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Theoretical and experimental studies of solar catalytic power plants based on reversible reactions with participation of methane and synthesis gas
Int. J. Hydrogen Energy, Vol. 15, No. 4, pp. 275-286, 1990. Printed in Great Britain.
0360-3199/90 $3.00 + 0.00 Pergamon Press pie. © 1990 International Association for Hydrogen Energy.
THEORETICAL A N D EXPERIMENTAL STUDIES OF SOLAR CATALYTIC POWER PLANTS BASED ON REVERSIBLE REACTIONS WITH PARTICIPATION OF M E T H A N E A N D SYNTHESIS GAS V. I. AN1KEEV,V. N. PARMON,V. A. KIRILLOVand K. I. ZAMARAEV Institute of Catalysis, Novosibirsk 630090, U.S.S.R. (Received for publication 4 October 1989) Abstract--The results of complex theoretical and experimental investigations of applicatibility of the thermochemical cycles based on reversible catalytic reactions of the "Methane reforming-methanation process" for the needs of concentrated solar energy utilization are presented. The data obtained evidence for practical possibility to obtain sufficiently high efficiency for both primary conversion of the concentrated solar light into well-stored energy of chemical bonds and conversion of the latter into high-potential heat. The experimentally registered values of efficiency exceeding 40% for the first process and 23% for the closed-loop system which allow obtaining heat at more than 700°C.
INTRODUCTION Conversion of energy in closed thermochemical cycles, based on conducting a direct endothermic catalytic reaction via reactant heating by the primary heat source and a back exothermic catalytic reaction with producing the heat needed for the user, can be accomplished with a high efficiency. This as well as the possibility of an unlimited length for storing the converted energy in the form of chemical energy of energy-rich products of endothermic reactions gave rise to great interest in such cycles as an efficient tool for smoothing some instabilities between energy supply and consumption. Of special interest are theoretical and experimental analyses of the perspectives to utilize closed thermochemical cycles for solar energy conversion, that is, for the situation where a concentrated solar flux serves as a primary heat source. A preliminary theoretical analysis [1] has shown that the thermodynamically allowed efficiency, r/c, of thermochemical conversion of solar energy into Gibbs' free energy of endothermic reaction products (in other words, into work) can be estimated via a simple expression: r/o ~ (1 - To~T)(1 -- ~,T/Ts),
(1)
where To, Ts and T are, respectively, temperatures of environment (ca 300 K), the sun (ca 5800 K) and a converter heated by solar light; ~ ~> 1 is a dimensionless coefficient that depends upon the shape of a receiver of the concentrated solar light. From expression (1) it follows that the maximum value r/G ~ 50-60% (i.e. a very high value even in comparison with the efficiency of the best of the known quantum solar energy converters into electricity or chemical energy) can be expected for processes that
occur at T ~ 900-1100 K. Note, that the same temperature region is also of greatest interest for thermochemical conversion of heat produced by nuclear energy sources (see, for example [2-5]). One may, therefore, believe that the processes of steam and carbon dioxide reforming of methane into synthesis gas (endothermic step), which are comprehensively studied and already tested for heat energy conversion, CH4 + H 2 0 ~ C O + 3H2,
AH2°gs= 206.25 kJ mo1-1 (2)
CH4 + C O 2 ~ 2 C O + 2H2,
AH~9s = 247.4 kJ mo1-1 (3)
will be very promising for thermochemical conversion of solar energy too. The reactions (2) and (3) are reversible catalytic processes, which are accompanied by the easily catalysed "water gas shift" reaction. CO + H20~-~CO2 + H2, AH~gs = -41.03 k J m o l - k
(4)
In the presence of suitable catalysts all these processes occur at high rates at temperatures from 800 to 1000 K and were already regarded as candidates for thermochemical solar energy conversion [6-13]. In this work a complex theoretical and experimental analysis of the efficiency of conversion of concentrated solar energy is given for simple steady-state closed-loop thermochemical cycles based on reactions (2)-(4) and for solar catalytic reactors of the cavity type. POSSIBLE TECHNOLOGICAL SCHEMES FOR CLOSED-LOOP THERMOCHEMICAL POWER PLANTS Figure 1 displays a principal scheme of a simple possible closed-loop thermochemical power plant which
275
276
V.I. ANIKEEV et al.
made it possible to convert concentrated solar energy into chemical energy of products of the endothermic catalytic reaction, transfer this converted energy (without its long storage) to the user and evolve it in the form of heat. The scheme consists of reforming (I), compression (II) and methanation (III) units. A mixture of methane with steam, preheated in the heat-exchanger 2 by the heat of the converted methane-water mixture, is directed (at pressure Px) to the catalytic reactor 1 heated by the concentrated solar energy flux W. In this reactor, endothermic reactions (2) and (3) (the latter--since at a steady-state operation the mixture can contain also an admixture of CO2) of the methane reaction as well as the accompanying slightly exothermic reaction (4) are completed. The products of the high-temperature catalytic reaction of methane pass from reactor 1 to the countercurrent heat-exchanger 2, in which their high potential heat is used to evaporate the water and superheat the gas which enters into the reactor. As a result, the products of the methane reaction are cooled. In particular cases this cooling can lead to condensation of residual water vapor-gas mixture at the heat-exchanger outlet. The dried and cooled gaseous mixture passes (e.g. at a great distance) from the reforming unit into the adiabatic compressor 3, where its pressure is raised to P2 > PI. In the adiabatic methanation reactor 5, due to a shift of the equilibrium of reactions (2) and (3) to the left as well as due to the pressure increase and water removal, a highly exothermic catalytic reaction of methanation occurs to give methane and water. The heat evolved during the reaction is passed to the user in device 6. A gaseous mixture free from a water condensate is throttled in 7, passes through heat-exchangers 4 and 2, and then is again saturated with water and directed onto the inlet of reactor 1. Note that heatexchanger 4 is included in the scheme (Fig. 1) in order to balance the heat evolution at adiabatic gas compression with its consumption when heating the throttling gas without a heat-exchange with the environment. Evidently, this heat-exchanger is not necessary when the
,
I I
heating of the throttling gas occurs in the presence of heat exchange with the environment. The scheme in Fig. 1 also allows it to operate in a mode with immediate cooling of the catalyst bed inside the reactor 5. In this case, some portion of the evolved heat (WI) is removed directly from nonadiabatic methanation reactor 5 at a fixed temperature TM, along with the subsequent utilization of the heat of gaseous products (W2) in heatexchanger 6. While in the first version of the scheme the temperature TM, attained in the methanation reactor, is determined by only the composition of the inlet mixture (and thus actually by the temperature in the catalytic reactor of conversion unit) and by pressure /'2, in the second version of the scheme, TM is determined by conditions of heat removal and, thus, can remain unchanged upon variation of the composition of the gas flow at the inlet of the methanation reactor, that is, upon variation of conditions of conversion unit operation. However, in the latter case the power of the heat flow (W~) directed to the user at a fixed temperature level will be changed and, in some cases, even be negative (see further discussion and Fig. 9). The season and day-to-night variations of the solar energy flux and that of its consumption necessitate an accumulation of the energy. This can be done by using a technological scheme shown in Fig. 2. The scheme allows accumulation of periodical solar energy in the form of energy-rich products and then to use this accumulated energy for obtaining high-potential heat. Operation of the power plant in Fig. 2 can also be explained using reversible reaction (2) as an example. Accumulation of the products of the steam reforming of methane (H2, CO and CO2) in gas-holder A occurs during recirculation of the mixture in the methanation unit. Simultaneously, in order to provide a continuous operation of the solar reactor of methane reforming, methane is directed from gas-holder B to reforming unit I. As soon as the necessary depth of H2 and CO conversion into CH4 is attained, circulation is stopped; methane passes to the reforming reactor through throttle
j _ ,
....
I
''
#,
j
ir°2' 3'1
Fig. 1. Principal scheme (scheme I) of a closed-loop thermochemical power plant for solar energy conversion using catalytic processes of steam reforming of hydrocarbons. I--reforming unit; II-----compressionunit; III--methanation unit; 1--solar catalytic reactor for hydrocarbon reforming; 2,4--heat-exchangers; 3---compressor; 5-----catalytic methanation reactor; ~-user of the heat; 7--throtde turbine.
SOLAR CATALYTIC POWER PLANTS
~ IW -
B
l,Wg.]U._~_rt
Fig. 2. A possible technological scheme (scheme II) of a closed-loop thermochemical solar power plant with periodic accumulation of the products of the catalytic reactions. valve (or turbine) and heat exchangers, and the products of the methane reforming pass to the methanation reactor after some period of time, both the recirculation of the gas mixture in the methanation reactor and the accumulation of Hz, CO and CO2 in gas-holder A are re-initiated. During the time with a low level of solar irradiance or during the night time, that is, when it is not possible to produce H 2 and CO in the methanereforming reactor, and the necessity to receive heat still exists, these energy-rich components are directed into the methanation reactor from gas-holder A. Methane produced is pumped into gas-holder B. A simple periodic operation is also possible, that is, H2 and CO are accumulated during the day time, and during any other part of a daily cycle the energy-rich mixture is converted into CH 4 and the received heat is transferred to the user. T H E O R E T I C A L ESTIMATION O F THE E F F I C I E N C Y O F THE SOLAR E N E R G Y CONVERSION IN A CLOSED T H E R M O C H E M I C A L CYCLE Efficiency of the solar energy conversion in a closed thermochemical cycle can be characterized by coefficient q, which we define as a ratio of the useful heat power received by the user in unit III (Win) minus power Wn, consumed for the gas mixture compression in unit II, to the total power W of the concentrated solar flux entering the cavity of the solar catalytic reactor: rt = ( w i n -
WH)IW.
Here q ' = 1 - To~T6 is the highest possible efficiency of conversion of the heat with temperature T6 = TM, corresponding to the temperature of the heat-carrier at the inlet of heat-exchanger 6, into work; TO is the temperature of environment; Gc and G t stand for the mass flow of the gas mixture through the compressor and the throttle valve (turbine), respectively; Lc and Lt are the specific powers consumed by the compressor and produced by the turbine, respectively; r/c and r/t are efficiencies of the compressor and the turbine. Obviously, the value of q for the technological scheme with the accumulation of the energy (Fig. 2) will be somewhat smaller than that for the scheme in Fig. 1 because of the existence of additional compressors and devices providing the cycle operation. Thus, a strict theoretical estimation of r/ requires consideration of many parameters, which depend on constructive peculiarities of all units and devices included in the technological schemes (Figs 1 and 2). A detailed estimation of these parameters is rather time-consuming and presents a special chemical and engineering problem. F o r this reason, in analysis of a closed system operated under steady-state conditions, it seems reasonable to restrict ourselves to a series of simplifying assumptions, the main of which are: (1) A mass-balance with respect to the flow of carbon in a gaseous mixture is kept in all devices. (2) The gas mixture composition at the outlets of the methane-reforming and methanation reactors corresponds to thermodynamical equilibrium for the temperature and pressure at the outlets of the reactors as well as for the mixture composition at the inlets of the reactors. (3) The pressure drop in the methane-reforming and methanation reactors is taken equal to 10% of the pressure at the inlets. In this work, we calculate the conversion efficiency, r/, for only schemes shown in Fig. 1. Let us designate the total weight consumption of the mixture through reforming reactor-solar energy receiver 1 and heatexchanger 2 by G; concentration of all mixture components at the outlet and inlet of reactor 1 by Y~jand Y2j (index j from j = 1 to j = 5 refers to CH4, CO, H2, CO and H20, respectively), and pressure-independent specific enthalpies of these components at the outlet and inlet of the heat-exchanger (that is at fixed temperatures, T01 and T0, respectively, see Fig. 1) by HIj and H21. Thus, we have
(5) G
Taking into account that W n is actually the work, which can be performed using part of the heat, evolved to the user in heat-exchanger 6 (Fig. 1), and that gas expansion in the turbine (throttle valve) 7 allows one to perform the work, which can be used for compressing part of the gas, the minimal needed value of WH can be described via the expression w . = [ G c L c l n c - GtLt q,]ln'.
