Water-soluble fractions of biomass and biomass ash and their

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Water-soluble fractions of biomass and biomass ash and their significance for biofuel application Stanislav V. Vassilev, and Christina Vassileva Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.9b00081 • Publication Date (Web): 16 Mar 2019 Downloaded from http://pubs.acs.org on March 17, 2019

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Energy & Fuels

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Water-soluble fractions of biomass and biomass ash

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and their significance for biofuel application

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Stanislav V. Vassilev*‡, Christina G. Vassileva‡

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Institute of Mineralogy and Crystallography, Bulgarian Academy of Sciences, Acad. G. Bonchev

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Street, Block 107, Sofia 1113, Bulgaria

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Keywords: Biomass; Biomass ash; Water-soluble components; Chemical composition; Mineral

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composition

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ABSTRACT

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An overview of the water-soluble fractions of biomass and biomass ash (BA) and their

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significance for solid biofuel application was conducted based on reference peer-reviewed data

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plus own investigations. Characteristics such as fluid matter and moisture of biomass,

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composition and properties of water-soluble fractions isolated from biomass and BA are

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considered including fraction yield, content of water-soluble elements, phase-mineral

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composition, pH, and electrical conductivity. It was found that the water-soluble fraction of

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biomass and BA is highly enriched in Cl, S, K, Na, N and P, and some hazardous trace elements

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with unfavorable modes of element occurrences such as alkaline chlorides, sulphates, nitrates,

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carbonates, oxalates, and some oxyhydroxides, phosphates and amorphous material. These

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compounds provoke the most critical technological (slagging, deposit formation, fouling, and

ACS Paragon Plus Environment

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corrosion) and environmental (fine particle partitioning, volatilization of hazardous air pollutant

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elements, contamination of air, water, soil and plant) challenges during the thermo-chemical

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conversion of biomass and BA processing. The reduction or immobilization of the undesirable

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water-soluble components in salt-tolerated biomass and BA can be achieved by feedstock

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selection, modification of harvesting and fertilization practices, natural or industrial water

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washing, fuel blending, and use of additives before processing. On the other hand, the water-

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soluble fraction leached from biomass and BA can be utilize for the recovery of some elements,

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synthesis of some minerals and production of soil amendments and different materials, whereas

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the water-soluble components in BA can contribute for capture and storage of atmospheric CO2.

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1. Introduction

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The bioenergy production has increased considerably at present because of global warming

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problems caused by fossil fuels combustion. About 15% of the world energy supplies as

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electricity, heat and transportation fuels nowadays are based on biomass,1 and it is expected that

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up to 50% of the world’s primary energy consumption would be met by biofuels in 2050.2 The

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annual world production of biomass with potential energy application at present is approximately

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7 billion tonnes mostly among biodiversity groups such as woody, agricultural and herbaceous

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biomasses, and semi-biomass including contaminated biomass and industrial organic wastes such

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as municipal solid waste, refuse-derived fuel, sewage sludge and others.3 Additional large spread

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biodiversity groups, namely aquatic biomass (algae) and animal and human wastes are also

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perspective for bioenergy application.3,4 The direct combustion of biomass is the dominant

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conversion process currently used for bioenergy production. As a result, about 480 million

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tonnes of biomass ash (BA) are probably generated annually worldwide3 and this quantity is


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similar to that of coal ash (780 million tonnes).5 The large-scale combustion and gasification of

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natural biomass and its co-combustion and co-gasification with semi-biomass and solid fossil

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fuels in short- to medium-term future provoke new challenges. These challenges are related to

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some potential technological problems and environmental risks during the thermo-chemical

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conversion of biomass, especially salt-tolerated varieties, and use of the resulted combustion and

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gasification ash residues.

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The knowledge on chemical and phase composition of biomass and BA defines the properties,

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quality and application perspectives related to their innovative and sustainable utilization.

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Reference peer-reviewed data and own studies were used recently to elucidate the composition

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and properties of both biomass and BA systems and behavior of biomass during combustion, as

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well as technological and ecological advantages and disadvantages related to biomass and BA

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utilization.3,4,6-13 These investigations emphasized that the water-soluble components have a key

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importance for the biomass and BA application. However, the knowledge about the composition

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and properties of these compounds occurring in various biomasses and different biodiversity

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groups is still limited.

