Fate of Trace Elements during the Combustion of Phytoremediation

Article pubs.acs.org/est

Fate of Trace Elements during the Combustion of Phytoremediation Wood Michel Chalot,*,†,‡ Damien Blaudez,‡ Yann Rogaume,§ Anne-Sonia Provent,∥ and Christophe Pascual∥ †

Laboratoire Chrono-Environnement, Université de Franche-Comté, UMR 6249, Place Leclerc, 25030 Besançon, France Interactions arbres-microorganismes, Université de Lorraine, UMR 1136, 54506 Vandoeuvre-les-Nancy, France § ENSTIB/LERMAB, Université de Lorraine, 88000 Epinal, France ∥ Cylergie, Centre de Recherche de COFELY, L’Orée d’Ecully - Bât C - Chemin de la Forestière, 69130 Ecully, France ‡

ABSTRACT: We investigated the fate of trace elements (TE) in poplar wood on the conversion of biomass to heat in a 0.2 MW combustion unit equipped with a fabric filter. The phytoremediation wood was harvested from a TE-contaminated agricultural site planted with a high-density poplar stand. The combustion technology used in the present experiment allows for an efficient separation of the various ash fractions. The combustion process concentrates Cu, Cr, and Ni in the bottom ash, heat exchanger ash, and cyclone ash fractions. Therefore, the impact of the fabric filter is negligible for these elements. Conversely, Cd, Pb, and Zn are significantly recovered in the emission fraction in the absence of the fabric filter above the emission limits. The use of a fabric filter will allow the concentration of these three TEs in the ashes collected below the filter, thus complying with all regulatory thresholds, i.e., those from the large combustion plant EU directive. Because the TE concentrations in the different fractions differed significantly, it is recommended that these fractions be treated separately, especially when recycling of ashes from phytoremediation wood through application in agriculture is envisaged.

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energy by 2020.8 However, when using contaminant enriched biomass crops for energy purposes, the impact of metals on conversion efficiency, as well as the energy needed to properly use or dispose the rest product after conversion need to be assessed. This has been recently calculated extensively for a few crops, such as willow, rapeseed, and silage maize.9 Willow reached the same energy production and same CO2 abatement per hectare per year as silage maize when its biomass reached higher yields. Furthermore, in bioenergy production from woody biomass, the fate of TE in the biomass needs to be known and managed. It is important to understand which ash fractions are formed and how trace elements will behave during this process. This knowledge becomes crucial for the subsequent handling of the produced ash fractions.10 Usually, the bulk of the materials accumulated in ash from biomass combustion plants, namely, the nutrients calcium, potassium, magnesium, and phosphorus, should be recycled at the highest possible rate.11 However, the recycling of these nutrients using ash as fertilizer is impeded by the accompanying trace elements. The use of such fertilizing material is subject to strict legal requirements within EU countries11 and USA.12 These directives establish limits for concentrations of trace elements in soil and sludge and for the maximum annual quantities of trace elements that may be introduced into the soil.

INTRODUCTION Because more than 3 million sites within the EU are suspected of being contaminated by trace elements (TE),1 the management of TE-polluted soils is of major concern for most industrialized countries. The ubiquity of TEs, their environmental persistence, and their hazardous effects on the environment and human health represent major threats.2−4 Various remediation methods, such as land filling, fixation, and leaching, may be beneficial to the restoration of TE-polluted soils. However, these methods are usually expensive, and some of them could have adverse effects on the biological activity, structure, and fertility of soils.5 A recent study compared the impacts of remediation through Salix viminalis cultivation with traditional excavation-and-landfill remediation using a life cycle assessment (LCA) methodology.1 The biomass remediation had significant environmental advantages over ex situ excavation remediation, except for land use. Land use is indeed a major concern because of increasing competition for land resources. In recent years, the use of trees in managing contaminated sites has attracted considerable interest.6 This approach may facilitate revegetation programs of disturbed sites for decreasing the potential migration of contaminants through dust or leaching. There are, therefore, potential economic opportunities for biomass production associated with phytoremediation,6 including bioenergy and traditional industrial products, such as paper, solid wood products, and reconstituted products (i.e., paper, chipboard, laminated beams, extruded trim).7 Energy production from biomass currently has high political priority, as shown by the European Union target of a 20% share of renewable © 2012 American Chemical Society

