Combined electrification and carbon capture for low-carbon cement: Techno-economic assessment of different designs

Combined electrification and carbon capture for low-carbon cement: techno-economic assessment of different designs Leonardo Varniera,b, Federico d’Amorea, * , Kim Clausenb, Georgios Melitosa,c, Bart de Grootc, Fabrizio Bezzoa,* a. CAPE-Lab – Computer-Aided Process Engineering Laboratory Department of Industrial Engineering University of Padova Via Marzolo 9, 35131 Padova (Italy) b. FLSmidth Cement A/S Vigerslev Allé 77, 2500 Valby, Copenhagen (Denmark) c. Siemens Industry Software Limited Hammersmith Grove 26-28, W6 7HA London (United Kingdom) * To whom all correspondence should be addressed: federico[email protected] (Federico d’Amore) fabrizio[email protected] (Fabrizio Bezzo) Abstract The cement industry is a major contributor to global CO2 emissions. Among the various decarbonisation strategies, heat demand electrification and carbon capture technologies offer promising solutions for reducing both processand fuel-related emissions. This study investigates the potential of low-carbon cement plants that combine calciner electrification with amine-based carbon capture on rotary kiln emissions. A techno-economic analysis is conducted on four process alternatives, differing in the type of electrified calciner – entrainment vs. drop tube – and the heat recovery strategy for the hot CO2 produced, in the EU context. The four low-carbon processes are benchmarked against a reference plant without mitigation measures. Drop tube calciner configurations show better energy efficiency than entrainment calciner alternatives although their environmental performance is comparable. When renewable electrical energy is supplied to the plants, the CO2 avoidance rates exceed 98%, making these options competitive with other decarbonisation technologies such as oxyfuel and calcium looping. Economic viability remains challenging under current EU prices and carbon intensity of imported electricity. The entrainment calciner configuration that uses pure CO2 to preheat raw materials emerges as the most favourable, with a cost of avoided CO2 of 217.4 €/tCO2, compared to 231-234 €/tCO2 for the other options. To ensure cost-effectiveness, electricity prices would need to remain below approximately 90 €/MWhel when low-carbon electricity is supplied. Keywords: cement production, decarbonisation, electrification, calciner, carbon capture, technoeconomic assessment. 1. Introduction The cement industry is responsible for approximately 2.5 Gt of CO2 emissions annually, primarily from clinker production, accounting for 7% of global emissions (Marmier, 2023). Cement is classified as a “hard-to-abate” sector because around 60% of its CO2 emissions are process-related, originating from the calcination reaction of limestone (CaCO3), which decomposes into lime (CaO) and CO2. The remaining CO2 emissions are generated by the combustion of fossil fuels, with coal being the primary fuel used in most industrial applications (Cavalett et al., 2024). Given these two major sources of CO2 emissions, it is essential to explore various complementary mitigation approaches (Strunge et al., 2024). Existing measures include carbon capture and storage (CCS) (Hills et al., 2016; Plaza et al., 2020), electrification of the process heat demand (Madeddu et al., 2020), fossil fuel substitution with alternative fuels (Nhuchhen et al., 2021; Yang et al., 2021), and clinker substitution with supplementary cementitious materials (Miller et al., 2016). CCS technologies are widely recognised as crucial for addressing the process-related emissions from cement plants (IEA, 2018; GCCA, 2021) and numerous studies have explored various techniques to abate such CO2. The CEMCAP project (CEMCAP, 2015) assessed and benchmarked several technologies, including monoethanolamine (MEA) absorption, oxyfuel combustion, calcium looping, membrane-assisted CO2 liquefaction and chilled ammonia. From these alternatives, oxyfuel and calcium looping emerged as promising technologies, encouraging further research in subsequent projects aimed at advancing their maturity (CLEANKER, 2017; AC2OCEM, 2019; Catch4Climate, 2020). The LEILAC project (LEILAC, 2016) investigated the potential for direct separation of process emissions through indirect heating of limestone in a drop tube (DT) calciner. This system enables to keep separated the flue gas from combustion and capturing the pure CO2 from calcination. Pilot plant results demonstrated successful CO2 separation without compromising the energy efficiency of the calciner, leading to continued development in the LEILAC2 project (LEILAC2, 2020). 