545934 Low-Temperature CO2 Methanation Catalyst for Power-to-Gas

Wednesday, June 5, 2019: 2:57 PM
Republic ABC (Grand Hyatt San Antonio)
Audrey Waldvogel1, Sébastien Thomas2 and Anne Cecile Roger2, (1)ICPEES, Strasbourg, France, (2)ICPEES, Université de Strasbourg, Strasbourg, France

Low-temperature CO2 methanation catalyst for Power-To-Gas

Audrey Waldvogel, Sébastien Thomas and Anne-Cécile Roger

Institut de Chimie et Procédés pour l’Energie, l’Environnement et la Santé, ICPEES, UMR CNRS 7515, Université de Strasbourg, 67087 Strasbourg, FRANCE

 

 

1.   Introduction

Renewable energies (RE) are the heart of the main energy transition scenario. In 2014, the objective of 20% share of RE in the total E.U. energy for 2020 was raised to 27% for 2030 [1]. The involvement of France is even more ambitious as it reaches 32% according to the French law on energy transition [2]. This objective corresponds to 40% of RE in the French electricity mix. However, the intermittency of solar and wind electricity can slow down their penetration in the coming decade. The “Power to Gas” (P2G) technology has the potential to overcome this limitation by coupling electric and gas grids, consuming excess of electricity while producing a storable and transportable fuel. The P2G technological chain allows to store the overproduction of renewable energies thanks to their transformation into H2 by electrolysis based technology (through water conversion by electricity) or in synthetic methane. Then, H2 or synthetic CH4 produced can be stored or injected into the natural gas network and supplied in the cities for sustainable mobility means, for electricity and heat needs. Thus, consumers are provided with energy from renewables at any time. Deployment of the P2G technology, currently still in the development stage, is expected from 2025 in France according to the ADEME [3], with the installation of power units of the scale of 1-50 MW.

The CHOCHCO project (Optimization of a flexible technological chain of CO2/H2O co-electrolysis and CO hydrogenation into synthetic CH4) [4], run from 2013 to 2017 in the frame of ANR (French National Research Agency), developed and evaluated an integrated “Power to Methane” chain based on: 1) a high temperature co-electrolyzer for electrochemical reduction of steam and CO2 into syngas; 2) a modular catalytic methanation reactor and 3) a methane upgrading /drying unit, to reach specifications for further injection into the natural gas grid.

2.   Catalyst development

A series of Ni/CZP (10 wt% of Ni° on mixed oxide ceria-zirconia-praseodymia) catalysts has been synthesized in this purpose. The Ni metallic phase has been impregnated on different mixed oxides of same composition Ce1.31Zr2.52Pr0.17O7.97, prepared by batch coprecipitation from nitrate salts.

In order to improve nickel dispersion, the way to increase specific surface of the CZP support has been investigated. Thus, the optimization of the coprecipitation synthesis, initially with NaCO3, by the combination of the precipitating salt (NH4)2CO3 and addition of a surfactant (CTAB) has been studied.

Table 1. Catalyst characterization.

Ni/CZP(Na)

Ni/CZP(NH4)

Ni/CZP(NH4,T)

SBET (m2/g)

51

69

84

Vpore (cm3/g)

0.07

0.11

0.18

dpore (nm)

5

5

8

SNi° (m2/g)

4.3

6.8

7.7

The optimized catalysts exhibited enhanced properties, as presented in Table 1. The use of ammonium carbonates instead of sodium carbonates as precipitating agent is favorable to the specific surface area of the oxide support, allowing a better dispersion of nickel after impregnation. The addition of CTAB as surfactant in the precipitation medium further improves porosity, BET surface and Ni metal surface. The characterization of the surface by thermoprogrammed desorption (TPD) of CO2 revealed that the use of ammonium carbonates led to a decrease of the mean basic strength : for CZP(NH4) and CZP(NH4,S) the main CO2 desorption occurs around 170 °C whereas it is around 280 °C for CZP(Na). This medium basicity promotes reversible interaction between CO2 and the catalytic support.

3.   Catalytic results

The catalytic activity has been evaluated in a fixed bed reactor on reduced (1h at 400°C under H2/N2) catalysts in powder (sieved between 125 and 200 μm), at atmospheric pressure, with a GHSV of 50.000 h-1 (STP), under a gas mixture which mimics the composition of the outlet flow of the co-electrolyzer in the P2G process studied in this project (H2/CO/CO2/H2O/CH4 70/12/8/5/5 in mole).

The increase of the specific surface and Ni metal areas along with a medium basicity, were shown to be responsible for the enhanced catalytic performance (see Figure 1) especially at low temperature (250 °C). The catalyst synthesized with ammonium carbonate in the presence of a surfactant is especially active at low temperature : almost 80 % of COx are converted into methane at 250°C, whereas  the two other catalysts exhibited almost no activity.

Even at much higher GHSV values (180.000 h-1, STP), the optimized catalyst still exhibits satisfactory methane yields (75% at 250°C).

 

Figure 1. Methane yield vs temperature for the three catalysts. Patm, inlet composition H2/CO/CO2/CH4/H2O of 70/12/8/5/5, GHSV (STP) = 50.000 h-1.

When submitted to successive  ‘start and stop’ conditions under various atmospheres, the catalyst CZP(NH4,S) showed a high resistance to carbon deposition and sintering deactivation, which is a key point for the intermittent working conditions of the process.

4.   Kinetic study

A kinetic model, of the Langmuir-Hinshelwood type, was developed for the first time on a Ni/CZP type catalyst based on reaction mechanism previously reported [5]. CO methanation, CO2 methanation, Water Gas Shift and the 3 corresponding reverse reactions were considered. Firstly, activity at low conversions was used to evaluate roughly the kinetic parameters which were then adjusted with data at high conversion by considering plug flow reactor by minimization between experimental and modeled values.

The obtained activation energies are consistent with values reported in the literature [6]. The developed model is accurate to predict the experimental methane yield, as shown in Figure 2.

 

Figure 2. Parity plot for methane yield for the optimized catalyst (Ni/CZP(NH4,S)).

5.   Conclusions

The use of (NH4)2CO3 instead of Na2CO3 as precipitating agent allows to obtain an oxide with enhanced textural properties, leading to a better dispersion of Ni°. Syntheses with (NH4)2CO3 induce more basicity of weak strength, favorable to the reversible adsorption of CO2The addition of CTAB as a surfactant further improves the dispersion of nickel.

Their high catalytic efficiency (methane yield > 75% at GHSV 180.000 h-1) at low temperature and atmospheric pressure was demonstrated under representative co-electrolysis exit gas composition, and their stability under intermittent conditions have been proven.

A complete kinetic study has been done. The experimental conditions have been extended from low (250 °C) to high (450 °C) reaction temperatures. The excellent activity was modeled with a reliable kinetic model. The activation energy of the reaction of CO2 methanation has been shown to be strongly decreased for the optimized catalyst Ni/CZP(NH4,S).

6.   References

[1]       Communication of 22/01/2014 of the European Commission.

[2]       Law N°2015-992 of 17th August 2015 relative to “la transition énergétique pour la croissance verte”.

[3]       ADEME report May 2014, Contribution de l’ADEME à l’élaboration de visions énergétiques 2030-2050.

[4]       www.chochco.fr

[5]       Ussa Aldana P.A., Ocampo F., Kobl K., Louis B., Thibault-Starzyk F., Daturi M., Thomas S., Roger A.C., Catalysis Today 2013, 215, 201‑207.

[6]       Van Herwijnen T., Van Doesburg H., De Jong W.A., Journal of Catalysis 1973, 28(3), 391‑402.

 


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