277
(6)
Y I j H I j -1
Y2jH2]
= ~T
W
(7)
j=l
where qr is the coefficient of conversion of the concentrated solar energy into the heat with temperature TR. The value qT depends on constructive peculiarities of the solar catalytic reactor and can easily be estimated for the most promising construction of the latter which is an ideal isothermic reactor of the cavity type [14-16, 19] with the temperature of the cavity TR. From an analysis
278
V.I. ANIKEEV et al. J~conv.
/
Thus equations (7) and (8) relate two independent variables G and Trt. Using the temperature dependence of the equilibrium constants of reactions (2), (3) and (4), one can easily find Yl: for given TR, Y2j, P1 and P1 out by the known procedure [21]. From the enthalpy balance in unit II it is possible to determine temperature T4 and the related enthalpy of a dried mixture after its compressing and passing through heat-exchanger 4:
~..-.w'--.~
@
4
Gc
Z
4
r"ljH4j = Gc
j=l
E
Y:Ho2j
j=l 4
Fig. 3. The structure of heat losses from the cavity-type catalytic reactor heated by concentrated solar energy flux.
+ GcLo - G, Lt - 6~ ~ Y,j(Ho,j- Ho,j)
(9)
j~l
where of heat losses from the cavity-type reactor (Fig. 3) we find r/r as follows: ~]T = [ W -
Wrad -- Wre fl -- W e o n v l / W
= [ W -- e a o ( T 4 -- T°)Si, -- (l -- e ) W -- ~ ( T R -- T o ) S w l / W ,
(8)
where wr~d, Wren and Wcon~are the energy losses through radiation of the cavity, reflection of the incident light and convection of the air; a 0 is the Stephan-Boltzmann coefficient for the ideal black body; Sin and S w are the surface areas of the inlet aperture and the inner walls of the cavity. From equation (8) it follows that the amount of energy absorbed by the cavity depends on self-radiation of the cavity walls at temperature TR through an aperture [the second term in numerator of (8)], on the value of a reflected flux, the third term and on losses by air convection. An effective coefficient of the cavity blackness, e, has been calculated for conic, cylindric and spheric shapes of the cavity [17]. The coefficient of convective heat-exchange ct was determined by the work [18]. Convective heat losses W~o~ from the reactor cavity depend on its geometry, area of the inlet aperture and inclination angle of the reactor relative to its vertical position; for an average cavity temperature higher than 1000 K convective losses W~o~,were shown to be much less than radiative losses Wr~d [19]. From an analysis of equation (8) it follows that with increasing power W of the incident solar energy flux at the fixed value of TR the contribution of losses via natural convection and re-radiation decreases, whereas value r/T approaches a limiting value. In Ref. [20] in order to diminish convective losses from the cavity-type receiver, its inlet aperture was closed by a quartz window; as a result, convective losses were decreased, but there appeared losses of the same order caused by reflection of the solar light from the quartz window. Using equation (8) it is possible to find the useful heat power, Wu~eml= r/~ W which can be converted into heat with temperature TR and then utilized to provide the endothermic catalytic reaction.
Y~j = Y , j / ( 1 -
Y , , ) , G t = Q(1 -- Y55)
= G(1 -- Y55)(1 -- Y1,), provided that the mixture, cooled in the heat-exchanger, has been completely dried, and H4: , H1j, H04i and H03j correspond to reagent enthalpies at temperatures T4, To2 = To4 = To and To3 (see Fig. 4). The values Lc can easily be calculated by the known equations [18]. L¢ = ~ - -K- ~ Ro To2 F(P2"~ ~ -
L\E)
where
11
(10)
4
g= Z ry,. j=l
R 0 is the universal gas constant, K: is the adiabatic index of the j t h component. L t is calculated by the equation similar to (10). Note that in calculations of the second version of scheme I (Fig. 1), the last two terms in the right-hand side equation (9) are equal to zero. By assuming that in catalytic methanation reactor 5 there is a thermodynamic equilibrium, determined by the composition of the inlet mixture, Y~j, temperature TM at the reactor outlet and pressures P2 and P2out (Fig. 1), from the enthalpy balance in this reactor 5
4
ac E Y5jn5J(TM)= Gc E Yijn4j(T4) j=l
(11)
j=|
and the known temperature dependence of reagent enthalpies and equilibrium constants of reactions (2), (3) and (4), one can find the value TM and concentrations of components Ysj, related to their concentrations at the methane reforming reactor 1 inlet through the following expression: (1 + Y25)Y2: = Ysfl(1 -- Yss), where j = I~IL For the methanation reactor with a fixed temperature (the second version of scheme I), the form of equation (l l) will be changed and allows determination of the heat power Wl : W I = Gc
Y ~ j H 4 y ( T 4 ) -j
1
YsjHsj(TM)
.
(12)
j~l
Thus, by fixing a degree of saturation of the mixture, directed to the methane reforming reactor 1, with water
SOLAR CATALYTIC POWER PLANTS (expressed, for example, as the ratio of molar portions of water and methane, fl, at the reactor 1 inlet) and choosing the corresponding values of T01, T03, T02 = / 0 4 = To, P1, e l out, t)2,192 out, w , Sin, Sw, £ and ~, we obtain a closed system of equations with respect to T R and TM. F r o m the values of TM and the concentrations Ysj of the components at the outlet of reactor 5 one can find the useful heat flow, W2, which is removed from heatexchanger 6 and transferred to the user, as well as the heat flow W l that is removed with a fixed temperature TM from the methanation reactor in the second version of scheme I:
2(4.1MPa) 0.8
0.7
0.6
I 700
I 900
I 1100
TR(K)
w~= wl + w2 W 2 = G¢
279
Fig. 5. Plot of q~ vs temperature in the methane reforming reactor Ta: 1--P 2 = 0.1 MPa; 2---4.1 MPa. The first version of scheme I.
Ysj [Hsj(TM) -- Hsj(To3)] J
+ Y55[H55(T,~) - n55(Ts) + Os(P2)
+ H~I~q) (Ts)
--
H(l~q)(To3)] t .
(13)
This expression gives a detailed description of the phenomena that occur in heat-exchanger 6 on cooling a v a p o r - g a s flow and on condensing water vapor. Here Qs stands for the heat of the water evaporation at pressure P2out; Ts is the boiling temperature. If the total value of heat transferred to a consumer is defined by Wz, it is evident that for the first version of scheme I W~ = I412, for the second version of scheme I Wz = W1 + W2. RESULTS OF CALCULATION Joint solution of the equations of the mathematical model discussed was done numerically with a view to examine dependence of r/ on parameters T R and P2 as well as on ratio fl = n~x2o/n~n4. In the calculation, the other parameters were Pl = 0.11 MPa, T01 = T03 = 350 K, T02 = T04 = 300 K, W/Sin = 1.6 103 kW m -2,
I (0.1 MPa)
SwlSi, = 30, E = 0.9, ~/¢ = /~t = 0.7, W = 8 kW, ~ = 2.2 W m 2 K - 1. Figures 4 and 5 show the dependence of ~/and n] = W z / W o n T R at different pressures P2 in the methanation reactor. It can be seen that efficiency of the concentrated solar energy conversion (both r/ and q]) passes through a maximum located in the temperature region T R ~ 900 K. The absolute values of r/ tend to decrease with increasing pressure P2, whereas the values of ql tend to increase (Fig. 5). The increase in ~/and n l with increasing TR for TR ~< 900 K is caused by a considerable temperature rise in the methanation reactor (Fig. 6); indeed, the value of TM is determined primarily by the degree of the methane conversion, Xcu4=(Y21-Yn)/Y2], In the methane reforming reactor (Fig. 7, curve 1). The decrease in r/ and ql with increasing TR for TR > 900 K is due both to a noticeable decrease in methane consumption during a cycle (Fig. 7, curve 2), which is needed for accumulating power qx W, and to a fall in rH (see Fig. 4, curve 4) despite the subsequent increase in TM. In Fig. 6 it can be also seen that TM is raised with increasing pressure P2, most markedly in the pressure range from 0.1 to 2 MPa.
]0.90
5 ( 1 1 0 0 K) 4 (1000 K)
1
0.7
3 ( 9 0 0 K)
1ooo
- 0.85 "qr 2 ( 8 0 0 K)
0.5
800 0.80
03
I 700
I I 900 11OO T~ (K) Fig. 4. Changes in conversion effÉciency, r/, (curves 1-3) and efficiency of the cavity-type receiver, qr (curve 4) vs temperature in the methane reforming reactor TR: l--P2=0.1 MPa; 2--1.1 MPa; 3--4.1 MPa. The first version of scheme I.
1
/ 600 0
I 1
I 2
I 3
I 4
P2 (MPa)
Fig. 6. Plot of temperature T M in the methanation reactor vs pressure P2: 1--TR = 700 K; 2--800 K; 3--900 K; 4--1000 K; 5--1100 K. The first version of scheme I.
