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.
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 of 3.15 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 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.