Produktivitet vid stubblyftning
Stump wood was used between 1850 and 1950 for production of tar and as firewood. In the 1950s the use decreased because of the introduction of the cheap fossil oil. In the 1970s stump wood became an interesting issue again as a raw material for the pulp industry. Stump lifting has today become a possible source for bioenergy. The objectives with this study were to investigate the productivity of stump lifting and stump extraction, and to analyse the economy. Another objective was to compile existing knowledge of stump lifting.
The SCA-stands that were used for the study were situated in Medelpad and Västerbotten. The stands were dominated by spruce and had rather high stem volume. A follow up on production data was done in five stands. In one of the stands a time study and a measure of stump biomass and moisture content was done. A Hyundai excavator (23.5 ton) with a Pallari stump-head was used for the stump lifting and a Timberjack 1710 was used for the extraction of stumps.
Time needed for lifting one stump (lifting, shaking, splitting and moving stump to heap) with 33 cm diameter was 84.1 s for pine, 78.,8 s for spruce and 69.6 s for birch. To get the total time, 13.2 s has to be added when harvesting sites with terrain conditions GYL 211. The corresponding additional time for sites with terrain conditions GYL 233 was 24.5 s. The time needed to lift stumps and to make additional soil preparation on one ha was 10 G0-h. The time needed to extract stumps from one ha was 3.5 G0-h, and the average load was 12.9 tons. The additional volume from the stumps corresponds to 30% of the stem volume. That is equivalent to approximately 40 raw ton/ha or 125 MWh/ha on productive sites in southern Norrland. 1 m³f stem volume corresponds to approximately 0.21 ton of raw stump wood. There is a small difference in extra yield between the conifers (spruce and pine) but a rather big difference between the conifers and birch. A pine stump with a stump diameter of 33 cm gives 72 kg dry-weight, a spruce stump gives 77 kg dry-weight and a birch stump gives 58 kg dry-weight.
In 2005 SCA harvested approximately 8.9 milj m³fub. If 50% of that volume is from spruce areas it corresponds to approximately 1.5 milj m3fub stump wood. Assuming that 55% of that volume could be stump-lifted means that approximately 1.8 TWh may be utilised each year. The conclusion is that stump lifting is a big and so far unused source of biofuel.
The productivity at stump lifting increases with 7% if stumps with a diameter less than 15 cm are avoided. To shake the stumps, to sort out soil and stones, is time consuming, it takes 40% of total work time. That is why technical improvements primarily should be focused on the shaking function. The cost with today?s technique and working methods to get 1 MWh to roadside is approximately 84 SEK/MWh. If basic development is carried out time consumption would be reduced with at least 20%. That would result in a cost reduction to 70 SEK/MWh. Based upon today?s high energy prices stump lifting will result in a larger amount of biofuel as well as possible increased profitability for the forest owner. Stump lifting is also positive from the other points of view. It decreases the risk of attack by root rot and pine weevil..