## 5 Discussion  
The process of biofuel production is highly complex and encompasses a series of operations, including raw material harvesting and seasoning, transportation and handling, grinding, semi-drying (if necessary), and removal of mineral impurities, followed by densification (briquetting, pelleting) and cooling. Among these biomass processing steps, grinding is one of the most critical stages, as it significantly influences the energy intensity of solid biofuel production. This step is especially crucial from the perspective of enhancing the competitiveness of solid biofuels and optimising subsequent production stages (by minimising energy consumption and maximising the quality of the produced chips). The findings of this study are intended to provide valuable insights for designers and operators of biomass grinding equipment. The results also aim to facilitate optimisation of the chipping process, ensuring high-quality chip production while minimising energy inputs. Quality requirements for fuel chips vary depending on their intended application (direct combustion or processing into densified fuels). Biomass for energy applications is subject to standardisation. It must comply with the quality standards defined by national legislation [88]. Legal provisions address various aspects, including the content of halogenated compounds or heavy metals in biomass, contamination with peat and carbonised fractions, the presence of non-biodegradable substances, and fractions derived from hardwood. However, Poland lacks uniform quality requirements for specific biomass types used in commercial power generation. Typically, quality requirements are determined by the respective energy group (buyer), which adheres to company-specific quality standards. These standards depend on the requirements and parameters defined by manufacturers of biomass combustion systems, biomass feeding systems, and the available biomass storage capacity at power plant facilities. Company-specific quality standards for solid biomass are typically based on the PN-EN ISO 17225-4:2014-07 standard [89]. For biomass in the form of wood chips, size parameters, including both minimum and maximum dimensions, are crucial. This applies to chips from wood, forestry, and energy willow, as well as residues from agricultural and orchard production. The authors have been publishing the results of studies on energy plant parameters for many years. In previous studies, the lowest energy demand was observed at high humidity and in the upper stem zones where shoots are least woody. In the process of obtaining biomass from willow trees, the stems are subjected to many loads related to bending, cutting and crushing. Obtaining the lowest possible energy expenditure in these processes involves knowing the properties of the plants. Knowing them makes it possible to understand the behaviour of the material in relation to different operations. Therefore, determining the effect of humidity and sampling location on the amount of cutting energy for willow stems is extremely important [90]. In the authors' previous studies, the results of a multifactor analysis of variance showed that both the plant species and the height zone from which the stem samples were taken had a highly statistically significant impact on the values of cutting stresses and unit cutting energy. The interactions of these main factors also proved important [91]. The results of the authors' previous research on the strength of Virginia Mallow show that in the shearing zone, at an altitude of about 0.10 m from the ground, the stalks had the greatest susceptibility to bending, with the greatest resistance to cutting and compression, because they were the most wooden. The lower sections of the Pennsylvania stalk had the lowest elasticity, with the lowest modulus of elasticity for cutting, compression and bending, and they had the most favourable values of the strength parameters at a humidity of about 20% [92].