4 While carbon capture remains essential for reducing process emissions, electrification of heat demand offers a promising solution to reduce fuel-related CO2 emissions, which constitute about 40% of total direct emissions (Schneider et al., 2023). Cement production can be electrified through plasma technology, induction heating, resistive electrical heating and microwave heating (Antunes et al., 2022). However, only the direct electrification of the heat duty of the calciner appears a viable shortterm solution. This unit operates at approximately 900°C, and technologies that supply heat below 1000°C are well-established and require minimal retrofitting (Madeddu et al., 2020). An electrified calciner offers two key advantages for a low-carbon cement plant: it removes the fuel-related CO2 emissions and produces a pure CO2 stream from the calcination reaction, ready for utilisation or storage. In this context, Tokheim et al., (2019) assessed the technical and economic feasibility of a rotary calciner with resistance-based heating. Jacob and Tokheim, (2023) compared the energy demand of a cement plant using different electrified calciner designs, all employing a gas-gas heat exchanger for heat recovery from the pure CO2 stream. They modelled various designs, including entrainment (Ent) calciner, fluidised bed calciner, and rotary calciner, with varying degrees of CO2 recycling. Other studies by the same group concluded that a full-scale electrified rotary calciner is not recommended due to its required size, while electrifying an entrainment calciner with heating rods was found to be technically feasible (Jacob and Tokheim, 2021; Jacob et al., 2023). Quevedo Parra and Romano (2023) compared the technical and economic performance of various configurations for partial or full electrification of a cement plant. Two alternatives involved an electrified DT-calciner using resistive elements, where they considered utilising the hot CO2 stream to preheat the raw meal in a separate cyclone tower. In one scenario, MEA-based capture was used to address residual emissions from the kiln. A recent report from LEILAC2 (LEILAC2, 2023) also assessed the technical and economic feasibility of an electrified DT-calciner, confirming that this technology is being developed to operate on multiple energy sources, including electricity. 5 In summary, most research on cement sector decarbonisation has focused on evaluating carbon capture technologies and electrifying heat demand separately. Only few techno-economic studies explored the combination of calciner electrification with CO2 abatement from the rotary kiln, with a primary focus on processes involving a DT-calciner, in which the hot CO2 stream is used to preheat the raw meal (LEILAC2, 2023; Quevedo Parra and Romano, 2023). Moreover, there is a lack of economic analyses exploring alternative heat recovery methods for pure CO2 or the electrification of an Ent-calciner. Consequently, the full potential of these approaches and the most effective implementation strategies remain unclear. To address this gap, this study evaluates the techno-economic performance of different low-carbon cement processes that combine calciner electrification and amine-based capture applied to emissions from the rotary kiln, in the EU context. Specifically, four alternatives are designed, which differ in the type of electrified calciner – either Ent or DT – and the heat recovery strategy for the pure CO2 stream exiting the calciner. The key contributions of this study are as follows: • Comparing the low-carbon alternatives within a consistent framework of technical and economic assumptions (calciner temperature, calcination extent, equipment cost estimates, etc.), allowing for the identification of strengths, weaknesses, and trade-offs between the technologies. • Evaluating the economic feasibility of low-carbon cement plants with an electrified Entcalciner alongside alternative heat recovery strategies for pure CO2 exiting the calciner. • Employing response surface models that leverage data generated in-silico through Design of Experiments techniques to assess the sensitivity of economic performance to key parameters (e.g., electricity price, grid carbon intensity) so as to evaluate the combined effect of multiple factors. • Adjusting the raw meal composition to account for variations in fuel ash in the low-carbon plants, which is overlooked in other studies and may lead to incorrect clinker chemistry. 6 The article is structured as follows. Section 2 details the methods for designing and modelling the reference cement plant and the low-carbon alternatives, along with the methodology for the economic assessment and the definition of key performance indicators (KPIs). Section 3 covers the technical results (Section 3.1) and the economic analysis (Section 3.2), including a sensitivity analysis of the economic outcomes using response surface models to evaluate the impact of key parameters. Finally, Section 4 provides the conclusions and key takeaways of the study. 