The majority of woodfuel systems have a very specific range of fuel requirements if they are to operate reliably, efficiently, with low levels of emissions and without blockage of the fuel feed system.
Matching supplies to fuel requirements is critical
There is a fairly widespread belief that any batch of wood makes acceptable woodfuel. There are some generally relatively large scale systems that can accept a wide range of woody material as fuel. However the majority of biomass systems have a very specific range of requirements and are designed to shut down automatically if the fuel leads to operating parameters outside a predetermined acceptable range.
The majority of problems with biomass installations in the UK are associated with the fuel. With biomass a relatively novel fuel in the UK, every difficulty can be perceived as a failure of the biomass concept and can attract disproportionate negative publicity. This can damage the biomass market and hinder the development of the biomass industry.
Mismatches between fuel requirements and fuel supplies can arise for a number of reasons:
- The system installer not ensuring a local fuel supply, or not fully informing the user of what is required
- The user not understanding the importance of correct fuel specifications, or attempting to economize
- The fuel supplier not understanding the importance of correct fuel specifications, the fuel specifications required, or mis-selling or misrepresenting the fuel.
It is therefore the responsibility of all links in the chain to ensure they, and those they are dealing with, know the specification required for a given system. This is the reason for a suite of internationally agreed solid biofuel standards covering many different forms of woodfuel and biomass.
For those receiving payments under the RHI, there is a database of approved suppliers of biomass (Biomass Suppliers List, BSL)who have demonstrated the sustainability of their fuel. It is obligatory to use a supplier from this list if you are receiving support under the domestic RHI, unless you are a self supplier.
Supplying different kinds of fuels
Specifications for solid biofuels are set out in the upcoming European Standard, EN335, and cover many fuels and many parameters including ash content, calorific value, contents of various elements, and technical standards for determining these parameters are also published.
Further information:
Fuel supply models
Self-supply
The owner of the combustion or energy system supplies their own fuel from their own estate. They are then responsible for ensuring adequate supply of fuel to the appropriate standard. It is therefore particularly important to ensure that the equipment installed is appropriate to use the fuel available, and that there will be sufficient fuel available. If necessary it might be possible to supplement with fuel from another local supplier.
Co-operative
A number of producers of biomass collaborate on the supply of fuel. This can be an efficient model that allows specialized equipment such as a chipper or delivery lorry to be shared between members of the co-operative, and collaboration on marketing and administration. It can also help to ensure that there is always fuel available to meet demand even if a particular member may be unable to supply at a particular time.
Fuel merchant
The fuel supplier produces and/or buys fuel for sale to customers. They may offer different grades or specifications of fuel for different customers or applications.
The fuel broker sources fuel from a number of suppliers for purchase by customers. They may identify different grades or specifications of fuel for different customers or equipment, allowing optimum use of biomass available.
Energy Service Company (ESCO)
An ESCO is contracted to supply heat rather than fuel. They may also own the combustion equipment, may be contracted to maintain it, or may lease it from another company, but are paid on the basis of heat delivered. This is metered on the basis of water flow rate and temperature differential between inflow and outflow, rather than fuel supplied. This is a popular model in Europe, particularly for district, site and building heating systems.
Supplying wood chips
For most users and suppliers of wood chips the two parameters that are critical to efficient, trouble free operation are moisture content and chip sizes. Other parameters such as ash content and contamination are also important.
Moisture content
Measuring moisture content
Specialist moisture probes are available to measure average moisture content in a pile of wood chips, and these are widely used in Europe to check deliveries. A more rigorous technique, that requires no specialist equipment, is to weigh a sample of chips, dry it out thoroughly in an oven, and then weigh it again, the difference being the water lost. This can be performed by either chip merchant or customer.
Meeting the fuel standards
To specify solid biofuels to BS EN 14961, a suite of standards have been published, including methods of determining various parameters.
- BS EN 14774-1:2009 Solid biofuels – Determination of moisture content – Oven dry method – Part 1: Total moisture – Reference method
- BS EN 14774-2:2009 Solid biofuels – Determination of moisture content – Oven dry method – Part 2: Total moisture – Simplified method
- BS EN 14774-2:2009 Solid biofuels – Determination of moisture content – Oven dry method – Part 3: Moisture in general analysis sample
With experience, however, a fuel supplier will also learn the average moisture content of a batch of chips from a particular timber type, form and characteristics following a given period of seasoning under a given set of conditions.
