Fuel Processing Technology 96 (2012) 48–55

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Fuel Processing Technology

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Chemical and ecotoxicological properties of ashes produced in the co-combustion ofcoal and meat and bone meal in a fluidized bed reactor

Rui Barbosa a,⁎, Nuno Lapa a, Helena Lopes b, Benilde Mendes a

a Universidade Nova de Lisboa, Faculdade de Ciências e Tecnologia, Departamento de Ciências e Tecnologia da Biomassa, UBiA, Ed. Departamental, gab 377, 2829–516 Caparica, Portugalb Laboratório Nacional de Energia e Geologia (LNEG), Unidade de Emissões Zero (UEZ), Ed. J., Estrada do Paço do Lumiar, 22, 1649–038 Lisboa, Portugal

⁎ Corresponding author. Tel./fax: +351 212948543.E-mail addresses: rfb@fct.unl.pt, r_p_f_b@hotmail.co

0378-3820/$ – see front matter © 2011 Elsevier B.V. Alldoi:10.1016/j.fuproc.2011.12.013

a b s t r a c t

a r t i c l e i n f o

Article history:Received 1 September 2011Accepted 12 December 2011Available online 13 January 2012

Keywords:CombustionCoalMeat and bone mealAshesChemical propertiesEcotoxicological properties

The co-combustion of coal and meat and bone meal (MBM) is a possible energetic valorization route for thisresidue. Nevertheless, the properties of ashes produced need to be studied. To evaluate these properties,three combustion tests were performed in a fluidized bed reactor: 1) coal combustion; 2) coal+MBM(85%+15%) co-combustion; 3) MBM combustion. The characterization of ashes was focused on the followingaspects: (1) Determination of bulk content of Cr, Zn, Ni, Cu, Pb, Cd, Hg, As, Ba, Mo, Sb, Se, Ca, Na, Mg, Fe, Al andK; (2) Leaching properties of ashes based on the European Standard EN12457-2. The eluates were character-ized for some of the metals referred above and for Cr VI, CN−, pH, Cl−, F−, SO4

2−, dissolved organic carbon andtotal dissolved solids. The eluates were also characterized for ecotoxicological levels by using the followingbio-indicators: bacterium V. fischeri, microalgae S. capricornutum and microcrustacean D. magna. The ashesproduced in the combustion of coal and co-combustion of coal+MBM have not shown evidences of ecotoxi-city, while the ashes produced in the combustion of MBM were classified as ecotoxic. An assessment of therelationship between the chemical and the ecotoxicological properties of the ashes was performed. pHseemed to be the chemical parameter that most influences the ecotoxicological level of ashes.

© 2011 Elsevier B.V. All rights reserved.

1. Introduction

The replacement of fossil fuels by renewable sources of energycan contribute to improve the environmental performance of theelectricity and vapor production and sustainability [1,2]. The experi-ence has shown that the availability of alternative fuels can be anobstacle for its extensive use for energy production. The use ofnon-hazardous wastes may be an alternative to biomass, if they areeconomically unattractive for recycling or if they have a high costfor landfilling [3]. Co-firing non-hazardous wastes with coal is,therefore, a subject of great interest for the sustainability of energyproduction and for the reduction of the emissions of fossil green-house gases [4]. One possible solution for MBM treatment isincineration [5,6]. The relatively high LHV of MBM is a characteristicof this material which allows classifying it as interesting for com-bustion [7]. Nevertheless, the use of MBM for energy is promisingif it combines well with other fuels during the conversion processfor energy and doesn't have negative effect on the combustion sys-tem, ash quality and gaseous emissions [8,9]. The thermal valoriza-tion of organic residues with considerable quantities of mineralmatter content may lead to the production of ashes with different

m (R. Barbosa).

rights reserved.

properties. The ashes produced during the co-combustion of coalwith MBM should be carefully studied, since the substitution ofcoal by MBM can produce ashes with high content of contaminants,such as heavy metals [10–15]. The properties of the ashes are partic-ularly important, since differences in these properties may requiredifferent management strategies of the ashes produced by the facil-ity that performs the thermal valorization of MBM.

The main objectives of the work is to assess the chemical and eco-toxicological properties of the ashes produced during the combustionof coal with those produced during the co-combustion of coal andMBM and to classify these ashes according to their ecotoxicity. Oneadditional goal is to assess the relationship between the chemicaland the ecotoxicological properties of the ashes.

