Ashes from straw and wood-chip fired plants for agricultural usage. Pilot project

The content of nutrients and heavy metals in ashes from the combustion of straw and wood chips at district heating plants is studied, on the basis of results of analyses from Danish municipalities, to determine whether such ashes are suitable for use as fertilizers. Results of the analysis of ashes from 9 wood-chip fired and 26 straw-fired plants are presented. They show significant variations in nutrient and heavy metal content which could be caused by combustion and operational conditions and/or testing methods. On condition that the phosphorous content of straw and wood-chip ashes amount to 1% of the dry matter, 50%-75% of the straw ashes and under 50% of wood chip ashes analyses are within the limit for cadmium stipulated in the Danish Ministry of the Environment's Executive Order no. 736 on the use of wastes for agricultural purposes. This is found to be unsatisfactory. It is suggested that a closer investigation should be undertaken in order to determine which amount of straw and wood-chip ashes can be accepted for use as fertilizers in consideration of the stipulated limits for cadmium content of wastes to be used for agricultural purposes. In addition the technological and economic potentials of dosing of these ashes for this use should be investigated. Fly ash and slag were also included in the analysis results studied and it was found that the cadmium content of slag did not prevent its use as fertilizer, but that the distribution of cadmium in slag, in fly ash and in slam from flue gas cleaning systems related to the combustion of wood chips should be further investigated. (AB)