Different types of wood explained
Three main types of wood fuel are best suited for heating & hot water – logs, wood chip and wood pellets. Charcoal is also a wood fuel, used primarily for cooking on barbecues. All these wood fuel types are produced locally in the Surrey Hills AONB.
Are a uniform fuel that can be fed to a boiler, gasifier or other conversion system as a steady flow using an auger feed or a conveyor. With a large surface area to volume ratio they can also be burned very efficiently.
The characteristics of wood chips will depend both on the chipper and the material from which they are made. They can be divided into the following groups:
- Forest chips – including:
- Log chips – from delimbed stem wood
- Whole tree chips – from all the above-ground biomass of a tree
- Logging residue chips – from branches, brash, etc
- Stump chips – from stumps
- Wood residue chips – from untreated wood residues, recycled wood and off-cuts
- Sawing residue chips – from sawmill residues
- Short rotation coppice / short rotation forestry chips – from the respective energy crops
Depending on the equipment they are to be used with, wood chips typically have a longest dimension from 20-50 mm, though larger chips (known as hog fuel), and chunks can be 100 mm or more. Long thin pieces (slivers) amongst the chips should be avoided as they can cause bridging and blockages in a chip feed system.
Wood chips should be stored under cover to prevent wetting, however good airflow is necessary to disperse water vapour and minimize the chance of composting and mould formation.
In addition, stack height should be kept below 10 m to prevent heat build up from composting and spontaneous combustion.
Wood chips may have a bulk energy density of about 50% of that of the solid wood.
Wood chips for energy applications should meet an appropriate quality standard if they are to be used reliably in combustion equipment, especially small scale and domestic equipment.
Physical parameters, such as maximum size and absence of slivers or fines (sawdust), and maximum moisture content are important to allow reliable operation and prevent feed blockages. Also levels of contaminants and ash content will have an impact on emissions and maintenance schedules.
The upcoming European standard for solid biomass, including wood chips, is being drafted by CEN/TC 335 that allows all of these parameters, and acceptable ranges, to be defined.
As with wood pellets modern wood chip boilers can provide a high level of automation and convenience for wood fuelled space heating. Wood chip systems generally have an output of greater than 20 kW (suitable for a large farmhouse or larger) and are not cost effective or appropriate for typical domestic scale applications.
Extensive fuel handling systems and fuel storage facilities are required for automated operation. It is important to be able to source a steady supply of woodchip with a consistent size and moisture content suitable for burning in a boiler, as not all chips are suitable for burning.
Wood chip is the cheapest form of wood fuel suitable for automated boilers. The cost of heating with wood chip is typically between 2-3p per kWh; to see how this compares with other fuels see fuel costs.
There are a growing number of suppliers of fuel grade wood chip in the UK, including several based in Surrey.
Wood pellets are made of highly compressed waste sawdust by-product. They can be used to produce heat in a specially designed stove or boiler. In addition, some existing solid fuel and oil boilers can be converted to make use of wood pellets.
Due to their low moisture content (about 6% to 10%) pellets have a high energy content, similar to high quality coal. Only minor energy losses are experienced burning off the moisture content.
The use of wood pellets for heating is well established in countries such as North America, Sweden, Austria and Denmark and there is a rapidly growing pellet industry in the UK. Pellets are now produced and/ or imported by a number of companies in the UK.
Wood pellets for fuel are mostly manufactured from waste products from sawmills and other wood processing industries. The materials used include ground woodchips produced from off cuts and waste wood and dry sawdust by-product. No chemical additives are needed, the natural lignin of the wood itself serving as a binder. With smaller pellet presses, which cannot achieve the same degree of pressure as larger ones, small quantities of natural binders are added as well.
Pellet equipment sales in the UK are increasing and tied to the rate of growth is the price and availability of pellet supply. There is now an emerging wood pellet fuel industry in parts of the UK-focussed around pellet production. There are a growing number of medium and small size plants now starting production.
There are currently three main routes to setting up pellet production in the UK.
- Purpose built ‘large scale’ wood pellet facility
Large scale pellet production plants may only be appropriate in a limited number of situations in the UK due to variations in local supplies of biomass material suitable for pelletising. Reliable supplies of local, good quality sawdust or other waste wood suitable for processing are essential.