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The impact of water leaching on biomass and BA properties has been studied to some extent. It

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was shown that such pre-treatment of biomass: reduces the ash yield and contents of water-

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soluble K, Na and Cl bound in salts; increases the heating value; and modifies the ash chemistry

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and ash-forming processes.14,15 The reference investigations reveal that the water-soluble

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components in biomass and BA may cause: low ash-fusion temperatures; increased

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agglomeration, sintering, fouling, slagging, and corrosion; enhanced volatilization of hazardous

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compounds; increased fine particulate emissions; acidic to highly alkaline character of water

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leachates; contamination of air, water, soil and plants by mobile toxic elements; deleterious


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effects in construction materials incorporating BA; and other technological and environmental

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problems, and health risks.14-29 It is commonly accepted in these studies that the concentration

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and behaviour of elements such as K, Na, Cl and S are mostly responsible for the above

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obstacles during biomass processing. However, it was recently highlighted that the high contents

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of specific water-soluble modes of occurrence of these and other elements may provoke the most

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important challenges during biomass and BA processing, and especially thermo-chemical

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conversion of solid biofuels.3,6-13 In contrast, the water-soluble fraction of biomass and BA could

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also have some beneficial applications in the industry.7,13 Hence, such mobile modes of element

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occurrence in biomass and BA require more detail investigations.

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The major purpose of the present overview is to evaluate the composition, properties and

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significance of water-soluble compounds in biomass and BA during their processing. Some

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potential technological and environmental challenges related to the water-soluble fraction during

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biomass and BA applications are also focused.

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2. Materials, methods and data used

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Extensive reference data and own investigations were used in the present overview. Eight

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biomass samples such as plum pits (PP), corn cobs (CC), walnut shells (WS), beech wood chips

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(BC), rice husks (RH), sunflower shells (SS), switchgrass (SG), and marine macroalgae (MM)

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were collected and studied (Table 1). The samples selected have highly variable composition and

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properties that fit to different biodiversity groups and sub-groups4 and specific organic6 and

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inorganic (Figure 1) types and sub-types. The samples collected were between 2 and 5 kg in

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weight and they were air dried under ambient conditions during a period of over 12 months.

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Then visible contaminants such as soil, sand or shell particles were eliminated manually from


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MM and RH. After that, the samples were crushed/cut and ground to a particle size of “C”

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type > “S” type (Figure 1). The “K” type BA is again the most abundant in DWR similarly to

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biomass, while the order for the other three BA types is opposite to that for biomass (see

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Paragraph 3.1.2). The reason for such differences is the diverse phase-mineral transformations

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among the inorganic biomass types during biomass combustion and they have been described in

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detail earlier.8,9 For instance, it can be seen that the low-acid BAs from “K”, “C” and “CK” types

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(CC > WS > MM > SS > BC) produce the highest yields when compared to other BAs (Figures

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1 and 3). BAs among these three types are enriched in sulphates, chlorides, carbonates,

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phosphates and oxyhydroxides, and depleted in silicates and inorganic amorphous material in

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comparison with “S” type BA (Table 1 and Figure 4). Finally, it was also found that DWR yields

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of eight BA samples correlate positively and significantly with the sum of K, Na, S, Cl and P

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components in BA (Figure 2), which is also in accordance with the above observations and

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phase-mineral composition of BA (see Paragraph 3.2.3).


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Figure 3. Comparison between the DWR yields generated from eight biomasses and their ashes

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produced at 500°C.

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Figure 4. Mean distribution of inorganic amorphous matter (IAM) and mineral classes in

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biomass ash types based on eight biomass ashes produced from corn cobs, marine macroalgae


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and sunflower shells (‘‘K” type), plum pits and walnut shells (‘‘CK” type), beech wood chips

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(‘‘C” type), and rice husks and switchgrass (‘‘S” type) at 500°C.

