- Email: [email protected]
A solar thermochemical fuel production system integrated with fossil fuel heat recuperation
Accepted Manuscript A Solar Thermochemical Fuel Production System Integrated with Fossil Fuels Hui Kong, Yong Hao, Hongsheng Wang PII: DOI: Reference:
S1359-4311(16)30478-1 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.170 ATE 8038
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
25 February 2016 31 March 2016
Please cite this article as: H. Kong, Y. Hao, H. Wang, A Solar Thermochemical Fuel Production System Integrated with Fossil Fuels, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2016.03.170
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Solar Thermochemical Fuel Production System Integrated with Fossil Fuels Hui Konga,b , Yong Haob*, Hongsheng Wanga,b a
b
University of Chinese Academy of Sciences, No.19A Yuquan Rd, Beijing, 100049, P. R. China.
Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd., Beijing, 100190, P. R. China. * Corresponding author; email: [email protected]
Abstract: Thermochemical cycling (TC) is a promising means of converting solar energy into chemical fuels. Isothermal solar TC has the potential of achieving high solar-to-fuel efficiencies contingent on effective gas-phase heat recovery at high temperatures. We propose an approach of simultaneously recovering waste heat and unreacted gas species (e.g. H2O or CO2) downstream the isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH 4). Such comprehensive utilization and the syngas produced enable the establishment of a polygeneration system for simultaneous power and methanol production with the possibility of eliminating the selfgeneration power-plant that is conventionally needed for methanol production. A scheme is conceptually proposed based on the type of splitting reaction in isothermal TC, and optimization of the polygeneration system is discussed with solar conversion efficiency as the objective. Fossil fuel consumption for the production of a unit mass of methanol is about 22GJ/ton, lower than typical values in current industrial processes. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream.
Keywords: Isothermal, Polygeneration, Solar Fuel, Syngas, Thermochemical Cycling, Methanol. 1
1. Introduction Conversion of sunlight into energy of storable forms such as latent heat or chemical energy of fuels carries special significance in addressing unfavourable natures of solar energy such as sparsity and intermittency. Solar energy conversion routes producing chemical fuels (especially H2O or CO2 splitting for H2 or CO) are of particular long-term interest with their great potential in meeting worldwide energy demand and tremendously reducing our dependence on fossil fuels. Electrochemical, photochemical and thermochemical are three basic pathways for such purposes [1, 2]. While some of these routes may benefit from the combination of more than one converted form of solar energy (e.g., solar-driven electrolyzer [3]), the thermochemical approach is driven by only concentrated solar heat and has the advantage of making use of the entire solar spectrum. Thermochemical cycling (TC) is a technology for solar energy storage via the synthesis of chemical fuels by utilizing concentrated solar heat alone to drive endothermic chemical reactions. Two-step TC of ceria and its derivatives attracts much attention for efficient splitting of H2O (or CO2) for H2 (or CO), a valuable feedstock to product ammonia, methanol, or liquid hydrocarbon. Ceria-based H2O and CO2 splitting processes are described as follows: oxygen is first released when solar energy is used to reduce the metal oxide at a high temperature T Re; then the reduced metal/metal oxide is re-oxidized with water or CO2 at a lower temperature TOx, yielding H2 or CO. Reduction (at TRe): 1CeO2- 1CeO2- + 0.5 O2 , f
i
Oxidation (at TOx): 1CeO2 H2O 1CeO2 H2 , or i
f
1CeO2i CO2 1CeO2f CO ,
Combined: H2O H2 0.5 O2 , or
(1) (2a) (2b) (3a)
CO2 CO 0.5 O2 ,
(3b)
where stands for oxygen nonstoichiometry; i and f correspond to values at the beginning and the end of the oxidation step, respectively ( i 2
f
), and i f stands for the net change
in oxygen nonstoichiometry; TOx and TRe are the temperatures at which oxidation and reduction reactions occur, respectively. Ceria-based TCs have been demonstrated to deliver stable fuel production at temperatures up to 1600°C due to the absence of phase change of ceria and relatively low tendency of sintering of ceria microstructure under typical TC conditions [4, 5].
Although theoretical solar-to-fuel efficiency of solar-driven TC could be high, low achievable solarto-fuel efficiency has been the major barrier against the practicality of solar-driven TC in general [4]. Steinfeld et al. recently achieved 1.73% (average) and 3.53% (peak) solar-to-fuel efficiencies with ceria (CeO2) packed bed of reticulated porous ceramic (RPC) [6]. Heat recovery from solidphase oxygen carrier (e.g., ceria) and reactor remains a key issue against efficiency enhancement [7]. A conventional two-temperature TC works with a large temperature swing, which potentially leads to a significant thermal shock to the reactor and results in the loss of a considerable portion of the solar energy input in the form of waste heat to the environment. In the last three years, several research groups [8-10] reported isothermal TC, in which reduction and oxidation reactions occur at identical temperatures. The elimination of temperature difference minimizes heat loss from solid phases and their thermal shock, but shifts the burden of heat recovery to the gas phase (including reactant and product species). Comparatively, high-efficiency heat recovery is considered easier to achieve with gases than with solids. Thus isothermal TC offers the possibility of making simple and highly efficient solar TC reactors that convert H2O or CO2 back to fuel [10, 11].
3
Figure 1. Conceptual illustration of an isothermal thermochemical water splitting TC integrated with methane heat recovery; SRM, CDR and RWGS stand for steam reforming of methane, carbon dioxide reforming, and reverse water-gas shift, respectively.
Effective heat recovery and energy cost reduction are essential for enhancing the efficiency of isothermal TC. The solar-to-fuel efficiency of isothermal splitting of CO2 is much higher than isothermal water splitting process (e.g., three times higher at 1500°C without heat recovery) [11] due to the favorable thermodynamic nature of CO2 and the lack of phase change. Direct use of oxidation products and unreacted CO2 can reduce additional energy consumption for CO2 separation, which eventually cuts down solar energy input to the system. The gas mixture downstream the oxidation process (Eq. 2a and 2b) contains a large amount of sensible heat, the recovery of which can be used to heat up fresh CO2 or H2O on the feed side. However, heat exchangers are immature above the temperature of 1100°C [12]. Instead of direct heat recovery via a heat exchanger, we propose an approach that simultaneously recovers waste heat and unreacted gas species (e.g. H2O or CO2) downstream an isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels [13] (e.g. CH4). The temperature of the gaseous exhaust of isothermal oxidation reaction (Eq. 2a and 2b) can be reduced to 600-850°C and the calorific value of which can also be improved. The scheme is shown in Fig. 1 (taking isothermal H2O splitting TC for example). Although isothermal co-splitting of CO2 and H2O can obtain syngas alone, the integration with fossil fuel downstream of an isothermal TC reactor can further enhance syngas yield and utilization of solar energy. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream. At the end of this paper, a solar thermochemical polygeneration system integrated with methane for methanol synthesis is proposed to further prove the benefit of this approach.
4
2. Analytical Methodology The thermodynamic efficiency of a conventional two-temperature, two-step solar TC for converting solar energy and H2O (or CO2) to H2 (or CO) can be defined as [11]:
HHV
(Qredu Qheat
HHV , Qceria ) / abs Qpump
(4)
where HHV is the higher heating value of one mole of H2 (or CO). The denominator represents the total solar energy input consisting of four enthalpy terms for extracting 1 mole of O atoms from ceria (considering re-radiation losses): that required to reduce ceria from CeO 2f to CeO 2i at TRe (denoted as Qredu), that required to heat water (or CO2) from 25°C to TOx (denoted as Qheat), that required to heat ceria from TOx to TRe (denoted as Qceria), and the energy needed to obtain a lower oxygen partial pressure in the reduction step with a vacuum pump (denoted as Qpump) [14]. For isothermal TC, Qceria vanishes for TRe=TOx=TH ; The denominator is the minimum solar energy needed for producing one mole of H2 (or CO), and solar energy absorption efficiency
abs
T4 1 , IC
(5)
is defined for an ideal blackbody reactor [15] in which the cycling reactions are assumed to take place,
is Stefan-Boltzmann’s constant, T is the temperature of the reactor, I is solar irradiation,
and C is the concentration ratio of solar energy. Our proposed approach recovers waste heat and utilizes unreacted gas species (e.g. H2O or CO2) downstream an isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH4), mainly the steam reforming of methane (SRM) , the carbon dioxide reforming of methane reaction (CDR) and the reverse water gas shift reaction (RWGS): CH4 H2O CO 3H2
H25C 205.9kJ / mol ,
(6)
CH4 CO2 2CO 2H2
H25C 247.0kJ / mol ,
(7)
H2 CO2 CO H2O
H25C 41.1kJ / mol .
(8)
5
Therefore, through the combination of these reactions, the unreacted gases in the exhaust and its high temperature sensible heat are rationally used, and the final products after reforming are a mixture of H2, CO, CO2, H2O (with residual CH4). The conversion of solar energy to chemical energy for an isothermal TC integrated with fossil fuels [16] is gauged by solar-to-syngas efficiency: ηsol-to-syngas =
n H2 HHVH2 n CO HHVCO n CH4 ,react HHVCH4 , Qsolar
(9)
where Qsolar is the total solar energy delivered through the receiver’s aperture considering the solar absorption efficiency. nCH4,react, nH2 and nCO are the amounts of CH4 reacted, H2 and CO produced after the reforming process, respectively. HHVH2, HHVCO and HHVCH4 are the higher heating values of H2, CO and CH4, respectively. The energetic upgrade factor is defined as the ratio of the higher heating value of syngas to that of feedstock processed in a similar form to Ref. [16] (but with HHV instead of LHV): U=
n syngas HHVsyngas , n feedstock HHVfeedstock
(10)
The calorific value per mole of the gas mixture downstream the oxidation reactor is defined by:
HHVmix
(XCO HHVCO X H2 HHVH2 XCH4 ,residue HHVCH4 ) , XCO X H2 X CO2 X H2O X CH4 ,residue
(11)
where XCO, XH2, XCO2, XH2O, XCH4 ,residue are the mole fractions of CO, H2, CO2, H2O and the remaining CH4, respectively. Another important parameter is the net carbon emissions per unit calorific value, defined as:
C
(n CH4 ,feed n CH4,residue ) 44g / mol , n H2 HHVH2 n CO HHVCO
(12)
where n CH4 ,feed and n CH4, residue are the amounts of CH4 input and residue for SRM or CDR process, respectively. The net carbon release is traced back to the reacted CH4 instead of CO2 input of the isothermal TC for that CO2 is released again after it goes through the entire process. 6
3. Results and Discussion 3.1. Isothermal splitting of CO2 or H2O For the convenience of discussions, the ratios of CO2 and H2O to available oxygen vacancies in ceria upon reduction are defined as rCO2 n CO2 / ( i n Ceria ) and rH2O n H2O / (i n Ceria ) , respectively. The high temperature thermolysis of CO2 or H2O will lead to a higher oxygen partial pressure (than that of the reduction step), which provides a driving force for oxygen intake by ceria during the oxidation (i.e. splitting) step. The net conversion of CO2 to CO is shown in Fig. 2 for different isothermal reaction temperature T H. For CO2 splitting isothermal TC at 1600°C, the net conversion of CO2 is 11.4% while the HHV efficiency is 21.5% (without heat recovery) for rCO2=0.01.
Figure 2. Percentage of CO2 converted to CO and HHV for different TH from 1500 to 1900°C.
