Dependence of Chemical Components in Woody Biomass Combustion Ash on the Particle Size and Its Utilization for the Fertilizer

Woody biomass is a growing renewable energy source, and the emitted combustion ash may serve as valuable materials.

In this study, the mass balance of the ashes and their potassium component was investigated when different biomass fuels were incinerated using a fluidized bed furnace and stoker furnace, respectively. Very little K is exhausted in the flue gas, and nearly all is included in the ashes. The stoker furnace is thought to be more appropriate than the fluidized bed furnace to obtain bag filter ash, which has the highest potassium concentration.

The relationship between the potassium concentration in combustion ash and the particle size was investigated for several types of biomass ashes with different morphologies. The form of the potassium components in the ash was also analyzed in order to examine this relationship. From the results, a method to estimate the degree of potassium enrichment was proposed. The potassium concentration first decreases rapidly with increasing mass median diameter and then continues to decrease gradually, although this relationship depends upon the properties of the combustion ash. It was also found that almost all the potassium component in the ash existed as relatively small particles composed of crystallized hydro-soluble potassium compounds, such as KCl, K₂SO₄, and so on, although the particle size and crystalline phases varied according to the type of power plants.

The classification process was implemented in an actual biomass power plant, and demonstrated to produce combustion ash with enriched potassium. The enrichment factor calculated from the size distributions of the potassium component particles and other components qualitatively agreed with the enrichment factor obtained experimentally for all types of ashes. Ash with potassium concentration above 300 mg/g could be successfully acquired on an industrial scale by collecting the finer ash selectively, using a size classification equipment constructed at the biomass power plant.

The enriched potassium ash cannot be utilized as raw material for the fertilizer when it contains harmful heavy metal components at concentrations in the higher than the regulatory limits. Hence, the concentrations of heavy metal components of biomass combustion ash was analyzed. It was also confirmed that the enriched potassium combustion ash, and the chemical fertilizer manufactured from it, completely satisfy the official specifications fixed by the Fertilizers Regulation Act in Japan.