electrified methane reforming: elucidating transient phenomena

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Page 1: Electrified methane reforming: Elucidating transient phenomena

General rights Copyright and moral rights for the publications made accessible in the public portal are retained by the authors and/or other copyright owners and it is a condition of accessing publications that users recognise and abide by the legal requirements associated with these rights.

Users may download and print one copy of any publication from the public portal for the purpose of private study or research.

You may not further distribute the material or use it for any profit-making activity or commercial gain

You may freely distribute the URL identifying the publication in the public portal If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim.

Downloaded from orbit.dtu.dk on: Nov 07, 2021

Electrified methane reformingElucidating transient phenomena

Wismann, Sebastian T.; Engbæk, Jakob S.; Vendelbo, Søren B.; Eriksen, Winnie L.; Frandsen, Cathrine;Mortensen, Peter M.; Chorkendorff, Ib

Published in:Chemical Engineering Journal

Link to article, DOI:10.1016/j.cej.2021.131509

Publication date:2021

Document VersionPublisher's PDF, also known as Version of record

Link back to DTU Orbit

Citation (APA):Wismann, S. T., Engbæk, J. S., Vendelbo, S. B., Eriksen, W. L., Frandsen, C., Mortensen, P. M., &Chorkendorff, I. (2021). Electrified methane reforming: Elucidating transient phenomena. Chemical EngineeringJournal, 425, [131509]. https://doi.org/10.1016/j.cej.2021.131509

Page 2: Electrified methane reforming: Elucidating transient phenomena

Chemical Engineering Journal 425 (2021) 131509

Available online 8 August 20211385-8947/© 2021 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY license (http://creativecommons.org/licenses/by/4.0/).

Electrified methane reforming: Elucidating transient phenomena

Sebastian T. Wismann a,b, Jakob S. Engbæk c, Søren B. Vendelbo c, Winnie L. Eriksen b, Cathrine Frandsen a, Peter M. Mortensen b,*, Ib Chorkendorff a,*

a Technical University of Denmark, Fysikvej 312, 2800 Kgs. Lyngby, Denmark b Haldor Topsoe A/S, Haldor Topsøes Alle 1, 2800 Kgs. Lyngby, Denmark c Danish Technological Institute, Gregersensvej 1, 2630 Tåstrup, Denmark

A R T I C L E I N F O

Keywords: SMR Electrification Industrial catalysis Transient

A B S T R A C T

Increasing implementation of renewable energy requires an infrastructure compatible with the intermittent production of green electricity. Herein we show the flexibility of electrically heated steam methane reforming with integrated ohmic heating, through a combination of CFD modelling and lab scale reactor tests. It is shown how start-up from an idle state to operation conditions can be achieved with instantaneous application of the full power required for a steady state conversion of 80%, with initial heating rates exceeding 50 ◦C/min. The initial heating rate is correlated to the thermal mass of the reactor, with the endothermic reaction governing the temperature profile. Cyclical operation displays no apparent delay between the change in temperature and methane conversion. The highest thermal gradient across the washcoat is predicted at steady state, with no increase during start-up despite the higher heating rates. The highest risk of carbon formation is predicted at the inlet at steady state operation. A temporarily peak in the equilibrated carbon potential is predicted near the outlet during start-up and shutdown between 500 and 600 ◦C, governed by the thermodynamics of the feed composition. Integrated ohmic heating supports steam methane reforming scalable to industrial conditions, operating closer to thermodynamic limits for carbon formation, and potentially based on the access to inter-mittent excess of renewable energy.

1. Introduction

Improving efficiency and a quickly growing global capacity for renewable energy has resulted in economically competitive prices for green electricity compared to fossil fuels [1–3]. Without efficient solu-tions for large-scale energy storage, the intermittent nature of renewable energy creates a demand for technologies compatible with the excess production [4]. Production of synthesis gas (syngas) through the endo-thermic steam methane reforming process (SMR) is a relevant approach for utilizing excess renewable electricity in a transition phase. Syngas is a mixture of hydrogen and carbon oxides, and serves as platform for production of many chemicals, including ammonia, methanol, and synthetic fuels [5]. The syngas composition is governed by the methane reforming reaction (Eq. (1)) and the water–gas-shift (WGS, Eq. (2)).

