Trace element concentrations and associations in some biomass ashes

Fuel 129 (2014) 292–313

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Trace element concentrations and associations in some biomass ashes Stanislav V. Vassilev a,⇑, Christina G. Vassileva a, David Baxter b a b

Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Block 107, Sofia 1113, Bulgaria Institute for Energy and Transport, Joint Research Centre, European Commission, P.O. Box 2, NL-1755 ZG Petten, The Netherlands

h i g h l i g h t s  Trace element contents in some biomass ashes and their types were studied.  Correlations and associations among major, minor and trace elements were described.  The leading importance of modes of trace elements occurrence was emphasized.  Some challenges related to trace elements in biomass ash were discussed.

a r t i c l e

i n f o

Article history: Received 11 March 2014 Received in revised form 28 March 2014 Accepted 1 April 2014 Available online 18 April 2014 Keywords: Biomass Biomass ash Trace elements Composition Classifications

a b s t r a c t The phase-mineral and chemical composition of biomass ashes (BAs) produced from 8 biomass varieties, namely beech wood chips, corn cobs, marine macroalgae, plum pits, rice husks, switchgrass, sunflower shells and walnut shells, was studied for the elucidation of trace element (TE) concentrations and their associations. For that purpose the contents of 60 major, minor and TEs in high-temperature BAs produced at 500 °C were determined by inductively coupled plasma and laser ablation mass spectrometry, as well as scanning electron microscopy equipped with an energy dispersive X-ray analyser. The phase-mineral composition of BAs was studied by light microscopy, powder X-ray diffraction, differential-thermal and thermo-gravimetric analyses plus some leaching, precipitation, ashing (500–1500 °C) and other procedures. The composition and properties of the biomass fuels studied and their ashes were characterized, and some general considerations about TEs in them are described. An elucidation of the TE contents in BAs and their ash types was conducted and a comparison with coal ashes was also performed. An explanation of the correlations and clarification of the associations of TEs with major and minor elements, as well as with different constituents (organic matter, inorganic matter, cellulose, hemicellulose, lignin, inorganic amorphous matter, silicates, phosphates, carbonates, oxides, hydroxides, sulphates and chlorides) in biomass and BAs are given. It was revealed that the greatest ecological challenges related to some TEs in biomass and BA include their: (1) high concentrations; (2) unfavourable modes of occurrence; (3) enhanced volatilization and limited retention and capture performance during biomass combustion; and (4) increased leaching behaviour during biomass and BA processing or storage. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction Extensive reference peer-reviewed data plus own investigations for both biomass and biomass ash systems were used recently to perform several extended and consecutive overviews related to: (1) chemical composition of biomass [1]; (2) organic and inorganic phase composition of biomass [2]; (3) phase-mineral and chemical composition of biomass ash (BA) [3]; (4) potential utilization, technological and ecological advantages and challenges of BA [4]; and (5) behaviour of biomass during combustion, namely ⇑ Corresponding author. Tel.: +359 2 9797055; fax: +359 2 9797056. E-mail address: [email protected] (S.V. Vassilev). http://dx.doi.org/10.1016/j.fuel.2014.04.001 0016-2361/Ó 2014 Elsevier Ltd. All rights reserved.

phase-mineral transformations of organic and inorganic matter [5] and ash fusion and ash formation mechanisms of biomass types [6]. New classifications based on data from proximate, ultimate, ash, structural and mineralogical analyses, and ash-fusion tests of biomass or BA have also been introduced therein [1–6]. It was highlighted in the above overviews that despite the low contents of trace elements (TEs) in biomass and BA they have very important ecological and technological impacts during sustainable utilization of solid biofuels and their products. Numerous studies related to TE concentrations in biomass and BA and their behaviour during thermo-chemical conversion including combustion, pyrolysis, gasification and liquefaction, as well as co-combustion, co-pyrolysis and co-gasification of coal

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293

Nomenclature BA BC CC daf db DTA DWR EDX FC IAM ICP IM LA MM

biomass ash beech wood chips corn cobs dry, ash-free basis dry basis differential-thermal analysis dry water-soluble residue energy dispersive X-ray analyser fixed carbon inorganic amorphous matter inductively coupled plasma inorganic matter laser ablation marine macroalgae

with biomass have been performed worldwide ([7–52] among others). Consequently, significant data for contents of TEs in biomass, biochar and BAs such as low-temperature and high-temperature laboratory ashes and industrial bottom ashes, slags and fly ashes, along with behaviour of TEs during thermal treatment of different biomass varieties have been generated. However, serious shortcomings related to TE studies have been identified and most of the short-comings are similar to those determined for chemical and phase investigations of biomass and BA [1–6]. Therefore, an attempt to summarize these problems was undertaken and is described below. The long term experience and knowledge gained for TEs in the most studied solid fuels and their ashes, namely for coal, peat, petroleum coke and municipal solid waste or refuse-derived fuel have not been applied successfully enough in the field of biomass and BA. For example, the methods for TE investigations of biomass and BA have not been refined and implemented thoroughly. The detailed and complete data sets from simultaneous investigations on TEs and bulk chemical and phase-mineral composition for numerous biomass varieties and their solid combustion products are also scarce. Additionally, many findings about the behaviour of various TEs during thermo-chemical conversion of biomass are based only on theoretical thermodynamic, equilibrium and stoichiometric calculating tools (modelling) of chemical databases. These indirect investigations may be unrealistic for actual predictions in a multicomponent system like that for biomass products and this subsidiary procedure cannot replace the most important and real approach, namely direct (input, output) studies of the systems themselves. Furthermore, sequential chemical fractionation is mostly used to distinguish the modes of occurrence of elements (speciation or chemical forms of combination for individual element) in biomass fuels and their products. However, this indirect procedure cannot be applied to identify the actual modes of occurrence of elements in a multicomponent system. Thus, limited systematic interpretations can be made from such investigations as pointed out in previous papers [1–6]. It is accepted that BA does not contain toxic metals like in the case of coal ash ([53–56] among others). However, certain results for BAs show enhanced concentrations of many elements including also potentially detrimental levels of toxic metals. For example, the reference investigations show that very high maximum contents of elements such as Ag (18 ppm), Al (28.3%), As (0.16%), Au (25 ppm), Ba (2.07%), Ca (59.6%), Cd (657 ppm), Cl (14.2%), Cr (0.20%), Cu (1.0%), Fe (25.4%), Hg (8.9 ppm), K (52.8%), Mg (9.8%), Mn (12.0%), Mo (121 ppm), Na (22.1%), Ni (0.11%), P (17.9%), Pb (5.0%), S (10.3%), Sb (362 ppm), Se (86 ppm), Si (44.1%), Sn (552 ppm), Th

MS OM PP R2 RH SEM SG SS TE TGA VM WS XRD %

mass spectrometry organic matter plum pits correlation coefficient rice husks scanning electron microscopy switchgrass sunflower shells trace element thermo-gravimetric analysis volatile matter walnut shells X-ray powder diffraction weight%

(112 ppm), Ti (16.5%), Tl (49 ppm), U (42 ppm), V (0.10%) and Zn (16.4%) were detected in some BAs, particularly filter fly ashes ([1,4] and references therein). Most of these concentrations are much greater than in coal ash [57,58] and many of them even have a unique resource recovery potential. Additionally, numerous TEs in biomass and BA tend to occur in much more mobile and hazardous compounds than in coal and coal ash ([1,3,4] and references therein). Finally, the common scientific approach used is to study the concentration and behaviour of individual elements in order to explain and evaluate different technological and environmental problems related to BAs. However, the actual reasons for such problems in a multicomponent system are most likely connected with modes of element occurrences, namely specific phases or minerals that contain such elements, similar to coal ash ([3–6] and references therein). Therefore, some of the major and still open questions are related to the limited knowledge concerning: (1) the identification, content and origin of modes of TE occurrence; (2) the correlation of TEs with major and minor elements and other important characteristics; and (3) the association of TEs with different organic and inorganic constituents (structural components, inorganic matter types, mineral classes, groups and specific species) of biomass and BA. Elucidation of the above topics is essential and can assist directly or indirectly in different technological and environmental challenges associated with biomass utilization. Unfortunately, systematic studies on TEs in biomass fuels and their BAs are only at an early stage and a lot of future work is needed, especially in: (1) the development and standardization of reliable approaches and methods for TEs determination; (2) combined chemical, phase and mineral characterization for identification, speciation and elucidation of modes of TE occurrence; (3) behaviour of TEs during biomass processing; and (4) fate of TEs after formation and collection, and during utilization and storage of the waste products generated from solid biofuels. There are thus ample opportunities for new research in this area. The above listed problems show that additional, systematic and detailed studies of TEs in biomass and BA based on proven, improved or new approaches and methods are required to reduce uncertainties. The major aims of the present study are: to supply additional results and to systematize the data; to describe some basic findings; and to clarify certain problems related to TEs. The present work includes own investigations of 8 biomass varieties and an attempt is undertaken to address the following objectives: (1) Characterization of the composition and properties of biomass fuels and their ashes.

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(2) Determination and elucidation of the contents for 60 major, minor and trace elements in BAs and their ash types. (3) Explanation of the correlations and clarification of the associations between TEs and different constituents and major and minor elements in biomass and BA. (4) Indication of some potential environmental challenges related to TEs during combustion of biomass fuels, as well as application and storage of their BAs. Some summarized data from peer-reviewed publications were also used for clarification.

2. Materials, methods and data used Eight biomass samples were collected and studied for determination and elucidation of TE concentrations and associations. These are the same samples that have been used earlier for clarification of the chemical and phase-mineral composition of biomass and BA, and behaviour of biomass during combustion [2–6]. They include beech wood chips (BC), corn cobs (CC), marine macroalgae (MM), weathered plum pits (PP), rice husks (RH), switchgrass (SG), sunflower shells (SS) and walnut shells (WS) belonging to different biomass groups and sub-groups specified by origin (biodiversity and source) [1]. The majority of investigations up to now have been concentrated on woody biomass and wheat straw as solid biofuels.

Most of the 8 samples are herbaceous and agricultural residues and algae (sea lettuces) and much less is known about them. The selection of these samples is also based on their variable: (1) proximate (Fig. 1a), ultimate (Fig. 1b) and structural (Fig. 1c) composition; and especially (2) inorganic chemical composition (Table 1 and Figs. 1d and 2). Another big advantage of these samples is that they belong to: (1) different organic structural types (‘‘CHL’’, ‘‘LCH’’ and ‘‘HCL’’ in Fig. 1c); (2) all inorganic chemical types (‘‘S’’, ‘‘C’’, ‘‘K’’, and ‘‘CK’’ types in Fig. 1d) and different sub-types (Fig. 1d); and (3) all inorganic acid tendencies (high, medium and low in Fig. 1d); that have been specified recently for biomass and BA [1–3]. Additionally, five of the samples fit in the low acid ‘‘K’’ and ‘‘CK’’ types and sub-types (Fig. 1d), which are among the most problematic biomass resources from technological and environmental points of view [1–6]. Finally, the composition of these 8 biomass varieties differs widely in comparison with coals (Table 1 and Figs. 1 and 2) and they are particularly suitable for comparative investigations. The biomass samples were investigated using methods such as light microscopy, powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), and differential-thermal (DTA), thermo-gravimetric (TGA) and chemical analyses, as well as some leaching, precipitation, ashing (500–1500 °C) and other procedures described in detail earlier [2,3,5,6]. The contents of 60 elements in biomass ashes produced at 500 °C for 2 h were determined by inductively coupled plasma – mass spectrometry (ICP–MS), laser ablation ICP–MS (LA–ICP–MS) and SEM equipped with an energy

Fig. 1. Positions of 8 biomass varieties in classification systems based on data from: (a) proximate; (b) ultimate; (c) structural; and (d) ash analyses, wt.%. Abbreviations: B, bituminous coal; BC, beech wood chips; CC, corn cobs; daf, dry, ash-free basis; db, dry basis; FC, fixed carbon; L, lignite; MM, marine macroalgae; P, peat; PP, plum pits; RH, rice husks; S, sub-bituminous coal; SG, switchgrass; SS, sunflower shells; VM, volatile matter; WS, walnut shells.

Table 1 Data for eight biomass samples (air-dried basis), wt.% (indicated otherwise). Rice husks

Switchgrass

Corn cobs

Plum pits (weathered)

Marine macroalgae

Beech wood chips

Walnut shells

Sunflower shells

Sample code Organic type Inorganic type Inorganic sub-type Source

RH CHL S S–HA Kovachevo, Bulgaria

SG CHL S S–MA Mead, Nebraska, USA

08.2007 Cleaned from sand and soil grains <0.5

2003 Two mixed harvests (50:50 vol.%) <0.5

08.2009 Cleaned from sand and shell grains <0.5

BC CHL C C–LA JRS Rosenberg, Germany 01.2009 Produced size is 1– 4 mm <0.5

WS LCH CK CK–LA Debnevo, Bulgaria 08.2008

SS CHL K K–LA Billa Sofia, Bulgaria 08.2008

<1

PP LCH CK CK–LA Debnevo, Bulgaria 08.2008 Weathered for 1 year <1

MM HCl K K–LA Tsarevo, Bulgaria

Collection time Note

CC CHL K K–LA Debnevo, Bulgaria 08.2008

<1

<0.5

62.4 19.1 18.5 100.0 49.3 43.6 6.1 0.8 0.08 0.12 100.00 43.8 31.6 24.6 100.0 6.6 1.5 6.66

80.4 14.5 5.1 100.0 49.6 43.4 6.1 0.7 0.11 0.08 99.99 48.7 38.4 12.9 100.0 8.0 7.8 5.20

81.1 16.8 2.1 100.0 47.8 45.6 5.9 0.5 0.01 0.23 100.04 48.1 37.2 14.7 100.0 7.0 0.8 5.32

81.4 17.9 0.7 100.0 49.9 42.4 6.7 0.9 0.08 0.01 99.99 23.7 22.0 54.3 100.0 5.4 0.3 5.57

47.4 24.4 28.2 100.0 41.7 44.3 6.0 2.1 2.51 3.34 99.95 28.5 65.5 6.0 100.0 15.1 15.1 6.75

81.6 17.1 1.3 100.0 47.2 46.6 6.1 0.1 0.01 0.01 100.02 45.2 32.7 22.1 100.0 1.1 1.4 5.15

60.5 38.6 0.9 100.0 49.8 42.4 6.2 1.4 0.09 0.15 100.04 28.1 26.6 45.3 100.0 2.7 0.9 5.28

76.1 20.9 3.0 100.0 50.4 42.9 5.5 1.1 0.03 0.10 100.03 56.5 28.0 15.5 100.0 2.6 2.9 5.12

3.9 8.1

23.3 9.8

45.1 11.3

9.4 8.9

42.8 8.4

20.2 12.1

41.1 10.5

29.8 12.9

89.86 1.40 4.16 0.60 1.04 0.49 0.41 1.31 0.23 0.02 0.21 0.27 100.00 10.85

62.90 7.69 18.77 2.44 0.89 4.20 0.42 2.17 0.11 0.04 0.23 0.14 100.00 1.81

36.67 2.12 47.57 4.82 0.81 1.97 1.50 3.44 0.28 0.06 0.69 0.07 100.00 0.65

12.01 43.95 16.29 10.17 2.57 4.18 1.75 7.72 0.79 0.16 0.27 0.14 100.00 0.21

5.60 22.06 7.56 0.97 0.98 7.69 0.69 28.49 14.07 0.03 11.84 0.02 100.00 0.27

5.48 62.58 18.00 2.49 0.49 7.49 0.36 0.77 0.11 0.03 0.09 2.11 100.00 0.07

2.84 38.11 49.33 2.37 0.52 3.80 0.40 2.03 0.12 0.05 0.38 0.05 100.00 0.04

1.45 15.57 49.84 6.54 0.12 11.87 1.32 11.39 0.22 0.01 1.60 0.07 100.00 0.03

Size reduction (mm) 1. Biomass VM (db) FC (db) A (db) Sum C (daf)a O (daf)a H (daf)a N (daf)a S (daf)a Cl (daf)a Sum Cel (daf)a Hem (daf)a Lig (daf)a Sum Ext (daf)a DWR pH of leachate (solid–liquid = 1:10 mass ratio) 2. Biomass ash (500 °C) DWR pH of leachate (solid–liquid = 1:50 mass ratio) SiO2 CaO K2O P2O5 Al2O3 MgO Fe2O3 SO3 Na2O TiO2 Cl2O MnO Sum DAI

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Characteristic

(continued on next page) 295

Abbreviations: A, ash yield; Cel, cellulose; daf, dry ash-free basis; DAI, detrital/authigenic index (Si + Al + Fe + Na + Ti oxides/Ca + Mg + Mn + K + P + S + Cl oxides); db, dry basis; DWR, dry water-soluble residue; Ext, extractives; FC, fixed carbon; Hem, hemicellulose; Lig, lignin; VM, volatile matter. a Based on reference data ([1,2] and references therein).

