Review article
A review of mechanisms responsible for changes to stored woody
biomass fuels
Sally Krigstin
,
, Suzanne Wetzel
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 uti-
lize 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 res-
piration, (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 avail-
able 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.
Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
2.
Mechanisms of change. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
2.1.
Living cell respiration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 76
2.2.
Biological degradation. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 77
2.3.
Thermo-chemical oxidative reactions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 78
2.4.
Moisture evaporation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.
Mass loss. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.1.
Implication of storage form . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 79
3.2.
Implications of temperature . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
3.3.
Implications of moisture content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
http://dx.doi.org/10.1016/j.fuel.2016.02.014
0016-2361/
Ó 2016 Elsevier Ltd. All rights reserved.
⇑
Tel.: +1 613 866 6108 (S. Wetzel), +1 416 946 8507 (S. Krigstin).
E-mail addresses:
(S. Krigstin),
(S. Wetzel).
Contents lists available at
Fuel
j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c o m / l o c a t e / f u e l
4.
Moisture losses. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 80
4.1.
Transpirational drying. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 81
4.2.
Moisture loss in covered storage versus uncovered storage. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
5.
Energy content . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 82
6.
Inorganic constituents . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 83
7.
Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
Acknowledgements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 84
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 produc-
tion. The alteration of biomass characteristics on storage has been
shown to be influenced by storage time, climatic conditions, spe-
cies 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 chem-
icals. ‘‘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
. Fluctuating supply often necessi-
tated inventory levels ranging from 3 to 26 weeks of supply
Later, beginning with the oil crisis of the 1970s, a body of literature
developed examining the best characteristics for wood as indus-
trial fuel and the best ways to store wood to achieve those charac-
teristics. 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 num-
ber 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.
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 stor-
age. 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 conver-
sion. 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 degrada-
tion, (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.
.
C
6
H
12
O
6
þ 6O
2
! 6CO
2
þ 6H
2
O
þ
DH
combustion
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 (C
6
H
12
O
6
) has been found to be 263,000 cal/
mol glucose (1100 kJ/mol)
.
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 con-
sumes 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
. In typical Great
Lakes-St. Lawrence hardwood species (Acer rubrum, Fraxinus amer-
icana, Quercus rubra) and softwood species (Pinus strobus, Tsuga
76
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
canadensis) higher respiration rates were found in the youngest
sapwood in material isolated from living trees
. 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 rela-
tive volume of parenchyma cells in the wood tissue. With about
1/3 the volume of parenchyma cells, softwoods show a lower over-
all respiration rate based on total tissue volume of 0.4–0.6
l
mol O
2
cm
3
h
1
as compared to 0.3–9.5
l
mol O
2
cm
3
h
1
for hardwoods
. This suggests that hardwoods would have a tendency to accel-
erate heating in storage over softwoods. Typically, respiration in
fresh cut chip piles can last from 10 to 40 days given suitable con-
ditions
and when stored in log form can continue for up to
6 months
.
High temperatures will result in inactivation of cellular
enzymes and reduce the ability of the parenchyma cells to respire
. 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 tempera-
ture conditions (21
°C)
. Both trembling aspen and Douglas-fir
showed a decline in activity above 42
°C and termination of activ-
ity at 60
°C
. 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
. Thus, wood harvested and stored
over the winter and chipped in the spring may still have viable par-
enchyma cells that would readily respire as the ambient tempera-
ture increases.
While respiration in the living parenchyma cells is responsible
for a relatively small mass loss in woody biomass through the cata-
bolic processing of stored starch and glucose, it has a greater influ-
ence 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 (O
2
) con-
sumed
. 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)
. Assuming
the specific heat of wood at 17
°C and moisture content of 20% is
1.8 J/g
°C
, then the temperature of one gram of wood chips
could theoretically rise by 26
°C over a 24-h period from respira-
tion 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
. The increased temperature of
the woody biomass promotes direct oxidation of wood con-
stituents and also provides a preferential environmental for micro-
bial 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
. Very high bacteria counts of up to
7.4
10
6
cells/odg to 2
10
9
cells/odg have been observed in
tropical wood chip piles as well as in stored agricultural crops such
as alfalfa hay and oat straw
. These early colonizers generally
metabolize only the starch stores in the freshly cut wood
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.
