, ▁where ▁it ▁is ▁he ated ▁to ▁ 8 5 0 - 9 5 0   ° C ▁to ▁cause ▁it ▁to ▁therm ally ▁de compose ▁into ▁g ase ous ▁carbon ▁dio x ide ▁and ▁solid ▁calci um ▁ox ide ▁( Ca O ). ▁The ▁almost - p ure ▁stream ▁of ▁CO 2 ▁is ▁then ▁removed ▁and ▁pur ified ▁so ▁that ▁it ▁is ▁suitable ▁for ▁storage ▁or ▁use . ▁This ▁is ▁the ▁' forward ' ▁reaction ▁in ▁the ▁equation ▁above . ▁ ▁Car bon ation : ▁The ▁solid ▁Ca O ▁is ▁removed ▁from ▁the ▁calc iner ▁and ▁fed ▁into ▁the ▁carbon ator . ▁It ▁is ▁co o led ▁to ▁approximately ▁ 6 5 0   ° C ▁and ▁is ▁brought ▁into ▁contact ▁with ▁a ▁fl ue ▁gas ▁containing ▁a ▁low ▁to ▁medium ▁concentration ▁of ▁CO 2 . ▁The ▁Ca O ▁and ▁CO 2 ▁react ▁to ▁form ▁Ca CO 3 , ▁thus ▁reducing ▁the ▁CO 2 ▁concentration ▁in ▁the ▁fl ue ▁gas ▁to ▁a ▁level ▁suitable ▁for ▁emission ▁to ▁the ▁atmosphere . ▁This ▁is ▁the ▁' back ward ' ▁reaction ▁in ▁the ▁equation ▁above . ▁ ▁Note ▁that ▁carbon ation ▁is ▁calc ination ▁in ▁reverse . ▁ ▁Wh ilst ▁the ▁process ▁can ▁be ▁theoret ically ▁performed ▁an ▁infinite ▁number ▁of ▁times , ▁the ▁calci um ▁ox ide ▁sor b ent ▁de gr ades ▁as ▁it ▁is ▁cy c led . ▁For ▁this ▁reason , ▁it ▁is ▁necessary ▁to ▁remove ▁( pur ge ) ▁some ▁of ▁the ▁sor b ent ▁from ▁the ▁system ▁and ▁replace