Formation of alkali salt deposits in biomass combustion

In the present work, the alkali salts deposit formation on cooled surfaces in biomass-fired boilers has been studied. Different deposition tests with two real biomasses (orujillo and cardoon) as well as with several synthetic materials have been conducted in an entrained flow (pilot scale) reactor for a range of conditions representative of those prevailing at different locations inside industrial boilers. A number of parameters, such as gas temperature around the probe or surface temperature, have been changed in order to analyze their influence on the characteristics of the ash deposits (morphology, thickness, composition…). The deposits' morphologies observed in the experimental samples indicate that the dominant deposition path for alkali salts (in the conditions explored in this work) is thermophoresis of aerosols formed either in the thermal boundary layer next to the deposition surface or already in the external gas stream (in case gas conditions allow for their formation). The contribution of each possible deposition mechanism has also been studied by comparing the obtained experimental results with theoretical predictions from a deposition model developed by Castillo and Rosner. Experimental trends have usually been in good agreement with those calculated according to the mechanisms of arrival of small particles to the deposition surface. Furthermore, the agreement found between calculations and measurements has been good enough to support the applicability of the model due to Castillo and Rosner to the quantitative prediction of deposit formation rates.