The most prevalent type of biomass fuel in Scotland & the UK is woody biomass (ideally sourced from local forestry) or energy crops such as willow coppice. Waste wood by-products like forestry residues or sawdust from sawmills can also be used, subject to quality.
The most common and convenient forms of wood biomass for domestic heating are wood chips, wood pellets and logs.
Wood chips are small pieces of wood which have been part seasoned to reduce the moisture content. Wood chips contain more moisture than wood pellets and have a lower bulk density; therefore they require a larger storage area than pellets.
Small wood chip boilers require high quality chips with low moisture content and a well-defined size. These are best obtained using air dried feedstock and a chipping machine designed specifically for wood chip production. It is also important to ensure that the wood chipper’s blades are kept sharp in order to achieve a consistently sized wood chip. The accompanying animation demonstrates the chipping process of hard wood stock from factory & sawmill offcuts.
Wood pellets are usually formed by compacting sawdust, such as from sawmills, using equipment that forces the saw-dust through a die under pressure, forcing the lignin in the wood to liquefy. When the pellet begins to cool, this lignin sets; binding the dense saw-dust together. These small, regularly shaped pellets facilitate automatic and regulated fuel delivery for wood pellet biomass boilers and can be less prone to clogging than wood chip feeding systems. Wood chip biomass pellets are also less prone to fungal growth than wood chips due to the lower moisture content but they must be kept dry or will absorb moisture, break down and swell causing blockages and damage.
The industrial process required to manufacture wood pellets results in a higher price per unit of energy.
There are different quality pellets for domestic and commercial use.
Traditional logs are generally only suitable for manually fed appliances and require a sufficient area for storage due to the longer period of seasoning required. All woods burn better when well seasoned and some burn better when split. Different species of wood burn slightly differently which can be significant in your purchasing decision, although contrary to popular opinion, all wood species actually have a similar calorific value per unit weight (provided the moisture content is the same). The key difference is moisture content when felled (high in alder, low in ash).
Biomass briquettes/compressed logs
Wood briquettes & compressed logs are reconstituted wood fuel, similar to pellets but comparable to log fuel in size. However their size means that they must be burn on the same appliances and installation systems as logs.
Choosing the right type of wood fuel
Your choice of fuel depends on a number of factors. For example, pellets often suit customers with limited storage space and a need for a fully automated system, whilst logs are ideal for a rural setting where people want to use their own timber resource. Key factors to consider include:
- Availability – local sources, on site, reliability of supply
- Cost, including processing and delivery
- Convenience and potential for automation
- Storage space availability
- Cost of combustion equipment and fuel handling
|Wood chips||Easy to produce locally from woodland thinnings etc.
Cheaper fuel costs
Market development can benefit immediate local economy
|Can require greater user input, depending on quality of fuel
System has higher capital cost due to larger storage capacity and robust fuel handling equipment
Need a specialist chipper
Most suitable for larger installations e.g. over 25kW output
|Wood pellets||User input reduced to basic cleaning and ash removal (this can be automated to some degree in many models)
Cheaper capital costs due to the drier and more homogeneous nature of the fuel
Denser fuel means reduced storage space and easier transport
Suitable for small to medium appliances eg under 300kW
|More expensive fuel costs
Supply is less likely to be localised and so may not provide local economic impacts
|Logs||Logs can be stored and transported conveniently when stacked
Ease of air passage through a log pile allows good drying compared with chip
Can be effectively produced on site or very locally
|Less suitable for automated systems (although some do exist)
Large storage space required to allow 1 – 2 years for seasoning
|Briquettes||Denser fuel means reduced storage space and easier transport
Less preparation than logs
Cheaper capital costs due to the drier and more homogeneous nature of the fuel
|More expensive fuel costs
Supply is less likely to be localised and so does not provide local economic impacts
Less suitable for automated systems (although some do exist)
Must be kept very dry in storage
Moisture content of fuel
Freshly felled wood may have a moisture content of 40% – 60%, depending on the species of tree. The useful energy available varies with the moisture inherent in the wood. Ideally, the wood should be as dry as possible, as water contributes nothing to the stored energy of the fuel and will reduce its useful energy.
