MEASURING TECHNOLOGY 158 60 LANDTECHNIK 3/2005 Bulk Density Determination of Solid Biofuels

Thorsten Böhm, Hans Hartmann, Straubing
B ulk density is an important quality parameter for determining storage and transportation room demands of pourable biofuels. Also the design of feeding systems depends on the fuel's bulk density. Furthermore it is a key property for volume based payments and it influences the readings from many physical principles for rapid moisture content determination [1]. Bulk density is calculated by the quotient of mass of a sample material filled into a measuring container and its known volume. Although
more » ... bulk density is mostly regarded as an easily determinable parameter, the applied methods are highly inconsistent in practice. E.g. the shape and size of the measuring container often vary. Moreover the application of shock impact on the material filled container and the fuel's moisture content (MC) can influence the bulk density result. In the following the mentioned influencing factors are analysed. Influencing factors Four containers of different sizes (15, 50, 100 l) and shapes (cube, cylinder) were tested in a European round robin ( Table 1) . The container was filled by pouring the sample material from a shovel until a cone of maximum possible height was formed. The pouring height was 100 to 200 mm above the upper rim of the container. Surplus material was removed using a scantling, which was shuffled over the container's edge in a sawing-like movement. The samples were weighed to the nearest 10 g on a platform balance. All measurements were conducted in two variants with three replications each, first by simple container filling ("without shock") and second by dropping the material filled container three times from 150 mm height ("with shock"). In order to evaluate the influence of shrinkage effects with decreasing moisture contents, several fresh wood chip samples were dried down stepwise. Thus, uniform fuel samples with six to eight ap-proximately equidistant moisture content steps were produced covering the natural fuel moisture range. All original bulk density data were initially calculated to a dry matter basis (0 % moisture). Work plan The biofuel samples were coniferous (14) , deciduous (16) and mixed wood chips (4); also bark (3), wood pellets (2) , herbaceous pellets (4), sawdust (2) and three other fuels (chopped miscanthus, grain kernels, peat) were applied. A total number of 8184 bulk density measurements using 341 samples were conducted in six European laboratories. Influence of container shape and size For wood chips the shape of the container was accountable for around 1,5 % differences in the measured bulk density, where the 100-l-cube produced always lower values than the 100-l-cylinder (Fig. 1) . The observed deviation was consistent for all tested fuels, but it was higher for bark (2,0 %) and lowest for high density fuels with a more homogeneous size distribution such as grain kernels (0,9 %) or wood pellets (0,7 %). The results are also influenced by the container size. However, the differences between the 50 and 100-l-container (cylinder) were small and statistically not significant (deviations of only + 0,4 to -0,5 %), a further reduction of the container size to only 15 l leads to an obvious underestimation of the bulk density (-1,4% for wood chips and -3,0 % for bark). Even for fuels with a more homogeneous size distribution such as grain kernels or wood pellets this observation was made (Fig. 1) . Several test methods and procedures for calculating the bulk density of solid biofuels were analysed. Cylindrical measuring containers proved to more advantageous than a cuboid shape. A 50 l volume seems to be sufficient for most fuels, while smaller containers are not recommendable. A defined application of shaking increases the bulk density value by 6 % (wood pellets) and 18 % (chopped miscanthus). When fuel moisture is below 25 % (w.b.), measurements with varying moisture content are only comparable, if the shrinkage is accounted for by a correction factor. Dipl.-Ing.agr. Thorsten Böhm is member of the scientific staff and Dr. Hans Hartmann leader of the working group "Solid biofuels" at the Technology and Support Centre (TFZ) within the Competence Centre for Renewable Raw Materials,