Piston engines are typically considered devices converting chemical energy into mechanical power via internal combustion. But more generally, their ability to provide high-pressure and high-temperature conditions for a limited time means they can be used as chemical reactors where reactions are initiated by compression heating and subsequently quenched by gas expansion. Thus, piston engines could be "polygeneration" reactors that can flexibly change from power generation to chemical synthesis, and even to chemical-energy storage. This may help mitigating one of the main challenges of future energy systemsaccommodating fluctuations in electricity supply and demand. Investments in devices for grid stabilization could be more economical if they have a second use.This paper presents a systematic approach to polygeneration9 in piston engines, combining thermodynamics, kinetics, numerical optimization, engineering, and thermo-economics. A focus is on the fuel-rich conversion of methane as a fuel that is considered important for the foreseeable future. Starting from thermodynamic theory and kinetic modeling, promising systems are selected. Mathematical optimization and an array of experimental kinetic investigations are used for model improvement and development. To evaluate technical feasibility, experiments are then performed in both a single-stroke rapid compression machine and a reciprocating engine. In both cases, chemical conversion is initiated by homogeneous-charge compression-ignition. A thermodynamic and thermo-economic assessment of the results is positive. Examples that illustrate how the piston engine can be used in polygeneration processes to convert methane to higher-value chemicals or to take up carbon dioxide are presented. Open issues for future research are addressed.
The production of chemical energy carriers utilizing electrical energy from renewable sources is essential for the future energy system. A motored piston engine may be used as a reactor to convert mechanical to chemical energy by the pyrolysis of methane and ethane; this is analyzed here. The piston engine is modeled as a compression–expansion cycle with detailed chemical kinetics. The main products are hydrogen and high‐energy hydrocarbons such as acetylene, ethylene, and benzene. To reach the required high temperatures for conversion after compression, the educt is diluted with argon. The influence of the operating conditions (temperature, pressure, dilution) on the product gas composition, the stored exergy, and the ratio of exergy gain to work input (efficiency) is investigated. A conversion of >80% is predicted for an argon dilution of 93 mol% at inlet temperatures of 573 K (methane) and 473 K (ethane), respectively. A storage power of 7.5 kW (methane) and 6 kW (ethane) for a 400 ccm four‐stroke single‐cylinder is predicted with an efficiency of 75% (methane) and 70% (ethane), respectively. Conditions are identified, where high yields of the target species are achieved, and soot formation can be avoided.
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