Research paperEffects of storage on dry matter, energy content and amount of extractives in Norway spruce bark
Introduction
The generation of energy from biomass has a key role in current international strategies to mitigate climate change, reduce CO2 emissions and enhance energy security. Biomass from forests is one of the main sources of energy s to help countries meet their long-term renewable energy targets [1]. In Finland, the proportion of renewable energy was set to be increased to 38% of total energy consumption by 2020 [2], and this goal was actually reached in 2014 [3]. This 38% goal is mainly reached by increasing the use of various biomasses, especially forest chips and forest industry by-products in energy generation. The latest statistics [4] state that the use of bark in heat and power production was 7.9 million m3 (49,4 TJ) in 2019. Bark is a by-product from the mechanical and the chemical forest industry. Debarking residues from coniferous roundwood are the most common bark used as fuel.
The first process step in mechanical and chemical forest industry is usually debarking of the harvested timber assortments. The aim of debarking is to separate the bark from wood, to avoid many harmful effects which bark has on the following process stages [5]. Two common methods are used for debarking; drum debarking and rotor debarking. Wood loss in the debarking drum depends mainly on the end product and the quality of the raw material, i.e. its species, temperature (especially when below zero degrees), cutting season, log freshness and the dimensions of the logs [5]. Koskinen [6] estimates normal wood loss to be in the range of 1–3% and Agin and Svensson [7] indicate that a wood loss of 5–6% is normal. The debarking result is a compromise between wood loss and bark removal: high bark removal requires the logs to have a long residence in the drum and wood loss may become high [8].
The value chain of forest biomass for energy always includes storage of the biomass. Due to an imbalance between bark production and the demand from the heating sector, the surplus of bark has become an issue following the initial processing step of debarking at mill sites in period of low demands for bark. As supply volumes and commercial values of biomass have increased, the economic losses associated with poor storage management of biomass have become obvious. Energy yields per unit of delivered biomass can be maximized, and emissions minimized through careful establishment and location of storage, prediction and measurement of changing moisture content, and the ability to match supply with demand [9]. Storage of bark, which is one of the major feedstocks in bioenergy scenarios, has been overlooked in the scientific literature to-date [10]. Research has focused on studying stem wood, whole wood chips and forest residues. Storage of bark is associated with problems like heat development, biomass losses, and the reduction of fuel quality [11,12], due to certain processes such as biological and chemical degradation. Microbial activity in stored bark pile is a major cause of decomposition of the materials leading to heat buildup and potentially high temperatures in the pile. Dry matter losses (DML) between 0.4 and 10.2% per month have been observed in bark storage [11,12]. During this fast decomposition, there exist the risks of emissions, energy losses and fires [13]. Depending on the type of bark, the moisture content can vary substantially, normally between 40 and 65% [14,15].
Bark has a distinctive chemical and anatomical makeup compared to wood. Bark has a higher proportion of parenchyma cells, implying a higher store of easily accessible sugars which gives rise to higher and longer respiration periods and causes greater heat generation [10]. Bark is currently used mainly for producing heat and power but extracting valuable components before burning is an interesting option for bark utilization [16,17]. After extraction of valuable extractives compounds rest of the material, i.e., the solid extraction residue, can still be utilized for bioenergy production. A considerable amount of bark could be used for manufacturing various value-added products. Bark contains two to six times more extractives than stem wood, and it is an abundant but unvalued feedstock for lignocellulosic biorefineries. The total content of both lipophilic and hydrophilic extractives usually corresponds to 20–40% of the dry weight of bark [18]. For this reason, it can be considered a potential raw material for refining.
According to Nurmi [19], the content of extractives in Norway spruce bark is 26.4%. Halmemies et al. [20] detected an even higher extractive content in fresh Norway spruce bark. According to their study, the content of lipophilic extractives was 4.8% and hydrophilic extractives 29.9%. Spruce bark extractives are potential raw material for the production of a range of added value-products, for example, pharmaceuticals or cosmetic ingredients, platform and specialty chemicals, dietary supplements, biopolymers, bioplastics, foams/emulsions, and coatings [17,[21], [22], [23], [24]].
