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Green Christmas. Happy Hirtzel/Shutterstock

New recycling process could help your Christmas tree lead a surprising second life

It wouldn’t be Christmas without a tree, but which is more sustainable – a real tree or a plastic one?

You might expect anything plastic to be the least environmentally friendly option. It’s true that manufacturing plastic trees consumes a lot of energy, and so does shipping them, to the UK from where they’re commonly manufactured in China, for example. Although you can use a plastic tree for many years, most aren’t recyclable and ultimately still end up in landfill.

However, real trees aren’t necessarily the greener option. The UK uses as many as 8m natural Christmas trees during the festive period each year and sadly, about 7m of these are discarded. The other million are mainly used as compost, though many people avoid this based on the assumption that the low pH of pine needles (between 3.2 and 3.8) will make the soil acidic.

Christmas trees such as the Norway spruce and Nordman fir have hundreds of thousands of pine needles which take a long time to decompose compared to other tree leaves. When they rot, they emit huge quantities of greenhouse gases. According to The Carbon Trust, the carbon footprint of a 2m-tall real Christmas tree is equivalent to 16kg of CO₂ if it ends up in landfill. That’s 100,000 tonnes of greenhouse gases from the 7m trees that end up languishing in landfills every year.

Pine needles take a long time to rot, releasing methane and carbon dioxide as they do. Kroshanosha/Shutterstock

A tree is for life, not just for Christmas

A better solution would be to reuse the pine needles and the trees. My research at the University of Sheffield has been investigating whether there are useful products that we can get from pine needles and how to produce these.

Like most plant biomass, 85% of a pine needle is a structurally complex polymer known as lignocellulose, which is rich in carbohydrate and aromatic compounds. The structural rigidity of lignocellulose makes it unattractive and useless in most industrial processes because of the high energy intensity needed to break it down.

Read more: Now that Christmas is over, what are you going to do with your tree?

My research is focused on how the complex structure of this polymer can be broken down into simple industrial chemical feedstocks of high value and low molecular weight, such as sugars, organic acids and phenolics – chemicals which are important raw materials in industrial manufacturing.

By a process called liquefaction which uses moderate temperatures and environmentally-friendly solvents like glycerol or water, the pine needles are converted into a liquid with a solid byproduct called bio-char. The warm solvent helps to break down the complex chemical structure of pine needles into smaller chemical molecules, which make up the liquid.

The reaction vessel for the liquefaction process. Cynthia Kartey, Author provided

This liquid product typically results in glucose, acetic acid and phenol. Glucose is used in the production of sweeteners for food, acetic acid in making paint, adhesives and even vinegar, and phenol in the manufacture of medicines.

None of the products from this process are wasted – even the bio-char can be used as a catalyst for other chemical reactions.

The tree doesn’t need to be fresh either, as the process applied in this work can effectively handle both dry and wet biomass, eliminating the need for an expensive drying process. This is a key advantage of the liquefaction technique over traditional technologies such as combustion and gasification, whose efficiency depends on the moisture content of the biomass.

This method also works well with other forms of biomass waste and can be used for any species of pine. An industry built on this process could convert much of the available biomass waste from food crops and forestry management into vital products.

A selection of biomass waste which can be saved from landfill. Cynthia Kartey, Author provided

Aside from converting biomass waste into precious materials, this process adds value to otherwise less useful solvents such as crude glycerol – an unwanted byproduct from the biodiesel manufacturing industry. Using glycerol increases how much of the biomass waste can be converted to liquid product compared to the commonly used water process, known as hydrothermal liquefaction. More than 90% of pine needle mass is converted in the presence of glycerol compared to only 60% with water.

The benefits of this research are huge. It can help reduce carbon emissions by decreasing dependence on imported artificial Christmas trees and limiting the amount of biomass sent to landfill. If commercially feasible, this could make industrial processes more sustainable by creating new products from something that was previously considered waste. Long after the festive period is over we could continue using this method to recycle forest and agricultural waste on a much larger scale, bringing greater benefits throughout the year.

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