Graph showing the relationship between moisture content and calorific value of soft wood
The moisture content influences the combustion process. Wet wood needs longer residence time within the furnace for drying before combustion takes place, which means a larger grate is required. Energy from the fire bed is used to drive off the moisture therefore less energy is available for space heating. In addition, high percentages of water vapour contained in flue gasses will result in excessive condensation in the flue system wherever the flue gasses come into contact with cool flue surfaces. Such instances can lead to the creation of a ‘creosote’ deposit within the chimney.
Moisture is also a factor in transport costs. Since one tonne of wet wood costs the same as one tonne of dry wood, it is preferable to transport as little water as possible. The consistency of moisture content is important for efficient appliance operation.
Different appliances can tolerate different ranges of moisture content. Generally, larger appliances are less sensitive to changes in moisture levels whilst domestic size appliances often have stricter limits on moisture. Appliance manufacturer’s guidelines should always be followed; typically this is less than 25% for logs and less than 35% for chips.
There are two methods used to calculate the moisture content, ‘Wet Basis’ and ‘Dry Basis’. The most common method in energy terms is wet basis, whilst foresters tend to use the dry basis. It is important to note that the two methods will give a different result for the same piece of wood.
A quantity of wood has a total mass of 10kg. It is dried in an oven so that all water is removed and then weighed.
Its new mass is 8kg. The moisture content is calculated as follows:
WET BASIS moisture (water) content (MC) = mass of water (2kg)/mass of wet wood (10kg) = 20%
DRY BASIS moisture (water) content (MC) = mass of water (2kg)/mass of wet wood (8kg) = 25%
Wet vs Dry
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Moisture content at any stage of the drying process can be measured in several ways:
a) By an electronic moisture meter which reads out the % figures on a screen
b) By artificially drying a sample and comparing the weight before and after
c) By visual examination, splits, peeling bark etc. Approximately.
d) By the ‘feel’ of handling the wood in an experienced hand (very approximately only)
The first method uses a hand-held instrument that measures the current between two steel probes pushed into the wood just after splitting; this provides an instant result and is useful for assessing wood just before purchase. However, it is less accurate compared to the second method which requires an oven, an accurate pair of scales and plenty of time! The ratio of weight before and weight after dying completely gives you an idea of the percentage content of water. The least accurate methods are the last two which rely on the assessors experience, but once the buyer is familiar with a certain species of tree, it is usually accurate enough for household decisions.
The energy content per volume or mass unit is known as the calorific value. Calorific value is usually expressed as MJ, GJ or kWh per kg. There are two different ways of expressing the energy content of wood. These are given below. When referring to manufacturer’s guidance or when talking to a fuel supplier it is important to note which value is being used and whether the calorific value refers to wet wood or oven dry wood.
- Gross calorific value or Higher Heating value (HHV). This is the method normally used in the UK and refers to the heat produced by the complete combustion of the wood at constant pressure with the condensation of the original moisture content of the wood and the water vapour formed during combustion.
- Net calorific value or Lower Heating Value (LHV). This method is more commonly used in mainland Europe and refers to the heat produced by the complete combustion of the fuel with the vapour formed during combustion in a gaseous state. This means that the recovery of heat by condensing the vapour in the flue gas is not included and results in a figure of efficiency that is inflated in relation to the actual one achieved in operation.
Calorific values and bulk densities of a range of fuels
|Fuel||Energy by mass GJ/tonne||Energy by mass kWh/kg||Bulk density kg/m3||Energy by volume MJ/m3||Energy by volume kWh/m3|
|Wood chips (Very dependent on moisture content)||7.15||2.4||175-350||2,000-3,600||600-1,000|
|Log wood (stacked – air dry 20% moisture content)||15||4.2||300-550||4,500-8,300||1,300-2,300|
|Wood (solid – oven dry)||18-21||5-5.8||450-800||8,100-16,800||2,300-4,600|
|Coal (lignite to anthracite)||20-30||5.6-8.3||800-1,100||16,000-33,000||4,500-9,100|
Constituents of wood
During the combustion process heat breaks down large molecules of lignin and cellulose to form volatile organic compounds (VOC’s). These volatiles components make up about 80% per dry mass of wood. Volatiles ignite at about 350°C and full combustion of the volatiles occurs at above 600°C.
Wood fuel contains incombustible constituents known as ash. These are mainly potassium (K), sodium (Na), phosphorus (P), calcium (Ca) and silicon (Si). Typically, wood contains between 0.5% and 2% ash. However this level of deposit can only be achieved at very high combustion temperatures and efficiencies.