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 con-
sumption 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
. It has also been observed that
anaerobic bacteria do not cause a significant amount of heating in
wood chip piles, and CO
2
levels greater than 32% limit viability of
these microorganisms
Temperature is reported to be the most important factor in
determining the number and type of microorganisms inhabiting
stored chip piles
. In controlled experiments, which monitored
chip pile bacteria counts and temperature over time, it was
observed that the two variables are highly correlated. Simultane-
ous increases in bacteria counts and temperature were observed
during the first 7–10 days and then again after 45 days
. Higher
amounts of mesophilic bacteria were identified at the onset, before
the temperature of the pile increased significantly, with thermo-
tolerant bacteria strains becoming dominant once higher tempera-
tures were reached. Bacteria themselves have low tolerance to
temperatures above 82–94
°C
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 con-
sequently 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 mois-
ture level (30–50% MC), a specific temperature regime, and a suit-
able oxygen level to thrive. Nitrogenous compounds, vitamins, and
essential elements are also necessary for fungal growth and subse-
quent 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 mole-
cules, and (3) serves to swell the small capillaries in the cell wall
which enables penetration of fungal digestive enzymes into the
substrate
. 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
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
. Thermophilic
species demonstrate optimal growth at 40–50
°C and are found
in high proportion during self-heating of piles. However, most fun-
gal activity will cease at temperatures above 60
°C
. While
most wood inhabiting fungi are of the mesophilic type, there are
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
77
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
. Also both
thermophilic and thermotolerant species were identified by
Shields
in the microorganism communities in chip piles in
Quebec, Nova Scotia, and New Brunswick. He deduced that the var-
ious microorganisms present in different areas of the wood pile
were determined not only by specific temperature but also by pH
conditions. Wiselogel
suggested that most of the microorgan-
isms found in stored material fed on free sugars, rather than decay-
ing 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 con-
centration of oxygen in the environment, with some organisms
able to function at 1% oxygen concentration in air, but most prefer-
ring levels above 20% for optimal growth and decay development.
Ernstson et al.
demonstrated that reduced oxygen levels les-
sen 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 toler-
ance to lower O
2
and higher CO
2
concentrations in the air by speci-
fic fungal species, so it is important to understand the specific
fungal populations present in any stored biomass
The comparatively low nitrogen content of wood limits fungal
growth, as fungi require nitrogen to produce the enzymes respon-
sible for the degradation of the wood polymers. Suadicani and
Heding
noted that the highest quantity of fungal spores and
fungal degradation were found in samples containing bark and in
piles of comminuted stored wood
, as a result of higher avail-
able nutrients in small branches and foliage. Mixed biomass pro-
vides 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 sig-
nificant dry matter loss in woody materials; (1) brown rot fungi,
(2) white rot fungi, and (3) soft rot fungi. Staining fungi and bacte-
ria have been shown to metabolize the easily degradable fatty
acids, triglycerides, and simple carbohydrates contained in the par-
enchyma and resin canals, but do not readily degrade wood’s major
polymers
. 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 com-
prehensive literature review Hellenbrand and Reade
report
that Hoover-Litty and Hanlin
found the seven most common
genera to colonize wood piles were (Tricoderma, Fusarium, Chaeto-
miu, Aspergillus, Rhizopus, Graphium, and Penicillium) with Trico-
derma (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 pop-
ulations were also found to change over storage time and also by
location within the pile. Assarson et al.
specified optimal con-
ditions 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
. Farrell et al.
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.
demon-
strated 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.
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 stor-
age, 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 mem-
branes may also restrict oxygen and retard fungal growth
.
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
. Addi-
tionally, higher rates of decay (as measured by lower specific grav-
ity) were noted on the surface of hardwood chip and hardwood
bark piles than within
. 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 meta-
bolic energy generation has occurred
. At temperatures in
excess of 80
°C exothermic oxidation reactions contribute to self-
heating
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
. The oxygen consumptions of these reactions were measured at
0.280 and 0.079 (ml/h)/odg for fresh aspen and Douglas-fir, respec-
tively, and the rate of oxygen consumption along with temperature
continued to increase through a 120 h time period
. Correspond-
ingly, 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
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
. At temperatures of 130
°C
and higher, thermal degradation of woody material becomes sub-
stantial. 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
78
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
pyrolytic degradation of wood can be found in Beall and Eickner
. 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 hemicel-
luloses are comprised of xylan (5 carbon sugar) chains while soft-
wood polyoses are comprised primarily of glucomanan (6 carbon
sugar) chains. The xylan chains of hardwood polyoses are irregu-
larly branched and many of the C2 and C3 hydroxyl groups of
the xylose unit are substituted with O-acetyl groups. At tempera-
tures 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
. 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 mole-
cules 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 lay-
ers of the chip pile and be deposited with evaporating moisture,
leading to a lower than average pH layer
. White et al.
found no change in pH of water soluble extract in piles of hard-
wood 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
did find that the pH of residues decreased over a 5 month stor-
age 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
.