280
V.I. ANIKEEV et al. 1 (0.1MPo) 1.0 15
0.8 # ~ 0.6
_ lO ~ = , d %
0.6t 0.4
0.4
5 0.2
0.2 0
I 700
I 900
I 1100
0
TR(K)
Fig. 7. Plot of the degree of methane conversion, XcH4, in the methane reforming reactor (curve 1) and its consumption in the cycle vs temperature TR of the reactor's cavity. The first version of scheme I; /'2 = 4 MPa. In large-scale industrial reactors, the steam reforming of hydrocarbons, in particular, methane, in the presence of an active catalyst is performed, water vapor is fed into the reactor in amounts 2-4 times larger than a stoichiometric a m o u n t in order to diminish carbon deposition. Our calculations indicate that an increase in fl with the other constant parameters does not lead to a significant change in t/ (Fig. 8) over the range of parameters studied. Only f o r / ' 2 = 4 M P A and T R = 700 K (Fig. 8, curve 1) there is a pronounced increase in t/when fl varies from 2 to 4. An analysis of the second version o f scheme I (Fig. 1) was made mainly for the estimation o f the fraction of the heat power W~ in the total heat flow W,- = W 1 + W 2 transferred to the user. F o r example, Fig. 9 displays variations in r / a n d ~ = WI W over the pressure P2 range from 0.1 to 4 . 1 M P a as a function of TR for fixed TM = 800 K and fl = 2. One can see that the value of W1 in some cases can be negative; this means that an
o
zoo/
900
looo
11oo
/ T R (K)
/
I
Fig. 9. Region of ~/variations as a function of TR restricted by curves 1 (P2 = 0.1 MPa) and 1' (P2 = 4.1 MPa) and region of variations as a function of TR restricted by curves 2 (/'2=0.1MPa) and 2' (/'2=4.1MPa); TM = 800K; /3 =2. additional amount of heat (WI) has to be supplied to the catalytic reactor in order to maintain the needed level of temperature. F o r example, this is the matter when one has to ensure T M = 8 0 0 K at T R < 7 4 0 K (and P2 = 4.1 MPa). The figure shows also that an increase in TR leads to an increase in ~ over almost the whole range of the temperature variations. Figure 10 demonstrates the regions of t/ and r7 = IV]/W2 variations for 0.1 ~< P2 ~< 4.1 M P a as functions of the fixed temperature TM in the methanation reactor. The maximum efficiency of energy conversion,
0.8
-
0.6 -
4
~
)
N
3
6 ( 9 0 0 K)"I 5 ( 8 0 0 K);~ 0.1 MPo 4 (700 K ) J
0.7
3 ( 9 0 0 K)" I 2 ( 8 0 0 K) / 0.5
0.3
aoo
Po)
4.1 MPo
1 ( 7 0 0 K)J
I 2
I 3
0.2
I 4
0 I
400
B Fig. 8. Plot o f q vs the ratio /3: 1 - - T ~ = 7 0 0 K ;
2--800K;
3 - - 9 0 0 K (for curves 1-3 P 2 = 4 . 1 M P a ) ; 4 ~ T R = 7 0 0 K ; 5-800 K; 6--900 K (for curves 4-6 P= = 0.1 MPa). The first
version of scheme I.
2(0.1 MPa) ~
I
1
I
600
800
o
looo
T u (K)
Fig. 10. Plot of q (curves 1 and l') and ~ (curves 2 and 2') vs TM at TR= II00K, fl =2; 1,2--P 2=0.1MPa; 1',2'-/'2 = 4.1 MPa.
281
SOLAR CATALYTIC POWER PLANTS
1.5
Table 1. Some summarized data on the best experimental runs of the SCR-3 solar catalytic reactor; Tat stands for the averaged temperatures of the catalyst's bed
-
5 (1100 K) 4 (1000 K) 1.0
3 (90OK)
T=,
Woh~/W Wohm
Process tested
(K)
(%)
(kW)
CH 4 + H20 CH 4 + CO 2 (0.17 C3Hs + 0.83 C4Hl0) + H20
8333 933 853
43 34 10
2.0 1.43 0.38
0.5
2 (800K) ]//f'~'~21 31
~
-0.5
I
4
Pa(MPo) 1 ( 7 0 0 K)
Z
Fig. 11. Plot of ~ vs pressure P2 at different Tc : 1--T R = 700 K; 2--800 K; 3--900 K; 4--1000 K and 5---1100 K. q, is attained for TM ~ 7 0 0 K Trt = 1100 K, fl = 2) and P2 = 0.11 MPa. Besides, within the TM range from 400 to 900 K, at/)2 = 0.11 M P a , the absolute values of q exceed at P2 > 0.11 M P a because of power consumptions for compressing the gas mixture by the compressor. Moreover, as it follows from the dependence o f ~ on TM, at TM < 800 K, the a m o u n t of heat W 1, removed from the methanation reactor, is higher (almost by the factor of 4 at T M = 400 K and P2 = 4.1 MPa) then W 2 transferred to the user through heat-exchanger 6. Finally, Fig. 11 illustrates the dependence of q on pressure P2 over the range of T R variations from 700 to 1100 K at TM = 800 K and fl = 2. F r o m the shape of the
curves it follows that at the fixed values of TM and fl the heat power W~ can exceed I4/2 only at TR > 1000 K and /)2 > 1.0 MPa. Thus, the estimates obtained for the efficiency of the energy conversion via the proposed schemes of the closed-loop thermochemical cycle with the usage of the steam reforming of methane allow the selection of optimal operational conditions for the methanereforming and methanation reactors, which will provide a needed potential o f utilized heat and a high energy conversion coefficient. The high theoretical efficiency of the thermochemical conversion of solar energy into heat (about 70%) in a closed-loop cycle has stimulated experimental works on the designing of a system with such a cycle and attaining a stationary operation of this system. EXPERIMENTAL STUDY OF A SMALL SOLAR CATALYTIC POWER PLANT BASED ON THE CLOSED-LOOP THERMOCHEMICAL CYCLE In order to verify the predictions of the above discussed theory, an experimental power plant based on the closed-loop thermochemical cycle has been designed to be used for conversion of the concentrated solar energy [22]. Experiments were performed in September 1984-1986 in the Crimea. A reflector of the solar energy concentrator S G U - 7 [23] served as a source of the concentrated solar light (Fig. 12). At the first step of the experimental studies we examined both thermophysical properties of the designed solar catalytic reactors [16] and the efficiency of
9
w
A
8
. oog,s,s .'l
I
2
Fig. 12. Reflector concentrator of the device SGU-7 with the reactor SCR-5 in operation.
Fig. 13. The principal scheme of the tested experimental solar catalytic power plant based on the thermochemical cycle. 1--solar catalytic reactor for hydrocarbon reforming SCR-5; 2,3,7--heat-exchangers; 4,8--water settling tank; 5-~ompressor; 6~methanation reactor CRM-2; 9--throttle valve.
282
V.I. ANIKEEV
et al.
Fig. 14. A general view of the methanation reactors CRM-1 (a) and CRM-2 (b). the conversion of the solar energy into the energy of chemical bonds through the steam and carbon dioxide reforming of saturated hydrocarbons (both of methane and of a propane-butane mixture) [24]. A cavity-type solar catalytic reactor SCR-3, heated by concentrated solar light, was used for these purposes [25]. As a measure of the efficiency of the solar energy conversion, we used a ratio of the gas mixtures enthalpy increase Wchem(per unit time) due to formation of the energy-rich chemical products to the total power of the solar flux falling inside the reactor cavity (Table 1). One can see that the construction of the SCR-3 solar catalytic reactor tested allows to reach a high (more than 40%) efficiency of the solar energy conversion into the energy of chemical bonds when the steam reforming of methane is performed. The second step of our experiments was testing the closed-loop thermochemical system which is schematically shown in Fig. 13. The system is represented by the solar catalytic reactor SCR-5 (a modified version of the reactor SCR-3) and the catalytic reactor ofmethanation CRM-1 or CRM-2 (see Fig. 14), all designed at the Institute of Catalysis of the Siberian
Branch of the U.S.S.R. Academy of Sciences. The catalytic reactors were removed from each other by more than 20 m. The solar catalytic reactor SCR-5 (Fig. 15) contains in the insulated housing a water evaporator, an overheater of steam, a solar-flux receiving cavity around which the catalyst bed is located, and counter-current heatexchangers for recuperation of the heat of reaction products. Water evaporation and overheating of steam to the temperature of the catalyst bed are accomplished solely at the expense of solar flux energy. The amount of the granulated catalyst load equals 2 dm 3. The diameter of the SCR-5 external housing is 42 cm, its height is 34 cm. The weight of all reactors, including the catalyst does not exceed 15 kg. The inlet diaphragm of the SCR cavity (160 mm in diameter) is located in the plane of the focal spot of the reflector concentrator of solar light, the diameter of the spot being 170 ram. The catalytic reactor of methanation CRM-2 (see its flow-sheet in Fig. 16) contains several tubes with the catalyst and can operate in two regimes of heat removal. The dry cooled mixture of H2, CO, CO2 and CH4 is passed from the methane reforming reactor into
SOLAR CATALYTIC POWER PLANTS
Fig. 15. Solar catalytic reactor SCR-5 after exploiting under action of concentrated solar energy flux. compressor 1, where it is compressed to pressure P2 and then is directed onto the catalyst bed. Prior to the operation of the system, the reaction mixture passes through furnace 3 in order to preheat the catalyst bed to the temperature the reaction starts. Then by switching valve 2, a cool gas mixture is fed onto the bed of catalyst 4. The catalyst bed in the tube is surrounded by heatexchanger 5. Hot products formed during the methanation reaction are directed to heat-exchanger 6, where
---- × 4 ~
8
Fig. 16. Flowsheet of the H 2 and CO in the methanation reactor CRM-2. 1---compressor; 2,9--valves; ~,,atalytic reaction ignition furnace; 4--catalyst bed; 5,6---heat-exchangers; 7--settling tank; 8--throttle-valve.
283
they transfer their heat to the heat-carrier. The condensed water is removed from the gaseous mixture in the settler 7, and then the gas passes through a throttle valve 8 and line-pipes to the solar methane reforming reactor. A commercial catalyst N K M - 4 with granules c a 2-3 mm in diameter was loaded into the methanation reactor. The scheme is intended for operation of the methanation reactor in (i) adiabatic regime (when the heatcarrier does not pass through heat-exchanger (5) and (ii) in regime of the external cooling of the catalyst bed. The latter cooling can be organized either autonomously by special gas or liquid (parallel cooling) or by the heatcarrier preheated in heat-exchanger 6 (successive cooling). Note that results of the energy plant tests reported in this work have been obtained at CRM-2 operation in the adiabatic regime. The circulation of a gas mixture and an excess pressure in the methanation reactor were provided by an electric compressor. The frequency of the gas recirculations in the closed-loop circuit of this experimental power plant was about 200 cycles per hour, that was obviously enough to create a steady-state during the typical experimental run (the latter usually of 3-4 h). The steady-state of a thermochemical cycle was established in less than 20 rain. The dry gas mixture consumption, IrA, in the methane-reforming reactor was measured by a flowmeter; the liquid water consumption VH2o was also monitored. Water was evaporated directly in the solar catalytic reactor by the heat of the concentrated solar light. Initially the system was filled by pure methane. The total light power W collected by the reflector in the reactor cavity was monitored by an actinometer and was equal c a 5 kW. The reaction of the reforming of methane in reactor 1 (Fig. 13) was carried out from 0.18 to 0.28 MPa; the pressure P2---0.6 MPa was maintained in methanation reactor 6 using compressor 5 and valve 9. The temperatures in the catalyst bed of the methane-reforming (TR) and methanation (TM) reactors and the temperature of a vapor-gas mixture (Tout) at the reactor 1 outlet after some heat recuperation were measured by a set of thermocouples. The reaction products dried from water were analysed chromatographically through a dosing valve directly from the reaction cycle (Fig. 13), that is, after the outlets of the methane-reforming and methanation reactors. Taking into account that chromatographically obtained data gave an imbalance less than _+ 10%, the sum of measured volume concentrations of the gas components in the dry mixture was reduced to 100% (see Table 2). The experimentally measured steady-state contents of the dry gas mixture at points A and B of the scheme on Fig. 13, volumes VA and Vn2o, over the set of thermocouples averaged temperature of the catalyst bed in CRM-2 (at the outlet maximum), as well as total light flux power W for some experimental runs are listed in Table 2. Given are also calculated values of the contact time, z (from the gas volume flow at the CRM-2 inlet).
