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In the energy section

A layperson's guide to efficient, modern production & use of small-scale woodfuel systems

Crown Copyright 2001

NB: You can also download this report on using wood as a fuel as a Word document [5.9Mb]. Ymddiheurwn dros beidio cyfieithu'r adroddiad hyn. We apologise that there is no Welsh version.


  1. Section 1 Introduction.

  2. Section 2 Modern Small-scale Woodfuel Usage

  3. Section 3 Woodchip Fuel

  4. Section 4 Log Fuel

  5. Section 5 Reconstituted Fuelwood

  6. Section 6 Woodfuel - Value and Sale

  7. Section 7 Safety

  8. Section 8 Conclusions

  9. Section 9 Quality Woodfuel Supply in the Dyfi Eco-partnership Area

  10. Section 10 Acknowledgments

  11. Appendix 1 Realisable Fuel Values

  12. Appendix 2 Some Suppliers of Chip-fuelled Equipment

  13. Local Woodfuel Interests Questionnaire


Plate 1 - Extracting Fuelwood from Broad-leaved Thinnings with simple Farm-based Equipment

1. Introduction

Feedback from local public meetings on the subject of Woodfuel as a renewable resource, has highlighted a shortfall in understanding and up to date knowledgeof small-scale heating technology.

To help address this problem Ecodyfi, working with the Technical Development Branch of Forest Research, is producing this document to: -

  • Describe the available systems. Their advantages and disadvantages and to give indications of current installation and running costs, when compared with traditional fossil fuel systems.

  • Discuss the need for and indicate optimal wood fuel qualities.

  • Describe suitable machinery that could meet the needs of local wood fuel demand.

  • Gain an essential feedback from potentially interested local people so that future development in this subject may be better co-ordinated and directed.

This document has been supported by the Forestry Commission and the Organisation for the Promotion of Energy Technologies, (OPET).

2. Modern small-scale Woodfuel usage

Heating with Woodfuel has long been practisedin Scandinavia, (Sweden; Norway; Finland; Denmark), the Baltic countries and Central Europe. Some fifteen years ago the momentum for continuing development to keep pace with oil and gas fuels began. Today in many of these countries, wood fired systems are now widely understood, available and supported by an established localised service infrastructure. The Danish Center for Biomass Technology quotes around 81% of all small wood fuelled heating equipment, (including chip and pellet fired) to be small log-fuelled 'stoves', (1999 statistics). Information from Austria [1] (1994) gives about 60% of total fuel biomass consumed in traditional forms of wood stoves'. A similar Woodfuel equipment pattern seemsto be emerging in the UK. [2]

Within the European Union there is now noshortage of hardware suppliers for a full range of quality tested small-scale Woodfuel burning and producing equipment. General technological development inWoodfuel consumption supports three main fuel types: -

  • Chipped wood fuels

  • Traditional log fuels

  • Reconstituted fuels, i.e. Pellets and Briquettes

The main positive benefit is the CO2 neutral character of wood energy, as long as the rate of harvest equals, or is less than the rate of tree re-growth.

3. Woodchip Fuel

Whilst modern woodchip fuelledsystems offer the attraction of largely automated operation, significant UK 'domestic' uptake is not yet evident. Modern woodchip systems are available down to 20 kilowatts, (kW),which is still an over-large installation for many domestic situations. An assurance of quality woodchip supply, burner equipment capital costs and theneed for a bigger wood-store, are all seen as problem areas. Broadly speaking 20 kW will provide for aboutten to fifteen radiators, (e.g. a large farmhouse). Automatically stoked woodchip burning equipment is available in a range from c. 20 kW up to as largeas desired, but for the purposes of this document a reasonable upper limit for 'small-scale' interests could be 500kW.

3.1. Woodchip fuelsystems and "how they work".

Modern small-scale woodchip fuel systems may beset to automatically produce heat 'on demand', rather similar to oil or gas systems. Similar to, but not quite the same. Small-scale modern oil and gas systems can 'idle' for very long periods, or shutdown until required. Then they have the ability to almost instantly fire up to maximum efficient output to match the demand, and then immediately shut down again. Because woodchip is a solid fuel it cannot exactly duplicate this performance. Modern small-scale woodchip appliances overcome this by automatically keeping a small nucleus offire-bed continuously alight. The heat produced from this minimal operation is a reason why woodchip heat appliances have a c. 20kW lower size limitation.

Problems can arise in periods where total heat demand during the day is insufficient to keep the Woodfuel burner going. A way of overcoming sporadic low heat demand is to add a large,"accumulator" heat store (e.g. insulated hot water tanks). However even for small installations these may have to be in the order of 800 to 1000 litres (recommendations may be found in the manufacturer's literature), and low summer demand may not support the wood-chip burner at all. To meet low summer demands most modern small-scale systems have an in-built facility for heating domestic hot water, (DHW) by electricity, or the setup can include a small fossil fuelled installation for low demand periods.

Possible ways around the constraints imposed by individual demands below the c. 20kW smallest sized woodchip burner system,are: -

  • Users forming a small co-operative in one local area heating set-up

  • Establishing several small domestic heat user groups, each serviced by a local woodchip fuel supplier, (i.e., a 'cluster').

