Biomass ash and options for utilisation - VGBtio fly ash/bottom ash. Grate-fired boilers and stoves generates relatively less fly ash and more bottom ash than fluidised bed boilers

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Biomass ash and options for utilisation VGB PowerTech 10 l 2018

Biomass ash and options for utilisation

Authors

Kurzfassung

Biomasseasche und Nutzungsmöglichkeiten

In einer Reihe von EU-Mitgliedstaaten wird Bio-masse zunehmend in der Strom- und Wärmeer-zeugung eingesetzt. Zum einen ist Bomasse eine Option für eine CO2-neutrale Erzeugung und zum anderen für die Wärmeversorgung in Win-termonaten. Brennstoff- und Verbrennungs-technik sowie bei den Verwertungsmöglichkei-ten der anfallenden Asche sind in den einzelnen Mitgliedstaaten unterschiedlich. Biomassea-sche fällt bei der Verbrennung einer Vielzahl von Biomassearten in verschiedenen Kesselty-pen unterschiedlicher Kesselleistung an. Dies führt zu Biomasseaschen mit unterschiedlichen Eigenschaften, die für ihren (potenziellen) Ein-satz in den jeweiligen Anwendungen entschei-dend sind. Eine Umfrage zu möglichen Anwen-dungen von Biomasseaschen und deren Umset-zung wurde durchgeführt. Aus mehreren Gründen werden aufgrund unterschiedlicher regulatorischer Vorschriften nur begrenzt An-wendungen in großem Umfang und in einzel-nen Ländern realisiert. Biomasseaschen werden in der Forstwirtschaft, der Landwirtschaft (Düngemittel, Bodenkalkung), bei Baustoffen (z.B. Asphalt) und im Tiefbau (z.B. Straßen-bau) eingesetzt. Anwendungen werden in eini-gen Regularien für Düngemittel (meist grober Anteil der Holzasche) und in Gesamtnormen behandelt. Neben den Anforderungen in regula-torischen Vorgaben und Normen müssen auch Umweltkriterien erfüllt sein. Die wichtigste Schlussfolgerung ist, dass die Verwertung von Biomasseaschen auf EU-Ebene am Anfang des Umwandlungsprozesses von Abfällen zu einem nützlichen Produkt steht. Die Erfahrungen mit diesem Prozess für Kohleflugasche und die Er-fahrungen mit der Verwertung von Biomassea-sche z.B. in skandinavischen Ländern können sehr hilfreich sein, um diesen Prozess in den Mitgliedsstaaten insgesamt zu beschleunigen. l

Dr ing. Angelo Sarabèr, MScSenior product manager at Vliegasunie Culemborg, The NetherlandsDr.-Ing. Hans-Joachim FeuerbornVGB PowerTech e.V. Essen Germany

Biomass ash and options for utilisationAngelo Sarabèr and Hans-Joachim Feuerborn

Introduction

Biomass is increasingly used for heat and power production in Europe. In 2016, the primary production of renewable energy was 211 million tons of oil equivalent (toe) within the 28 EU member states. The total quantity of renewable energy in EU-28 member states increased by about 67 % be-tween 2006 and 2016, equivalent to an av-erage increase of 5.3 % per year. Among renewable energies, the most important source in the EU-28 was wood and other solid biofuels as well as renewable wastes, accounting for 49.4 % of primary renew-ables production in 2016 (see F i g u r e 1 ) [1]. The situations of using biomass for energy are different in the EU countries regarding fuel type, boiler type and capacity. Biomass is a synonym for different types of biofuels, from the well-known wood to different types of sludge or agricultural residues like straw. The biomass use results in increas-ing amounts of ash which are either pro-duced in small units leading to small amounts or in higher amounts in e.g. con-verted formerly coal-fired units with up to 50,000 tons of biomass ash in one plant. The use of biomass for sustainable energy encompasses many economic, social and environmental aspects. One aspect is how to deal with the residues from biomass combustion. In the past, the utilisation of coal ashes has been developed from being

a waste to a valuable secondary raw mate-rial for use in building materials, road con-struction and civil engineering. The chal-lenge is to create a comparable develop-ment for biomass ashes, preferably in a shorter period. For the ash management requirements by the Industrial Emissions Directives as well as best practices for utili-sation according to the Waste Hierarchy in the Waste Directive have to be considered by the producers. On the other hand tech-nical and environmental requirements for the application of such materials have to be met and end-users need reliability in avail-ability, quality and finally also on legal safety for use. In this article several aspects of utilisation of biomass ashes will be discussed, espe-cially circularity aspects, ash management and properties of biomass ashes. The re-sults of a survey will be given on basic op-tions for utilisation, with the status of the development. Two cases are of special in-terest, namely the use of ashes for nutrient recycling in the forest and as a fertilizer for agriculture.