284
V.I. ANIKEEV
et al.
Table 2. Parameters of the steady-state operation of the experimental solar catalytic power plant on the basis of methane-reforming and methanation reactions Composition of a dry gas mixture at the inlet and outlet of the methanation reactor (% by volume) TM inlet outlet VA Va~o bl(H2) b2(CO)b3(CH4) b4(CO2) al(H2) a2(CO) a3(CH4) a4(CO2) (m3h -I) (kgh -l) (°c)
"c
W
(s) (kW)
Wchem Wehem"}- Wout
W
w
0.21
0.38
0.22
0.34
4.33
0.23
0.42
0.563
0.262
0.0
0.175
0.24
0.15
0.26
0.35
1.32
0.69
630 3.77 4.07 7OO
0.613
0.283
0.01
0.1
0.31
0.161
0.269
0.26
1.29
0.51
__655
0.735
0.149
0.01
0.106
0.389
0.107
0.294
0.21
1.29
0.92
650 2.9 730
0.68
0.23
0.0
0.09
386
0.113
0.303
0.198
1.29
0.6
--
650 3.59 4.33 730
0.23
0.34
0.683
0.225
0.016
0.076
0.394
0.111
0.313
0.182
1.27
0.6
655 7~0 3.55 4.33
0.21
0.34
0.725 0.179
0.01
0.09
0 . 4 5 5 0.075
0.291
0.179
1.34
1.8
610 - - 3.42 4.33 705
0.22
0.57
0.745
0.01
0.139
0.465
0.062
0.298
0.175
1.34
1.8
630 - - 3.18 4.33 690
0.21
0.54
0.735 0.169
0.0
0.096
0.247
0.0
0.553
0.173
0.38
0.5
520 - - 7.63 4.27 725
0.12
0.2
0.728 0.189
0.027
0.056
0.229
0.0
0.597
0.174
0.39
0.45
520 - - 7.46 4.27 725
0.13
0.21
0.749
0.032
0.047
0.272
0.0
0.648
0.08
0.36
0.45
520 7.95 4.27 725
0.12
0.19
0.112
0.172
The volume flow VB of the dry gas at the outlet of reactor 1 can be easily determined from the known compositions of the dry gas at points A and B of the scheme and from value VA: VB = VA
a/ i
)
bi i
(14)
V~2o = VB(2bl -- b2 + 4b3 - 2b4) V A ( 2 a l - - a2 + 4a3 - -
3.66 4.1
Thus the power W~h~maccumulated by methane-reforming products (or evolved in the methanation reactor), which is defined as a difference between the enthalpies of the gas mixture at the outlet and inlet per unit time at the same temperature To = 298 K, can be expressed in the following manner: 4
(the notation is given in Table 2). Expression (14) is derived from an elementary balance of carbon in the absence of coke formation since no free carbon was detected on the catalyst during experiments. The latter is confirmed also by the constancy of the parameters measured; the pressures Pa and P2, volume gas flow Va and the contents of the gas mixture during the cycle stationary operation (from 2 to 3.5 h) in each experimental run. The a m o u n t of water, Vn~o, consumed during methane reforming or evolved during methanation at the steady-state operation of the system was determined according to an elementary mass balance by the following equation:
-
710
2a4)
(15)
Wchem = VB E b ~ ' H ~ 9 . i=1 4 -
vA ~ a:I-I,~,~- Vh2o.H~2o.29~ (16) i=1
where H~9" is the specific enthalpy of formation of the ith component at standard conditions. The values of Wchom calculated by this equation for each experimental run as well as the ratio W c h e m / W (i.e. the efficiency of the solar energy conversion into chemical energy) are compiled in Table 2. As is seen from the Table, the ratio W ~ h e m / W did not exceed 23% which is less than what was received during the solar catalytic reactor SCR-3 open-circuit tests (see Table 1). The latter is due to the fact that, at the steady-state closed-loop operation the content of gas mixture at the inlet of SCR differs strongly from that of open-circuit tests.
SOLAR CATALYTIC POWER PLANTS Indeed, from the tabulated results it follows that reforming of methane in the solar catalytic reactor is almost complete at any regime (temperature of the gas mixture at the catalyst bed outlet changes within 650-700°C), and only the mixture of H2, CO and CO 2 passes to the CRM inlet. However, the depth of one-step methanation in the reactor operated under close to adiabatic conditions is insufficiently high. As the result, methane is strongly diluted with H2 and CO2 at the inlet of the solar catalytic reactor. For this reason the ratio between Wchemand W due to energy enrichment of the products of the endothermic catalytic reaction is here less than in experiments without dilution of methane and does not exceed 23% at the close-loop experiments. Thus, in order to find further ways for increasing the efficiency of the energy system, it is necessary to know everything about physico-chemical processes that occur in the methanation reactor. Using an external cooling of the catalyst bed of the methanation reactor, it is possible to decrease the bed temperature, and thus increase the yield of methane from CRM. This will lead consequently, to an increasing efficiency of the solar methane reforming reactor and of the whole energy system. One can also see from Table 2, that a sufficiently highlevel temperature (up to 700°C) was achieved inside the methanation reactor catalyst bed during the steady-state experiments. This evidences for a real possibility to use the closed-loop solar thermochemical systems for some practical needs (e.g. for further electricity production via an intermediate overheated steam generation). In order to estimate the obtained energetic efficiency of the methanation reactor, we calculated the heat power WM, evolved in heat-exchanger 7 as a result of the gas-mixture heat utilization at the methanation reactor outlet. Since this heat was removed with water, that cooled the evolved gas down to 30-35°C, the values of WM were readily calculated as the enthalpy change (per unit time) of the gas mixture at the heat-exchanger outlet and inlet. As shown in [22], the values of Wra/Wch~m obtained appear to be sufficiently high (up to 65%) for the primitive construction of the methanation reactor tsted and thus indicate the possibility to design the reactors with more efficient conversion of the energy of the exothermic methanation reactor into heat energy transferred to the user. Table 2 gives also estimations of the total efficiency of accumulation in the solar catalytic energy reactor, the ratio (Wchem+ Wout)/W, which take into account power Wout obtained from utilization of the heat of the vaporgas mixture at the SCR-5 reactor outlet (the temperature of this mixture was ca 300°C). Note that only a small portion [see expression (15)] of the introduced water is consumed in the methane-reforming reactions. CONCLUSION The theoretical analysis and experimentations on the steady-state operation of the closed-loop thermochemical solar power plants on the basis of reversible
285
catalytic reactions of the methane reforming and syn-gas methanation evidence for fairly good practical potentialities of these power plants, the first generation of the experimental plants of this type having demonstrated a satisfactory efficiency of the concentrated solar energy storage, transmission and conversion into a high-potential heat. The data obtained allow further improvement of both the solar methane-reforming reactor and methanation reactor, as well as the choice of optimal conditions of the total system operation. One could expect that in the near future the solar catalytic power plants of the considered type will excite a commercial interest. REFERENCES 1. V. N. Parmon, Maximum possible efficienciesof conversion of solar energy into chemical energy. In K. I. Zamaraev (ed.), Photocatalytic Solar Energy Conversion, pp. 42-57. Part I, Nauka, Novosibirsk. 2. H. Fedders and B. Hohlein, Int. J. Hydrogen Energy 7, 793-800 (1980). 3. E. Nazarov and A. Stolyarevskii, in V. A. Legasov (ed.), Atomic Hydrogen Energetics and Engineering, 3, pp. 5-57, Atomizdat, Moscow. 4. N. N. Ponomarev-Stepnoi, A. N. Protsenko, A. Ya. Stolyarevskii, Voprosy Atomnoi Nauki i Tekhniki, ser. Atomnovodorodnaya Energetika i Tekhnologiya, 1, 74-86 (1978). 5. E. Shpilrain, S. P. Malyshenko and G. G. Kuleshov, Introduction in Hydrogen Energy, Energoatomizdat, Moscow (1984). 6. V. I. Anikeev, V. N. Parmon, V. A. Kirillov and K. I. Zamaraev, Application of mathematical modelling to calculation of catalytic reactors for thermochemicalconversion of solar energy, Italian-Soviet Seminar Catalysis Application to Energy Problem Messina 1-23 (1984). 7. R. B. Akhmedov and M. A. Berchenko, Conversion and storage solar energy via simple thermochemical reactions. In K. ~I. Zamaraev (ed.), Photocatalytic Solar Energy Conversion, Part I, pp. 58~8, Nauka, Novosibirsk. 8. T. A. Chubb, Solar Energy 17, 129-136 (1975). 9. T. A. Chubb, J.- J. Nemecek and D. E. Simmons, Solar Energy 20, 219-224 (1978). 10. T. A. Chubb, Solar Energy 24, 341 (1980). 11. J. H. McGrary, G. S. McGrary, T. A. Chubb, J. J. Nemecek and D. E. Simmona, Solar Energy 29, 141-151 (1982). 12. Y. Villermaux, Entropie 85, 25-31 (1979). 13. K. I. Zamaraev and V. N. Parmon, Chemical method of solar energy conversion, in K. I. Zamaraev (ed,), Photocatalytic Solar Energy Conversion, Part I, pp. 7-42, Nauka, Novosibirsk (1985). 14. V. A. Grilikhes, V. M. Matveev and V. B. Poluektov, High-Temperature Solar Heat Sources for Cosmic Apparatus, Mir, Moscow (1983). 15. C. W. Stephers and A. H. Haiz, J. Rokets Technik 7, 51 (1961). 16. V. I. Anikeev, V. A. Kuzmin, V. A. Kirillov and V. N. Parmon, Experimental study of heat losses and temperature distribution within cavity-type catalytic reactor, Geliotekhnika, No. 6, 45-49 0987). 17. E. H. Sparrow and R. D. Cess, Radiation Heat Transfer, Brooks Cole, Belmont, California 0970). 18. A. G. Kasatkin, Fundamental Processes and Apparatus of Chemical Technology, Khimiya, Moscow (1960).