The ash contained in wood comes from soil and sand trapped in the bark. A minor proportion also comes from salts absorbed during the period of growth. These salts are important in the combustion process. Ash may also contain heavy metals derived from undesirable environmental exposure, but this content is normally much lower than in other mineral fuels.
A special characteristic of ash is its heat conservation properties. For wood stoves, an ash layer at the bottom of the stove transfers heat to the final burnout of the char. For appliances incorporating a grate, ash is important in protecting the fire-bars from excessive temperatures.
At very high temperatures ash can fuse to form clinker (750 – 900°C). Some fuels with high ash content and excess silica can form clinker at lower temperatures (450 – 750°C) which may create difficulties for the user. Straw and energy grasses such as Miscanthus are typical examples.
Quality and standards
The quality of wood is critical to ensure the reliable and efficient operation of a wood fuelled heating system.
A specification, or standard, for wood fuel provides reassurance to the end user that a consignment of wood with a given description will be of consistent quality within a range of tolerances. This enables a supplier to decide on the species and size of tree and processing machinery best suited to produce the fuel required. In addition the buyer can more accurately assess the quantity of wood needed to generate the heat required.
There is a variety of guidelines and standards on particle size, moisture content, ash content and level of contaminants etc for wood chips and pellets. Equipment manufacturers also issue fuel specifications for their appliances. Using poor quality products may invalidate guarantees, impair performance and damage components. Consistency of fuel quality is very important in order to maintain reliable performance and the specification should be agreed with the supplier when any contract is made.
Generally, the smaller the boiler, the better the fuel quality required as a lower tolerance for variation in specification is likely. Therefore, only good quality fuel should be supplied for small domestic scale appliances.
CEN/TC 335 is the Technical Committee developing standards to describe all forms of solid biofuels within the European Union, including wood chips, wood pellets and briquettes, logs, sawdust and straw bales.
The European Standards specifications include:
- Origin – Generally, it will be specified that clean/untreated wood products from the agricultural, forestry and timber processing sectors must be used and be free of contamination. Expert advice should be sought before using any other material.
- Particle size (P15/P30/P50/P100) – percentage (wet basis)
- Ash content (A0.5/A1/A3/A6/A10 – percentage and chemical content
CEN-TS-14588 Solid Biofuels – ‘Terminology, Definitions and Descriptions’ defines various terms such as biomass, densified biofuel and biofuel pellet. DD CEN/TS 14961:2005. ‘Solid biofuels, Fuel specifications and classes’ provides further information.
CEN-TS-14774-1, 14774-2, 14774-3 deal with methods for wood fuel moisture content determination. Until CEN/TC 335 Standard is ratified and widely accepted, the most common specification for pellets and chips is the Austrian ÖNORM Standard.
Wood chips have three specifications (material, particle size and moisture content). The particle size distribution of chipped wood is normally quoted according to the table below:
|Max cross sectional area (cm2)||3||5||100|
|Max Length (long thin interval) (cm)||8.5||12||25|
|Coarse material (max 20%) (retained by sieve mesh width, mm)||16||31.5||63|
|Main material (60-20%) (retained by sieve mesh width, mm)||2.8||5.6||11.2|
|Fine material (max 20%) (retained by sieve mesh width, mm)||1||1||1|
|Dust (max 4%) (passing sieve mesh width, mm)||1||1|
Contaminants: Wood fuel should be free from contaminants. Factors such as the source of the wood, methods of harvesting and handling the product will determine the proportion of contaminants present in the wood. Pure ash can be 0.5% to 2.5% of oven dry weight wood.
European standards relating to wood pellets:
SS 1871 23Fuel briquettes; three classes
|Austria||ÖNORM M 7135-1||Wood pellets, briquettes and sticks|
|ÖNORM M 7135-2||Bark pellets, briquettes and pieces|
|ÖNORM M 7136||Wood pellets, quality assurance, transport logistics and storage logistics|
|Germany||DIN 51731 – category HP 5||Wood briquettes (including pellets) length <50mm; width 4 to 10mm|
|Sweden||SS 1871 20||Fuel pellets; three classes|
|Europe||CEN/TS 14961||Solid organic fuels|
There has been a UK Voluntary Code of Good Practice formerly issued by British Biogen for pellets and sub 25kW appliances which remains operative until the British or EN standard becomes available.
The Austrian ÖNORM 7135-1 Standard is commonly recommended by imported pellet appliance manufacturers. The standard requires that pellets or briquettes must only be made from pure wood and only 2% of natural binding agents are allowed.
