The 71 MWe PFBC combined cycle power plant at Wakamatsu, Electric Power Development Company (EPDC) (funded by the Ministry of International Trade and Industry (MITI) and the Center of Coal Utilization Japan (CCUJ)) achieved 11,500 h accumulative operation while establishing extremely low dust emission (< 1 mg/m(3) N) with a hot gas cleaning system combining cyclones and ceramic tube filter (CTF). Two different configurations of cyclones and CTF were examined in Phases 1 and 2, where several coals and domestic limestone as absorbents were used. The pressure drop across the filter (Pc) was continuously observed to predict its maximum value (DeltaP(max)) under operating conditions. The DeltaP(max) value consisted of irreversible (DeltaP(base)) and reversible (DeltaP(base)) drops with cleaning as the following equation shows, DeltaP(max) DeltaP(base) + DeltaP(own) = f(face velocity, gas viscosity, thickness/permeability, dust concentration) + d Delta Pc/dt x t where: d Delta Pc/dt, mean rate of Delta Pc increase during cleaning interval; t, reverse cleaning interval time. The pressure drop across the regenerator (DeltaP(regenerator)) was monitored to calculate the changing face velocity of each compartment according to the Fanning equation. Both pressure drops (DeltaP(base) and DeltaP(own)) were successfully estimated by the face velocity calculated for both phases. Dust concentration to the filter in both phases was found to improve the predictability of the base line one (DeltaP(base)). The conditioned filter material was proven to be stable, showing a constant drop depending on the operating condition. The particle size of ashes on the filter must be considered in order to obtain better prediction of the maximum pressure drop in the future.