RESEARCH ARTICLE | JULY 25 2019

Novel concept for indirect solar-heated methane reforming

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Zahra Mahdi  ; Carlos Rendón; Christian Schwager; Cristiano Teixeira Boura; Ulf Herrmann

AIP Conf. Proc. 2126, 180014 (2019)

https://doi.org/10.1063/1.5117694

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24 July 2024 13:05:31

 Novel Concept for Indirect Solar-Heated Methane
Reforming
Zahra Mahdi1, a), Carlos Rendón1, b), Christian Schwager1,
Cristiano Teixeira Boura1, Ulf Herrmann1
1
Solar-Institut Jülich of FH Aachen University of Applied Sciences, 52428 Jülich, Germany.
a)
Corresponding author: mahdi@sij.fh-aachen.de
b)
rendon@sij.fh-aachen.de

Abstract. A model to investigate an indirectly solar-heated bayonet-tube reactor for converting methane to synthetic gas
(syngas) through combined steam reforming and dry reforming is presented. Concentrated solar radiation, as generated in
solar power towers is capable of efficiently providing heat for this process. Different concepts of reforming reactors have
been analyzed and assessed under the following considerations: The risk of carbon deposition at low-temperature regimes
in the reactor, the possibility of heat recovery from the syngas, maximized heat extraction for the air stream to improve the
receiver efficiency and flexibility. As a result, a novel bayonet-tubes reactor design has been developed. Different

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simulation software tools have been applied for this purpose. Simulations in EBSILON®Professional show that the heat
recovery from the syngas allows a 28 % higher syngas production (8.42 kg/s instead of 6.59 kg/s) based on the same solar
resource, since the required heat for the methane reforming is simultaneously transferred from both air and syngas. In the
system simulations, the syngas cools down from 900 °C to about 451 °C while the air is cooled down from 930 °C to approx.
220 °C. A one-dimensionally discretized model of a single bayonet-tube reactor was simulated in Dymola to corroborate
that the reactor design provides sufficient temperature gradients for the heat transfer from air and syngas to the reactant
flow. Further thermal and fluid mechanical analysis were performed in ANSYS® Fluent as preparation for building a first
prototype.

INTRODUCTION
A solar power tower is a suitable high-temperature heat sorce for thermochemical processes. The reforming of
methane offers a flexible and effective way of chemical storage of solar energy through an industrially established
process. Of particular interest in this work is the development and simulation of an indirectly solar-heated reforming
reactor system. The design of a reforming reactor shall enable an efficient synthesis gas (syngas) production. The
syngas is the product of the mixed reforming and is a mixture of hydrogen, carbon monoxide and other components
formed by the endothermic reaction between water, carbon dioxide and methane molecules. By supplying solar-
thermal energy at temperatures up to 1000 °C, the calorific value of methane can be increased by approximately 31 %.
The input of CO2 as educt for the dry reforming reaction can be provided from energy-intensive combustion processes
and thus be recycled. The produced syngas and its components are valuable intermediate resources for the production
of hydrogen, ammonia, methanol and synthetic hydrocarbon fuels. [1]

The Solar-Institut Jülich (SIJ) developed a process model to simulate continuous operation of the reforming system
and estimate the production of syngas using hot air at 930 °C as heat transfer fluid. In EBSILON®Professional the
reforming reactor system is modeled with and without heat recovery from the syngas to analyze the thermodynamics
inside the reactor under consideration of the expected chemical reactions. The assumptions regarding the heat transfer
in the reactor model is verified by a more detailed reactor model in Dymola using Modelica® language. The reforming
reactor with heat recovery is based on a bayonet-tubes design whereas the one without heat recovery is based on a
shell and tube concept. The boundary conditions and the inlet composition for the reforming reactor (CO2/CH4=0.33,

SolarPACES 2018
AIP Conf. Proc. 2126, 180014-1–180014-7; https://doi.org/10.1063/1.5117694
Published by AIP Publishing. 978-0-7354-1866-0/$30.00

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H2O/CH4=1.33) were taken from preliminary simulations performed by the project partner German Aerospace Center
(DLR) [2]. Additional CFD simulations have been performed to investigate the appropriate design and dimensions of
a small-scaled bayonet-tube reactor prototype for the experimental validation of simulation data. An initial test and
demonstration phase with this reactor-prototype is planned in the near future.

