Advantages and disadvantages of composition and properties of biomass in comparison with coal: An overview

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Contents lists available at ScienceDirect

Fuel journal homepage: www.elsevier.com/locate/fuel

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Review article

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Advantages and disadvantages of composition and properties of biomass in comparison with coal: An overview

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Stanislav V. Vassilev a,⇑, Christina G. Vassileva a, Vassil S. Vassilev b

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a b

Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Block 107, Sofia 1113, Bulgaria Space Research and Technology Institute, Bulgarian Academy of Sciences, Acad. G. Bonchev Street, Block 1, Sofia 1113, Bulgaria

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h i g h l i g h t s

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 Composition and properties of biomass were summarised.

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 Comparative characterization between biomass and coal was given.

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 Disadvantages of biomass composition and properties were discussed.

 Advantages of biomass composition and properties were described.

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a r t i c l e 2 5 2 3 23 24 25 26 27 28 29 30 31 32 33 34

i n f o

Article history: Received 23 February 2015 Received in revised form 22 April 2015 Accepted 19 May 2015 Available online xxxx Keywords: Biomass Coal Ash Composition and properties Advantages and disadvantages

a b s t r a c t An extended overview of the advantages and disadvantages of biomass composition and properties for biofuel application was conducted based on reference peer-reviewed data plus own investigations. Initially, some general considerations and comparisons about composition and properties of biomass and coal as the most popular solid fuel are addressed. Then, some of the major advantages related to the composition and properties of biomass and/or biomass ash (BA) are discussed. They include: (1) high values of volatile matter, H, structural organic components, extractives and reactivity of biomass, water-soluble nutrient elements and alkaline-earth elements in biomass and BA, and pH of BA; and (2) low values of C, fixed C, ash, N, S, Si and initial ignition and combustion temperatures of biomass, and low contents of many trace elements including hazardous ones in biomass and BA. Further, some of the major disadvantages connected with the composition and properties of biomass and/or BA are described. They comprise: (1) high values of moisture and O in biomass, water-soluble fraction, alkaline and halogen elements, and some hazardous trace elements in biomass and BA; (2) low values of energy density (bulk density and calorific value), pH and ash-fusion temperatures of biomass, and bulk density and size of BA; (3) highly variable composition and properties of biomass and BA; and (4) indefinite availability of sustainable biomass resources for production of biofuels. Finally, a discussion about the availability of sustainable biomass resources for production of biofuels and biochemicals is given. It was found that the disadvantages of biomass for biofuel and biochemical applications prevail over the advantages; however, the major environmental, economic and social benefits appear to compensate the technological and other barriers caused by the unfavourable composition and properties of biomass. Ó 2015 Published by Elsevier Ltd.

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Contents 1. 2. 3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Materials, methods and data used . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Results and discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. General considerations and comparisons about composition and properties of biomass and coal . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Advantages of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.1. Volatile matter, combustion temperatures and reactivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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⇑ 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.2015.05.050 0016-2361/Ó 2015 Published by Elsevier Ltd.

Please cite this article in press as: Vassilev SV et al. Advantages and disadvantages of composition and properties of biomass in comparison with coal: An overview. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.05.050

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3.2.2. Carbon, fixed carbon and hydrogen. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.3. Structural organic components . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.4. Extractives. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.5. Ash yield and inorganic matter . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.6. Water-soluble nutrient elements. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.7. Alkaline-earth elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.8. Nitrogen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.9. Sulphur . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.10. Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.11. Trace elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2.12. pH . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Disadvantages of biomass . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.1. Moisture . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.2. Alkaline and halogen elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.3. Hazardous trace elements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.4. Ash-fusion temperatures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.5. Oxygen . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.6. Energy density, bulk density and calorific value . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.7. Size . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.8. Variable composition and properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3.9. Sustainable biomass resources. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00 00

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Nomenclature A AFT BA daf db DTA DWR EDX FC HHV IAM ICP

ash yield ash-fusion temperature biomass ash dry, ash-free basis dry basis differential-thermal analysis dry water-soluble residue energy dispersive X-ray analyser fixed carbon higher heating value inorganic amorphous matter inductively coupled plasma

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1. Introduction

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Biomass can be converted into solid, liquid and gaseous biofuels for generating bioenergy, as well as into some chemicals. It is widely accepted that biofuels combustion does not contribute to the greenhouse effect due to the CO2 neutral conversion thanks to the renewability of biomass. The focus on bioenergy as an alternative to fossil energy has increased tremendously in recent times because of global warming problems originating mostly from fossil fuels combustion. Therefore, extensive investigations have been carried out worldwide recently to enhance biomass use instead of fossil fuels for energy production ([1–7] and references therein). Numerous biomass varieties among biomass groups, namely wood and woody biomass, herbaceous and agricultural biomass, aquatic biomass, animal and human biomass wastes, semi-biomass (contaminated biomass and industrial biomass wastes such as municipal solid waste, refuse-derived fuel, sewage sludge, demolition wood and other industrial organic wastes) and their biomass mixtures can be used for biofuels and biochemicals [1,2]. In total about 95–97% of the world’s bioenergy is currently produced by direct combustion of biomass and the perspective of increasing large-scale combustion of natural biomass and its co-combustion with semi-biomass and solid fossil fuels (coal, peat, petroleum coke) seems to be one of the main drivers for biofuel promotion

93 94 95 96 97 98 99 100 101 102 103 104 105 106 107 108 109 110 111 112 113

IM LA M MS OM SEM TE TGA VM XRD %

inorganic matter laser ablation moisture mass spectrometry organic matter scanning electron microscopy trace element thermo-gravimetric analysis volatile matter X-ray powder diffraction weight%

in many countries worldwide in the near future ([3] and references therein]). Two fundamental aspects related to biomass use as fuel are: (1) to extend and improve the basic knowledge on composition and properties; and (2) to apply this knowledge for the most advanced and sustainable utilisation of biomass. The fuel composition is a fundamental code that depends on various factors and definite properties, quality and application perspectives, as well as different technological and environmental problems related to any fuel [1]. Therefore, 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 utilisation, technological and ecological advantages and challenges of BA [4]; and (5) behaviour of biomass during combustion, namely 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 been introduced therein [1–6]. Additional investigations on trace element concentrations and associations in biomass and BA have also been conducted

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[7]. It was highlighted in the above studies that there are different advantages and disadvantages related to biomass composition and properties for fuel application and they have very important ecological and technological impacts during sustainable utilisation of biomass fuels and their products. Studies connected with advantages and disadvantages of using biomass fuels for thermochemical (combustion, pyrolysis, gasification and liquefaction) and biochemical (anaerobic digestion, alcoholic fermentation, aerobic biodegradation) conversions, as well as co-combustion, co-pyrolysis and co-gasification of biomass with other solid fuels have been performed worldwide ([1–48] among others). As a result, substantial data for the composition and properties of biomass, biochar and BAs such as low-temperature and high-temperature laboratory ashes and industrial bottom ashes, slags and fly ashes, along with the behaviour of different biomass varieties during their thermal treatment have been generated and summarised recently [1–7]. Some comparative characterizations between biomass and other fossil fuels have also been given in some of the above investigations. It is well known that ‘‘the methodology and logic from coal experiments can be applied to biomass’’ [8]. However, parallel and detail comparisons between biomass and coal as the most popular solid fuel, as well as their respective conversion products based on numerous characteristics, namely: (1) chemical composition (major, minor and trace elements); (2) phase-mineral composition of organic matter (cellulose, hemicellulose, lignin, extractives, petrographic ingredients, char) and inorganic matter (mineral classes, groups and species, and inorganic phases); and (3) various properties (volatile matter, fixed C, moisture, ash yield, ash-fusion and combustion temperatures, density, pH, calorific value and water-soluble components); are still limited. Therefore, an attempt to summarise the advantages and disadvantages of the above characteristics based on the data from numerous combined investigations for both biomass and coal was undertaken and is described below. The major aims of the present overview are: (1) to systematize and summarise the peer-reviewed data; (2) to supply additional own results; (3) to describe some basic findings; and (4) to clarify the advantages and disadvantages related to the biomass composition and properties compared to coal as a traditional and conventional fossil fuel. Indications of some potential technological and environmental challenges during processing of biomass fuels, as well as application of their products are also addressed.

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2. Materials, methods and data used

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The summarised data for the composition and properties of biomass and coal based on own data-base including numerous mostly peer-reviewed publications (totally more than 840 references) have been used for the present and former overviews ([1–6] and references therein). Additionally, eight biomass samples (beech wood chips, corn cobs, marine macroalgae, plum pits, rice husks, switchgrass, sunflower shells and walnut shells) were collected and studied in detail for the determination and elucidation of many characteristics related to the chemical and phase–mineral composition of biomass and BA, and behaviour of biomass during combustion [2–7]. The samples investigated belong to different biomass groups and sub-groups classified by origin (biodiversity and source), as well as to variable organic and inorganic types and sub-types specified for biomass and BA [1–3,6]. These biomass samples were studied using methods such as light microscopy, powder X-ray diffraction (XRD) and scanning electron microscopy (SEM), and differential-thermal (DTA), thermo-gravimetric (TGA) and different chemical analyses, as well as some leaching, precipitation, ashing (500–1500 °C) and other procedures described in detail earlier [2,3,5–7].