2. Methods This work investigates CO2 emissions abatement strategies for cement plants from a techno-economic perspective. The plants were simulated by means of the gPROMS Process 2023.2.0 software (Siemens, 2024), by solving steady-state material and energy balances. The Peng-Robinson cubic equation of state was employed to calculate the thermodynamic properties of both the gas and liquid phases. The solid phases, including natural minerals and clinker phases, are assumed to be ideal solid solutions. The correlations used for the estimations of thermodynamic properties were rigorously reviewed and sourced from various references (Bonnickson, 1954; Bonnickson, 1955; Haas et al., 1981; Hanein et al., 2020; Linstrom and Mallard, 2001; Mcbride et al., 2002), as detailed in the Supplementary Material. For pure water present throughout the process, the IAPWS-95 thermodynamic model (Wagner and Pruß, 2002) was applied, as it is the standard reference for calculating water and steam thermodynamic properties. Both custom models and gPROMS built-in models were deployed to simulate the process units of the plants (details are in the Supplementary Material). The following sections describe the plant setup of the reference (i.e., unabated) cement plant, and of the low-carbon configurations (i.e., cement plants decarbonised via electrification and carbon capture). 7 2.1 Reference cement plant configuration The reference cement plant (Figure 1) employs a state-of-the-art, highly efficient dry process with a five-stage preheater-calciner kiln system equipped with a grate cooler. The plant is coal-fired and has a production capacity typical of a mid-sized European facility, manufacturing 3024 t/d of clinker, corresponding to about 1 Mt/y when assuming a yearly operation of 8000 h/y. This type of process represents the Best Available Technique (BAT) as indicated in the European BREF document (Schorcht et al., 2013). Figure 1. Reference cement plant scheme: modern process with a five-stage preheater-calciner system. The raw material, referred to as raw meal, consists of CaCO3, SiO2, Al2O3, Fe2O3 and MgCO3. After being dried and milled in the raw mill, the raw meal enters the five-cyclone preheating tower (stream #2 in Figure 1). Within the tower, exhaust gases from the calciner flow counter-currently with the descending solids through risers and cyclones, cooling to approximately 330°C, while the solids are 8 heated to around 800°C before being transferred to the calciner (stream #6). The cooled gases (stream #20) are then directed to the raw mill to dry the raw meal. Within the calciner, the hot tertiary air from the clinker cooler and the kiln exhaust gas are used to burn roughly 60% of the total coal (stream #9) providing the required temperature (870°C) and heat to achieve a calcination rate of 92% (Hewlett and Liska, 2019). The material leaving the lowest cyclone stage (stream #7) descends into the kiln where clinker formation occurs. In this unit, the remaining 40% of the coal (stream #10) is burned with secondary air from the clinker cooler, gradually heating the materials to a peak temperature of 1450°C. The clinker is cooled to 95°C in the clinker cooler (stream #8), preserving its microstructure and recovering heat for preheating the secondary and tertiary air used in the kiln and calciner. The key assumptions used for modelling the reference cement plant are provided in the Supplementary Material, along with validation against the plant simulated in Campanari et al. (2016). 2.2 Low-carbon cement plant configurations The low-carbon configurations are modelled to produce the same output quantity of clinker, namely 3024 t/d. They replace the traditional coal-fuelled calciner with an electrified one, which can be either an Entor DT-calciner, as anticipated in Section 1. The Ent-calciner is a vertical tube where calcination occurs as solids are carried by gas flow along its length from the bottom to the top. It is assumed to be directly electrified through the insertion of heating rods, while the necessary gas flow is provided by recycling a portion of the pure CO2 produced. Conversely, the DT-calciner is based on the design investigated by LEILAC2 (2023), where calcination occurs within an inner tube, and the heat is transferred indirectly through the calciner wall by an electrical energy source. Here, the solid material moves downward, avoiding the need for CO2 recycling. Both electrified calciners are assumed to achieve a calcination rate of 92%, as in the reference plant, by heating the raw meal to 920°C. This is to account for the increased CO2 partial pressure, following 9 similar studies on oxyfuel or electrified calciners (De Lena et al., 2019; Quevedo Parra and Romano, 2023). The electrical power required for the calciner is calculated assuming a 95% electricity-to-heat efficiency (Wilhelmson et al., 2018). The remaining calcination and clinker formation reactions occur in a coal-fired kiln, which maintains the same design and secondary air flowrate as the reference plant. This setup enables the capture of CO2 released during calcination, which can then be directed to a CO2 compression and purification unit (CPU) before transportation and storage. To maintain the purity of the CO2-rich stream from the electrified calciner, modifications to the typical system are necessary. Since fuel combustion is no longer needed, the supply of coal and tertiary air to the calciner (stream #9 and stream #13 in Figure 1) is removed. The supply of exhaust gas from the kiln (stream #12) is also eliminated because it would merely dilute the CO2 stream. The absence of coal combustion alters the fuel ash contribution to clinker composition in the low-carbon scenarios. To maintain consistent clinker chemistry with the reference case, the raw meal chemistry is adjusted accordingly, as presented in Table 