PLANT CONFIGURATION
Figure 1 shows the plant layout of the solar-heated syngas reactor system with heat recovery as implemented in
EBSILON®Professional. The thermal power of the solar receiver is set to 50.5 MWth. For an optimal balance between
high heat extraction from the air stream and great heat recovery it is assumed that in each of the 12 discretized reactor
elements 77 % of the heat is delivered by air and 23 % by syngas. This ratio results from the maximization of the
syngas production constrained by minimum temperature difference of 7.9 K. The same temperature difference was
considered for the reactor model without hear recovery.

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FIGURE 1. Configuration of the solar-heated syngas reactor system

The solar-heated syngas reactor system consists of an air and reactant/syngas loop. Solar operation takes place
when the heliostat field concentrates the solar irradiation onto the surface of the open, volumetric air receiver. Under
nominal conditions the air is taken from the ambient and heated up to 930 °C. The thermal storage, which is based on
a porous ceramic structure, is deployed for storing heat to enable night operation of the system. Additionally, a back-
up unit enables continuous operation of the reactor, especially for long periods with impaired solar radiation. The hot
air (red line) flows into the reactor from bottom to top and provides heat to the reaction. The warm air (orange line) is
collected at the top of the reactor and used for preheating the reactants and evaporating the feed water. The green lines
depict the mixture of the reactants. Before entering in the reactor, the reactants are preheated to 300 °C using the
residual heat of the air by means of a plate heat exchanger. The temperature of the syngas is set to 900 °C at the end
of the active reactor part. During the backflow through the inner tube, the syngas is cooled down and collected at the

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top of the reactor and can be used for methanol production. After passing the reactor, the heat exchanger and the feed
water evaporator, the cooled air flow is redirected to the receiver to be heated up again.

Figure 2 presents the resulting T-Q diagrams of the two simulated reforming reactor systems. The reactor
configuration without heat recovery in Fig. 2 (a) uses 37.4 MWth i.e. 74 % of the thermal power provided by the solar
receiver for the reforming reaction. 2.8 MWth of the residual power is deployed for reactant preheating and 7.5 MWth
for feed water evaporation. By this, the air (gray line) is cooled from 930 °C down to about 261 °C. The reactants (blue
line) are preheated from 113 °C to 300 °C before entering the reactor. The further trend of the blue line represents the
corresponding reactant temperature profile in the reactor and as the syngas is not recuperated, its outlet temperature
remains at 900 °C.
In contrast, the reactor configuration with heat recovery has the advantage, that the reforming reaction gains
additional heat from the syngas flow, as indicated by the dashed orange line in Fig. 2 (b). Due to the syngas
recuperation inside the reactor, 10.6 MWth of additional heat is available, allowing a higher reactant mass flow
(8.42 kg/s instead of 6.59 kg/s) and consequently higher syngas production by circa 28 %. As a result, for the reactor
with recuperation a thermal power of 47.8 MWth is required. 3.6 MWth of the residual power is deployed for reactant
preheating and 9.7 MWth for feed water evaporation. The syngas cools down from 900 °C to about 451 °C while the
air is cooled down from 930 °C to approximately 220 °C. Nevertheless, the decrease of the air outlet temperature not
only benefits the receiver efficiency but also causes an increase of the thermal input power by 2.8 MWth. In conclusion,
the reactor with recuperation produces 0.016 kg/kJ syngas in comparison to 0.013 kg/kJ without recuperation, which
results in an actual efficiency improvement of 23 %.

1000

800
Temperature [°C]

600

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400
Air mass flow 64.31 kg/s
CH4 mass flow 1.94 kg/s
200 Reactant mass flow 6.59 kg/s
Air Temperature after Evaporation 261.15 °C
0
-10 0 10. 20 30 40 50
Preheating Q Reactant [MW]
Syngas-Reactor Heated only with Air
Evaporation
Reactant Air
(a) (b)
FIGURE 2. T-Q diagrams of the reforming reactor system without heat recovery (a) and with heat recovery (b)

Furthermore, the SIJ has verified the molar composition of the simulated syngas production at 900 °C based on the
results from a model implemented in the chemical process simulation software Aspen. The composition of the syngas
in both process simulations is very similar. The deviation of the molar content varies from -2.3 % to +0.68 %, which
is within an acceptable range (see Table 2).