Received: Revised: Accepted: Published: 13361

May 4, 2012 September 10, 2012 November 15, 2012 November 15, 2012 dx.doi.org/10.1021/es3017478 | Environ. Sci. Technol. 2012, 46, 13361−13369

Environmental Science & Technology

Article

Figure 1. A. Picture of the apparatus showing the sampling locations for the different fractions (ash, particle, and gas) indicated with numbers. B. Details of the various fractions collected: (1) total entry from biomass; (2) bottom ashes; (3) furnace ashes; (4) heat exchanger ashes; (5) cyclonefly ashes; (6) particulate fraction upstream the filter; (7) gas fraction upstream the filter; (8) filter-fly ashes; (9) particulate fraction downstream the filter; (10) gas fraction downstream the filter.

fly ashes compared with the bottom ashes. A study also reported environmental and resource implications of metal occurrence in recovered construction and demolition waste wood that is used as an energy source in biofuel boilers.18 Cd, Hg, Pb, and Zn were found to be enriched in the cyclone- and filter-ash fractions, whereas As, Cr, Cu, and Ni were more concentrated in the bottom ash fraction. Concerning poplar short rotation coppice (SRC), it is widely used for energy production in Europe and is also largely planted across Europe for phytoremediation purposes.19−24 However, no study has yet addressed the question of TE emissions upon the conversion of contaminated poplar biomass. Based on the report by Pitman,25 we made the hypothesis that TE accumulated in the phytoremediation poplar biomass will mostly enrich the fly ash and filter fractions upon combustion at high temperature. All the studies quoted by Pitman concerned biomass collected from nonpolluted areas, and the rationale behind the present work was to determine whether the presence of high TE concentrations would be likely to influence the distribution of metals in these fractions. Additionally, modern waste-to-energy incinerators

One study described a two-reactor method for predicting the fate of eight trace elements (As, Cd, Hg, Ni, Pb, Se, V, and Zn) from coal in combustion processes. Most TEs (As, Cd, Pb, Ni, V, Zn) were captured to some extent in the bottom ash and were enriched in the fly ash fraction.13 The model predicted that Se would be completely volatilized, and neither the bottom ash nor the fly ash captured any Se. Hg was reported to be captured to 10% in the fly ash fraction, and the rest was present as gas. In contrast to coal, however, where most inorganic elements are mineral bound, biomass has the advantage that most of the ash-forming elements are organically or ionically bound in the fuel biomass.14,15 A comparison of coal and biomass is presented elsewhere16 including the chemical structures and the modes of occurrence of inorganic material in the fuels. The major mechanisms of ash deposition during combustion of coal and biomass are related to the types of inorganic material in the fuel and the combustion conditions. Data are also available on the fate of TEs in willow wood upon conversion of the biomass to electricity and heat in a small-scale fixed-bed downdraft gasifier.17 This study showed that Cd, Zn, and Pb were enriched by a factor of 7−100 in the 13362

