- Email: [email protected]
Energetic aspects of the syngas production by solar energy: Reforming of methane and carbon gasification
Solar & Wind Technology Vol. 6, No. 2, pp. 101-104, 1989 Printed in Great Britain.
0741-983X/89 $3.00+.00 Pergamon Press pie
ENERGETIC ASPECTS OF THE SYNGAS PRODUCTION BY SOLAR ENERGY: REFORMING OF METHANE A N D CARBON GASIFICATION L. D'ALESSIO a n d M. PAOLUCCI Dipartimento di Chimica, Universita di Roma "La Sapienza", Rome, Italy
(Received 15 April 1988 ; accepted 22 July 1988) Al~traet--The syngas (CO + H2) production from methane or carbon has been studied from the energetic point of view for solar energy storage applications. Results indicate that, at operating temperatures about 900-1000 K a net enthalpy change, or stored energy, of 192 KJ (CH4+H20) or 272 KJ (CH4+CO2) is obtainable, per mol of reformed methane. For the carbon gasification a maximum enthalpy change of 131 KJ is aspected. Further developments are submitted to the choice of more selective catalysts and to the building of suitable chemical reactors-heat exchangers designed for solar plants.
of performing the syngas production employing concentrated sun radiation as a high temperature energy source, in the l kW solar furnace operating at the University of Rome [5].
INTRODUCTION
The conversion of solar energy into other kinds of energy, suitable for storage and transportation, has been of growing interest in recent years and systems have been proposed based upon sensible heat, latent heat and reaction heat [1]. The latter appears very attractive because chemical reactions display characteristics that are more advantageous than in other systems (high energy density and storage at room temperature), which indicate that these systems are excellent candidates for large energy plants. Among the available chemical reactions, the syngas production from methane [2] or coal [3] has been studied for solar energy applications since the technological know-how in this field is considerable because the reforming of hydrocarbons and the coal gasification are commonly carded out in the synthesis gas industry. Since these processes are endothermic, when they are performed in conventional plants, heat must be supplied, usually burning part of the fuel which increases the cost of these operations. A new and interesting approach to syngas production is the use of solar energy to furnish the reaction heat, as well as the heat needed to raise the reactants' temperature to the reaction temperature. The advantages are : (i) the possibility of solar energy storage as chemical energy, and (/i) to obtain an energy enriched fuel at low cost. In addition, the syngas is of great importance in chemical industry since it is the raw material for further transformations into chemicals [4]. In this case the use of solar energy allows a saving of part of the fuel needed to run the plant. The purpose of this work is to explore the feasibility
REFORMING OF METHANE
The syngas production from methane [6] may be carded out catalytically by means of two processes, namely the steam reforming: CH4 + H20 -* CO + 3H2 AH2°98= 206.22 kJ/mol
(1)
CO + H20 --~ CO2 + H2 AH~98 --- --41.18 kJ/mol
(2)
and the carbon dioxide reduction : CH4+CO
2 --~ 2 C O +
H20
AH~9g = 247.40 kJ/mol
(3)
H2+ CO2 ~ 2CO + 2H 2 AH~98 = 41.18 kJ/mol.
(4)
These reactions may be accompanied by side reactions [7] that lead to the formation of carbon which deactivates the catalyst. To avoid these side reactions, operations must be carded out using an excess of steam or carbon dioxide.
