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Waste Disposal & Sustainable Energy

, Volume 1, Issue 1, pp 53–65 | Cite as

Combining CO2 capture and catalytic conversion to methane

  • Paulina Melo Bravo
  • Damien P. DebeckerEmail author
Review

Abstract

Considering the global objective to mitigate climate change, import efforts are made on decreasing the net emission of CO2 from gas effluents. On the one hand CO2 capture—for example by adsorption onto solid basic materials—allows to withdraw CO2 from the waste gas streams emitted by incinerators, cement manufacture plants, combustion plants, power plants, etc. On the second hand, CO2 can be converted to useful chemicals—e.g. hydrogenation to methane—using appropriate heterogeneous catalysts. A relatively innovative strategy consists in combining both technologies by designing materials and processes which can switch between capture and methanation modes cyclically. This allows treating complex waste gas effluents by selectively and reversibly capturing CO2, and to perform the catalytic hydrogenation in appropriate reaction conditions. This short review presents the main strategies recently reported in the literature for such combined CO2 capture and methanation (CCCM) processes. We discuss the different types of reactor configurations and we present the formulations used in this context as adsorbent, as methanation catalysts, and as “dual functional materials”.

Keywords

CO2 capture Methanation Sabatier reaction Catalytic hydrogenation Adsorption Methane Dual functional materials 

Introduction

Global emissions of carbon dioxide (CO2) have been increasing steadily in the last few decades and are recognized as one of the main anthropic cause of global warming [1]. In this context, the recent Paris Agreement sets the aim to hold the global warming well below 2 °C and to pursue efforts to limit it to 1.5 °C [2]. Among the range of different options that can help towards this target, carbon capture and storage (CCS) [3, 4] and carbon capture and utilization (CCU) [5, 6] are the focus of intense research efforts.

To mitigate emissions from large and stationary gas effluents sources, CO2 should be captured from the effluents emitted from combustion plants (fuel, coal, municipal waste and biomass combustion) and from gasification or thermal power generation plants. CO2 capture from waste gases includes absorption, adsorption, membrane processes, hydrate and cryogenic process [3, 7, 8]. In particular, adsorption technologies are receiving a lot of attention due to their relatively lower energy consumption [9]. In addition to the economical challenge that must be overcome before it can be deployed on an industrial scale (large capital investments are required), the technical challenge of CCS is related to the development of solids with high CO2 adsorption capacity, selectivity and thermal stability. The nature and properties of the adsorption material obviously play a key role in the process. Thus, research is focused on the development of innovative sorbents, using various types of materials such as, for example, basic zeolites, carbon-based materials, amine-functionalized materials, basic clays, alkali metal oxides, etc. [9, 10, 11, 12, 13, 14, 15, 16]. Adsorption capacity for solid sorbents can reach more than five \( {\text{mmol}}_{{{\text{CO}}_{ 2} }} /{\text{g}} \), for example, for NaCO3 or for amine-functionalized silica supports [17, 18]. An emerging class of sorbents for CO2 capture is represented by hybrid materials like metal–organic frameworks (MOFs) and covalent organic frameworks (COF) [19, 20, 21, 22, 23, 24, 25]. After adsorption, the CO2-saturated sorbent can either be disposed or regenerated. The latter option is obviously more attractive because it permits the reuse of the sorbent and the further conversion of CO2. Yet, in this case, CO2 adsorption has to be reversible so that subsequent desorption is facile, to avoid excessive energy consumption [18, 26].

In fact, upon desorption, a relatively pure and concentrated stream of CO2 can be produced, which allows envisaging further use. Indeed, carbon capture and utilization (CCU) can in principle turn waste CO2 into valuable products. The conversion of CO2 by chemical means is a mushrooming topic in the current scholarly publication landscape [27]. CO2 utilization encompasses a wide range of possible targeted molecules and industrial applications in the sectors of chemistry, energy, food, pharmaceuticals, pulp and paper, etc. [3, 28]. In terms of CO2 valorization, the most sustainable solution could be the catalytic conversion into chemical fuels and value-added chemicals [29]. In particular, the hydrogenation of CO2 towards methane [30, 31, 32, 33, 34, 35, 36, 37] methanol [38, 39, 40] or higher hydrocarbons [41, 42, 43, 44] is a topical research field [45, 46, 47].