dx.doi.org/10.1021/es3017478 | Environ. Sci. Technol. 2012, 46, 13361−13369


Article

continuous filtering of the fumes, whose temperatures vary between 120 and 170 °C. This fabric filter was specifically designed to fit with the combustion unit used in this study, but it includes all the features and mode of operation of conventional filters. Wood Collection and Properties. The contaminated wood came from field trials within the framework of the PHYTOPOP project. It is located in the northwest suburban area of Paris, in the vicinity of the city Pierrelaye (department 95). Control wood was harvested from a noncontaminated area. Raw wastewater containing much salt and organic matter was spread for more than 100 years on about 1200 ha of sandy soil, which was thereafter used for market gardening. During the 1950s, 1960s, and 1970s, when there were no guides for micropollutants, significant amounts of trace elements entered the soil, which are given in a previous study.26 A poplar stand consisting of various poplar clones was installed in April 2007 and was harvested in February 2010. The wood was chipped into 40−60 mm pieces in a traditional forestry chipper. Eight combustion assays were performed within the 3-year project. However, since the parameters were adjusted after each assay to reach an optimal recovery of TEs, only the data from the last assay are provided in the present paper. Optimization is currently done by experience with the aim to optimize boiler efficiency and reduce emissions of unburned (CO and VOC). This optimization leads to a boiler efficiency of about 85% and CO emissions below 500 mg/Nm3. Collected TE Fractions. As shown in Figure 1b, various ash and wood fractions detailed below were collected and further analyzed for TE content. (1) Total entry from biomass. (2) Bottom ashes; the ash fraction collected in an ashtray placed under the main oven. (3) Furnace ashes, collected during cleaning of the furnace (including ashes collected onto the unit wall and under the arch) at the end of the combustion assay. Because of the short duration of the assays, these ashes may be separated from the bottom ash fraction. However, this fraction, if the assay were to last longer, will end it up in the bottom ash fraction. (4) Heat exchanger ashes produced in the combustion tests collected on the arch and on the heat exchanger tubes. (5) Cyclone-fly ashes; ash particles collected in an ashtray placed behind a cyclone. (6) Particulate and (7) gas fractions collected upstream the fabric filter. (8) Filter-fly ashes; finer fly ash fraction collected in an ashtray placed behind the fabric filter. (9) Particulate and (10) gas fractions collected downstream the fabric filter. (11) The total TE upstream the fabric filter is the sum of (2, 3, 4, 5, 6, and 7 fractions). (12) The total TE downstream the fabric filter is the sum of (2, 3, 4, 5, 8, 9, and 10 fractions). Determination of Analytical Parameters. All analytical parameters provided in Table 1 were measured according to the standard methods previously described.15 The furnace, heat exchanger, and filter temperatures were measured using type K thermocouples. To obtain a measure of mass flow of gas flowing through the boiler, the temperatures and gas velocities were measured in pipes of known diameter. A pitot tube and a temperature-corrected differential pressure sensor were used to measure the flow of smoke, consisting of hot gas (150 °C on average, varying up to 250 °C) loaded with solid particles. For

address these concerns with sophisticated pollution-control devices. Scrubbers, electrostatic precipitators, and fabric filters remove much of the heavy metals and fly ashes from an incinerator’s air emissions. We therefore put forward the hypothesis that a combustion unit fully equipped with similar filtering devices would reduce emissions in compliance with regulatory thresholds. The objective of the whole investigation was to solve emission issues associated with the combustion of phytoremediation wood harvested from a contaminated agricultural site planted with a high-density poplar stand by focusing on (1) particulate and gas emissions by characterizing the various ash and gas fractions upon the conversion of poplar woody biomass to heat in a 0.2 MW combustion unit, (2) optimization of the filtering step by comparing trace element fluxes in the presence or absence of a fabric filter, and (3) comparing our data to existing regulatory thresholds, i.e., those from the large combustion plant EU directive. Such knowledge will be needed to optimize biofuel production systems in order to achieve a sustainable, environmentally safe, and friendly production of energy from otherwise derelict land.

