Treatment of wood from landscape conservation and maintenance work for fuel
After the amendment of the German Federal Immission Control Act related to new and automatic firing systems wood chips and wood pellets have to be more environmentally friendly and meet stricter emission threshold values since the beginning of 2015. The fuel quality of wood chips from landscape conservation and maintenance work (LCMW) is lower compared to forest wood because of higher bark and fine material contents. Therefore, the use of wood chips from LCMW as fuel necessitates certain treatment.
In addition to the design and settlement of the boiler, the fuel plays a major role for the quality of firing installations. Whereas wood pellets are available as a standardised fuel according to DIN- or EN- Norms, the standardization of wood chips has not yet been completed. Since 2014 the DIN-EN-ISO 17225 has been in force and replaced the Austrian Ö-Norm which can often still be found in brochures of boiler manufacturers. The new norm solely relates to the use of wood chips in small firing installations (< 1000 kW) and contains other classifications of sizes, which allow better grading of bulk material.
In the DIN-EN-ISO 17225 the used raw materials (e.g. full trees or waste wood) and physical fuel properties (water and ash contents) are described in four quality classes. The fuel classification is not only important for trading but also for the fuel admission of the boiler manufacturer.
In addition to the quality of the fuel, primary measures of the boiler manufacturer like construction and operation of the firing, grinding and segregation effects of the feeding system or cleaning and service intervals have a big influence on the emissions of a firing system. In case the demanded values cannot be met, separators or filters become necessary which are currently available for example as electrostatic separators and range in price from 20 to 200 €/kW. For separate filters, additional design approvals are necessary while separators, which are integrated into the boiler, are recorded by the type approval of the boiler.
Technical solutions for fuel treatment
As described above, wood chips from LCMW have a lower fuel quality compared to wood chips from forests because of the higher contents of bark and fine material. In most cases additional treatment of the raw material is required.
During the LIGNA 2015 (trade fair for woodworking industry) a technological line created by the Chamber of Agriculture of Lower Saxony (technical partner in greenGain, COALS) showed practical examples of how wood chips can be produced as high-quality fuel. Companies such as Brucks, Eschlböck, Greentec, Jenz and Pezzolato presented chippers which are increasingly used by contractors because of their size and performance.
Leaves, needles, bark, attached humus and sand particles are dust generating elements in wood chips which can be found mainly in the fine share of the fuel. The company MVD showed a star-roll separator to sieve wood chips in stationary operation. With different spaces between the star-rolls three different fractions per run can be separated. The performance indicated is up to 80 m3/ hour. For the mobile and inter-company use of a sieving system Terra Selecta showed a machine with a drum sieve which usually is used for compost preparation. Depending on the material and the quality of the sieve, the same high performance of 80 m3/ hour may be reached.
Drying of wood chips
While storage of rough wood chips is rather unproblematic fine material tends to warm-up and grow mould when the water content is higher than 35 %. This happens especially when the raw material is contaminated (e.g. by sand) and/or has a high content of bark as well as at compression-horizons and separation-zones with a high amount of very fine material. Wood chips from LCMW often show these properties. Ventilation is recommended if moist and fine wood chips are to be deposited higher than 50 cm in a storage room. Constructions with good air circulation such as sheds should be preferred to closed rooms. Because moist wood chips in storage heat up on their own (up to 60° C) a natural supply of ambient air can already have a drying effect. The air distribution in the storage room can be achieved by drainage tubes, underfloor ducts or other simple air channels. A fan is not necessary because air, warmed up by the moist wood chips, rises and causes cold air to follow from bellow.
The longer the wood is left to dry after the logging and before the chipping, the lesser are the problems later during storage. It is recommended to log the wood in winter and to chip it in late summer when the water content is already reduced to about 35 %. In this case the wood chips can be stored outdoors on a pile and be covered by a fleece. After 100 to 120 days the decomposition process stops and the wood chips have dried to a water content of around 20 %. This process consumes energy thus the energy content of the wood chips drops for about 15%. However, because no further technical methods are needed for this process this loss is tolerable.
Wood with the best quality results when it is dried for example with the exhaust air of thermal power stations of biogas plants. For this, mobile and stationary treatments have been developed in the past years which can be used according to local situations.
Summary: Preparation of wood from landscape conservation and maintenance work to fuel
A special exhibition at the LIGNA 2015 showed solutions for quality improvement of wood chips. Since January 2015 the amendment of the Federal Immission Control Act for new and automatic firing systems requests more environmentally friendly wood chips and wood pellets in Germany to meet stricter threshold values. The fuel quality of wood chips from landscape conservation and maintenance work (LCMW) is lower compared to forest wood chips because of higher bark and fine material contents. Therefore, the use of wood chips from LCMW as fuel necessitates certain treatments.
Photos: C. Brüggemann
Author: Carsten Brüggemann