Biomass ashes and circularity

The most popular definition of sustainabil-ity originates from the Brundtland Report of 1987, which states that: sustainable de-velopment is development that meets the needs of the present without compromis-ing the ability of the future generations to

Year

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e

225

200

175

150

125

100

75

50

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0

Wood & other solid biofuels BiogasWind power Solar energy

Liquid biofuelsGeothermal energy

Hydro powerRenewable wastes

1990 1992 1994 1996 1998 2000 2002 2004 2006 2008 2010 2012 2014 2016

Fig. 1. Renewable energy produced in the EU in 1990 to 2016 [1].

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VGB PowerTech 10 l 2018 Biomass ash and options for utilisation

meet their own needs. This encompasses both, environmental, social and economic aspects. In the national and in the EU poli-cy, circularity is one of the ways to create a more sustainable society [2]. In the con-cept of a circular economy the value of products and materials is maintained as long as possible. In fact, waste generation and down cycling (Definition: process of recycling whereby the second utilization has less quality than its first utilization.) have to be avoided as much as possible. The concept of a Circular Economy distin-guishes two basic cycles, namely the bio-logical and the technological cycle [3]. The technological cycle involves the manage-ment of finite stock materials like iron, alu-mina, granite etc. The biological cycle in-volves basically renewable materials like wood and grass and includes the return of elements to the biosphere (especially re-turn of nutrients). In a circular economy, biomass will be used according to the cas-cading approach. See also F i g u r e 2 . En-ergy conversion by combustion is mostly the last step within this cascade, whereby ashes will be generated.

Management of biomass ashes

In most cases, ashes from biomass combus-tion are seen as a waste, which has to be discarded. The Waste Frame Directive [4] contains a waste hierarchy as a priority order in waste prevention and management legislation and policy (see F i g u r e 3 ):

– prevention, – preparing for re-use, – recycling, – other recovery, e.g. energy recovery, – disposal.

However, from the point of view of circu-larity, it is proposed that recycling as part of the priority order is distinguished fur-ther:

– nutrient recycling, by returning biomass ash to the same forest where the biomass originates from,

– nutrient recycling, by returning biomass ash to the forest, but elsewhere or in ag-riculture,

– utilisation whereby the application is based on substitution of elements like P, K or Ca,

– utilisation whereby the application is based on other properties (as volume or grain size).

This approach pays more attention to the role of nutrients in biomass ashes and can

be used in discussions about utilisation of biomass ashes.The management of ashes from biomass combustion has to meet at least the waste priority order of the Waste Framework Di-rective [4]. Key themes to create a success-ful ash management are:

– removal of by-products from the power plants must be assured; otherwise stor-age capacity will be the determining fac-tor for the operational period of a power plant. Therefore, assurance of utilisation is an important aspect. In practice, a bal-ance will be found between financial benefits and assurance. Assurance of uti-lisation can be promoted by closing long-term contracts with customers and risk spreading by having different applica-tions and/or customers.

– the by-product chain from generation at the power plant to end-user must comply with the current regulations. This ap-plies to regulations concerning trans-port, waste legislation, health and safety, environment and technical aspects.

– utilisation must be sustainable. – utilisation must generate maximum fi-

nancial benefits (or minimum of costs). – utilisation options must be accepted by

society and must comply with the power brand (green power, grey power).

Properties of biomass ashes

The properties of biomass ashes depend on three main factors: the origin of biomass, the combustion process and the ash remov-al system. Generally, the presence of ash forming matter in biomass is based on the uptake of inorganic elements by the plant due to its need for growth. There are fifteen inorganic elements which are essential for growth in higher plants. These can be di-vided (based on average concentration) in primary macro nutrients (N, P, K), second-ary macro nutrients (Ca, Mg, S) and micro-nutrients (Fe, Mn, Zn, Cu, B, Mo, Cl, Ni, Co). The macronutrients are present in concentrations of 0.2 to 5 % or even higher, while micronutrient concentrations may be present at levels of 0.1 to 100 mg/kg [5]. The concentration of these elements in plants depends on several factors like the species, season of year, part of the plant and soil conditions. Furthermore, it is in-fluenced by the way of harvesting (contam-ination with soil minerals), processing and pre-treatment. Due to these influence fac-tors biomass shows a wide range of compo-sitions. When biomass is first used for other purposes (construction wood, paper), the ash forming matter is dominated by fillers and contaminations. Ashes from wood combustion in large scale combustion plants contain high amounts of calcium, silicon and potassium (see Ta -b l e 1 ). The living parts of the trees (leaves, needles, twigs) contain more po-tassium, phosphorus and magnesia than

Biological materials Technical materials

Partsmanufacurer

Productmanufacturer

Service provider

Energy recovery

Landfill

Biochemicalfeedstock

Farming/collection

Restoration

Biogas

Anaerobicdigestion/

composting

Extraction ofbiochemical feedstock

Cascades

Collection Collection

Leakage – tobe minimized

MaintenanceReuse/redistribute

Refurbish/remanufacture

Recycle

Mining/materialsmanufacturing

UserConsumer

Biosphere

Fig. 2. The use of biological materials in the concept of a circular economy [3].

