Boiler Fuel Types

ETA Information on Fuel

Suitable Fuels

The preferred fuel is coarse wood chips with a low fine content
The length of the individual wood chips should be between 30 and 50 mm, to ensure that air can circulate well among the stored chips, water can escape from the pile and the tendency to rot and turn mouldy is minimal.

If the fine content (sawdust, bark, needles, soil, sand) is too high, it can obstruct the airways. Water evaporates inside the warm wood chip pile. If the vapour cannot escape, it will condense in the upper part of the heap. Some of the wood chips will then rot and turn to compost with no heating value.

Avoid Green, Moist Wood Chips
Only wood chips that are sufficiently dry (water content below 25%) can be stored in a concrete bunker without problems. Moist, coarsely chopped chips stored in an open hall exposed to the wind quickly reach water content under 35%.

Be Especially Careful With Waste Wood
When waste wood is on offer, it may be rotten with no heating value; only buy waste wood by weight and with limited water content (25% maximum). Also ensure that the wood contains no contaminants. The applicable laws only allow the use of untreated wood without foreign substances for heating purposes.

Wood-Processing Facilities May Burn Chipboard If It Contains No Halogens Or Wood Preservatives.
Binding agents containing chlorine used to frequently be used in wood products, if these are burned in a boiler without appropriate cleaning of the flue gas, contaminants detrimental to health will be emitted. Apart from this, very high chlorine content will have a severe negative effect on the life of the boiler. Only burn wood products or painted, coated or varnished wood if you are sure these contain no wood preservatives or heavy metals.

Nails and Stones
Nails and stones shouldn’t bring the boiler to a halt, but they cause increased wear on the screws and the blades in the rotary valve.

Impurities in the Fuel
Impurities in the fuel result in higher ash content and are frequently responsible for a lower ash melting point. If the ash melting point is exceeded, the ash fuses (slag formation). It is therefore in the plant operator’s interests to keep impurities to a minimum.

Types of Wood Chips
The best quality wood chips are finely chopped (G30) hardwood, without bark, with low dust content and water content below 20%. With the highest heating value and the lowest storage volume requirement, this would be the optimum fuel for any boiler.

Fine wood chips (G30 or finer) can be made from dry wood, enabling a higher storage density. Even if wood is stored dry, the energy content wanes, and the heating value of dry wood decreases by 10% over 10 years.

Wood chips from dry branches and shrubbery cuttings have higher bark content and usually also large amounts of dirt and decayed material, resulting in more ash. If they were chopped from dry wood, there will be no storage problems and the reduction in boiler output is low.

Forest-chopped wood chips from fresh branches are not fit for storage and reduce the achievable boiler output significantly.

Wood that has been stored in damp conditions for a long time (10 years) has lost up to 50% of its heating value. Chips from such wood significantly reduce the achievable boiler output.

The final product from wet wood is compost, which can be disposed of in a refuse incinerator but can hardly be burned in a normal boiler. Ensure good ventilation during storage. The higher the water content of the wood, the more coarsely it should be chopped. Never store more than a year’s supply of wood chips (round logs can be stored more easily and densely).

Be very careful where wood chips from waste wood are on offer at especially low prices; the fraction of decayed material and perhaps also the amount of foreign objects (nails, wood preservatives, sand, stones) can be very high.

 

According to ONORM M7133
Total weight = 100% G30 G50 G100
Coarse content maximum 20% Maximum cross section in cm2 3 5 10
Maximum length in mm 85 120 250
Nominal mesh size of coarse-mesh screen in mm 16 31.5 63
Main content 60-100% Nominal mesh size of medium-mesh screen in mm 2.8 5.6 11.2
Maximum Fine content 20% (of which maximum 4% dust) Nominal mesh size of fine mesh screen in mm 1 1 1

 

Moist Fuel

 

Decay and Mould
Up to a water content of approx. 25%, the water is bound up in the wood fibres. Above 25%, water is found between the fibre cells in cavities and capillary vessels. This unbound water is a habitat for, and also a basis for the reproduction of, microbes and fungi, which can penetrate the tree through wounds in the wood structure, particularly cuts or breaks. These microbes convert cellulose and lignin into the basic building blocks carbon dioxide and water.