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 con-
trolled 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, fol-
lowed at a much slower rate by the bound water, which is chem-
ically 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%
.
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
. 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
. 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 simultane-
ously condenses and vapourizes. The result is that the heat given
off to the woody material on condensation of water vapour is used
to provide the heat necessary to vapourize liquid water, theoreti-
cally 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 con-
densation and evaporation.
3. Mass loss
The quantity of biomass material available for energy is ulti-
mately 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 bio-
ethanol favours biomass with a high percent of polysaccharides
and higher water content.
Dry matter loss can be largely affected by biological degrada-
tion. 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 oxy-
gen 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
reported a
general dry matter loss of 1% per month for pulp chips stored out-
doors in North America, which closely agreed with a mass loss of
1.2% per month reported for birch chips stored under cover in Nor-
way
and Wiselogel’s
findings of 1% mass loss per month.
Gjølsjø’s
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
trials had similar out-
comes with an average monthly dry matter loss of 2% for Sitka
spruce chips and 1.7% for chunks under covered storage. In uncov-
ered storage, the losses were greater at 4% and 10% per month for
chips and chunks respectively. Assarson et al.
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 har-
vested form of chips/particles
. 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
. No significant difference was found in dry
matter loss over the entire storage period (22% for fine chips and
21% for the coarse chips)
. 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
. 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.
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
79
A number of researchers have noted that there is a greater rate
of weight loss for forest residues at the initiation of the storage per-
iod, especially when stored on the harvest site
. Assarson
et al.
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
. Andersson et al.
noted that forest residues stored in small piles at the harvest-
ing site lose more foliage than windrows stored at roadside. Flink-
man et al.
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
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 scenar-
ios, has been largely overlooked in the scientific literature to-date.
Research has focused on clean chips and whole tree chips and for-
est residues. Bark has a distinctive chemical and anatomical make-
up 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 fun-
gal invasion, as studies have found that parenchyma facilitate the
spread of fungi
. This is backed somewhat by evidence pre-
sented by Thörnqvist
, 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
. Furthermore, needles and
bark were found to have higher degradation rates than sapwood
. However, bark of certain tree species can contain anti-
fungal agents that can resist fungal invasion during storage
Therefore, while bark-chip piles have similar dynamics to wood-
chip 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 bio-
mass 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, Chrysospo-
rium lignorum, were prevalent or combustion took place. Ernstson
et al.
simulated field storage conditions and tested degrada-
tion rates of needles, bark, and sapwood fractions at various
temperatures (15–55
°C). They found the maximum rate of degra-
dation occurred at 25
°C for all three biomass materials. Almost no
degradation occurred at the 15
°C minimum or the 65 °C maxi-
mum. 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.
also showed that material col-
lected in the winter months showed no degradation, most likely
due to an absence of microorganisms. White et al.
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. Accumula-
tion 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 com-
pared to the outside (52%)
. 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
. Quillin
suggests that
shorter pile height (50
0
) can lead to less compaction and slower
rate of heat build up, while larger piles (100
0
) have high com-
paction 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 bio-
mass, energy produced upon combustion is consumed to evaporate
moisture from the material, thus reducing the usable energy avail-
able or the fuel’s net heating value
. 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
. This point is illus-
trated in a study on mass loss in chip piles in Sweden
. Chips
with initial moisture contents of 20%, 32%, 42%, 51% and 58% exhib-
ited 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 com-
ponents 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
.
Biomass will dry under natural conditions to an equilibrium
moisture content (EMC), which is determined by ambient temper-
ature and humidity conditions. Regionally specific and species
specific tables have been developed which show the relationship
of temperature and humidity to EMC
. 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 commer-
cial lumber have been well established and reported upon. Simp-
son and Hart
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 com-
position, 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 pre-
sent microorganisms generate CO
2
, H
2
O, and heat. Agblevor et al.
found that pockets of wet and warm bagasse, present in the
predominantly dry interior of piles, were directly above microbial
80
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
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
found that the season of harvest and
storage did influence the final moisture content of harvest resi-
dues. 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.
of the Nether-
lands 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 mois-
ture 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 pro-
tection of the bark, and its low diffusivity, the increase was short-
lived.