lO1
102
L. D'ALEsSIO and M. PAOLUCCI SC _
I
3
E
2
800
IOCO Temperature
1'
1200
(K)
Fig. 1. Standard free energy of the reactions involved in the reforming of methane vs temperature. In Fig. 1 the free energy of the main reactions (1), (3) and the secondary reactions (2), (4) are reported as a function of temperature: from the diagram it appears that an increase of temperature favours reactions (1), (3), (4) and hinders reaction (2) accordingly with the exothermal character of the last one. Reactions (1) and (3) occur in an appreciable amount only if the operating temperature is kept above 1000 K, which is a temperature readily available in a solar furnace [8]. The catalysts needed to obtain reactions (1)-(4) are well known in the industrial field: they are redox catalysts made of metallic Ni supported on A1203. In this work we have used 15% nickel on 2-3 mm alumina pellets. A prototype of solar reactor has been designed and constructed in our laboratory with the purpose of carrying out a small-scale test of the solar syngas production. Details of the reactor have been described elsewhere [9]. It consists (see Fig. 2) of a cylindrical cavity receiver, which traps the focusing radiation, surrounded by two annular interspaces, the inner one filled by the catalyst. In front of the opening of the reactor diaphragms can be placed to control the energy losses in order to adjust the reaction temperature. The thermal exchange between the entering gases and those leaving the reactor is ensured by a steel tube-in-tube exchanger. The composition of the inlet and outlet gas has been determined by standard chromatographic methods. A number of test runs have been performed at atmospheric pressure under different temperatures and feed gas composition. Some results are reported in Figs 3 and 4, for the steam reforming and the carbon dioxide reduction respectively. From the diagram it is apparent that the syngas composition is quite independent from the reaction temperature. In the CHa/H20 reforming experiments the mean H2 to CO ratio of the exit gas approaches the value of 5.15, instead of 3 which is
4
Fig. 2. Outline of the cylindrical solar reactor showing the inner gas path. I00
1
f
r
d
I
(
I
I I
--c
HzO
°'"-
e
~
-I
.......
8
o
CI-44
- o
i •
r /i
CH4~ I 960
~ . . . _ _ . . - - c - o - . o - ~
_
O~.o~o
--
-"'-'-- CO~..-:~_ g~ e'e-e--e,e
I O~'DT~---"}
I I000
980 Temperature
I-;
0'=1'0° 1020
(K)
Fig. 3. Feed gas composition (©) and dry reformed gas composition (0) vs temperature in the CH4/H20 reforming experiments. I00|
I
1
P
J
1
J~
(.9
J~- H~ Jg
CH4 ~
I '
7
~..~
•
jo~_
e~
• Temperature
•
..
_
CH4 •
(K)
Fig. 4. Feed gas composition (0) and dry reformed gas composition (0) vs temperature in the CH4/CO2reforming experiments.
Syngas production by solar energy
,oo1\ , / , 50
-Ioo
-
I
I \
I
15130 Teml~r oture (K)
I0(0)
I
2000
Fig. 5. Standard free energy of the reactions involved in the steam carbon gasification vs temperature. aspected if only reaction (1) occurs, and this means that about 35% of the CO undergoes reaction (2). In the C H 4 / C O 2 experiments the above ratio is 0.537, instead of 1 as foreseen for reaction (3), and in this case 30% of the H 2 undergoes reaction (4). From this it follows that the overall energy requirement to run the processes, i.e. the energy stored at room temperature, is about 191.85 kJ for the steam reforming and 272.19 kJ for the CO2 reduction, per mol of reformed methane.
The gasification of coal and carbonaceous materials is a well known technology [10] for converting a solid fuel into a gaseous one, which is easy to store and transport. The steam carbon gasification [11] may be represented by the main reaction : --~ C O + H 2 AH~98 =
131.33 kJ/mol
(5)
followed by two side reactions, namely the shift reaction : C O + H 2 0 --~ C O 2 + H2 AH~98 = -41.18 kJ/mol
(6)
and the methane formation reaction : C + 2H~ --* C H
4
AH~98 = - 7 4 . 8 9 kJ/mol
