Potential and challenges for CDR in the European pulp and paper sector

Abstract

The European pulp and paper sector has an estimated potential for large-scale, permanent Carbon Dioxide Removal (CDR) of 50 Mtpa, through the implementation of CO₂ capture and storage (CCS). CCS has been explored for this sector in literature, but results are limited compared to many other sectors and show substantial variability in costs and performance across mill types, operating conditions and locations. Several assessments highlight large kraft pulp mills in Northern Europe as among the most cost-effective opportunities. However, the characteristics of pulp mill black liquor recovery boiler flue gases raise concerns regarding the interaction with CO₂ capture, due to elevated levels of alkali species, sulfur compounds, chlorides, trace metals and other constituents. Potential challenges include solvent degradation, foaming, fouling, corrosion and operational instability. Supported by results from pilot testing on two black liquor recovery boiler flue gases at a Swedish kraft pulp mill, this Perspective argues that more research is required in the public domain to understand the physico-chemical mechanisms involved in alkaline solvent CO₂ capture from black liquor flue gases. Unleashing the CDR potential in the pulp and paper sector also requires a supportive regulatory framework and beneficial financial conditions, including buyers of CDR certificates on an megatonne scale. In view of the long investment cycles (up to 40 years) this Perspective calls for the development of scenarios for a stable, sustainable, climate-neutral economy well beyond 2050.

1 Introduction

Carbon Dioxide Removal (CDR) plays an essential role in achieving net-zero CO₂ emissions targets and is a central component of IPCC pathways toward carbon neutrality (Calvin et al., 2023) as the remaining carbon emission budget is rapidly decreasing (Forster et al., 2025). In the IEA’s Net Zero Emissions scenario, overshoot of the 1.5 °C target is considered to be inevitable, with a projected return below 1.5 °C by 2100 made possible by a rapid transformation of the energy sector and widespread deployment of CDR (World Energy Outlook, 2025). Also, the EU recognizes in its Industrial Carbon Management strategy that CDR will be required for reaching climate neutrality by 2050 (European Commission, 2024a).

A main avenue for permanent, quantifiable CDR is BioCCS – CO₂ capture and storage from partly or fully biogenic emission sources. In this context, pulp and paper mills often constitute large emission points with up to 100% biogenic CO₂ emissions_, largely arising from combustion of biomass-derived by-products in black liquor recovery boilers. Excess heat is available in many mills (Cruz et al., 2021), with potential to reduce additional fuel requirements for CO₂ capture. The pulp and paper sector therewith has a potential for high capture efficiencies while limiting additional pressure on land use, biodiversity, and biomass supply chains, compared to most other CDR options.

This Perspective briefly reviews the CCS and CDR potential in the pulp and paper sector and black liquor recovery boiler flue gas compositions and impurities, and potential challenges that may arise when these flue gas impurities interact with alkaline CO₂ capture systems. To date, the open scientific literature contains no detailed reports from pilot- or large-scale testing of post-combustion CO₂ capture applied directly to black liquor recovery boiler flue gases, although a limited number of industrial pilots have been announced. This constrains the ability to assess technical risks, identify critical impurities, and develop effective mitigation strategies. A summary is therefore provided of CO₂ capture pilot testing at a Swedish kraft pulp mill after which we argue that the challenges encountered during the tests call for further research on flue gas compositions as well as flue gas-alkaline solvent interaction. We conclude by discussing the requirements that need to be fulfilled for CCS implementation in the pulp and paper sector.

2 The potential for CCS and CDR from the pulp and paper sector

Since the pioneering work of Möllersten et al. (2003a), Möllersten et al. (2003b), Möllersten et al. (2004), Möllersten et al. (2006), implementation of BioCCS in the pulp and paper industry has been increasingly recognized as a promising pathway for cost-effective CDR. Several studies have explored the sector’s potential to contribute to permanent CDR (a.k.a. negative emissions) e.g. in Europe (Rosa et al., 2021; Kuparinen et al., 2019; Jönsson and Berntsson, 2012) and the United States (Sagues et al., 2020; Charles et al., 2024). These studies emphasize the sector’s heavy reliance on biomass fuels derived as by-products in the wood preparation and pulp production processes, resulting in significantly higher emissions of biogenic CO₂ compared to fossil CO₂. For example, in the United States, the pulp and paper industry currently emits 35 Mt/a of fossil CO₂ while biogenic emissions reach 119 Mt/a (Charles et al., 2024).