7
60
Conversion (%)
50
H2O splitting isothermal TC CO2 splitting isothermal TC
40 30 20 10 0 1200
1300
1400
1500
1600
1700
1800
1900
o
Isothermal TC Temperature TH ( C)
Figure 3. The conversion of CO2 and H2O for isothermal oxidation reaction with TH in the range of 1200-1900°C. The thermodynamic efficiency as defined by Eq. 4 only takes into account the minimum energy required for a redox cycle of isothermal TC to complete. At a given temperature TH and oxygen partial pressure of reduction (e.g. 10-5 atm), the thermodynamic efficiency of the isothermal TC (Eq. 4) is a fixed number for a specific steam/oxide ratio. Details of efficiency calculations similar to what we present in this work can be found in our previous work for a closed system [10]. For producing one mole of CO or H2, the value of rCO2 or rH2O is varied in a wide range between 10-4 and 104 so that the maximum HHV efficiency (and correspondingly the minimum energy input) of the upstream isothermal TC can be found in this interval. The conditions under which the minimum energy input occurs and the corresponding gas mixture compositions are then used for the calculation of the methane-incorporated heat recovery downstream of the reactor. The output of the oxidation reaction of isothermal TC is a mixture of CO and unreacted CO 2 (at TH) for CO2 splitting process, while for H2O splitting process, the output is a mixture of H2 and unreacted H2O (at TH). The conversion rates of CO2 to CO and H2O to H2 for independent CO2 or H2O splitting isothermal TC are shown in Fig. 3, for which rCO2=rH2O=0.01. The oxide/steam ratio designated by rCO2 or rH2O can be achievable in practice where the volume of CO2 or H2O is close to the catalyst (50% porosity is assumed) at temperatures of 1500°C or above. Separation of H2 from H2O consumes no
8
additional energy due to condensation of the latter, while separation of CO from CO2 requires extra energy to obtain pure CO.
3.2. Isothermal TC integrated with downstream methane reforming Methane is added into the mixture of CO and CO2 downstream of an isothermal reactor (1600°C, pO2=10-5 atm for thermal reduction) to both lower the temperature and completely convert unreacted CO2 to CO and H2 (via CDR, Eq. 7). The initial flow rate of CO2 into the reactor is assumed to be 106 mol/h. With the continuous increase of methane, the gas mixture achieves thermodynamic equilibrium without additional heat input, and the mole fraction of the gas mixture is shown in Fig. 4. CH4/CO2 ratio is the molar ratio of CH4 to CO2 input of isothermal TC. The mixing temperature decreases rapidly until CH 4 cannot be consumed further and begins to accumulate with a noticeable amount (i.e. mole fraction > 10-3) at a CH4/CO2 ratio of 0.1562 (marked as CH4/CO2 ratio-x). It is worth mentioning that the reverse water gas shift reaction and other chemical reactions occur simultaneously to gain a minimum Gibbs free energy. 100 TH=1600°C
Temperature (°C)
1400
80
1200 60 1000 CO2
800 689.1
600 400 0.00
40
CO H2
20
Mole Fraction (%)
1600
H2 O CH4
0.05
0.10
0.15
0.20
0.25
0 0.30
CH4/CO2 ratio
Figure 4. Isothermal CO2 splitting process integrated with downstream methane-integrated heat recovery process at TH=1600°C.
9
0.16
70
(a)
60
CO2
0.15
CO
0.14
50 COCDR
40
0.13
30
0.12 H2
0.11
H2,CDR
20
Mole Fraction (%)
Optimal CH4/CO2 ratio - x
0.17
10
0.10
H2 O
0 1200 1300 1400 1500 1600 1700 1800 1900 Isothermal TC Temperature TH (℃)
H2 H 2O
60 50
H2,SRM
0.16
40 30
0.14
20
0.12 0.10 1200
CO
CO2
CH4
COSRM
Mole Fraction (%)
Optimal CH4/H2O ratio - y
0.18
70
(b)
0.20
10 0
1300
1400
1500
1600
1700
1800
1900
Isothermal TC Temperature TH (℃)
Figure 5. CH4/CO2 ratio-x or CH4/H2O ratio-y and the mole fractions of various gas species after methane reforming process (a)Isothermal CO2 splitting process integrated with CDR, TH=12001900°C; (b) Isothermal H2O splitting process integrated with SRM, TH=1200-1900°C.
For temperatures of isothermal CO2 splitting in the range of 1200-1900°C, the optimal CH4/CO2 ratio (denoted as x) for the mole fraction of CH4 to reach 10-3 is shown in Fig. 5(a). With the increase of TH, the CH4/CO2 ratio-x increases mainly because the sensible heat of the oxidation products of isothermal TC does so. The mole fraction of CO is roughly twice as much as that of H2 in a temperature range of 1200-1900°C, in which the main reactions are CDR (Eq. 7) and RWGS (Eq. 8). The reaction of CDR has a trend to occur easily because the partial pressure of CO2 downstream the oxidation reactor and the thermodynamic driving force are the greatest among CDR, SRM and RWGS above 817°C from the data of HSC once methane is added. The H2 produced in the CDR reacts with CO2 to obtain CO and H2O in the following step above the equilibrium 10
temperature of 817°C where the Gibbs free energy change of RWGS is about zero, and only a small amount of produced water reacts with methane because that the content of CO2 in the gas mixture is much larger than H2O even at equilibrium. With the increase in temperature, the thermodynamic equilibrium constants for both CDR and RWGS increase but the former grows faster, such that the ratio of CO/H2 decrease (the mole fractions of CO and H2 after a reforming process without recovery of CO from an isothermal TC are shown in dashed line in Fig. 5(a)). The gap between CO and H2 content enlarges with the increase of isothermal temperature due to higher CO2 to CO conversion by isothermal TC. More CO from the isothermal TC is added into the methane reforming process, which slightly influences the RWGS reaction.
As for water splitting with isothermal TC, the H2 content in the oxidation reaction varies with respect to TH at fixed rH2O. SRM and RWGS are the main endothermic reactions when methane is added to the gas mixture containing the oxidation products. Fig. 5(b) shows the optimal CH4/H2O ratio-y and the equilibrium mole fraction of each component after the reforming process, and hydrogen takes up a large portion in the gas mixture. Here, CH4/H2O ratio-y is the optimal molar ratio of CH4 to H2O input of the isothermal TC when the mole fraction of CH4 is 10-3 after the reforming process. The red solid and dashed lines represent the contents of H2 in the gas mixture of the proposed system and that of the methane reforming only system at the same temperature, respectively. More H2 will be produced in the isothermal TC with the increase of TH and it will influence the component ratio of the proposed system. The mole fraction of H2 of the proposed system has a larger increase trend than that of a methane reforming only system with the increase of T H.
11
35
0.18
30
0.15 T0
25
0.12
CO2 dash line H2O solid line
20 1200
1300
1400
1500
1600
1700
1800
0.09 1900
800
750
700
650
Mixing temperature T0 (℃)
0.21
CH4/CO2 ratio-x or CH4/H2O ratio-y
Carbon emissions per unit calorific value (g/MJ)
40
600
Isothermal TC Temperature TH (℃)
Figure 6. The comparation of isothermal CO2 and H2O splitting process integrated with downstream methane reforming at TH=1200-1900°C. Fig. 6 shows the comparison of H2 and CO for TH ranging from 1200 to 1900°C where the dashed and solid lines represent CO2 splitting and water splitting isothermal TC integrated with methane, respectively. We assume that the initial flow rate inputs of CO2 and H2O for isothermal TC are both 1000kmol/h. The sensible heat of the gas mixture after H2O splitting isothermal oxidation reaction is larger than CO2 and the enthalpy change of SRM is smaller than CDR for the same amount of methane. That is why the CO2 splitting process needs less CH4 input than water splitting process and the mixing temperature T0 is higher. It is also worth pointing out that due to the large excess of water or CO2 in the exhaust of an isothermal TC reactor, carbon deposition is disfavored thermodynamically, especially for exhaust of isothermal water splitting. Our thermodynamic calculation shows that the dashed red line in Fig. 6 approximately corresponds to the threshold temperature below which carbon deposition could occur, and methane-integrated heat recovery could use a slightly less amount of methane than previously calculated for each isothermal TC temperature shown in Fig. 6, thereby both avoiding carbon deposition and still reducing the temperature of the gas mixture down to a range (e.g., < 1100°C) where mature heat exchanger technology applies.
12
T0
H2
37 H2 O
30
36 35
20
CO2 34
10
CO CH4
0 0.0
0.5
1.0
33 TH=1600C
1.5 2.0 2.5 H2O/CO2 ratio
3.0
3.5
32
4.0
705
700
695
Mixing temperature T0 (°C)
Mole Fraction (%)
40
Carbon emissions per unit of calorific value (g/MJ)
38
690
Figure 7. Isothermal co-splitting CO2 and H2O splitting process integrated with methane varying with the H2O/CO2 molar ratio.
At a given isothermal splitting temperature of CO2 or H2O alone, the percentage of each gas species in the mixture after the refoming process is fixed, but co-splitting of CO2 and H2O can obtain varying molar ratios of CO/H2. Fig. 7 shows the mole fraction of the gas mixture after the reforming process. The CH4/CO2 ratio-x and CH4/H2O ratio-y are optimal values shown in Fig. 5 and the isothermal TC temperature is 1600°C. No H2O exists when H2O/CO2 ratio is 0, and the mole fraction of the gas mixture is the same as that shown in Fig. 5 (a) when the isothermal temperature is 1600°C. With the increase of H2O/CO2 ratio, the percentage of H2 increases rapidly, for SRM plays an increasingly important role, producing more H2. The mole fraction of H2 is lower than CO before the H2O/CO2 ratio reaches 0.56. The mixing temperature T 0 after reforming is in the range of 685°C to 705°C. When the H2O/CO2 ratio is about 1.39, the CO/H2 ratio is about 0.5, which is an important parameter for many chemical synthesis processes, such as methanol and dimethyl ether. Both the isothermal TC and methane reforming process store part of the concentrated solar energy. To reveal the proportion of storage capability by isothermal TC, we define Riso,st as the isothermal TC heat storage rate: R iso,st
n CO,iso HHVCO n H2 ,iso HHVH2 , n CO HHVCO n H2 HHVH2 n CH4 ,react HHVCH4
13
(13)
where nCO,iso and nH2,iso are the amounts of CO and H2 produced during isothermal TC, respectively, with subscript ‘iso’ standing for products of isothermal TC (rather than those of downstream methane reforming); n CH 4 ,react is the amount of CH4 consumed in the methane reforming process. The isothermal TC stores energy in the form of chemical energy of CO or H2, while the reforming process convert sensible heat of the gas mixture downstream the oxidation reactor into the promoted chemical energy by the enthalpy change of SRM or CDR and RWGS. Fig. 8(a) shows the isothermal heat storage rate of isothermal TC, where the capacity of CO2 splitting process is higher than that of water splitting process. The calorific value per mole of the gas mixture is an important parameter indicating the ability of gas mixture in converting chemical potential to internal energy when burning, which has been defined in Eq. 11 and showed in Fig. 8(a). According to the calorific value of the gas mixture, a proper combined-cycle power plant can be established to generate electricity. To get a higher calorific value per mole gas mixture, physical condensation can be used to remove H2O. The calorific value is also an important parameter for the modification of existing combined cycle power plants. Extra fossil fuels can be directly added into the combustion chamber of a gas turbine in order to improve the calorific value per mole if the power generated from the newly proposed system is not enough for methanol synthesis.
14
40
160
20
120
60
40
80
(b) CO2 dash line
60
H2O solid line
sol-to-syngas,CO2
40
sol-to-syngas,H2O
20
Riso,st,CO2
HHV,CO2
20
Riso,st,H2O HHV,H2O
0 1200
1300
1400
1500
1600
1700
1800
Isothermal TC Temperature TH (℃)
0 1900
0.95
Solar absorption efficiency abs
0 80 1200 1300 1400 1500 1600 1700 1800 1900 Isothermal TC Temperature TH (℃)
Calorific value per mole HHV mix(kJ/mol)
H2O solid line
Isothermal TC Heat Storage Rate R iso,st (%)
Isothermal TC Heat Storage Rate R iso,st (%)
200
CO2 dash line
60
80
Efficiency (%)
240
(a)
80
0.90
0.85
0.80
0.75
Figure 8. (a)Isothermal TC heat storage rate and the calorific value per mole of gas mixture for CO2 and H2O splitting integrated with methane reforming downstream. (b)The efficiency, isothermal TC heat storage rate and solar absorption efficiency varied with TH.