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19. Yu. I. Aristov, V. I. Anikeev, V. A. Kirillov, V. N. Parmon and K. I. Zamaraev, Thermochemical Conversion of Solar Energy in Catalytic Reactors, Potentialities of Catalysts, Institute of Catalysis, Novosibirsk (1986). 20. Direct Flux Solar Chemical Reactors, Project 61065, Final Report (1984). 21. Nitrogenist's Handbook, Vol. 1, Khimiya, Moscow (1967). 22. V. I. Anikeev, V. A~ Kuzmin, V. A. Kirillov, I. I. Bobrova, V. V. Pasichnyi, V. N. Parmon and K. I. Zamaraev, Experimental studies of a solar catalytic energy plant based
on closed thermochemical cycle. Dokl. Akad. Nauk SSSR 293, 1427-1432 (1987). 23. V. S. Dvernyakov, Kinetics of High-Temperature Metal Destruction, Naukova Dumka, Kiev (1981). 24. V. I. Anikeev, V. N. Parmon, Yu. I. Aristov, V. I. Zheivot, V. A. Kirillov and K. I. Zamaraev, Dokl. Akad. Nauk SSSR, (transl.: Soviet Phys. Dokl.) 289, 158-162 (1986). 25. V. I. Anikeev, V. N. Parmon, V. A. Kirillov and K. I. Zamaraev, Catalytic Chemical Reactor, Authorship Certificate No. 1190157 (1985).
0360-3199/90 $3.00 + 0.00 Pergamon Press pie. © 1990 International Association for Hydrogen Energy.
THEORETICAL A N D EXPERIMENTAL STUDIES OF SOLAR CATALYTIC POWER PLANTS BASED ON REVERSIBLE REACTIONS WITH PARTICIPATION OF M E T H A N E A N D SYNTHESIS GAS V. I. AN1KEEV,V. N. PARMON,V. A. KIRILLOVand K. I. ZAMARAEV Institute of Catalysis, Novosibirsk 630090, U.S.S.R. (Received for publication 4 October 1989) Abstract--The results of complex theoretical and experimental investigations of applicatibility of the thermochemical cycles based on reversible catalytic reactions of the "Methane reforming-methanation process" for the needs of concentrated solar energy utilization are presented. The data obtained evidence for practical possibility to obtain sufficiently high efficiency for both primary conversion of the concentrated solar light into well-stored energy of chemical bonds and conversion of the latter into high-potential heat. The experimentally registered values of efficiency exceeding 40% for the first process and 23% for the closed-loop system which allow obtaining heat at more than 700°C.
INTRODUCTION Conversion of energy in closed thermochemical cycles, based on conducting a direct endothermic catalytic reaction via reactant heating by the primary heat source and a back exothermic catalytic reaction with producing the heat needed for the user, can be accomplished with a high efficiency. This as well as the possibility of an unlimited length for storing the converted energy in the form of chemical energy of energy-rich products of endothermic reactions gave rise to great interest in such cycles as an efficient tool for smoothing some instabilities between energy supply and consumption. Of special interest are theoretical and experimental analyses of the perspectives to utilize closed thermochemical cycles for solar energy conversion, that is, for the situation where a concentrated solar flux serves as a primary heat source. A preliminary theoretical analysis [1] has shown that the thermodynamically allowed efficiency, r/c, of thermochemical conversion of solar energy into Gibbs' free energy of endothermic reaction products (in other words, into work) can be estimated via a simple expression: r/o ~ (1 - To~T)(1 -- ~,T/Ts),
(1)
where To, Ts and T are, respectively, temperatures of environment (ca 300 K), the sun (ca 5800 K) and a converter heated by solar light; ~ ~> 1 is a dimensionless coefficient that depends upon the shape of a receiver of the concentrated solar light. From expression (1) it follows that the maximum value r/G ~ 50-60% (i.e. a very high value even in comparison with the efficiency of the best of the known quantum solar energy converters into electricity or chemical energy) can be expected for processes that
occur at T ~ 900-1100 K. Note, that the same temperature region is also of greatest interest for thermochemical conversion of heat produced by nuclear energy sources (see, for example [2-5]). One may, therefore, believe that the processes of steam and carbon dioxide reforming of methane into synthesis gas (endothermic step), which are comprehensively studied and already tested for heat energy conversion, CH4 + H 2 0 ~ C O + 3H2,
AH2°gs= 206.25 kJ mo1-1 (2)
CH4 + C O 2 ~ 2 C O + 2H2,
AH~9s = 247.4 kJ mo1-1 (3)
will be very promising for thermochemical conversion of solar energy too. The reactions (2) and (3) are reversible catalytic processes, which are accompanied by the easily catalysed "water gas shift" reaction. CO + H20~-~CO2 + H2, AH~gs = -41.03 k J m o l - k
(4)
In the presence of suitable catalysts all these processes occur at high rates at temperatures from 800 to 1000 K and were already regarded as candidates for thermochemical solar energy conversion [6-13]. In this work a complex theoretical and experimental analysis of the efficiency of conversion of concentrated solar energy is given for simple steady-state closed-loop thermochemical cycles based on reactions (2)-(4) and for solar catalytic reactors of the cavity type. POSSIBLE TECHNOLOGICAL SCHEMES FOR CLOSED-LOOP THERMOCHEMICAL POWER PLANTS Figure 1 displays a principal scheme of a simple possible closed-loop thermochemical power plant which
275
276
V.I. ANIKEEV et al.
made it possible to convert concentrated solar energy into chemical energy of products of the endothermic catalytic reaction, transfer this converted energy (without its long storage) to the user and evolve it in the form of heat. The scheme consists of reforming (I), compression (II) and methanation (III) units. A mixture of methane with steam, preheated in the heat-exchanger 2 by the heat of the converted methane-water mixture, is directed (at pressure Px) to the catalytic reactor 1 heated by the concentrated solar energy flux W. In this reactor, endothermic reactions (2) and (3) (the latter--since at a steady-state operation the mixture can contain also an admixture of CO2) of the methane reaction as well as the accompanying slightly exothermic reaction (4) are completed. The products of the high-temperature catalytic reaction of methane pass from reactor 1 to the countercurrent heat-exchanger 2, in which their high potential heat is used to evaporate the water and superheat the gas which enters into the reactor. As a result, the products of the methane reaction are cooled. In particular cases this cooling can lead to condensation of residual water vapor-gas mixture at the heat-exchanger outlet. The dried and cooled gaseous mixture passes (e.g. at a great distance) from the reforming unit into the adiabatic compressor 3, where its pressure is raised to P2 > PI. In the adiabatic methanation reactor 5, due to a shift of the equilibrium of reactions (2) and (3) to the left as well as due to the pressure increase and water removal, a highly exothermic catalytic reaction of methanation occurs to give methane and water. The heat evolved during the reaction is passed to the user in device 6. A gaseous mixture free from a water condensate is throttled in 7, passes through heat-exchangers 4 and 2, and then is again saturated with water and directed onto the inlet of reactor 1. Note that heatexchanger 4 is included in the scheme (Fig. 1) in order to balance the heat evolution at adiabatic gas compression with its consumption when heating the throttling gas without a heat-exchange with the environment. Evidently, this heat-exchanger is not necessary when the
,
I I
heating of the throttling gas occurs in the presence of heat exchange with the environment. The scheme in Fig. 1 also allows it to operate in a mode with immediate cooling of the catalyst bed inside the reactor 5. In this case, some portion of the evolved heat (WI) is removed directly from nonadiabatic methanation reactor 5 at a fixed temperature TM, along with the subsequent utilization of the heat of gaseous products (W2) in heatexchanger 6. While in the first version of the scheme the temperature TM, attained in the methanation reactor, is determined by only the composition of the inlet mixture (and thus actually by the temperature in the catalytic reactor of conversion unit) and by pressure /'2, in the second version of the scheme, TM is determined by conditions of heat removal and, thus, can remain unchanged upon variation of the composition of the gas flow at the inlet of the methanation reactor, that is, upon variation of conditions of conversion unit operation. However, in the latter case the power of the heat flow (W~) directed to the user at a fixed temperature level will be changed and, in some cases, even be negative (see further discussion and Fig. 9). The season and day-to-night variations of the solar energy flux and that of its consumption necessitate an accumulation of the energy. This can be done by using a technological scheme shown in Fig. 2. The scheme allows accumulation of periodical solar energy in the form of energy-rich products and then to use this accumulated energy for obtaining high-potential heat. Operation of the power plant in Fig. 2 can also be explained using reversible reaction (2) as an example. Accumulation of the products of the steam reforming of methane (H2, CO and CO2) in gas-holder A occurs during recirculation of the mixture in the methanation unit. Simultaneously, in order to provide a continuous operation of the solar reactor of methane reforming, methane is directed from gas-holder B to reforming unit I. As soon as the necessary depth of H2 and CO conversion into CH4 is attained, circulation is stopped; methane passes to the reforming reactor through throttle
j _ ,
....
I
''
#,
j
ir°2' 3'1
Fig. 1. Principal scheme (scheme I) of a closed-loop thermochemical power plant for solar energy conversion using catalytic processes of steam reforming of hydrocarbons. I--reforming unit; II-----compressionunit; III--methanation unit; 1--solar catalytic reactor for hydrocarbon reforming; 2,4--heat-exchangers; 3---compressor; 5-----catalytic methanation reactor; ~-user of the heat; 7--throtde turbine.
SOLAR CATALYTIC POWER PLANTS
~ IW -
B
l,Wg.]U._~_rt
Fig. 2. A possible technological scheme (scheme II) of a closed-loop thermochemical solar power plant with periodic accumulation of the products of the catalytic reactions. valve (or turbine) and heat exchangers, and the products of the methane reforming pass to the methanation reactor after some period of time, both the recirculation of the gas mixture in the methanation reactor and the accumulation of Hz, CO and CO2 in gas-holder A are re-initiated. During the time with a low level of solar irradiance or during the night time, that is, when it is not possible to produce H 2 and CO in the methanereforming reactor, and the necessity to receive heat still exists, these energy-rich components are directed into the methanation reactor from gas-holder A. Methane produced is pumped into gas-holder B. A simple periodic operation is also possible, that is, H2 and CO are accumulated during the day time, and during any other part of a daily cycle the energy-rich mixture is converted into CH 4 and the received heat is transferred to the user. T H E O R E T I C A L ESTIMATION O F THE E F F I C I E N C Y O F THE SOLAR E N E R G Y CONVERSION IN A CLOSED T H E R M O C H E M I C A L CYCLE Efficiency of the solar energy conversion in a closed thermochemical cycle can be characterized by coefficient q, which we define as a ratio of the useful heat power received by the user in unit III (Win) minus power Wn, consumed for the gas mixture compression in unit II, to the total power W of the concentrated solar flux entering the cavity of the solar catalytic reactor: rt = ( w i n -
WH)IW.
