Admixture of biofuel ash into liquid biogas digestate

The increased use of bio-materials for energy purposes is generating large amounts of ash. In most cases, e.g. during incineration of straw and grain, bottom ash contains small amounts of heavy metals, but significant amounts of the plant nutrients phosphorus (P) and potassium (K). Bottom ash also contains some micronutrients (e.g. copper and zinc) that are good for plants in small doses, together with generally small amounts of undesirable heavy metals, including cadmium.

Biogas plants are becoming increasingly common in Sweden. They produce considerable amounts of digestate, a liquid fertiliser with a high content of nitrogen (N) comparable with that of pig slurry but lower contents of P and K. High quality biogas digestate may now be applied to arable land.

The objective of this project was to investigate techniques and systems for blending and spreading liquid digestate with ash admixture. The physical and alkaline properties and solubility in water of some ash types were therefore studied at laboratory scale, as were the buffering properties of some digestates. The influence of ash admixture on digestate pH and ammonia emissions was also studied. At pilot scale, the properties of different ash/digestate mixtures were studied and a cell wheel for liquid manure was tested for adding the ash. Three types of ash (from oats, straw and wood pellets) and two types of digestate were studied.

Publications:

Bernesson S., Olsson, J., Rodhe, L., Salomon, E., Hansson, P.-A. 2008. Inblandning av aska från biobränslen i flytande biogasrötrest. Admixture of biofuel ash into liquid biogas digestate. SLU, Institutionen för energi och teknik, Rapport 002, Uppsala.