2. Materials and methods

2.1. Fluidized bed reactor, fuels and combustion conditions

The combustion and co-combustion tests were performed, by UEZteam, in a pilot scale bubbling fluidized bed reactor (FBR) of LNEG/UEZ. This equipment includes the reactor and the auxiliary systems:fuel feed-in system; air fans for primary and secondary air injectionand for exhaustion of gases, two heat exchangers and two cyclonesfor fly ash removal. The reactor has an internal square section with0.3 m×0.3 m and of 5 m height. The internal chamber is of refractorysteel (AISI 310). The bottom of the reactor is settled over a wind box

Highest TU tested

Lowest TU tested

Fig. 1. Methodology for the definition of the NEL based on the TU obtained during theecotoxicological characterization of the eluates.

Table 2Average bulk content and elemental composition of coal and MBM (n=2; ±SD).

Parameter Unit Coal MBM

C % db (m/m) 66.4 (±5.0) >>39.2 (±4.1)H % db (m/m) 4.7 (±0.4) >>4.8 (±0.4)N % db (m/m) 1.4 (±0.1) >>8.5 (±1.0)S % db (m/m) 0.96 (±0.01) >>0.43 (±0.04)Cl % db (m/m) 0.07 (±0.001) >>0.29 (±0.03)Ca % db (m/m) 0.23 (±0.02) >>13.4 (±1.1)K % db (m/m) 0.24 (±0.02) >>0.26 (±0.03)Na % db (m/m) 0.08 (±0.01) >>0.63 (±0.07)P % db (m/m) 0.004 (±0.0003) >>5.0 (±0.51)Fe % db (m/m) 0.64 (±0.04) >>0.02 (±0.003)Al % db (m/m) 1.29 (±0.1) >>0.08 (±0.001)Mg % db (m/m) 0.004 (±0.0003) >>5.0 (±0.51)Ba mg/kg db b3.6 (n.a.) >>452 (±35.5)Sb mg/kg db b0.07 (n.a.) >>0.13 (±0.01)Mo mg/kg db b22.4 (n.a.) >>117 (±15.5)Se mg/kg db b0.21 (n.a.) >>0.30 (±0.02)As mg/kg db b0.73 (n.a.) >>b0.65 (n.a.)Hg mg/kg db b0.27 (n.a.) >>b0.24 (n.a.)Cd mg/kg db b7.3 (n.a.) >>b6.5 (n.a.)Pb mg/kg db b22.8 (n.a.) >>b20.4 (n.a.)Cu mg/kg db b9.4 (n.a.) >>9.9 (±1.2)Ni mg/kg db b14.4 (n.a.) >>b14.0 (n.a.)Zn mg/kg db 36.8 (±4.9) >>94.3 (±17.1)Cr mg/kg db 33.5 (±4.1) >>b10.2 (n.a.)

n: number of replicates; SD: standard deviation; db: dry basis; n.a.: not applicable.

49R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

chamber with 0.3 m×0.3 m and with 0.2 m height. The primary air isinjected through the wind box and is distributed at the bottom ofthe reactor through an air distribution plate. The fuel feed-in pipe islocated 500 mm above the air distribution plate. The secondary airis injected 1100 mm above the air distribution plate. The heat ex-changers are located close to the bed of the reactor and at the top ofthe combustor chamber. The bottom ashes are collected through thewind box and the fly ashes are collected in the bottom of twosequential cyclones. The average temperatures in the first and secondcyclones were of 300 and 150 °C, respectively. The flue gases areexhausted through a fan located in the bottom of a vertical stack.Silica sand was used as the fluidization agent in the bed. Further de-tails of this FBR are shown in Gulyurtlu and Monteiro [16] andGulyurtlu et al. [17]. Three combustion tests were performed: 1) com-bustion of coal; 2) co-combustion of coal (85%) and MBM (15%);3) combustion of MBM. Each combustion test produced three typesof ashes: bottom ashes and two cyclone ashes (1st and 2nd cycloneashes). The fossil fuel used was a bituminous coal from the open pitmine of El Cerrejón, in Colombia. MBM was produced in slaughterhouses of Germany.

