Ash Utilisation

Ash utilisation

Download PDF Presentation Ash utilisation

Since its foundation one of BIOS’ main focuses has been on the sustainable utilisation of biomass ashes and the consequent development and improvement of ash utilisation processes. The comprehensive Know-How and experience gained guarantees a successful execution of R&D and engineering projects in this field.

BIOS is a competent partner for the management and execution of R&D projects regarding biomass ash utilisation and the planning and realisation of biomass ash utilisation concepts.

Fields of activity

BIOS offers the following services related to ash utilisation:

Performance of R+D projects for the sustainable utilisation of biomass ash
Chemical analysis of biomass ashes, characterization and evaluation of possibilities of ash utilisation
Implementation of concepts for fractionated heavy metal separation in biomass combustion and gasification plants
Preparation of ash utilisation and logistics concepts for biomass heating and biomass CHP plants
Development and planning of the process technology related to a sustainable ash utilisation in biomass combustion and gasification plants (ash handling, ash treatment, ash storage, ash distribution / utilisation)

Industry sectors

BIOS provides services related to ash utilisation for the following clients / customers:

Municipalities and local authorities
Utilities and municipal energy suppliers
Heat supply companies
Sawmills and wood industry
Window, door and furniture industry
Hotels
Waste treatment, recycling and disposal companies
Composting plants
Industries interested in ash utilisation (e.g. cement production plants)

Background

In Austria the energetic utilisation of woody biomass in heating and combined heat and power plants increased by about 39% from 2005 to 2015 (source: Statistik Austria). The same is true for other European countries like Germany, Italy or Scandinavia. The promotion of energy production from biomass has also led to a strong increase in the amount of combustion residues, i.e. ashes. Therefore, increasing importance is attached to a sustainable and cost efficient utilization of wood ashes and other biomass ashes.

At present, a large fraction of the biomass ashes produced is disposed of in landfills. Considering the fact that biomass ashes are a valuable secondary raw material for fertilizing and soil improving measures as well as soil stabilization the current situation is far from satisfying.

According to a report of the Environment Agency Austria (Umweltbundesamt) from 2016 (Biomass ash mass flows in Austria) the thermal utilisation of biomass resulted in 133,000 tons of ash (excluding small-scale applications) in Austria in 2013. Almost 50% (58,300 tons) of the ash were disposed of in landfills in 2013. In Austria disposal costs range from 60 to 100 €/t on average, but up to 200 €/t are possible in worst case. Accordingly, the annual costs for ash disposal are significant. An increased utilisation of biomass ash is therefore of high interest both for the economy and the closing of the mineral cycle.

Biomass ash – a valuable secondary raw material

Ash from natural solid biomass fuels contains valuable plant nutrients such as K, P, Mg and Ca. Thus, their application on agricultural or forest soils can significantly contribute a closure of the mineral cycle. The rather high content of Ca in wood ashes (between 20 and 35 wt%, dry basis) facilitates a utilization as a binding agent for soil stabilization (similar to burnt lime).

However, biomass ashes also contain significant amounts of heavy metals, which have to be considered for the utilisation strategy applied. Therefore, only the fractions rich in nutrients or other valuable components and poor in heavy metals (bottom ash and coarse fly ash) shall be used for fertilizing, soil improvement or soil stabilization purposes. The fraction rich in heavy metals and rather poor in nutrients or other valuable components have to be disposed of. The distribution of nutrients, other valuable components and heavy metals over the individual ash fractions depends on the type of combustion technology applied:

Ashes from grate furnaces show increasing concentrations for most of the environmentally relevant heavy metals (Zn, Pb, Cd, Hg) with decreasing particle sizes of the ash fractions.
The contents of Cadmium (Cd) and to a lesser extent Zink (Zn) are usually the most problematic in terms of environmental impacts. Based on the current state-of-the-art of grate furnaces about 35 to 65% of the total amount of Cd and 35 to 55% of Zn in the ash is concentrated in the fine fly ash fraction, which accounts for only 5 to 10% of the total amount of ash generated in a biomass combustion plant.
The mixture of bottom and coarse fly ash accounts to approx. 80 to 95% of the total amount of nutrients (Ca, Mg, K, P) included in the fuel.
Ashes from fluidised bed furnaces also show increasing concentrations for Zn, Pb, Cd, Hg with decreasing particle sizes of the ash fractions, but the difference between the individual ash fractions is significantly smaller. This is mainly due to the lower temperatures in the combustion zone, which reduce the release of volatile heavy metals in the flue gas and thus their accumulation in the fly ash fractions. Moreover, parts of the bed material (usually quartz sand) are also entrained with the flue gas, which “dilutes” the fly ash fractions and reduces the concentrations of all elements except Si. Due to these reasons the heavy metal concentrations in fly ashes from fluidized bed furnaces are usually lower than in fly ashes from grate furnaces.
The bottom ash from fluidised bed furnaces features the lowest heavy metal concentrations but given the high share of bed material in this ash fraction, the concentrations of nutrients and other valuable elements are rather low, too. Thus, a utilization of this fraction may not be reasonable in every case. In contrast to bottom ash from grate furnaces, the share of the bottom ash (including bed material) is only between 10 and 35 wt%.
The main share of the ash generated in fluidised bed furnaces is fly ash (boiler and filter fly ash). Boiler fly ashes feature rather low heavy metal contents and rather high contents of nutrient and other valuable materials and are suitable for recycling. 60 to 90 wt% of the valuable materials but also 60 to 85 wt% of the heavy metals can be found in the fly ashes. Thus, a fractionation of fractions with high nutrient/valuable material contents and low heavy metal contents and vice versa are not possible. However, compared to fly ashes from grate furnaces, the heavy metal contents in fly ashes from fluidised bed furnaces are generally lower.

Considering the points listed above, bottom ash and coarse fly ash from grate furnaces as well as fly ash from fluidised bed furnaces (and under certain circumstances also bottom ash) are the fractions most suitable for further utilisation.

Current status of biomass ash utilisation in Austria

The ash utilisation strategies currently possible in Austria are described briefly below. For a general overview of biomass utilisation in Europe see Download #003: Possibilities of Ash Utilisation from Biomass Combustion Plants

Application of ashes on agricultural and/or forest land:

The basis for the utilisation of biomass ashes on agricultural and/or forest land is, besides the Waste management law and its ordinances (waste monitoring ordinance and waste balance ordinance) together with the federals state laws for soil protection; the Guideline for the proper utilisation of biomass ashes on agricultural and forest soils (Biomass ash guideline, in German “Richtlinie für den sachgerechten Einsatz von Pflanzenaschen zur Verwertung auf land- und forstwirtschaftlich genutzten Flächen”), which entered into force as from 01.01.2011. According to this guideline, the application of biomass ashes is allowed under the following framework conditions:

Only ash from the combustion of natural biomass fuel sources can be used for fertilizing and soil improvement purposes. Ashes from the combustion of waste wood or chemically treated wood must not be applied on soils due to their high heavy metal contents.
Bottom ashes, boiler fly ashes and coarse fly ashes may be used as individual fractions or as mixtures, if the individual fractions and the mixtures meet the limiting values for harmful substances (heavy metals and organic harmful substances) defined in the biomass ash guideline. For grate furnaces, the use of a mixture of bottom and coarse fly ash is recommended, for fluidised furnaces the utilisation of the boiler fly ash is usually the best choice.
If any ash fractions are collected and stored separately, a mixture is only allowed if all fractions that are mixed meet the limiting values of the biomass ash guideline.
Fine fly ashes (as a single fraction or in mixture with other fractions) must not be used.
The amount of ash to be applied depends on the nutrient contents of the ash and the nutrient demand of the crop. The maximum amount defined in the biomass ash guideline depends on the type of soil and varies from 2,000 kg/ha every 20 years (forest soils) and 1,000 kg/ha and year (agricultural soils).

The application of ashes on agricultural or forest land requires an appropriate ash treatment in order to meet the requirements of the ash application process intended by the end user. The following tasks have to be considered prior to the application (for ash utilisation in Austria):

Proper mixture of the usable ash fractions (if a mixture is necessary/planned).
Preferably dust free ash handling. Avoidance or minimisation of dust formation during ash handling.
Appropriate dimensioning of the ash storage facilities at intermediate storage sites is usually necessary (the main application period is in spring/summer, so enough storage capacity is needed to store the ash produced during the rest of the year).
The ash must be provided in a spreadable particle size (i.e. free of slag and other particles larger than 1.0 cm). Subsequently, a screening of the ash and also a metal precipitation are usually necessary.
Selection of the suitable ways of transport (big bags, silo wagon etc.).
Suitable spreaders are spreaders that are also used for lime and compost application.

