Wood pellets and wood chips in automatic combustion plant


This paper is the English version of the booklet with the same title published by the Centre for Biomass Technology, as part of the information campaign "Small Woodstoves and Wood Boilers......an information Campaign - FIRE AWAY" (1996).

The campaign was granted by the EU (contract no. 4.1030/A/94-121) and The Danish Energy Agency (contracts no. 51161/95-0018 and 51161/95-0054).

Text: Søren Houmøller,
Centre for Biomass Technology
(FORCE Technology, Gladsaxe Møllevej 15, DK 2860 Søborg, Denmark, phone +45 39 69 65 11, fax +45 39 69 60 02)


This booklet tells you about automatic combustion plant for burning wood pellets and wood chips as fuel.

Why use wood as a fuel?

If you scrap your oil-fired central heating unit in favor of an automatic one burning wood, you can save up to 20% on your heating bills when you take account of the cost of investment and running the unit. This is because pellets and wood-chips cost about half what oil fuel does. In addition to which, the Energy Agency grants up to 30% of the cost of the unit.

Woodburning units are eco-friendly. They only emit the same amount of the greenhouse gas CO2 as the tree absorbed when it was growing. So burning wood does not contribute to the greenhouse effect. Less timber is felled every year in Denmark than is planted. This means that our forests are getting bigger year by year, absorbing more CO2 than they emit.

What is an automatic combustion unit?
Manual boilerThe automatic boiler is connected to the central heating system in exactly the same way as an oil-fired one. The heat of combustion is transferred to water, which is heated up and carried round the house to the radiators. The automatic boiler thus supplies heat to all the radiators in the house, unlike a woodburning stove, which really only heats the room it is in.

Pellets and wood-chips are of a size and shape that make them ideal for automatic boilers, since they can be fed in directly from a bunker. This makes it much easier to stoke, since the bunker only needs filling up once or twice a week. In hand-fired units like woodburning boilers, one has to stoke up several times a day - though they are usually cheaper to buy than automatic ones.

Wood pellets
Wood pelletsWood pellets are a comparatively new and attractive form of fuel. Pellets are usually made out of waste (sawdust and wood shavings), and are used in large quantities by district heating works. The pellets are made in presses, and come out 1-3 cms long and about 1 cm wide. When you burn wood pellets, you are utilising an energy resource that would otherwise have gone to waste or been dumped in a landfill.

There are different kinds of pellets. Some manufacturers use a bonding agent to extend the life of the pellets; others make them without it. The bonder used often contains sulphur, which goes up the chimney on burning. Sulphate pollution contributes to acid rain and chimney corrosion, so it is best to buy pellets without a bonding agent.

You can collect the pellets from a merchant or manufacturer yourself, or have them delivered. If you fetch them yourself, they are cheapest sold loose into your own trailer. Plastic bags holding 25 to 40 Kg cost a bit more.

Pellet factories are quite irregularly distributed about the country, so it can be a long way to the nearest one. If you don't use very much, it might be an idea to have the pellets delivered by carrier, perhaps in bags stacked on pallets.

The cheapest way of having wood pellets delivered is in loose weight by tank lorry, for pouring straight into your bunker. The pellets are the same size as farmers use for fodder, so they can be handled in the same way. Assuming, of course, one has enough storage space.

Wood pellets have a low moisture content (under 10% by weight), giving them a higher combustion value than other wood fuels. The fact that they are pressed means they take up less space, so they have a higher volume energy (more energy per cubic yard).

Production of wood chipsWood-chips are made of waste wood from the forests. Trees have to be thinned to make room for commercial timber (beams, flooring, furniture). Wood-chips are thus a waste product of normal forestry operations.

Wood is cut up in mechanical chippers. The size and shape of the chips depends on the machine, but they are typically about a centimetre thick and 2 to 5 cms long. The water content of newly felled chips is usually about 50% by weight, but this drops considerably on drying.

Most of the chips currently produced are burnt in Denmark's 30 or so wood-chip fired district heating stations. They are usually delivered by road, so there must be facilities for storing at least 20 m3 of chips under cover if they are to be used in an automatic burner.

Various types of unit
Automatic furnaces come in three types:-

Compact units
Compact unit
In compact units the fuel is fed into the fire from the bunker by an automatic feeder. The rate at which fuel is fed in is determined by a thermostat, which puts less in when the water is hot and more in when it is cold.

Compact units are excellent for wood pellets, but not for wood-chips. This is due to the lower volume energy of chips, so that stoking has to be more frequent.