2.2. Bulk characterization of fuels and ashes

The quantification of Cr, Zn, Ni, Cu, Pb, Cd, Ba, Mo, Sb, Se, As and Hgwas performed over samples submitted to an acidic digestion accord-ing to the USEPA Method 3051A (HNO3/HCl). The quantification ofMg, Al, Fe, Ca, Na and K was performed over samples submitted toan acidic digestion according to the European Standard EN13656(HF/HNO3/HCl). The digestion was developed in microwave oven(Milestone Ethos 1600) using closed vessels and with controlled tem-perature (175±5 °C, 10 min). The quantification of metals wasachieved through AAS (Thermo AAS, M series). Phosphorous contentwas determined through UV/Vis. The C, H, N, S and Cl content in fuelswas determined according to ASTM and CEN/TS standards.

Table 1Proximate analysis of the fuels used in the combustion and co-combustion tests.

Fuels Watercontent[wt.% ar]

LHV[GJ/t ar]

Ashes(wt.% db)

Volatile matter(wt.% db)

Fixed carbon(wt.% db)

Bituminous coal 13.0 24.79 12.6 37.0 50.4MBM 2.9 13.10 28.8 63.1 8.1

LHV: low heating value; ar: as received base; db: dry basis.

2.3. Leaching test, chemical and ecotoxicological characterization ofeluates

The ashes were submitted to the leaching test described in theEuropean Standard EN12457-2. The eluates were characterized forAs (EN ISO 11969), Hg (ISO 5666/1), Cd, Cu, Ni, Pb, Zn (ISO 8288),Cr (ISO 9174), Cr VI (NF T90-043, 1988), Se (ISO 9965, 1993), Ba,Mo, Sb, (APHA/AWWA/WPCF, 1996), Ca (ISO 7980, 1986), Na (ISO9964–1, 1993), K (ISO 9964–2, 1993), pH, DOC, CN−, SO4

2−, F−, TDS(APHA/AWWA/WPCF, 1996), Cl− (ISO 9297, 1989), phenol com-pounds (ISO 6439, 1990). The eluates were also characterized forthe following ecotoxicological parameters: a) Luminescence inhibi-tion of the bacteria Vibrio fischeri (ISO 11348–3, 2003); b) Mobilityinhibition of the crustacean Daphnia magna (ISO 6341); and c) grow-ing inhibition of the algae Pseudokirchneriella subcapitata (ISO 8692).The evaluation of the ecotoxic properties (property H14 of CouncilDirective 91/689/EEC) of bottom and fly ashes was based on theCriterion and Evaluation Methods for Waste Ecotoxicity (CEMWE).The original CEMWEmethodology was adapted according to the dis-cussion previously shown in Lapa et al. [18].

2.4. Chemical index

All materials were ranked according to a chemical index based onthe chemical composition of eluates and the limit values defined inCEMWE. This chemical index was based on the following steps:

(a) Calculation of the toxicity equivalents (TE): The TE was calcu-lated by the conversion of the CEMWE limit value, of each pa-rameter, from mg/L to μmol/L. Then, it was calculated the ratiobetween the parameter with the highest toxicity, i.e., the pa-rameter with the lowest limit value (expressed as μmol/L),and the limit value (expressed as μmol/L) of each chemicalparameter.

(b) Calculation of the relative toxicity (RT): RT for each chemicalparameter was obtained through the product of TE by the con-centration determined in the eluates.

(c) Calculation of the toxicity level (TL): TL was calculated for eachsample as the sum of all RT calculated for each chemicalparameter.

Table 3Bulk composition of the bottom, 1st and 2nd cyclone ashes (mg/kg db).

Parameter Bottom ashes 1st cyclone ashes 2nd cyclone ashes

Coal Coal+MBM MBM Coal Coal+MBM MBM Coal Coal+MBM MBM

K 4016 8070 5705 14,082 14,442 9583 14,735 17,890 27,016Na 3129 7731 8121 6778 8585 15,544 6733 9300 23,236Ca 48,056 18,078 129,617 15,880 51,336 238,378 9185 16,463 210,427Cr 172 162 133 313 308 572 59 292 4800Zn 18.3 28.6 128 148 178 233 167 234 1495Ni 69.6 30.3 43.5 298 173 202 156 158 3828Cu b8.4 b10.4 b9.3 47.8 49.9 81.1 68.7 73.4 470Pb b17.4 b18.9 b17.6 b26.2 b22.5 81.1 44.7 35.6 470Cd 19.5 22.5 b0.70 b9.1 b18.6 177 19.8 19.7 5,7Ba b10.4 133 3110 1238 1608 485 1086 1428 1782Mo b34.8 b37.7 b35.2 b37.8 73.3 140 90.3 102 508Sb b0.10 b0.11 b0.11 b0.11 b0.11 b0.11 b0.11 b0.11 b0.11Se b0.70 b0.75 b0.70 32 1.9 b0.73 9.7 12.9 b0.73Hg b0.42 b0.45 b0.42 b0.45 b0.61 b0.44 b0.44 b0.75 0.9As 1.4 0.89 b0.70 3.5 3.4 b0.73 6 6.2 4.8

Coal: combustion of coal; coal+MBM: co-comb. of coal and MBM; MBM: comb. of MBM.