CH4 +H2O⇌CO+ 3H2ΔH◦

r = 206kJ/mol (1)

CO+H2O⇌CO2 +H2ΔH◦

r = − 41kJ/mol (2)

Today, combustion of fossil fuels provide heat in conventional SMR plants to drive the strongly endothermic reaction, resulting in emissions of large amounts of CO2-containing flue gas. A typical fired SMR process based on natural gas emits 7–11 kg CO2/kg H2, depending on efficiency and fuel [6–8]. The global production of syngas accounts for nearly 3% of global CO2 emissions [9,10].

A typical large-scale industrial syngas plant for hydrogen production contains several unit operations; a desulphurization section, a pre- reformer, the primary reformer, and a WGS section [5,11]. The pri-mary reformer is by far the largest, most complex, and most expensive unit operation for a large-scale syngas plant, and it is the only unit operation considered in the following [12]. For a typical large-scale reformer more than hundred reactor tubes, in one or more rows, are heated by combustion in a large furnace box. Performance is typically limited by heat transfer, and relatively narrow tubes (100 to 200 mm in external diameter, 10–13 m in length) are used to minimize gradients across the catalyst bed in the individual tubes [11]. To avoid thermal stress or formation of hotspots in the reactor tubes, it is imperative to

* Corresponding authors. E-mail addresses: [email protected] (P.M. Mortensen), [email protected] (I. Chorkendorff).

Contents lists available at ScienceDirect

Chemical Engineering Journal

journal homepage: www.elsevier.com/locate/cej

https://doi.org/10.1016/j.cej.2021.131509 Received 17 May 2021; Received in revised form 21 July 2021; Accepted 24 July 2021

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balance heat supply to the energy consumption by the endothermic re-action. Safe start-up procedures can require several days to gradually increase reactant flowrate and heat flux before reaching a steady state for operation [5]. It is critical that the heat distribution between the hundreds of reactor tubes in the furnace box is uniform, as low flux limits performance, and too high flux for a single tube can lead to hot spots or hot banding – leading to carbon formation and demanding shutdown of the entire reformer [5,13].

Electrification of the reforming technology can substantially reduce CO2 emissions, and in addition address key limiting aspects of the conventional process. Induction heating of ferromagnetic catalyst par-ticles enables heat delivered directly to the catalytic site, as demon-strated by Vinum et al. [14]. However large scale is required to attain competitive energy efficiency, as showed by Almind et al. [15]. Meloni et al. demonstrated similar inversion of the thermal gradient using mi-crowaves to heat a structured Ni/SiC catalyst [16]. This was also demonstrated by Renda et al., heating a catalyzed SiC element by ohmic resistance [17]. A highly robust system was demonstrated by Surov et al. utilizing a high temperature plasma torch to drive a reforming reaction without a catalyst [18].

Integrated ohmic heating enables substantial process intensification, reducing reactor volume by up to two orders of magnitude compared to equivalent fired reformer [10]. The short characteristic length scales inverts typically limiting heat transfer to become the least limiting re-action step [19].

This work investigates the potential for cold-starting a resistance- heated reformer, the transient response to fluctuations in supplied power to mimic intermittent operation, and how it influences system gradients. The work is based on a laboratory scale reformer with inte-grated ohmic heating, analyzed with detailed CFD model to elucidate the occurring phenomena. The typical limiting phenomena, carbon deposition and thermal gradients, are assessed, providing suggestions towards the possibility for the transient reforming of methane.

