Method for determining bridging properties of biomass fuels—experimental and model approach

A test apparatus enabling to determine the length of a fuel bridge over an opening was developed in order to describe the physical–mechanical fuel property of bridging. Two such testers were applied by six European partners who applied a uniform test procedure on a total of 85 large biomass fuel samples of 2 m³ volume. All samples were further analysed for angle of repose, moisture content, bulk density and image analysis parameters (e.g. particle size distribution, particle shape factor and length/diameter ratio). The opening width (i.e. maximum bridge length) of all tested fuels ranged from 0.8 cm (grain kernels) to 99 cm (unchopped grass materials). For the developed test method, the mean coefficient of variation of the replications was between 5.4% (wood pellets) and 20% (sawdust). The also measured angle of repose showed no satisfying correlation to the opening width from the bridging tests. But other influencing parameters for bridging were regarded in a multiple linear regression model. It was shown that for wood samples (chips, hog fuel), the parameters mean particle size, mean particle shape factor and mean length/diameter ratio are highly capable of explaining or predicting any unfavourable bridging phenomena. In this model, there was no significant influence of bulk density in bridging.