where the standard free energy is reported vs T. From the diagram it is clear that an increase of temperature favours reaction (5) and hinders reactions (6) and (7). At 1200 K reaction (5) sharply prevails. The overall process is endothermal, and it may be carried out in two ways [12] : (i) the autothermic reaction, when part of the fuel is burned out to furnish the energy required, and (ii) the allotbermic one, when the heat necessary is supplied externally. The allothermic reaction appears interesting for solar applications since a concentrating device can release the energy needed to a suitable solar gasifier [13]. In relation to the energetic implications of the process, an equilibrium configuration calculation has been performed in order to estimate the influence of the reaction temperature on the syngas composition. It has been assumed that reactions (5)-(7) equilibrate simultaneously at each temperature. In addition the behaviour of all gases has been considered ideal and the solid carbon activity equals one unit. Under this hypothesis the equilibrium molar fractions of CO, CO2, H20, H2 and CH4 are expressed by: xCO xCO2 xH20 xH2
= = = =
xCH 4
=
(x-y)~(1 + x - z ) y/(1 + x - z ) (1 - x - y ) ~ ( 1 + x - z ) (x + y - 2z)/(1 + x - z) z/(1 +x-z)
where x, y, z are the solutions of the simultaneous equations :
CARBON GASIFICATION
C+H20
103
(7)
Since the side reactions are exothermal, they lead to a depletion of the converted energy, whose amount depends upon the extent of such reactions. The relative importance of reactions (5)-(7) is a function of temperature, as can be seen from Fig. 5,
F1 = KI(1 --x--y)(1 + x - z ) -- P ( x - - y ) ( x + y - - 2 z ) = 0
F2 = K2(x--y)(1 + x - y ) - y ( x + y - - 2z) = 0 F3 = K3P(x+ y - 2 z ) 2 - - z ( 1 + x - - z ) = 0 in which K1, K2, K3 stand for the equilibrium constants of reactions (5), (6) and (7) respectively, at the considered temperature, and P is the total pressure. The solution of the system is the absolute minimum of the function : F = F12+F22+F32. TO find this solution we have employed two distinct BASIC programs for function minimization, implemented on an Olivetti M20 ST personal computer. The first program is based on an algorithm known as the simplex method [14]. The second one is based on the maximum slope method [15], known also as the gradient method. Results obtained by the two programs are consistent and are reported in Fig. 6, where the chemical composition (mol fractions) at atmospheric pressure of the gas phase in equilibrium with solid carbon is plotted against temperature. From the knowledge of the degrees of reaction x, y, z, we have
L. D'ALESSIO and M, PAOLUCCt
104 I
I
]
r
I
600
800 T(K)
Acknowledgements--This work has been performed with the financial support of C.N.R.-E.N.E.A. under the contract, "Progetto Finalizzato Energetica 2. Sottoprogetto Trasporto, Accumulo e Distribuzione del Calore".
I
80
0.~
~ 60 <1 40
REFERENCES "~0.6
400
{
i000
t200_
° I-t2
0.2
430
600
8O0 Temperoture (K)
I000
r~O0
Fig. 6. Equilibrium composition at atmospheric pressure for the allothermic steam carbon gasification vs temperature. Inside the box is reported the net enthalpy change per mol of gasified carbon.
calculated the net enthalpy change of the process per mol of gasified carbon, as shown in the box of Fig. 6. It is apparent that when the temperature increases, reaction (5) tends to dominate and the net AH approaches the 131.33 kJ/mol value.
CONCLUSIONS Our study of the energetics of the syngas production has shown that solar energy can be utilized with good results. An improvement of the process may be obtained using more selective catalysts, also for coal gasification, which are to be experimented on a micro pilot plant. These catalysts should be F e / N i mixtures for the CH4 reforming and K2CO3 or CaCO3 for the coal gasification. M o r e o v e r the design of high efficiency solar receivers-chemical reactors plays a decisive role, especially for coal gasification, where a continuous feed reactor must be tested. Economics of the processes may be improved taking into account the global energy balance and the connection between the solar energy facility and other energy supply or energy consuming devices.
