Decarbonisation Technology - February 2023

oxygen transport capacities, costs, and tendencies over coke deposition, agglomeration, and attrition. OCs for CLC can be divided into single oxides, mixed oxides, natural minerals, spinel from mixed metal oxides, and perovskite (Qasim, et al ., 2021). CLC processes can be classified in two groups: systems with OCs (Shen and Huang, 2018), and carbonate looping systems with OCs (Martinez, et al ., 2011) (Luo, et al., 2018). Lyngfelt and colleagues reported a study in which 46 smaller chemical looping combustors operated for more than 11,000 hours (Lyngfelt and Thunman, 2005). The progress in developing CLC technology over a short time, including such experience with continuous operation of pilot plants, is encouraging a shift towards commercialisation (Nandy, et al ., 2016). So far, CLC has focused mainly on gaseous fuels using metal oxides as carriers, with further development needed to extend the application to solid fuels, such as coal. This will require higher combustion efficiencies and more effective metal oxides. Co-production of hydrogen and CO 2 capture The IEA considers the key pillars of decarbonising the global energy system: CCUS, energy efficiency, electrification, renewable energy sources, hydrogen, hydrogen‐based fuels, and behavioural change. The importance of hydrogen in the IEA’s net-zero emissions scenario (NZE) is evident from its increasing share in cumulative emission reductions. The use of hydrogen and hydrogen-based fuels could result in reduced emissions of up to 60 Gt CO2 by 2050, or 6% of total cumulative emissions reductions in the NZE scenario (IEA, 2022a). Oil refining was the single largest consumer of hydrogen in 2020 (close to 40 Mt hydrogen). The main production units within a refinery are the catalytic reformer, where hydrogen is a by- product from gasoline production and dedicated units, such as steam methane reformers. Additionally, integrated petrochemical refineries can source hydrogen from the naphtha cracker, while any remaining hydrogen demand may be met from merchant hydrogen. Several refineries in China have gasification units (for heavy residues or coal), which provide almost 20% of hydrogen production (IEA, 2022b).

In 2005, Shell installed a unit at its Pernis refinery to capture CO2 from the heavy-residue gasification units. Since then, six facilities combining CCUS with hydrogen production have been commissioned, with the most recent at the North West Sturgeon refinery in Canada (Shell, 2005). Another 30 projects to retrofit CCUS on current fossil-based hydrogen production units have been announced. Chlapik et al. present advanced reforming technologies for retrofitting existing refinery hydrogen plants to achieve long-term CO2 capture while minimising site space requirements and capital (Chlapik, Winch and Dierking, 2022). Streb et.al have characterised new vacuum pressure swing adsorption (VPSA) cycles for high recovery of high-purity CO2 co-produced with H2 from a ternary (multi-component) feed stream with a significant amount of impurity. The first publication identifies two cycles of the process that can purify CO2 up to 95% with recoveries greater than 90% while co-producing hydrogen (Streb, Hefti, et al., 2019). Their second publication assesses the performance of VPSA for co-purifying H2 and CO 2. VPSA allows for the integration of two separation tasks, which simplifies the coupling of H2 production with CCS. Performance of five different VPSA cycles with four different feeds, typical for steam methane reforming (SMR) and autothermal reforming (ATR) of natural gas or biomethane, at two different H2 purity levels (99.9 and 99.97%) were compared. Three out of the five cycles met the higher H2 purity level with a recovery rate of 90% for CO2, which met the specifications for CCS (Streb and Mazzotti, 2020). Summary Herein, an attempt has been made to review innovations in the three main approaches for carbon capture: pre-combustion, post- combustion, and oxy-fuel combustion. The techniques are absorption, membranes separation, adsorption, and cryogenics. Absorption is an established CO2 separation procedure that divides into two classes: physical, reliant on both temperature and pressure, and chemical, where absorption of CO2 is reliant upon neutralising acid-base response. Remarkable of the favoured solvents are amines (mono ethanol