Volume 5: Innovative Solutions for Energy Transitions: Part IV

Modelling and Experimental Validation of Deposit Formation for  a Woodchip Fired Grate Boiler Based on a Mechanistic Fouling Model Zhimin Zheng, Wenming Yang*, Hui Wang, Anqi Zhou, Yongtie Cai, Guang, Zeng, Hongpeng Xu

Abstract

Woodchip as an alternative fuel is widely fired to generate electricity and steam. However, fouling is a major issue for woodchip fired boilers. It is important to develop mathematical models and predictive tools to understand and predict ash deposition behavior. A mechanistic fouling model considering build‐up and removal mechanisms during ash deposition is developed in this work. In the models, the effect of surface roughness on ash deposition was also considered. Meanwhile, the fouling model was implemented into the ANSYS FLUENT and was combined with the discrete particle model (DPM), heat transfer model, and dynamic mesh model to predict ash deposition behavior on a deposition probe. The simulation result was validated against the experimental data obtained from a lab‐scale experimental setup. The simulated trend of the deposit thickness as a function of time shows good agreement with the experimental results. Based on the developed model, the effects of the removal model and flue gas velocity were investigated. The results show that it is necessary to consider the removal mechanism even when the flue gas is low. The deposition mass presented a significant decrease with the increase of flue gas velocity. An asymptotic trend for the deposition mass was observed for the cases considering the removal model. The research shows that the mechanistic fouling model coupled with CFD is a promising tool to predict the ash deposition behavior in low‐temperature conditions for the woodchip fired grate boilers.

Keywords ash deposition; grate boiler; woodchip;  dynamic mesh; dynamic simulation; surface roughness