The combined CO2 capture and utilization is a response to two technical challenges. On the one hand, most energy-intensive industrial processes release gas effluents containing CO2 plus a complex mixture of other components arising from combustion (H2O, N2, unreacted O2, dust, sulphur and nitrogen oxides) [48]. Under such conditions, CO2 hydrogenation catalysts would deactivate rapidly [49]. So, the direct application of CO2 hydrogenation in those streams is not viable and more advanced processes have to be developed [48, 50, 51]. On the second hand, clean hydrogen production processes based on renewable energy (i.e. water electrolysis via renewable electricity [52]) are being developed, but remaining issues are the intermittence of the production and the difficult storage and transportation of H2 as a fuel [53]. The combination of CO2 utilization and energy storage can be realized on-site, using the “Power to Fuel” technology: CO2 is first captured and then released in the form of a concentrated and “clean” stream, mixed with hydrogen obtained via renewable sources, compressed, and converted in catalytic reactors [48, 54]. As it will be discussed below, processes where both the capture and the hydrogenation occur in the same unit can be designed too. Thus, such power-to-gas technologies enable the storage of surplus electricity from fluctuating renewable sources [52]. Power to synthetic-natural-gas (SNG), also called “Power to methane”, targets the production of methane in the catalytic reactor. The SNG produced can be used as fuel for mobility, in the residential sector, for power generation (at times when the power demand surpasses the power supply), and as raw material in the industry [55]. Arguably, in the current environmental, legal and economic landscape, the capture of CO2 by adsorption and its subsequent hydrogenation to CH4 by catalytic methanation is a promising line of research and development.

The objective of this contribution is to present a brief overview of the recent scholarly reports where this strategy of coupling CO2 adsorption and methanation is developed. Indeed, in the last few years, progress has been made both in the chemical engineering aspects and in the development of innovative and effective adsorbents and catalysts. Interestingly, bifunctional materials—coined “dual functional materials” (DFM) and able to carry out both the adsorption and the hydrogenation of CO2—are currently being intensely studied. We highlight the types of formulations of adsorbents, catalysts and dual functional materials that have been proposed, some basic process engineering aspects, and the reported performance. This overview will hopefully guide the community by highlighting the technological locks that still have to be opened to develop effective processes that are applicable to CO2-emitting plants.

Options for combining CO2 capture and methanation

The efforts of the scientific community to tackle the global issue around CO2 emissions are both intense and diverse. In the case of the combined CO2 capture and methanation (CCCM), three main process configurations can be envisaged (Fig. 1).
Fig. 1

Configurations of CO2 adsorption coupled to methanation process. The “CO2 source” is the complex flue gas from a combustion plant, for example a Separated units for the adsorption of CO2 and for its methanation, b adsorbent and catalyst placed together in the same unit (either mechanically mixed, in multilayers or as two separate beds), c dual functional material able to perform both adsorption and methanation loaded in the same unit

In a first configuration (Fig. 1a), capture and methanation steps are physically separated. Adsorption is carried out in an adsorption unit, using a solid adsorbent. Treated flue gas exiting the adsorption unit is ideally CO2-free and can be released in the environment or further treated if needed. When the adsorbent is saturated, working conditions are changed—e.g. the temperature is increased—to provoke CO2 desorption. The desorbed CO2—ideally in high concentrations and in a “clean” stream—is then mixed with H2, pressurized, and sent to another unit to be converted to methane, using a methanation catalysts classically based on Ru or Ni or other noble metals [34, 56, 57, 58]. The advantage of this configuration is the possibility to optimize separately the two processes of capture and methanation, for example in terms of temperature and pressure.

In a second configuration (Fig. 1b), capture and methanation are carried out in the same physical unit, using two distinct solids: one adsorbent and one methanation catalyst. The two solids can be physically mixed to form only one fixed bed or separated to form distinct fixed beds in the same reactor. The stream of gases always flows through the whole set-up, in contact with both the adsorbent and the catalyst. The system operates alternatively in adsorption mode (releasing CO2-free effluents) and in methanation mode (releasing a CH4-rich gas). The switch between adsorption mode and methanation mode is made by changing the conditions (temperature, injection of gas effluents or pure H2). The advantage of this configuration is the lower investment cost (only one reactor for both modes). Also, the preparation of both types of solids can be optimized separately, in a search for high adsorption or methanation performance.

In a third configuration, capture and methanation are performed by a single solid which features both adsorption and reduction capabilities (Fig. 1c). The solid is loaded in one unit, forming one fixed bed, and is denoted “dual functional material”. In this case, the focus of the research is mainly on the development of new types of advanced materials formulations, as it is discussed hereunder. Here, a challenge is to develop such bifunctional solids exhibiting both high adsorption capacity and high methanation activity. However, as described below, it is claimed that the close proximity between the two phases (basic phase for adsorption and metallic nanoparticles for hydrogenation) present on the same solid results in enhanced performance in the regeneration/methanation step.

