Oxy-fired boiler unit with three combustion zones, and method of operating the same

An oxy-combustion boiler unit is disclosed which includes a furnace (100) for combusting fuel and for emitting flue gas resulting from combustion. The furnace has first (100a), second (100b) and third (100c) combustion zones, and an air separation unit (106) for separating oxygen gas (118) from air and providing a first portion (119) of the separated oxygen to a first oxidant flow (115), a second portion (120) to a second oxidant flow (117), and a third portion (121) of the separated oxygen gas to the first (100a), second (100b), and third (100c) combustion zones of the furnace (100). A controller together with control valves and measuring units cause the separated oxygen gas (118) to be distributed so that the first (115) and second (117) oxygen flows have a desired oxygen content, and so that the first (100a), second (100b), and third (100c) zones of the furnace (100) receive a desired amount of oxygen based on a combustion zone stoichiometry control. The flue gas oxygen content is determined in order to co