Diminishing of the contents of many extractives starts immediately after tree felling and this degradation continues during storage [[25], [26], [27], [28]]. During storage the chemical composition of the extractives-based fraction changes gradually. It has been observed in previous studies that the content of softwood bark extractives decreases significantly during the first weeks of storage. In the case of softwood bark (mixture of Norway spruce and Scots pine pulp mill barks), the biggest extractives losses occurred during the first week of storage and the total gravimetric extractives content was roughly halved after four weeks storage [29]. Halmemies et al. [20] studied the effects of pile storage on spruce (Picea abies) sawmill bark. The content of total dissolved solids in sawmill bark was 37.6% (of dry matter) before storage. After four week storage period the extractives content varied between 18.4% and 24.7% (of d.m.) depending on sampling location in pile.
It also should be pointed out that at the same time with the changes in gravimetric extractives amount the composition of the obtained extract changes. Living cell respiration, biological degradation, and thermo-chemical oxidative reactions are the key mechanisms which are responsible of major changes during storage of woody biomass [10]. Extractives are also lost due to leaching of compounds when bark is exposed to weathering, and by evaporation of volatile compounds [30,31] During storage wood components are decomposed and complex structures are degraded which leads to formation of more simple compounds. As the chemical composition of bark changes, it has an inevitable effect to the composition of the obtained extract. More volatile compounds, and compounds that are more easily extracted, are formed as a result of degradation of bark components. On the other hand, it has been earlier observed that extractives may undergo also polymerization reactions [27] during storage, which also may have an effect to their solubility in the chosen extraction solvent. For example, condensation reactions of tannins and polymerization of resin acids have been reported to occur during weathering [27,31].
The aim of this study was to determine the changes in physical and chemical properties during the storage of Norway spruce (Picea abies) bark piles. Firstly, the changes were measured in the properties (moisture and ash content, calorific value) which are considered important regarding the bioenergy utilization of bark. In addition, the extractives contents were determined after two, four and eight weeks of storing to study the effect of storage time on the magnitude and rate of the losses in industrial scale storage pile. This information is essential when planning on utilization of bark as a feedstock for extractives-derived compounds.
Section snippets
Experimental and treatment design
The storage experiment was carried out from August 23rd, to October 18th, 2018 in Puhos (62° 10′ N, 29° 55′E), Eastern Finland. One pile of fresh Norway spruce (Picea abies) bark was established at Kitee sawmill. Logs were harvested with a harvester around two weeks before using the mechanized cut to length method. Accumulated rainfall for the two month follow-up period was 172 mm and the mean air temperature was 10.8 °C.
Fresh wood was debarked with a Valon kone VK-820 rotor debarking machine
Temperature
After establishment of the pile, the temperature in the spruce bark pile reached 50 °C within one week in the top and middle of the pile, and within two weeks in the bottom of the pile (Fig. 3). The temperature in the bottom level rose slower than in the middle or in the top of the pile, but at the end of the eight-week study period, the temperatures were between 55 and 61 °C throughout the undisturbed pile. Unloading the pile when taking the samples caused a dramatic decrease in temperatures;
Discussion
The bark was stored from August to October, which is quite a short storage period in an industry with year-round production. During the storage period, the ambient temperature and total amount of precipitation were slightly higher than long term period average (1981–2010) obtained from FMI [38]. The temperature in the spruce bark pile rose to around 60 °C and stayed there for the rest of the two-month storage period. Temperature development in the undisturbed part of the bark pile was very
Conclusions
Forest industry produce remarkable amount of bark which is currently used mainly for producing heat and power but extracting valuable components before burning could create new business models and improve the material circulation in bioeconomy. The use of a relatively inexpensive side-stream such as bark as a feedstock for biorefining has gained lots of interest recently. Despite this, the effect of storage on the chemical composition of bark is not well covered in scientific literature. This
Funding
The authors gratefully acknowledge financial support from the Natural Resources Institute Finland Harvesting alternatives for land clearing wood and buffer storing of comminuted wood biomass- HaBu-project and from European Regional Development Fund, European Union, Local Divided Bioenergy as Solution -project [grant number: A71770].
Acknowledgements
We greatly acknowledge the technical assistance of Perttu Anttila, Jaakko Miettinen, Matti Savinainen, Mikko Karjalainen and Reetta Kolppanen from Luke. We are particularly grateful to Jorma Kaukoaho and Mauno Väyrynen of Stora Enso Kitee Sawmill for organizing the storage experiment.
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