Ashes from biomass heat (and power) plants that apply untreated woody biofuels may be suitable for use as fertilizers if certain requirements regarding pollutant and nutrient contents are met. The aim of this study was to examine if both bottom and cyclone ashes from 17 Bavarian heating plants and one ash collection depot are suitable as fertilizers (n = 50). The range and average values of relevant nutrients and pollutants in the ashes were analyzed and evaluated for conformity with the German Fertilizer Ordinance (DüMV). Approximately 30% of the bottom ashes directly complied with the heavy metal limits of the Fertilizer Ordinance. The limits were exceeded for chromium(VI) (62%), cadmium (12%) and lead (4%). If chromium(VI) could be reduced by suitable treatment, 85% of the bottom ashes would comply with the required limit values. Cyclone ashes were high in cadmium, lead, and zinc. The analysis of the main nutrients showed high values for potassium and calcium in bottom ashes, but also relevant amounts of phosphorus, making them suitable as fertilizers if pollutant limits are met. Quality assurance systems should be applied at biomass heating plants to improve ash quality if wood ashes are used as fertilizers in agriculture.

Fuel qualities of wood chips and energy consumption during their production vary depending on raw materials and machine settings. Thereby, requirements for small combustion units <1 MW are rarely met. Therefore, the effects of raw material and machine settings on chipping performance were investigated in numerous field trials (n = 34). Raw materials were derived from different tree parts of various species (forest residues and “energy round wood,” i.e. small-sized delimbed stem wood). Chipper settings varied in knife sharpness, screen size and discharge systems. All samples were analyzed following European standards. In addition, particle size distribution and particle shape were analyzed using a continuously measuring image analysis device. Classification followed ISO standards 17225-1 and 17225-4. Fuel consumption during chipping was measured and related to volume and weight of the bulk material. Ash content of wood chips ranged from 0.4–1.7 weight.-% (w.-% dry basis) for energy round wood and from 1.0–5.4 w.-% for forest residues. Particle size distribution of all samples could be classified according to ISO 17225-1 but only 30% could be classified according to ISO 17225-4. These requirements were often met when using energy round wood and sharp knives. Moreover, particle shape strongly related to knife sharpness. Specific fuel consumption during chipping ranged from 0.17–0.70 litres per m³ bulk volume and was high for forest residues, small screen sizes or blunt knives. Overall energy consumption of the chipping process was <1% of the energy stored within the produced bulk material.

Technical drying of harvested wood fuels is heat and energy consuming, while natural pre-drying in the forest, e.g., in stacks or storage piles, is accompanied by energy losses through natural degradation processes. Dewatering of energy wood by mechanical pressing is an innovative method to reduce the moisture content prior to thermal drying while producing press waters (PW, also referred to as wood juice) as a by-product. To date, the characteristics and utilization potentials of PW are largely unknown. In this study, three different spruce- and poplar-based PW were analyzed for their characteristics such as dry matter (DM), organic dry matter (oDM) concentration, pH-value, element concentration or chemical compounds. Additionally, they were used for anaerobic digestion (AD) experiments with digested sewage sludge (DSS) serving as inoculum. The fresh matter-based DM concentrations of the PW were between 0.4 and 3.2%, while oDM concentrations were between 87 and 89%_DM. The spruce-based PW were characterized by lower pH-values of approx. 4.4, while the poplar-based PW was measured at pH 8. In the AD experiments, DSS alone (blank variant) achieved a specific methane yield of 95 ± 26 mL/g_oDM, while the mixture of spruce-based PW and DSS achieved up to 160 ± 12 mL/g_oDM, respectively. With further research, PW from wood fuel preparation offer the potential to be a suitable co-substrate or supplement for AD processes.