The solar-to-syngas efficiency of the solar-driven isothermal TC integrated with methane is shown in Fig. 9(a). It is worth mentioning that the product of Riso,st and ηsol-to-syngas is just ηHHV .With the increase of TH, the solar-to-syngas efficiencies of the newly proposed system for both CO 2 and H2O splitting are higher than the ηHHV of isothermal TC without heat recovery (Fig. 8(b)), because of the rational use of waste heat from T0 to TH. After 1700°C, the solar-to-syngas efficiency of CO2 splitting system integrated with methane begins to decrease. The maximum upgrade factors of Eq. 6 and Eq. 7 driven by solar energy are 1.281 and 1.277 (ratio of the heating value of products to reactants), respectively and the values are close to the ugrade factors of isothermal H 2O and CO2 splitting TC at 1200°C (Fig. 9(a)), for only a small amount H2 or CO is produced in the isothermal 15
TC. With the increase of the isothermal TC temperature, the upgrade factor of CO2 splitting process integrated with methane has a larger growth trend than H2O for the conversion of CO2 to CO is
50
3.0
(a) CO2 dash line
45
H2O solid line
2.5
40 2.0 35 1.5
30
Upgrade Factor
Solar to Syngas Efficiency sol-to-syngas (%)
larger than that of H2O to H2 in an isothermal TC at the same TH (Fig. 3).
25 1.0 1200 1300 1400 1500 1600 1700 1800 1900 Isothermal TC Temperature TH (℃) 80
HHV Efficiency (%)
(b) 60
Proposed system sol-to-syngas
(methane+(f25C-T0=80%))
40
Real isothermal TC HHV (f25C-1100C=80%) Potential isothermal TC HHV (f25C-TH=80%)
CO2 dash line
20
H2O solid line isothermal TC HHV (fequivalent)
0 1200
1300
1400
1500
1600
1700
1800
Isothermal TC Temperature TH (℃)
1900
Figure 9. (a) Solar-to-syngas efficiency soltosyngas and Upgrade Factor for isothermal TC integrated with methane as a function of TH (b) Efficiency of isothermal TC and proposed system with different heat recovery varied with isothermal TC temperature. After methane reforming process, the heat of the gas mixture ranging from 25°C to T0 is recovered with a 80% rate to reheat the input room-temperature H2O or CO2. The solar to syngas efficiency for the proposed system is shown in black lines with solid symbols in Fig. 9(b), where the dashed and solid lines represent CO2 and H2O splitting isothermal TC integrated with methane, respectively. Therefore, the proposed system recovers two parts of thermal energy: the total sensible heat of the 16
mixed gases after the oxidation reaction during the temperature swing of T H and T0 (by methane reforming) and 80% sensible heat during the temperature swing of T 0 to 25°C (by a heat exchanger), so that the total heat recovery rate is in a range of approximately 86-92%. For comparation, the green lines in the chart have a equivalent recovery rate as the proposed system but only for an isothermal TC without methane heat recovery. We also calculate the isothermal TC efficiencies without methane reforming process in blue and red lines (Fig. 9(b)), which represent a 80% heat recover rate of the mixed gases (CO/CO2 or H2/H2O) downstream the oxidation reactor from 25°C to TH and 25°C to 1100°C, respectively. The efficiencies of isothermal TC with a potential 80% heat recorvery of the mixed gases (i.e. potential isothemal TC efficiencies) are shown in blue lines with half open symbols, while in the mean time, the mixed gases are cooled from T H to room temperature. Because of the immaturity of heat exchanger for temperatures above 1100°C, only 80% heat recovery of the mixed gases from 1100°C to 25°C is calculated, and the efficiencies of which (i.e. real isothemal TC efficiencies) are shown in red lines with open symbols (Fig. 9 (b)). The efficiencies of the proposed system for both CO2 and H2O splitting isothermal TC integrated with CH4 are higher than the potential and ideal efficiencies of isothermal TC. This new approach has the potential to produce high-quality syngas and lower specific CO2 emissions per mole of feedstock at the same time. The carbon emission per unit calorific value of fuels obtained after the reforming process of this new approach is calculated according to Eq. 12, compared with direct combustion of methane or direct combustion of syngas after solar reforming (Fig. 10). The heat needed for the enthalpy change of the CDR and SRM reactions is from the waste heat downstream the oxidation reactor, and then the products are combusted to calculate the carbon emissions per unit calorific value. Both the isothermal splitting of CO2 and H2O integrated with methane lower the specific CO2 emissions as the isothermal TC is carbon-neutral and the calorific value of the methane is upgraded through the solar energy input by the enthalpy change of endothermic reactions (SRM, CDR and RWGS). The proportion of CO in the isothermal oxidation gas mixture and the isothermal TC heat storage rate improves with the increase of isothermal 17
temperature, explaining the decrease in specific CO2 emissions of this new system. The carbon emissions per calorific value is lower for CO2 splitting than that of H2O splitting isothermal TC integrated with methane although the value of CDR is a bit higher than SRM shown in Fig. 10,
Carbon emissions per unit of calorific value (g/MJ)
mainly because of a higer conversion of CO2 to CO than H2O to H2 at the same TH. (a) CH4(g) + 2O2(g) = CO2(g) + 2H2O(l)
50
49.4
40
1200℃ 1300℃
38.58
1400℃
30
CH4(g) + H2O(g) = CO(g) + 3H2(g)
1500℃ 1600℃ 1700℃ 1800℃ 1900℃
20 H2O:1000kmol/h oxidation products of TC+CH4 reforming
10 0 0
50
100
150
200
250
300
Carbon emissions per unit of calorific value (g/MJ)
CH4(kmol/h) (b) CH4(g) + 2O2(g) = CO2(g) + 2H2O(l)
50
49.4
40
CH4(g) + CO2(g) = 2CO(g) + 2H2(g)
1200℃
38.68
1300℃ 1400℃ 1500℃ 1600℃ 1700℃ 1800℃
30 20
1900℃
10
CO2:1000kmol/h Oxidation products of TC+CH4 reforming
0 0
50
100
150
200
250
CH4(kmol/h)
Figure 10. Carbon emissions per unit calorific value of fuel obtained of the polygeneration system with isothermal TC splitting water (a) or CO2 (b) TC integrated with CH4 heat and gas recovery downstream. The specific CO2 emissions of the CO2 and H2O co-splitting isothermal TC process integrated with methane are also calculated in Fig. 7 in the front part. The carbon emissions per unit calorific value of fuel obtained are in the range of 32.7 to 36.1 g/MJ for T H=1600°C, which is between those of the splitting of H2O and CO2 integrated with methane independently. With the increase of H2O/CO2 ratio, the specific CO2 emissions increase, for the H2O splitting isothermal TC integrated with 18
methane has a much larger specific CO2 emission (Fig. 10(a)) than CO2 (Fig. 10(b)) and more methane is needed to obtain nearly the same calorific value of syngas.
3.3. Solar thermochemical polygeneration system integrated with CH4 A polygeneration system is proposed based on analyses on recovery of waste heat and utilization of unreacted gas species (e.g. H2O or CO2) with methane downstream an isothermal solar TC reactor. It is capable of simultaneous power and methanol production with the advantage of eliminating selfgeneration power plant which is conventionally needed for methanol production. 90 °C Exhaust Gas CO2
①
Power
CH4 1600 °C,1bar
Oxidation Reaction of Isothermal TC
CO/CO2
Reforming
Power Gas Turbine Unit
HRSG
A
H I Recycled Gas
G Unreacted Gas Discharge
CeO2
CeO2-δ CH4 1600 °C,PO2=10-5 bar 600-800 °C Solar Energy Reduction Reaction O2 (1 bar) Partial Heat B of Isothermal TC Oxidation Recovery
25 °C D
②
CeO2-δ
③
H2O
H2/H2O
CO2 Seperation
E
Compression Unit
F
Methanol Synthesis Unit
Flashing
Distillation Unit
Methanol
H2O
CeO2 1600 °C,1bar
Oxidation Reaction of Isothermal TC
225 °C,80bar
Reforming C CH4
Figure 11. Simplified process of solar thermochemical polygeneration system integrated with methane
A scheme [17] is conceptually proposed based on the splitting reaction in isothermal TC (Fig. 11), of which solar energy and methane are the main input energy and the purpose of the system is to generate power and methanol. The polygeneration system mainly consists of four parts: isothermal splitting of CO2 and H2O, methane reforming process, methanol synthesis reaction and combined cycle for power. The isothermal oxidation reaction of CO 2 or H2O splitting TC produces CO or H2 respectively with a large amount of CO2 or H2O remaining, while the reduction reaction produces pure oxygen. SRM, partial oxidation of methane and CDR are the main reactions occurring when methane is added to the products of isothermal TC, shown as A, B and C in Fig. 11, respectively. 19
Before the mixing of these three streams (point D in Fig. 11), the latent heat is recovered and water is removed independently. Selexol method [18] is used to absorb CO2 and the syngas is then compressed to a pressure of 80 bar. Methanol is synthesized at 225°C and purified in the distillation unit [19]. The unreacted gas and exhaust gas are then combusted to produce power in a gas and steam combined cycle. The inputs of the polygeneration system are the concentrated solar energy and complementary CH 4 and the outputs of which are power and methanol. We define a series of formula to evaluate the performance and energy consumptions in the following part: The Solar Energy Consumption is the ratio of solar energy to the mass of synthesized methanol:
Esol Qsolar / mCH3OH ,
(14)
where mCH3OH is the mass flow rate of synthesized methanol, Qsolar (kW) is the solar energy input considering the solar absorption efficiency in Eq. 5. The Fossil Fuel Consumption is the ratio of lower heating value of methane to the mass of synthesized methanol without considering the solar energy input, which is an important index indicating the utilization of CH4 input: Efos (n CH4 ,init LHVCH4 ,init W / 0 ) / mCH3OH ,
(15)
where W is the net power output of the system, more methane will be directly added into the combined cycle to make sure that the value of W is positive; 0 is the heat to power efficiency of the gas and steam combined cycle. If W / 0 is positive, it represents the superfluous methane for the polygeneration system and if it is negative it represents the extra needed methane to produce enough power for methanol production except the initial input methane for reforming process. The power generated by the polygeneration system is much smaller than the heating value of synthesized methanol from the calculation of the system in Fig. 11. First, we assume that the power of the polygeneration system comes from the CH4 input (extra CH4 is needed to make sure that the 20
power output of the system is enough for the methanol production) instead of solar energy. Therefore, the system can be simplified to have one output (methanol) and two inputs (solar energy and CH4 deducted the portion to generate power). Then the system is compared with state-of-the-art industrial methanol energy consumption making a further simplification to obtain a one input (solar energy) and one output (methanol deducted the portion from CH 4) system. Then we define
Solar CH OH as the heating value of methanol to the solar energy input of this one input and one 3
output model:
Solar CH OH (R CH OH (QCH 3
3
4
W
0
)
3.6 ) LHVCH3OH / (Qsolar 3600) , E
(16)
where R CH3OH (kg/h) is methane productivity; Q CH (kW) is the total heating value of CH4 input; E 4
(GJ/ton), is the industrial energy consumption of CH4 to produce methanol. LHVCH3OH (kJ/kg) is the lower heating value of methane. The Net Solar Energy Consumption is defined as the ratio of the solar energy to the mass of methanol of the simplified one input and one output model: Esol,net Qsolar 3600 / [QCH3OH (QCH4
W
0
)
3.6 ] E
(17)
In the configuration of water spliting and CO2 splitting oxidation reaction, the ratio of CO2 to H2O is adjusted so that the optimal (nH2-nCO2)/(nCO+nCO2) molar ratio (usually 2.05~2.15 in actual methanol synthesis process) [20] of the fresh gas mixture at point E in Fig. 11 is produced to synthesize methanol. The (nH2-nCO2)/(nCO+nCO2) ratio in this paper is chosen to be 2.05 rather than 2.0 (the ideal stoichiometric number) [21,22] and a little higher amount of H2 has good benefit for the activity of catalyst and avoiding carbon deposition. Note that the CO/H2 ratio at equilibrium of partial oxidation of methane (CH4+0.5O2=CO+2H2) is exactly 1:2 at point B in Fig. 11. The temperature of isothermal oxidation products is 1600°C, which decreases to 600-850°C after the reforming process of methane. WGS reaction (CO+H2O=CO2+H2) occurs and more hydrogen will be generated at the expense of CO with the decrease of the temperature. The three pathways are not mixed before heat recovery process, for the reason that there is still a large amout of water after 21
SRM (pathway ③), which will react with the produced CO in pathway ①. The separation of water before mixing at point D in Fig. 11 is necessary to improve the yield of methanol. An advantage of the polygeneration system is that syngas is produced from the isothermal TC and methane
11 10 9
1.1 Solar Energy
Energy Consumpution for CH3OH Production(GJ/t)
0.150
0.175
50
0.200 0.225 CH4/H2O ratio - y
8 0.250
40 44.0
30
43.5
Solar Energy Consumption
43.0
20
10 0.150
42.5
0.175
0.200 0.225 CH4/H2O ratio - y
4
2.0 630
1.5 627 Work Output of Combined Cycle CO2 Seperation and Gas Compression Work
624
1.0
Net Work Output
0.5 0.150
44.5
Net Solar Energy Consumption CH4 Consumption
2.5 633
621
45.0
(c)
(b)
636
42.0 0.250
0.175
0.200 0.225 CH4/H2O ratio - y
1.45
0.250
(d)
1.40 H2O/CO2 ratio- w
1.2
CH3OH Production (kmol/h)
12 1.3
3
13
CH4 heat value
Energy Input (x10 kW)
1.4
3.0
14
(a)
Solar to CH3OH Efficiency Solar-CH3OH (%)
H2O/CO2 ratio- w
1.5
Work Input and Output (x10 kmol/h)
reforming process without either WGS or RWGS reactors.