41.4 0 53.8 3.6 1.2 0 100.0 50.6 0.4 41.8 7.2 0 0 100.0 57.0 1.0 12.2 0 16.0 13.8 100.0 51.7 44.3 0 0 2.7 1.3 100.0 3. Biomass ash (500–1500 °C) Inorganic amorphous matter Silicates Carbonates, oxides and hydroxides Phosphates Sulphates Chlorides Sum

64.0 26.5 7.0 0 2.5 0 100.0

59.5 18.5 19.5 0 1.7 0.8 100.0

43.8 6.0 23.2 23.6 3.4 0 100.0

Walnut shells Beech wood chips Marine macroalgae Plum pits (weathered) Corn cobs Switchgrass Rice husks Characteristic

Table 1 (continued)

49.8 1.8 18.2 2.4 27.8 0 100.0

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Sunflower shells

296

dispersive X-ray analyser (EDX). The ICP–MS and LA–ICP–MS measurements were carried out, respectively, following: (1) Acid digestion of the samples by HCl, HNO3 and HF and subsequent measurement of the solutions (for B, Bi, Li and Mn) by Perkin Elmer ELAN DRC-e mass spectrometer. (2) Preparation of discs after fusion of the samples with Li2B4O7 at 1000 °C and subsequent measurement of the glass discs (for Ag, Al, As, Au, Ba, Be, Ca, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, Fe, Ga, Gd, Ge, Hf, Ho, K, La, Lu, Mg, Mo, Na, Nb, Nd, Ni, P, Pb, Pr, Rb, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Th, Ti, Tm, U, V, W, Y, Yb, Zn, Zr) by the above apparatus combined with New Wave UP193FX excimer laser ablation system. NIST SRM glasses 610 and 612 were used as external calibration standards. The contents of S and Cl in the above ashes were determined by SEM–EDX of carbon coated powder material. The elemental spectrum of an area has been taken for all finely ground samples under identical conditions (area of 160  250 lm, magnification of 370 and accelerating voltage of 15 kV) using internal standards. The above chemical and mineral data were subjected to the Pearson’s correlation test to calculate correlation coefficient values among 81 variables for these biomass and BA samples. Due to the huge amount of data obtained, preferably the most informative characteristics and statistically significant or strong relationships, namely positive and negative correlation coefficient values for TEs are commonly used and interpreted in the present work.

3. Results and discussion 3.1. General considerations about trace elements in biomass and biomass ash The composition of biomass and BA is highly variable because each biomass and BA variety has specific origin and formation conditions, which can cause enrichment or depletion of different elements [1–6]. Therefore, the element contents in biomass and BA are also variable and depend on many factors including: (1) biomass resource (type of biomass, plant species or part of plants, growing processes and conditions, age of the plants, fertilizer and pesticide doses used, harvesting time, collection technique, transport, storage, processing, others); (2) biomass combustion (fuel preparation, combustion technology and conditions, collection and cleaning equipments); and (3) transport and storage of BA. A clarification of certain terminology, definitions and specifications related to the element contents in biomass and BA is useful herein due to some misunderstandings and unclear interpretations in many references. For example, the elemental composition of biomass and BA may potentially include the entire periodic table and comprise: (1) major (>1%); (2) minor (1–0.1%); and (3) trace (<0.1%) elements; according to their contents (Table 2). Inappropriate terms such as ‘‘minor elements’’ or ‘‘microelements’’ instead ‘‘trace elements’’ are occasionally reported in the literature and even prescribed in some standards. It is also commonly accepted to use the term ‘‘heavy metals’’ instead ‘‘trace elements’’ during ecological investigations. It should be stated that the former term is not fully correct because some elements with environmental concerns can also be light metals or heavy non-metals and semi-metals (Tables 2 and 3). As a general trend (Table 2) most of the elements determined in biomass are TEs excluding C, Ca, Cl, H, K, Mg, N, Na, O, P and S

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297

Fig. 2. Areas of 86 biomass varieties plus algae and 38 solid fossil fuels in the chemical classification system of biomass ash, wt.%.

based on the Clarke contents (worldwide average concentrations) for angiospermous and gymnospermous plants worldwide [8] and world reference plant (worldwide averages of all parts of all plants) [23]. On the other hand, most of the elements detected in BA (Table 2) are also TEs excluding Al, Ca, Cl, Fe, K, Mg, Mn, Na, O, P, Rb, S, Si, Sr and Zn based on the Clarke contents for plant ashes [9] and world reference plant ash [23]. Nevertheless, there are many individual cases among biomass varieties and their ashes where the concentrations for certain major, minor and trace elements are interchanged. A comparison (Table 2) between the above mean element contents in biomass and BA and those in coal and coal ash, namely Clarke contents for coals and coal ashes worldwide [58] and US coals and coal ashes [57], shows: (1) Higher concentrations of Ag, B, Br, Ca, Cl, H, I, K, Mg, Mn, N, Na, O, P, Rb, Ru and Zn in biomass and lower contents of Al, As, Au, Ba, Be, Bi, C, Cd, Ce, Co, Cr, Cs, Cu, Dy, Er, Eu, F, Fe, Ga, Gd, Ge, Hf, Hg, Ho, In, Ir, La, Li, Lu, Mo, Nb, Nd, Ni, Os, Pb, Pd, Pr, Pt, Rh, S, Sb, Sc, Se, Si, Sm, Sn, Sr, Ta, Tb, Te, Th, Ti, Tl, Tm, U, V, W, Y, Yb and Zr in biomass in comparison with coal. (2) Higher concentrations of Ag, B, Br, Ca, Cl, Cr, Cu, Ga, Hg, I, K, Mg, Mn, Mo, Na, P, Rb, Sr, Te and Zn in BA and lower contents of Al, As, Au, Ba, Be, Bi, Cd, Ce, Co, Cs, Dy, Er, Eu, F, Fe, Gd, Ge, Hf, Ho, In, Ir, La, Li, Lu, Nb, Nd, Ni, Os, Pb, Pd, Pr, Pt, S, Sb, Sc, Se, Si, Sm, Sn, Ta, Tb, Th, Ti, Tl, Tm, U, V, W, Y, Yb and Zr in BA in comparison with coal ash. Hence, most TEs are commonly more enriched in coal and coal ash than in biomass and BA, respectively, and the reason for that is mostly associated with higher amount of inorganic matter with authigenic and especially detrital nature in coal than in biomass [1–3]. For example, it is well known that the bulk amount of ashforming constituents in biomass (based on ash yield) is generally much lower (0.1–46%, mean 6.8% on dry basis) in contrast to coal (6–52%, mean 21% on dry basis) [1]. However, the concentrations of some TEs in BA can be very high due to the enhanced enrichment factor of such elements in the combustion residue due to the high contents of organic matter in biomass. For instance, Ag, B, Br, Cr, Cu, Ga, Hg, I, Mo, Rb, Sr, Te, Zn and others in BA can exhibit much higher enrichment factors in comparison with coal ash

(Table 2). These observations have a primary importance for evaluating different technological and environmental problems related to the behaviour of TEs during biomass combustion and particularly co-firing of coal with biomass. The bulk chemical composition of biomass and BA is an important characteristic, but it is quite insufficient for a reliable explanation of elemental behaviour during biomass combustion and BA utilization. For that purpose, knowledge about the modes of element occurrence in biomass and BA is required. The elements in biomass and BA can occur in both organic matter (OM) and inorganic matter (IM) as each element has dominant associations and affinities to specific components, phases or minerals [2,3]. The occurrence of major elements and most of the minor elements in biomass and BA are relatively well known, in contrast to TEs [1–3]. Nevertheless, the data indicate that the modes of TE occurrence in biomass and BA [2,3] can be similar to coal and coal ash, namely: (1) Element-organic compounds and impurities in organic phases and minerals. TEs can be adsorbed in/on OM and covalently or ionically bound in OM such as organo-metallic complexes and certain specific functional groups. TEs may also occur as isomorphic substitution or in defect sites of the crystal lattices of organic minerals (oxalates), and as solid solutions in organic phases. (2) TEs as impurities in inorganic mineral matter. These elements may occur isomorphously or in defect sites of the crystal structures, as well as ion-exchanged and adsorbed elements in the mineral matrix or on mineral surfaces. (3) TEs as discrete inorganic phases and minerals. Some of them can be present even as sub-micrometre and nanometre inclusions in larger mineral matrices. (4) TEs as impurities in inorganic amorphous and semi-crystalline matter. This matter consists of amorphous and poorly crystallized mineraloids of cryptomere, metamict, metacolloid and gel species with amorphous character or imperfect crystal lattices. (5) TEs in fluid constituent of both OM and IM. They are present mostly as dissolved cations, anions or non-charged species in the moisture (mineralized water solution). Subsequently, these TEs commonly occur as impurities in sulphates, chlorides, nitrates, oxyhydroxides, carbonates and phosphates

Element

a

World reference plantc

0.095 15,000b 8.3a 0.0037a 52a 150a 1.6a 0.97a 5.2a 630,000b 4600b 0.22a 23a 180a 5.1a 16a 1.0a 16a 2.1a 0.93a 0.47a 88a 13,000b 5.8a 2.7a 2.2a 52,000b 1.2a 0.10a 0.54a 1.9a 0.031a 0.002a 1800b 11a 12a 0.20a 1100b 86a 2.2a 13,000b 800b 3.7a 12a 13a 160,000b <0.001b 230a 7.8a 0.0074a

0.2 80 0.1 0.001 40 40 0.001 0.01 4 445,000 10,000 0.05 0.5 2000 0.2 1.5 0.2 10 0.03 0.02 0.008 2 150 0.1 0.04 0.01 65,000 0.05 0.1 0.008 3 0.001 0.0001 19,000 0.2 0.2 0.003 2000 200 0.5 25,000 150 0.05 0.2 1.5 425,000 0.000015 2000 1 0.001

3.5a 0.035a 14a <0.001b

0.05 0.00005 50

Clarke for angiospermous plantsd 0.06 550 0.2 <0.00045 50 14 <0.1 0.06 15 454,000 18,000 <0.64 <34 2000 0.48 0.23 0.2 14

0.021 0.5 140 0.05

Clarke for gymnospermous plantse 0.07 65

63 63

450,000 6500 <0.24

0.2 0.16 15

130 <0.07

0.015 0.4

14,000 0.085 0.1

6300

3200 630 0.9 30,000 1200 0.3 <24 2.7 410,000

1300 330 0.13 32,000 340 0.3

2300 2.7

2900 1.8

1.9 440,000

Clarke for coal ashesf,g

World reference plant ashh

Clarke for plant ashesi

0.61 114,500g 47f 0.022f 335f 940f 9.4f 5.9f 32f

4.1 1633 2.0 0.02 816 816 0.02 0.2 82

1.0 14,000 0.3

35,100g 1.2f 130f 1440f 32f 100f 6.6f 92f 14f 5.5f 2.5f 605f 99,200g 33f 16f 15f

204,082 1.0 10 40,816 4.1 31 4.1 204 0.6 0.4 0.2 41 3061 2.0 0.8 0.2

30,000 0.01

8.3f 0.75f 4.0f 12.6f 0.16f 0.010f 13,700g 69f 66f 1.2f 8400g 490f 14f

1.0 2.0 0.2 61 0.02 0.002 38,7755 4.1 4.1 0.06 40,816 4082 10

0.05 0.001

6100g 20f 67f 76f

3061 1.0 4.1 31

20,000 0.5

<0.008g 1350f 47f 0.037f

0.0003 40,816 20 0.02

0.0005 70,000 10

20f 0.13f 79f <0.008g

1.0 0.001 1020

f

400 100 2.1 0.0005 150

10,000 15 250 2.0 200

10 10,000 50

50 0.05 0.000005 30,000 11 70,000 7500 20

50

0.0005

20

0.000005 100 0.000005

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

Ag Al As Au B Ba Be Bi Br C Ca Cd Ce Cl Co Cr Cs Cu Dy Er Eu F Fe Ga Gd Ge H Hf Hg Ho I In Ir K La Li Lu Mg Mn Mo N Na Nb Nd Ni O Os P Pb Pd Pa Pr Pt Rb Re

Clarke for coalsa,b

298

Table 2 Clarke contents (worldwide average element concentrations) of biomass, biomass ash, coal and coal ash, ppm on dry basis.

a b c d e f g h i

Clarke Clarke World Clarke Clarke Clarke Clarke World Clarke

<0.001b <0.001b 18,000b 0.92a 3.9a 1.3a 27,000b 2.0a 1.1a 110a 0.28a 0.32a <0.1b 3.3a 800a 0.63a 0.31a 2.4a 25a 1.1a 8.4a 1.0a 23a 36a

0.0001 0.0001 3000 0.1 0.02 0.02 1000 0.04 0.2 50 0.001 0.008 0.05 0.005 5 0.05 0.004 0.01 0.5 0.2 0.2 0.02 50 0.1

0.005 3400 0.06 0.008 0.2 200 0.0055 0.3 26

1100

0.24

<0.0015

1 0.0015 0.038 1.6 0.07 <0.6 <0.0015 160 0.64

<0.35 0.69 <0.24 26 0.24

for coals worldwide [58]. for US coals [57]. reference plant (worldwide average of all parts of all plants) [23]. for angiospermous plants worldwide [8]. for gymnospermous plants worldwide [8]. for coal ashes worldwide [58]. for US coal ashes recalculated from coal data for 13.1% ash yield [57]. reference plant ash (worldwide average of all parts of all plants) [23], recalculated from plant data for 4.9% ash yield [1]. for plant ashes [9].

<0.008g <0.008g 137,400g 6.3f 23f 8.8f 206,100g 13f 6.4f 740f 1.7f 2.1f <0.8g 21f 4650f 4.9f 2.0f 16f 155f 6.9f 51f 6.2f 140f 210f

0.002 0.002 61,224 2.0 0.4 0.4 20,408 0.8 4.1 1020 0.02 0.2 1.0 0.10 102 1.0 0.08 0.2 10 4.1 4.1 0.4 1020 2.0

0.00005 0.00005 50,000 0.05 0.5 150,000 5.0 30 0.005

0.5 1000 0.005 0.5 61 0.005 10 900

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Rh Ru S Sb Sc Se Si Sm Sn Sr Ta Tb Te Th Ti Tl Tm U V W Y Yb Zn Zr

299

300

Table 3 Contents of 60 elements in eight biomass ashes (500 °C/2 h), ppm (indicated otherwise). Switchgrass SG

Corn cobs CC

Plum pits PP

Marine macroalgae MM

Beech wood chips BC

Walnut shells WS

Sunflower shells SS

Mean

EDFa

1. Non-metal elements B 335b Cl (%) 0.14b P 1350b S (%) 13.74c Si (%) 20.61c

4101 0.15 2299 0.46 36.86

7106 0.15 8320 0.68 23.01

6619 0.40 14,880 0.97 12.11

11,311 0.10 19,571 1.36 2.47

8653 8.32 3668 9.84 2.26

10,257 0.05 7012 0.20 1.66

8908 0.20 6758 0.53 0.87

5900 0.96 21,001 3.35 0.50

7857 1.29 10,439 2.17 9.97

23.45 9.21 7.73 0.16 0.48

2. Lithophile elements Al (%) 11.45c Ba 940b Be 9.4b Ca (%) 3.51c Ce 130b Cs 6.6b Dy 14b Er 5.5b Eu 2.5b Gd 16b Hf 8.3b Ho 4.0b K (%) 1.37c La 69b Li 66b Lu 1.2b Mg (%) 0.84c Mo 14b Na (%) 0.61c Nb 20b Nd 67b Pr 20b Rb 79b Sm 13b Sr 740b Ta 1.7b Tb 2.1b Tm 2.0b W 6.9b Y 51b Yb 6.2b Zr 210b

0.48 101 33.6 0.88 2.93 0.54 0.71 0.80 0.46 1.35 0.63 0.20 3.02 1.30 2.0 0.20 0.26 4.35 0.15 0.54 1.27 0.38 35 1.42 47 0.26 0.17 0.25 1.16 0.62 1.10 5.9