Here q ' = 1 - To~T6 is the highest possible efficiency of conversion of the heat with temperature T6 = TM, corresponding to the temperature of the heat-carrier at the inlet of heat-exchanger 6, into work; TO is the temperature of environment; Gc and G t stand for the mass flow of the gas mixture through the compressor and the throttle valve (turbine), respectively; Lc and Lt are the specific powers consumed by the compressor and produced by the turbine, respectively; r/c and r/t are efficiencies of the compressor and the turbine. Obviously, the value of q for the technological scheme with the accumulation of the energy (Fig. 2) will be somewhat smaller than that for the scheme in Fig. 1 because of the existence of additional compressors and devices providing the cycle operation. Thus, a strict theoretical estimation of r/ requires consideration of many parameters, which depend on constructive peculiarities of all units and devices included in the technological schemes (Figs 1 and 2). A detailed estimation of these parameters is rather time-consuming and presents a special chemical and engineering problem. F o r this reason, in analysis of a closed system operated under steady-state conditions, it seems reasonable to restrict ourselves to a series of simplifying assumptions, the main of which are: (1) A mass-balance with respect to the flow of carbon in a gaseous mixture is kept in all devices. (2) The gas mixture composition at the outlets of the methane-reforming and methanation reactors corresponds to thermodynamical equilibrium for the temperature and pressure at the outlets of the reactors as well as for the mixture composition at the inlets of the reactors. (3) The pressure drop in the methane-reforming and methanation reactors is taken equal to 10% of the pressure at the inlets. In this work, we calculate the conversion efficiency, r/, for only schemes shown in Fig. 1. Let us designate the total weight consumption of the mixture through reforming reactor-solar energy receiver 1 and heatexchanger 2 by G; concentration of all mixture components at the outlet and inlet of reactor 1 by Y~jand Y2j (index j from j = 1 to j = 5 refers to CH4, CO, H2, CO and H20, respectively), and pressure-independent specific enthalpies of these components at the outlet and inlet of the heat-exchanger (that is at fixed temperatures, T01 and T0, respectively, see Fig. 1) by HIj and H21. Thus, we have
(5) G
Taking into account that W n is actually the work, which can be performed using part of the heat, evolved to the user in heat-exchanger 6 (Fig. 1), and that gas expansion in the turbine (throttle valve) 7 allows one to perform the work, which can be used for compressing part of the gas, the minimal needed value of WH can be described via the expression w . = [ G c L c l n c - GtLt q,]ln'.
277
(6)
Y I j H I j -1
Y2jH2]
= ~T
W
(7)
j=l
where qr is the coefficient of conversion of the concentrated solar energy into the heat with temperature TR. The value qT depends on constructive peculiarities of the solar catalytic reactor and can easily be estimated for the most promising construction of the latter which is an ideal isothermic reactor of the cavity type [14-16, 19] with the temperature of the cavity TR. From an analysis
278
V.I. ANIKEEV et al. J~conv.
/
Thus equations (7) and (8) relate two independent variables G and Trt. Using the temperature dependence of the equilibrium constants of reactions (2), (3) and (4), one can easily find Yl: for given TR, Y2j, P1 and P1 out by the known procedure [21]. From the enthalpy balance in unit II it is possible to determine temperature T4 and the related enthalpy of a dried mixture after its compressing and passing through heat-exchanger 4:
~..-.w'--.~
@
4
Gc
Z
4
r"ljH4j = Gc
j=l
E
Y:Ho2j
j=l 4
Fig. 3. The structure of heat losses from the cavity-type catalytic reactor heated by concentrated solar energy flux.
+ GcLo - G, Lt - 6~ ~ Y,j(Ho,j- Ho,j)
(9)
j~l
where of heat losses from the cavity-type reactor (Fig. 3) we find r/r as follows: ~]T = [ W -
Wrad -- Wre fl -- W e o n v l / W
= [ W -- e a o ( T 4 -- T°)Si, -- (l -- e ) W -- ~ ( T R -- T o ) S w l / W ,
(8)
where wr~d, Wren and Wcon~are the energy losses through radiation of the cavity, reflection of the incident light and convection of the air; a 0 is the Stephan-Boltzmann coefficient for the ideal black body; Sin and S w are the surface areas of the inlet aperture and the inner walls of the cavity. From equation (8) it follows that the amount of energy absorbed by the cavity depends on self-radiation of the cavity walls at temperature TR through an aperture [the second term in numerator of (8)], on the value of a reflected flux, the third term and on losses by air convection. An effective coefficient of the cavity blackness, e, has been calculated for conic, cylindric and spheric shapes of the cavity [17]. The coefficient of convective heat-exchange ct was determined by the work [18]. Convective heat losses W~o~ from the reactor cavity depend on its geometry, area of the inlet aperture and inclination angle of the reactor relative to its vertical position; for an average cavity temperature higher than 1000 K convective losses W~o~,were shown to be much less than radiative losses Wr~d [19]. From an analysis of equation (8) it follows that with increasing power W of the incident solar energy flux at the fixed value of TR the contribution of losses via natural convection and re-radiation decreases, whereas value r/T approaches a limiting value. In Ref. [20] in order to diminish convective losses from the cavity-type receiver, its inlet aperture was closed by a quartz window; as a result, convective losses were decreased, but there appeared losses of the same order caused by reflection of the solar light from the quartz window. Using equation (8) it is possible to find the useful heat power, Wu~eml= r/~ W which can be converted into heat with temperature TR and then utilized to provide the endothermic catalytic reaction.
Y~j = Y , j / ( 1 -
Y , , ) , G t = Q(1 -- Y55)
= G(1 -- Y55)(1 -- Y1,), provided that the mixture, cooled in the heat-exchanger, has been completely dried, and H4: , H1j, H04i and H03j correspond to reagent enthalpies at temperatures T4, To2 = To4 = To and To3 (see Fig. 4). The values Lc can easily be calculated by the known equations [18]. L¢ = ~ - -K- ~ Ro To2 F(P2"~ ~ -
L\E)
where
11
(10)
4
g= Z ry,. j=l
R 0 is the universal gas constant, K: is the adiabatic index of the j t h component. L t is calculated by the equation similar to (10). Note that in calculations of the second version of scheme I (Fig. 1), the last two terms in the right-hand side equation (9) are equal to zero. By assuming that in catalytic methanation reactor 5 there is a thermodynamic equilibrium, determined by the composition of the inlet mixture, Y~j, temperature TM at the reactor outlet and pressures P2 and P2out (Fig. 1), from the enthalpy balance in this reactor 5
4
ac E Y5jn5J(TM)= Gc E Yijn4j(T4) j=l
(11)
j=|
and the known temperature dependence of reagent enthalpies and equilibrium constants of reactions (2), (3) and (4), one can find the value TM and concentrations of components Ysj, related to their concentrations at the methane reforming reactor 1 inlet through the following expression: (1 + Y25)Y2: = Ysfl(1 -- Yss), where j = I~IL For the methanation reactor with a fixed temperature (the second version of scheme I), the form of equation (l l) will be changed and allows determination of the heat power Wl : W I = Gc
Y ~ j H 4 y ( T 4 ) -j
1
YsjHsj(TM)
.
(12)
j~l
Thus, by fixing a degree of saturation of the mixture, directed to the methane reforming reactor 1, with water
SOLAR CATALYTIC POWER PLANTS (expressed, for example, as the ratio of molar portions of water and methane, fl, at the reactor 1 inlet) and choosing the corresponding values of T01, T03, T02 = / 0 4 = To, P1, e l out, t)2,192 out, w , Sin, Sw, £ and ~, we obtain a closed system of equations with respect to T R and TM. F r o m the values of TM and the concentrations Ysj of the components at the outlet of reactor 5 one can find the useful heat flow, W2, which is removed from heatexchanger 6 and transferred to the user, as well as the heat flow W l that is removed with a fixed temperature TM from the methanation reactor in the second version of scheme I:
2(4.1MPa) 0.8
0.7
0.6
I 700
I 900
I 1100
TR(K)
w~= wl + w2 W 2 = G¢
279
Fig. 5. Plot of q~ vs temperature in the methane reforming reactor Ta: 1--P 2 = 0.1 MPa; 2---4.1 MPa. The first version of scheme I.
Ysj [Hsj(TM) -- Hsj(To3)] J
+ Y55[H55(T,~) - n55(Ts) + Os(P2)
+ H~I~q) (Ts)
--
H(l~q)(To3)] t .
(13)
This expression gives a detailed description of the phenomena that occur in heat-exchanger 6 on cooling a v a p o r - g a s flow and on condensing water vapor. Here Qs stands for the heat of the water evaporation at pressure P2out; Ts is the boiling temperature. If the total value of heat transferred to a consumer is defined by Wz, it is evident that for the first version of scheme I W~ = I412, for the second version of scheme I Wz = W1 + W2. RESULTS OF CALCULATION Joint solution of the equations of the mathematical model discussed was done numerically with a view to examine dependence of r/ on parameters T R and P2 as well as on ratio fl = n~x2o/n~n4. In the calculation, the other parameters were Pl = 0.11 MPa, T01 = T03 = 350 K, T02 = T04 = 300 K, W/Sin = 1.6 103 kW m -2,
I (0.1 MPa)
SwlSi, = 30, E = 0.9, ~/¢ = /~t = 0.7, W = 8 kW, ~ = 2.2 W m 2 K - 1. Figures 4 and 5 show the dependence of ~/and n] = W z / W o n T R at different pressures P2 in the methanation reactor. It can be seen that efficiency of the concentrated solar energy conversion (both r/ and q]) passes through a maximum located in the temperature region T R ~ 900 K. The absolute values of r/ tend to decrease with increasing pressure P2, whereas the values of ql tend to increase (Fig. 5). The increase in ~/and n l with increasing TR for TR ~< 900 K is caused by a considerable temperature rise in the methanation reactor (Fig. 6); indeed, the value of TM is determined primarily by the degree of the methane conversion, Xcu4=(Y21-Yn)/Y2], In the methane reforming reactor (Fig. 7, curve 1). The decrease in r/ and ql with increasing TR for TR > 900 K is due both to a noticeable decrease in methane consumption during a cycle (Fig. 7, curve 2), which is needed for accumulating power qx W, and to a fall in rH (see Fig. 4, curve 4) despite the subsequent increase in TM. In Fig. 6 it can be also seen that TM is raised with increasing pressure P2, most markedly in the pressure range from 0.1 to 2 MPa.
]0.90
5 ( 1 1 0 0 K) 4 (1000 K)
1
0.7
3 ( 9 0 0 K)
1ooo
- 0.85 "qr 2 ( 8 0 0 K)
0.5
800 0.80
03
I 700
I I 900 11OO T~ (K) Fig. 4. Changes in conversion effÉciency, r/, (curves 1-3) and efficiency of the cavity-type receiver, qr (curve 4) vs temperature in the methane reforming reactor TR: l--P2=0.1 MPa; 2--1.1 MPa; 3--4.1 MPa. The first version of scheme I.
1
/ 600 0
I 1
I 2
I 3
I 4
P2 (MPa)
Fig. 6. Plot of temperature T M in the methanation reactor vs pressure P2: 1--TR = 700 K; 2--800 K; 3--900 K; 4--1000 K; 5--1100 K. The first version of scheme I.