Utilisation as an additive in composting

The basis for the utilisation of biomass ashes as an additive in composting is, besides the Waste management law and its ordinances (waste monitoring ordinance and waste balance ordinance), the composting ordinance, which entered into force as from 2001. According to the composting ordinance, the utilisation of biomass ashes is allowed under the following framework conditions:

Only ash from the combustion of natural biomass fuel sources can be used for fertilizing and soil improvement purposes. Ashes from the combustion of waste wood or chemically treated wood must not be applied on soils due to their high heavy metal contents.
Bottom ashes, boiler fly ashes and coarse fly ashes may be used as individual fractions or as mixtures, if the individual fractions and the mixtures meet the limiting values for harmful substances (heavy metals and organic harmful substances) defined in the composting ordinance. For grate furnaces, the use of a mixture of bottom and coarse fly ash is recommended, for fluidised furnaces the utilisation of the boiler fly ash is the usually the best choice.
If any ash fractions are collected and stored separately, a mixture is only allowed if all fractions that are mixed meet the limiting values of the biomass ash guideline.
Fine fly ashes (as a single fraction or in mixture with other fractions) must not be used.
A maximum of 2 wt% (wet basis) biomass ash can be added to the compost material.

Indirect ash application via composting features some advantages compared to a direct application of ash to agricultural and forest land:

No problems of dust formation.
No pre-treatment (screening, metal precipitation) of the ash is usually necessary, since these impurities are sorted out during the composting process (turning of compost material at regular intervals).
The addition of ash can reduce the time required for the composting process and can improve the compost structure.
An addition of ash to the compost reduces the pH-level of the ash due to the carbonisation process occurring.

Research results from the FFG industry project (see section FFG collective research project „ Development of innovative processes for wood ash recycling“) show that additions of up to 8 wt% (wet basis) are ecologically sensible.

As of 2020, an amendment to the Compost Ordinance is in progress. At EU level, the inclusion of ash, biochar and struvite (a phosphate-containing mineral recovered from wastewater) as permitted starting materials for the production of fertilizers (for details see section Registration as a fertiliser below) in the EU Fertilizer Product Ordinance (FPR) is currently being discussed. The inclusion of these secondary raw materials as starting materials for fertilizers could be decided in 2021. These changes would then also have to be taken into account in an amendment to the Compost Ordinance.

Registration as a fertiliser

Since the latest amendment of the Austrian fertiliser law in April 2017, combustion residues (i.e. ashes) may be used as fertilisers or additives to fertilisers, if the regulations of the fertiliser law and its related ordinances are met. The application for approval can be filed by the producer of the ash itself (if an individual ash fraction shall be approved) or by any fertiliser producer (if the ash fraction is used as a raw material for fertiliser production).

During the approval procedure the chemical properties of the ash (heavy metals, nutrients, total organic carbon, organic harmful substances) and the source of the ash (only ash from chemically untreated biomass is allowed) are checked. Ashes which meet the limiting values of the biomass ash guideline may also be eligible for approval as a fertiliser. Additionally, at least one primary (N, K, P) or secondary nutrient (Ca, Mg, Na, S) must be contained in relevant amounts. The minimum contents of nutrients defined in the approval and the limiting values of harmful substances defined in the fertiliser law must be kept at all times by the approved ash fraction.

The main advantage of the registration of an ash fraction as a fertiliser is the fact that this fraction is no longer considered as a waste. This facilitates the utilisation, since farmers and forest owners that use the approved ash are exempt from the reporting and recording duties that are required for any kind of waste utilisation.

On July 16, 2019, the new EU fertilizer regulation ((EU) 2019/1009) came into force (national implementation by July 16, 2022). On the basis of the regulation, the Commission can include biomass ash and other potential raw materials (e.g. struvite, a phosphate-containing mineral recovered from wastewater, or biochar) as permitted raw materials for fertilizers (listed in Annex II of the regulation) within the framework of delegated acts. The delegated acts for biomass ash, struvite and biochar were presented to the fertilizer expert group in the Commission in autumn 2019. There is currently a discussion about the inclusion of these raw materials as permitted starting materials for the production of fertilizers in the EU Fertilizer Product Ordinance (FPR). The inclusion of these secondary raw materials as starting materials for fertilizers at EU level could be decided in 2021

Utilisation as an additive in the cement industry

The basis for the utilisation of biomass ashes as an additive in the cement industry is, besides the Waste management law and its ordinances (waste monitoring ordinance and waste balance ordinance), the plant approvals of the individual cement factories. The framework conditions for the utilisation of biomass ashes depend on the conditions defined in the plant approval and may differ between different cement factories.

