Wood chip is a processed form of wood fuel that can be used in automatic boiler equipment without the high degree of processing required to manufacture pellets.
Within biomass heating, wood chip is often used in boilers, ranging from 25 kilowatts to many megawatts for Combined Heat and Power (CHP) and district heating plants.
Wood chip boilers are available for all scales of operation. As with wood pellets, modern wood chip boilers can provide a high level of automation and convenience for wood fuelled space heating.
Due to oil and gas price rises over the last three years, wood chip is a very cost-competitive fuel, with the less processed forms such as logs and chips typically undercutting all other domestic fuels. The equipment to burn chip must be rather heavy-duty and tends therefore to be a little more expensive and space extensive than its pellet burning counter-parts. However, with the recent introduction of various grant schemes such as the RHI to help with the capital costs the scales are more balanced, particularly as the variable costs are lower and more predictable.
Wood chips present no risk if accidentally released into the environment, unlike oil which is a serious pollutant and gas which can explode. There are no harmful by-products. The flue gas is smoke-free and the ash content of between 0.5% and 3% by volume (depending on material), is minimal. Unlike coal ash, wood ash is an excellent fertiliser and can be used in the garden or returned to the forest as bio-char. Modern appliances burn very cleanly with minimal smoke. The majority of woodchip boilers currently being installed in the UK require fuel to comply with the Austrian ONORM M 7133 specification.
This wood should then be carefully stacked allowing good airflow and kept covered on top (typically with a breathable membrane sheet) allowing natural air to flow and moisture to evaporate through the membrane. After being left in situ in log form in this way for approximately 9 months to one year after felling, the wood will typically dry naturally to around 20-25% moisture content and will then produce a good, high quality chip.
Woodchip can then be stored under cover ready for use, but again it is advisable to store in a well-ventilated, yet reasonably weather proof enclosure to help avoid stagnation and moisture absorption. If wood chip is turned occasionally in this situation, this will help create the ideal chip.
SUITABLE WOOD CHIP SPECIFICATION:
- Woodchips used in smaller installations should have a homogeneous particle size. Avoid pieces of width less than 8mm or length greater than 50mm.
- Ideally wood chip shall be sized: 10mm width x 30 (G30)-40mm length (in the direction of the wood fibre).
- Moisture Content: between 20 – 30% (W30), ideally 20-25%.
- Limits: no less than 15% moisture content ideally (W20) (the moisture actually assists in improving combustion efficiency.
- No greater than 35% moisture content (W35) (excess moisture leads to product clumping, insufficient combustion chamber temperatures, excessive boiler condensation and overall poor energy value for a given volume of product.
- Unknown objects: Every effort should be made to avoid the introduction of foreign bodies such as sand or metal. Sand and grit can become embedded within bark if logs are dragged prior to stacking (and can in some instances where exposure to sand, dust or grit is possible) grow within the bark . De-barking is therefore a practice that should be considered for ideal chip production.
The moisture content determines the choice of equipment used.
For a domestic stoker boiler, the moisture content should not be higher than 20-25%.
For ceramic lined retort or heavy-duty stepped grate boilers the maximum moisture content should be no higher than 35%.
Purpose made ‘wet fuel’ boilers (where combustion chambers are lined with refractory bricks for example) can typically handle moisture contents between 35% and 55%. These types of boilers are not usually then able to fire ‘dry’ fuels as their combustion chamber temperatures become excessive on dry fuel.
Please see Table 3 within the “Biomass Energy Content & Price” section of the website for further details on wood chips.
