Background

Under the EU Renewable Energy Directive the UK has committed to allocating 15% of electricity supplies from renewable energy by the year 2020; therefore there is mounting interest in the use of energy crops for power production.

Energy crops are typically densely planted, high yielding crop species that will be burnt to generate power. Recognised biomass sources include forest and woodland products, short rotation coppice (SRC), short rotation forestry (SRF), forest residues and arboricultural arisings. Woody crops such as Willow or Poplar are widely utilised, as well as tropical grasses such as Miscanthus and Pennisetum purpureum, due to their ability to grow fast and to provide high output of biomass per hectare.

Very high outputs per hectare can be achieved from energy crops such as wheat, which typically yields 7.5-8 tonnes of grain per ha in the UK, in addition to which there is typically 3.5-5 tonnes per ha of straw. The grain could be used to produce liquid transport fuels and the straw could be burned to produce heat or electricity.

Opportunities

Sustainably managed SRC provides a source of renewable energy with virtually no net carbon emissions (i.e. no increase in atmospheric carbon).

• Stems are usually harvested from SRC plantations every 3–5 years
• Coppice stools remain productive for up to 30 years before they require replacing
• Coppice stems are usually cut and chipped by a dedicated SRC harvester in a single operation.

The establishment of the perennial energy crops miscanthus and short rotation coppice of willow and poplar, are funded as part of the Energy Crops Scheme under the Rural Development Regulation. All plantings will be supported at a rate of 40% of actual costs. Information on opportunities for siting of SRC and optimum conditions can be obtained from Natural England on Environmental planning of short rotation coppice and miscanthus.

Practical considerations

To achieve high yields from energy crops, high levels of fertiliser inputs are required. For example, high nitrogen fertiliser levels of 100-200 kg/ha are typically applied for wheat crops in the UK. However, this can lead to emissions of ammonia or NOx into the air and nitrogen compounds into groundwater, and can release carbon dioxide into the atmosphere. This is both environmentally undesirable and expensive, especially with increasing natural gas prices.

It is therefore highly preferable for energy crops to be developed that can achieve high levels of biomass output with minimal or zero inputs.

Case studies

In England a short rotation forestry (SRF) trial is underway in order to determine the feasibility of SRF as a viable renewable energy source. The purpose of the research is to establish the sustainability of SRF as a renewable energy source in England using a range of fast-growing species that have shown potential to be used as a biomass energy fuel.

Sites, which are generally on “marginal” agricultural land and include former arable and pasture, have been selected across a range of geographical areas in England with a variety of climatic and soil conditions in:

• Northern Cumbria, close to the Scottish border
• West Yorkshire, near Wetherby
• South Lincolnshire, near Stamford
• Oxfordshire, near Didcot
• West Sussex, near Horsham
• South Devon, near Totnes
• Kent, near Westerham.

Forest Research will assess the environmental impacts, biomass production and operational techniques for all sites.

Services

Forest Research is able to provide advice and guidance on a wide range of biomass fuels and conversion technologies. This initiative has been undertaken in support of Government’s response to recommendations made by the Biomass Task Force.

The initial focus will be on woodfuel and some energy crops, drawing on results and knowledge accumulated over many years by staff linked to the Woodfuel Research Centre.