Abstract
Retrofitting existing emission sources with carbon capture systems can alter the emission profile. Emissions of some pollutants[1] may decrease, others may increase, and new substances could be introduced into the environment.[2] Established permitting frameworks, measurement techniques, and emission control approaches in other industrial contexts provide a basis for regulatory assessment of post combustion CO2 capture (PCC) in industrial processes*.
Previous comprehensive literature reviews on the impact of carbon capture and storage (CCS) deployment on air quality have primarily focused on the coal and natural gas power industries or have not addressed the impacts on co-pollutants (SEPA, 2015; Buvik et al., 2021; Gibbins, Lucquiaud and Samson, 2024; Rochelle, 2024)[3]. The industrial sector is a significant contributor to global greenhouse gas emissions, contributing around 25 % (GCCSI, 2016), with key industries such as cement, iron and steel, and refining being responsible for a broad spectrum of pollutants beyond carbon dioxide (CO2).
Therefore, there is a clear need for a comprehensive study such as this one. A team of Independent experts from Imperial College London, led by Prof Paul Fennell, were commissioned via Imperial Consultants to undertake this work, which assesses the air pollutant implications of CCS retrofit in heavy industry by comparing baseline emissions from representative host facilities in the cement, iron and steel, and refining sectors prior to capture with emissions expected following CCS integration. A gap analysis was also conducted to identify research areas that are currently underexplored, and the study investigates potential synergies between existing air quality control technologies and CCS systems.
[1] Such as sulphur dioxide, nitrous oxides, and particulate matter.
[2] Pollutants other than CO2 are referred to as “co-pollutants”.
[3] The authors are unaware of existing literature reviews examining the impact of CCS deployment in industries and projects that combust biomass or materials with high biogenic content, such as waste-to-energy (WtE) and bioenergy with carbon capture and storage (BECCS). This report does not specifically address biomass combustion. However, studies in this field were included when relevant for context.
Key Findings
- While CCS is primarily deployed to capture (and store) CO₂ emissions, it can also deliver important co-benefits. Studies suggest that reductions in NOₓ, SO₂ and PM₂.₅ associated with CCS can improve material public health benefits, which may improve the overall societal value of CCS deployment, although outcomes and the extent of any health-related savings are site specific.
- Severe health effects associated with SOx and NOx often arise from their atmospheric conversion into secondary PM2.5 and from ozone formation. Regulatory frameworks generally emphasise controlling these precursors through the use of best available technologies (BATs).
- Solvent developers are creating formulations and mitigation methods that tolerate higher concentrations of co-pollutants absorbed into or interacting with the solvent during capture, which can influence the extent of upstream flue gas cleaning required and the potential for solvent-related emissions. However, the specific tolerance limits remain proprietary and are not disclosed to the academic community.
- The role of the amine unit in PM control is context dependent. When stringent upstream PM controls are not in place, the amine unit functions as a PM removal device. Conversely, with highly effective upstream controls, the effect of condensable PM (CPM) formation may dominate. Even without upstream controls, CPM formation is still possible but is overshadowed by overall filterable PM (FPM) removal.
- NO₂ is the primary reactive NOₓ species in amine systems, absorbed mainly through hydrogen abstraction to form amine radicals and nitrite. NO shows little reactivity with monoethanolamine (MEA), although amine radicals can react directly with NO to formnitrosamines, a route that is generally minor in overall NOₓ reactions with amines.
- SO₂ is readily absorbed by amine solvents, forming heat-stable salts (HSS) that do not degrade under normal operating temperatures. Operators typically rely on upstream flue gas desulfurisation (FGD) to prevent HSS build-up, as solvent reclamation is energy intensive. This ensures SO₂ concentrations in the absorber outlet gas exiting the amine units are usually below 1 ppmv.
- Nitrosamines (NSAs) and nitramines (NAs) attract significant regulatory and research attention, given their carcinogenic and mutagenic potential because detected levels are often at or near instrument detection limits.
- In plume modelling, interactions with emissions from nearby stacks are important, particularly in industrial clusters where NO₂ from secondary sources can react with amine emissions, potentially compounding impacts at downwind receptors.
- Permit approval for CCS plants is typically contingent upon robust modelling demonstrating that predicted ground-level concentrations of these compounds remain below regulatory thresholds.
- Air quality assessments of CCS in industry should also consider indirect co-pollutant emissions from added energy use, as well as liquid and solid waste streams.
- While CO₂-side waste streams are generally smaller in volume and lower in contaminant concentration than flue-gas-side streams, they should not be assumed to pose negligible environmental risk. For instance, a short vent stack on the TEG regenerator could lead to localised ground-level impacts if amines or other volatile contaminants are present.
- Solid waste streams are less documented, with the only reference found in the 2015 Scottish Environment Protection Agency (SEPA) review.[4]
- Air quality implications and health co-benefits of post-combustion capture in industry are site- and technology-dependent. Implications are typically influenced by plant type, age, capture media, baseline emission controls, fuel characteristics, and proximity to population centres, with newer facilities or those already upgraded to BAT controls showing smaller incremental improvements relative to older, higher-emitting plants.
- Evidence base is still emerging. While studies indicate that amine CCS in heavy industry can deliver reductions in SO₂ and PM and generate large health benefits (marked reductions in population health burden within the local exposure catchment area), methodological uncertainties mean findings should be seen as indicative rather than definitive. Robust, site-specific analysis is needed to underpin reliable health and environmental assessments.
- Typical continuous emission monitoring systems report a single NOx value as NO2 equivalent and are optimised for total NOx compliance, rather than robust NO and NO2 speciation. Inventory reporting therefore, often relies on assumed or default NO and NO2 splits.
- This matters because nitrosamine formation depends on NOx speciation and nitrosating chemistry, so the NO and NO2 balance can materially influence impact assessments. Assumptions that were previously acceptable for compliance purposes may no longer be adequate for assessing nitrosamine risk.
- SO3 is generally not included in emission inventories, despite its strong influence on amine aerosol formation and solvent loss. Available SO3 measurements are limited to a small number of pilot or site-specific studies, creating uncertainty around typical levels and variability across the industry.
- There is a notable lack of publicly available data on air quality implications of PCC in heavy industries such as cement, steel & iron production.
- To bridge these knowledge gaps, ongoing research projects, such as AURORA[5], will provide valuable data to the research community. Furthermore, the U.S. Department of Energy (DOE) is deploying a mobile emissions testing unit and funding front-end engineering design (FEED) studies as well as pilot plant studies for PCC deployment in cement plants and steel mills.[6] It is hoped that data from these efforts will be publicly shared to benefit the entire research and industrial community.
[4] SEPA (2015) Review of amine emissions from carbon capture systems. Scottish Environment Protection Agency. Available at: https://www.sepa.org.uk/media/141046/review_of_amine_emissions_from_carbon_capture_systems.pdf (Accessed: February 19, 2025).
[5] About us | aurora” (2023), 19 June. Available at: https://aurora-heu.eu/about-us/ (Accessed: February 10, 2025)
[6] US DOE (2024) Multi-Year Program Plan: Point Source Carbon Capture. US Department of Energy.