As a general principle there are fewer problems with woodchip fuel appliances serving larger heat demands.

Diagram1 - Small-ScaleWoodchip Burner Unit

Modern small-scale woodchip fuelled heating systems are simple in design, (Diagram 1). A feature common to them all is a woodchip storage hopper, and a supply pipe to the burner unit, which is attached to the boiler, (heat exchanger).For efficiency, fuel burning takes place with small amounts of fuel and at high temperatures. The burner unit itself may be a variation on one of three common small-scale designs: -

3.1.1. The most common type is a 'pre-furnace burner', (Plate 2). This is a small ceramic or cast iron lined box that works rather like a woodchip blowlamp. Woodchip fuel is metered in along the feedpipe and mixed with a separately introduced forced combustion air supply. Burning takes place inside the ceramic box and the resultant flames and hot gases are forced out of a nozzle into a boiler water jacket heat exchanger. This boiler unit is separable from the pre-furnace burner / feedpipe / hopper unit, and may not even be from the same manufacturer. Pre-furnace burners are the design usually favoured by the smallest woodchip fuel units and because of their design, the type most able to react quickly to heat demands. Control of the automated system is via a central panel, with electric powered sensors, and motors for the various feed and air functions.

Plate 2 - Pre-Furnace Chip-wood Burner

3.1.2.Another type, a 'stoker-burner', (Plate 3) has a small self-contained firebed at the end of the fuel feed pipe, which is fixed into the base of a boilerwater jacket heat exchanger. Forced combustion air is supplied to the small heap of Woodfuel that wells up out of the feedpipe. This is an earlier development to the 'pre-furnace burner', simpler in construction, but not ideally suited to the very smallest of woodchip fuel demands.

Plate 3 - Chip-wood Stoker-Burner Unit

3.1.3.The least common type is a 'moving grate' appliance, (Plate 4). This designmirrors furnaces traditionally designed for coal firing. Woodchip fuel is fedin via the same hopper and feedpipe, onto an inclined sectional firegrate. This firegrate gently oscillates to move the woodchip down and along as it burns. Forced combustion air is fed in underneath the grate. All combustion happens athigh temperatures within a ceramic chamber, with a multiple pass boiler heatexchanger above this firebox. Appliances like this are made in the full range from 20kW upwards. A feature of this design is its ability to self-condition the fuel by drying it before actual combustion. Therefore it is less sensitive to fuel moisture content and to a degree fuel quality. Of the three general designs this type is the most complex in construction, and generally the most costly.

Plate 4 - A "Moving-grate" Chip-wood Burner (complete with feed hopper)

3.2. Manufacturers and Suppliers

Many of the existing manufacturers, (Appendix 2) are based in the Nordic countries although well-established British companies and suppliers are developing in this sector.

3.3. Indicative Costs

These costs should be taken as approximate onlyand would best be used for comparative purposes. Most small-scale woodchip appliances are of foreign manufacture and prices have tended to fluctuate, particularly with exchange rates.

Table1 Woodchipburners - indicative costs

Burner Type

Size, (kW)

Indicative Cost (£)


Minimum Accumulator Size (Litres)




Complete with boiler & 500 litre fuel hopper

800 litres

Moving grate



Complete, as above

Ditto, as above




c/w 1000 litre hopper

1600 litres




c/w 8m3 hopper, installed & commissioned.

No accumulator necessary as demand is reasonably constant.




Quoted cost covers the installed system +25m3 fuel silo.

No accumulator necessary as demand is reasonably constant.

Hot water storage, (accumulators), may cost from £300 to £700 dependant on size. A rough guide to the required size, inlitres, is about 40 to 60 times the boiler heat output in kW.

Apart from the higher capital costs of the hopper/burner/boiler, domestic sized installation costs for the rest of the system should be the same as fossil fuels.

Chimney insulation would most definitely be a point to check on for older properties. The 'yellow pages' directory contains particulars of specialist engineers; or contact your local heating appliance suppliers. Indicative costs for this service are difficult as each situation tends to be different and access is often a problem. Individual estimates are advised beforehand, but for a small property costs could be around £500.

Fuel hopper size or adjacent fuel storage capacity is a point of consideration. This will also be influenced by the likely frequency of fuel deliveries, and haulage on-costs for small deliveries.As a rough guide a 20kW system could use about 0.6m3 of chips per day in the heating season. Specific amounts for each setup will depend on variables such as demand, fuel moisture content, building insulation, etc. It would be worth assessing each potential setup to be able to specify the optimal hopper /storage size required.

3.4. Payback

Generally the 'cost payback' term will dependon the initial capital cost of installation and the amount of annual usage the system gets. To give some idea of chipfuel economy a Swedish appliancemanufacturer [3] states that 12 m3 of (fresh)chips can give similar warmth as I m3of oil fuel, (1,000 litres). Approximately 2.5m3 of woodchip may beproduced from 1 m3 of roundwood, (Approximately equivalent to 1tonne of Greenwood). Calculations show that wood fuelled systems can break evenwith alternative systems in approx. six to nine years, at about half expectedequipment life. [4] Some comparison with fossil fuels and alternative forms of wood fuel, are shown in appendix 1.