50 R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

In this work, since in the leaching assays most of the results arelower than the quantification limits (QL), two values of TL areshown for each sample. The lowest one, named as “Lowest ToxicityLevel”, was calculated assuming a concentration value of zero whenthe concentration was below the QL. The highest value of TL, namedas "Highest Toxicity Level", was calculated assuming the value of QL,for each parameter, when the concentration was below the QL.

2.5. Ecotoxicological index

In order to assess the ecotoxicological level of each sample, it wasdefined three normalized ecotoxicity levels (NEL). These NEL weredefined taking into account the following: 1) average values of toxicityunits (TU) obtained in D. magna, V. fischeri and P. subcapitata assays;2) average values of the TU obtained in D. magna and P. subcapitataassays; 3) TU obtained in V. fischeri assay. The TU were calculatedaccording to Eq. (1).

TU ¼ Ef f ective Concentration %ð Þ100%

ð1Þ

To define NEL, three threshold limits were considered: a) the low-est TU that was determined; b) the limit value of the ecotoxicologicalparameter indicated in CEMWE; and c) the highest TU that wasdetermined.

Table 4Chemical characterization of the eluates produced by the ashes (pH: Sorensen; other speci

Bottom ashes 1st cyclone ashes

Coal Coal+MBM MBM Coal Coa

pH 11.5 9.7 8.0 10.5SO4

−2 1580 2897 1863 18,925 18,DOC 54.2 77.4 b0.99 4.2TDS 4652 4775 11,685 26,401 31,CN− b0.13 b0.13 b0.13 0.30Cl− 98.5 b25.0 993 179F− 95.7 1.5 79 135K 52 153 2986 650Na 127 244 2310 781Ca 757 799 113 1939 2Cr b0.49 b0.50 2.0 b0.51CrVI b0.49 b0.50 1.6 b0.51Ni b0.20 b0.20 b0.20 b0.20Ba b1.6 6.0 b1.6 4.5Mo 6.9 6.1 b0.97 33.3Se 0.19 b0.009 0.50 29.7Hg b0.01 b0.01 b0.01 b0.01As b0.03 b0.03 b0.03 b0.03

The conditions for the definition of the NEL were the following:

a) The NEL values range from 0 to 100;b) NEL was scored as 0, for TU lower than the lowest TU that was

determined;c) For TU between the lowest TU that was determined and the

CEMWE limit value, the NEL ranged between 0 and 50 and itwas assumed a linear relation between TU and NEL;

d) For TU equal to CEMWE limit value it was attributed the scoreof 50;

e) For TU between the CEMWE limit and the highest TU that couldbe determined, the NEL ranged between 50 and 100, and it wasassumed a linear relation between TU and NEL;

f) For TU higher than the highest TU that was determined, the scoreof 100 was attributed to NEL.

Fig. 1 shows an example of conversion of TU to NEL, for V. fischeri.

3. Results and discussion

3.1. Bulk characterization of the fuels

Table 1 shows the proximate analysis of the fuels used in the com-bustion and co-combustion tests. The coal used on the combustiontests was a bituminous coal. This type of coal presents a high LHV,which is in agreement with that found by Wagland et al. [19], of

es: mg/kg db).

