High-pressure densification of wood residues to form an upgraded fuel

High-pressure binderless compaction of wood processing residues and other biomass waste materials, including hardwood, softwood, and bark in the forms of sawdust, mulches, and chips, were studied. A piston-and-mold (punch-and-die) process was used to produce densified logs (slugs) under room temperature and at pressures ranging from 34 to 138 MPa. The properties of the logs including density, abrasion resistance, impact resistance, compressive strength, water resistance, and long-term performance were tested. The effects of moisture content, compaction pressure, compaction speed, pressure holding time, particle size and particle shape were studied. It was found that the necessary moisture for producing good-quality logs ranges from 5 to 12% for all the woody materials studied, and the optimum moisture content is in the neighborhood of 8%. It was also found that mulch is the easiest form to be compacted into dense and strong logs, sawdust is the second, and chips the last. For the mulches, a compaction pressure of 70 MPa can produce high-quality logs. For sawdust, a minimum pressure of 100 MPa is needed to form good logs. And for chips, no good logs can be made even at pressure as high as 138 MPa. The logs produced under optimal conditions had dry densities near or higher than 1 g/cm3. Such high density facilitates storage, handling and transportation of biomass. The dense logs also have high-energy content per unit volume, making it easier to be co-fired with coal in power plants.