3.5. Woodchip fuel quality

Ideal woodchip fuel should: -

  • Be of an average woodchip size suited to the heating appliance. Woodchip fuelledappliances will usually perform best on a quality fuel where chip size isbetween 2 and 25mm.

  • Be uniform in size. Pieces should be at or close to the average size recommended for the appliance. Most small-scale woodchip fuelled equipment could be expected to operate best on 'super' or 'fine' grades of chip (See table 2).

  • Be free of oversize slivers or pieces of twig that could block the automatic feed system.

  • Have a low dust / "fines" content. Small airborne particles are injurious to health, reduce the airflow through the chip medium, and tend to create morefly-ash.

  • Be of low moisture content. Burning wet fuel decreases heat recovery efficiency (the wood must dry before it combusts). Woodchip systems are not as sensitive to this as log fuel systems. Woodchip moisture contents below 30% are usually fine, but some small-scale appliances are more sensitive.

Not all chipping machines are designed to give a Woodfuel quality. Many examples, especially the smaller machines, are only intended for waste-wood bulk reduction, or to produce "mulch" quality chips.

Woodchip fuel quality also depends on the"branchiness" of the wood material fed into the chipper. A lot of twiggy, leafy material will tend to produce a lot of slivers and fines and havea greater size inconsistency.

Quality not only depends on the machine design but also on its maintenance. Poorly maintained machines will produce greater inconsistency with more slivers and fines being produced.

Larger woodchip fuel appliances are generally less sensitive to fuel quality than smaller. Some small woodchip appliance designs are less sensitive to woodchip quality than others, (e.g. the moving grate types). Within the parameters of fuel supply quality there will be some variation, but generally speaking systems can be adjusted to optimise performance within this variation.

Table 2 Woodchip size grades [5]

Woodchip size


2 -25mm

25 - 50mm

50 - 100mm

100 - 200mm







Super Grade

< 15%





Fine Grade

< 15%


< 10%

< 2%


Coarse Grade

< 15%



< 30%

< 2%

In addition there should be no more than 5% of 'foreign' material, and no stones. Oversize and sliver material is assumed to be long and thin; material greater than 50mm square is unacceptable.

3.6.Woodchip machinery; a brief description

Compared with equivalent small-scale woodlog producing machinery, wood chippers may be more than three times the capital cost. Therefore, unless the intention is to produce in bulk or to supply a large woodchip burner, purchase may not be recommended, (Plate 5).

Plate 5 Sasmo HP25 chipping Sweet Chestnut Coppice for Fuelwood

Machine design falls into three types. In all types wood is cut across the grain. Two types chip the wood by the action of a set of knives, set round the surface of either a disc or a drum.The third has a continuous knife formed in the shape of a spiral cone, (ie. a"screw" chipper). All cut the wood by trapping it between the knife,and an anvil, set to give a small working clearance for knife rotation. Cut chips are ejected forcibly out of a directable spout. Wood feed is either by hand or by a hydraulic loader. Drum and disc chippers have hydraulic powered feeding rollers but screw chippers usually do not, depending on the coarseness of screw thread, ('pitch') for feeding rate, and therefore chip size. Different screw pitches are available. The rotation rate of any hydraulically powered feed rollers is usually adjustable. Given a constant chipper drum/disc speed, feed speed adjustment alters the average size of the chip produced.

One well set-up chipper machine can supply a large number of small-scale woodchip heating installations. Table 3 gives an idea of the outputs and costs involved. Because bulk fuel storage facilities will probably be limited at small-scale woodchip fuel installations, a centralised bulk storage facility will be necessary if the chipper machinery is to be fully utilised. For efficiency and competitive supply costs, the above considerations should be combined at one, or a limited number of locations.

Table 3 Some typical tested woodchipmachines, outputs, and costs [6]


Sasmo HP25SS

Gravely Pro-chip

Greenmech MT 252

Gandini 007 TPS

Morbark EZ - Beaver 10

Capital cost £

14 000

14 950

19 300

13 600

14 000

Operating hourly cost * £

26.38 #



26.30 #


Material chipped **

Dry Oak

Green Oak

Dry Oak

Green Oak

Dry Oak

Green Oak

Dry Oak

Moisture content %, (wet basis)








Input m3/shr solid wood








Output m3/shr loose chips








Cost per m3 of loose chips output £








* Hourly operating costs include charges for a 2 man team, repairs and maintenance, fuel and a calculated allowance for rest and other work. A high yearly usage of 1667 hours is assumed together with a machine economic life of 5000 hours.

# Includes hourly charges for a Tractor as thepowering unit, all others self powered.

** All material chipped was branchwood, ( twigs or leaves).

NB."Loose chips" means as produced by the chipper. Significant settling occurs in transport.

4. Log Fuel

Plate 6 A Traditional "Tiled Stove" Design

4.1. Modern Systems for burning log fuel

Simple and efficient systems for logfuelled space heating have existed for over a hundred years, and are still viable, (Plate 6). Developments have produced efficient log fuelled systems for hot water central heating and domestic hot water (DHW), some combined with a cooking facility as well. Many are designed to run for long periods on a single charge of fuel. There may also be an in-built facility for a fossil fuel alternative, which can be programmed to operate automatically.

