From waste to raw material -- The route from biomass to wood ash for cadmium and other heavy metals

Energetic utilization of biomass is considered an environmentally safe way of providing energy, especially for process heat and district - heating purposes. The main advantage of energy from biomass is the CO-neutrality of this energy-production process. However, this process produces a solid by-products, namely ash, that has to be considered. This ash contains nutrients like calcium, potassium and phosphorus that should be recycled to forest or agricultural soils, thus closing not only the carbon cycle but also the fluxes of mineral materials caused by these technologies. The problem is, however, that besides nutrients, the ash also contains heavy metals. Cadmium poses a special risk to the use of wood ash in agriculture. It pollutes a large fraction of the ash generated in a biomass combustion plant, namely the cyclone fly-ash and, to an even higher degree, the filter fly-ash or (where flue gas condensation is installed) the condensation sludge. A medium-term solution to the recycling of solid residues from biomass combustion is blending cyclone fly-ash and bottom ash and using the mixture in agriculture. Although a large part of nutrients might be recycled in this manner, care has to be taken of the relatively high amount of cadmium in this material. A new technology currently under development takes advantage of the different temperatures in a biomass combustion plant. This technology enables concentration of cadmium (and other volatile heavy metals) in a very small portion of the whole ash flux from a plant and the concentrations of environmentally relevant substances in the remainder of the ash is kept low. In this manner, wood ash from the process industry or district heating systems might be transformed from waste to raw material for agricultural use