- Email: [email protected]
A plasmochemical concept for thermochemical hydrogen production
Int. J. Hydrooen Eneroy Vol. 5, pp 1-6 Pergamon Press Ltd 1980. Printed in Great Britain International Association for Hydrogen Energy
0360-3199/80/0201 1~)01 $02.00/0
A P L A S M O C H E M I C A L CONCEPT FOR T H E R M O C H E M I C A L HYDROGEN PRODUCTION I. G. BELOUSOV,V. A. LEGASOVand V. D. RUSANOV I.V. Kurchatov Institute of Atomic Energy, Moscow, U.S.S.R.
(Receivedfor publication 19 September 1979) Abstract--This paper discusses plasmochemical methods of water decomposition to produce hydrogen. Both the direct decomposition of water and a cycle involving the decomposition of carbon dioxide are discussed. A comparison of the relative merits of several methods of hydrogen production using a nuclear energy source is presented.
INTRODUCTION MANY methods for the conversion of atomic energy into portable energy forms convenient for mass production are known [1-4]. These include thermochemical, electrochemical, plasmochemical and radiolysis, all of which are technically adapatable to large scale production. Each of these methods occupies a place in the overall energy conversion scheme. The problem lies in developing the optimum strategy for industrial production and mass distribution. The solution cannot be single-valued either globally or within a particular country since it is dependent upon the regional industrial potential and natural conditions. It is therefore useless to seek a single global technology. Our aim is to consider some of the problems concerning hydrogen production from water on the basis of a nuclear energy source. The peculiarity of the scheme suggested is its technical readiness for commercial production at a lower capital expenditure compared with electrolysis and, perhaps, thermolysis. In this case it is important to realize that the thermodynamic limit of efficiency for complete cycles of the thermochemical, electrolysis and plasmochemical methods of nuclear fuel conversion into the chemical energy of hydrogen fuel is identical and equal to the Carnot-Funk limit, since the maximum temperature of the primary heat source and the minimum temperature for heat rejection are alike in all three cases. The theorem of equivalence [3] is directly applicable to cycles of these three methods. The radiolysis method for the direct conversion of the kinetic energy of nuclear fuel fission fragments into the chemical energy of substances is also restricted by the Carnot-Funk limit. The practical limit of its possible efficiency depends upon molecular peculiarities of the substance subjected to radiolysis and there is a lot of research work to do before the required clarity will be established.
THEORY The essence of the plasmochemical method considered consists in consecutively carrying out two reactions: 2 C O 2 - ' 2 C O * - ' 2 C O + 02, AHef = 67.6 kcal/mol,
(1)
2CO + 2H20(g)-*2CO 2 + 2H2,AH = - 9 . 6 k c a l / m o l .
(2)
The first reaction, non-equilibrium plasmochemical destruction, is performed in a gas-discharge reactor chamber to provide the electric energy of the discharge in an atmosphere of carbon dioxide. The second reaction involves the standard "shift" process already mastered by the chemical industry. The experiment shows that the energy efficiency for the plasmochemical process of carbon dioxide destruction defined by the ratio of effective enthalpy of reaction (1) to actual expenditure of electric energy, reaches 80 ~. The heat from the exothermic reaction (2) can be used for steam production or to supply energy for reaction (1).
I. G. BELOUSOV, V. A. LEGASOV AND V. D. RUSANOV H20(g), AH = - 58.0 kcal/mol (3)
H z + ~-O-.j2 H20(I), AH = - 68.3 kcal/mol.
