Layer Formation Mechanism of K-feldspar in Bubbling Fluidized Bed Combustion of Phosphorus-lean and Phosphorus-rich Residual Biomass

Abstract

The use of phosphorus-rich fuels in fluidized bed combustion is one probable way to support both heat and power production and phosphorus recovery. Ash is accumulating in the bed during combustion and interacting with the bed material to form layers and/or agglomerates possibly removing phosphorus from the bed ash fraction. To further deepen the knowledge about the differences in mechanisms behind the ash chemistry of phosphorus-lean and phosphorus-rich fuels, experiments in a 5 kW bench-scale-fluidized bed test-rig with K-feldspar as bed material were conducted with bark, wheat straw, chicken manure, and chicken manure admixtures to bark and straw. Bed material samples were collected and studied for layer formation and agglomeration phenomena. It was observed that the admixture of phosphorus-rich chicken manure to bark changed the layer formation mechanism, shifting the chemistry to the formation of phosphates rather than silicates. The admixture of chicken manure to straw reduced the ash melting and agglomeration risk, making it possible to increase the time until defluidisation of the fluidized bed occurred. The results also highlight that an increased ash content does not necessarily lead to more ash melting related problems as long as the ash melting temperature is high enough.