Overview of the literature

Table 1 presents a list of recent contributions where the capture by adsorption of CO2 is combined with its subsequent utilization by means of catalytic methanation, together with the types of formulations used as adsorbent, catalyst or DFM, the operating conditions, and the performance (Table 1). These examples are discussed in the next sections.
Table 1

Overview of the reported studies on the combined use of CO2 capture and CO2 methanation (combined CO2 capture and methanation, CCCM)

Material

CO2 adsorption

CO2 methanation–adsorbent regeneration

System configuration

References

Adsorbent

Methanation catalyst

Operation conditions

CO2 adsorption capacity [molCO2/kg]

Operation conditions

CH4 production [\( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kg]

Two solids in two separate units

 22.1% K2CO3/Al2O3

NKM-2 V (24% Ni, 13.3% Al, 7.36% Ca, 0.57% Si)

Wet air (380–430 ppm CO2, 20–30% H2O), 20–25 °C, 14 h

Not reported

100 ml/min H2 (420 °C) 2 h (adsorption unit set at 325 °C)

Not reported

Two reactors in series

[59]

 22.1% K2CO3/Al2O3

4% Ru/γ-Al2O3

Wet air (380–430 ppm CO2, 20–30% H2O), 20–25 °C, 8 h

0.91 ± 0.08

100 ml/min H2 (200–400 °C) 2 h (adsorption unit set at 325 °C)

0.89 ± 0.08

Two reactors in series

[60]

One unit and two solids

 10% CaO/γ-Al2O3

10% Ru/γ-Al2O3

10% CO2/N2, 320 °C, 30 min

Not reported

26 ml/min 4% H2/N2, 2 h at 320 °C, 1 atm

0.12

Fixed bed in microreactor

[61]

 Commercial hydrotalcite: Pural MG 30 K (17% K2CO3; Mg/Al = 0.5)

Ni-based catalyst pelletized

15% CO2/N2, 320–350 °C, 1.34–2.5 bar

Max: 0.3 (350 °C and 0.2 bar \( {\text{p}}_{{{\text{CO}}_{ 2} }} \))

100 ml/min of 10% H2/N2

at 320–350 °C, 1.34–2.5 bar

2.36 \( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kg.h(a)

Layered bed reactor (17 cycles)

[62]

 22.1% K2CO3/Al2O3

4% Ru/γ-Al2O3

400–550 ppm CO2, 20–30% H2O, 25 °C, 8 h

0.82–0.85

100 ml/min H2 (300 and 350 °C) 2 h

0.80 (300 °C)

0.82 (350 °C)

Cylindrical reactor with two fixed beds

[63]

One unit with a dual functional material

 10%Ru/γ-Al2O3

10% CO2/N2, 320 °C, 30 min

Not reported

4% H2/N2, 320 °C, 2 h or 5% H2/N2, 320 °C, 20 min/cycle

0.10

Fixed bed in microreactor

[61]

 10%Ru-1%CaO/γ-Al2O3

  

0.19

 10%Ru-5%CaO/γ-Al2O3

  

0.27

 10%Ru-10%CaO/γ-Al2O3

  

0.30

 1.1%Ru-10%CaO/γ-Al2O3

  

0.27

 2%Ru-10%CaO/γ-Al2O3

   

0.35

 5%Ru-10%CaO/γ-Al2O3

   

0.50

 6.8%Ru-10%CaO/γ-Al2O3

   

0.44

 10.6%Ru-10%CaO/γ-Al2O3

   

0.46

 10%Rh/γ-Al2O3

10% CO2/N2, 320 °C, 30 min

Not reported

4% H2/N2, 320 °C, 2 h

0.3

Fixed bed in microreactor

[64]

 10%Rh-10%CaO/γ-Al2O3

  

0.9

 5%Rh-10%CaO/γ-Al2O3

  

0.7

 1%Rh-10%CaO/γ-Al2O3

  

0.7

 0.1%Rh-10%CaO/γ-Al2O3

  

0.4

 5%Ru-10%CaO/γ-Al2O3

  

0.5

 5%Ru-10%K2CO3/γ-Al2O3

  

0.91

 5%Ru-10%Na2CO3/γ-Al2O3

  

1.05

 5%Ru-6.1%Na2O/γ-Al2O3 (tablets)

7.5% CO2/N2, 320 °C, 30 min

0.40

5% H2/N2, 320 °C, 30 min

0.24

Scaled-up fixed bed reactor

[65]

 5%Ru-10%CaO/Al2O3 tableted powder (BASF)

7.5% CO2 + 15% H2O + 4.5% O2 + 73% N2, 320 °C, 20 min

0.23

5% H2/N2, 320 °C,

0.13

Fixed bed reactor (11,236 h−1)

[66]

 5%Ru-10%CaO/Al2O3 pellets (TH200)

 

0.13

60 min

0.14

 5%Ru-10%CaO/Al2O3 pellets (TH100)