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MATERIALS AND METHODS Description of the Industrial Boiler. The device used consisted of an industrial boiler (Compte.R., Arlanc, France) with a power of 200 kW equipped with a conveyor belt and an inlet for continuous feeding. Figure 1a shows schematically the experimental apparatus as a whole, consisting of a storage silo, then a transfer system worm, the boiler (furnace + heat exchanger), and the fabric filter, which can be bypassed based on requirements and tests. The control device of the boiler can be operated in various modes and management programs as needed. In the general case, the mode is that encountered in the vast majority of industrial facilities, i.e., a simple control injection of fuel and primary air to the needs and the burden of the boiler and an independent regulation of the secondary air flow to maintain a selected oxygen content in the flue gas (usually 10%). At the same time, a measurement of the pressure in the home keeps it at 100 Pa of depression. The whole system is instrumented with multiple sensors to continuously monitor various temperatures, fume compositions, flows in and out (air, fuel, smoke), the power generated by the system, and all the parameters taken into account by the automated control. After the fire, the smoke is channeled to a heat exchanger tube and then passed through a multicyclone device for trapping large particles. At the end of this set, the contents of the fume particles are between 50 and 200 mg/Nm3 based on the tests. All the measurements are managed via an acquisition system that collects data every 20 s during the test period. Two levels of sampling upstream and downstream are implemented in the installation to characterize the operation of the fabric filter. This setup allows the progress of the combustion in the operating conditions or the type of fuel used to be fully followed. The relatively low inertia of the boiler requires trial periods of a few hours to validate the type of operation. However, to account for even low variability in fuel characteristics, the tests were conducted over periods of operation from 12 h. At the outlet of the boiler, the fumes are routed directly into the chimney or through a fabric filter designed specifically for this installation. The fabric filter is equipped with Tecfidis (Meximieux, France) brand sleeves (500 g/m2) that have been treated by a PTFE full bath to allow 13363



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The ash TE contents were determined by the CARSO laboratory according to the AFNOR standard NF M 03-052 by a plasma emission method. Data provided are the mean of 3 to 5 measurements. The detection limit was 1 μg for filter fractions and for solution fractions. There was at least 2 mg of material collected on the filter and 750 mL of solution, which contained at least 150 to 200 μg of TE.

Table 1. Combustion Parameters during the Assay, Measured Upstream and Downstream the Fabric Filter, As Illustrated in Figure 1a O2 CO2 CO NO NO2 furnace temperature heat exchanger temperature flue temperature fabric filter (entry) temperature fabric filter (internal) temperature fabric filter (out) temperature combustion yield global yield dissipated power biomass power wet flue gas flow dry flue gas flow moist wood flow dry wood flow a

upstream

downstream

12.9% 7.9% 1864 mg/Nm3 281 mg/Nm3 72.4 mg/Nm3 509 °C 441 °C 158 °C 145 °C 37 °C 141 °C 87.2% 86.7% 120 kW 139 kW 433 N m3/h 389 N m3/h 67.9 kg/h 33.9 kg/h

11.4% 9.3% 260 mg/Nm3 315 mg/Nm3 30.6 mg/Nm3 540 °C 453 °C 164 °C 136 °C 119 °C 105 °C 88.7% 83.5% 133 kW 160 kW 432 N m3/h 381 N m3/h 78.2 kg/h 39.0 kg/h

