Productivity and costs of stump harvesting for bioenergy production in Latvian conditions

The aim of this paper is to summarize results of two research projects implemented as collaborative actions of Joint stock company Latvijas Valsts meži (LVM), SKOGFORSK, The Forestry Research Institute of Sweden (Skogforsk), Latvian State Forestry Research Institute Silava (Silava) and State limited company VSIA Vides projekti (Vides projekti). The first project, Forest energy from small-dimension stands, infrastructure objects and stumps (LVM, Skogforsk and Silava), has the aim to estimate productivity and prime costs of stump extraction in Latvian conditions; the second project, Biomasas izmantošanas ilgtspejibas kriteriju pielietošana un pasakumu izstrade (Application of sustainability criteria of utilization of biofuels and elaboration of preventive actions to secure sustainable use of biofuels, Silava and Vides projekti), besides other tasks has targeted to estimate environmentally sustainable resources of stumps for biofuel in Latvian forests. The results of the first project demonstrated, that harvestable amount of stumps in calculation to dry tons is 12% of harvested volume of roundwood in cubic meters under bark. This assumption corresponds to average Swedish and Finnish conditions, but it should be evaluated further in Latvia, taking in account different dimensions of trees and composition of forest stands. The productivity of two key elements of stump harvesting, extraction and forwarding is, respectively, 5.2 and 5.1 tdry E0-h (dry tons per efficient hour) with one way terrain transport distance 500 m. The total time consumption to produce and supply 1 LVm3 (LV – loose volume) of stump chips is about 12 minutes in calculation to E0-h, if one way terrain transport distance is 500 m, road transport distance of stumps – 7 km and road transport distance of chips – 50 km. The prime cost of stump biofuel production under the same conditions is 4.89 LVL LVm3 in calculation to current fuel price. This cost includes soil preparation for forest regeneration. The total potential of stumps in clear-cuts according to the Forest inventory data about harvesting in 2007 was 1350 th.tdry. A part of those stumps are located in poor forest types on sandy soils, where stump extraction is not recommended, therefore, from environmental point of view the available resources are about 1228 th.tdry. Taking in account technological losses, technically available resources reduce to 737 th.tdry yearly, corresponding to about 3903 th.MWh of net energy. It should be taken in account, that no technical or economical limitations are included in this estimation. The total carbon (C) emissions during harvesting and supply of stump biofuel as well as utilization of wood ash as a compensatory fertilizer corresponds to 5.6 % of carbon content in biofuel.