2. Experimental setup

A custom build resistance-heated plug flow SMR bench scale reactor setup was used for the experiment. The principle reactor was made of a FeCrAl tube. The resistivity of FeCrAl is practically independent of temperature, and a 6 mm tube (Goodfellow) of 50 cm in length was coated with a 130 µm porous zirconia washcoat on the internal surface. An 11 cm section of the coat was removed from each end through drilling, prior to impregnation with a nickel nitrate solution. A thin, residual layer of catalytically active coat was present towards the outlet of the reactor (39–50 cm) [10]. The reactor was reduced in H2 and passivated at room temperature. Several 0.25 mm K-type thermocouples was spot-welded to the external surface to enable accurate measurement of the temperature profile (SR630 Thermocouple monitor). The reactor was insulated by high temperature granular insulation (FreeFlow). The reactor configuration is shown in Fig. 1.

The reactor was reduced in the ohmic heating setup prior to catalytic tests. A feed gas of methane, steam, and hydrogen (30/60/10) was used for all experiments. Flowrate and composition was controlled by flow-meters (SLA850, Brooks digital flow controller). The feed gas was pre-heated to 105 ◦C to prevent condensation prior to the reactor. The applied current was measured using a current clamp kept perpendicular to the current (Keysight 1146B), and the potential via an oscilloscope (Agilent infiniiVision DSO-X 2014A). Contact resistance was measured before and after by a source meter at 1A (Keithley 2400 Sourcemeter). Outlet gas composition was measured by gas chromatography (Agilent 7890, TCD/FID) or mass spectrometry (MS, Spectra Microvision plus). Timescale for the MS data is calibrated against temperature measurements.

The reactor was heated by transforming 220–240 V from the grid using a set of coils (N = 23), with the applied AC potential to the reactor controlled by a variotransformer. The AC potential was applied along

the reactor, using two copper sockets at opposite ends of the reactor (Fig. 1). The current is governed by the electrical resistance according to Ohm’s law, resulting in a heat loss proportional to the square of the applied potential;

P = U2RMS/RΩ

where URMS is the root mean square potential (RMS), and RΩ the elec-trical resistance for the system, including wiring to the transformer coil and the connections (contact resistance). The heat loss over the reactor accounts for ca. 98% of the total electrical losses based on the measured contact resistance. This does not include transformation losses. The root- mean-square (RMS) potential is calculated assuming an ideal sinusoidal wave.

URMS = UPeak/2

3. CFD implementation

Full fluid dynamic equations for momentum, mass transfer, heat transport and electric fields was implemented in an axisymmetric ge-ometry in Comsol 5.2a (Fig. 1). Flow is at all times laminar (Re less than 1100). Consequently, radial mass transfer depends on molecular diffu-sion. A fully developed laminar flow was defined at the inlet, and reference pressure at the outlet. The fluid momentum is defined by the full Navier-Stokes equation for a compressible fluid.

ρ ∂u∂t

+ ρ(u∙∇) = − ∇p+∇∙[

μmix∇u −23μmix(∇∙u)I

]

In the porous washcoat, the brinkman modification is used [20]. Heat transfer is defined by the standard equation [21]

ρCp

(∂T∂t

+u∙∇T)

= keff∇2T+Qh +Qr

where Qh is the heatsource from ohmic heating and Qr the heat source of the reactions. The measured temperature was defined for the copper sockets, as their geometry was distorted in the axial-symmetrical approach. Heat losses were defined as a fixed temperature dependent flux on external surfaces, with additional radiation losses from the un-

Fig. 1. Reactor geometry Experimental reactor cross section, showing the relevant elements. Each red line represents a thermocouple. The illustration is not to scale.

S.T. Wismann et al.