It is easy to see that expenditure of electric energy in reaction (1) at an efficiency of 8 0 ~ is equivalent to water electrolysis at a voltage of 1.83 V. In other words, the energy efficiency of the plasmochemical reaction is somewhat higher than for good modem electrolyzers if work of separation is ignored. Thus, the question of implementation of the plasmochemical technology for large scale hydrogen production using a nuclear primary heat source resolves itself into a purely technical estimation of profitability and practical accessibility on the necessary scale. Comparative estimates indicate that large scale hydrogen production by electrolysis will require five to seven times the capital expenditure needed for plasmochemical. This is connected with the simplicity of the plasmochemical production flowsheet, a high intensity of carbon dioxide destruction in the electric discharge and the rapid rate of the chemical shift reaction with a suitable catalyst. The kinetic features of the plasmochemical cycle makes it especially attractive for large scale production. Stability of the plasmogenerator operation over a wide range of capacities provides a flexibility in the process control under conditions of electric load irregularities that, in its turn, offers an interesting scope for a wide use of cheap electric energy generated by atomic power plants during daily and seasonal variation of general power consumption. Production of carbon monoxide in the first stage of the plasmochemical process makes it easy to combine plasmochemical technology and a broad spectrum of chemical and metallurgical productions. This is the qualitative feature of the plasmochemical technology which, generally speaking, the standard electrolysis or thermochemical technology does not possess. So it is clear that plasmochemical technology is multipurpose both in respect to the quality and quantity ofelectric energy utilized and in the spectrum of the reduced gases produced. An estimate of the relative merits of thermochemical and plasmochemical cycles can be obtained by comparing their energy efficiencies and capital cost requirements. In a common case, as had already been mentioned, the limiting efficiencies of the thermochemical and plasmochemical cycles under similar temperature conditions are the same. The technically attainable efficiencies of both cycles prove also to be close. Indeed, bench data on the hybrid sulfur cycle give an estimate of efficiency of 48 %. Here the maximum cycle temperature is 900°C and the minimum is 50°C. The plasmochemical technology of the dioxide-carbon cycle with the cited interval of temperatures also has an efficiency of 48 %. This results, for instance, from the simple combination of triplet power installation (potassium, diphenyl, water) [-12] with extreme temperatures of the cycle of 900°C and 50°C, the total efficiency of 60 % and the efficiency of the plasmochemical chain being 80 %. The possibility of practical realization of the triplet installation with a 60 % efficiency does not raise doubts. The question is in the economic expediency of its construction for reasons of complexity of the thermophysical scheme and the manufacture of the potassium and oliphenyl turbines has not yet been mastered by industry. The hybrid sulfur cycle technology for hydrogen production from water is also complicated and includes many different components. These include high temperature reactors and heat exchangers operating in a corrosive atmosphere and the electrolysis installation for sulfurous gas oxidation into sulfuric acid with expensive electrodes at a low capacity. The electroenergetic part has to be included in the complete hybrid sulfur cycle technology as well as in the plasmochemical one. Estimates of capital construction costs of large scale hydrogen production from water on the basis of primary nuclear energy sources for the hybrid sulfur cycle and plasmochemical technology have not yet been made and a quantitative comparison of their merits is difficult. However, it is thought that the capital cost of the electrolysis component alone for the hybrid sulfur cycle will be about the same as the common electrolysis component for water electrolysis since the processes in both cases are performed with approximately identical current densities. Hence, the hybrid sulfur cycle technology of hydrogen production from water will not, generally speaking, be competitive with the plasmochemical technology. One can draw analogous conclusions considering the processes of plasmochemical destruction of water and carbon dioxide at the molecular level. Let's analyze in more detail the plasmochemical method of hydrogen production from water. The direct reduction reaction in a low temperature plasma can be accomplished in two ways; first, by thermal decomposition of steam at temperatures over 3000 K; second, it can occur at low
A PLASMOCHEMICAL CONCEPT FOR THERMOCHEMICAL HYDROGEN PRODUCTION
3
gas temperature by the thermally non-equilibrium interactions of plasma electrons with molecules of water. In the first case, the simplest event for optimal temperatures requires a superfast hardening that is not technically realized; within the low temperature range, the equilibrium degree of decomposition is small and the efficiency of the process is very low. Therefore, the thermal destruction of water molecules is not considered and the gas temperature is held to be low. In such a thermally nonequilibrium medium, the main active agent is the electron gas whose temperature, T~, is 10-100 times higher than that of the neutral particles, To. For such a formulation of the problem, an analysis of the elementary act of electron interaction with molecular water becomes the main question. It is known that the principal processes of nonelastic interaction at an electron energy of 1-7 eV are the vibrational excitation and the dissipative sticking [8]; as for losses due to rotational excitation of molecules, their level does not exceed Joule's ones and therefore will not be considered to be significant. The excitation of the electron structure of a water molecule has a threshold of 7 eV and is non-effective below this energy level. The vibrational excitation is characterized by the cross section 10-17 cm 2 and has a maximum within the energy range 1-3 eV ['8]. When the electron energy increases to 6 eV, the process of dissociative attachment arises according to the scheme ['8]. /,
H- +OH
e + H20
(4) O-+H
2.
With the cross section maximum of 10-17 cm 2, the major reaction is the first one, in which H ions are generated. The relatively small energy of electron affinity to atomic hydrogen (EA(h) = 0.75 eV) permits the reverse process of H - ionization by electron impact to be effective: H - + e--*H + 2e,
(5)
which is characterized in the maximum by the cross section value a ~ 10-14 cm 2 ( k d = 10-6 cm 3 sec- 1). In this manner there appears the possibility of multiple electron utilization in reactions (4) and (5), if the velocity of chain termination due to recombination of negative ions H - and by the ion-molecular reaction (Ki,) H - + H20--*H 2 + O H -
(6)
does not exceed the velocity of process (5). More precisely: no ~
kd"
(7)
from which at ka ~ 10 -6, kin ~ 10 -9 we have the hard limitation on the ionization degree ne/[-H20 ] > 10 -3.
Similarly, at lower electron energies when the dissociative attachment does not take place, their energies are transferred to oscillating degrees of freedom of the water molecules, according to the equation e + H20--~HzO* + e,
(9)
which in its turn can lead to the elementary act of dissociation H20* + H20*--*-H + O H + H20.