Prevention

Preparing for re-use

Recycling

Recovery

Disposal

Waste

Product (Non-Waste)

Fig. 3. Waste priority order according to the Waste Frame Directive [4].

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stem wood. Therefore, the way of harvest-ing (whole tree or conventional harvest-ing) influences the composition of the ashes (see chapter nutrient recycling in the forests). Also bark has a higher ash content and different ash composition compared to stem wood and the living parts of the trees. Ashes from combustion of agro-residues differs from wood by its very high potassi-um content, although specific residues like rice husks have very high silicon contents and low potassium contents. All these ash-es have one parameter in common: the alu-mina content is very low compared to coal fly ashes. Higher alumina contents are mostly caused by contamination. Examples are contamination with soil minerals dur-ing harvesting or during first use (like waste wood and paper sludge).A second important factor is the combus-tion process. The combustion temperature and residence time influences the ash for-mation. The higher the combustion tem-perature the more elements will be vola-tised and are able to react with other gase-ous and non-gaseous components. The boiler type determines also the volume ra-tio fly ash/bottom ash. Grate-fired boilers and stoves generates relatively less fly ash and more bottom ash than fluidised bed boilers and pulverised fuel boilers. At the same time these fly ashes have higher con-centrations of elements like potassium, ar-senic, cadmium and zinc, which are volatil-ised during combustion and condensates before the fly ash is removed from the flue gases. A third factor is the ash removal sys-tem, included whether bottom ashes and fly ashes are mixed together.It can be imagined that the chemical and physical properties of biomass ashes can be highly different from source to source. This is illustrated by Ta b l e 1 , which shows the concentration of macro elements and sev-eral trace elements in biomass fly ash and coal fly ash, and F i g u r e 4 , which shows the different morphology of fly ash parti-cles from wood and coal combustion.

Basic options for utilisation

As wood ash is one of the first human re-lated mineral products, it can be imagined that during many centuries wood ash is used for all kind of applications like ferti-lizer, raw material for soap production ice remover etc. If we take a closer look there is a wide range of more or less sound applica-tions, like use for tooth paste, egg preserva-tion, skunk smell remover from sprayed pets etc. A survey of more serious options for utilisation of biomass ashes has been performed, using databases of internation-al journals, interviews, internet and own research work. The results are presented in Ta b l e 2 . In this survey only options are mentioned which are at least investigated on a laboratory scale with biomass ash. In-vestigations with coal ash are excluded.

ForestryAshes from combustion of clean wood (in-cluding forest residues, bark, cut-offs and saw dust) when not contaminated are basi-cally suitable for recycling to the forests to restore the nutrient balance of the forest and also for soil liming to counteract acidi-fication of the soil (both are closely relat-ed). These applications are practiced in e.g. Scandinavian countries and Germany [20; 21]. See further the case Nutrient re-cycling in forests. The utilisation of wood ashes in forestry is covered by several na-tional fertilizer regulations whereby also some requirements for trace element con-centrations are given (see Ta b l e 3 ). AgricultureDue to its content of nutrients, biomass ashes can be used in agriculture. There are three main applications:

– fertilizer; directly or indirectly (as raw material for fertilizer production),

– soil improvement by improving the structure of the soil,

– addition to compost to improve the com-posting process and the final quality of the compost.

Ashes from clean wood are already used as fertilizer in Germany and Denmark (also straw ash), based on existing fertilizer acts. Ashes from combustion of poultry litter are used as fertilizer in UK.

Regulations for use of secondary materials as fertilizer exist on EU level and national level. If the use of biomass ash is covered in national acts, than it is mostly on wood ash and for the coarse ashes only (bottom ash and/or the first filter step). Ta b l e 3 gives a compilation of requirements on biomass ashes for fertilizers in several EU member states. It can be derived that the national regulations for use of biomass ash as ferti-lizer differs strongly (both limit values and parameters).

Tab. 1. Concentration of macro elements and trace elements in biomass fly ashes [6] and coal fly ash.

