Bioenergy frequently asked questions (FAQs)
Welcome to our newly added FAQs section. We will be adding to this knowledge base over-time, so if your visiting our site and you need to find out something quick, be sure to re-visit this page, (better still, add it to your bookmarks).
How much energy/fuel can I get from wood?
Wood can be converted into a number of different fuel and energy forms. The values in the table below provide broad estimates of the likely amounts of energy, or fuel, which can be delivered from a cubic metre of solid wood.
One cubic metre of solid wood at 50% m.c (wet basis) can create:
|
Heat |
6.6 GJ @ 80% eff |
|
Electricity |
1.64GJ or 455kWh @ 20% eff |
|
Heat & Electricity via CHP1 |
5.7 GJ heat (60% eff) and 228 kWh electricity (10% eff) |
|
Biodiesel (31 MJ/l) |
100 (90 to 106) litres* |
|
Ethanol (24 MJ/l) |
148 (140 to 152) litres* |
|
Bio-oil (21 MJ/l) |
300 (270 to 330) litres* |
|
Wood gas (5MJ/m³) |
1130m³ (requires drying to 15% m.c. wet basis) |
*assumes 1 cubic metre contains 0.440 oven-dry tonnes of wood
In comparison diesel has a calorific value of 39 MJ/l and natural gas is 36MJ/m2. These values are based on Pinus radiata, as it is the most commonly available wood in New Zealand.
For the heat content at other moisture contents use the calorific value calculator
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What are the greenhouse gas, or CO2, emissions from burning wood?
The short answer is zero. If the wood is from a sustainably managed plantation forest, the CO2 which is released when wood is burnt is reabsorbed by the next rotation of trees, so burning biomass for energy is regarded as carbon neutral. In reality, there is an expenditure of energy (often fossil fuels) in growing, transporting, and processing biomass (this has been calculated for wood pellets at this link ). link
You can work out the greenhouse gas emissions of burning fossil fuels by using the CO2 emission calculator.
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What do I need to know to burn firewood effectively?
To burn firewood cleanly and effectively requires just a few steps:
- Always burn dry firewood: The magic number is approximately 20% moisture content (wet basis). It’s not that simple to work this out but, as a rule of thumb, you should collect your firewood and let it season (dry) for at least 12 months before burning.
- Firewood needs to be the correct size for the fireplace: Smaller pieces are better and try to avoid over‑filling the fireplace with wood, as this restricts the airflow and slows down the combustion rate.
- Do not dampen the fireplace down: The more modern fireplaces have had the dampener feature removed. When the airflow into the fireplace is restricted it creates very poor combustion characteristics which will, in turn, create very high air emission (creating smog). It will also increase the creosote build up in the chimney and this could ultimately lead to a chimney fire.
To dry your firewood you will need to first split the wood into smaller pieces to increase the surface area of the pieces to be dried. This will reduce the drying time. It is best to stack the wood in rows, with enough room between them for the air to flow freely. The more airflow you have, the faster the wood will dry. For most tree species it is a good idea to give your firewood some shelter. Sheets of corrugated iron held on top of the rows of firewood work well.
Please read this wood guide for more information
Useful links
Summer is the best time to buy firewood, find out why at this link.
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What are the advantages of using wood pellets?
Unlike gas, coal, and other fossil fuels, wood pellets are a renewable carbon‑neutral fuel. They are made by compressing waste sawdust and shavings from wood processing operations that use wood from New Zealand plantation forests.
Air emissions from the combustion of wood pellets are very low compared with those from traditional log/wood burners. The air emissions from burning wood pellets are comparable with diesel heaters (although higher than gas). Wood pellet burners are therefore very suitable technologies for heating in urban areas.
The combustion system for a wood pellet fire is much more advanced than a typical fireplace. The wood pellets are fed in automatically and the feed rate varies with the required heat output. Wood pellet fuel is very consistent in quality so that a pellet fire can adjust the air to match the fuel, making sure that the wood is completely combusted even on low settings. This leads to an increase in efficiency and lower emissions.
