(529c) Insights into Gas-Phase Methane Pyrolysis for Hydrogen Production and Carbon Capture

Among technological processes in chemical industry for reducing greenhouse gas emissions, methane pyrolysis for large-scale hydrogen (H₂) production from methane (CH₄) is considered an auspicious alternative approach to water electrolysis [1, 2]. Herein, gaseous CH₄ originating from natural gas or biogas is converted into gaseous H₂ and solid carbon at high temperatures. Particularly the use of renewable biogas as feed and sustainable energy from wind and solar for setup operation would allow for achieving a net-zero emission process or even negative emissions, making methane pyrolysis a process of high relevance in both academia and industry. Despite its great potential as a CO₂-free and low-energy H₂ production route, no commercial large-scale methane pyrolysis processes have been realized so far. In addition to political aspects, primarily economic considerations and technical challenges require research and development: Efficient energy supply at high temperatures, use of materials that are suitable for high temperatures, handling of carbon deposits forming during the process and utilization of accruing carbon are only few facets that come into play in the context of methane pyrolysis. After shining a light on the boundary conditions that determine a successful development and implementation at industrial scale, the present contribution focuses on gas-phase methane pyrolysis in order to provide an overview on process parameters governing high methane conversion and hydrogen selectivity as well as on the underlying detailed chemistry.

Materials and Methods

By variation of the inlet feed gas mixture, the temperature of operation and the pressure, a systematic experimental measurement campaign was conducted using a continuous flow high-temperature, high-pressure alumina reactor (hot zone: 0.4 m) that was developed in-house. A mass spectrometer provides on-line information on the composition on the effluent product gas stream concentrations, which serve as a basis for numerical simulations with the DETCHEM^PFR reactor code. Assuming steady-state and isothermal conditions, numerical simulations were performed using a detailed gas-phase reaction mechanism by Appel et al. comprising a set of pyrolysis reactions of C1 and C2 species [3]. Varying the temperature between 1000 and 1600 °C, the H₂/CH₄ molar ratio in the feed between 1 and 4, the residence time between 3 and 7 s and the pressure between 1 and 8 bar provides information of the impact of the different process parameters on the pyrolysis reaction by means of both, experiments and simulations.

Results and Discussion

The experiments in our in-house developed continuous flow high-temperature, high-pressure alumina reactor coupled with a mass spectrometer revealed that gas phase methane pyrolysis takes place for temperatures above 1000 °C if using CH₄ and H₂ as feed gases. Increasing temperatures and a lower H₂/CH₄ molar ratio in the gas feed benefit CH₄ conversion and high H₂ yield (Fig. 1a). Low H₂/CH₄ feed gas ratios promote CH₄ conversion particularly between 1000 °C and 1300 °C, whereas a further temperature increase results in almost complete CH₄ conversion, irrespective of the reaction conditions. Equally important, higher residence times facilitate high H₂ yields. Among the byproducts, the numerical simulations suggest C₂H₂, C₂H₄, C₂H₆ and C₆H₆ as most important species and point to their prominent role during carbon formation in the gas phase. Although the experimental data support these findings, it becomes also clear that such intermediate gas-phase species can only survive at moderate temperatures (Fig. 1b). An increase of the pressure seems to be a suitable approach for suppressing byproduct formation, however, at the cost of lower CH₄ conversion rates.

While simulations and experiments are in good agreement at less severe conditions (i.e. low temperature and high H₂/CH₄ ratio), an almost inverse trend is observed for severe conditions. This behavior can be explained by the carbon that forms and deposits in-situ during the course of the experiment. Due to adsorption and desorption processes on the carbon layers formed, the presence of carbon significantly influences the gas-phase concentrations [4]. Future research will aim at linking the type of carbon formed and the corresponding reaction conditions, which will not only provide additional mechanistic insights, but which will also allow for optimizing the process parameters in order to produce well-defined carbon that can be utilized in industry.

References

[1] Ashik, U. P. M., Wan Daud, W. M. A., Abbas, H. F, Renewable Sustainable Energy Rev., 44 (2015) 221–256.

[2] Machhammer, O., Bode, A., Hormuth, W. Chem. Eng. Technol., 39 (2016) 1185-1193.

[3] Appel, J., Bockhorn, H., Frenklach, M., Combust. Flame, 121 (2000) 122-136.

[4] Li, A., Deutschmann, O., Chem. Eng. Sci. 62 (2007) 4976-4982.

Figure 1: a) Comparison of experiment and pure gas-phase kinetic simulations for gas-phase methane pyrolysis. b) Experimental concentration profile of gas-phase species for H₂/CH₄ = 2, 1 bar, Ï„ = 3 s.