RCM studies on CO2 utilization by dry reforming
Session chaired by Dr. Ruben Van de Vijver
The conversion of carbon dioxide (CO2) and methane (CH4) to syngas under internal combustion engine (ICE) conditions is studied in a Rapid Compression Machine (RCM). The conversion (dry reforming) according to the overall reaction CO2 + CH4 ↔2 CO + 2 H2 has to overcome large activation energies, since CO2 and CH4 are quite stable species at ambient conditions. We present a methodology that allows reaching high temperatures in an ICE environment, and thus, to overcome the activation energy. A key factor is the selection of a suitable initial mixture composition, which can be determined by an optimization procedure, involving numerical simulations. Experimental studies in an RCM show CO2 conversion levels of up to 50 %. Thermodynamic analysis shows that this operation, besides converting CO2 and producing syngas, can also deliver a net output of mechanical work. Experiments with CO2/CH4 mixtures are conducted in an RCM, which is a piston-cylinder device that rapidly compresses a gas mixture in a near adiabatic fashion. At top dead center, the piston is stopped, granting isochoric conditions for the cylinder load. The operation is therefore similar to that of a piston engine; however, the RCM offers better defined initial conditions with respect to temperature, mixture composition and flow field. To initiate reaction among the quite unreactive species CO2 and CH4, several measures are taken, including dilution by Argon to increase the heat capacity ratio of the mixture, and adding small amounts of DME as a reaction enhancer and oxygen to allowing exothermal reactions to run in parallel to the CO2 dry reforming process. The achievable conversion depends on the initial conditions, and in particular, on the initial composition. Without addition of oxidizer, the temperature levels remain too low for significant conversion. Too much oxidizer, while providing sufficiently high temperatures for reaction, will cause some of the formed CO and H2 to be oxidized to CO2 and water (H2O), thus diminishing conversion. In previous work [1] we show an optimization of this conversion processes with the aim to maximize the CO2 conversion C, defined as C= 1 − n1/n0 = 1 − m1/m0. Here, n denotes the amount of substance and m the mass in the system for the species CO2. Indices 0 and 1 refer to the state before and after reaction, respectively. In an attempt to maximize the conversion, an optimization procedure was applied which involves joint experiment and numerical simulations. The optimization treated the initial mixture composition as an independent variable, and the conversion C (for given compression ratio and initial temperature and pressure) was the objective function. CO2 mole fractions in the initial mixture were varied between 10 mol-% to 50 mol-% in the total mixture, which included also CH4, Ar, O2, and DME. We found conversions up to C= 51% with an initial CO2 mole fraction XCO2,0 ≈ 20 mol−% and an overall initial fuel-air equivalence ratio (including CH4, DME and O2) of ϕ = 2.8. Both, the initial CO2 mole fractions and the conversions of CO2 C of the optimized mixtures, agreed well in the simulation and in experiment. Analysis of the process shows that the initiation of the reaction is kinetically controlled. For high temperatures (exceeding about 1500 K), activation energies can be overcome, and the system approaches chemical equilibrium within the time scales of an ICE compression cycle (some milliseconds). At the compressed state with high temperatures, the equilibrium composition is shifted towards syngas production and thus, CO2 will be converted. At expanded conditions (low pressure, low temperature), equilibrium composition is on the side of CO2 and CH4, and therefore unfavorable. If the gas is allowed to attain equilibrium states at any time, any cooling of the reacted mixture would then lead to a back-formation to CO2 and CH4. However, the system can be hindered forming again CO2 and CH4 from syngas by reaching the low-temperature equilibrium states by cooling down quicker than CO2 will be formed. This can be achieved by heat losses and/or volume expansion. This essentially exploits that also the backwards reactions are subject to strongly temperature dependent reaction speeds, and cannot proceed at low temperatures. Since the amount of CO2 decreases from the initial state to the state after reaction/combustion, this process essentially converts CO2, and could be used in CO2 removal and negative emission technologies Acknowledgments Financial support by the Deutsche Forschungsgemeinschaft within the framework of the DFG research group FOR 1993 “Multi-functional conversion of chemical species and energy” (No. SCHI647/3-2) is gratefully acknowledged. References [1] H. Gossler, S. Drost, S. Porras, R. Schießl, U. Maas, and O. Deutschmann. The internal combustion engine as a CO2 reformer. Combustion and Flame, 207, 2019
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