Investigations of the Use of Invasive Plant Biomass as an Additive in the Production of Wood-Based Pressed Biofuels, with a Focus on Their Quality and Environmental Impact

The present study investigates the suitability of the invasive herbaceous species Sosnowsky's hogweed (<i>Heracleum sosnowskyi</i>) and giant knotweed (<i>Fallopia sachalinensis</i>), together with reed (<i>Phragmites australis</i>), as feedstock for pressed biofuel pellets used alone and as additives to pinewood. Biomass of the three herbaceous species and pinewood was harvested, dried, chopped, milled, and pelletized through a 6 mm die to obtain pure pellets and binary mixtures of each herbaceous biomass with pinewood (25, 50, and 75% by weight of herbaceous share). The pellets were characterized for physical and mechanical properties, elemental composition, calorific value, combustion emissions, and life cycle impacts per 1 GJ of heat. Pellet density ranged from 1145.60 to 1227.47 kg m^-3, comparable to or higher than pinewood, while compressive resistance satisfied solid biofuel quality requirements. The lower calorific values of all herbaceous and mixed pellets varied between 16.29 and 17.78 MJ kg^-1, with increased ash and nitrogen contents at higher herbaceous shares. Combustion tests showed substantially higher CO and NO_x emissions for pure invasive and reed pellets than for pinewood, but all values remained within national regulatory limits. Life cycle assessment indicated the highest global warming and fossil fuel depletion potentials for reed systems, followed by Sosnowsky's hogweed and giant knotweed, with pinewood consistently exhibiting the lowest impacts. Overall, invasive plants and reed are technically suitable as partial pinewood substitutes in pellet production, supporting simultaneous invasive biomass management and renewable heat generation.