Power-to-Gas through High Temperature ... - ACS Publications

Subscriber access provided by Kaohsiung Medical University

Process Systems Engineering

Power-to-gas through high temperature electrolysis and carbon dioxide methanation: reactor design and process modeling Emanuele Giglio, Fabio Alessandro Deorsola, Manuel Gruber, Stefan Raphael Harth, Eduard Alexandru Morosanu, Dimosthenis Trimis, Samir BENSAID, and Raffaele Pirone Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.8b00477 • Publication Date (Web): 07 Mar 2018 Downloaded from http://pubs.acs.org on March 9, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Industrial & Engineering Chemistry Research

Power-to-gas through high temperature electrolysis and carbon dioxide methanation: reactor design and process modeling. Emanuele Giglioa, Fabio Alessandro Deorsolaa, Manuel Gruberb, Stefan Raphael Harthb, Eduard Alexandru Morosanua, Dimosthenis Trimisb, Samir Bensaida,* and Raffaele Pironea. a

Department of Applied Science and Technology (DISAT), Politecnico di Torino, Corso Duca degli

Abruzzi 24, 10129 Torino, Italy. b

Engler-Bunte-Institute, Division of Combustion Technology, Karlsruhe Institute of Technology,

Engler-Bunte-Ring 7, 76131 Karlsruhe, Germany. *Corresponding author. E-mail address: [email protected]

Abstract This work deals with the coupling between high temperature steam electrolysis using solid oxide cells (SOEC) and carbon dioxide methanation to produce a synthetic natural gas (SNG) directly injectable in the natural gas distribution grid via a Power-to-gas (P2G) pathway. An intrinsic kinetics obtained from the open literature has been used as the basis for a plug flow reactor model applied to a series of cooled multi-tube fixed bed reactors for methane synthesis. Evaporating water has been considered as coolant, ensuring a high heat transfer coefficient within the shell side of the reactor. Methanation section has been designed and optimized in order to moderate the maximum temperature within the catalytic bed and to minimize the catalyst load. Then, process modeling of a plant coupling high temperature electrolysis and methanation is presented: the main goal of this analysis is the calculation of overall plant efficiency (in terms of electricity-to-SNG chemical energy). Plant size has been set considering a 10 MWel SOEC-based electrolysis unit; heat produced from the exothermal methanation is entirely used for water evaporation before the steam electrolysis. Heat exchangers network (HEN) has been designed in order to reduce the number of components, resulting in an external heat requirement equal to 185 kW (≈ 1.9% of the electrolysis power). The SOEC-based power-to-gas system presented a HHV-based efficiency equal to ≈ 86 % (≈ 77 % if evaluated on LHV basis).

Keywords: power-to-gas; carbon dioxide methanation; solid oxide cells; process modeling; system integration.

1 ACS Paragon Plus Environment

Industrial & Engineering Chemistry Research 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

1. Introduction World electricity production is still strongly dependent on fossil fuels: in 2014, roughly 66% of generated electricity came from coal (40.7%), natural gas (21.6%) and oil (4.3%). On the other hand, renewable energy sources (RES) were the second largest contributors to global electricity production, accounting for 22.3% of world generation in 2014. Since 1990, RES have grown at an average annual rate of 2.2%: solar and wind power penetration increased with average annual rates of 46.2% and 24.3%, respectively1. The International Energy Association (IEA) estimates that the increasing penetration of RES to produce power and heat will drive up their share of total primary energy supply (TPES) from 13% to 19% over the period 2012-2040; renewable installed capacity for electric production will represent 42% of the overall capacity and will generate 33% of the total electricity2. In 2015 electricity production from RES reached about 23% (≈ 2500 TWh) of the overall production in OECD countries (the highest level ever reached). Between 2014 and 2015 there was a decrease in electricity production from coal (-7.6%) and oil (-10.6%). Meanwhile, electricity production from natural gas (+7.2%) and RES as wind (+16.0%) and solar (+17.8%) increased3. In 2015, much of the new installed electric capacity in Europe came from RES: wind power was the technology with the highest installation rate (12.8 GW, accounting for 44% of all new power), followed by solar PV with 8.5 GW (29% of 2015 installations)4. In EU, wind share of total capacity has increased six-fold in the period 2000-2015 (from 2.4% to 15.6%) overtaking hydroelectricity as the third largest power generation capacity. Over the same period, RES increased their share from 24% to 44% of total installed capacity. The management of intermittent RES and their integration with the electric grid is not straightforward: higher integration costs may occur as their penetration level increases5. The difficulty in grid management because of sudden and fluctuating RES availability should be solved for widespread diffusion of wind and solar energy. The need for a high storage capacity represents a key challenge to stimulate a larger use of RES within the energy mix. Pumped hydroelectricity is the dominant way for the electricity storage, but the potential of this technology is limited by the topography of the region where it is installed. Compressed-air storage seems to be an alternative, but it requires large volumes for the energy storage6. Batteries represent a future perspective for the electrochemical energy storage, even though this technology is still characterized by low capacity and degradation challenges. In order to store large amounts of surplus electricity for long periods and to balance seasonally RES electric production, chemical storage/conversion in synthetic fuels might represent one of the most convenient ways7–11. According to the Power-to-Gas (P2G) pathway, low-priced surplus electricity is used to produce H2 via water/steam electrolysis. Hydrogen is then mixed with carbon dioxide: a catalytic conversion of 2 ACS Paragon Plus Environment

Page 2 of 37

Page 3 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the syngas, leading to the production of a synthetic natural gas (SNG) is pursued afterward12. Carbon dioxide methanation (Sabatier reaction) is an exothermic reaction where H2 and CO2 react to form CH4 and H2O: 4 + ⇄ + 2 ℎ . = −165 /

(1)

