Dan Bergström

Simulation of harvester productivity in selective and boom-corridor thinning of young forests

Abstract Forest management practices may change in the future, due to increases in the extraction of forest fuel in first thinnings. Simulation models can be used to aid in developing new harvesting systems. We used such an approach to assess the productivity of innovative systems in various thinnings of young stands with wide ranges of mean breast height diameter (1.5–15.6 cm), stems per hectare (1000–19,100), and mean height (2.3–14.6 m). The results show that selective multiple-tree-handling increases productivity by 20–46% compared to single-tree-handling. If the trees are cut in boom-corridors (10×1 or 2 m strips between strip roads), productivity increases up to 41%, compared to selective multiple-tree-handling. Moreover, if the trees are felled using area-based felling systems, productivity increases by 33–199%, compared to selective multiple-tree-handling. For any given harvesting intensity, productivity increased the most in the densest stands with small trees. The results were used to derive time consumption functions. Comparisons with time study results suggest that our simulation model successfully mimicked productivity in real-life forest operations, hence the model and derived functions should be useful for cost calculations and evaluating forest management scenarios in diverse stands.

Path tracking in forest terrain by an autonomous forwarder

Abstract Autonomous navigation in forest terrain, where operation paths are rarely straight or flat and obstacles are common, is challenging. This paper evaluates a system designed to autonomously follow previously demonstrated paths in a forest environment without loading/unloading timber, a pre-step in the development of fully autonomous forwarders. The system consisted of a forwarder equipped with a high-precision global positioning system to measure the vehicles heading and position. A gyro was used to compensate for the influence of the vehicles roll and pitch. On an ordinary clear-cut forest area with numerous stumps, the vehicle was able to follow two different tracks, three times each at a speed of 1 m s−1, with a mean path tracking error of 6 and 7 cm, respectively. The error never exceeded 35 cm, and in 90% of the observations it was less than 14 and 15 cm, respectively. This accuracy is well within the necessary tolerance for forestry operations. In fact, a human operator would probably have a hard time following the track more accurately. Hence, the developed systems function satisfactorily when using previously demonstrated paths. However, further research on planning new paths in unknown unstructured terrain and on loading/unloading is required before timber transports can be fully automated.

Compression Processing and Load Compression of Young Scots Pine and Birch Trees in Thinnings for Bioenergy

Abstract In many boreal countries forest biomass from dense young stands has great potential to meet growing demands for bioenergy. However, there is currently no cost-effective operational system for harvesting such stands. When harvesting, it would be desirable to compress trees and leave some of the foliage and fine branches in the stand in order to reduce extraction of nutrients and improve the harvested material’s fuel properties from a reduced ash content point of view. The objectives were to evaluate experimental systems (a boom-tip mounted unit for processing bunches, and a small prototype forwarder for compressing loads) in terms of their utility for compressing and partially debranching fresh and stored bunches of Scots pine trees and compressing forwarder loads of fresh Scots pine and birch trees. Processing of fresh bunches resulted in mass losses of about 10% (dbh-class 5–8 cm; not significant) to 15% (dbh-class 12–15 cm; significant) with 35 to 50% reductions in ash contents and increases in bulk and net energy density of about 80 to 160%. In-stand storage of bunches before processing generally yielded no significant advantages except for a 10%-unit reduction in moisture content. Compression of forwarder loads resulted in 16 (pine) to 32% (birch) increases in load densities, with a significant increase for birch trees. The described techniques facilitate productivity increases of off-road and road transport of tree parts, and a substantial proportion of the ash-rich materials such as needles and fine branches are left in the stand. If both tested techniques are applied during the harvesting of young trees the benefits would be substantial.

