انرژی زیستی از نازک شدن جنگل: انتشار کربن، توازن انرژی و تجزیه و تحلیل هزینه

The growing demand for bioenergy in Sweden has drawn attention to the potential of forest thinning as bioenergy feedstock. There are, however, concerns regarding the cost effectiveness and environmental challenges of harvesting and processing forest thinnings into bioenergy. It is against this background that cost, energy and carbon balances were analysed to evaluate some of the economic and environmental sustainability issues of forest thinning based bioenergy systems. Primary data was collected from two thinning operations in two forest plots comprising spruce and birch stands. One operation involved the use of the conventional two machines (one separate machine for cutting or felling and another for forwarding felled trees) for the thinning work. The second operation involved a harwarder, which combines tree felling/cutting and forwarding in one unit machine. The results showed that forest thinnings provide a potential resource for the sustainable production of bioenergy.

Energy derived from biomass sources could play an important role in reducing emissions of greenhouse gasses and providing an alternative source of energy in Sweden and elsewhere. In Sweden bioenergy share has been growing at an increasing rate over the years owing to increasing demand of bioenergy in the construction, district heating, electricity generation and the transport sector [1]. The bio-based final energy use share grew from 9% in 1970 to 27% in 2006 [2]. This growth has been attributed to the bioenergy promotional instruments being implemented in Sweden. The dominating instruments include market support (e.g. national authorities spearheading use of flex fuel vehicles), green certificates, carbon trading, heavy carbon taxes, subsidies on clean energy development and climate change investment programmes [3] and [4]. The growing demand for biomass based energy sources and the need to replace more fossil fuels have prompted increasing search for more sources of bioenergy feedstock. One source gaining increasing attention is forest thinnings. Commercial thinning for bioenergy use, like in commercial logging of fully grown trees, involves felling trees (removal of trees growing too close to each other); forwarding felled trees (primary/in-stand transportation); chipping (or any other thinned materials handling method); transportation to processing facility; and processing into biofuel often in the form of wood chips [5]. A number of questions have, however, been asked regarding the potential of forest thinnings to provide an efficient low cost and environmentally friendly bioenergy system. The economic and environmental performance of forest thinnings based bioenergy system could be determined by a number of factors including the thinning technology, stem volume, bucking, stand density, removal per site, handling unit, forwarding distances, terrain, load capacity, machine operator skills and work environment among other determinants [6], [7] and [8].

The two machine system provided a more efficient energy system compared to the harwarder system in both economic and environmental terms. The better operational efficiency of the two machine system was evident in its ability to complete a bigger thinned area, use shorter time of thinning, lower in-stand primary energy use per hectare of thinning and lower operational costs per hectare. It should, however, be noted that the hawarder system provided a more effective way of achieving energy recovery per hectare and therefore better environmental performance by maximising biomass recovery. This result shows that biomass recovered per unit area of thinning plays an important role in determining the overall environmental sustainability of thinning systems. It also directly affects energy recovered, energy balances and carbon emissions displaced by the replacement of fossil energy. However, just the maximisation of biomass recovery alone does not achieve an efficient forest thinning bioenergy based system. Inefficient biomass recovery contributed to the high costs of energy production as noted in this study. The study shows the importance of the machine operator in achieving the overall performance and efficiency in forest thinning systems. This issue is often overlooked in a field dominated by studies on the technical improvements of machine productivity. It is therefore vital to establish non machine technical aspects of forest thinning and harvesting systems such as machine operator productivity and efficient work methods which significantly determine the overall performance of forest fuel systems. The machine operator skills and different work patterns are not only important for minimising forest fuel systems’ costs, maximising energy output and minimising CO2 emissions as reflected in this study, but also essential for saving important trees for maximum growth, preventing damages to land and residual trees, and therefore reduces forest maintenance costs. These issues need further research and development. The study demonstrates the potential ability of forest thinnings to provide a sustainable energy system. Factors such as the quantity of biomass and the rate of removals per unit area, the optimal machine size for a given forest density, and machine operator’s experience will influence primary energy use, energy recovery and balances, and therefore economic and environmental sustainability of the system.