Impact of operating parameters on CO₂ capture using carbon monolith by Electrical Swing Adsorption technology (ESA)

Highlights

• MAST carbon monolith experimentally assessed for its CO₂ capture performance.

• CO₂ capture studied over a range in electrification conditions and flow rates.

• The optimal current for maximum purity was 12 A.

Abstract

Electrical Swing Adsorption (ESA) technology is an interesting option for rapid temperature swing adsorption to capture CO₂ due to its highly efficient and rapid regeneration with the Joule effect. Electric current, electrification time and N₂ purge rate have strong effects on ESA processing performance. In this study, these operating parameters were investigated experimentally and theoretically, for their effect on CO₂ product purity, recovery, harvest time and energy consumption. For a feed stream of 15% CO₂ balanced with N₂ using a MAST© carbon monolith, CO₂ purity and energy consumption significantly increased as electric current and electrification time increased; CO₂ recovery increased as N₂ purge rate increased; these three factors all impacted on CO₂ productivity positively, that is, productivity increased with increase of each of electric current, electrification time and N₂ purge rate. The optimal conditions obtained for CO₂ capture using MAST© monolith in this study are electric current of 12 A, electrification time of 80 s and N₂ purge rate of 600 ml/min (0.088 m/s). Under such optimal condition, a CO₂ purity of 52%, and a recovery of 76% was achieved with an energy consumption of 5.64 MJ/kg.CO₂.

Introduction

Carbon dioxide (CO₂), a major greenhouse gas, is emitted from a range of industries such as power plants, iron and steel, refining, petrochemical, and cement manufacturing. If the emission trend continues, the average global temperature will increase by around 6 °C in the long term [8], [9].

Carbon capture and storage CCS) is one potential technology to help mitigate these Greenhouse Gas GHGs) emissions [12]. There are at least five technologies which have been explored for post-combustion CO₂ capture – solvent absorption, solid adsorption, cryogenics technology, membrane separation, and microbial/algal systems [13], [19]. Solvent absorption, solid adsorption, and membrane technology are the most mature technologies for CO₂ capture, with solvent absorption currently deployed in capture plants at Boundary Dam and Petra Nova. Although solvent absorption is readily scaled up, it has a high energy demand for regeneration and also poses problems such as solvent loss by evaporation, equipment corrosion and formation of potentially carcinogenic nitrosamines [11]. Compared with absorption, adsorption-based technology offers a number of advantages such as simplicity of operation, relatively low energy requirement and ease of retrofitting to existing facilities [20]. This technology, which has been developed over the past decades, includes Pressure Swing Adsorption (PSA), Vacuum Swing Adsorption (VSA), Temperature Swing Adsorption (TSA) and Electrical Swing Adsorption (ESA). ESA has recently attracted interest because it can quickly and directly heat the adsorbent for regeneration via the Joule effect, resulting in a short heating time and high regeneration efficiency [18]. Thus, higher gas throughput, high purity and recovery can be achieved by ESA, compared to conventional TSA in which gases are used to heat the bed.

The adsorbent in ESA must also be electrically conducting, so activated carbon in a continuous form such as a monolith) is a common adsorbent for this technology [18]. Compared with activated carbon beads and fiber cloth, activated carbon in monolith form is preferred in the ESA process [10], due to its lower pressure drop, higher permeability, higher electrical resistivity and lower cost. Yu et al. [24] first proposed electrothermal desorption on an activated carbon monolith using the Joule effect. They controlled the desorption rate by maintaining electrical current and a specific purge gas flow rate in removal of toluene. Subsequently, they conducted a series of experiments and theoretical simulations on the removal of toluene by the ESA process using activated carbon monolith and found that the maximal desorbed concentration of toluene and recovery increased almost linearly with current intensity and purge flow rate [23]. In 2008, Grande et al., simulated CO₂ removal from a mixture of low concentrated CO₂ balanced with Helium. From their simulation, a CO₂ purity of 16% with a recovery of 89% was obtained using a feed gas with 4.5% CO₂ at an adsorption temperature of 293 K and desorption temperature of 423 K [6]. They studied the effect of electrode materials on electrothermal performance and found brass electrodes presented lower energy losses in the contact between the electrode and the surface of monolith [16]. They also designed a novel ESA processing cycle with seven steps including feed, rinse with recycled gas, internal rinse, electrification, depressurization and purge to separate CO₂ from a mixture of low concentrated CO₂ balanced with N₂. A CO₂ purity of 89.8% with a recovery of 72.0% was achieved from their simulation [4]. Furthermore, in order to improve adsorption characteristics of activated carbon monolith on CO₂, a hybrid zeolite 13X monolith system was employed, CO₂ product with purity of 46.6% and recovery of 80% was obtained experimentally from the six step ESA cycle and the estimated energy consumption could be reduced to 6 GJ/tonne CO₂ [17].

Research on CO₂ capture from flue gases by ESA technology is still in its infancy. It is important therefore to systematically investigate the influence of ESA operating parameters on ESA performance, specifically CO₂ purity, recovery and energy efficiency. The major variables affecting these performance metrics are electrification time, current intensity, and nitrogen purge. In this study, a four–step ESA cycle was used, and included adsorption, electrification, N₂ purge and cooling steps. In order to reduce the amount of experiments, a central composite design (CCD) was employed to analytically design a series of experiments with various parameters [7] using the Design Expert Design of Experiments software. In this software package, response surface methodology (RSM) [2], [3], [14], was applied to analyse the experimental data and to determine how important these factors (current intensity, electrification time and purge rate) were in affecting CO₂ purity, recovery, CO₂ harvest time and energy consumption. Furthermore, optimal values of the operating parameters, such as electric current, electrification time, N₂ purge rate and time for CO₂ capture process, at fixed electric current, were determined and confirmed by experimental data and simulation results.