Decarbonisation Technology - February 2023

(BPMED), and redox active molecules that undergo proton-coupled electron transfer (PCET) or capacitive deionisation. The processes do not require external sources of heat or high pressures/vacuum for operation, nor degradation of the sorbent material is expected (Shaw and Hatton, 2020). In such systems, the electric potential gradient is the main driving force, which can be precisely controlled to drive chemical reactions isothermally. (Stern, Simeon and Herzog, 2013), (Wang, Herzog and Hatton, 2020), (M. C. Stern, et al., 2011). Several bench-scale demonstrations of carbon capture via electrochemical pH-swing have achieved CO 2 capture and release at the promising value of ∼ 100 kJ mol −1 CO 2 (Datta, et al., 2013), (Eisaman, et al., 2011). An additional feature of electrochemical methods is their ability to integrate CO 2 capture with utilisation (De Mot, et al ., 2019). For instance, a modular electrochemical pH- swing system coupled with a CO 2 reduction cell converts the captured CO 2 to carbon monoxide (CO) (Digdaya, et al., 2020), (Smith, et al., 2019). In addition, reactive CO 2 capture, in which the capturing medium pre-concentrates the CO2, has become an emerging field for integrated CO 2 capture and conversion (Li, et al., 2019), (Lees, et al., 2020). Two new carbon capture developments include:  Carbon capture from seawater using bipolar membrane electrodialysis (BPMED) can provide a sustainable route to capture dissolved inorganic carbon (DIC). An in-situ pH-swing inside of the BPMED cell reduces the energy consumption of oceanic-DIC capture (Sharifian, Boer, et al. 2022)  Heavy-duty road and marine shipping are two segments in the transport sector where switching to battery electric power trains is problematic. This has stimulated interest in using on-board carbon capture to capture the CO 2 from the combustion of hydrocarbon fuels from fossil or renewable sources. Storage volumes and mass requirements have been calculated for a wide range of vehicle types, and prototype systems have been successfully trialled (Mandra, 2022), (Schmauss and Barnett, 2021). Chemical looping combustion for CO 2 capture Chemical looping combustion (CLC) enables the

Me O X Y

CO, HO

Air

MeO oxidation Air reactor

MeO reduction Fuel reactor

Me O X Y–1

Fuel

Figure 2 Block diagram of chemical-looping combustion process

burning of hydrocarbon fuels while dramatically reducing emissions of CO 2 into the atmosphere (Sreenivasulu, et al., 2013). CLC is a two-step cyclic process (see Figure 2 ) comprising the fuel reactor (FR), where combustion takes place, and the air reactor (AR), where oxygen carriers (OC) are regenerated (Alalwan and Alminshid, 2021), (Czakiert, et al., 2022). The inherent separation that takes place results in a highly reduced internal load of the plant. These interconnected reactors facilitate alternate oxidation and reduction of the OC, as shown in Equations 1 and 2 (Adanez, et al., 2012). (1) Fuel reactor: (2n+m)MyOx + CnH2m → (2n+m)MyOx- 1+mH 2 O+nCO 2 (2) Net reaction of CLC: CnHm + ½ (2n+m) O 2 → n CO 2 +m H 2 O + heat (3) Air reactor: MxOy-1+1/2 O 2 → MyOx The OCs circulating through the CLC unit require a higher particle residence time for reduction than for oxidation (Lyngfelt and Thunman, 2005), (Ryu, Jin and Yi, 2005). There are two crucial benefits of OCs in a power generation system: the elimination of conventional energy-consuming air separation units and the inherent separation of flue gases (mainly CO 2 and H 2 O) from the air supplied to the combustion process. As a result, the flue gas that leaves the fuel reactor is free of atmospheric nitrogen, and thus the CO 2 is almost ready for geological sequestration or commercial utilisation (Anthony, 2010). The exhaust gas from the air reactor consists essentially of nitrogen and oxygen, both environmentally benign gases. The reactive species in OCs are oxides of transition metals, such as Ni, Fe, Cu, and Mn. Ni-based OCs exhibited the best reactivity and stability during multiple redox cycles. Criteria for selecting OCs include their oxygen vacancies,