Chip off the old block: Exploring the chemical composition of wood chips and pellets

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  • Published: Aug 13, 2012
  • Author: Jon Evans
  • Channels: Ion Chromatography
thumbnail image: Chip off the old block: Exploring the chemical composition of wood chips and pellets

Chips or pellets?

Chip off the old block: Exploring the chemical composition of wood chips and pellets

In the search for more sustainable alternatives to coal and gas for generating energy, wood chips and wood pellets are an increasingly popular option. According to IEA Bioenergy, the global production of wood pellets more than doubled between 2006 and 2010, to over 14 million tons.

Wood chips are produced by simply chopping up wood into tiny pieces using a wood chipper. In contrast, the production of wood pellets is slightly more complicated, requiring the wood to be mashed up and then squeezed through holes to produce small pellets with a diameter of around 6mm. Each has certain advantages over the other.

Wood chips are cheaper than wood pellets and require less energy to produce. Wood pellets are much denser than wood chips, meaning they contain more energy; they also contain less moisture, as much of it is squeezed out during the production process; and their regular size make them easier to transport and automatically feed into a burner.

 

Virgin or recycled?

Both wood chips and pellets can be produced from either virgin or recycled wood, but both these sources have their problems. When produced from virgin wood, questions have been asked about how sustainable wood chips and pellets actually are. If the wood is derived from trees that are replaced, then the chips and pellets should be carbon-neutral, but only if you don’t count the energy used to grow, harvest and transport the wood.

Producing the chips and pellets from recycled wood does away with these concerns, but at the expense of new concerns about toxic emissions. If the recycled wood is painted or has been treated with some kind of preservative, then the chips and pellets produced from them could contain toxic compounds such as heavy metals. When the chips and pellets are burnt, these toxic compounds will either be released into the air or end up in the leftover ash, which can be problematic as this nutrient-rich ash is often used as a fertilizer.

In practice, most commercially-sold wood chips and pellets are produced from a mixture of virgin and recycled wood. So a team of US scientists led by Philip Hopke at Clarkson University in Potsdam, New York, decided to conduct the first ever large-scale study into the chemical composition of wood chips and pellets.

 

Paint and preservative

Using a variety of analytical techniques, Hopke and his team analyzed the chemical composition of the ash produced from burning 132 samples of commercially-available wood pellets, representing 100 different brands, and 23 wood chip samples. They used ion chromatography to determine the concentration of chlorides and sulphates, which can cause fouling and corrosion at high concentrations, and inductively coupled plasma mass spectrometry to determine the concentration of trace elements, including heavy elements. They also measured the gross calorific value and moisture content of the wood chips and pellets.

As expected, they found that the wood chips contained around five times more moisture than the wood pellets, and that this high moisture content combined with their lower density meant the wood chips produced less energy when burnt. In contrast, both the wood chips and pellets had similar concentrations of chlorides, sulphates and trace elements, with the major trace elements being calcium, potassium and magnesium. Also generally present, although at lower concentrations, were manganese, iron, aluminium and sodium.

Although heavy elements such as cadmium, arsenic and chromium were usually present at concentrations below 0.1%, they were present at much higher concentrations in a few of the samples. This was probably due to these samples being produced from recycled residential timber that had been treated with wood preservatives containing copper chromium arsenate. Hopke and his team also detected high concentrations of lead in some of the wood pellet samples, probably due their production from recycled wood containing a coating of lead-based paint.

This all suggests that wood pellets contain more recycled material than wood chips, and that this can occasionally result in wood pellets containing relatively high concentrations of heavy elements and lead. Hopke therefore warns that manufacturers of wood pellets should be more discriminating about the kind of recycled wood they use to produce their wood pellets.

Related Links

Energy & Fuels (Article in Press): "Chemical composition of wood chips and wood pellets"


Article by Jon Evans

The views represented in this article are solely those of the author and do not necessarily represent those of John Wiley and Sons, Ltd.

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