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RESULTS AND DISCUSSION Operational Parameters. The wood consumption during the test, which lasted 4.7 h, was approximately 172 kg at a rate of approximately 36 kg h−1 (rate of dry wood without ash). The lower heating value of the wood (LHV) was 3.65 MWh/t. The combustion of this biomass produced 385.5 N m3/h dry fumes and generated 126.5 kWh/t. The detailed combustion parameters are provided in Table 1. During this assay, the combustion of poplar wood resulted in the production of 22.5% (dry weight ashes/dry weight biomass) bottom ashes (including furnace ashes collected by cleaning), 0.95% cyclone ashes (including heat exchanger ashes collected by cleaning), and 0.05% fabric filter ashes. An estimated 23.5% of the weight of the processed wood was thus recovered as ashes during this assay. Wood Characteristics. Table 2 shows the bulk content and elemental composition of the woody biomass. The TE (Cd, Pb, and Zn) concentrations present in the wood were significantly higher than those of the TEs in the reference wood, while the concentrations of As and Hg, in most cases, were below the limit for quantification by the ICP-AES analysis and were, therefore, excluded. These values are very similar to those reported earlier for the phytoremediation of Salix wood harvested on contaminated sites.17 The TE concentrations measured in poplar cultivars collected from this contaminated stand are 5−10 times lower than those reported in one of our previous (Metaleurop) studies.22 Although the total TE concentrations in the soils are within the same range for the two poplar stands, the TE bioavailable fraction of the Metaleurop site is much higher, i.e., 10% for Cd, as compared to ca. 1% for the present site (unpublished data). Consequently, the amount of TE taken up by Salicaceous species is much greater, which finally explains the difference of TE content in the biomass from the two sites. The Fate of Major Trace Elements in the Various Ash Fractions. In agreement with the flow of TE described by Pitman,25 we found that TE present in the contaminated woody biomass were distributed across the different ash, particulate and gas fractions as a result of combustion (Figure 2). Cr, Cu, and Ni will be concentrated mostly in the furnace ash and, to a lesser extent, in the bottom and heat exchanger ash fractions. Other environmentally relevant trace elements, such as Cd, Pb, and Zn, will be similarly recovered in these 3 ash fractions, but most importantly, these three TEs were also present in the particulate and gas fractions that came out of the cyclone. The enrichment of TE in the fly ash fraction (including the heat exchanger fractions) as opposed to the bottom ash fraction is due to the combustion temperature of

CO, NO, and NO2 values are corrected for a 10% O2 concentration.

flue gas emission analysis, CO, CO2, O2, and NO were measured with a portable gas analyzer (Testo 350 xl) equipped with electrochemical cells. The gas is drawn directly into the chimney, where the sample temperature is measured. The sample gas is first dried with a Peltier-effect module, which cools the gas at a temperature between 4 and 8 °C. The sample gas is then dusted in a filter to finally be injected into the measurement cells. The wood humidity and density were determined according to the NF B 51-004 and NF B 51-005 standards, respectively. The heating value of the wood was determined using a bomb calorimeter according to a method described by the NF M03-005 standard. Data provided are the mean of 3 to 5 measurements. The fine particulate matter was first extracted isokinetically from the duct and was then led to the fly ash measuring equipment. Quartz filters were used to collect and measure the total mass concentration of the particulate fractions. The filters were gravimetrically analyzed to estimate particle loading. Gases were further collected in hydrogen peroxide and nitric acid solutions to capture the TE of the gaseous fractions. These solutions were further analyzed for TE contents. Determination of TE Contents. The determination of the total TE concentrations in the biomass was performed by the accredited CARSO laboratory (Villeurbanne, France) according to the NF EN ISO 11885 standard. Briefly, the samples were first dried for 48 h at 50 °C, ground into powder, and subsequently mineralized with 5 mL of HNO3 70% (w/v) in a microwave oven. After dilution to a final concentration of 3.5% HNO3, the TE content was finally determined using Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES).

Table 2. TE Content (mg/kg D Wt) of Woody Biomass for the 2010 Combustion Assays wood source

Cd

Cr

Cu

Ni

Pb

Zn

reference wood phytoremediation wood

0.80 5.89

1.91 10.80

4.23 19.47

6.05 13.06

1.11 20.61

33.24 288.53

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Figure 2. Trace element concentrations in ashes and emission fractions during combustion of phytoremediation wood. Data are expressed as mg/kg of dry woody biomass. Fractions are as follows: (1) total entry from biomass; (2) bottom ashes; (3) furnace ashes; (4) heat exchanger ashes; (5) cyclone-fly ashes; (6) particulate fraction upstream the filter; (7) gas fraction upstream the filter; (8) filter-fly ashes; (9) particulate fraction downstream the filter; (10) gas fraction downstream the filter; (11) total TE upstream the filter is the sum of (2, 3, 4, 5, 6, and 7) fractions; (12) total TE downstream the filter is the sum of (2, 3, 4, 5, 8, 9, and 10) fractions.

