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  • Author: Peeter Muiste x
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Abstract

In wood moisture contents above fibre saturation point (FSP) the measuring accuracy of resistance type sensors starts decreasing and several side effects related to measuring wood moisture begin to occur. These side effects can be taken into account in the measuring process and can be eliminated. This research examined wood charging and discharging in the process of measurements, phenomena related to repeated measurements in both a single electrode insertion spot and different insertion spots in the specimen made of pine (Pinus sylvestris L.) sapwood and black alder (Alnus glutinosa (L.) Gaertn.). The research presented approaching equations separately for the voltage and current of the polarization and depolarization process for the initial phase of the process. Empirical equations were found for wood electrical resistance and electrical capacitance in the polarization process. For black alder possibilities for calibration of a resistance meter and resistance type electrodes were explored. It was found that to improve the calibration accuracy of resistance type measuring electrodes an additional individual calibration with regard to the average moisture content of the specimen may be carried out with the electrodes above FSP. Research results can be implemented in monitoring the wood drying process using resistance type wood moisture sensors, but also in determining the electrical parameters of growing trees.

Abstract

The properties of biomass-based fuel and combustion tests showed that logging residues are promising renewable energy sources. The data used in this study were collected from four clear-felling areas in Järvselja Training and Experimental Forest Centre, Southeast Estonia in 2013–2014. Logging was carried out by harvesters in Scots pine (Pinus sylvestris L.), Norway spruce (Picea abies [L.] Karst.), silver birch (Betula pendula Roth.) and black alder (Alnus glutinosa L. Gaertn.) dominated stands with a small admixture of other tree species according to the cut-to-length method and logging residues were placed in heaps. The aim of this research is to assess different characteristics of logging residues (quantity, moisture content, energetic potential, ash content and amount) in clear-felling areas. The highest load of slash was measured on the birch dominated study site, where the dry weight of the logging residues was 29 t ha−1. Only the branch fraction moisture content on the black alder dominated site (35.4%) was clearly different from respective values on other sites (21.6–25.4%). The highest calorific value of the residues was assessed with the residues from the birch dominated site, where in moist sample it was 365 GJ ha−1 and in dry matter 585 GJ ha−1. The heating value of the fresh residues is highest in coniferous trees. The highest ash content in branch segments was registered for the black alder dominated site. Järvselja data indicate higher quality in conifer dominated sites, yet a higher load of logging residues in broadleaf dominated stands.

Abstract

In Estonia up to now the producers of wood fuels have not yet adequately analysed the production and transport costs although it is known that the share of these costs form up to half of the final price of wood chips or hog fuel. With this in mind the goal of the case study was defined – to analyse the price formation of comminuted wood fuels in Estonian conditions. Within one year of the survey the initial data was collected from the company SLG Energy. Different types of machines (7 in total) were observed – wood chippers Jenz HEM 561, Jenz HEM 582, Heinola 910ES, and Doppstadt DH 608. All machines were equipped with a GPS tracking device which recorded the route and the fuel consumption and during the observation period all costs and revenues related to the concerned machines were accounted. It was determined that due to frequent repairs the maintenance costs of older machines were higher and productivity lower compared with new machines. The biggest item of expenditure turned out to be the fuel consumption. The second was the cost of repairs in the case of older machines and price in the case of new machines. Also the type of chipper had an impact on the production cost. Under the cumulative effect of various factors the production cost turned out to vary widely from 1.03 to 2.38 €/ml 3.

Abstract

Due to practical need to improve the accuracy of measurement of wood electrical resistance via creating multivariate calibration models the study of electrode effects was carried out. The experiments were made in DC mode and five types of electrode effects were measured: electrical resistance of the galvanic contact between the measuring electrode and wood, corrosion of measuring electrodes in wet wood, polarization and depolarization of the double layer forming on the surface of the measuring electrodes, residual polarization voltage between measuring electrodes and effect of repeated use of measuring electrodes. For multi-channel measurement mode effect of neighbouring electrodes was measured. Behind each of these factors is a complex transfer mechanism of free and bounded charge carriers from wood to measuring electrodes. Among the factors, polarization and depolarization have an immediate effect (duration of the processes in seconds) while resistance of electrode/wood contact, corrosion of electrodes and residual polarization have a slow effect (duration of processes in hours). For pine sapwood, coefficients of the Stamm formula were found in moisture contents (MC) above the fibre saturation point. It was determined that wood polarization and depolarization indicators are dependent on wood moisture content as is the case with wood electrical resistance. Compared to wood electrical resistance, wood polarization proved approximately four times and depolarization approximately eight times less sensitive to moisture content variation.