Log fuelled systems are applicable to all domestic sized demands, and heating fairly large spaces, (e.g. village halls and buildings up to c. 500m2). Efficient, purpose-built modern equipment covers a range from 2 to 75kW,"multi-fuel" [7] boiler equipment from 20 to over 100kW.

In Scandinavia and parts of Europe, woodlogs are still the most common form of woodfuel. Mainly because the technology and equipment for production and use is readily available and reasonably well established. Table 4 gives basic information for wood log burning appliances.

Table 4 - Wood log burning equipment - Basic Details


Example makes

Heat output forms

Indicative Max. efficiency *



J£tul;Morso; Charnwood; Coalbrookdale; Franco-Belge

Mainly space heating; (Some models D.H.W. also ***).

Up to 70% [8]

Probably the most common small domestic Woodfuel equipment

'Tiled' Stove

Tulikivi;Cronspisen; Warmo

Space heating only

Up to 85 to 90% [9]

A traditional form of heat storage stove, still popular in some countries [10]

"Over-fired"Boilers **

Wamsler;Franco-Belge; KY Warmservice.

Radiator systems and DHW

c.60 to 70%

Asimpler form of combustion, now being superceded.

"Under-fired"Boilers **

Eryl;Veto; Arimax.

Radiator systems and DHW

c.65 to 75%

Amore efficient form of combustion, allowing 'hopper' feeding.

"Reverse-draught"boilers **

CTCParka; TTM Euronom; Hertz; Laka; Varmebaronen; H£val: Fr£ling: HS Perifal.

Radiator systems and DHW

c. 75 to 90%

Themost efficient of all modern log-fuel designs, but adequate flue draught is essential.

"Combination"boilers **

CTCParka; Varmebaronen

Radiator systems and DHW

c.65 to 75% from woodlogs,

65 to 80% from alternative fossil fuels

Purposemade for smaller domestic demands with alternative fossil fuel running for summer use or lengthy unattended operation.


Franco-Belge; Wamsler;Tirolia

Radiator systems, cooking and DHW

50 to 60%

Similar to Aga/Rayburn to look at.

* In comparison, a standard open fireplace can not achieve more than 30% efficiency.

** Most boiler models have an option for a fossil fuel burner, (oil or gas) fitted to the same heat exchanger, together with an automated control 'package' to suit.

*** All water-heating units can be combined with other fossil-fuelled units to form an integrated system.Control packages are commercially available.

4.2. Efficiency and Manually Fired Equipment

Many factors influence burning efficiency: -

  • Burner equipment design

  • Equipment choice and size requirements

  • Equipment installation

  • Woodfuel 'quality'

  • Operating technique

Details in table 4 show varying indicative efficiencies for the range of equipment, assuming optimum conditions. A principal rule for the optimal operation of log-burners is that efficient combustion is achieved at or near the rated output, (i.e. full load). Some highly developed units have probes in the flue gas to measure oxygen content, so allowing automated control to operate efficiently down to c. 50% of rated output. Log-fuel boilers will require heat storage tanks, (accumulators), and systems for the controlled release of the heat between firing intervals, (Diagram 2). Heat storage is in-built into the design of traditional tiled stoves. Many box stoves have a cast iron or firebrick lined construction for a similar reason.

Diagram 2 - Typical Wood-burner / Accumulator System

Developments within the last decade have resulted in an extensive range of woodlog boilers with very efficient under-fired or reverse-fired design, (Diagrams 3 and 4). Many also have a large fuel hopper capacity. When properly installed and operated, such equipment is able to reduce re-fuelling to once or twice a day, dependant upon heat demand.

Diagram 3 - An "Under-fired" Boiler

Diagram 4 A "Reverse-fired" Boiler

Equipment with basic 'over-fired' [11] technology, (Plate 7) will require more frequent stoking. Undesirable emissions of incompletely burnt gases will result if large amounts of fuel are suddenly loaded. These will tend to condense in any colder parts of the equipment or chimney. Good operational technique involves 'building-up' a hot fire by stoking at regular intervals. If 'idling' is required, it should only be done after a hot fire has built a good base of 'char' and then combustion closed down to c. 50%, or whatever the equipment manufacturer advises.

Plate 7 A Typical "Box" Stove

4.3. Log-wood fired equipment- 'fitting in with a modern lifestyle'

Modern lifestyles often require domestic heating to take care of itself for periods when the property is unoccupied. Often these periods are irregular and have no set pattern. Fossil fuelled and woodchip fired equipment are able to accommodate this as they can run for long periods unattended. Developments in the design of log-fuelled equipment and systems have addressed this problem.

Reverse and under-fired boilers not only have a large in-built fuel hopper, but usually a facility for adding a fossil fuelled (oil or gas) burner. This can be programmed in the usual way and can take over from the log burner when that is exhausted.

There are several small 'combination' woodfuel boilers on the market specifically designed for the needs of the smaller domestic user. These units have a built-in dual fuel function for woodlogs and a fossil fuel (oil or gas), and can be programmed as flexibly as the above. Woodlog hopper capacities on these are generally smaller because of a need for overall compactness.