Compared to coal, which is often used in commercial and industrial installations, wood pellets have 3 distinct advantages:
- lower air emissions
- cleaner fuel handling
- less ash for disposal.
Did you know - the ash from wood pellets can simply be spread on the garden as fertiliser.
A useful link for more info.
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How are wood pellets made?
A wood pellet is a wood ‑based heating fuel. Basically the wood pellet is made up of lots of smaller pieces of wood that is a by‑product of the wood processing industry, e.g., wood shavings and sawdust.
A wood pellet is a very high‑quality fuel, which means that the material going into the manufacturing process is also very clean. No soils, rocks, or bark.
There are several steps to producing wood pellets.
- Particle size. The material needs to be a particular size. To get larger material reduced in size it is run through a hammer mill, which thrashes the material into a fine, uniform particle size.
- Drying. The material needs to be very dry. 10% moisture content (wet basis) is fairly common. There are several ways to dry wood fibre and the heat required for drying can also come from many sources. Dry wood shavings are already in the correct moisture range and may not require any additional drying.
- Pelletising. The soft, dry material is then extruded at very high pressure through a ring die, which has thousands of very small holes drilled through the circumference. The wood fibre will flow like a liquid through these holes and the lignin, which is the wood’s natural glue, binds the wood pellet together.
- Cooling and sieving. The pellets are usually dropped through a cooling unit which removes the heat that is picked up during the extrusion process. There is also a sieve that removes the fines, or smaller pieces, that are not bound to the pellets. Bagging and distribution. The pellets are either bagged in 20kg bags for domestic use or stored in silos for the larger user where bulk deliveries are required.
- Bagging and distribution. The pellets are either bagged in 20kg bags for domestic use or stored in silos for the larger user where bulk deliveries are required.
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What is the energy yield ratio of wood pellet manufacturing?
The energy yield ratio for production of wood pellets (i.e. the ratio of energy input to processing and the energy value of the produced pellets) depends on a number of factors, the most important being the moisture content of the feedstock as wet feed stock requires drying. Most parts of the pellet manufacturing process require electrical energy for running hydraulic pumps, hammer mills, and blowers.
If the feedstock material for producing wood pellets is dry, for example dry shavings, then the energy consumption for producing the wood pellets will be approximately 5% of the fuel value of the pellet. If the feedstock material is wet, for example wet sawdust, then the energy consumption for producing the wood pellets will increase to 14% of the fuel value of the pellet. Hence the energy yield ratio is 20:1 using dry material and 7:1 when using wet material.
- Example 1: To produce 1 tonne of wood pellets from dry shavings requires approx 250kWh’s of electrical energy. One tonne of wood pellets contains 5000kWh’s of energy. For every 1 unit of energy put in there are 20 units out. This gives an energy yield ratio of 20:1, or 5%.
- Example 2: To produce 1 tonne of wood pellets from wet sawdust (50% m.c), requires 300kWh’s of electrical energy and 400kWh’s of heat energy. One tonne of wood pellets contains 5000kWh’s of energy. For every 1 unit of energy in there are 7 units out. This gives an energy yield ratio of 7:1 or 14%.
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Where can I get wood pellets?Typically, manufacturers sell wood pellets by the 10 or 20kg bag or in bulk.
Bulk can be by the 1 tonne bag or by the truck load. In the near future we should see conveyer trucks and blower trucks for bulk pellet delivery
The following is a list of the wood pellet manufacturers in New Zealand:
Natures Flame
PO-Box 11-259
Christchurch
Tel: 03-342-9920
Physical address of mill:
Hoskins Rd, Rolleston
Firelogs (NZ) LtdPO Box 2401
Stortford Lodge,
Hastings
Tel: 06-879-8151
Physical address of mill:
Manchester Street
Natures FlamePO-Box 11-259
Christchurch
Tel: 0800 735538 (pellet)
Physical address of mill:
Vaughan Rd, Rotorua
Wood Pellet FuelsPhysical address of mill:
364 Lumsden Rd
Ohinewai
Huntly
Tel: 09-2360075
Wood pellets are also sold by retailers. Some retailing chains, such as the
BBQ Factory, sell wood pellets, but this varies from store to store.
