A review of mechanisms responsible for changes to stored woody biomass fuels

Fuel 175 (2016) 75–86

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Review article

A review of mechanisms responsible for changes to stored woody biomass fuels Sally Krigstin a,⇑, Suzanne Wetzel b,⇑ a b

University of Toronto, Faculty of Forestry, 33 Willcocks Street, Toronto, Ontario M5S 3B3, Canada Canadian Wood Fibre Centre, 580 Booth St., Ottawa, Ontario K1A 0E4, Canada

h i g h l i g h t s
Stored biomass change agents are cellular respiration, microbial, thermo-chemical.
Storage affects biomass fuel characteristics such as moisture, energy, inorganics.
Storage design can limit feedstock loses, reduce moisture and reduce inorganics.

a r t i c l e

i n f o

Article history: Received 17 October 2015 Received in revised form 29 January 2016 Accepted 5 February 2016 Available online 10 February 2016 Keywords: Woody biomass Storage Degradation Mass loss

a b s t r a c t Large scale bioenergy facilities require vast amounts of biomass materials and take advantage of a variety of woody materials in various forms including logs, hog fuel, bark, forest harvest residue, short-rotation hardwoods and whole tree chips. Development of the supply chain logistics necessary to deliver and utilize these material in a cost effective manner is well underway but is strongly dependent on forest type, regional and local harvesting practices as well as location, size and design of storage facilities available. Storage of woody biomass is necessary at various points along the supply chain but the effect of storage on woody biomass is complex and not fully understood. The key mechanisms responsible for major changes to woody biomass on storage are (i) living cell respiration, (ii) biological degradation, and (iii) thermo-chemical oxidative reaction. All three mechanisms involve mass to energy conversion and contribute to self-heating of piles and dry matter losses. Living cell respiration is a short term effect that lasts only several weeks while starch and sugar are readily available and adequate temperature and oxygen levels are present. Biological degradation is caused by a large variety of organisms from bacteria to wood degrading fungi and function best under specific moisture, temperature and oxygen conditions. Finally, thermo-chemical oxidative reactions can contribute to excessive dry matter loss once elevated temperatures have been attained in the pile as a consequence of the first two mechanisms. This review paper discusses the science behind the mechanisms of change to biomass on storage, and draws examples from experimental research to support the explanations. Ó 2016 Elsevier Ltd. All rights reserved.

Contents 1. 2.

3.

Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanisms of change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.1. Living cell respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.2. Biological degradation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.3. Thermo-chemical oxidative reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2.4. Moisture evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mass loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.1. Implication of storage form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.2. Implications of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3.3. Implications of moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

⇑ Tel.: +1 613 866 6108 (S. Wetzel), +1 416 946 8507 (S. Krigstin). E-mail addresses: [email protected] (S. Krigstin), [email protected] (S. Wetzel). http://dx.doi.org/10.1016/j.fuel.2016.02.014 0016-2361/Ó 2016 Elsevier Ltd. All rights reserved.

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4.

5. 6. 7.

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Moisture losses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.1. Transpirational drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 4.2. Moisture loss in covered storage versus uncovered storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Energy content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Inorganic constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

1. Introduction Review of the literature pertaining to storage of woody biomass, in piles of chips, chunks, logs and coppice stems, indicated that considerable research has been undertaken to understand the changes that take place in woody biomass when stored. Early work has focused on clean chip piles, logs, and storage of forest harvest residues intended for use in pulp manufacturing or energy production. The alteration of biomass characteristics on storage has been shown to be influenced by storage time, climatic conditions, species composition, and form of the biomass, as well as geometry and structure of the storage pile. Although numerous studies have been performed under controlled and monitored conditions, results are still plagued with discrepancies. For this reason it is very helpful to understand the fundamental processes that can affect biomass in storage so that well thought out storage regimes can be designed based on theory that can be applied to specific conditions and situations. This review includes pertinent literature on the influence of storage conditions on biomass characteristics and the mechanisms responsible for their alteration. Understanding the mechanisms and their effects can inform optimal storage design with an aim to beneficially pre-process biomass for its intended end-use, whether for pellets, direct combustion, or bioconversion to chemicals. ‘‘Best” storage practices must consider material losses, safety, cost and quality. Conversion of biomass to energy or bio-products, such as pulp, necessarily requires some period of storage to enhance properties, i.e. reduce moisture or to ensure that sufficient material is on hand to cover periods where harvesting is limited. Pulp mill personnel and researchers, searching for the most effective pulp chip yard management strategies, conducted early examinations into the effects of storage on wood [1]. Fluctuating supply often necessitated inventory levels ranging from 3 to 26 weeks of supply [2]. Later, beginning with the oil crisis of the 1970s, a body of literature developed examining the best characteristics for wood as industrial fuel and the best ways to store wood to achieve those characteristics. Findings agree that during storage wood moisture content will change and dry matter losses due to biological and chemical degradation are likely. There are numerous factors that have been identified which influence the change in woody biomass on storage. A partial list of the variables would include (i) local weather conditions, (ii) form of material (ie logs to particles) (ii) pile density, (iii) pile size and geometry, (iv) species, (v) covered or uncovered, (vi) initial moisture content and (vii) season of harvest. Due to the vast number of alternative storage scenarios and inherently inhomogeneous nature of woody biomass, it is extremely difficult to test the quality changes that take place over time. Therefore, it is the intent of this review to survey the body of literature for information that explains the mechanisms for the changes that affect biomass fuels when stored over time. Once understood, these general principles can be applied to ensure the management of biomass during the storage stage is safe, cost effective and delivers the required quality to energy generating facilities and biorefineries.

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This review examines the mechanisms responsible for changes which take place in biomass on storage and the impacts of these mechanisms on moisture content, chemical composition and dry matter loss. 2. Mechanisms of change The quantity of biomass material available for energy and other bio-products is ultimately affected by the material mass loss (water, dry matter, volatile chemicals) that takes place during storage. Loss of mass can be considered positive or negative depending on the anticipated end-use application. For example, moisture loss is advantageous for reducing transportation costs in terms of $/GJ-km when the material is intended for direct energy conversion. However, water removal may not be an advantage for biomass used for bio-ethanol production where water is added in downstream processing. Valuable products that can be refined from biomass such as volatile oils, tall oils, and turpentine may be lost completely depending on storage time and conditions. Furthermore, dry matter loses will affect all utilization strategies in terms of increasing cost for material procurement per unit dry weight. Mass loss in stored chips or biomass material can occur through five mechanisms; (1) living cell respiration, (2) biological degradation, (3) chemical reactions, (4) moisture evaporation, and (5) material handling. 2.1. Living cell respiration Parenchyma cells, which comprise a relatively small proportion of the secondary xylem tissue and larger proportions of bark and foliage, are responsible for respiration in plants. Respiration is a relatively minor process in plants but is required to provide energy for cellular processes. Respiration is a catabolic process that is summarized by Eq. (1).

