Experimental study on the fouling behaviour of an underfeed fixed-bed biomass combustor

Abstract

One of the most important problems occurring in biomass combustion is the appearance of fouling on heat exchangers which reduces the efficiency and lifetime of the facility. In this experimental work, wood pellets were burned in a small-scale biomass underfed fixed-bed combustor (12kWth) using low primary to secondary air ratios (15/85, 20/80, 25/75, 30/70). A water-refrigerated sampling probe was placed in the cross-flow above the bed to measure the fouling layer build-up rates. The water temperature inside the tube was varied between 20 and 95°C. This research proposes a test methodology to mimic the behaviour of a real heat exchanger of a commercial boiler.The measured deposition rates fluctuated between 7 and 28g/m2h. A greater amount of total airflow, an increase in the primary air inlet and a lower water temperature increased the deposition mass. In addition, the deposition rate decreased with time and did not appear to follow a linear pattern, which likely indicates that after a strong initial fouling rate, removal and collapse mechanisms begin to counter-balance the arrival of particles, which stabilizes the fouling layer.