Experiments and modelling of potassium release behavior from tablet biomass ash for better recycling of ash as eco-friendly fertilizer

Returning of biomass ash as fertilizer to field plays great role in sustainable utilization of biomass as energy. In the fertilizer application, release rate of nutrients from ash should be controlled to avoid nutrients shock to plants. Making ash into briquettes (granulated ash, pellet, tablet) is a good choice to slow down the release rate of nutrients and prevent dust formation. However, experimental or theoretical analysis of the release behavior of nutrient from ash briquette is scarce. In this paper, different sizes (from Ф6 mm × 2 mm to Ф15 mm × 5 mm) of tablets of biomass ash were made and their nutrient release behavior was investigated experimentally. Results showed that the release rate of potassium in tablet ash was about 1/20-1/60 of that in the powder ash. And that changing tablet size in the normal size range could not decrease the nutrient release rate to the value required by the norm for slow-release fertilizer. Based on the experiments, the mathematical description of the nutrients release process was established and a Matlab code was written for modelling. The theoretical model was validated and parameters required to make slow release fertilizer from the ash were predicted. It showed that, in order to produce slow release potassic fertilizer, much bigger tablet size (from Ф15 mm × 5 mm to Ф210 mm × 70 mm), much lower saturated concentration (from 0.17 to 0.17/142 kg/kg water for potassium), or much smaller effective diffusion coefficient (from 3.2 × 10−10 to 1.6 × 10−12) are necessary. The theoretical model can be generalized to describe the release behavior of any nutrient and can be used in the parameters optimization for producing slow/control-release fertilizer.