2nd cyclone ashes

l+MBM MBM Coal Coal+MBM MBM

9.6 7.4 11.3 10.8 7.3734 1786 13,531 10,320 1338129 12.8 b1.0 98.9 72.3519 23,056 23,955 35,098 120,056

0.47 0.21 b0.13 b0.13 0.25206 1559 103 156 302108 52.3 110 95.4 641610 3852 341 1033 2430958 3782 658 2302 22,739880 1610 2953 1234 6621b0.51 4.6 b0.51 b0.52 3.3b0.51 1.8 b0.51 b0.52 1.7b0.20 b0.20 b0.20 b0.20 17.2b1.6 6.5 b1.6 2.7 4.118.2 46.1 71.3 79.7 35.50.10 0.09 0.82 9.6 0.28

b0.01 b0.01 0.05 b0.01 b0.01b0.03 b0.03 0.12 b0.03 0.17

0.0 0.0

0.50

0.08 0.12

1.22

0.080.0

0.76

0.47 0.47

0.82

0.51 0.56

1.51

0.53 0.47

1.04

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

Bottom ashes 1st cyclone ashes 2nd cyclone ashes

Tox

icit

y L

evel

Lowest Toxicity Level

Highest Toxicity Level

Fig. 2. Lowest and highest toxicity levels based on the ecotoxicological characterization of the eluates and the limits defined in CEMWE.

Table 5TU limits defined in CEMWE and TU of the eluates.

Parameter CEMWE limit Bottom ashes 1st cyclone ashes 2nd cyclone ashes

Coal Coal+MBM MBM Coal Coal+MBM MBM Coal Coal+MBM MBM

D. magna 10 1.95 b1.05 b1.05 b1.05 b1.05 1.57 b1.05 b1.05 3.39V. fischeri 10 4.59 b1.01 2.58 b1.01 b1.01 2.35 b1.01 b1.01 b1.01P. subcapitata 1000 4.63 1.91 b1.05 b1.05 b1.05 b1.05 28.6 1.31 2.53

51R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

26.5 GJ/t. The LHV of MBM was similar to those observed by Senneca[10] (14.47 GJ/t) and Gulyurtlu et al. [17] (13.06 GJ/t), but lower thanthat found by Beck et al. [20] (15.7 GJ/t), Conesa et al. [21] (17 GJ/t)and Cummins et al. [5] (18.19 GJ/t).

Table 2 shows the metals bulk composition of the fuels for a setof metals. The concentration of Cr in coal was higher than thosefound by López-Antón et al. [22] (19.9 mg/kg), Lopes et al. [8](12.2 mg/kg), Miller et al. [23] (24.0 mg/kg) and Ward et al. [24](11.0 mg/kg). Miller et al. [23] and Reddy et al. [25] have found con-centrations levels of Cr and Zn similar to those found in the presentwork. Nevertheless, these authors have found higher levels ofAs (1.83 mg/kg and 41.9 mg/kg), Cu (25.0 and 43.0 mg/kg), Pb(34.0 and 29.4 mg/kg) and Se (2.4 and 12.1 mg/kg). Lopes et al. [8]

8.4

0.01

2.9

19.9

0.0

8.7

0.0 02.6

0.02

0.00

5

10

15

20

25

Coal CoalCoal + CMBM

MBMM

Nor

mal

ized

Eco

toxi

city

Lev

els Average

Average

Average

Bottom ashes 1st cycl

Fig. 3. NEL of th

and Ward et al. [24] have found similar levels of As and Cu to thosedetermined in this work. The concentrations of Mg, Al, Fe, Ca, K, Naand P found in coal were higher than those found by Gulyurtluet al. [17]. The differences in the composition of coal are probablyassociated with the differences in the geological properties wherethe samples were collected.

Generally, MBM has shown the highest concentrations of theheavy metals. MBM has presented high levels of Ba and Mo, whichcan be explained by the fact that these elements are rapidly trans-ported in blood plasma and accumulated in bones, through ingestionor airborne exposition [26,27]. The concentrations of K, Fe, Mg andNa in MBM analyzed in the present work were similar to thosefound by Dybowska et al. [28]. MBM has presented, namely, the

3.5

0.5

0.00

4 4.4.0

7.5

0.0

0.0

0.01.5

0.7

0.00

7 6.6

oal + MBM Coal Coal + MBMBM MBM

-Marine + Freshwater organisms

-Marine organisms

-Freshwater organisms

one ashes 2nd cyclone ashes

e eluates.

0

4

8

12

16

20

0 3.5 7 10.5 14

Nor

mal

ized

Eco

toxi

city

Lev

els

pH

Average -Marine + Freshwater organisms

Fig. 4. Relationship between pH and NEL based on the average of the ecotoxicity levelsof the three bio-indicators.

Average -Freshwater organisms

0

4

8

12

16

20

Nor

mal

ized

Eco

toxi

city

Lev

els

0 3.5 7 10.5 14pH

Fig. 6. Relationship between pH and NEL based on the average of the ecotoxicity levelsof the freshwater bio-indicators.

