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Fuel quality

Fuel quality is crucial for reliable operation of a woodchip boiler. The key parameters are material, size and moisture content.

Material

Forestry chips from air dried roundwood are ideal. Tops and brash may also be suitable although high bark content can cause problems with ash formation. Sawmill co-product such as slabwood is also suitable.

Reclaimed timber from pallets and construction can be used, but requires close attention to ensure there is minimal contamination from metals and plastics. In general, fresh timber will be preferred and will benefit existing timber operations by providing a local market for low value products.

Chip size

Consistency of chip size is critical for the reliable operation of a small boiler fuel feed system. Indeed, fuel blockages are often the main cause of system shut down on many small systems, primarily due to poor fuel quality. For small systems, chips should be no larger than 50mm whilst the fines fraction (< 2mm) should not be more than 15%. Optimum size is generally 8 to 30 mm. Long slivers can be especially problematic - many chippers produce these as the end of the log passes through the blades.

Moisture content

The moisture content of the fuel has a considerable effect on its useful heat yield as higher water content means more energy is consumed to boil off the moisture. This often affects optimum combustion conditions leading to higher emissions and losses. Any fuel supply contract should make specific reference to the acceptable moisture content.

All boilers have an operating range of acceptable moisture content, depending largely on the type of grate or burner head. Larger boilers with moving grates tend to prefer wetter fuel. At the small end of the market below 100kW, stoker heads are typical and work best with fuel between 20% and 30% moisture. Since freshly felled timber is 40% - 60 % moisture, it is necessary to air-dry timber prior to chipping. This is easier than drying chip and at its simplest involves stacking roundwood and covering the top with a tarpaulin or plastic sheet for a few months.

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Fuel quality Fuel quality is crucial for reliable operation of a woodchip boiler. The key parameters are material, size and moisture content. Material Forestry chips from air dried roundwood are ideal. Tops and brash may also be suitable although high bark content can cause problems with ash formation. Sawmill co-product such as slabwood is also suitable. Reclaimed timber from pallets and construction can be used, but requires close attention to ensure there is minimal contamination from metals and plastics. In general, fresh timber will be preferred and will benefit existing timber operations by providing a local market for low value products. Chip size Consistency of chip size is critical for the reliable operation of a small boiler fuel feed system. Indeed, fuel blockages are often the main cause of system shut down on many small systems, primarily due to poor fuel quality. For small systems, chips should be no larger than 50mm whilst the fines fraction (< 2mm) should not be more than 15%. Optimum size is generally 8 to 30 mm. Long slivers can be especially problematic - many chippers produce these as the end of the log passes through the blades. Moisture content The moisture content of the fuel has a considerable effect on its useful heat yield as higher water content means more energy is consumed to boil off the moisture. This often affects optimum combustion conditions leading to higher emissions and losses. Any fuel supply contract should make specific reference to the acceptable moisture content. All boilers have an operating range of acceptable moisture content, depending largely on the type of grate or burner head. Larger boilers with moving grates tend to prefer wetter fuel. At the small end of the market below 100kW, stoker heads are typical and work best with fuel between 20% and 30% moisture. Since freshly felled timber is 40% - 60 % moisture, it is necessary to air-dry timber prior to chipping. This is easier than drying chip and at its simplest involves stacking roundwood and covering the top with a tarpaulin or plastic sheet for a few months. <<< Previous: Supply options Next: >>> Classification	SEARCH Find a supplier In this section Technical Wood fuel for a better landscape Sustainability Local jobs for local people The cleanest fuel Where to buy wood fuel How to burn wood Chimneys Wood fuel costs Logs & Firewood Wood Chip Supply options Fuel quality Classification Boilers Fuel handling Fuel storage Wood pellets Storage and Delivery Electricity Contact us Unit 7 Dyfi Eco Park, Machynlleth, Powys SY20 8AX (+44) (0)845 456 0342
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