May-2023

Carbon utilisation to accelerate the transition to net zero

Captured carbon can be converted into valuable products, enabling circularity and decarbonisation of hard-to-abate industries.

Cecilia Mondelli Sulzer Chemtech
Brent Konstantz Blue Planet Systems

Article Summary

Global decarbonisation targets are driving the demand for carbon capture, utilisation, and storage (CCUS) solutions that can be applied within power generation and manufacturing facilities to slash CO2 emissions. As the CCUS offering continues to expand, it is important for companies to be able to recognise the technologies that are ready and feasible for commercial use and best suited to drive more sustainable practices while maximising profitability and efficiency.

Since the beginning of the first Industrial Revolution in 1750, more than 1.7 trillion tonnes of CO2 have been emitted, most of which (87%) were due to the combustion of fossil fuels from the second half of the 20th century onwards (Friedlingstein, et al., 2022). On average, global emissions increased by nearly 2.8% annually from 1950 to 2021, and are now more than 63% above the levels of 1990 (Friedlingstein, et al., 2022), the reference year for the Kyoto Protocol and the Paris Agreement.

This continuous growth is in stark contrast to worldwide efforts to limit global warming to well below 2°C, preferably to 1.5°C, compared to pre-industrial levels. In fact, meeting these targets would require a linear decrease in anthropogenic CO2 emissions by about 1.4 billion tonnes of CO2 each year (Friedlingstein, et al., 2022).

To meet this challenge, energy and manufacturing players worldwide need to adopt suitable mitigation strategies and solutions. Industrial GHG emissions comprise 29% of global emissions. Eliminating the emissions from the use of energy in industry and direct industrial processes would curtail global emissions by 14.7 billion tonnes per annum (Ritchie, Roser, & Rosado, 2020).

Fortunately, industrial players can employ a range of technologies to reduce their carbon emissions. One of the most popular and effective groups of technologies currently available is CCUS, which embraces carbon capture and storage (CCS) and carbon capture and utilisation (CCU, see Figure 1). CCUS techniques within the energy sector are generally classified based on how emissions are captured and then processed, as outlined below.

Carbon capture methodologies
When looking at capturing methodologies, these are generally discerned within the energy sector as pre-, post-, and oxy-fuel combustion solutions, depending on whether CO2 is captured during intermediate reactions, from waste streams, or in processes involving the burning of fuels with diluted or concentrated oxygen.

While all these carbon capture methods have been applied to different setups, post-combustion strategies offer the most mature and easy-to-implement technologies. They are the most common solution utilised in commercial-scale energy plants, and similar approaches are being applied to waste-to-energy and biomass-to-energy facilities as well as to reduce carbon emissions from waste gas effluents in chemicals, metals, and cement production plants.

By contrast, pre-combustion involves high capital expenditure (Capex) and operational expenses (Opex) due to the complexity of its processes and the number of operating units required. Oxy-fuel frameworks are generally economical; however, current applications have failed to offer feasible options for large-scale facilities, mainly because of various operational challenges (Kheirinik, Ahmed, & Rahmanian, 2021).

Post-combustion technologies
Currently, post-combustion technologies feature different technology readiness levels. The most commonly discussed methods leverage absorption or adsorption technologies to remove CO2 from flue gases, whereas fewer approaches rely on permeation principles (membrane separation).

The mechanisms behind these three alternatives can differ greatly. Membranes for carbon capture are designed so that they are highly permeable for CO2. In this case, the specific properties of the membrane determine the efficiency of the separation process. In adsorption methods, CO2 binds to the surface of a solid sorbent, such as hydrotalcites, lithium zirconate, active carbon, molecular sieves, calcium oxides, or zeolites. Following this step, CO2 is desorbed via pressure or temperature swings.

Absorption techniques, on the other hand, use liquid solvents, such as aqueous monoethanolamine, diethanolamine, piperazine, or potassium carbonate, rather than solids. These absorb the CO2 gas through the formation of a chemical bond with the dissolved reactant. The CO2-rich liquid solvent is then processed to release the CO2 (stripping) and regenerated, returning as a CO2-lean chemical for reuse in more cycles, in line with circular practices. To date, absorption is the most mature solution for post-combustion installations, as it has been extensively researched and commercialised.

Absorption for large-scale CO2 capture
This method offers optimum separation efficiencies, which can reach 90% and above. To achieve peak performance, it is important to leverage the most effective column components for absorption and stripping, such as bed packings and other internals. Specifically, while the number of beds and their height play a key role in the separation process, their design also heavily influences the possible outcomes, with the geometrical structure of the packings defining the hydraulic and mass transfer properties.