Integrated P2G process
The overall objective of the project HELMETH, which is co-funded by the European Commission in the FP7 framework for the Fuel Cells and Hydrogen Joint Technology Initiative, is to provide “proof of concept” for a novel electricity storage process based on Power-to-Gas (PtG) technology. The proposed concept (Figure 1) targets at increasing the efficiency of the PtG process via the combination of pressurized high temperature steam electrolysis with carbon dioxide methanation.
High temperature electrolysis in the HELMETH process is carried out with the use of Solid Oxide Electrolyzer Cells (SOECs) and is thermally coupled with the exothermic carbon dioxide methanation via the steam produced by the later:
|CO2 + 4H2 ↔ CH4 + 2H2O
|ΔrH298 = -165 kJ/mol
Compared with common PtG plants that use low temperature (LT) electrolysis systems based either on Alkaline or Proton Exchange Membrane (PEM) technologies, the HELMETH concept is expected to improve drastically the efficiency of the PtG plant from approx. 61% up to > 85% (Figure 2). It should be pointed out that both the SOEC module as well as the complete HELMETH PtG plant constitute first prototypes the functionality of which, is going to be demonstrated by the research and development activities that take place within the project.
The novel pressurized high temperature SOEC module is shown in Figure 3. A detailed explanation about the working principle of high temperature electrolysis can be found in the SOEC section.
Key performance data for the high temperature electrolysis module are:
- Operation temperature: 800°C
- Current densities > 1 A/cm2
- Investigation of the co-electrolysis mode (simultaneous feed of steam and CO2)
As far as the second key process is concerned, the methanation, this is carried out with the use of two reactors (Figure 4) in order to achieve a synthetic natural gas (SNG) quality that fulfils existing and future regulations for the natural gas grid. As described in the methanation process section, the gas quality is dependent on reaction temperature and pressure. Since the CO2-methanation reaction is favoured at high pressures, an operation of up to 30 bar pressure is going to be realized. Controlling the temperature of the reaction is a major challenge and can only be achieved with proper reactor design for efficient heat removal. Therefore, the methanation module is designed to operate at near isothermal conditions with boiling water as cooling medium for highly intensive heat removal. Two separate reactors with a condensation stage in between are linked by the boiling water cooling system. The condensation stage separates the water product from methanation and shifts the chemical equilibrium in the second reactor towards higher CH4-yields. If the process was carried out in one isothermal reactor operating at 300°C, the quality of the produced SNG would be marginal. In order to achieve high efficiencies and to maintain high gas quality, a sophisticated heat exchanger network is used for recuperating most of the reaction heat.
Key performance data of the methanation module are:
- Pressurized operation at up to 30 bar
- Produced SNG quality that fulfils (current) and future natural gas grid quality standards. Especially low hydrogen contents
- Multistep module with condensation of product water between
- Steady steam production
- Load modulation from 20 - 100 %
As earlier mentioned the methanation module is thermally integrated with the SOEC module via the steam produced during methanation. A pressurized operation of both modules reduces the compression power required to feed the produced methane into the natural gas grid. The most efficient way to pressurize the system is before steam generation via the water pump. The planed pressure level of 30 bar is a compromise between technical feasibility, process efficiency and integration into the existing natural gas grid. Finally, these two features, the thermal integration of the two modules and the pressurized operation lead to the increased PtG efficiency of HELMETH compared to state of the art technology (Figure 2).
Efficiency of the HELMETH PtG prototype:
→ 76 % with the potential for 80 % in industrial scale
Definition of PtG efficiency:
Experimental test results as input for the efficiency calculation:
- Reactant conversion in CO2-methanation module: X ≥ 98.958 %
- Energy consumption of SOEC: 3.37 kWh/m3 (H2, NTP)
- Steam conversion SOEC: SC = 70 %
- From CO2-methanation module produced steam amount: as measured
- Energy consumption of remaining steam production (Twater,in to Tevap)
1) Trimis, D., Anger, S., Potenzial der thermisch integrierten Hochtemperaturelektrolyse und Methanisierung für die Energiespeicherung durch Power-to-Gas (PtG), gwf-Gas; Erdgas, Januar/Februar 2014, pp. 50–59