In the UK many of our heathland and wetland sites are routinely managed to preserve these unique habitats. This provides a big opportunity for the biomass sector as the harvested plant material could be used as a renewable fuel. This material, termed conservation arisings, is potentially a huge resource as there is currently over 4 million hectares of this type of land in the UK and much of the biomass is burnt in situ.
There are 1.3 million hectares of heathland and 2.8 million hectares of open wetlands in the UK. Plants that are removed from these habitats include heather, gorse, bracken, reed and rush. Yields vary greatly depending on the sites but are around 3 tonnes per hectare for bracken, 5 t/ha for reed, 2 t/ha for heather and frequently over 10 t/ha for gorse.
There are many challenges to successfully harnessing this resource. Most of the sites where conservation work is carried out are remote, terrain can be exposed and precarious and access is not always easy. Therefore, harvesting the biomass and getting it to a place where it can be processed and stored must be worked out to make it practically and economically feasible.
Herbaceous fuels tend to have a high ash content (for instance bracken has an ash content of 5% compared to 0.5% for roundwood chip) and are more likely to have higher emissions of nitrogen oxides and particulates when combusted. This has implications on the boiler used, which may require aftermarket emissions abatement technology such as bag and cyclonic filters.
Many of the hurdles are already being overcome. Brackenburn is a company that produces briquettes from bracken harvested in the Mendip Hills and West of England and some interesting trials of fuels derived from conservation arisings have been carried out by the Royal Society for the Protection of Birds (RSPB). The RSPB is especially interested in these fuels because the land area which they manage has high biomass potential.
So how much of this material is there? Supposing that just 10% of the available resource could be acquired, at a modest yield of 2 t/ha per year this would yield some 800,000 tonnes to be used as a renewable biomass fuel. The significance of this can be seen when comparing against latest Forestry Commission estimates of 1.6 million tonnes of recycled wood and 720,000 tonnes of sawmill residues that were used for biomass fuel in 2016.