REACTOR MODELLING AND DESIGN

Based on the previous simulations a reforming reactor with bayonet-tubes promises an effective and novel
alternative for producing syngas using solar energy. To investigate the heat transfer rate and heat transfer mechanisms
taking place in the reaction zone, a one-dimensionally discretized model of a bayonet-tube reactor was implemented
in Modelica®/Dymola. The aim of this simulation is to determine, whether the heat flux through the tube walls from
the air and syngas sides into the reaction fulfills the assumption taken for the previous simulations. Figure 3 shows a
cross and longitudinal sectional view of a single bayonet-tube reactor. The bayonet-tube is based on a coaxial tubes
arrangement (inner and outer tube) inside a shell. The hot air stream (gray arrows) flows in the annular gab between
shell and outer tube, heating up the reactants stream (blue arrow), which is flowing in the opposite direction through

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the outer tube. With help of catalyst pellets, the reaction takes place in the outer tube, where syngas is formed. The
flow-back of the syngas through the inner tube acts as a recuperator before leaving the reforming reactor. The hot air
enters the shell at 930 °C and atmospheric pressure with a mass flow rate of 15 g/s, while the reactants enter the outer
tube at 300 °C and a pressure of 2.5 bar with a mass flow rate of 1.21 g/s.

FIGURE 3. Schematic of a model of a single bayonet-tube arrangement and the flow direction of each fluid

1-D Reactor Modelling

By simulating the reactor model of a single bayonet-tube in Dymola the heat transfer mechanisms by means of the
geometry of the reactor is calculated according to appropriate correlations [3, 4]. The model considers local heat
convection between the different gas streams and the tube wall as well as the conduction in the tube walls. Figure 4
shows the T-Q diagram of a single bayonet-tubes reactor simulated in Dymola. It visualizes the local temperatures of
the reactants, syngas and air stream in relation to the transferred heat into the reaction region. The reactants (blue line)

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enter in the reaction annular gap at 300 °C and leave as syngas at about 906 °C. The syngas stream flowing through
the inner tube is cooled down to about 673 °C whereas the air (gray line) is cooled from 930 °C down to about 635 °C.
1000

800
Temperature [°C]

600

400

Reactants
200 Syngas
Air
0
0 1000 2000 3000
. 4000 5000 6000 7000 8000
Q Reactant [W]
FIGURE 4. T-Q diagram of a single bayonet-tube reactor as result from the simulation in Dymola

In conclusion, the parallel heat transfer from the air and the syngas side balance each other out, so that both
temperature lines come close together. The air still cools down quicker due to the larger heat transfer area. The wide
gap between those two line and the temperature line of the reactant indicate that the reactant mass flow can be higher.
Optimizing the reactant/air mass flow ratio will further increase the efficiency and lower both outlet temperatures,
which will be discussed in future publications.

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 CFD Simulation and Design of the Test Reactor
In the context of designing a first prototype for the bayonet reactor, CFD simulations with ANSYS®Fluent has
been performed to investigate the detailed characteristics of the reaction and the 3-dimensional air/syngas flow through
the reactor. The CFD model consists of a single bayonet-tube reactor, analogue to the investigated design in Dymola
(see Fig. 3). The boundary conditions are set up as no-flux (at shell walls), equal flux (at the fluid-solid interfaces)
and symmetry (at the channel centers). It is assumed that the following reactions are occurring in the reactor (outer
tube):

Steam methane reforming (SMR):

CH4 +H2 O ⇆3H2 +CO ∆H0298 K = +206 KJ/mol (1)

Dry methane reforming (DMR):

CH4 +CO2 ⇆2H2 +2CO ∆H0298 K = +247 KJ/mol (2)

