Critical aspects of biomass ashes utilization in soils: Composition, leachability, PAH and PCDD/F

Abstract

a b s t r a c tBottom and fly ashes streams collected along a year in several biomass thermal plants were studied. The bulk composition of ashes and other chemical characteristics that may impact soil application showed a high variability depending on the ash stream, combustion technology and ash management practice at the power plants. The acid neutralization capacity (ANC) and metal's availability for leaching at fixed pH 7 and 4 was performed according with EA NEN 7371, as a quick evaluation method to provide information on the long-term behavior of ashes, regarding heavy metals and also plant nutrients release. Also the pH dependence leachability study was performed according to CEN/TS 14429 for predicting the leaching behavior under different scenarios. Leachability profiles were established between pH 3 and 12, allowing to distinguish different solubility control phenomena of toxic heavy metals (Cu, Cr, Mn, Ni, Zn, Pb) as well as other salts (Ca, K, Mg, Na, Cl). The ANC of fly ashes at pH 4 (3.6-9.6 molH + /kg) were higher than that observed for the bottom ashes (1.2-2.1 molH + /kg). Ashes were also characterized for persistent organic pollutants (POP), such as polycyclic aromatic hydrocarbons (PAH) and paradibenzodioxines and furanes (PCDD/F). Contents were found to be much higher in fly ash than in bottom ash streams. None of the PAH levels did reach the current national limit value of sewage sludge application in soils or the guide value for ash in north European countries. However, PCDD/F contents, which are not regulated, varied from non-detectable levels to high amounts, regardless the level of loss on ignition (LOI) or unburned carbon content in fly ashes. Given the current ash management practices and possible use of blends of bottom and fly ash streams as soil conditioners resembles clear the urgent need to regulate ash utilization in soils, incorporating limit values both for heavy metals, PAH and PCDD/F.

Evaluation of biomass quality of selected woody species depending on the soil enrichment practice

“…Still, compared to coal, ash content in woody biomass can be up to a dozen times lower (Bowen and Irwin, 2008;Dincer and Zamfirescu, 2014). This results in higher proportions of energy obtained from this fuel, and the remaining ash which meets the quality standards can be used as valuable soil enrichment (Freire et al, 2015). With regard to the HHV, only the harvest cycle did not have a significant impact on the results.…”

Mineralogy, chemical composition and leachability of ash from biomass combustion and biomass–coal co-combustion

“…In the case of the biomass ash coming from the fluidised bed boiler in one of the power plants (sample CZ_FLY), the content of SiO 2 is very high because of the admixture of quartz sand from the bed, whereas in the ash sample from the other fluidised bed boiler (PO_FLY) the SiO 2 content is significantly lower. An increase of SiO 2 content in ash from fluidised bed boilers due to the admixture of material from the bed is often noted (Steenari et al 1999;Freire et al 2015). The variation of SiO 2 content in ash from the co-combustion of biomass and coal is lower (from ca 41 to 56 wt%) compared with the biomass ash, which can be related to differences in coal composition, differences in the biomass burned and different biomass share in the fuel blend.…”

Column leaching from a Danish forest soil amended with wood ashes: fate of major and trace elements

“…Likewise, ash composition has also been shown to vary, depending on the type of fuel, the combustion technology and the boiler operating parameters (e.g. [9][10][11]).…”