Fuel Unit Wood Wood Rice husks Mixed biomass1

Coal3

Boiler type2 % m/m PFB GFB Stove FBB PFB

Al2O3 % m/m 4.76 1.73 0.30 7.65 27

CaO % m/m 29.7 44.5 32.8 4.5

Cl % m/m 0.42 1.45 0.80 <0.01

Fe2O3 % m/m 2.45 8.41 0.52 2.04 7.3

K2O % m/m 12.3 16.5 3.29 2.05 1.6

MgO % m/m 5.65 3.93 1.71 1.5

Na2O % m/m 1.19 0.09 0.87 0.6

P2O5 % m/m 2.86 4.97 1.11 0.77

SO3 % m/m 5.78 18.2 0.47 2.70 0.7

SiO2 % m/m 29.7 12.9 88.0 36.4 55.0

TiO2 % m/m 0.32 0.09 0.93

Cu mg/kg 143 160 509 100

Mn mg/kg 11,000 1,100 1,160 630

Pb mg/kg 22.3 100 3,150 54

Zn mg/kg 1,040 5,340 3,180 190

1) mixed biomass is a mix of paper sludge, waste wood and green wood2) boiler type: PFB = pulverized fuel boiler FBB = fluidized bed combustion boiler GFB = grate fired boiler3) average values of Dutch coal fly ashes

Fig. 4. Morphology of coal fly ash (left) and wood fly ash (right). Both from pulverized fuel boilers (scanning electron photograph) [7].

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Building materialsCoal fly ash is widely used for the produc-tion of cement and concrete since decades. Also furnace bottom ash is used in concrete as a fine aggregate, especially in masonry blocks. Depending on properties and espe-cially the chemical and mineralogical com-position of the ashes, biomass ash can be used in building materials for production of cement, alternative binders and con-crete. The technical suitability and feasibil-ity depends highly on the properties of the ashes. Examples of applications which are at least investigated at real-scale are:

– bottom ash from biomass fired fluidised bed combustion boilers for concrete blocks, especially when quartz sand is used as bed material. The utilisation of coarse and fine biomass ashes as inert aggregate or filler is already covered by the aggregate standards for different ap-plications (e.g. EN 12620 Aggregate for concrete).

– fly ash from paper sludge combustion as binder for non-structural concrete and sand-lime brick production. Important is to steer the combustion temperature to have a sufficient reactive lime.

– fly ash from wood combustion as raw material for clinker production. Howev-er, the addition is limited due to the con-tent of undesired components, like alka-lis and sulphur

– fly ash from wood combustion produc-tion of sand-lime bricks. Real-scale tests have been performed to use wood ash as partial replacement of lime and fine sand in sand-lime bricks. the addition was limited to less than 5 % m/m. The imple-mentation was put on hold due to the high transport costs.

Coal fly ash from co-combustion with bio-mass is used as reactive (pozzolanic) filler in concrete in several countries. The Euro-pean technical standard EN 450-1 Fly ash for concrete [24] allows co-combustion up to 50 % m/m fuel based for virgin wood only, whereby the ash amount of the co-combustion material does not exceed 30 % m/m ash based. The latter is normally no bottle-neck due to the low ash content of wood pellets compared to coal. Co-com-bustion of other biomass is allowed up to 40 % m/m (included mixes with virgin wood), if the biomass is listed in the EN 450 (see Ta b l e 4 ) and all product re-

Tab. 1. Survey of (potential) applications for biomass ashes [8 to 23].

Sector Application Function Level Ash types

ForestryNutrient recycling Restoring nutrient balance IM Wood ash

Soil liming Counteract acidification IM Wood ash

Agriculture

Fertilizer Ca, P, K, Mg, S fertilizer IM Wood ash

Raw material fertilizer

Ca, P, K, Mg, S fertilizer IM Wood ash

Compost Improving composting process and quality

IM Wood ash

Soil improvement Improving structure soil IM Wood ash

Civil engineering

Infrastructural works (embankments, fillings)

Filling material IM Wood ashes

Road Construction material

Binder/filling material IM Wood fly ash

Asphaltic filler Raw material IM biomass fly ashes

Sludge stabilisation Binder IM Wood ash

Soil stabilisation Filler/lime substitution IM Biomass fly ash

Mining Back-filling Filler IM Waste wood fly ash

IndustryPhosphor production

Phosphor source RS Sewage sludge ash

Zeolites Raw material LS Biomass ash

Energy

Biogas purification

Reagents PS Wood ash

Synthesis of bio diesel

catalyst LS Wood ash, rice husk ash

Flue gas cleaning desulphurisation sorbent IM Paper sludge ash

LS = investigated at lab scalePS = investigated at pilot-scaleRS = investigated at real-scaleIM = implemented

Tab. 3. Compilation of requirements on biomass ashes for fertilizers and nutrient recycling in several EU member states [mg/kg] [22].