1. Table 1. Raw meal composition after drying for the reference plant and the low-carbon designs. Species Reference plant (% mass wet) Low-carbon designs (% mass wet) CaCO3 78.91 78.45 SiO2 13.74 13.98 Al2O3 3.32 3.52 Fe2O3 2.00 2.01 MgCO3 1.53 1.54 H2O (Moisture) 0.50 0.50 Another major modification regards the heat recovery strategy for the hot CO2 produced, leading to a significant distinction in configuration. Following Quevedo Parra and Romano (2023) and Jacob and Tokheim (2023), two design options can be envisaged: the heat content of the CO2-rich stream can either be used to preheat the raw meal (RMP configuration) or be transferred to a fraction of the vent air from the clinker cooler via a gas-gas heat exchanger (GGHX configuration). The two alternatives are detailed in the following: 16 2.3 KPIs and economic analysis The environmental and economic performance of the investigated processes are evaluated using a series of KPIs, as outlined in Equations 1-7. The equivalent specific CO2 emissions (𝑒𝑒𝑞,𝑐𝑙𝑘 [kgCO2tclk ⁄]) are calculated as: 𝑒𝑒𝑞,𝑐𝑙𝑘 = 𝑒𝑐𝑙𝑘+𝑒𝑒𝑙,𝑐𝑙𝑘. (1) where 𝑒𝑐𝑙𝑘 represent direct CO2 emissions (Scope 1) and 𝑒𝑒𝑙,𝑐𝑙𝑘 reflects indirect CO2 emissions associated to the electricity supplied to the plant (Scope 2). The latter is computed as the sum of the electrical energy consumption of the process (𝑃𝑒𝑙,𝑐𝑙𝑘 [MWhel tclk ⁄ ]) and the carbon intensity of the imported electricity (𝑒𝑒𝑙 [kgCO2,eq MWhel ⁄]): 𝑒𝑒𝑙,𝑐𝑙𝑘 =𝑃𝑒𝑙,𝑐𝑙𝑘∙𝑒𝑒𝑙 (2) The carbon capture rate (𝐶𝐶𝑅 [%]) indicates the CO2 capture efficiency by measuring the amount of carbon effectively captured (and sent to storage) relative to the total amount generated by the process, excluding indirect emissions. It is defined as: 𝐶𝐶𝑅= 𝑒𝑐𝑎𝑝𝑡,𝑐𝑙𝑘 𝑒𝑐𝑎𝑝𝑡,𝑐𝑙𝑘+𝑒𝑐𝑙𝑘 ∙100 . (3) where 𝑒𝑐𝑎𝑝𝑡,𝑐𝑙𝑘 [kgCO2tclk ⁄] is the amount of CO2 captured. To include indirect emissions in the analysis, the equivalent CO2 avoided (𝐴𝐶𝑒𝑞 [%]) metric evaluates CO2 reduction by comparing equivalent emissions between the low-carbon plant and the reference plant: 𝐴𝐶𝑒𝑞 = (1−𝑒𝑒𝑞,𝑐𝑙𝑘 𝑑𝑒𝑐𝑎𝑟𝑏 𝑒𝑒𝑞,𝑐𝑙𝑘 𝑟𝑒𝑓 )∙100. (4) where the superscript "decarb" refers to the plant with decarbonisation measure, and "ref" to the unabated reference plant. Two scenarios are evaluated for the carbon intensity of imported electricity. The first scenario reflects the EU-27 energy mix, based on the latest available data from 2022, with 𝑒𝑒𝑙 = 258 kgCO2,eq/MWhel (EEA, 2024). The second scenario assumes electricity supplied entirely from non-combustible 17 renewables (wind, solar and hydro), thus setting 𝑒𝑒𝑙 = 0 kgCO2,eq/MWhel. Notably, these carbon intensity values exclude upstream emissions associated with the electricity supply chain. The economic analysis methodology presented in this study follows best practices for cost estimation in CCS projects, as outlined by Rubin et al. (2013) and van der Spek et al. (2019). The assessment requires estimating the capital costs (CAPEX) and operating costs (𝑂𝑃𝐸𝑋 [€ y ⁄ ]) for the investigated processes, resulting in two economic KPIs: the cost of clinker (𝐶𝑂𝐶 [€ tclk ⁄ ]) and the cost of avoided CO2 (𝐶𝐴𝐶 [€ tCO2 ⁄]). They are defined as follows: 𝐶𝑂𝐶=𝑇𝐴𝐶+𝑂𝑃𝐸𝑋 𝑚󰇗𝑐𝑙𝑘 (5) 𝐶𝐴𝐶=𝐶𝑂𝐶𝑑𝑒𝑐𝑎𝑟𝑏−𝐶𝑂𝐶𝑟𝑒𝑓 𝑒𝑒𝑞,𝑐𝑙𝑘 𝑟𝑒𝑓 −𝑒𝑒𝑞,𝑐𝑙𝑘 𝑑𝑒𝑐𝑎𝑟𝑏 , (6) where 𝑇𝐴𝐶 [€ y ⁄ ] is the total annualised CAPEX and 𝑚󰇗𝑐𝑙𝑘 [tclk y ⁄ ] is the annual clinker productivity. All costs are expressed in €2024, adjusted using the CEPCI for January 2024, which is 795.4 (Chemical Engineering, 2024), and assuming a US Dollar to Euro conversion rate of 0.91. The baseline CAC calculations assume a grid carbon intensity of 258 kgCO2,eq/MWhel for the EU-27 in 2022. A bottom-up approach, typical of preliminary cost analysis is employed for the evaluation of CAPEX (Rubin et al., 2013). This involves computing the total overnight cost (𝑇𝑂𝐶 [€]) of the plant, starting from the equipment cost of individual units (𝐸𝐶 [€]), as illustrated in Figure 6 and explained in the following. The estimation of EC for a generic unit 𝑗 is based on its key features derived from process simulations and design criteria, using various cost correlations (Cinti et al., 2018; d’Amore et al., 2023; De Lena et al., 2019; Gardarsdottir et al., 2019; IEAGHG, 2013; Magli et al., 2022; Manzolini et al., 2015; Mastropasqua et al., 2019; NETL, 2019; Turton et al., 2018). A suitable installation factor (𝐼𝐹 [−]) is applied to EC to determine the bare erected cost (𝐵𝐸𝐶 [€]) of the equipment, accounting for the installation cost. Depending on the maturity of the technology and the level of detail in the equipment list, a process contingency is applied as a percentage of the 18 BEC, yielding the total direct cost (𝑇𝐷𝐶 [€]) of the unit. A detailed survey of all cost correlations, installation factors and process contingencies used in this study is provided in the Supplementary Material. Figure 6. Diagram of calculation procedure to estimate plant CAPEX. To obtain the total plant cost (𝑇𝑃𝐶 [€]) of each item, indirect costs and project contingencies are added as a percentage of TDC. Based on Gardarsdottir et al. (2019), indirect costs are set at 14% of TDC, and project contingencies are 15% of TDC for the reference plant. For low-carbon cases, project contingencies are increased to 