 

0.16

 

0.20

 5%Ru-10%CaO/Al2O3 (SAS200)

 

0.17

 

0.12

 15%Ni/ZrO2

 15%Ni-5%K/ZrO2

 15%Ni-5%La/ZrO2

4.7% CO2/He, 250–450 °C, 5 min

Not reported

Pure H2, 250–450 °C, 5 min

Not reported

Fixed bed reactor (6 cycles)

[67]

If not stated otherwise, the systems were operated at atmospheric pressure

aThis value is not a CH4 production per cycle, but an average CH4 productivity per hour under a long-term CO2 adsorption–methanation experiments

Two solids in two separate units

The combined CO2 adsorption and methanation was studied by Veselovskaya et al. [59] using two distinct reactors in series (Fig. 1a). A potassium carbonate-based adsorbent (K2CO3/Al2O3 with 22.1 wt% K2CO3 impregnated onto 1–2 mm-sized cylindrical pellets of mesoporous Al2O3) was inserted in the first reactor and a Ni-based methanation catalyst (granulated commercial “NKM-2 V catalyst”; 23.4 wt% Ni, 13.3 wt% Al, 7.36 wt% Ca, 0.57 wt% Si) was placed in the second reactor. In the first step of each cycle, the sorbent was saturated at room temperature with CO2 using a flow of humid air. Then, the system was purged for 10 min under H2 and the catalytic reactor was preheated up to 420 °C. After that, the sorbent was heated to 325 °C to trigger desorption (regeneration of the sorbent, 2 h) and the desorbed CO2 was carried towards the reactor with the H2 flow to undergo catalytic methanation at 420 °C. Finally, the system was cooled down for 2 h before starting another cycle. Under this configuration and operational conditions, it was demonstrated that CO2 uptake over the adsorbent (K2CO3/Al2O3) is directly dependent on the regeneration temperature; that is, increasing the regeneration temperature results in an increase in the total CO2 uptake. The outflows of the catalytic reactor contained CO2 only in the ppm range (~ 300 ppm), and around 99% of the CO2 desorbed from the adsorption step was converted to CH4 [59]. The same authors have also studied the use of a 4% Ru/γ-Al2O3 methanation catalyst in combination with a K2CO3/Al2O3 adsorbent applied for such “direct air capture for methanation” [60]. These authors also discuss how the energy produced by the methanation reactor can reduce the operating cost mainly associated with regeneration in the capture step.

One unit and two solids

A combined reactor was developed by Miguel et al. [62] to carry out both the CO2 capture and CH4 production in the same unit. The reactor contained several alternated layers of (i) a commercial potassium-promoted hydrotalcite for capturing CO2 and (ii) a commercial Ni-based catalyst for the CO2 methanation (Fig. 1b). Before the adsorption-methanation experiments, the layered bed was activated using a flow of 10% H2/N2 for 1 h at 320 °C, and then, the bed was filled with N2. Each cycle of adsorption-methanation experiment consisted in two steps. The sorption step was carried out using a flow of 15% CO2/N2 at either 300 °C or 350 °C. The regeneration and methanation step was performed by injecting pure H2 at the same operating temperatures. These two steps were repeated several times. After six cycles, the adsorption capacity was decreased from 0.52 to 0.32 mmol/g. Yet, applying again the activation procedure, the initial bed sorption capacity could be recovered. The temperature effect on methane productivity was negligible with a maximum value of 2.32 \( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kg/h and 2.36 \( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kg/h at 350 °C and 300 °C, respectively. Modifying the total pressure in the system, the sorption capacity increased from 0.38 to 0.43 \( {\text{mmol}}_{{{\text{CO}}_{ 2} }} /{\text{g}} \) at 1.34 and 2.5 bar, respectively. Finally, working under optimized conditions (1.34 bar and 300 °C), CO2 conversion was almost complete (99%) and CO was not detected. However, the CH4 purity at the reactor outlet was unsatisfactory (30%, taking into account the actual composition of the outlet, including H2, N2, and CO). So, parameters such as catalyst/sorbent ratio, bed layout, H2 feed mode, for example, should be improved.

