(36d) Utilizing Piston Reactor for Chemical Energy Storage from Renewable Energy

Energy storage solutions are increasingly being utilized to address the substantial variations in energy generation and supply caused by the inherent intermittency of renewable sources [1]. Among these, chemical energy storage shows particular promise for long-term, high-capacity needs [2]. The primary methods explored for transforming renewable electricity into chemical energy include electrochemical and electro-thermal techniques [1]. An emerging yet relatively unexplored approach is the electro-mechanical pathway. The electrically-driven piston reactor introduces a novel approach for storing electrical energy as chemical bonds [3]. This system combines simplicity, safety, and a compact design, operating similarly to an internal combustion engine but aiming to produce valuable chemicals instead of emitting waste. The process involves introducing a feed gas or gas-liquid mixture into a cylinder, where a piston driven by electricity provides compression. This compression generates extremely high temperatures and pressures within milliseconds, allowing endothermic reactions with gases such as but not limited to methane, ethane, and propane. Despite its promise, piston reactor technology is still in its early stages, with current studies primarily examining methane or natural gas partial oxidation to produce hydrogen or synthesis gas (H₂ and CO) [4]. Research into its application for broader endothermic reactions remains limited.

This work to aims explore the potential of the piston reactor for endothermic reactions using a thermodynamic model. The model incorporates gas-phase mechanistic models from literature to predict transient behavior. The initial case study focuses on highly endothermic methane steam reforming (SMR) and propane pyrolysis. A base-case scenario of a 400-cc piston reactor (with a compression ratio of 20) operating under SMR conditions is shown in Figure 1(a). A minimum intake temperature of 1100 K was required to initiate the SMR reaction, resulting in only 12% CH₄ conversion. In contrast, adding oxygen to the feed mixture to initiate methane oxidation and drive the highly endothermic SMR reaction under autothermal conditions has proven to be an attractive option for hydrogen production, as shown in Figure 1(b). The piston reactor achieved a methane conversion of 89%, operating in the gas phase without the need for a heterogeneous catalyst. This was achieved with a lower intake temperature of 673 K and an intake pressure of 1 bar, compared to the higher temperatures and pressures typical of conventional reactors. Based on techno-economic analysis, the process of producing hydrogen from methane by coupling the SMR reaction with methane oxidation demonstrated favorable economics, even at a production capacity as low as 25 tons/day of hydrogen. A similar approach was demonstrated for propane pyrolysis, as shown in Figure 2(a). With a high intake temperature of 950 K, propane consumption remained relatively low, resulting in only 2% of propane converted per cycle. However, the addition of triggers, such as oxygen, yielded promising results leading to conversions exceeding 90% as shown in Figure 2(b). A diverse range of products are generated, including hydrogen, carbon monoxide, ethylene, and propylene. The solution space explored was then evaluated to identify economically viable regions, considering the economic value of the generated product mixtures at an early stage.

This study represents a significant advancement in the development of piston reactor technology for chemical energy storage solutions. It underlines the importance of a systematic model-driven approach to investigate the potential and limitations of carrying out endothermic reactions in the piston reactor, identify promising candidates, complemented by an economic value analysis that should be considered when assessing traditional reactor performance indicators, such as conversion, yield, and selectivity.

Figure 1. Evolution of species and in-cylinder temperature as a function of crank angle for (A) case of SMR at N = 3000 RPM, T_intake = 1100 K, P_intake = 1 bar, H₂O/CH₄ = 3.561, (B) case of SMR with oxygen as a trigger at N = 3000 RPM, T_intake = 673 K, P_intake = 1 bar, H₂O/CH₄ = 0.75, O₂/CH₄ = 0.6

Figure 2. Evolution of species and in-cylinder temperature as a function of crank angle for (A) propane alone at N = 3000 RPM, T_intake = 950 K, P_intake = 1 bar, (B) case of propane with oxygen as a trigger at N = 3000 RPM, T_intake = 773 K, P_intake = 1 bar, O₂/C₃H₈ = 0-2, O₂/C₃H₈ = 0-2

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).

References:

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