Article pubs.acs.org/EF
Behavior of Heavy Metals during Fluidized Bed Combustion of Poultry Litter Deirdre Lynch,†,‡ Fiona Low,‡ Anne Marie Henihan,† Alberto Garcia,† Witold Kwapinski,† Lian Zhang,‡ and James J. Leahy*,†,§ †
Department of Chemical and Environmental Science, University of Limerick, Limerick, Ireland Department of Chemical Engineering, Monash University, Clayton, VIC 3800, Australia § Materials and Surface Science Institute, University of Limerick, Limerick, Ireland ‡
S Supporting Information *
ABSTRACT: In this study, we have examined the behavior of heavy metals during fluidized bed combustion of poultry litter. Heavy metals examined include As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, V, and Zn. Solid and gaseous streams were analyzed and compared with relevant guidelines to determine the potential environmental impact of combustion and subsequent land spreading or landfill of the resulting ash. The majority of heavy metals were associated with the solid ash fraction, with low gaseous emissions. Pb and As were concentrated in the fine baghouse ash (160 °C) due to their volatility. The remaining heavy metals, excluding Cd, were enriched in the heat exchangers and cyclone, where flue gas temperatures ranged from 580 to 220 °C. Under the waste acceptance criteria, all samples of process ash, excluding bed ash, exceeded the limits for nonhazardous landfill waste, as a result of high levels of water-soluble Cr. Water-soluble Cr indicated the presence of Cr(VI), and its presence was confirmed using X-ray absorption near-edge structure spectroscopy (18.4% to 38.3%). The source of Cr was identified as the bedding material (wood shavings), and its conversion to Cr(VI) was temperature-dependent and could be facilitated by the high alkali content found in poultry litter.
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INTRODUCTION Global energy concerns have led to a shift toward combustion of biomass to provide both heat and power. The resulting ash essentially concentrates the inorganic nutrients present in the original biomass, in a homogeneous, sterile, and easily transportable form. Accordingly, biomass ash has begun to receive more attention with respect to its potential end use. Two areas of growing interest are fertilizer substitution1−3 particularly for ash with high phosphorus content and remediation of acidic soils4,5 due to the inherent alkalinity of biomass ash. However, biomass from either virgin or waste feedstocks may contain significant levels of undesirable substances or heavy metals (HMs) including Cd, Cr, Cu, Ni, and Zn.4,6,7 These have the potential to limit the possible end uses of the ashes; e.g. Denmark has banned the application of wood ash to soils due to concern over Cd contamination.6 Poultry litter (PL), resulting from the production of broiler chickens, has significant potential as a biomass resource; however, its combustion behavior has scarcely been studied in the literature. The ash from combustion is sterile and contains 28 wt % K2O, 21 wt % CaO, and 19 wt % P2O5,8 making it an attractive fertilizer substitute. However, PL is known to contain significant levels of HMs, namely Cu and Zn,9 which are both added as enzyme cofactors,10,11 and may impede its use as a fertilizer. Furthermore, the combustion process can affect the speciation of some heavy metals, which can influence the behavior and toxicity of the metal in the environment.12 The release of these metals in the environment is of key concern as they are often chemically stable and can persist in aquatic and terrestrial environments.13 They can affect crop © 2014 American Chemical Society
production and lead to the transfer of HMs to humans through the consumption of plants and animals.14 Xenobiotic elements such as Pb and Hg are known to exert toxic effects at any level of exposure, while Cd and As are thought to pose significant threats to humans.13 HMs in soils can also be transported offsite and into nearby water ways via leaching and runoff.15 The level of HM leaching is determined by a number of physical and chemical parameters, including particle size, contact time, porosity, temperature, and pH.16 Furthermore, the degree of leaching experienced is not only related to the concentration of HMs present in the original residues but also can depend on the particular matrix in which they are present.15,17 As a result, several leaching protocols have been developed which aim to replicate typical leaching environments; e.g. EN 12457-2,18 which simulates batch stage leaching with a solid to liquid ratio of 10:1 and is comparable to the real leaching effect occurring by rainfall penetrating a landfill over several hundred years.19 In a combustion environment, the migration behavior of elements is affected by vaporization/condensation of volatile species, physical adsorption as a function of specific surface area of the ash, and chemisorption.20 Process conditions−temperature, boiler design, residence time, and air supply−can also influence the behavior of HMs.6,21 The behavior of HMs during combustion of PL has not been reported in the literature, and few studies exist on potential Received: April 30, 2014 Revised: June 12, 2014 Published: July 1, 2014 5158