I. C. Wyman, J. Castle and F. Kreith, A review of collector and energy storage technology for intermediate temperature applications. Sol. Energy 24, 517 (1980). 2. J. H. McCrary, G. E. McCrary, T. A. Chubb, J. J. Nemecek and D. E. Simmons, An experimental study of the CO2-CH, reforming-methanation cycle as a mechanism for converting and transporting solar energy. Sol. Energy 29, 141 (1982). 3. D.W. Gregg, R. W. Taylor, J. H. Campbell, J. R. Taylor and A. Cotton, Solar gasification of coal, activated carbon, coke and coal and biomass mixtures. Sol. Energy 25, 353 (1980). 4. Extensive bibliography in !. Pasquon, La nuova chimica del gas di sintesi. III. Aspetti economico-industriali. Prospettive. La Chimica e l'Industria 66, 776 (1984). 5. G. De Maria, L. D'Alessio, E. Coffari, M. Paolucci and C. A. Tiberio, Thermochemical storage of solar energy with high-temperature chemical reactions. Sol. Energy 35, 409 (1985). 6. L. D'Alessio and M. Paolucci, Gas di sintesi da reforming catalitico del metano utilizzando energia solare, per una possibile transforrnazione in composti organici. Atti del XVII Convegno Nazionale della Divisione di Chimica Organica, p. 391, Fiuggi, 13-18 September (1987). 7. C. A. Bernardo, I. Alstrup and J. R. Rostrup-Nielsen, Carbon deposition and methane steam reforming on silica-supported Ni~Cu catalysts. J. Catalysis 90, 517 (1985). 8. T. Sakurai, Solar furnaces, in Solar Energy Engineering (A. A. M. Sayigh ed.), p. 233. Academic Press (1977). 9. G. De Maria, C. A. Tiberio, L. D'Alessio, M. Piccirilli, E. Coffari and M. Paolucci, Thermochemical conversion of solar energy by steam reforming of methane. Energy It, 805 (1986). tO. W. P. M. Van Swaij, Gasification. The process and the technology. Resources Conservation 7, 333 ( 1981). 11. G. K. Vick, Review of coal gasification technologies for the production of methane. Resources Conservation 7. 202. 12. F. Kaptein and J. A. Moulijn, Kinetics of catalysed and uncatalysed coal gasification. NATO ASI Carbon and Coal Gasification. Science and Technology, Alvor (Portugal), 2~31 May (1985), 13. A. P. Brucker, Continuous duty solar coal gasification system using molten slag and direct-contact heat exchange. Sol. Energy 34, 239 (1985). 14. J. A. Nedler and R. Mead, A simplex method for function minimization, Comput. J. 7, 308 (1964). 15. F. Scheid, Numerical Analysis, Chap. 25. McGraw-Hill (1968).
0741-983X/89 $3.00+.00 Pergamon Press pie
ENERGETIC ASPECTS OF THE SYNGAS PRODUCTION BY SOLAR ENERGY: REFORMING OF METHANE A N D CARBON GASIFICATION L. D'ALESSIO a n d M. PAOLUCCI Dipartimento di Chimica, Universita di Roma "La Sapienza", Rome, Italy
(Received 15 April 1988 ; accepted 22 July 1988) Al~traet--The syngas (CO + H2) production from methane or carbon has been studied from the energetic point of view for solar energy storage applications. Results indicate that, at operating temperatures about 900-1000 K a net enthalpy change, or stored energy, of 192 KJ (CH4+H20) or 272 KJ (CH4+CO2) is obtainable, per mol of reformed methane. For the carbon gasification a maximum enthalpy change of 131 KJ is aspected. Further developments are submitted to the choice of more selective catalysts and to the building of suitable chemical reactors-heat exchangers designed for solar plants.
of performing the syngas production employing concentrated sun radiation as a high temperature energy source, in the l kW solar furnace operating at the University of Rome [5].