C6 H12 O6 þ 6O2 ! 6CO2 þ 6H2 O þ DHcombustion glucose  2; 805 kJ=mol ð1Þ However, in the aerobic catabolism of glucose in plant cells there are also a number of endergonic reactions assist in the production of adenosine tri phosphate (ATP), a form of stored energy. Therefore, the net energy that is liberated as heat energy from the biochemical processing of glucose (C6H12O6) has been found to be 263,000 cal/ mol glucose (1100 kJ/mol) [3]. When wood is harvested and comminuted the parenchyma cells, which were previously confined within the xylem tissue, are exposed to air (oxygen). Exposure to the ambient air facilitates respiration in the live parenchyma (primarily ray cells), which consumes stored sugars and causes a minor mass loss. The respiration reaction also generates heat that further catalyzes chemical and biological degradative reactions. Respiration has been observed and measured in fresh cut wood chips [4–7]. In typical Great Lakes-St. Lawrence hardwood species (Acer rubrum, Fraxinus americana, Quercus rubra) and softwood species (Pinus strobus, Tsuga

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canadensis) higher respiration rates were found in the youngest sapwood in material isolated from living trees [8]. Hence, forest residue biomass piles comprised of small twigs, tree tops, bark, and foliage would be expected to have a relatively high proportion of viable parenchyma cells and correspondingly high respiration rate and, congruently, higher heat generation. Respiration rates are species dependent, influenced by the relative volume of parenchyma cells in the wood tissue. With about 1/3 the volume of parenchyma cells, softwoods show a lower overall respiration rate based on total tissue volume of 0.4–0.6 lmol O2 cm3 h1 as compared to 0.3–9.5 lmol O2 cm3 h1 for hardwoods [8]. This suggests that hardwoods would have a tendency to accelerate heating in storage over softwoods. Typically, respiration in fresh cut chip piles can last from 10 to 40 days given suitable conditions [4,6,9] and when stored in log form can continue for up to 6 months [10]. High temperatures will result in inactivation of cellular enzymes and reduce the ability of the parenchyma cells to respire [6,7]. In fresh cut sapwood of Populus tremuloides (trembling aspen) and Pseudotsuga menziesii (Douglas-fir), respiration was observed to cease completely as temperatures in excess of 55–60 °C were reached or after 32 days under moderate temperature conditions (21 °C) [4,5]. Both trembling aspen and Douglas-fir showed a decline in activity above 42 °C and termination of activity at 60 °C [7]. Respiration rate is also sensitive to low temperature and will reduce activity at 4 °C. However if logs are stored at low temperatures (2 °C) for prolonged time their parenchyma will lose viability at a very slow rate [4,8]. Thus, wood harvested and stored over the winter and chipped in the spring may still have viable parenchyma cells that would readily respire as the ambient temperature increases. While respiration in the living parenchyma cells is responsible for a relatively small mass loss in woody biomass through the catabolic processing of stored starch and glucose, it has a greater influence on mass loss of the biomass by creating environmental conditions which are favourable to microbial colonization and chemical oxidative reactions. In fact, biochemical studies on heat generated from the aerobic respiration process indicate that 4.82 kcal (20.17 kJ) of heat is released per litre of oxygen (O2) consumed [6]. Measurements of oxygen consumption by fresh cut aspen and Douglas-fir chips (free of microorganisms) were 0.102 and 0.051 (ml/h)/odg, respectively, which represent a heat release of 0.49 and 0.25 (cal/h)/odg (2.06 and 1.05 (J/h)/odg) [6]. Assuming the specific heat of wood at 17 °C and moisture content of 20% is 1.8 J/g °C [11], then the temperature of one gram of wood chips could theoretically rise by 26 °C over a 24-h period from respiration activity alone. Empirical observations of this nature are readily available and reports of pile temperatures reaching 49–82 °C after a 7-day period are common [9,10]. The increased temperature of the woody biomass promotes direct oxidation of wood constituents and also provides a preferential environmental for microbial growth of bacteria and wood degrading fungi. 2.2. Biological degradation Numerous types of microorganisms will colonize wood chips as conditions turn favourable for their growth. These conditions include suitable temperature (15–60°), moisture level above fibre saturation point (fsp), and appropriate oxygen and carbon dioxide concentrations. The first groups of organisms that are believed to colonize wood chip piles are aerobic bacteria but their contribution to mass loss is believed to be minimal [5]. Very high bacteria counts of up to 7.4  106 cells/odg to 2  109 cells/odg have been observed in tropical wood chip piles as well as in stored agricultural crops such as alfalfa hay and oat straw [5]. These early colonizers generally

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metabolize only the starch stores in the freshly cut wood [10] and hence their contribution to elevating pile temperature is believed to be short-lived, as is the influence from parenchyma cell respiration. Ferrero et al. [13] observed that microbial activity was responsible for the initial temperature increase in stored pine chips/sawdust but ceased after a few days due to the diminished supply of easily digestible sugar materials. The rate of oxygen consumption of the bacteria (0.076 (ml/h)/odg wood) was similar to the respiration rate of parenchyma cells (0.102 (ml/h)/odg wood), as mentioned previously. Hence, a similar heat release to cellular respiration can be expected for the bacterial respiration (0.37 (cal/h)/odg) occurring in the pile [5]. It has also been observed that anaerobic bacteria do not cause a significant amount of heating in wood chip piles, and CO2 levels greater than 32% limit viability of these microorganisms [4]. Temperature is reported to be the most important factor in determining the number and type of microorganisms inhabiting stored chip piles [12]. In controlled experiments, which monitored chip pile bacteria counts and temperature over time, it was observed that the two variables are highly correlated. Simultaneous increases in bacteria counts and temperature were observed during the first 7–10 days and then again after 45 days [5]. Higher amounts of mesophilic bacteria were identified at the onset, before the temperature of the pile increased significantly, with thermotolerant bacteria strains becoming dominant once higher temperatures were reached. Bacteria themselves have low tolerance to temperatures above 82–94 °C [10] and they require high levels of moisture. In summary, bacteria colonization is not directly responsible for significant mass loss in stored biomass but these organisms contribute to the initial increase in the temperature of a pile and consequently accelerate chemical oxidation processes and/or make the environment suitable for other, more destructive microorganisms. Fungal degradation will readily occur in stored woody biomass when favourable conditions are realized. All wood-destroying organisms use the degradation products of wood (e.g. glucose) as their energy source, however they also require an adequate moisture level (30–50% MC), a specific temperature regime, and a suitable oxygen level to thrive. Nitrogenous compounds, vitamins, and essential elements are also necessary for fungal growth and subsequent biomass decay. Biomass or wood with an extreme moisture condition, such as being dry or water-saturated, will seldom decay; however most moisture levels above the fibre saturation point (25–32%) are favorable for fungal growth. Water in the biomass is compulsory for fungal growth and performs a number of functions. Water (1) is one of the reactants used in the enzyme catalyzed hydrolysis reaction to break down the glycosidic bonds between adjoining glucose molecules in the cellulose or hemicellulose polymers, (2) serves as a diffusion medium for enzymes and solubilized molecules, and (3) serves to swell the small capillaries in the cell wall which enables penetration of fungal digestive enzymes into the substrate [14]. Moisture levels of 30–50% in biomass are ideal for fungal growth, but studies have shown that fungi can actually grow on wood with moisture content as low as 10%, given a high humidity in the air around the pile [15]. Fungi are well adapted to metabolize and thrive under a wide range of temperatures. One main classification of fungal organisms is based on the temperature range in which they grow. Mesophilic fungi are comprised of species that exhibit optimal growth in the temperature range of 20–30 °C and are predominant when the storage pile is near ambient temperature [16,17]. Thermophilic species demonstrate optimal growth at 40–50 °C and are found in high proportion during self-heating of piles. However, most fungal activity will cease at temperatures above 60 °C [18]. While most wood inhabiting fungi are of the mesophilic type, there are

