PRODUCTION OF WOOD CHIPS

Sources of wood chips Wood fuel arises from multiple sources including forests, forest plantations, other wooded land and trees outside forests, by-products from wood processing, demolition wood and processed wood-based fuels. In forest the major resources are: firewood; logging residues (branches, tops, stems) from final fellings; small-dimension trees from cleanings and thinnings; other wood not demanded by the industry; stumps. Usually logging residues and stumps make up forest fuels, called energy wood. Most often forest fuels arise in clearcuts done by harvester. Forest fuel as whole trees, as roughly delimbed tree parts (energy wood) and in combination with pulpwood can also be obtained in thinning young, dense stands. Small-scale use of stumps is tested by a couple of companies, but there is still a need for research and development to secure sustainable forest management and reduce costs.

Production cycle Sustainable forest management provides different assortments of timber, as well as resources for energy production. Forest energy production comprises several steps: forest management planning and sustainable forest harvesting, fuel production, transporting and heat production. Recently the importance of ash recycling is especially increasing and today ash is already used as fertilizer in forest.

Procurement systems The production of wood chips can be divided into several stages: felling for chipping; chipping; off-road hauling; storage of energy wood; road transport. The choice of logging residue procurement system is largely determined by the customer’s reception facilities and fuel requirements, as well as from available equipment and services. Five main systems with several alternatives for logging residue procurement can be identified. The two most common systems are chipping at the landing and trucking of chips to the customer and the deliveries of loose logging residues.

Quality of wood fuel It depends mainly on: moisture content; particle size distri- bution; tree species; bulk density; amount of dust and fungal spores in the fuel; ash content. Most important quality aspects can be taken into account in the whole chain of custody from forest to end user of heat or electricity: supply (selection of raw material, harvesting time, proportion of soiling admixed); processing (soiling, selection of technologies); storage & logistics (drying, ventilation, transport system); combustion (compliance to boiler requirements, dimensions, thermal efficiency). The quality requirements for wood chips depend on the size of the installation in which it is used – smaller boilers have more restrictions than bigger ones.

Production process Wood chips production is complicated. It starts from forest owner and forests management specialist followed by the operator of harvester, forwarder, chipper and truck driver who delivers wood chips to a heating plant or other customer. Heat or electricity are supplied to end users. During the whole process wood is converted not only in shape, but also in measurement units: the boiler plants use MWh; the hauling operators talk in terms of tonnes; wood merchants mainly use volumes, such as solid or loose cubic meter of chip wood; forest owner and end users are talking about the money they get or have to pay for energy wood or wood energy.

Importance of drying Freshly felled wood has a moisture content of 40-60%. Leaving logging residues in forest for one summer can reduce the moisture content by about 10 to 15%. An additional benefit is that needles will drop off which reduces corrosion risk in the boiler and retains nutrients in the forest. Moisture content of around 30% can be achieved for wood stored as roundwood in a covered pile at the roadside or in a yard. Natural drying can reduce moisture content to a minimum of about 20%.

Wood chips Wood chips are a medium-sized solid material made by crushing or chipping, larger pieces of stemwood and branches. There is a difference between wood chips from fresh and dried logging residues. Forwarding fresh logg- ing residues frees up the Cutover at once, facilitates planning, increases harvest- and improves forwarding productivity. Yet, it deple- tes nutrients from forest, and increases the volumes and costs of transportation. Chipping of dried logging residues means longer time from harvesting to chipping, but these chips have higher quality and thermal efficiency.

Use of wood chips Traditional use of wood chips is as solid fuel for heating buildings or in energy plants for generating electric power from renewable energy sources. In a number of cases coal power plants have been converted to run on wood chips. This is fairly straightforward to do, since they both use an identical steam turbine heat engine, and the costs of wood chips are comparable to those of coal. In Sweden the energy produced from logging residues by district heating companies has increased over the past years.