.0
76
75
1.3"10
-6 -6 -5 -5 -5 -4
covery, even for large bubbles. We were surprised, however, to uncover an anomaly in the kinetics, making the rates of some hydrogen depletion actually increase during cooling. We develop the explanation for this as follows. The rate of a hydrogen depletion half-reaction, for bimolecular collisions may be written R = Ae°/Tclc2
(1)
where 0 = E a / R is the activation temperature, and Cl and C2 are the concentrations of the two species involved. In a constant pressure model, the volume of the bubble is directly proportional to the temperature so that c, and c2 vary inversely with T. Then the temperature dependence of the rate is given by k e - OlT
R -
Tz
(2)
where k incorporates all numbers that do not depend on T. Equation (2) has a maximum at T = 0/ 2. If the activation temperature is large, the rate decreases during cooling. If however the activation temperature is less than 2T, the rate increases during cooling. It is the free radical reactions that have low activation temperatures and increase in rate during injection quenching. The principal hydrogen depletion reaction is H2 + OH--> H20 + H
(3)
Reaction (3) has an activation temperature of 2600 K, so the rate does not begin to decrease until the temperature is below 1300 K. At 700 K, the temperature factor of eqn (2) is still greater than at 2500 K. The equilibrium ratio of OH to Hz is 0.56 at 2500 K, so that if all the atomic hydrogen produced by reation (3) goes to molecular hydrogen, about 28% of the H2 will be lost. Additionally, the equilibrium ratio of H to H2 at 2500 K is 0.12, which can supply another 6% molecular hydrogen. Considering only reaction (3) and recombination of atomic hydrogen, one expects to lose about 22% of the molecular hydrogen during injection quenching. The results of simulations of the bubble collapse are given in Table 1. The quantity A(OH) is the accommodation coefficient for OH. When A(OH) = 1, all OH that strikes the bubble wall is depleted, and removed from the bubble. When A(OH) is zero, OH rebounds from the bubble wall. It is not possible to calculate A(OH), so we carried out the calculations for both extremes. It is seen that, usually, about 24% of the hydrogen is lost, so that the explanation given above accounts for most of the depletion. In order to verify our picture, we repeated the calculations with a fixed bubble volume, and in all cases found nearly complete hydrogen recovery. The minimum recovery ratio, for bubble radius about 0.1 cm, seems to be due to recombination of atomic hydrogen with atomic oxygen, to produce more OH, and increase the degree of depletion of H2. In larger bubbles, the atomic oxygen usually combines to give molecular oxygen. To heat the water from room temperature to 2500
Injection quenching and the high temperature water-splitting reactor K, and react, 3.82 x 103 kJ/moleH2 is required. Of this, about 134 kJ/moleH2 can be recovered from the coolant, assuming it is raised to 80°C by the quenching. With 76% hydrogen recovery and an enthalpy of combustion of hydrogen of 247 kJ/ moleH2, the efficiency will be about 5%. Additionally, Bilgen[4] has suggested a 30% loss in the solar collectors and reactor, so that the overall efficiency will be about 3.5%. Although this is an encouraging value for a fuel-producing solar device, a more appealing feature is the technological simplicity of the injection quenching process. Acknowledgements--This work was supported by Contract No. 5083-260-0834 with the Gas Research Institute, Chicago, Illinois. REFERENCES
1. J. Led6, F. Lapieque and J. Villermaux, Production of hydrogen by direct thermal decomposition of water. Int. J. Hyd. Energy 8, 675 (1983).
537
2. Richard B. Diver, Stephen Pederson, Todd Kappauf and Edward A. Fletcher, Hydrogen and oxygen from water-VI. Quenching the effluent from a solar furnace. Energy 8, 947 (1983). 3. E. Bilgen, M. Ducarrioer, M. Foex, F. Sibleude and F. Trombe, use of solar energy for direct and two-step water decomposition cycles. Int. J. Hyd. Energy 2, 251 (1977). 4. E. Bilgen, Solar hydrogen production by direct water decomposition process: a preliminary engineering assessment. Int, J. Hyd. Energy 9, 53 (1984). 5. S. Ihara, Int. J. Hyd. Energy 3, 287 (1978) and Direct thermal decomposition of water. In Solar-Hydrogen Systems (T. Ohta, Ed.), Chapter 4. Pergamon Press, Oxford (1979). 6. H. H. G. Jellinek and H. Kachi, The catalytic thermal decomposition of water and the production of hydrogen. Int. J. Hyd. Energy 9, 667 (1984). 7. J. W. Warner and R. Stephen Berry, The hydrogen separation problem in the direct high-temperature splitting of water. Int. J. Hyd. Energy (in press). 8. Theodore T. Robin and Nathan W. Snyder, Theoretical analysis of bubble dynamics for an artificiallyproduced vapor bubble in a turbulent stream. Int. J. Heat and Mass Transfer 13, 523 (1970).