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The Kraft recovery boiler air system consists of three levels of air ports: Primary, secondary, and tertiary that allow the heated air to enter the boiler for combustion. The strong black liquor is sprayed into the boiler between the secondary and tertiary ports through the liquor gun ports located at about 15 feet above the furnace bottom. First, the liquor droplets dry as they fall through the hot combustion gases, then the easily volatilized components are released from the liquor droplets and burned. Next, the remaining char which is mostly carbon and the sodium compounds falls unto the smelt bed where the char burns off leaving the smelt. The smelt then flows out of the bottom of the furnace and into the dissolving tank to become green liquor completing the recovery boiler processing of the strong black liquor.