1.35 1.30 CH4/CO2 ratio - x
1.25 1.20 1.15
0.25 0.17 0.16 0.15
1.10 1.05 0.150
0.175
0.200 0.225 CH4/H2O ratio - y
0.250
Figure 12. (a) The optimal H2O/CO2 ratio and energy input varied with CH4/H2O ratio-y (b) Methanol production and work output varied with CH4/H2O ratio-y (c) Energy consumption for CH3OH production varied with CH4/H2O ratio-y (d) The optimal H2O/CO2 ratio-w for various CH4/H2O ratio-y and CH4/CO2 ratio-x. The CH4/CO2 ratio-y of (a)(b)(c) is fixed to 0.21.
We assume that the CH4/CO2 ratio (pathway①) is fixed to 0.21 and the molar flow rate of CO2 is 1000 kmol/h, while the CH4/H2O ratio (pathway ③) changes from 0.15 to 0.25, therefore a series of H2O/CO2 ratio will be obtained to make sure that the (nH2-nCO2)/(nCO+nCO2) ratio before methanol synthesis at point E in Fig. 11 is 2.05, shown in Fig. 12(a). The heating value of CH4 and 22
concentrated solar energy needed by the polygeneration system are also shown. The methanol production and work output are shown in Fig. 12(b), where the net work output is the work output of combined cycle unit deducting the CO2 separation work and the compression work of the gas mixture before methanol synthesis. The net work output may be negative because the exhaust gas is not enough for the combined cycle to produce a large amount of power and it will be positive with the increase of CH4/CO2 ratio-x. The minimum net solar energy consumption of methanol is about 44.5 GJ/ton and maximum Solar CH3OH is about 44.8% (Fig. 12(c)). The net solar energy consumption is much higher than solar energy consumption because the conversion of fossil fuel to methanol synthesis is deducted. The Fossil Fuel Consumption is about 22 GJ/ton, lower than typical
45.0
45.5
(b)
44.5 45.0 CH4/H2O ratio - y= 0.22 44.0 44.5 43.5 0.14
0.16
0.18 0.20 0.22 CH4/CO2 ratio - x
0.24
Net Solar Energy for CH 3OH (GJ/t)
Solar to CH3OH efficiency Solar-CH3OH, max (%)
values (about 28-40 GJ/ton) [23] in current industrial processes.
0.26
Figure 13. (a)The Net Solar Energy Consumption for CH3OH with CH4/CO2 ratio-x and CH4/H2O ratio-y ranging from 0.15-0.25 (b) The maximum Solar CH3OH and minimum Net Solar Energy Consumption for CH3OH production CH4/CO2 ratio-x is changed from 0.15 to 0.25 with CH4/H2O ratio-y being changed simultaneously, and the H2O/CO2 ratio-w is optimized to make sure that the (nH2-nCO2)/(nCO+nCO2) ratio of the syngas before methanol synthesis is 2.05 (Fig. 12 (d)). The net solar energy consumption for CH3OH is shown in Fig. 13(a) with a variety of CH4/H2O ratio-y and CH4/CO2 ratio-x. For each point of CH4/CO2 ratio-x from 0.15 to 0.25, the solar to CH3OH efficiency reaches the maximum
23
value when CH4/H2O ratio-y is around 0.22. Note that the operational and optical losses are not taken into account when calculating the solar to CH3OH efficiency.
5. Conclusions In this paper, we proposed an approach of simultaneously recovering waste heat and unreacted gas (e.g. H2O or CO2) downstream an isothermal thermochemical cycling (TC) reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH 4). It has the potential to produce high-quality syngas and lower the temperature of isothermal oxidation products to the region of 600-850°C where heat recuperation technology is mature. The solar-to-syngas efficiencies of the proposed system for CO2 and H2O splitting at 1600°C are 45.7% and 38.1%, respectively, while the efficiencies of isothermal TC without CH4 integration are only 21.0% and 6.4% without heat recovery. The isothermal TC heat storage rates of the proposed system for CO 2 and H2O splitting processes are 46.1% and 15.7% at 1600°C, respectively. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream. Cosplitting of CO2 and H2O integrated with methane can obtain adjustable CO/H2 ratio for various chemical synthesis process. A scheme of polygeneration of methanol and power system is proposed as an example of this approach. It is possible to eliminate the self-generation power-plant which is conventionally needed for methanol production and no WGS or RWGS reactor is needed. Fossil fuel consumption for the production of a unit mass of methanol is about 22GJ/ton, lower than typical values in current industrial processes and the optimal solar to methanol efficiency can be more than 44%.
Acknowledgments This work was funded by the Recruitment Program of Global Experts of China, the Chinese Academy of Sciences Innovative and Interdisciplinary Team Award, and Natural Scientific Foundation of China under contract No. 51236008. 24
References [1] Steinfeld A., Solar thermochemical production of hydrogen-a review. Solar Energy 2005; 78(5): 603-615. [2] Smestad G.P., Steinfeld A., Review: Photochemical and Thermochemical Production of Solar Fuels from H2O and CO2 Using Metal Oxide Catalysts. Industrial & Engineering Chemistry Research 2012; 51(37): 11828-11840. [3] Jensen S.H., Larsen P.H., Mogensen M., Hydrogen and synthetic fuel production from renewable energy sources. International Journal of Hydrogen Energy 2007; 32(15): 3253-3257. [4] Chueh W.C., Falter C., Abbott M., Scipio D., Furler P., Haile S.M., Steinfeld A., High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 2010; 330(6012): 1797-1801. [5] Venstrom L.J., De Smith R.M., Hao Y., Haile S.M., Davidson J.H., Efficient Splitting of CO2 in an Isothermal Redox Cycle Based on Ceria. Energy & Fuels 2014; 28(4): 2732-2742. [6] Furler P., Scheffe J., Gorbar M., Moes L., Vogt U., Steinfeld A., Solar thermochemical CO2 splitting utilizing a reticulated porous ceria redox system. Energy & Fuels 2012; 26(11): 70517059. [7] Felinks J., Brendelberger S., Roeb M., Sattler C., Pitz-Paal R., Heat recovery concept for thermochemical processes using a solid heat transfer medium. Applied Thermal Engineering 2014; 73(1): 1006-1013. [8] Muhich C.L., Evanko B.W., Weston K.C., Lichty P., Liang X., Martinek J., Musgrave C.B., Weimer A.W., Efficient generation of H2 by splitting water with an isothermal redox cycle. Science 2013; 341(6145): 540-542. [9] Roeb M., Sattler C., Isothermal water splitting. Science 2013; 341(6145): 470-471. [10]
Hao Y., Yang C.K., Haile S.M., High-temperature isothermal chemical cycling for solar-
driven fuel production, Physical Chemistry and Chemical Physics 2013; 15(40): 17084-17092. 25
[11]
Hao Y. Solar thermal dissociation of CO2: towards higher efficiencies. Proceedings of the
9th International Green Energy Conference (IGEC-IX); 2014; Tianjin, China. [12]
De Mello P.E.B., Monteiro D.B., Thermodynamic study of an EFGT (externally fired gas
turbine) cycle with one detailed model for the ceramic heat exchanger. Energy 2012; 45(1): 497-502. [13]
Kim K.T., Dae H.L., Kwon S., Effects of thermal and chemical surface-flame interaction on
flame quenching. Combustion and Flame 2006; 146(1-2): 19-28. [14] Wang H.S., Hao Y., Kong H., Thermodynamic study on solar thermochemical fuel production with oxygen permeation membrane reactors. International Journal of Energy Research 2015; 39(13): 1790-1799. [15] Fletcher E.A., Moen R.L., Hydrogen and oxygen from water. Science 1977; 197(4308): 10501056. [16] Piatkowski N., Wieckert C., Weimer A.W., Steinfeld A., Solar-driven gasification of carbonaceous feedstock – a review. Energy & Environmental Science 2011; 4(1): 73-82. [17] Kong H., Hao Y., Wang H.S., Ji X.Z., Jin H.G., A Solar Thermochemical Polygeneration System Integrated With Methane. Journal of Engineering Thermophysics 2015; 36(4): 691697. (in Chinese) [18] Chiesa P., Consonni S., Kreutz T., Williams R., Co-production of hydrogen, electricity and CO2 from coal with commercially ready technology. Part A: Performance and emissions. International Journal of Hydrogen Energy 2005; 30(7): 747-767. [19] Greeff I.L., Visser J.A., Ptasinski K.J., Janssen F.J.J.G., Utilisation of reactor heat in methanol synthesis to reduce compressor duty-application of power cycle principles and simulation tools. Applied Thermal Engineering 2002; 22(14): 1549-1558.
26
[20] Liu L.N., Li H., Wang D., Coal, Oil Field Gas and Residual Oil Joint Production of Methanol to Realize Resources Complementary, Energy Conservation and Emissions Reduction. Chemical Production and Technology 2012; 19(4): 45-50. [21] Lange J.P., Methanol synthesis: a short review of technology improvements. Catalysis Today 2001; 64(1-2): 3-8. [22] Luu M.T., Milani D., Abbas A., Analysis of CO2 utilization for methanol synthesis integrated with enhanced gas recovery. Journal of Cleaner Production 2016; 112(4): 3540-3554. [23] Dave N., Foulds G.A., Comparative-assessment of catalytic partial oxidation and steam reforming for the production of methanol from natural-gas. Industrial & Engineering Chemistry Research 1995; 34(4): 1037-1043.
27
Highlights
Methane reforming proves effective for heat recovery of isothermal solar reactors; Solar-to-syngas efficiencies for CO2 and H2O splitting at 1600°C are 45.7% and 38.1%, respectively; Carbon emission per calorific value of solar syngas can be significantly lowered.
