The paper investigates agglomerate formation in the world biggest pressurized fluidized bed combustor at Karita Power Station, Japan. It was found that significant amount of agglomerates was formed at an increased boiler load bringing about a significant decrease of bed density. The agglomerates had a glassy-like molten surface and consisted of a mixture of ash from char fines and limestone particles. The main components found in the agglomerate cross-section were Ca, Al and Si oxides, all quite uniformly distributed. Agglomerate formation occurred particularly when the unit was fired with a porous Blair Athol coal that was found to produce a porous char. Firing the boiler with other coal (Nanton) that produced less porous char prevented the formation of agglomerates and enabled to maintain stable operation of the combustor. A model was developed to calculate the horizontal distribution of char surface temperature in the PFBC based on a quasi-steady heat balance for a burning char by taking into consideration the distribution of volatiles above fuel feeding nozzles, as well as char porosity. In order to take into consideration the effect of porosity on combustion of a porous char a completely new expression to estimate the reaction rate was proposed. The calculation results indicated that the agglomerates were mainly formed due to the combustion of highly porous Blair Athol chars in the poorly fluidized areas in the bed, where air-to-fuel ratio became larger. The combustion rate of less porous Nanton char was much slower then that of the Blair Athol. Accordingly, combustion temperature of its char was lower bringing about no formation of agglomerates.

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