Green liquor dregs for carbon capture, utilization, and storage: initial LCA and economic analysis

2.1 Materials

The analyzed GLD material is provided by a pulp mill located in northern Sweden, producing around one million tonnes pulp annually. The composition of the GLD varies depending on the type of feedstock, the process technology, operating conditions, and the properties of the pulp. Commonly, GLD consists of a diverse array of elements, such as calcium (Ca), sodium (Na), magnesium (Mg), sulphur (S), carbon (C), iron (Fe), manganese (Mn), potassium (K), phosporus (P), silicon (Si), aluminium (Al), and copper (Cu) (Kinnarinen et al. 2018). On average, Sweden produces approximately 12 million tonnes of pulp annually. Approximately 4–20 kg of GLD per tonne of pulp is produced in mills, depending on the size of the mill (Kinnarinen et al. 2018). These pulp mills typically also produce flue gas that can serve as a source of the CO₂ required for the carbonation reaction. The carbonation process leads to the formation of carbonated material that can potentially be utilized in a variety of ways depending on the properties of the final material.

The investigated options for carbonates’ utilization are 1) landfilling, and their use as: 2) replacement of natural fine aggregates in road construction; and 3) replacement of a portion of ordinary Portland cement. By replacing part of these materials with carbonates, the pressure on natural resource sources could be reduced (National Academies of Sciences 2019). Currently, the primary source of aggregates consists of mined virgin materials such as sand, gravel, and crushed rock. Another possible utilization alternative is the use of carbonated GLD as a substitute for cement. The production of commonly used ordinary Portland cement has a significant impact on climate change due to its substantial release of CO₂ emissions (Turner and Collins 2013). Therefore, replacing a portion of ordinary Portland cement production with an alternative material might result in significant CO₂ emissions reduction. A study conducted by Batuecas et al. (2021) has shown that replacing 2 % of cement with nano CaCO₃ can result in a reduction of over 60 % in CO₂ emissions from cement plants. However, the effectiveness of these substitutions is strongly influenced by the properties of the carbonated GLD, and the performance of the carbonated material may be inferior compared to that of the substituted material (Benhelal et al. 2018; Ostovari et al. 2020). For example, concrete produced of cement with added calcium carbonate showed reduced compressive strength at the mature stage (Ali et al. 2015). Additionally, the substitution of the construction materials by the carbonated residual materials might be hindered by regulations and stakeholders’ acceptance (Ramírez 2022).

2.2 LCA

2.2.1 Goal and scope definition

The analysis seeks to address the following question: Does utilizing GLD for carbon capture, storage, and utilization offer environmental benefits? The goal of the LCA is to assess use of GLD for carbon capture, storage, and utilization. The assessment compares landfilling of the GLD (base case) and GLD carbonation with three alternative utilization routes of the carbonated material: (1) CO₂ storage through landfilling of the carbonates; (2) CO₂ storage in carbonates utilized as construction aggregates, and (3) CO₂ storage in carbonates utilized as a cement substitution. The steps of the life cycle that influence the results the most are identified. Additionally, environmental performance related to the use of carbonated GLD as construction materials instead of being sent to landfills is explored.

In LCA, the environmental impact of the studied system is related to a reference unit representing the system’s function, the functional unit (Baumann and Tillman 2004). However, the function of an industrial system can be defined differently depending on the needs and point of view of the main decision-maker. Since the current study supports experimental work on alternative utilization of GLD as a carbon capture, storage, and utilization material, the functional unit is selected to be 1 tonne of GLD entering the process.

The carbonation process is primarily focused on CO₂ reduction, consequently, the study’s focus is on the impact category of climate change. IPCC 2013 GWP 100a was used as the life cycle impact assessment method. Other environmental aspects like depletion of natural resources and leaching of heavy metals are discussed qualitatively. The geographical scope is defined to be Sweden, with the underlying assumption that the dregs undergo carbonation at the pulp production site.

The modeled systems in the comparison are presented in Figure 2. Construction of the carbonation plant is not included in the calculations; however, the environmental impact of the plant’s construction is described under uncertainties of the analysis in the Discussion. In cases when the final carbonated product is utilized as some form of construction material, the system is credited with the avoided environmental load of the averted production of natural aggregates and of cement binder for the construction sector.