Carbon dioxide can be collected from exhaust gas in industrial plants13. CO2 capture from air can be considered as a future perspective10, even though at the present state it seems to be an expensive process14. Biogas-to-biomethane upgrading15,16 can be also considered as carbon source: if electricity for electrolysis comes from RES, the integration with CO2 collection from biogas plants would lead to a fully renewable power-to-SNG pathway. In Europe, there is a significant potential for CO2 coming from biogas: in 2013 the overall European production was 13379 ktoe17. Reaction (1) is thermodynamically favored at low temperature and high pressure. Due to kinetic limitations, a catalyst is required18. Reviews of several catalysts used for methanation are available in the literature19–21. Nickel based catalysts are the most common studied for CO2 methanation because of their high activity and low price19. The hydrogenation of carbon dioxide (and carbon monoxide) producing methane has been widely investigated over Ni/Al2O3, which represents a sort of baseline catalyst family for methanation reactions22. In order to improve the catalyst performance, addition of oxides as CeO223,24, ZrO225 and TiO226 to γ-Al2O3 is frequently reported27. Nickel-based hydrotalcites (HTLCs) have recently gained interest as alternative catalysts for their activity in carbon dioxide methanation28–32. In Europe, there is a great interest in power-to-gas to couple electricity from wind with electrolysis and synthetic fuels production33: Audi established in Werlte (Germany) a 6 MWel plant for SNG production based on electrolysis34. The advantage of SNG production through P2G instead of hydrogen production consists in the presence of an already existing storage and transport infrastructure: the natural gas distribution grid35. Natural gas (NG) plays a crucial role in global economy: NG share in TPES increased continuously in the last four decades passing from 16% in 1973 to 21.2% in 201436. IEA estimated that within 2040 NG will overtake coal becoming the second-largest fuel in global energy mix, after oil2. NG is mainly consumed in the industrial and residential/commercial sector. Thus, the simultaneous increasing share of natural gas and renewables in the energy mix represent a synergy that could drive a future development of Power-to-gas pathway. Electrolysis technologies can be grouped into two main categories: low temperature and high temperature. Alkaline and PEM (Proton Exchange Membrane) electrolyzers belong to the first type: their operating temperature usually stays in the range 50-80 °C37. High temperature electrolysis through solid oxide electrolysis cells (SOEC) represents an alternative technology: in this case 3 ACS Paragon Plus Environment


steam (instead of liquid water) electrolysis is carried out. By using SOEC, the reacting water should be vaporized in a separated device before electrolysis section. Potential benefits of high temperature electrolysis (HTE) are summarized in the literature38. SOEC operating temperature stays in the range between 700 and 900 °C. Reviews of materials and cell configuration are available in the open literature39,40. Concerning the thermal balance of a SOEC stack, an interesting operating point is thermoneutral voltage: in this condition heat generated via irreversible phenomena equals the thermal energy required for the endothermal steam reduction and the SOEC device can operate isothermally without energy supply/rejection. A deeper investigation on thermodynamics of high temperature electrolysis can be found in the literature37,41. The coupling between SOEC and methanation seems to be promising due to the use of reaction heat (coming from the methanation reaction) for steam production41,42 as outlined in Figure 1. This work is focused on the integration between SOEC and methanation. Intrinsic kinetics obtained from literature data has been used in a 1D model evaluating the profile of several parameters (e.g., temperature and conversion) along the axial coordinate of a packed bed reactor filled with catalyst particles at pellet size and cooled with evaporating water. The model provided a guideline for the design of the methanation unit consisting in a series of cooled reactors. Then, process modeling of a plant coupling high temperature electrolysis and methanation is presented, including the calculation of the overall plant efficiency (in terms of electricity-to-SNG heating value) and the evaluation of the produced SNG quality (i.e., composition and heating value).

2. Methodology In this section, the model of a multi-tube fixed bed reactor filled with catalyst particles at pellet size and cooled with evaporating water is described. Intrinsic kinetics of the catalysts has been taken from a recent kinetic study (Koschany et al,43) on a nickel-based catalyst (Ni-Al hydrotalcite) with experimental investigation of CO2 methanation at high pressure. The reaction mechanism that provided the best fit of experimental data is based on dissociative chemisorption of both hydrogen and carbon dioxide; activation energy resulting from the aforementioned study is equal to 77.5 kJ/mol. The integration between SOEC-based steam electrolysis and the methanation unit is then presented.

2.1. Reactor design The chosen methanation concept consists in a series of cooled reactors with an eventual intermediate condensation stage in order to remove the produced water. This configuration has been preferred to a more conventional one with adiabatic reactors because the outlet adiabatic temperature within the first reactor could reach too high values. Such a high temperature can lead to 4 ACS Paragon Plus Environment

Page 4 of 37

Page 5 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


nickel sintering or, more in general, to catalyst structural damages. In order to ensure the proper heat transfer between reacting gas and coolant, ‘shell and tube’ reactor has been considered for methanation: tubes are filled with catalyst packed pellets, while in the shell the coolant circulation takes place. Considering the thermal coupling between exothermal methanation and steam production (required if the synthetic gas production system is integrated with high temperature steam electrolysis), evaporating (saturated) water has been chosen as cooling fluid. A onedimensional plug flow reactor (PFR) model has been built in MATLAB™ environment. Reactor axial coordinate has been discretized (using a discretization step ∆z equal to 1 mm): the overall control volume is thus divided into finite cylindrical volumes. For each volume, the bulk gas temperature, pressure and composition are considered constant, while the transport phenomena between catalyst pellet and gas have been taken into account. For each elementary cell, reaction is locally treated as first-order type to simplify the model mathematics.

≅ ′!

(2)

CA is the concentration of the key reactant (CO2) Thus, the distance from chemical equilibrium and the effect of hydrogen partial pressure are englobed inside the pre-exponential factor of the new kinetic constant k’. For this reason, the pre-exponential factor will slightly change for each elementary cell, even though it is, from a theoretical standpoint, a characteristic constant of the catalyst not affected by the gas composition, pressure and temperature. The error on rate estimation decreases if the spatial discretization is denser. Such simplifying assumption enables to consider a pseudo-first order kinetics: an analytical expression for the concentration along the pellet radial coordinate can be derived and used for the estimation of transport phenomena limiting the reaction rate. Solid catalyst will be considered as spherical pellet with a diameter of 5 mm. The single catalyst particle is considered as a porous sphere (characterized by a porosity εi) and will be treated as pseudo-homogeneous, i.e. as consisting of a single gaseous phase with an effective diffusivity (De) defined as follows:

"# =

"!$ %& '&

(3)

Where τi is the pellet tortuosity. Catalyst porosity and tortuosity have been set equal to 0.3 and 2.5 according to literature data44. DAm is the mean binary diffusivity of the gas mixture: it has been calculated according to the Wilke equation, considering the binary diffusivity of A (carbon dioxide) through the other involved gases (hydrogen, steam and methane). Both porosity and tortuosity contribute to the diffusivity drop: De will be lower than DAm. In this work, each binary diffusion coefficient has been calculated according to the empirical method proposed by Fuller et al.45,46. 5 ACS Paragon Plus Environment


Page 6 of 37

Viscosity (µ) and thermal conductivity (λ) of the gas mixture should also be calculated. Viscosity of pure gases was calculated through the first order Chapman-Enskog equation; Reichenberg method47 was used to determine the gaseous mixture viscosity. Thermal conductivity of pure gases was calculated by using the method suggested by Chung et al.48,49. Wassiljewa equation47 was used for the calculation of the gas mixture value. For the design of the fixed bed reactor mass and energy balances within the gas phase have been firstly considered:

()! * + = ,! - (.