(10)
In this case the chain character of the process is also possible if the reactions are assumed to be H + H20*--~H 2 + OH
(11)
OH + H20*---~ H 2 0 z + H.
(12)
4
I.G. BELOUSOV, V. A. LEGASOV AND V. D. RUSANOV
The chain termination in this case has to be mostly connected with the triple act H + OH + H20--~2H20.
(13)
An indispensable condition of realization of such a process is the obvious requirement that reaction (10) take place with more probability than VT-relaxations relieving the vibrational excitation of water molecules attained through the primary reaction (9) and further that the Boltzman distribution is established over vibrational levels due to V V-relaxation, from which one can obtain the inequality: ne key. [ H 2 0 ] ~> kvT'
(14)
where key is the velocity constant of process (9), and k v r is the velocity constant of oscillating-translatory relaxation. Because of specific features of the water molecule, the value of k~r is anomalously high and attains a value of 10- t 2 cm a sec- 1 [9]. This makes the condition (14) rigid since key for water does not usually exceed 10 -9 cm a sec- 1 Consequently, both condition (14) and the limitation (8) formulate essentially a single requirement for a high degree of ionization in a discharge independent of the exact nature of the physical process of destruction. The boundary value, T, where the high temperature process of dissocitaive attachment has to convert into the low temperature oscillating excitation, is given by the expression oln - I
Fk~'~ ~ 1
05)
and equals 1.7 eV. The value ea ~ 6 eV corresponds to the minimum electron attachment cross section; k~"x ~ 10- 9 cm 3 sec- 1 is the value of the corresponding rate constant; hto ~ 0.2 eV is the value of an oscillating quantum of a water molecule. Therefore, under typical conditions of non-equilibrium discharge (T~ ~ 1 eV, To ~ 600 K), the mechanism of oscillating excitation prevails. In the case of hotter electrons (T~ ~ 3 eV) dissociative attachment becomes the main channel of electron energy losses. As a whole, the efficiency value of the process related, mainly due to losses to the VT-relaxation and finite length of chemical chain, amounts to 60~o assuming that [ H 2 0 ] ~ 3 x 10 is cm -3, T e ~ 1 eV, and the energy contribution in the discharge assigned to one molecule is 1.2 eV. A somewhat higher value, ~/~ 70 ~/~ is attained in the high temperature region (Te ~ 3 eV) according to the calculation. In both cases the capacity amounts to about 20 ~ at the given parameters. The details of the efficiency calculation are given in [6]. Difficulties associated with realization of the discharge with the high electron concentration ( n J n o ) m i . ~ l0 -4 to 10 -3 and low neutral particle temperature predetermine a search for alternative plasmochemical systems where these contradictions are removed. The two stage cycle can become such a scheme, in which the high temperature stage is replaced by the non-equilibrium plasmochemical cycle and the low temperature one is carried out under equilibrium conditions in the traditional chemical manner. In principle, it is possible to arrange the second stage to also be plasmochemical using an anomalously high rate constant for the process CO + e ~ C O * + e reaching 10- 7 cm a sec- 1. The most promising system is the scheme consisting of the cycle of CO 2 decomposition according to the expression CO*--*CO + ~O 1 2,
(16)
with subsequent conversion of the CO into hydrogen in reaction (2). According to (9) the first stage of the cycle (16) is connected with the vibrational excitation of molecular carbon dioxide by electron impact e + CO2--~CO ~' + e,
(17)
that in the case of CO 2 is essentially facilitated by the very high efficiency of this process (k~o(CO2) ~ 3 × 10 - s cm a sec-1). Together with this, the constant of VT-relaxation for CO 2 molecules is significantly less than that of water k~r(CO2) ~ 10-15 cm sec-x which reduces the permissible degree of ionization nJ[CO2] ~ 10 -7 and facilitates sharply an organization of the chemically active discharge in CO 2.
A PLASMOCHEMICAL CONCEPT FOR THERMOCHEMICAL HYDROGEN PRODUCTION
5
I00
x
60 X 40
20 0
,
I
j
I
2
~
4
I
I
6
8
eV E v [~"ot]
Fro. 1. Dependence of efficiency (1) and conversion coefficient (2) at CO 2 dissociation in thermally non-equilibrium plasma. - - E s t i m a t e d curve; x experimental data with UHF-discharge: Q experimental data with HF-discharge in a magnetic field. The interaction of oscillatorily excited C O 2 molecules has to result in a non-equilibrium disproportionation by two alternative channels [-10]:
1.
CO* + CO*--~CO20 + CO
(18)
CO~ Jr- CO20"-~CO -~- CO 2 + 02 2.
CO~ + COW--CO + O + CO 2
(19)
0 -~- CO~-~CO --F 02 which have similar energy characteristics. The calculated efficiency in this case is between 70 and 80 % which is equivalent to 3.7 to 3.8 eV m o l - 1 decomposed. However, the cycle requires operations for the
IOOO 9oo 8oo
700
.£ a."