280
V.I. ANIKEEV et al. 1 (0.1MPo) 1.0 15
0.8 # ~ 0.6
_ lO ~ = , d %
0.6t 0.4
0.4
5 0.2
0.2 0
I 700
I 900
I 1100
0
TR(K)
Fig. 7. Plot of the degree of methane conversion, XcH4, in the methane reforming reactor (curve 1) and its consumption in the cycle vs temperature TR of the reactor's cavity. The first version of scheme I; /'2 = 4 MPa. In large-scale industrial reactors, the steam reforming of hydrocarbons, in particular, methane, in the presence of an active catalyst is performed, water vapor is fed into the reactor in amounts 2-4 times larger than a stoichiometric a m o u n t in order to diminish carbon deposition. Our calculations indicate that an increase in fl with the other constant parameters does not lead to a significant change in t/ (Fig. 8) over the range of parameters studied. Only f o r / ' 2 = 4 M P A and T R = 700 K (Fig. 8, curve 1) there is a pronounced increase in t/when fl varies from 2 to 4. An analysis of the second version o f scheme I (Fig. 1) was made mainly for the estimation o f the fraction of the heat power W~ in the total heat flow W,- = W 1 + W 2 transferred to the user. F o r example, Fig. 9 displays variations in r / a n d ~ = WI W over the pressure P2 range from 0.1 to 4 . 1 M P a as a function of TR for fixed TM = 800 K and fl = 2. One can see that the value of W1 in some cases can be negative; this means that an
o
zoo/
900
looo
11oo
/ T R (K)
/
I
Fig. 9. Region of ~/variations as a function of TR restricted by curves 1 (P2 = 0.1 MPa) and 1' (P2 = 4.1 MPa) and region of variations as a function of TR restricted by curves 2 (/'2=0.1MPa) and 2' (/'2=4.1MPa); TM = 800K; /3 =2. additional amount of heat (WI) has to be supplied to the catalytic reactor in order to maintain the needed level of temperature. F o r example, this is the matter when one has to ensure T M = 8 0 0 K at T R < 7 4 0 K (and P2 = 4.1 MPa). The figure shows also that an increase in TR leads to an increase in ~ over almost the whole range of the temperature variations. Figure 10 demonstrates the regions of t/ and r7 = IV]/W2 variations for 0.1 ~< P2 ~< 4.1 M P a as functions of the fixed temperature TM in the methanation reactor. The maximum efficiency of energy conversion,
0.8
-
0.6 -
4
~
)
N
3
6 ( 9 0 0 K)"I 5 ( 8 0 0 K);~ 0.1 MPo 4 (700 K ) J
0.7
3 ( 9 0 0 K)" I 2 ( 8 0 0 K) / 0.5
0.3
aoo
Po)
4.1 MPo
1 ( 7 0 0 K)J
I 2
I 3
0.2
I 4
0 I
400
B Fig. 8. Plot o f q vs the ratio /3: 1 - - T ~ = 7 0 0 K ;
2--800K;
3 - - 9 0 0 K (for curves 1-3 P 2 = 4 . 1 M P a ) ; 4 ~ T R = 7 0 0 K ; 5-800 K; 6--900 K (for curves 4-6 P= = 0.1 MPa). The first
version of scheme I.
2(0.1 MPa) ~
I
1
I
600
800
o
looo
T u (K)
Fig. 10. Plot of q (curves 1 and l') and ~ (curves 2 and 2') vs TM at TR= II00K, fl =2; 1,2--P 2=0.1MPa; 1',2'-/'2 = 4.1 MPa.
281
SOLAR CATALYTIC POWER PLANTS
1.5
Table 1. Some summarized data on the best experimental runs of the SCR-3 solar catalytic reactor; Tat stands for the averaged temperatures of the catalyst's bed
-
5 (1100 K) 4 (1000 K) 1.0
3 (90OK)
T=,
Woh~/W Wohm
Process tested
(K)
(%)
(kW)
CH 4 + H20 CH 4 + CO 2 (0.17 C3Hs + 0.83 C4Hl0) + H20
8333 933 853
43 34 10
2.0 1.43 0.38
0.5
2 (800K) ]//f'~'~21 31
~
-0.5
I
4
Pa(MPo) 1 ( 7 0 0 K)
Z
Fig. 11. Plot of ~ vs pressure P2 at different Tc : 1--T R = 700 K; 2--800 K; 3--900 K; 4--1000 K and 5---1100 K. q, is attained for TM ~ 7 0 0 K Trt = 1100 K, fl = 2) and P2 = 0.11 MPa. Besides, within the TM range from 400 to 900 K, at/)2 = 0.11 M P a , the absolute values of q exceed at P2 > 0.11 M P a because of power consumptions for compressing the gas mixture by the compressor. Moreover, as it follows from the dependence o f ~ on TM, at TM < 800 K, the a m o u n t of heat W 1, removed from the methanation reactor, is higher (almost by the factor of 4 at T M = 400 K and P2 = 4.1 MPa) then W 2 transferred to the user through heat-exchanger 6. Finally, Fig. 11 illustrates the dependence of q on pressure P2 over the range of T R variations from 700 to 1100 K at TM = 800 K and fl = 2. F r o m the shape of the
curves it follows that at the fixed values of TM and fl the heat power W~ can exceed I4/2 only at TR > 1000 K and /)2 > 1.0 MPa. Thus, the estimates obtained for the efficiency of the energy conversion via the proposed schemes of the closed-loop thermochemical cycle with the usage of the steam reforming of methane allow the selection of optimal operational conditions for the methanereforming and methanation reactors, which will provide a needed potential o f utilized heat and a high energy conversion coefficient. The high theoretical efficiency of the thermochemical conversion of solar energy into heat (about 70%) in a closed-loop cycle has stimulated experimental works on the designing of a system with such a cycle and attaining a stationary operation of this system. EXPERIMENTAL STUDY OF A SMALL SOLAR CATALYTIC POWER PLANT BASED ON THE CLOSED-LOOP THERMOCHEMICAL CYCLE In order to verify the predictions of the above discussed theory, an experimental power plant based on the closed-loop thermochemical cycle has been designed to be used for conversion of the concentrated solar energy [22]. Experiments were performed in September 1984-1986 in the Crimea. A reflector of the solar energy concentrator S G U - 7 [23] served as a source of the concentrated solar light (Fig. 12). At the first step of the experimental studies we examined both thermophysical properties of the designed solar catalytic reactors [16] and the efficiency of
9
w
A
8
. oog,s,s .'l
I
2
Fig. 12. Reflector concentrator of the device SGU-7 with the reactor SCR-5 in operation.
Fig. 13. The principal scheme of the tested experimental solar catalytic power plant based on the thermochemical cycle. 1--solar catalytic reactor for hydrocarbon reforming SCR-5; 2,3,7--heat-exchangers; 4,8--water settling tank; 5-~ompressor; 6~methanation reactor CRM-2; 9--throttle valve.
282
V.I. ANIKEEV
et al.
Fig. 14. A general view of the methanation reactors CRM-1 (a) and CRM-2 (b). the conversion of the solar energy into the energy of chemical bonds through the steam and carbon dioxide reforming of saturated hydrocarbons (both of methane and of a propane-butane mixture) [24]. A cavity-type solar catalytic reactor SCR-3, heated by concentrated solar light, was used for these purposes [25]. As a measure of the efficiency of the solar energy conversion, we used a ratio of the gas mixtures enthalpy increase Wchem(per unit time) due to formation of the energy-rich chemical products to the total power of the solar flux falling inside the reactor cavity (Table 1). One can see that the construction of the SCR-3 solar catalytic reactor tested allows to reach a high (more than 40%) efficiency of the solar energy conversion into the energy of chemical bonds when the steam reforming of methane is performed. The second step of our experiments was testing the closed-loop thermochemical system which is schematically shown in Fig. 13. The system is represented by the solar catalytic reactor SCR-5 (a modified version of the reactor SCR-3) and the catalytic reactor ofmethanation CRM-1 or CRM-2 (see Fig. 14), all designed at the Institute of Catalysis of the Siberian
Branch of the U.S.S.R. Academy of Sciences. The catalytic reactors were removed from each other by more than 20 m. The solar catalytic reactor SCR-5 (Fig. 15) contains in the insulated housing a water evaporator, an overheater of steam, a solar-flux receiving cavity around which the catalyst bed is located, and counter-current heatexchangers for recuperation of the heat of reaction products. Water evaporation and overheating of steam to the temperature of the catalyst bed are accomplished solely at the expense of solar flux energy. The amount of the granulated catalyst load equals 2 dm 3. The diameter of the SCR-5 external housing is 42 cm, its height is 34 cm. The weight of all reactors, including the catalyst does not exceed 15 kg. The inlet diaphragm of the SCR cavity (160 mm in diameter) is located in the plane of the focal spot of the reflector concentrator of solar light, the diameter of the spot being 170 ram. The catalytic reactor of methanation CRM-2 (see its flow-sheet in Fig. 16) contains several tubes with the catalyst and can operate in two regimes of heat removal. The dry cooled mixture of H2, CO, CO2 and CH4 is passed from the methane reforming reactor into
SOLAR CATALYTIC POWER PLANTS
Fig. 15. Solar catalytic reactor SCR-5 after exploiting under action of concentrated solar energy flux. compressor 1, where it is compressed to pressure P2 and then is directed onto the catalyst bed. Prior to the operation of the system, the reaction mixture passes through furnace 3 in order to preheat the catalyst bed to the temperature the reaction starts. Then by switching valve 2, a cool gas mixture is fed onto the bed of catalyst 4. The catalyst bed in the tube is surrounded by heatexchanger 5. Hot products formed during the methanation reaction are directed to heat-exchanger 6, where
---- × 4 ~
8
Fig. 16. Flowsheet of the H 2 and CO in the methanation reactor CRM-2. 1---compressor; 2,9--valves; ~,,atalytic reaction ignition furnace; 4--catalyst bed; 5,6---heat-exchangers; 7--settling tank; 8--throttle-valve.
283
they transfer their heat to the heat-carrier. The condensed water is removed from the gaseous mixture in the settler 7, and then the gas passes through a throttle valve 8 and line-pipes to the solar methane reforming reactor. A commercial catalyst N K M - 4 with granules c a 2-3 mm in diameter was loaded into the methanation reactor. The scheme is intended for operation of the methanation reactor in (i) adiabatic regime (when the heatcarrier does not pass through heat-exchanger (5) and (ii) in regime of the external cooling of the catalyst bed. The latter cooling can be organized either autonomously by special gas or liquid (parallel cooling) or by the heatcarrier preheated in heat-exchanger 6 (successive cooling). Note that results of the energy plant tests reported in this work have been obtained at CRM-2 operation in the adiabatic regime. The circulation of a gas mixture and an excess pressure in the methanation reactor were provided by an electric compressor. The frequency of the gas recirculations in the closed-loop circuit of this experimental power plant was about 200 cycles per hour, that was obviously enough to create a steady-state during the typical experimental run (the latter usually of 3-4 h). The steady-state of a thermochemical cycle was established in less than 20 rain. The dry gas mixture consumption, IrA, in the methane-reforming reactor was measured by a flowmeter; the liquid water consumption VH2o was also monitored. Water was evaporated directly in the solar catalytic reactor by the heat of the concentrated solar light. Initially the system was filled by pure methane. The total light power W collected by the reflector in the reactor cavity was monitored by an actinometer and was equal c a 5 kW. The reaction of the reforming of methane in reactor 1 (Fig. 13) was carried out from 0.18 to 0.28 MPa; the pressure P2---0.6 MPa was maintained in methanation reactor 6 using compressor 5 and valve 9. The temperatures in the catalyst bed of the methane-reforming (TR) and methanation (TM) reactors and the temperature of a vapor-gas mixture (Tout) at the reactor 1 outlet after some heat recuperation were measured by a set of thermocouples. The reaction products dried from water were analysed chromatographically through a dosing valve directly from the reaction cycle (Fig. 13), that is, after the outlets of the methane-reforming and methanation reactors. Taking into account that chromatographically obtained data gave an imbalance less than _+ 10%, the sum of measured volume concentrations of the gas components in the dry mixture was reduced to 100% (see Table 2). The experimentally measured steady-state contents of the dry gas mixture at points A and B of the scheme on Fig. 13, volumes VA and Vn2o, over the set of thermocouples averaged temperature of the catalyst bed in CRM-2 (at the outlet maximum), as well as total light flux power W for some experimental runs are listed in Table 2. Given are also calculated values of the contact time, z (from the gas volume flow at the CRM-2 inlet).