CZ D NL Se DK (new) FIN

Fertiliser1 Farming/forestry

Fertiliser Forest

As mg/kg 10 40 75 to 375 30 25 40

B mg/kg 800

Ba mg/kg

Cd mg/kg 2 1,5 6.3 to 31.3 30 5 straw 20 wood

5 mix

2,5 25

Cr mg/kg 100 375 to 1875 100 100 300 300

CrVI mg/kg 2

Cu mg/kg 100 375 to 1875 400 600 700

Hg mg/kg 1,0 1,0 3.8 to 18.8 3 0,8 1 1

Mo mg/kg 50

Ni mg/kg 300 80 150 to 750 70 60 100 150

Pb mg/kg 100 150 333 to 2,500 300 120 250 wood ash

for forest

100 150

Se mg/kg 20

Tl mg/kg 1,0

V mg/kg 70

Zn mg/kg 1,500 to 7,500 7,000 1,500 4,500

PFT mg/kg 0,1

Dioxins ng/kg 30 3.8 to 19

PCB mg/kg 75 to 375

PAH mg/kg 2,300 to 11,5001 the concentrations shall be calculated to the ‘value determining’ component of the fertilizer/biomass ash

like phosphorus, potassium or magnesium. The limit values itself depends also on the ‘value determining’ component.

Tab. 4. Types of co-combustion materials according to EN 450-1 [24].

1 Solid Bio Fuels conforming to EN 14588:2010 including animal husbandry residues and excluding waste wood

2 Animal meal (meat and bone meal)

3 Municipal sewage sludge

4 Paper sludge

5 Petroleum coke

6 Virtually ash free liquid and gaseous fuels

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quirements are met. Waste wood is exclud-ed and thereby not allowed for certification according to EN 450-1. A European Assessment Document (EAD) is in preparation to cover fly ash from com-bined firing of wood and biomass. Thereby 4534% ash amount from the co-combustion material is allowed, with no limit value for co-combustion fuel based [25]. However, most of the chemical and physical param-eters for the fly ash are the same. The EAD will be used to support the utilisation of this so-called bio-coal fly ash in non-struc-tural concrete. The chemical requirements, as set in the EN 450-1, together with the range for coal fly ash and wood fly ash are presented in Ta b l e 5 . It shows that wood fly ash is unable to meet the criteria for coal fly ash in EN 450-1.

Civil engineeringFly ash can be used as a substitute for sand in embankments and structural fillings (earth works), road construction material, soil stabilisation and production of asphal-tic fillers. Implemented applications in civil engineering are:

– ash from paper sludge combustion is used as lime replacement for soil stabili-sation to improve the geo-mechanical properties of soils (bearing capacity, plasticity).

– fly ashes from combustion of sewage sludge, paper sludge and waste wood are used as raw material for composite fillers for asphalt, especially in the Nether-lands. Most important properties of bio-mass fly ashes for this application are water solubility, bitumen binding capac-

ity and content of hazardous compo-nents. For this application the biomass fly ashes are mixed with other ingredi-ents and grinded to the required grain size. Basic requirements for fillers for as-phalt are given in the standard EN 13043 Aggregates for bituminous mixtures and surface treatments for roads, airfields and other trafficked areas [18].

– use of biomass ashes in road construc-tion (see F i g u r e 5 ) as binder compo-nent in base course stabilisation [23].

– bottom ashes from waste wood combus-tion are mixed with MSW bottom ash and used as road construction base ma-terial

– sludge stabilisation and neutralisation, before landscaping. Biomass ashes are used to stabilise sludge to improve the geo-mechanical consistency for handling and application.

MiningCoal fly ash and also biomass ashes are used for infill, which means filling of voids, mine shafts and subsurface mine workings.

IndustryDue to dependency of imports of phospho-rus in most European countries there are several efforts and initiatives to satisfy the future use considering the processing of sewage sludge and also the reuse of ashes. Phosphorus is an essential element for to-day’s life. It is used in essential applications like fertilizers, steel production and pesti-cides.In November 2013, the German Phospho-rus Platform was found as a network of stakeholders from science, industry and public bodies to establish sustainable phos-phorus management in Germany through a more efficient use of phosphorus as well as effective recycling and reprocessing. Core to the approach of DPP is identifying and implementing innovative technologies that allow economically feasible recycling of phosphorus on a broad scale but also through improved use of phosphorus in al-ready existing manufacturing processes. This includes increases in production effi-ciency as well as re-using production waste as input material were possible [27].

In October 2017, Germany has release a new sewage act. By this, after an interim period, no sewage sludge from medium and big sewage plants are allowed for ferti-lisation and phosphorus has to be recycled from the sewage sludge [28]. Also in France it is expected that in 2020 100,000 tons/year of sewage sludge ash will be used to replace phosphate ore for the production of phosphorus [17]. In Japan the option of the application of biomass fly ash for the synthesis of zeolites was reported. Potassium was wet extracted from biomass fly ash and the aqueous solu-tion was used for hydrothermal synthesis of K-zeolites [13]. Zeolites are used for many applications like absorption, in de-tergents, and for cracking of crude oil.

EnergyThe use of biomass ash for energy produc-tion is very specific. A few studies have been reported about the use of wood ash and rice husk ash as a catalyst to produce biodiesel from Jatropha oil [16] or palm oil [11] respectively. Another study reported the use of wood ash for the purification of biogas by trapping CO2 and H2S [12]. Fly ash from paper sludge combustion is used as desulphurisation sorbent in some com-bustion processes.