I
600 500 400 300
200 I00
I
600
I
700
800
900 I000 T, K
I100
1200
FIG. 2. Conditions of product stability relative to reverse reactions. Solid curve--pkTk for the system H2-O 2. I. l = 10%; II. Z = 1%; III. Z < 1%. Dotted curve pkTk for the system
CO-O 2.
I. G. BELOUSOV, V. A. LEGASOV AND V. D. RUSANOV separation of CO from 0 2 and for the CO conversion into n 2 in the reaction with water. The destruction process of CO 2 molecules in the thermally non-equilibrium discharge was experimentally investigated in H F and U H F versions within a pressure range of 1-500 torr [7]. The experiments with U H F discharge were carried out at a frequency of 2300 M G and with H F discharge at 21 MG. In both cases the power input into the discharge did not exceed 1.5 kW. The gas flow rate was varied between 5 x 104 and 0.71-atm sec- 1. The degree of discharge non-isothermality was Te/To ~ 15-100, and the value of n J n o ~ 5 x l 0 -7 to l0 -a. The calculated results of the efficiency, r/, and degree of decomposition, ;C, are shown on the plot in Fig. 1. It is seen that the maximum value of the efficiency is 80 % while the degree of conversion exceeds 85 % but at a lower efficiency. In the thermally non-equilibrium discharge, highly effective CO 2 destruction is indeed possible. However, the total efficiency of the process as a whole, ~/~, according to estimates, reduces to approximately 65-75 % which is close to the calculated efficiency of the direct destruction of water cycle. It should be noted that the direct destruction of water, as is noted above, is connected with difficulties in sustaining the low temperature regime for discharge at a high degree of gas ionization since the excess of pkTk critical parameters can stimulate a chain reaction of the gas mixture. Regions of permissible concentrations and temperatures are shown in Fig. 2 by a solid curve. The bottom branch of the curve corresponds to a chain mechanism of explosion and the upper branch corresponds to the "thermal" character of burning. The dotted curve on the same coordinates restricts the chain mechanism region for explosion in the mixture C O - O 2. As is seen from the figure, requirements for parameters P and T are essentially mild here. The thermal mechanism of reaction in the pure mixture is hindered and the burning can be accomplished in a catalytic m a n n e r only in the presence of water and hydrogen at a low rate. Thus, it has been shown that the electric means of water decomposition give similar values of energy output and the main difference lies in the technological design of the process. NOMENCLATURE kvv kvr
V-V vibrational relaxation rate constant V-T vibrational relaxation rate constant
ki. ne no
ion-molecular reaction (6) rate constant electron concentration neutral gas concentration
key k~m~
vibrational excitation rate constant dissociative attachment rate constant.
REFERENCES 1. First World Hydroyen Energy Conf., 1-3 March 1976, Miami Beach, Florida, U.S.A. 2. Proc. Hydrogen Economy, Miami Energy Conference (THEME). 3. I. V. KURTSCHATOVA,Collection of "Questions Related to Atomic Science and Technique", Hydrogen Energy Series, No. l, 1(2) 0976, 1977). 4. G. DE BENI, Trans. Am. nucl. Soc. 20, 719 (1975). 5. I.G. BELOUSOV,V. A. LEGASOVand V. D. RUSANOVIn Voprosy atomnoi nauki i tecniki, Set. Atomno-vodorodnaia energetica 2, (2), Moscow, U.S.S.R. 0977). 6. V. P. BECnIN,V. A. LEGASOV,V. D. RUSANOV, A. A. FRIDMANand G. V. SHOLIN,ibid. 1, (2) 0977); V. A.
LEGASOVe.a. DAN 237, 6 (1977). 7. R. I. ASlSOV, V. K. ZHIVOTOV,M. F. K.ROTOV, V. D. RUSANOVand Yu. V. TARASOV,III Int. Symp. Plasma Chemistry, Limoge, France, G 5.1 (1977). B. I. PATRUSHEV,G. V. RUCKUNOVand A. M. SPECTOR,IIl Int. Syrup. Plasma Chemistry, Limoge, France, G. 2.18 (1977). 8. E. McDANIEL, Collision Phenomena in Ionized Gases. N.I.-L. (1964). 9. L. S. POLAK, Teoreticheskaia i prikladnaia plasmachimia, "Nauka", Moscow, U.S.S.R. (1975). 10. V. A. LEGASOV,V. D. RUSANOV,A. A. FPaDMANand G. V. SHOLIN,Hint. Syrup. Plasma Chemistry, Limoge, France, G. 5.18 (1977). 1I. E. E. NIKITINand V. N. KONDRATIEV,Kinetica i mekanism gasofaznyh reactsii, "Nauka", Moscow (1975). 12. G. E. RJOAKOVlCS,Energy Conversion Process with about 60%0Efficiency for Central Station. 9th Intersoc. Energy Conversion Engno Conf., 1974, p. 1100.
