Energy and pressure requirement for compression of Miscanthus giganteus to an extreme density

To achieve optimal use of transportation infrastructure in general, the bulk density of the material being transported must be such that the containment structure reaches its weight and volume limits simultaneously. As an example, “gondola type” coal railcar dimensions are designed to precisely accommodate the density of coal in a pile, which is 850 kg m−3. Compressing biomass to the bulk density of coal would have the advantage of potential utilisation of the existing coal logistics infrastructure, as well as allowing co-combustion by adding biomass directly to the coal stream. However, the required material density could be as high as 1657 kg m−3, owing to post-compression rebound and a limited particulate packing density. This paper describes an experiment in which a sample of biomass (Miscanthus giganteus) weighing 230 g was compressed to 1767 kg m−3, at an applied pressure of 519 MPa. The test was conducted using a very large Universal Testing Machine, capable of generating a force of 13 MN. The sample was further compressed to a pressure of 756 MPa, shortly after which an explosion occurred, presumably caused by ignition of volatile gases generated by localised pyrolysis through frictional heat generation in the biomass. At the point of explosion, the height of the sample was 8.9 mm. Upon reducing the load to zero, the sample was retrieved and the height was measured at 10.7 mm, yielding an instantaneous rebound percentage of 20.2%. After three weeks, the measured sample height was 11.8 mm, yielding a long-term rebound of 32.6%.

Graphical abstractFigure optionsDownload full-size imageDownload as PowerPoint slideHighlights► The research aim was to achieve a biomass bulk density equal to that of coal in a pile. ► Miscanthus was compressed to 1767 kg m−3 at 756 MPa and a force of 6.45 MN. ► The instantaneous material rebound was 20.2% and the long-term rebound was 32.6%. ► At a pressure of 756 MPa, the material extruded and self-combusted.