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some soft-rot fungi which demonstrate exceptional tolerance to heat. Thermophilic species (Humicola lanuginose, Talaromyces emersonii, Thermoascus aurantiacus) were found in self-heating environments of stored chip piles in California [18,19]. Also both thermophilic and thermotolerant species were identified by Shields [20] in the microorganism communities in chip piles in Quebec, Nova Scotia, and New Brunswick. He deduced that the various microorganisms present in different areas of the wood pile were determined not only by specific temperature but also by pH conditions. Wiselogel [21] suggested that most of the microorganisms found in stored material fed on free sugars, rather than decaying the wood. However, decay agents including fungi and some bacteria were present and degraded the wood at a rate of about 1% dry matter per month. They were found to be most active at the 30–45 °C temperature range and 40–100% moisture content. Most wood degrading fungi require oxygen for their metabolic process (aerobic respiration). There are however some fungi, such as yeasts, which require very low oxygen levels to ferment sugars and others that require no oxygen, but instead use oxygenated compounds in their anaerobic respiration process. However, for the most part, the growth of decay fungi is influenced by the concentration of oxygen in the environment, with some organisms able to function at 1% oxygen concentration in air, but most preferring levels above 20% for optimal growth and decay development. Ernstson et al. [22] demonstrated that reduced oxygen levels lessen the degradation rate of woody biomass samples. This infers that structuring a storage pile with reduced air permeability will retard fungal degradation. There are definite differences in tolerance to lower O2 and higher CO2 concentrations in the air by specific fungal species, so it is important to understand the specific fungal populations present in any stored biomass [14]. The comparatively low nitrogen content of wood limits fungal growth, as fungi require nitrogen to produce the enzymes responsible for the degradation of the wood polymers. Suadicani and Heding [23] noted that the highest quantity of fungal spores and fungal degradation were found in samples containing bark and in piles of comminuted stored wood [24], as a result of higher available nutrients in small branches and foliage. Mixed biomass provides higher levels of nitrogen and other nutrients for the microorganisms than clean chips/wood and is therefore a preferred resource for accelerating degradation in stored biomass piles. There are three broad classifications of fungi that can cause significant dry matter loss in woody materials; (1) brown rot fungi, (2) white rot fungi, and (3) soft rot fungi. Staining fungi and bacteria have been shown to metabolize the easily degradable fatty acids, triglycerides, and simple carbohydrates contained in the parenchyma and resin canals, but do not readily degrade wood’s major polymers [25,26]. White rots consume both major wood polymers (holocellulose and lignin) while brown rots consume primarily polysaccharides (cellulose and hemicelluloses), leaving behind a predominately lignin-based residue. Soft rot fungi are capable of degrading cellulose but leave lignin largely untouched. In a comprehensive literature review Hellenbrand and Reade [17] report that Hoover-Litty and Hanlin [27] found the seven most common genera to colonize wood piles were (Tricoderma, Fusarium, Chaetomiu, Aspergillus, Rhizopus, Graphium, and Penicillium) with Tricoderma (soft rot) being the most prevalent. They identified a plethora of species, 221 in total, but reported that generally the fungal populations were dominated by a few species. Fungal populations were also found to change over storage time and also by location within the pile. Assarson et al. [28] specified optimal conditions under which 20 species of fungi consumed aspen, birch, pine, and spruce, and further diagrammed the position in a birch chip pile at which they were most likely to be found. Microbial degradation of wood extractives has been observed with wood inhabiting fungi [29]. Farrell et al. [12] reported on a

number of studies involving the degradation of extractives by a variety of Basidiomycetes species (Phanerochaete chrysosporium, P. subacida, P. gigantean, P. tremellosa, and Hyphodonia setulosa) on a number of softwood species (spruce, yellow pine, loblolly pine). Reduction in extractive weights from 40% to 50% over a two-week period were observed. Sap-stain fungi also have the ability to metabolize extractives. Ceratocystis adipose, Ophiostoma piliferum, and Ophiostoma piceae showed reductions of 25–40% over a two-week incubation period. Josefsson et al. [25] demonstrated that triglycerides were rapidly degraded while steryl esters and fatty and resin acids remained unchanged in Scots pine pulp chips inoculated with O. piliferum. They surmised that the fungal enzymes hydrolyze the triglycerides to fatty acids and glycerol providing the fungal mycelium with a usable source of food. Wang et al. [29] were interested in the detoxifying effect of various fungi on resin acids in lodgepole pine sapwood and found that O. piliferum, O. picea, Lecythophora sp. and Ophiostoma ainoae were all effective at reducing resin acid concentrations. Removal of extractives from biomass makes the material more susceptible to wood decaying organisms since a number of extractive compounds contain anti-microbial qualities. Fungal degradation of stored biomass is a standard occurrence. By understanding and controlling the conditions of biomass storage, fungal growth can be accelerated or limited. For example, increased particle surface area and increased bark/foliage will enhance fungal growth. Limiting the permeability of the pile through compaction and thereby restricting air flow will retard or deter fungal growth. Encasing chip piles in unbreathable membranes may also restrict oxygen and retard fungal growth [4]. Other findings suggest that chips harvested in the summer are more susceptible to microbial growth and drying wood prior to chipping make them less susceptible to fungal infection [17]. Additionally, higher rates of decay (as measured by lower specific gravity) were noted on the surface of hardwood chip and hardwood bark piles than within [30]. This could be a result of better oxygen availability for the fungi on outside of pile. 2.3. Thermo-chemical oxidative reactions Direct chemical oxidation of wood constituents can occur at elevated and even ambient temperatures. It is important to note that the oxidative reactions are responsible for temperature rise in wood chip piles, especially at higher temperatures, once metabolic energy generation has occurred [31]. At temperatures in excess of 80 °C exothermic oxidation reactions contribute to selfheating and possible auto-ignition. Oxidative reactions in microorganism-free chip piles of aspen and Douglas-fir were observed at 40 °C and became the primary oxygen consuming reactions at 50 °C as the respiration of ray cells ceased functioning [7]. The oxygen consumptions of these reactions were measured at 0.280 and 0.079 (ml/h)/odg for fresh aspen and Douglas-fir, respectively, and the rate of oxygen consumption along with temperature continued to increase through a 120 h time period [7]. Correspondingly, the rate of heat release for the oxidation reaction was found to be 1.35 (cal/h)/odg and 0.38 (cal/h)/odg for the aspen and Douglas-fir as above, much higher than that given off during the bacterial or parenchyma respiration phases [6]. High temperatures catalyze oxidative reactions associated with the various wood components. Mass loss resulting from thermal oxidative reactions in wood has been reported to initiate at 120 °C with prolonged exposure [32]. At temperatures of 130 °C and higher, thermal degradation of woody material becomes substantial. Generally at temperatures between 100 and 200 °C the fibres dehydrate and water vapour is generated. The pyrolytic breakdown of wood results in numerous degradation products and volatile organic compounds (VOCs). Detailed discussion of