A large share of the biogenic emissions originates from relatively large point sources, notably from the combustion of black liquor in kraft mill recovery boilers. Kuparinen et al. (2019) estimated that global energy generation from black liquor in recovery boilers amounts to approximately 500 TWh/a, ranking black liquor as the fifth most important fuel globally after coal, oil, natural gas and gasoline. Top-down assessments conducted within the EU-funded ACCSESS project (Karlsson et al., 2025) suggest that in Europe, mills emitting more than 250 kt/a of CO₂ collectively account for around 55 Mt/a of biogenic emissions. Remarkably, this potential is concentrated in just 58 of the over 850 operational mills, since most mills are non-energy-intensive paper mills that primarily rely on electricity and/or fossil fuels. Notably, Sweden and Finland alone contribute approximately 40 Mt/a of biogenic CO₂ emissions from the pulp and paper sector (Kuparinen et al., 2019; Johnsson et al., 2020).

Available excess heat from the kraft mill process could be utilized to reduce additional energy requirements for CO₂ capture and thereby potentially lower the costs (Lacaze-Masmonteil et al., 2025). However, most of the excess heat available in such mills is at relatively low temperatures, typically in the range of 60–100 °C (Cruz et al., 2021). Heat at this temperature level is insufficient for amine solvent regeneration without the use of heat pumps but could be a promising heat source for capture technologies that operate at lower temperatures, such as advanced solvents or solid sorbents.

Previous research on CO₂ capture in the pulp and paper industry includes detailed integration studies (Onarheim et al., 2017a; Skoglund et al., 2023; Hektor and Berntsson, 2007) as well as techno-economic assessments (Lacaze-Masmonteil et al., 2025; Hektor and Berntsson, 2009; Kuparinen et al., 2021; Kuparinen et al., 2023; Nwaoha and Tontiwachwuthikul, 2019; Onarheim et al., 2017b; Skagestad et al., 2018; Stenström et al., 2025) across various mill types. These studies indicate clearly that costs may differ substantially between different types of mills and geographical locations, with large kraft pulp mills in Southern Sweden representing some of the most promising opportunities in Europe (Karlsson et al., 2025).

Several studies position BioCCS as one of several pathways for the sector to contribute to CDR (Joyo et al., 2025; Lipiäinen et al., 2022; Lipiäinen et al., 2023; Jönsson and Algehed, 2010). Others discuss the implications of CO₂ capture on, for example, biomass use (Karlsson et al., 2021) or operational flexibility in relation to electricity markets (Ingvarsson et al., 2025). Certain publications have focused on CO₂ capture from lime kilns, e.g., Lefvert and Grönkvist (2023), which offers cost advantages due to higher CO₂ concentrations, but does not unlock the substantial potential associated with recovery boilers. Some authors have investigated alternative capture technologies requiring significant modifications to the recovery boiler itself, such as calcium looping (Santos et al., 2021) or oxyfuel combustion (Sewring et al., 2025).

Common for all the references cited above is that they are desktop studies. To our knowledge, there are no scientific reports from large-scale testing of post-combustion CO₂ capture from black liquor recovery boilers that report the technical feasibility of BioCCS for this emission source. A pilot plant utilizing Andritz capture technology is currently in operation at Metsä Group’s Rauma mill in Finland. While initial reports suggest that the system “works well with the pulp mill’s flue gases” (Metsä, 2025), no detailed results have been publicly disclosed. A press release from November 2025 informs that SLB Capturi and CO280 have successfully completed CO₂ capture pilot testing at a pulp mill in the U.S (SLB Capturi, 2025).