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3. Results and discussion

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3.1. General considerations and comparisons about composition and properties of biomass and coal

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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 and phases [1–7]. For example, the composition and properties of biomass and BA 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, contamination, transport, storage, processing, others); (2) biomass fuel preparation (pre-treatment and upgrading such as drying, cutting-grinding, densification, and torrefaction); (3) biomass combustion (combustion technology and conditions, collection and cleaning equipment); and (4) transport and storage of BA. It should be stated that the composition and properties of biomass and BA are much more variable than those of coal and coal ash (Tables 1 and 2 and [1–7]). The elemental composition of biomass and BA may potentially include the entire periodic table and comprises: (1) major (>1%); (2) minor (0.1–1%); and (3) trace (<0.1%) elements; according to their contents (Tables 1 and 2). As a general trend (Table 2), most of the elements determined in biomass are trace elements (TEs) excluding C, Ca, Cl, H, K, Mg, N, Na, O, P and S based on the Clarke contents (worldwide average concentrations) for angiospermous and gymnospermous plants [49] and world reference plant (worldwide averages of all parts of all plants) [16]. 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 [50] and world reference plant ash [16]. Nevertheless, there are many individual cases among biomass varieties and their ashes where the concentrations for certain major, minor and TEs are interchanged. A summarised comparison between the mean values of elements and some properties for biomass and BA and those for coal and coal ash, respectively (Tables 1 and 2, and [1–7,16,49–52]), are listed in Table 3. As a general trend, it can be seen from the above data that most non-metals and especially halogen non-metals (B, Br, Cl, H, I, O, P), alkaline and alkaline-earth lithophile elements (Ca, K, Mg, Na, Rb, Sr), and only some chalcophile (Cu, Ga, Hg, Te, Zn), siderophile (Cr, Mn), noble (Ag, Ru) and other lithophile (Mo) elements are commonly more enriched in biomass and BA than in coal and coal ash, respectively. The reason for that is mostly associated with higher amount of organic matter (OM) and limited inorganic matter (IM) with authigenic and, to a lesser extent, detrital origin in biomass than in coal [1–3,7]. In contrast, most elements among lithophile, siderophile, chalcophile, radioactive and noble elements, and only some non-metals (C, F, N, S, Si) are normally more enriched in coal and coal ash than in biomass and BA, respectively (Table 3). The reason for that is mainly associated with higher amount of IM with detrital and, to a lesser extent, authigenic nature in coal than in biomass [1–3,7]. For example, it is well known that the bulk amount of ash-forming constituents in biomass based on ash yield is generally much lower in contrast to coal (Table 1). However, the concentrations of some elements 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 (cellulose, hemicellulose and lignin) in biomass. For instance, Ag, B, Br, Ca, Cr, Cu, Ga, Hg, I, K, Mg, Mo, Na, P, Rb, Sr, Te, Zn and others in BA can exhibit much higher enrichment factors in comparison with coal ash (Tables 1 and 2, and [7]). The bulk chemical composition of biomass and BA is an important characteristic, but it is quite insufficient for a reliable

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Table 1 Mean and range values for common characteristics of coal, biomass, coal ash and biomass ash based on [1–7] and references therein, wt.% on air-dried basis. Coal ash

Biomass ash

EDFa biomass/coal

Characteristic

Coal

Biomass

Proximate analysis VM FC M A Sum

30.8 (12.2–44.5) 43.9 (17.9–70.4) 5.5 (0.4–20.2) 19.8 (5.0–48.9) 100.0

64.4 (30.4–79.7) 16.0 (6.5–35.3) 14.7 (2.5–62.9) 4.9 (0.1–34.3) 100.0

2.09 0.36 2.67 0.25

Ultimate analysis C (daf) O (daf) H (daf) N (daf) S (daf) Sum

78.2 (62.9–86.9) 13.6 (4.4–29.9) 5.2 (3.5–6.3) 1.3 (0.5–2.9) 1.7 (0.2–9.8) 100.0

51.1 (42.2–60.5) 41.4 (20.8–49.0) 6.2 (3.2–10.2) 1.1 (0.1–12.2) 0.20 (0.01–1.69) 100.0

0.65 3.04 1.19 0.85 0.12

EDFb biomass ash/coal ash

Ash analysis SiO2 CaO K2O P2O5 Al2O3 MgO Fe2O3 SO3 Na2O TiO2 Sum

54.06 (32.04–68.35) 6.57 (0.43–27.78) 1.60 (0.29–4.15) 0.50 (0.10–1.70) 23.18 (11.32–35.23) 1.83 (0.31–3.98) 6.85 (0.79–16.44) 3.54 (0.27–14.42) 0.82 (0.09–2.90) 1.05 (0.62–1.61) 100.00

29.14 (0.02–94.48) 25.99 (0.97–83.46) 19.40 (2.19–63.90) 5.92 (0.54–40.94) 4.49 (0.10–15.12) 5.60 (0.19–16.21) 3.41 (0.22–36.27) 3.27 (0.01–14.74) 2.54 (0.09–29.82) 0.24 (0.01–2.02) 100.00

0.54 3.96 12.13 11.84 0.19 3.06 0.50 0.92 3.10 0.23

Ash fusion test DT (°C) HT (°C) FT (°C)

1251 (1105–1525) 1388 (1200–1575) 1411 (1205–1585)

1103 (670–1565) 1319 (975–1665) 1354 (1000–>1700)

0.88 0.95 0.96

800 (400–1100)

194 (80–430)

10.0 (6.2–12.5) 1.6 (0.2–7.2)

10.3 (8.1–12.9) 27.0 (3.9–45.1)

Other characteristics Bulk density (kg m 3) HHV (MJ kg 1) Initial ignition by DTA–TGA (°C) Peak combustion by DTA–TGA (°C) pH of leachate DWR

1250 (1100–1300) 25.0 (16.0–34.0) 256 (190–322) 508 (415–600) 5.9 (2.2–7.5) 2.1 (0.2–8.4)

563 (250–954) 18.0 (14.0–22.0) 167 (144–190) 339 (324–351) 5.6 (5.1–6.8) 3.8 (0.3–15.1)

0.45 0.72 0.65 0.67 0.95 1.81

0.24

1.03 16.88

Abbreviations: A, ash yield; daf, dry ash-free basis; db, dry basis; DT, deformation temperature; DTA, differential-thermal analysis; DWR, dry water-soluble residue; FC, fixed carbon; FT, fluid temperature; HHV, higher heating value; HT, hemispherical temperature; M, moisture; TGA, thermo-gravimetric analysis; VM, volatile matter. a Enrichment/depletion factor (EDF) is defined as a ratio of the value for biomass to the respective value for coal. b Enrichment/depletion factor (EDF) is defined as a ratio of the value for biomass ash to the respective value for coal ash.

265 266 267 268 269 270 271 272 273 274 275 276 277 278 279 280 281 282 283 284 285 286 287 288 289

explanation of elemental behaviour during biomass and BA utilisation because the knowledge about the modes of element occurrence in biomass and BA is also required for that purpose. 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 above issue. The elements in biomass and BA can occur in both OM and IM as each element has dominant associations and affinities to specific components, phases or minerals [7]. Therefore, the phase–mineral composition of biomass [2] and BA [3] has been summarised recently. The data indicate that the modes of element occurrence in biomass and BA can be similar to coal and coal ash [2,3,7]. It was found that numerous minerals and phases identified in coal and coal ash were also present in biomass and BA. However, many mineral species among Ca–K–Mn silicates, Ca–Al–Mn oxides, K–Na–Ca chlorides and K–Ca–Mg–Na carbonates, sulphates and phosphates were normally not identified in coal and coal ash. The reason for that could be the enrichment of elements such as Ca, Cl, K, Mg, Mn, Na and P in biomass and BA compared to coal and coal ash. In contrast, many Al-, Fe-, Si- and Ti-bearing minerals, typical for coal and coal ash, were not identified in biomass and BA probably due to limited contents of these elements in biomass and BA [2,3]. The above-listed observations on the composition, as well as some properties given in Table 3 (see below) have a primary importance for evaluating different technological and environmental

problems or benefits related to thermochemical and biochemical conversions of biomass, coal and other solid fuels [4–7].

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3.2. Advantages of biomass

292

The major advantages and disadvantages related to the use of biomass as fuel are listed in Tables 4 and 5. It should be noted that the benefits in some cases can be barriers in other circumstances and vice versa during biomass and biofuel application. The highly variable composition and properties of biomass have been described in detail [1–7]. Therefore, the evaluation of some key advantages and disadvantages related to biomass and BA composition and properties in the present study is based on the mean values of many characteristics to assess the general trends compared to coal and coal ash (Tables 1–3). Most of the common advantages of biomass and biofuel related to environmental, technological, economic and social issues (Table 4) have been described relatively well, while the benefits connected with the composition and properties of biomass and biofuel have been discussed to a lesser extent in the literature [1–48]. Some of the major advantages related to the composition and properties of biomass and/or BA include: (1) high values of volatile matter (VM), H, structural organic components, extractives and reactivity of biomass, water-soluble nutrient elements and alkaline-earth elements in biomass and BA, and pH of BA; and

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S.V. Vassilev et al. / Fuel xxx (2015) xxx–xxx Table 2 Clarke contents (worldwide average element concentrations) of coal, biomass, coal ash and biomass ash based on [7], ppm on dry basis. Element

Clarke for coalsa,b

World reference plantc

Clarke for angiospermous plantsd

Clarke for gymnospermous plantse

Clarke for coal ashesf,g

World reference plant ashh

Clarke for plant ashesi

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 Rh Ru S Sb Sc Se Si Sm Sn Sr Ta Tb Te Th Ti Tl Tm U

0.095a 15,000b 8.3a 0.0037a 52a 150a 1.6a 0.97a 5.2a 630000b 4600b 0.22a 23a 180a 5.1a 16a 1.0a 16a 2.1a 0.93a 0.47a 88a 13000b 5.8a 2.7a 2.2a 52000b 1.2a 0.10a 0.54a 1.9a 0.031a 0.002a 1800b 11a 12a 0.20a 1100b 86a 2.2a 13000b 800b 3.7a 12a 13a 160000b <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

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

0.07 65

0.61f 114500g 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 99200g 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

8.3f 0.75f 4.0f 12.6f 0.16f 0.010f 13700g 69f 66f 1.2f 8400g 490f 14f

1.0 2.0 0.2 61 0.02 0.002 387,755 4.1 4.1 0.06 40,816 4082 10

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

3.5a 0.035a 14a <0.001b <0.001b <0.001b 18000b 0.92a 3.9a 1.3a 27000b 2.0a 1.1a 110a 0.28a 0.32a <0.1b 3.3a 800a 0.63a 0.31a 2.4a

