Potential greenhouse gas benefits of transatlantic wood pellet trade

Abstract

Power utility companies in the United Kingdom are using imported wood pellets from the southern region of the United States for electricity generation to meet the legally binding mandate of sourcing 15% of the nation's total energy consumption from renewable sources by 2020. This study ascertains relative savings in greenhouse gas (GHG) emissions for a unit of electricity generated using imported wood pellet in the United Kingdom under 930 different scenarios: three woody feedstocks (logging residues, pulpwood, and logging residues and pulpwood combined), two forest management choices (intensive and non-intensive), 31 plantation rotation ages (year 10 to year 40 in steps of 1 year), and five power plant capacities (20–100 MW in steps of 20 MW). Relative savings in GHG emissions with respect to a unit of electricity derived from fossil fuels in the United Kingdom range between 50% and 68% depending upon the capacity of power plant and rotation age. Relative savings in GHG emissions increase with higher power plant capacity. GHG emissions related to wood pellet production and transatlantic shipment of wood pellets typically contribute about 48% and 31% of total GHG emissions, respectively. Overall, use of imported wood pellets for electricity generation could help in reducing the United Kingdom's GHG emissions. We suggest that future research be directed to evaluation of the impacts of additional forest management practices, changing climate, and soil carbon on the overall savings in GHG emissions related to transatlantic wood pellet trade.

Full text

Environmental Research Letters
Environ. Res. Lett. 9(2014) 024007 (11pp) doi:10.1088/1748-9326/9/2/024007
Potential greenhouse gas benefits of
transatlantic wood pellet trade
Puneet Dwivedi1, Madhu Khanna2, Robert Bailis3and Adrian Ghilardi3
1Warnell School of Forestry and Natural Resources, University of Georgia, Building #4, Room #114,
180 E Green Street, Athens, GA 30602-2152, USA
2Energy Biosciences Institute, University of Illinois at Urbana-Champaign, Urbana, IL 61801, USA
3School of Forestry & Environmental Studies, Yale University, New Haven, CT 06511, USA
E-mail: puneetd@uga.edu and puneetdwivedi@gmail.com
Received 21 August 2013, revised 9 December 2013
Accepted for publication 6 January 2014
Published 18 February 2014
Abstract
Power utility companies in the United Kingdom are using imported wood pellets from the
southern region of the United States for electricity generation to meet the legally binding
mandate of sourcing 15% of the nation’s total energy consumption from renewable sources by
2020. This study ascertains relative savings in greenhouse gas (GHG) emissions for a unit of
electricity generated using imported wood pellet in the United Kingdom under 930 different
scenarios: three woody feedstocks (logging residues, pulpwood, and logging residues and
pulpwood combined), two forest management choices (intensive and non-intensive), 31
plantation rotation ages (year 10 to year 40 in steps of 1 year), and five power plant capacities
(20–100 MW in steps of 20 MW). Relative savings in GHG emissions with respect to a unit of
electricity derived from fossil fuels in the United Kingdom range between 50% and 68%
depending upon the capacity of power plant and rotation age. Relative savings in GHG
emissions increase with higher power plant capacity. GHG emissions related to wood pellet
production and transatlantic shipment of wood pellets typically contribute about 48% and 31%
of total GHG emissions, respectively. Overall, use of imported wood pellets for electricity
generation could help in reducing the United Kingdom’s GHG emissions. We suggest that
future research be directed to evaluation of the impacts of additional forest management
practices, changing climate, and soil carbon on the overall savings in GHG emissions related
to transatlantic wood pellet trade.
Keywords: electricity generation, European markets, greenhouse gas emissions, life-cycle
assessment, southern United States, wood pellets
SOnline supplementary data available from stacks.iop.org/ERL/9/024007/mmedia
1. Introduction
Global demand for wood pellets is increasing as several power
utility companies in the Europe are using or planning to utilize
them as a feedstock for electricity generation [1]. It is expected
that the utilization of wood pellets will help in meeting national
mandates, where a certain percentage of total energy consumed
within the country needs to come from various renewable
Content from this work may be used under the terms of
the Creative Commons Attribution 3.0 licence. Any further
distribution of this work must maintain attribution to the author(s) and the
title of the work, journal citation and DOI.
energy sources, including biomass, by the end of 2020 [2,3]. In
recent years, the southern United States has become a major
exporter of wood pellets to several European countries [1].
Exports of wood pellets from this region are forecasted to
increase from 1.5 to 5.2 million metric tons between 2012 and
2015 [4].
