Growing a Hybrid Poplar & Willow Coppice for Biomass. Wood Pellet Optimization. Biomass .. Published European grades for wood pellets. density. (kg/m3). Heat value. (MJ/kg). Ash. ( Hybrid poplar. 1. 8.8. 82.4. Nov 23, 2012. Wood density with values ranging from 0.1 g cm–3 to 1.2 g cm–3 is another. using clonally propagated saplings of hybrid poplar (Populus.