Thermodynamic Study of Solar Thermochemical

Available online at www.sciencedirect.com

ScienceDirect Energy Procedia 105 (2017) 1980 – 1985

The 8th International Conference on Applied Energy – ICAE2016

Thermodynamic study of solar thermochemical methane steam reforming with alternating H2 and CO2 permeation membranes reactors Hongsheng Wanga,b, Yong Haoa,* a

Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd., Beijing 100190, P. R. China b University of Chinese Academy of Sciences, No.19A Yuquan Rd., Beijing 100049, P. R. China

Abstract Hydrogen production by way of methane steam reforming based on solar energy is a promising means of solar energy harvesting and storage. In this work, a concept of solar-driven thermochemical hydrogen derivation by way of alternating hydrogen and carbon dioxide permeation membranes is proposed, which is capable of driving methane steam reforming reaction towards a virtual completion isothermally in the temperature range of 400-700°C. Thermodynamic analysis is performed on a one-dimensional model of a reactor consisting of alternate H2 and CO2 permeation membranes. At 400°C, both the conversion rates of water and methane can reach 99.9%, while solar-to-fuel efficiency is 61.7% when the solar-to-electricity efficiency is 15%. Major advantages of this approach include high efficiency, isothermal and continuous operation and low theoretical energy penalties for hydrogen and carbon dioxide separation. This study brings new insights into efficient low/mid-temperature solar fuel production. ©©2017 Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 2016The The Authors. Published by Elsevier Ltd. (http://creativecommons.org/licenses/by-nc-nd/4.0/). Selection and/or peer-review under responsibility of ICAE Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy.

Keywords: Solar, methane reforming, hydrogen production, thermochemical, membrane, isothermal

1. Introduction Solar energy, which is clean and renewable, is abundant and widespread for facing global energy demand in an environmentally friendly and sustainable manner. One method of solar energy utilization is converting solar radiation into chemical fuels for further application, which is relatively easy storage and transportation of solar energy. As hydrogen has a high specific energy and can be drawn from water, which is plentiful on the earth, it is an ideal medium for solar storage. Currently, more than 95% of the hydrogen is produced by hydrocarbon steam reforming [1], and methane steam reforming (MSR) is the most economic method for hydrogen production [2]. But the enthalpy for reforming reaction and the heat input to raise the temperature of reactants from room

* Corresponding author. Tel.: +86-10-82543150; fax: +86-10-82543019. E-mail address: [email protected].

1876-6102 © 2017 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license (http://creativecommons.org/licenses/by-nc-nd/4.0/). Peer-review under responsibility of the scientific committee of the 8th International Conference on Applied Energy. doi:10.1016/j.egypro.2017.03.570

1981

Hongsheng Wang and Yong Hao / Energy Procedia 105 (2017) 1980 – 1985

temperature to reaction temperature, which is about 850°C for a high conversion rate, usually derived from fuel combustion, and the separation and purification of hydrogen also require additional energy costs, which lead to a low energy conversion efficiency. One effective approach to address this problem is using membrane reactor based on solar energy to realize MSR [3], and high purity hydrogen can be obtained. A few groups have used palladium (Pd) or palladium alloy membrane to separate hydrogen during the reaction [4-6], and the temperature of their reaction usually not less than 500°C, as a long enough Pd membrane, which is rare and expensive, is needed for even lower reaction temperature. For a higher reactants conversion rate and a lower reaction temperature, Wu et al. [7] proposed a dual-enhanced steam methane reforming by combining reactive sorption complex catalyst with palladium membrane to separate CO2 and H2 from reaction equilibrium simultaneously, which has an obvious effect, but the reaction cannot be continuous as the sorption catalyst has to be exchanged when it is saturated. In this work, we propose a conceptual membrane reactor capable of thorough conversion of methane at low temperatures by way of continuous separation of H2 and CO2 from methane reforming products with alternating permeation membranes corresponding to either gas. The material of carbon dioxide permeation membrane could be the dual-phase ceramic-carbonate mixed carbonate-ion and oxide-ion conductor (MOCC) (e.g. Ce0.8Sm0.2O1.9 (SDC) with a carbonate melt of Li2CO3 and Na2CO3 (52:48 in mol%) [8]), while that for hydrogen permeation remains Pd. The alternating permeation concept is not only capable of decreasing the length of Pd membrane, but can also reduce reforming temperature or boost conversion rate of methane, resulting in enhanced solar-to-fuel conversion efficiencies. 2. Theoretical Formulation A concept of solar thermochemical H2 and CO2 permeation membranes reactor with solar energy collector is illustrated in Fig. 1.