0.05 0.00005 50

20f 0.13f 79f <0.008g <0.008g <0.008g 137400g 6.3f 23f 8.8f 206100g 13f 6.4f 740f 1.7f 2.1f <0.8g 21f 4650f 4.9f 2.0f 16f

1.0 0.001 1020

0.021 0.5 140 0.05

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

400 100 2.1 0.0005 150 30,000 0.01 10,000 15 250 2.0 200

10 10,000 50

0.05 0.001 50 0.05 0.000005 30,000 11 70,000 7500 20

50

0.0005

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

20

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

<0.35

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

0.000005 100 0.000005 0.00005 0.00005 50,000 0.05 0.5 150,000 5.0 30 0.005

0.5 1000 0.005 0.5 (continued on next page)

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Table 2 (continued)

a b c d e f g h i

Element

Clarke for coalsa,b

World reference plantc

Clarke for angiospermous plantsd

Clarke for gymnospermous plantse

Clarke for coal ashesf,g

World reference plant ashh

Clarke for plant ashesi

V W Y Yb Zn Zr

25a 1.1a 8.4a 1.0a 23a 36a

0.5 0.2 0.2 0.02 50 0.1

1.6 0.07 <0.6 <0.0015 160 0.64

0.69

155f 6.9f 51f 6.2f 140f 210f

10 4.1 4.1 0.4 1020 2.0

61 0.005 10

Clarke Clarke World Clarke Clarke Clarke Clarke World Clarke

<0.24 26 0.24

900

for coals worldwide [52]. for US coals [51]. reference plant (worldwide average of all parts of all plants) [16]. for angiospermous plants worldwide [49]. for gymnospermous plants worldwide [49]. for coal ashes worldwide [52]. for US coal ashes recalculated from coal data for 13.1% ash yield [51]. reference plant ash (worldwide averages of all parts of all plants) [16], recalculated from plant data for 4.9% ash yield [1]. for plant ashes [50].

Table 3 Summarised comparisons between the mean values of elements and some properties for biomass and biomass ash and those for coal and coal ash, respectively (based on Tables 1 and 2, and [1–7,16,49–52]).

313 314 315 316 317 318 319 320

Biomass compared to coal

Biomass ash compared to coal ash

Higher contents or values of:  Ag, B, Br, Ca, Cl, H, I, K, Mg, Mn, Na, O, P, Rb, Ru, Zn  Carbohydrates  Carbonates  Chelates  Chlorides  Dry water-soluble residue  Extractives  Light hydrocarbons  Moisture  Organically bound inorganic elements  Oxalates  Oxygen-containing functional groups (hydroxyl, carboxyl, ether and ketone groups) with highly reactive functionalities (–COOH, –OCH3 and –OH)  Phosphates  Volatile matter  Water-soluble components Lower contents or values 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, N, 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, Zr  Aromaticity  Ash yield  Bulk density  Fixed C  Friability  Functionalities  Higher heating value  Hydrocarbons  Initial ignition temperature  Inorganic matter  Peak combustion temperature  Ph  Oxy hydroxides  Silicates  Sulphates–sulphides

Higher contents or values of:  Ag, B, Br, Ca, Cl, Cr, Cu, Ga, Hg, I, K, Mg, Mn, Mo, Na, P, Rb, Sr, Te, Zn  Dry water-soluble residue  pH  Carbonates  Chlorides  Oxy hydroxides  Phosphates  Water-soluble components

(2) low values of C, fixed C (FC), ash yield (A), N, S, Si and initial ignition and combustion temperatures of biomass, and low contents of many TEs including hazardous ones in biomass and BA (Tables 1–3). 3.2.1. Volatile matter, combustion temperatures and reactivity The high VM content (especially combustible VM) and low initial ignition and combustion temperatures (Table 1 and Fig. 1) are among the greatest advantages of biomass for thermochemical

Lower contents or values 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, Zr  Ash-fusion temperatures  Bulk density  Silicates  Sulphates-sulphides

conversion because they are criteria for the highly reactive nature of this fuel [19,53,54]. Biomass volatiles mainly consist of different: (1) combustible species such as CH4, C2H2, CO, H2, H2S, tars and other mainly light hydrocarbons; and (2) incombustible components, namely CO2, HCl, H2O, N2, NH3, NOX (NO, NO2), N2O, KCl, KOH, NaCl, NaOH, SOX (SO2, SO3), others [10,17,28,54–59]. Additionally, biomass is enriched in carbohydrates and H instead of hydrocarbons [60]. For example, biomass has a VM/FC ratio typically >4.0 and frequently exceeding 5.0, while for coal VM/FC ratio

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S.V. Vassilev et al. / Fuel xxx (2015) xxx–xxx Table 4 Major advantages of biomass and biomass fuels.                

Renewable energy source for natural biomass CO2 neutral conversion and climate change benefits Transition to low carbon economy, namely from hydrocarbon to carbohydrate and H resources Use of nonedible biomass Conservation of fossil fuels Low contents of ash, C, FC, N, S, Si and most trace elements High concentrations of volatile matter, Ca, H, Mg and P, structural organic components, extractives, water-soluble nutrient elements Biodegradable resource with great reactivity and low initial ignition and combustion temperatures during conversion Huge and cheap resource for production of biofuels, sorbents, fertilizers, liming and neutralizing alkaline agents, building materials, synthesis of some minerals and recovery of certain elements and compounds Reduction of biomass residues and wastes Decrease of hazardous emissions (CH4, CO2, NOX, SOX, toxic trace elements) Capture and storage of toxic components by ash Use of oceans, seas, low-quality soils and non-agricultural, degraded and contaminated lands Restoration of degraded and contaminated lands Diversification of fuel supply and energy security Rural revitalization with creation of new jobs and income

Table 5 Major disadvantages of biomass and biomass fuels.                     

Incomplete renewable energy resource for biofuels with respect to complete life cycle assessment Competition with edible biomass (food, feed), fibre and biomaterial productions Damage of natural ecosystems (water, soil, land use changes, deforestation, biodiversity, land degradation, fertilizers, pesticides, contaminants) Insecurity of biomass feedstock supply Indefinite availability of sustainable biomass resources for production of biofuels and chemicals Omission of sustainable criteria for production of biomass resources for biofuels and chemicals Lack of global monitoring and control of biofuels production with certification of origin and source Miss of accepted terminology, methodologies, standards and classification and certification systems Insufficient knowledge and variability of composition, properties and quality for assessment and validation High contents of moisture, water-soluble fraction, Cl, K, Na, O and some trace elements (Ag, Br, Cd, Co, Cr, Cu, Hg, Mn, Ni, Pb, Se, Tl, Zn, others) Low energy density (bulk density and calorific value) Low pH and ash-fusion temperatures Low bulk density and fine size of ash with increased dust inhalation risk Technological problems during processing (agglomeration, deposit formation, slagging, fouling, corrosion, erosion) Odour, emission and leaching of hazardous components during disposal and processing Use of extra water, fertilizers and pesticides Great growing, harvesting, collection, transportation, storage and pre-treatment costs Regional and seasonal availability and local energy supply Limited practical experience in biofuel production and unclear utilisation of waste products Miss of developed biomass markets High investment cost

WWB - wood and woody biomass HAB - herbaceous and agricultural biomass HAG - herbaceous and agricultural grass HAS - herbaceous and agricultural straw HAR - herbaceous and agricultural residue AB - animal biomass MB - mixture of biomass CB - contaminated biomass AVB - all varieties of biomass P - peat L - lignite S - sub-bituminous coal B - bituminous coal A - algae

Fig. 1. Mean proximate composition of the biomass groups and sub-groups, and four solid fossil fuel types based on 87 biomass varieties and 38 solid fossil fuels [1], wt.%.

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S.V. Vassilev et al. / Fuel xxx (2015) xxx–xxx

is virtually always <1.0 [61]. Therefore, the high content of VM (more typical of wood and woody biomass and herbaceous and agricultural biomass in Fig. 1 and [1]) causes: (1) low ignition temperature [41,57,62,63]; (2) easier and rapid ignition, devolatilisation and burning [10,62]; (3) higher production of combustible gas and inorganic vapours [8,41,63]; (4) faster oxidation of VM than char [53,54] or more ‘‘flaming combustion’’ and less ‘‘char combustion’’ [64]; (5) improved combustion resulting in a better and quicker burnout with lower unburned C in the ash [19,62,65] when the fuel particle size is suitable; and (6) formation of biochar with high specific surface area [4,12]. Finally, there is a synergic benefit during co-firing of coal with biomass because the high VM contents in biomass provide a more stable flame for the mixed fuel [66]. On the other hand, the combustion of high VM biomass is rapid and difficult to control, and requires a bigger reactor volume and optimum design in order to achieve complete combustion and low pollutant emissions such as CO, unburned products, polycyclic aromatic hydrocarbons and others [10,64,67]. 3.2.2. Carbon, fixed carbon and hydrogen The low values of C (more typical of algae and herbaceous and agricultural biomass in Fig. 2 and [1]) and FC (more characteristic of semi-biomass, animal biomass, straws and grasses in Fig. 1 and [1]) have significant benefit for reducing CO2 emissions during biomass conversion. Carbon dioxide is a primary agent of global warming among the greenhouse gas emissions despite that CH4 and N2O have 25 and 300 times stronger global warming potential than CO2, respectively [30]. As a result CO2 constitutes 72% of the total anthropogenic greenhouse gases, causing between 9% and 26% of the greenhouse effect [68]. Carbon dioxide is a major combustion product from all biomass fuels and its emissions are regarded as being CO2-neutral with respect to the greenhouse gas effect and this is considered to be the main environmental benefit of biomass combustion [23]. However, the incomplete biomass combustion can also lead to emissions of unburnt C-based pollutants such as CO, methane, polycyclic aromatic hydrocarbons, dioxins, furans, tar, soot and other hydrocarbons, and special emission reduction measures are applied [23,69]. It is well known that a fundamental difference between biofuels and fossil fuels is the lower amount of C and the higher proportion of O and H in biomass

(Fig. 2). This reduces the energy value of biofuel due to the lower energy contained in C–O and C–H bonds than in C–C bonds [54]. The typical increased H/C and O/C ratios in biomass imply decreasing aromaticity and increasing role of the oxygen-containing hydroxyl, carboxyl, ether and ketone functional groups [8]. On the other hand, FC represents the fraction of fuel which will undergo heterogeneous combustion reactions [54]. The FC, C and H contents have a direct positive relationship with the calorific values of biomass as latter are generally higher in terrestrial, perennial, forestry and woody biomass, than in aquatic, annual, herbaceous and agricultural biomass [2,69–73]. Finally, the high content of H in biomass (more characteristic of animal biomass, semi-biomass and herbaceous and agricultural residues in Fig. 2 and [1]) is an advantage (see also above) due to: increased role of combustible H2, H2S, carbohydrates, hydrocarbons and H-containing functional groups, greater volatility and highly reactive nature of this fuel [8,19,53,54].

