Wood pellets - a fuel for the 21st century
From Building for a Future Autumn 2003
In January 2003 , the Pilkington Energy Efficiency Trust and Envirogen Sustainable Resources Ltd. commissioned architectural consultants XCO2 to carry out a study on the benefits of heating with wood pellet fuel. The study aims to inform key decision makers about the considerable benefits of wood pellet heating – a fuel for the 21st century.
The burning of fossil fuel is damaging our environment by unlocking sequestered carbon and releasing it into the atmosphere in the form of CO2. Wood fuel on the other hand is ‘carbon neutral’ – a tree will absorb as much carbon during its lifetime as it gives off when it is burnt meaning there is no net gain in CO2 emissions. Wood and wood pellets are essentially a form of stored solar energy.
For thousands of years man has been burning wood as a form of energy. Wood pellets are the modern form of wood heating which arguably has more potential to cut CO2 emissions than any other source of renewable energy currently available, and yet there are still many specifiers, architects and house builders who are not aware of the benefits of incorporating heating systems that run on wood pellets.
By using a sustainable, carbon neutral source of fuel such as biomass wood pellets to heat our buildings, we can drastically cut our CO2 emissions and in some cases save money too.
What are wood pellets?
Pellets are a form of biomass material (usually wood), dried and processed into easily combustible pellet form. They are clean to handle and their ‘free flowing’ characteristics make them ideal for automatic heating systems.
How are they made?
Biomass material is milled onto fine sawdust; this is often derived from recycled pallets. It is then fed into a pelletising machine where it is compressed under immense pressure and extruded through a die. During this process the lignin, naturally contained in the biomass, melts and acts as a 'binder' stitching together the fibres to form an extruded pellet section and helping to keep the shape of the pellets as they dry. The lignin also gives rise to the 'glaze' found on the exterior of the pellets. The pellets are extruded at around 90oC, the lignin then cools and the pellets harden and set. No other additives are used. After cooling the pellets are either bagged up for domestic use or loaded into container lorries for bulk distribution. Pellets supplied by the bag are currently up to twice the cost of pellets bought in bulk (delivered by the tonne).
Pellets can be made from various biomass materials. One of the most common is excess wood from the joinery industry in the form of sawdust, or recycled pallets. Pellets are also made from specially grown crops such as willow coppice or even from straw.
Pellets vs. wood chips?
Wood chips (at 25% moisture content) are cheaper than pellets per unit of energy delivered. However, they are not always suitable for domestic or intensive heating uses since they require considerably more storage space and their energy content is less predictable. The volumetric bulk density (weight per unit volume) of wood pellets is significantly higher than that of wood chips; this figure depends on the moisture content of the wood chips but as a guide:
• wood pellets: 650kg/m3 (based on wood pellets <10% moisture: source RHPL Ltd)
• wood chips: 200kg/m3 (based on wood chips 25% moisture: source RHPL Ltd)
Wood chips require at least three times the storage space as wood pellets for the same energy content.
Energy density
The term 'energy density' refers to the energy contained in a fuel per unit weight; figures are usually quoted in GJ/tonne. The energy density of fossil fuels is considerably higher (up to three times) than that of biomass fuels such as pellets or wood chips.
* assumes 10% moisture
** assumes 25% moisture
Moisture content has a drastic effect on energy density for all biomass materials. Wood pellets below 10% moisture (as stipulated by the British BioGen code of good practice), have an energy density of around 16.8 GJ per tonne. Wood chips can vary from 14.3 GJ/tonne at 25% moisture to 9.5 GJ/tonne at 50% moisture, making wood chips a more un-predictable form of fuel.
Embodied energy of wood pellets
If dry wood is being used (eg: dry wood shavings or recycled pallets), the embodied energy will be considerably lower than that of newly harvested willow coppice, which requires a higher degree of drying.
embodied energy expressed as % of total produced:
• dry wood ~ 1.62%. (1)
• damp wood ~ up to 17.6%
Embodied energy figures for wood pellets derived from short rotation coppice should take account of cultivation and harvesting inputs, these can be as high as 30%* however, the equivalent value for fossil fuels such as refining and transport are rarely considered.
Energy costs
Calculations as to the cost of pellets vs. other fuel types should take account of energy density and cost per delivered unit. Different fuel types are delivered in different ways eg: oil in pence/litre or pellets in £/tonne. For this reason it is best to compare costs in pence per kWh - that is to say the cost of 'delivered energy'
Prices are based on domestic tariffs, larger users such as schools or businesses can buy wood pellets considerably cheaper. (2)
• delivered LP gas is the most expensive
• wood chips are the same price as mains gas
• wood pellets are cheaper than delivered LP gas
• wood pellets are only 0.1 pence/kWh more expensive than heating oil
At today's prices wood pellet fuel is competitive with delivered oil and significantly cheaper than delivered gas, however these figures don’t account for delivery charges which can be higher for wood pellets since there are less retail outlets nation wide.
With the price of oil and gas likely to rise, and the cost of pellets likely to fall with increased demand, these cost savings could become even greater in the future.
 story continues on page 2
Author: Andrew Cox XC02
All pictures/illustrations courtesy of Xco2 conisbee
References
1, *expressed as % of total energy produced. source: Biomass Tank THERMIE B Project STR-0940,
2. Heat Loads (from BRE) UK Victorian
- Water heating : 60kWh/m2 per year
- Space heating: 164 kWh/m2 per year
Part L 2002 - Water heating : 50kWh/m2 per year
•Space heating: 71 kWh/m2 per year
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