Figure 2:

Life cycle of the GLD carbonation system with the compared options: (1) landfilling of GLD – base case; (2) GLD carbonation cases: the carbonates landfilling case; the aggregates case – use of the carbonated material to substitute natural aggregates; and the binder case – use of the carbonated material to substitute cementitious binder in construction.

For the calculations, experimental data for the GLD carbonation process are scaled up to be representative of a middle-size industrial plant. For scaling up the site-specific data for pulp and paper production plants in Sweden, and a scale-up framework for chemical processes presented by Piccinno et al. (2016) are used. For the background data, e.g., the impact of electricity fuels, and cement production average values are collected from the Ecoinvent 3.8 database (Ecoinvent 2024) and relevant literature.

2.3 Economic analysis

The economic analysis is made for a preliminary estimation of operational costs and revenues for the carbonation system in comparison to the GLD landfilling option. This is an initial analysis focusing on operational costs and largely excludes capital costs. Key data used for economic analysis are presented in Table 2.

The GLD landfilling scenario only includes the transportation of the GLD. The analysis of the carbonation options includes costs for electricity usage of the carbonation process, transportation costs, and potential revenue generated from the utilization of the carbonated material. In the case of carbonates landfilling, revenue is generated by selling surplus carbon permits, while in the aggregates and binder cases, revenue can potentially be generated from both selling emission rights and carbonates as a product. Transportation costs are estimated based on the 2021 report on the Swedish road freight sector by the French National Road Committee (Comité national routier (CNR), 2021).

While electricity usage plays a pivotal role in all process stages, the price of electricity can vary significantly due to factors such as supply and demand, fuel, and CO₂ prices, the output of renewable energy, and nuclear power (Hellström et al. 2012; Wang et al. 2022). These variations can significantly impact the economic performance of the process. Consequently, the study explores the potential profitability and costs associated with different scenarios, considering a best, an intermediate, and a worst-case scenario. Regarding prices, the best, intermediate, and worst-case scenarios encompass fluctuations in electricity and carbonates prices (see Table 2). The potential profit is assessed by subtracting the operational costs of the carbonation process from the revenue at various electricity and carbonates selling prices. As for the process data, the same data used in the LCA process are applied for the best- and worst-case scenarios (see Table 1), while average values are utilized for the intermediate scenario.

3.1 Life cycle assessment

The findings depicted in Figure 3 suggest that opting for carbonation of GLD instead of landfilling could be advantageous, especially if the carbonates can be utilized as construction materials. Depending on the scenario, the environmental benefit is estimated to fall within the range of 142–686 kg CO₂e/tonne of GLD. The substitution of construction materials by the carbonates may lead to additional benefits in the form of a substitution credit: up to 22 kg in the case of aggregates and up to 533 kg in the case of the binder. When carbonates are utilized as aggregates or cement substitutes, CO₂ is not only stored in the carbonates but also CO₂ and other greenhouse gas emissions are avoided following the substitution of natural aggregates and ordinary Portland cement.

Figure 3:

Environmental impact (climate change) of the compared scenarios.

The difference in environmental impact between the best and worst cases primarily emerges during the transportation and drying stages of the life cycle, with the latter being particularly significant for utilization scenarios. The environmental impact difference between the best and worst cases is relatively modest in comparison with the potential of emissions reductions due to the CO₂ capture and substitution of conventional products. Nevertheless, it remains important to limit greenhouse gas emissions from the added process steps.

Figure 4 illustrates CO₂e emissions from different stages of the carbonation process. It becomes clear that in the carbonation phase, under the worst-case scenario with the assumed linear scale-up of lab data, the majority of emissions can be attributed to the electricity demand for stirring. In the best-case scenario, using axial or radial flow impellers for stirring leads to significantly lower energy consumption. Energy consumption continues to be the primary contributor to the overall impact, extending into the subsequent post-processing and utilization steps. Notably, the filtration process exhibits a significantly higher impact in the worst-case scenario, where filtration of smaller carbonate particles is assumed. In all utilization cases, carbonates drying emerges as the main contributor to the impact. Lower levels of moisture in the final product result in higher energy consumption compared to values that permit higher moisture content in the carbonates. Transportation impact becomes notably significant under the worst-case scenario when fossil-based transportation modes are utilized, despite employing EURO6 diesel-powered lorries in these calculations. In contrast, Figure 4 shows that the electric mode of transportation in the best-case scenario with a carbon footprint around 60 times lower than that of the diesel lorry.