INTRODUCTION
The conversion of solar energy into other kinds of energy, suitable for storage and transportation, has been of growing interest in recent years and systems have been proposed based upon sensible heat, latent heat and reaction heat [1]. The latter appears very attractive because chemical reactions display characteristics that are more advantageous than in other systems (high energy density and storage at room temperature), which indicate that these systems are excellent candidates for large energy plants. Among the available chemical reactions, the syngas production from methane [2] or coal [3] has been studied for solar energy applications since the technological know-how in this field is considerable because the reforming of hydrocarbons and the coal gasification are commonly carded out in the synthesis gas industry. Since these processes are endothermic, when they are performed in conventional plants, heat must be supplied, usually burning part of the fuel which increases the cost of these operations. A new and interesting approach to syngas production is the use of solar energy to furnish the reaction heat, as well as the heat needed to raise the reactants' temperature to the reaction temperature. The advantages are : (i) the possibility of solar energy storage as chemical energy, and (/i) to obtain an energy enriched fuel at low cost. In addition, the syngas is of great importance in chemical industry since it is the raw material for further transformations into chemicals [4]. In this case the use of solar energy allows a saving of part of the fuel needed to run the plant. The purpose of this work is to explore the feasibility
REFORMING OF METHANE
The syngas production from methane [6] may be carded out catalytically by means of two processes, namely the steam reforming: CH4 + H20 -* CO + 3H2 AH2°98= 206.22 kJ/mol
(1)
CO + H20 --~ CO2 + H2 AH~98 --- --41.18 kJ/mol
(2)
and the carbon dioxide reduction : CH4+CO
2 --~ 2 C O +
H20
AH~9g = 247.40 kJ/mol
(3)
H2+ CO2 ~ 2CO + 2H 2 AH~98 = 41.18 kJ/mol.
(4)
These reactions may be accompanied by side reactions [7] that lead to the formation of carbon which deactivates the catalyst. To avoid these side reactions, operations must be carded out using an excess of steam or carbon dioxide.
lO1
102
L. D'ALEsSIO and M. PAOLUCCI SC _
I
3
E
2
800
IOCO Temperature
1'
1200
(K)
Fig. 1. Standard free energy of the reactions involved in the reforming of methane vs temperature. In Fig. 1 the free energy of the main reactions (1), (3) and the secondary reactions (2), (4) are reported as a function of temperature: from the diagram it appears that an increase of temperature favours reactions (1), (3), (4) and hinders reaction (2) accordingly with the exothermal character of the last one. Reactions (1) and (3) occur in an appreciable amount only if the operating temperature is kept above 1000 K, which is a temperature readily available in a solar furnace [8]. The catalysts needed to obtain reactions (1)-(4) are well known in the industrial field: they are redox catalysts made of metallic Ni supported on A1203. In this work we have used 15% nickel on 2-3 mm alumina pellets. A prototype of solar reactor has been designed and constructed in our laboratory with the purpose of carrying out a small-scale test of the solar syngas production. Details of the reactor have been described elsewhere [9]. It consists (see Fig. 2) of a cylindrical cavity receiver, which traps the focusing radiation, surrounded by two annular interspaces, the inner one filled by the catalyst. In front of the opening of the reactor diaphragms can be placed to control the energy losses in order to adjust the reaction temperature. The thermal exchange between the entering gases and those leaving the reactor is ensured by a steel tube-in-tube exchanger. The composition of the inlet and outlet gas has been determined by standard chromatographic methods. A number of test runs have been performed at atmospheric pressure under different temperatures and feed gas composition. Some results are reported in Figs 3 and 4, for the steam reforming and the carbon dioxide reduction respectively. From the diagram it is apparent that the syngas composition is quite independent from the reaction temperature. In the CHa/H20 reforming experiments the mean H2 to CO ratio of the exit gas approaches the value of 5.15, instead of 3 which is
4
Fig. 2. Outline of the cylindrical solar reactor showing the inner gas path. I00
1
f
r
d
I
(
I
I I
--c
HzO
°'"-
e
~
-I
.......
8
o
CI-44
- o
i •
r /i
CH4~ I 960
~ . . . _ _ . . - - c - o - . o - ~
_
O~.o~o
--
-"'-'-- CO~..-:~_ g~ e'e-e--e,e
I O~'DT~---"}
I I000
980 Temperature
I-;
0'=1'0° 1020
(K)
Fig. 3. Feed gas composition (©) and dry reformed gas composition (0) vs temperature in the CH4/H20 reforming experiments. I00|
I
1
P
J
1
J~
(.9
J~- H~ Jg
CH4 ~
I '
7
~..~
•
jo~_
e~
• Temperature
•
..
_
CH4 •
(K)
Fig. 4. Feed gas composition (0) and dry reformed gas composition (0) vs temperature in the CH4/CO2reforming experiments.