0.37 386 31.7 4.30 4.55 0.84 1.05 0.53 0.48 1.30 0.63 0.17 12.15 2.20 14.8 0.20 1.98 5.43 0.07 0.80 2.11 0.58 54 1.55 241 0.25 0.21 0.21 1.11 1.42 1.37 18.2

0.30 47 23.4 1.07 3.31 0.87 0.60 0.41 0.26 0.97 0.37 0.15 27.79 1.53 30.2 0.11 0.84 5.71 0.15 0.88 1.17 0.31 233 1.04 72 0.20 0.10 0.11 0.75 17.51 0.64 9.0

0.60 136 13.0 13.82 6.61 0.40 0.50 0.27 0.16 0.44 0.71 0.09 5.93 3.16 36.7 0.05 1.11 2.69 0.26 1.34 3.17 0.67 19 0.71 326 0.09 0.07 0.05 0.57 2.72 0.40 23.6

0.45 43 8.7 13.61 3.15 0.26 0.21 0.17 0.15 0.34 0.20 0.06 5.40 1.13 12.9 0.09 4.00 1.60 9.00 0.14 1.37 0.31 25 0.55 1142 0.05 0.04 0.06 0.31 1.22 0.29 3.5

0.17 1786 20.9 28.96 3.52 4.98 0.49 0.32 0.21 0.62 0.44 0.10 9.64 2.41 37.3 0.08 2.92 2.06 0.05 0.45 1.79 0.32 453 0.57 324 0.15 0.09 0.09 0.53 3.18 0.79 6.2

0.18 113 27.4 17.82 1.52 0.44 0.76 0.41 0.38 1.22 0.77 0.16 26.69 0.74 19.9 0.15 1.50 5.55 0.06 0.43 0.94 0.20 64 1.29 465 0.21 0.19 0.16 0.99 32.22 1.34 3.7

0.05 46 25.1 8.18 0.78 0.51 0.36 0.47 0.26 0.77 0.40 0.09 30.29 0.39 14.2 0.09 5.26 8.95 0.12 0.29 0.61 0.12 278 0.83 448 0.13 0.10 0.09 0.70 0.29 0.70 2.4

0.33 332 23.0 11.08 3.30 1.11 0.59 0.42 0.30 0.88 0.52 0.13 15.11 1.61 21.0 0.12 2.23 4.54 1.23 0.61 1.55 0.36 145 1.00 383 0.17 0.12 0.13 0.77 7.40 0.83 9.1

0.03 0.35 2.45 3.16 0.03 0.17 0.04 0.08 0.12 0.06 0.06 0.03 11.03 0.02 0.32 0.10 2.65 0.32 2.02 0.03 0.02 0.02 1.84 0.08 0.52 0.10 0.06 0.07 0.11 0.15 0.13 0.04

3. Siderophile elements Co 32b Cr 100b Fe (%) 9.92c Mn 490b Ni 76b Sc 23b Ti 4650b V 155b

2.1 348 0.25 1825 107 4.1 128 10.8

1.3 49 0.23 872 37 4.8 196 10.5

2.6 750 0.74 404 286 2.6 211 13.1

3.4 160 0.54 466 64 1.4 432 11.8

3.5 66 0.42 167 42 1.3 174 32.8

4.2 24 0.16 10,595 36 2.1 91 8.9

1.4 167 0.18 202 60 3.5 157 7.2

4.2 1504 0.68 365 574 2.4 27 17.6

2.8 384 0.40 1862 151 2.8 177 14.1

0.09 3.84 0.04 3.80 1.99 0.12 0.04 0.09

9.4 0.49 9.8 66

6.4 0.50 5.1 139

2.6 2.48 3.5 636

5.6

4.5

5.0 36

4.9 98

7.5 0.49 5.9 477

5.2 0.49 4.4 331

6.3 0.74 5.9 232

0.13 0.13 4.92 2.52

4. Chalcophile As Bi Cd Cu

Coal ash Clarke

elements (metals and semi-metals) 47b 9.4 5.9b 0.01 b 1.2 8.2 b 92 76

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

Rice husks RH

Element Sample code

301

3.48 18.18

formed from the crystallization of this water solution during biomass and BA storage. Some TEs may also occur in gas and gas–liquid inclusions which are typical of apatite, quartz, zircon, glass and other minerals or phases [59–61]. Therefore, the knowledge about the abundance, origin and behaviour of modes of element occurrence (minerals and phases) in biomass and BA is a leading factor to fully understand the elemental behaviour during biomass combustion and BA utilization. Note: The mineralogical classification of elements [62] was used. The bold and bold-italic fonts show elements with enrichment/depletion factor (EDF) P 5.00 and EDF = 2.00–4.99, respectively. a Enricment/depletion factor (EDF) is defined as a ratio of the element content in biomass ash to the respective Clarke value in coal ash. b Clarke for coal ashes worldwide [58]. c Clarke for US coal ashes [57].

2.12 0.40 1.98 0.30 2.57 0.52 1.08 0.37 0.90 0.16 1.00 0.20 1.91 0.32 4.13 0.62 6. Noble elements Ag 0.61b Au 0.022b

3.37 0.73

0.02 0.01 0.42 0.22 0.16 0.11 0.30 0.16 0.27 0.11 0.23 0.43 0.98 0.29 0.42 0.20 0.42 0.28 0.57 0.18 5. Radioactive elements Th 21b U 16b

Ga Ge Pb Sb Se Sn Zn

33b 15b 47b 6.3b 8.8b 6.4b 140b

3.6 21.2 7.4 1.89 35.3 4.1 103

8.8 27.0 3.0 1.60 37.1 3.6 103

2.2 13.2 68.3 1.38 15.7 5.2 1232

4.1 6.4 32.4 0.63 10.5 5.3 779

8.9 6.1 2.7 0.59 9.8 0.9 52

37.7 11.1 38.7 0.71 16.4 2.1 213

3.1 19.8 11.7 1.54 28.9 6.6 110

1.6 12.6 1.5 1.22 19.0 2.1 338

8.8 14.7 20.7 1.20 21.6 3.7 366

0.27 0.98 0.44 0.19 2.45 0.58 2.61

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

3.2. Trace element contents in some biomass ashes The bulk phase-mineral and chemical composition of the 8 biomass samples studied and their ashes has been described earlier [2–6]. The contents of 60 elements in these BAs are provided in Table 3 and some specific issues related to the concentration trends, correlations, associations and potential modes of occurrence of TEs in biomass and especially BA are characterized in the present study. Most of the elements detected in the BAs studied are TEs (<0.1%) excluding Al, B, Ba, Ca, Cl, Cr, Fe, K, Mg, Mn, Na, O, P, S, Si, Sr and Zn (Table 3). For better differentiation in the present study, the major and minor elements in BA are limited to Al, Ca, Cl, Fe, K, Mg, Mn, Na, O, P, S and Si, while TEs are the other elements described throughout the text. It can be seen that the concentration of TEs in these BAs is highly variable due to the different origin and composition of biomass species (Tables 1 and 3, and Fig. 1). The TE contents in the BAs studied are normally close to those for world reference and Clarke plant ashes, excluding some much higher or lower concentrations of elements such as As, Au, B, Be, Bi, Cd, Cr, Ge, Li, Mo, Ni, Pb, Rb, Sc, Se, Ta, Zn and Zr in certain BAs (Tables 2 and 3). It is interesting to note that Ag, Au, B, Be, Ca, Cd, Cl, Cr, Cu, K, Mg, Mn, Na, Ni, P, Rb, Se and Zn in some of these BAs can show higher contents than the respective Clarke values for coal ashes (Table 3). This comparison is made using the enrichment/depletion factor (EDF), defined as a ratio of the element content in BA to the respective Clarke value in coal ash. For example, the decreasing order of elements with over Clarke concentrations based on the mean EDF values for 8 BAs (Table 3) is: B (23.5) > Au (18.2) > K (11.0) > Cl (9.2) > P (7.7) > Cd (4.9) > Cr@Mn (3.8) > Ag (3.5) > Ca (3.2) > Mg (2.7) > Na@Zn (2.6) > Be@Cu@Se (2.5) > Ni (2.0) > Rb (1.8). It can be seen that TEs among the above listed elements belong mostly to chalcophile, siderophile and lithophile groups and, to a lesser extent, noble and non-metal groups according to the mineralogical classification of elements [62]. Despite the limited BAs studied such a preliminary geochemical comparison indicates elements that may have some environmental impacts and/or industrial potential for BA. For instance, these initial data (Table 3) point out that the high contents of some regulated toxic and potentially toxic TEs [63,64] in BA such as Ag, Be, Cd, Cr, Cu, Ni, Se and Zn might cause some environmental pollution of the air, water, soil and plants during BA utilization. On the other hand, the concentrations and modes of occurrence of some TEs such as Ag, Au, B, Cr and others may have a resource recovery potential for certain BAs. Hence, the enriched and/or hazardous TEs in BA should be subject to more detailed environmental and economical assessments. 3.3. Trace element contents in biomass ash types The mean contents of major, minor and TEs for four BA types based on 8 BAs are provided (Fig. 1d and Table 4) and compared. It can be seen that the high acid rice husk and medium acid switchgrass BAs from ‘‘S’’ type show enrichments in lithophile (Al, Be, Ta, W and rare earth Dy, Er, Eu, Gd, Ho, Lu, Pr, Sm, Tb,

302

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

Table 4 Mean contents of 60 elements in biomass ash types based on eight biomass ashes (500 °C/2 h), ppm (indicated otherwise). Element

S type (RH, SG)

C type (BC)

K type (CC, MM, SS)

CK type (PP, WS)

1. Non-metal elements B Cl (%) P S (%) Si (%)

5604 0.15 5310 0.57 29.94

10,257 0.05 7012 0.20 1.66

7057 3.23 13,183 4.72 4.96

10,110 0.15 13,165 0.95 1.67

2. Lithophile elements Al (%) Ba Be Ca (%) Ce Cs Dy Er Eu Gd Hf Ho K (%) La Li Lu Mg (%) Mo Na (%) Nb Nd Pr Rb Sm Sr Ta Tb Tm W Y Yb Zr

0.43 244 32.7 2.59 3.74 0.69 0.88 0.67 0.47 1.33 0.63 0.19 7.59 1.75 8.4 0.20 1.12 4.89 0.11 0.67 1.69 0.48 45 1.49 144 0.26 0.19 0.23 1.14 1.02 1.24 12.1

0.17 1786 20.9 28.96 3.52 4.98 0.49 0.32 0.21 0.62 0.44 0.10 9.64 2.41 37.3 0.08 2.92 2.06 0.05 0.45 1.79 0.32 453 0.57 324 0.15 0.09 0.09 0.53 3.18 0.79 6.2

0.27 45 19.1 7.62 2.41 0.55 0.39 0.35 0.22 0.69 0.32 0.10 21.16 1.02 19.10 0.10 3.37 5.42 3.09 0.44 1.05 0.25 179 0.81 554 0.13 0.08 0.09 0.59 6.34 0.54 5.0

0.39 125 20.2 15.82 4.07 0.42 0.63 0.34 0.27 0.83 0.74 0.13 16.31 1.95 28.3 0.10 1.31 4.12 0.16 0.89 2.06 0.44 42 1.00 396 0.15 0.13 0.11 0.78 17.47 0.87 13.7

3. Siderophile elements Co Cr Fe (%) Mn Ni Sc Ti V

1.7 199 0.24 1349 72 4.5 162 10.7

4.2 24 0.16 10,595 36 2.1 91 8.9

3.4 773 0.61 312 301 2.1 137 21.2

2.4 164 0.36 334 62 2.5 295 9.5

4.5 4.9 98 37.7 11.1 38.7 0.71 16.4 2.1 213

5.7 0.33 4.8 169 4.2 10.6 24.2 1.06 14.8 2.7 541

5.1 1.49 4.7 557 3.6 13.1 22.1 1.09 19.7 6.0 445

4. Chalcophile elements (metals and semi-metals) As 9.4 Bi 0.25 Cd 9.0 Cu 71 Ga 6.2 Ge 24.1 Pb 5.2 Sb 1.75 Se 36.2 Sn 3.9 Zn 103 5. Radioactive elements Th U

0.50 0.23

0.27 0.11

0.27 0.25

0.64 0.23

6. Noble elements Ag Au Samples

3.75 0.68 2

1.08 0.37 1

1.60 0.26 3

1.79 0.36 2

Tm and Yb), chalcophile (As, Cd, Ge, Sb, Se) and noble elements (Ag, Au), and some non-metal (Si) and siderophile (Sc) elements. The phase-mineral composition of these two ‘‘S’’ type BAs comprise:

inorganic amorphous matter (IAM) > silicates (cristobalite, quartz, leucite, tridymite, kalsilite) > carbonates (calcite) > sulphates (anhydrite, arcanite) > chlorides (sylvite) [6]. It should be noted

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

that IAM is represented mainly by glass and, to a lesser extent, non-glass amorphous material in BAs produced at 500–1500 °C [5,6]. Hence, the above element enrichments associate mostly with these phases or minerals. The low acid beech wood chip BA from ‘‘C’’ type reveals enrichments in lithophile (La and alkaline-earth and alkaline Ba, Ca, Cs, Li, Mg and Rb) and chalcophile (Ga, Pb) elements, and some non-metal (B) and siderophile (Mn) elements. The phase-mineral composition of this ‘‘C’’ type BA comprises: IAM > carbonates (calcite) > oxyhydroxides (portlandite, periclase, lime) > phosphates (whitlockite, apatite) > silicates (quartz) [6]. Therefore, the above element enrichments associate mainly with these phases or minerals. The low acid corn cob, marine macroalgae and sunflower shell BAs from ‘‘K’’ type show enrichments in lithophile (Mo and alkaline-earth and alkaline K, Mg, Na, Rb and Sr), siderophile (Co, Cr, Fe, Ni, V) and non-metal (Cl, P, S) elements, and some chalcophile (Zn) and radioactive (U) elements. The phase-mineral composition of these three ‘‘K’’ type BAs comprises: IAM > sulphates (arcanite, anhydrite) > carbonates (kalicinite, calcite, fairchildite) > silicates (leucite, quartz, kalsilite) > oxyhydroxides (periclase, portlandite, lime) > chlorides (halite, sylvite) > phosphates (apatite) [6]. Hence, the above element enrichments associate mostly with these phases or minerals. Finally, the low acid plum pit and walnut shell BAs from ‘‘CK’’ type reveal enrichments in lithophile (Ca, Ce, Hf, K, Nb, Nd, Y, Zr) and chalcophile (Bi, Cu, Sn, Zn) elements, and some non-metal (B, P), siderophile (Ti) and radioactive (Th) elements. The phasemineral composition of these two ‘‘CK’’ type BAs comprises: IAM > carbonates (calcite, fairchildite, butschliite, kalicinite) > phosphates (apatite) > oxyhydroxides (portlandite, periclase, lime) > silicates (quartz, plagioclases) > sulphates (anhydrite) [6]. Therefore, the above element enrichments associate mainly with these phases or minerals. The present results show that there is some preferable association of lithophile (‘‘S’’, ‘‘C’’ and ‘‘CK’’ types), chalcophile (‘‘S’’ and ‘‘CK’’ types), siderophile (‘‘K’’ type), non-metal (‘‘K’’ and ‘‘CK’’ types), noble (‘‘S’’ type) and radioactive (‘‘K’’ and ‘‘CK’’ types) TEs to different BA types depending on the concentration of their bearing phases in such BA types. These data also indicate that BAs from ‘‘K’’ and ‘‘CK’’ types and, to some extent, ‘‘S’’ type can show enhanced environmental problems and/or potential recovery benefits for TEs in comparison with ‘‘C’’ type. 3.4. Trace element associations in biomass The statistically significant and insignificant correlation coefficient values for the common chemical characteristics of 8 biomass varieties and their ashes are listed in Table 5. The compositional similarities between different solid biomass fuels are not common and their identification requires some special attention. Such correlation data provide initial and valuable information for understanding some fundamental trends, relationships and associations in biomass and BA because their occurrence is statistically confirmed. For instance, the positive correlations of some TEs with certain components or major and minor elements indicate relationships and specify associations of such TEs with phases containing these components or major and minor elements in biomass or BA. However, such correlation results should be used with caution because they are not exclusive and future use of a larger number of biomass samples or different set of biomass varieties would very likely lead to some changes in the system (see below). Furthermore, it should be stated that the significant correlations among elements and phases or minerals are a result of both direct and indirect genetic associations in biomass, similar to coal [65,66]. The direct genetic associations comprise: (1) mineral parageneses,