284
V.I. ANIKEEV
et al.
Table 2. Parameters of the steady-state operation of the experimental solar catalytic power plant on the basis of methane-reforming and methanation reactions Composition of a dry gas mixture at the inlet and outlet of the methanation reactor (% by volume) TM inlet outlet VA Va~o bl(H2) b2(CO)b3(CH4) b4(CO2) al(H2) a2(CO) a3(CH4) a4(CO2) (m3h -I) (kgh -l) (°c)
"c
W
(s) (kW)
Wchem Wehem"}- Wout
W
w
0.21
0.38
0.22
0.34
4.33
0.23
0.42
0.563
0.262
0.0
0.175
0.24
0.15
0.26
0.35
1.32
0.69
630 3.77 4.07 7OO
0.613
0.283
0.01
0.1
0.31
0.161
0.269
0.26
1.29
0.51
__655
0.735
0.149
0.01
0.106
0.389
0.107
0.294
0.21
1.29
0.92
650 2.9 730
0.68
0.23
0.0
0.09
386
0.113
0.303
0.198
1.29
0.6
--
650 3.59 4.33 730
0.23
0.34
0.683
0.225
0.016
0.076
0.394
0.111
0.313
0.182
1.27
0.6
655 7~0 3.55 4.33
0.21
0.34
0.725 0.179
0.01
0.09
0 . 4 5 5 0.075
0.291
0.179
1.34
1.8
610 - - 3.42 4.33 705
0.22
0.57
0.745
0.01
0.139
0.465
0.062
0.298
0.175
1.34
1.8
630 - - 3.18 4.33 690
0.21
0.54
0.735 0.169
0.0
0.096
0.247
0.0
0.553
0.173
0.38
0.5
520 - - 7.63 4.27 725
0.12
0.2
0.728 0.189
0.027
0.056
0.229
0.0
0.597
0.174
0.39
0.45
520 - - 7.46 4.27 725
0.13
0.21
0.749
0.032
0.047
0.272
0.0
0.648
0.08
0.36
0.45
520 7.95 4.27 725
0.12
0.19
0.112
0.172
The volume flow VB of the dry gas at the outlet of reactor 1 can be easily determined from the known compositions of the dry gas at points A and B of the scheme and from value VA: VB = VA
a/ i
)
bi i
(14)
V~2o = VB(2bl -- b2 + 4b3 - 2b4) V A ( 2 a l - - a2 + 4a3 - -
3.66 4.1
Thus the power W~h~maccumulated by methane-reforming products (or evolved in the methanation reactor), which is defined as a difference between the enthalpies of the gas mixture at the outlet and inlet per unit time at the same temperature To = 298 K, can be expressed in the following manner: 4
(the notation is given in Table 2). Expression (14) is derived from an elementary balance of carbon in the absence of coke formation since no free carbon was detected on the catalyst during experiments. The latter is confirmed also by the constancy of the parameters measured; the pressures Pa and P2, volume gas flow Va and the contents of the gas mixture during the cycle stationary operation (from 2 to 3.5 h) in each experimental run. The a m o u n t of water, Vn~o, consumed during methane reforming or evolved during methanation at the steady-state operation of the system was determined according to an elementary mass balance by the following equation:
-
710
2a4)
(15)
Wchem = VB E b ~ ' H ~ 9 . i=1 4 -
vA ~ a:I-I,~,~- Vh2o.H~2o.29~ (16) i=1
where H~9" is the specific enthalpy of formation of the ith component at standard conditions. The values of Wchom calculated by this equation for each experimental run as well as the ratio W c h e m / W (i.e. the efficiency of the solar energy conversion into chemical energy) are compiled in Table 2. As is seen from the Table, the ratio W ~ h e m / W did not exceed 23% which is less than what was received during the solar catalytic reactor SCR-3 open-circuit tests (see Table 1). The latter is due to the fact that, at the steady-state closed-loop operation the content of gas mixture at the inlet of SCR differs strongly from that of open-circuit tests.
SOLAR CATALYTIC POWER PLANTS Indeed, from the tabulated results it follows that reforming of methane in the solar catalytic reactor is almost complete at any regime (temperature of the gas mixture at the catalyst bed outlet changes within 650-700°C), and only the mixture of H2, CO and CO 2 passes to the CRM inlet. However, the depth of one-step methanation in the reactor operated under close to adiabatic conditions is insufficiently high. As the result, methane is strongly diluted with H2 and CO2 at the inlet of the solar catalytic reactor. For this reason the ratio between Wchemand W due to energy enrichment of the products of the endothermic catalytic reaction is here less than in experiments without dilution of methane and does not exceed 23% at the close-loop experiments. Thus, in order to find further ways for increasing the efficiency of the energy system, it is necessary to know everything about physico-chemical processes that occur in the methanation reactor. Using an external cooling of the catalyst bed of the methanation reactor, it is possible to decrease the bed temperature, and thus increase the yield of methane from CRM. This will lead consequently, to an increasing efficiency of the solar methane reforming reactor and of the whole energy system. One can also see from Table 2, that a sufficiently highlevel temperature (up to 700°C) was achieved inside the methanation reactor catalyst bed during the steady-state experiments. This evidences for a real possibility to use the closed-loop solar thermochemical systems for some practical needs (e.g. for further electricity production via an intermediate overheated steam generation). In order to estimate the obtained energetic efficiency of the methanation reactor, we calculated the heat power WM, evolved in heat-exchanger 7 as a result of the gas-mixture heat utilization at the methanation reactor outlet. Since this heat was removed with water, that cooled the evolved gas down to 30-35°C, the values of WM were readily calculated as the enthalpy change (per unit time) of the gas mixture at the heat-exchanger outlet and inlet. As shown in [22], the values of Wra/Wch~m obtained appear to be sufficiently high (up to 65%) for the primitive construction of the methanation reactor tsted and thus indicate the possibility to design the reactors with more efficient conversion of the energy of the exothermic methanation reactor into heat energy transferred to the user. Table 2 gives also estimations of the total efficiency of accumulation in the solar catalytic energy reactor, the ratio (Wchem+ Wout)/W, which take into account power Wout obtained from utilization of the heat of the vaporgas mixture at the SCR-5 reactor outlet (the temperature of this mixture was ca 300°C). Note that only a small portion [see expression (15)] of the introduced water is consumed in the methane-reforming reactions. CONCLUSION The theoretical analysis and experimentations on the steady-state operation of the closed-loop thermochemical solar power plants on the basis of reversible
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catalytic reactions of the methane reforming and syn-gas methanation evidence for fairly good practical potentialities of these power plants, the first generation of the experimental plants of this type having demonstrated a satisfactory efficiency of the concentrated solar energy storage, transmission and conversion into a high-potential heat. The data obtained allow further improvement of both the solar methane-reforming reactor and methanation reactor, as well as the choice of optimal conditions of the total system operation. One could expect that in the near future the solar catalytic power plants of the considered type will excite a commercial interest. REFERENCES 1. V. N. Parmon, Maximum possible efficienciesof conversion of solar energy into chemical energy. In K. I. Zamaraev (ed.), Photocatalytic Solar Energy Conversion, pp. 42-57. Part I, Nauka, Novosibirsk. 2. H. Fedders and B. Hohlein, Int. J. Hydrogen Energy 7, 793-800 (1980). 3. E. Nazarov and A. Stolyarevskii, in V. A. Legasov (ed.), Atomic Hydrogen Energetics and Engineering, 3, pp. 5-57, Atomizdat, Moscow. 4. N. N. Ponomarev-Stepnoi, A. N. Protsenko, A. Ya. Stolyarevskii, Voprosy Atomnoi Nauki i Tekhniki, ser. Atomnovodorodnaya Energetika i Tekhnologiya, 1, 74-86 (1978). 5. E. Shpilrain, S. P. Malyshenko and G. G. Kuleshov, Introduction in Hydrogen Energy, Energoatomizdat, Moscow (1984). 6. V. I. Anikeev, V. N. Parmon, V. A. Kirillov and K. I. Zamaraev, Application of mathematical modelling to calculation of catalytic reactors for thermochemicalconversion of solar energy, Italian-Soviet Seminar Catalysis Application to Energy Problem Messina 1-23 (1984). 7. R. B. Akhmedov and M. A. Berchenko, Conversion and storage solar energy via simple thermochemical reactions. In K. ~I. Zamaraev (ed.), Photocatalytic Solar Energy Conversion, Part I, pp. 58~8, Nauka, Novosibirsk. 8. T. A. Chubb, Solar Energy 17, 129-136 (1975). 9. T. A. Chubb, J.- J. Nemecek and D. E. Simmons, Solar Energy 20, 219-224 (1978). 10. T. A. Chubb, Solar Energy 24, 341 (1980). 11. J. H. McGrary, G. S. McGrary, T. A. Chubb, J. J. Nemecek and D. E. Simmona, Solar Energy 29, 141-151 (1982). 12. Y. Villermaux, Entropie 85, 25-31 (1979). 13. K. I. Zamaraev and V. N. Parmon, Chemical method of solar energy conversion, in K. I. Zamaraev (ed,), Photocatalytic Solar Energy Conversion, Part I, pp. 7-42, Nauka, Novosibirsk (1985). 14. V. A. Grilikhes, V. M. Matveev and V. B. Poluektov, High-Temperature Solar Heat Sources for Cosmic Apparatus, Mir, Moscow (1983). 15. C. W. Stephers and A. H. Haiz, J. Rokets Technik 7, 51 (1961). 16. V. I. Anikeev, V. A. Kuzmin, V. A. Kirillov and V. N. Parmon, Experimental study of heat losses and temperature distribution within cavity-type catalytic reactor, Geliotekhnika, No. 6, 45-49 0987). 17. E. H. Sparrow and R. D. Cess, Radiation Heat Transfer, Brooks Cole, Belmont, California 0970). 18. A. G. Kasatkin, Fundamental Processes and Apparatus of Chemical Technology, Khimiya, Moscow (1960).
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