Syngas production by solar energy
,oo1\ , / , 50
-Ioo
-
I
I \
I
15130 Teml~r oture (K)
I0(0)
I
2000
Fig. 5. Standard free energy of the reactions involved in the steam carbon gasification vs temperature. aspected if only reaction (1) occurs, and this means that about 35% of the CO undergoes reaction (2). In the C H 4 / C O 2 experiments the above ratio is 0.537, instead of 1 as foreseen for reaction (3), and in this case 30% of the H 2 undergoes reaction (4). From this it follows that the overall energy requirement to run the processes, i.e. the energy stored at room temperature, is about 191.85 kJ for the steam reforming and 272.19 kJ for the CO2 reduction, per mol of reformed methane.
The gasification of coal and carbonaceous materials is a well known technology [10] for converting a solid fuel into a gaseous one, which is easy to store and transport. The steam carbon gasification [11] may be represented by the main reaction : --~ C O + H 2 AH~98 =
131.33 kJ/mol
(5)
followed by two side reactions, namely the shift reaction : C O + H 2 0 --~ C O 2 + H2 AH~98 = -41.18 kJ/mol
(6)
and the methane formation reaction : C + 2H~ --* C H
4
AH~98 = - 7 4 . 8 9 kJ/mol
where the standard free energy is reported vs T. From the diagram it is clear that an increase of temperature favours reaction (5) and hinders reactions (6) and (7). At 1200 K reaction (5) sharply prevails. The overall process is endothermal, and it may be carried out in two ways [12] : (i) the autothermic reaction, when part of the fuel is burned out to furnish the energy required, and (ii) the allotbermic one, when the heat necessary is supplied externally. The allothermic reaction appears interesting for solar applications since a concentrating device can release the energy needed to a suitable solar gasifier [13]. In relation to the energetic implications of the process, an equilibrium configuration calculation has been performed in order to estimate the influence of the reaction temperature on the syngas composition. It has been assumed that reactions (5)-(7) equilibrate simultaneously at each temperature. In addition the behaviour of all gases has been considered ideal and the solid carbon activity equals one unit. Under this hypothesis the equilibrium molar fractions of CO, CO2, H20, H2 and CH4 are expressed by: xCO xCO2 xH20 xH2
= = = =
xCH 4
=
(x-y)~(1 + x - z ) y/(1 + x - z ) (1 - x - y ) ~ ( 1 + x - z ) (x + y - 2z)/(1 + x - z) z/(1 +x-z)
where x, y, z are the solutions of the simultaneous equations :
CARBON GASIFICATION
C+H20
103
(7)
Since the side reactions are exothermal, they lead to a depletion of the converted energy, whose amount depends upon the extent of such reactions. The relative importance of reactions (5)-(7) is a function of temperature, as can be seen from Fig. 5,
F1 = KI(1 --x--y)(1 + x - z ) -- P ( x - - y ) ( x + y - - 2 z ) = 0
F2 = K2(x--y)(1 + x - y ) - y ( x + y - - 2z) = 0 F3 = K3P(x+ y - 2 z ) 2 - - z ( 1 + x - - z ) = 0 in which K1, K2, K3 stand for the equilibrium constants of reactions (5), (6) and (7) respectively, at the considered temperature, and P is the total pressure. The solution of the system is the absolute minimum of the function : F = F12+F22+F32. TO find this solution we have employed two distinct BASIC programs for function minimization, implemented on an Olivetti M20 ST personal computer. The first program is based on an algorithm known as the simplex method [14]. The second one is based on the maximum slope method [15], known also as the gradient method. Results obtained by the two programs are consistent and are reported in Fig. 6, where the chemical composition (mol fractions) at atmospheric pressure of the gas phase in equilibrium with solid carbon is plotted against temperature. From the knowledge of the degrees of reaction x, y, z, we have
L. D'ALESSIO and M, PAOLUCCt
104 I
I
]
r
I
600
800 T(K)
Acknowledgements--This work has been performed with the financial support of C.N.R.-E.N.E.A. under the contract, "Progetto Finalizzato Energetica 2. Sottoprogetto Trasporto, Accumulo e Distribuzione del Calore".