A similar strategy was proposed for the consecutive CO2 adsorption directly from the air at room temperature and CO2 desorption/methanation in H2 [63]. A cylindrical reactor was used with two separate gas permeable compartments (Fig. 2). The potassium carbonate-based adsorbent (22.1 wt% K2CO3 impregnated onto 1–2 mm-sized cylindrical pellets of mesoporous Al2O3) was inserted in the lower compartment and a Ru-based methanation catalyst (4 wt%. Ru) was placed in the upper compartment. In the first step of each cycle the sorbent was saturated at room temperature with CO2 using a flow of air. Then, the system was purged for 10 min under H2 at room temperature. After that, the unit was preheated at 300 or 350 °C. Finally, the CO2 desorption/methanation step was carried out at 300 or 350 °C under H2 atmosphere. CO2 adsorbent and methanation catalyst showed stable performances after ten cycles. CO2 adsorption capacity results were in the range from 19.7 to 20.5 \( {\text{mL}}_{{{\text{CO}}_{2} }} \)/g independent of the regeneration temperature (300 and 350 °C). Besides, thanks to the high H2 partial pressure in the methanation step, the total conversion of carbon dioxide to methane was ~ 94% at 300 °C and ~ 96% at 350 °C.
Fig. 2

Direct air capture/methanation (DACM) set-up. Cylindrical reactor with 19 mm of inner diameter with two separate gas permeable compartments (upper compartment: 0.57 g of methanation catalyst, lower compartment: 4.58 g of CO2 adsorbent); 1300 ± 50 ml/min of air (CO2 concentration: 400–550 ppm, relative humidity: 20–30%) during 8 h at 25 °C. CO2 desorption—methanation: 100 ml/min of H2 during 2 h at 300 and 350 °C.

Reproduced from Veselovskaya et al. [63] with authorization from Springer

One unit and one bifunctional solid (dual functional materials)

With the emergence of innovative materials able to perform both the CO2 adsorption and its methanation, new processes of CO2 capture and methanation are being developed (Fig. 3). These solids, coined “dual functional materials” (DFM), feature two key capabilities: (i) basicity to allow CO2 adsorption, and (ii) redox catalytic sites to carry out the hydrogenation of CO2. Thus, a single solid is loaded in one reactor (Fig. 1c) and the operating conditions are modified at regular intervals to switch between capture and methanation modes. Practically speaking, one would need two parallel units to be able to treat effluents in continuous mode by switching the respective units between the capture and methanation modes.
Fig. 3

Process diagram of CO2 capture and CH4 production in one reactor simulating an industrial application. In step 1 (adsorption), flue gases loaded with CO2 and other gases are fed in the reactor and CO2 is adsorbed over the adsorbent until saturation. A CO2-free flue gas can be released or further treated. In step 2 (regeneration/methanation), hydrogen is fed in the reactor and CO2 is desorbed from the adsorbent and converted to CH4. These steps are repeated cyclically

DFM-based processes can be performed isothermally; the exothermic nature of the methanation reaction, supplies the necessary energy for CO2 desorption from the adsorption sites [61]. In fact, for each mol of CO2 that is converted to methane, 165 kJ are generated and, therefore, available to promote CO2 desorption from the solid. In these processes, heat management is crucial to avoid catalyst and sorbent damage and to utilize the released heat effectively [68]. Another important aspect is that the methanation catalyst should be highly active at moderate temperature (e.g. below 350 °C) to maximize the yield of methane (at higher temperature, the thermodynamics would be in favour of larger proportions of CO). Finally, it is important that the catalyst is stable against deactivation caused by sintering, especially in the context of a reactor that is essentially used in cyclic mode, where the atmosphere is changed at regular intervals and where local temperature variations can be encountered.

Formulations of dual functional materials

The Farrauto group is pioneering the research on dual functional materials for capture and methanation of CO2. Starting with Ru as the active phase for methanation reaction, and CaO as the adsorbing agent, they prepared DFM with various Ru loading (1–10%) and various amounts of CaO as a adsorbent agent (1–10%) on \( \gamma \)-Al2O3 [61]. The reversible CO2 adsorption/desorption and CH4 production occurred in the same reactor at the same temperature (320 °C) and pressure (1 atm). When feeding H2, methane production took place with CO2 chemisorbed on Ru sites. This drives CO2 desorption from dispersed CaO sites resulting in further methanation. Interestingly, the impregnation of Ru onto CaO/\( \gamma \)-Al2O3 gave better performance as compared to the catalysts where CaO was impregnated on Ru/\( \gamma \)-Al2O3. Increasing the CaO:Ru ratio was shown to be favourable for the production of CH4 because more CO2 molecules can be adsorbed and then spillover towards Ru for methanation. However, if the Ru loading is too low, the generated heat is not sufficient to drive CO2 desorption from CaO sites, which also lowers the methanation capacity; so an optimum has to be found [61]. The best sample in this study was 5%Ru-10%CaO/\( \gamma \)-Al2O3 with a CO2 capacity capture of 0.41 \( {\text{mol}}_{{{\text{CO}}_{2} }} \)/kgDFM and 0.31 \( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kgDFM methanation activity.