369

3.2.3. Structural organic components It is well known that dependent upon its structural composition (cellulose, hemicellulose and lignin), each variety of biomass can be better suited to specific pre-treatment and conversion processes [14,47,74,75]. For example, the high contents of cellulose (more typical of wood stems and herbaceous and agricultural stalks and fibres [2]) and hemicellulose (more characteristic of wood twigs, leaves and barks, algae and some grasses and grains [2]) in biomass can be leading features for considering suitable processing technologies due to: (1) increased reactivity, hydrophility, hydrolysis, oxidation, volatility, crystallinity, sugar and inorganic matter; and (2) decreased calorific value, aromaticity, density and char yield ([2] and references therein). On the other hand, the high contents of lignin (more typical of agricultural shells, husks and pits, and certain wood barks [2]) in biomass can be leading features for considering suitable processing technologies due to: (1) increased calorific value, density, hydrophobicity, mechanical strength, binder properties, char yield, aromaticity, soot formation, variety of functional groups, as well as greater resistance to natural degradation, biological digestion and many chemical agents; and (2) decreased inorganic matter, oxidation, reactivity, volatility and crystallinity ([2] and references therein). Therefore, the

386

WWB - wood and woody biomass HAB - herbaceous and agricultural biomass HAG - herbaceous and agricultural grass HAS - herbaceous and agricultural straw HAR - herbaceous and agricultural residue AB - animal biomass MB - mixture of biomass CB - contaminated biomass AVB - all varieties of biomass P - peat L - lignite S - sub-bituminous coal B - bituminous coal A - algae

Fig. 2. Mean ultimate composition of the biomass groups and sub-groups, and four solid fossil fuel types based on 87 biomass varieties and 38 solid fossil fuels [1], wt.%.

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structural composition of biomass varieties among biomass groups and sub-groups were specified for application purposes into six different structural types (‘‘CHL’’, ‘‘CLH’’, ‘‘HCL’’, ‘‘HLC’’, ‘‘LCH’’, ‘‘LHC’’) based on the contents of cellulose, hemicellulose and lignin (Fig. 3 and [2]). Finally, the dominant occurrence of specific structural components in biomass can also be an advantage as precursors for the production of high quality biochars with high surface area and specially tailored active sorption properties [2,4]. For example, biomass varieties enriched in lignin from ‘‘LCH’’, ‘‘LHC’’ and ‘‘HLC’’ structural types and from inorganic low acid ‘‘K’’ type can favour the production of such advanced biochars [2]. 3.2.4. Extractives The bulk extractives of biomass consist of various organic and inorganic components extracted individually or sequentially by different polar or non-polar solvents, namely water, ethanol, benzene and toluene, and occasionally acetone, dichloromethane, ethers, heptane, hexane, methanol, methylene chloride, petroleum spirit and their mixtures [2]. The high content of extractives in biomass (more typical of herbaceous and agricultural fibres, grasses and residues, and algae [2]) is an advantage or strong indicators for: (1) potential production of biodiesel, bioethanol, biomethanol, bio-oil and other biofuels and biochemicals; (2) occurrence and potential recovery and use of lipids, proteins, fats, oils, terpenes, tannins, resins, sugars, starches, organic acids, inorganic salts and other organic compounds; (3) increased proportions of water-soluble fraction and extractable inorganic matter, and higher heating value; and (4) decreased oxidation [2,17,53,57,76– 80]. 3.2.5. Ash yield and inorganic matter The low ash content of biomass (more typical of wood and woody biomass and some herbaceous and agricultural biomass varieties in Fig. 1 and [1]) is more desirable and has a great benefit for higher fuel quality [19,81] due to: (1) increasing heating value

9

[20]; (2) easier thermochemical and biochemical conversions [69,82,83]; (3) less fouling, deposition, agglomeration, slagging, corrosion and erosion problems [28,41,69,82]; (4) decreasing operating costs concerning biomass harvesting, transport and processing, gas-cleaning technologies, as well as ash transport, disposal and utilisation [3,4,69,82,84]; (5) less fuel contamination by soil, dirt, rainfall, wind, fertilizers, pesticides and additives during biomass growing, harvesting, transport and processing [1,53,85]; (6) assessing the best time for biomass harvesting [44,86,87] and others. It is well known that the biggest technological and environmental challenges that biofuel faces today are mostly related to the occurrence, proportion, origin and behaviour of inorganic matter in biomass [2–7,28]. The inorganic constituents in biomass are normally much less than in solid fossil fuels, excluding aquatic biomass, animal biomass and some varieties from herbaceous, agricultural and contaminated biomass (Table 1, Fig. 1 and [1,2]). It should be noted that the ash contents in certain seaweeds, municipal solid wastes, paper sludge and sewage sludge can even reach up to 50–70% [16,88–92]. The inorganic matter in biomass and BA includes mineral matter, namely mineral species and poorly crystallized mineraloids from different mineral groups and classes, as well as amorphous inorganic phases with different origin [2,3]. It is supposed that the genetic inorganic types in biomass and BA may have a leading importance for technological and environmental problems similar to coal and coal ash [2–7]. Certain major associations related to the occurrence, content and origin of inorganic elements and phases were identified in BA system and they include: (1) Si–Al–Fe–Na–Ti; (2) Ca–Mg–Mn; and (3) K–P–S–Cl [1–4]. These associations were applied for classifications of IM in biomass (Fig. 4) and BAs (Fig. 5) to four types (‘‘S’’, ‘‘C’’, ‘‘K’’ and ‘‘CK’’) and six sub-types (‘‘S-HA’’, ‘‘S-MA’’, ‘‘C-MA’’, ‘‘C-LA’’, ‘‘K-MA’’ and ‘‘K-LA’’). It was found that these systematic associations have a key importance, namely their potential application for classification and indicator purposes connected with innovative and sustainable processing of biomass and BA [4–7]. For example,

WWB - Wood and woody biomass WWS - Stems WWBA - Barks WWT - Twigs WWL - Leaves WWO - Others HAB - Herbaceous and agricultural biomass HAG - Grasses HAS - Straws HAST - Stalks HAF - Fibers HASH - Shells and husks HAP - Pits HAR - Other residues AB - Animal biomass CB - Contaminated biomass AVB - All varieties of biomass NB - Natural biomass

Fig. 3. Mean structural composition of the biomass groups and sub-groups based on 93 biomass varieties [2], wt.%.

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WWB - Wood and woody biomass HAB - Herbaceous and agricultural biomass HAG - Herbaceous and agricultural grass HAS - Herbaceous and agricultural straw HAR - Herbaceous and agricultural residue AB - Animal biomass MB - Mixture of biomass CB - Contaminated biomass AVB - All varieties of biomass P - Peat L - Lignite S - Sub-bituminous coal B - Bituminous coal A - Algae

Fig. 4. Areas of 87 biomass varieties and 38 solid fossil fuels in the chemical classification system of inorganic matter in biomass [2], wt.%.

WWB - Wood and woody biomass HAB - Herbaceous and agricultural biomass HAG - Herbaceous and agricultural grass HAS - Herbaceous and agricultural straw HAR - Herbaceous and agricultural residue AB - Animal biomass MB - Mixture of biomass CB - Contaminated biomass AVB - All varieties of biomass P - Peat L - Lignite S - Sub-bituminous coal B - Bituminous coal A - Algae

Fig. 5. Areas of 87 biomass varieties and 38 solid fossil fuels in the chemical classification system of biomass ash [1,3,7], wt.%.

476 477 478 479 480 481 482

the concept of dividing IM into detrital, authigenic and technogenic types of biomass (Fig. 4 and [2]), and original (primary) or newly formed secondary and tertiary types of BA (Fig. 5 and [3–6]) has both fundamental and applied aspects. The detrital minerals (silicates and oxyhydroxides) are commonly stable during weathering, less mobile (water-insoluble) and less reactive, and with high-melting temperatures during biomass processing. In contrast,

the authigenic minerals (opal, oxalates, carbonates, phosphates, sulphates, chlorides and nitrates) are normally unstable during weathering, highly mobile (water-soluble) and reactive, and with low decomposition or melting temperatures during biomass processing. Further, the technogenic IM includes various mineral species with more variable properties and behaviour in comparison with the natural inorganic constituents. Therefore, IM types,

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S type

% 60 50 40 30 20 10 0

C type

% 60 50 40 30 20 10 0 % 60

K type

50 40 30 20 10 0

CK type

% 60 50 40 30 20 10 0

Four types

% 60 50 40 30 20 10 0 Amorphous matter

Oxyhydroxides

Crystalline matter

Phosphates

Carbonates

Sulphates

Silicates

Chlorides

Fig. 6. Mean distribution of amorphous matter, crystalline matter and mineral classes in biomass ash types based on eight biomass ashes produced at 500, 700, 900, 1100, 1300 and 1500 °C [6], wt.%.