By further performing a weighted average of the flows in and out of the boiler on all periods of testing, the balance between the input/output of TEs was coherent and properly balanced for Ni, Pb, and Zn (Figure 2, compare fraction 1 to fraction 11 or 12) and less well balanced for Cu. A maximum of 50% Cd is recovered in the various fractions. We may hypothesize that either the input term is systematically maximized or that part of the incoming Cd is stored temporarily in the boiler (the shortterm tests do not achieve a stationary phase where everything comes out of the boiler) or, finally, that the outputs are not properly accounted for. The latter hypothesis is the least likely because, over the trials, we gradually established a protocol that allowed us to recover material either continuously or at the end of testing by cleaning all the ash and by flue gas analysis. The first hypothesis that all analysis may generate a systematic error may be excluded, first because all analyses have been done by an accredited laboratory that ensures the reliability of the measurements, second because a subset of samples were

800 °C in the boiler being high enough to vaporize the elements.27 The further enrichment of Cd, Pb, and Zn in the fractions downstream the cyclone is presumably caused by the higher volatilization of these elements from the woody fuel during combustion, subsequently forming fine particles in the flue gases, as already documented during the combustion of wood pellets.28 We also calculated the total amount of TE recovered in the various fractions in the presence (total TE fraction 12) or absence (total TE fraction 11) of the fabric filter. Although statistical tests could not be applied to these data, the difference between these two fractions was never greater than 10% (Figure 2), demonstrating the effectiveness of the sampling and analyses made during the combustion process. The enrichment of the fractions downstream the fabric filter is highlighted when the data are expressed as the percent of total recovered TE (Figure 3). 13365



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(LCPs) in power stations, petroleum refineries, and steelworking and other industrial processes running on solid, liquid, or gaseous fuel. As a further hypothesis, we put forward the potential of using adequate devices to remove trace elements in emissions produced by a combustion unit burning phytoremediation wood. The measurements performed with the 0.2 MW equipment were therefore compared with emission limits, either from the industrial emission directive (IED)31 or from the incineration of waste directive (IWD)30 in Europe. Data show that the emission values measured upstream the fabric filter for Cd (IED1 or IWD1), Pb (IED2), and for the sum of 9 (IED3) or 8 (IWD2) TEs are lower or slightly higher than the emission limits, although within the same order of magnitude (Table 3). The IED3 emission limit is much greater than the IWD2 emission limit, but it includes Zn, which is a major contaminant of woody biomass (see Figure 2). The IED emission limits concern large units (>50 MW). However, there are no TE emission limits for lower combustion units, so we used the IED values to make comparisons. Remarkably, when measured downstream the fabric filter unit, the TE emissions were significantly reduced, well below the emission limits from the EU directives. For example, Pb was reduced approximately 50-fold (IED2), and Cd was reduced by 15-fold (IED1). The function of the fabric filter unit is confirmed by the flue dust emissions measured downstream and upstream the fabric filter. The data given in Table 4 show that filtration through the

Figure 3. Distribution of major trace elements in ash and emission fractions from phytoremediation wood. Data are expressed as % of recovered trace elements in the various fractions: (2) bottom ashes; (3) furnace ashes; (4) heat exchanger ashes; (5) cyclone-fly ashes; (8) filter-fly ashes; (9) particulate fraction downstream the filter; (10) gas fraction downstream the filter.

