Garbage in, graphite out — plus green hydrogen

Thursday, 15 October, 2015
It may look like rubbish, but food waste does have its uses. It could even be converted into valuable graphite and hydrogen. An EU-funded project has set out to do just that. The process it is developing combines anaerobic digestion with microwave plasma technology to transform trash into treasure.

The PlasCarb project intends to produce graphitic carbon — graphite — and green hydrogen, cost-effectively, from biogas. It is building a plant that could provide a blue-print for industrial-scale roll-out. One year into the project, the partners have already made considerable headway.

Synthetic graphite and sustainably produced hydrogen are substances of particular interest for the future. A cost-effective process to generate them from food waste such as potato peel and coffee grounds would solve two problems at once, securing a steady supply of valuable materials and helping to eliminate substantial amounts of waste.

Bountiful biogas

“The objective of PlasCarb is twofold,” says project coordinator Neville Slack from the Centre for Process Innovation in the UK. “One objective is to see how we could utilise food waste instead of sending it to landfill or just putting it through an anaerobic digester to generate electricity. The second is looking at potentially critical outputs.”

And green hydrogen and synthetic graphite, particularly of the outstanding quality the partners aim to generate, certainly are valuable products. Natural graphite, a high grade of coal, is 1 of 20 raw materials that the EU has identified as critical to its economy and for which it is largely dependent on imports.

Graphite is already widely used, for example in electronics and in batteries, including those of electric cars. It can also be transformed into graphene, the strongest, thinnest material known to science. This substance, currently being developed, potentially holds the key to a wide variety of innovative applications such as flexible electronics and ultrafast battery charging.

A reliable source of synthetic graphite could help to secure Europe’s supply. Hydrogen produced sustainably from a renewable source would meet another need, as this substance is derived mainly from fossil fuels at the moment. Access to substantial amounts of green hydrogen would help to minimise the environmental footprint of many promising applications, notably in transport.

Making more of methane

The PlasCarb partners are planning to develop a plant that will integrate the whole process, from the production of biogas to the isolation and purification of graphitic carbon and hydrogen.  

The biogas is generated from food waste, which is broken down in an anaerobic digester. The aim is then to split this gas into its two main components — methane and carbon dioxide — using a filtration process developed by the project, Slack explains. The methane (CH4), once isolated, is injected into the plasma reactor, where it is heated using low-energy microwave technology until the molecules come apart, forming graphitic carbon (C) and hydrogen (H2).

One of the challenges, says Slack, lies in ensuring that the process delivers carbon in the desired form. Another arises from the complexity of separating the two substances. Further steps are required to divide the two and eliminate impurities.

Bin bonanza?

Getting this facility up and running will be the culmination of nearly three years of joint research into the necessary technologies and processes. By the time the project ends in November 2016, the partners hope to have operated this plant for at least one month, processing over 150 tonnes of food waste into more than 25 000 cubic metres of biogas.

Whether the approach would be profitable is another question the partners intend to address in PlasCarb. “The project will not just look at building this piece of equipment; it will also perform the associated life cycle analyses and assess the business case. It may well be that you need to build a bigger plant or indeed a smaller plant for the process to be viable,” says Slack. Time will tell.

Read more: article on PlasCarb in The Guardian

Innovative plasma based transformation of food waste into high value graphitic carbon and renewable hydrogen
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