Modeling, assessment, and optimization of the indirectly heated carbonate looping process for CO₂ capture from lime plants

Lime is an essential raw material for iron and steel production, in construction and agriculture, in civil engineering, in environmental protection, and in manifold chemical manufacturing processes. To solve the problem of unavoidable process CO₂ emissions associated with the production of lime, efficient and affordable capture technologies need to be developed and implemented. The indirectly heated carbonate looping (IHCaL) process is a promising technology for capturing CO₂ from lime and cement production. This process can exploit synergies with these industries, achieving low penalties in terms of economics and energy utilization. However, this requires integration measures that have not been undertaken until now. Furthermore, experimental efforts to develop IHCaL technology have not been matched by modeling work. Yet, accurate models are crucial for interpreting experiments and scaling up this technology for commercial operation. In this study, we introduce concepts for efficiently integrating the IHCaL process into lime production. The main configurations are a tail-end solution suitable for retrofitting lime plants and a fully integrated configuration for greenfield projects. Furthermore, we present different fueling and heat integration strategies. We assessed these concepts in terms of energy efficiency, net CO₂ emissions, and CO₂ avoidance costs (CAC). The processes were simulated with Aspen Plus, including customized reactor models. EBSILON professional was used to model the steam cycles for heat recovery and power generation, and ECLIPSE was used for the economic analysis. The results show a considerable increase in fuel consumption, but approximately 30 % of the entire heat input can be converted into electric power via heat recovery steam generation. The critical parameters influencing the process efficiency are the preheating temperature of the combustion air, the sorbent temperature at calciner inlet, and the sorbent circulation rate. Utilizing the sorbent purge as lime product is necessary for the feasibility of the proposed concepts. The heat recovery strategy can be optimized to achieve tailored outcomes, such as reduced fuel requirement or increased power production. Net-negative CO₂ emissions higher than −1.8 tCO₂/tCaO can be obtained when firing waste-derived fuels. Fuels with a high biogenic fraction and low specific CO₂ emissions, such as solid recovered fuels (SRFs) with a high calorific value, perform remarkably well in terms of energy utilization (SPECCA < 1.5 MJLHV/kgCO₂,av) and cost efficiency (CAC < 20 €/tCO₂,av). We present a novel carbonator model for CO₂ capture using IHCaL technology. Our model is based on a systematic literature review, and experimental results from pilot tests and thermogravimetric analysis (TGA). The model features a reactor sub-model to simulate flow patterns and gas-solid contact, and a particle sub-model to deal with sorbent deactivation and reaction kinetics. We follow an original approach to non-ideal calcination and introduce a new calculation methodology for sorbent aging that considers fluctuations in sorbent circulation and make-up rates. We highlight that modeling assumptions commonly found in the literature can result in a significant overestimation of the carbonator performance. Additionally, we offer guidelines for the appropriate selection of these assumptions. Finally, we introduce a novel calciner model by combining a particle sub-model with a onedimensional reactor sub-model. The model was validated with results from experimental campaigns conducted at two different pilot plants (300-kWth and 1-MWth). The predictions of the model were interpreted using a stochastic methodology and novel dimensionless numbers. Based on the model, we introduce a three-step approach to designing calciners for CO₂ capture. Calciners with oxy-fuel combustion should be operated at 930–965 °C to achieve sufficient sorbent regeneration. For indirectly heated calciners, an operating temperature of 950 °C is necessary for high performance, but lower temperatures (e.g., 900 °C) are also possible using steam for fluidization. Considerations regarding particle residence time are also discussed. Our guidelines are straightforward and enable the design of a calciner with simple calculations. Overall, our study provides valuable information on the potential of IHCaL technology for reducing CO₂ emissions in the lime industry and demonstrates how process configurations, fuel properties, operating parameters, and components can be optimized to exploit this potential. Our results reduce design uncertainties, avoidance costs, and energy penalties, thereby bringing this technology closer to commercialization.