490 491 492 493 494 495 496 497 498 499

mineral classes and groups and specific mineral species are likely to be the major reasons for many problems during biomass processing (similar to coal [2]). For example, the authigenic minerals can be highly responsible for enhanced leaching behaviour, low-temperature transformations, partitioning behaviour and emission (or capture) of many volatile elements and hazardous components, corrosion, agglomeration, deposits formation, slagging, fouling, bed defluidization and composition of residues during biomass processing. The detrital minerals can be important for enhanced abrasion-erosion (hard and angular quartz, feldspars,

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rutile, corundum), formation of some low-temperature eutectics, partitioning element behaviour and for decreasing combustion efficiency and increasing operating costs for the handling of inert materials during biomass processing. The technogenic minerals can also be responsible for many of the above listed problems plus enhanced pollution by heavy metals, and this is because most semi-biomass fuels contain high levels of TEs [2]. Organic matter and authigenic minerals in biomass are intimately mixed with each other and their physical isolation (in contrast to chemical leaching), namely separation by screening, dense media treatment or flotation can be difficult during biomass upgrading. In contrast, detrital minerals in biomass occur as physically more easily separable particles similar to coal [2]. The unusually high ash yield determined in some biomass varieties can be a very strong indicator for contamination of biomass by detrital and technogenic materials [1,2]. Hence, many of the key constraints in the efficient thermal treatment of biomass may arise mostly from authigenic minerals. However, such minerals and some inorganic elements in biomass can also have a catalytic effect on thermochemical conversion. Many inorganic elements in biomass are also bound in OM and these organically associated elements and their phases react during the thermal treatment of biomass and appear dominantly as newly formed IM in residues ([2] and references therein). Finally, the newly formed secondary and tertiary minerals and phases in the thermochemical products generated from biomass (various silicates, oxyhydroxides, carbonates, chlorides, phosphates, sulphates, glass and char) also have specific impacts on the different technological and environmental problems related to the sustainable utilisation of biofuels (Figs 5–7 and [3–7]).

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3.2.6. Water-soluble nutrient elements The high contents of water-soluble macronutrient and micronutrient elements (B, Ca, Cl, Cu, H, K, Mg, Mn, Mo, N, O, P, S, Zn, others) in biomass and BA (Tables 1 and 2, Fig. 8 and [7]) are a big advantage. It was found that biomass and BA contain significant amounts of water-soluble components of 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) (Fig. 8, [7] and references therein). For example, the content of water-soluble fractions in biomass and BA are much higher than in coal and coal ash, respectively (Table 1 and [7,93,94]). Therefore, the water-soluble fractions play a very important role for biomass (particularly for algae and some herbaceous and agricultural biomass) and especially for all BAs [7]. Most of the above mobile elements associate preferably with OM in biomass and water-soluble minerals and phases in biomass and BA (chlorides, sulphates, oxides, hydroxides, oxalates, and nitrates plus some carbonates, bicarbonates, phosphates, silicates and amorphous material) [6]. Hence, significant portions of the above listed water-soluble nutrient elements in biomass and BA are bioavailable and can be used as fertilizers or for soil improvements supplying plant-growing, nutrient and essential elements for improving the natural balance in the system. Finally, some valuable water-soluble elements in biomass and BA are easily recoverable and can find another industrial utilisation [4]. However, it should be stated that the future large-scale biomass and BA application may create new environmental and technological concerns related to the fate of some water-soluble phases. For instance, these issues include potentially serious problems related to: (1) unavailable nutrients as water insoluble glass, silicates, phosphates and char; (2) hazardous TEs in/on mobile inorganic phases; (3) polluted char with polychlorinated dioxins and furans, and polycyclic aromatic hydrocarbons; (4) groundwater contamination; (5) pH shock and chemical burning damage to plants; (6) disturbing the microorganisms; (7) extra salinity problems; (8) dust emissions; (9) ash swelling and obstructing soil pores ([4,6] and references therein).

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Fig. 7. Positions of eight biomass varieties in the phase–mineral classification of biomass ash, based on mean contents of amorphous matter, silicates and others (carbonates + oxyhydroxides + phosphates + sulphates + chlorides) for biomass ashes produced at 500, 700, 900, 1100, 1300 and 1500 °C [6], wt.%. Abbreviations: BC, beech wood chips; CC, corn cobs; MM, marine macroalgae; PP, plum pits; RH, rice husks; SG, switchgrass; SS, sunflower shells; WS, walnut shells.

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Additionally, the extra accumulation of P, N, K and other plant nutrients into water is a natural aging process known as eutrophication. This anthropogenic process is a serious problem resulting in accumulation of large concentrations of specific plant nutrients in a relatively short amount of time [95]. 3.2.7. Alkaline-earth elements The high contents of alkaline-earth elements (Ca, Mg) in biomass and BA (Tables 1 and 2 and Figs. 4 and 5) are a big advantage for: (1) soil amendment and fertilization; (2) production of construction materials, adsorbents and ceramics; (3) synthesis of minerals; (4) recovery of valuable components; (5) multicomponent utilisation; and (6) reduction of some technological and environmental problems ([4] and references therein). Numerous biomass fuels especially among wood and woody biomass, algae, semi-biomass, animal biomass and some perennial herbaceous and agricultural biomass, are normally enriched in these elements ([1] and Figs 4 and 5). Proportions of alkaline-earth elements in biomass occur in mobile (water-soluble) and bioavailable Ca- and Mg-bearing chlorides, sulphates, oxalates and nitrates plus some carbonates and phosphates, as well as amorphous material with both organic and inorganic character [2]. These organic and inorganic Ca–Mg-containing phases in biomass form active and semi-active newly formed Ca–Mg minerals and phases in BA such as lime–portlandite, periclase–brucite, anhydrite–bassanite–gypsum, feldspars and other silicates, glass, carbonates, phosphates and chlorides. These reactive substances in water cause hardening and binder effects in the system during evaporation of water. Such effects are a result of formation of new and relatively more stable silicates, sulphates, carbonates, hydrates and oxyhydroxides containing water molecules and/or hydroxyl groups during hydration–dehydration, hydroxylation–dehydroxylation and subsequent carbonation processes of BA. These newly formed crystalline and amorphous products bind the pozzolanic (glass) and inert phases

(inactive to less active quartz, cristobalite, tridymite, mullite, other silicates and some char) relatively quickly in such multicomponent systems [6]. The above-mentioned complex processes play a leading role in the production of construction materials and some other applications, and the bulk chemical and phase–mineral composition of BAs provides vital information for that purpose. Therefore, it is possible to classify BAs to phase–mineral types and sub-types with dominant pozzolanic, inert or active properties (Fig. 7 and [4,6]) for different innovative and sustainable utilisation of BAs, as well as reducing some technological and environmental problems related to BA [4,6]. For example, biomass fuels with higher Ca contents exhibit more manageable slagging, fouling and corrosion problems [96,97]. The higher Ca contents in some biomass (mostly woody biomass) can reduce the lime/calcite usage and plant operation costs of the installed flue gas desulphurisation systems during the acid gas abatement ([2,4] and references therein). Additionally, some biomass fuels highly enriched in Ca and Mg could produce suitable bed materials that may reduce the need to use additives in fluidized-bed combustion chambers [4]. This type of combustion requires additional use of quartz, other silicates or carbonates as a bed material. Hence, biomass highly enriched in Ca and Mg (wood biomass and animal biomass) and silica (straws, grasses and rice husks) may reduce such a need [2]. Finally, the reason for increased capture of some volatile hazardous air pollutant elements in BAs is caused mostly by some Ca- and Mg-bearing minerals or phases with well-known sorbent properties, which are present in the combustion residues such as carbonates, oxyhydroxides, phosphates, sulphates, chlorides and amorphous matter [4,6]. The increased capture and immobilisation of some volatile and hazardous elements (C, S, Cl and in particular TEs) in combustion residues is phenomenon and it is known as the concept of ‘‘self-cleaning fuels’’ ([2,4,6] and references therein). Additionally, the data [2–4,6] also indicate that the biomass energy can be not only carbon-neutral, but also with some extra

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Fig. 8. Mean and range contents of water-soluble elements leached from: (a) biomass; and (b) biomass ash [6], wt.%.

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carbon-capture and storage potential due to fixation and immobilisation of CO2 in the combustion residues. For example, the mineral composition of BAs clearly shows the intensive formation of various newly formed carbonates. These carbonates are a result of solid–gas reaction between the volatile CO2 (released from biomass or occurring naturally in the atmosphere) and Ca and Mg oxyhydroxides formed during biomass combustion [2–4,6]. The last phenomenon is actual extra CO2 capture and storage mechanisms. Hence, there are potential possibilities for reduction of harmful emissions by modification of the feed fuel composition through tailored and self-cleaning fuel mixtures during co-firing or co-gasification of biomass with fossil fuels [2,3]. Finally, various Ca–Mg minerals in BA like Ca silicate hydrate gel, Ca aluminosilicate hydrate, portlandite, calcite and ettringite can be formed in the ash disposals (under high pH) and such crystallisations may reduce mobility either by physically reducing the porosity of the ash or by chemically binding the elements ([4] and references therein). On the other hand, the high contents of alkaline-earth and alkaline oxides in ash may cause burning damage on human and plant tissues [98]. 3.2.8. Nitrogen The low N content in biomass (especially for wood and woody biomass in Fig. 2 and [1]) is a big advantage due to decreased NOX and ammonia (HN3) emissions, acid precipitation, ozone

pollution, photochemical smog and corrosion problems during thermochemical conversion [23,54,71,72,99,100]. Therefore, the concentration of N (together with S, Cl, some trace elements, plus dioxins and furans) in biomass and/or BA needs to be regulated within fairly stringent limits to maintain an acceptable feedstock quality [23,44,69]. On the other hand, the high N concentration is characteristic of certain biomass (semi-biomass, animal biomass, algae, others) and it is also an indication for some eutrophication and use of N-bearing fertilizers, additives in semi-biomass or substances in the pelletisation process [1,23,72,95,101]. The NOx emissions may decrease by: (1) application of low N fuels; (2) emission reduction techniques such as staged combustion and injection of NH3 or urea [10]; and (3) coal and biomass blending because biomass produces NO destroying species such as ammonia and hydrocarbon radicals in the furnace [102].