- Conversion of animal feed or grass mills
This option represents a relatively low cost way to set up pellet production since most of the machinery and auxiliary equipment (including driers) already exists. Grass has similar characteristics to wood for pelletisation and as grass mills are used only from April to October they offer a good opportunity for wood pellet production during the winter.Other advantages to this approach are that it would need a relatively low capital cost – especially as production of wood pellets could start modestly and increase as market demand grows. Some disadvantages could be that grass/animal feed pelletising equipment has a lower output rate than machinery specially designed to pelletise wood. Modifications to method and material handling would be needed.
- Small scale pellet machines
The advantages of this approach to pellet production are that it allows for relatively low capital cost, there is a fairly low power requirement (50kVA supply) and there is a short project set up period. Delivery is 10-12 weeks, commissioning 2-6 weeks. Total project 12-18 weeks from order. The typical cost for fully installed and commissioned system ‘pellet factory’ is between £75k-95k depending on options. Typical production output is 250-300kg per hour, and often it is possible to run the plant on 24 hour shifts, giving a capacity of 1,500 – 2,000 tonnes per annum.With small scale pellet production it is easier to find a supply of raw material locally and to supply pellets more locally, keeping both production and transport costs down and ensuring the least environmental impact.
Being manufactured to a consistent size (usually about 2cm long with a diameter of 6 or 8mm), low moisture content and high density means that wood pellets can be used in automatic clean-burn heating appliances. It also means that the boiler response time is fast and the technology is controllable without increasing the load on the environment.
This and the fact that they are clean and easy to handle make them particularly suitable for domestic use. Being compressed also means that they take up less room than other forms of wood fuel.
Pellets have the following advantages over other types of wood fuel:
- Less volume to transport and store (due to higher energy density)
- Fewer deliveries
- Consistent size and moisture content
- Versatility – can be used in stoves and boilers
- Less ash and emissions
- Pellets are dry and can be stored without degrading
- Flow like a liquid and can be used in automatic machinery
- Easier to handle
- Easier to ignite
The best solution for delivering pellets to a pellet boiler, allowing for easy and convenient pellet handling, is to install a pellet store designed to receive pellets delivered by bulk tanker. The pellet store can be built either outside the house or inside and needs only slightly more space than the equivalent oil or LPG tank if deliveries are scheduled regularly. The pellets are transported to the boiler via special feed systems consisting either of an auger inside a tube or a vacuum transfer system.
The alternative system for the storage of pellets is to buy them by the bag. Although this is more expensive than purchasing pellets in bulk, it is the main method for the delivery of pellets to domestic boilers in the UK at the present time. Purchasing by the bag is the most convenient way of storing pellets to be used in a pellet stove.
Wood pellets are burned in specially designed pellet boilers and stoves, many brands of which are available in the UK – the most well established being the Swedish, Danish and Austrian makes (there are very few UK manufacturers of wood pellet systems). Using a ‘pellet basket’ pellets can also be burnt directly in your existing hearth or solid fuel burner, or even used outside in the garden.
The growth of the wood pellet market will be relative to customer confidence in availability and quality of supply. A pan European standard CEN/TC 335 for biomass is also being developed, under which the fuel specifications and classes for all solid biofuels are set out in CEN/TS 14961:2005.
These are the specifications which customers should look for when buying premium wood pellets:
- Diameter between 6 & 25mm & length = 5 x diameter
- Ash content =0.7%
- Moisture content below 10%
- Calorific value > 4.7kWh/kg (net)
- Bulk density > 600kg/m
- Nitrogen =0.3%
- Sulphur content < 0.08%
- Chlorine < 0.03%
- Additives < 2% natural (from agricultural and forestry biomass, unmodified).
- Durability =97.5% (i.e. less than 2.5% break off as fines under stress-testing)
- Fines =1.0% (at the factory gate)
The cost of wood pellets fluctuates according to supply and demand and such factors as the availability of sawdust, the cost of importing and delivery costs. However the cost of heating with pellets is usually on a par with or better than the cost of oil, LPG or mains gas. As the price of these fossil fuels increase, so too do the economic benefits of using wood pellets. For further information on the cost of wood fuel see fuel costs.
Pellets can be delivered in one tonne loads in sacks of 10- 20kg on a pallet; in bulk bags of up to 1,250kg; or in bulk by blower lorry, in the same way as oil. Pellets can be collected by the bag (more common for pellet stoves as opposed to boilers). Pellets bought by the bag are more expensive and more handling is required.
Logs are produced in many places in the Surrey Hills. To find out more about logs as a fuel source choose from the following:
Logs are usually produced from small round wood, which is delimbed, often in the forest, and cut into logs of typically 2-3 m in length. These may be stacked at ‘rideside’ for subsequent collection following a period of drying. On average 1 m3 of roundwood requires 1 linear metre of space.