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3.2.2. Water-soluble elements

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The water-soluble elements in various BAs have also been studied.15,17,25-29,43,57, 117-134 The

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reference data show (Table 2) that the mobile elements (based on mean contents) leached from

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BAs and generated dominantly from woody and straw biomasses are mainly Cl, S, K, Na, Sr,

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Mn, and Ni, and to a lesser extent, Si, Li, Mo, Co, Cr, Cd, and Zn, and some Br, P, Al, Ba, Ca,

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Mg, Fe, Cu, Hg, Pb, Sb, and Se.15,17,43,57,117,119-123,126-130 It can be seen that besides ash-forming

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non-metals and alkaline and alkaline-earth elements, many TEs among lithophile, chalcophile

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and siderophile groups are also water-soluble in BA in contrast to biomass. The reason for that is

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the intensive formation of more mobile modes of TE occurrences in BAs produced at 500-900

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°C than in biomass.8,9,13

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3.2.3. Phase-mineral composition

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The own (Table 3) and reference data3 show that the water-soluble phases in BA mostly include

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species of alkaline and alkaline-earth elements such as: (1) highly soluble nitrates, chlorides

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(sylvite, halite), sulphates (arcanite, syngenite, picromerite, ettringite), carbonates and

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bicarbonates (fairchildite, natrofairchildite, kalicinite, natrite), and oxides (lime); (2) less soluble

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sulphates (gypsum, anhydrite), carbonates (calcite, dolomite), hydroxides (portlandite), and

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phosphates (phosphorites); and (3) slightly soluble to insoluble phosphates (apatite), silicates (Ca

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silicates, feldspars) and glass. Minerals such as calcite, gypsum, portlandite, ettringite, and Ca

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silicate hydrate were also observed to precipitate from BA water leachates at alkaline pH.17,117

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The amount of DWR separated from the eight BAs studied, excluding these from CC, MM and


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WS, was not enough for further XRD and ICP analyses. However, XRD of the above three

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DWRs indicated the occurrence of K-Na chlorides, K-Ca carbonates, and Ca sulphate and

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hydroxide. More detailed studies regarding the mineral and chemical composition of the present

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eight DWRs will be performed in future using a large amount of BAs for leaching.

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The reference data show that some water-soluble phases in biomass transform to more stable

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acid soluble and insoluble forms in BA at high temperatures.57 For instance, bicarbonates and

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carbonates are typically generated at combustion temperatures below 500–800 °C, whereas

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silicates and oxides are characteristic minerals at higher temperatures (above 800–1000 °C) and

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longer heating times.79,80,117,135,136 The phase and mineral transformations during combustion of

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present eight biomasses at 500-1500 ºC have been studied in detail recently by the present

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authors.8,9 These investigations explain the conversion of water-soluble compounds to less

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soluble components in BA. For example, it was identified therein that the sequential phase-

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mineral transformations in biomass with increasing combustion temperature include: (1) initial

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formation of intermediate, less stable and more water-soluble chlorides, carbonates, sulphates,

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oxyhydroxides, phosphates, oxalates, nitrates and inorganic amorphous (non-glass) material in

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BA originated from both organic and inorganic matter of biomass (normally at 500-900 °C); and

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(2) subsequent transformation of the above minerals to more stable and less water-soluble

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silicates, glass and some oxides and phosphates in BA (commonly at 900-1500 °C.) This is the

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reason for the higher DWR yields of all eight BAs produced at 500 °C in comparison with the

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respective DWR yields of biomass samples (Table 1). Hence, the biomass burning temperatures

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are very important for the amount and properties of the water-soluble fraction in BA.

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Table 3. Major (M), minor (Mi) and accessory (A) inorganic phases and minerals in eight

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biomass ashes produced at 500°C, based on XRD data.9


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Phase, mineral 1. Inorganic amorphous matter 2. Inorganic crystalline matter 2.1. Chlorides Halite Sylvite

Formula

NaCl KCl

2.2. Sulphates Anhydrite Arcanite

CaSO4 K2SO4

2.3. Carbonates Butschliite Calcite Fairchildite Kalicinite

K2Ca(CO3)2 CaCO3 K2Ca(CO3)2 KHCO3

2.4. Oxides and hydroxides Periclase Portlandite

MgO Ca(OH)2

2.5. Phosphates Apatite Whitlockite

Ca(PO4)3(Cl,F,OH,CO3) Ca3(PO4)2

2.6. Silicates Kalsilite Leucite Quartz

KAlSiO4 KAlSi2O6 SiO2

CC M M

MM M M

Mi

M A

A

M

M M

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WS M M

SS M M

SG M M

BC M M

PP M M

RH M M

Mi

A

M M M Mi

A M

A

Mi Mi Mi

M

A Mi

M

M A

Mi Mi

Mi Mi

Mi Mi A

A

Mi Mi

A

Mi

Mi

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3.2.4. pH and electrical conductivity

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The reference data for pH values of numerous BA water leachates (103 samples) generated

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dominantly from woody, herbaceous and agricultural biomasses vary in the range of 4.5-13.5

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(mean 11.3).17,43,111,113,117-119,121,122,128-132,136-150 These solutions have acidic to highly alkaline

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character; however, the BA leachates are mostly alkaline, whereas the low pH values are

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characteristic of BAs enriched in unburnt char probably due to the occurrence of some residual

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organic acids. In contrast, the pH values of water leachates from coal ashes are less alkaline,

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namely 6.2–12.5 (mean 10.0),114 and fall in a relatively narrower range because the latter ashes

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are normally depleted in salts.


