"Bioenergy has huge potential to reduce carbon emissions but this can be at the expense of increasing other environmental impacts," Thornley told environmentalresearchweb. "In particular, many facilities in the UK have struggled with objections related to traffic and transport of biomass material."
But Thornley’s model showed that the traffic and transport of biomass to thermal conversion plants makes only a very minor contribution to overall airborne emissions.
"Despite being highly visible and a frequent source of concern for local communities its environmental impact is actually quite low," she explained. "However, there is still the potential for significant quantities of NOx and particulates – both of which have significant local air quality implications – to be emitted during fuel production and processing. These are primarily connected to crop harvesting and tractor haulage over relatively short distances."
Thornley modelled the emissions of carbon monoxide, nitrogen oxides, particulates and volatile organic compounds from eight biomass-based power generation systems, using different methods and scales of transport, drying and burning for either willow or miscanthus.
She found that carbon monoxide and hydrocarbon emissions arose primarily at the conversion plant but that up to 44% of NOx and 70% of particulate emissions could be released earlier in the process, mainly by crop harvesting and tractor haulage. Improved practices such as using lower emission engines and avoiding tractor haulage, where practical, in favour of larger purpose-built haulage vehicles could help lower these emissions.
"Of course, we have to remember that if farmers were not growing energy crops they would be growing something else which would also impact on the environment, but it has been useful to pinpoint the activites that contribute most," said Thornley. "Another interesting finding was that the smaller systems which are frequently preferred by local communities frequently perform worse in terms of overall airborne emissions than do large centralised plants – even taking into account long lorry trips to bring material to the centralised facility."
Thornley’s work was part of the Supergen bioenergy consortium, which focused on production of electricity from biomass. The consortium has now been renewed for a further four years. "We will be exploring a range of technical, economic, environmental and social implications for a wider range of bioenergy systems, including heating – from domestic scale upwards – and production of transport fuels," she said. "This promises to bring a whole new set of questions to answer: are there significant local air quality impacts associated with widespread deployment of wood-fired domestic boilers, which types of biofuel are most likey to have negative social impacts in production countries and how can biomass help the UK meet its carbon reduction targets in the power, heat and transport sectors most economically?"
Thornley reported her research in Environmental Research Letters.