Rising from the Waste to Energy Ashes

Resource efficiency, sustainable growth and climate protection are important challenges for European society.

Across Europe some 70 million tonnes of waste were incinerated in waste to energy facilities during 2009. Despite best efforts to process only residual waste and divert recyclables, large quantities of metals and minerals remain in the resulting 16 million tonnes of bottom ash. As commodity prices remain high, Carsten Spohn and Marta Gurin explore some of the recycling opportunities presented.

Resource efficiency, sustainable growth and climate protection are important challenges for European society. With growing demand for virgin and recycled materials it is becoming increasingly important to make the best use of our resources. Waste management has a role to play in meeting these challenges.

In Europe we can observe steady improvements in waste treatment and reduction of impact from waste disposal, growing rates of waste recycling and efficient recovery of energy from the remaining waste. Policy initiatives also aim at reducing waste generation through prevention. Today waste is increasingly viewed as a resource, and using unavoidable waste as a valuable resource wherever possible is a must for today's society.

Waste to Energy (WtE) plants play an important role in the environmentally sound treatment of waste and in improving Europe's resource efficiency. The facilities produce sustainable energy from the treatment of mixed municipal and similar waste which remains after waste prevention and recycling.

Furthermore valuable parts of the bottom ash, which is the residue from the combustion process can be recycled. For example, the remaining ferrous and non-ferrous metals in waste can be extracted from the bottom ashes and recycled into new products such as aluminium castings for the automotive industry. Other remaining minerals can be used as secondary aggregates, e.g. in road construction or in building products.

Resource efficiency of bottom ash

Bottom ash consists of non-combustible materials, and is the residual part from the incineration of waste. It is collected at the end of the grate in a WtE plant and represents the largest mass flow of residues from the thermal treatment of waste, accounting for 20% to 25%.

A special plant for ash treatment operates at a Indaver site in Doel, Belgium

Taking into account the amount of waste incinerated with energy recovery in Europe during 2009 (around 70 million tonnes of household and similar waste) it is estimated that 16 million tonnes of bottom ash was produced that year.

The table below presents the amounts of bottom ash produced in several European countries:

Raw bottom ash is a granular material that consists of a mix of inert materials such as sand, stone, glass, porcelain, metals such as iron, steel, aluminium, copper, zinc and ash from burnt materials. It is worth mentioning that the combustion process cleans and separates metals and inerts from mixed waste, which could not otherwise be recycled. As a result further recycling of these materials is possible.

The composition of bottom ash varies with the input of a WtE plant. As a rule of thumb in 1000 kg of bottom ash there are the following main fractions:

Mineral fraction 80% - 85 Metals 10% - 15 % Non-ferrous metals 3% - 5 % Unburnt material < 3%

The feasibility of recovering ferrous and non-ferrous metals from bottom ash is very much linked to the prices of the raw materials on the market. Today the price levels for metals are quite high and are likely to increase in the future due to the growing global demand for raw materials. An increasing number of waste to energy plants and bottom ash processors are investing in the latest innovative sorting technologies, supported by metal prices and the growing need for raw materials in Europe. It is worth noting that, with existing conventional technologies, there is already potential for recycling metals and obtaining aggregates from bottom ash treatment.

Both ferrous and non-ferrous metals can be recovered from WtE bottom ash

A good example regarding the quantities of the metals extracted from the bottom ash comes from the Netherlands, where 119,000 tonnes of ferrous metals and 21,900 tonnes of non-ferrous metals were extracted from bottom ash and used in metal production in 2009.

Different options, one goal

In the conventional method (used in most of the WtE plants in Europe) bottom ash is removed from the grate by a wet discharge and follows a dry treatment process. In this approach the final goal is to achieve a high quality material that can be used as a secondary construction material in selected applications.Raw bottom ash is collected from the WtE plant and taken to a special reprocessing facility. Ferrous and non-ferrous metals are separated and all particles above a certain size are broken down in a crushing facility. The remaining combustible material is also removed and returned to the WtE plant. In the reprocessing plant ferrous metals are extracted magnetically.

The Alkmaar WtE facility treats upto 27.5 tonnes per hour of household waste. In the Netherlands bottom ash is used for road construction and landfill recovery

Non-ferrous metals are sorted using the eddy current technique, which is based on the phenomenon that changing magnetic fields create small currents in metal objects. They are then further refined and put back onto the market. Ferrous metals are then sold to iron producers. The inert fraction of bottom ash is stored for ageing in order to improve its quality as a construction material.

One innovative approach to bottom ash treatment removes the bottom ash from the grate by a wet discharge and follows a wet treatment process. The goal is to further improve the quality of the secondary construction material and recyclability of the metals. Examples of this process can be found in Belgium, Netherlands, Germany and Italy.

In another promising new approach bottom ash is removed from the grate by dry extraction (two WtE plants in Switzerland: KEZO, Hinwill and SATOM, Monthey) and follows a dry treatment process. Using this approach better metal separation and higher metal quantities are obtained. However, a significant disadvantage of this process are high dust emissions of the mineral fraction and the limited numbers of application areas of the mineral fraction.

From ashes to resources

Currently WtE plants and the recycling industry are the keystones in modern waste management systems. Due to the extra quantities of raw materials recovered from bottom ashes, WtE plants contribute further to an environmentally sound recycling society and thus help to improve Europe's resource efficiency, using unavoidable waste as a valuable resource wherever possible. Both ferrous and non-ferrous metals are extracted and recycled, and the other remaining materials are used as a secondary aggregate in road construction and building products.

Carsten Spohn is managing director of ITAD and chairman of CEWEP WG Residues
Marta Gurin is the technical and scientific officer at CEWEP (Confederation of European Waste to Energy Plants)

Proceedings from the seminar From ashes to metals - Bottom ash from Waste to Energy plants as a material resource, are available from: http://cewep.eu/events/pastcewepevents/workshops/561

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