Water sorption enhanced CO₂ methanation process: Optimization of reaction conditions and study of various sorbents

Highlights

• Good thermal control was achieved (T < 315 °C), enhancing water sorption.

• Commercial Ni-based catalyst mixed with La₂O₃ or zeolite 4A showed the best results.

• 92–94% H₂ conversions with 100% selectivity were achieved working at kinetic regime.

• In-situ H₂O sorption enhances Sabatier’s reaction shortening the initial transitory.

Abstract

Excess renewable energy can produce hydrogen and together with carbon dioxide a closed energy loop can be formed through methanation (Power to Gas), which can be enhanced by in situ water sorption. Sorption causes a shift in equilibrium to favor methane production, while also decreasing secondary reactions and allowing operation at lower temperatures. In order to get suitable conditions for water sorption operating parameters were adjusted. The optimum temperature was 290 °C, avoiding high temperatures inside the catalytic bed due to the high exothermicity of the methanation; moreover, 15 bar of total pressure, 1.4 g catalyst and intermediate feedflows were found the best ones to obtain conversion close to equilibrium without external mass transfer resistance. The achieved hydrogen conversion was 92–94% with high selectivity, as no CO was detected and methane yield matched hydrogen conversion. The positive effect of water sorption was proved with Zeolite 4A, La₂O₃ and CaO sorbents.

Introduction

The present global energy system is still mainly based on fossil fuels (Eurostat - European Union, 2019), but the increasing warning about climate change urges a revolution. A way to reduce CO₂ emissions, is to transform it into an energy carrier employing the excess renewable electricity (chemical storage). The most favorable option is to use the surplus renewable energy for the electrolysis of water producing hydrogen that, along with captured CO₂, can be converted to methane through methanation reaction (Sabatier’s reaction). This concept is known as Power to Gas, and consists of a thermo-catalytic route for the conversion of H₂ and CO₂ at temperatures about 150–500 °C and pressures about 1–100 bar, using a metal-based catalysts (Younas et al., 2016).

$${\mathit{CO}}_2+4H_2\leftrightarrow {\mathit{CH}}_4+2H_{2}O\mathrm{\Delta }{H}^o=-165kJ/mol$$

There are two main paths proposed for the methanation process. The first, and most popular, is the dissociation of CO₂ to CO and later hydrogenation of CO to CH₄; and the second is the direct hydrogenation of CO₂ to CH₄ (Baraj et al., 2016, Frontera et al., 2017, Lehner et al., 2014, Younas et al., 2016). One of the main secondary reactions that may occur is CH₄ cracking reaction, which is proposed to be the main cause of carbon deposition (Jürgensen et al., 2015).

$${\mathit{CO}}_2+H_2\leftrightarrow CO+H_{2}O\mathrm{\Delta }{H}^o=41kJ/mol$$

$$\mathit{CO}+3H_2\leftrightarrow {\mathit{CH}}_4+H_{2}O\mathrm{\Delta }{H}^o=-206kJ/mol$$

Ni and Ru are the most studied transition metals in CO₂ methanation, being Ni the most interesting one because it is cheaper and offers higher activity and selectivity (Stangeland et al., 2017). A broad range of Ni content has been studied in the literature, being from 10 to 25 wt% the preferred content (Younas et al., 2016). It produces almost exclusively CH₄, while other metals like Pd, Pt, Rh, Mo, Re and Au produce simultaneously CH₃OH and CO (Stangeland et al., 2017). With regards to the support, the use of Al₂O₃ (especially the γ-phase), SiO₂, TiO₂, ZrO₂ and zeolites have been reported, being γ-Al₂O₃ the most used one (Stangeland et al., 2017, Younas et al., 2016). Some Katalco catalysts (Ni on Al₂O₃ support) have also been reported (Falcinelli et al., 2017, Miguel et al., 2018) showing good results.

There are already some Pilot Plants / Commercial Scale projects for CO₂ methanation around the world (Younas et al., 2016). Three main units compose it: an electrolyzer, a CO₂ provider and a chemical reactor. When these facilities are going to be used for periodic surpluses of electricity, they should be highly flexible and have a quick start-up time, which is specially challenging in methanation, due to the required operating temperatures (Younas et al., 2016). It is still difficult to stablish the most appropriate combination of the operating parameters, feed flowrates and reactor design, allowing operation at the lowest possible temperature, with high selectivity to methane and low CO production (Bailera et al., 2017, Rönsch et al., 2016). This is especially critical due to the high exothermicity of the CO₂ methanation trying to reach high energy efficiency. The operating conditions could be softer if the equilibrium of the reversible methanation reaction is displaced to the products generation, and this may be achieved adsorbing water in-situ inside the reactor.

Water sorption at low temperatures (<100 °C) is widely employed/studied in several technologies such as cooling (Ye et al., 2014) or in ceramics (Zolfaghari et al., 2017). However, water sorption at higher temperatures, such as those required in the methanation reaction, need a much more complex material development. Due to the exothermic character of the adsorption, it is thermodynamically favored at low temperatures, but high temperatures are required for sufficiently fast methanation kinetics (Stangeland et al., 2017). Water retention can occur by different mechanisms. Zeolites vary their water content according to changes in temperature or pressure and the type of cations. Other compounds retain water in the form of salt hydrates or water can also be directly adsorbed (Vieillard, 2012). The porous structure and the presence of inorganic salts in the pores of mesoporous or microporous matrices can affect their sorption capacity (Hauta et al., 2013, Soboleva et al., 2011).

Few studies have been found in which water sorption is used to displace the methanation reaction. The first of these studies was that of Walspurger et al., where they used a Ni catalyst and zeolite 4A as a water adsorbent at atmospheric pressure and 250 °C. They observed a conversion close to 100% operating under highly diluted reactants. However, they observed a decrease of 15–20% in the adsorption capacity of the zeolite because it also adsorbed CO₂ (Walspurger et al., 2014). Borgschulte et al. impregnated Ni directly over Zeolite 5A sorbent, demonstrating the enhancement of CO₂ methanation by water sorption enhanced catalysis (Borgschulte et al., 2013). More recently Delmelle et al., impregnated Ni over zeolite 5A and zeolite 13X (Delmelle et al., 2016), achieving nearly pure CH₄ at 300 °C with both sorbents, being Zeolite 13X the one that showed a higher sorption capacity.

The main objective of this work was the optimization and understanding of the operating conditions for a sorption enhanced methanation process. Especial attention was focused on the temperature profile inside the reactor, since it is a key parameter at such high temperatures for water sorption purposes. Optimization of space velocity is also a key parameter that determines not only the temperature profile in the reactor but also the transport and the required amount of sorbent. This work is aligned with recent theoretical studies of sorption enhanced methanation process that state the need of experimental work to study transport and fluid-dynamic limitations, which together with temperature profile are restrictive in the design of industrial methanators (Catarina Faria et al., 2018, Massa et al., 2020). According to these objectives, three different sorbents were chosen for water sorption: Zeolite 4A, CaO and La₂O₃. Lanthanum oxide has been reported as an interesting catalyst support in CO₂ methanation process by several authors (He et al., 2015, Mansir et al., 2017, Song et al., 2010, Younas et al., 2016), but it has not been discussed its water sorption capacity. The three sorbents were deeply characterized, their water sorption stability at high temperatures was analyzed, and their effect in shifting methanation reaction was tested, once adjusted the above-mentioned operating conditions.