▁indirect ly . ▁ ▁Direct ▁provision ▁of ▁heat ▁involves ▁the ▁comb ust ion ▁of ▁fuel ▁in ▁the ▁calc iner ▁itself ▁( fl uid ised ▁bed ▁comb ust ion ). ▁This ▁is ▁generally ▁assumed ▁to ▁be ▁done ▁under ▁o xy - f uel ▁conditions ; ▁i . e . ▁o xygen ▁rather ▁than ▁air ▁is ▁used ▁to ▁burn ▁the ▁fuel ▁to ▁prevent ▁dil ution ▁of ▁the ▁CO 2 ▁with ▁nit ro gen . ▁The ▁provision ▁of ▁o xygen ▁for ▁the ▁comb ust ion ▁uses ▁lots ▁of ▁electric ity ; ▁other ▁air ▁separation ▁processes ▁are ▁being ▁developed . ▁ ▁Ind irect ▁provision ▁of ▁heat ▁to ▁the ▁calc iner ▁involves ▁either : ▁ ▁Com b ust ion ▁of ▁fuel ▁outside ▁the ▁vessel ▁and ▁condu ction ▁of ▁energy ▁in ▁to ▁the ▁vessel ▁ ▁Com b ust ion ▁of ▁fuel ▁in ▁another ▁vessel ▁and ▁use ▁of ▁a ▁heat ▁transfer ▁medium . ▁ ▁Ind irect ▁methods ▁are ▁generally ▁less ▁efficient ▁but ▁do ▁not ▁require ▁the ▁provision ▁of ▁o xygen ▁for ▁comb ust ion ▁within ▁the ▁calc iner ▁to ▁prevent ▁dil ution . ▁The ▁fl ue ▁gas ▁from ▁the ▁comb ust ion ▁of ▁fuel ▁in ▁the ▁indirect ▁method ▁could ▁be ▁mixed ▁with ▁the ▁fl ue ▁gas ▁from ▁the ▁process ▁that ▁the ▁Ca L ▁plant ▁is ▁attached ▁to ▁and ▁passed ▁through ▁the ▁carbon ator ▁to ▁capture ▁the ▁CO 2 . ▁ ▁Rec overy ▁of ▁energy ▁from ▁the ▁carbon ator ▁ ▁Although ▁the ▁heat ▁from ▁the ▁carbon ator ▁is ▁not ▁at ▁a ▁high ▁enough ▁temperature ▁to ▁be ▁used ▁in ▁the ▁calc iner