The Catalyst Review April 2024 - 12

Microplasmas and hot-spots due to arcing may accidentally
appear in catalysts heated with microwaves, as hypothesized
by Zhang et al. 2003 and Fidalgo et al. 2008, but the
purposeful use of plasmas solely as a method of catalyst
heating cannot be technical and economically justified.
However, there are examples of integrated two-stage
plasma-catalytic reactors, where the heat generated in the
first, plasma-stage is carried downstream to the second,
catalytic stage. For instance, Deliconstantis et al. 2018
proposed a two-stage reactor for methane conversion
to ethylene, where acetylene formed by the plasma
cracking of methane was hydrogenated to ethylene in an
integrated fixed bed of a catalyst (Figure 3). Lian et al. 2019
took advantage of the energy from plasma in methanol
reforming, employing warm plasma followed by a Fe-Cu
catalyst. Under the combined action of steam reforming and
WGS, the methanol conversion nearly doubled, compared
to a plasma-only process.
Figure 3. Two-stage plasma-catalytic reactor for methane conversion to
ethylene proposed by Delikonstantis, et al. 2018.
Induction heating exploits an alternating electromagnetic field to create eddy currents inside an electrically conductive material, which is
heated by the Joule effect. An induction coil, typically wound around the heated object, is connected to a high-frequency, high voltage
generator. In ferromagnetic materials, such as iron, heat is also generated by magnetic hysteresis losses. Induction heating has been
proposed and demonstrated for several endothermic heterogeneous catalytic processes (Fireteanu et al. 2005; Chatterjee et al. 2015;
Bordet et al. 2016). However, catalysts susceptible to induction by hysteresis heating require ferromagnetic components with a Curie
temperature (i.e., the temperature above which those materials lose their permanent magnetic properties) compatible with the reaction
temperature: only iron, cobalt, nickel, as well as their alloys, are ferromagnetic above room temperature. This remains one of the biggest
limitations for direct induction heating of catalysts. One solution is that of mixing the catalysts with other ferromagnetic components,
which function as induction susceptors and afterwards transfer the thermal energy to the catalytic sites (Mortensen et al. 2017; Vinum et
al. 2021).
Concerning the reactor design, quartz or glass can be used as reactor materials at the lab scale, so that the ferromagnetic catalyst
particles inside the reactor are directly heated by induction heating. As an alternative, a stainless-steel tubular reactor can also be
adopted: in this case the reactor tube is directly heated by induction heating, driving the catalytic reaction on the catalyst inside the
reactor tubes. During energy transfer to the target materials, energy losses occur via various pathways. Even though such losses can
be minimized by improving the design of the reactor, e.g., by using high radio frequency and long / narrow coils, the energy efficiency
reported so far for induction heating of catalytic processes remains low (below 25%). According to theoretical estimation by Almind et al.
2020, however, energy efficiencies up to 80% could be expected upon upscaling.
In Joule heating, also known as resistive or ohmic heating, electric energy is transformed into thermal energy when an electric current
flows across an electrical conductor due to a voltage difference. The electric field accelerates charge carriers: when the charged particles
collide with ions in the conductor, kinetic energy is transformed into thermal energy. Since all the electric power can be converted to heat,
an energy efficiency of 100% is theoretically attainable (Dincer 2018).
The general concept of Joule heating of catalytic
reactors has been implemented using a variety of
materials, including metals, electrically conductive
ceramics and carbon-based materials. The
Joule heated substrate needs to be electrically
continuous, therefore the direct Joule heating
of packed beds of catalyst pellets is incredibly
challenging since it is difficult to control the contact
area of conductive particles (Lu et al. 2021). On
the other hand, the Joule heating concept is well
applicable to structured catalysts and reactors
(Kapteijn and Moulijn 2022), where the heating
elements can be catalyst coated reactor walls
(Wismann et al. 2019), resistive heaters (Lu et
al. 2022; Renda et al. 2022), open-cell foams
(Badakhsh et al. 2021; Zheng et al. 2022, 2023)
or heating wires inserted into modular reactors
or honeycomb catalysts (Balakotaiah et al. 2022;
Pauletto 2021).
Figure 4 Joule heated SiC open cell foam in flowing N2
, quartz reactor.
a) room temperature, b) 700°C. Solution tested at Politecnico di Milano
for the electrification of methane steam and dry reforming, reverse
water gas shift and ammonia cracking Source: Zheng et al. 2023c.
12
The Catalyst Review
April 2024

The Catalyst Review April 2024

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