By 2020 the UK has to source 15% of the nation’s total energy consumption from renewables. Recently the country started taking significant steps to meet this legally binding mandate. In April 2013 the coal-fired Drax power station, which typically produces around 7% of the UK’s electricity, converted one of its generating units to run on biomass alone. By 2016 three units will have been converted – subject to Drax securing the rights to a sustainable source of biomass – and the power station will burn nearly 7 million tonnes of biomass every year. Much of this biomass is expected to come from wood pellet plants in Louisiana and Mississippi, in the southern United States.

“The US has a large wood and paper industry and much of the low-grade wood currently goes to waste,” said Matt Willey, a spokesperson for Drax. An estimated 93 million tonnes of usable wood remnants were left on forest floors across the US in 2006, for example.

So will the imported wood pellets really reduce greenhouse gas emissions, compared with burning fossil fuels? To answer this question, Puneet Dwivedi from the University of Georgia calculated the greenhouse gas emissions associated with generating a unit of electricity under 930 different scenarios of wood pellet production. He compared this with emissions produced for a unit of electricity generated by burning fossil fuels at the grid level. The analysis included different kinds of woody feedstock (logging residues and pulpwood), forest management choices (intensive and non-intensive), plantation rotation lengths (between 10 and 40 years) and power plant capacities (–100 MW).

Dwivedi’s calculations, which are published in Environmental Research Letters (ERL), showed that burning the imported wood pellets saves between 50 and 68% of the greenhouse gas emissions from electricity derived from fossil fuels in the UK. “If the wood pellets could be sourced locally you could further increase savings in greenhouse gas emissions because transatlantic shipment of wood pellets contributes about 30% of total emissions related to consumption of wood pellets in England, but it is very difficult to produce this volume of wood pellets in Europe,” said Dwivedi.

The greatest greenhouse gas savings were achieved using a high-capacity wood pellet power plant (like those being installed at the Drax power station) because of the higher conversion efficiencies of these plants. Meanwhile, the type of forest management (intensive versus non-intensive) and the type of feedstock (logging residues or pulpwood) made little difference to the greenhouse gas emissions. “Our study shows that, contrary to previous beliefs, using imported wood pellets from the southern United States for electricity generation in the UK can help reduce the greenhouse gas emissions of the UK,” said Dwivedi.

However, that doesn’t necessarily mean that the UK should be converting all of its coal-fired power stations to burn wood pellets. The study did not take into account the impact of increasing demand for wood pellets on land use or how the yields of timber products might change over time. And the impact that wood pellet production might have on the environment was not included in this study. “There are biodiversity issues that need to be considered, particularly with large-scale wood pellet production plants,” said Dwivedi.

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