(4)

/01 (2 + ,! )−Δℎ4 + - (. = 567 )28 − 2+

(5)

TC is the temperature of coolant fluid, s is the cross section of the tube, AL is the lateral surface of the cylindrical tube and U is the overall heat transfer coefficient. RA considers heat and mass transport phenomena between catalysts and gas. Kinetic equation is thus expressed as:

,! = 9 : !& )1 − % +

(6)

ε0 is the void fraction of the solid bed, i.e. the volume fraction that is not occupied by the catalyst. η is the effectiveness factor, defined as the ratio between the amount of element A effectively reacting within the particle and the amount of A that would react if the concentration profile through the particle would be constant. If the solid particle is considered isothermal, effectiveness for a first order reaction can be expressed as:

9=

3 < = − 1A < >?@ℎ)O > >ℎO O?0> 2>? a@O> _` ?>O

(14)

In this case, second reactor is strictly required to consume the remaining part of carbon dioxide. Figure 3 presents the effect of χ on temperature profile for a methanation section with two reactor 9 ACS Paragon Plus Environment


and intermediate condensation. When χ increases, peak temperature within the first reactor drops (as expected), while the maximum T in the second reactor grows. No acceptable configurations were found by varying χ, TC and NT: variation of these parameters in reasonable intervals did not lead to a SNG production respecting the constraints in terms of maximum temperature, pressure drop or methane content. A methanation configuration without condensation between two reactors has been then considered, but in this case the second reactor is too long (implying a huge amount of catalyst and a high pressure drop) and a SNG with 95 mol.-% of CH4 is not easily reached due to same thermodynamic aspects already mentioned for the reactor fed with stoichiometric mixture. Thus, a methanation section with three reactors and multiple carbon dioxide split has been considered. Water condensation does not occur after the first reactor in order to moderate the temperature rise within the second reactor: this choice slightly limits the thermodynamically achievable conversion, but steam acts as thermal sink enhancing temperature control. Figure 4 presents a scheme of the final methanation configuration, enabling the respect of all constraints (maximum temperature below 550 °C and SNG with 95 mol.-% of methane). χ, TC and NT have been then adjusted in order to minimize the overall catalyst amount. Table 2 summarizes the methanation section main results. Gas hourly space velocity (GHSV) is calculated using inlet volume flow in normal condition (0 °C, 1 atm). GHSV is higher (i.e., residence time is lower) for the first reactor because average reaction rate is greater (due to higher reactants partial pressure). Coolant temperature (resulting from the optimization) is equal to 207 °C: corresponding pressure for saturated water is 17.9 bar. Figure 5 show gas and solid temperature profile for the three reactors. Bulk gas temperature reaches its maximum when heat rejected by the cooling fluid is equal to heat generated through the exothermal reaction, according to the energy balance. As expected, pellet temperature is greater than bulk gas value, due to exothermal reaction and to gas-solid energy balance. Figure 6 presents CH4 mole fraction (on dry basis) profile over the entire methanation unit. Slight decrease between two consecutive reactors is due to the CO2 addition/injection. The final value is, as expected, 95 vol.-%.

3.2. System integration In order to perform pinch analysis, thermal data must be extracted from the process. Table 3 summarizes the streams involved in the analysis for the considered SOEC-based power-to-gas system (R1, R2 and R3 stay for first, second and third reactor, respectively). R2 inlet (i.e., R1 outlet mixed with CO2) and R2 outlet temperature are equal to 289 and 290 °C, respectively. R1 and R2 outlet temperatures resulting from reactor design section are higher, but these values represent the temperature at the end of the catalytic bed. To ensure sufficient heat flow for water evaporation, gas is further cooled down to 290 °C into the empty tubes (i.e., not filled with catalyst pellets). 10 ACS Paragon Plus Environment

Page 10 of 37

Page 11 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Water evaporation and reactor cooling are excluded from the analysis because they are perfectly matched. Heat capacity flow rate (C, measured in kW/K) is the product between mass flow rate (in kg/s) and specific heat capacity (in kJ/kg/K). Processes involving water condensation (i.e., cathode and reactors outlet) have been analyzed dividing them into two streams, because specific heat capacity is approximated as constant during the thermal integration procedure. When gas mixture reaches the dew point, specific heat significantly changes due condensation latent heat. A glide occurs because only one component (water) of the gas mixture condensates: process is not isothermal, but the strong increase of heat capacity flow reflects condensation. C of these streams is calculated considering enthalpy flow balance and inlet/outlet temperatures. Figure 7 shows composite curves for the analyzed power-to-gas system. External heat requirement has been minimized, resulting equal to 37 kW (i.e., about 0.4% of the total electrolysis power). The external heat minimization leads to a complicated heat exchanger network (HEN): several devices and stream splits are required to fulfill the target. For this reason, the number of heat exchangers has been reduced. Hot streams of SOEC section have been used only for the heating of SOEC section cold streams; similarly, in the methanation unit hot streams heat up R1 and R3 inlet mixtures (R2 inlet is a hot stream because its temperature is higher than 250 °C and it must be chilled). Thus, the two sections are thermally independent, except for the coupling between steam generation (SOEC) and reactor cooling (methanation)57. This aspect can become useful if the power-to-gas system works in an intermittent way: the two sections may operate at different partial loads and stronger thermal integration between SOEC and methanation could lead to regulation and/or operation issues. Such modification of the HEN leads to an increase of external heat requirement from 37 kW (energy optimization scenario) to 185 kW (system modification with reduced number of devices). The external energy input increase is considered as acceptable, passing from ≈ 0.4% to ≈ 1.9% of electric input (10 MW). The new configuration will lead to slight decrease of the achievable efficiency, but the system would be less complex and more flexible: the number of heat exchangers is thus reduced with a minimal increase of external heat. In Table 4 the main parameters of heat exchanger network are summarized, while in Figure 8 an overview of the system putting in evidence the thermal integration is presented. In HX1 and HX6 the steam contained into hot stream starts to condensate during the heat exchange. When pinch analysis has been performed each one of these hot streams was split, but in real HEN a single heat exchanger can be used. HX4 and HX5 (steam super-heaters) heat up as much as possible the steam before entering the SOEC. As discussed, a final heating step by using external source (electricity) is required to ensure isothermal operation of the electrolysis unit. Instead of electric heater, a possibility consists in operating above the thermoneutral point. Heat produced by irreversible 11 ACS Paragon Plus Environment