20% of TDC to account for the higher risks associated with emerging CCS technologies. The summation of TPC of each unit represents the plant CAPEX including direct and indirect costs, to which the owner's costs (e.g., land, inventory and start-up capital) must be added. Owner’s costs of 10% of the TPC are applied to obtain the TOC. Finally, the TOC are annualised to TAC with Equation 10, assuming a plant operational life (𝑛) of 25 years and a discount rate (𝑖) equal to 8%: 𝑇𝐴𝐶=𝑇𝑂𝐶∙ 𝑖(1+𝑖)𝑛 (1+𝑖)𝑛−1 . (7) The OPEX are divided into fixed and variable costs, with the assumptions for their calculations summarised in Table 4. The operating labour cost (𝑂𝐿 [€/y]) is calculated by multiplying the labour cost by the number of employees at the plant. OL and maintenance labour (𝑀𝐿 [€/𝑦]), which constitutes 40% of the total maintenance cost, contribute to the estimation of administrative/support labour costs. 19 Table 4. Assumptions for OPEX calculation Fixed OPEX Reference Local taxes/insurance %TPC/y 2.0 Gardarsdottir et al. (2019) Maintenance %TPC/y 2.5 " Personnel in reference plant - 100 " Additional personnel in capture plant - 20 " Labour Cost €/year per personnel 60000 " Maintenance labour (ML) % Maintenance 40 " Administrative/support labour % (OL + ML) 30 " Variable OPEX Reference Raw Meal €/tRaw Meal 3.012 Gardarsdottir et al. (2019) Coal €/GJLHV 3.00 Gardarsdottir et al. (2019) Electricity €/MWhel 125.0 Eurostat (2024) Steam (from waste heat) €/MWhth 8.5 Gardarsdottir et al. (2019) Cooling water (30-40°C) €/m3 0.02 Cormos (2022) Refrigerated water (15-25°C) €/m3 0.14 Estimated Process water €/m3 6.65 Gardarsdottir et al. (2019) Ammonia SNCR €/tNH3 130.0 Gardarsdottir et al. (2019) MEA €/tMEA 1450.0 Gardarsdottir et al. (2019) NaOH DeSOx €/tNaOH 370.0 Gardarsdottir et al. (2019) CO2 Transport and Storage €/tCO2,capt 35.0 d’Amore et al. (2021) It is important to note that no carbon tax (𝑐𝐶𝑂2 [€ tCO2 ⁄]) on the emitted CO2 is applied in the baseline analysis. However, this study assesses sensitivity to this parameter. When a carbon tax is applied, the electricity cost (𝑐𝑒𝑙 [€ MWhel ⁄ ]) is updated as: 𝑐𝑒𝑙 =𝑐𝑒𝑙,𝑏𝑎𝑠𝑒+𝑐𝐶𝑂2∙𝑒𝑒𝑙 1000 (11) where the base cost of electricity (𝑐𝑒𝑙,𝑏𝑎𝑠𝑒) is the EU-27 average value for non-household consumers (IG band) in the second semester of 2023 (Eurostat, 2024). 3. Results and discussion 3.1 Techno-environmental results The heat and electrical energy demands for the reference plant and the low-carbon scenarios are compared in Table 5. The heat duty for the calciner increases in all the low-carbon cases compared to the reference (between +12% and +32%), as the temperature needed to achieve a 92% calcination 20 rate is raised from 870°C to 920°C in a pure CO2 atmosphere. Consequently, the heat requirements of the rotary kiln decrease (between -15% and -11%) because the calcined meal enters the kiln at a higher temperature with the same calcination rate. Table 5. Summary of fuel consumptions, heat and electricity demands for reference cement plant and low-carbon configurations. DT-RMP = drop tube calciner with pure CO2 preheating raw meal; Ent-RMP = entrainment calciner with pure CO2 preheating raw meal; DT-GGHX = drop tube calciner with pure CO2 preheating vent air via a gas-gas heat exchanger; Ent-GGHX = entrainment calciner with pure CO2 preheating vent air via a gas-gas heat exchanger. Reference plant DT-RMP Ent-RMP DT-GGHX Ent-GGHX Heat duty cement process Heat duty calciner GJLHV/tclk 1.89 2.15 2.44 2.11 2.50 Heat duty kiln GJLHV/tclk 1.23 1.06 1.04 1.09 1.09 Direct fuel consumption GJLHV/tclk 3.13 1.06 1.04 1.09 1.09 Change w/r to reference plant - -66.2% -66.6% -65.2% -65.0% Heat duty MEA capture Heat duty MEA regeneration GJLHV/tclk - 0.50 0.48 0.56 0.55 Waste heat availablea GJLHV/tclk - 0.21 0.51 0.25 0.66 Net heat duty MEA regeneration GJLHV/tclk - 0.29 -b 0.31 -b Total heat duty GJLHV/tclk 3.13 3.51 3.48 3.51 3.60 Change w/r to reference plant - 12.1% 11.4% 12.1% 15.0% Overall electrical energy demand Electrified calcinerc GJel/tclk - 2.27 2.57 2.22 2.63 MEA heat pumpd GJel/tclk - 0.15 0.00 0.15 0.00 MEA auxiliaries GJel/tclk - 0.02 0.02 0.04 0.04 CPU GJel/tclk - 0.23 0.24 0.23 0.24 Cement plant auxiliaries GJel/tclk 0.47 0.48 0.49 0.49 0.50 Total electricity GJel/tclk 0.47 3.15 3.31 3.14 3.41 MWel 16.58 110.30 115.91 109.95 119.40 Change w/r to reference plant - 565.1% 599.0% 563.0% 620.0% a Assumed to be recovered from hot streams down to 160°C b Waste heat available is sufficient to cover the MEA regeneration heat duty entirely. The excess waste heat is minimal and will not significantly impact the technical analysis. c Computed assuming electricity-to-heat efficiency of 95% d Computed assuming a COP equal to 2 The DT-calciner configurations exhibit a lower calciner heat demand than Ent-calciner ones (2.11 and 2.15 against 2.44 and 2.50 GJLHV/tclk) due to the energy penalty of re-heating the CO2 flow recycled to lift the solids. However, a larger amount of waste heat can be recovered when the Entcalciner is installed, which is sufficient to cover the entire thermal requirements of the MEA capture. 