303

namely simultaneous biogenic and detrital (pre-syngenetic) mineral influx or authigenic mineral formations; and (2) mineral generations such as subsequent authigenic (syngenetic, epigenetic) and technogenic (post-epigenetic) formations of minerals at different stages during biomass history and processing [1,2,5]. On the other hand, the indirect genetic associations include only the coexistence of mineral assemblages in this complex biomass system. Similar considerations are also applicable to the original (primary) and newly formed (secondary and tertiary) phases identified in BA [3,5,6]. Hence, additional studies and subsidiary literature data about the modes of occurrence of elements, phases or minerals in biomass and BA should always be used together with such correlation tests for better explanation. For example, a strong positive correlation between an element and phase does not always imply that this phase is the dominant form of that element in fuel or fuel ash [65,66]. Finally, some statistically insignificant correlations could be important, while other significant correlations could not be explained by the present knowledge of biomass and BA, or alternatively, they could simply be artefacts. The present data reveal some important associations for 12 major and minor elements in 8 BAs according to the positive and negative correlation values among them (Table 5). For instance, three important elemental associations were found in this set of samples and they include: (1) SiAAlATi; (2) CaAMn; and (3) KAPAMgAFeASAClANa. These associations are similar to those that have been found earlier for BAs generated from 78 natural biomass varieties [1]. The only exclusions are Mg, Fe and Na that tend to show stronger positive correlations with the most mobile elements in the system such as Cl, S, K and P. The reason for that seems to be mainly the algae biomass in this suite of samples which is highly enriched in Cl, S, Na and Mg. Nevertheless, the above listed associations are important in terms of the concentrations and modes of occurrence of TEs in biomass and BA because it is well known that TEs commonly follow the behaviour of their geochemical analogues among the major and minor elements during fuel combustion ([61] and see also below). The present study shows that most TEs together with C, Ca, Fe, H, K, Mn, P and Ti (hence their bearing phases) correlate positively with OM of biomass based on ash yield data (Table 5). In contrast, only TEs such as Ag, As, Cd, Co, Er, Lu, Sr, Tm, U and V plus Al-, Cl-, Mg-, N-, Na-, O-, S- and Si-containing phases associate preferably with IM of biomass (Table 5). The content of water-soluble fraction in biomass varieties is between 0.3% and 15.1% (Table 1) and these values are much higher than in coals (0.2–8.4%) [67]. Therefore, the water-soluble components play a significant role for biomass. The summarized reference data reveal that the water-soluble elements leached from different biomass varieties include Al, As, Ba, Ca, Cl, Co, Cu, Fe, K, Mg, Mn, Mo, N, Na, P, Pb, S, Si, Sr, Ti, V and Zn [6]. The present study (Table 5) shows that most TEs are prone to correlate positively with the water-insoluble fraction in biomass, while only As, Cd, Co, Ga, Lu, Pr, Sr, U and V plus Al-, Cl-, Mg-, N-, Na- O-and S-containing phases tend to associate with the water-soluble fraction in biomass. The latter fraction for the biomass samples studied contains chlorides, sulphates, oxalates and nitrates plus some carbonates and amorphous material with both inorganic and organic character [2]. Hence, correlation is confirmed for the mobile occurrence of different major and minor elements, and some trace elements (Al, As, Cl, Co, Mg, N, Na, S, Sr and V) in biomass. Most TEs such as Ag, As, Au, Ba, Be, Cd, Co, Cr, Cs, Ga, Ge, Mo, Ni, Pb, Rb, Sb, Sc, Se, Ta, W, Zn and rare earth elements (Dy, Er, Eu, Gd, Ho, Lu, Sm, Tb, Tm and Yb, excluding only the light La, Ce, Pr and Nd) together with C-, Fe-, K-, Mg-, Mn-, O-, P- and Si-bearing phases correlate positively with cellulose (Table 5). In contrast, hemicellulose is the structural component with the fewest TE associations, excluding As, Cd, Co, Ga, Sr, U and V plus Al-, Cl-, Mg-, N-,

304

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Table 5 Significant positive (+) and negative (-) correlation coefficient values (R2) at 95% confidence levels (bold font)a and insignificant (normal font) R2 values between different characteristics and element contents for 8 biomass samples and their ashes. Characteristic 1. Biomass Volatile matter (VM)

Correlation coefficient value with (+) Nb(0.64) P(0.63) Li(0.61) C(0.60) La(0.55) Zn(0.55) Zr(0.55)Pb(0.54) Rb(0.50) Cel(0.46) Bi(0.42) Cs(0.40) Nd(0.40) Phos(0.39) Ba(0.38) Ce(0.38) Pr(0.35) Dy(0.32) Mn(0.32) Th(0.31) Mo(0.26) Fe(0.25) Ni(0.25) Ga(0.23) Hf(0.23) K(0.23) Be(0.22) Cr(0.22) Cu(0.21) O(0.21) Sn(0.21) Ta(0.21) Ti(0.20) B(0.16) Lig(0.16) Co(0.15) W(0.12) Au(0.10) IAM(0.10) Ge(0.09) Ho(0.09) COH(0.08) Er(0.08) H(0.08) Sc(0.08) Tb(0.08) Yb(0.07) Gd(0.06) Sil(0.05) Sm(0.03) Eu(0.00) ( ) N( 0.84) A( 0.81) Chl( 0.77) ClB( 0.76) SB( 0.76) Cl( 0.74) Na( 0.74) Sr( 0.68) S( 0.67) FC( 0.61) Hem( 0.61) V( 0.60) DWRB( 0.59) Ext( 0.58) U( 0.47) DWRA( 0.29) As( 0.26) Sul( 0.24) Lu( 0.19) Al( 0.17) Mg( 0.14) Y( 0.13) Tm( 0.09) Ag( 0.07) Cd( 0.07) Si( 0.02) Ca( 0.01) Sb( 0.01) Se( 0.01)

Fixed carbon (FC)

(+) Y(0.74) COH(0.62) N(0.59) DWRA(0.45) Sr(0.40) Cu(0.39) Lig(0.37) Sn(0.35) K(0.34) Ca(0.30) Hf(0.26) Yb(0.25) ClB(0.19) Tb(0.19) SB(0.18) Cl(0.17) Na(0.16) B(0.15) Chl(0.15) S(0.15) Gd(0.12) Sb(0.11) Mo(0.10) As(0.09) Sm(0.08) Se(0.07) Eu(0.06) Lu(0.06) W(0.06) Mg(0.05) Sul(0.05) Ge(0.04) A(0.03) H(0.03) Ho(0.02) Tm(0.01) V(0.01) Ag(0.00) Sc(0.00) ( ) VM( 0.61) IAM( 0.60) La( 0.56) Ce( 0.53) Pr( 0.51) Cel( 0.50) Zr( 0.48) Nd( 0.44) Nb( 0.42) O( 0.42) Sil( 0.42) Si( 0.38) Zn( 0.34) Pb( 0.30) Th( 0.30) Al( 0.29) Co( 0.29) Cs( 0.28) Ba( 0.27) Fe( 0.27) Mn( 0.27) Rb( 0.26) Ga( 0.25) P( 0.24) Li( 0.18) Cd( 0.16) Er( 0.15) Ti( 0.15) Bi( 0.12) Ext( 0.10) Ni( 0.10) Phos( 0.10) Cr( 0.08) Dy( 0.06) U( 0.06) Ta( 0.05) C( 0.03) Hem( 0.03) Au( 0.02) Be( 0.02) DWRB( 0.01)

Ash yield (A)

(+) Chl(0.86) ClB(0.82) Ext(0.82) Na(0.82) SB(0.82) Cl(0.81) Hem(0.80) V(0.76) DWRB(0.75) S(0.74) U(0.64) N(0.63) Sr(0.57) Al(0.42) IAM(0.32) Si(0.31) As(0.27) Sul(0.27) Sil(0.25) Cd(0.21) Lu(0.19) Mg(0.14) Tm(0.11) Ag(0.08) DWRA(0.04) O(0.04) FC(0.03) Co(0.02) Er(0.02) ( ) VM( 0.81) C( 0.73) Li( 0.64) P( 0.61) COH( 0.56) Cu( 0.55) K( 0.55) Sn( 0.53) Nb( 0.50) Lig( 0.49) Hf( 0.48) Pb( 0.46) Bi( 0.45) Zn( 0.45) Rb( 0.44) Phos( 0.41) Mo( 0.40) Y( 0.38) Dy( 0.35) Zr( 0.34) B( 0.31) Cs( 0.30) Ba( 0.28) La( 0.28) Yb( 0.28) Be( 0.26) Tb( 0.24) Ni( 0.23) Ta( 0.23) Ca( 0.22) Cr( 0.22) Cel( 0.21) Mn( 0.20) W( 0.20) Nd( 0.18) Th( 0.17) Gd( 0.16) Ge( 0.14) Ti( 0.14) H( 0.13) Ho( 0.13) Au( 0.12) Fe( 0.12) Ga( 0.11) Sc( 0.10) Sm( 0.10) Ce( 0.09) Sb( 0.07) Pr( 0.06) Eu( 0.04) Se( 0.04)

C

(+) Hf(0.76) W(0.67) Be(0.66) Mo(0.63) Sn(0.60) VM(0.60) Tb(0.59) Dy(0.57) Gd(0.56) Er(0.56) Ta(0.56) Yb(0.55) Sb(0.54) Sm(0.54) Cu(0.52) Lig(0.52) Eu(0.51) Ho(0.51) Ge(0.50) Sc(0.50) Se(0.50) Ag(0.49) Au(0.48) P(0.47) Nb(0.46) Bi(0.40) Tm(0.40) K(0.37) Cr(0.34) Th(0.33) Ni(0.32) Zr(0.32) Cel(0.30) Lu(0.27) Phos(0.27) Sil(0.26) Si(0.21) Cd(0.19) Y(0.18) Zn(0.17) As(0.15) Pr(0.13) COH(0.12) H(0.09) La(0.08) Nd(0.08) Ti(0.06) Li(0.05) Rb(0.04) Fe(0.02) Pb(0.01) ( ) Chl( 0.93) ClB( 0.92) Hem( 0.92) Na( 0.92) SB( 0.91) Cl( 0.90) S( 0.81) V( 0.81) DWRB( 0.77) Sr( 0.75) Ext( 0.74) A( 0.73) U( 0.66) N( 0.50) O( 0.47) DWRA( 0.44) IAM( 0.35) Mg( 0.31) Co( 0.30) Ga( 0.27) Ca( 0.22) Al( 0.21) B( 0.19) Sul( 0.15) Mn( 0.10) Cs( 0.09) Ba( 0.08) FC( 0.03) Ce( 0.02)

O

(+) Cs(0.75) Ga(0.71) Mn(0.71) Rb(0.70) Ba(0.67) Pb(0.59) IAM(0.43) Li(0.37) Co(0.36) Cel(0.34) Hem(0.34) Ca(0.27) VM(0.21) Zn(0.21) La(0.19) DWRA(0.18) Chl(0.13) ClB(0.11) Na(0.10) Cl(0.09) COH(0.07) SB(0.07) B(0.06) V(0.06) A(0.04) DWRB(0.04) Mg(0.04) Ce(0.01) Ext(0.00) ( ) Cu( 0.65) Hf( 0.59) Lig( 0.51) Bi( 0.50) N( 0.49) C( 0.47) FC( 0.42) Sn( 0.39) Mo( 0.37) Sm( 0.36) W( 0.35) Tb( 0.33) Th( 0.33) Ag( 0.30) Ti( 0.29) Phos( 0.28) Se( 0.28) Eu( 0.27) P( 0.26) Sul( 0.26) Sb( 0.25) Zr( 0.25) H( 0.23) Yb( 0.23) Ge( 0.22) Sc( 0.21) Al( 0.20) Gd( 0.20) Dy( 0.20) Lu( 0.19) Er( 0.18) Nb( 0.18) Tm( 0.17) Pr( 0.16) Au( 0.15) U( 0.15) Cr( 0.14) Be( 0.13) Ho( 0.12) Y( 0.12) As( 0.10) Nd( 0.10) Ni( 0.10) Cd( 0.09) K( 0.09) Sr( 0.09) Fe( 0.04) Sil( 0.03) S( 0.02) Ta( 0.02) Si( 0.01)

H

(+) Ti(0.85) Nd(0.84) Th(0.82) Ce(0.80) Pr(0.77) La(0.76) Lig(0.74) Phos(0.74) Al(0.72) Zr(0.71) Nb(0.68) Bi(0.66) B(0.64) Hf(0.60) Sn(0.49) Cu(0.47) Li(0.42) U(0.34) Ca(0.26) Dy(0.23) COH(0.18) Pb(0.18) Zn(0.10) C(0.09) Y(0.09) Ba(0.08) Ga(0.08) VM(0.08) Ho(0.05) Mn(0.04) FC(0.03) Sil(0.03) Ext(0.02) Si(0.02) Tb(0.01) Cs(0.00) ( ) Cel( 0.77) Ni( 0.75) Cr( 0.73) Sul( 0.66) Mo( 0.64) Mg( 0.62) K( 0.57) Rb( 0.46) DWRA( 0.44) IAM( 0.37) Fe( 0.33) V( 0.31) Be( 0.27) Sb( 0.26) As( 0.25) Hem( 0.25) S( 0.25) Co( 0.24) O( 0.23) Er( 0.21) DWRB( 0.20) Cl( 0.17) Ag( 0.14) Gd( 0.14) A( 0.13) ClB( 0.12) Ge( 0.12) Lu( 0.12) Sc( 0.12) Chl( 0.10) Eu( 0.10) Sr( 0.10) P( 0.09) Na( 0.08) Se( 0.08) Ta( 0.08) N( 0.07) SB( 0.07) Cd( 0.06) Tm( 0.06) Yb( 0.04) Au( 0.02) Sm( 0.01) W( 0.01)

N

(+) Sr(0.83) S(0.81) Cl(0.79) ClB(0.78) SB(0.78) Na(0.77) Chl(0.75) V(0.73) DWRB(0.66) A(0.63) Ext(0.63) U(0.63) FC(0.59) Hem(0.56) Sul(0.51) DWRA(0.42) Mg(0.36) Al(0.17) Cu(0.11) Y(0.10) Fe(0.08) Ti(0.03) B(0.02) As(0.00) ( ) VM( 0.84) Cs( 0.65) Mn( 0.62) Ba( 0.61) Rb( 0.61) Pb( 0.58) Cel( 0.52) C( 0.50) La( 0.50) O( 0.49) Ta( 0.48) Ga( 0.47) Li( 0.47) Nb( 0.45) Be( 0.43) Dy( 0.40) Zn( 0.37) Ho( 0.36) Er( 0.34) Au( 0.33) Sil( 0.30) Zr( 0.30) W( 0.28) Gd( 0.27) Si( 0.27) Ce( 0.26) Sc( 0.26) Yb( 0.26) Ge( 0.25) Nd( 0.25) Pr( 0.24) Hf( 0.23) Tb( 0.22) Th( 0.21) Tm( 0.21) Eu( 0.20) Se( 0.20) P( 0.19) Sn( 0.19) Sb( 0.18) Ag( 0.16) Phos( 0.16) Sm( 0.15) Cd( 0.14) IAM( 0.13) COH( 0.08) Lig( 0.08) Mo( 0.08) H( 0.07) Lu( 0.07) Bi( 0.05) Ca( 0.05) Co( 0.05) K( 0.04) Ni( 0.03) Cr( 0.02)

S (SB)

(+) ClB(1.00) Na(1.00) Chl(0.99) Cl(0.99) S(0.95) V(0.92) Hem(0.91) DWRB(0.89) Sr(0.89) Ext(0.84) A(0.82) U(0.80) N(0.78) DWRA(0.40) Mg(0.40) Sul(0.36) Al(0.29) IAM(0.25) Co(0.20) FC(0.18) B(0.13) Ca(0.10) O(0.07) Fe(0.01) Ti(0.01) ( ) C( 0.91) VM( 0.76) Be( 0.66) Hf( 0.63) Ta( 0.62) W( 0.59) Sn( 0.58) Dy( 0.56) Ho( 0.54) Gd( 0.53) Er( 0.52) Tb( 0.51) Yb( 0.51) Mo( 0.49) Nb( 0.48) Sb( 0.48) Au( 0.46) Sc( 0.45) Ge( 0.44) Eu( 0.43) Lig( 0.43) Sm( 0.43) Cel( 0.42) Se( 0.42) P( 0.40) Ag( 0.39) K( 0.36) Cu( 0.35) Tm( 0.35) Rb( 0.34) Pb( 0.33) Zn( 0.32) Li( 0.28) Th( 0.28) Bi( 0.27) Cr( 0.27) Sil( 0.27) Zr( 0.27) Ni( 0.25) Cs( 0.24) COH( 0.23) Phos( 0.23) Si( 0.22) Ba( 0.21) Mn( 0.21) Y( 0.21) La( 0.20) Lu( 0.20) Cd( 0.14) As( 0.10) Pr( 0.09) Nd( 0.08) H( 0.07) Ce( 0.02) Ga( 0.01)