Case: Nutrient recycling in forests

In a natural situation most of the biomass (leaves, needles, branches and finally the stem) will fall on the ground near the tree or plant. In this way the nutrients are kept within the ecosystem. The nutrient balance of an ecosystem is influenced by several factors especially by wash-off of com-pounds and soil erosion (weathering of minerals) whereby nutrients (like K, Ca, Mg and P) become available for organisms and by deposition from the atmosphere [29]. The latter is mainly relevant for nitro-gen. Nitrogen can also be bound from the atmosphere by specific plants. F i g u r e 6 gives a simplified illustration of the recy-cling of nutrients and inputs and outputs of a forest ecosystem. When biomass is harvested, the ecosystem is not closed anymore as nutrients are tak-en away together with the biomass. Alterra Wageningen UR assessed the nutrient bal-ance on the sustainability of different tim-ber harvesting scenarios in the Netherlands in relation to the nutrient balance for Ca, Mg, K and P [30]. Nitrogen was excluded in this study as no depletion is expected due to the high level of deposition in the Neth-erlands. A negative nutrient balance is hardly calculated for richer soil types (loamy and clayey soil types), but for poor-er soils (sandy) depletion occurs, especial-ly for Ca and K, for all growth classes. For some situations depletion of these ele-ments was calculated even after one har-

Tab. 5. Chemical requirements of EN 450-1 and some chemical properties of coal fly ash and wood fly ash, generated in pulverized fuel boilers [7].

Component Unit Wood fly ash Coal fly ash EN 450 Criteria

Parameter

CaO % m/m 30 to 33 4.2 ±1.2 ≤10.0

Cl % m/m 0.16 to 0.42 <0.01 ≤0.10

CaOfree % m/m 6.1 to 10.3 0.2 ±0.3 ≤1.51

P2O5 % m/m 2.5 to 2.9 0.58 ±0.31 5.0

SO3 % m/m 3.6 to 5.8 0.68 ±0.18 ≤3.0

Na2Oeq % m/m 7.6 to 9.3 1.5 ±0.5 ≤5.0

Al2O3+Fe2O3+SiO2 % m/m 28 to 36 88 ±2 ≥701 If this limit value is exceeded the Le ‘Chatelier test has to be performed.

Fig. 5. Example of use of biomass ash in road construction [23].

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vesting. It was remarked by the authors that the calculation shows high uncertain-ties, especially about the relation between uptake of nutrients in parts of the trees and the availability of nutrients and further the impact of deposition and weathering for phosphorus on the phosphorus balance. Ta b l e 6 gives an indication of the removal of macro-nutrients N, P, K and Ca (data about the other macro-nutrients Mg and S were not given) due to extraction of bio-mass in several forests in the United States [30]. Two types of harvesting were com-pared, namely conventional harvesting (stem only) and so-called ‘whole tree har-vesting’, whereby the complete tree includ-ing branches, leaves etc. is removed from the forest. As most nutrients are present in the living parts of the tree, whole tree har-vesting leads to considerably higher nutri-ent loss than conventional harvesting where just the stems are taken away [31-33]. In several countries, like Denmark, Sweden and Finland, spreading wood ash in the

forests is practiced to recycle nutrients or as liming agent. In this way macro- and mi-cronutrients are recycled, with exception of nitrogen as this compound is not bound in the fly ash during combustion. However in many regions in the developed coun-tries there is significant contribution to the nutrient balance by atmospheric deposi-tion. Besides the nutrient balance, recycling of ashes in the forests encompasses many as-pects to be taken into account, especially

– biomass fly ashes have a very high pH, which may influence the flora and fauna of the top soil in the forest. Pretreatment by carbonation in open air may reduce this effect.

– content and bio-availability of heavy metals. When the ashes are returned to the same area where the biomass origi-nated from, the heavy metal burden is not influenced on a macro level. Howev-er, the bio availability may differ due to different speciation.

– legislation. The spreading of wood ash can be seen as dispersion of waste, which is not allowed

– contamination. If the biomass is first used for other applications according the cascade approach (which is the idea of a circular economy), the biomass may be contaminated with fillers, coating etc. These contaminations are also spread in the forest, which may have a negative impact on the ecosystem

– logistics. When the power plant is not situated near the forest it may not be ef-ficient to recycle the ashes back to the forests.

The above mentioned aspects shows that the evaluation for an effective and/or effi-cient return of ashes to the forest is very complex and needs to be evaluated within the context of a circular economy.

