Wood Chippers and Crushers in Wood Energy
Tekes, Finland for OPET - Organisations for the Promotion of Energy Technologies - February 2001
This study addresses the problem of converting underutilized reserves of biomass for use in wood energy, and cellulose to energy projects.
Developing technology for large-scale production of forest chips
Wood Energy Technology Programme 1999–2003
Pentti Hakkila, VTT Processes
From the Forward
Finland is the world leader in utilisation of bioenergy. About 20 % of the primary en-ergy is derived from wood-based fuels, a higher proportion than in any other indus-trialized country. Finnish forest industry has the central role in converting wood-based residues into heat and power.
However, meeting the challenges of the mitigation of climate change has lead to the commitment to double the use of the renewable energy sources by 2025, as com-pared to the situation in 1995. The main focus is on bioenergy. As all industrial wood residues are in use either as raw material or to produce energy, any increase must be based on the recovery of unutilised biomass in the forests: forest chips from logging residues and small-sized trees. The limiting factor is not the utilisation but rather the production of the fuel: the cost of production must be reduced, its supply must be re-liable, and the quality of the fuel must be improved.
Unutilized Reserves of Biomass
For silvicultural reasons, large quantities of unmerchantable small-sized trees should be re-moved from young stands even though there is no demand for this low-quality biomass (Figure 7). Another unutilized reserve, although offering less silvicultural incentives, is the biomass residues composed of crown mass, unmerchantable stem parts and even stump-root systems from the clear-cutting areas of mature forests.
A forest fuel production system is built around the comminution phase. The position of the chipper or crusher in the procurement chain largely deter-mines the state of biomass during transportation and, consequently, whether subsequent machines are dependent on each other. Comminution may take place at the source, at the road side or landing, at a terminal, or at the plant where the chips are to be used. Four alternative production systems have been studied in the Wood Energy Technology Programme (Figure 14).
Comminution at the source
, or in the terrain, re-quires a highly mobile chipper suitable for cross-country operations and equipped with a tippable 10–20 m3 chip container. The chipper moves in the terrain on strip roads and transfers the biomass with its grapple loader to the feeder of the chipping device. When the chipper container fills up, the load is hauled to the road side and tipped into a truck container, which may be on the ground or on a truck trailer
As a single machine carries out both the comminution of biomass and the off-road transport of chips, the cost of shifting machines from site to site is re-duced, and smaller logging sites become commer-cially viable. The use of containers weakens the in-terdependence between the chipper and the truck, although it is not entirely removed. Large landing areas are not needed, but a level and firm site is nec-essary for the truck containers.
Comminution at a landing
is performed in smaller operations with farm tractor-driven chippers and in large-scale operations primarily with heavy truck-mounted chippers or crushers. The biomass is hauled with forwarders to the landing and bunched onto 4 to 5 m high piles. This facilitates operation in difficult terrain and in winter condi-tions and allows longer off-road hauling distances. The forwarder operates independently of the chip-per. The comminuted biomass from the chipper is blown directly into a 100 to 130 m3 trailer truck, a process that makes the system hot and vulnerable, i.e. subsequent machines are dependent on each other. A wider landing area is required than in the alternative systems because of the large road-side inventories of biomass and the simultaneous pres-ence of the chipper and the truck.
Landing chippers do not operate off road and can therefore be heavier, stronger and more efficient than terrain chippers. If the biomass, such as stump and root wood, is contaminated by stones and soil, it is possible to use crushers that are more tolerant instead of chippers
Comminution at a terminal
or plant means that road transportation of the biomass takes place be-fore the size reduction. The biomass is transported to the terminal or plant in the form of undelimbed tree sections, whole small-trees, loose logging res-idues or bundles. Low bulk density restricts the op-eration radius, unless the biomass is bundled.
At large plants, comminution can be performed with efficient stationary crushers at low cost. At satellite terminals or smaller plants, the use of transportable chippers or crushers is more feasible, although the productivity of comminution is lower and the cost higher.
Truck transport of forest chips
Truck transport is the largest single cost factor in the procurement of logging residue chips, constituting up to one third of the total cost at the plant. As the use of forest fuels grows, the average distance and the cost of transport will also grow further.
The project was put together with the support of
Tekes, Finnish Funding Agency for Technology and Innovation