Limit values of chemical and physical parameters:
|Austrian ÖNORM 7135-1||wood pellet standard|
|Length||5 x diameter (6mm)|
|Unit density||> 1.12kg/dm3|
|Calorific value||> 18MJ/kg|
|Diameter||4mm < D < 10mm (6mm)|
|Abrasion/Durability||<2.3% (in Lignotester)|
|Binding agents||<2% (only natural)|
The DIN Plus standard combines the Austrian ÖNORM M7135 and German DIN 51731 standards in a common, single, highest quality standard.
|Moisture content||Volume of fuel required, storage considerations and dry-matter losses, calorific value of fuel, self-ignition, suitable appliance design|
|Calorific value||Volume of fuel required, appliance design|
|Volatiles content||Thermal decomposition and behaviour during combustion, calorific value of fuel|
|Ash content||Dust emissions, requirement for ash removal|
|Ash-melting behaviour||Clinker, operational safety, combustion technology, process control|
|Bulk density||Storage space, transport and handling|
|Particle density||Combustion rate|
|Physical dimensions||Hoisting and conveying, combustion technology, bridging of fuel|
|Size distribution||Operational safety, ease of drying, fuel transfer, combustion control|
|Fine parts (wood pressings)||Storage volume, transport losses, dust information|
All combustion requires three elements: Fuel, an Oxidiser and a Source of Heat. The fuel is the wood, the oxidiser is air and the source of heat is the means by which the fire is lit. Understanding the combustion process is essential in understanding how to get the most useful heat energy from wood fuel.
The combustion process
The combustion of solid biomass consists of five main stages: drying, pyrolysis, gasification, oxidisation of gasses and oxidisation of char.
Drying: The drying stage is influenced by the size of the fuel and the moisture content. Heat is supplied from the flames and energy reflected from the walls, forming the combustion chamber.
Pyrolysis and Gasification: As the biomass becomes almost completely dry, the temperature increases to about 200°C and pyrolysis (inside the wood) and gasification (on the surface) starts. During pyrolysis, the volatile matter is released in the form of vaporised tars and oils.
Oxidisation of volatiles and char: Beyond 400°C the fuel begins to oxidise (burn). At this stage heat is released from the combustion process. The heat released ignites the volatiles which give rise to yellow flames burning above the fuel. As the volatiles are exhausted, the remaining solid material (char) oxidises at temperatures of 800°C to 1000°C.
Combustion efficiency refers to how well heating equipment converts the energy in the fuel into heat energy for collection. Complete combustion efficiency (100%) would extract all the energy available in the fuel, though this is not realistically achievable due to the inability of combustion air to mix intimately with all the fuel throughout the burning process.
However, the overall appliance efficiency calculations (system efficiency), the ability of the appliance to burn the fuel and extract the heat from it into useful energy, assume complete fuel combustion and the following factors:
- The chemistry of the fuel (proportions of hydrogen, carbon, oxygen and other compounds) and how much energy is chemically bound in the fuel.
- The net temperature of the flue gasses or how much heat goes up the chimney
- The percentage of oxygen (O2) or carbon dioxide (O2) by volume after the combustion process or how much O2 was used in the combustion process.
The overall efficiency of the combustion process depends on the characteristics of the fuel:
- Moisture content: A high moisture content leads to longer drying time and wasted heat used to evaporate the water.
- Calorific value of dry matter
- Fuel size – large pieces of fuel have a low surface to volume ratio, therefore drying takes longer.
Efficient combustion relies on 3 T’s –
Temperature, Time and Turbulence
For combustion to take place, the following conditions must be fulfilled:
The whole process requires optimum combustion temperature –
- For combustion to start, the fuel must be heated to a temperature high enough for ignition to occur.
- Too low a combustion temperature results in incomplete combustion and high carbon monoxide emission.
- Too high a combustion temperature causes problems of slagging (clinker build up on firebox surfaces).
The combustion temperature is affected by:
- Combustion air supply
- Flue gas recirculation
- Cooled surfaces
The combustion temperature will also depend on fuel composition (moisture content and volatile content), the primary and secondary air ratio and the temperature at which the combustion air is supplied.
• The mixture of fuel and air requires a sufficiently long dwell time in the reaction zone to ensure complete combustion. This is determined by the combustion chamber design.
