General information concerning ash utilisation

Ash from natural solid biomass fuels contains valuable plant nutrients such as K, P, Mg and Ca, but also contains significant amounts of heavy metals, which have to be considered for the utilisation strategy applied. Therefore, only the nutrient rich and rather heavy metal poor fractions (bottom ash and coarse fly ash) shall be used for fertilizing and soil improvement purposes. The heavy metal rich fine fly ash fraction, which represents the smallest fraction, must be industrially utilized or disposed of.

Generally, the following principles apply for biomass combustion plants:

• Especially the content of volatile heavy metals such as Zn, Pb and Cd increases with decreasing particle size.
• 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 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.

The following general principles for sustainable ash utilisation arise from the general principles listed above in Austria:

• Bottom ash or a mixture of bottom and coarse fly ash proportional to the actual amount generated at the combustion plant shall be recycled to agricultural or forest land in order to contribute to a closure of the mineral cycle. Since the combustion of straw and other herbaceous fuels leads to ashes with high nutrient and low heavy metal contents (compared to ash from ligneous fuels), a mixture of bottom ash and coarse fly ash shall be used in any case.
• If possible, the ash should be returned to the soil where the fuel has been cultivated (i.e. wood ash to forest land, ash from straw and other herbaceous fuels to grass or agricultural land).
• Fine fly ash (or condensation sludge), which represents the smallest and heavy metal richest ash fraction, shall be utilised in industrial processes (e.g. heavy metal recovery) or disposed of.
• The cut between coarse and fine fly ash is the ecologically and economically most efficient. Therefore, modern biomass combustion/CHP plants shall be equipped with a two-stage dust precipitation system (cyclone and filter) and an applicable ash handling system to allow the separate collection of the different fly ash fractions.
• 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.
• In the late 1990’s, based on 4 year application trials performed on agricultural and forest soils, the expert advisory committee for soil fertility and soil protection of the Austrian Ministry of Agriculture, Forest, Environment and Water issued guidelines for the appropriate ash application on agricultural and forest land, which define limits for heavy metals, quantity limits for ash application as well as general requirements and recommendations for an appropriate ash utilisation.

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 selected. The following tasks have to be considered prior to the application:

• 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 1.5 to 2 cm) if direct ash application on soils is planned. Subsequently, a screening of the ash and also a metal precipitation are usually necessary (except for indirect application).
• Preparation of the treated ash for transport (big bags, silo wagon etc.).
• Decision between direct or indirect ash application.

The utilisation of biomass ash as a secondary raw material with for fertilizing and soil improving measures can be distinguished between direct (with spreading devices) and indirect (via compost production) ash application.

The methods of direct ash application vary depending on whether the ash is spread on agricultural or forest land and on the topographic conditions of the area to be fertilised.

The direct application on agricultural land can be performed by conventional fertiliser spreading equipment. However, since fertiliser spreaders are not optimised for the application of ash, problems like the erosion of the spreading devices and the generation of dust during application can occur. Screw spreaders and pendulum spout spreaders showed the best results in the comprehensive spreading tests performed and are therefore recommended for ash application on agricultural land.

The direct application of ash on forest land is more difficult since the accessibility to forest roads is often limited. Blowing devices can be used from forest roads but their range is limited to a maximum of 50 m into the forest. In addition, the blowing of ash causes a lot of dust which remains in the air for a long time and thus this application method cannot be used in close proximity to residential areas. As an alternative, the application of ash to forests can be performed by helicopter.

Apart from the appropriate application technique optimised storage and transport logistics are inevitable for an efficient and economically feasible ash application. Generally, the installation of easily accessible intermediate storage areas close to the area selected for ash application is recommended in order to reduce transport routes and thus minimising the lag time between filling of the spreading device and ash application.

In addition to the selection of the ash application method, the local regulations regarding ash application need to be considered. The approach required in Austria for direct ash application based on the current guidelines for ash application on agricultural and forest land is described below:

• Analysis of the nutrient and heavy metal content of the ash prior to the first soil application and repetition of the analysis in regular intervals according to the guideline.
• The maximum quantity of ash applied per ha has to be observed (1,000 kg/ha*a for agricultural land and 750 kg/ha*a for grass land if one or more of the heavy metal limits is reached, respectively higher quantities are allowed if all heavy metal contents are below the limits; 3,000 kg/ha once in 50 years for forest land). The amount of ash to be spread on forest soils was determined by considering the Cadmium balance for the ecosystem wood (since Cd is the most critical heavy metal in biomass ashes) as well as the removal of nutrients after harvesting [22].
• Selection of the appropriate time for ash application.
• Consideration of the ban of ash application for designated areas.
• Selection of areas suitable for ash application (e.g. soils with a low pH-level and/or high nitrogen reservoirs) and consideration of general fertilizing guidelines (e.g. determination of the application quantities based on the nutrient demand of the plants to be fertilised).

Besides the direct application of ash on agricultural and forest land, the ash can also be used as an additive in compost production.
Indirect ash application 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.

Based on the Austrian “Kompostverordnung” a maximum of 2 % (w/w, wet basis) biomass ash can be added to the compost material.
Apart from the sustainable utilisation in agriculture and forestry, biomass ash can also be utilised in road and surface construction or in the cement industry. However, technical and legal standards and regulations are currently missing, so except for the utilisation in the cement industry these alternative utilisation paths are not applicable in Austria.
The following picture gives an overview of the possibilities for biomass ash utilisation.