Improving performance of induction-heated steam methane reforming
Graphical abstract
Introduction
Today, a range of methods exists for delivering energy to endothermic chemical catalytic reactions. One such is pre-heating coupled with an adiabatic reactor, where the reactant is heated prior to entering the reactor and the temperature decreases towards the thermodynamic equilibrium temperature of the given reaction in the catalyst bed [1]. Other methods include heat-exchanging technologies, where a hot fluid is in heat exchange contact with the reaction zone of a reactor to provide energy for the reaction [2,3]. For strongly endothermic processes requiring high reaction temperatures, fired reactors are a preferred solution. In this case, burners are used to heat up the catalytic reactor to the desired reaction temperature. These are used extensively for steam methane reforming [4].
Almost half of the world’s hydrogen demand is produced through the steam methane reforming reaction [5], where a mix of steam and methane is converted into carbon monoxide and hydrogen by Reaction (1) [4,6,7]. This is accompanied by the reverse water gas shift in Reaction (2) [4,6,7]:
Typically, industrial steam methane reforming reactors operate around 25 bar, which requires temperatures of 800-950 °C to obtain sufficient conversion of the natural gas feedstock. In industrial reactors, this heating is typically done in a tubular reformer configuration, which includes the reformer furnace, and a waste-heat section, in addition to several reformer tubes placed centrally in the furnace [4]. These tubes are usually 100-200 mm in diameter, and 10-13 m long. This geometry is used to optimize the heat transfer from the reformer burners [4]. However, only around 50% of the heat supplied by the reformer burners is transferred to the reforming reaction in the catalyst bed, the rest of the heat is recovered in the waste-heat section and used for preheating and steam production [4]. As a consequence, the technology relies on economy of scale to make tubular reformers energy efficient [4]. Additionally, in order to allow for controlled material expansion, the reformer tubes are heated slowly to the reaction temperature, and hence start-up times are long, often on the order of days.
Induction heating offers an alternative route to supply energy to the steam methane reforming reaction that may solve some of the challenges of tubular reformers. Induction-heated steam methane reforming will require a coil outside the catalyst bed, while an inductor material (a magnetic material) is incorporated into the reactor [8]. When passing an alternating current through the coil, it generates an alternating magnetic field, which enables local heating of the catalytic material inside the reformer, either by magnetic hysteresis heating (dominating in nanoparticles) [9] or by resistive heating due to eddy currents (dominating in macroscopic conducting materials), and efficient heat transfer to the reaction zone is possible. The direct heating would allow for high reaction rates in the catalyst material, and by extension, more compact reformer plants. Induction heating could also have the added benefit of lowering the start-up time, as induction heating, in general, provides a fast and more homogeneous heating response [10,11]. Lowering start-up times would also make the induction-heated steam methane reformers more flexible, allowing for on demand production of hydrogen.
Induction heating (by hysteresis) of nanoparticles has been studied extensively as a way to treat cancer (magnetic hyperthermia) [12], but efforts were also made to do hysteresis heated catalysis [9]. Previously, induction (hysteresis) heating of magnetic particles in chemical reactors was pioneered by Ceylan et al. [13] on liquid organic chemical synthesis using modifiable silica-coated iron oxide particles as the heatable media. Chatterjee et al. [14] applied induction heating for fast and isothermal heating of a micro-reactor to 80-100 °C, using nickel-ferrite material as susceptor, and Bordet et al. [15] demonstrated induction heating for continuous CO2 hydrogenation using iron carbide nanoparticles.
Within this topic, our group has demonstrated steam methane reforming on CoNi nanoparticles [16,17], which are both catalytically active for induction-heated steam methane reforming and can be heated to above 800 °C using only induction heating. In this configuration it is desirable to maximize the hysteresis heating of the nanoparticles, while also ensuring high catalytic activity, which must be secured in the synthesis of the catalyst [17]. The previous work showed that the direct delivery of heat to the active site lead to efficient transfer of energy to the chemical reaction [17]. Consequently, the catalyst activity was identified as the limiting factor in the reaction test [17]. In contrast, industrial scale tubular reformers are typically limited by heat transfer rates [4].
To utilize induction heating for industrial scale steam methane reforming, it is important to address the energy required to make an alternating magnetic field, which is yet unaddressed, but the topic of this article. We focus on optimizing the performance of induction-heated steam methane reforming by manipulating two key parameters: the frequency of the alternating magnetic field in the coil and the coil geometry. We also establish a simple theoretical framework to explain general trends in the performance as a function of these parameters, with the aim of using the framework to study the prospective performance of induction-heated steam methane reforming at larger scale.
Section snippets
Material and experimental methods
The catalytic material used for this study consisted of CoNi nanoparticles on a porous samarium aluminium oxide support (CoNi/Sm2O3-Al2O3). The nanoparticles of this system act both as catalyst for steam methane reforming (due to the Ni) and as susceptor for magnetic hysteresis heating (due to the ferromagnetic properties of the metallic CoNi nanoparticles). The sample was synthesized as described by Vinum et al. [17], and then reduced in H2 at 850 °C for 4 h, followed by passivation in 1% O2
Modelling
The power losses in the induction-heated reactor system can be modelled aswhere Pin is the power delivered to the power supply and equals the total power loss in the system. The power loss Preaction is the power consumed by the steam reforming reaction (Eqs. (1) and (2)) and the heating of the gas to the required temperature, Pcoil is the loss due to resistive heating of the copper coil, Piso is the energy loss through the insulation, and Prest is any remaining
Hysteresis heating
Initially the catalyst’s magnetic properties were investigated to verify its applicability for induction-heated steam reforming. The mass-specific magnetization of the catalyst was measured as a function of applied field and temperature in the VSM. These data were used to calculate the hysteresis area as a function of magnetic field and temperature. Fig. 2 shows the hysteresis area (Ahys) of the sample as function of magnetic field amplitude (Bmax) and temperature. The data show an almost
Conclusions
We studied the performance increase of induction-heated steam methane reforming introduced by changing the frequency and geometry of the induction coil. Specifically by using a narrower coil with less insulation around the sample and 3-fold higher frequency, we approximately doubled the maximum energy transfer efficiency from 11% to 23%. This increase has been attributed to a higher hysteresis input into the magnetic material by increasing the frequency, and a lower resistive loss in the coil
Acknowledgments
This work was funded by Innovation Fund Denmark (IFD) under File No. 5160-00004B.
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