Existing studies [5–7] demonstrate that the GHG intensity
of a unit of electricity generated in European countries using
imported wood pellets from the United States and Canada is
about 65%–80% lower than the GHG intensity of a unit of
grid electricity depending upon whether natural gas or wood
residues were used to dry wood pellets in a wood pellet plant.
1748-9326/14/024007+11$33.00 1 c
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Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Table 1. Distances traveled to source required feedstock for manufacturing of wood pellets.
Serial no. Details and distances References
1 Wood is transported to a wood pellet plant from forested area (81 km) [5]
2 Wood is transported to a sawmill from forestlands (110 km) [6]
Sawmill residues are transported to a wood pellet plant (27 km)
3 Wood is transported to a sawmill (75 km) [7]
Wood is transported to a chip mill (50 km)
Sawmill residues are transported to a wood pellet plant (0 km)
Wood chips are transported to a wood pellet plant (75 km)
These studies typically assume that the feedstock needed for
manufacturing of wood pellets was sourced from a nearby
forest area or a wood processing facility which was located at
a fixed distance from the wood pellet plant (table 1).
This is a limiting assumption for three main reasons. First,
several harvest tracts are needed to supply required wood
to a nearby wood pellet plant on an annual basis depending
upon the capacity of the wood pellet plant; second, wood is
immediately transported from a harvested tract to a wood pellet
plant by loggers to meet the daily needs of the wood pellet
plant and therefore, distance between each harvested tract and
a wood pellet plant must be summed for ascertaining GHG
emissions related with the transportation of required wood
for wood pellets production; and third, harvested tracts are
distributed across a landscape and chances that all harvested
tracts are located at a fixed distance from a wood pellet plant
are practically nil. These points are particularly valid for
the southern United States as 87% of forestland is privately
owned in this region [8], considerable variability exists among
forestland owners about the objectives of forest management
[9], and forestlands are part of a heterogeneous landscape
where competing land uses coexist [10].
Furthermore, existing studies consider only one harvest
cycle while determining GHG savings of electricity generated
from imported wood pellets in Europe. This raises concerns
among environmentalists and other stakeholders [11]. Again,
this is a limiting assumption as forestland owners repeatedly
use their forestlands for raising plantations especially in the
southern United States. Therefore, average annual quantities
of feedstocks available over time instead of quantities of
feedstocks available at the time of harvest should be considered
for determining GHG savings of electricity generated using
imported wood pellets.
This study combines a simulation-based landscape ap-
proach with life-cycle assessment [12,13] to ascertain rela-
tive savings in GHG emissions when imported wood pellets
from the southern United States are used as a feedstock for
electricity generation at a power plant located in Selby, United
Kingdom [14]. The largest coal-fired power plant in the United
Kingdom is situated at this location. Recently, the management
of this power plant decided to generate about 1000 MW of
electricity using imported wood pellets from the southern
United States [14].
The following seven steps were part of the transatlantic
wood pellet trade supply chain: (a) production of woody
feedstocks; (b) transportation of woody feedstocks from har-
vested tracts to a wood pellet plant using log-trucks; (c)
manufacturing of wood pellets at a wood pellet plant; (d)
transportation of wood pellets from a wood pellet plant to
New Orleans Seaport in the United States using railroads;
(e) transatlantic shipment of wood pellets from New Orleans
Seaport in the United States to Immingham Seaport in the
United Kingdom; (f) transportation of wood pellets from
Immingham Seaport in the United Kingdom to Selby, United
Kingdom using railroads; and (g) burning of wood pellets to
generate electricity at a power plant located in Selby, United
Kingdom. The functional unit selected for this study was a unit
of electricity generated from wood pellets at the power plant
located at Selby, United Kingdom. Individual GHG emissions
for each step present in the supply chain were summed up
and then divided by the total electricity generated at the power
plant to estimate GHG intensity of electricity generated using
imported wood pellets in the United Kingdom. This GHG
intensity was compared with the average GHG intensity of a
unit of grid electricity derived from fossil fuels in the United
Kingdom to determine relative savings in GHG emissions.
2. Methods
This study assumed that biomass obtained from slash pine
(Pinus elliottii) plantations was used for manufacturing wood
pellets. Slash pine is a popular commercial forest species of the
southern United States. In 2007, longleaf-slash pine occupied
about 5.2 million hectares in this region [8]. This species also
reflects general constitution of southern forest resources where
pine forests (planted and natural) occupy about 32% of total
forestland [8]. This study also assumed that the annual quantity
of wood pellets needed by the power plant was sourced from
a wood pellet plant located in the southern United States.