3 Capturing CO₂ from black liquor combustion flue gases: potential challenges identified in literature

As illustrated in Figure 1, there are three major sources of CO₂ emissions from kraft pulp mills: black liquor combustion in the recovery boilers, calcination of CaCO₃ to CaO in the lime kiln, and auxiliary conventional power boilers burning bark and other biomass residues (Kuparinen et al., 2019; Onarheim et al., 2017a). CO₂ capture from calcination is similar to CO₂ capture from cement and lime, where the technology is under development and implementation, especially through chemical solvent-based capture at high TRL (Voldsund et al., 2019), while CO₂ capture from the auxiliary conventional boilers should be fairly straightforward. Hence, the main challenge in the pulp and paper sector would be the technical feasibility of CO₂ capture from black liquor recovery boilers, which is also the biggest emission source in a kraft pulp mill, typically representing ~75% of CO₂ emissions. Black liquor recovery boilers recover chemicals and generate energy from black liquor, a kraft pulping process byproduct that contains lignin, hemicellulose and spent cooking chemicals. In the boiler, black liquor is burned to produce heat while reducing sodium sulfate to sodium sulfide for chemical recycling. The combustion process occurs in multiple zones (Figure 2, left), balancing oxidation and reduction reactions to maintain the correct chemical makeup for recycling in the pulping cycle.

Figure 1

Figure 2

3.1 Main flue gas components and potential impurities

Black liquor recovery boiler flue gas typically contains 13–15% CO₂ and 3–5% O₂ (dry basis) after combustion with slight excess air. The CO₂ concentration is favorable for CO₂ capture, as it is slightly higher than for typical coal-fired boilers (12–14% CO₂) or natural gas boilers (4–10% CO₂) (Suhr et al., 2015; Hupa, 2007; Vakkilainen, 2005). However, the recovery boiler flue gas has a unique composition of high sodium and potassium content, sulfur compounds, chlorides, and potential trace contaminants. This presents distinct challenges that warrant careful consideration in the design and operation for CO₂ capture, compared to flue gases in coal-fired power plants and industrial processes such as cement kilns, where most practical CO₂ capture experience has been gained.

A literature review was conducted on impurities in black liquor combustion, including Best Available Technology (BAT) documents (Suhr et al., 2015) that describe appropriate primary and secondary measures to limit emissions. Black liquor contains, in addition to C, H, O, and N, typically 15–20 wt% sodium (Na) on dry basis, as well as significant quantities of potassium (K), sulfur (S), and chlorine (Cl) (Hupa, 2007; Adams et al., 2023). Emissions into the atmosphere may vary with the properties of the black liquor, operating conditions and flue gas-cleaning processes. Consequently, black liquor combustion can produce a range of impurities. Emissions of known pollutants are measured according to national and EU regulations, as well as site-specific permits from authorities.

The main regulated flue gas pollutants include SO₂, Total Reduced Sulfur (TRS), NOx and particulates. TRS is a gaseous mixture of sulfur compounds in their reduced state, primarily composed of hydrogen sulfide (H₂S), methyl mercaptan (CH₃SH), dimethyl sulfide ((CH₃)₂S), and dimethyl disulfide ((CH₃)₂S₂). Particulates are mainly comprised of sodium sulfate (Na₂SO₄). Modern recovery boilers are equipped with electrostatic precipitators (ESPs) that operate at high efficiencies (>99.5%) to control particulate emissions. To decrease SO₂ emissions, some installations are equipped with a flue-gas scrubber. Staged air combustion (Figure 2, left), where primary, secondary, and sometimes tertiary air ports create fuel-rich and fuel-lean zones, is the main approach to reduce NOx formation, as SCR and SNCR are unsuitable for use in this type of boilers.

Emission permits are issued for average emission levels, although process stability and emissions can fluctuate in practice. Especially, CO levels can vary significantly depending on operating conditions, particularly during operation below 70% of maximum continuous rating, boiler start-up/shut-down conditions or process disturbances. Properly operated recovery boilers maintain low CO levels, but fluctuations can indicate combustion instability and potential formation of unburnt carbon particles. This could affect the performance and operation of downstream flue gas cleaning equipment and ultimately the CO₂ capture system. According to the literature, the presence of unburnt carbon particles or carbon-rich particulates is uncommon in properly operated boilers but can occur during upset conditions (Adams et al., 2023).

In-depth studies have shown that additional compounds that could affect carbon capture systems might be found in the flue gas. This includes trace metals (such as sodium, potassium, zinc, lead, and iron), acid gases (HCl, SO₂), polycyclic aromatic hydrocarbons (PAH), and potentially dioxins/furans (Adams et al., 2023).