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pyrolytic degradation of wood can be found in Beall and Eickner [33]. The components in the biomass that are least thermally stable are the volatiles (essential oils) and the polyoses (hemicellulose). Hemicellulose is a complex polysaccharide and its chemical composition is species-dependent. In general, hardwood hemicelluloses are comprised of xylan (5 carbon sugar) chains while softwood polyoses are comprised primarily of glucomanan (6 carbon sugar) chains. The xylan chains of hardwood polyoses are irregularly branched and many of the C2 and C3 hydroxyl groups of the xylose unit are substituted with O-acetyl groups. At temperatures of 60–70 °C the acetyl groups will cleave and form acetic acid and or formic acid. This is an exothermic reaction that produces heat and lowers the pH of the material [10]. The decreasing pH and increasing temperature creates conditions that enhance the reaction rate of acetic acid production. The low pH also creates a condition which encourages the hydrolysis of the cellulose molecules creating shorter chains and auto-oxidation reactions leading to higher pile temperatures of 80–90 °C. Much of the acetic acid formed from deacetylation will migrate to the outer and upper layers of the chip pile and be deposited with evaporating moisture, leading to a lower than average pH layer [28]. White et al. [30] found no change in pH of water soluble extract in piles of hardwood whole tree chips (4.1) and hardwood bark (3.8) stored for a period of 50 weeks, but in an earlier study, White and DeLuca [34] did find that the pH of residues decreased over a 5 month storage period, following a trend with increasing temperature. Loss of acetyl groups derived from the hemicelluloses may be correlated with position within the pile, temperature at that location, and degree of compaction. Monitoring for low pH in water runoff from piles may indicate the early on-set of thermal degradation [1]. 2.4. Moisture evaporation Fresh woody biomass contains a large proportion of water. The weight of water in living trees exceeds the dry matter weight and hence the moisture content on a dry weight basis can frequently be over 100%. Water in the wood cells is held as ‘‘free water” in the cell lumen or as ‘‘bound water” within the microfibrilar structure of the cell wall. Removal of water from woody type biomass is controlled by environmental factors such as relative humidity and temperature and by morphological and chemical characteristics of the biomass. As wood dries the free water is first to leave, followed at a much slower rate by the bound water, which is chemically bound by the hygroscopic cell wall components. The point at which there is no water remaining in the cell lumens but the cell walls are fully saturated is called the fibre saturation point (fsp). The moisture content at fsp for most Eastern Ontario woods is 25–30% [11]. There are two predominant mechanisms by which moisture can contribute to self-heating of biomass piles: (1) through the heat released on moisture absorption (heat of wetting) by the biomass and (2) through the heat of condensation. The heat given off when 1 g of liquid water is absorbed by wood material is 1170 J/g at 20 °C, and decreases at higher temperature. The heat of wetting is a small contributor to the overall heating of biomass, as only 80 J/g of energy would be released on complete wetting of totally dry material [35]. Conversely, the heat released on condensation of water vapour onto the biomass material is more significant. The heat of condensation can vary between 2440 J/g of water at 20 °C to 2265 J/g of water at 100 °C [35]. Practically speaking, and ignoring any heat transfer, 5% absorption of ambient (20 °C) water vapour by woody material can raise the temperature by 80 °C, given a specific heat of wood of about 1.3 J/g °C. However, as water vapour travels throughout piled biomass it simultaneously condenses and vapourizes. The result is that the heat given off to the woody material on condensation of water vapour is used

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to provide the heat necessary to vapourize liquid water, theoretically maintaining a relatively constant temperature within the pile. However, factors such as initial moisture content, pile compaction, pile geometry, prevailing winds and pile coverings can influence the localized heating in a biomass storage pile from vapour condensation and evaporation. 3. Mass loss The quantity of biomass material available for energy is ultimately affected by the material mass losses during storage. Loss of mass can result from loss of moisture, loss of volatile chemicals, and loss of dry matter. Depending on the intended end use of the biomass, the mass losses may be beneficial or not. For example, loss of moisture and a drier material is preferred for direct energy conversion applications, while the process for conversion to bioethanol favours biomass with a high percent of polysaccharides and higher water content. Dry matter loss can be largely affected by biological degradation. The feedstock’s chemical composition, the specific biological agents, accessibility of the feedstock to the organisms, as well as environmental conditions such as humidity, temperature and oxygen levels all play a factor in the rate of dry matter loss that occurs on storage. 3.1. Implication of storage form Studies that monitor dry matter loses have been carried out on a number of different forms of biomass in various parts of the world. Climatic conditions and weather regimes can greatly affect dry matter loss of materials stored outdoors. Quillin [1] reported a general dry matter loss of 1% per month for pulp chips stored outdoors in North America, which closely agreed with a mass loss of 1.2% per month reported for birch chips stored under cover in Norway [36] and Wiselogel’s [21] findings of 1% mass loss per month. Gjølsjø’s [36] work demonstrated that when material was stored in larger pieces such as firewood size, a lower dry matter loss of 0.07% per month was observed. Mitchell’s [37] trials had similar outcomes with an average monthly dry matter loss of 2% for Sitka spruce chips and 1.7% for chunks under covered storage. In uncovered storage, the losses were greater at 4% and 10% per month for chips and chunks respectively. Assarson et al. [28] reported that losses in round wood storage were about 1.1% for one summer and 1.7% over two warm seasons for pine in Scandinavia, which differed from higher losses of 6% for pine in the southern United States. Climatic conditions are believed to be responsible for the divergent observations. Whole tree chipping systems and mower-chippers for short rotation hardwoods necessitate the storage of biomass in its harvested form of chips/particles [84]. Industrially scaled experiment on storage of short rotation poplar using fine chips and coarse chips produced from two different harvesting systems was carried out in Germany [83]. No significant difference was found in dry matter loss over the entire storage period (22% for fine chips and 21% for the coarse chips) [83]. Both piles showed rapid degradation during the first 3 months, once the temperature of the pile was above 0 °C, however particle size proved not to be a factor in either dry matter loss or moisture loss [83]. In theory, larger particles, and lower compaction, in a pile should facilitate heat transfer and thereby prevent excessive heating of the pile, which would other wise provide the perfect environment for fungal and chemical degradation of the biomass. However this phenomena may be offset by the fact that the larger particles also allow for more rapid gas exchange, providing the oxygen required for higher fungal metabolic processes.

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A number of researchers have noted that there is a greater rate of weight loss for forest residues at the initiation of the storage period, especially when stored on the harvest site [24,28]. Assarson et al. [28] estimated that half of the loss occurred during the first month resulting from loss of low molecular weight carbohydrates, resins, and acetic acid components. However, others have shown that the major mass losses upon storage of forest residues is due to foliar and moisture losses. When forest residues were stored on the harvesting site a loss of 11% was recorded over the summer, with greater losses occurring in larger piles [24]. Andersson et al. [38] noted that forest residues stored in small piles at the harvesting site lose more foliage than windrows stored at roadside. Flinkman et al. [39] reported that loss of forest residues stored in heaps on the harvest site could be attributed to needle loss – most of which dropped between March, when the site was harvested, and July. Nurmi [40] calculated the dry matter loss of needles stored on the harvesting site versus being brought to landing for storage. Needle composition fell from 27.7% to 6.9% for the biomass left on site compared to 18.9% for biomass stored on the landing. Storage of bark, which is a major feedstock in bioenergy scenarios, has been largely overlooked in the scientific literature to-date. Research has focused on clean chips and whole tree chips and forest residues. Bark has a distinctive chemical and anatomical makeup as compared to wood. Bark generally has higher proportion of parenchyma cells, implying higher store of easily accessible sugars which gives rise to higher and longer respiration period and causes greater heat generation. Bark may also be more susceptible to fungal invasion, as studies have found that parenchyma facilitate the spread of fungi [85]. This is backed somewhat by evidence presented by Thörnqvist [24], where piles containing bark compared to those that were bark-free, contained more fungal spores and exhibit higher degradation. In addition to anatomical differences, there are significant chemical differences between wood and bark with bark typically containing higher concentrations of lignin, extractives and inorganic metals [72]. Furthermore, needles and bark were found to have higher degradation rates than sapwood [22]. However, bark of certain tree species can contain antifungal agents that can resist fungal invasion during storage [86]. Therefore, while bark-chip piles have similar dynamics to woodchip piles, it is important to keep in mind that there are inherent differences between the two feedstocks which can affect the dry matter losses.

