Trying to find wood to use as an energy source is becoming more challenging. One potential 
source is stump wood. This article looks at the productivity of the equipment used in the harvesting of stump wood.
The study was carried in Norway spruce (Picea abies), with the objective of creating productivity models for stump wood extraction, forwarding and site preparation, as well as looking at improvement opportunities. The article was published by Juha Latila, Tapio Ranta and Antii Asikainen in the International Journal of Forest Engineering (IJFE 2008: 19(2): 37-47) titled “Productivity of Stump Harvesting for Fuel”.
Stumps that have a high wood content and are loosely anchored into the ground are ideal (such as Norway spruce). Stumps are also often preferred as a wood source as they do not absorb as much water as other harvesting residue. However, stones and sand can cause problems during processing and combustion. The stumps usually need to be split to increase drying rates and bulk density during transport. Removing stumps can make soil preparation for reestablishment easier, especially for mechanised planting.
In this study, the stumps were uprooted and broken up/ split using a 17 tonne excavator equipped with a special stump rake extraction-splitting device. The extracted stumps were split and shaken to remove debris and then placed in small piles and left for a period of time so that rain can wash them clean and they can dry out. The excavator would also carry out the site preparation, which consisted of mounding of soil with the upper tongue of the extraction-splitter device, and then smoothing over the stump holes before moving to the next stump. The stumps were then forwarded to roadside for storage using a 12 tonne forwarder where they were left for further seasoning before final transport.
For stump extraction from the ground, the main productivity factors were stump diameter and number of stumps per hectare. The volume of stump wood removed per hectare and the extraction distance were found to be the main productivity factors for the forwarder. Some of the key productivity results are as follows:
- Removing 350 stumps/hectare with an average stump diameter of 40 cm = 7.9 m3/machine hour.
- Removing 800 stumps/hectare with the same diameter of 40 cm = 10.8 m3/machine hour.
- Splitting of stumps and shaking off impurities took 42% of the excavator’s time, stump lifting 18%, site preparation 15%, creating small stump heaps 11%, with the remaining time being activities such as positioning the boom and smoothing stump holes.
- Bigger stumps (>35cm diameter) require a larger excavator (21 tonne) to maintain productivity.
- Forwarder productivity over 250 m with a 7 tonne payload and removing 60 m3/hectare = 7.8 m3/machine hour.
- Loading took 57% of the forwarders time with unloading 25%.
Please access the International Journal of Forest Engineering for a detailed and more complete account of the research. Juha Latila and Antii Asikainen are from the Finnish Forest Research Institute and Tapio Ranta from the Lappeenranta University of Technology in Finland.
Source: www.lib.unb.ca/Texts/IFE/July08
Picture source: www-nlfak.slu.se/pressbilder/stubbar/stubbrytning_von_h.jpg