Moisture content determination of prices
As the energy content of a load of wood chips is heavily dependent upon moisture content many agreements for wood chip supply include a sliding cost scale. This allows the price to be varied depending upon the moisture content, between limits agreed as acceptable, so that the cost of the energy content remains approximately constant.
Particle size (chip size distribution)
Measuring chip size distribution
Chip size distributions are measured using screens, and again methods for determination are set out in BS EN standards:
- BS EN 15149-1:2010 Solid biofuels. Determination of particle size distribution. Oscillating screen method using sieve apertures of 1 mm and above
- BS EN 15149-2:2010 Solid biofuels. Determination of particle size distribution. Vibrating screen method using sieve apertures of3.15 mm and below
The most important consideration for most installations, however, is that there be no long slivers over a certain size as these can cause bridging in fuel feed systems, leading to blockages, and jamming in augers. The system installer should tell the customer of the maximum allowable length.
In addition too many fines (small particles and sawdust) can also cause difficulties as they can build up in places in the fuel feed system and compact over time, potentially leading to obstructions or blockages.
Other parameters
Other parameters of wood chips, set out in EN 335, can also cause problems occasionally:
- Excessive ash content, possibly as a result of a high proportion of bark or leaves
- Contamination of the fuel with excessive levels of heavy metals, alkali metals, sulphur or chloride from soil or air or inclusion of dirt, soil or stones.
The wood species used will have a bearing on the calorific value of the chips, with hardwood chips being higher than softwood.
Supplying wood pellets
Manyissues for suppliers of wood pellets are similar to those for wood chips. With pellets however composition, calorific value, ash content, contamination and mechanical durability are much more significant than with chips.
Moisture content
In general moisture content of wood pellets is less of an issue than for wood chips, though should still be specified, being typically 8-10%.
Pellet size
Particle size also tends to be less of an issue than for wood chips, provided pellets of the right size are requested. Typically these are:
- 6 mm or 8 mm for domestic and small scale systems
- 10 mm or 12 mm for larger and industrial systems
Contamination issues
A high proportion of bark in the sawdust used to produce the pellets will lead to higher ash content.
The presence of minerals such as heavy metals, high levels of alkali and alkaline earth metals, sulphur, etc. from the soil and air of the growth environment, or a range of potential contaminants from the use of recycled timber can potentially be an issue.
Calorific value
The mix of timber used, the presence of any additives such as binders, the quality of grinding and pressure used in production and hence final density, as well as the moisture content will all have an influence on the calorific value.
Wood pellets can absorb moisture from the air if stored in a damp environment, and this can lead to them becoming crumbly and breaking down to sawdust. The mechanical durability of wood pellets is important, especially if they are to be subjected to more than a few handling steps. This can particularly be the case with pellets imported from overseas which may have been subject to many handling processes during loading and unloading.
Mechanical durability
Mechanical durability of pellets is specified in EN 335, with determination by standard BS EN 15210-1:2009.
Pellets that break up into sawdust during handling or storage can cause difficulties in fuel feeds and also the much lower effective fuel energy density of sawdust may require the fuel feed to be recalibrated, or cause the combustion system to trip out.
Supplying firewood logs
The two most important parameters for firewood supply are moisture content (degree of seasoning) and wood species (hardwood or softwood). Log size is also important.
Moisture content
Firewood should always be seasoned before use, usually before supply, preferably for 1-2 years, depending on when it was cut. If a tree is felled in spring or summer it will have a much higher moisture content than if it is felled in autumn or winter.
Small round wood (SRW) should be dried by stacking on bearers in a sunny place, exposed to the prevailing wind. Most moisture loss is through the ends of logs, and these should be open and exposed, but air flow through the stack should be unhindered. Bark may be scored or partially or fully removed to assist drying, and the stack should be covered to prevent direct re-wetting from rain, with either a roof, or waterproof or semi-permeable sheeting.
Logs that are well seasoned and fully air dry (typically around 25% moisture content) have cracks and splits across the grain, and the bark comes off easily.
Wood species
In general hardwoods make better firewood, as theyare denserand burn more steadily. Owing to the higher density, for a given moisture content a given volume of hardwood will be heavier, and contain more energy, than the same volume of softwood. Softwoods can be easier to light, however, and burn more quickly. They also tend to be marginally higher calorific value for the same moisture content owing to the higher lignin content. Softwoods can be useful for kindling.
Log size
Provided they are of suitable size to fit in the firebox, feed hopper or grate, log sizing is relatively straightforward. If larger than 10-15 cm diameter they should be split.