Cl (ClB)

(+) Chl(1.00) Na(1.00) SB(1.00) Cl(0.99) S(0.95) V(0.93) Hem(0.92) DWRB(0.88) Sr(0.88) Ext(0.84) A(0.82) N(0.78) U(0.78) DWRA(0.45) Mg(0.40) Sul(0.37) IAM(0.27) Al(0.26) Co(0.20) FC(0.19) O(0.11) B(0.10) Ca(0.07) Fe(0.06) ( ) C( 0.92) VM( 0.76) Hf( 0.67) Be( 0.65) Ta( 0.60) W( 0.59) Dy( 0.57) Sn( 0.57) Ho( 0.53) Gd( 0.52) Er( 0.52) Tb( 0.52) Yb( 0.52) Nb( 0.50) Au( 0.47) Lig( 0.46) Mo( 0.46) Sb( 0.46) Ge( 0.45) Sc( 0.45) Eu( 0.43) Se( 0.43) Sm( 0.43) Ag( 0.40) Cel( 0.39) P( 0.39) Cu( 0.37) Tm( 0.36) Zr( 0.32) K( 0.31) Rb( 0.31) Th( 0.31) Bi( 0.30) Pb( 0.29) Li( 0.28) Zn( 0.28) Phos( 0.27) Sil( 0.26) La( 0.25) Cs( 0.24) Ba( 0.23) COH( 0.23) Cr( 0.23) Mn( 0.22) Si( 0.22) Ni( 0.21) Lu( 0.20) Y( 0.18) Cd( 0.15) Pr( 0.14) Nd( 0.13) H( 0.12) As( 0.09) Ce( 0.06) Ga( 0.03) Ti( 0.02)

Cellulose (Cel)

(+) Mo(0.64) Ni(0.64) Cr(0.62) Be(0.58) Rb(0.58) Er(0.51) IAM(0.50) VM(0.46) Sb(0.43) Ta(0.43) K(0.40) Sc(0.40) Ge(0.39) Gd(0.38) Ag(0.37) Au(0.35) Eu(0.35) O(0.34) As(0.33) Se(0.33) Sil(0.33) Cd(0.32) Si(0.32) Tm(0.31) W(0.31) C(0.30) Lu(0.29) Mg(0.28) Sul(0.28) Yb(0.27) Ho(0.26) Cs(0.25) Sm(0.25) Tb(0.24) Fe(0.22) Mn(0.21) Ba(0.19) Dy(0.19) P(0.19) Co(0.12) Ga(0.10)

305

S.V. Vassilev et al. / Fuel 129 (2014) 292–313 Table 5 (continued) Characteristic

Correlation coefficient value with Zn(0.07) Pb(0.04) ( ) H( 0.77) Ti( 0.66) B( 0.65) Lig( 0.61) Al( 0.57) U( 0.55) Phos( 0.54) N( 0.52) FC( 0.50) Cu( 0.49) Bi( 0.46) Nd( 0.46) Sr( 0.46) Th( 0.46) Ce( 0.42) SB ( 0.42) Na( 0.41) ClB( 0.39) Chl( 0.38) Ca( 0.36) Cl( 0.34) Pr( 0.34) COH( 0.32) Sn( 0.31) Ext( 0.30) La( 0.29) Y( 0.29) Hf( 0.28) S( 0.28) Zr( 0.26) Li( 0.22) Nb( 0.22) A( 0.21) V( 0.17) DWRB( 0.15) Hem( 0.12) DWRA( 0.02)

Hemicellulose (Hem)

(+) Chl(0.92) ClB(0.92) DWRB(0.92) Cl(0.91) Na(0.91) SB(0.91) Ext(0.86) V(0.84) S(0.83) A(0.80) U(0.72) Sr(0.71) IAM(0.59) N(0.56) DWRA(0.48) Mg(0.34) O(0.34) Sul(0.25) Al(0.19) As(0.13) Cd(0.10) Ga(0.09) Co(0.08) Fe(0.00) Lu(0.00) ( ) C( 0.92) Hf( 0.74) Lig( 0.71) Cu( 0.66) Sn( 0.63) VM( 0.61) Nb( 0.51) Bi( 0.50) Phos( 0.50) P( 0.49) Be( 0.44) W( 0.44) Th( 0.43) Mo( 0.41) Er( 0.37) Ta( 0.37) Dy( 0.36) Tb( 0.36) Yb( 0.36) COH( 0.35) Ho( 0.34) Gd( 0.33) Li( 0.31) Zr( 0.30) K( 0.29) Sb( 0.29) Sm( 0.26) Zn( 0.26) Au( 0.25) Cr( 0.25) H( 0.25) Se( 0.25) Ag( 0.24) Eu( 0.24) Ge( 0.23) Ni( 0.23) Sc( 0.23) Y( 0.23) La( 0.21) Pb( 0.20) Nd( 0.18) Rb( 0.18) Tm( 0.17) Ti( 0.15) Cel( 0.12) Mn( 0.11) Pr( 0.11) Ba( 0.10) Cs( 0.10) Sil( 0.07) B( 0.06) Ce( 0.06) Ca( 0.05) FC( 0.03) Si ( 0.01)

Lignin (Lig)

(+) Cu(0.87) Hf(0.79) Phos(0.78) H(0.74) Bi(0.72) Sn(0.72) Th(0.66) Ti(0.59) Nb(0.56) C(0.52) B(0.51) COH(0.51) Nd(0.47) Zr(0.42) Li(0.40) Y(0.39) FC(0.37) La(0.37) Ce(0.34) Pr(0.33) Ca(0.30) Al(0.25) P(0.25) Dy(0.16) VM(0.16) Zn(0.16) Pb(0.13) W(0.13) Tb(0.12) Yb(0.10) Ho(0.09) Sm(0.03) Gd(0.00) ( ) IAM( 0.83) Hem( 0.71) DWRB( 0.62) Cel( 0.61) V( 0.55) O( 0.51) A( 0.49) Cl( 0.48) Ext( 0.47) Mg( 0.47) Chl( 0.46) ClB( 0.46) S( 0.46) Na( 0.43) SB( 0.43) Sul( 0.40) DWRA( 0.37) As( 0.34) Cd( 0.31) Ni( 0.27) Rb( 0.27) Cr( 0.24) Sr( 0.24) Lu( 0.21) Si( 0.21) Sil( 0.18) U( 0.18) Co( 0.15) Fe( 0.15) Ga( 0.14) Mo( 0.12) Cs( 0.10) Sc( 0.10) Ge( 0.09) Tm( 0.09) N( 0.08) Ag( 0.07) Sb( 0.07) Be( 0.06) Er( 0.06) Eu( 0.06) Mn( 0.06) Au( 0.05) Ba( 0.05) K( 0.05) Se( 0.04) Ta( 0.02)

Extractives (Ext)

(+) U(0.92) Chl(0.86) Hem(0.86) ClB(0.84) DWRB(0.84) Na(0.84) SB(0.84) A(0.82) Cl(0.82) V(0.79) S(0.76) N(0.63) Al(0.58) IAM(0.56) Sr(0.56) DWRA(0.27) Pr(0.23) Ti(0.23) Ce(0.22) Si(0.20) Cd(0.18) Sil(0.18) As(0.17) Sul(0.17) Fe(0.14) Lu(0.10) Nd(0.08) Mg(0.05) Zr(0.05) H(0.02) Th(0.01) O(0.00) ( ) C( 0.74) COH( 0.59) Rb( 0.59) VM( 0.58) Cs( 0.48) Hf( 0.48) Lig( 0.47) Mn( 0.46) Ba( 0.45) Cu( 0.44) Be( 0.42) K( 0.42) Li( 0.39) Mo( 0.39) Ca( 0.37) Yb( 0.37) P( 0.36) Sn( 0.36) Cel( 0.30) Ta( 0.30) Phos( 0.29) Cr( 0.28) Ga( 0.28) Ni( 0.28) Tb( 0.28) W( 0.27) Y( 0.27) Gd( 0.24) Er( 0.24) Pb( 0.22) Dy( 0.21) Au( 0.20) Ho( 0.19) Ge( 0.17) Sb( 0.17) Se( 0.17) Sc( 0.15) Bi( 0.14) Eu( 0.14) Nb( 0.14) Co( 0.13) B( 0.12) FC( 0.10) Zn( 0.10) Ag( 0.08) Sm( 0.07) Tm( 0.06) La( 0.05)

Dry water-soluble residue of biomass (DWRB)

(+) Hem(0.92) SB(0.89) Cl(0.88) ClB(0.88) Na(0.88) Chl(0.87) S(0.85) Ext(0.84) V(0.83) Sr(0.79) U(0.78) A(0.75) N(0.66) IAM(0.53) Mg(0.48) Sul(0.39) DWRA(0.35) Al(0.21) Cd(0.19) As(0.14) Lu(0.05) O(0.04) Co(0.03) Pr(0.02) Ga(0.01) ( ) C( 0.77) Sn( 0.66) Lig( 0.62) VM( 0.59) Hf( 0.58) Cu( 0.51) Pb( 0.50) Nb( 0.48) Zn( 0.45) Be( 0.41) Li( 0.41) P( 0.40) Ta( 0.40) COH( 0.39) Ho( 0.39) Phos( 0.39) Th( 0.37) W( 0.37) Er( 0.36) Y( 0.36) Bi( 0.35) K( 0.33) Rb( 0.33) Gd( 0.32) Mo( 0.31) Sb( 0.29) Dy( 0.27) Yb( 0.26) Cr( 0.25) Tb( 0.24) Cs( 0.23) Mn( 0.22) Ni( 0.22) H( 0.20) Sm( 0.19) Ag( 0.18) La( 0.18) Ba( 0.17) Au( 0.16) Cel( 0.15) Eu( 0.15) Zr( 0.15) Sc( 0.14) Se( 0.14) Tm( 0.14) Ge( 0.13) Sil( 0.12) Ti( 0.10) Fe( 0.08) Nd( 0.06) Si( 0.05) Ca( 0.04) B( 0.01) Ce( 0.01) FC( 0.01)

2. Biomass ash Dry water-soluble residue of biomass ash (DWRA)

(+) K(0.63) Y(0.56) Hem(0.48) Sr(0.47) Cl(0.45) ClB(0.45) FC(0.45) S(0.43) N(0.42) Na(0.40) SB(0.40) V(0.40) Chl(0.39) DWRB(0.35) Mg(0.34) COH(0.33) Fe(0.30) Ext(0.27) Sul(0.23) IAM(0.21) Ni(0.21) Mo(0.20) Cr(0.18) O(0.18) Pb(0.17) U(0.17) Zn(0.17) Rb(0.11) Li(0.06) B(0.05) A(0.04) Ca(0.04) P(0.01) As(0.00) ( ) Th( 0.62) Pr( 0.56) Nd( 0.54) La( 0.52) Hf( 0.49) Si( 0.49) Sil( 0.49) Er( 0.48) Ce( 0.47) Phos( 0.46) Zr( 0.45) Al( 0.44) C( 0.44) H( 0.44) Nb( 0.40) Lig( 0.37) Ag( 0.32) Bi( 0.32) Tm( 0.32) W( 0.30) Au( 0.29) Mn( 0.29) Se( 0.29) VM( 0.29) Be( 0.25) Eu( 0.24) Ho( 0.24) Ba( 0.23) Cd( 0.23) Ti( 0.23) Dy( 0.22) Ta( 0.21) Sc( 0.19) Cs( 0.17) Tb( 0.17) Ga( 0.16) Lu( 0.16) Ge( 0.15) Sm( 0.15) Yb( 0.13) Gd( 0.12) Cu( 0.12) Sb( 0.10) Co( 0.05) Sn( 0.03) Cel( 0.02)

Inorganic amorphous matter (IAM)

(+) Hem(0.59) Ext(0.56) Cd(0.53) DWRB(0.53) Cel(0.50) As(0.44) Si(0.44) O(0.43) Sil(0.43) Lu(0.36) U(0.36) A(0.32) V(0.31) Sc(0.30) Ge(0.29) Chl(0.28) ClB(0.27) Au(0.25) Cl(0.25) Na(0.25) SB(0.25) Dy(0.23) Eu(0.23) Sm(0.23) Ta(0.23) Tm(0.23) DWRA(0.21) S(0.21) Ag(0.20) Pr(0.18) Se(0.18) Gd(0.17) Sb(0.17) Ho(0.15) Be(0.14) Fe(0.13) Ce(0.12) Er (0.12) Al(0.10) VM (0.10) Zr(0.09) Tb(0.08) W(0.08) Mg(0.06) Pb(0.06) Zn(0.06) La(0.05) Ga(0.04) Yb(0.03) Mo(0.02) Rb(0.02) Sul(0.01) Cs(0.00) Ni(0.00) ( ) Lig( 0.83) Cu( 0.80) COH( 0.63) FC( 0.60) Phos( 0.59) Hf( 0.50) Ca( 0.49) B( 0.41) Bi( 0.41) Sn( 0.41) H( 0.37) Y( 0.37) C( 0.35) Th( 0.25) Li( 0.24) P( 0.20) Co( 0.18) Ti( 0.15) N( 0.13) K( 0.10) Mn( 0.06) Nb( 0.06) Nd( 0.05) Sr( 0.03) Cr( 0.02) Ba( 0.01)

Silicates (Sil)

(+) Si(0.99) Er(0.84) Ag(0.82) Tm(0.82) Ho(0.79) Lu(0.76) Cd(0.74) Sb(0.73) Ta(0.73) As(0.72) Eu(0.72) Sm(0.72) W(0.72) Sc(0.69) Gd(0.68) Se(0.68) Au(0.66) Be(0.65) Ge(0.63) Dy(0.57) Tb(0.53) IAM(0.43) Al(0.42) Yb(0.42) Pr(0.34) Cel(0.33) Th(0.32) C(0.26) Hf(0.26) A(0.25) Nb(0.24) Sn(0.19) Zr(0.19) Ext(0.18) Ce(0.16) Mo(0.11) La(0.07) VM(0.05) H(0.03) Nd(0.03) Ti(0.01) ( ) Ca( 0.74) B( 0.71) COH( 0.70) Mg( 0.62) Sr( 0.61) Co( 0.54) Li( 0.53) DWRA( 0.49) Cu( 0.42) FC( 0.42) S( 0.36) Rb ( 0.34) Sul( 0.34) K( 0.33) P( 0.33) Phos( 0.33) Cl( 0.30) N( 0.30) Ga( 0.28) Na( 0.28) SB( 0.27) V( 0.27) ClB( 0.26) Y( 0.25) Ba( 0.23) Cs( 0.23) Chl( 0.20) Bi( 0.18) Lig( 0.18) Mn( 0.16) Fe( 0.14) DWRB( 0.12) Ni( 0.11) Hem( 0.07) Cr( 0.06) Pb( 0.06) O( 0.03) U( 0.01) Zn( 0.01)

Carbonates, oxides and hydroxides (COH)

(+) Ca(0.76) Y(0.70) FC(0.62) B(0.61) Li(0.57) Lig(0.51) Cu(0.48) Cs(0.42) Ba(0.41) K(0.40) Rb(0.38) Ga(0.36) Mn(0.36) Sn(0.35) DWRA(0.33) Hf(0.27) Phos(0.27) Pb(0.26) H(0.18) Yb(0.14) C(0.12) Sr(0.09) Bi(0.08) VM(0.08) Co(0.07) O(0.07) P(0.06) Mg(0.05) Tb(0.00) Zn(0.00) ( ) Sil( 0.70) Si( 0.68) IAM( 0.63) Ext( 0.59) A( 0.56) Al( 0.50) Cd( 0.44) Er( 0.43) U( 0.41) Ag( 0.39) DWRB( 0.39) V( 0.38) As( 0.36) Hem( 0.35) Lu( 0.35) Pr( 0.34) Cel( 0.32) Tm( 0.32) Chl( 0.28) Fe( 0.27) S( 0.27) Sul( 0.26) Sb( 0.25) Sm( 0.25) Cl( 0.24) Eu( 0.24) ClB( 0.23) Na( 0.23) SB( 0.23) Sc( 0.22) Ce( 0.21) Se( 0.21) Th( 0.20) Zr( 0.20) W( 0.18) Ho( 0.17) Cr( 0.16) Ge( 0.16) Au( 0.15) Ni( 0.15) Be( 0.14) Gd( 0.12) Ta( 0.12) Nb( 0.09) Nd( 0.09) N( 0.08) Mo( 0.06) Dy( 0.05) Ti( 0.05) La( 0.02)