dx.doi.org/10.1021/ef500981k | Energy Fuels 2014, 28, 5158−5166
Energy & Fuels
Article
Figure 1. Schematic of process. reagents and any potential contamination. Once cooled, the digest was carefully transferred to volumetric flasks and made up to the mark with deionized water. The digestate was analyzed using inductively coupled plasma - mass spectroscopy (ICP-MS) (Agilent 7500). Water-soluble HMs were determined following BS EN 12457-2. The resulting leachate was acidified to between pH 1 and 2 using HNO3 and analyzed using atomic absorption spectroscopy (AAS) (Varian Spectra AA-220) and inductively coupled plasma - optical emission spectroscopy (ICPOES) (PerkinElmer Optima 7000-DV) following an established method.23 Particle size analysis was conducted utilizing either a “Malvern Mastersizer” particle size analyzer, or sieves for BA, and specific surface area (SSA) was determined using a “Micrometrics Tri-Star” BET analyzer, according to ISO 9277. Conductivity and pH measurements were conducted following ENV 13370 and ENV 12506, respectively. FactSage6.1 was used to predict the gaseous and particulate inorganic species liberated during the combustion of PL. The program uses the method of minimization of the total Gibbs free energy of the system, and thermodynamic data taken from the Fact database included real gases, pure liquids, and solids as well as nonideal phases. Further details of the procedure are available elsewhere,22 and input conditions are given in the SI (Table SI-1). Thermodynamic calculations assume infinite residence time and perfect mixing and do not account for reaction kinetics or transport limitations.24 As a result, the calculated equilibrium results are not always comparable with real data. To validate the projections, the predicted results were compared with experimental data. The Cr oxidation-state speciation was determined using Cr K-edge XANES. Spectra were collected in Hutch B of the X-ray absorption spectroscopy (XAS) beamline at the Australian Synchrotron. Samples were measured in fluorescence mode at room temperature using a 100-element Ge fluorescence detector. Full technical details of this beamline can be found elsewhere.25 To obtain an acceptable signal/ noise ratio, a minimum of two replicate scans were conducted for each sample depending on its Cr concentration, with up to nine scans performed for the lowest case. Raw data conversion was performed using “Average” software prior to merging replicate scans, and data was normalized using “Athena” software. Cr(VI) quantification was performed using the method described by Huggins et al.26 The least-squares fitting was conducted using “Origin 8.1”. Emissions Monitoring. Particulates in the flue gas were measured in two 1 h sampling runs, during which an average of 0.8 m3 gas was sampled. A borosilicate, glass impinger, with a 47 mm quartz fiber filter was used, and the concentration of particulates was determined gravimetrically (BS EN 13284-1). HMs were also collected during the sampling runs, with an average 0.8 m3 gas sampled, using 47 mm quartz fiber filters. Sample collection and analysis was undertaken in
toxicity of the resulting ashes, with respect to established leaching methodologies. Therefore, in this study, we have analyzed the process ash (PA) streams from a 200 kWth atmospheric bubbling fluidized bed combustor (FBC), using 100% PL as its feed material and silica sand as the bed material. Gaseous emissions of HMs were analyzed, and the resulting PA streams were analyzed for the total content of 12 HMs (As, Cd, Co, Cr, Cu, Hg, Mn, Mo, Ni, Pb, V, and Zn), and their leaching potential was established. Thermodynamic equilibrium predictions of the volatilization and speciation of HMs upon combustion was undertaken and compared with experimental results, and nondestructive synchrotron X-ray absorption nearedge structure spectroscopy (XANES) was used to quantify the fraction of toxic Cr(VI) in PA streams. Finally, these results were compared with relevant legislation to determine the impact of using PL as a fuel and the resulting ash as a fertilizer. This knowledge is pivotal for safe utilization of PL in combustion processes and to ensure that valorization of PL ash as soil amendments is environmentally benign.
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EXPERIMENTAL SECTION
Sample Collection/Generation. A schematic of the FBC unit is given in Figure 1; PL composition and full process conditions have been detailed in an earlier publication.22 The baghouse filter was composed of plate filters with PTFE material and were rated with an efficiency of 99.999% for 0.5 μm. The system generated five separate PA streams: bed ash (BA) which was a mixture of bed material (sand), coated bed material, agglomerated bed material and ash; ash from heat exchangers 1 and 2 (denoted as HE 1 and HE 2, respectively), cyclone ash (CYC), and baghouse ash (BH). Further to the five PA samples, a sixth representative sample was collected of ash to be used as a fertilizer substitute or sent for landfill (FERT). This fraction consisted of ash from HE 1, HE 2, CYC, and BH; the BA fraction was recycled so this was not included. Laboratory generated ash (GA) was prepared by heating samples of PL (dried and ground to