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insulated, lower part of the reactor (Fig. 1). Radiation from internal surfaces had no influence on the model predictions due to confined view factors in the narrow tube. Electrical heating was defined according to Ohm’s law;

Qh = E∙J

where E is the electric field [V/m], and J the current density vector [A/ m2]. A contact resistance was implemented equivalent to heat transfer coefficients [10,19]

n∙J = hc∙(Vi − Vj)

where n is the normal vector, (Vi − Vj) the potential difference across the surface, and hc a resistance factor. Mass transfer was implemented on a mass fraction basis

ρ ∂ωi

∂t+∇∙ji + ρ(∇∙u)ωi = Ri

where Ri is the reaction rate, and ji is the mass flux for the i’th specie, defined as;

ji = −

(

ρDi∇ωi + ρDiωi∇Mi

Mi

)

Reaction kinetics was implemented for SMR, WGS, and reverse methanation [22]. The reaction rates were implemented as a function of approach to equilibrium. For the reforming reaction it is defined as;

R1 = k1pCH4 pH2O

p2.5H2

(1 − β1)

where (1 − β) is the approach to equilibrium, which for SMR is defined as;

β1 =pCOp3

H2

pCH4 pH2OKeq

where Keq is an equilibrium constant [5]. For fluid domains, a triangular mesh with a maximum resolution of

0.1 mm was employed, with improved resolution in the boundary layer domain. A quadratic mesh was used for the catalytic coat and reactor wall, with 12 elements across the coat. Remaining domains employed a triangular mesh with a resolution of 5 mm or less. Further details on the implementation of equations, mesh sensitivity analysis, and model fitting are described elsewhere [10,19].

4. Carbon activity calculations

Low thermal conductivity across the catalyst bed and reactor wall creates steep thermal gradients, increasing risk of phenomena detri-mental to reformer lifetime occurring, such as carbon formation, or thermal stress. A net positive rate for carbon formation at any point in a reactor is detrimental to catalyst lifetime and would be incompatible with industrial operation [5,11]. The prevalent reactions for carbon formation are the Boudouard reaction (Eq. (3)), CO reduction (Eq. (4)), methane decomposition (Eq. (5)), and decomposition of higher hydro-carbons (Eq. (6)), here with enthalpy given with respect to formation of graphitic carbon;

2CO⇌C+CO2(ΔH◦

r = − 172kJ/mol)

(3)

CO+H2⇌C+H2O(ΔH◦

r = − 131kJ/mol)

(4)

CH4⇌C+ 2H2(ΔH◦

r = + 75kJ/mol)

(5)

CnHm→nC+ A½mH2(ΔH◦

r > 0kJ/mol)

(6)

Formation of carbon occurs on the catalyst surface and is dependent

on the kinetic balance of adsorbed species against the reaction rates. Higher hydrocarbons display a high tendency towards forming carbon by irreversible decomposition on nickel catalysts [5]. However, a pre- reformer can convert practically all higher hydrocarbons (C2+) at suf-ficiently low temperatures to kinetically suppress formation of carbon, and carbon formation from higher hydrocarbons is thus not addressed further in this work [5,13,23].

The prevalent mechanism for carbon formation for the conditions relevant to methane reforming in the primary reformer is formation of whisker carbon [24,25]. The characteristic carbon fibers of whisker carbon (nanotubes) nucleate on the more reactive step sites [5,24]. If formed, the high mechanical strength of the carbon whiskers will break apart the catalyst pellets, leading to an increasing pressure drop, which can alter flow patterns leading to formation of hotspots, requiring a complete shutdown and replacement of the catalyst bed [26].

The risk of carbon forming reactions can be assessed by the carbon activity. If the carbon activity exceeds unity, there is a thermodynamic potential for the formation of carbon. However, catalysts with high intrinsic activity can operate at conditions where the apparent ther-modynamic carbon activity in the bulk phase exceeds unity assuming the catalytic surface is in equilibrium. A critical carbon activity can be estimated from the local concentration of species equilibrated with respect to SMR and WGS (Eq. 1–2), reflecting the expected composition at the catalyst surface. The critical carbon activity, here for the methane decomposition reaction, can be calculated as;

aC,eq = KeqpCH4

p2H2

eq

aC,eq > 1⇒Carbon

where pi,eq is the partial pressure of the equilibrated process gas, and Keq an equilibrium constant for the formation of carbon which depends on the mechanism of the formed carbon.[27] The equilibrated carbon po-tential is generally always below the thermodynamic potential for the bulk composition. While graphitic carbon is the thermodynamically favorable form of carbon, it is kinetically suppressed, and less stable whisker carbon is formed at typical SMR conditions which can be taken into account by using a lower equilibrium constant.