The same group performed a screening of different active phases for methanation (Ru, Rh, Pt, Pd, Ni, Co) and adsorption (MgO, K2O3, Na2CO3) [64]. A simple methanation test was first used to select the best active phase, supported on alumina. 10%Ru/\( \gamma \)-Al2O3 was clearly the most active methanation catalysts, but at 320 °C 10%Ru/\( \gamma \)-Al2O3 was also able to reach the equilibrium conversion in the conditions of the test, while other catalysts were significantly less active. Thus, Rh was selected as the active phase and a series of DFM was prepared by impregnation of 0.1–10 wt% of Rh on 10%CaO/\( \gamma \)-Al2O3. The DFM were reduced at 320 °C in 4% H2/N2 under 1 atm. Then, maintaining the temperature and the pressure constant, CO2 capture–methanation cycles were carried out first exposing the sample to a 10% CO2/N2 flow and then to a 4% H2/N2 mixture. For 10%Rh-10%CaO/Al2O3 the methanation capacity reached 0.9 \( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kgDFM which is three times higher compared to 10%Rh/\( \gamma \)-Al2O3 (no adsorbent phase) and two times higher than the previously reported 5%Ru-10%CaO/\( \gamma \)Al2O3. The latter formulation remains economically more viable; it was selected a starting point for the next screening, where the sorbent component was changed (with MgO, K2CO3 or Na2CO3). All samples showed partially reversible CO2 capture behaviour under isothermal CO2 partial pressure swing conditions, being able to release more than 60% of CO2 captured. The highest uptake was observed for Na2CO3 and K2CO3. Thus, 5%Ru-10%Y/Al2O3 DMF with Y = CaO, K2O3 or Na2CO3 were tested and followed the trend Na2CO3 > K2O3 > CaO (reaching 1.05, 0.91 and 0.5 \( {\text{mol}}_{{{\text{CH}}_{ 4} }} \)/kgDFM, respectively). Stability tests were performed for the best sample over three cycles of CO2 capture and methanation. More CO2 was captured in the first cycle compared to the cycle 2 and cycle 3, probably because the methanation step did not allow the full regeneration of the adsorption sites.

Using a 5%Ru-6.1%“Na2O”/Al2O3 formulation, Proano et al. [69] clarified that CO2 can also adsorb both on Ru active sites and on the alumina surface (hydroxyls on Al2O3). However, in the DFM, the sodium carbonate precursor used in the preparation is converted to a “Na2O” phase, which exhibits strong and abundant adsorption sites (Al–O–Na+ sites) onto which large amounts of CO2 adsorb in the form of bidentate carbonates. Using in situ DRIFTS it was shown that these adsorbed carbonates spill over from the adsorbent to the Ru-support interface where the hydrogenation to methane takes place, via the intermediate formation of adsorbed formates.

Hu and Urakawa studied nickel-based catalysts as a DFM for CO2 capture and methanation [67]. Ni catalysts (15%) supported on ZrO2 were prepared by impregnation and compared to K- and La-promoted formulations (5% of promoter). 1 g of catalyst was pelletized crushed and sieved and loaded into a tubular reactor (Fig. 1c). First, the sample was pre-reduced in H2 at 450 °C for 1 h and then cooled down in He before starting the cycles of capture and desorption/methanation. Each cycle consisted in a CO2 capture step (5 min) run with a 4.7% CO2/He mixture and a methanation step (5 min) run by feeding pure H2. Both steps were carried out at the same temperature, in the 250–450 °C range. The quantity of CO2 captured on the catalyst during the whole capture phase was markedly (and expectedly) higher when using promoted catalysts (Fig. 4). Consistently, the amount of CH4 produced in the methanation step was higher for the promoted catalysts as compared to the pristine Ni/ZrO2. La-doped catalyst was more effective in the methanation reaction, allowing a more efficient exploitation of the catalyst capture capacity. However, a relatively higher proportion of unreacted CO2 was released from this catalyst during the reduction step.
Fig. 4

Results reported by Hu and Urakawa on the combined CO2 capture and methanation using Ni-based formulations (figure reproduced from Ref. [67] with authorization from Elsevier). The plot reports the effluent concentration profiles in two consecutive steps of CO2 capture (white area) and methanation (grey area), using an empty reactor (blank) or a Ni/ZrO2 catalyst or its La- or K-doped DFM versions. Colour code: CO2 (black), CH4 (blue), CO (red)

Interestingly, authors also discussed the importance of the relative duration of each step (capture and methanation) in the perspective of process optimization. Ideally, in a scenario where two identical reactors are used for continuous CO2 capture and reduction by switching functions (between capture mode and methanation mode), the two durations should be matching [67]. So this requires a fine tuning of the actual sorption and methanation capacity of these solids.

