Heat can be provided for the endothermic calcination step either directly or indirectly.
Direct provision of heat involves the combustion of fuel in the calciner itself (fluidised bed combustion). This is generally assumed to be done under oxy-fuel conditions; i.e. oxygen rather than air is used to burn the fuel to prevent dilution of the CO with nitrogen. The provision of oxygen for the combustion uses much electricity; other air separation processes are being developed.
Indirect provision of heat to the calciner involves either:
* Combustion of fuel outside the vessel and conduction of energy in to the vessel
* Combustion of fuel in another vessel and use of a heat transfer medium.
Indirect methods are generally less efficient but do not require the provision of oxygen for combustion within the calciner to prevent dilution. The flue gas from the combustion of fuel in the indirect method could be mixed with the flue gas from the process that the CaL plant is attached to and passed through the carbonator to capture the CO.