This assumption is valid for two reasons. First, the capacity
of wood pellet plants is rising in the southern United States.
For example, German Pellets announced a plan to build a new
wood pellet plant with an annual capacity of one million metric
tons at Urania, Louisiana [15]; and second, several European
power utilities are opening their own wood pellet plants in the
southern United States to ensure consistent supplies of wood
pellets for their power plants located in Europe [16].
A total of 930 different scenarios were selected for this
study: three feedstocks (logging residues only, pulpwood only,
and logging residues and pulpwood combined), two forest
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Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
management choices (intensive and non-intensive), 31 rotation
ages (year 10 to year 40 in steps of 1 year), and five power
plant capacities (20–100 MW in steps of 20 MW). Unlike
non-intensive forest management, herbicides (at plantation
establishment year) and fertilizers (at 2nd and 12th year of
plantation) were applied under intensive forest management.
Utilization of pulpwood for manufacturing of wood pellets was
considered as evidence suggests that pulpwood is increasingly
being utilized for manufacturing of wood pellets to meet rising
export demand [17]. Total availability of timber products at a
harvest age was divided by the harvest age itself to determine
average annual availability of timber products over time. This
was done to consider the impact of multiple harvest cycles at a
given rotation age on the availability of feedstocks. Biogenic
GHG emissions related to burning of bark and wood pellets
were not considered under the assumption that harvested
tracts were immediately planted after harvest. This assumption
is valid as this study uses average annual availability of
feedstocks for wood pellet production. This study does not
consider above- and below-ground carbon sequestered on
forestlands assuming that a forestland owner will continue
to follow the same rotation age.
Procedures adopted for determining GHG emissions of
all the steps present within the supply chain of transatlantic
wood pellets trade are explained below. All distances used in
this study are approximate.
2.1. Production of woody feedstocks
A growth and yield model of slash pine was used to estimate
availability of three timber products: sawtimber, chip-n-saw,
and pulpwood, from a hectare of plantation under inten-
sive and non-intensive forest management choices [18]. The
availability of logging residues at a harvest age was calculated
as the difference between total biomass available in harvested
logs and total biomass present in sawtimber, chip-n-saw, and
pulpwood at the stand level plus 20% of all biomass present
in sawtimber, chip-n-saw, and pulpwood at the same harvest
age [19]. The additional 20% biomass was added as a proxy for
biomass available in branches and tree tops [19]. Total GHG
emission related to plantation management under intensive
forest management choice was 4803 kg CO2e ha−1when
the harvest age was equal to or greater than 12 years4. It
was 2431 kg CO2e ha−1when the harvest age was 10 and
11 years [5]5. For non-intensive forest management choice,
total GHG emission was 2200 kg CO2e ha−1for the selected
range of harvest ages [5]6. An updated value of nitrous oxide
emissions was used based on GREET [20]. These GHG
emissions were divided by the harvest age and then allocated to
available timber products by the percentage weight contributed
4GHG emission related to site preparation, fertilizer application, and
harvesting was 1127.4 kg ha−1, 2541.7 kg ha−1and 1134.2 kg ha−1,
respectively.
5GHG emission related to site preparation, fertilizer application, and
harvesting was 1127.4 kg ha−1, 170.3 kg ha−1and 1134.2 kg ha−1,
respectively.
6GHG emission related to site preparation and harvesting was
1065.5 kg ha−1and 1134.2 kg ha−1, respectively.
by each timber product towards the combined weight of timber
products available at that harvest age. Percentages were based
on average annual availability of timber products. Collection
efficiency of logging residues was taken as 70% only [21].
No GHG emissions were allocated to logging residues when
only pulpwood was used as a feedstock under the assumption
that logging residues were left in field when not used as a
feedstock.
2.2. Transportation of selected feedstocks
The annual quantity of wood pellets required (WPreq in Mg
yr−1) by the power plant was estimated using equation (1):
WPreq = [P∗(CF/100)∗24 ∗365 ∗1000]/[(CVWP/3.6)
∗(CE/100)](1)
where Pis the capacity of the power plant in MW, CF is
the power plant capacity factor in percentage, CVWP is the
calorific value of wood pellets in MJ kg−1, and CE is the
percentage efficiency of converting heat into electricity. For
CF, a value of 66.9% based on the average capacity utilization
of the power sector in the United Kingdom was used [22].
