Deciding whether to run a biomass boiler on woodchips or pellets can make a big difference to the viability of installations. Paul Spackman examines the pros and cons of each.
Look beyond fuel costs and consider logistics to see how a biomass boiler might fit with the farm business.
The three key areas to focus on are fuel sourcing, storage, and supply to the boiler, says David Turley of the National Non-Food Crops Centre (NNFCC).
“If you’re looking at supplying your own fuel from an area of woodland then woodchips could be the way to go.”
As well as needing enough woodland, self-supply requires labour to process timber, plus sufficient space for storing seasoned wood and chips, he notes. “Chips have about one-third of the energy density of pellets, so storage can be an issue.”
Farmers also need to factor in the frequency of loading fuel hoppers, says Knight Frank’s Paddy Hoare. Pellet-fuelled boilers could be a better option where self-supply is not possible or where limited boiler accessibility restricts access for tractor-mounted loaders, he says.
“It is much easier to arrange regular deliveries of wood pellets which can be blown 20-30m straight into the feed hopper using a piped pneumatic supply.”
Pellets are also a more concentrated fuel with known calorific value, which can give greater efficiency than woodchips, he says.
“If you’re buying-in woodchip, make sure it’s assured to British Standard. Cheap, non-assured chip could contain too much bark, dust, or oversized chip, which reduces energy output and causes problems with the feed auger.”
While pellets may seem a more efficient, hassle-free option, cost is typically double that of woodchips. Prices vary considerably, with bulk pellet prices about £185-190/t, up to £200-210/t (excluding VAT) during peak demand, the NNFCC says. Bagged pellet prices for the domestic sector are £223-284/t (including VAT and delivery).
Woodchip prices are more difficult to obtain as much trade is done in bulk on bespoke quotes, but Mr Turley says prices range from £90-120/t (excluding VAT) for a 10t load. Spot price for imported chip at a UK port is about £58/t. This compares with £25-30/t for fresh (about 50% moisture) home-produced whole-tree chip.
Multifuel boilers are a good compromise as they give farmers the flexibility to change between fuel types relatively easily, says Mr Hoare. “You could potentially start off using pellets to ease yourself into management of the boiler, then switch over to woodchip or another fuel, such as miscanthus or grain, once the system is established.”
Dry, well-ventilated storage is crucial, in anything from converted sheds or feed silos to adapted shipping containers or purpose-built stores. All must meet key criteria:
- Watertight from rain and groundwater
- Adequate ventilation to prevent condensation, aid drying, stop harmful gases or dust build-up and reduce risk of moulds developing
- Allow regular turning of fuel in larger stores to reduce the risk of degradation or “hotspots” developing
- Be large enough to allow space to tip trailers or operate handling equipment. Large stores also allow bulk orders, which can reduce unit cost and means longer periods between fuel deliveries.
|Lower cost a tonne (equivalent to about 3.1p/kWh)||Bulky material adds to transport cost and requires more storage space, as large as possible and ideally 25cu m|
|Empty sheds can be used for fuel storage||More frequent loading of boiler/feed hoppers|
|Home-sourced woodchip can use farm labour and woodland||Risk of microbial degredation in store where moisture exceeds 30% – fire risk and may release harmful spores|
|Chips in long-term storage need turning to aid drying and stop microbial acrivity|
|Lower calorific value, 3,500kWh/t, and higher moisture (about 30). Consistency less certain than pellets|
|Higher energy density than chips and a more consistent producy (typically 8-10% moisture)||More expensive (Equivalent to about 4.4p/kWh) – exposure to future price increases|
|High-bulk density means less storage space is needed – 12-16cu m||Dust risk from overhandling pellets – health and safety risk and can cause handling/combustion difficulties|
|Great flexibility: Pellets can be used in most woodchip systems, whereas chips cannot be used in pellet boilers||Pellets can give off carbon monoxide in storage – adequate ventilation is needed|
|Generally, the most responsive and closest to fossil fuel-based boilers in their maintenance and operation||Pellets less robust than chip – stores need to be watertight to avoid pellets absorbing moisture and disintergrating|
|Pneumatic delivery reduces the need for vehicle access|