It is possible to fit any water heating, woodlog appliance into a domestic sized system, in tandem with any other water heating fossil-fuelled equipment. Connecting devices and programmable control systems are commercially available.

Even the smallest space -heating wood stove can fit into a flexible lifestyle arrangement, but obviously it cannot be automated. However it may be effectively used to 'top-up' a level of background heat supplied by conventional fossil fuel.

4.4 Good Log Fuel Characteristics.

For optimal efficiency it stands to reason that a good log fuel quality is needed. In several respects the definition of quality fuel depends upon the specifications laid down by the equipment manufacturer. Factors effecting fuelwood quality are: -

  • Moisture content

  • Mean (Average) piece length

  • Mean piece thickness

  • Consistency ofspecifications

  • Negligible contamination with other materials, (e.g.sawdust; soil ).

  • Bark content

  • Hardwood or softwood

  • Species

Log-fuel moisture content shouldideally be 20% (wet basis) or lower. Some modern equipment may accommodatehigher moisture contents. Burning a wetter fuel than specified may corrode theequipment and chimney, give rise to chimney fire problems, reduce heat outputs and generally give poor economic returns for an investment in woodfuel technology.

Firewood lengths range from 15 to 50cm, depending upon burner requirements.Ideal firewood width is from 6 to 8cm. Size consistency is important for uniform combustion and being able to load the fire to achieve an even burn rate. Fuelwood contamination and/or a high bark content will reduce the burn rate and increase emissions and ash.

In terms of heat value per unit weight of dry matter, there is not much difference between hardwood and softwood. The greater density of some hardwoods means that more heat content can be loaded at a single charge. All current log-fuel burning equipment can operate well on either hardwood or softwood logs, or a mixture. The hopper capacity of many boilers is specifically arranged to accommodate softwood fuel.

4.5. Log-fuel Storage

There is little point in specifying dry fuel if it is stored in a position where it will be exposed to wet weather. Fuelwood stores need not be complicated; a good roof with sufficient overhang to keep the sides dry is required. Sides may be completely open to allow for good air circulation. A dry base is also important, but not essential, logs could be stacked on bearers or old pallets.

The size of the fuel store is often a big consideration. Ideally the size should reflect the total heating requirement for the cold season, but often this is impractical for smalldomestic dwellings. The alternative then is to rely upon regular deliveries of quality fuel from a reliable supplier. Annual fuelwood supply contracts are a good way of doing this, to the benefit of both supplier and user.

4.6. Firewood Production

Producing wood fuel will be of most interest to those who have access to the raw material, or who own woodland. Studies have indicated that woodfuel production can be an economic way of managing what otherwise would be disadvantaged woodland. [12] Cutting by chainsaw and splitting by axe is physically demanding and requires certain specialist skills for safety reasons. It would only be relevant to those with a 'hobby' interest and who only have a small domestic woodfuel requirement.

There is a wide range of machinery available for the production of cut and split wood fuel (Plate 8).Most are designed for attachment to a standard tractor 3-point linkage and power take-off (PTO). Forest Research's Technical Development Branch hasevaluated a representative sample from the UK available range. [13] Table 5 and 6 are summaries of data, outputs and costs cutting 20cm long logs from small roundwood, (still the most common UK length specification for small-scale use).

Plate 8 - The Nokka PK200 Firewood processor in Broadleaved Thinnings

Table 5 Firewood production machinery - Data


Japa 100

Technorton 'Compact'

Japa 600

Nokka PK 200

Cutting Mechanism

Circular Saw

Circular Saw

Circular Saw

Shear Mechanism

Splitting Mechanism


Hydraulic,2 or 4 knives

Hydraulic,2 or 4 knives

Wedge, (on shear)

Min. Power Required





Max. Log Dia. Capacity





Max. Log Length Capacity





Automatic splitting





Conveyor included





Safety Stopping Devices

Tractorstop cord

Drive disconnect pedal & brake

Tractorstop cord & guarding interlocks

Driveshaft overload clutch

Table 6 Firewoodproduction machinery - Outputs & Costs



Average piece size

Output (m3/ [14]

Cost (£/hour, men + machine)

Cost (£/ m3 of output)

Japa 100

1man working





Technorton 'Compact'

1man working

2men working









Japa 600

1man working





Nokka PK200

1man working

1man working

2men working













Each machine is representative of its 'type'. The Technorton is typical of a farmer's 'universal sawbench' and is not limited only to firewood production. The 'cubic meter', (m3) is a volume measure and is therefore not affected by moisture content, as is weight measure.

5. Re-constituted Fuelwood

Reconstituted wood-fuels take theform of briquettes or pellets, and are usually formed from wood processing industry by-products, (e.g. sawdust, shavings etc.). Of the two categories, wood pellets are the most common. Pellets are manufactured mainly in one of two processes, in the form of either small cylindrical pieces or small round balls.

Wood pellets have several important characteristics: -

  • A very consistent density, therefore even heat content,(19 to 17GJ/tonne).

  • A very small moisture content, (5 to 10% (wet basis))

  • A consistent size

  • A clean fuel, lacking the dust, bark pieces andcontaminants of other wood fuels.