21 This is not the case for DT-RMP and DT-GGHX cases, where the available waste heat meets 42% and 45% of thermal demand for MEA regeneration, respectively, necessitating a heat pump to provide the remaining duty to operate the MEA PCC. As expected, the low-carbon processes demonstrate a substantial increase in electrical energy demand, with the electricity consumption significantly higher than the reference cement plant by factors of 5.6, 5.7, 6, and 6.2 for DT-GGHX, DT-RMP, Ent-RMP, and Ent-GGHX, respectively. This increase is due to the electrification of the calciner (71-78% of total electrical demand), MEA capture (1-6% of total electrical demand), and CPU sections (7% of total electrical demand). Among the low-carbon processes, the major difference in electrical requirements is related to the different calciner duties. The estimated electrical power at the calciner is 79.3 MWel for DT-RMP, 89.9 MWel for Ent-RMP, 77.8 MWel for DT-GGHX, and 92.2 MWel for Ent-GGHX. Additionally, DT-RMP and DT-GGHX configurations require additional electrical power for the heat pump (5.15.4 MWel), while the CPU shows similar energy requirements for all cases (8.2-8.4 MWel). Detailed stream tables for the reference plant and the low-carbon processes are provided in the Supplementary Material. The environmental KPIs, computed under the two carbon intensity scenarios for electricity as described in Section 2, are summarised in Table 6. All configurations exhibit very high CCR of approximately 97.5%. When considering the same heat recovery strategy for the CO2-rich stream, alternatives employing DT-calciner show better energy efficiency compared to those with Entcalciner. Specifically, DT-calciners achieve a comparable CCR with similar fuel consumption while requiring less electricity. Under an EU-27 energy mix scenario, ACeq ranges between 70 and 72%, with only minor differences observed among the low-carbon configurations. DT-calciner designs demonstrate slightly better performance than Ent-calciner ones, primarily due to their lower electrical power demand, which translates into reduced indirect emissions. Overall, under these assumptions, the ACeq results for all the configurations are not competitive with those of an oxyfuel cement plant (ACeq = 81.5%) or an 22 integrated calcium looping technology (ACeq = 88.3%) (Voldsund et al., 2019). This is notable considering that the investigated low-carbon scenarios achieve a significantly higher CCR (about 97.5% in this study, compared to 90% for oxyfuel and calcium looping in Voldsund et al., 2019). Table 6. Summary of environmental key performance indicators for reference cement plant and low-carbon configurations under different scenarios for the carbon intensity of electricity. DT-RMP = drop tube calciner with pure CO2 preheating raw meal; Ent-RMP = entrainment calciner with pure CO2 preheating raw meal; DT-GGHX = drop tube calciner with pure CO2 preheating vent air via a gas-gas heat exchanger; Ent-GGHX = entrainment calciner with pure CO2 preheating vent air via a gas-gas heat exchanger. Reference plant DT-RMP Ent-RMP DT-GGHX Ent-GGHX Direct fuel consumption GJLHV/tclk 3.13 1.06 1.04 1.09 1.09 Electric power consumption (𝑃𝑒𝑙,𝑐𝑙𝑘) MWhel/tclk 0.132 0.875 0.920 0.873 0.948 CO2 captured (𝑒𝑐𝑎𝑝𝑡,𝑐𝑙𝑘) kgCO2/tclk 0.0 630.6 630.0 633.4 634.3 Direct CO2 emissions (𝑒𝑐𝑙𝑘) kgCO2/tclk 833.1 16.4 15.5 15.8 15.5 CO2 outlet purity % mol (dry) - 99.0% 97.7% 99.4% 98.6% Carbon capture rate (𝑪𝑪𝑹) % - 97.5% 97.6% 97.6% 97.6% EU-27 energy mix in 2022 ( 𝑒𝑒𝑙 =258kgCO2,eq MWhel ⁄ ) Indirect CO2 emissions (𝑒𝑐𝑙𝑘,𝑒𝑙) kgCO2/tclk 34.0 225.9 237.3 225.1 244.5 Equivalent CO2 emissions (𝑒𝑐𝑙𝑘,𝑒𝑞) kgCO2/tclk 867.1 242.3 252.8 241.0 260.0 Equivalent CO2 avoided (𝑨𝑪𝒆𝒒) % - 72.0% 70.8% 72.2% 70.0% Non-combustible renewables ( 𝑒𝑒𝑙 =0kgCO2,eq MWhel ⁄ ) Indirect CO2 emissions (𝑒𝑐𝑙𝑘,𝑒𝑙) kgCO2/tclk 0.0 0.0 0.0 0.0 0.0 Equivalent CO2 emissions (𝑒𝑐𝑙𝑘,𝑒𝑞) kgCO2/tclk 833.1 16.4 15.5 15.8 15.5 Equivalent CO2 avoided (𝑨𝑪𝒆𝒒) % - 98.0% 98.1% 98.1% 98.1% Due to the absence of indirect emissions, the ACeq index improves substantially in a scenario where the plant electrical demand is supplied by renewable energy, with values above 98% and identical for all the configurations. In this context, the proposed processes are competitive with oxyfuel plants and plants with integrated calcium looping technology, which have ACeq of 89.4% and of 93.2%, respectively (Voldsund et al., 2019). The contribution of direct, indirect and captured CO2 emissions to the equivalent emissions under the two electricity carbon intensity scenarios is illustrated in Figure 7. 