As the carbon needs to nucleate, a critical particle diameter is required, completely suppressing formation of carbon for small nano-particles [27,28]. From a practical perspective the high temperatures, metal loading, and steam content facilitate sintering, resulting in par-ticle diameters quickly exceeding the critical threshold for nickel par-ticles [5,11]. Consequently, carbon activity in industrial reformers is controlled by operating at a high S/C and optimized reactor geometry [11].

5. Results

5.1. Cold start of SMR reactor

The intermittent production of renewable electricity can change significantly throughout a day. This has generated a demand for compatible technologies with a fast temporal response. Safe start-up procedures for current industrial fired reformers typically require days to avoid thermal stress or formation of hotspots.[5] With integrated ohmic heating, fast temporal response is feasible, as the electrical con-ductivity of FeCrAl is practically independent of temperature [29]. This enables homogeneous heating along the entire reactor length and alle-viates the risk of transient hotspot formation.

Fig. 2 shows the temperature development of the maximum tem-perature for the lab scale reactor heated from inlet conditions (105 ◦C, S/C 2, 1 atm) to operation conditions (80% methane conversion), by instantaneous application of the steady state potential to a “cold” reactor. The “cold” reactor starts at 15% of the steady state power to

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prevent condensation of steam. Both experimental data and simulations are combined in the chart. At 15% power the maximum temperature is 275 ◦C, and there is no quantifiable or expected conversion of methane. To limit uncertainty related to the CO and CO2 signal of the MS, Fig. 2 is based on the methane content in the dry gas. The transient model pre-dicts a slightly slower heating of the reactor, and consequently lower initial conversion, but generally predict the maximum temperature within ±10οC.

Initial start-up time is correlated to the applied power and the thermal mass of the reactor wall, catalyst, and insulation, based on the near linear increase in temperature the first 2 min. Above 600 ◦C, the reaction rate of the strongly endothermic reaction becomes significant, substantially changing the slope. The reactor reaches 95% of the thermal steady state within 12 min, while nearly 30 min are required to reach 95% of steady state conversion. The equilibration time is primarily dependent on the thermal diffusivity of the insulation. Initial heating rate is practically independent of flow rate as the thermal mass of the flow is more than two orders of magnitude lower than that of the reactor and the temperatures are insufficient for a substantial contribution from the endothermic reaction. Increasing flowrate or conversion requires a higher heat flux, and concomitantly an increase in the applied potential, resulting in faster start-up as the thermal mass is nearly constant. Heating rates coinciding with the characteristic timescales are practi-cally impossible for the given design, given the high thermal mass of the reactor [19].

Heating substantially faster does not bring further benefits for ap-plications utilizing an intermittent supply of renewable energy, as the grid is typically regulated on an hourly basis [30].

Due to the temperature-independent electrical resistance, a large part of the reactor is initially nearly uniform in temperature (Fig. 3). After 1 min, the temperature is too low for the reaction rate to signifi-cantly influence the heat losses, resulting in a near constant temperature along the insulated part of the reactor, due to the inherent uniform ohmic heating. As the temperature increases, the exponentially depen-dent reaction rate will consume more heat, limiting the increase in temperature. As the reaction proceeds, higher temperatures are required to drive it further, shaping the temperature profile. The small step at 39 cm (Fig. 3) is due to insufficient catalytic activity in a thin residual layer

of catalyst, as explained in previous work [10,19]. Fig. 4 shows the radial temperature gradient at the top of the coat (z

= 11 cm) where the largest difference is predicted due to an edge effect, and 5 mm below as indicated in the insert. The gradient across the washcoat reaches a maximum at steady state where the highest tem-perature, and consequently reaction rate occurs. The slope of the tran-sient gradient across the washcoat is correlated to the reaction rate, as it can be seen to level off after 3 min, corresponding to the onset of the endothermic reaction (Fig. 2). The thermal difference across the coat peaks at the ends of the coat due to edge effects, as relative more cata-lytic area is exposed facilitating additional cooling due to endothermic reactions.