52 R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

highest concentrations of Ca and P. This fact may explain the highlevels of some metals in the ashes produced during the combustiontest. The concentrations of As, Hg, Cd, Pb and Ni were below the QL.

3.2. Bulk characterization of ashes

Table 3 shows the bulk composition of ashes. Generally, the metalcontent was higher in fly ashes. The substitution of coal by MBM haspromoted, generally, a higher concentration of metals in ashes. Theconcentrations of Cr, Ni and As were similar in bottom ashes. The2nd cyclone ashes, especially those produced in the combustion ofMBM, have presented the highest concentration of Cr, Zn, Ni, Cu andPb, which can be attributed to the lower particle size of the ashesthat usually present enrichment in heavy metals due to volatiliza-tion/condensation phenomena, especially in the presence of highlevels of Cl [29–31]. Ba and Mo were also found in high concentra-tions in the ashes from the combustion tests in which MBM wasused as fuel. This behavior is probably associated with the high con-centrations of these elements in MBM. The 1st and 2nd cycloneashes, produced in the combustion of coal and co-combustion test, haveretained As and Se in higher levels than those observed in the sametype of ashes produced in the combustion of MBM, although the levelswere insignificant in the fuels. The same behavior was observed for Crand Cd. The relative high concentration of Cd in ashes may be explainedby two reasons: 1) the enrichment factor in the ashes; and 2) the immo-bilization of Cd in ashes as Ca10−xCdx(PO4)6(OH)2 or as CdCO3

Marine organism

0 3.5 7 10.5 14pH

0

4

8

12

16

20

Nor

mal

ized

Eco

toxi

city

Lev

els

Fig. 5. Relationship between pH and NEL based on the ecotoxicity levels of the marinebio-indicator (V. fischeri).

(otavite) as it was observed by Coutand et al. [11]. According to Dybow-ska et al. [28], the apatites present in the ashes may remove Cd, Pb, Znand Cu, through the chemical precipitation of these metals with phos-phates as Pb hydroxylapatite, Zn phosphate (hopeite), Cd phosphateand Cu phosphate (libenthenite).

3.3. Leaching behavior of ashes

3.3.1. Chemical characterization of the eluatesTable 4 shows the release of chemical species from the ashes

under the leaching test conditions. The concentrations of Sb, Zn, Ni,Cu, Pb, Cd and phenolic compounds were below QL values. pH valuesof the eluates produced by bottom ashes were between 8.0 and 11.5,which can be attributed to the high level of alkaline oxides in the bot-tom ashes. pH values of eluates produced by 1st cyclone ashes wereslightly lower (7.4 and 10.5) than those of bottom ashes. pH levelsof eluates from the 2nd cyclone ashes were similar to those from1st cyclone ashes (7.3 to 11.3). The decrease of pH levels fromthe eluates of bottom to fly ashes are, probably, associated to thepresence of acidic condensates from the flue gases [32,33]. The con-centration of Cr VI was below QL, except in the eluates produced bythe ashes from the combustion of MBM. The concentration of Cl−

was, generally, higher in the eluates produced by ashes of co-combustion test and in the combustion of MBM, which can be dueto the high concentration of this element in MBM [34]. The

0 20 40 60 80 100

Mo (mg/kg)

Average -Marine + Freshwater organisms

0

4

8

12

16

20

Nor

mal

ized

Eco

toxi

city

Lev

els

Fig. 7. Relationship between the concentration of Mo and NEL based on the average ofthe ecotoxicity levels of the three bio-indicators.

Marine organism

0

4

8

12

16

20N

orm

aliz

ed E

coto

xici

ty L

evel

s

0 20 40 60 80 100

Mo (mg/kg)

Fig. 8. Relationship between the concentration of Mo and NEL based on the ecotoxicitylevels of the marine bio-indicator (V. fischeri).

0.0

0.5

1.0

1.5

2.0

0 5 10 15 20 25

Nor

mal

ized

Eco

toxi

city

Lev

el

Lowest Toxicity Level

Average -Marine + Freshwater organisms

Fig. 10. Relationship between the lowest toxicity level and NEL based on the average ofthe Ecotoxicity Levels of the three bio-indicators.