Figure 4:

Greenhouse gas emissions in CO₂e from various stages of the carbonation system.

The assessment indicates that the most effective strategy for improving environmental impact involves utilizing carbonates as replacements for materials with a high carbon footprint, such as cement. However, in practice, substituting cement may prove more challenging due to stricter functional requirements placed on such a material compared to the replacement of aggregates. While replacing aggregates might result in a significantly lower environmental benefit, it could be easier to achieve. Further improvement in the carbon footprint of the process could be attained by adopting alternative energy sources, such as utilizing waste heat for drying carbonates and enhancing capturing yield, for example.

3.2 Economic analysis

The economic analysis suggests that the profitability of the CCUS process is influenced by specific conditions due to the additional process steps involved in various carbonation scenarios (see Figure 5). The profitability of GLD carbonation is largely determined by the electricity price and the amount of electricity utilized in the process. Landfilling of carbonated GLD may become a more attractive choice if electricity (or alternative energy source) prices for drying exceed 0.09 EUR per kWh, and the selling price of carbonates remains below 60 EUR. With lower electricity prices (best case scenario), selling the carbonates as construction material might be profitable if the product prices are lower than 30 EUR per tonne of carbonates.

Figure 5:

Revenue and profit flows of the carbonation process at different electricity and carbonates prices.

Figure 6 shows a breakdown of the costs associated with various steps of the GLD carbonation scenarios. In the scenario where carbonates are utilized as construction material, the sum of operational costs may surpass 100 EUR per tonne of GLD, primarily driven by the elevated energy demand of the drying process, especially when electricity prices are high, as observed in the worst-case.

Figure 6:

Comparison of the production costs of different steps of the GLD carbonation scenario.

Given the considerable costs associated with drying, it becomes important to search for more favorable alternatives, either opting for carbonates that require minimal drying or accomplish drying using an alternative, cost-effective energy source.

The costs of the equipment necessary for constructing the carbonation process have not been factored into the calculations. However, including them in the assessment could significantly reduce or even negate the benefits of the carbonation process. Therefore, to maximize its advantages, retrofitting previously constructed equipment is an option worth exploring.

Since electricity prices fluctuate, the production costs of GLD carbonates varies but these variations are easier to foresee than future selling prices of GLD carbonates since these are greatly affected by the properties and the demand for such carbonated materials. Uncertainties persist regarding its material properties and the anticipated demand for it, but the market for such materials is expected to grow (Rousseau et al. 2024). Currently, material standards, availability, and costs of production hinder the utilization of alternative cementitious materials (Rintala et al. 2021).

While GLD is currently exempt from landfill tax in Sweden (Skatteverket 2024), this may change in the future. Rising costs related to landfilling taxes may drive further development of alternatives for utilizing GLD and GLD-derived-carbonates. For the present analysis, data on operational landfilling costs was not available, but adding these to the tax-related landfill costs would likely further drive the exploration of utilization cases.

The effects of also considering the carbon credits obtained through the substitution of the cementitious material sold on the voluntary carbon market (VCM) could enhance the economic feasibility of the analyzed cases. Nevertheless, the potential effects remain uncertain. The price of the VCM credits is influenced by various factors, including the nature of the project, its scale, geographical location, associated co-benefits, and the year in which they were generated (Carbon Credits 2025; Streck 2021). At the time of writing this paper, the voluntary carbon offsetting market is small and not centralized, but the increasing need to reduce the greenhouse gas emissions of companies and individuals could drive the growth of the VCM significantly (Streck 2021).

4.1 Limitations and uncertainties

The findings reveal uncertainties due to the restricted availability of data across various system steps. The lab data utilized is specific to GLD from a particular site. While this can suffice to evaluate if pursuing CCUS using GLD can be worthwhile, the results should not be interpreted as definite. Since the composition of GLD, and consequently, the carbonation efficiency, may differ for different GLD samples and pulp mills. Assessing GLD samples from various pulp mills and establishing a carbon capture range for the material would be beneficial.

