A Full-Scale Experimental and Modeling Investigation of Dust Explosions in a Roller Mill

Combustion of biomass is becoming an increasingly important energy source. This is especially true for wood pellets, as several power companies have decided to use this fuel instead of coal. In this process the wood pellets are ground in big mills and, from there, pneumatically transported to the burners in the boiler where they are consumed. The grinding of biomass and coal represents an explosion hazard, which can potentially result in considerable damage to the power plant upon propagation of the explosion from the mill into other parts of the installation.

An experimental investigation was performed aiming at understanding the potential damage that can occur due to an accidental dust explosion in a mill under worst case conditions. The tests were performed in a roller mill (capacity 36 metric tons coal/hour; internal volume 23 m3) connected to a coal feeder located on top of the mill. The roller mill was made available by Vattenfall and DONG Energy. The full-scale explosion experiments were performed at GexCon's test site on the island of Sotra, west of Bergen, Norway.

Test conditions were predefined using the CFD-based dust explosion simulation tool DESC. DESC was used to estimate the flow conditions inside the roller mill during normal operating conditions. In the full-scale tests these flow conditions have been simulated by the use of a dust injection system.

In total 26 tests were performed and certain parameters were varied, namely dust type (wood dust and coal dust), dust concentration and dust cloud location. The paper reports on both the DESC simulations performed and an overview of the main results obtained in the dust explosion experiments performed in the mill-coal feeder combination. This study shows how large-scale experiments can provide valuable information for evaluating potential hazards associated various types of processes. It further highlights how DESC can be a powerful tool in improving methods for dust explosion prevention and mitigation.