Phosphates (Phos)

(+) Bi(0.89) Lig(0.78) Nd(0.78) Cu(0.76) Th(0.75) H(0.74) Ti(0.74) B(0.73) La(0.70) Nb(0.68) Ce(0.67) Li(0.66) Zr(0.66) Pr(0.56) P(0.50) Hf(0.41) Al(0.40) Ca(0.39) VM(0.39) Co(0.32) Zn(0.32) Sn(0.29) C(0.27) COH(0.27) Pb(0.24) Ba(0.13) Fe(0.11) Mn(0.10) Ga(0.09) Cs(0.08) U(0.06) (continued on next page)

306

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

Table 5 (continued) Characteristic

Correlation coefficient value with ( ) As( 0.74) Lu( 0.61) IAM( 0.59) Cd( 0.55) Sb( 0.55) Cel( 0.54) Tm( 0.51) Gd( 0.50) Ge( 0.50) Hem( 0.50) Sc( 0.50) Ag( 0.49) Eu( 0.49) Be( 0.47) DWRA( 0.46) Se( 0.46) Ta( 0.44) Au( 0.42) A( 0.41) Sm( 0.41) Er( 0.40) DWRB( 0.39) Si( 0.39) Yb( 0.39) Ho( 0.38) Tb( 0.36) Mo( 0.35) Sil( 0.33) W( 0.33) Ext( 0.29) K( 0.28) O( 0.28) Chl( 0.27) ClB( 0.27) Cl( 0.26) SB( 0.23) V( 0.23) Na( 0.22) Cr( 0.21) Ni( 0.20) S( 0.20) Sul( 0.19) Dy( 0.18) Mg( 0.18) N( 0.16) FC( 0.10) Y( 0.10) Rb( 0.08) Sr( 0.07)

Sulphates (Sul)

(+) Mg(0.84) Ni(0.72) Cr(0.70) V(0.64) S(0.63) Co(0.53) Fe(0.51) N(0.51) Sr(0.51) Cl(0.46) Mo(0.45) DWRB(0.39) P(0.39) ClB(0.37) Na(0.37) SB(0.36) Chl(0.35) K(0.30) Cel(0.28) A(0.27) Hem(0.25) DWRA(0.23) Ext(0.17) Rb(0.08) U(0.06) FC(0.05) Cu(0.02) IAM(0.01) ( ) H( 0.66) Dy( 0.61) Sn( 0.59) La( 0.58) Ho( 0.56) Hf( 0.53) Nb( 0.51) Ta( 0.51) Ce( 0.50) Pr( 0.49) Pb( 0.48) Th( 0.45) Nd( 0.44) Au( 0.43) Ti( 0.42) Zr( 0.41) Lig( 0.40) Tb( 0.40) Yb( 0.40) Y( 0.38) Gd( 0.37) Li( 0.37) Tm( 0.37) W( 0.37) Sm( 0.36) Al( 0.35) Ba( 0.35) Ge( 0.34) Sc( 0.34) Si( 0.34) Sil( 0.34) Cs( 0.33) Mn( 0.33) Cd( 0.32) Eu( 0.31) Lu( 0.31) Se( 0.30) Ga( 0.29) B( 0.26) COH( 0.26) O( 0.26) Be( 0.25) VM( 0.24) Ag( 0.23) As( 0.23) Sb( 0.21) Phos( 0.19) Zn( 0.16) C( 0.15) Er( 0.15) Ca( 0.13) Bi( 0.11)

Chlorides (Chl)

(+) ClB(1.00) Na(1.00) Cl(0.99) SB(0.99) S(0.94) Hem(0.92) V(0.92) DWRB(0.87) A(0.86) Ext(0.86) Sr(0.85) U(0.78) N(0.75) DWRA(0.39) Mg(0.36) Sul(0.35) Al(0.31) IAM(0.28) Co(0.20) FC(0.15) O(0.13) B(0.06) Fe(0.05) Ca(0.04) ( ) C( 0.93) VM( 0.77) Hf( 0.66) Be( 0.63) Sn( 0.58) Ta( 0.58) Dy( 0.57) W( 0.57) Yb( 0.53) Tb( 0.52) Gd( 0.51) Ho( 0.50) Mo( 0.49) Nb( 0.49) Er( 0.47) Au( 0.46) Lig( 0.46) Ge( 0.45) Sb( 0.44) Sc( 0.44) Eu( 0.42) P( 0.42) Sm( 0.42) Se( 0.41) Cu( 0.40) Cel( 0.38) K( 0.37) Ag( 0.36) Rb( 0.32) Tm( 0.32) Bi( 0.31) Li( 0.31) Zr( 0.31) COH( 0.28) Pb( 0.28) Zn( 0.28) Phos( 0.27) Th( 0.27) Cr( 0.24) Cs( 0.24) Ba( 0.23) La( 0.23) Ni( 0.23) Y( 0.22) Mn( 0.21) Sil( 0.20) Lu( 0.18) Si( 0.15) Cd( 0.13) Nd( 0.12) Pr( 0.12) H( 0.10) As( 0.07) Ce( 0.04) Ga( 0.02) Ti( 0.02)

Si

(+) Sil(0.99) Tm(0.86) Ag(0.85) Er(0.85) Lu(0.81) Cd(0.80) Ho(0.79) As(0.77) Eu(0.75) Ta(0.74) Sb(0.73) Sc(0.73) Se(0.73) Sm(0.73) W(0.73) Au(0.71) Gd(0.70) Be(0.67) Ge(0.67) Dy(0.59) Tb(0.57) Yb(0.48) IAM(0.44) Al(0.40) Cel(0.32) Pr(0.32) A(0.31) Hf(0.25) Th(0.25) C(0.21) Ext(0.20) Nb(0.15) Zr(0.14) Ce(0.13) Sn(0.13) Mo(0.07) La(0.05) H(0.02) Nd(0.01) U(0.00) ( ) B( 0.70) Ca( 0.68) COH( 0.68) Li( 0.58) Mg( 0.58) Co( 0.56) Sr( 0.56) DWRA( 0.49) Cu( 0.48) P( 0.44) Phos( 0.39) FC( 0.38) K( 0.38) Rb( 0.35) Sul( 0.34) S( 0.33) N( 0.27) Bi( 0.26) Cl( 0.26) Y( 0.26) Fe( 0.25) V( 0.25) Na( 0.23) ClB( 0.22) SB( 0.22) Ga( 0.21) Lig( 0.21) Cs( 0.19) Ba( 0.17) Ni( 0.17) Chl( 0.15) Pb( 0.14) Zn( 0.14) Cr( 0.13) Mn( 0.11) DWRB( 0.05) Ti( 0.05) VM( 0.02) Hem( 0.01) O( 0.01)

Ca

(+) B(0.78) COH(0.76) Ga(0.75) Ba(0.72) Cs(0.69) Mn(0.68) Li(0.56) Rb(0.45) Co(0.44) Phos(0.39) Sr(0.37) Mg(0.31) FC(0.30) Lig(0.30) O(0.27) H(0.26) La(0.24) Cu(0.22) Nd(0.18) Y(0.12) Na(0.10) SB(0.10) Cl(0.07) ClB(0.07) Pb(0.05) Chl(0.04) DWRA(0.04) Ce(0.03) S(0.03) Bi(0.00) Hf(0.00) ( ) Sil( 0.74) Si( 0.68) Sb( 0.64) Ag( 0.61) Sm( 0.60) Er( 0.59) As( 0.55) Ho( 0.52) Lu( 0.52) W( 0.52) Gd( 0.50) IAM( 0.49) Mo( 0.49) Tm( 0.49) Eu( 0.48) Sc( 0.47) Cd( 0.46) Fe( 0.45) Ta( 0.45) Be( 0.43) Ge( 0.41) Cr( 0.40) Se( 0.39) Ext( 0.37) Cel( 0.36) Ni( 0.36) Au( 0.32) Dy( 0.32) Tb( 0.31) Al( 0.29) Zn( 0.29) A( 0.22) C( 0.22) Nb( 0.22) Sn( 0.20) Th( 0.18) U( 0.16) Yb( 0.15) Zr( 0.14) K( 0.13) Pr( 0.13) Sul( 0.13) P( 0.12) V( 0.07) Ti( 0.06) Hem( 0.05) N( 0.05) DWRB( 0.04) VM( 0.01)

K

(+) Mo(0.80) Ni(0.70) Cr(0.69) DWRA(0.63) Y(0.59) P(0.50) Fe(0.45) Cel(0.40) COH(0.40) C(0.37) Rb(0.35) Zn(0.35) FC(0.34) Sul(0.30) Sn(0.29) Sb(0.28) Be(0.26) Cu(0.25) Mg(0.25) Gd(0.23) VM(0.23) Pb(0.21) Yb(0.19) Tb(0.16) Ta(0.15) Sm(0.13) W(0.13) Ge(0.12) Li(0.09) Sc(0.09) Eu(0.06) Ho(0.06) Dy(0.04) As(0.01) Ag(0.00) Er(0.00) Se(0.00) ( ) Al( 0.76) Ce( 0.63) Pr( 0.63) Nd( 0.61) H( 0.57) La( 0.57) A( 0.55) U( 0.51) Th( 0.47) Ext( 0.42) Si( 0.38) Chl( 0.37) SB( 0.36) Ti( 0.36) Zr( 0.36) Na( 0.35) DWRB( 0.33) Ga( 0.33) Sil( 0.33) ClB( 0.31) Hem( 0.29) Cl( 0.28) Phos( 0.28) Mn( 0.27) Ba( 0.24) Cd( 0.22) B( 0.20) S( 0.19) V( 0.19) Cs( 0.15) Nb( 0.15) Sr( 0.15) Ca( 0.13) IAM( 0.10) Tm( 0.10) O( 0.09) Bi( 0.08) Lu( 0.06) Co( 0.05) Lig( 0.05) Hf( 0.04) N( 0.04) Au( 0.03)

P

(+) Fe(0.77) Bi(0.67) Ni(0.67) Cr(0.65) Zn(0.65) VM(0.63) Cu(0.61) K(0.50) Phos(0.50) Mo(0.49) C(0.47) Nb(0.47) Li(0.45) Co(0.41) Sul(0.39) Zr(0.35) Pb(0.30) Ti(0.28) Th(0.26) Lig(0.25) Mg(0.24) Rb(0.22) Cel(0.19) B(0.17) Nd(0.17) Sn(0.16) La(0.13) Ce(0.12) COH(0.06) Pr(0.06) Hf(0.02) DWRA(0.01) ( ) A( 0.61) As( 0.59) Lu( 0.57) Cd( 0.56) Tm( 0.52) Hem( 0.49) Au( 0.44) Si( 0.44) Chl( 0.42) Se( 0.42) DWRB( 0.40) SB( 0.40) ClB( 0.39) Eu( 0.39) Ag( 0.37) Na( 0.37) Yb( 0.37) Ext( 0.36) Ge( 0.36) Ho( 0.36) Sc( 0.36) Sil( 0.33) Gd( 0.32) Cl( 0.32) Ta( 0.32) Tb( 0.32) Ga( 0.31) Sm( 0.28) O( 0.26) Mn( 0.25) Sb( 0.25) FC( 0.24) U( 0.23) W( 0.23) Ba( 0.22) Dy( 0.22) Er( 0.22) Be( 0.20) IAM( 0.20) Sr( 0.20) N( 0.19) Al( 0.18) Cs( 0.17) S( 0.14) Ca( 0.12) H( 0.09) V( 0.09) Y( 0.07)

Al

(+) Pr(0.80) Th(0.79) Ce(0.78) Ti(0.78) H(0.72) Nd(0.70) U(0.70) Zr(0.63) Ext(0.58) La(0.57) Nb(0.55) Bi(0.46) A(0.42) Sil(0.42) Phos(0.40) Si(0.40) Chl(0.31) Na(0.29) SB(0.29) ClB(0.26) Lig(0.25) Cl(0.21) DWRB(0.21) Hem(0.19) Hf(0.19) V(0.18) N(0.17) B(0.16) S(0.16) Sn(0.15) Cd(0.13) Zn(0.13) IAM(0.10) Dy(0.06) Ho(0.06) Cu(0.05) Lu(0.05) Sm(0.05) Tm(0.05) Ag(0.03) Pb(0.02) Sr(0.02) ( ) K( 0.76) Rb( 0.71) Mo( 0.60) Cel ( 0.57) Ni( 0.56) Mg( 0.54) Cr( 0.53) COH( 0.50) DWRA( 0.44) Cs( 0.36) Sul( 0.35) Be( 0.31) Y( 0.31) Ba( 0.30) Ca( 0.29) FC( 0.29) Mn( 0.28) Yb( 0.28) Ga( 0.23) C( 0.21) Co( 0.21) O( 0.20) P( 0.18) VM( 0.17) Gd( 0.15) Tb( 0.15) Ge( 0.14) Sb( 0.14) Ta( 0.12) Sc( 0.10) Se( 0.08) Au( 0.07) Eu( 0.06) Li( 0.04) As( 0.03) W( 0.03) Er( 0.02) Fe( 0.02)

Mg

(+) Sul(0.84) Sr(0.64) Co (0.63) S(0.60) V(0.58) Cl(0.48) DWRB(0.48) Ni(0.48) Cr(0.43) Na(0.41) ClB(0.40) SB(0.40) Rb(0.38) Chl(0.36) N(0.36) DWRA(0.34) Hem(0.34) Ca(0.31) Cel(0.28) K(0.25) P(0.24) Mo(0.22) Fe(0.21) Ga(0.19) A(0.14) Ba(0.12) Cs(0.11) Mn(0.08) B(0.06) IAM(0.06) COH(0.05) Ext(0.05) FC(0.05) O(0.04) U(0.02) ( ) Sn( 0.76) Ho( 0.72) Th( 0.66) Nb( 0.63) H( 0.62) Sil( 0.62) Hf( 0.60) Dy( 0.59) Si( 0.58) Ta( 0.58) Al( 0.54) Sm( 0.54) W( 0.54) Ti( 0.52) Gd( 0.51) Pr( 0.50) Tm( 0.50) Lig( 0.47) Ce( 0.46) Ag( 0.45) Sb( 0.45) Zr( 0.44) Tb( 0.43) Au( 0.42) Er( 0.42) Eu( 0.42) Pb( 0.42) La( 0.41) Lu( 0.41) Sc( 0.40) Ge( 0.37) Se( 0.37) Nd( 0.36) Be( 0.35) As( 0.34) Cd( 0.34) Y( 0.34) Yb( 0.34) Zn( 0.33) C( 0.31) Bi( 0.26) Phos( 0.18) VM( 0.14) Cu( 0.11) Li( 0.09)

Fe

(+) P(0.77) Zn(0.76) Ni(0.73) Cr(0.72) Sul(0.51) K(0.45) Co(0.40) Mo(0.40) Pb(0.38) Bi(0.35) V(0.35) DWRA(0.30) S(0.25) VM(0.25) Nb(0.24) Cel(0.22) Cu(0.22) Mg(0.21) Ti(0.18) Li(0.17) Ext(0.14) IAM(0.13) Phos(0.11) Cl(0.10) Th(0.10) Rb(0.09) N(0.08) U(0.07) Zr(0.07) ClB(0.06) Chl(0.05) Na(0.05) C(0.02) SB(0.01) Hem(0.00) ( ) Yb( 0.62) Au( 0.60) Se( 0.55) Cd( 0.54) Tb( 0.53) Ga( 0.51) Tm( 0.51) Ba ( 0.50) Lu( 0.49) Ge( 0.48) Eu( 0.47) Mn( 0.47) Dy( 0.46) Sc( 0.46) Ca( 0.45) As( 0.43) Hf( 0.43) Cs( 0.40) Ta( 0.38) Gd( 0.36) Ho( 0.36) Ag( 0.35) W( 0.35) H( 0.33) Be( 0.32) Sm( 0.32) COH( 0.27) FC( 0.27) Si( 0.25) Er( 0.21) Sb( 0.19) B( 0.15) Lig( 0.15) La( 0.14) Sil( 0.14) Pr( 0.13) A( 0.12) Nd( 0.11) Y( 0.09) DWRB( 0.08) O( 0.04) Al( 0.02) Ce( 0.02) Sn( 0.02) Sr( 0.02)