Size of the chip storage pile has been reported to have a signif-
icant effect on moisture reduction with storage time. In fact, in
studies by Thörnqvist
and Koch
, an absolute reduction
in moisture in stored chip piles was found to depend on pile size.
Piles smaller than 120 m
3
exhibited similar moisture losses to that
reported by Nurmi and Hillebrand
, while those above 600 m
3
exhibited redistribution, rather than a reduction, in average mois-
ture content. These observations were supported by Acquah et al.’s
study which showed that the moisture content in the pile con-
tinued 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.
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
reported that the average moisture con-
tent of residues stored in piles was strongly dependent on the com-
position 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, tran-
spirational 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.
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 condi-
tions. Lawrence
concluded that diffuse-porous species and
softwoods experienced a faster drying rate than ring porous spe-
cies 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
. The sapwood of Douglas-fir lost more moisture
than the heartwood in the Saralecos et al.
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 transpira-
tional drying occurs directly after harvest. Garret
noted that
most of the moisture loss occurred within 36 h of felling. Saralecos
et al.
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
. In Garret’s New England study
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
Table 1
Moisture content (total weight basis) of some Eastern Ontario tree species.
Common name
Moisture content percent (wet basis)
Heartwood
Sapwood
Inner bark
Outer bark
Other
Paper birch
43–47
42
40.4
18.4
–
Trembling aspen
62
53
39.4–40.2
–
–
Black spruce
52
,
–
–
Red maple
41.1
42.7
White ash
32.2
Norway spruce logging residue
–
–
–
–
56
–
–
–
–
42.4
Forest harvest residue (ON, Canada)
a
Value represents total bark
.
b
.
c
.
d
.
e
.
f
.
g
Value represents stem wood.
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
81
diameters, was 4% moisture over a two-week period during the
summer
.
Models of transpirational drying rates of various southern U.S.
species over different drying seasons have been compiled in the
Stokes et al.
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 con-
tent of 45–60% od
. Andersson et al.
reported that transpi-
rational 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 effi-
ciently than windrows and that covering the piles with imperme-
able coverings increased moisture loss and equalized moisture
content throughout the pile. Their work confirmed earlier findings
by Gislerud
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
. Gjølsjø
found that stor-
age 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.
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
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
found that red oak and hick-
ory chip piles attained minimum moisture content of 29% after
151 days of storage
. Nurmi
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 mate-
rial saw an increase in moisture of 16.4%. Factors responsible for
the higher moisture content after uncovered storage were attribu-
ted to metabolic activity of microbes that produce CO
2
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 complica-
tion 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
. Several generalities may be made regarding moisture content
changes in biomass with storage. Forest residue stored in the sim-
plest form will result in moisture reduction. Chip storage in piles,
on the other hand, is complicated by heat development and mois-
ture redistribution rather than simply loss and is greatly influenced
by pile size and permeability. The foregoing illustrates that mois-
ture content changes are not a straightforward issue but are influ-
enced 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
. Kipping et al.
describes a model for drying of particulate wood fuels during stor-
age. 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 tempera-
ture. 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 mois-
ture 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.
). This represents the maxi-
mum 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.
. 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 data-
bases available on the energy value of specific tree species.
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 con-
tent which is less oxygenated than the carbohydrate components.
White et al.
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
found that the carbon content (50.03%) of stored hammer-
milled logging residue significantly increased (51.33%) on storage
Table 2
The higher heating value (on dry basis) of some selected biomass.
Fuel type
Higher heating value
(MJ/kg)
Thinnings
20.0
Forest residue
,
19.7
20.0
Sugar maple branchwood
20.4
Aspen branch
19.5
Birch stem bark
24.0
Birch stem bark
,
22.7
Balsam fir twig
21.1
Fresh chipped forest residue (wood, needles, bark,
20.26, 20.97, 20.62,
21.40
Mixed forest harvest residue (ON Canada)
18.7–19.0
A
Lower heating value (LHV) (on dry basis).
a
b
c
d
e
f
g
82
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
for 10 months, while the H content decreased, likely due to a loss of
volatile compounds. Ragland et al.