3.2. Implications of temperature Mass loss is indirectly influenced by the temperature of a biomass pile as higher temperatures create conditions amenable to wood degrading microorganisms and chemical oxidation reactions. Higher mass losses have been observed where the temperature was optimum for fungal activity (i.e. 20–30 °C), with lower losses observed in warmer piles unless the thermophilic fungi, Chrysosporium lignorum, were prevalent or combustion took place. Ernstson et al. [22] simulated field storage conditions and tested degradation rates of needles, bark, and sapwood fractions at various temperatures (15–55 °C). They found the maximum rate of degradation occurred at 25 °C for all three biomass materials. Almost no degradation occurred at the 15 °C minimum or the 65 °C maximum. Of the three fractions, the needles and bark (0.37 and 0.50 kg/kg dry matter-month) showed an order of magnitude higher rate of degradation than the sapwood (0.043 kg/kg dry matter-month). Ernstson et al. [22] also showed that material collected in the winter months showed no degradation, most likely due to an absence of microorganisms. White et al. [30] concurred with this observation noting that frozen hardwood biomass stored over the winter had no dry matter loss. In White’s work dry matter

loss only occurred once ambient temperature increased to about 20 °C. Self-heating of piles and the resulting degradation of material can be somewhat controlled by pile structure and size. Accumulation of fines and the associated compaction, which can occur in piles due to particle settling or grinder performance, can lead to areas where reduced heat dissipation and increased self-heating rates are likely. A study of ground forest harvest residues in the Great Lakes-St. Lawrence region stored for 1 year in large piles at roadside showed a distinct change in compaction of the pile over time with a higher amount of fines on the inside (17.6%) as compared to the outside (52%) [41]. Experimental data from Ferrero et al. (2009), showed that the internal temperature of a sawdust pile reached approximately 70 °C, while a chip pile reached only 50 °C, under near identical conditions [13]. Quillin [1] suggests that shorter pile height (500 ) can lead to less compaction and slower rate of heat build up, while larger piles (1000 ) have high compaction and an increased probability of self-ignition. 3.3. Implications of moisture content Reducing moisture content is an important objective for storing biomass that is destined for direct energy generation. In wet biomass, energy produced upon combustion is consumed to evaporate moisture from the material, thus reducing the usable energy available or the fuel’s net heating value [41]. The net heating values for freshly ground, mixed forest harvest residues are reported between 10.2 and 8.7 MJ/kg. Another important reason to reduce moisture content is to make the biomass less hospitable to decay agents, which will also suppress dry matter loss [28,42]. This point is illustrated in a study on mass loss in chip piles in Sweden [43]. Chips with initial moisture contents of 20%, 32%, 42%, 51% and 58% exhibited monthly dry matter losses of 0.23–0.35%, 1.03%, 1.1%, 2.2% and 2.6%, respectively. 4. Moisture losses Water is contained in all components of a growing tree from the roots to the foliage. The amount of moisture contained in a plant depends on soil moisture level, the plant’s physiology, season, and medium term weather conditions. Water content of tree components decreases from the outside of the tree inwards, i.e. crown and roots to heartwood. Typical moisture contents for some Great Lakes-St. Lawrence Forest species are illustrated in Table 1. Biomass will dry under natural conditions to an equilibrium moisture content (EMC), which is determined by ambient temperature and humidity conditions. Regionally specific and species specific tables have been developed which show the relationship of temperature and humidity to EMC [47]. Once a tree is cut, the free water in the wood cells will move towards a lower humidity environment if available. The quickest path for the water to move is through the cell lumen, parallel to the wood grain. Therefore it makes sense that the shorter the path the water must travel, the quicker the drying rate. Estimates of air-drying times for commercial lumber have been well established and reported upon. Simpson and Hart [48] present a comprehensive guide to yearly drying times for specific species and locations across the U.S.A. Moisture losses in stored biomass are dependent on local climatic conditions (precipitation and temperature), material composition, form of biomass, pile size and permeability (compaction), and the activities that affect the internal temperature of the pile. As noted earlier, respiratory activities of living cells and naturally present microorganisms generate CO2, H2O, and heat. Agblevor et al. [18] found that pockets of wet and warm bagasse, present in the predominantly dry interior of piles, were directly above microbial

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S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86 Table 1 Moisture content (total weight basis) of some Eastern Ontario tree species. Common name

Paper birch Trembling aspen Black spruce Red maplef White ashf Norway spruce logging residuec Birchd Forest harvest residue (ON, Canada) a b c d e f g

Moisture content percent (wet basis) Heartwood

Sapwood

Inner bark

Outer bark

Other

43–47 62 52b 41.1g 32.2g – –

42 53 113b

40.4 39.4–40.2a 0.32a,b 42.7a 40.6a – –

18.4 – –

– – –

– –

56 42.4 40.1–47.5e

– –

Value represents total bark [44]. [11]. [40]. [45]. [41]. [46]. Value represents stem wood.

degraded material. Heat generated through these activities also serves to increase the rate of moisture evaporation, however, removal of water vapour from the pile is dependent on the pile’s permeability and air flow dynamics. Nurmi and Hillebrand [49] found that the season of harvest and storage did influence the final moisture content of harvest residues. Residues harvested and stored in uncovered piles early in the spring and allowed to dry over the summer reached the lowest moisture content by the end of the summer; however, if they were stored over the following winter the moisture content increased. Similar findings were reported by Gigler et al. [50] of the Netherlands in natural drying of willow stems in piles that had a very low air resistance as compared to chip piles. Accelerated drying rates were noted at the beginning of drying due to the high moisture gradient between the biomass and the relatively low air humidity. Over a one-year period, the moisture content in the stored willow stems was reduced from about 100% to 20–30% (dry weight basis) despite rainfall. The precipitation was found to cause a slight increase in moisture content but because of the protection of the bark, and its low diffusivity, the increase was shortlived. Size of the chip storage pile has been reported to have a significant effect on moisture reduction with storage time. In fact, in studies by Thörnqvist [24] and Koch [51], an absolute reduction in moisture in stored chip piles was found to depend on pile size. Piles smaller than 120 m3 exhibited similar moisture losses to that reported by Nurmi and Hillebrand [49], while those above 600 m3 exhibited redistribution, rather than a reduction, in average moisture content. These observations were supported by Acquah et al.’s [41] study which showed that the moisture content in the pile continued to stratify over 2 years of storage, with the outer layer decreasing from 28.9% after 1 year of storage to 19.3% after the 2nd year of storage while the inner layer increased from 67.8% to 73.1%. The initial moisture content of the material was 40.3%. White et al. [52] made conflicting observations in that piles with greater height (ranging from 10 ft to 20 ft) had lower average moisture content after 360 days of storage because they were less affected by precipitation and had a higher dry core volume than smaller piles. White and DeLuca [34] reported that the average moisture content of residues stored in piles was strongly dependent on the composition of the material. The moisture content of pine bark decreased by 52% over a five-month period while hardwood and softwood sawdust declined by only 15% over the same time period and under the same climatic conditions. Even more surprising, a hardwood bark pile showed reverse effect, gaining 47% moisture

