Calcium carbonate looping (CCL) is a promising 2nd generation technology for low-cost post combustion CO2 capture for fossil fuels which has recently been tested in pilot scale. Previous assessments of the CCL process propose a low efficiency penalty (including CO2 compression) and low CO2 avoidance costs, i.e. far below that of 1st generation capture processes, but were mainly based on thermogravimetric data and lab-scale experiments. The principle of CCL follows the scheme of Figure 1.
After the flue gas stream has been cleaned by conventional pollution control equipment (e.g desulphurization), the CO2 in the flue gas is absorbed by CaO in a fluidized bed reactor operating at temperatures of about 650 °C. As the CaO is carbonated by the CO2 in the flue gas, CaCO3 is formed in an exothermic reaction. The CaCO3 is then fed to a calciner where, at temperatures above 900 °C, the CO2 is released through an endothermic reaction, and CaO is transported back to the carbonator. The calciner is also a fluidized bed type reactor. Fuel is burned with oxygen in the calciner to supply the heat needed for the calcination reaction of the CaCO3 and to obtain a CO2-rich stream at the calciner outlet.
As this carbon capture process operates at very high temperatures, the heat of the off-gas streams from carbonator and calciner can be utilized to produce high temperature steam for electricity generation in a water steam cycle when combined with a conventional fossil fuel steam generation system. This is a key differentiator with respect to many first generation carbon capture processes, in which significant portions of energy are at temperature levels that allow no useful recovery but require degrading of energy by dumping into cooling systems.
To overcome the uncertainties of small scale tests as a basis for scale-up scenarios, the extensive long-term pilot testing under realistic operating conditions is required. Previous tests performed with a 1 MWth scale pilot plant at TU Darmstadt have confirmed the feasibility of the technology in short operation phases, while giving a solid ground to assess the required next steps of follow-up projects, aiming mainly at conversion improvement and reliability. Given the results obtained at TU Darmstadt and given that the next logical step of CCL technology industrialization is to move to larger demonstrations – in a scale of 10 MWth or more, the following key issues were identified at the project start:
The process of CO2 removal by carbonation and calcination is new and therefore the key process variables and control strategies must be determined for a wide range of operating conditions (e.g. using different fuels in the calciner) by operating an upgraded pilot plant.
The CO2 capture is strongly dependent on the reactor configuration, residence time of solids, and system thermal operation, and this dependence has to be modelled through process and 3D computational fluid dynamics (CFD) simulations for scale-up.
More reliable experimental data, obtained from a wider set of measurements, taken on an upgraded version of the technology that is closer to realistic full operation conditions with different fuels, are necessary for the validation of models and assumptions on which the scale-up to larger units will be based.
Economic evaluations, a thorough understanding of environmental effects and a methodical assessment of risks are required to give confidence for the scale-up of the technology.
The major goal of the SCARLET project was to obtain reliable information and tools for the scale-up and pre-engineering of a 20 MWth CCL plant by continuous self-sustaining operation of an upgraded 1 MWth pilot plant at TU Darmstadt. The project should provide technical, economic and environmental assessments of this promising technology, as well as the fundamental expertise needed for the scale-up and integration of pre-commercialisation CCL facilities. By addressing these key challenges and demonstrating the upgraded technology at the TU Darmstadt pilot scale of 1 MWth, the project should give confidence for investments into a larger-scale unit, i.e. 20 MWth. After successful demonstration of units of this size, the technology will be ready for design and installation of commercial size units on utility boilers and other sources where CO2 reduction is required. The following key objectives were addressed for the SCARLET project:
Identification of the key process parameters and control strategies for the 1 MWth CCL pilot plant fuelled by hard coal and by lignite.
Development of scale-up tools and guidelines for CCL reactor design and process layout, validated by experimental data of 1 MWth pilot plant)
Design, cost estimation, and health, safety and technical risk assessment of a 20 MWth CCL pilot plant to be built at the Emile Huchet Site of Uniper France.