Process parameters for capture and methanation in dual functional systems

Using a 5%Ru-10%CaO/γ-Al2O3 formulation, Zheng et al. presented a parametric study, to evaluate the effect of the adsorption time, feed composition, operating temperatures, flow rates and Al2O3 support pellet shape (tableted powder, cylindrical pellets or spherical pellets) [66]. The latter two parameters did have a marked impact, showing clearly that the optimization of mass transfer and fluid dynamics issues have to be addressed when designing such process.

Tests were performed in a fixed bed flow reactor at 320 °C and 1 bar, with a high space velocity (11,236/h−1), using a feed which simulates the flue gas content of natural gas fired power plant. In all cases, the CH4 production in the first methanation step was lower than the amount of CO2 adsorbed in the corresponding capture step. This suggests the formation of unreactive carbonate on CaO, which could possibly be exploited only when using longer methanation times or higher H2 pressures. During the first 15–20 min of the capture step, CO2 adsorption is rapid. Also, Ru nanoparticles get rapidly but partly oxidized due to the presence of molecular oxygen in the feed (Fig. 5). In the regeneration/methanation step, CH4 production is relatively slower (40–60 min). In that timespan, three phenomena occur: (i) reduction of the layer of Ru oxide to get back to metallic Ru nanoparticles, (ii) CO2 spillover from CaO to Ru and (iii) CO2 methanation on Ru to form CH4 and H2O (Fig. 5). Studying the effect of temperature in the 280–350 °C range, an optimum was found at 320 °C. Since both adsorption and methanation are exothermic, both are thermodynamically favoured at low temperature. Low temperature allows capturing more CO2. However, kinetics are slow at low temperature. So, low operating temperatures result in low methanation rate and limit the productivity of the process. High temperatures on the other hand allow to boost methanation rate but decrease the capture capacity and also provoke excessive oxidation of Ru during the adsorption step which delays the onset of methanation in the regeneration/methanation step. This explains why an optimum has to be found.
Fig. 5

Schematic view of the working mechanism for dual functional materials. In Step 1 (adsorption), the adsorbent is progressively saturated with CO2. In these conditions, the metal nanoparticles may be partially oxidized due to the presence of O2 (among other possible gases) in the treated effluent, resulting in the formation of a passivation layer. In Step 2 (methanation) hydrogen is fed. The metal nanoparticles are reduced back to the metallic state and become active for the methanation reaction. CO2 molecules adsorbed on the adsorbent desorb or spillover towards the metal nanoparticles and are hydrogenated to methane. These steps are cyclically repeated

Another possible issue is the release of unreacted CO2 from the DFM at the beginning of the regeneration/methanation step. This may be caused by the large exothermicity of the methanation reaction, causing a sudden burst of CO2 desorption which the catalytic part of the DFM is unable to convert quantitatively. To tackle this issue, one possibility is to use an additional methanation catalytic bed after the main bed containing the DFM [65]. Otherwise, additional purification of the methane stream will be required.

Effects of O2 and H2O in the effluent

The impact of the presence of O2 in the feed on the performance of DFM was studied by Duyar et al. [61]. In this study 100 mg of 5%Ru-10%CaO/γ-Al2O3 powder was placed in a packed bed reactor and pre-reduced under a 5% H2/N2 mixture. Then, a 20 cycles experiment was conducted, each cycle consisting of an adsorption step, a purge in He, and then a methanation step. In the CO2 adsorption step, a flow of 10% CO2/air was applied for 20 min. In the methanation step a flow of 5% H2/N2 was applied, also for 20 min. The presence of O2 was shown to decrease methane production, due to the oxidation of Ru sites, leading to the formation of a surface layer of RuOx, which is inactive in CO2 methanation [70]. In another work on 5%Ru-6.1%NaO/γ-Al2O3, CO2 adsorption and methanation cycles were conducted at 320 °C using an O2-containing feed [65]. CO2 adsorption capacity was significantly reduced compared to the O2-free process. In fact, the presence of O2 in actual flue gases can limit the effectiveness of Ru (both for adsorption and methanation) due to its oxidation, but thanks to the ease of reduction of RuOx [71], this apparent deactivation can be easily reversed at the cost of a more prolonged exposure to H2. Alternatively, higher H2 partial pressure can be used in the regeneration step [65].

Since flue gases typically contain water vapour, the performance of DFM should also be studied in the presence of steam because it can provoke deeper oxidation and sintering. Upon exposure to steam (8% CO2/21% H2O/air) the production of methane from 5%Ru-10%CaO/γ-Al2O3 was shown to be rather stable for 19 cycles of adsorption and methanation [61]. Yet, in terms of CO2 capture, a slight decreasing trend was observed over the cycles. Again, this can be overcome by an extended exposure to the H2-containing flow.