The wood rots, hollows out and becomes brittle, ultimately losing all of its heating value. When a tree is felled, the race between drying and decay begins. As the water content decreases, the living conditions for microbes worsen until they die off when the water content drops below 25%. The faster the drying process, the more heating value remains in the wood.

For thin branches, the microbes have a very large contact surface in relation to the wood volume. No matter how neatly the branches are stacked, losses in heating value over 25% are the rule (and considerably more in wet weather). This is why forestry businesses don’t even take part in this race when branches are smaller than 3 to 5 cm; instead, the material is left behind as nutrient material for the forest.

Easy Recognition of Moist or Dry Wood
Even through professionals who work daily with wood chips only trust the oven test for measurements of the exact water content, there is still a very simple way to distinguish moist from dry. Chips that feel dry in the hand have a water content below 25% and can be stored without problems. If they feel wet, the water content is sure to be above 35%. If the wood chips are dark brown, light and already crumbly, then you are holding rotten wood in your hands, wood that has already lost the majority of its heating value. From such “compost” you can expect only problems, but not boiler output.

Do Not Store Moist Wood Chips without Ventilation
Wood chips can only be stored without ventilation (for example in a concrete cellar) if their water content is no more than 30%. If moist wood chips from a sawmill are to be used anyway, then no more than three weeks’ supply should ever be kept in an unventilated bunker. An air inlet and an outlet fan can remove water vapour and at least limit mould growth.

Store Moist Wood Chips in a Well-Ventilated Area.
During the decay process, moist wood chips release heat that drives out water. Wet areas form on the surface, and rising water vapour may also be visible. If the wind is given the chance to transport the water away, coarse wood chips will dry while decay and fungi will stay within acceptable limits. Best is a flying roof separate from living and working quarters, which keeps rain out but lets the wind work unhindered. At least one side of the storage hall should be completely open. Additional openings in all other walls improve the storage conditions.

Water Content and Moisture
Water content has become the established measurement for wood used for generating energy; in the lumber business, the wood’s moisture is usually stated.

Water content (%) =                              water in the wood (kg) x 100
total weight of wood (kg)

Moisture content (%) =                          water in the wood (kg) x 100
dry weight of the wood (kg)

Water content (%) =                                      moisture (%) x 100
100% + moisture (%)

When the boiler is commissioned, J L Phillips configures the combustion parameter to the commissioning fuel. It is extremely important that the commissioning fuel is representative of the fuel that is to be used by the customer.

If the fuel changes significantly from one delivery to the next, the combustion parameters will need to be adjusted to suit the new fuel.

Wood chips are naturally variable in their nature (density, water content, size, shape etc) and the boiler controller will self-adjust the combustion parameters to suit the fuel.

The range of self-adjustment will cope with a variability of +/- 5% water content. If the water content varies by more than +/-5% from the commissioning fuel then the boiler will need re-commissioning to suit the new fuel.

Highly variable fuel particle size can also have the effect of upsetting the boiler combustion and may also require boiler re-commissioning.

It is important to note the effect that changing the particle size has on the combustion. For instance, moving from G30 to G50 will increase the air gaps in the fuel hence reducing the volume throughput and the boiler power output. Boiler power can be increased by re-commissioning.

 

Wood Chip Specification for Herz Biomass Boilers;

 

Boiler Type Chip Size Water Content Ash Content Wood Density
Herz Firematic(1 phase) G30 or G50 Up to W35 Up to A0.5 200-250kg/m3
Herz Firematic(3 phase) G30 or G50 Up to W35 Up to A0.5 200-250kg/m3
Herz Biomatic G30 or G50 Up to W35 Up to A0.5 200-250kg/m3
Herz Biofire G30 or G50 Up to W40 Up to A0.5 200-250kg/m3

 

If you are planning to use recycled wood in your boiler, J L Phillips and Rural Energy will require a fuel sample before approving its use. Failure to supply this could invalidate your warranties.

