Fly ash classification efficiency of electrostatic precipitators in fluidized bed combustion of peat, wood, and forest residues

Abstract

The increasing use of biomasses in the production of electricity and heat results in an increased amount of burning residue, fly ash which disposal is becoming more and more restricted and expensive. Therefore, there is a great interest in utilizing fly ashes instead of just disposing of it. This study aimed to establish whether the utilization of fly ash from the fluidized bed combustion of peat, wood, and forest residues can be improved by electrostatic precipitator separation of sulfate, chloride, and some detrimental metals. Classification selectivity calculations of electrostatic precipitators for three different fuel mixtures from two different power plants were performed by using Nelson's and Karnis's selectivity indices. Results showed that all fly ashes behaved similarly in the electrostatic separation process SiO resulted in coarse fractions with Nelson's selectivity of 0.2 or more, while sulfate, chloride, and the studied detrimental metals (arsenic, cadmium, and lead) enriched into fine fractions with varying selectivity from 0.2 to 0.65. Overall, the results of this study suggest that it is possible to improve the utilization potential of fly ashes from fluidized bed combustion in concrete, fertilizer, and earth construction applications by using electrostatic precipitators for the fractionating of fly ashes in addition to their initial function of collecting fly ash particles from flue gases. The separation of the finer fractions (ESP 2 and 3) from ESP 1 field fly ash is recommended.

Mechanically Treated Fly Ash from Fluidized Bed Combustion of Peat, Wood, and Wastes in Concrete

“…When comparing these results to earlier published work [9,16,29,35], it can be seen that the achieved activity indexes are lower for classified fly ashes (Fig. 7) than for original fly ashes.…”

“…However, this procedure requires water and energy for drying. Therefore, mechanical classification is one interesting option to improve fly ash chemical composition without the use of water, as there is a relationship between fly ash particle size and heavy metal, Cl − and SO 4 2 content [23][24][25][26][27][28][29][30][31]. It has been found that the utilization potential of fly ash from biomass and waste combustion can be significantly improved with mechanical classification [30].…”

Utilization of Fly Ashes from Fluidized Bed Combustion: A Review

“…These are the maximum allowed total concentrations of harmful elements (As, Cd, Cr, Cu, Ni, Pb, and Zn) and minimum concentrations of nutrients (Ca, K + P) using nitric-acid digestion. Most of the studies on ash fertilizers were conducted in Nordic countries, almost exclusively in Finland [48][49][50][51][52][53][54][55][56][57][58][59][60][61][62]. The typical viewpoint of most studies was to study the amount of pollutants in potential ash fertilizers and assess their environmental risks.…”

“…The fractionation of fly ash to improve its quality for fertilizing purposes has been studied in a few papers with the target to separate harmful substances from the fly ash [48][49][50][51]58,61,76]. FBCFA is typically collected using electrostatic precipitators (ESP); therefore, Ohenoja et al [58], Dahl et al [49], and Orava et al [51] studied the use of electrostatic precipitators as classifiers of ash. Ohenoja et al [58] found that the harmful elements (As, Cd, and Pb) were enriched in the finest ash fraction collected from second and third ESP fields (particle size d 50 < 18 µm); therefore, the coarse ash from first field (d 50 > 18 µm) would be better suited for fertilizer use.…”

“…FBCFA is typically collected using electrostatic precipitators (ESP); therefore, Ohenoja et al [58], Dahl et al [49], and Orava et al [51] studied the use of electrostatic precipitators as classifiers of ash. Ohenoja et al [58] found that the harmful elements (As, Cd, and Pb) were enriched in the finest ash fraction collected from second and third ESP fields (particle size d 50 < 18 µm); therefore, the coarse ash from first field (d 50 > 18 µm) would be better suited for fertilizer use. However, the authors did not study the behavior of nutrients such as K and P, but they noted that Ca was enriched to some extent in the fine ash fractions.…”