Mechanical durability and water absorption of pellets made from different tree species – a case study
¹Latvian State Forest Research Institute ‘Silava’, 111 Riga street, LV–2169 Salaspils,
²LTD NewFuels, 169a Atbrivosanas aleja, LV – 4604 Rezekne, Latvia
Seven different tree species (coniferous and broad leaved) were selected for small scale pelletizing tests: birch (Betula sp.), aspen (Populus tremula L.), grey alder (Alnus incana L.), poplar (Populus sp.), European larch (Larix decidua Miil.), pine (Pinus sylvestris) and lodgepole pine (Pinus contorta). Tree species were mixed in different combinations and proportions. Wood mixture from one tree specie (several tree species were tested as base material during study) was used as base material with volume share in the mix at least 70% and mixtures from other tree species were used as additives. In total 49 different tree mixes were tested in pellet production where mechanical durability and water absorption was later measured for each sample. Mechanical durability where grey alder was mixed with pine was 98.8% (fulfils ENplus quality class). Poplar also showed high results and in some mixes meet the criteria for mechanical durability with best result 99% in mixes with European larch and lodgepole pine (proportions 80:10:10). From 9 different poplar mixes 7 of them showed mechanical durability higher than 97.5%. In tests where no additives was added (100% poplar), poplar pellets mechanical durability was 98.8%. Other mixes with birch, aspen and grey alder when they were taken as base material for pellet production (base material wood volume share in the mixture at least 70%, where remaining 30% consists of other tree specie mixtures) didn’t meet the mechanical durability limit for ENplus quality classes and it was lower than 97.5%. Also in samples where birch and grey alder were used without adding other tree species durability was under 97.5%. European larch was the only one from coniferous trees was tested as base material and the best results in mechanical durability showed in mixes with lodgepole pine (proportion 70:30). From 9 different European larch mixes 7 of them showed mechanical durability higher than 97.5%, which is suitable for ENplus certification. Water absorption in pellets with different tree species composition does not change significantly and ranges from 0.70 to 0.73 ml g-1 when in commercially available litter material it is 0.75–0.8 ml g-1. Water absorption tests leads to a conclusion that if pellets mechanical durability is not sufficient to sell it as combustion material in could be sold as litter material for animals.