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

307

Table 5 (continued) Characteristic

Correlation coefficient value with

S

(+) V(0.99) Cl(0.98) ClB(0.95) Na(0.95) SB (0.95) Chl(0.94) Sr(0.91) DWRB(0.85) Hem(0.83) N(0.81) Ext(0.76) A(0.74) U(0.71) Sul(0.63) Mg(0.60) DWRA(0.43) Co(0.37) Fe(0.25) IAM(0.21) Al(0.16) FC(0.15) B(0.07) Ni(0.04) Ca(0.03) Cr(0.02) ( ) C( 0.81) Ta( 0.72) Hf( 0.71) Dy( 0.69) Be( 0.68) Ho( 0.67) VM( 0.67) W( 0.66) Sn( 0.65) Yb( 0.62) Gd( 0.61) Tb( 0.61) Au( 0.59) Er( 0.54) Ge( 0.54) Sc( 0.54) Eu( 0.52) Sm( 0.52) Nb( 0.51) Sb( 0.51) Se( 0.51) Tm( 0.48) Ag( 0.46) Lig( 0.46) Pb( 0.37) Sil( 0.36) Lu( 0.33) Si( 0.33) Th( 0.33) Cs( 0.32) La( 0.32) Zr( 0.32) Ba( 0.31) Mn( 0.31) Li( 0.29) Y( 0.29) Cd( 0.28) Cel( 0.28) COH( 0.27) Mo( 0.27) H( 0.25) Rb( 0.25) Cu( 0.24) Zn( 0.23) As( 0.22) Pr( 0.21) Phos( 0.20) Bi( 0.19) K( 0.19) Nd( 0.17) Ce( 0.14) P( 0.14) Ga( 0.13) Ti( 0.07) O( 0.02)

Na

(+) Chl(1.00) ClB(1.00) SB(1.00) Cl(0.99) S(0.95) V(0.93) Hem(0.91) Sr(0.89) DWRB(0.88) Ext(0.84) A(0.82) U(0.79) N(0.77) Mg(0.41) DWRA(0.40) Sul(0.37) Al(0.29) IAM(0.25) Co(0.23) FC(0.16) B(0.14) Ca(0.10) O(0.10) Fe(0.05) Ti(0.01) ( ) C( 0.92) VM( 0.74) Be( 0.68) Hf( 0.66) Ta( 0.64) W( 0.62) Dy( 0.59) Sn( 0.59) Gd( 0.56) Ho( 0.56) Tb( 0.55) Yb( 0.55) Er( 0.54) Mo( 0.50) Sb( 0.50) Au( 0.49) Ge( 0.48) Nb( 0.48) Sc( 0.48) Sm( 0.47) Eu( 0.46) Se( 0.45) Lig( 0.43) Ag( 0.42) Cel( 0.41) Tm( 0.38) P( 0.37) K( 0.35) Cu( 0.34) Rb( 0.31) Pb( 0.30) Zn( 0.29) Sil( 0.28) Zr( 0.28) Th( 0.27) Bi( 0.26) Li( 0.26) Cr( 0.25) COH( 0.23) Cs( 0.23) Lu( 0.23) Ni( 0.23) Si( 0.23) Phos( 0.22) Y( 0.22) Ba( 0.21) La( 0.20) Mn( 0.20) Cd( 0.17) As( 0.13) Pr( 0.10) H( 0.08) Nd( 0.08) Ce( 0.02) Ga( 0.01)

Ti

(+) Th(0.87) Ce(0.86) Bi(0.85) H(0.85) Nb(0.84) Zr(0.84) Nd(0.83) Pr(0.81) Al(0.78) Phos(0.74) La(0.72) Lig(0.59) B(0.54) Cu(0.52) U(0.52) Sn(0.47) Zn(0.47) Li(0.46) Hf(0.37) Pb(0.33) P(0.28) Ext(0.23) VM(0.20) Fe(0.18) Dy(0.10) C(0.06) N(0.03) Y(0.03) Na(0.01) SB(0.01) Sil(0.01) ( ) Cel( 0.66) Rb( 0.53) Mg( 0.52) Ni( 0.44) Be( 0.43) Mo( 0.43) Cr( 0.42) Sul( 0.42) As( 0.37) K( 0.36) Er( 0.33) Sb( 0.32) Mn( 0.31) Yb( 0.31) Cs( 0.30) Se( 0.30) O( 0.29) Gd( 0.28) Ge( 0.28) Lu( 0.28) Ag( 0.27) Eu( 0.27) Sc( 0.27) Tm( 0.27) Au( 0.25) Ba( 0.25) Ga( 0.25) Ta( 0.24) DWRA( 0.23) Cd( 0.21) Tb( 0.19) Co( 0.16) FC( 0.15) Hem( 0.15) IAM( 0.15) A( 0.14) W( 0.14) DWRB( 0.10) Ho( 0.10) Sm( 0.08) V( 0.08) S( 0.07) Sr( 0.07) Ca( 0.06) Cl( 0.06) COH( 0.05) Si( 0.05) Chl( 0.02) ClB( 0.02)

Cl

(+) Chl(0.99) ClB(0.99) Na(0.99) SB(0.99) S(0.98) V(0.95) Hem(0.91) Sr(0.90) DWRB(0.88) Ext(0.82) A (0.81) N(0.79) U(0.75) Mg(0.48) Sul(0.46) DWRA(0.45) Co(0.27) IAM(0.25) Al(0.21) FC(0.17) Fe(0.10) B(0.09) O(0.09) Ca(0.07) ( ) C( 0.90) VM( 0.74) Hf( 0.70) Be( 0.66) Ta( 0.65) Dy( 0.62) Sn( 0.62) W( 0.62) Ho( 0.58) Gd( 0.56) Tb( 0.56) Yb( 0.55) Er( 0.53) Nb( 0.53) Au( 0.51) Sc( 0.49) Ge( 0.48) Lig( 0.48) Sb( 0.48) Sm( 0.47) Eu( 0.46) Se( 0.46) Ag( 0.42) Mo( 0.41) Tm( 0.40) Cu( 0.35) Cel( 0.34) Th( 0.34) Zr( 0.33) P( 0.32) Pb( 0.32) Sil( 0.30) Bi( 0.29) Li( 0.29) K( 0.28) La( 0.28) Rb( 0.28) Zn( 0.28) Phos( 0.26) Si( 0.26) Cs( 0.25) Ba( 0.24) COH( 0.24) Lu( 0.24) Mn( 0.23) Y ( 0.23) Cd( 0.19) Pr( 0.18) H( 0.17) Nd( 0.16) Cr( 0.15) As( 0.13) Ni( 0.13) Ce( 0.10) Ti( 0.06) Ga( 0.04)

Mn

(+) Ba(0.98) Cs(0.98) Ga(0.96) Rb(0.75) O(0.71) Ca(0.68) Li(0.44) Co(0.42) La(0.37) COH(0.36) VM(0.32) B(0.31) Pb(0.27) Cel(0.21) Nd(0.13) Phos(0.10) Mg(0.08) Ce(0.07) H(0.04) Au(0.03) Yb(0.02) Be(0.00) Ta(0.00) ( ) N( 0.62) Fe( 0.47) Ext( 0.46) U( 0.43) Mo( 0.41) Sm( 0.35) Sn( 0.33) Sul( 0.33) Bi( 0.31) S( 0.31) Ti( 0.31) V( 0.31) Cr( 0.30) Cu( 0.30) Sb( 0.30) DWRA( 0.29) Al( 0.28) FC( 0.27) K( 0.27) Ni( 0.27) P( 0.25) Ag( 0.24) Cl( 0.23) As( 0.22) ClB( 0.22) DWRB( 0.22) W( 0.22) Chl( 0.21) SB( 0.21) A( 0.20) Na( 0.20) Y( 0.20) Lu( 0.19) Th( 0.18) Zn( 0.18) Gd( 0.17) Eu( 0.17) Sil( 0.16) Sr( 0.16) Nb( 0.15) Tb( 0.13) Zr( 0.13) Ho( 0.12) Sc( 0.12) Ge( 0.11) Hem( 0.11) Hf( 0.11) Si ( 0.11) C( 0.10) Er( 0.09) Se( 0.09) Tm( 0.09) Cd( 0.08) Dy( 0.08) IAM( 0.06) Lig( 0.06) Pr( 0.05)

Abbreviations: A, ash yield of biomass; Cel, cellulose in biomass; Chl, chlorides in biomass ash; ClB, Cl in biomass; COH, carbonates, oxides and hydroxides in biomass ash; DWRA, dry water-soluble residue of biomass ash; DWRB, dry water-soluble residue of biomass; Ext, extractives in biomass; FC, fixed carbon of biomass; Hem, hemicellulose in biomass; IAM, inorganic amorphous matter in biomass ash; Lig, lignin in biomass; Phos, phosphates in biomass ash; SB, S in biomass; Sil, silicates in biomass ash; Sul, sulphates in biomass ash; VM, volatile matter of biomass. a The significant R2 values at 95% confidence level are: P0.63 and 6 0.63 for 8 variables.

Na-, O- and S-containing phases (Table 5). On the other hand, lignin has moderate and intermediate TE associations between the above listed organic components and it correlates positively with B, Bi, Cu, Hf, Li, Nb, Pb, rare earth elements (Ce, Dy, Ho, La, Nd, Pr, Sm, Tb, Yb and excluding Er, Eu, Gd, Lu, Tm), Sn, Th, W, Y, Zn and Zr plus Al-, C-, Ca-, H-, P- and Ti-bearing phases (Table 5). It can be seen that the above element associations found for IM are quite similar to those for hemicellulose and the water-soluble fraction in biomass. This observation indicates some analogous elemental relationships among these three characteristics. In contrast, the element associations for OM are quite similar to those for cellulose and water-insoluble fraction in biomass. Hence, there are certain highly informative differentiations for the elemental associations in biomass that include: (1) OM – cellulose – waterinsoluble fraction; and (2) IM – hemicellulose – water-soluble fraction. It is interesting to note that similar associations between IM, hemicellulose and highly mobile elements in biomass such as S, Cl, N and K has been identified earlier for more (28) varieties of natural biomass [2]. It seems that hemicellulose in plants plays a role of conducting and concentrating tissue for mineralized solutions abundant in mobile sulphates, chlorides, nitrates and silicic acid [2]. This structural component tends to be relatively poor in TEs, but associates with certain harmful and mobile elements

such as As, Cd, Cl, Co, N, S, U and V (Table 5). It is well known that hemicellulose is mostly abundant in annual and fast-growing twigs and leaves of trees, grasses and straws among biomass sub-groups [2]. In contrast, cellulose and lignin in biomass show associations with different non-mobile and slightly, to highly mobile phases [2] enriched in much more TEs. The present data (Table 5) reveal some very strong and significant correlations between the TE concentrations in BA and contents of some common characteristics for biomass, namely: (1) Positive associations for Cl, N, S, dry water-soluble residue and hemicellulose with SrAUAV; ash yield and extractives with UAV; H and BABiACeALaANbANdAPrAThAZr; lignin and BiACuAHfASnATh; O and BaACsAGaARb; C and BeAHfAMoAW; cellulose and MoANi; volatile matter (VM) and Nb; and fixed carbon (FC) and Y. (2) Negative associations for H and CrAMoANi; C and SrAUAV; Cl and S with BeAHf; hemicellulose and CuAHfASn; cellulose and B; VM and Sr; ash yield and Li; dry water-soluble residue and Sn; N and Cs; and O and Cu. Unfortunately, the present knowledge for TEs in biomass is not enough for appropriate explanations of many of these associations and thus future detailed studies are required.

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3.5. Trace element associations in biomass ashes Biomass ash has totally different composition in comparison with the respective biomass fuel [1–4]. Therefore, the systematization of physico-chemical transformations during biomass combustion has been described in detail earlier [5,6] and the phasemineral composition of the 8 BAs (500–1500 °C) studied has also been given therein. The above data [5,6] show that the original OM and IM in biomass transform: (1) initially to devolatilization of OM and burning of combustible gases and char with formation of intermediate and less stable oxalates, nitrates, chlorides, hydroxides, carbonates, sulphates and inorganic amorphous (non-glass) material; (2) subsequently to more stable silicates, phosphates and oxides; (3) then to melting accompanied by dissolution of the refractory minerals; with increasing combustion temperatures in the system; and (4) followed by crystallization of melt and formation of glass accompanied by some salt condensation and hydroxylation, hydration and carbonation of newly formed phases during cooling of BA. Finally, some post-combustion transformations of the newly formed minerals and phases to stable species during weathering among silicates, hydroxides, phosphates, sulphates, carbonates, chlorides and nitrates also occur due to their hydration, hydroxylation and carbonation by moisture and CO2 in the air through storage of BA [6]. The mineral classes and IAM found in the BAs studied are given in Table l. The content of water-soluble fraction in BAs (3.9–45.1%) is high (Table 1), while the same value for coal ashes (0.2–7.2%) is much less [68]. Hence, the water-soluble components play a very important role for BA. The present data (Table 5) show that most TEs simultaneously with ash-forming Al-, Mn-, Si- and Ti-bearing phases correlate positively with water-insoluble fraction in BAs. In contrast, TEs such as B, Cr, Li, Mo, Ni, Pb, Rb, Sr, U, V, Y and Zn plus ash-forming Ca-, Cl-, Fe-, K-, Mg-, Na-, P- and S-containing phases associate preferably with the water-soluble fraction of BA. The summarized reference data reveal that the water-soluble elements leached from different BAs include Al, Ba, Br, Ca, Cd, Cl, Co, Cr, Cu, Fe, Hg, K, Li, Mg, Mn, Mo, Na, Ni, P, Pb, S, Sb, Se, Si, Sr and Zn [6]. Hence, correlation is confirmed for most major and minor elements, and some trace elements (Ca, Cl, Cr, Fe, K, Mg, Na, P, Li, Mo, Ni, Pb, S, Sr, Zn) in BAs. The above trends are a strong indication that the nutrients and some dangerous trace elements can associate preferentially with the water-soluble fraction of BAs. It can be seen that most TEs (mainly lithophile, noble and radioactive elements plus As, Cd, Ga, Ge, Pb, Sb, Sc, Se, Sn, V and Zn) together with ash-forming Al, Cl, Fe, Mg, Na, S, Si and Ti correlate positively with IAM and silicates. A significant number of TEs also associate with phosphates (B, Ba, Bi, Ce, Co, Cs, Cu, Ga, Hf, La, Li, Nb, Nd, Pb, Pr, Sn, Th, U, Zn, Zr) and carbonates, oxides and hydroxides (B, Ba, Bi, Co, Cs, Cu, Ga, Hf, Li, Pb, Rb, Sn, Sr, Y, Yb). In contrast, relatively few TEs correlate positively with sulphates (Co, Cr, Cu, Mo, Ni, Rb, Sr, U, V) and chlorides (B, Co, Sr, U, V), which are typically water-soluble phases in BAs. The association of TEs with different mineral classes in BA confirmed most of the relationships found between TEs and major and minor elements in BA (Table 5). For example, the present data (Table 5 and Figs. 3 and 4) reveal some very strong and significant correlations between the TE concentrations in BA and contents of some common characteristics for BA, namely: (1) Positive associations for silicates and Si with AgAAsAAuABeACdAErAEuAGdAGeAHoALuASbAScASeA SmATaATmAW; phosphates and BABiACeACuALaALiA NbANdAThAZr; Ti and BiACeALaANbANdAPrAThAZr; Al and CeANdAPrAThAUAZr; Ca and BABaACsAGa; P and BiACrANiAZn; Mn and BaACsAGaARb; chlorides, Cl, S and