0360-3199/80/0201 1~)01 $02.00/0
A P L A S M O C H E M I C A L CONCEPT FOR T H E R M O C H E M I C A L HYDROGEN PRODUCTION I. G. BELOUSOV,V. A. LEGASOVand V. D. RUSANOV I.V. Kurchatov Institute of Atomic Energy, Moscow, U.S.S.R.
(Receivedfor publication 19 September 1979) Abstract--This paper discusses plasmochemical methods of water decomposition to produce hydrogen. Both the direct decomposition of water and a cycle involving the decomposition of carbon dioxide are discussed. A comparison of the relative merits of several methods of hydrogen production using a nuclear energy source is presented.
INTRODUCTION MANY methods for the conversion of atomic energy into portable energy forms convenient for mass production are known [1-4]. These include thermochemical, electrochemical, plasmochemical and radiolysis, all of which are technically adapatable to large scale production. Each of these methods occupies a place in the overall energy conversion scheme. The problem lies in developing the optimum strategy for industrial production and mass distribution. The solution cannot be single-valued either globally or within a particular country since it is dependent upon the regional industrial potential and natural conditions. It is therefore useless to seek a single global technology. Our aim is to consider some of the problems concerning hydrogen production from water on the basis of a nuclear energy source. The peculiarity of the scheme suggested is its technical readiness for commercial production at a lower capital expenditure compared with electrolysis and, perhaps, thermolysis. In this case it is important to realize that the thermodynamic limit of efficiency for complete cycles of the thermochemical, electrolysis and plasmochemical methods of nuclear fuel conversion into the chemical energy of hydrogen fuel is identical and equal to the Carnot-Funk limit, since the maximum temperature of the primary heat source and the minimum temperature for heat rejection are alike in all three cases. The theorem of equivalence [3] is directly applicable to cycles of these three methods. The radiolysis method for the direct conversion of the kinetic energy of nuclear fuel fission fragments into the chemical energy of substances is also restricted by the Carnot-Funk limit. The practical limit of its possible efficiency depends upon molecular peculiarities of the substance subjected to radiolysis and there is a lot of research work to do before the required clarity will be established.
THEORY The essence of the plasmochemical method considered consists in consecutively carrying out two reactions: 2 C O 2 - ' 2 C O * - ' 2 C O + 02, AHef = 67.6 kcal/mol,
(1)
2CO + 2H20(g)-*2CO 2 + 2H2,AH = - 9 . 6 k c a l / m o l .
(2)
The first reaction, non-equilibrium plasmochemical destruction, is performed in a gas-discharge reactor chamber to provide the electric energy of the discharge in an atmosphere of carbon dioxide. The second reaction involves the standard "shift" process already mastered by the chemical industry. The experiment shows that the energy efficiency for the plasmochemical process of carbon dioxide destruction defined by the ratio of effective enthalpy of reaction (1) to actual expenditure of electric energy, reaches 80 ~. The heat from the exothermic reaction (2) can be used for steam production or to supply energy for reaction (1).
I. G. BELOUSOV, V. A. LEGASOV AND V. D. RUSANOV H20(g), AH = - 58.0 kcal/mol (3)
H z + ~-O-.j2 H20(I), AH = - 68.3 kcal/mol.
It is easy to see that expenditure of electric energy in reaction (1) at an efficiency of 8 0 ~ is equivalent to water electrolysis at a voltage of 1.83 V. In other words, the energy efficiency of the plasmochemical reaction is somewhat higher than for good modem electrolyzers if work of separation is ignored. Thus, the question of implementation of the plasmochemical technology for large scale hydrogen production using a nuclear primary heat source resolves itself into a purely technical estimation of profitability and practical accessibility on the necessary scale. Comparative estimates indicate that large scale hydrogen production by electrolysis will require five to seven times the capital expenditure needed for plasmochemical. This is connected with the simplicity of the plasmochemical production flowsheet, a high intensity of carbon dioxide destruction in the electric discharge and the rapid rate of the chemical shift reaction with a suitable catalyst. The kinetic features of the plasmochemical cycle makes it especially attractive for large scale production. Stability of the plasmogenerator operation over a wide range of capacities provides a flexibility in the process control under conditions of electric load irregularities that, in its turn, offers an interesting scope for a wide use of cheap electric energy generated by atomic power plants during daily and seasonal variation of general power consumption. Production of carbon monoxide in the first stage of the plasmochemical process makes it easy to combine plasmochemical technology and a broad spectrum of chemical and metallurgical productions. This is the qualitative feature of the plasmochemical technology which, generally speaking, the standard electrolysis or thermochemical technology does not possess. So it is clear that plasmochemical technology is multipurpose both in respect to the quality and quantity ofelectric energy utilized and in the spectrum of the reduced gases produced. An estimate of the relative merits of thermochemical and plasmochemical cycles can be obtained by