Introduction
The decarbonisation of hard-to-abate industrial sectors such as cement, lime, and steel production requires CO2 capture technologies capable of operating efficiently under high-temperature, dust-laden, and compositionally variable flue gas conditions, while simultaneously delivering CO₂ streams of sufficiently high purity for transport, storage, or utilisation. Calcium looping (CaL) is a promising CO₂ capture technology due to the fast kinetics, low cost, and wide availability of CaO-based sorbents. However, conventional CaL concepts face major challenges associated with the high energy demand of CaCO₃ calcination and the need for downstream CO₂ purification. This work presents the experimental validation of a novel packed-bed calcium looping process incorporating steam-enhanced calcination, designed to exploit the heat released during carbonation to regenerate the sorbent via a rapid CO₂ partial-pressure swing, enabling the direct production of ultra-high-purity CO₂ without costly purification steps.
Methods
The proposed process was experimentally investigated in a laboratory-scale packed-bed reactor (1.2 m bed height, 50 mm internal diameter) loaded with up to 4.4 kg of natural limestone as CaO precursor. The reactor was operated in cyclic carbonation–calcination mode. During carbonation, CO₂-containing gas mixtures (15–40 vol% CO₂ in air) were fed at temperatures between 500 and 750 °C, allowing exothermic CaO carbonation to proceed and store heat within the solid bed. Calcination was subsequently initiated by injecting preheated steam, inducing a rapid decrease in CO₂ partial pressure and triggering CaCO₃ decomposition. Gas compositions were continuously monitored, and axial temperature profiles were measured using a multipoint thermocouple. Additional experiments integrated chemical looping combustion (CLC) stages using CuO-based oxygen carriers to provide in situ heat generation under conditions where carbonation alone could not achieve sufficiently high temperatures.
Results
The experimental results demonstrate that the heat released during carbonation can raise the bed temperature well above 800 °C when CO₂ concentrations exceed approximately 25 vol%, enabling rapid and effective sorbent regeneration during subsequent steam-assisted calcination. CO₂ capture efficiencies above 95% were consistently achieved during carbonation for feed gases containing up to 40 vol% CO₂, with gas residence times of only a few seconds. During calcination, the injection of steam produced outlet gas streams composed of virtually 100% CO₂ on a dry basis, confirming that high-purity CO₂ can be obtained without downstream purification beyond water condensation. Temperature profiles showed sharp and uniform drops during calcination, indicating effective decomposition of CaCO₃ throughout the bed. Multi-cycle experiments confirmed stable operation over successive carbonation–calcination cycles, with capture efficiencies remaining between 85% and 95%, despite the non-adiabatic limitations of the laboratory-scale setup. Integration of CLC stages successfully increased bed temperatures beyond 850 °C, improving calcination rates and enabling robust operation even with low CO₂ inlet concentrations or highly cycled sorbents.
Conclusions
This study provides experimental proof of concept for a novel packed-bed calcium looping process with steam-enhanced calcination capable of delivering ultra-pure CO₂ while significantly reducing the energy penalty associated with sorbent regeneration. By exploiting the heat stored during carbonation and combining it with rapid CO₂ partial-pressure swings and optional in situ heat generation via chemical looping combustion, the process enables near-autothermal operation and eliminates the need for costly CO₂ purification units. The results highlight the strong potential of this technology for decarbonising hard-to-abate industrial sectors and provide a solid experimental basis for further scale-up, optimisation of multi-cycle stability, and future techno-economic assessment.
