Classification performance analysis of a louver classifier with a tilted sidewall and its application to the separation of unburned carbon from biomass combustion ash

• Analyzed the classification performance of a louver classifier with a tilted sidewall. • Applied classifier for separating unburned carbon from biomass combustion ash. • Separation performance of louver was improved by tilted sidewall addition by increasing gas flow downward velocity. • The modified louver design improved the RUC of ash from 12.7 to 26.9 %. • A blow-up ratio of 30 vol% increased the RUC in the modified louver to 83.8 %. Separating unburned carbon (UC) from biomass combustion ash is necessary to achieve high-quality ash as recyclable raw material and improve energy efficiency by reburning the UC in the furnace. In this study, a novel louver classifier geometry with tilted sidewall was proposed to separate the UC from biomass ash and its classification performance was analyzed via experiments and simulations. The louver classifier with a tilted sidewall provided a smaller 50% cut size and classification accuracy index indicating that the separation performance is improved. The CFD simulations revealed that tilted sidewall in the louver increased gas flow downward velocity in the classification region, increasing the inertia of the particles, and causing more particles to leave gas flow and deposit into the collection box as coarse ash. The UC of classified combustion ash increased with a decrease in the median mass diameter, indicating that the classification process for combustion ash produced coarse ash with a lower UC. The tilted sidewall in the modified louver classifier increased the reduction of unburned carbon (RUC) in ash up to 83.8%. The RUC increased with a cut size of the classified ash which was achieved when a high inlet flow velocity and high blow-up ratio are applied.