comparing their energy efficiencies and capital cost requirements. In a common case, as had already been mentioned, the limiting efficiencies of the thermochemical and plasmochemical cycles under similar temperature conditions are the same. The technically attainable efficiencies of both cycles prove also to be close. Indeed, bench data on the hybrid sulfur cycle give an estimate of efficiency of 48 %. Here the maximum cycle temperature is 900°C and the minimum is 50°C. The plasmochemical technology of the dioxide-carbon cycle with the cited interval of temperatures also has an efficiency of 48 %. This results, for instance, from the simple combination of triplet power installation (potassium, diphenyl, water) [-12] with extreme temperatures of the cycle of 900°C and 50°C, the total efficiency of 60 % and the efficiency of the plasmochemical chain being 80 %. The possibility of practical realization of the triplet installation with a 60 % efficiency does not raise doubts. The question is in the economic expediency of its construction for reasons of complexity of the thermophysical scheme and the manufacture of the potassium and oliphenyl turbines has not yet been mastered by industry. The hybrid sulfur cycle technology for hydrogen production from water is also complicated and includes many different components. These include high temperature reactors and heat exchangers operating in a corrosive atmosphere and the electrolysis installation for sulfurous gas oxidation into sulfuric acid with expensive electrodes at a low capacity. The electroenergetic part has to be included in the complete hybrid sulfur cycle technology as well as in the plasmochemical one. Estimates of capital construction costs of large scale hydrogen production from water on the basis of primary nuclear energy sources for the hybrid sulfur cycle and plasmochemical technology have not yet been made and a quantitative comparison of their merits is difficult. However, it is thought that the capital cost of the electrolysis component alone for the hybrid sulfur cycle will be about the same as the common electrolysis component for water electrolysis since the processes in both cases are performed with approximately identical current densities. Hence, the hybrid sulfur cycle technology of hydrogen production from water will not, generally speaking, be competitive with the plasmochemical technology. One can draw analogous conclusions considering the processes of plasmochemical destruction of water and carbon dioxide at the molecular level. Let's analyze in more detail the plasmochemical method of hydrogen production from water. The direct reduction reaction in a low temperature plasma can be accomplished in two ways; first, by thermal decomposition of steam at temperatures over 3000 K; second, it can occur at low
A PLASMOCHEMICAL CONCEPT FOR THERMOCHEMICAL HYDROGEN PRODUCTION
3
gas temperature by the thermally non-equilibrium interactions of plasma electrons with molecules of water. In the first case, the simplest event for optimal temperatures requires a superfast hardening that is not technically realized; within the low temperature range, the equilibrium degree of decomposition is small and the efficiency of the process is very low. Therefore, the thermal destruction of water molecules is not considered and the gas temperature is held to be low. In such a thermally nonequilibrium medium, the main active agent is the electron gas whose temperature, T~, is 10-100 times higher than that of the neutral particles, To. For such a formulation of the problem, an analysis of the elementary act of electron interaction with molecular water becomes the main question. It is known that the principal processes of nonelastic interaction at an electron energy of 1-7 eV are the vibrational excitation and the dissipative sticking [8]; as for losses due to rotational excitation of molecules, their level does not exceed Joule's ones and therefore will not be considered to be significant. The excitation of the electron structure of a water molecule has a threshold of 7 eV and is non-effective below this energy level. The vibrational excitation is characterized by the cross section 10-17 cm 2 and has a maximum within the energy range 1-3 eV ['8]. When the electron energy increases to 6 eV, the process of dissociative attachment arises according to the scheme ['8]. /,
H- +OH
e + H20
(4) O-+H
2.
With the cross section maximum of 10-17 cm 2, the major reaction is the first one, in which H ions are generated. The relatively small energy of electron affinity to atomic hydrogen (EA(h) = 0.75 eV) permits the reverse process of H - ionization by electron impact to be effective: H - + e--*H + 2e,
(5)
which is characterized in the maximum by the cross section value a ~ 10-14 cm 2 ( k d = 10-6 cm 3 sec- 1). In this manner there appears the possibility of multiple electron utilization in reactions (4) and (5), if the velocity of chain termination due to recombination of negative ions H - and by the ion-molecular reaction (Ki,) H - + H20--*H 2 + O H -
(6)
does not exceed the velocity of process (5). More precisely: no ~
kd"
(7)
from which at ka ~ 10 -6, kin ~ 10 -9 we have the hard limitation on the ionization degree ne/[-H20 ] > 10 -3.
Similarly, at lower electron energies when the dissociative attachment does not take place, their energies are transferred to oscillating degrees of freedom of the water molecules, according to the equation e + H20--~HzO* + e,
(9)
which in its turn can lead to the elementary act of dissociation H20* + H20*--*-H + O H + H20.
