Waste to Energy is a major growth sector, and it has a lot of potential. While it’s obviously preferable to recycle waste wherever possible a percentage inevitably gets incinerated, and waste to energy (WtE) schemes do this in a more efficient way. By collecting heat from the incineration process power can be generated or combustible fuels such as ethanol produced. However WtE produces its own waste in the form of ash. Usually this goes to landfill, because it doesn’t have any further uses, but it can contain valuable materials in amounts that can be economically recovered. Because WtE often uses mixed waste a variety of materials can be involved, mostly metals.
The USA has a number of WtE projects but is lagging behind in reprocessing the ash. At the moment most of it is simply sent to landfill with no further processing once it’s been burned, but this is missing an opportunity to recover metals. In Europe, especially Scandinavia, there’s more of a focus on resource extraction and WtE technology is more advanced. Now the same processes are starting to be used in some US plants, with adaptations to suit local practices.
One feature of US WtE is that all the ash from the process is disposed of together. In Europe the lighter fly ash – the percentage recovered from the smokestacks – and the heavier bottom ash are separated. This makes reprocessing easier because the bottom ash tends to have higher levels of recoverable metals, so dealing with the ash from a US plant needs more efficient extraction. Now Danish company Meldgaard has come up with workable solutions and is using them at three separate locations.
Meldgaard has 20 years of experience in WtE recovery but up to now it’s all been in Europe. However improved technology means they can now handle mixed ash, especially when it comes to recovering non-ferrous metals. Ferrous metal recovery is relatively simple, because ferrous slag can be collected by magnets, but metals such as copper often need chemical recovery.
The first two US operations set up by Meldgaard collect ash from various plants and stockpile it for processing; the actual processes they use vary, because each WtE plant produces ash with distinct characteristics and material content, so it’s important to keep track of which plant a batch of waste came from. Their newest plant is different. It’s integrated with the WtE scheme, so ash is delivered straight from the incinerator to the recycling system. It’s then processed immediately, and the residue sent for landfill as usual. Because the ash has come directly from the burners it’s still moist; current US recyclers can’t deal with ash unless it’s dry, but the new process allows record recovery levels. Currently this operation can handle up to 500 tons per day, with recovered metals being shared between Meldgaard and the WtE operator.
As WtE becomes a more common solution for reducing landfill volume there will be increased potential for recycling the ash. It’s vital that new plants are set up to separate the ash streams, making recycling more efficient, and that recovery technology is regularly updated to maximize extraction. As well as reducing volume even more this could make WtE much more cost-effective.