phenomena is greater than energy required by endothermal electrochemical reactions: heat surplus can be used for the final steam heating. This possibility requires a careful stack design, because steam enters the electrolyser at 701 °C, receives the surplus heat and then reacts. From a thermodynamic standpoint, there is no difference with the thermoneutral operation: external heat (185 kW) is electrically provided in both cases. The different SOEC operating point affects only the current density, i.e. the active area. In solid oxide cells a too high current density may lead to unexpected degradation challenges and thus it should not exceed a limit value (usually specified by the manufacturer). Focusing on steam production, it should be noticed that saturation temperature within the reactor cooling system (207 °C) is slightly different from the inlet steam temperature in HX3 (204 °C): the steam produced within the reactors is pressurized (≈ 18 bar) whereas the SOEC operating pressure is 15 bar. Saturated steam exiting the reactors is expanded through a valve in adiabatic throttling process: Joule-Thomson coefficient is positive in that point, leading to a temperature decrease because of lamination. Such pressure difference between produced and reacting steam is an opportunity for future development of power-to-gas systems. In case of intermittent operation of electrolysis and methanation sections, a steam buffer could be required to manage transients and fluctuations. All the hot streams must be further chilled after integration with cold streams: a corresponding number of coolers involving external cold source (e.g., water) is required. For the anode outlet cooling, a final temperature of 30 °C has been set. Concerning the condensation of reactors and cathode outlets, a different temperature would mean a different H2O content in the vapor phase: it is thus preferable to avoid any adjustment of condensation temperature. A possible alternative to conventional condensation with a shell-and-tube heat exchanger could be a mixing/spray condenser, in which cooling water is injected within the condensing mixture. This solution ensures a huge equivalent heat exchange area: lower ∆Tmin would not represent a challenge in terms of heat exchanger cost (the choice of the minimum temperature difference is usually related to an economic evaluation). With reference to Table 3 and Table 4, R1 and R3 inlet temperature is higher than 30 °C because syngas from condensation is mixed with CO2 coming from the compressor (i.e., warmer). If carbon dioxide is already available at high pressure compression could be avoided, but additional heating duty would be required. Evaporation duty accounts for ≈ 1645 kW resulting from the sum of rejected heat flows within the three reactors. Internally exchanged heat flow (obtained by summing the heat exchangers duties) is ≈ 2060 kW. This value represents ≈ 20.6 % of electrolysis power (i.e., the plant size), putting in evidence the importance of thermal integration. 12 ACS Paragon Plus Environment

Page 12 of 37

Page 13 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


3.3. System energy performance Overall plant efficiency is calculated as the ratio between the SNG chemical power and total electric power input, including electricity fed to the SOEC for electrochemical water/steam splitting, electric power for reactant pre-heating (resulting from thermal integration with the reduced number of heat exchangers) and electricity to drive pumps and compressors. Efficiency can be thus expressed as follows (power is obtained by multiplying the heating value and the SNG flow):

9I =

b`OcdB TO eZe

(15)

Table 5 outlines the results concerning energy performance evaluation: a LHV-based electricity-toSNG efficiency of 77.4% has been calculated (86.1% on HHV basis). Compression power takes into account the electricity needed for both water pump and CO2 compressor. Due to the strong thermal integration, external heat requirement is very low (≈ 2% of the electrolysis power), leading to high electricity-to-SNG conversion efficiency. Final SNG dry composition is: CH4=95 vol.-%, H2=4 vol.-% and CO2=1 vol.-%; according to the assumptions made in reactor design section. Wobbe Index (WI) or Wobbe number is an indicator of the interchangeability of fuel gases such as natural gas and is frequently defined in the specifications of gas supply and transport utilities:

Tf =

g

(16)

hLK

HHV is the SNG higher heating value (usually in MJ/Sm3, i.e. evaluated at 1 atm and 15 °C). Gs is the gas gravity, calculated as the ratio between SNG and air standard densities. Wobbe Index is used to compare the combustion energy output of fuel gases with different composition. If two fuels have identical Wobbe Indices, then for given pressure and valve settings the energy output will also be identical. Variations of up to 5% are typically allowed. Wobbe Index is a critical factor to minimize the impact of the changeover, e.g., when analyzing the use of substitute natural gas. Each country has its own prescriptions for gas injection into the pipeline: according to Italian normative a gas with a Wobbe Index of 49.2 MJ/Sm3 can be fed to the natural gas grid. Energy requirement (in terms of kWhel/kgSNG) for the analyzed system can be compared with the performance of the above-mentioned 6 MW Audi e-gas plant, based on alkaline electrolyser technology. For this system an efficiency of 70% (on HHV basis) is reported35. Assuming an SNG composition equal to that obtained in this work, an energy requirement of 21.0 kWhel/kgSNG has been calculated. This value is higher than 17.5 kWhel/kgSNG obtained for the SOEC-based system, meaning the high-T electrolysis system potentially yields a lower energy requirement than integrated low-T electrolysis methanation processes producing the same amount of SNG.