Challenges and opportunities

The combined CO2 capture and methanation (CCCM) is currently emerging as a vibrant field of research. Innovative approaches in process engineering and materials science will be the key to open up new perspectives to transfer the technology from the laboratories to the actual plants. On the first hand, reactors and processes will have to be designed so as to cope with the stringent specificities of such process: unsteady-state operating mode, synchronicity of the reactor switches, variable feed composition, heat transfer management, etc. On the second hand, advanced catalysts preparation methods, including bottom-up approaches [72], will offer new routes for the design of highly efficient materials, especially in the context of dual functional materials, where two distinct functionalities have to be carried out by the same material.

In CCCM processes, it should be noted that, in addition to the produced methane, other gases will be present in the methane stream (unreacted CO2 and hydrogen, mainly). Thus, an additional question that has to be addressed is the purification of these streams before their addition to the natural gas distribution pipes, for example by membrane technologies [73, 74, 75].

Also, a practical challenge is to evaluate the best suitable gas treatment technologies, looking at the specificities of each process. CCCM is certainly not a panacea, and precise comparisons have to be made on the technological and energetic aspects. For example for the capture step, a choice has to be made between absorption and adsorption. For post-combustion plants, due to the large flue gases flow rates and the low CO2 concentration, chemical absorption in solvents seems more suitable than adsorption for CO2 capture [8]. Yet, the cost for the thermal regeneration is high, and the rates of equipment corrosion and solvent degradation can be prohibitive [7]. During regeneration, attention must be paid to the potentially negative environmental impact of solvent emission. As a matter of fact, the combination with a subsequent CO2 methanation step is not evident. Physical adsorption, on the other hand, is most suitable for CO2 capture at high pressures and low temperatures. As compared to liquid solvents, solid adsorbents can be used over a wider temperature range, yield less waste during cycling, and the spent solid can be disposed of without excessive environmental precautions. Yet these sorbents tend to have a lower CO2 selectivity and adsorption rate; and the pressure drop caused by the fixed bed can be excessive, for example in flue gas applications. The cost for regeneration is lower and the combination with a subsequent methanation step is more straightforward. All in all, informed decision will have to be made, based on the materials available as solvents or adsorbents, and on the specifications of the gas effluent which has to be treated.

Even if this short review is deliberately circumscribed to the combination of CO2 capture and methanation, it should be mentioned that such combined approach is of interest for other similar cases. For example, the combination of CO2 capture with the synthesis of syngas can be implemented using similar combined processes, with other reducing feeds and other catalysts. Thus, Al Mamoori et al. have demonstrated the use of DFM for the combined capture of CO2 and subsequent dry ethane reforming to produce syngas [76]. Bobadilla et al. have developed a “CO2 capture-reduction” process for syngas synthesis, using a DFM based on a hydrotalcite as a support, on FeCrCu as the reducing phase, and on K as a CO2 capture component [77]. K- and Ba-promoted Cu/Al2O3 materials [78] and Ni over Ce-modified CaO sorbents [79] were also investigated recently for the combined CO2 capture and reduction to CO.

Conclusion

This short review shows that, in the context of CO2 emissions mitigation, strategies based on the combined CO2 capture on solid adsorbents and CO2 methanation on metal-based catalysts are both pertinent and practically achievable. Both fields of CO2 adsorption and catalytic upgrading have made enormous progress over the last decades, and high-performance materials are now available both for capture and for methanation. Designing combined processes with distinct units for capture and for methanation is a first option that will mostly require process optimization. It allows to treat complex gas effluents on the one hand and to perform the methanation under “clean” conditions on the second hand. Processes where the two types of solids (adsorbent and catalyst) are combined in the same adsorption/methanation unit constitute a second option. In these cases, the catalyst not only has to exhibit high performance in its normal operating conditions (during methanation), but also has to withstand the operating conditions that are applied during capture (and vice versa concerning the adsorbent). Finally, a third strategy which is being intensely investigated is the use of dual functional materials, bearing both a capture function and a reduction function. The associated processes benefit from the proximity between the adsorption sites and the reduction sites, between which CO2 spillover can occur. Yet again, the development of efficient formulations is conditioned to the stability of both types of sites in the full cycle of capture and methanation.

All in all, the recent scholarly literature tends to show that CCCM could emerge as one of the practical solutions for the combined CO2 capture and utilisation, presumably for effluents with high CO2 concentrations, operated under moderate flow rates, and where the production of “green” H2 is available on-site.

Notes

Acknowledgements

Paulina Melo Bravo thanks the CONICYT for her PhD fellowship (BECAS CHILE 2018).

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© Zhejiang University Press 2019

Authors and Affiliations

  1. 1.Institute of Condensed Matter and Nanosciences (IMCN)UCLouvainLouvain-La-NeuveBelgium

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