(a)

(b)

Fig. 1. Illustration of the solar H2 and CO2 permeation membranes reactor concept; (a) trough and (b) dish solar energy collector.

The membrane tube reactor consists of two concentric tubes, and the inner tube is made of permeation membrane, while the exterior tube is impermeable. H2 permeation membranes and CO2 permeation membranes are arranged alternately, so that the H2 and CO2 can be drawn from the inner tube in turn to shift the equilibrium of MSR (Eq. 1) forward to promote conversion of methane.

CH4 +2H2 O=CO2 +4H2

(1)

1982


The reactor is exposed to concentrated sunlight as shown in Fig. 1, which illustrates two types of solar energy collectors. In trough type solar energy collector, the alternate membranes lie in a line, and the axes of the tubes locate in the focal line of the collector to absorb more heat. Several division plates are placed along the radial direction in the chamber, which is between the inner tube and exterior tube, to separate the room into isolated space to keep each membrane having its own chamber, and each chamber connects with its vacuum pump to keep a low partial pressure outside the membrane. While in dish or tower type solar energy collector, as the reactor should be situated in the focal point, the tubes are arranged as a coiled pipe, and each gas membrane also has its chamber, which has one side clogged and the other side links to its vacuum pump. The dimensions of the reactors are illustrated on Fig. 1. Ri and Ro are the inner and outer radii of the internal tube, respectively, which are assumed to be 0.1 cm and 0.11cm, respectively. The length of each tube is 10cm, and the dimensions are the same for both H2 and CO2 permeation membranes. In this study, the hydrogen permeation membrane is made of palladium, and the hydrogen flux is determined by the diffusion coefficient, the concentration gradient and the thickness of membrane [9]:

k PHn2 ,in PHn2 ,out

J H2 k

dM

(mol H /(m2 s))

(2)

2

§ · 13410 J/mol 0.62 3.21u108 exp ¨¨ ¸¸ (mol H2/(m s Pa )) © 8.314 J/mol/K u T (K) ¹

(3)

where k is the rate constant of Arrhenius’s Equation; n is an exponent, and it equals to 0.62 which fits well with the experiment in the temperature range of 350°C to 900°C [9]. The material of CO2 permeation membrane has many types [10]. Taken the reaction temperature into account, we choose the dual-phase MOCC, whose permeation flux is [8]: cc 2 PCO § H · RT MV c 1 M V o (4) J CO2 ¨ ¸ 2 ln c2 © W ¹ 4 F L MV c 1 M V o PCO where H and W are the porosity and the tortuosity of the porous SDC, and the values are 0.321 and 26.1 [8], respectively; R is the gas constant; T is the absolute temperature; F is Faraday’s constant; L is the thickness of the membrane; M is the volume fraction of the molten carbonate (MC) phase, which is 0.3 in cc 2 are the partial pressures at the feed and permeate sides in Pa, c 2 and PCO this simulation [8]; PCO respectively; V c and V o are the ionic conductivities of carbonate-ions and oxide-ions in S/cm, respectively. The equations of V c [11] and V o [12] are:

Vc

83.8e

3716.7 T

(5)

9051.5 T

Vo

(6) 224.6e The solution process of this model is implemented by marching the differential equations in time using a Runge-Kutta method of the fourth order with a variable time step. The theoretical maximum first-law thermodynamic efficiency of MSR is defined as: KHHV

K

1 abs

Q

CH 4

nH2 HHVH2

QH2 O Qth Kso1 e WPH WPCO 2

2

n

CH 4

HHVCH4

(7)

where nH2 and nCH4 are the molar amounts of H2 generation and CH4 consumption, respectively; HHVH and HHVCH are the molar higher heating value of H2 and CH4, respectively; Kabs is the absorption efficiency of the reactor: V TH4 (8) Kabs 1 2

4

I C

1983


where V is Stephen-Boltzmann’s constant; I is the solar irradiation at earth’s surface and is conventionally taken as 1000 W/m2; TH is the reaction temperature; s e is the solar-to-electricity efficiency; QCH4 and QH2 O are the heat input for raising the temperature of methane and water from 25°C to TH, respectively, whereas Qth is the total heat absorbed by reactant gas for its reforming: T (9) QCH nCH ³ Cp,CH g dT H