Characteristics of Swedish forest biomass terminals for energy

Forest biomass terminals provide diverse services to the forest industry: buffer storage, transfers of material between different modes of transport, raw material upgrading, etc. Terminals’ operational costs are highly sensitive to their layout and design. In order to design efficient terminals, it will be essential to understand the current state of forest terminals. To this end, a survey was sent out to companies operating forest terminals for energy in Sweden. The respondents were asked to provide information about their terminals’ areas, volumes of material handled, equipment, inventory methods and age. The terminals were grouped into four size classes according to their surface area. Most terminals covered < 5 ha; terminals in this class accounted for 65% of the country’s total terminal area. In addition, more than half the country’s total forest biomass output was handled at terminals of <2 ha. The extent of paving at terminal size classes varied widely from 28 to 60%. Studied terminals handled 14 different assortments; on average, each individual terminal handled 2.4–4.0 assortments. The most widespread assortment was energy wood which accounted for 63% of the total volume handled. Larger terminals were older, often had better measurement equipment than smaller ones and relied more heavily on third parties to perform inventories. Conversely to big terminals, smaller terminals were more likely to have mobile machinery. Our results provide a detailed overview of the state of Sweden’s forest terminals and will be useful in the design of improved biomass terminals in future.

Analyses of a single-machine system for harvesting pulpwood and/or energy-wood in early thinnings

Harwarder systems used in roundwood harvesting can be useful at sites where there are low volumes per hectare to be removed or a small total removal volume and where frequent or long-distance machine relocations are required. As well as being suitable for conventional roundwood harvesting, a multi-purpose forest machine can have its configuration changed, so that it can function as a harvester, forwarder, or harwarder, and thus can be considered viable for use in young stands when harvesting pulpwood or energy-wood or combining assortments. Accordingly, in this study we compare the time consumption and productivity of a Ponsse Dual machine system with two harvesting heads and one loading grapple (three machine configurations) when harvesting and forwarding pulpwood and/or energy-wood during early thinning at sites in northern Sweden. Our aim is to analyze costs and profitability under a range of site conditions. Working as a harvester, the machine reached the same level of productivity (trees per hour) whether it was operating with an accumulating felling head or a harvester head able to accumulate some trees; productivity was also independent of work method. The forwarding efficiency was highly correlated with biomass concentration and forwarding distance; whole-tree (tree section) removal over short forwarding distances resulted in the highest productivity. The system achieved the greatest cost-effectiveness when only energy-wood was removed. At current market prices, pulpwood extraction produced the highest net income per hectare, but even this was economically unprofitable. The Ponsse Dual achieves similar productivities to specialized harvesters and forwarders during thinning operations, but at a much higher cost per operating hour. Thus, a single-machine system would need to have ca. 20–30% lower operational costs than the Ponsse Dual to be an economically viable option for early thinning.

Simulation of boom-corridor thinning using a double-crane system and different levels of automation

This study evaluates the productivity of a harvester equipped with a double-crane system for thinning with continuous felling and accumulation of whole small-diameter trees for bioenergy at different levels of automation. The simulations were performed using a discrete event simulation tool that has been developed recently and is specifically designed for simulations in forestry, incorporating spatial awareness of the simulated world. The study shows that introducing boom-corridor thinning with a semi-automatic double-crane system can significantly increase productivity compared to conventional thinning and harvesting. For the specific harvester model used in this study, the modification that yielded the greatest productivity increase was automating the release and placement of the harvested trees. Studies on the effects of implementing automation for other forest machine operations could be analyzed using a similar approach.

The state of the art in woody biomass comminution and sorting in Northern Europe

This review provides a survey of the literature on methods for comminuting and sorting woody biomass. The reported methods were classified according to their basic operating principles: methods of comminution were separated based on whether they use sharp, semi-sharp or blunt tools; methods of discrimination were classified according to the parameters they use. The identified methods of comminution differ significantly in terms of productivity and energy use. The differences between individual biocomponents with respect to the parameters used for discrimination (such as solid density, or particle size and shape) are often rather small and species dependent, which makes it difficult to efficiently sort woody biomass from mixed species. Other parameters (e.g. optical and near-infrared properties) can be used as inputs for active sorting on larger scales. Several methods for tree processing and chip sorting that were originally developed for pulp-chip production now see little use because they are not considered cost-effective. However, the economic importance of fuel products has increased substantially in recent years. Fuel can be produced using material with a higher bark content than is tolerated in pulp chips, so it may be worth re-evaluating some of the methods that were previously discarded. Examples include tree-processing machines such as drums and cradles for partial foliage removal, processes such as delimbing and debarking (which facilitate downstream sorting of the comminuted material), and screens and wind classifiers for processes such as separating fines (often ash-rich) from comminuted material. The available data on the performance, required investment, and operating costs for these methods are incomplete and often not comparable. This issue must be addressed to facilitate the development of optimized fuel supply chains.