28
S1359-4311(16)30478-1 http://dx.doi.org/10.1016/j.applthermaleng.2016.03.170 ATE 8038
To appear in:
Applied Thermal Engineering
Received Date: Accepted Date:
25 February 2016 31 March 2016
Please cite this article as: H. Kong, Y. Hao, H. Wang, A Solar Thermochemical Fuel Production System Integrated with Fossil Fuels, Applied Thermal Engineering (2016), doi: http://dx.doi.org/10.1016/j.applthermaleng. 2016.03.170
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
A Solar Thermochemical Fuel Production System Integrated with Fossil Fuels Hui Konga,b , Yong Haob*, Hongsheng Wanga,b a
b
University of Chinese Academy of Sciences, No.19A Yuquan Rd, Beijing, 100049, P. R. China.
Institute of Engineering Thermophysics, Chinese Academy of Sciences, 11 Beisihuanxi Rd., Beijing, 100190, P. R. China. * Corresponding author; email: [email protected]
Abstract: Thermochemical cycling (TC) is a promising means of converting solar energy into chemical fuels. Isothermal solar TC has the potential of achieving high solar-to-fuel efficiencies contingent on effective gas-phase heat recovery at high temperatures. We propose an approach of simultaneously recovering waste heat and unreacted gas species (e.g. H2O or CO2) downstream the isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH 4). Such comprehensive utilization and the syngas produced enable the establishment of a polygeneration system for simultaneous power and methanol production with the possibility of eliminating the selfgeneration power-plant that is conventionally needed for methanol production. A scheme is conceptually proposed based on the type of splitting reaction in isothermal TC, and optimization of the polygeneration system is discussed with solar conversion efficiency as the objective. Fossil fuel consumption for the production of a unit mass of methanol is about 22GJ/ton, lower than typical values in current industrial processes. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream.
Keywords: Isothermal, Polygeneration, Solar Fuel, Syngas, Thermochemical Cycling, Methanol. 1
1. Introduction Conversion of sunlight into energy of storable forms such as latent heat or chemical energy of fuels carries special significance in addressing unfavourable natures of solar energy such as sparsity and intermittency. Solar energy conversion routes producing chemical fuels (especially H2O or CO2 splitting for H2 or CO) are of particular long-term interest with their great potential in meeting worldwide energy demand and tremendously reducing our dependence on fossil fuels. Electrochemical, photochemical and thermochemical are three basic pathways for such purposes [1, 2]. While some of these routes may benefit from the combination of more than one converted form of solar energy (e.g., solar-driven electrolyzer [3]), the thermochemical approach is driven by only concentrated solar heat and has the advantage of making use of the entire solar spectrum. Thermochemical cycling (TC) is a technology for solar energy storage via the synthesis of chemical fuels by utilizing concentrated solar heat alone to drive endothermic chemical reactions. Two-step TC of ceria and its derivatives attracts much attention for efficient splitting of H2O (or CO2) for H2 (or CO), a valuable feedstock to product ammonia, methanol, or liquid hydrocarbon. Ceria-based H2O and CO2 splitting processes are described as follows: oxygen is first released when solar energy is used to reduce the metal oxide at a high temperature T Re; then the reduced metal/metal oxide is re-oxidized with water or CO2 at a lower temperature TOx, yielding H2 or CO. Reduction (at TRe): 1CeO2- 1CeO2- + 0.5 O2 , f
i
Oxidation (at TOx): 1CeO2 H2O 1CeO2 H2 , or i
f
1CeO2i CO2 1CeO2f CO ,
Combined: H2O H2 0.5 O2 , or
(1) (2a) (2b) (3a)
CO2 CO 0.5 O2 ,
(3b)
where stands for oxygen nonstoichiometry; i and f correspond to values at the beginning and the end of the oxidation step, respectively ( i 2
f
), and i f stands for the net change
in oxygen nonstoichiometry; TOx and TRe are the temperatures at which oxidation and reduction reactions occur, respectively. Ceria-based TCs have been demonstrated to deliver stable fuel production at temperatures up to 1600°C due to the absence of phase change of ceria and relatively low tendency of sintering of ceria microstructure under typical TC conditions [4, 5].
Although theoretical solar-to-fuel efficiency of solar-driven TC could be high, low achievable solarto-fuel efficiency has been the major barrier against the practicality of solar-driven TC in general [4]. Steinfeld et al. recently achieved 1.73% (average) and 3.53% (peak) solar-to-fuel efficiencies with ceria (CeO2) packed bed of reticulated porous ceramic (RPC) [6]. Heat recovery from solidphase oxygen carrier (e.g., ceria) and reactor remains a key issue against efficiency enhancement [7]. A conventional two-temperature TC works with a large temperature swing, which potentially leads to a significant thermal shock to the reactor and results in the loss of a considerable portion of the solar energy input in the form of waste heat to the environment. In the last three years, several research groups [8-10] reported isothermal TC, in which reduction and oxidation reactions occur at identical temperatures. The elimination of temperature difference minimizes heat loss from solid phases and their thermal shock, but shifts the burden of heat recovery to the gas phase (including reactant and product species). Comparatively, high-efficiency heat recovery is considered easier to achieve with gases than with solids. Thus isothermal TC offers the possibility of making simple and highly efficient solar TC reactors that convert H2O or CO2 back to fuel [10, 11].
3
Figure 1. Conceptual illustration of an isothermal thermochemical water splitting TC integrated with methane heat recovery; SRM, CDR and RWGS stand for steam reforming of methane, carbon dioxide reforming, and reverse water-gas shift, respectively.
Effective heat recovery and energy cost reduction are essential for enhancing the efficiency of isothermal TC. The solar-to-fuel efficiency of isothermal splitting of CO2 is much higher than isothermal water splitting process (e.g., three times higher at 1500°C without heat recovery) [11] due to the favorable thermodynamic nature of CO2 and the lack of phase change. Direct use of oxidation products and unreacted CO2 can reduce additional energy consumption for CO2 separation, which eventually cuts down solar energy input to the system. The gas mixture downstream the oxidation process (Eq. 2a and 2b) contains a large amount of sensible heat, the recovery of which can be used to heat up fresh CO2 or H2O on the feed side. However, heat exchangers are immature above the temperature of 1100°C [12]. Instead of direct heat recovery via a heat exchanger, we propose an approach that simultaneously recovers waste heat and unreacted gas species (e.g. H2O or CO2) downstream an isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels [13] (e.g. CH4). The temperature of the gaseous exhaust of isothermal oxidation reaction (Eq. 2a and 2b) can be reduced to 600-850°C and the calorific value of which can also be improved. The scheme is shown in Fig. 1 (taking isothermal H2O splitting TC for example). Although isothermal co-splitting of CO2 and H2O can obtain syngas alone, the integration with fossil fuel downstream of an isothermal TC reactor can further enhance syngas yield and utilization of solar energy. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream. At the end of this paper, a solar thermochemical polygeneration system integrated with methane for methanol synthesis is proposed to further prove the benefit of this approach.
4
2. Analytical Methodology The thermodynamic efficiency of a conventional two-temperature, two-step solar TC for converting solar energy and H2O (or CO2) to H2 (or CO) can be defined as [11]:
HHV
(Qredu Qheat
HHV , Qceria ) / abs Qpump
(4)
where HHV is the higher heating value of one mole of H2 (or CO). The denominator represents the total solar energy input consisting of four enthalpy terms for extracting 1 mole of O atoms from ceria (considering re-radiation losses): that required to reduce ceria from CeO 2f to CeO 2i at TRe (denoted as Qredu), that required to heat water (or CO2) from 25°C to TOx (denoted as Qheat), that required to heat ceria from TOx to TRe (denoted as Qceria), and the energy needed to obtain a lower oxygen partial pressure in the reduction step with a vacuum pump (denoted as Qpump) [14]. For isothermal TC, Qceria vanishes for TRe=TOx=TH ; The denominator is the minimum solar energy needed for producing one mole of H2 (or CO), and solar energy absorption efficiency
abs
T4 1 , IC
(5)
is defined for an ideal blackbody reactor [15] in which the cycling reactions are assumed to take place,
is Stefan-Boltzmann’s constant, T is the temperature of the reactor, I is solar irradiation,
and C is the concentration ratio of solar energy. Our proposed approach recovers waste heat and utilizes unreacted gas species (e.g. H2O or CO2) downstream an isothermal TC reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH4), mainly the steam reforming of methane (SRM) , the carbon dioxide reforming of methane reaction (CDR) and the reverse water gas shift reaction (RWGS): CH4 H2O CO 3H2
H25C 205.9kJ / mol ,
(6)
CH4 CO2 2CO 2H2
H25C 247.0kJ / mol ,
(7)
H2 CO2 CO H2O
H25C 41.1kJ / mol .
(8)
5
Therefore, through the combination of these reactions, the unreacted gases in the exhaust and its high temperature sensible heat are rationally used, and the final products after reforming are a mixture of H2, CO, CO2, H2O (with residual CH4). The conversion of solar energy to chemical energy for an isothermal TC integrated with fossil fuels [16] is gauged by solar-to-syngas efficiency: ηsol-to-syngas =
n H2 HHVH2 n CO HHVCO n CH4 ,react HHVCH4 , Qsolar
(9)
where Qsolar is the total solar energy delivered through the receiver’s aperture considering the solar absorption efficiency. nCH4,react, nH2 and nCO are the amounts of CH4 reacted, H2 and CO produced after the reforming process, respectively. HHVH2, HHVCO and HHVCH4 are the higher heating values of H2, CO and CH4, respectively. The energetic upgrade factor is defined as the ratio of the higher heating value of syngas to that of feedstock processed in a similar form to Ref. [16] (but with HHV instead of LHV): U=
n syngas HHVsyngas , n feedstock HHVfeedstock
(10)
The calorific value per mole of the gas mixture downstream the oxidation reactor is defined by:
HHVmix
(XCO HHVCO X H2 HHVH2 XCH4 ,residue HHVCH4 ) , XCO X H2 X CO2 X H2O X CH4 ,residue
(11)
where XCO, XH2, XCO2, XH2O, XCH4 ,residue are the mole fractions of CO, H2, CO2, H2O and the remaining CH4, respectively. Another important parameter is the net carbon emissions per unit calorific value, defined as:
C
(n CH4 ,feed n CH4,residue ) 44g / mol , n H2 HHVH2 n CO HHVCO
(12)
where n CH4 ,feed and n CH4, residue are the amounts of CH4 input and residue for SRM or CDR process, respectively. The net carbon release is traced back to the reacted CH4 instead of CO2 input of the isothermal TC for that CO2 is released again after it goes through the entire process. 6
3. Results and Discussion 3.1. Isothermal splitting of CO2 or H2O For the convenience of discussions, the ratios of CO2 and H2O to available oxygen vacancies in ceria upon reduction are defined as rCO2 n CO2 / ( i n Ceria ) and rH2O n H2O / (i n Ceria ) , respectively. The high temperature thermolysis of CO2 or H2O will lead to a higher oxygen partial pressure (than that of the reduction step), which provides a driving force for oxygen intake by ceria during the oxidation (i.e. splitting) step. The net conversion of CO2 to CO is shown in Fig. 2 for different isothermal reaction temperature T H. For CO2 splitting isothermal TC at 1600°C, the net conversion of CO2 is 11.4% while the HHV efficiency is 21.5% (without heat recovery) for rCO2=0.01.
Figure 2. Percentage of CO2 converted to CO and HHV for different TH from 1500 to 1900°C.