Techno-economic analysis of CCL application to hard coal and lignite fired power plants as well as cement and steel industry at commercial full-scale.
Environmental impact analysis of CCL application to hard coal and lignite fired power plants as well as cement and steel industry at commercial full-scale
The work plan was divided up in 9 comprehensive work packages (see Figure 2).
WP1 focused on long-term tests in an upgraded 2 MWth pilot plant. Four comprehensive test campaigns of 4 weeks each for hard coal and lignite were conducted to investigate the long-term behavior of the process focused on sorbent stability and reactivity. In-furnace measurements (gas extraction probe for in-bed gas analysis, capacitive probe for measuring solid load and velocity) were carried out to further validate the developed models with these experimental data. Solid samples extracted while testing were analyzed to complete the set of data. Additionally, a modified solids flow measuring system was tested to create a reliable method for measuring solids mass flow at the high temperatures of the process.
For WP2, the main objectives were the development of steady-state and unsteady process models as well as CFD models for three different approaches. For the process model development the heat and mass balances of CCL plants were calculated. CFD model development was focused on the discrete element method (DEM), the two fluid model utilizing the energy-minimization multi-scale method (EMMS) and stochastic collision detection for particle/particle interaction. Experimental data were used to validate the models by simulations performed for selected operating conditions. Additionally, concepts for improved design were elaborated with particular attention to the geometry.
WP3 was focused on the design and engineering of a 20 MWth pilot plant utilizing the operational and design experience from the 1 MWth pilot plant tests (WP1), the design models and scale-up tools developed and validated (WP2). The workload compromised the definition of the process configuration, the design and engineering of reactor and auxiliary system, the definition of a measurement plan and operation procedures for various scenarios as well as detailed planning for operation and logistics. A health, safety and technical risk analysis was carried out for identification of potential risks and thereby supporting the initial permission actions. All information acquired were used to calculate the overall investment costs (CAPEX), operational costs (OPEX) and maintenance costs expected for the scaled-up 20 MWth pilot.
The objectives for WPs 4-6 were the thermodynamic, economic and environmental evaluation for hard coal, lignite, steel and cement host plants. Therefore, existing plants were selected to provide the basis of design and the boundary conditions for the work to be carried out. The defined boundary conditions for the host plants were reconciled to guarantee the comparability of the results for the different plants considering the European Benchmarking Task Force definitions. This allowed the comparison of CCL with other capture technologies. Based on the host plant data, the thermodynamics (i.e. heat and mass balances, energy penalties) were calculated with the validated and scaled-up process model (WP2). With this input data, techno-economic analyses for the identification of cost of electricity (CoE) and cost of CO2 avoided to evaluate CCL technology compared to other CCS solutions were carried out. In addition, the assessment of the environmental impact of CCL systems was conducted by a life cycle analysis (LCA).
All these tasks are accompanied by project management, dissemination and technical coordination activities in WP7, WP8, and WP9, respectively.
The SCARLET consortium consisting of 11 international members (see Table 1) including two universities, a research organization and 8 industrial partners with an excellent industrial support provided a strong platform to address the key challenges in scaling-up the CCL technology.
Table 1: Details of the SCARLET consortium
|Technische Universität Darmstadt||TUD||DE||University|
|GE Carbon Capture GmbH||GECC||DE||Technology provider|
|Centre for Research & Technology Hellas||CERTH||EL||Research institute|
|University of Ulster||ULster||UK||University|
|Lhoist Recherche et Développement||LRD||BE||Material supplier|
|RWE Power AG||RWE||DE||Utility|
|Steinmüller Babcock Environment GmbH||SBE||DE||Technology provider|
|ArcelorMittal Maizières Research SA||AM||FR||Steel producer|
|Cemex Research Group AG||CEMEX||CH||Cement producer|
|SWR Engineering Messtechnik GmbH||SWR||DE||Equipment provider|