Tendency to bridge over openings for chopped Phalaris and straw of Triticum mixed in different proportions with wood chips

Solid biofuels with poor flow properties can cause problems in heating and power plants. Bridging over openings is one problem that was studied for chopped reed canary grass (Phalaris arundinacea) and wheat straw (Triticum aestivum) mixed in different proportions with wood chips. The aim was to determine whether the bridging tendency in chopped reed canary grass and in chopped wheat straw depends on the particle size, and if it would be reduced if grass and straw were mixed with dry wood chips. The bridging tendency was defined as the slot opening when a “bridge” of material formed over the opening breaks, and was measured with equipment developed and tested for wood fuels. Earlier studies showed that long and thin particles have a high tendency to bridge. This was confirmed for the straw and grass studied. Their tendency to bridge was 6–8 times higher than that of wood chips. Mixing wood chips in grass or straw decreased the bridging tendency, but only marginally. Even when 80% of the dry matter was wood chips the mixtures had a bridging tendency that was 3–5 times higher than that of pure wood chips. The results indicate that mixing wood chips into chopped reed canary grass or straw is not a promising concept to decrease their bridging tendencies. The lack of suitable methods to classify particle shape in mixtures of wood chips and straw or grass was an obstacle to a closer analysis of how the particle shape affects the bridging tendency.