4

4

QH2O Qth

nH2O

25q C

³

4

100q C

25q C

Cp,H2 Ol dT 40.872 ³

TH

100qC

Cp,H2 O g dT

TH 'SMSR

where 'SMSR is the entropy change during the reaction; WPH and WPCO 2 2 the separation of H2 and CO2, respectively: WH2

WCO2

nH2 RT0 ln P0 / PH2 ,out

nCO2 RT0 ln P0 / PCO2 ,out

(10) (11) are the vacuum pump works for (12)

(13)

where T0 is room temperature (25°C); nCO2 is the molar amount of CO2; PH2 ,out and PCO2 ,out are the hydrogen partial pressure and carbon dioxide partial pressure outside the permeation membranes, respectively; P0 is atmospheric pressure. The efficiency as defined by Eq. 7 takes into account the energy costs of reradiation from the solar thermal collector (Eq. 8, which inherently assumes infinitely fast heat transfer processes), heating the reactant gas (Eq.s 9-10), reaction heat upon dissociation (Eq. 11) and vacuum pump work for maintaining low partial pressures. However, part of H2 is produced from methane, and the efficiency contains contributions from both solar energy and fossil fuel. For a better evaluation of the contributions from solar energy, a net solar-to-fuel efficiency is defined as: nH HHVH nCH HHVCH (14) K 2

HHV,net

2

4

4

1 Kabs QCH QH O Qth Kso1 e WP WP 4

2

H2

CO2

In this thermodynamic analysis, these assumptions are given: (1) the whole system is under steadystate conditions; (2) the time for MSR is ignored; (3) the flow resistance is not taken into account; (4) the backward diffusion of gas species in the tubes is negligible. For calculations and results to be discussed below, no heat recovery from the gas phase is enforced; the solar concentration ratio is fixed at C=97 [13,14] in Eq. (8) in consistency with the solar parabolic trough concentrators. For each simulation case, the system remains isothermal at a specified temperature TH unless stated otherwise. All thermodynamic properties are calculated with HSC software [15]. 3. Results and discussion Numerical simulation was accomplished with the model described above. The thickness of both membranes is assumed to be 0.01cm. As the partial pressure of H2 is higher than CO2 at equilibrium and H2 permeation flux is much greater than CO2 (Fig. 2(b)), the hydrogen partial pressure on the permeate side (outside of the membrane) is fixed at a relative high pressure, which is PH ,out 103 atm , and the CO2 partial pressure is fixed at PCO ,out 105 atm . We calculated the conversion rates of water and methane, the theoretical maximum first-law thermodynamic efficiency and net solar-to-fuel efficiency as functions of temperature TH and length of the flow channel. As the analyses for the thermodynamic progresses in trough type solar energy collector and dish type solar energy collector are the same, we just take parabolic trough collectors as an example. The analysis results of the H2 and CO2 permeation membranes reactor are shown in Fig. 2. 2

2


1.0

0

40

80

120

160

200 100

3.0x10-7

H2O

0.8

80 CO2

0.6

60

0.4

40

H2O CH4

0.2

20

H2 CO

0.0 0

40

80

120

Conversion Rate(%)

Partial Pressure(atm)

CH4

0 200

160

Permeation Flux(mol/(cm2·s))

1984

2.5x10-7

H2

2.0x10-7 1.5x10

-7

1.0x10-7 5.0x10-8

CO2 0.0 0

50

100

(a)

(b) 100 0q C

H2

H2O

3.72 CH4

1.24

CO

CO2 200

300

400

500

600

700

C 0q

80

45

C 0q 40

60 40

H2O

80

450

0

500

550

Temperature(qC)

CH4

0

800

120

40 400

20

Temperature(qC)

C 0q

160

Length(cm)

6.20

0.00 100

50

55

7.44

Conversion Rate(%)

Molar Flow Rate(Pmol/s)

8.68

2.48

200

Length(cm)

Length(cm)

4.96

150

40

80

120

160

200

Length(cm)

(c)

(d)

Fig. 2. Analyses of the H2 and CO2 permeation membranes reactor; (a) Variation of partial pressures and conversion rates along the flow channel (400°C); (b) Variation of H2 and CO2 permeation fluxes (400°C); (c) Variation of MSR molar flow rate at thermodynamic equilibrium without membrane reactor; (d) The conversion rates versus temperature and flow channel length.

