We can help you decide on the most appropriate wood fuel heating system from wood chip, wood pellet and log systems for you, its location and most importantly your heating requirements.
Wood fuel heating systems are best suited to providing a continuous heat load and burn most cleanly and efficiently when working at their maximum output. There are three main fuels types; wood chip, wood pellets and logs. Wood chip is most commonly used for commercial use because of the amount of space required for the equipment and storage of the fuel. Where as wood pellets are generally used in domestic properties, as wood pellet systems and storage take up less space.
Therefore, it is best not to over-specify a wood fuel boiler but to choose one which is sized to meet your average heating requirements with additional heating sources to provide extra heat on the coldest days. A good way to achieve optimum firing is to use an accumulator tank which allows the boiler to operate at maximum output to heat a large reservoir of water that is stored in an insulated container until required.
Unlike oil and gas, wood, in whatever form, does not achieve an optimum burn instantaneously and wood fuel boilers are not designed to continually cycle on and off. Therefore, if a continuous, low-level heat output is routinely required biomass may be the best choice.
80% of the whole life cost of a heating system is the fuel, and the availability of an affordable, secure supply of appropriate wood fuel nearby (preferably within 20-30 miles) is most important. This should be integral to the planning of any wood fuel heating system, and the equipment must be specified accordingly.
The storage of wood fuel must be well designed and constructed in order to keep the fuel in a good condition, particularly protecting it from moisture. It must also be possible to deliver the fuel to the boiler conveniently and efficiently. Each type of wood fuel therefore requires a different design of store. For further information, see the individual sections on types of wood fuel.
Wood chip is a uniform fuel that can flow and 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 it can also be burned very efficiently.
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 chip is the cheapest form of wood fuel suitable for automated boilers. Please contact us to enquire about our wood chip prices and delivery areas.
The characteristics of wood chips will depend both on the chipper and the material from which they are made. They can be divided into Forest Chips or Stump Chips.
Depending on the equipment wood chip is 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.
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.
Please contact us to enquire about our engineering services; design, installation, servicing and maintenance.
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.
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 produced and/ or imported in to the UK.
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).
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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:
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 currently the main method for the delivery of pellets to domestic boilers in the UK. Purchasing by the bag is the most convenient way of storing pellets to be used in a pellet stove.
Visit our online store to purchase bagged wood pellets and please contact us to enquire regarding bulk wood pellet deliveries. 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.
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.
An efficient alternative to an open fire 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. Please contact us to enquire about our engineering services; design, installation, servicing and maintenance.
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 but can sometimes be difficult to burn, so may be worth combining with another type of wood. Softwoods are easy to light and burn quickly. 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.
Logs are usually produced in the forest from small round wood and cut into logs of typically 2-3 m in length. These may be stacked for subsequent collection following a period of drying. On average 1 m3 of round wood requires 1 linear metre of space.
To make a good fuel, wood needs to be dried or seasoned to reduce the moisture content, as trying to burn wet wood will produce steam, less heat (as so much of it is being used to dry the wood), cause problems with the chimney and create 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 around 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. 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.
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.
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