Pellets are suitable for a widesize range of burner equipment, and can be used in boilers designed for coalfiring. The most popular use is for heating domestic properties. Pelletised fuel characteristics make it the closest to fossil fuel of all the alternative formsof wood fuels. There is also a good range of pellet burner units specifically designed for small modern domestic heating requirements (Plate 9) as well as a range of boilers and boiler conversions.

Developments in pelletised woodfuel present a definite advantage for efficient, automatic small domestic heating systems and avoid many of the problems associated with woodchip and logfuel. But an assured local fuel availability is not yet very evident in the UK.

Plate 9 - Pellet Stove, with in-built hopper

6. Wood-fuel - Value and Sale

The theoretical maximum 'point ofuse' value for woodfuel is determined by its realisable heat content per unit cost compared to an equivalent for fossil fuel, usually oil or natural gas. Realisable heat can be described as that which the consumer is able to utilisewith their equipment. This can differ quite a lot according to the efficiencyof the equipment and installation. Woodfuel heat potential also varies, mostlyaccording to its moisture content. The table in appendix 1 illustrates the realisable value of all types of wood fuel, and makes comparisons with fossilfuels.

Consequently it is better to buy fuelwood by volume than by weight. The weight of a fuelwood delivery always includes an amount of water. As prepared fuelwood sold by volumetric measure always contains some air spaces, the following average conversions to a solid volume measure may be applied.

Solid Volume

'Bulk' Volume of chopped firewood [15]

'Stacked' Volume of chopped firewood15

'Loose' Volume of Fuelwood Chips [16]

1 cubic metre (m3).




"Bulk volume", relates to chopped firewood that has been loosely heaped or tipped. "Stacked volume", relatesto chopped firewood that has been neatly and closely stacked. "Loose chips", describe fuel chips that have been blown into a container by the chipper. This is best measured before transport, as some settlement will take place.

Re-constituted fuels, (e.g. pellets) containsuch a small amount of moisture that it presents no problem to purchase by a weight measure.

7. Safety

7.1. Woodfuel Procurement and Processing

Working or even being near wood processing or forest machinery operations is potentially dangerous for those who are not adequately trained or informed. Annually, accident incidents continue to reinforce this point. Responsible people will ensure that they are adequately trained and informed, and take proper account of others who may come near.

Simple hand-tools such as the bowsaw, axe orother wood splitting equipment can also give considerable injury if improperly used. Similar safety precautions apply, even for domestic situations, and a safe working area is essential.

The chainsaw bears special mention as probablythe piece of woodcutting equipment requiring the greatest safety attention andcareful use.

7.2 Woodchip

Woodchip boiler equipment should be installedin an outhouse, or in a separate room. This is as a result of size as much asany overriding safety consideration. Modern equipment is very safe. In those countries where woodchip burning is common there are now stringent independent safety tests for woodchip boiler manufacturers' products. The most important designed-in safety features are systems that prevent the fuel from burning back along the feedpipe to the hopper. The distance separating the hopper from the burner is also a safety feature. On larger systems, with correspondingly larger hoppers, many equipment manufacturers recommend that the hopper be located in a separate room, or sectioned off from the burner unit.

Stored woodchip can give rise to dust, and spores from decomposition. Breathing these in can be harmful, especially in small confined spaces with poor ventilation. Working in confined spaces has its own safety hazards, particularly when this is near automated machinery. Therefore it is essential that the system installation is competently designed, installed and run by adequately qualified and informed people.

7.3. Woodlog

Logfuelled equipment should be properly installed to avoid fire hazard. A common mistake is to underestimate the protection required from radiant heat, not only for nearby persons but also of the building fabric and furnishings. Manufacturer's recommendations should be stringently followed.

Equipment operation should always follow the manufacturers and heating system installer's instructions.

For those who like to watch a log fire burning the advice is, to purchase woodburning equipment designed to enable that. Do not, on any account operate equipment with its fire-doors open when it is intended that they be shut. Stove equipment intended for (occasional) viewing of naked flames will have suitable spark arrestors available. If in doubt, ask the equipment manufacturer.

The woodlog store-shed may have its own safety problems. Children love to play in there and climb the log pile. An axe may b e kept near a chopping block to make kindling. For safety's sake keep the door secured and restrict access to all except responsible people.

7.4. Chimney

As modern systems are very efficient in recovering maximum heat, insulation is essential to prevent moisture in the waste gases condensing and causing chimney problems. Neglecting adequate chimney insulation in the long term, is a very expensive mistake. The main riskis an increased likelihood and severity of chimney fires. For small domestic appliances in recently built properties, modern building regulations should have specified an appropriate level of chimney insulation, but it is as well to check with the architect or builder.

Similar chimney problems can be caused by wet fuel. Always stick to the fuel moisture recommendations advised by the appliance manufacturer.

8. Conclusions

8.1. Woodfuel Advantages

Wood fuel, appropriately managed, can be a sustainable resource. For example ithas been estimated [17] that 4.5 ha. (c. 11 acres) of high yielding poplar, grown on a short rotation forestry (SRF) system with a 10 year rotation could sustainably supply a 50 kW woodchip burner/boiler.

Woodfuel is virtually 'carbon neutral'. For sustainably managed energy woodlands on balance the carbon dioxide, (a "greenhouse gas") released on burning is equal to that taken up by new growth [18] .

