Cement sector is currently responsible for approximately 5% of the global CO2 emissions. CO2 originates principally from the raw meal calcination stage and conventional fuel (e.g. coal) combustion for the thermal needs of the process. Carbon capture and storage (CCS) is among the examined technologies for mitigating CO2 emissions generated in a cement plant. A very competitive technology for CO2 capture from flue gases appears to be Calcium Looping (CaL). The process is realized in a dual fluidized bed system where CO2 is absorbed by CaO in the first reactor (carbonator), and the produced CaCO3 is regenerated in the second oxy – fired reactor (calciner). During calcination, CO2 is released from the sorbents, purified, compressed and finally led to storage. Among the advantages of CaL when compared against other CO2 capturing technologies that could be applied in the cement industry, are the familiarity of the sector with the management (extraction, storage, feeding, etc.) of CaO-bearing materials and the prospect of reusing purge CaO in cement making as it is chemically compatible with cement raw meal.
This study describes the process modeling of the CaL implementation on a typical (no by-pass) five-stage preheater with pre-calciner cement plant as a retrofit option, in order to capture the CO2 produced through the clinker production. The process simulations were performed with the commercial software ASPEN Plus™ in conjunction with house-built models for the CaL process itself. A detailed description of the process configuration of the CO2 capture unit including its integration with the CO2 purification scheme and the basic parameters for the clinker production line are presented. Simulation results revealed that high S content in the supplementary fuel affects negatively the CaL performance and the quality of purge CaO in terms of replacing limestone in the kiln feed. Using low-sulfur coal for a 90% capture rate, a total purge CaO utilization can be achieved along with a raw limestone substitution of 8% and a net electricity yield of 426.66 kWh/t clinker. The economic evaluation of the proposed concept was also performed, estimating that the cost for CO2 avoidance equals approx. 68.75 €/tCO2. In terms of economic efficiency, CaL was found to be comparable to amine scrubbing. Give that CaL has not been examined so extensively as MEA, it has the potential for technical improvements in order to become more competitive by the adoption of novel concepts with smaller equipment cost.