3.2 Potential challenges for amine-based CO₂ capture

To the authors’ knowledge, no previous studies in literature have specifically focused on black liquor flue gases characteristics in relation to CO₂ capture, which underscores the need for further investigations. There is however general knowledge in the literature related to flue gas impurities interactions with amine solvents, as briefly summarized below:

• SO₂ and NOx: These acid gases react with amine solvents, leading to the formation of heat-stable salts (HSS), irreversible solvent degradation, loss of CO₂ absorption capacity, and increased corrosion (Supap et al., 2009; Fostås et al., 2011; Zhou et al., 2012; Vega et al., 2014). SO₂ has been shown to accelerate oxidative degradation of amines dramatically (Uyanga and Idem, 2007; Gao et al., 2011). Target SO₂ concentrations below 10–50 ppm have been recommended for amine-based capture systems (Rochelle, 2009).

• O₂: Oxygen in flue gas (typically 3–5% in kraft recovery boilers) causes oxidative degradation of amines, resulting in solvent loss and formation of degradation products (Goff and Rochelle, 2004; Vevelstad et al., 2013; Voice and Rochelle, 2013). While oxygen cannot be economically removed from flue gas, its impact can be mitigated through solvent selection, the use of inhibitors, and process optimization (Buvik et al., 2021).

• Particulates and trace metals: Fly ash and metal contaminants (particularly iron, copper, vanadium, chromium, and nickel) can act as catalysts, accelerating amine degradation rates (Sexton and Rochelle, 2009; Goff, 2005; Nikolic et al., 2015). These metals may originate from the black liquor itself or from corrosion of equipment. Efficient particulate removal via ESPs is critical (Suhr et al., 2015).

• Chlorides: Chlorine compounds in the flue gas can contribute to corrosion issues and the formation of deposits in both the recovery boiler and potentially in CO₂ capture equipment (Hupa et al., 1987; Tran et al., 1999).

4 Results from pilot testing at a kraft pulp mill

A 2 tonnes-per-day CO₂ capture pilot unit was tested at a kraft pulp mill in Skutskär, Sweden, from early 2024 to early 2025 (Figure 2, right). The main objective was to verify the technical feasibility of post-combustion CO₂ capture from black liquor recovery boiler flue gases. To the authors’ knowledge, at the time this represented the world’s first CO₂ capture tests with flue gas from black liquor combustion.

The CO₂ capture pilot used Saipem’s proprietary enzyme-enhanced potassium carbonate solvent (Saunier et al., 2019), rather than amine technology, for two reasons: First, the reduced environmental impact due to the use of a non-toxic, non-volatile solvent, avoiding emissions of harmful compounds to atmosphere or water. Second, solvent regeneration occurs at 70–80 °C, which is beneficial for kraft pulp mill integration (ref. to section 2). The pilot had previously operated successfully with Waste-to-Energy and refinery flue gases and has since successfully operated with cement kiln flue gases, within the same project.

During the experimental campaign on the black liquor combustion flue gas at the kraft pulp mill, unexpected excessive foaming and fouling was observed within the alkaline solvent circuit; an issue never experienced during tests at other sites. This phenomenon interfered with the solvent’s ability to capture CO₂ and prevented the achievement of stable operating conditions; additionally, it also led to significant fouling of the pilot equipment. The phenomenon was observed for two different black liquor recovery boilers at the kraft pulp mill. Despite differences in combustion principles and pretreatment configurations for the two boilers, the observations of foaming and fouling were consistent, suggesting that this was inherent to the flue gas generated by this specific biogenic source rather than to a specific boiler configuration. Notably, the interaction was detected even using a solvent consisting solely of water and potassium carbonate (i.e., prior to the active ingredient addition: the enzyme), indicating that the behavior was intrinsically linked to the flue gas contact with an alkaline solvent. Although findings from flue gas and solvent analyses provided useful results, representative data applicable for, e.g., further CO₂ capture scale-up were limited.