For CE, steam cycle conversion efficiencies for 20, 40, 60,
80, and 100 MW power plants were taken as 23.4%, 26.9%,
29.1%, 30.6%, and 31.7%, respectively [23]. For CVWP, an
average value of 18.5 MJ kg−1was used [24]. Total quantity
of green biomass required (GBreq in Mg yr−1) was estimated
using equation (2):
GBreq =WPreq ∗(1−MCWP/100)∗(1+(MCSP /100))
∗(100/(100 −BK)) (2)
where MCWP is the percentage moisture content of wood
pellets on oven dry basis, MCSP is the percentage moisture
content of slash pine on oven dry basis, and BK is the
percentage of bark weight. The values of MCWP, MCSP,
and BK were taken as 5% [5], 69% [25], and 18% [26],
respectively.
This study assumed that the shape of a harvest tract was
square. Total number of harvest tracts (HTtot) needed to supply
required wood was calculated using equation (3):
HTtot =GBreq/(WA ∗HTsize)(3)
where WA is woody feedstock available for wood pellet
production (Mg ha−1yr−1) and HTsize is the average harvest
tract size in hectares. The average harvest tract size was
36.5 ha for the southern United States [27]. The value of
HTtot was rounded to the nearest greater integer. The value
of available feedstock was dependent on the type of timber
products considered for wood pellet production, harvest age,
and forest management choice.
This study assumed that the wood pellet plant was
located in the center of a woodshed surrounded by contiguous
harvest tracts of similar characteristics (species, age, forest
management choice, and size). A woodshed was defined as an
area from which wood was sourced for manufacturing of wood
pellets by an owner of a wood pellet plant. The shape of the
3

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
woodshed was assumed to be square as well. The side length of
this woodshed (WSside), in terms of number of harvest tracts,
was determined using equation (4):
WSside =sqrt(HTtot)∗2+1.(4)
The value of WSside was rounded to the nearest greater integer
and then squared to ascertain total number of contiguous har-
vest tracts present in the woodshed. Then, Euclidian distances
between the wood pellet plant and all other harvest tracts
(WSside ∗WSside −1) present in the woodshed were estimated
and multiplied by 1.35 individually [28]. This multiplication
was necessary to consider the impact of local terrain on the
distance between wood pellet plant and any harvest tract.
This study assumed that an owner of the wood pellet
plant will attempt to reduce total distance traveled to transport
required quantities of feedstocks from surrounding harvest
tracts present in the woodshed. Therefore, an owner would
follow the optimization rule given in equation (5) to select
required number of harvested tracts out of all harvest tracts
present in the woodshed:
min
i=k
X
i=1
XiDi,such that Xiis binary (0,1)
and XiBi≥GBreq (5)
where k=WSside ∗WSside −1,Bi=WAi∗HTsize, and Dis
the distance of each harvest tract from the wood pellet plant
located in the center of the woodshed. An algorithm programed
in MS Excel c
was used to implement equation (5).
Total biomass availability on a harvested tract was di-
vided by the capacity of a log-truck (22.7 Mg) and rounded
off (upward) to estimate total trips needed for transporting
available feedstock from a harvested tract to a wood pellet
plant. The distance between a harvested tract and wood pellet
plant was multiplied with number of trips required and divided
by the fuel economy of a loaded log-truck (1.91 km l−1) to
estimate total diesel consumption. The same procedure was
adopted to determine fuel consumption related to return trips.
The fuel economy of a returning unloaded log-truck was taken
as 2.34 km l−1. Total diesel consumption (loaded and unloaded
log-truck) was added and multiplied with a GHG emission
factor (2.68 kg CO2e l−1, [29]) to estimate GHG emissions
related to transportation of biomass from a harvested tract
to a wood pellet plant. This procedure was repeated for all
harvested tracts. Finally, all GHG emissions were added to
determine total GHG emissions related to the transportation of
a feedstock to a wood pellet plant.
2.3. Manufacturing of wood pellets
Total quantities of wood pellets produced were multiplied with
a GHG emission factor (155.7 g CO2e kg−1, [5]) to ascertain
total GHG emissions related with wood pellet production.
Non-biogenic GHG emissions related with bark burning at
the wood pellet plant were also considered (34.4 g CO2e kg−1
of burned material, [30]).
2.4. Transportation of wood pellets in the United States
The average distance between a wood pellet plant and New
Orleans Seaport in the United States was 150 km [14].