Comparison of energy-wood and pulpwood thinning systems in young birch stands

Abstract In early thinnings, a profitable alternative to pulpwood could be to harvest whole trees as energy-wood. In theoretical analyses, we compared the extractible volumes of energy-wood and pulpwood, and their respective gross values in differently aged stands of early birch thinnings at varying intensities of removal. In a parallel field experiment, we compared the productivity at harvest of either pulpwood or energy-wood, and the profitability when the costs of harvesting and forwarding were included. The theoretical analyses showed that the proportion of the total tree biomass removed as pulpwood increased with increasing thinning intensity and stem size. The biomass volume was 1.5–1.7 times larger than the pulpwood volume for a 13.9 diameter at breast height stand and 2.0–3.5 times larger for a 10.4 diameter at breast height stand. In the field experiment, the harvested volume per hectare of energy-wood was almost twice as high as the harvest of pulpwood. The harvesting productivity (trees Productive harvesting Work Time-hour−1) was 205 in the energy-wood and 120 in the pulpwood treatment. The pulpwood treatment generated a net loss, whereas the energy-wood treatment generated a net income, the average difference being €595 ha−1. We conclude that in birch-dominated early thinning stands, at current market prices, harvesting energy-wood is more profitable than harvesting pulpwood.

Continuous felling of small diameter trees in boom-corridors with a prototype felling head

Abstract Simulations of the continuous felling of trees in boom-corridors have suggested that it may be possible to increase harvester productivity by a factor of 2.4 when thinning young dense stands. A prototype boom-tip mounted felling head for harvesting in this fashion was therefore built and tested in the field. Using the new head, stands with a density of 10,000 trees ha−1 and a mean diameter at breast height (dbh) of 7 cm were felled at a speed of 0.4 m s−1, with an efficiency of 3.5 s tree−1. The felling speed was limited by the speed of movement of the harvester cranes boom. However, additional tests suggested that the new head could be operated at felling speeds of up to 1.3 m s−1 when cutting trees with dbh values of up to 8 cm. The simulation that prompted this study focused on multiple felling with the crane in continuous motion at speeds of 1 m s−1; the results obtained in this work suggest that the new felling head is capable of sustaining these speeds and so further technical development of the design (e.g. to incorporate accumulating functionality) is warranted.

Comparison of the cost and energy efficiencies of present and future biomass supply systems for young dense forests

The objective was to study the effect of future harvesting and handling technologies on the cost and energy efficiency of supply chains for young dense thinnings. The system costs and energy requirements were modeled using type of stands, products delivered, and transport distances as variables. In total, 14 systems were analyzed, of which five represented future systems. The effects of increasing the payloads of off-road and road transportation of whole tree (WT) parts by 10%, 20%, and 30% were also analyzed. If boom-corridor thinning technologies, optimized bundle-harvesters, and load-compression devices are developed, on average, costs are reduced by 12–27% and 11–30% less energy is required when compared with present systems. For example, at an average harvested tree size of 22 dm3, the supply of WT using future technologies would reduce the cost by up to 15% and the energy requirements by 21%. These effects increase with reduced tree sizes and increased transportation distances. The effects of future technologies are especially significant for tree sizes below 30 dm3, representing a significant part of the potential that could be harvested annually in Sweden. Thus, there should be increased research and development of boom-corridor felling technology, bundle-harvesters, and load-compression devices.