Evaluation and conclusions

Due to the increase of energy production from biomass, more biomass ashes are gen-erated in the EU than recent decades. Ac-cording to the EU policy, generation of waste has to be avoided. Therefore, utilisa-tion of biomass ashes has to be developed further as a significant part of the biomass ashes are disposed. The re-use of these bio-mass ashes is also relevant because bio-mass ashes contain nutrients, which are taken away with the biomass. A lot of potential applications for biomass ashes have been identified. However, prac-tice is stubborn and only a limited amount is implemented on a large scale. This can be attributed to several aspects as relatively small volumes per source, variations in (chemical) properties, lack of knowledge and the non-maturity of the market (not in all countries). The logistic chain for biomass ashes is a very important aspect in the development of utilisation due to the relatively small vol-umes per plant compared to coal ashes. This chain may also include stockpiling and processing before supply to end user. Processing of biomass ash may help to im-prove specific properties and to create a more consistent and uniform quality. Ex-amples are:

– carbonation to lower the pH by weather-ing

– forced leaching to remove heavy metals and/or nutrients

– granulation to get ashes which can be spread in the forests without dust and acts as a slow release fertilizer

– blending, which is performed on a rou-tine base with coal fly ash

– Milling to create a more reactive biomass ash.

Another positive influence to promote the utilisation of biomass ashes is to share the existing knowledge and experience in dif-ferent countries. Especially regarding the

Dry and wet deposition

Transport within the tree

Soil erosion

Uptake

Wash off

Fall of organic material

Fig. 6. Simplified model for nutrient cycling in forest ecosystem [29].

Tab. 6. Removal of N, P, K and Ca due to whole tree harvesting and conventional harvesting in forests on poor (P) and rich (R) soils in the USA (harvested after 55 years of growth) [31].

Conventional harvesting Whole tree harvesting

Tree EXT* N P K Ca EXT. N P K Ca

ton/ha kg/ha kg/ha kg/ha kg/ha ton/ha kg/ha kg/ha kg/ha kg/ha

Douglass (R) 281 478 56 225 23 318 728 96 326 411

Douglas (P) 134 161 27 121 - 165 325 56 140 -

High alder (R) 137 287 41 151 388 147 347 47 174 426

Low alder (P) 111 311 22 122 - 120 242 27 143 -

*EXT.=extraction of wood

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use of wood ashes for nutrient recycling and soil liming in the Scandinavian countries. Main conclusion is that on EU scale, utili-sation of biomass ashes is at the beginning of the transformation process from waste into a useful product. The experience with this process for coal fly ash and the experi-ence with biomass ash utilisation in coun-tries like in Scandinavia may be very help-ful to accelerate this process in the single countries.

Literature[1] EUROSTAT Renewables Energy statistics

(http://ec.europa.eu/eurostat/statistics-explained/ index.php/Renewable_ener-gy_statistics#Renewable_energy_pro-duced_in_the_EU_increased_by_two_thirds_in_2006-2016).

[2] European Commission, 2015. Closing the loop; Commission adopts ambitious new Cir-cular Economy Package to boost competive-ness, create jobs and generate sustainable growth. Press Release December 2nd 2015.

[3] Ellen Macarthur Foundation, 2015. To-wards a circular economy: Business ration-ale for an accelerated transition.

[4] WFD. Waste Framework Directive - Directive 2008/98/EC of the European Parliament and of the Council of 19 November 2008 on waste and repealing certain Directives.

[5] Alley M.M. and VanLauwe, B., 2009. The role of fertilizers in integrated plant nutrient management, International Fertilizer In-dustry Association. page 12.

[6] Sarabèr, A.J. Suitability of biomass fly ash as binder in concrete (in Dutch). 2016. Final report. TKI-Project co-financed by Dutch Ministry of Economic Affairs.

[7] Sarabèr, A.J., 2017. Fly ash from coal and biomass for use in concrete. PhD Disserta-tion Technical University Delft.

[8] Asquer, C. et al., 2017. Biomass ash reutili-zation as an additive in the composting pro-cess of organic fraction of municipal solid waste. In: Waste Management, Volume 69, November 2017, pp 127-135.

[9] Ban, C. et al., 2015. The hybridization od coal fly ash and wood ash for the fabrication

of low alkalinity geopolymer load bearing block cured at ambient temperatures. In: Construction and building materials, Vol-ume 88, July 2015, pp 41-55.

[10] Bougnom, B.P. et al., 2011. Possible use of wood ash and compost for improving acid tropical soils. In: Recycling of biomass ash-es. Edited by Heribert Insam et al. Springer Verlag.

[11] Chen, G-Y, et al., 2015. Transesterification of palm oil to biodiesel using rice husk ash-based catalysts. In: Fuel Processing Tech-nology, Volume 133, May 2015, pp 8-13.

[12] Fernandez-Delgado Juarez, M. et al 2018. Biogas purification with biomass ash. In: Waste Management, Volume 71, January 2018, pp 224-232.

[13] Fukasawa, T. et al., 2017. Utilization of in-cinerator fly ash from biomass power plants for zeolite synthesis from coal fly ash by hy-drothermal treatment. In: Fuel Processing Technology, Volume 167, December 2017, pp 92-98.

