Particulate fouling in waste incinerators as influenced by the critical sticking velocity and layer porosity

Fouling of heat transfer surfaces introduces a major uncertainty into the design and operation of heat exchange equipment. Fouling layers as observed on the tube bundles of the economizer in a Dutch waste incinerator were thin and powdery. The fouling layer showed an asymptotic growth rate with a levelling off increase of the thickness. In this study, the influence of the critical sticking velocity on the growth rate of particulate fouling layers is described. The critical sticking velocity of an incident particle hitting a powdery layer is defined as the maximum impact speed at which the particle will stick to the layer. Since the critical sticking velocity is a key parameter in the deposition mechanism, a well-defined experimental set-up has been built to assign it. Experimental results showed that the critical sticking velocity increases with the porosity of the fouling layer. Literature shows that the porosity of a thin sintered powdery layer changes with the layer thickness. Based on the experimental results and the variation of porosity with thickness for a thin sintered powdery fouling layers, a correlation is developed which shows that the sticking velocity decreases exponentially as the fouling layer thickness increases. Therefore, fewer particles are likely to stick as the fouling layer builds up and consequently the deposition rate decreases. The change in the critical sticking velocity as the fouling layer builds up contributes to the explanation of the asymptotic growth of particulate fouling layers on the tube bundle of waste incinerators.