23 Figure 7. Breakdown of CO2 emissions for reference cement plant and low-carbon configurations under different scenarios for the carbon intensity of electricity. EU-27: carbon intensity electricity = 258 kgCO2/MWel; Renew: carbon intensity electricity = 0 kgCO2/MWel. DT-RMP = drop tube calciner with pure CO2 preheating raw meal; Ent-RMP = entrainment calciner with pure CO2 preheating raw meal; DT-GGHX = drop tube calciner with pure CO2 preheating vent air via a gas-gas heat exchanger; Ent-GGHX = entrainment calciner with pure CO2 preheating vent air via a gas-gas heat exchanger. The captured and direct emissions from low-carbon processes remain constant across both scenarios. Notably, under the EU-27 energy mix, the total emissions of low-carbon plants are slightly higher than those of the unabated plant, despite a 66% reduction in fuel combustion. This is due to the high indirect emissions associated with carbon-intensive electricity, resulting in still significant equivalent emissions (242-260 kgCO2/tclk). This underscores that these processes only reduce emissions when low-carbon electricity is utilised. In fact, when renewable energy sources are employed, the total emissions in low-carbon configurations decrease by an average of 22% compared to the unabated plant, with equivalent emissions dropping as low as 16 kgCO2/tclk. 3.2 Economic results The total plant cost (TPC) breakdown across different equipment categories is illustrated in Figure 8. The low-carbon configurations exhibit a marked increase in TPC compared to the reference plant, with costs rising from 98 M€ for Ent-RMP (+36%) up to 224 M€ for DT-GGHX (+83%). This cost increase is primarily driven by the addition of units such as the MEA capture system, the CPU, and 24 the electrified calciner, which are the major contributors. At the same time, the new low-carbon designs involve some costs reduction associated with the coal mill and SNCR due to the reduced coal consumption. However, these savings are not sufficient to offset the increase in TPC of low-carbon alternatives compared to the reference unabated cement plant. Figure 8. Breakdown of total plant cost for reference cement plant and low-carbon configurations. DT-RMP = drop tube calciner with pure CO2 preheating raw meal; Ent-RMP = entrainment calciner with pure CO2 preheating raw meal; DTGGHX = drop tube calciner with pure CO2 preheating vent air via a gas-gas heat exchanger; Ent-GGHX = entrainment calciner with pure CO2 preheating vent air via a gas-gas heat exchanger. DT-calciner configurations are the most capital-intensive, mainly due to two key factors: (i) the DTcalciner itself is significantly more expensive, with an estimated TPC of 120 M€, compared to 24 M€ for the Ent-calciner; (ii) the DT-calciner designs require the installation of an 11 M€ electric heating system based on heat pumps, which is not needed in the Ent-calciner processes, as the latter can recover sufficient heat internally to the process. Comparing processes with the same type of calciner, RMP configurations exhibit lower capital expenditures than GGHX ones due to the more cost-effective MEA capture process. In fact, the CO2 stream sent to MEA capture, while similar in flowrate, is more concentrated, leading to smaller and less expensive equipment. Other process sections do not exhibit significant cost differences among the alternatives. 25 The breakdown of the cost of clinker (COC) for the reference cement plant and the low-carbon configurations is reported in Figure 9a. Variable OPEX is a major contributor, accounting for 63-70% of the total COC, depending on the configuration. The total annualised CAPEX (TAC) represents approximately 18% of the COC in Ent-calciner processes and around 22% in DT-calciner processes. The remaining share is attributed to fixed OPEX, which follows the same trend as CAPEX since it is calculated based on TPC. Figure 97. Breakdown of (a) cost of clinker and (b) cost of avoided CO2 for reference cement plant and low-carbon configurations. DT-RMP = drop tube calciner with pure CO2 preheating raw meal; Ent-RMP = entrainment calciner with pure CO2 preheating raw meal; DT-GGHX = drop tube calciner with pure CO2 preheating vent air via a gas-gas heat exchanger; Ent-GGHX = entrainment calciner with pure CO2 preheating vent air via a gas-gas heat exchanger. Figure 9a clearly shows that electricity costs dominate the total COC, due to both the high baseline electricity prices assumed and the significant electrical energy demand of the processes. Electricity costs account for 49%, 54%, 48%, and 54% of the total COC for DT-RMP, Ent-RMP, DT-GGHX, and Ent-GGHX, respectively. Additionally, CO2 transportation and storage costs have a significant impact on the COC, accounting for about 10% of the total. Other costs are related to raw meal purchase and the various utilities required for plant operations, such as cooling, refrigerated and process water, steam management, and make-ups for NH3, MEA, and NaOH, and account for up to 4% of the total. 32 In scenarios where electricity remains very expensive and sourced, for instance, from natural gas (Figure 11c), a broader trade-off emerges between Ent-RMP and DT-RMP. In these cases, the uncertainty in the electrified calciner cost plays a significant role in determining which alternative is more economically favourable. Further research, particularly focused on the electrified calciner design, will be essential to provide clearer expectation regarding the actual performance and cost, thereby increasing the confidence in this technology. 