over the same storage period. In this study it was observed that all residue pile surfaces remained saturated with moisture through the 6 weeks of the study, illustrating the effect that local weather can have on uncovered stored biomass. 4.1. Transpirational drying For biomass harvested in the spring and summer seasons, transpirational drying has proven to be an effective means of reducing moisture content. Transpirational drying occurs when foliage is left on trees, tops, or branches after the felling operation. In a living plant, as water is evaporated from the leaf surface, the transpired water is replaced by water held within the xylem, causing a rapid removal of water. Stokes et al. [53] reviewed a number of research studies on transpirational drying and reported that the important variables are the felling season, species, and diameter. In a study looking at different species and their response to transpirational drying, red oak (ring porous species) harvested with foliage intact showed no significant difference in moisture content after drying for 3 weeks as opposed to bolts that were immediately bucked and stacked. White birch (diffuse porous species), on the other hand, showed a significant loss of moisture under the same conditions. Lawrence [54] concluded that diffuse-porous species and softwoods experienced a faster drying rate than ring porous species and could attain minimum moisture content of 40% (od basis). Another study, done in New Zealand, found that younger trees experienced greater moisture loss than older trees, and the older trees showed no real moisture change if left to dry with their leaves intact [55]. The sapwood of Douglas-fir lost more moisture than the heartwood in the Saralecos et al. [56] study, suggesting younger trees (which have a greater proportion of sapwood) will be more beneficially affected by transpirational drying. This study also showed that smaller diameter (12.7–25.4 cm) trees had a greater initial rate of moisture loss than large stems (38.2– 50.8 cm). The most significant loss of moisture resulting from transpirational drying occurs directly after harvest. Garret [57] noted that most of the moisture loss occurred within 36 h of felling. Saralecos et al. [56] noted the initial losses for the large trees were about 15% over the 6 days following harvest and for the smaller diameter trees was about 35%. After this time the rate of moisture loss decreased significantly and moisture content stabilized at around 40–50 days, and sometimes as quickly as 20 days, post-harvest [56]. In Garret’s New England study [57] the hardwoods stabilized after 40 days and pine after 50 days. The average moisture loss for a number of hardwood and softwood species, with a wide range of

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Table 2 The higher heating value (on dry basis) of some selected biomass.

A a b c d e f g

Fuel type

Higher heating value (MJ/kg)

Thinningsa Forest residueb,A Maplec Sugar maple branchwoodd Aspen branchd Birch stem barkd Birch stem barke,A Balsam fir twigd Fresh chipped forest residue (wood, needles, bark, limbs)f Mixed forest harvest residue (ON Canada)g

20.0 19.7 20.0 20.4 19.5 24.0 22.7 21.1 20.26, 20.97, 20.62, 21.40 18.7–19.0

Lower heating value (LHV) (on dry basis). [62]. [63]. [64]. [65]. [66]. [24]. [41].

diameters, was 4% moisture over a two-week period during the summer [57]. Models of transpirational drying rates of various southern U.S. species over different drying seasons have been compiled in the Stokes et al. [53] review article. The models were derived from weight loss and clearly show that summer drying attains lower moisture content than winter drying. The stabilization time for summer drying is about 50 days, while the winter stabilization was in excess of the 50-day test. Maximum weight loss for the summer drying was about 62%. For hardwood species the weight reduction stabilized after 30–40 days, reaching final moisture content of 45–60% od [53]. Andersson et al. [38] reported that transpirational drying in temperate climates reduced wet basis moisture content by 20–30% if forest residues were left in piles or windrows during the summer. They noted that small piles dried more efficiently than windrows and that covering the piles with impermeable coverings increased moisture loss and equalized moisture content throughout the pile. Their work confirmed earlier findings by Gislerud [42] that forest residues harvested and stored with foliage dried more rapidly through transpiration than residues without attached foliage. 4.2. Moisture loss in covered storage versus uncovered storage A number of studies done in Europe specifically evaluated the influence of storage site, including covered versus uncovered piles and various particle sizes, ranging through sawdust, chips, chunk wood, and firewood [36,37,40,45,58]. Gjølsjø [36] found that storage under cover resulted in a greater reduction in moisture than storage without cover for birch chips, chunk wood, and firewood. A similar study by Mitchell et al. [37] done in the U.K. concurred with these findings. The moisture content of the chips stored under cover in Mitchell’s study was reduced from 45.5% to 30%, while the moisture content on the chunks went from 44% to 29.5%. All authors have found that larger pieces dry more quickly and attained lower moisture levels than small material over similar time periods, with the exception of Arola [58] who found that final moisture loss is the same for both sizes of material but the drying time was prolonged. The moisture content of Arola’s sugar maple chunks changed from 34% to 17%, and the chips from 34% to 15% over the 61-day drying period. The higher rate of moisture loss was attributed to the increased spaces between larger particles allowing for better airflow. Koch [51] found that red oak and hickory chip piles attained minimum moisture content of 29% after

151 days of storage [53]. Nurmi [40] reported that over a one year storage period, logging residue left in the cutover had a reduction in moisture of 27.5%, while the same residue piled at roadside lost only 13.8%. Even worse results were observed when the material was hammer-milled and stored at an off-site terminal. This material saw an increase in moisture of 16.4%. Factors responsible for the higher moisture content after uncovered storage were attributed to metabolic activity of microbes that produce CO2 and water on respiration, and local weather conditions. As can be seen from the review of literature, there is agreement that mass loss of stored biomass material is usual. The complication comes in determining the exact cause of the mass loss and whether it is moisture, dry matter, or chemical losses. Common agreement is that mass loss on stored material is 1–5% per month [59]. Several generalities may be made regarding moisture content changes in biomass with storage. Forest residue stored in the simplest form will result in moisture reduction. Chip storage in piles, on the other hand, is complicated by heat development and moisture redistribution rather than simply loss and is greatly influenced by pile size and permeability. The foregoing illustrates that moisture content changes are not a straightforward issue but are influenced by geographical location, season of harvest, period of storage, storage configuration, and biomass composition. Wood removal from site can be delayed for 7–10 days after felling for the most significant moisture reduction [53]. Kipping et al. [60] describes a model for drying of particulate wood fuels during storage. They identified fuel particle geometry, drying air temperature, moisture carrying capacity, and airflow as the variables of most importance for predicting the drying rate of a biomass pile. 5. Energy content Higher heating value (HHV) is defined as the total energy released when a substance is burned in an oxygenated atmosphere and the beginning and end of the reaction are at room temperature. All water is re-condensed to a liquid state and therefore there is no loss of latent or sensible heat. The HHV is an indicator of the value of a material as a direct energy resource; however, the moisture content of biomass has a marked influence on its usable energy. The actual usable energy in a fuel is often referred to as the net heating value (NHV) (Eq. (2)). This represents the maximum potential energy available in an as-received biomass fuel and accounts for the double energy loss associated with moisture, i.e. mass [(1  MC/100)] and energy [(670 ⁄ (MC/100)] as in Eq. (2) [52]. Hence, any change in moisture content of the biomass on storage will directly affect its potential energy value.