A comprehensive troubleshooting activity to determine the root cause(s) included a deep analysis of recovery boiler flue gas composition, analysis of the solvent quality and the generated agglomerates. A review of boiler operating conditions and flue gas pretreatments upstream the capture unit was also performed (section 3). Several critical impurities, that could potentially react with or interfere with the alkaline capture process, were identified. The presence in the flue gas sent to the pilot of some of these components was evaluated through targeted flue gas analyses. Given the wide range of possible analyses for flue gases and the need to limit their scope and duration, only the most relevant tests were conducted, providing first indications of what would be the necessary flue gas treatment for stable operation of the CO₂ capture pilot. In sum, although the test campaign was less fruitful than anticipated up front, it generated substantial, valuable learnings on the way toward large-scale CO₂ capture in kraft pulp mills and highlighted the importance of on-site CO₂ capture pilot testing before scaling up.

5 Discussion

The theoretical potential for CDR in the pulp and paper sector exceeds 50 Mtpa in Europe alone. However, specific challenges with capturing CO₂ from black liquor recovery boilers need to be addressed for this potential to materialize. During 2025, industry reported instances of successful CO₂ capture pilot testing with amine technology but without providing further details (Metsä, 2025; SLB Capturi, 2025), while the literature indicates potential flue gas/solvent interaction challenges with CO₂ capture for amines in contact with impurities whose presence cannot be excluded for recovery boiler flue gases. The above-mentioned pilot testing of Saipem’s proprietary, enzyme-enhanced potassium carbonate solvent experienced challenges with excessive foaming and fouling when tested on black liquor recovery boiler flue gas, while the technology has been successfully tested on other industrial flue gases with the same CO₂ capture pilot and the same solvent technology. Our findings indicate that CO₂ capture from black liquor recovery boilers may not be straightforward to implement, due to the properties of the flue gas. A hypothesis based on the findings presented here is that there may be trace amounts of unburnt particles and other components, that are not measured on a regular basis today, that may be fluctuating due to process instabilities, and that are not removed with current flue gas cleaning steps. These trace amounts may have a practical impact on CO₂ capture implementation, not only for solvents but also for membranes or MOFs. Components might even be present under the detection limit with current measurement technologies.

Overall, we argue that further research is required, including dedicated flue gas analyses and characterization as well as pilot-scale projects, to assess different capture technologies for the pulp and paper sector. This will build understanding of the physico-chemical mechanisms involved and enable identification of potential problems with different technologies. This would, in turn, pave the way for capture-technology tailored flue gas cleaning trains downstream black liquor recovery boilers, which could enable CDR deployment in the sector. It also needs to be verified if the issues experienced in Skutskär were site-specific.

In addition to solving technological challenges, the realization of the significant CDR potential in the pulp and paper sector requires stable, voluntary or policy-driven market demand. Biogenic CO₂ emissions are typically not covered by carbon pricing mechanisms, but there are other options for incentivizing BioCCS, such as reversed auctions, where Sweden has been a frontrunner. There are several existing certification schemes for CDR on the Voluntary Carbon Market (VCM) (Arcusa and Sprenkle-Hyppolite, 2022), and also the EU Carbon Removals and Carbon Farming Certification (CRCF) regulation currently targets the VCM (European Commission, 2024b). CRCF methodologies were presented in February 2026 for permanent carbon removals, including BioCCS (European Commission, 2026). The integration of carbon removals into the EU-ETS and the associated risks have been studied, suggesting that a potential way forward could be a gradual introduction of CDR over time (Verbist et al., 2025; European Environment Agency, 2024). It has been announced that the revision of the EU-ETS in 2026 will include an assessment of the inclusion of CDR (European Parliament, 2026).

Deployment of CCS in the pulp and paper sector could balance significant residual hard-to-abate European fossil emissions, therewith contributing to climate neutrality by 2050, as enshrined in European law (European Commission, 2021). However, realizing CDR at scale in the pulp and paper sector currently requires that enough buyers are willing to enter into long-term purchase agreements for large quantities of CDR on the VCM. From the sector perspective, long-term foreseeable revenue streams, up to 30–40 years ahead, may be required to make an investment decision, which is an obvious hurdle to overcome. In sum, in view of this long-time horizon for climate technology investments, we call for the development of scenarios far beyond year 2050 for a healthy, sustainable, climate-neutral economy.

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