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The own data (Table 1) reveal that the pH values of eight BA leachates are 8.1-12.9 (mean 10.3)

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and they have slightly to highly alkaline character similarly to the reference data. The decreasing

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order of pH values for these BA solutions is SS, BC, CC, WS, SG, PP, MM, and RH. It is widely

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accepted that the production of water-soluble Ca, Mg, K and Na oxides, hydroxides, carbonates

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and bicarbonates in BA and the loss of organic acids during biomass burning are the major

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reasons for increasing pH in BA.117,118,121,137 It was also found that the alkalinity of BA decreases

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with increasing combustion temperature and period of storage.121 The decreasing pH tendency

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with increasing storage time of BA is caused by the transformation of hydroxides to

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carbonates.119 It was identified that the solubility in water for most elements in BAs and coal

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ashes is pH sensitive. For example, the more alkaline character of BAs causes greater release of

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oxyanionic-forming species of elements, namely B, F, Mo, W, Cr, V, As, Sb, and Se, and lower

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mobility of a large number of species containing siderophile and chalcophile elements such as

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Co, Fe, Mn, Ni, Ti, Cd, Cu, Hg, Pb, Sn, and Zn.15,51,145,151,152 It was also found that the soluble

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fraction of wood BA increases dramatically with decreasing pH, whereas the extraction rate is

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mostly independent on pH.117

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The reference data for the electrical conductivity of water leachates (19 samples) from woody,

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straw and cereal BAs varies between 3.0 and 49.5 mS cm-1 (mean 20.2). 43,112,132,143,144,150 It can

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be seen that these values are much higher than those for biomass. The explanation of this

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observation is similar to that for pH in biomass and BA.

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3.3. Technological and environmental challenges related to water-soluble components in

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biomass and biomass ash


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It is commonly accepted that the content and behaviour of elements such as S, Cl, K, Na, Ca, P,

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Si, and some TEs are responsible for many environmental and technological problems during

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biomass processing. However, recent studies show that these problems are mostly related to the

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concentration and behaviour of modes of element occurrence represented by specific phases,

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minerals or mineral classes in solid fuels and their products.10,12,13

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The reference chemical data and own phase-mineral and chemical investigations demonstrate

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that numerous biomass varieties and their ashes, especially among herbaceous, agricultural,

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aquatic and semi-biomass groups, are highly enriched in water-soluble species of chlorides,

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sulphates, nitrates, oxalates, carbonates, oxyhydroxides, phosphates, and amorphous material

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abundant in Cl, S, N, K, Na, Ca, Mg, P, and some TEs. Such salt-tolerated biomass diversities

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can be responsible for enhanced leaching behavior, low-temperature ash fusion, partitioning

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behavior, emission of volatile and hazardous elements, corrosion, agglomeration, deposits

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formation, slagging, fouling, and bed defluidization during thermo-chemical conversion of

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biomass.3,6,7,10-29 Hence, the reduction or immobilization of the water-soluble fraction from salt-

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tolerated biomasses is a very important topic for their less problematic utilization.

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The reduction or immobilization of undesirable water-soluble Cl-, S-, K-, Na-, P- and N-bearing

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components in salt-tolerated biomass and BA can be achieved by: (1) selection of feedstock; (2)

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alteration of fertilization process; (3) modification of harvesting practices; (4) natural or

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industrial water washing in disposals or installations, respectively; (5) blending with other fuels

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such as wood and woody biomass, peat, coal and petroleum coke; and (6) use of additives before

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processing. For instance, the reduction of detrimental water-soluble Cl-, S-, K-, and Na-bearing

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phases can be realized by the selection of feedstock because the wood and woody biomass is

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depleted in such components in contrast to agricultural, herbaceous and aquatic biomass (Table 1


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and reference data4,12). The decrease of the above mobile components can also be achieved by

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the alteration of fertilization process using lesser fertilizers or fertilizers with low contents of

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such elements. Additionally, the reduction of detrimental water-soluble Cl-, S-, K-, Na-, P- and

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N-bearing phases in salt-tolerated biomass can be realized by the modification of harvesting time

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of biomass, when such components occur in minimum concentrations.