NHV ¼ HHVð1  MC=100Þ  ½670  ðMC=100Þ

ð2Þ

NHV – net heating value (kcal/kg); HHV – higher heating value (kcal/kg); MC – moisture content (wet basis) There is a tremendous body of literature and numerous databases available on the energy value of specific tree species. Table 2 provides a sampling of the HHV of some relevant Great Lakes-St. Lawrence species and components. In general terms, softwoods have higher HHV than hardwoods because of their greater amounts of energy-rich extractives (resins) and higher lignin content which is less oxygenated than the carbohydrate components. White et al. [30] measured the higher heating value of each of four hardwoods and softwoods and compared those measurements with the chemical composition of the woods. He found that there was a positive correlation between HHV and lignin content of extractive-free wood and that, in fact, HHV was consistent with carbon content of lignin, extractives, and carbohydrates. Nurmi [40] found that the carbon content (50.03%) of stored hammermilled logging residue significantly increased (51.33%) on storage

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for 10 months, while the H content decreased, likely due to a loss of volatile compounds. Ragland et al. [61] reported average, ultimate, and proximate values for 11 American hardwoods and 9 American softwoods. Average values for hardwood were: carbon – 50.2%, hydrogen – 6.2%, oxygen – 43.5%, nitrogen – 0.1% and sulphur – not detected. Average values for softwood were: carbon – 52.7%, hydrogen – 6.3%, oxygen – 40.8%, nitrogen – 0.2%, and sulphur – 0.0%. The ultimate analysis of heterogeneous forest harvest residues freshly harvested and ground from Great Lakes St. Lawrence forest had carbon – 49.7–51.7%, hydrogen – 5.6–5.9%, oxygen – 42.5–44.68%. After one-year storage on landing, there was no measurable change in elemental composition. The same observation was made after 2 years of storage [41]. Any changes to the proportion of components in the biomass on storage will have a distinct effect on the material’s energy value. For example, loss of extractives on storage due to volatilization and oxidation will diminish the energy content, whereas an increase in the proportion of lignin due to the preferential biological degradation of the carbohydrate polymers will create a higher energy fuel. Inconsistent and contradictory results have been published on the influence of storage on the energy value of hogged fuels and sawdust [24,37,52,53]. White et al. [52] reported on a series of publications where storage of green sawdust and hog fuels showed various results ranging from no significant change in HHV after 3 years to 8.7% decrease after 5 years. One of the authors reported a loss in extractives and an increase in fixed carbon but no change to the material’s HHV. Studies by White and DeLuca [34] found that HHV increased 7–9% near the centre of piles of hardwood bark, hardwood sawdust, and pine bark during 5 months of storage. A study on the influence of pile height on the HHV of stored hardwood whole tree chips demonstrated that pile height ranging from 10 ft to 20 ft had no influence on average HHV. However, the HHV of all materials remained consistent for 160 days and then started to gradually decline, leveling off at a 9% loss after 360 days [52]. A similarly constructed hardwood bark pile had a similar response with a 7% loss in HHV, while a hardwood sawdust pile experienced only a 3% loss in HHV over the same storage time. The obsevations in the sawdust pile were slightly different, with a gradual loss in HHV from the start of storage. The effects of storage on the NHV of biomass fuels are quite profound as it is affected by moisture gain or loss in the material. Reisinger and Kluender [67] found that the NHV of whole tree chips stored outside immediately decreased upon harvest and continued to decline for approximately 120 days (25% loss of heat value) after which losses were negligible [53]. From an energy utilization perspective, the NHV of whole tree chips in the smallest pile (10 ft height) decreased more on storage than the material contained in the largest piles (20 ft height). After the first 4 months of storage the change to NHV was minimal, but between 20% and 40% lower than the original NHV. This change was attributed mainly to the increased moisture content of the material. Similar observations were made on the NHV of stored hardwood bark and sawdust with losses of 50% and 40%, respectively. Mitchell et al. [37] also reported on the percentage change in the net calorific value (NCV) of the material including both dry matter loss and moisture change, and reported that covered chips and chunk wood had minimal losses in NCV (1.2% and 0.8%) while uncovered chips had 5.0% and 2.3% losses over the 6 month trial period. Acquah et al. [41] reported a change in NHV on storage that was strongly influenced by the position of the material within the storage pile. The material on the outside of the pile showed an increase in NHV of 30% and 48% and that on the inside showed a decrease of 59% and 68% over 1 and 2 years of outdoor storage. Thörnqvist [24] reported the average heating value for specific components of freshly chipped forest residues (0.26 MJ/kg od for

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wood, 20.97 MJ/kg od for needles, 20.62 MJ/kg od for bark, and 21.40 MJ/kg od for limbs). He contrasted these values with 21.3 MJ/kg od for forest residues that had been stored at the harvest site for two summers, indicating that storage regime plays a role. He also noted that a 4% energy gain had been found by his colleagues for residues stored 9 months in small piles, while 3% loss was observed for residues stored 6 months in large piles. The effect on energy content of comminuted forest residue storage was found to be a loss of 6.8% to 21.4% over 6–9 months, depending upon particle size, proportions of foliage, bark, and wood, and initial moisture content. In logging residues windrowed from May to September after a February harvest, Jirjis and Lehtikangas [68] reported an increases of 4% in energy value for covered sections of the windrow and losses of 10% in uncovered portions. The energy quality of biomass can be positively or negatively affected by changes that take place on storage. Reduction of moisture is obviously a positive change that will increase the material’s NHV, increase its energy density, and hence reduce the unit energy cost for transportation. Depending on the nature of the changes to the chemical characteristics of the biomass on storage, changes may increase the material’s HHV or in some cases actually decrease it. Biological degradation that reduces the carbohydrate portion of the material and causes a subsequent increase in the lignin component will result in a material that is less oxygenated and therefore possesses a higher HHV. Jirjis and Theander [69] observed a higher proportion of lignin in compacted forest residue chips stored for 8 months, suggesting a loss of carbohydrates. This is confirmed by work done by Bergman and Nilsson [70] who observed a faster decomposition of carbohydrates compared to lignin in samples taken from the centre of a pile of Scots pine chips [69]. However, as noted throughout this review, many factors influence changes to biomass on storage and other authors have found no changes in lignin/carbohydrate amounts (hardwood chips) or proportions (for white spruce and lodgepole pine wood) after 6 months and 24 months of outdoor storage [69].