7
60
Conversion (%)
50
H2O splitting isothermal TC CO2 splitting isothermal TC
40 30 20 10 0 1200
1300
1400
1500
1600
1700
1800
1900
o
Isothermal TC Temperature TH ( C)
Figure 3. The conversion of CO2 and H2O for isothermal oxidation reaction with TH in the range of 1200-1900°C. The thermodynamic efficiency as defined by Eq. 4 only takes into account the minimum energy required for a redox cycle of isothermal TC to complete. At a given temperature TH and oxygen partial pressure of reduction (e.g. 10-5 atm), the thermodynamic efficiency of the isothermal TC (Eq. 4) is a fixed number for a specific steam/oxide ratio. Details of efficiency calculations similar to what we present in this work can be found in our previous work for a closed system [10]. For producing one mole of CO or H2, the value of rCO2 or rH2O is varied in a wide range between 10-4 and 104 so that the maximum HHV efficiency (and correspondingly the minimum energy input) of the upstream isothermal TC can be found in this interval. The conditions under which the minimum energy input occurs and the corresponding gas mixture compositions are then used for the calculation of the methane-incorporated heat recovery downstream of the reactor. The output of the oxidation reaction of isothermal TC is a mixture of CO and unreacted CO 2 (at TH) for CO2 splitting process, while for H2O splitting process, the output is a mixture of H2 and unreacted H2O (at TH). The conversion rates of CO2 to CO and H2O to H2 for independent CO2 or H2O splitting isothermal TC are shown in Fig. 3, for which rCO2=rH2O=0.01. The oxide/steam ratio designated by rCO2 or rH2O can be achievable in practice where the volume of CO2 or H2O is close to the catalyst (50% porosity is assumed) at temperatures of 1500°C or above. Separation of H2 from H2O consumes no
8
additional energy due to condensation of the latter, while separation of CO from CO2 requires extra energy to obtain pure CO.
3.2. Isothermal TC integrated with downstream methane reforming Methane is added into the mixture of CO and CO2 downstream of an isothermal reactor (1600°C, pO2=10-5 atm for thermal reduction) to both lower the temperature and completely convert unreacted CO2 to CO and H2 (via CDR, Eq. 7). The initial flow rate of CO2 into the reactor is assumed to be 106 mol/h. With the continuous increase of methane, the gas mixture achieves thermodynamic equilibrium without additional heat input, and the mole fraction of the gas mixture is shown in Fig. 4. CH4/CO2 ratio is the molar ratio of CH4 to CO2 input of isothermal TC. The mixing temperature decreases rapidly until CH 4 cannot be consumed further and begins to accumulate with a noticeable amount (i.e. mole fraction > 10-3) at a CH4/CO2 ratio of 0.1562 (marked as CH4/CO2 ratio-x). It is worth mentioning that the reverse water gas shift reaction and other chemical reactions occur simultaneously to gain a minimum Gibbs free energy. 100 TH=1600°C
Temperature (°C)
1400
80
1200 60 1000 CO2
800 689.1
600 400 0.00
40
CO H2
20
Mole Fraction (%)
1600
H2 O CH4
0.05
0.10
0.15
0.20
0.25
0 0.30
CH4/CO2 ratio
Figure 4. Isothermal CO2 splitting process integrated with downstream methane-integrated heat recovery process at TH=1600°C.
9
0.16
70
(a)
60
CO2
0.15
CO
0.14
50 COCDR
40
0.13
30
0.12 H2
0.11
H2,CDR
20
Mole Fraction (%)
Optimal CH4/CO2 ratio - x
0.17
10
0.10
H2 O
0 1200 1300 1400 1500 1600 1700 1800 1900 Isothermal TC Temperature TH (℃)
H2 H 2O
60 50
H2,SRM
0.16
40 30
0.14
20
0.12 0.10 1200
CO
CO2
CH4
COSRM
Mole Fraction (%)
Optimal CH4/H2O ratio - y
0.18
70
(b)
0.20
10 0
1300
1400
1500
1600
1700
1800
1900
Isothermal TC Temperature TH (℃)
Figure 5. CH4/CO2 ratio-x or CH4/H2O ratio-y and the mole fractions of various gas species after methane reforming process (a)Isothermal CO2 splitting process integrated with CDR, TH=12001900°C; (b) Isothermal H2O splitting process integrated with SRM, TH=1200-1900°C.
For temperatures of isothermal CO2 splitting in the range of 1200-1900°C, the optimal CH4/CO2 ratio (denoted as x) for the mole fraction of CH4 to reach 10-3 is shown in Fig. 5(a). With the increase of TH, the CH4/CO2 ratio-x increases mainly because the sensible heat of the oxidation products of isothermal TC does so. The mole fraction of CO is roughly twice as much as that of H2 in a temperature range of 1200-1900°C, in which the main reactions are CDR (Eq. 7) and RWGS (Eq. 8). The reaction of CDR has a trend to occur easily because the partial pressure of CO2 downstream the oxidation reactor and the thermodynamic driving force are the greatest among CDR, SRM and RWGS above 817°C from the data of HSC once methane is added. The H2 produced in the CDR reacts with CO2 to obtain CO and H2O in the following step above the equilibrium 10
temperature of 817°C where the Gibbs free energy change of RWGS is about zero, and only a small amount of produced water reacts with methane because that the content of CO2 in the gas mixture is much larger than H2O even at equilibrium. With the increase in temperature, the thermodynamic equilibrium constants for both CDR and RWGS increase but the former grows faster, such that the ratio of CO/H2 decrease (the mole fractions of CO and H2 after a reforming process without recovery of CO from an isothermal TC are shown in dashed line in Fig. 5(a)). The gap between CO and H2 content enlarges with the increase of isothermal temperature due to higher CO2 to CO conversion by isothermal TC. More CO from the isothermal TC is added into the methane reforming process, which slightly influences the RWGS reaction.
As for water splitting with isothermal TC, the H2 content in the oxidation reaction varies with respect to TH at fixed rH2O. SRM and RWGS are the main endothermic reactions when methane is added to the gas mixture containing the oxidation products. Fig. 5(b) shows the optimal CH4/H2O ratio-y and the equilibrium mole fraction of each component after the reforming process, and hydrogen takes up a large portion in the gas mixture. Here, CH4/H2O ratio-y is the optimal molar ratio of CH4 to H2O input of the isothermal TC when the mole fraction of CH4 is 10-3 after the reforming process. The red solid and dashed lines represent the contents of H2 in the gas mixture of the proposed system and that of the methane reforming only system at the same temperature, respectively. More H2 will be produced in the isothermal TC with the increase of TH and it will influence the component ratio of the proposed system. The mole fraction of H2 of the proposed system has a larger increase trend than that of a methane reforming only system with the increase of T H.
11
35
0.18
30
0.15 T0
25
0.12
CO2 dash line H2O solid line
20 1200
1300
1400
1500
1600
1700
1800
0.09 1900
800
750
700
650
Mixing temperature T0 (℃)
0.21
CH4/CO2 ratio-x or CH4/H2O ratio-y
Carbon emissions per unit calorific value (g/MJ)
40
600
Isothermal TC Temperature TH (℃)
Figure 6. The comparation of isothermal CO2 and H2O splitting process integrated with downstream methane reforming at TH=1200-1900°C. Fig. 6 shows the comparison of H2 and CO for TH ranging from 1200 to 1900°C where the dashed and solid lines represent CO2 splitting and water splitting isothermal TC integrated with methane, respectively. We assume that the initial flow rate inputs of CO2 and H2O for isothermal TC are both 1000kmol/h. The sensible heat of the gas mixture after H2O splitting isothermal oxidation reaction is larger than CO2 and the enthalpy change of SRM is smaller than CDR for the same amount of methane. That is why the CO2 splitting process needs less CH4 input than water splitting process and the mixing temperature T0 is higher. It is also worth pointing out that due to the large excess of water or CO2 in the exhaust of an isothermal TC reactor, carbon deposition is disfavored thermodynamically, especially for exhaust of isothermal water splitting. Our thermodynamic calculation shows that the dashed red line in Fig. 6 approximately corresponds to the threshold temperature below which carbon deposition could occur, and methane-integrated heat recovery could use a slightly less amount of methane than previously calculated for each isothermal TC temperature shown in Fig. 6, thereby both avoiding carbon deposition and still reducing the temperature of the gas mixture down to a range (e.g., < 1100°C) where mature heat exchanger technology applies.
12
T0
H2
37 H2 O
30
36 35
20
CO2 34
10
CO CH4
0 0.0
0.5
1.0
33 TH=1600C
1.5 2.0 2.5 H2O/CO2 ratio
3.0
3.5
32
4.0
705
700
695
Mixing temperature T0 (°C)
Mole Fraction (%)
40
Carbon emissions per unit of calorific value (g/MJ)
38
690
Figure 7. Isothermal co-splitting CO2 and H2O splitting process integrated with methane varying with the H2O/CO2 molar ratio.
At a given isothermal splitting temperature of CO2 or H2O alone, the percentage of each gas species in the mixture after the refoming process is fixed, but co-splitting of CO2 and H2O can obtain varying molar ratios of CO/H2. Fig. 7 shows the mole fraction of the gas mixture after the reforming process. The CH4/CO2 ratio-x and CH4/H2O ratio-y are optimal values shown in Fig. 5 and the isothermal TC temperature is 1600°C. No H2O exists when H2O/CO2 ratio is 0, and the mole fraction of the gas mixture is the same as that shown in Fig. 5 (a) when the isothermal temperature is 1600°C. With the increase of H2O/CO2 ratio, the percentage of H2 increases rapidly, for SRM plays an increasingly important role, producing more H2. The mole fraction of H2 is lower than CO before the H2O/CO2 ratio reaches 0.56. The mixing temperature T 0 after reforming is in the range of 685°C to 705°C. When the H2O/CO2 ratio is about 1.39, the CO/H2 ratio is about 0.5, which is an important parameter for many chemical synthesis processes, such as methanol and dimethyl ether. Both the isothermal TC and methane reforming process store part of the concentrated solar energy. To reveal the proportion of storage capability by isothermal TC, we define Riso,st as the isothermal TC heat storage rate: R iso,st
n CO,iso HHVCO n H2 ,iso HHVH2 , n CO HHVCO n H2 HHVH2 n CH4 ,react HHVCH4
13
(13)
where nCO,iso and nH2,iso are the amounts of CO and H2 produced during isothermal TC, respectively, with subscript ‘iso’ standing for products of isothermal TC (rather than those of downstream methane reforming); n CH 4 ,react is the amount of CH4 consumed in the methane reforming process. The isothermal TC stores energy in the form of chemical energy of CO or H2, while the reforming process convert sensible heat of the gas mixture downstream the oxidation reactor into the promoted chemical energy by the enthalpy change of SRM or CDR and RWGS. Fig. 8(a) shows the isothermal heat storage rate of isothermal TC, where the capacity of CO2 splitting process is higher than that of water splitting process. The calorific value per mole of the gas mixture is an important parameter indicating the ability of gas mixture in converting chemical potential to internal energy when burning, which has been defined in Eq. 11 and showed in Fig. 8(a). According to the calorific value of the gas mixture, a proper combined-cycle power plant can be established to generate electricity. To get a higher calorific value per mole gas mixture, physical condensation can be used to remove H2O. The calorific value is also an important parameter for the modification of existing combined cycle power plants. Extra fossil fuels can be directly added into the combustion chamber of a gas turbine in order to improve the calorific value per mole if the power generated from the newly proposed system is not enough for methanol synthesis.
14
40
160
20
120
60
40
80
(b) CO2 dash line
60
H2O solid line
sol-to-syngas,CO2
40
sol-to-syngas,H2O
20
Riso,st,CO2
HHV,CO2
20
Riso,st,H2O HHV,H2O
0 1200
1300
1400
1500
1600
1700
1800
Isothermal TC Temperature TH (℃)
0 1900
0.95
Solar absorption efficiency abs
0 80 1200 1300 1400 1500 1600 1700 1800 1900 Isothermal TC Temperature TH (℃)
Calorific value per mole HHV mix(kJ/mol)
H2O solid line
Isothermal TC Heat Storage Rate R iso,st (%)
Isothermal TC Heat Storage Rate R iso,st (%)
200
CO2 dash line
60
80
Efficiency (%)
240
(a)
80
0.90
0.85
0.80
0.75
Figure 8. (a)Isothermal TC heat storage rate and the calorific value per mole of gas mixture for CO2 and H2O splitting integrated with methane reforming downstream. (b)The efficiency, isothermal TC heat storage rate and solar absorption efficiency varied with TH.