53R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

concentrations of F− and SO42− were higher in the ashes resulting

from the combustion tests in which coal was used as fuel. Generally,Cl−, F−, and SO4

2− were found in higher concentration in fly ashes,which may be associated with the accumulation of soluble particleswith high content of these species and to the presence on acidiccondensates [31]. The combustion tests in which MBM was usedas fuel have produced ashes with higher concentration of TDS, spe-cially the fly ashes retained in the 2nd cyclone. Relative high con-centration of As was also found in the eluate of the 2nd cycloneashes of the combustion of MBM. This behavior was also observedby McDonnell et al. [35]. This fact may be associated with highercontents of soluble species in these particles [31].

Fig. 2 shows the Lowest and Highest Toxicity Levels of eluates.Ashes produced in the combustion of MBM have presented the high-est TL. Among these ashes, the increasing order of toxicity is bottomashesb2nd cyclone ashesb1st cyclone ashes. The TL in the eluates ofthe ashes produced during the combustion of MBM are related withthe leachability of Cr and Cr VI. The ashes produced during the com-bustion of coal and co-combustion of coal and MBM have presentedsimilar TL values.

3.3.2. Ecotoxicological characterization of eluatesTable 5 shows the TU obtained in eluates of ashes. The eluates

have presented low ecotoxicological levels which were even below

Average -Freshwater organisms

0

4

8

12

16

20

Nor

mal

ized

Eco

toxi

city

Lev

els

0 20 40 60 80 100

Mo (mg/kg)

Fig. 9. Relationship between the concentration of Mo and NEL based on the average ofthe ecotoxicity levels of the freshwater bio-indicators.

the limit values defined in CEMWE. According to CEMWE, all asheswere classified as non-ecotoxic. The bottom ashes produced duringthe combustion of coal have presented higher ecotoxicity levels prob-ably due to the high pH levels [31,36,37] or the synergic effect of pHand solubility of heavy metals. The 2nd cyclone ashes have producedeluates with the highest ecotoxicological levels, especially those pro-duced in the combustion of coal. The P. subcapitata was particularsensitive to the eluate produced by the 2nd cyclone ashes from thecombustion of coal. Further studies are needed to justify thisbehavior.

Fig. 3 shows the NEL. Generally, the ashes produced during thecombustion of MBM have presented NEL higher than the otherashes, except for the bottom ashes produced in the combustion.

3.4. Assessment of factors that affect the biological response of thebio-indicators

It is difficult to indicate a definitive explanation for the biologicalresponses, since the eluates of ashes are a very complex matrix.Nevertheless, it is proposed a methodology to identify which pa-rameters have significantly conditioned the biological responses ofthe bio-indicators. An analysis of possible relationships betweenthe chemical and ecotoxicological characterizations was performed.The assessment was developed through the application of disper-sion graphs to a set of parameters.

0.0

0.5

1.0

1.5

2.0

Nor

mal

ized

Eco

toxi

city

Lev

el

Marine organism

0 5 10 15 20 25

Lowest Toxicity Level

Fig. 11. Relationship between the lowest toxicity level and NEL based on the ecotoxi-city levels of the marine bio-indicator (V. fischeri).

0.0

0.5

1.0

1.5

2.0

Nor

mal

ized

Eco

toxi

city

Lev

el Average -Freshwater organisms

0 5 10 15 20 25

Lowest Toxicity Level

Fig. 12. Relationship between the lowest toxicity level and NEL based on the average ofthe ecotoxicity levels of the freshwater bio-indicator.

54 R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

It was decided to consider the NEL for the three bio-indicatorstested and the NEL for freshwater organisms and the NEL of the ma-rine organism, in an independent approach. The main reason forthis approach is related with the possibility of occurring different bi-ological responses by those two groups of organisms. For example,the organisms belonging to a marine environment can be more resis-tant to higher concentrations of Cl−, and SO4

2− than those belongingto freshwater environment [38].

Figs. 4 to 6 show the relationship between the pH and the NELof the three bio-indicators, the NEL based on the results of themarine bio-indicator and the NEL based on the results of the fresh-water bio-indicators, respectively. As it was indicated before, theresults seem to indicate a relationship between the pH of the elu-ates and the biological responses: pH levels close to the interval8 to 10 have promoted low NEL, while pH levels above or belowthis interval have promoted an increase of this ecotoxicologicalparameter.

