Christmas trees could provide annual low-cost fuel bonanza

Prof Jenny Jones and colleagues from the School of Process, Environmental and Materials Engineering at Leeds have been experimenting with the uses of a process called torrefaction, which has so far mainly been useful in processing coffee beans.

Plant material is roasted at about 300C in an airtight container which stops short of turning it into charcoal but drives out moisture and begins a chemical change which makes the material easy to crumble – like paper browned but not burnt.

Biomass is moist and bulky which makes it expensive to transport and carries the risk of going mouldy if stored for long periods. Mills designed to break up coal for use in power stations have difficulty chewing up the fibrous plant matter.

Researchers at Leeds University have already shown that torrefied material retains most of the energy potential of raw biomass but is much lighter and easier to chip, keeps for much longer and can easily be crumbled into dust, like coal is, for feeding into power station burners.

Now the Engineering and Physical Sciences Research Council is funding three years of research to pave the way for industrial-scale experiments to see if the use of biomass in power stations can be made more cost-effective if it is pre-roasted like coffee beans.

The work is being carried out in collaboration with energy firms Alstom Power, Drax Power, EON and RWE nPower.

The researchers need to find out what the byproducts of the roasting process are and to assess the danger of explosions from the dust generated by the roasting and milling processes.

"We will be carrying out experiments to characterise the explosibility of biomass and torrefied biomass powder so that appropriate safety features can be designed into industrial-scale powder handling and power generation plants," said University of Leeds researcher Dr Roth Phylaktou, an expert on fire and explosion safety engineering and a co-investigator on the project

The Leeds team is working with a range of different materials that could potentially replace coal. These include energy crops such as willow and miscanthus, which are grown specifically for making "green" fuel and research so far has shown miscanthus suits torrefaction better than willow.

"These are all materials that grow well in the UK but not at the expense of food crops," said Prof Jones. "We do not want farmers to have to choose between planting a field of wheat or barley and a field of willow. Ultimately, this is all about providing a secure energy supply for the future and one that is sustainable on all levels. "

The scientists will go on to try other possibilities including forestry trimmings and harvest straw, and try to work out whether it might pay suppliers of biomass to invest in torrefaction if the power stations paid a premium for the improvement in performance of the fuel.

Prof Jones says: "If we can show that torrefaction is feasible on an industrial scale then we would hope to end up with a demonstration plant here in the UK. We already know that many more farmers would be interested in growing energy crops on poorer quality soil if the economic barriers were lowered and the power companies could use more biomass without losing out financially."

Many UK coal-based power stations, including Drax, at Selby, are already burning a proportion of biomass, co-firing with coal, to reduce their carbon footprints. Unlike fossil fuels, biomass, such as willow, miscanthus and poplar, is seen as a virtually carbon-neutral source of energy since the carbon dioxide emitted when they burn is absorbed during photosynthesis by the next batch of 'energy crops' planted in their place.

In Sweden, nearly all used Christmas trees are collected and burnt in power stations.

Process offers cut in coal use

The Leeds University report in the latest issue of the scientific journal Fuel says: "The process of torrefaction alters the physical properties of biomass, reducing its fibrous tenacious nature.

"This could allow increased rates of co-milling and therefore co-firing in coal fired power stations, which in turn would enable a reduction in the amount of coal used and an increase in the use of sustainable fuels, without the need for additional plant."