analyzed by a second laboratory that produced no statistical differences between the two set of data. The second hypothesis is the most likely because it is possible that Cd was adsorbed by the boiler (including metal alloys) during a transitional period of up to several days. However, this hypothesis, already proposed in the past for waste incinerators, remains to be demonstrated. Cr always appears in surplus and is found at higher concentrations in the outflows. The presence of this TE in the bars and walls of the boiler may explain the enrichment of the exchanger ashes. Although the ratio of trace elements between the various ash fractions greatly depends on the type of boiler, the operating conditions, and the fuel mix,11 the distribution patterns among the bottom, cyclone, and fabric filter ash fractions are similar to those reported earlier11,29 for these major contaminants. However the study by Narodoslawsky11 relates only to Cd distribution pattern (from willow biomass), and data provided by Ginsberg29 have been acquired by modeling. Our set of data is therefore quite unique, as compared to the existing literature. The Fate of Major Trace Elements in Emission Fractions. In this study, our interest was also to assess the efficiency of the fabric filter in regulating emissions. Because the emission of TEs to the environment is subject to strict legal requirements due to health risks, we investigated the behavior of TEs in the gaseous fractions sampled and measured upstream and downstream the fabric filter and then compared these data to those from the European Council directives,30,31 of the revised EU Large Combustion Plant Directive (LCPD). The LCPD aims to reduce acidification, ground level ozone, and particles throughout Europe by controlling emissions of sulfur dioxide (SO2), nitrogen oxides (NOx), and dust (particulate matter (PM)) from large combustion plants

Table 4. Flue Dust Emission (Expressed as mg/Nm3) from Contaminated Poplar Woody Biomass Compared with Emission Limits for Solid Fuel Use in Large Combustion Plants (LCP) from the EU Directive32 dust emission (mg/Nm3) emission limit for LCP 50 to 100 MWth emission limit for LCP < 50 MWth TE emission upstream the filter TE emission downstream the filter

30.0 50.0 105.03 1.84

fabric filter allowed a reduction of dust emissions of 98%, ensuring weight index values less than 20 mg/Nm3. The fabric filter, in all cases, drastically reduces emissions in compliance with all regulatory thresholds (Table 4), including those from the large combustion plant EU directive.32 The combustion of woody biomass from contaminated soils generates airborne emissions and solid residue products in the form of ashes. Figure 4 summarizes the distribution of six major contaminants in these two major fractions and in ashes from a 200 kW biomass boiler equipped with a fabric filter device. The combustion process concentrates Cu, Cr, and Ni in the bottom ash (included heat exchanger) and cyclone-ash fractions. Therefore, the impact of the fabric filter is negligible for

Table 3. TE Emission (Expressed as μg/Nm3) from Contaminated Poplar Woody Biomass Compared with Emission Limits, Either from the Industrial Emission Directive (IED)31 or from the Incineration of Waste Directive (IWD) 30 in Europea directive

IED1

IED2

IED3

IWD1

IWD2

TE considered emission limit TE emission upstream the filter TE emission downstream the filter

Cd 50 57.0 3.7

Pb 1000 200.9 6.0

sum of Cr, Co, Cu, Mn, Ni, Sn, Sb, V, Zn 10000 4678.2 560.6

sum of Cd, Ti 50 60.3 6.9

sum of As, Cr, Co, Cu, Mn, Ni, Pb, Sb 500 305.0 61.2

a

Data are the average of 3 replicates. 13366



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Figure 4. Impact of the bag filter unit on the quality of emissions from the biomass combustion unit. The arrow schemes included for each TE show their distribution over the two major ash fractions and the emission as the percentage of the total TE input from poplar wood. The data are expressed in mg/Nm3.

Table 5. Trace Element Concentration in Ashes (mg kg−1 of Dry Matter) in Comparison with Limit Values (Expressed As Fluxes and As Maximal Concentrations) for Sewage Sludge Applications in Agriculture from the EU34 and US-EPA12a trace element

Cd

Cu

Ni

Pb

Zn

EU directive maximal concentration (mg/kg) EU directive annual limit (kg/ha/year) US directive maximal concentration (mg/kg) US directive annual limit (kg/ha/year) bottom ashes cyclone-fly ashes fabric filter-fly ashes

20−40 0.15 39 1.9 1.2−10 48−530 140−550

1000−1750 12 1500 0.7 52−110 52−110 100−120

300−400 3 420 21 11−33 11−35 4.4−5.4

750−1200 15 300 15 6.4−42 40−780 420−2200

2500−4000 30 2800 140 160−810 1600−3200 14000−40000

a

The ash data are given as concentration ranges measured in a series of 8 assays, included the data presented in Figure 2. The chromium concentration is not provided in the US or EU directives.