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 resi-
dues 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 mea-
surable change in elemental composition. The same observation
was made after 2 years of storage
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 biolog-
ical 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
. White et al.
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
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 hard-
wood 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
. 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 pro-
found as it is affected by moisture gain or loss in the material.
Reisinger and Kluender
found that the NHV of whole tree
chips stored outside immediately decreased upon harvest and con-
tinued to decline for approximately 120 days (25% loss of heat
value) after which losses were negligible
. From an energy uti-
lization 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.
also reported on the percentage change in the net calori-
fic 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.
reported a change in NHV on storage that was
strongly influenced by the position of the material within the stor-
age 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
reported the average heating value for specific
components of freshly chipped forest residues (0.26 MJ/kg od for
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 har-
vest site for two summers, indicating that storage regime plays a
role. He also noted that a 4% energy gain had been found by his col-
leagues 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 par-
ticle size, proportions of foliage, bark, and wood, and initial mois-
ture content. In logging residues windrowed from May to
September after a February harvest, Jirjis and Lehtikangas
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 mois-
ture 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 compo-
nent will result in a material that is less oxygenated and therefore
possesses a higher HHV. Jirjis and Theander
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
who observed a faster
decomposition of carbohydrates compared to lignin in samples
taken from the centre of a pile of Scots pine chips
. 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
.
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.
. The general conclusion was that combus-
tion 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 com-
bustion 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
. Chlorine on its own
does not cause slagging or fouling problems, but rather its propen-
sity to react with alkali metals. Therefore, biomass that contains
both alkali metals and chlorine (and/or sulfur) in a ratio which pro-
vides 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
, chlorine is involved in oxygen splitting dur-
ing photosynthesis
and as one of the major counterions active
in cell division in leaves, shoots, roots
, and stomatal openings
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
83
. 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
. Wood only occasionally contains these nutrients
in sufficient concentration to cause boiler problems
, but con-
centrations 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
. Some of these compounds are present as wood extrac-
tives, which are distributed throughout wood and bark (which
generally has higher extractives content)
. One group
examined the elemental composition of wood pellets produced
from separated bark and stems of several species and the emis-
sions 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 con-
centration of ash and chlorides in plant matter and its loss can rep-
resent an increase in the quality of residue for fuel. Thörnqvist
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.
found that the ash content of stored
hardwood chips increased from 1.2% to 2% during weeks 25–50.
Acquah et al.
found that the ash content of ground forest har-
vest 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 identi-
fying the ideal location for storage. The literature was examined for
effect of leaching on biomass nutrients, including chloride. Nurmi
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
mea-
sured 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 nitro-
gen but was a minor source of organic nitrogen, as well as phos-
phorous, calcium, and magnesium, and a significant source of
potassium. Similarly, Gotou and Nishimura
examined leaching
of nutrients from chips placed around stems of trees. They found
that phosphorous, potassium, calcium, and magnesium were read-
ily released to the soil, but chips retained nitrogen. Sander
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
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 litera-
ture examining leaching of biomass studied return of growth-
limiting 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 har-
vest 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
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 stor-
age 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 stor-
age pile influence the alteration of biomass characteristics on stor-
age. 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 quan-
tities. Although many studies have been performed under con-
trolled 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 evap-
oration. These factors have specific effects on the biomass, but also
have strong interaction and dependency on one-another that influ-
ences 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 bioen-
ergy supply chain.
Acknowledgements
The authors wish to acknowledge Dr. C. Ledger and Kaho Haya-
shi for their assistance in collecting information. Financial support
from Natural Resources Canada is gratefully acknowledged.
[1]
.
[2]
.
[3]
[4]
.
[5]
[6]
Springer EL, Hajny GJ. Spontaneous heating in piled wood chips. Tappi 1970;53
(1):85–6
.
[7]
.
[8]
.
84
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
.
Zabel RA, Morrell JJ. Wood microbiology: decay and its prevention. Academic
Press; 1992
.
.
.
.
Shields JK. Microflora of Eastern Canadian wood chip piles. Mycologia 1969;61
(6):1165–8
Ernstson M-L, Jirjis R, Rasmuson A. Experimental determination of the
degradation
temperatures and oxygen concentrations. Scan J Forest Res 1991;6:271–87
Suadicani K, Heding N. Wood preparation, storage, and drying. Biomass
Bioenergy 1992;2(1–6):149–56
.
