Inblandning av aska från biobränslen i flytande biogasrötrest

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. This study investigated 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. The ash produced so much dust that dust reduction measures and breathing masks proved necessary during handling. It also contained so much abrasive material that the risk of wear to pumps, pipes and other parts of the spreading equipment was rated high. It is therefore best if the ash can be added as late as possible to equipment for spreading on arable land, and also it can be prevented from passing through pumps and other moving parts. Only a minor proportion of the oat ash and straw ash was soluble in water. All three types of ash were alkaline and therefore increased the pH when mixed with water. However, the digestate had a high buffering capacity, so the overall effect on pH when moderate amounts of ash were included was slight. When oat ash was mixed into digestate, equilibrium concentrations of ammonia did not increase. There is thus no risk of increased ammonia emissions as long as the mixture does not contain more than 5% ash. In fact, ammonia concentrations were lower when ash was added, possibly because of ammonia binding to some metal ions. In practice, the SNFS 1994:2 limit values for P and heavy metals mean that the test ash-digestate mixtures with 1% by weight of ash can be applied to the same field at most twice in a three-year period. Mixtures with 3% by weight of ash can be spread at most every third year and mixtures with 5% by weight of ash at most every fifth year. Batch admixture of ash into liquid digestate is unsuitable, because the majority of the ash settles and forms sediment in the bottom of the tank, despite efficient stirring. For continuous dosage of finely pulverised ash into digestate, a cell wheel made for dosage of liquid manure can be used. However, for dosage into a flow under pressure a sluice is necessary to prevent digestate being pressed up into the ash container. Admixture of ash into solid digestate is another possible alternative, e.g. during movement of the solid fraction from a digestate separator to a store