Practically no gradient is predicted across the reactor wall, and less than 2 ◦C across the catalytic coat at the conditions of Fig. 4. The thermal gradient is correlated nearly linearly to the inward heat flux, thus depending on operation conditions and increasing with flowrate or conversion.

Fig. 2. Cold start of electrified methane reformer at ambient pressure. Maximum temperature and outlet conversion as a function of time. Power was 15% of the steady state conditions before t = 0. Open circles show experimental data and full lines show model predictions. The temperature position is illus-trated in the insert, 7.1 cm from the outlet. Conditions: inlet temperature 105 ◦C, feed composition CH4:H2O:H2 (30/60/10), ambient pressure, 920 Nml/min.

Fig. 3. Transient temperature profiles at ambient pressure. Simulated (full lines) and experimental (dots) initial temperature profiles for an electrically heated reformer initiated from inlet conditions (cf. Fig. 2).

5 mm from edge

Coat edge

0

0.5

1

1.5

2

2.5

0 10 20 30 40 50 60

ΔT C

oat[K

]

Time [min]

Coat

Reactor wall00.5

11.5

22.5

10 20 30 40

ΔT C

oat [

K]

Axial position [cm]

Fig. 4. Modelled transient temperature gradient. Predicted radial temperature gradient across the catalytic washcoat evaluated at the start of the coat and 5 mm below. The insert shows the radial thermal gradient at steady state con-ditions along the catalytic washcoat. The arrows indicate the position for the transient profile. The timescale corresponds to the results in Fig. 2. 920 Nml/ min, CH4:H2O:H2 (30:60:10), 1 atm.

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5.2. Transient response

Adding on the understanding of the transient response of the system, also thermal cycling of the system was simulated to understand the time- wise response to electricity availability as e.g. induced by intermittent production of renewable electricity. Fig. 5 shows the response to rapid changes in energy input for the electrically heated reformer. Starting from a steady state at 80% methane conversion at an applied potential of 2.9 V (t = 0 s), the RMS potential is abruptly changed every 2 min, between 1.3 and 2.9 V. At 1.3 V the corresponding steady state con-version is less than 20%. The transient operation in Fig. 5 was modelled in a transient CFD model approach. The model accurately predicts the behavior at high temperature (T ±10 ◦C, XCH4±2%), with slightly increasing deviation over time for low conversion (T ±23 ◦C), as illus-trated by the difference between full lines and points in Fig. 5. Consid-ering the accumulating uncertainties related to time, temperature, potential, and material parameters, the deviation is considered acceptable.

Decreasing the potential from 2.9 to 1.3 V corresponds to an 80% decrease in supplied energy (Eq. (9)). The endothermic reaction facili-tates an initial steep drop in temperature consuming the latent energy. As the temperature decreases, the reaction rate slows to the point where cooling is predominantly through external losses. Fig. 5B shows the measured, modelled and equilibrium dry methane fraction, increasing as the temperature drops, as less methane is converted. The equilibrium fraction is calculated based on the measured maximum temperature and is at all points below the measured and predicted fraction, illustrating the endothermic driving force of the reactions towards cooling the sys-tem. At low temperatures the deviation between experimental and

equilibrium fraction is greater as the reaction is kinetically limited due to insufficient temperatures.

The model predicts slightly faster cooling of the reactor, correlated to an overprediction of the methane fraction, as the temperature decreases. The peak temperature decreases over time, indicating the system does not reach a steady state. This is correlated to the thick insulation, as the characteristic thermal timescale for the reactor and catalyst is within seconds, while hours for the insulation [19].

No distinguishable transient deviation between temperature and reaction kinetics (methane fraction) is observed on the given timescale. This is not unexpected given the inherent uniform heat source from ohmic heating for a material with electrical conductivity independent of temperature, and characteristic timescales below seconds for the cata-lytic processes [19]. As the applied potential is decreased, the inward heat flux is uniformly lowered, resulting in a downward shift of the entire temperature profile (Fig. 6).