• It is important to ensure there is sufficient mixing of primary and secondary air with the fuel and asses, both in the correct proportion and ratio, so that complete combustion results.
Combustion air and excess air ratio
Combustion air is supplied as primary, secondary and in some cases tertiary air:
Primary air is delivered under the fire bed and is used to dry fuel, sustain initial combustion and the burn out of char, plus cooling the grate, if fitted.
Secondary air supplied above the fire bed and may be pre-heated. The secondary air provides oxygen for combustion of the gasses above the fire bed.
Tertiary air may be added at the end of the secondary combustion phase to ensure complete combustion of volatiles.
The excess air ratio (λ) is a measure for the air supplied to the combustion process compared with the minimum air demand for a complete oxidisation of the fuel.
- For stoichiometric oxidation of the fuel ie the theoretical exact amount of oxygen supplied for complete combustion, λ = 1.
- When less air is supplied than required for complete oxidisation, 0 < λ < 1.
- Where there is no available oxygen, λ = 0. This is pyrolysis (thermo chemical conversion in the absence of an oxidising agent).
- λ > 1 refers to the situation where there is more air (excess air) than is needed for complete oxidisation. These conditions are usually maintained during combustion.
Excess air or lambda required for optimum combustion of wood fuel is between 1.4 and 1.6 but depends on the appliance design and fuel. Typical figures are given below:
|Appliance||Excess air ratio, λ||Resulting oxygen content in the flue gas, dry %|
Most wood burning stoves are provided with controls for primary and secondary air supply whilst some automatically vary the ratio of one to the other. With some sophisticated boilers the air supply is electronically adjusted using a lambda sensor.
There are two forms of pollutants associated with wood fuel: atmospheric emissions and leachates.
Atmospheric emissions from incomplete combustion include carbon monoxide (CO), soot (C), hydrocarbons, tar compounds, sulphur dioxide, particulates (dust), carbon dioxide, organic compounds, nitrogen oxides, water vapour and unburned particles. These substances include carcinogens and are harmful but also cause sooting of the flue and firebox. They should be avoided by ensuring optimum combustion conditions, and more importantly complete evacuation of these products of combustion to the outside atmosphere. Emissions breathed in from any fire must be avoided. The particulates in emissions are sometimes invisible to the eye and can lodge deep inside the lungs.
Emissions from complete combustion include carbon dioxide and nitrogen oxides (NOx). The latter are formed from both nitrogen in the wood and in the combustion air and are considered undesirable due to the risk of acid rain and smog. NOx emissions increase with excess air ratio, high nitrogen content of the fuel, high combustion temperatures and long residence times. There is therefore a fine balance between achieving full combustion and reducing NOx emissions.
NOx emissions can be limited by the following measures:
- A staged injection of primary and secondary combustion air in separate zones.
- Air ratio in the primary combustion chamber between λ = 0.6 and 0.8.
- Residence time of the flue gasses in the primary combustion chamber of approx. 0.3 – 0.5 seconds.
- Low excess air ratio in the secondary combustion chamber.
Other possible emissions include hydrogen chloride (HCI) which can cause corrosion. Most wood has very low chlorine content so HCI emissions are not a problem. However, some biofuels, such as grain, have enough chlorine to cause corrosion problems in the combustion chamber, heat exchanger and flue system. Sulphur oxides (SOx) should not normally be a problem as most wood has a very low sulphur content. Other harmful emissions, such as polychlorinated dioxins (PCD) may be formed if the wood contains paint or other chemicals. For this reason only uncontaminated wood should be used for heating.
Leachate can be produced during the timber harvesting and storage processes. When it rains on stored branchwood and residues, the rain can pick up contaminants, leachates, which can enter groundwater or watercourses. However, once the wood fuel has been air dried, prepared and delivered to the end user, there will be no risk of leachate if it is kept dry and under cover. Keeping the fuel dry also means that it will burn more efficiently.
Ash is not normally considered a pollutant. Ash from wood fuel combustion is deposited in two locations: ‘bottom ash’ which collects in the combustion unit (the residue that is left in the combustion chamber) and ‘fly’ ash collected from the flue gases either in the boiler flues or the chimney itself. Ash has the potential to form a valuable by-product as a low nitrate fertiliser for use in areas high in nitrogen.
The system efficiency refers to the ratio between the chemical energy of the fuel to the useful heat output from the appliance less the heat loss from the combustion air supply if this is drawn from a heated room.
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