Processes for controlling afterburn in a reheater and loss of entrained solid particles in reheater flue gas are provided. Carbonaceous biomass feedstock is pyrolyzed using a heat transfer medium forming pyrolysis products and a spent heat transfer medium comprising combustible solid particles. The spent heat transfer medium is introduced into a fluidizing dense bed. The combustible solid particles of the spent heat transfer medium are combusted forming combustion product flue gas in a dilute phase above the fluidizing dense bed. The combustion product flue gas comprises flue gas and solid particles entrained therein. The solid particles are separated from the combustion product flue gas to form separated solid particles. At least a portion of the separated solid particles are returned to the fluidizing dense bed.