When efficiently run, woodfuel systems produce little smoke emissions.

Wood fuel production can be a cost-effectiveway of managing neglected woodlands [19] ,allowing for timber production and woodland and wildlife diversity. Alternative markets for small low-grade roundwood have current values of c. £16/m3 to £22/m3 delivered. Fuelwood is a potentially competitive market, (Appendix 1) but only in a local sense, the costs of haulage quickly outweighing economic benefits of production. Woodfuel, from a local woodland resource, helps to keep money circulating in the local economy and can be acompetitive heating solution.

For large domestic woodfuel heat requirements(> c.50kW) woodchip is probably the best solution. Although there are plenty of large woodlog appliances designed for infrequent stoking, the more automaticnature of woodchip is its biggest attraction.

The decision to opt for woodchip fuel is verymuch the individuals choice. Major considerations will be: -

  • the investment in supporting machinery
  • fueltype availability.

The 'overlap' in choice between woodchip and purely woodlog appliances runs from c.20kW to 75kW, (although multi-fuel systems, incorporating woodlog fuel, run to over 100kW).Woodfuel heat demands below 20kW would be for log fuel, and above 75 kW it is likely to be woodchip.

Modern pellet fuel systems hold considerable attraction for woodfuel use at the smallest end of the domestic range. Well-established burner units are available but as yet reliably consistent fuel supplies are a problem.

Woodfuel is less susceptible to fuel priceincrease than fossil fuels, especially if it comes from your own resource.

8.2. Wood fuel disadvantages

Compared to fossil fuels, more space is neededfor the installation. More space is also required for fuel storage at site. Exactly how much will depend on fuel delivery frequency and consumption. For most small-scale installations wood fuel deliveries are usually needed more frequently than an equivalent fossil fuel installation, for example a 20kW system could use up to 0.6m3 of woodchip per day. Improper woodchip storage can cause health and safety, and operational problems.

Compared to fossil fuel heating systems,installation costs are initially higher, although this can be 'clawed' back bylower running costs. Woodfuel installations should be considered as a long-terminvestment. The build quality of most modern equipment supports this strategy.

The cost of producing woodfuel can be high. As is the investment needed to learn the skills necessary for its safe andefficient use, and to gain the knowledge to be able to manage your resourcesustainably.

When purchasing woodfuel, there will be a needfor a continuing supply at a competitive price compared to heat derived from fossil fuels, and of a suitable quality for your particular system. Woodfuel quality must be suited to the appliance, otherwise smooth running and efficiency could be compromised.

Compared to fossil fuels a little more consumer time must be spent on the smooth running of the system. For example, making sure the hopper does not run empty or clog, stove cleaning and emptying the ashes. Judging when to switch over to fossil fuels for summer heat demands.

9. Quality woodfuel supply within the Dyfi Eco Partnership area

It is one of the aims of the Dyfi EcoPartnership to help co-ordinate local interest in woodfuel production, in order to achieve an optimal supply position. Your responses to the questionnaire at the end of this document would help in this assessment.

Potentially larger wood fuel users, especially those with their own woods resource, may wish to hold their own production machinery. It would then be in their interests to join in with any local supply quality designation. This would provide them with a contingency supply of fuelor a local market for any surplus.

10. Acknowledgements

Information contained in this document has been gained over many years from numerous U.K. & European Forest Industry contacts and sources, and from Technical Development Branch, (TDB) studies, often working with other UK organisations in partnership. Input from present and former colleagues within TDB is gratefully acknowledged.

Pictures used appear by facility of:

  • Fuelwood Harvesting, Abbey St Bathans, Duns,Berwickshire; Tel. 01361 840251; Fax.01361 840248.

  • Maskinfabrikken REKA A/S, Vestvej 7, DK-9600 Aars, Denmark; Tel. 0045 98624011; Fax. 0045 98624071.

  • CronspisenKakelugnsmakeri AB, Box 43, S-361 01 EMMABODA; Tel 0046 471 136 10.

  • A J Wells& Sons, Bishops Way, Newport, Isle of Wight PO30 5WS; Tel. 01983 527552.

  • SkandinaviskaPelletskaminer AB, Box 43, S-361 21 EMMABODA; Tel. 0046 471 110 88.

Several illustrations were also obtained in the course of other TDB studies and projects.

David H Jones, Forestry Commission, Forest Research, Technical Development Branch, Wales.

July 2001

Technical Development publications referred to, are available from Technical Development Branch, Ae Village, Dumfries DG1 1QB, Tel. 01387 860264/348

Appendix 1

'Realisable'fuel values

Fuel Type & cost

Calorific Value per unit

Net cost of 1 Gigajoule, (GJ)

Appliance Efficiency Range

Value of Heat Realized, per GJ.