Thörnqvist T. Drying and storage of forest residues for energy production.
Biomass 1985;7:125–34
.
.
Assarson A, Croon I, Frisk E. Outside chip storage (OCS). Sven Papperstidning
1970;73(16):493–501
.
.
.
.
reactions. Berlin: Walter de Gruyter; 1983. p. 321
[33] Beall FC, Eickner HW. Thermal degradation of wood components: a review of
literature. Forest Service Research Paper FPL; 1970. 130, 0363–6259.
.
.
.
.
[40]
Nurmi J. The storage of logging residue for fuel. Biomass Bioenergy
1999;17:41–7
[41]
.
[42]
Gislerud O. Drying and storing of comminuted wood fuels. Biomass
1990;22:229–44
.
[43]
[44] Wilson PL, Funch JW, Avery RB. Fuelwood characteristics of northwestern
conifers and hardwoods (updated). U.S. Department of Agriculture, Forest
Service; 2010. PNW-GTR-810.
[45]
.
[46]
[47] Forest Products Laboratory. Wood handbook—Wood as an engineering
material. General Technical Report FPL-GTR-190. Madison (WI): U.S.
Department of Agriculture, Forest Service, Forest Products Laboratory; 2010.
p. 508.
[48] Simpson WT, Hart CA. Estimates of air drying times for several hardwoods and
softwoods. Madison (WI): U.S. Department of Agriculture, Forest Service,
Forest Product Laboratory; 2000; Gen Tech Rep FPL-GTR-121. p. 70
[49]
[50]
[51]
.
[52]
.
[53] Stokes BJ, McDonald TP, Kelley T. Transpirational drying and costs for
transporting woody biomass – A preliminary review. In: Timothy P, Kelley T,
editor. Proceedings of IEA/BE Conference. Task IX/Activity 6: Transport and
Handling; 1993 May 16–25; New Brunswick, Canada.
[54] Lawrence WE. Field drying logging residues as an industrial fuel. M.S. thesis.
Blacksburg, VA: Virginia Polytechnic Institute and State University; 1981.
109p.
[55] Wells GC, Booker RE. Drying logs in the forest. New Zealand: Logging Industry
Research Association Report; 1981. 6(2). 4p.
[56]
.
[57]
.
[58]
[59]
.
[60]
.
[61]
[62]
.
[63]
.
[64]
theory. Oxford (UK): Elsevier; 2010
.
[65]
[66]
[67] Reisinger TW, Kluender RA. Storage and handling of whole tree chips and
residues for fuel. APA Paper No. 83-A-2. Washington, DC: American Pulpwood
Association, Southwide Energy Committee; 1982. p. 5.
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
85
[68]
[69]
[70]
.
[71]
.
[72]
[73]
[74]
.
[75]
Hopkins WG. Introduction to plant physiology. 2th ed. New York: John Wiley
and Sons; 1999
.
[76]
.
.
.
.
.
.
Schwarze F. Wood decay under the microscope. Fungal Biol Rev 2007;21
(4):133–70
86
S. Krigstin, S. Wetzel / Fuel 175 (2016) 75–86
Document Outline
- A review of mechanisms responsible for changes to stored woody biomass fuels
Wyszukiwarka
Podobne podstrony:
Język angielski What do you think of AIDS victims?gging for money to live
Systematic Review of Randomized Trials for
Review of Orthomolecular Medicine for Everyone
systematic review of Mckenzie thateraphy for
Bondeson; Aristotle on Responsibility for Ones Character and the Possibility of Character Change
Principles of Sigma Delta Conversion for Analog to Digital Converters
Applications of polyphase filters for bandpass sigma delta analog to digital conversion
A review of molecular techniques to type C glabrata isolates
Short review of the book entitled E for?stasy
Student Roles and Responsibilities for the Masters of Counsel
Part8 Review of Objectives, Points to Remember
Differential Heat Capacity Calorimeter for Polymer Transition Studies The review of scientific inst
0620 Removal and installation of control unit for airbag seat belt tensioner Model 126 (to 08 87)
Principles of Sigma Delta Conversion for Analog to Digital Converters
Applications of polyphase filters for bandpass sigma delta analog to digital conversion
Reviews and Practice of College Students Regarding Access to Scientific Knowledge A Case Study in Tw
Plebaniak, Robert On best proximity points for set valued contractions of Nadler type with respect
więcej podobnych podstron