Plant optimisation and ash recycling in fluidised bed waste gasification

Abstract

We studied the gasification of two wastes in a fluidised-bed pilot plant aiming at finding sustainable methods for the utilisation of the ash produced. Two objectives were pursued: (1) optimisation of the operating conditions to achieve better ash quality while keeping reasonably high energy efficiency and (2) assessment of the ash quality in order to explore potential utilisation options for the ash. An extensive characterisation of the raw ash produced was carried out, finding that, in most cases, the existing fly ash utilisation methods are not applicable. The high concentrations of unburned carbon and harmful soluble compounds restrict ash utilisation or normal disposal, making pretreatment necessary prior to use in agriculture and construction. The only alternative that allows material recycling and energy recovery without the need of pretreatment is the use of the ash in cement kilns. However, competition with other fuels makes this option uncertain. Therefore, we further investigated more innovative utilisation concepts where pretreatment is not necessary, like the addition of ash in the manufacture of lightweight wall board and bricks with special properties. These utilisation options are based on low-cost preparation methods using ash in significant proportions, yielding a notably high-value product. We show that they are promising new applications with high market potential.