The lower heat flux is most visible near the inlet, between the copper socket and catalytically activity region (6–11 cm), where the slope de-creases substantially. The slope along the catalytic washcoat is nearly the same, indicating the supplied heat is consumed by the reaction, keeping the reaction near equilibrium.

5.3. Carbon formation risk

Design and operation of industrial reformers is to a large extend constrained by the need to avoid formation of carbon. Carbon deposition is detrimental to SMR catalysis, and results in deterioration of the catalyst, large pressure drops, formation of hotspots, and in worst case, rupture of the reactor tube [5,26]. Fig. 7 shows the temperature and carbon activity for the experimental reactor, with the carbon activity evaluated for the actual gas with respect to graphite (the worst-case scenario). Of the three carbon forming reactions evaluated (Eq. 3–5), the conditions in the actual gas determines which is prevalent. The Boudouard and CO reduction reaction have a similar thermodynamic behavior, and consequently the CO reduction reaction is not illustrated in Fig. 7. The carbon activity greatly exceeds unity for the methane decomposition reaction (Eq. (5)) near the top of the washcoat, corre-lated to the fast pre-heating in the region between the copper socket and catalytic washcoat. A similar increase, though significantly lower (aMDC 1.1) is observed below the washcoat, for the region with a thin, residual

washcoat. The un-insulated part of the reactor heat exchanges with the surroundings towards the lower copper socket, swiftly cooling the CO rich process gas, resulting in a steep increase in the thermodynamic potential for the exothermic Boudouard reaction.

The carbon activity in the catalytic washcoat initially exceeds unity due to preheating, but quickly decrease as the reaction proceeds towards

Fig. 5. Transient response of electrified methane reformer. Measured and modelled transient response. A) Peak Temperature (z = 44.5 cm). B) Methane fraction in dry gas. C) Applied RMS potential over reactor. Conditions. Inlet 105 ◦C, feed composition CH4:H2O:H2 (30/60/10), ambient pressure, 680 Nml/ min. MS time resolution is calibrated against temperature measurements.

Fig. 6. Transient temperature profiles. Modelled and measured temperature profiles at 0, 120, and 240 s respectively (cf. Fig. 5). Full lines are model pre-dictions, with experimental values as circles.

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equilibrium as there is no carbon risk for the equilibrated gas. The carbon activity drops across the coat as the reaction is driven close to equilibrium. Despite the initial high carbon activity in the inlet section, there is no formation of carbon due to the equilibrated gas carbon ac-tivity is below 1 throughout the reactor (as shown in Fig. 8), which means that the overall reaction scheme is guided towards the syngas product due to the high partial pressure of steam and the SMR reaction being kinetically favorable [5]. This was further evidenced in end-of-run inspection of the washcoat in the top section, where no traces of carbon were found. The small thermal gradients across the washcoat keeps the entire catalytic region near equilibrium, suppressing carbon formation, enabling operation at harsher conditions than feasible for conventional pellet-based catalyst [31,32].

No formation of graphitic carbon is expected in the gas phase at the

conditions of SMR without higher hydrocarbons [33]. Fig. 8 shows the temperature and carbon activity along the catalytically active reactor surface. It is observed how the initial peak in carbon activity for methane decomposition (eq. (5)) decrease along the surface, as the process gas shifts towards equilibrium. Based on equilibrated gas, there is no carbon potential along the catalytic surface. The thermodynamic carbon ac-tivity for the equilibrated gas is independent of reaction. As there is no CO in the feed gas, the activity of the Boudouard and CO reduction re-action is zero before the catalytic zone.