Focusing on SOEC, at the anode side pure oxygen production was assumed. Challenges related to oxygen hazard at high temperature are sometimes highlighted in the literature37. Also, in some studies the use of a sweep gas (usually air, steam or nitrogen) at anode side has been modeled 58. This would mean to re-consider evaluation of plant efficiency, because the sweep agent must be heated up to cell temperature and compressed to the same operating pressure. It is also worth to notice that electrolyzers operate in DC: if electricity is coming from the external grid, the AC/DC conversion system efficiency should be considered. Rectifier devices can reach high efficiency values (up to 98%), but almost all the electric input (except for compression) is required in DC. Thus, also high AC/DC conversion efficiency means an efficiency decrease of ≈ 2-3 %. Coelectrolysis of steam and carbon dioxide can represent a possible alternative to steam electrolysis: in this case CO2 is directly fed to SOEC and split into CO and oxygen. The simultaneous presence of H2, CO and CO2 within the nickel-based cathode may enable methane production already during electrolysis (especially at high pressure), reducing heat production in the subsequent methanation process. On the other hand, strongly exothermal methanation can lead to hot spots within the cathode and poses further challenges related to the temperature profile over the electrode channel. Furthermore, also carbon monoxide hydrogenation should be considered within the methanation reactor(s)42. SNG may be further compressed before the injection into the natural gas distribution grid. The size of this compression section depends on the final pressure, but it may imply a slight efficiency drop, because the manometric ratio should be very low even though the discharge pressure would be equal to 60 bar, i.e., the usual pressure of a main pipeline. Liquefied natural gas (LNG) production can represent an alternative to the pipeline injection, but it would require an energy-intensive cryogenic system to chill synthetic gas down to ≈ -165 °C. Power-to-gas (P2G) pathway has the purpose to store surplus coming from renewable sources (RES), but also to balance the fluctuating electricity production from wind and solar. For this reason, the functioning of a P2G SOEC-based plant under intermittent operation should be investigated in the near future. Systems based on high-T electrolysis ensure, as seen, higher conversion efficiency than low-T plants. On the other hand, thermal transient management of a stack operating at 800 °C is not straightforward; in addition, variable-load operation effects on SOEC stacks durability must be further investigated. Intermittent functioning of the system can be managed by operating SOEC and methanation section at different partial loads. SOEC is based on modular technology: an electrolysis section made by several stacks linked in parallel can allow the partial-load. Multi-tubular methanation reactor is a solution that could probably ensure higher rangeability than a conventional industrial reactor (usually in the interval 60-110 % of the nominal size). Different partial-load for the two section implies the presence of a hydrogen storage, 14 ACS Paragon Plus Environment

Page 14 of 37

Page 15 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


buffering the difference between H2 production (electrolysis) and demand (methanation). In addition, a steam storage system can be involved when steam production (methanation) is higher than demand (SOEC). Management of this second buffer is more complicated due to the phase change (condensation) but can avoid alternative ways to produce steam (e.g., combustion of a little SNG fraction) lowering the efficiency and the economic profitability.

4. Conclusion The production of synthetic natural gas (SNG) via high temperature electrolysis and catalytic methanation has been analyzed. In electrolysis unit hydrogen production takes place, with subsequent CO2 injection before methanation. Kinetic parameters obtained from the open literature have been used for the methanation unit modeling. A model for the design of a multi-tubular cooled reactor has been developed. Results showed that the main concern is related to the reactor thermal management: with a strongly active catalyst the temperature increase due to exothermal reaction is not easily controllable, even with a cooling system characterized by high heat transfer coefficient. Among the strategies for controlling the temperature increase along the reactor, split of CO2 overall inlet flow (stoichiometric with hydrogen) has been adopted in this work. Furthermore, condensation between first and second reactor has been avoided in order to better control the temperature increase. Optimization of the methanation section has been carried out to minimize the catalyst load. Maximum reaction temperature < 550 °C and outlet CH4 molar fraction equal to 95 mol.-% were the minimization constraints. 35 %, 46 % and 19% of the total CO2 inlet flow resulted as feeding for first, second and third reactor, respectively. The optimal coolant temperature was 207 °C (evaporating water at ≈ 18 bar). Number of tubes for each reactor has also been adjusted during the optimization procedure. Results from the reactor modeling have been used for the subsequent analysis of a synthetic methane production plant via high temperature steam electrolysis and catalytic methanation. The heat exchange network has been designed in order to decouple the SOEC-based electrolysis unit and the methanation section, except for the water vaporization heat (required from the SOEC) entirely provided through the methanation reactor cooling. Thermal integration led to promising overall system HHV-based efficiency equal to ≈ 86 % (≈ 77 % if evaluated on LHV basis). The integrated process is characterized by external energy input of about 185 kW against an overall required thermal input equal to ≈ 4 MWth, confirming the importance of strong thermal match. Without any integration (i.e., if process cooling and heating duties are provided via external sources) the system HHV-based efficiency would drop down to ≈ 72 % (63 % avoiding also steam production through the reactors chilling). Future work can be focused on improvement of the 15 ACS Paragon Plus Environment


process at several scales. Innovative reactor concepts could be investigated in order to improve the heat exchange between the gas and the coolant or, more in general, to mitigate the maximum achieved temperature. In addition, transient behavior and intermittent functioning of both electrolysis and methanation must be analyzed, in order to fully integrate a power-to-gas system with electric grids characterized by higher fluctuations due to an increasing penetration of renewables.

Supporting Information The Supporting Information is available free of charge on the ACS Publications website. Table S1 and equation S1-S3 describe the kinetic model used in this work.

Acknowledgment The research leading to these results has received funding from the European Union's Seventh Framework Programme for the Fuel Cells and Hydrogen Joint Technology Initiative under grant agreement n° 621210 for the HELMETH project (Integrated High-Temperature Electrolysis and Methanation for Effective Power to Gas Conversion).

Nomenclature Acronyms and abbreviations AC = alternating current DC = direct current HEN = heat exchange network HTLC = hydrotalcite HX = heat exchanger IEA = International energy agency LHHW = Langmuir-Hinshelwood-Hougen-Watson NDIR = non-dispersive infrared sensor NG = natural gas P2G = power-to-gas PEM = proton exchange membrane PFR = plug flow reactor PV = photovoltaic RES = renewable energy sources SNG = synthetic (or substitute) natural gas SOEC = solid oxide electrolysis cell 16 ACS Paragon Plus Environment

Page 16 of 37

Page 17 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TPES = total primary energy supply Parameters a = pellet surface-to-volume ratio (m2 m-3) Ain = internal heat exchange surface (m2) Aext = external heat exchange surface (m2) AL = lateral surface of the cylindrical tube (m2) C = heat capacity flow rate (W K-1) CA = reactant mole concentration (mol m-3) cp = specific heat capacity (J mol-1 K-1 or J kg-1 K-1) DAm = gas binary diffusivity (m2 s-1) Dp = pellet diameter (m) DR = single tube diameter (m) F = gas volume flow (m3 s-1) G = mass flux (kg m-2 s-1) Gs = gas gravity h = specific enthalpy (J mol-1 or J kg-1) hf = heat transfer coefficient between gas and pellet (W m-2 K-1) HHV = higher heating value (MJ Sm-3 or MJ kg-1) k = reaction rate constant k’ = auxiliary kinetic constant (s-1) kG = mass transfer coefficient between gas and pellet (m s-1) LHV = lower heating value (MJ Sm-3 or MJ kg-1) / = mass flow rate (kg s-1) NT = number of tubes RA = effective reaction rate in the reactor (mol m-3 s-1) RU = Reactant utilization / steam conversion s = cross section of a tube (m2) T = temperature (°C or K) U = overall heat transfer coefficient (W m-2 K-1) Wel = electric power (kW) WI = Wobbe Index (MJ Sm-3) y = molar fraction z = axial coordinate (m) αi = tube internal heat transfer coefficient (W m-2 K-1) αe = tube external heat transfer coefficient (W m-2 K-1) 17 ACS Paragon Plus Environment