The solar-to-syngas efficiency of the solar-driven isothermal TC integrated with methane is shown in Fig. 9(a). It is worth mentioning that the product of Riso,st and ηsol-to-syngas is just ηHHV .With the increase of TH, the solar-to-syngas efficiencies of the newly proposed system for both CO 2 and H2O splitting are higher than the ηHHV of isothermal TC without heat recovery (Fig. 8(b)), because of the rational use of waste heat from T0 to TH. After 1700°C, the solar-to-syngas efficiency of CO2 splitting system integrated with methane begins to decrease. The maximum upgrade factors of Eq. 6 and Eq. 7 driven by solar energy are 1.281 and 1.277 (ratio of the heating value of products to reactants), respectively and the values are close to the ugrade factors of isothermal H 2O and CO2 splitting TC at 1200°C (Fig. 9(a)), for only a small amount H2 or CO is produced in the isothermal 15
TC. With the increase of the isothermal TC temperature, the upgrade factor of CO2 splitting process integrated with methane has a larger growth trend than H2O for the conversion of CO2 to CO is
50
3.0
(a) CO2 dash line
45
H2O solid line
2.5
40 2.0 35 1.5
30
Upgrade Factor
Solar to Syngas Efficiency sol-to-syngas (%)
larger than that of H2O to H2 in an isothermal TC at the same TH (Fig. 3).
25 1.0 1200 1300 1400 1500 1600 1700 1800 1900 Isothermal TC Temperature TH (℃) 80
HHV Efficiency (%)
(b) 60
Proposed system sol-to-syngas
(methane+(f25C-T0=80%))
40
Real isothermal TC HHV (f25C-1100C=80%) Potential isothermal TC HHV (f25C-TH=80%)
CO2 dash line
20
H2O solid line isothermal TC HHV (fequivalent)
0 1200
1300
1400
1500
1600
1700
1800
Isothermal TC Temperature TH (℃)
1900
Figure 9. (a) Solar-to-syngas efficiency soltosyngas and Upgrade Factor for isothermal TC integrated with methane as a function of TH (b) Efficiency of isothermal TC and proposed system with different heat recovery varied with isothermal TC temperature. After methane reforming process, the heat of the gas mixture ranging from 25°C to T0 is recovered with a 80% rate to reheat the input room-temperature H2O or CO2. The solar to syngas efficiency for the proposed system is shown in black lines with solid symbols in Fig. 9(b), where the dashed and solid lines represent CO2 and H2O splitting isothermal TC integrated with methane, respectively. Therefore, the proposed system recovers two parts of thermal energy: the total sensible heat of the 16
mixed gases after the oxidation reaction during the temperature swing of T H and T0 (by methane reforming) and 80% sensible heat during the temperature swing of T 0 to 25°C (by a heat exchanger), so that the total heat recovery rate is in a range of approximately 86-92%. For comparation, the green lines in the chart have a equivalent recovery rate as the proposed system but only for an isothermal TC without methane heat recovery. We also calculate the isothermal TC efficiencies without methane reforming process in blue and red lines (Fig. 9(b)), which represent a 80% heat recover rate of the mixed gases (CO/CO2 or H2/H2O) downstream the oxidation reactor from 25°C to TH and 25°C to 1100°C, respectively. The efficiencies of isothermal TC with a potential 80% heat recorvery of the mixed gases (i.e. potential isothemal TC efficiencies) are shown in blue lines with half open symbols, while in the mean time, the mixed gases are cooled from T H to room temperature. Because of the immaturity of heat exchanger for temperatures above 1100°C, only 80% heat recovery of the mixed gases from 1100°C to 25°C is calculated, and the efficiencies of which (i.e. real isothemal TC efficiencies) are shown in red lines with open symbols (Fig. 9 (b)). The efficiencies of the proposed system for both CO2 and H2O splitting isothermal TC integrated with CH4 are higher than the potential and ideal efficiencies of isothermal TC. This new approach has the potential to produce high-quality syngas and lower specific CO2 emissions per mole of feedstock at the same time. The carbon emission per unit calorific value of fuels obtained after the reforming process of this new approach is calculated according to Eq. 12, compared with direct combustion of methane or direct combustion of syngas after solar reforming (Fig. 10). The heat needed for the enthalpy change of the CDR and SRM reactions is from the waste heat downstream the oxidation reactor, and then the products are combusted to calculate the carbon emissions per unit calorific value. Both the isothermal splitting of CO2 and H2O integrated with methane lower the specific CO2 emissions as the isothermal TC is carbon-neutral and the calorific value of the methane is upgraded through the solar energy input by the enthalpy change of endothermic reactions (SRM, CDR and RWGS). The proportion of CO in the isothermal oxidation gas mixture and the isothermal TC heat storage rate improves with the increase of isothermal 17
temperature, explaining the decrease in specific CO2 emissions of this new system. The carbon emissions per calorific value is lower for CO2 splitting than that of H2O splitting isothermal TC integrated with methane although the value of CDR is a bit higher than SRM shown in Fig. 10,
Carbon emissions per unit of calorific value (g/MJ)
mainly because of a higer conversion of CO2 to CO than H2O to H2 at the same TH. (a) CH4(g) + 2O2(g) = CO2(g) + 2H2O(l)
50
49.4
40
1200℃ 1300℃
38.58
1400℃
30
CH4(g) + H2O(g) = CO(g) + 3H2(g)
1500℃ 1600℃ 1700℃ 1800℃ 1900℃
20 H2O:1000kmol/h oxidation products of TC+CH4 reforming
10 0 0
50
100
150
200
250
300
Carbon emissions per unit of calorific value (g/MJ)
CH4(kmol/h) (b) CH4(g) + 2O2(g) = CO2(g) + 2H2O(l)
50
49.4
40
CH4(g) + CO2(g) = 2CO(g) + 2H2(g)
1200℃
38.68
1300℃ 1400℃ 1500℃ 1600℃ 1700℃ 1800℃
30 20
1900℃
10
CO2:1000kmol/h Oxidation products of TC+CH4 reforming
0 0
50
100
150
200
250
CH4(kmol/h)
Figure 10. Carbon emissions per unit calorific value of fuel obtained of the polygeneration system with isothermal TC splitting water (a) or CO2 (b) TC integrated with CH4 heat and gas recovery downstream. The specific CO2 emissions of the CO2 and H2O co-splitting isothermal TC process integrated with methane are also calculated in Fig. 7 in the front part. The carbon emissions per unit calorific value of fuel obtained are in the range of 32.7 to 36.1 g/MJ for T H=1600°C, which is between those of the splitting of H2O and CO2 integrated with methane independently. With the increase of H2O/CO2 ratio, the specific CO2 emissions increase, for the H2O splitting isothermal TC integrated with 18
methane has a much larger specific CO2 emission (Fig. 10(a)) than CO2 (Fig. 10(b)) and more methane is needed to obtain nearly the same calorific value of syngas.
3.3. Solar thermochemical polygeneration system integrated with CH4 A polygeneration system is proposed based on analyses on recovery of waste heat and utilization of unreacted gas species (e.g. H2O or CO2) with methane downstream an isothermal solar TC reactor. It is capable of simultaneous power and methanol production with the advantage of eliminating selfgeneration power plant which is conventionally needed for methanol production. 90 °C Exhaust Gas CO2
①
Power
CH4 1600 °C,1bar
Oxidation Reaction of Isothermal TC
CO/CO2
Reforming
Power Gas Turbine Unit
HRSG
A
H I Recycled Gas
G Unreacted Gas Discharge
CeO2
CeO2-δ CH4 1600 °C,PO2=10-5 bar 600-800 °C Solar Energy Reduction Reaction O2 (1 bar) Partial Heat B of Isothermal TC Oxidation Recovery
25 °C D
②
CeO2-δ
③
H2O
H2/H2O
CO2 Seperation
E
Compression Unit
F
Methanol Synthesis Unit
Flashing
Distillation Unit
Methanol
H2O
CeO2 1600 °C,1bar
Oxidation Reaction of Isothermal TC
225 °C,80bar
Reforming C CH4
Figure 11. Simplified process of solar thermochemical polygeneration system integrated with methane
A scheme [17] is conceptually proposed based on the splitting reaction in isothermal TC (Fig. 11), of which solar energy and methane are the main input energy and the purpose of the system is to generate power and methanol. The polygeneration system mainly consists of four parts: isothermal splitting of CO2 and H2O, methane reforming process, methanol synthesis reaction and combined cycle for power. The isothermal oxidation reaction of CO 2 or H2O splitting TC produces CO or H2 respectively with a large amount of CO2 or H2O remaining, while the reduction reaction produces pure oxygen. SRM, partial oxidation of methane and CDR are the main reactions occurring when methane is added to the products of isothermal TC, shown as A, B and C in Fig. 11, respectively. 19
Before the mixing of these three streams (point D in Fig. 11), the latent heat is recovered and water is removed independently. Selexol method [18] is used to absorb CO2 and the syngas is then compressed to a pressure of 80 bar. Methanol is synthesized at 225°C and purified in the distillation unit [19]. The unreacted gas and exhaust gas are then combusted to produce power in a gas and steam combined cycle. The inputs of the polygeneration system are the concentrated solar energy and complementary CH 4 and the outputs of which are power and methanol. We define a series of formula to evaluate the performance and energy consumptions in the following part: The Solar Energy Consumption is the ratio of solar energy to the mass of synthesized methanol:
Esol Qsolar / mCH3OH ,
(14)
where mCH3OH is the mass flow rate of synthesized methanol, Qsolar (kW) is the solar energy input considering the solar absorption efficiency in Eq. 5. The Fossil Fuel Consumption is the ratio of lower heating value of methane to the mass of synthesized methanol without considering the solar energy input, which is an important index indicating the utilization of CH4 input: Efos (n CH4 ,init LHVCH4 ,init W / 0 ) / mCH3OH ,
(15)
where W is the net power output of the system, more methane will be directly added into the combined cycle to make sure that the value of W is positive; 0 is the heat to power efficiency of the gas and steam combined cycle. If W / 0 is positive, it represents the superfluous methane for the polygeneration system and if it is negative it represents the extra needed methane to produce enough power for methanol production except the initial input methane for reforming process. The power generated by the polygeneration system is much smaller than the heating value of synthesized methanol from the calculation of the system in Fig. 11. First, we assume that the power of the polygeneration system comes from the CH4 input (extra CH4 is needed to make sure that the 20
power output of the system is enough for the methanol production) instead of solar energy. Therefore, the system can be simplified to have one output (methanol) and two inputs (solar energy and CH4 deducted the portion to generate power). Then the system is compared with state-of-the-art industrial methanol energy consumption making a further simplification to obtain a one input (solar energy) and one output (methanol deducted the portion from CH 4) system. Then we define
Solar CH OH as the heating value of methanol to the solar energy input of this one input and one 3
output model:
Solar CH OH (R CH OH (QCH 3
3
4
W
0
)
3.6 ) LHVCH3OH / (Qsolar 3600) , E
(16)
where R CH3OH (kg/h) is methane productivity; Q CH (kW) is the total heating value of CH4 input; E 4
(GJ/ton), is the industrial energy consumption of CH4 to produce methanol. LHVCH3OH (kJ/kg) is the lower heating value of methane. The Net Solar Energy Consumption is defined as the ratio of the solar energy to the mass of methanol of the simplified one input and one output model: Esol,net Qsolar 3600 / [QCH3OH (QCH4
W
0
)
3.6 ] E
(17)
In the configuration of water spliting and CO2 splitting oxidation reaction, the ratio of CO2 to H2O is adjusted so that the optimal (nH2-nCO2)/(nCO+nCO2) molar ratio (usually 2.05~2.15 in actual methanol synthesis process) [20] of the fresh gas mixture at point E in Fig. 11 is produced to synthesize methanol. The (nH2-nCO2)/(nCO+nCO2) ratio in this paper is chosen to be 2.05 rather than 2.0 (the ideal stoichiometric number) [21,22] and a little higher amount of H2 has good benefit for the activity of catalyst and avoiding carbon deposition. Note that the CO/H2 ratio at equilibrium of partial oxidation of methane (CH4+0.5O2=CO+2H2) is exactly 1:2 at point B in Fig. 11. The temperature of isothermal oxidation products is 1600°C, which decreases to 600-850°C after the reforming process of methane. WGS reaction (CO+H2O=CO2+H2) occurs and more hydrogen will be generated at the expense of CO with the decrease of the temperature. The three pathways are not mixed before heat recovery process, for the reason that there is still a large amout of water after 21
SRM (pathway ③), which will react with the produced CO in pathway ①. The separation of water before mixing at point D in Fig. 11 is necessary to improve the yield of methanol. An advantage of the polygeneration system is that syngas is produced from the isothermal TC and methane
11 10 9
1.1 Solar Energy
Energy Consumpution for CH3OH Production(GJ/t)
0.150
0.175
50
0.200 0.225 CH4/H2O ratio - y
8 0.250
40 44.0
30
43.5
Solar Energy Consumption
43.0
20
10 0.150
42.5
0.175
0.200 0.225 CH4/H2O ratio - y
4
2.0 630
1.5 627 Work Output of Combined Cycle CO2 Seperation and Gas Compression Work
624
1.0
Net Work Output
0.5 0.150
44.5
Net Solar Energy Consumption CH4 Consumption
2.5 633
621
45.0
(c)
(b)
636
42.0 0.250
0.175
0.200 0.225 CH4/H2O ratio - y
1.45
0.250
(d)
1.40 H2O/CO2 ratio- w
1.2
CH3OH Production (kmol/h)
12 1.3
3
13
CH4 heat value
Energy Input (x10 kW)
1.4
3.0
14
(a)
Solar to CH3OH Efficiency Solar-CH3OH (%)
H2O/CO2 ratio- w
1.5
Work Input and Output (x10 kmol/h)
reforming process without either WGS or RWGS reactors.