(10)
In this case the chain character of the process is also possible if the reactions are assumed to be H + H20*--~H 2 + OH
(11)
OH + H20*---~ H 2 0 z + H.
(12)
4
I.G. BELOUSOV, V. A. LEGASOV AND V. D. RUSANOV
The chain termination in this case has to be mostly connected with the triple act H + OH + H20--~2H20.
(13)
An indispensable condition of realization of such a process is the obvious requirement that reaction (10) take place with more probability than VT-relaxations relieving the vibrational excitation of water molecules attained through the primary reaction (9) and further that the Boltzman distribution is established over vibrational levels due to V V-relaxation, from which one can obtain the inequality: ne key. [ H 2 0 ] ~> kvT'
(14)
where key is the velocity constant of process (9), and k v r is the velocity constant of oscillating-translatory relaxation. Because of specific features of the water molecule, the value of k~r is anomalously high and attains a value of 10- t 2 cm a sec- 1 [9]. This makes the condition (14) rigid since key for water does not usually exceed 10 -9 cm a sec- 1 Consequently, both condition (14) and the limitation (8) formulate essentially a single requirement for a high degree of ionization in a discharge independent of the exact nature of the physical process of destruction. The boundary value, T, where the high temperature process of dissocitaive attachment has to convert into the low temperature oscillating excitation, is given by the expression oln - I
Fk~'~ ~ 1
05)
and equals 1.7 eV. The value ea ~ 6 eV corresponds to the minimum electron attachment cross section; k~"x ~ 10- 9 cm 3 sec- 1 is the value of the corresponding rate constant; hto ~ 0.2 eV is the value of an oscillating quantum of a water molecule. Therefore, under typical conditions of non-equilibrium discharge (T~ ~ 1 eV, To ~ 600 K), the mechanism of oscillating excitation prevails. In the case of hotter electrons (T~ ~ 3 eV) dissociative attachment becomes the main channel of electron energy losses. As a whole, the efficiency value of the process related, mainly due to losses to the VT-relaxation and finite length of chemical chain, amounts to 60~o assuming that [ H 2 0 ] ~ 3 x 10 is cm -3, T e ~ 1 eV, and the energy contribution in the discharge assigned to one molecule is 1.2 eV. A somewhat higher value, ~/~ 70 ~/~ is attained in the high temperature region (Te ~ 3 eV) according to the calculation. In both cases the capacity amounts to about 20 ~ at the given parameters. The details of the efficiency calculation are given in [6]. Difficulties associated with realization of the discharge with the high electron concentration ( n J n o ) m i . ~ l0 -4 to 10 -3 and low neutral particle temperature predetermine a search for alternative plasmochemical systems where these contradictions are removed. The two stage cycle can become such a scheme, in which the high temperature stage is replaced by the non-equilibrium plasmochemical cycle and the low temperature one is carried out under equilibrium conditions in the traditional chemical manner. In principle, it is possible to arrange the second stage to also be plasmochemical using an anomalously high rate constant for the process CO + e ~ C O * + e reaching 10- 7 cm a sec- 1. The most promising system is the scheme consisting of the cycle of CO 2 decomposition according to the expression CO*--*CO + ~O 1 2,
(16)
with subsequent conversion of the CO into hydrogen in reaction (2). According to (9) the first stage of the cycle (16) is connected with the vibrational excitation of molecular carbon dioxide by electron impact e + CO2--~CO ~' + e,
(17)
that in the case of CO 2 is essentially facilitated by the very high efficiency of this process (k~o(CO2) ~ 3 × 10 - s cm a sec-1). Together with this, the constant of VT-relaxation for CO 2 molecules is significantly less than that of water k~r(CO2) ~ 10-15 cm sec-x which reduces the permissible degree of ionization nJ[CO2] ~ 10 -7 and facilitates sharply an organization of the chemically active discharge in CO 2.
A PLASMOCHEMICAL CONCEPT FOR THERMOCHEMICAL HYDROGEN PRODUCTION
5
I00
x
60 X 40
20 0
,
I
j
I
2
~
4
I
I
6
8
eV E v [~"ot]
Fro. 1. Dependence of efficiency (1) and conversion coefficient (2) at CO 2 dissociation in thermally non-equilibrium plasma. - - E s t i m a t e d curve; x experimental data with UHF-discharge: Q experimental data with HF-discharge in a magnetic field. The interaction of oscillatorily excited C O 2 molecules has to result in a non-equilibrium disproportionation by two alternative channels [-10]:
1.
CO* + CO*--~CO20 + CO
(18)
CO~ Jr- CO20"-~CO -~- CO 2 + 02 2.
CO~ + COW--CO + O + CO 2
(19)
0 -~- CO~-~CO --F 02 which have similar energy characteristics. The calculated efficiency in this case is between 70 and 80 % which is equivalent to 3.7 to 3.8 eV m o l - 1 decomposed. However, the cycle requires operations for the
IOOO 9oo 8oo
700
.£ a."
