A process and apparatus for fluidized bed combustion using a dense phase combustion zone over a dilute phase combustion zone. When used to regenerate FCC catalyst containing coke with relatively large amounts of NO.sub.x precursors, the catalyst is added to and partially regenerated in the dense phase zone and then falls down into the dilute phase zone. Most of the combustion air is added to the dilute phase and rises into and fluidizes the dense phase zone. NO.sub.x formed in the dilute phase is reduced to N.sub.2 in the dense phase.