The reaction rate exponentials are assumed to be first order with respect to the reactants in both reactions. Both
are highly endothermic. Therefore, the heating value of the product is greater than the heating value of the reactants
and both reactions are favored by high temperatures, as industrial reforming processes are carried out between 800
and 1000 °C [1].
Moreover, it is assumed that the reactants have been premixed before entering the reactor. The reaction channel
i.e. the annular gap between inner and outer tube is defined to be filled with a porous material (catalyst pellets). The
characteristics of the porous material are chosen based on literature [5]. The cross-section areas of the inner tube and
the annular gap of the bayonet-tube were defined with the constraint that the effective cross-section area through the
gaps between the pellets should be the same as in the inner tube. The minimum width of the annular gap was considered
greater than four times the diameter of a pellet (36 mm). The smaller the inner diameter is, more turbulent the flow

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and more effective is the heat transfer to the reaction-side.
The investigated geometries for the reactor tubes, which are based on a combinations of available commercial
diameter size, that promise efficient reforming reactions based on the CFD simulations, are given in Table 1.
Additionally, the outlet temperature of the air and syngas as well as the temperature of the reactant flow at 2.5 m are
listed.
TABLE 1. Investigated dimensions of the reactor tubes (inner/outer/shell) and resulting temperatures

Air outlet Syngas outlet Reactant

Alternative Nominal diameters Wall thicknesses
temperature temperature temperature @ 2.5 m
[mm] [mm]
[°C] [°C] [°C]

1 33.4 / 114.3 / 168.3 1.65 / 2.11 / 2.77 677.1 545.5 905.9

2 48.3 / 141.3 / 168.3 1.65 / 2.77 / 2.77 594.9 530.2 912.1

3 33.4 / 141.3 / 168.3 1.65 / 2.77 /2 .77 614.9 609.9 925.1

In alternative 1 and 2 the syngas stream is cooled down to 545.5 ºC and 530.2 ºC respectively, while in alternative 3
the syngas outlet temperature reaches about 610 °C. The lowest air outlet temperature is reached in alternative 2 with
594.9 ºC followed by alternative 3 with 614.9 ºC. The CFD simulations of the different geometries of the bayonet-tube
reactor have shown that alternative 3 reaches the highest reaction temperature at 925.1 ºC. Moreover, under equal
operation conditions alternative 3 enables a highly heat transfer inside the reactor boosting the temperature in the outer
tube, which ensures complete reforming within a minimum length of 2.5 m. Figure 5 presents the cross-sectional mean
temperatures of the reactants, syngas and the air stream along the reactor length for alternative 3. The temperature
profile of air (gray line) and syngas (orange line) are almost identical along the entire reactor length, meaning that the
heat transfer and the heat input in the reaction region is very similar from both air-side and syngas-side. This slightly
differs from the result in the Dymola simulation, since the CFD model considers the porous material properties.

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 1000

900

Temperature [°C]
800

700

600

500

400

300
0 0,5 1 1,5 2 2,5 3
Reactor Length [m]

Reactants Air Syngas

FIGURE 5. Temperature profile of the gases along the bayonet-tube reactor for the cross-section of alternative 3

In addition, the reactant mass flow rate is varied between 90 % and 120 % (100 % =1.216 g/s) for Alternative 3
while keeping constant the air mass flow rate at nominal 15 g/s. Figure 6 shows the resulting temperatures in terms of

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the relative mass flow rate. The outlet temperature of the air (gray line) and syngas (orange line) decreases with
increasing reactant mass flow rate. The reactor temperature at 2.5 m (blue line) shows the same tendency with less
sensitivity to the reactant mass flow rate. For the first prototype a higher reactant mass flow will be preferred as the
reaction still reaches a temperature over 920 °C and the air outlet temperature is cooled down further. This results
more favorable for the efficiency of the system.
700 950

Reactor temperarue [°C]

Outlet temperature [°C]

650 900

600 850
Air
Syngas
Reactor @ 2.5 m
550 800
85% 90% 95% 100% 105% 110% 115% 120% 125%
Relative mass flow rate
FIGURE 6. Simulated reactor outlet temperature after varying the nominal reactant mass
flow rate by -10%, +10% and +20%

The molar fraction composition of the produced syngas obtained in the CFD simulation with ANSYS®Fluent and
EBSILON®Professional is compared with the syngas composition determined by the process simulation software
Aspen used commonly for extensive chemical processes. The deviation between the compositions of the syngas is
shown in parenthesis in Table 2. The reactant composition for the reactor inlet consist of CH4=37.6 %, H2O=50.0 %,
CO2=12.4 %.

