Testing Different Sorbents for Sorption-Enhanced Methanation in a Dual Fluidized Bed System

The increasingly growing debate on climate change is pushing our society toward the development of innovative technologies for limiting the emission of greenhouse gases deriving from the exploitation of fossil resources. On the one side, Carbon Capture and Storage (CCS) and Carbon Capture and Utilization (CCU) technologies are intended to exploit sustainably fossil fuels, and represent the transition solution toward the complete conversion of our society to renewable energy (RE). On the other hand, the scientific community is working hard to find solutions to the typical problems of REs, which are mainly represented by their intrinsic intermittent nature. In particular, the concept of energy storage is a key feature for the development of a fully renewable energy systems. Currently, energy storage techniques are mostly based on the accumulation of sensible or latent heat with different materials, either liquid or solid. Unfortunately, these techniques suffer in general of a low storage energy density, and, in some cases, problems related to the chemical stability of the storage medium. For these reasons, in the last years the concept of chemical storage is gaining an increasing interest as a valid energy accumulation system: the idea is based on the utilization of solar radiation to produce fuels, named solar fuels (SF). Among them, hydrogen can be considered as the simplest SF or as the precursor of other SFs, because it can be produced by photochemical or thermochemical electrolysis of water (Smestad and Steinfeld 2012).

In this framework, methanation can assume an important role because it can be strictly linked to the concepts of CCU, solar fuels and chemical storage. In particular, catalytic methanation with hydrogen of either CO (Eq.1) or CO₂ (Eq.2) occurs exothermically following the reactions:

3H₂+CO=CH₄+H₂O -206kJ mol-1 (@298K) (1)

4H₂+CO₂=CH₄+2H₂O -164kJ mol-1 (@298K) (2)

Thus, methane could represent the final product for the storage of solar energy in the framework of the Power-to-Gas (PtG) technology (Götz et al. 2016), initially converted into hydrogen by water splitting with photochemical or thermochemical processes. Specifically, CO₂ methanation could be considered as a process for the utilization of CO₂ coming from combustion of fossil fuels, while CO methanation could be the final conversion step of syngas produced by coal or biomass gasification.

Methane, contrary to hydrogen, is already extensively used in the civil and industrial sectors, as well as in the automotive industry, and as a consequence has a well-developed distribution and storage infrastructure in many countries, and finally benefits from a relatively large public acceptance.

The commercially available methanation plants consist of a cascade of fixed bed reactors, operated at high pressure, and equipped with gas recycling and intermediate cooling, for the management of the heat of reaction (Rönsch et al. 2016). However, to reduce investment and operational efforts, there is large scope in finding new solutions to improve process temperature control, enhancement of efficiency and flexibility.

Some research groups (Borgschulte et al. 2013; Walspurger et al. 2014) recently proposed and demonstrated, in a lab-scale fixed bed apparatus, the concept of Sorption-Enhanced Methanation (SEM), where the steam generated during methanation is continuously captured by means of a suitable regenerable sorbent (Eq. 3):

Sorbent + H₂O= Sorbent(H₂O) (3)

In this way, the equilibrium of reactions (1) or (2) can be shifted toward higher methane yields, allowing to lower the operating pressures.

In this work an innovative configuration system for the methanation process based on the technology of dual interconnected fluidized beds is presented, coupling the concepts of SEM and of chemical looping.

The idea is to employ one fluidized bed, named Methanator/Hydrator, for the catalytic methanation reaction with inherent capture of steam by a selective sorbent at low or atmospheric pressure, in order to drive the equilibrium towards product formation; while the regeneration of the hydrated sorbent takes place in a second fluidized bed, named Dehydrator, operated at higher temperature. In particular, this study was focused on the evaluation of the suitability of two different sorbents to capture and release water at different temperatures and reaction environments relevant for catalytic methanation. The two sorbents used are CaO, derived from natural limestone, and a commercial Zeolite 3A. The CaO sorbent was obtained by fluidized bed calcination at 850 °C of an Italian limestone named Massicci.

The tests were carried out in a lab-scale apparatus called Twin Beds (TB) (Coppola et al. 2017) consisting of two identical bubbling beds operated batch-wise and connected by a rapid solid transfer line. Transfer of sorbent is accomplished pneumatically by using a valve system arranged along the duct and at the outlet of the reactors. The two reactors were employed as hydrator and dehydrator respectively.

Each test consisted of 10 complete cycles, with a fluidization velocity fixed at 0.5m/s, while the time of each hydration or dehydration step was fixed at 10 min.

The main operating conditions were: hydration with 10% steam (balance air) and dehydration in nitrogen or in air. An initial sensitivity analysis on temperature was carried out fixing the hydration temperature at 250 or 300°C and varying the dehydration temperature at 350, 400 and 450°C in air.

Generally, the results for CaO showed, for all conditions investigated, that the H₂O capture capacity (γ), expressed as the mass of captured water per mass of initial sorbent, increases after the first cycle, with a maximum around the 2^nd-3^rd cycle, and successively it tends to decrease with the number of cycles, reaching an asymptotic value after approximately the 5^th-6^th cycle. The increase of the dehydration temperature entails an increase of γ. In fact, for the tests with hydration at 300°C the asymptotic values of γ were 0.04, 0.13 and 0.15 g/g for a dehydration temperature of 350, 400 and 450 °C, respectively. This behavior could be determined by a different change of the sorbent microstructure (pore structure), with a consequent variation of the sorbent capacity. A positive effect on γ was also registered with the rising of the hydration temperature.

As opposed to CaO, for the Zeolite 3A no decay effect of γ with the number of cycles was observed. However, the hydration temperature has an opposite effect on the Zeolite as for CaO: the higher the hydration temperature, the lower the sorbent capacity. On the contrary, the dehydration temperature seems to have no remarkable effect on the zeolite performance. In general, the value of γ fluctuates from 0.02 up to 0.05 g/g which are on average lower with respect to CaO.

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