To make a good fuel wood needs to be dried or seasoned to reduce its moisture content, which in freshly felled wood constitutes between 35% and 60% of the weight.
Trying to burn wet wood will produce steam, less heat (as so much of it is being used to dry the wood) problems with the chimney (see below) and pollution.
Wood felled during one winter should be seasoned until the next before it is burned. Trees felled during the Spring/Summer will have a very high moisture content compared to those felled in late Autumn/Winter, therefore whilst a log first cut in January may be ready to burn within say, a year, it is necessary for a log cut in May to be seasoned for at least two years. Scoring or partial removal of bark helps to accelerate drying, as does splitting logs over 15 cm in diameter.
Whilst seasoning logs should be stored under cover in an airy place such as an open sided lean-to. If well stacked, a pile of round and split logs can show a bulk density 70% of that of the solid wood, though if loose this can drop to only 40% or less.
Wood should be burned when the moisture content is below 25% – ‘air-dry’ – the bark will come away easily in the hand and the log will have splits across the grain. Logs to be burned in an open fire or log burner should be around 15-50 cm long (25-30 cm is the optimum) and split if greater than 10 cm diameter.
In terms of what type of wood to burn it is worth bearing in mind the heavier and therefore denser the wood, the higher its calorific value and therefore the longer it will burn. Hardwoods tend to be denser than softwoods such as pine and spruce and some of the densest are oak and beech.
However, some of the very dense hardwoods like oak and elm can be difficult to burn, so it is usually best to burn them with another type of wood as well. Softwoods tend to be easy to light and to burn quickly (making them good kindling). Some species like spruce and horse chestnut spit badly making them a hazard in an open fire. Some of the best types of wood for burning are ash, beech, hornbeam, hawthorn, crab apple and wild cherry.
Like other fuels, wood needs plenty of air to burn well. It is best to allow a fresh change of logs to burn freely until they almost turn to charcoal and only then to ‘damp’ down the fire by reducing the air supply. Filling a stove with logs and damping it down straight away, stops the ‘volatiles’ from burning and generates a lot of smoke and tar that is bad for health and the environment.
To produce heat for one or more rooms logs can be burned on an open fire. These look nice, but tend to have low efficiencies – about 80-85% of the heat goes up the chimney.
A more efficient alternative is a wood burning stove; modern stoves have efficiencies in excess of 70%. With the right stoves wood can be burned in a smokeless zone. Some stoves can also be fitted with back boilers to heat one or more radiators or domestic hot water.
Logs can also be burned in a ceramic stove. These have extensive internal flues and can retain heat for up to 24 hours after the last firing. They can be around 90% efficient and some can be used in smokeless zones. Another option is a range. Ranges can be used for cooking, hot water and central heating.
There are also many domestic scale log central heating boilers available. Using wood to run the central heating system is often a cheaper option although the initial cost of the boiler will be more than a conventional one.
Although the price of logs has increased considerably in the last few years it is still one of the least expensive forms of heating available, especially when burnt in a modern, efficient wood fuel burner.
At the time of writing (January 2009) the average UK cost of a stacked cubic metre of seasoned hardwood was £70-80. The energy density of well seasoned hardwood is 4 kWh per kg, which equates to an energy cost of approximately 4.5-5 pence per kWh. This compares favourably with the present cost of bulk deliveries of other fuel types but using logs is a much more manual process. To see how this compares with other fuels see fuel costs.
Suppliers of wood should be questioned to ascertain:
- That the wood is from a sustainable source
- It has been seasoned and for how long
- The type of wood being sold
- Whether the price quoted includes delivery
- The length and general size
- Quantity by volume or by weight
- Some suppliers can specify what species of wood they are selling although often, especially in the case of wood sold by tree surgeons, it will be a mixed load containing soft and hardwoods.
Although competitive for small scale users with stoves and wood fired cookers, buying in loads of wood is still relatively expensive for a large (boiler) user. It is much cheaper to buy unprocessed wood in bulk, at typically a quarter of the above cost, giving an energy cost of approximately 1 pence per kWh.
This requires work to cut, split and stack the wood and time and space for seasoning but despite the extra effort required this is likely to be the cheapest way to heat a rural home in the UK. It also enables the user to ensure the proper seasoning of their wood. Farmers and landowners who manage larger areas of woodland have even greater potential to reduce energy costs making wood-firing a highly attractive option.