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Frequently asked questions

Some of the more frequent queries submitted to woodenergy.ie are listed below.

What is energy, what is wood energy, and how can it be translated into typical household use?

In the natural sciences, energy can be described as the capacity of a system to do work. The SI unit of energy is the joule. Energy cannot be created or destroyed, but only converted from one form into another. The total energy of the universe always remains constant.

Wood biomass is a carbon neutral, renewable energy source. Wood is said to be an environmentally friendly form of heating since the combustion of a tree releases the same amount of carbon dioxide as is bound up by a growing tree.

Wood is made up of Carbon, Nitrogen and Oxygen. It has a typical calorific value of 19.2 GJ/tonne dry matter. One tonne of wood fuel with a moisture content of 45% has a calorific value of 9.42 giga joules.

Common forms of wood fuel are pellets, wood chip and firewood.

The typical house needs 14 m3 loose volume of wood chip, or 2200 kg of wood pellets, to replace 1000 litres of oil for heating and utilities.

 


What is the energy content of wood?

The energy content of wood depends mainly on the moisture content. Bone dry wood of any species has about the same calorific value of ca 19 GJ/oven dry tonne. Use the following formula to calculate the lower calorific value of wood:

GJ/tonne = 19.2 - (0.2164*MC), where MC is the moisture content in percent of total weight.

What is the moisture content of wood?

Freshly felled coniferous wood has a moisture content of between 55 and 60%, one summer seasoning reduces the moisture content by 10 to 15%. Freshly felled ash has a low moisture content of between 35 and 40%, other hardwood species have moisture content between 45 and 50%.


How is the moisture content expressed?

The moisture content of wood for energy is expressed as a percentage of total weight according to the following formula:

(wet weight-dry weight)/wet weight*100= MC in %


What is the moisture content of wood pellets?

Wood pellets have a normal moisture content of 8-10% of total weight. If the moisture content gets too high, pellets will start to disintegrate. Pellets have to be protected against contact with moisture, like rain, condensation etc. Normal air humidity will not harm the pellets.


I want to start pellet production - how do I begin?

The most important factor is to secure a long term supply of raw material used to produce the pellets. Remember that for every tonne of pellets 2 tonnes of wet sawdust (55% MC) are needed.

A normal pellet production plant has a capacity of at least 25.000 tonnes annually with one press.

All materials handling and preparation, like storage, drying, hammer-milling, cooling of the pellets, will cost about the same for one or two presses.

Handling these large volumes of materials (50.000 tonnes of wet sawdust in, 25.000 tonnes of pellets out) will require a good infrastructure.

See also COFORD Connects note: The production of wood pellets (pdf 738Kb) .


I want to heat my house with a pellet or wood chip boiler - where do I find more information?

Information on boilers is available from Sustainable Energy Ireland (www.sei.ie).

To store the fuel for a wood chip boiler, one needs much more room than for storage of wood pellets. Wood chips will weigh ca 200-250 kg/ m3 loose volume, wood pellets ca 650 kg/m3 loose volume.

What moisture content fuel do I need for my boiler?

For a domestic boiler (up to 100 kW) one needs a relatively dry fuel of 20-25% MC on total weight basis.

For a medium size boiler (100 kW to 1000 kW) one needs a moisture content of less than 35%.

Large boilers (>1 MW) can handle fuel up to 55 % or even 60% MC.

Variation of the moisture content between loads of fuel has to be small though.

What is the price of wood chip?

The price of wood chip depends on several factors such as:

- moisture content (the lower the MC, the higher the price)
- the volume bought (the larger the volume, the lower the price)
- the volume per load (small loads are more expensive than big loads)
- the method of delivery (tipped off is cheaper than blown into a silo)

Given the early stage of development of the wood chip sector, price guides are not yet available.

How is wood chip paid for?

In small deliveries one usually pays for the volume delivered and one has to rely on the supplier that the moisture content is as agreed. The volume is measured on the vehicle, since measuring a heap of chip is almost impossible.

For larger deliveries, the best way is to pay by the energy content. The chips are weighed on a weighbridge and a sample is taken to calculate the moisture content.

Payment by tonne alone is not recommended. The weight of chip not only depends on the moisture content but also on the tree species and how the chips are produced. Hardwood species have a much higher wight/volume ratio than coniferous species.

See also COFORD Connects note: Ordering and receiving wood chip fuel (pdf 122Kb) .

How are wood pellets delivered?

Wood pellets can be delivered in several ways:

- in bulk tipped off
- in bulk, blown into a silo
- in big bags of 1 tonne each
- in small bags on a pallet of ca 1 tonne

The usual delivery in bulk is 20 tonne tipped off or 3 tonnes blown in.

Big bags and pallets with small bags can be delivered individually.

See also COFORD Connects note: Delivery and storage of wood chip fuel (pdf 625Kb) .

Copyright COFORD 2006.