Behavior of Heavy Metals during Fluidized Bed Combustion of Poultry Litter

Abstract

In this study, we have examined the behavior of heavy metals during fluidized bed combustion of poultry litter. 10Heavy metals examined include As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, V, and Zn. Solid and gaseous streams were analyzed 11 and compared with relevant guidelines to determine the potential environmental impact of combustion and subsequent land 12 spreading or landfill of the resulting ash. The majority of heavy metals were associated with the solid ash fraction, with low 13 gaseous emissions. Pb and As were concentrated in the fine baghouse ash (160°C) due to their volatility. The remaining heavy 14 metals, excluding Cd, were enriched in the heat exchangers and cyclone, where flue gas temperatures ranged from 580 to 220°C. 15 Under the waste acceptance criteria, all samples of process ash, excluding bed ash, exceeded the limits for nonhazardous landfill 16 waste, as a result of high levels of water-soluble Cr. Water-soluble Cr indicated the presence of Cr(VI), and its presence was 17 confirmed using X-ray absorption near-edge structure spectroscopy (18.4% to 38.3%). The source of Cr was identified as the 18 bedding material (wood shavings), and its conversion to Cr(VI) was temperature-dependent and could be facilitated by the high 19 alkali content found in poultry litter. 20■ INTRODUCTION 21 Global energy concerns have led to a shift toward combustion 22 of biomass to provide both heat and power. The resulting ash 23 essentially concentrates the inorganic nutrients present in the 24 original biomass, in a homogeneous, sterile, and easily 25 transportable form. Accordingly, biomass ash has begun to 26 receive more attention with respect to its potential end use. 27 Two areas of growing interest are fertilizer substitution 1−3 28 particularly for ash with high phosphorus content and 29 remediation of acidic soils 4,5 due to the inherent alkalinity of 30 biomass ash. However, biomass from either virgin or waste 31 feedstocks may contain significant levels of undesirable 32 substances or heavy metals (HMs) including Cd, Cr, Cu, Ni, 33 and Zn. 4,6,7 These have the potential to limit the possible end 34 uses of the ashes; e.g. Denmark has banned the application of 35 wood ash to soils due to concern over Cd contamination. 6 36 Poultry litter (PL), resulting from the production of broiler 37 chickens, has significant potential as a biomass resource; 38 however, its combustion behavior has scarcely been studied in 39 the literature. The ash from combustion is sterile and contains 40 28 wt % K 2 O, 21 wt % CaO, and 19 wt % P 2 O 5 , 8 making it an 41 attractive fertilizer substitute. However, PL is known to contain 42 significant levels of HMs, namely Cu and Zn, 9 which are both 43 added as enzyme cofactors, 10,11 and may impede its use as a 44 fertilizer. Furthermore, the combustion process can affect the 45 speciation of some heavy metals, which can influence the 46 behavior and toxicity of the metal in the environment. 12 47The release of these metals in the environment is of key 4...

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