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Can my coal boiler be converted to wood pellets?
In simple terms, yes! Most coal boilers have the basic setup to operate on wood pellets. The feed system is easily switched over to wood pellets. The boiler itself will sometimes require a greater level of modification to burn the pellets in the most effective way possible. A simple scenario is to introduce an over‑fire air supply which burns off the remaining volatiles that create the poor emissions (smog).
One major issue that needs to be addressed is the weather‑proofing of the fuel storage area. Wood pellets must be kept very dry or they will expand and lose their pellet form.
There are a number of coal boilers in schools in New Zealand that have already been converted to wood pellets. Most of these conversions also include a degree of automation that reduces the time needed to attend to the operation of the boiler. This new fuel also produces one‑tenth of the ash that coal produces. The new wood‑based ash is also compost-able and can be spread on the garden.
For more detailed information...click here.
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What can I do with my forest residues?
There are many uses for leftover forest residue. Forest residue refers to the woody material that is left on the ground after log harvesting. This can be in the form of full, tree‑length stems, small off-cuts after trimming, and branches after delimbing or even the tree roots themselves if they are extracted.
The process of extracting these residues is quite simple. They are usually collected and left in a pile to dry. This can also assist in removing some of the residual soil from the woody residue. There are several types of machine that can be used to turn this material into a hog fuel, and some of these machines have contamination removal devices installed. In most cases, the material is loaded into the hogger with a grapple on an excavator base. Chip freighters, or bin‑wood trucks, are commonly used to transport the hog fuel to the heat plant.
If you are on a site that needs forest residues removed please contact one of the following companies for further information.
|
Contractor |
Contact |
Phone |
Location |
Details |
|
Central Wood Recyclers |
Noel Richmond |
(0274)813140 |
Tokoroa |
Hogging/energy contractor |
|
Ernslaw Bio-Energy
|
Murray Cowan
|
(021) 393 141
|
Central Otago
|
Bioenergy supplier
|
|
Materials Processing |
Peter Frederickson |
(07)8230086 |
Hamilton |
Energy contractor |
|
Plateau Bark |
Terry Robinson |
(027)2709647 |
Kawerau |
Energy contractor |
|
WastePro |
Theo Greyling
|
(07)3490940 |
Waipa |
Energy contractor |
|
Wholesale Landscapes |
Ferg Brewerton |
(03)5475300 |
Nelson |
Energy contractor |
back to top...What is a good industrial grade wood fuel?
The recovery of wood-waste from the forest is becoming more common. As the cost of fossil fuels rise the recovery of forest residues is becoming more attractive. There are several operations throughout New Zealand which are collecting, chipping, and transporting woody‑based fuels.
The types of material that can be collected for processing can be quite varied. These can range from suitable recycled building materials, waste panel‑board material, skid‑site waste, and thinning, over-sized pulp logs, sawmill off-cuts, dairy conversion waste, etc.
These materials are put through a chipper/hogger to produce a usable woody‑based fuel. However, not all these fuels are of the same value. There are several key components that make up a high‑quality fuel.
- Low fuel contamination: This means keeping the fuel clean. Reducing contact with soil is paramount.
- Low moisture content: The drier the material the higher the calorific value. Most basic boiler designs work best with a drier fuel.
- Particle size and distribution: This is also very important: Specific boilers require specific fuel dimensions. Most feed/conveyer systems are size‑distribution specific.
The current system for purchasing wood‑based fuels is commonly based on the tonne, or cubic metre, of product. This system does not include any of the above three principles. In the future there is likely to be a demand for a purchasing system which relates to the fuel value and effectiveness of the fuel, not just its weight.
For more information...click here
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