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Drying of biomass harvested and left in the field for a prolonged period of time causes an

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extensive salt formation and enhanced leaching of water-soluble Cl, S, N, K, Na, Ca, Mg P, and

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other elements from biomass. 4,15,16,45-52 Washing to remove water-soluble phases prior to use of

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biomass and BAs is a beneficial approach that may reduce many technological and

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environmental problems. For example, the extraction of K, Na and Cl was mostly complete after

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the application of simulated rainfall over uniformly spread rice straw.14 However, such future

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industrial and large-scale washing of biomass and BA may induce new environmental issues

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related to the fate of hazardous elements associated with the water-soluble phases. Hence, the

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concentrations of mobile Cl, S, N, K, Na, and P plus some toxic TEs in biomass and BA need to

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be regulated within strict limits to maintain an acceptable feedstock quality.

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It is well-known that most of the serious deposit formation, slagging and fouling problems

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during biomass combustion are a result of low ash-melting temperatures. Such temperatures are

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typical of salt-tolerated biomasses due to the occurrence of initial low-temperature (≤800 °C)

410

melts formed from alkaline chlorides, carbonates and sulphates. These active and low viscosity

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melts dissolute the refractory minerals and induce subsequent melt crystallization followed by

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abrupt glass formation during cooling at lower temperatures.8 Therefore, the maximum

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temperature for the thermo-chemical conversion of such biomasses should not reach 800 °C.

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However, it was found that the reduction of water-soluble components by water washing of


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different biomasses increases the ash-fusion temperatures significantly.14,15,26,63,73,106

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Additionally, the combination of high K and Si contents in salt-tolerated biomass (particularly

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grasses and straws) contribute to extremely low ash-fusion temperatures. For example, the high

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contents of Si and K in BAs with SiO2/K2O ratio in the range of 1.3-4.0 (especially 2.1-2.3) are

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critical for the formation of low-temperature eutectics and active melts.9 Hence, the reduction of

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K concentrations by water washing of biomass will modify this ratio and avoid the intensive

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formation of such eutectics. The use of solid fuel blends, alternative bed materials for fluidized

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bed combustion or additives with high-melting temperatures (kaolinite, mullite, quartz, basic

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plagioclases, Al and Fe oxyhydroxides, calcite, dolomite, others) is another beneficial approach

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to prevent the agglomeration, sintering and slagging caused by the low ash-fusion temperatures

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of salt-tolerated biomass. Finally, there are many solid fuels which are naturally abundant in

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refractory minerals and their appropriate blending can avoid the use of non-fuel and expensive

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additives, as well as extra handling.

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It was also found that the water-soluble alkaline chlorides, sulphates and carbonates are volatile

429

species and they form fine ash particles after condensation and may form deposits in the

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superheater and economizer surfaces of the boilers.67 It is widely accepted that the water-soluble

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alkaline chlorides and sulphates strongly accelerate the oxidation rate of Fe and steels during

432

biomass combustion and these compounds are responsible for the corrosion of metal surfaces in

433

industrial facilities.153 Hence, the removal of water-soluble fraction by water washing of biomass

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can contribute for reducing corrosion, ash deposition and acid gas emissions.15,73

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The potentially hazardous TEs in solid fuels and/or their combustion products normally comprise

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F, Ba, Be, Th, U, Co, Cr, Mn, Ni, V, As, Cd, Cu, Hg, Pb, Sb, Se, Sn, Tl, Zn, and Ag. 13 It was

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emphasized that the greatest ecological challenges related to TEs in biomass and BA include


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their high contents, unfavourable modes of occurrence, high volatilization potential and limited

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retention during biomass combustion and increased leaching behaviour during biomass and BA

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processing or storage.13

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The water-soluble alkaline chlorides and sulphates cause particle partitioning and formation of

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fine particles in BA responsible for different environmental and health problems. For example,

443

these alkaline phases and some TEs associated with them are normally enriched in the finest fly

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ash fraction (

Water-soluble fractions of biomass and biomass ash and their

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