Analysis of fouling in domestic boilers fueled with non-woody biomass

The greater sustainability of agricultural and industrial wastes (non-woody biomass) when compared to the widely used high-quality forestry biomass encourages their usage in small-scale thermal devices (residential and domestic applications). However, the typical significant ash and alkali-compounds content of agro-industrial wastes may form undesirable deposits over the heat exchange surfaces that could require additional cleaning systems (thus increasing the cost and complexity of the boilers). This work analyses the fouling tendency of exhausted olive cake and almond shells when used in a biomass boiler (with a nominal thermal capacity of 55 kW) at partial and nominal load, trying to highlight the main mechanism leading to the deposits growth depending on the fuel properties and load. Inertial impaction of coarse fly ash governs deposition at partial load for almond shells, this is attributed to the low K, Cl and S content and the lower bed temperature reached (431 °C vs 528 °C at nominal load). Among the deposition mechanisms associated to K-salts condensates, direct condensation on the incipient layers dominates over thermophoresis of submicron particles at nominal load. Although no additional maintenance procedures seem to be required when using almond shells, it is highly recommended to remove K/Cl components from exhausted olive cake through prior water leaching or by using additives to retain these species in the bottom ash.