Using recycled wood which has been shredded instead of chipped (using a fuel wood chipper) will require authorisation by the Local Authority and may cause the following issues;

  • The full nominal boiler power may not be achievable due to reduced flow characteristics
  • It may not be possible to obtain good combustion parameters. Incorrect combustion can reduce equipment life and would invalidate the boiler warranty
  • There may be problems with bridging over the auger in the wood chip store

 

Size Classification;

 

Chip Designation

Maximum % Particle Size

Maximum Extremes

<4%

<20% 60 -100% <200% CS Area cm2

Length cm

G30

<1mm

1-3mm 3-16mm >16mm 3 8.5
G50

<1mm

1-6mm 6-32mm >32mm 5

12

G100

<1mm 6-32mm 12-63mm >63mm 10

25

 

Moisture Classification;

 

W20

W30 W35 W40

W50

<20

20-30 30-35 35-40

40-50

 

The moisture content of the wood should be calculated using the ‘wet basis method’ as described below;

Moisture content (%) = 100 x weight of wood when wet – weight of the wood when dry

Weight of wood when wet

For comparison, fresh timber will be in the region of 50% moisture, well-seasoned logs will be in the region of 30% moisture, and standard delivery pallets are usually around 20% moisture content.

 

Ash Content;

 

Ash Content (by weight)
A0.5 <0.5% low ash content
A1 0.5-1% medium ash content
A2 1% -2% high ash content

 

Factors affecting the ash content include the bark content and impurities such as glues.

 

Wood Density;

 

Wood Density kg/m3
S160 <160 Low density
S200 160-250 Medium density
S250 >250 High density

 

The main factor affecting the wood density is the wood species, i.e. hard woods are more dense than soft woods.

 

Combustion Analysis

When the boiler is commissioned, J L Phillips configures the combustion parameter to the commissioning fuel. It is extremely important that the commissioning fuel is representative of the fuel that is to be used by the customer.

If the fuel changes significantly from one delivery to the next, the combustion parameters will need to be adjusted to suit the new fuel.

Wood chips are naturally variable in their nature (density, water content, size, shape etc) and the boiler controller will self-adjust the combustion parameters to suit the fuel.

The range of self-adjustment will cope with a variability of +/- 5% water content. If the water content varies by more than +/-5% from the commissioning fuel then the boiler will need re-commissioning to suit the new fuel.

Highly variable fuel particle size can also have the effect of upsetting the boiler combustion and may also require boiler re-commissioning.

It is important to note the effect that changing the particle size has on the combustion. For instance, moving from G30 to G50 will increase the air gaps in the fuel hence reducing the volume throughput and the boiler power output. Boiler power can be increased by re-commissioning.

See also ‘R.H.I Commercial Fuel Log

 

WOODCHIP/ WOOD PELLET SUPPLIERS

Biomass Boiler Installations | WoodchipTo find a reputable woodchip or pellet supplier in your area, who will be supplying a quality product, meeting the R.H.I standards, visit the B.S.L (Biomass Suppliers List) website at the link below. You can simply type in your post code and search for suppliers in your area or you can contact them;

Tel: 0207 090 7769 Email:  bslhelpdesk@gemserv.com

http://biomass-suppliers-list.service.gov.uk/

J L Phillips Renewable Energy is happy to recommend the following suppliers of woodchip and pellet;

Forest Fuels

Woodchip & Pellet suppliers covering the East Midlands & National

Tel: 01409 281977 Email:info@forestfuels.co.uk

http://www.forestfuels.co.uk/

Forever Fuels

Wood Pellet Suppliers covering the East Midlands & National

Tel: 01628 509690 Email:info@forever-fuels.com

http://www.forever-fuels.com/

MWF Sustainable Wood Heat

Wood Chip Suppliers covering the East Midlands & National

Tel: 01952 621030 Email: info@wood-fuel.co.uk

http://www.wood-fuel.co.uk/

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J L Phillips Renewable Energy Ltd, Fen Lane, Long Bennington, Newark, NG23 5ED