[14] Havukainen, J. et al., 2016. Potential of phosphorus recovery from sewage sludge and manure ash by thermochemical treat-ment. In: Waste Management, Volume 49, March 2016, pp 221-229.

[15] Kizinievic, O. and Kizinievic, V., 2016. Uti-lisation of wood ash from biomass for the production of ceramic products. In: Con-struction and building materials. Volume 127, 30 November 2016, pp 264-273.

[16] Sharma, M. et al., 2012. Wood ash as a po tential heterogeneous catalyst for biodiesel synthesis. In: Biomass and Bioenergy, Vol-ume 41, June 2012, pp 94-106.

[17] SNB, 2018. Oral information by Luc Sijster mans, July 17th 2018.

[18] Sibelco, 2018. Oral information by Kees van der Plas, July 17th 2018.

[19] Ottosen, L.M. et al., 2016. Wood ash as partly sand and/or cement replacement in mortar. In: International Journal of Sus-tainable Development and Planning. 11(5), pp 781-791.

[20] Swedish Forest Agency, 2006. Internation-al Handbook from Extraction of Forest Fuels to Ash Recycling.

[21] Wilpert, K. von, (FVA) 2015. RAL Quality mark for wood ash for forest fertilization.

Presentation at VGB Biomass Ash Work-shop, April 29th 2015.

[22] VGB, 2017. Biomass ash (BMA) in Europe. A report on the situation of properties and use of Biomass Ash from power plants of different technology burning biomass only. draft – 23.05.2017. Not publicly available.

[23] Lahtinen, P., 2015. Utilization of biomass ashes in infrastructure construction in Fin-land. Presentation at VGB Biomass Ash Workshop, April 29th 2015.

[24] EN 450-1:2012. Fly ash for concrete, Defini-tion, requirements and conformity criteria.

[25] KIWA, 2018. European Assessment Docu-ment Bio-coal Fly ash for Concrete. EAD DP 17-26-0021-03.01. Fourth Draft version.

[26] VGB, 2018. Report on the situation with bio-mass ash in Europe, draft (not publicly available).

[27] [DPP : https://www.deutsche-phosphor-plattform.de/english/].

[28] Verordnung über die Verwertung von Klär-schlamm, Klärschlammgemisch und Klär-schlammkompost (Klärschlammverord-nung - AbfKlärV): 09.2017. (Decree about the utilisation of sewage sludge, mixes of sewage sludge and mixes of sewage com-post).

[29] Paré D. and Thiffault, E., 2016. Nutrient budgets in forests under increased biomas harvesting scenarios. In: Current Forestry Reports (2016) 2:pp 81-91.

[30] Alterra, Wageningen UR, 2015. Houtoogst en bodemvruchtbaarheid. Een modelstudie naar duurzaamheid op Nederlandse bos-groeiplaatsen.

[31] Mann L.K. et al, 1998. Effects of Whole-Tree and Stem-only Clearcutting om posthar-vest Hydrologic Losses, Nutrient Capital and Regrowth. In: Forest Science, volume 34 Number 2, 1 june 1988 pp 412-418 (17).

[32] De Schrijver A. et al, 2012. Koolstof- en nu-triëntenkringlopen. In: Bosecologie en Bos-beheer, pp 167-175. Uitgeverij Acco, Leu-ven.

[33] Alterra, Wageningen UR, 2011. Effecten van de oogst van takhout op de voedingstoe-stand en bijgroei van het bos. Alterra-rap-port 2202. l

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Biomass plants are operated as heating, power and combined heat and power plants. Their fundamental difference from fossil fired power plants is the nature of the fuel. This also entails certain special features with regard to fire protection. In principle, VGB Guideline VGB-R 108 also applies to power plants of this type, but the specific characteristics of the biomass require a number of additions to that guideline. This standard therefore serves the purpose of making the experience of the recent past available to all VGB members and the interested public.This VGB-Standard presents methods of using operational and adapted technical measures to keep the risks of fire and explosion as low as possible. In this way, potential hazards can be avoided by the deployment of suitable operational workflows. Fires can be detected without delay and combatted effectively under the special conditions of the type of plant concerned. The formation of potentially explosive atmospheres can be limited by operational and technical measures.

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The electricity sector

at a crossroads

The role of renewables energy

in Europe

Power market, technologies and acceptance

Dynamic process simulation as an engineering tool

European Generation Mix

Flexibility and Storage

1/2

2012




The electricity sector

at a crossroads

The role of

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in Europe

Power market,

technologies and

acceptance

Dynamic process

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engineering tool

European

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Storage

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Documents

Biomass ash and options for utilisation - VGBtio fly ash/bottom ash. Grate-fired boilers and stoves generates relatively less fly ash and more bottom ash than fluidised bed boilers