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3.2.9. Sulphur The low S content in biomass (particularly for wood and woody biomass in Fig. 2 and [1]) is favourable for: (1) lower SOx emissions, smoke type smog, acid precipitation; and (2) limited generation of fine particulates, less stable and mobile sulphates, deposit formation, agglomeration, slagging and corrosion; during thermochemical conversion [3–7,23,54]. The low S concentration is also an indication of decreased contamination of biomass by additives, fertilizers, pesticides, adhesives, glues, lacquer, dyestuff or

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preservatives [72] which are typical for semi-biomass and agricultural biomass residues. On the other hand, the S contents can be high in some biomass varieties (semi-biomass, animal biomass and certain herbaceous and agricultural biomass [1]) and can cause problems regarding SOX emissions, generation of fine particles, deposit formation and corrosion [23,69,72]. However, the alkaline and alkaline-earth compounds in biomass and BA are effective capture agents of S in newly formed sulphates during thermochemical conversion of biomass [5,6]. 3.2.10. Silicon The low Si and silicate contents (especially for wood and woody biomass, animal biomass and algae in Figs 4 and 5, and [1]) in biomass and BA is an advantage for less erosion–abrasion and slagging problems during processing of biomass and BA. For example, high contents of hard (harder than the steels and refractory materials), coarser-grained and angular mineral particles of silica minerals (quartz, cristobalite, tridymite), feldspars and other silicates plus glass, rutile and corundum identified in biomass and BA can be a reason for increased wear of the metal equipment surfaces [4]. Additionally, the low Si contents are an advantage for a limited formation of low-temperature alkaline silicates and glass, as well as fine and respirable crystalline silica minerals (especially cristobalite) in BA that can present slagging and health risks ([4] and references therein). 3.2.11. Trace elements The low contents of many TEs including hazardous ones in biomass and BA (Table 2 and [7]) are favourable for less environmental problems related to the emissions of such elements and formation of dangerous components during biomass conversion (see above). On the other hand, the high concentrations and modes of occurrence of some TEs (Ag, Au, B, Be, Cd, Cr, Cu, Mn, Ni, Se, Zn, others) may have a resource recovery potential for certain biomass varieties and BAs and an economical assessment is required in such cases [4,7]. For example, it was found that some ashes enriched in P (from sewage sludge, meat-bone meal, olive residue, poultry litter and peach pits) or Cu and other trace elements (from municipal solid waste and sewage sludge) are very perspective for that purpose [103–105]. However, this topic is still at initial stage of investigations [7]. 3.2.12. pH The high pH of BA (Table 1) is favourable for soil amendment and fertilization due to the alkaline character (liming effect) that promotes: (1) base cations; (2) acid neutralizing potential; (3) higher salinity (electrical conductivity); (4) mineral weathering (dissolution of aluminosilicates, clay dispersion and illuviation); (5) lower acidic leaching of hazardous elements from soil into water stream resulting in their less mobilisation and bioavailability; (6) reduced Al, Mn and Fe plant toxicity by decreasing the exchangeable contents of these ions in acidic soils; (7) enhanced biological activities and better environment to some microorganisms; and (8) improved texture, aeration and water holding capacity ([4] and references therein). It is well known that the 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); however, such conditions enhance release of oxyanionic-forming species of As, B, Cr, F, Mo, Sb, Se, V and W ([4,7] and references therein). Unfortunately, prolonged leaching during BA weathering in disposal sites could provoke a decrease of pH and significant release of many TEs from BA ([4,7] and references therein). In contrast, the low pH values of biomass are a disadvantage because the solubility of most elements is markedly pH sensitive and most elements are much more mobile under acidic than alkaline conditions ([4,7] and references therein). The pH values of water leachates for

biomass are slightly acidic to neutral with salty to brine total mineralisation (Table 1 and [2]) and variable electrical conductivity [2]. As a result, different elements in biomass tend to be mobile (see above) and they are prone to pose environmental concerns [7].

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3.3. Disadvantages of biomass

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The major disadvantages related to the use of biomass as fuel are listed in Table 5. Most of the common barriers related to environmental, technological, economic and social issues have been described relatively well, while the obstacles connected with the composition and properties of biomass and biofuel have been discussed to a lesser extent in the literature [1–48]. Some of the major disadvantages related to the composition and properties of biomass and/or BA include: (1) high values of moisture (M) and O in biomass, water-soluble fraction, alkaline and halogen elements, and some hazardous TEs in biomass and BA; (2) low values of energy density (bulk density and calorific value), pH and ash-fusion temperatures (AFTs) of biomass, and bulk density and size of BA; (3) highly variable composition and properties of biomass and BA (Tables 1–3); and (4) indefinite availability of sustainable biomass resources for production of biofuels and chemicals.

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3.3.1. Moisture The high moisture content in biomass is one of the major disadvantages. This content normally varies in the interval of 3–63% (Table 1) and it can even reach up to 91% [16]. In contrast, the moisture occurrence in peat and coal is commonly in the more narrow range up to 20% [1]. Moisture in biomass is a mineralised aqueous solution containing various cations, anions or non-charged species [2]. The high M content has numerous negative effects such as: (1) problems during biofuel pre-treatment, preparation and upgrading (drying, grinding, separation, baling, pelletization, briquetting, torrefaction, others); (2) complications during biomass conversion (excluding liquefaction, gasification, alcoholic fermentation and anaerobic digestion), namely lowering calorific value and grinding capacity (non-friable properties), poor ignition, reducing the combustion temperature and combustion efficiency, longer residence time in combustion units, flame stability problems, delaying the release of volatiles, forming a large quantity of flue gas and fumes, incomplete cracking of the hydrocarbons, increasing unburned C levels in ash, and larger equipment dimensions; (3) precipitation of chlorides, carbonates, sulphates, nitrates and phosphates during biomass drying; (4) enhanced leaching (Ca, Cl, K, Mg, Na, P, S, TEs) during biomass storage and processing; and (5) deterioration of biofuel due to the microbial activity and promoting health risk resulted from fungus and moulds proliferation in biomass [1,10,1 4,18,19,28,39,53,54,56,64,79,83,106–108]. It is well known that the raw biomass with >30–50% moisture content does not burn before the partial evaporation of some moisture [83]. On the other hand, when the fuel is air-dried the stored moisture equilibrates with the ambient relative humidity and this equilibrium (saturation point in air-dried fuel) is normally 20–30% for woody biomass [19,57,109] and 6–10% for other biomass fuels [110].

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3.3.2. Alkaline and halogen elements The high contents of alkaline and halogen elements such as K, Na, Cl and occasionally Br, Cs, F, I, Li and Rb (Tables 1 and 2, and [7]) with unfavourable modes of occurrences (organic matter, chlorides, sulphates, carbonates, oxalates, nitrates, oxyhydroxides, phosphates, amorphous material, others), are among the major technological and environmental challenges of biomass conversion because they cause: (1) increased volatilization and formation of many dangerous components (Cl2, HCl, HBr, HF, dioxins, furans, TEs, others); (2) enhanced fine particulate emissions; (3) greater quantity of water-soluble fraction; (4) generation of low-melting

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eutectic phases and low ash-fusion temperatures; (5) higher amount of active melts with low viscosity; (6) increased deposit formation, fouling, agglomeration, slagging and corrosion; (7) enhanced deleterious effects on the cement-based construction materials incorporating BA; (8) greater deterioration rates and accelerated deactivation of catalysts used for selective catalytic reduction of NOX and SOX; (9) fouling of the solid oxide fuel cells anodes in gasifiers; and (10) burning damage on human and plant tissues by alkaline oxides in BA ([2–7] and references therein). Numerous biomass fuels among agricultural and herbaceous biomass, annual plants, algae, semi-biomass, animal biomass and wood branches and leaves are normally enriched in alkaline and halogen elements (Figs 2–5, and [1]). The technological countermeasures include fuel selection and blending, fuel pre-treatment (mostly water washing) and application of appropriate additives, temperature and ash cleaning systems. For example, biomass harvested and left in the field for a prolonged period of time (natural rain washing) or industrial biomass washing to remove water-soluble phases prior to use of biomass fuels may reduce some technological and environmental problems. However, such future large-scale leaching may create new environmental concerns related to the fate of water-soluble Cl-, K-, Mg-, N-, Na-, Pand S-bearing phases and different hazardous and mobile TEs associated with them (Fig. 8 and [1–7] and references therein). Additionally, the highly variable phase and chemical composition of biomass fuels gives the possibility of reducing high contents of alkaline and halogen elements and improving the fuel performance by modification of the feedstock composition through tailored fuel mixtures ([6] and see below). On the other hand, the high contents of alkaline and halogen elements can have some positive effects during biomass conversion. For example, some alkaline inorganic constituents of biomass (like KCl, NaCl and others) can act as catalysts or catalyst precursors for thermochemical conversion of biomass [111–113]. In some cases, the high alkali and Cl contents in biomass lead to the enhanced formation of KCl and NaCl in ash and this can lower the level of gaseous Cl available for the synthesis of dioxins and furans [23]. Moreover, the alkaline BAs have been used for neutralization of wastes, as a source for production of potash, liming and tannin neutralizing agents, fertilizers, glass, glaze, soap, detergents, composites, synthesis of minerals and other applications [4,114–116]. Finally, some Br and Cl enriched biofuels could also play the role of ‘‘self-cleaning fuels’’ for the successful capture and immobilisation of Hg ([4–6] and references therein). The use of such advanced approaches is of particular interest because the improved and/or cleaner fuel blends may contribute for reducing or avoiding many technological or environmental problems during biomass thermochemical conversion.

