(338c) Piston Reactor Capabilities to Drive Endothermic Gas Reactions – a Modeling Study

In the future, it is predicted that electricity derived from renewable sources such as wind, geothermal and solar will become one of the most widespread, cheap, and accessible forms of energy [1]. However, the intermittency of such sources will impose considerable fluctuations in the generation and supply on various timescales [2]. To tackle the aforementioned shortcomings, the concept of energy storage has been increasingly implemented [3, 4]. Amidst the various energy storage routes available, chemical energy storage appears to be the most promising technology to provide long-term high-capacity energy storage [3].

The utilization of an electrical-driven piston reactor is an emerging concept to store electrical energy in the form of chemical bonds [5]. It offers a simple, safe, and compact design to carry out chemical reactions. This reactor concept operates similar to a conventional internal combustion engine (ICE); however, it focuses on producing value-added chemicals instead of power. The piston reactor operates via inducting a feed gas or gas-liquid mixture into the reactor cylinder. Upon compression, high temperature and pressure pulses are attained within very short time scales in the range of milliseconds. The high cylinder temperatures and pressures attained due to compression heating can be used to drive endothermic gas reactions of interest with gas feedstocks such as methane, ethane, and propane. Examples of these reactions are methane steam reforming (SMR), reverse water gas shift (WGS), dry reforming (DRM), methane cracking (MC), and ethane or propane oxidative dehydrogenation or cracking. The piston reactor technology is still at its early stage of development. Most of the reported work on the piston reactor studies methane or natural gas partial oxidation to produce hydrogen or synthesis gas (H₂ and CO) [6, 7]. However, less research has been dedicated to their use to carry out endothermic reactions.

This work aims to numerically investigate the potential of utilizing the piston reactor concept to carry out endothermic reactions. For this purpose, a single zone zero-dimensional time-dependent thermodynamic model is developed. The model incorporates available steady-state kinetic models from the literature to predict the transient kinetic behavior of the piston reactor. The first implemented case study represented the highly endothermic SMR reaction. The kinetics were predicted using the Xu and Froment kinetic model for SMR operating with a Ni/MgOAl₂O₃ catalyst. An H₂O/CH₄ feed ratio of 3.561 was used and adopted from Elnashaie et al. [8] The total volume of the simulated piston reactor was 425 cm³ based on the geometrical parameters reported by R. Hegner et al. [9]. A piston speed of 3000 RPM, intake temperature of 660 K and an intake pressure of 1 bar were used in this case study. The latter conditions are lower than typically used in industry (temperatures > 1100 K and pressures >3-25 bar [10-13]) to study the potential of utilizing compression heating to drive the SMR reactions.

Figure 1.A displays the reaction progress in terms of the mole fractions of chemical species contributing to the reaction and the temperature trace as a function of crank angle for the SMR base case scenario. The CH₄ consumption is relatively slow, leading to only 13% of CH₄ conversion. The maximum attained temperature in the piston reactor was approximately 1011 K, which seems insufficient to allow the SMR reactions to proceed at adequate speeds. High cylinder temperatures are desired to overcome the thermodynamic limitations and achieve high levels of CH₄ conversion. This implies that the thermal energy provided by the compression heating is insufficient on its own. One approach to overcome this limitation is to burn a part of the methane by adding an ignition source for instance oxygen. In the case of SMR, when oxygen is added, methane combustion takes place initially and the heat generated is utilized to drive the subsequent SMR. For a given composition, the reaction could operate under autothermal conditions (ATR) as shown in Figure 1.B. A maximum temperature of 2200 K was achieved in a short timeframe and full combustion products (H₂O and CO₂) and synthesis gas were formed. This resulted in around 94% methane conversion, which provides a 7-fold improvement compared to the SMR base case operated with an oxygen-free mixture. Another alternative approach to attain high cylinder temperatures is to alter the intake mixture's heat capacity through dilution with an inert gas. Argon can be added to decrease the heat capacity of the intake gas mixture and hence increase the temperature during the compression phase [14, 15]. A third approach is to add an external heating source to increase the thermal energy of the gas during the piston operation to drive the endothermic reaction forward. This resembles the idea typically used in conventional reforming reactors, where the required heat for the reaction is continuously supplied via a fired heater to maintain a high reactor temperature to achieve high conversions [8, 14]. The aforementioned approaches to overcome the thermodynamic limitations of the SMR case study will be presented in this work followed by similar studies for other industrially relevant endothermic gas-phase reactions. Full details and finding from these studies will be presented in this work.

Acknowledgments. This work was made possible by funding from Qatar National Research Fund (QNRF) project number NPRP12S-0304-190222 and co-funding by Qatar Shell Research and Technology Center (QSRTC). The statements made herein are solely the responsibility of the author(s).

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