I
600 500 400 300
200 I00
I
600
I
700
800
900 I000 T, K
I100
1200
FIG. 2. Conditions of product stability relative to reverse reactions. Solid curve--pkTk for the system H2-O 2. I. l = 10%; II. Z = 1%; III. Z < 1%. Dotted curve pkTk for the system
CO-O 2.
I. G. BELOUSOV, V. A. LEGASOV AND V. D. RUSANOV separation of CO from 0 2 and for the CO conversion into n 2 in the reaction with water. The destruction process of CO 2 molecules in the thermally non-equilibrium discharge was experimentally investigated in H F and U H F versions within a pressure range of 1-500 torr [7]. The experiments with U H F discharge were carried out at a frequency of 2300 M G and with H F discharge at 21 MG. In both cases the power input into the discharge did not exceed 1.5 kW. The gas flow rate was varied between 5 x 104 and 0.71-atm sec- 1. The degree of discharge non-isothermality was Te/To ~ 15-100, and the value of n J n o ~ 5 x l 0 -7 to l0 -a. The calculated results of the efficiency, r/, and degree of decomposition, ;C, are shown on the plot in Fig. 1. It is seen that the maximum value of the efficiency is 80 % while the degree of conversion exceeds 85 % but at a lower efficiency. In the thermally non-equilibrium discharge, highly effective CO 2 destruction is indeed possible. However, the total efficiency of the process as a whole, ~/~, according to estimates, reduces to approximately 65-75 % which is close to the calculated efficiency of the direct destruction of water cycle. It should be noted that the direct destruction of water, as is noted above, is connected with difficulties in sustaining the low temperature regime for discharge at a high degree of gas ionization since the excess of pkTk critical parameters can stimulate a chain reaction of the gas mixture. Regions of permissible concentrations and temperatures are shown in Fig. 2 by a solid curve. The bottom branch of the curve corresponds to a chain mechanism of explosion and the upper branch corresponds to the "thermal" character of burning. The dotted curve on the same coordinates restricts the chain mechanism region for explosion in the mixture C O - O 2. As is seen from the figure, requirements for parameters P and T are essentially mild here. The thermal mechanism of reaction in the pure mixture is hindered and the burning can be accomplished in a catalytic m a n n e r only in the presence of water and hydrogen at a low rate. Thus, it has been shown that the electric means of water decomposition give similar values of energy output and the main difference lies in the technological design of the process. NOMENCLATURE kvv kvr
V-V vibrational relaxation rate constant V-T vibrational relaxation rate constant
ki. ne no
ion-molecular reaction (6) rate constant electron concentration neutral gas concentration
key k~m~
vibrational excitation rate constant dissociative attachment rate constant.
REFERENCES 1. First World Hydroyen Energy Conf., 1-3 March 1976, Miami Beach, Florida, U.S.A. 2. Proc. Hydrogen Economy, Miami Energy Conference (THEME). 3. I. V. KURTSCHATOVA,Collection of "Questions Related to Atomic Science and Technique", Hydrogen Energy Series, No. l, 1(2) 0976, 1977). 4. G. DE BENI, Trans. Am. nucl. Soc. 20, 719 (1975). 5. I.G. BELOUSOV,V. A. LEGASOVand V. D. RUSANOVIn Voprosy atomnoi nauki i tecniki, Set. Atomno-vodorodnaia energetica 2, (2), Moscow, U.S.S.R. 0977). 6. V. P. BECnIN,V. A. LEGASOV,V. D. RUSANOV, A. A. FRIDMANand G. V. SHOLIN,ibid. 1, (2) 0977); V. A.
LEGASOVe.a. DAN 237, 6 (1977). 7. R. I. ASlSOV, V. K. ZHIVOTOV,M. F. K.ROTOV, V. D. RUSANOVand Yu. V. TARASOV,III Int. Symp. Plasma Chemistry, Limoge, France, G 5.1 (1977). B. I. PATRUSHEV,G. V. RUCKUNOVand A. M. SPECTOR,IIl Int. Syrup. Plasma Chemistry, Limoge, France, G. 2.18 (1977). 8. E. McDANIEL, Collision Phenomena in Ionized Gases. N.I.-L. (1964). 9. L. S. POLAK, Teoreticheskaia i prikladnaia plasmachimia, "Nauka", Moscow, U.S.S.R. (1975). 10. V. A. LEGASOV,V. D. RUSANOV,A. A. FPaDMANand G. V. SHOLIN,Hint. Syrup. Plasma Chemistry, Limoge, France, G. 5.18 (1977). 1I. E. E. NIKITINand V. N. KONDRATIEV,Kinetica i mekanism gasofaznyh reactsii, "Nauka", Moscow (1975). 12. G. E. RJOAKOVlCS,Energy Conversion Process with about 60%0Efficiency for Central Station. 9th Intersoc. Energy Conversion Engno Conf., 1974, p. 1100.