A 41 MWth oil shale fired demonstration power plant was built in 1989 by PAMA in Mishor Rotem, Negev, Israel. The raw material for the plant is the local "oil shale", which is in fact organic-rich mart. Since then, and until today, the unit is operated at high reliability and availability. At first, heavy soft fouling occurred due to the Circulating Fluidized Bed Combustion (CFBC) mode of operation, which caused a considerable reduction in the heat transfer coefficient of the heat exchangers. By going over to the Fluidized Bed Combustion (FBC) mode of operation the soft fouling phenomenon stopped at once, the heat transfer coefficient improved, and the power plant could be operated at its designed values. After five months of operation at the FBC mode the boiler had to be shut down because Hard Deposits (HD) blocked physically the passes in the boiler. These deposits could be removed only with the help of mechanical devices. During the first two years the boiler had to be stopped, at least, three times a year for deposit cleaning purposes. Research conducted at the plant and in the laboratories of the Geological Survey of Israel enabled us to understand the mechanism of formation of these deposits. The results showed that the HD are formed in two stages: (1) Deposition of very fine ash particles on the pipes of the boiler, as a result of the impact of larger particles on the pipes. The fine particles adhere to the pipes and to each other, and step by step build the deposit. The growth of the deposit on the pipe surface is always perpendicular to the particles flow direction. (2) The deposits harden due to chemical reactions. The joint experiments at the plant and at the laboratories of the Geological Survey showed: (A) The rate of deposition depends mainly on the lime concentration in the fly ash. (B) The lime concentration in the fly ash is a function of the clays concentration in the oil shale. (C) The increase and hardening of the deposit with time is due to solid-gas reactions within the deposit. At first recarbonation occurs, reaction between CaO in the deposit and CO2 (produced by the combustion) in the flue gas to form CaCO3 (bonded deposits), and then sulfatization; the reactions of the sulfatization are: (1) SO2 in the flue gas with CaO and CaCO3 in the deposit, leading to the formation of anhydrite CaSO4; and (2) SO in the flue gas with the amorphous silicates in the deposit forming hydroxylellestadite Ca-10(SiO4)(3)-(SO4)(3)(OH)(2). These minerals are the hard deposits. The conclusions following these findings for the combustion of oil shales with a significant Ca-carbonate content are: (A) The FBC is the preferred mode of combustion. (B) The rate of deposition in the boiler depends mainly on the lime (free CaO) concentration in the Fly Ash (FAS). (C) The ratio Ca-carbonates to silicates (At, Fe, etc.), in the oil shale feed, determines the concentration of lime in the FAS. (D) The rate of deposition in the boiler depends also on the geometry of the boiler and on the particles aerodynamic conditions in it. Following these conclusions, the plant was able to reduce the shutdowns to twice a year. Furthermore, based on the understanding of the deposit formation mechanism, it will be possible to minimize shutdowns, for deposit cleaning, to only once a year in future similar oil shale fuelled power plants. (c) 2007 Elsevier Ltd. All rights reserved.