At present, GLD is classified as waste rather than a by-product, and, as per ISO 14044, no environmental impacts are currently assigned to it. However, this status may evolve in the future with the increased utilization of GLD across various applications, potentially leading to its reclassification as a by-product. The calculations employed an impact factor for the Swedish electricity mix, but the carbonation system may operate on renewable energy. Additional waste management might be required for the flows of the post-processing step if the filtered water is not suitable for recycling. No flue gas pretreatment is expected before the carbonation reaction; however, experimental work might be required to assess the behavior of the system when the industrial flue gas is supplied to the carbonator. Although there are data uncertainties, there are also improvement possibilities including energy sources for transportation and drying.

A more important factor that could determine the net carbon balance is related to the extent of atmospheric carbonation of landfilled GLD. The high moisture content of GLD may induce self-carbonation under atmospheric conditions. Laboratory samples demonstrated the capture of 0.115 g CO₂/g of GLD within one year of storage compared to 0.177 g CO₂/g of GLD in experiments. This suggests that carbonation of GLD could also occur during landfilling, albeit at a much slower rate and only in superficial layers (Jin et al. 2024). However, while forced carbonation of GLD accelerates the uptake of CO₂, it also holds the potential to impact the mechanical and chemical properties of the resulting carbonated material, which could be beneficial for its utilization. The reduction of pH and immobilization of heavy metals would take place before the material is landfilled or utilized for other purposes.

Research addressing the substitution of traditional fossil-based products in construction reveals that substitutability depends not only on the technical properties of the new materials but is also influenced by factors such as price, acceptance, regulations of building materials and codes, and knowledge among stakeholders (Arehart et al. 2021; Howard et al. 2021). Thus, other utilization cases (e.g., in geopolymers) could be of interest for further research than those analyzed here. According to Zink et al. (2016), the complete displacement of conventional products necessitates special conditions and that it is likely that low or even zero displacements could occur. In such a scenario, a carbonation-derived product can be introduced to the market without displacing any of the existing fossil-based products. However, solid carbonates are already used in construction, mainly in the production of cement and concrete (Jin et al. 2024). Furthermore, the massive need for construction materials in the future – global demand is estimated to exceed 60 billion tonnes by 2030 (Da and Le Billon 2022; Statista 2024) – might facilitate their use in the construction sector. The substitution of cement and natural aggregates with carbonates would significantly alleviate the pressure on natural resources and ecosystems.

4.2 Recommendations for further analysis

The analysis identified potential methods to enhance the environmental performance of the carbonation system, which require practical testing and evaluation. One such method has been tested with promising results: Queiroz et al. (2025) replaced the fresh water used in the experiments by Leventaki (2023) with alkaline wastewater from the pulp and paper industry which led to enhanced CO₂ absorption. Nevertheless, further research is essential to evaluate other ways to enhance the carbon capture and storage efficiency of the entire process, with minimal or no additional environmental impact. This may entail investigating access to low/no-cost surplus energy at pulp mills and other alkaline material flows within other industries towards utilizing the carbonation process. Surplus energy could be examined for enhancing, for instance, carbonation efficiency and drying the carbonated material. Further improving the process of leaching alkaline metals and filtering leached metals before the carbonation reaction (indirect carbonation) could result in a higher-purity material with a broader range of potential applications. Assessment of the attributes of the carbonated material could unveil potential applications and determine the feasibility of utilizing carbonates for specific purposes. Additionally, comprehending the maximum theoretical capture rate of CO₂ by the GLD under various conditions is crucial for evaluating the benefits of the process and optimizing the efficiency of GLD carbonation.

Strong alkalinity, presence of heavy metals and water, and limited knowledge of the long-term stability, technical performance, and environmental impact of GLD are possible reasons for a lack of industrial-scale applications of the material (Quina and Pinheiro 2020) beyond landfilling. Carbonation of the GLD could provide a better solution to some of these problems and thereby offer a viable alternative towards GLD utilization. For this, improved understanding of the composition of the carbonates is needed as this would help identify suitable utilization cases.