6. Inorganic constituents Certain plant micronutrients have been found to adversely affect boiler operations when using biomass fuels. Research regarding chemical contaminants of biomass, their impacts on combustion systems, and means of mitigation were examined by Obernberger et al. [71]. The general conclusion was that combustion systems should be tailored for the type of fuel expected to be used. Plant inorganics of particular interest to combustion plant operators are nitrogen, chlorine, and sulfur, and, to a lesser extent, calcium, magnesium, and potassium. The role of chlorine in creating operational problems in combustion applications includes a number of complex phenomena that results in deposition of inorganic particles on the surface of the boiler (fouling and slagging). Slagging can occur when the ash particles reach a temperature above their softening or melting point (ash fusion) in the flue gas. The particles soften and become sticky which facilitates their sticking together (agglomerating) and adhering to cooler tubes or boiler sides [72]. Chlorine on its own does not cause slagging or fouling problems, but rather its propensity to react with alkali metals. Therefore, biomass that contains both alkali metals and chlorine (and/or sulfur) in a ratio which provides for the complex reaction to KCl and NaCl can be the most detrimental in combustion applications. Due to the impact of micro-nutrients on boiler operations, it is important to understand their role in plants. Generally present in plants as chloride [73], chlorine is involved in oxygen splitting during photosynthesis [74] and as one of the major counterions active in cell division in leaves, shoots, roots [75], and stomatal openings

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[73]. It is generally present not bound to organic molecules, but rather in plant moisture or loosely bound to organic molecules, and is highly mobile. Chlorine is found anywhere that moisture is present [73]. Wood only occasionally contains these nutrients in sufficient concentration to cause boiler problems [76], but concentrations are much higher in grasses, grains, and straw. Some 130 chlorinated organic compounds have been identified in higher plants, such as polyacetylenes, thiophenes, iridoids, sesquiterpene lactones, pterosinoids, diterpenoids, steroids, phenolics, and fatty acids [73]. Some of these compounds are present as wood extractives, which are distributed throughout wood and bark (which generally has higher extractives content) [77]. One group [78] examined the elemental composition of wood pellets produced from separated bark and stems of several species and the emissions from combustion of those pellets in a top-loading pellet stove. Ash, sulfur, and chlorine content were generally higher in the bark than the stem. While foliar loss contributes to dry matter loss and therefore total energy loss, foliage is also the component with highest concentration of ash and chlorides in plant matter and its loss can represent an increase in the quality of residue for fuel. Thörnqvist [24] noted that natural ash content may be augmented by sand and gravel introduced through handling, resulting in higher apparent ash and chloride contents. Storage of woody biomass, through loss of the foliar component, provides a means of reducing the ash and consequently the chloride content of forest biomass materials. However, storage of whole tree chips and bark piles generally results in increased ash content as the organic materials are degraded. White et al. [30] found that the ash content of stored hardwood chips increased from 1.2% to 2% during weeks 25–50. Acquah et al. [41] found that the ash content of ground forest harvest residue increased from 1.7% to over 2.6% after one year of storage. Leaching of nutrients and other chemicals from stored biomass impacts not only potential use of the biomass, but also soil nutrient levels and possible soil contamination, which may assist in identifying the ideal location for storage. The literature was examined for effect of leaching on biomass nutrients, including chloride. Nurmi [40] studied the chemical composition of needles from residues subjected to various storage treatments. He found no significant leaching of nutrients from needles during storage but concluded foliar loss through initial seasoning at the harvest site was the best method for promoting nutrient loss from residues. Wall [79] measured soil nutrient flux under three different treatments of forest residue. Residues were left in the harvest block, completely removed, and foliage was left in the harvest block. It was found that forest residue was not a significant source of inorganic nitrogen but was a minor source of organic nitrogen, as well as phosphorous, calcium, and magnesium, and a significant source of potassium. Similarly, Gotou and Nishimura [80] examined leaching of nutrients from chips placed around stems of trees. They found that phosphorous, potassium, calcium, and magnesium were readily released to the soil, but chips retained nitrogen. Sander [81] examined straw exposed to rain in the field after harvest and before drying and collection, and found losses of potassium and chlorine while calcium content was unaffected. Bakker and Jenkins [82] evaluated the feasibility of allowing nutrients to leach from rice straw before collection. They found levels of both potassium and calcium in rice straw decreased but that sulfur levels increased, possibly due to higher concentration in the remaining rice straw. With the exception of agricultural residues, most of the literature examining leaching of biomass studied return of growthlimiting soil nutrients rather than loss of chlorides and sulfates. Given the importance of chlorides and sulfates to slagging and corrosion during boiler operations there is potential for studies

examining influence of various storage practices on chloride and sulfate concentrations. Recent work monitoring chloride change on storage found that the chloride concentration of the forest harvest residue decreased from 274 ppm to 154 ppm after one year of outdoor storage. There was no further decline in chlorides after a second year of storage [41]. 7. Conclusion Large-scale utilization of biomass for fuel requires a smartly designed supply chain that will ensure biomass of specific quantity and quality on an ‘as needed’ basis. In North America the majority of forest harvesting is done in the summer season while energy requirements are highest throughout the winter season. Therefore, one very important stage in the supply chain is the storage phase. The time that the feedstock needs to be in storage depends on the requirements of the bioenergy facility as well as the availability and distance from the forest or feedstock suppliers, in addition to a number of other complicating factors. The storage regime will affect the biomass characteristics and will influence its suitability in the energy conversion processing. Storage time, climatic conditions, species composition, and form of the biomass, as well as geometry and structure of the storage pile influence the alteration of biomass characteristics on storage. Large-scale industrial evaluations of the influence of storage regimes on the biomass are difficult because of the heterogeneity of the material and the difficulty in sampling from such large quantities. Although many studies have been performed under controlled and monitored conditions, results are still plagued with discrepancies. For this reason it is very helpful to understand the fundamental processes that can affect biomass in storage so that well thought-out storage regimes can be designed based on theory and applied to specific conditions and situations. The main change agents for stored biomass are cell respiration, microbial activity, thermo-chemical reactions, and moisture evaporation. These factors have specific effects on the biomass, but also have strong interaction and dependency on one-another that influences the final biomass characteristics and mass loss. Control of storage conditions can influence the mechanisms of change and examples of this can be found throughout the review. Optimum storage design can limit feedstock losses and reduce moisture, ultimately reducing economic and efficiency losses in any bioenergy supply chain. Acknowledgements The authors wish to acknowledge Dr. C. Ledger and Kaho Hayashi for their assistance in collecting information. Financial support from Natural Resources Canada is gratefully acknowledged. References [1] Quillin S. Effective chip storage design reduces pulp variation, improves mill profits. Pulp Paper 1994;68:105–7. [2] Schmidt RL. The effect of wood chip inventory rotation policies on storage costs, chip quality and chip variability. Tappi 1990;73:211–6. [3] Brown WH. Introduction to organic and biochemistry. 4th ed. Monterey (California): Brooks/Cole Publishing Company; 1987. p. 547. [4] Feist WC, Springer EL, Hajny GJ. Encasing wood chips piles in plastic membranes to control chip deterioration. Tappi 1971;54(7):1140–2. [5] Feist WC, Springer EL, Hajny GJ. Spontaneous heating in piled wood chips— contribution of bacteria. Tappi 1973;56(4):148–51. [6] Springer EL, Hajny GJ. Spontaneous heating in piled wood chips. Tappi 1970;53 (1):85–6. [7] Springer EI, Hajny GJ, Feist WC. Spontaneous heating in piled wood chips II. Effect of temperature. Tappi 1971;54(4):589–91. [8] Spicer R, Holbrook NM. Parenchyma cell respiration and survival in secondary xylem: does metabolic activity decline with cell age? Plant, Cell Environ 2007;30:934–43.

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