Outlook

To some extent, the existing utilisation strategies are used, but still almost 50 wt% of the biomass ashes generated annually in Austria are landfilled. Moreover, interesting new utilisation options (utilisation as a building material for forest streets, utilisation as a binding agent for soil stabilisation) exist, that are currently not executed in Austria. This is due to the fact that not all ecological, logistic, technical, and economic aspects have been covered and investigated yet and that sufficient legislation is still lacking to a large extent.

In order to tackle these deficits, BIOS initiated a R&D project, funded by the Austrian Research Promotion Agency (FFG), with the goal to optimise existing biomass ash utilisation strategies and to research and develop new utilisations options. The main facts and findings of this project are presented below.

FFG collective research project „ Development of innovative processes for wood ash recycling“

The FFG collective research project was initiated by BIOS in cooperation with the trade association of the Austrian wood processing industry (FHP, platform for forest, timber and paper). The project consortium included renowned scientific partners and partners from different industry sectors.

Main facts of the project:

Duration: 4 years
Budget: more than 1.5 million EUR
Project coordination: FHP, trade association of the Austrian wood processing industry
Scientific project management: Dr. Ingwald Obernberger (BIOS)
Scientific partners: Department for forest and soil sciences, University of Natural Resources and Life Sciences, Vienna, Styrian Chamber of Agriculture, Bioenergy 2020+ GmbH
Industrial partners: 12 partners from the forest, wood processing, paper, energy and construction industry

The project aimed for the following overall goals:

The development and optimisation of ecologically meaningful, practicable and cost-efficient utilisation strategies for wood ash while considering results from national and international R&D projects.
Evaluation of the complete process chain from combustion technology to treatment, logistics, transport and recycling of the ashes with the aim to close the mineral cycle while considering environmental and economic feasibility.

Comprehensive evaluation and assessment of technological, agricultural as well as pedological aspects under consideration of the legal framework conditions and the economic feasibility in order to provide the basis for the implementation of the project results in legal guidelines, ordinances or laws.
Focus on realistic and rather simple recycling processes.

One main goal of the project was the development, evaluation and assessment of new and practicable recycling processes for wood ashes. In this regard the project focused on the evaluation as well as the techno-economic and environmental assessment of the utilization of wood ashes on short-rotation-coppice fields and as a binder in forest and conventional road construction. These new recycling processes show great potential but have not been investigated yet in detail in Austria.

Beside new innovative processes for wood ash utilization also already established recycling processes like the utilization of wood ashes as an additive in composting or the application on forest soils were investigated in detail.

In summary, the results of this project reveal that the utilisation of wood ashes in agriculture and forestry or in forest and conventional road construction for soil stabilization is technically and economically feasible and under consideration of the defined framework conditions also environmentally safe. As a conclusion, the preconditions for the increased utilization of wood ashes in Austria are given. If all possible utilization potentials are used up to 800,000 t of wood ash could be recycled every year, which exceeds the annual amount of wood ashes in Austria (some 133,000 t in 2013 according to the Report of the Environment Agency Austria 2016) several times. To increase the recycling of wood ashes significantly, a scientifically verified and uniform approach for the newly developed utilization options (forest and conventional road construction for soil stabilisation) has to be defined and existing legal guidelines like the Austrian Compost Ordinance have to be adapted accordingly.

In addition, FACT SHEETS have been prepared for relevant areas of the utilisation of wood ashes, which summarise the relevant Q&A for owners of biomass combustion and CHP plants and potential end users of the ash alike.

The final scientific report can be obtained directly from the Austrian Wood Industry Association (Requests to Dipl.-Ing. (FH) Rainer Handl via handl@holzindustrie.at).

Based on the results of the project a guideline for the proper utilization of wood ashes in forest and conventional road construction for soil stabilisation, which includes all relevant technical, economic and environmental framework conditions is under preparation. The main goal is the definition of a standardized approach for operators of biomass heating and combined heat and power plants as well as for users of the biomass ash, which is accepted by the relevant authorities. This way a short approval process shall be achieved and ashes that are used as building material or binding agents shall be considered as raw materials rather than waste.