∆h = reaction enthalpy (J mol-1) ∆Tmin = minimum temperature difference (°C or K) εi = pellet porosity ε0 = catalytic bed void fraction η = effectiveness factor ηg = plant efficiency λ = thermal conductivity (W m-1 K-1) µ = dynamic viscosity (Pa s) τi = pellet tortuosity ψ = Thiele modulus χ = split parameter

References (1)

International Energy Agency (IEA). Key Renewables Trends (Excerpt from Renewables Information); 2016.

(2)

International Energy Agency (IEA). World Energy Outlook; 2014.

(3)

International Energy Agency (IEA). Key Electricity Trends (Excerpt from Electricity Information); 2016.

(4)

European Wind Energy Association (EWEA). Wind in Power - 2015 European Statistics; 2016.

(5)

Albadi, M. H.; El-Saadany, E. F. Overview of Wind Power Intermittency Impacts on Power Systems. Electr. Power Syst. Res. 2010, 80, 627.

(6)

Maton, J. P.; Zhao, L.; Brouwer, J. Dynamic Modeling of Compressed Gas Energy Storage to Complement Renewable Wind Power Intermittency. Int. J. Hydrogen Energy 2013, 38, 7867.

(7)

Sun, X.; Chen, M.; Jensen, S. H.; Ebbesen, S. D.; Graves, C.; Mogensen, M. Thermodynamic Analysis of Synthetic Hydrocarbon Fuel Production in Pressurized Solid Oxide Electrolysis Cells. Int. J. Hydrogen Energy 2012, 37, 17101.

(8)

Becker, W. L.; Braun, R. J.; Penev, M.; Melaina, M. Production of Fischer-Tropsch Liquid 18 ACS Paragon Plus Environment

Page 18 of 37

Page 19 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Fuels from High Temperature Solid Oxide Co-Electrolysis Units. Energy 2012, 47, 99. (9)

Specht, M.; Baumgart, F.; Feigl, B.; Frick, V.; Stürmer, B.; Zuberbühler, U.; Sterner, M.; Waldstein, G. Storing Bioenergy and Renewable Electricity in the Natural Gas Grid. FVEE AEE Top. 2009, 69.

(10)

Graves, C.; Ebbesen, S. D.; Mogensen, M.; Lackner, K. S. Sustainable Hydrocarbon Fuels by Recycling CO2 and H2O with Renewable or Nuclear Energy. Renew. Sustain. Energy Rev. 2011, 15, 1.

(11)

Soltanieh, M.; Azar, K. M.; Saber, M. Development of a Zero Emission Integrated System for Co-Production of Electricity and Methanol through Renewable Hydrogen and CO2 Capture. Int. J. Greenh. Gas Control 2012, 7, 145.

(12)

Hoekman, S. K.; Broch, A.; Robbins, C.; Purcell, R. CO2 Recycling by Reaction with Renewably-Generated Hydrogen. Int. J. Greenh. Gas Control 2010, 4, 44.

(13)

Kuramochi, T.; Ramírez, A.; Turkenburg, W.; Faaij, A. Comparative Assessment of CO2 Capture Technologies for Carbon-Intensive Industrial Processes. Prog. Energy Combust. Sci. 2012, 38, 87.

(14)

Socolow, R.; Desmond, M.; Aines, R.; Blackstock, J.; Bolland, O.; Kaarsberg, T.; Lewis, N.; Mazzotti, M.; Pfeffer, A.; Sawyer, K.; et al. Direct Air Capture of CO2 with Chemicals: A Technology Assessment for the APS Panel on Public Affairs. APS physics 2011.

(15)

Ryckebosch, E.; Drouillon, M.; Vervaeren, H. Techniques for Transformation of Biogas to Biomethane. Biomass and Bioenergy 2011, 35, 1633.

(16)

Augelletti, R.; Conti, M.; Annesini, M. C. Pressure Swing Adsorption for Biogas Upgrading. A New Process Configuration for the Separation of Biomethane and Carbon Dioxide. J. Clean. Prod. 2017, 140, 1390.

(17)

Eurobserv’ER. Biogas Barometer. 2014. Https://www.eurobserv-Er.org/biogas-Barometer2014/. (Accessed December 2017).

(18)

Brooks, K. P.; Hu, J.; Zhu, H.; Kee, R. J. Methanation of Carbon Dioxide by Hydrogen Reduction Using the Sabatier Process in Microchannel Reactors. Chem. Eng. Sci. 2007, 62, 1161.

(19)

Aziz, M. A. A.; Jalil, A. A.; Triwahyono, S.; Ahmad, A. CO2 Methanation over 19 ACS Paragon Plus Environment


Heterogeneous Catalysts: Recent Progress and Future Prospects. Green Chem. 2015, 17, 2647. (20)

Wang, W.; Gong, J. Methanation of Carbon Dioxide: An Overview. Front. Chem. Eng. China 2011, 5, 2.

(21)

Gao, J.; Liu, Q.; Gu, F.; Liu, B.; Zhong, Z.; Su, F. Recent Advances in Methanation Catalysts for the Production of Synthetic Natural Gas. RSC Adv. 2015, 5, 22759.

(22)

Garbarino, G.; Riani, P.; Magistri, L.; Busca, G. A Study of the Methanation of Carbon Dioxide on Ni/Al2O3 Catalysts at Atmospheric Pressure. Int. J. Hydrogen Energy 2014, 39, 11557.

(23)

Tada, S.; Shimizu, T.; Kameyama, H.; Haneda, T.; Kikuchi, R. Ni/CeO2 Catalysts with High CO2 Methanation Activity and High CH4 Selectivity at Low Temperatures. Int. J. Hydrogen Energy 2012, 37, 5527.