Wood Combustion Ash Management. This is important to note that most of the European countries do not have a specific directive on wood ash, and only Germany and Austria have integrated wood ashes in their directive either in the case of broadcasting or for the manufacture of fertilizers (Germany) or for composting (Austria). Other countries such as Finland and Sweden have established recommendations but have no directive.33 There are 3 major utilization options of wood ashes.

these elements. Conversely, Cd, Pb, and Zn are significantly recovered in the emission fraction in the absence of the fabric filter above the emission limits (Table 3). The use of a fabric filter will allow the concentration of these three TEs in the ashes collected below the fabric filter. These data clearly illustrate the impact of the fabric filter on the quality of the emissions from wood collected on contaminated soils. 13367



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Fly and bottom ashes can be utilized in certain earth construction applications: public roads, streets, bicycle lanes, pavements, parking areas, sports areas, storage fields, and roads in industrial areas. There are limit values for total content of harmful substances and maximum values for their leaching for the use in covered and paved structures. Fly ashes may also be used in concrete production as binder material or as aggregates. Binder materials must have pozzolanic properties and aggregates suitable particle size distribution. The third option for ash use is through agriculture, as detailed below. Because wood ashes may be used as fertilizing materials we compare the loading limits for trace elements set in Europe,34 and in the USA,12 for sewage sludge application with those for potential wood ash additions (Table 5). These directives provide both annual and cumulative limits (expressed as kg/ha/ year) and maximum concentration of TE (expressed as mg/kg) on the soils. TE contents of bottom ashes are largely below the limit values and may thus be recycled through applications in agriculture. As detailed by Pitman,25 the effects of wood ash are primarily governed by application rate and soil type. The benefits are maximized at low dose rates, with possible toxicity from applications in excess of 10 t ha−1. Pitman also reported that applications of ash up to 10 t ha−1 from ordinary boilers result in trace elements soil levels still 2 orders of magnitude lower than the US-EPA advised loading. Since TE concentrations of bottom ashes in our study are in the same range than those reported by Pitman, these conclusions may be well applied to the type of phytoremediation wood used in our study. Conversely, cyclone-and filter-fly ashes are polluted to a degree that does not allow their use as fertilizing agents in agriculture. The Cd and Zn concentrations are indeed 10 to 30 times higher than the concentration limits. It is therefore recommended that these ash fractions must be treated separately. The fabric filter-fly ash fraction, which acts as a sink for Cd, Zn, and Pb, must be disposed in a safe way or industrially treated, potentially of interest for the recovery of TEs. In two previous studies, electrochemical remediation methods to reduce the Cd content in wood combustion fly ash - for the aim of recycling - are described.35,36 In both studies, approximately 70% Cd was removed from ashes. Given the high enrichment in Cd, Pb, and Zn of fly- and/or filter-ash fractions, these technologies might allow achieving a sustainable, environmentally safe, and friendly production of energy from otherwise derelict land.

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*Phone: +33-3-81-66-57-20. Fax: +33-3-81-66-57-97. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by an ANR PRECODD project (ANR-06-ECOT-O15-01). We also thank the ADEME for funding the fabric filter and the DRIAFF (Direction Régionale et Interdépartementale de l’Alimentation, de l’Agriculture et de la Forêt d’Ile de France) represented by Christian DRON for providing us with administrative support for access to the experimental field. 13368



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Fate of Trace Elements during the Combustion of Phytoremediation

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