Calcium Looping, CaL, is an emerging CO2 capture technology specially suited for existing coal power plants, with known theoretical advantages due to the high temperature operation of the reactors working in the CO2 capture loop: the carbonator at around 650ºC taking CO2 from the flue gases of the existing power plant and the calciner burning additional coal under oxy-combustion conditions. This allows for an effective heat recovery in a steam cycle, in order to generate more power. Therefore main efficiency penalty of the process comes from the O2 generation in the Air Separation Unit (ASU) unit. The total O2 required in the process depends on the heat input required in the calciner, existing a potential for efficiency penalty reduction and cost by reducing this energy requirements in the calciner. The best way to reach this target is to reduce the recirculation of CO2 to the calciner which brings to an increase of the concentration of O2 in the oxidant flow to the calciner. This measure:
- Reduces the heat requirements in the calciner and therefore the consumption of coal and O2.
- Reduces the calciner size for the same heat input (to keep similar gas velocities in the CFB calciner) and the size of the ASU, which implies a decrease of investment costs.
Simplified scheme of the proposed CaL process to be developed in this project, highlighting the reduction of the CO2 recirculated flow and the smaller calciner and Air Separation Unit (ASU) equipments.
The current energy efficiency penalty of the standard configuration of a CaL system has been estimated around 7-9 net points including CO2 purification compression. More than half of this penalty (4-5 net points) arise from the energy needs to separate O2 from air in the ASU unit and for the recycle of a large CO2 stream to moderate combustion temperatures in the oxy combustor and calciner. Basic mass and heat balance calculations reveal that the standard CaL system can reduce about 20-30% the energy requirements in the calciner by switching to a configuration as proposed in this CaO2 project.