In addition, the water content of wood-chips is often so high that compact units do not combust them properly.

Stoker-fired units
Stoker fired unit

In stoker-fired units too, the fuel is automatically fed into the boiler. This is a helical con-veyor which conveys the fuel from the bunker to the boiler. The fuel is fed in at the bottom of the grate, where it burns. As in compact units, feed-in is thermostatically controlled.

Wood pellets are best for stoker-fired units, but chips can also be used if the unit is designed for them. The chips must not be too moist, so they need drying first. The best way of doing this is to leave the trees outside to dry until they are put through the chipper. Chips can also be dried under cover after being cut up.

If wood-chips are used, they need drying under cover for at least two months. They also need a lot of storage space.

Boilers with Pre-furnace
Boiler with pre-furnace

In the third type of unit most of the combustion takes place at high temperature in a pre-furnace. The pre-furnace is earthenware-lined, allowing high temperatures to be maintained. A pre-furnace-mounted boiler is therefore highly suitable for burning wet wood-chips. Heat comes in from the pre-furnace and is transferred to the water in the boiler. Any gases not combusted in the pre-furnace are burnt off in the boiler.

Boilers fitted with pre-furnace are designed for burning wood-chips. Some can also burn pellets, though others would be damaged by the heat generated by the dry fuel. Ask the manufacturer before buying.

It costs more to buy an automatic stoker unit than a hand-fired one, because there are more bits and pieces in it. You really need quite a lot of heat to make it worth buying an automatic instead of shovelling the stuff in yourself. You have to be burning the equivalent of at least 3,000 litres of oil a year before you should consider an automatic unit. If you use less, it is better to buy a hand-fired unit burning firewood and wood briquettes.

If you already have a boiler that works perfectly well and you are thinking of buying an automatic unit, the cheapest thing is to invest in a separate stoker. In Denmark this sort of thing costs about DKK 20-25,000 to install, including VAT.

If you want a compact unit, a stoked unit or a pre-furnace boiler, you will have to invest at least DKK 50,000, including VAT. Despite this a woodburning unit pays in the long run, because the saving on fuel is of the order of DKK 2,000 for each 1,000 litres of oil you replace.

All heating units must be installed in accordance with current legislation, to ensure compliance with fire regulations and Danish law in general (see booklet entitled "Bygningsreglement for småhuse" (Building legislation for single family homes.)).

Maintenance is very important, otherwise you risk chimney fires and carbon monoxide poisoning. As well as which, a properly maintained fire utilises fuel better and gives better value for money. The working life of the unit also depends on maintenance. See what it says in the instructions; do what they say, and keep the chimney swept.

Up to 30% in State subsidy for automatic stoker units

Burning wood makes sense. Wood pellets or chips come in at only about half the cost of oil. On the other hand it costs a bit more to buy and maintain an automatic feed boiler than it does for an ordinary oil-fired central heating unit.

An automatic unit costs from DKK 30,000 and up, including Danish VAT, not including subsidy. Installation costs money as well: typically of the order of DKK 20,000 including Danish VAT.

The Danish Energy Agency grants a subsidy of 30% for boilers approved for burning bio-fuels such as wood and straw, if installed in areas outside those served by natural gas and/or district heating. The grant depends on the useful effect of the boiler and its emissions of dust and carbon monoxide.

Lists of approved units, including prices and subsidies, are available from the Information Secretariat for Sustainable Energy, which up-dates the lists as new units are approved.

Subsidies are also available for existing boilers, for attaching e.g. wood-chip or pellet stokers.

If you don't have central heating, you will have to have radiators and a boiler room put in. Talk to your local central heating experts about this, and get offers to suit your requirements.

If you live in a house heated by electricity, new regulations for conversion to central heating came into force on Jan. 1, 1996. Ask at your local Energy and Environmental Office.

To find out more about the use of bio-fuels for heating, read:-

* Firewood and heating: the forests have it

* Correct firing: how to utilise your fuel better

* Open fires and woodburning stoves
Building regulations for single family homes

* Boilers burning firewood and wood briquettes

* Danish Standards for woodburning stoves

* Standard grants for sustainable energy

* Subsidies for small-scale bio-fuel units

* Bio-fuel boilers: type-approval licensed units entitled to subsidy

* Manufacturers of open fires and woodburning stoves

All the above material is available (in Danish) from:-
  EnergiOplysningen, Teknikerbyen 45, 2830 Virum, Denmark. Phone: (+45) 70218010 Fax: (+45) 70218011 www.energioplysningen.dk



 FORCE Technology. Last update 03-05-00