I
80
0.~
~ 60 <1 40
REFERENCES "~0.6
400
{
i000
t200_
° I-t2
0.2
430
600
8O0 Temperoture (K)
I000
r~O0
Fig. 6. Equilibrium composition at atmospheric pressure for the allothermic steam carbon gasification vs temperature. Inside the box is reported the net enthalpy change per mol of gasified carbon.
calculated the net enthalpy change of the process per mol of gasified carbon, as shown in the box of Fig. 6. It is apparent that when the temperature increases, reaction (5) tends to dominate and the net AH approaches the 131.33 kJ/mol value.
CONCLUSIONS Our study of the energetics of the syngas production has shown that solar energy can be utilized with good results. An improvement of the process may be obtained using more selective catalysts, also for coal gasification, which are to be experimented on a micro pilot plant. These catalysts should be F e / N i mixtures for the CH4 reforming and K2CO3 or CaCO3 for the coal gasification. M o r e o v e r the design of high efficiency solar receivers-chemical reactors plays a decisive role, especially for coal gasification, where a continuous feed reactor must be tested. Economics of the processes may be improved taking into account the global energy balance and the connection between the solar energy facility and other energy supply or energy consuming devices.
I. C. Wyman, J. Castle and F. Kreith, A review of collector and energy storage technology for intermediate temperature applications. Sol. Energy 24, 517 (1980). 2. J. H. McCrary, G. E. McCrary, T. A. Chubb, J. J. Nemecek and D. E. Simmons, An experimental study of the CO2-CH, reforming-methanation cycle as a mechanism for converting and transporting solar energy. Sol. Energy 29, 141 (1982). 3. D.W. Gregg, R. W. Taylor, J. H. Campbell, J. R. Taylor and A. Cotton, Solar gasification of coal, activated carbon, coke and coal and biomass mixtures. Sol. Energy 25, 353 (1980). 4. Extensive bibliography in !. Pasquon, La nuova chimica del gas di sintesi. III. Aspetti economico-industriali. Prospettive. La Chimica e l'Industria 66, 776 (1984). 5. G. De Maria, L. D'Alessio, E. Coffari, M. Paolucci and C. A. Tiberio, Thermochemical storage of solar energy with high-temperature chemical reactions. Sol. Energy 35, 409 (1985). 6. L. D'Alessio and M. Paolucci, Gas di sintesi da reforming catalitico del metano utilizzando energia solare, per una possibile transforrnazione in composti organici. Atti del XVII Convegno Nazionale della Divisione di Chimica Organica, p. 391, Fiuggi, 13-18 September (1987). 7. C. A. Bernardo, I. Alstrup and J. R. Rostrup-Nielsen, Carbon deposition and methane steam reforming on silica-supported Ni~Cu catalysts. J. Catalysis 90, 517 (1985). 8. T. Sakurai, Solar furnaces, in Solar Energy Engineering (A. A. M. Sayigh ed.), p. 233. Academic Press (1977). 9. G. De Maria, C. A. Tiberio, L. D'Alessio, M. Piccirilli, E. Coffari and M. Paolucci, Thermochemical conversion of solar energy by steam reforming of methane. Energy It, 805 (1986). tO. W. P. M. Van Swaij, Gasification. The process and the technology. Resources Conservation 7, 333 ( 1981). 11. G. K. Vick, Review of coal gasification technologies for the production of methane. Resources Conservation 7. 202. 12. F. Kaptein and J. A. Moulijn, Kinetics of catalysed and uncatalysed coal gasification. NATO ASI Carbon and Coal Gasification. Science and Technology, Alvor (Portugal), 2~31 May (1985), 13. A. P. Brucker, Continuous duty solar coal gasification system using molten slag and direct-contact heat exchange. Sol. Energy 34, 239 (1985). 14. J. A. Nedler and R. Mead, A simplex method for function minimization, Comput. J. 7, 308 (1964). 15. F. Scheid, Numerical Analysis, Chap. 25. McGraw-Hill (1968).