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.

For five agricultural biogas plants with a high share of energy crops in the input material, a detailed balance of greenhouse gas emissions (GHG) and cumulated energy demand (CED) was calculated for the years 2007 and 2010. The results vary considerably between plants and over time. In 2010 compared with 2007, all of the five biogas plants reduced their impact on climate change and four of them also reduced their consumption of fossil energy. The strongest influence was from the enhanced utilization of surplus heat energy, whereas variations of environmental impact due to direct emissions from the biogas plants were less distinctive. Compared with a reference system based on fossil resources, electricity production in the biogas plants avoided GHG emissions of 603 g to 940 g carbon dioxide equivalents (CO₂-eq)•kilowatt hours electrical energy (kWh_el^-1) and saved 2.48 to 3.23 kilowatt hours primary energy from fossil energy carriers (kWh_fossil)•kWh_el^-1 CED (results for 2010).