4. Conclusions This study explored the potential for decarbonising the cement sector by electrifying the calciner and applying amine-based carbon capture on rotary kiln emissions. A techno-economic analysis was conducted on four distinct process alternatives, which differed by the type of calciner used – either entrainment or drop tube, and the heat recovery strategy for the hot CO2 produced in the electrified calciner. Low-carbon processes increased electrical energy demand by 5.6 to 6.2 times compared to the reference plant, with entrainment calciner configurations requiring more power due to the energy penalty from CO2 recycling to lift the solids. All alternatives achieved competitive capture rates of approximately 97.5%, and CO2 concentrations met the required quality specifications without the need for additional purification. Overall, drop tube calciner designs demonstrated better energy efficiency, achieving comparable capture rates with similar fuel consumption, but lower electrical demand compared to Ent-calciner processes. Under an EU-27 energy mix, the low-carbon processes reached a CO2 avoidance rate of 70-72%, which was lower than other promising decarbonisation technologies such as oxyfuel and calcium looping. However, when low-carbon electricity is supplied, the CO2 avoidance rate improved to 98%, making these processes competitive with both oxyfuel and calcium looping technologies. 33 From an economic perspective, entrainment calciner with raw materials preheating emerged as the most favourable configuration with a cost of clinker (COC) of 213.4 €/tclk and a cost of avoided CO2 (CAC) of 217.4 €/tCO2, primarily due to the lower CAPEX of the electrified calciner and amine capture system. The other scenarios showed comparable economic performance, with CAC values between 231-234 €/tCO2. The high CAC values observed in the baseline scenario revealed that these technologies are not yet economically viable, mainly due to the current electricity price in the EU. A sensitivity analysis through a response surface model highlighted that the competitiveness of lowcarbon cement plants is strongly influenced by electricity price and grid carbon intensity. Further research focusing on detailed calciner design, accurate cost estimation, and the optimisation of processes and energy systems is critical to enabling widespread adoption and ensuring economic viability. CRediT authorship contribution statement Leonardo Varnier: Conceptualisation, Formal analysis, Investigation, Methodology, Software, Visualisation, Writing – original draft. Federico d’Amore: Methodology, Supervision, Validation, Writing – review and editing. Kim Clausen: Methodology, Supervision, Validation, Writing – review and editing. Georgios Melitos: Methodology, Software, Writing – review and editing. Bart de Groot: Software, Writing – review and editing. Fabrizio Bezzo: Methodology, Supervision, Writing – review and editing, Funding acquisition. Declaration of competing interest The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: K.C. is an employee of FLSmidth Cement A/S and G.M and B.d.G are employees of Siemens Industry Software Limited. 34 Data availability Data will be made available on request. Acknowledgements This work has been funded by the European Union’s Horizon Europe research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No 101073547 “CO2Valorize”. FdA acknowledges of the support of the National Recovery and Resilience Plan (NRRP), Mission 4 Component 2 Investment 1.4 - Call for tender No. 3138 of December 16, 2021 of the Italian Ministry of University and Research, funded by the European Union - NextGenerationEU [Award Number: CNMS named MOST, Concession Decree No. 1033 of June 17, 2022, adopted by the Italian Ministry of University and Research, CUP: C93C22002750006, Spoke 14 ‘‘Hydrogen and New Fuels’’]. Nomenclature Acronyms BAT Best Available Technique BF Bag Filter CAPEX Capital cost CCS Carbon capture and storage COP Coefficient of performance CPU Compression and Purification Unit DT Drop tube ESP Electrostatic precipitator Ent Entrainment GGHX Gas-gas heat exchanger configuration KPIs Key performance indicators LHV Lower heating value MEA Monoethanolamine PCC Post-combustion carbon capture RMP Raw meal preheating configuration SNCR Selective non-catalytic reduction Symbols 𝐴𝐶𝑒𝑞 % Equivalent CO2 avoided 𝐵𝐸𝐶 € Bare erected cost 𝐶𝐴𝐶 € tCO2 ⁄ Cost of avoided CO2 𝐶𝐶𝑅 % Carbon capture rate 𝑐𝐶𝑂2 € tCO2 ⁄ Carbon tax 36 𝑐𝑒𝑙 € MWhel ⁄ Electricity cost (with carbon tax) 𝑐𝑒𝑙,𝑏𝑎𝑠𝑒 € MWhel ⁄ Base cost of electricity (without carbon tax) 𝐶𝑂𝐶 € tclk ⁄ Cost of clinker 𝐸𝐶 € Equipment cost 𝑒𝑐𝑎𝑝𝑡,𝑐𝑙𝑘 kgCO2tclk ⁄ Specific CO2 captured 𝑒𝑐𝑙𝑘 kgCO2tclk ⁄ Direct specific CO2 emissions 𝑒𝑒𝑙 kgCO2,eq MWhel ⁄ Carbon intensity of the imported electricity 𝑒𝑒𝑙,𝑐𝑙𝑘 kgCO2tclk ⁄ Equivalent specific CO2 emissions 𝑒𝑒𝑞,𝑐𝑙𝑘 kgCO2tclk ⁄ Equivalent specific CO2 emissions 𝑖 % Discount rate 𝐼𝐹 − Installation factor 𝑗 − Generic unit 𝑚󰇗𝑐𝑙𝑘 [tclk y ⁄ ] Annual clinker productivity 𝑀𝐿 €/y Maintenance labour cost 𝑛 y Plant operational life 𝑂𝐿 €/y Operating labour cost 𝑂𝑃𝐸𝑋 €/y Operative costs 𝑃𝑒𝑙,𝑐𝑙𝑘 MWhel tclk ⁄ Electrical energy consumption 𝑇𝐴𝐶 €/y Total annualised CAPEX 𝑇𝐷𝐶 € Total direct cost 𝑇𝑂𝐶 € Total overnight cost 𝑇𝑃𝐶 € Total plant cost Sub-/Superscripts clk Clinker 37 decarb Plant with decarbonisation measure el Electric eq Equivalent ref Reference unabated plant th Thermal References AC2OCEM. 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