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3.3.3. Hazardous trace elements The high contents of some toxic and potentially toxic TEs such as Ag, Be, Cd, Cl, Cr, Cu, Hg, Mn, Ni, Se, Zn, others in biomass and/or BA (Table 2 and [7]), and especially those with unfavourable modes of occurrences, are among the most serious disadvantages during biomass conversion. It was recently revealed that the greatest ecological challenges related to some TEs in biomass and BA include their: (1) high concentrations; (2) unfavourable modes of occurrence (organic matter, carbonates, chlorides, sulphates, oxalates, nitrates, oxyhydroxides, phosphates, amorphous material, others); (3) enhanced volatilization and limited retention and capture performance during biomass combustion; and (4) increased leaching behaviour during biomass and BA processing or storage [7]. It is well known that TE contaminants can be accumulated in biomass fuel throughout air, water, soil, pesticides, fertilizers and additives ([7] and references therein). It was found that the thermochemical conversion of biomass can increase the negative TE impacts because significant amounts of TEs (As, Cd, Cr, Cu, Hg, Ni, Pb, V and Zn) that have been mobilised from geochemically stable sources are remobilised during biomass conversion [101]. For instance, TEs such as As, Br, Cd, Cr, Hg, Pb, Sb, Se, V and Zn demonstrate the highest volatilization potential during biomass combustion, whereas TEs such as As, Ba, Br, Cd, Co, Cr, Cu, Hg, Li, Mn, Mo, Ni, Pb, Sb, Se, Sr, V and Zn show significant water-soluble occurrence in biomass and BA (Figs 8 and 9, and [4,7] and references therein). Additionally, the above TEs tend to occur in much more mobile and hazardous compounds than in coal and coal ash because they associate preferably with the easily decomposed OM and water-soluble minerals and phases in biomass and BA ([4,6,7] and references therein). These modes of element occurrence favour their high mobility (leaching and volatilization) during biomass and BA processing. Hence, some dangerous, volatilized and water-soluble elements might cause some environmental pollution of the air, water (surface and subsoil water), soil, flora and fauna with possible subsequent penetration into the food chain during biomass and BA utilisation [4]. Further, the observations that most of the hazardous TEs tend to have higher water leaching potential from BA than from biomass give important information for the sustainable utilisation of BA, especially for soil application and recovery of elements [6]. For example, it was found that filter fly ashes and ashes from semi-biomass should be avoided for soil application because they are commonly highly contaminated with hazardous TEs such as As, B, Ba, Cd, Cr, Cu, Hg, Mn, Mo, Ni, Pb, Se, Zn and others ([4] and references therein). Therefore, the concentration of hazardous TEs (plus Cl, N and S) in biomass and BA needs to be regulated within fairly stringent limits to maintain an acceptable quality [7,10,16,23,44,69,72,117–119] and highly efficient

Fig. 9. Mean and range contents of elements volatilized from biomass during combustion [6], wt.%.

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separation of these elements in biomass power plants is required [28,82]. On the other hand, the highly variable phase and chemical composition of biomass fuels gives the possibility of reducing the harmful emissions by modification of the feed fuel composition through tailored fuel mixtures ([6] and see above). For instance, some biofuels can be to some extent ‘‘self-cleaning fuels’’ because a number of their original constituents and their newly formed

minerals and phases are active during thermochemical conversion and can play more or less a capture and immobilisation role for the volatile hazardous elements. This is due to the enrichment of biomass in: (1) active alkaline-earth and alkaline constituents that form carbonates, oxyhydroxides, phosphates, sulphates, chlorides and amorphous matter; and (2) structural organic components that can produce unburned chars with high specific surface area [6].

Fig. 10. Mean ash-fusion temperatures for the biomass groups and sub-groups, and three solid fossil fuel types based on 90 biomass varieties and 37 solid fossil fuels in increasing order of the fluid temperatures [6], °C. Abbreviations: A, aquatic biomass; AVB, all varieties of biomass; B, bituminous coal; C, coal; CB, contaminated biomass; DT, initial deformation temperature; FT, fluid temperature; HAB, herbaceous and agricultural biomass; HAF, herbaceous and agricultural fibres; HAG, herbaceous and agricultural grasses and flowers; HAH, herbaceous and agricultural husks; HAPT, herbaceous and agricultural pits; HAR, other herbaceous and agricultural residues; HAS, herbaceous and agricultural straws; HASH, herbaceous and agricultural shells; HAST, herbaceous and agricultural stalks; HT, hemispherical temperature; L, lignite; NB, natural biomass; S, sub-bituminous coal; ST, spherical temperature; WWB, wood and woody biomass; WWBA, wood and woody barks; WWL, wood and woody leaves; WWO, other wood and woody biomass; WWR, wood and woody roots; WWS, wood and woody stems; WWST, wood and woody stumps; WWT, wood and woody twigs.

Fig. 11. Areas of low (<1200 °C), medium (1200–1400 °C) and high (>1400 °C) hemispherical (HT) ash-fusion temperatures for 60 biomass varieties in the chemical classification system of biomass ash [6], wt.%.

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3.3.4. Ash-fusion temperatures The low ash-fusion temperatures of biomass are a serious disadvantage. It was found that AFTs of biomass are more variable than those of coal, but they are normally lower, excluding certain lignites and biomass varieties from ‘‘S’’ and ‘‘C’’ ash types (Figs 10 and 11, and [3,6]). It is widely accepted that most of the severe deposit formation, slagging and fouling problems during biomass thermochemical conversion result from the low ash-melting temperatures [4,6]. Some indicative trends of natural biomass in comparison with coal were identified recently [2], namely the potential of biomass to have normally: (1) Higher values of Ca, Cl, K, Mg, Mn, Na, P, carbonates, chlorides, phosphates, organically bound inorganic elements and water-soluble components. (2) Lower values of Al, Fe, N, S, Si, Ti, inorganic matter, oxyhydroxides, silicates and sulphates–sulphides.

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Hence, such differences are reasons for the lower AFTs of biomass in comparison with coal. For instance, it was found that the high concentrations of K-, Si-, P-, S-, Fe-, Na- and Mg-containing minerals (excluding the highly enriched in Si biomass varieties) and low contents of Ca-, Al- and Ti-bearing minerals are commonly responsible for decreased AFTs of biomass (Fig. 12 and [6]). The low ash-fusion biomass varieties normally have high slagging propensity due to formation of low-temperature melts and their subsequent intensive melt crystallization followed by abrupt glass generation during cooling at relatively low temperatures [6]. The lower AFTs with short softening-melting range and high flow-dissolution rate (‘‘S-MA’’ inorganic sub-type) seem to be the worst case for slagging and fouling (Fig. 11 and [6]). A selection of optimal temperatures for thermochemical conversion is required to avoid the above problems. Another possibility is the preliminary biomass washing to eliminate the water-soluble components which contribute greatly for lower AFTs [97,120–123]. Additionally, a beneficial approach for problematic low ash-fusion biofuels or alternative bed materials for fluidized bed combustion is to use various additives, namely kaolinite, mullite, clinochlore, bentonite, K feldspar, plagioclase, olivine, quartz, lime, bauxite, gibbsite, diaspore, corundum, hematite, calcite, dolomite, magnesite, ankerite, sand, high alumina sand, limestone, diatomaceous earth, dicalcium phosphate, chalk, elemental S, peat, coal and coal ash ([6] and references therein). The application of such additives is to prevent the agglomeration, sintering and slagging tendencies by achieving higher ash-melting temperatures. The purpose of the above blending is to provoke the intensive formation of more refractory minerals (silica minerals, mullite, and Ca and Mg silicates, oxides and phosphates) and less fluxing phases (K and Na silicates, chlorides, sulphates and phosphates) in the system [6]. However, there are many solid fuels which are naturally abundant in these refractory minerals and appropriate blending of such fuels can avoid the use of expensive biomass pre-treatment procedures and non-fuel additives [6]. For example, the favourable or less problematic future fuel mixtures between biomass, coal and other solid fuels may include an adjustment of chemical and phase composition for fuel blends to fit preferentially some chemical sub-types of BA (Fig. 11 and [6]). Hence, the fuel mix can play a leading role to solve different technological and ecological problems (see also above). 3.3.5. Oxygen Biomass is highly oxygenated fuel due to the carbohydrate structure with respect to conventional fossil fuels with hydrocarbon structure [97]. The high O content in biomass (especially among agricultural and herbaceous biomass, wood and woody biomass and algae in Fig. 2 and [1]) is a disadvantage because of: (1)

% 60

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Hemispherical (HT) ash fusion temperature, C Fig. 12. Mean contents (wt.%) of the chemical components and mean hemispherical (HT) ash-fusion temperatures (1101, 1284 and 1520 °C) for three temperature ranges (<1200, 1200–1400 and >1400 °C) based on 60 biomass varieties [6].

reduced energy value [54]; (2) less predictable behaviour of highly oxygen-functionalized carbohydrates [60,124]; and (3) greater volatility [54], inorganic vapours [8], smoking [40] and soot formation [45] during thermochemical conversion. On the other hand, biomass has high reactivity as a result of significant concentrations of O in highly reactive forms such as –COOH, –OCH3 and –OH functional groups [61].