1.35 1.30 CH4/CO2 ratio - x
1.25 1.20 1.15
0.25 0.17 0.16 0.15
1.10 1.05 0.150
0.175
0.200 0.225 CH4/H2O ratio - y
0.250
Figure 12. (a) The optimal H2O/CO2 ratio and energy input varied with CH4/H2O ratio-y (b) Methanol production and work output varied with CH4/H2O ratio-y (c) Energy consumption for CH3OH production varied with CH4/H2O ratio-y (d) The optimal H2O/CO2 ratio-w for various CH4/H2O ratio-y and CH4/CO2 ratio-x. The CH4/CO2 ratio-y of (a)(b)(c) is fixed to 0.21.
We assume that the CH4/CO2 ratio (pathway①) is fixed to 0.21 and the molar flow rate of CO2 is 1000 kmol/h, while the CH4/H2O ratio (pathway ③) changes from 0.15 to 0.25, therefore a series of H2O/CO2 ratio will be obtained to make sure that the (nH2-nCO2)/(nCO+nCO2) ratio before methanol synthesis at point E in Fig. 11 is 2.05, shown in Fig. 12(a). The heating value of CH4 and 22
concentrated solar energy needed by the polygeneration system are also shown. The methanol production and work output are shown in Fig. 12(b), where the net work output is the work output of combined cycle unit deducting the CO2 separation work and the compression work of the gas mixture before methanol synthesis. The net work output may be negative because the exhaust gas is not enough for the combined cycle to produce a large amount of power and it will be positive with the increase of CH4/CO2 ratio-x. The minimum net solar energy consumption of methanol is about 44.5 GJ/ton and maximum Solar CH3OH is about 44.8% (Fig. 12(c)). The net solar energy consumption is much higher than solar energy consumption because the conversion of fossil fuel to methanol synthesis is deducted. The Fossil Fuel Consumption is about 22 GJ/ton, lower than typical
45.0
45.5
(b)
44.5 45.0 CH4/H2O ratio - y= 0.22 44.0 44.5 43.5 0.14
0.16
0.18 0.20 0.22 CH4/CO2 ratio - x
0.24
Net Solar Energy for CH 3OH (GJ/t)
Solar to CH3OH efficiency Solar-CH3OH, max (%)
values (about 28-40 GJ/ton) [23] in current industrial processes.
0.26
Figure 13. (a)The Net Solar Energy Consumption for CH3OH with CH4/CO2 ratio-x and CH4/H2O ratio-y ranging from 0.15-0.25 (b) The maximum Solar CH3OH and minimum Net Solar Energy Consumption for CH3OH production CH4/CO2 ratio-x is changed from 0.15 to 0.25 with CH4/H2O ratio-y being changed simultaneously, and the H2O/CO2 ratio-w is optimized to make sure that the (nH2-nCO2)/(nCO+nCO2) ratio of the syngas before methanol synthesis is 2.05 (Fig. 12 (d)). The net solar energy consumption for CH3OH is shown in Fig. 13(a) with a variety of CH4/H2O ratio-y and CH4/CO2 ratio-x. For each point of CH4/CO2 ratio-x from 0.15 to 0.25, the solar to CH3OH efficiency reaches the maximum
23
value when CH4/H2O ratio-y is around 0.22. Note that the operational and optical losses are not taken into account when calculating the solar to CH3OH efficiency.
5. Conclusions In this paper, we proposed an approach of simultaneously recovering waste heat and unreacted gas (e.g. H2O or CO2) downstream an isothermal thermochemical cycling (TC) reactor by taking advantage of endothermic reactions of certain fossil fuels (e.g. CH 4). It has the potential to produce high-quality syngas and lower the temperature of isothermal oxidation products to the region of 600-850°C where heat recuperation technology is mature. The solar-to-syngas efficiencies of the proposed system for CO2 and H2O splitting at 1600°C are 45.7% and 38.1%, respectively, while the efficiencies of isothermal TC without CH4 integration are only 21.0% and 6.4% without heat recovery. The isothermal TC heat storage rates of the proposed system for CO 2 and H2O splitting processes are 46.1% and 15.7% at 1600°C, respectively. Compared with direct solar reforming or direct combustion of the same fossil fuels, this new approach features lower carbon emissions per unit calorific value of fuel obtained due to the incorporation of the isothermal TC upstream. Cosplitting of CO2 and H2O integrated with methane can obtain adjustable CO/H2 ratio for various chemical synthesis process. A scheme of polygeneration of methanol and power system is proposed as an example of this approach. It is possible to eliminate the self-generation power-plant which is conventionally needed for methanol production and no WGS or RWGS reactor is needed. Fossil fuel consumption for the production of a unit mass of methanol is about 22GJ/ton, lower than typical values in current industrial processes and the optimal solar to methanol efficiency can be more than 44%.
Acknowledgments This work was funded by the Recruitment Program of Global Experts of China, the Chinese Academy of Sciences Innovative and Interdisciplinary Team Award, and Natural Scientific Foundation of China under contract No. 51236008. 24
References [1] Steinfeld A., Solar thermochemical production of hydrogen-a review. Solar Energy 2005; 78(5): 603-615. [2] Smestad G.P., Steinfeld A., Review: Photochemical and Thermochemical Production of Solar Fuels from H2O and CO2 Using Metal Oxide Catalysts. Industrial & Engineering Chemistry Research 2012; 51(37): 11828-11840. [3] Jensen S.H., Larsen P.H., Mogensen M., Hydrogen and synthetic fuel production from renewable energy sources. International Journal of Hydrogen Energy 2007; 32(15): 3253-3257. [4] Chueh W.C., Falter C., Abbott M., Scipio D., Furler P., Haile S.M., Steinfeld A., High-flux solar-driven thermochemical dissociation of CO2 and H2O using nonstoichiometric ceria. Science 2010; 330(6012): 1797-1801. [5] Venstrom L.J., De Smith R.M., Hao Y., Haile S.M., Davidson J.H., Efficient Splitting of CO2 in an Isothermal Redox Cycle Based on Ceria. Energy & Fuels 2014; 28(4): 2732-2742. [6] Furler P., Scheffe J., Gorbar M., Moes L., Vogt U., Steinfeld A., Solar thermochemical CO2 splitting utilizing a reticulated porous ceria redox system. Energy & Fuels 2012; 26(11): 70517059. [7] Felinks J., Brendelberger S., Roeb M., Sattler C., Pitz-Paal R., Heat recovery concept for thermochemical processes using a solid heat transfer medium. Applied Thermal Engineering 2014; 73(1): 1006-1013. [8] Muhich C.L., Evanko B.W., Weston K.C., Lichty P., Liang X., Martinek J., Musgrave C.B., Weimer A.W., Efficient generation of H2 by splitting water with an isothermal redox cycle. Science 2013; 341(6145): 540-542. [9] Roeb M., Sattler C., Isothermal water splitting. Science 2013; 341(6145): 470-471. [10]
Hao Y., Yang C.K., Haile S.M., High-temperature isothermal chemical cycling for solar-
driven fuel production, Physical Chemistry and Chemical Physics 2013; 15(40): 17084-17092. 25
[11]
Hao Y. Solar thermal dissociation of CO2: towards higher efficiencies. Proceedings of the
9th International Green Energy Conference (IGEC-IX); 2014; Tianjin, China. [12]
De Mello P.E.B., Monteiro D.B., Thermodynamic study of an EFGT (externally fired gas
turbine) cycle with one detailed model for the ceramic heat exchanger. Energy 2012; 45(1): 497-502. [13]
Kim K.T., Dae H.L., Kwon S., Effects of thermal and chemical surface-flame interaction on
flame quenching. Combustion and Flame 2006; 146(1-2): 19-28. [14] Wang H.S., Hao Y., Kong H., Thermodynamic study on solar thermochemical fuel production with oxygen permeation membrane reactors. International Journal of Energy Research 2015; 39(13): 1790-1799. [15] Fletcher E.A., Moen R.L., Hydrogen and oxygen from water. Science 1977; 197(4308): 10501056. [16] Piatkowski N., Wieckert C., Weimer A.W., Steinfeld A., Solar-driven gasification of carbonaceous feedstock – a review. Energy & Environmental Science 2011; 4(1): 73-82. [17] Kong H., Hao Y., Wang H.S., Ji X.Z., Jin H.G., A Solar Thermochemical Polygeneration System Integrated With Methane. Journal of Engineering Thermophysics 2015; 36(4): 691697. (in Chinese) [18] Chiesa P., Consonni S., Kreutz T., Williams R., Co-production of hydrogen, electricity and CO2 from coal with commercially ready technology. Part A: Performance and emissions. International Journal of Hydrogen Energy 2005; 30(7): 747-767. [19] Greeff I.L., Visser J.A., Ptasinski K.J., Janssen F.J.J.G., Utilisation of reactor heat in methanol synthesis to reduce compressor duty-application of power cycle principles and simulation tools. Applied Thermal Engineering 2002; 22(14): 1549-1558.
26
[20] Liu L.N., Li H., Wang D., Coal, Oil Field Gas and Residual Oil Joint Production of Methanol to Realize Resources Complementary, Energy Conservation and Emissions Reduction. Chemical Production and Technology 2012; 19(4): 45-50. [21] Lange J.P., Methanol synthesis: a short review of technology improvements. Catalysis Today 2001; 64(1-2): 3-8. [22] Luu M.T., Milani D., Abbas A., Analysis of CO2 utilization for methanol synthesis integrated with enhanced gas recovery. Journal of Cleaner Production 2016; 112(4): 3540-3554. [23] Dave N., Foulds G.A., Comparative-assessment of catalytic partial oxidation and steam reforming for the production of methanol from natural-gas. Industrial & Engineering Chemistry Research 1995; 34(4): 1037-1043.
27
Highlights
Methane reforming proves effective for heat recovery of isothermal solar reactors; Solar-to-syngas efficiencies for CO2 and H2O splitting at 1600°C are 45.7% and 38.1%, respectively; Carbon emission per calorific value of solar syngas can be significantly lowered.
28