Previous studies have shown that many biomass fuels result in critical agglomeration temperatures in the same range as typical operating process temperatures for fluidised beds. As soon as a sufficiently thick coating on the bed particles has been formed, the risk for severe agglomeration and defluidisation is often significant. Frequent bed change is therefore the normally method applied to ensure problem-free operation, but this is associated with additional costs and not sustainable on a long-term basis.
The objectives of the present work were therefore to; i) collect full-scale bed material samples to determine coating characteristics and growth as functions of time; ii) estimate the critical coating thickness/age of the bed particles by SEM/EDS analysis and experimental determinate the agglomeration temperatures for the collected bed material; iii) experimentally evaluate if size fractionation and mechanical treatment could be used to reduce the bed material consumption.
Bed material samples from two bubbling and two circulating full-scale fluidised bed boilers, with previous documented bed agglomeration problems, were collected. All plants used typical wood-based fuel mixtures. The coating formation rates on the bed particles were found to be significant, with an initial rate of some mm per day, but decreasing with time. The coating material was generally found to consist of Ca- and Mg-silicates as well as P, although the form in which it was present was not determined. The experimentally determined fuel specific agglomeration temperatures were found to agree well with the corresponding critical temperatures for the full-scale bed materials, as well as with the experiences reported by the different plant operators. The critical coating thickness was found to be relatively thin (<10 mm), and was reached within a few days. Unfortunately, regeneration by sieving and bed removal at different heights seemed relatively ineffective. Mechanical treatment (soft grinding) resulted in a breakdown of the original quartz bed material particles and the desired selective scavenging of the coating could not be obtained.