From the calculation, the conversion rates of CH4 and H2O are increasing along the flow channel and they reach 99% at the length of 161 cm at 400°C (Fig. 2(a)). As the permeation flux of H2 is much greater than the permeation flux of CO2 (Fig. 2(b)), the amount of CO2 remaining inside the membrane is rising, leading to the partial pressure of CO2 increasing and the others decreasing along axial direction. Since the temperature increases, the equilibrium of Eq. 1 shifts forward, and the partial pressures of H2 and CO2 are growing (Fig. 2(c)), which causes the chemical potential differences of each gas species across its corresponding membrane to increase. More products can be transport from the reactor per unit length, which will result in a shorter length demanded for a higher conversion rate (Fig. 2(d)). Based on the simulation, the alternate membranes reactor dramatically improves the conversion rates of CH 4 and H2O, and the efficiency of the alternate membranes reactor also calculated as shown in Fig. 3. 80

Efficiency(%)

60 s

e

15%

20%

25%

s

e

15%

20%

25%

30%

35%

40%

40

20 KHHV

30%

35%

40%

KHHV,net

0 400

450

500

550

Temperature(qC)

Fig.3 The first-law thermodynamic efficiency and net solar-to-fuel efficiency under different solar-to-electricity efficiency scenarios.


Based on Eq.s 7 and 14, we calculated first-law thermodynamic efficiency and net solar-to-fuel efficiency, both of which decrease as temperature increases, mainly due to the decrease in absorption efficiency ηabs, as well as the simultaneous increase of thermal energy inputs to raise the temperature of reactants to TH and for subsequent reforming reaction. As the conversion rates of reactants are almost 100% for temperatures above 400°C, the decrease in both efficiencies are clearly accounted for by the increase in energy penalties for hydrogen generation as stated above. As the net solar-to-fuel efficiency deducts the contributions from CH4 chemical energy, it is significantly lower than the first-law thermodynamic efficiency, but both of them have similar trends. 4. Conclusion This study focuses on a novel approach of clean and efficient hydrogen production by way of an alternate H2 and CO2 permeation membranes reactor. Theoretical framework and numerical model for thermodynamic analyses of the reactor have been established. First-law thermodynamic efficiency, net solar-to-fuel efficiency and conversion rates of water and methane are calculated. A crucial characteristic of the reactor is that H2 and CO2 are drawn from the equilibrium of reaction alternately, and the MSR can progress continuously and completely, which results first-law thermodynamic efficiency up to 61.7% and net solar-to-fuel efficiency up to 26.4% at 400°C with s e 15% (or 79.3% and 45.7% with 40% ), respectively, while conversion rates of both water and methane reach 100%. This study s e indicates a promising isothermal method for high-efficiency hydrogen generation based on solar energy. References [1] Collodi G, Wheeler F. Chem Eng Trans 2010; 19: 37-42. [2] Harrison DP. Ind. Eng. Chem. Res 2008; 47: 6486–6501. [3] Syed AS, David SS, Esmail MM et al.. Int. J. Hydrogen Energy 2015; 40: 3158-69. [4] Ryi SK, Park JS, Kim DK et al.. J Membrane Sci 2009; 339: 189-194. [5] Giole DM, Francesco S, Giampaolo M et al.. Int. J. Hydrogen Energy 2015; 40: 7559-7567. [6] Roses L, Gallucci F, Manzolini G et al.. Chem Eng J 2013; 222: 307-320. [7] Wu X, Wu C, Wu SF. Chem Eng Res Des 2015; 96: 150-157. [8] Zhang LJ, Xu NS, Li X et al.. Energ Environ Sci 2012; 5: 8310-8317. [9] Morreale BD, Ciocco MV, Enick RM et al.. J. Membrane Scl 2003: 212: 87-97. [10]Yeo ZY, Chew TL, Zhu PW et al.. J Porous Mater 2013; 20: 1457-1475. [11] Chung SJ, Park JH, Li D et al.. Ind Eng Chem Res 2005; 44: 7999–8006. [12] Mori T, Wang Y, Drennan J et al.. Solid State Ionics 2004; 175: 641–649. [13] Bader R, Pedretti A, Barbato M et al.. Appl Energ 2015; 138: 337-345. [14]Good P, Ambrosetti G, Pedretti Aet al.. Sol Energy 2015; 111:378-395. [15] HSC Chemistry 5.11, 2001-2002, Outokumpu Research Oy, Pori, Finland, A. Roine.

Biography Yong Hao is a professor at the Institute of Engineering Thermophysics, Chinese Academy of Sciences. He received his Ph.D. in Mechanical Engineering from California Institute of Technology. He then worked as a postdoctoral scholar and research scientist in the group of Prof. Haile in Materials Science at the same institution.

1985