The actual gas carbon activity exceeding unity outside the catalytic region in the top and bottom of the reactor as illustrated in Fig. 8 is a consequence of preheating taking place in the initial part of the reactor, and the rapid cooling due to heat exchange of the un-insulated part of the tube and are a compromise in the experimental setup (See Fig. 1). However, handling local actual gas activity exceeding unity is done in modern SMR based plants, and through use of pre-reforming technol-ogy, proper preheating, and syngas cooling technology, these aspects can all be controlled as long the equilibrated gas carbon potential stays below 1 [5,23,34].

5.4. Transient carbon activity

To investigate the transient carbon activity, start-up and shutdown of the reactor was modelled. The start-up was initiated from a steady state simulation with no applied power from a temperature equivalent to inlet conditions by an abrupt supply of the desired current for steady state operation at 80% methane conversion (Equivalent to Fig. 2). The shut-down is modelled equivalently by changing the applied current to zero from a steady state solution equivalent to 80% conversion. Fig. 9 shows the temperature and equilibrated carbon activity for start-up and shut-down of an electrically heated reformer. Initial change in temperature of the reactor wall is fast for both start-up and shutdown, where the low thermal diffusivity of the insulation results in slow thermal equilibration.

At no point during start-up or shutdown is the equilibrated carbon potential exceeding unity, indicating no formation of carbon (at given conditions). A transient peak in the carbon activity is observed both during start-up and shutdown near the outlet (z = 43 cm, Fig. 8). The peak occurs at 550 ◦C and is a thermodynamic effect, as reflected in the carbon potential for the equilibrium composition, indicating the reac-tion is close to equilibrium at the surface.

Fig. 7. Temperature and actual carbon activity at ambient pressure. Thermal contours and maximum carbon activity evaluated between the methance decomposition (aMDC) and Boudouard reaction (aBoud.) with respect to graphite. White lines mark the regions for carbon activity exceeding unity. Conditions: 1700 Nml/min, ambient pressure, CH4:H2O:H2 (30/60/10).

Fig. 8. Carbon activity and temperature on the internal surface. Carbon activity for the methane decomposition (aMDC) and Boudouard reaction (aBoud.) along the catalytic surface, including the equilibrated carbon activity (aEq.) for the catalytic region. Up to 11 cm, there is no catalytic activity. The equilibrium temperature is calculated w.r.t. SMR (Eq. (1)). Conditions: CH4:H2O:H2 (30/60/ 10), ambient pressure.

Fig. 9. Transient carbon activity during start-up and shutdown. Modelled equilibrated carbon activity and temperature evaluated at inlet and outlet (z =11 and 43 cm, cf. Fig. 7) during start-up from inlet conditions and shutdown. Conditions: 920 Nml/min, CH4:H2O:H2 (30/60/10), ambient pressure.

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Overall, the data evidence that transient carbon activities easily adjust to the chemical system during start-up and shutdown. This demonstrates a large agility of the technology and the prospect of rapid change of operating conditions. Overall, this points towards that the electrically heated steam reformer debottlenecks the slow response of the reforming step in syngas plants. Consequently, this can help bring syngas plants closer to become a balancing technology in the energy infrastructure.

6. Conclusions

Increasing implementation of renewable energy requires technolo-gies compatible with the intermittent nature of green electricity. Elec-trical heating of the strongly endothermic steam methane reforming process can contribute to a substantial reduction in CO2 emissions, and offers a prospective large off-taker of surplus electricity. Integrated ohmic heating inherently provides improved thermal control, and the intimate contact between catalyst and heat source, enables fast thermal response, such as start-up or shutdown within minutes without transient detrimental phenomena. With a transient response on a timescale of minutes rather than days, an electrically heated reformer is not a bottleneck for transient operation, in contrast to conventional fired re-formers. With minimal thermal gradients across the catalyst, carbon activity is governed by the equilibrium, enabling operation close to thermodynamic constrains. Electrically heated reformers consequently provide substantial flexibility towards intermittent operation and could be an enabling factor towards plant designs based on utilizing excess renewable energy for a greener production chemical according to availability of renewable electricity.

Funding

This work was supported by Innovation Fund Denmark (IFD) under file no. 5160-00004B and research grant 9455 from Villum Fonden.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

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