(24)

Liu, H.; Zou, X.; Wang, X.; Lu, X.; Ding, W. Effect of CeO2 Addition on Ni/Al2O3 Catalysts for Methanation of Carbon Dioxide with Hydrogen. J. Nat. Gas Chem. 2012, 21, 703.

(25)

Cai, M.; Wen, J.; Chu, W.; Cheng, X.; Li, Z. Methanation of Carbon Dioxide on Ni/ZrO2Al2O3 Catalysts: Effects of ZrO2 Promoter and Preparation Method of Novel ZrO2-Al2O3 Carrier. J. Nat. Gas Chem. 2011, 20, 318.

(26)

Liu, J.; Li, C.; Wang, F.; He, S.; Chen, H.; Zhao, Y.; Wei, M.; Evans, D. G.; Duan, X. Enhanced Low-Temperature Activity of CO2 Methanation over Highly-Dispersed Ni/TiO2 Catalyst. Catal. Sci. Technol. 2013, 3, 2627.

(27)

Abate, S.; Mebrahtu, C.; Giglio, E.; Deorsola, F.; Bensaid, S.; Perathoner, S.; Pirone, R.; Centi, G. Catalytic Performance of γ-Al2O3-ZrO2-TiO2-CeO2 Composite Oxide Supported Ni-Based Catalysts for CO2 Methanation. Ind. Eng. Chem. Res. 2016, 55, 4451.

(28)

Gabrovska, M.; Edreva-Kardjieva, R.; Crişan, D.; Tzvetkov, P.; Shopska, M.; Shtereva, I. Ni–Al Layered Double Hydroxides as Catalyst Precursors for CO2 Removal by Methanation. React. Kinet. Mech. Catal. 2012, 105, 79.

(29)

He, L.; Lin, Q.; Liu, Y.; Huang, Y. Unique Catalysis of Ni-Al Hydrotalcite Derived Catalyst in CO2 Methanation: Cooperative Effect between Ni Nanoparticles and a Basic Support. J. 20 ACS Paragon Plus Environment

Page 20 of 37

Page 21 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Energy Chem. 2014, 23, 587. (30)

Wierzbicki, D.; Debek, R.; Motak, M.; Grzybek, T.; Gálvez, M. E.; Da Costa, P. Novel NiLa-Hydrotalcite Derived Catalysts for CO2 Methanation. Catal. Commun. 2016, 83, 5.

(31)

Abelló, S.; Berrueco, C.; Montané, D. High-Loaded Nickel–alumina Catalyst for Direct CO2 Hydrogenation into Synthetic Natural Gas (SNG). Fuel 2013, 113, 598.

(32)

Abate, S.; Barbera, K.; Giglio, E.; Deorsola, F.; Bensaid, S.; Perathoner, S.; Pirone, R.; Centi, G. Synthesis, Characterization, and Activity Pattern of Ni–Al Hydrotalcite Catalysts in CO2 Methanation. Ind. Eng. Chem. Res. 2016, 55, 8299.

(33)

Ebbesen, S. D.; Graves, C.; Mogensen, M. Production of Synthetic Fuels by Co-Electrolysis of Steam and Carbon Dioxide. Int. J. Green Energy 2009, 6, 646.

(34)

Audi

Annual

Report

2015.

Https://www.audi-

Mediacenter.com/en/publications/corporate/audi-Annual-Report-2015-Tomorrow-Now-298. (Accessed December 2017). (35)

Gahleitner, G. Hydrogen from Renewable Electricity: An International Review of Power-toGas Pilot Plants for Stationary Applications. Int. J. Hydrogen Energy 2013, 38, 2039.

(36)

International Energy Agency (IEA). Key World Energy Statistics; 2016.

(37)

Nuclear Hydrogen Production Handbook; Yan, X. L., Hino, R., Eds.; Taylor & Francis Group, 2011.

(38)

Ferrero, D.; Lanzini, A.; Santarelli, M.; Leone, P. A Comparative Assessment on Hydrogen Production from Low- and High-Temperature Electrolysis. Int. J. Hydrogen Energy 2013, 38, 3523.

(39)

Ni, M.; Leung, M. K. H.; Leung, D. Y. C. Technological Development of Hydrogen Production by Solid Oxide Electrolyzer Cell (SOEC). Int. J. Hydrogen Energy 2008, 33, 2337.

(40)

Laguna-Bercero, M. A. Recent Advances in High Temperature Electrolysis Using Solid Oxide Fuel Cells: A Review. J. Power Sources 2012, 203, 4.

(41)

Giglio, E.; Lanzini, A.; Santarelli, M.; Leone, P. Synthetic Natural Gas via Integrated HighTemperature Electrolysis and Methanation: Part I-Energy Performance. J. Energy Storage 21 ACS Paragon Plus Environment


2015, 1, 22. (42)

Giglio, E.; Lanzini, A.; Santarelli, M.; Leone, P. Synthetic Natural Gas via Integrated HighTemperature Electrolysis and Methanation: Part II-Economic Analysis. J. Energy Storage 2015, 2, 64.

(43)

Koschany, F.; Schlereth, D.; Hinrichsen, O. On the Kinetics of the Methanation of Carbon Dioxide on Coprecipitated NiAl(O)x. Appl. Catal. B Environ. 2016, 181, 504.

(44)

Green, D. W.; Perry, R. H. Perry’s Chemical Engineers’ Handbook. 8th Edition.; McGrawHill, 2008.

(45)

Fuller, E. N.; Giddings, J. C. A Comparison of Methods for Predicting Gaseous Diffusion Coefficients. J. Chromatogr. Sci. 1965, 3, 222.

(46)

Fuller, E. N.; Schettler, P. D.; Giddings, J. C. A New Method for Prediction of Binary GasPhase Diffusion Coefficients. Ind. Eng. Chem. 1966, 58, 18.

(47)

Poling, B. E.; Prausnitz, J. M.; O’Connell, J. P. The Properties of Gases and Liquids. Fifth Edition; McGraw-Hill, 2001.

(48)

Chung, T.-H.; Lee, L. L.; Starling, K. E. Applications of Kinetic Gas Theories and Multiparameter Correlation for Prediction of Dilute Gas Viscosity and Thermal Conductivity. Ind. Eng. Chem. Fundam. 1984, 23, 8.

(49)

Chung, T. H.; Ajlan, M.; Lee, L. L.; Starling, K. E. Generalized Multiparameter Correlation for Nonpolar and Polar Fluid Transport Properties. Ind. Eng. Chem. Res. 1988, 27 (4), 671.

