Abstract Ash transformation and deposition during pulverized wood combustion in a full-scale power plant boiler of 800 MW th were studied with and without the addition of coal fly ash. The transient ash deposition behavior was characterized by using an advanced deposit probe system at two boiler locations with flue gas temperatures of about 1300 °C and 800 °C, respectively. The mechanisms of ash transformation and deposit formation were elaborated through a detailed characterization of the collected deposits and fly ashes. The results implied that during pulverized wood combustion, the formation of deposits at the location with high flue gas temperatures was characterized by a slow and continuous growth of deposits followed by the shedding of a large layer of deposits, while at the location with low flue gas temperature the deposit formation started with a slow build-up and the amount of deposits became almost constant after a few hours. The formed deposits, especially those at the location with low flue gas temperatures, contained a considerable amount of K 2 SO 4 , KCl, and KOH/K 2 CO 3 . With the addition of a large amount (about 4 times of the mass flow of wood ash) of coal fly ash to the boiler, these alkali species were effectively removed both in the fly ash and in the deposits. Although the ash deposition rate at the location with high flue gas temperature was increased with coal fly ash addition, the removability of the deposits was significantly improved, resulting in a more frequent shedding of the deposits. Overall, the results from this work suggest that coal fly ash can be an effective additive to minimize the possible ash deposition and corrosion problems during suspension-firing of wood.