Methanation is a chemical reaction that converts carbon monoxide and/or carbon dioxide to methane. The production of methane across the “Sabatier” reaction (1) is a well-known process for converting CO2 to a useful product and was proposed by Paul Sabatier and J.B. Sendersens in 1902. A large-scale production of methane on the basis of carbon dioxide has never been widely established and on basis of carbon monoxide just in a few plants. This is because of the up to date good availability and economy of natural gas. With the new development of the Power to Gas technology for an energy-efficient storage solution of renewable electricity, methanation gains a lot of importance.
|CO2 + 4H2 ↔ CH4 + 2H2O||ΔrH298 = -165 kJ/mol||(1)|
The reaction is thermodynamically favored at relatively low temperatures and high pressures and has been studied extensively for many years1,2,3,4,5, even if with purposes and under conditions very different from the present interest. Carbon monoxide (CO) conversion to methane is described by:
|CO + 3H2 ↔ CH4 + H2O||ΔrH298 = -206 kJ/mol||(2)|
that is namely the reverse Steam Reforming reaction or also called CO-methanation. In effect, the Sabatier reaction can be seen as the sum of the CO-methanation with the reverse WGS (Water Gas Shift reaction):
|CO2 + H2 ↔ CO + H2O||ΔrH298 = 41 kJ/mol||(3)|
The equilibrium concentration of the methanation reactions at 1 and 30 bar are shown in Figure 1 and Figure 2
Figure 1: Equilibrium concentration of a stoichiometric reactants mixture at 1 bar pressure
Figure 2: Equilibrium concentration of a stoichiometric reactants mixture at 30 bar pressure
Methanation is nowadays revamped in a lot of projects of syngas and carbon dioxide valorization. However, for CO2-concentrated feed (with no dilution, the stoichiometric feed implies 20 % vol of CO2 and 80 % vol. of H2) the thermodynamic limitations appear to be very strong. The reaction (1) is highly exothermal and the adiabatic temperature raise connected to the reaction progress is quite high. For example the adiabatic equilibrium temperature of a stoichiometric CO2 + 4H2 feed at 30 bar and 25°C would be 724°C. So the greatest challenge involved in methanation is the temperature control of the exothermic reactions, meaning an efficient heat removal, which is closely linked to reactor design.
The adiabatic fixed bed reactor represents the simplest reactor design option. The reactor is filled with catalytic pellets, and rather than being cooled its heat is instead used to increase the gas temperature. With that increased temperature the reaction rate is very high and therefore requires a relative small amount of catalyst. As temperature increases the chemical equilibrium of the methane formation process shifts towards the reactants and the reaction comes to an equilibrium state at high temperatures which is not optimal for the CO2 conversion depending on the prevailing pressure. However the adiabatic temperature might need to be limited to prevent catalyst destruction through thermal sintering. To limit the temperature raise, the reactor feed gas can be diluted. Either with inert gas, a surplus of one reactant or product gas recycle. For example reactor outlet gas can be recycled and shift the adiabatic temperature raise towards lower temperatures. The relationship between the CO2 conversion, pressure and temperature is shown in Figure 3.
Figure 3: Relationship between the CO2 conversion rate, pressure and temperature during methanation of a stoichiometric reactants mixture (including RWGS reaction)
As CO2 conversion rates are relatively low at high temperatures (approaching equilibrium, reverse reactions (1)-(3) become faster), a number of adiabatic reactors connected in series are necessary to reach the target conversion. The gas is subjected to intermediate cooling prior to each catalytic reactor with the advantage of using simple fixed bed reactors that presents very low complexity and easy constructional design thanks to the adiabatic conditions adopted. On the other hand, the basic limit of a series of adiabatic reactor with intermediate cooling is that a high number of stages are needed to achieve a satisfactorily conversion; when too many reactors are necessary, such a solution becomes complex and expensive, while using few in-series stages, the limitation to conversion could be important.
The opposite solution for such a classical problem of exothermal reactions engineering is the design of a cooled reactor setup (isothermal operation). In such a way, the highest methane conversion would be virtually reached. One suitable option comes in the form of catalyst-filled pipe bundles surrounded by a circulating cooling medium in order to carry off heat. The use of boiling water as a cooling medium is widespread (e.g. in Fischer-Tropsch reactors). Boiling water offers the advantages of highly intensive heat removal and therefore good isothermal conditions in the reactor tube, which in turn facilitate hot spot avoidance and a high level of control over cooling performance. One key disadvantage of using boiling water for the methanation process is the high boiling water pressure required, which is dependent on the reaction temperature targeted as shown in Figure 3.
Figure 4: Boilingpoint of water
Even more disadvantageous is the fact that an isothermal reactor does not provide high reaction rates for the CO2-methanation also it would lead to the highest CO2 conversion at equilibrium. The lower reactor temperatures lower the reaction rate with consequent increase of catalysts load, and reactor size in addition to the severely increased complexity of the inter-cooled system.
A reactor concept with more than one reactor gives the chance to remove reaction water and therefore shift the chemical equilibrium towards the products side.
Within the HELMETH project two promising approaches
- two near isothermal reactors
- adiabatic reactor with recirculation and near isothermal reactor afterwards
are going to be evaluated. Further requirements for the methanation process are the ability to produce enough pressurized steam for the electrolysis module and also to fulfill the required methane quality criteria.
Key characteristics of the final HELMETH CO2-methanation module:
- Multistep module with product water condensation
- Operating gas pressure: 10 - 30 bar
- Boiling water cooling: up to 300 °C (~ 87 bar)
- Continuous steam generation for input to SOEC module
- SNG output: 12 - 60+* kWHHV,CH4 (1.08 - 5.42 m3/h CH4, NTP)
- Modulation: 20 - 100+* %
- Final SNG-composition
- CH4: > 97 vol.-%
- H2: < 2 vol.-%
*The maximum throughput of the module was limited to the BoP (Balance of Plant) equipment size. Experimental results suggest a higher possible output.
1) Fujita, S., Nakamura, M., Doi, T., Takezawa, N., 1993. Mechanisms of methanation of carbon dioxide and carbon monoxide over nickel/alumina catalysts, Applied Catalysis, A: General 104, pp. 87–100, 1993.
2) Fujita, S., Terunuma, H., Nakamura, M., Takezawa, N., Mechanisms of methanation of CO and CO2 over Ni, Industrial & Engineering Chemistry Research 30, pp. 1146–1151, 1991.
3) Kopyscinski, J., Production of synthetic natural gas in a fluidized bed reactor, Dissertation ETH Zurich, 2010.
4) Gao, J., Wang Y., Ping, Y., et al., A thermodynamic analysis of methanation reactions of carbon oxides for the production of synthetic natural gas, RSC Advances 2, pp. 2358-2368, 2012.
5) Wang, W., Wang, S., Ma, X., et al., Recent advances in catalytic hydrogenation of carbon dioxide, Chem. Soc. Rev., 40, pp. 3703-3727, 2011.