(50)

Bird, R. B.; Stewart, W. E.; Lightfoot, E. N. Transport Phenomena - Second Edition; John Wiley and Sons, Inc., 2007.

(51)

Froment, G. F.; Bischoff, K. B.; De Wilde, J. Chemical Reactor Analysis and Design. 3rd Edition; John Wiley and Sons, Inc., 2011.

(52)

Leva, M.; Weintraub, M.; Grummer, M.; Clark, E. L. Cooling of Gases through Packed Tubes. Ind. Eng. Chem. 1948, 40, 747.

(53)

Whalley, P. B. Boiling, Condensation and Gas-Liquid Flow; Clarendon Press, 1987.

(54)

Linnhoff, B.; Hindmarsh, E. The Pinch Design Method for Heat Exchanger Networks. Chem. Eng. Sci. 1983, 38, 745. 22 ACS Paragon Plus Environment

Page 22 of 37

Page 23 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


(55)

Linnhoff, B. User Guide on Process Integration for the Efficient Use of Energy; The Institution of Chemical Engineers, 1994.

(56)

Gruber, M. Effizienzsteigerung Der Power to Gas Technologie Durch Thermische Integration von Hochtemperatur Dampfelektrolyse Und CO2-Methanisierung - Das HELMETH

Projekt,

Jahrestreffen

Der

ProcessNet

-

Gasreinigung

&

Energieverfahrenstechnik, Frankfurt, Germany. 2017. (57)

Brabandt, J.; Mittmann, F.; Blumentritt, R.; Posdziech, O.; Gruber, M. Pressurised High Temperature Co-Electrolysis as Effective Source for Power-to-X Applications. Presented at HydrogenDays, Prague, Czech Republic, May 2016.

(58)

Perdikaris, N.; Panopoulos, K. D.; Hofmann, P.; Spyrakis, S.; Kakaras, E. Design and Exergetic Analysis of a Novel Carbon Free Tri-Generation System for Hydrogen, Power and Heat Production from Natural Gas, Based on Combined Solid Oxide Fuel and Electrolyser Cells. Int. J. Hydrogen Energy 2010, 35, 2446.



Page 24 of 37

Tables Hydrogen inlet mole flow (mol/s)

40

Catalyst particle diameter (mm)

5

Tube-catalyst diameter ratio

10

Gas inlet pressure (bar)

15

Reactors inlet temperature (°C)

250

CH4 outlet concentration (vol.-%; dry basis)

95

Table 1. Data assumption for the reactor model.


Page 25 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Reactor 1

Reactor 2

Reactor 3

Number of tubes

15

23

15

Tube length (m)

1.35

2.12

2.48

GHSV (h-1)

88048

34809

19690

Total catalyst load (kg)

39.9

95.5

72.9

χ – CO2 fraction to reactor

0.35

0.46

0.19

Pressure drop (bar)

0.78

0.87

0.55

Table 2. Results for the three methanation reactors.



Page 26 of 37

Stream description

Fluid type

C [kW/K]

Tin [°C]

Tout [°C]

Water heating

Cold

3.93

15

207

Steam superheating

Cold

1.88

204

800

R1 inlet heating

Cold

1.48

59

250

R3 inlet heating

Cold

0.81

64

250

R2 inlet cooling

Hot

1.42

289

250

Cathode outlet cooling

Hot

1.48

800

125

Cathode outlet condensation

Hot

4.84

125

30

Anode outlet cooling

Hot

0.66

800

30

R2 outlet cooling

Hot

1.21

290

163

R2 outlet condensation

Hot

6.44

163

30

R3 outlet cooling

Hot

0.60

277

137

R3 outlet condensation

Hot

2.81

137

30

Table 3. Thermal data required for pinch analysis.


Page 27 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Tcold,in

Tcold,out

Thot,in

Thot,out

Duty

(°C)

(°C)

(°C)

(°C)

(kW)

R2 outlet

59

250

290

147

255

R3 inlet

R3 outlet

64

166

277

166

67

HX3

R3 inlet

R2 inlet

166

250

289

250

55

HX4

Steam

204

406

800

224

380

HX5

Steam

406

701

800

426

554

15

207

426

61

754

665

800

-

-

185

Heat

Cold

exchanger

stream

HX1

R1 inlet

HX2

HX6 Heater

Hot stream

SOEC anode outlet SOEC cathode outlet

Liquid

SOEC cathode

water

outlet

Steam

-

Table 4. Main results for the heat exchange network.



Page 28 of 37

Electrolysis power (kW)

10000

Compression/pumping power (kW)

112

External heat requirement (kW)

185

LHV of the SNG (MJ/kg)

48.7

HHV of the SNG (MJ/kg)

54.3

SNG mass flow rate (kg/s)

0.16

Energy requirement (kWhel/kgSNG)

17.5

Wobbe Index (MJ/Sm3)

49.2

SNG chemical power (MW) - LHV/HHV basis

8.0 / 8.9

Overall plant efficiency (LHV basis)

77.4 %

Overall plant efficiency (HHV basis)

86.1%

Table 5. Efficiency calculations for a Power-to-gas plant based on SOEC steam electrolysis.


Page 29 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figures

Figure 1. Integration between high temperature electrolysis and methanation: reaction heat coming from exothermal methanation is used for water vaporization before electrolysis.



Figure 2. Temperature profile for a reactor fed with a stoichiometric inlet mixture (H2/CO2=4): a) effect of number of tubes; b) effect of coolant temperature.


Page 30 of 37

Page 31 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 3. Effect of split parameter χ on temperature profile: a) first reactor; b) second reactor. TC=210 °C, NT=20 for both the reactors. Produced water within the first reactor is condensed and removed.



Figure 4. Methanation section adopted in this work. Red line is pure hydrogen, black lines refer to carbon dioxide and green lines represent synthesis gas with different composition: from H2+CO2 to SNG with high methane content.


Page 32 of 37

Page 33 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. Bulk and catalyst temperature profile for reactor 1 (a), reactor 2 (b) and reactor 3 (c).



Figure 6. CH4 mole fraction on a dry basis over the methanation section.


Page 34 of 37

Page 35 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. Composite curves for both hot and cold streams related to the analyzed system.



Figure 8. Overview of the 10 MWel SOEC-based power-to-gas system.


Page 36 of 37

Page 37 of 37 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


For Table of Contents Only


Power-to-Gas through High Temperature ... - ACS Publications

Recommend Documents