Na with SrAUAV; Fe and CrANiAZn; K and CrAMoANi; sulphates and CrANiAV; Mg and CoASr; and carbonates, oxides and hydroxides with Y. (2) Negative associations for S and BeADyAHfAHoASnA TaAWAYb; Mg and HoANbASnATh; Cl and Na with BeAHfATa; chlorides and BeAHf; K and CeAPr; Al and Rb; Ca and Sb; IAM and Cu; phosphates and As; silicates and Si with B. Unfortunately, the present knowledge for TEs in BA is also not enough for appropriate explanations of many of these associations and thus future detailed investigations are required. 3.6. Some environmental challenges related to trace elements in biomass and biomass ash The indication of some potential environmental and technological problems or advantages related to TEs in biomass and BA during combustion, as well as throughout application and storage of BA has been evaluated earlier [1–6]. It was noticed therein that the greatest ecological challenges related to some TEs in biomass and BA include their: (1) high concentrations; (2) unfavourable modes of occurrence; (3) enhanced volatilization and limited retention and capture performance during biomass combustion; and (4) increased leaching behaviour during biomass and BA processing or storage. The potentially hazardous TEs normally include Ag, As, Ba, Be, Cd, Co, Cr, Cu, F, Hg, Ni, Pb, Sb, Se, Sn, Th, Tl, U, V and Zn plus C, Cl, Mn, N, S and some of their compounds (CO, CO2, CH4, HCl, Cl2, HF, NOX, N2O, NH3, SOX, H2S, polycyclic aromatic hydrocarbons, polychlorinated dioxins and furans, and others) in solid fuels and/or their combustion products ([4,6] and references therein). The reference and present data show that many TEs can have high concentrations in different biomass varieties and especially their BAs (see Sections 1, 3.1 and 3.2) and such enhanced TE concentrations have potential environmental concerns. Therefore, the high contents of TEs and their modes of occurrence in biomass and BA, as well as their behaviour during combustion should be evaluated on a case-by-case basis for each biomass feedstock and produced BA. Subsequently, the reasons for such high TE concentrations should be systematically evaluated for potential ecological and/or technological applications. The volatilization, retention, capture and immobilization of potentially hazardous air pollutant elements is a serious challenge during biomass combustion and co-firing of biomass with other solid fuels [4,6]. For example, the summarized reference data show that elements such as As, Br, Cd, Cl, Cr, Hg, K, Na, Pb, S, Sb, Se, V and Zn demonstrate the highest volatilization potential during biomass combustion ([3,6] and references therein). It can be seen that most of the above elements associate preferably with OM (especially cellulose) in biomass and water-soluble minerals and phases (chlorides, sulphates, oxalates and nitrates plus some carbonates, phosphates and amorphous material) in biomass and BA (see Sections 3.4 and 3.5). Hence, such modes of TE occurrence in biomass and BA favour their high volatilization behaviour during biomass combustion and their low capture and retention action in BA. These volatilized elements present some environmental risk and potential health concern because they could contaminate the air, soil, water and plants in the areas surrounding biomass-fired power plants. Therefore, some biomass fuels and their BAs are the basis for potentially serious global and local contamination concerns during future large-scale combustion of biomass. Major and minor elements such as Al, C, Ca, Cl, Fe, K, Mg, Mn, N, Na, P, S, Si and Ti, plus many TEs (As, Ba, Br, Cd, Co, Cr, Cu, Hg, Li, Mo, Ni, Pb, Sb, Se, Sr, V, Zn) including also hazardous elements

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ppm

R2 0.82

Ag

ppm

ppm

R2 0.65

Be

35

5.0

R2 0.84

Er

1.0

25

2.5

0.5

15

0.0

0.0

5

0

10

20

30

40

50

0

10

Silicates, % ppm

20

30

40

R2 0.68

Gd

1.0 0.5

ppm

20

30

40

R2 0.79

Ho

2.0

0.1

1.0

R2 0.73

10

20

30

40

0

50

ppm

R2 0.82

Tm

1.5

0.2

0.2

1.0

0.1

0.1

0.5

0.0

0

ppm

20

30

Silicates, %

40

50

0

10

R2 0.67

Ce

20

30

40

50

0.0

ppm

R2 0.68

Nb

4.0

4.0

1.0

2.0

0.0 5

10

15

20

Phosphates, %

ppm

25

R2 0.75

Th

1.0

5

10

15

20

Phosphates, %

ppm

25

R2 0.66

Zr

0 5

10

15

20

5

10

ppm

R2 0.67

Be

ppm 0.8

20

0.4

0

0.0 0

10

20

30

40

15

Phosphates, %

Phosphates, %

40

50

R2 0.72

10

20

30

Silicates, %

40

50

R2 0.78

Nd

0

5

10

15

20

Phosphates, %

ppm

25

R2 0.85

Ag

0.0

0

25

40

2.5

10

0.0

30

5.0

20

0.5

20

0.0 0

30

0

0

ppm

2.0

0.0

R2 0.73

W

Silicates, %

8.0

0

10

ppm

0.3

10

50

Silicates, %

0.3

0

40

Sb

Silicates, %

Ta

30

0.0 0

Silicates, % ppm

ppm

0.2

50

20

Silicates, %

0

0.0 10

10

Silicates, %

1.5

0

0

50

20

25

10

20

30

40

Si, % R2 0.85

Er

0

ppm

R2 0.70

Gd

1.5 1.0 0.5 0.0

0

10

Si, %

20

30

Si, %

40

0

10

20

30

40

Si, %

Fig. 3. Some significant positive correlations between the concentrations of trace elements in BA and the contents of common characteristics for BA.

may occur in water-soluble phases in biomass and BA ([6] and references therein). Washing to remove water-soluble phases prior to use of biomass and BAs may reduce some environmental and technological problems. However, such future large-scale leaching may create new environmental concerns related to the fate of hazardous elements, which associate with these water-soluble phases [6]. It is well known that the solubility (mobility) of most elements is markedly pH sensitive. Most elements are mobile under acidic conditions. While alkaline environments suppress the release of a large number of elements (Al, Cd, Co, Cu, Fe, Hg, Mn, Ni, Pb, Sn Ti, Zn among others), they enhance release of oxyanionic-forming species of As, B, Cr, F, Mo, Sb, Se, V and W ([4] and references therein). The pH values of water leachates (Table 1) from the BAs studied are slightly alkaline to highly alkaline (8.1–12.9), while these values for the biomass varieties studied are slightly acidic

to neutral (5.1–6.8). Therefore, different elements in biomass and BA tend to be water-soluble (see Sections 3.4 and 3.5). For example, elements such as As, B, Ba, Cl, Cr, F, Mo, Pb, S, Sb, Se, V and W (characteristic mainly of oxyanionic species) in BA are mobile and they are prone to pose environmental concerns after BA disposal in landfills due to their mobility under alkaline conditions, which are typical for BA ([4] and references therein). On the other hand, various tertiary minerals like Ca silicate hydrate gel, Ca aluminosilicate hydrate, portlandite, calcite and ettringite can be formed in these ash disposals (under high pH) and such crystallizations may reduce mobility either by physically reducing the porosity of the ash or by chemically binding the elements. However, prolonged leaching during BA weathering in disposal sites could provoke a decrease of pH and significant release of many trace elements from BA ([4] and references therein).

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ppm

R2 0.79

Ho

ppm

0.2

2.0

0.1

1.0

0.0

0.0

0

10

20

30

40

0.1 0.0 10

20

30

R2 0.86

ppm 1.5

0.2

1.0

0.1

0.5 20

30

Si, %

ppm

R2 0.73

W

R2 0.80

20

30

40

ppm

R2 0.78

Ce

30

40

0

ppm

0.2

0.4

0

0.6

ppm

R2 0.70

U

0.5

0.3

2.5

0.0 0.2

0.4

0.6

R2 0.91

0.2

0.4

ppm

0.6

R2 0.99

V

40

2

4

6

8

0

10

2

4

ppm

R2 0.81

Pr

0.5 0.0

6

8

10

100

200

ppm

300

400

500

4

6

R2 0.86

Ce

0.0 0

100

200

ppm

R2 0.87

Th

300

400

500

Ti, ppm ppm

1.0

40

0.5

20

0.0 0

Mg, %

S, %

S, %

1.0

2

4.0

0

0 0

0

8.0

20

600

R2 0.63

Co

Al, %

Sr

0.6

0.0 0

Al, % ppm

0.4

ppm 5.0

1200

0.2

Al, %

0.6

0.0

R2 0.80

Pr

Al, % R2 0.79

30

20

Ca, %

0.0 0

1.0

0

10

0.4

K, % Th

R2 0.78

B

0.8

0.0 20

ppm

ppm 12000

4.0

10

40

4000 10

8.0

0

30

8000

0

40

5

0

20

Si, %

Mo

10

10

Si, %

0.0

0.0 10

0

40

Si, %

Tm

R2 0.74

Ta

0.2

0

0.3

0

ppm 0.3

Si, % ppm

R2 0.73

Sb

R2 0.95

V

0 0

100

200

Ti, ppm

300

Ti, ppm

400

500

0

2

4

6

Cl, %

8

10

Fig. 4. Some significant positive correlations between the concentrations of trace elements in BA and the contents of common characteristics for BA.

It is commonly accepted that the concentration and behaviour of elements such as Ca, Cl, K, Na, P, S, Si and TEs are mostly responsible for many environmental and technological problems during biomass processing. However, other studies of solid fuels emphasize that the actual reasons for such problems are most likely connected with the abundance and behaviour of certain modes of element occurrence (specific phases or minerals) in these fuels and their products ([1–6,61,67–72] and references therein). Additionally, regulations exist in some countries which specify the limiting and guiding values for the contents of some elements (Ca, Cd, Cl, Co, Cr, Cu, K, N, Ni, Pb, S, V, Zn) in biomass fuels and their BAs in respect of their unrestricted use. However, the bulk concentrations of these elements are less informative than the abundance and behaviour of the forms in which they are present ([1–4] and references therein).

The leading importance of modes of element occurrence in biomass and BA for such issue is also emphasized in the present study. Finally, the negative impacts of TEs associated with biomass can be significant because this fuel can contain amounts of elements accumulated from contaminated air and rainfall, as well as mobilized from previously geochemically stable sources. Such elements are subsequently remobilised during biomass burning and take part in the on-going biogeochemical cycles. Similar opinions about the mobilization and remobilization of Cd, Cr, Hg, Ni, Pb and V from natural biomass and As, Cd, Cr, Cu, Hg, Ni, Pb and Zn from semibiomass have been mentioned earlier ([4] and references therein). It should be stated that the global dispersion and redistribution of TEs from the Lithosphere and Biosphere onto the Earth’s surface by the fuel combustion could likely be the next environmental

S.V. Vassilev et al. / Fuel 129 (2014) 292–313

challenge following historical problems related to the hazardous emissions of SOX, NOX and CO2 [4]. Unfortunately, systematic studies about the concentration, modes of occurrence, behaviour and fate of hazardous phases and TEs in biomass fuels and their BAs are only at an initial stage of investigation and much more detailed work is required in this topic. 4. Conclusions Some conclusions based on the contents of 60 major, minor and trace elements in BAs produced from 8 biomass varieties belonging to four biomass ash types can be made: (1) The concentration of TEs in BAs is highly variable due to the different origin and composition of biomass species. Most of the elements detected in BAs are TEs (<0.1%) excluding Al, B, Ba, Ca, Cl, Cr, Fe, K, Mg, Mn, Na, O, P, S, Si, Sr and Zn. (2) As a general trend, most TEs are commonly less enriched in BA than in coal ash and the reason for that is mostly associated with lower amount of IM with authigenic and especially detrital nature in biomass than in coal. However, the concentrations of some TEs (Ag, Au, B, Be, Cd, Cr, Cu, Ni, Rb, Se, Zn, others) in BA can be very high due to the enhanced enrichment factor of such elements in the combustion residue due to the high contents of OM in biomass. (3) Elements such as Ag, Au, B, Be, Ca, Cd, Cl, Cr, Cu, K, Mg, Mn, Na, Ni, P, Rb, Se and Zn in some BAs can be present with higher concentrations than the respective Clarke values for coal ashes. The TEs with over Clarke concentrations in BAs belong mostly to chalcophile, siderophile and lithophile groups and, to a lesser extent, noble and non-metal groups. Such a preliminary geochemical comparison indicates that the enriched and/or hazardous TEs in BAs could lead to environmental concern and economic impact. (4) There is some preferable association of lithophile (‘‘S’’, ‘‘C’’ and ‘‘CK’’ types), chalcophile (‘‘S’’ and ‘‘CK’’ types), siderophile (‘‘K’’ type), non-metal (‘‘K’’ and ‘‘CK’’ types), noble (‘‘S’’ type) and radioactive (‘‘K’’ and ‘‘CK’’ types) TEs to different BA types depending on concentration of their bearing phases in such BA types. (5) Most TEs together with C, Ca, Fe, H, K, Mn, P and Ti (and their bearing phases) correlate positively with OM, whereas only TEs such as Ag, As, Cd, Co, Er, Lu, Sr, Tm, U and V plus Al-, Cl-, Mg-, N-, Na-, O-, S- and Si-containing phases associate preferably with IM of biomass. (6) Most TEs tend to correlate positively with the water-insoluble fraction in biomass, while only As, Cd, Co, Ga, Lu, Pr, Sr, U and V plus Al-, Cl-, Mg-, N-, Na- O-and S-containing phases tend to associate with the water-soluble fraction in biomass, which is abundant in chlorides, sulphates, oxalates and nitrates plus some carbonates and amorphous material with both inorganic and organic character. (7) There are certain highly informative differentiations for the elemental associations in biomass that include: (1) OM – cellulose – water-insoluble fraction; and (2) IM – hemicellulose – water-soluble fraction. Most TEs such as Ag, As, Au, Ba, Be, Cd, Co, Cr, Cs, Ga, Ge, Mo, Ni, Pb, Rb, Sb, Sc, Se, Ta, W, Zn and rare earth elements (Dy, Er, Eu, Gd, Ho, Lu, Sm, Tb, Tm and Yb, excluding only the light La, Ce, Pr and Nd) together with C-, Fe-, K-, Mg-, Mn-, O-, P- and Si-bearing phases correlate positively with cellulose. In contrast, hemicellulose is the structural components with the fewest TE associations, excluding As, Cd, Co, Ga, Sr, U and V plus Al-, Cl-, Mg-, N-, Na-, O- and S-containing phases. On the other hand, lignin

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has moderate and intermediate TE associations between the above listed organic components and it correlates positively with B, Bi, Cu, Hf, Li, Nb, Pb, rare earth elements (Ce, Dy, Ho, La, Nd, Pr, Sm, Tb, Yb and excluding Er, Eu, Gd, Lu, Tm), Sn, Th, W, Y, Zn and Zr plus Al-, C-, Ca-, H-, P- and Tibearing phases. (8) Most TEs simultaneously with ash-forming Al-, Mn-, Si- and Ti-bearing phases correlate positively with the waterinsoluble fraction in BAs. In contrast, TEs such as B, Cr, Li, Mo, Ni, Pb, Rb, Sr, U, V, Y and Zn plus ash-forming Ca-, Cl-, Fe-, K-, Mg-, Na-, P- and S-containing phases associate preferably with the water-soluble fraction of BA. The above trends are a strong indication that the nutrients and some potentially hazardous trace elements can associate preferentially with the water-soluble fraction of BAs. (9) Most TEs (mainly lithophile, noble and radioactive elements plus As, Cd, Ga, Ge, Pb, Sb, Sc, Se, Sn, V and Zn) together with ash-forming Al, Cl, Fe, Mg, Na, S, Si and Ti correlate positively with IAM and silicates in BA. A significant number of TEs also associate with phosphates (B, Ba, Bi, Ce, Co, Cs, Cu, Ga, Hf, La, Li, Nb, Nd, Pb, Pr, Sn, Th, U, Zn, Zr) and carbonates, oxides and hydroxides (B, Ba, Bi, Co, Cs, Cu, Ga, Hf, Li, Pb, Rb, Sn, Sr, Y, Yb) in BA. In contrast, relatively few TEs correlate positively with sulphates (Co, Cr, Cu, Mo, Ni, Rb, Sr, U, V) and chlorides (B, Co, Sr, U, V), which are typically water-soluble phases in BA. The association of TEs with different mineral classes in BA confirmed most of the relationships found between TEs and major and minor elements in BA. (10) It was revealed that the greatest ecological challenges related to some TEs in biomass and BA include their: (1) high concentrations; (2) unfavourable modes of occurrence; (3) enhanced volatilization and limited retention and capture performance during biomass combustion; and (4) increased leaching behaviour during biomass and BA processing or storage. The data indicate that BAs from ‘‘K’’ and ‘‘CK’’ types and, to some extent, ‘‘S’’ type can show enhanced environmental problems and/or potential recovery benefits for TEs in comparison with ‘‘C’’ type.

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