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 TABLE 2. Molar fraction deviation of the formed syngas at 900 °C compared to a reference simulation in Aspen
Aspen EBSILON
ANSYS
(reference) (with heat recovery)
[%]
[%] [%]
CO2 3.557 6.756 (+89.9 %) 3.542 (-0.4 %)
H2 60.465 68.770 (+13.7%) 60.402 (-0.1%)
H2O(g) 10.870 10.624 (-2.3%) 10.944 (+0.7%)
CH4 0.214 0.090 (-57.9%) 0.209 (-2.3%)
CO 24.880 13.760 (-44.69%) 24.904 (+0.1%)

The syngas composition in ANSYS®Fluent and EBSILON®Professional in comparison to the reference simulation
in Aspen shows molar fraction deviations from -57.9 % to +89.9 % and from -2.3 % to +0.7 % respectively. Due to
the high activation energy and the equilibrium reaction assumptions for the simulations in ANSYS, the CH4 is
consumed completely (0.090%) and a high molar fraction of H2 is produced (68.770 %). The consumption of almost
54.5 % of the CO2 input induces to the half production of CO (13.76 %) in relation to the result from Aspen. The
consumption of H2O is in all three simulations around 21 %. Nevertheless, the composition of the syngas in the process
simulation in EBSILON®Professional and Aspen are very similar.

CONCLUSION AND OUTLOOK

Simulations in EBSILON®Professional have shown that the reforming reactor with syngas heat recovery in
comparison to a reactor without heat recovery allows a higher reactant inlet mass flow rate and enhances the syngas
production by circa 28 % based on the same hot air mass flow. Simulations in Dymola corroborates the heat transfer
assumptions considered in the model in EBSILON®Professional.
It has been shown that a bayonet-tube reactor design provides an efficient alternative for producing syngas, since

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the required heat for the reforming is transferred simultaneously from both air and syngas. A single bayonet-tube with
a reactant/air mass flow ratio of 8.06 % is designed for a thermal power of about 6.79 kW. The reactor model in
Dymola can be used for further analysis and optimization of the mass flow ratio in future works.
The syngas molar composition at 900 °C in ANSYS is strongly determined by the inlet parameter specification set
for the simulation. While the molar composition of the syngas in ANSYS diverts significantly from the reference, the
syngas molar compositions in EBSILON®Professional and Aspen are very similar and deviates only between -2.3 %
to +0.7 %.
Based on CFD Simulations a prototype of a bayonet-tube reactor with a reactor length of 2.5 m will be constructed
and tested at the Synlight, research facility of the German Aerospace Center in Jülich (Germany).

ACKNOWLEDGMENT
The authors gratefully acknowledge the financial support from the LeitmarktAgentur.NRW and the state
government of North Rhine Westphalia

REFERENCES
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Hydrogen and Syngas Production – A Review”, in: Renewable and Sustainable Energy Reviews, Vol. 29,
pp. 656–682. (2014).
2. H. von Storch. “Methanol Production Via Solar Reforming of Methane”, Ph.D. thesis, Rheinisch-Westfälische
Technische Hochschule Aachen (RWTH Aachen University), Aachen, (2016).
3. Gnielinski, V. “Heat Transfer in Pipe Flow”, in VDI-Wärmeatlas, (Springer-Verlag, Berlin Heidelberg, 2013),
pp. 785-791.
4. Gnielinski, V. “Heat Transfer in Concentric Annular and Parrallel Plate Ducts”, in VDI-Wärmeatlas, (Springer-
Verlag, Berlin Heidelberg, 2013), pp. 793-780.
5. M. Wesenberg. “Gas Heated Steam Reformer Modelling”. Ph.D. thesis. Norwegian University of Science and
Technology (NTNU), Department of Chemical Engineering, Trondheim, (2006).

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