NUMERICAL PREDICTIONS OF THE CARBON BURNOUT PERFORMANCE OF COAL-FIREDNONSLAGGING VERTICAL CYCLONE COMBUSTORS

This paper describes the modifications and evolution of a finite-diffe rence solution procedure for predicting the aero and particle dynamics of a 500 kW coal-fired non-slagging vertical cyclone combustor incorp orating a fragmentation model and modified volatile burning law. Valid ation of the procedure was carried out from experimental data from a p rototype firing natural gas, where an algebraic stress model of turbul ence was needed to model cyclonic flows. The fragmentation model was b ased on experimental work indicating that fragmentation occurred early in the unit and at 80% of the coal diameter. The volatile-burning law was modified to allow for large amounts of soot, tars and higher hydr ocarbons formed by gasification within the unit. The programme investi gated wall ricochets/impacts of the burning coal particles and found t hat fragmentation after four ricochets for particles d greater than or equal to 50 mu m correlates well with the original 80%-diameter fragm entation concept. Investigation of scale, inlet velocity and chamber-w all roughness show that inlet velocities of 36 m s(-1) appear optimum for all sizes of unit; carbon burnout increases with scale, and the ef fect of chamber-wall roughness is reduced with scale.