INTRODUCTION - WOOD PELLETS

As part of a desire to move towards renewable fuels, there has been increasing interest in the conversion of timber co-products (i.e. sawdust, woodchips, wood shavings and off-cuts) into fuel pellets.


Fuel pellets are a convenient, high density fuel that can be used in purpose built stoves and boilers. The supply of pellets made in the UK has been developing slowly, with focus tending to be on medium to large scale production plants. For the purposes of this information page we will focus on small scale production systems.

The pellet burning stove is inexpensive to install and cheap to run. Currently, wood pellets are only available from large producers. Having a small scale wood pelletiser on site would minimise transport costs.

Coed Cymru has been carrying out trials with a small, 10HP pellet mill that will convert wood co-products into fuel pellets. Sawdust is forced through holes in a die-ring to form cylindrical pellets. The process consists of obtaining the raw material, shredding and granulating the raw material, pelleting in the pellet mill, bagging and feeding into appropriate burners.

THE RAW MATERIAL - It is possible to produce pellets from a wide range of materials. The operational flexibility of the mill means that it can accommodate different species and moisture contents. It is, therefore, particularly suitable for many wood using SMEs.

Species: Pellets have been made from different hardwoods (oak, beech) and softwoods (spruce, pine) without bark. All resulting pellets will have the same density, calorific values and ash (iroko has a high ash content).

Moisture content: It is important that it is relatively dry, but the moisture content can vary between 10 to 20% by weight.

Form: Starting material can be sawdust, wood chip, shavings and off-cuts derived from solid wood.
GRANULATION - The raw material must be converted to a homogeneous consistency prior to feeding the pellet mill. We have been using a shredder/granulator manufactured by Dragon Machinery in Pencader, Carmarthenshire (01559 384303). Ours is the smallest in the range and will process up to 60kg/hour of shavings, less for off-cuts. There is a range of similar equipment on the market although it tends to cater for large quantities of material. In the past, hammer mills have been used and are often recommended to break down raw material for pelleting. We have found the Dragon Machinery equipment satisfactory for all materials tested particularly for off-cuts.

THE PELLET MILL This mill has a 10HP motor geared down to cope with wood. It is manufactured by Farm Feed Systems in Cinderford (01594 825106), who produce a range of small to medium sized mills. We have been using two dies, one producing pellets of 8mm (for pellet stoves and boilers), the other pellets of 11mm which burn in unmodified solid fuel stoves. The mill operates at constant speed. The feed system has been designed for experimental use: it is envisaged that a permanent installation will have a much larger feed hopper fed by the granulator with level monitoring and automated controls. The scale and degree of automation can be tailored to the client's needs.


There is a range of other pellet mills on the market, but most are large scale originally developed for producing animal feeds. The smallest mill, from SPC, Sweden is rated 250kg per hour but we have no experience of this although a few have been imported into the UK.

OPERATION OF THE MILL
The operating conditions can be varied to suit the material being pelleted. Options and variables include:

For the addition of powdered pelleting aids (binders) there is a separate hopper with controllable feed rate

Good pellets can be made by selecting and regulating these different elements. Hardwoods tend to need oil to lubricate the flow, very dry material may need water or steam to help binding. Material of high moisture content may also need oil. If consistent material is being processed and/or if a certain level of automation were added then we envisage that the mill will run unattended. The aim would be pellets of under 10% moisture and a density of greater than 0.6. The throughput will depend on the material, hardwood or softwood, dense or light. The maximum obtained with softwood (large die) was 180kg/hr although up to 100kg/hr would seem to be realistic. Some hardwoods may only achieve half this.
For further information please contact David Jenkins at Coed Cymru, 01686 650777
(OR see Staff Page for email addresses and officers' telephone numbers).

Economics
With mill consumables (replacement die, rollers) £6.50/tonne pellets, rough estimates of cost would be £25/tonne (a very low 30kg/hour) to £12/tonne (120kg/hour). This excludes the granulator, labour, capital, oil (if used) and any raw material cost. At 30/120kg/hour output, a pelleter running 6 hours per day for 200 days per year will produce 36 to 144 tonnes per year equivalent to £4,320 or £17,280 per year using a selling price for pellets of £120 per tonne. A fully automatic system should run considerably longer per day with a significantly uncreased throughput.

Throughput	Hours/days	Days/year	Annual output (tonnes)	Value at £120/tonne
36	6	200	36	£4,230
120	6	200	144	£17,280

Those considering the manufacture of pellets need to satisfy themselves that a mill will be viable. The material has to be relatively dry and in sufficient quantity. Generally, woodworking businesses producing a few hundred tonnes of dry shavings and sawdust a year will be good candidates, particularly where the material is free or represents a cost to the business. A number of farmers have diversified into timber processing and there will be slabwood that could be chipped and shredded for pellets, as long as sufficient time is allowed for drying.

The market for pellets is growing steadily with a number of larger installations appearing in public buildings. I suspect that this growth is constrained by the current supply of pellets, and the reliance on imported pellets. The potential producer should explore the potential market. For further information it would be worth contacting the National Energy Foundation (www.logpile.co.uk) 01908 665555 or the British Pellet Club (www.british-pellet-club.org.uk) to discuss the market possibilities.

For further information on small scale pelleting contact Andy Stewart at Coed Cymru 01686 650777 (andys@coedcymru.org.uk) or the manufacturers of the mill and granulator. Coed Cymru, The Old Sawmill, Tregynon, Powys SY16 4NA

Coed Cymru, The Old Sawmill,
Tregynon, Newtown,
Powys SY16 3PL
Tel 01686 650 777

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