By Mark Kinver
Science and nature reporter, BBC News
Emissions from transporting wood chips were higher for smaller plants
Small-scale biomass power plants can have a greater environmental impact than other renewables, a study says.
UK researchers found that although the facilities offered carbon savings, they produced more pollutants per unit of electricity than larger biomass plants.
They suggested the way the feedstock was transported produced proportionally more pollutants than larger sites.
The findings challenged the view that such schemes offer an green alternative to grid-based electricity, they added.
Supporters of community biomass schemes say the power plants are sustainable because the fuel, such as wood chips, can be sourced from the local area.
Study co-author Patricia Thornley, from the Tyndall Centre for Climate Research at the University of Manchester, said the results did surprise the team.
"The fact that the carbon savings were pretty constant across the technologies, yet the emissions varied hugely was a surprise," she told BBC News.
The researchers examined 25 different biomass power generation systems, some of which were established technologies, while others were still at the development stage.
"Models were used to produce key indicators that summarised the performance of each bio-energy system," Dr Thornley explained.
Described as the most comprehensive study of its kind to date, four airborne pollutants - carbon monoxide, nitrogen oxides, particulates and volatile organic compounds - were tracked across each system's life cycle - from field to power plant.
There were three main reasons for small biomass plants (less than two megawatts (MW)) producing more pollutants per unit of electricity than facilities larger than 5MW.
"The efficiency of smaller plants is generally lower," Dr Thornley observed.
"Therefore they are producing less electricity for every unit of pollutant they emit."
She added that small-scale plants also tended to use gas engines in the generation process, which also emitted pollutants.
"Perhaps the most interesting aspect is that small plants tend to use smaller scale transport systems, like tractors.
"These generally produce higher levels of pollutants than specialised haulage vehicles per tonne of material moved.
"Hence, particularly for nitrogen oxide and particulates, a larger proportion of the additional emissions from small plants are upstream of the conversion plant and relate to short distance haulage.
"Each of these reasons carries for the different systems studied, but the general trend is that the small systems do tend to perform worse than the larger ones."
The researchers, who presented their findings to the UK Energy Research Centre's sustainable energy conference in Oxford, hoped their findings will help policy makers and planners.
"Every energy option has pros and cons and you have to choose the right application," Dr Thornley explained.
"Where local communities are considering bio-energy, it is important they know that it does really matter much which system they choose in order to save carbon or create jobs - they are all pretty even on those measures.
"However, people often assume that small plants are more environmentally friendly than larger ones, yet these results show that is not necessarily the case."
One possible use of biomass fuel is in power plants using a process such as the one shown above, the Combined Cycle
The fuel is turned into hot pressurised combustion gases, which are cleaned to prevent corrosion of the system
The clean gases are then burned with air before entering a turbine, generating electricity
Heat from the gases is recovered after the gas turbine using water in the heat exchanger
The combustion gases can then usually be vented from a stack without further cleaning
The only other by-product is non-toxic ash, which could, for example, be mixed with compost to help grow more biomass fuel