Woodchip [20] @ 30%MC(wb) & £35/tonne delivered

12.7 GJ/t.


70 to 85%

£3.94 to £3.25

Woodlogs @ 20% MC(wb) & £50/tonne delivered

15 GJ/t.


50 to 85%

£6.66 to £3.91

Pelletised Woodfuel @ £70/tonne [21]



70 to 85%

£5.26 to £4.33

Anthracite Coal @ £184/tonne

27.2 GJ/t.


60 to 85%

£11.27 to £7.96

Mains Gas @ £0.131/m3

39.5 MJ/m3


60 to 90%

£5.53 to £3.69

L.P.Gas @ £248/tonne

46.3 GJ/t.


60 to 90%

£8.93 to £5.95

Domestic Heating oil @ £0.19/litre

35.7 MJ/Lit.


60 to 85%

£8.87 to £6.26

Appendix 2

Some Suppliers of Chip-fuelledEquipment

Type/Model & Range

Manufacturer or Agent (Contact No.)



20 - 150kW

Fuelwood Harvesting UK Tel: 01361 840251,

Fax; 01361 840248


Chip-Fired & Wood-waste

25 - 1000kW

Talbotts Heating UK

Tel; 01785 213366

Fax; 01785 56418



40 - 400kW

Borgstroms Platindustri AB

Tel; 0046 278 50705 Sweden

Fax; 0046 27851573



40 - 70kW

KV Varmeservice A/S Denmark

Tel; 0045 8694 3800

Fax; 0045 86943035


Chip-Fired Systems

100 - 5000kW

Söderhamn Eriksson Ltd
TEL: 00 44 (0)161 429 9437


Chip-Fired, Moving Grate

20 - 3500kW

Maskinfabikken REKA A/S

Tel; 0045 9862 4011 Denmark

Fax; 0045 98624071


Chip & Integrated Log-Fired

20 - 320kW

Froling, Heizkessel-Und Behalterbau Ges.m.b.H. Tel; 00437248 606 Austria

Fax; 0043 724862387



The purpose of this questionnaire is to be better able to judge local woodfuel interests. Anyone can register. You do not necessarily have to be an existing user or producer of woodfuel. We would wish to hear from anyone, whatever their interests in woodfuel, especially from those who consider that they might have a woodfuel resource that could be utilised locally.

Your Name & Address:

Is your interest in woodlogs, woodchip or pellets? Please say whether you own woodlands, (area & approx. age).

Are you interested in partial heating with woodfuel (i.e. in combination with another fuel),

If you know it, please give an indication of the size of your heat loading, (kW or Btu)

Alternatively, describe the size of theproperty you wish to heat by woodfuel:

(detached/semi-detached; number of bedrooms/ground floor rooms, how many radiators do you now have, approx.floor area, )

Please describe your present heating system,(eg. gas. oil etc.). Give a brief description of the heating boiler, and if possible the make & model number, (eg. "LPG, Worcester Combi240").

If you presently burn woodfuel say whether logs or chips, and describe the appliance, (eg.Woodlogs; Charnwood 6kW.).

Can you make an assessment of how much woodfuel storage space you have/could have?

Thank you for your help. Please return this paper to Ecodyfi; Ty Bro Ddyfi, Y Plas, Machynlleth SY20 8ER.

[1] The Federal Ministry of Science & Research.

[2] Feedback from wood-burning equipment suppliers in Wales.

[3] A. B. Swebo Flis& Energi, Boden, Sweden, 1995.

[4] Forestry Research, Technical development Branch; Chart 2 of Technical Note 29/96, "WoodBurning Central Heating Systems" by D.H.Jones.

[5] This standardised retail grading system has been produced by British Biogen. It will be periodically reviewed to ensure it continues to meet supplier and user needs and for harmonisation with forthcoming EU standards.

[6] This table is a summary of more comprehensive data published in Technical Note 9/98; by C Reynolds, ForestResearch, Technical Development Branch 1998, "Woodfuel Chipping; FieldTrials". TDB research into chip quality is ongoing and the above is a small part of current work.

[7] Equipment that is designed to operate on an alternative fossil fuel. Automatic switchover gives an appeal to certain lifestyles and confers 'green' credentials.

[8] If properly used, with doors closed and dry wood. Under 30% with doors open.

[9] If properly used these designs will extract and store large amounts of combustion heat, releasing it slowly over a long period afterwards.

[10] Thesestoves have a considerable aesthetic appeal, but soapstone or ceramic constructiongives a heavy weight.

[11] Where the fire gases rises and burns up through the new fuel, laid on top.

[12] A conclusion of Forestry Research Technical Note No. 34/98.

[13] Forestry Research Technical Note No. 14/97, (1997 data.).

[14] A 'standardhour' includes a factor of 1.446 for Rest and Other Work.

[15] H Pirinen,1998, Jysky Polytechnic, Finland.

[16] C Reynolds, Forestry Commission ResearchDivision, Technical Development Branch, Report No 9/98, "Woodfuelchipping; Field Trials".

[17] G R Drake-Brockman, Forestry Commission Research Division, Technical Development Branch, Report 9/96, "A desk study on the supply and use of wood fuel from newwoodlands".

[18] British Biogen,1999; "Heating with Wood".

[19] D. H. Jones, Forestry Commission Research Division, Technical Development Branch, Report 34/98, "Woodfuel production from small, undermanaged woodlands".

[20] Assumed to be from delimbed roundwood.

[ 21] Issue No. 22; Biomass Farmer and User; October 1999. Quoted at £65/tonne delivered to an UK port, (distribution charges would be added to this forconsumers).


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