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3.3.6. Energy density, bulk density and calorific value The low energy density (both low bulk density and heating value) of biomass is a significant disadvantage for biomass conversion (Table 1). For example, the energy density of biomass is only

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10–40% of that of most fossil fuels [35,57] and biomass heating values generally are slightly over half that of coal [18]. Furthermore, the biomass particle densities are about half that of coal, whereas the biomass bulk densities are about one fifth that of coal [18]. The lower energy density of biomass requires a biomass resource close to the processing facility, high storage cost and minimal storage time because the weathering and bacteria can lower the energy quality of the biomass [13]. Additionally, the low bulk densities of biomass particles are prone to adhesion by the build-up of static electric charge [125]. Therefore, different pre-treatment procedures such as drying, baling, chopping, milling, screening, pelletisation, briquetting, torrefaction and others are applied to: (1) increase the energy density; (2) depress the self-ignition; and (3) improve the storage, resistance, grindability, hydrophobicity, biological degradation, strength, transportation, feeding and conversion of biomass. The torrefaction process is an advanced approach for that purpose [40]; however, the application of such low-temperature pyrolysis technology seems to be much more economically perspective to generate high-value products such as biochar (for soil amenders, fertilizers and C sequestration), adsorbents, activated carbons, catalyst supporters, metal reductants, biocarbon electrodes, additives in polymer/rubber composites and others [4,109,126] than to produce a simple and cheap solid biofuel just for combustion and gasification. On the other hand, the low particle densities help biomass particles to oxidize at rates much higher than coal [18]. 3.3.7. Size The fine size and low values of bulk density of BA (Table 1) are a disadvantage. For example, biomass fly ashes are light materials having bulk density mostly between 101 and 830 kg/m3 (mean 392 kg/m3), while the same value for coal ashes is much higher, namely 400–1100 kg/m3 (mean 800 kg/m3) ([4] and references therein). The size of biomass fly ash particles varies from 10 to 50 nm to more than 1–2 mm, but these powder materials are commonly very fine as their median size is dominantly below 10– 100 lm [4]. Therefore, biomass fly ashes tend to be lighter and finer than coal fly ashes and these characteristics increase health and safety risks during transport, storage and processing of biomass fly ashes due to particulate emissions and possible dust inhalation as the smallest particles are of the greatest concern due to their composition ([4] and references therein). This situation can be improved by fly ash compaction into pellets, briquettes and agglomerates or during storage. 3.3.8. Variable composition and properties The highly variable composition and properties of biomass and BA (Tables 1 and 2, and Figs 1–7) are serious barrier issues for the application of specific and suitable thermochemical and biochemical conversion technologies of biomass and utilisation of BA. These disadvantages include: (1) irregular quality; (2) possible interrupted processing; (3) less predictable behaviour of biofuels and their products; and (4) challenges to meet the technical and environmental requirements. On the other hand, this variation is an advantage for application of more diverse biomass conversion technologies, as well as more variable BA utilisation directions such as soil amendment and fertilization, production of different materials (construction materials, adsorbents, ceramics, synthesis of minerals), recovery of valuable components (char, water-soluble, cenosphere–plerosphere, magnetic and heavy fractions; and elements) and multicomponent utilisation [4]. Finally, the definitive utilisation, technological and environmental advantages and challenges related to biomass and BA associate preferentially with specific organic and inorganic types or sub-types (Figs. 3–7, 11 and 12) and they can be predictable by using the combined chemical and phase–mineral classification approaches [1–7].

3.3.9. Sustainable biomass resources It was highlighted that the utilisation of biomass resources will be one of the most important factors for environmental protection in the 21st century [25]; however, it is well known that ‘‘under current policies, the environmental effects from biofuel production might be worse than those from fossil fuels’’ [127]. Therefore, one of the biggest challenges for large-scale production of biofuels and biochemicals is the availability of sustainable biomass resources for such purposes. Natural biomass is a renewable energy source, while biofuel is still an incomplete renewable energy resource at present [1]. Unfortunately, the modern bioenergy chains are usually associated with additional use of fossil fuels during growing, fertilizing, harvesting, transportation, storage, pre-treatment and conversion of biomass, as well as discharge and handling of wastes, emissions and other environmental impacts, when considering the complete biomass life cycle assessment [23,29,30,101]. Furthermore, the biofuel demands should not compete with food and feed production. The conversion of biomass sources from natural ecosystems (forests, tundra, grasslands, prairies, pastures, peatlands, wetlands, rivers, lakes, seas and oceans) into energy resources may lead to serious environmental problems related to balance, regeneration, biodiversity, biocoenosis and life cycle assimilation in such systems [2]. Shortly, not enough knowledge is available to truly comprehend the importance of the natural ecosystems and given this lack of knowledge humans should be very careful not to further disturb and destroy these native systems. The natural ecosystems are relatively equilibrium habitats (despite the global and regional pollutions) and they should be avoided, to a maximum extent, as resources for biofuel or chemical production. There is balance in composition, modes of occurrence and behaviour of mobile elements (mostly nutrients) in biomass communities, species and plant parts when considering the biomass regeneration in natural ecosystems. For example, the ‘‘sustainable forest management’’ by clearing, thinning or trimming, and selective harvesting with removal of trees, shrubs, branches, twigs and foliage (alive or dead) can reduce significantly the nutrient capital and balance of the forest ecosystem because large amounts of the most mobile nutrients can be exported. It was emphasized that with increasing out-take of the most nutrient-rich parts of the trees as fuel, measures must be taken to ensure that the nutrient balance of the forest biotope is maintained and recycling of wood ash will thus be an important part of a sustainable forestry ecosystem [128,129]. However, the subsequent ‘‘sustainable’’ use of BAs as bioavailable nutrients back to the forest soil cannot solve the potential balance problem because these ashes have totally different phase composition, modes of nutrient and trace elements occurrence, properties and behaviour in comparison with the original biomass and associated soils [4]. Similar assumptions can also be valid for the other natural ecosystems. Therefore, the large-scale use of such unfavourable biomass resources will cause further disturbances and imbalances in the already stressed natural ecosystems and similar opinions have already been highlighted [30,127]. These observations lead to the conclusion that exploitation of natural ecosystems should remain undisturbed to a maximum extent in order to avoid adverse impacts. In the current trend towards large-scale production of fuel and energy from biomass resources it would thus be desirable to restrict the exploitation of natural ecosystems and instead derive the biomass from a limited number of feedstocks. The above reasons strongly indicate that the potential favourable biomass resources for biofuel production, despite some possible technological and contamination problems, should be focused (in decreasing order of significance) preferably on terrestrial and aquatic phytomass, zoomass and excreta such as: (1) Non-edible agricultural, forest, feed and food residues.

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(2) Semi-biomass (contaminated biomass and industrial biomass wastes). (3) Short-rotation energy crops such as specifically cultivated forest, grass and algae plantations, but grown only on existing low productive, degraded or contaminated non-arable land and in wastewater or contaminated ponds. (4) Animal and human wastes.

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Thus, the potential biomass resources for biofuel and biochemical production should always be divided initially into sustainable and unsustainable management resources under strictly specified environmental criteria. For example, the complete biomass life cycle assessment is very important tool for that purpose [23,27,29,30,43,48,101,130]. Additionally, the remote sensing methods and geographical information systems (satellite, aerial and ground-based remote sensing) have recently become critically important approaches for the environmental impacts, land-use change, site-specific management, production, estimation, monitoring, planning, control and policy implementations of sustainable biomass feedstock [131–133]. Subsequently, when there is confirmation of sustainable origin and/or production of biomass feedstock only after that biofuels can be assigned correctly to the currently used terms such as primary and secondary biofuels, biofuel generations (I–IV) and biofuel types (solid fuel, diesel, alcohols, syngas, hydrogen, chemicals, others) commonly used in practice. It is noted that this might very-well reduce the overall available and sustainable biomass resource due to insecurity of biomass feedstock supply. Therefore, it may be speculated that the huge contribution of biomass in the future energy mix is significantly overestimated if appropriate additional measures and corrections are not taken into considerations regarding the availability and management of unsustainable and unfavourable biomass resources for production of biofuels. Still, biomass could play an important role as a fuel and energy resource when combined with other efforts, namely other types of renewable energy and overall reducing the fuel and energy consumption.

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4. Conclusions

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Certain conclusions about the advantages and disadvantages of biomass composition and properties for biofuel application based on parallel and detail investigations of numerous characteristics for both biomass and coal and their conversion products can be made:

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(1) Some of the major advantages related to the composition and properties of biomass and/or BA include: (1) high values of volatile matter, H, structural organic components, extractives and reactivity of biomass, water-soluble nutrient elements and alkaline-earth elements in biomass and BA, and pH of BA; and (2) low values of C, fixed C, ash yield, N, S, Si and initial ignition and combustion temperatures of biomass, and low contents of many trace elements including hazardous ones in biomass and BA. (2) Some of the major disadvantages connected with the composition and properties of biomass and/or BA comprise:(1) high values of moisture and O in biomass, water-soluble fraction, alkaline and halogen elements, and some hazardous trace elements in biomass and BA; (2) low values of energy density (bulk density and calorific value), pH and ash-fusion temperatures of biomass, and bulk density and size of BA; (3) highly variable composition and properties of biomass and BA; and (4) indefinite availability of sustainable biomass resources for production of biofuels and chemicals.

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(3) The conversion of biomass sources from natural ecosystems into energy resources may lead to serious environmental problems. The native ecosystems should be avoided, to a maximum extent, as resources for biofuel production. The potential favourable resources for that purpose should be focused preferably on: (1) non-edible agricultural, forest, feed and food residues; (2) semi-biomass (contaminated biomass and industrial biomass wastes); (3) short-rotation energy crops such as specifically cultivated forest, grass and algae plantations, but grown only on existing low productive, degraded or contaminated non-arable land and in wastewater or contaminated ponds; and (4) animal and human wastes. The potential biomass resources for biofuel and biochemical production should always be divided initially into sustainable and unsustainable management resources under strictly specified environmental criteria. (4) It was found that the disadvantages of biomass for biofuel and biochemical applications prevail over the advantages; however, the major environmental, economic and social benefits appear to compensate the technological and other barriers caused by the unfavourable composition and properties of biomass.

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Please cite this article in press as: Vassilev SV et al. Advantages and disadvantages of composition and properties of biomass in comparison with coal: An overview. Fuel (2015), http://dx.doi.org/10.1016/j.fuel.2015.05.050