Figs. 7 to 9 show the relationship between Mo and the NEL ofthe three bio-indicators, the NEL based on the results of the marinebio-indicator and the NEL based on the results of the freshwater bio-indicators, respectively. In what concerns the remaining chemicalparameters, it was not observed any positive relationship betweenthe NEL and those parameters. In what concerns the remainingparameters, the results have demonstrated, generally, that the rela-tionship between the chemical parameters and the biological re-sponse was similar to that observed for Mo.

According to Komjarova and Blust [39] and Deleebeeck et al.[40], the concentrations of Ca, Na, Mg and pH can affect the kineticsof Cd, Cu, Ni, Pb and Zn uptake by D. magna. Those authors haveconcluded that increasing concentrations of Ca, Mg and Na, in anaqueous matrix submitted to ecotoxicological tests have had a pro-tective effect over the organisms. According to data shown inTable 3, the eluate of the fly ashes produced during the combustionof MBM has presented high levels of Ni, which could have promot-ed high levels of ecotoxicity. Since it was not observed high levelsof ecotoxicity, it is possible that the high levels of Na and Ca in

Table 6Classification of the ashes according to CD 2003/33/EC.

Classification Bottom ashes

Coal Coal+MBM MBM

N-H N-H N-H

Due to… Mo, Se, F−, SO42−, TDS Mo, SO4

2−, TDS Cr, Ni, Cl−, F−, SO42−, TDS

N–H: non hazardous; H: hazardous; DnA: deposition not allowed.

this eluate may have reduced the ecotoxic effects of Ni. Park et al.[41] have indicated that DOC and hardness can affect the toxicityto D. magna due to, respectively, Cu II and Cr VI. Nevertheless, itis necessary further studies to evaluate the relationship betweenthe chemical parameters and the NEL, i.e., it is necessary furtherstudies to identify how the chemical parameters affect the biolog-ical responses in this type of matrix. This study would be impor-tant in order to reduce the ecotoxicity of the eluates produced byashes.

Figs. 10 to 12 show the relationship between Lowest ToxicityLevels the NEL of the three bio-indicators, the NEL based on the re-sults of the marine bio-indicator and the NEL based on the results ofthe freshwater bio-indicators, respectively. The results indicate thatlow Lowest Toxicity Levels have promoted, generally, low NEL. Itwas identified two to three eluates in which the increasing of theLowest Toxicity Levels have promoted an increasing of the NEL. Itwas not identified the reason for this behavior, revealing the neces-sity of more studies in this area.

3.5. Classification of the ashes according to CEMWE and to the CouncilDecision 2003/33/EC

According to CEMWE, the ashes produced during combustion ofcoal and co-combustion of coal and MBM have not shown evidencesof ecotoxicity. All ashes produced during the combustion of MBMwere classified as ecotoxic, due to the chemical composition of theeluates.

Table 6 shows the classification of ashes according the CouncilDecision 2003/33/EC. Fly ashes need to be stabilized prior to landfill-ing, except the 1st cyclone ashes produced during the co-combustionof coal and MBM. This fly ash can be landfilled in a hazardous wastelandfill. The bottom ashes were classified as non-hazardous residue.

4. Conclusions

The leaching rates of metals were very low. The substitution ofcoal by MBM produced ashes with higher content of heavy metals.Nevertheless, the leaching rates of heavy metals were similar. Thebiological responses of the organisms tested are, probably, associatedwith the pH of the eluates. Nevertheless, it is necessary to developmore studies in order to identify the factors that promote the highlevels of ecotoxicity in some eluates.

According to CEMWE, the ashes produced during the combustionof coal and co-combustion test did not show evidences of ecotoxicity.All ashes produced during the combustion of MBM were classified asecotoxic due to the chemical composition of the eluates.

According to the Council Decision 2003/33/EC, all fly ashes needstabilization prior to landfilling, except the 1st cyclone ash producedin the co-combustion test that was classified as hazardous residue.The bottom ashes were classified as non-hazardous residues.

Despite the differences in the scope of CEMWE and CouncilDecision 2003/33/EC, it can be stated that its application to asheshas led to different conclusions about the classification of thesematerials.

1st cyclone ashes 2nd cyclone ashes

Coal Coal+MBM MBM Coal Coal+MBM MBM

DnA H DnA DnA DnA DnA

Mo, Se Mo, F− Mo Mo Mo, Se Mo, Ni, F−, TDS

55R. Barbosa et al. / Fuel Processing Technology 96 (2012) 48–55

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