The product of total distance traveled and total biomass
transported was multiplied with a GHG emission factor (0.022
kg CO2e Mg−1km−1, [31]) to determine GHG emission
related with the transportation of required wood pellets to
New Orleans Seaport in the United States using railroads.
2.5. Transatlantic shipment of wood pellets
The average distance between New Orleans Seaport in the
United States to Immingham Seaport in the United Kingdom
was 11050 km [32]. The product of total distance traveled
and total wood pellets transported was multiplied with a GHG
emission factor (0.009 kg CO2e Mg−1km−1, [33]) to ascertain
GHG emissions related with the transatlantic shipment of
wood pellets.
2.6. Transportation of wood pellets in the UK
The average distance between a power plant at Selby, United
Kingdom and Immingham Seaport in the United Kingdom was
112 km [14]. The product of total distance traveled and total
wood pellets transported was multiplied with a GHG emission
factor (0.022 kg CO2e Mg−1km−1, [33]) to determine GHG
emissions related with the transportation of wood pellets using
railroads.
2.7. Electricity generation
Non-biogenic GHG emissions related with wood pellet burn-
ing at the power plant was 34.4 g CO2e kg−1of burned material
[30]. The GHG intensity of a unit of grid electricity derived
from fossil fuels was taken as 690 g CO2e kWh−1in the United
Kingdom [22].
3. Results
The availability of logging residues was higher under intensive
than non-intensive forest management. The availability of
pulpwood was higher under intensive forest management only
until the 15th year of plantation with respect to non-intensive
forest management. However, the combined availability of
both pulpwood and logging residues was greater under inten-
sive than non-intensive forest management for all plantation
ages considered (figure 1(a)). Average annual availability of
pulpwood was higher at early plantation years and declined
with an increase in plantation age (figure 1(b)). Average annual
availability of logging residues initially increased with a rise
in plantation age but started to decrease with a further rise
in plantation age. The weight of logging residues relative to
the total weight of all timber products stabilized at about 12%
after the 12th year of plantation under both forest management
choices (figure S1 available at stacks.iop.org/ERL/9/024007/m
media). GHG emissions related with the production of woody
feedstocks were higher under intensive than non-intensive
forest management choice after the 12th year of plantation
4

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Figure 1. Availability of timber products with respect to plantation age. (a) shows quantities of timber products available at a harvest age. (b)
shows average availability of timber products per year for a rotation age. Values in (b) are obtained after dividing values in (a) by the
corresponding harvest age. Combined availability of pulpwood and logging residues is shown separately. Site index is about 21 m at 25th
year of a slash pine plantation. Initial plantation density is 1235 seedlings ha−1.
because of application of fertilizers at the same year under
intensive forest management (figure S2 available at stacks.iop
.org/ERL/9/024007/mmedia).
Total electricity generated was proportional to the ca-
pacity of the power plant (table 2). Total wood and wood
pellets needed to generate electricity were proportional to the
power plant capacity as well (table 3). For a given power
plant capacity, total number of harvested tracts was inversely
proportional to the average annual feedstock availability per
unit forestland—a function of rotation age, forest management
5

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Figure 2. Total distance covered to source required feedstock from surrounding harvested tracts. Extra distance covered to transport excess
feedstock from the last harvested tract is not considered. LR: logging residues; PW: pulpwood; intensive: intensive forest management; and
non-intensive: non-intensive forest management.
Table 2. Total electricity generated at a power plant. Reported numbers are based on the numerator in equation (1).
Power plant capacity (MW) →20 40 60 80 100
Electricity generated (million kWh yr−1) 117.2 234.4 351.6 468.8 586.0
Table 3. Quantities of required wood and wood pellets. Reported numbers are based on equations (1) and (2).
Power plant capacity (MW) →20 40 60 80 100
Wood pellets (1000 Mg yr−1) 97.5 169.1 235.4 298.6 359.7
Wood (green, 1000 Mg yr−1) 190.9 331.1 460.9 584.7 704.3
choice, and feedstock type considered for manufacturing of
wood pellets (figure S3 available at stacks.iop.org/ERL/9/02
4007/mmedia). For instance, total number of harvested tracts
for scenarios when only pulpwood was used as a feedstock was
higher under intensive than non-intensive forest management
choice from the 16th year of rotation age because the average
annual availability of pulpwood was lower under intensive than
non-intensive forest management choice after the 15th year of
plantation. Total number of trips needed to transport feedstocks
from a harvested tract to wood pellet plant decreased with a
decline in feedstock availability per unit forestland (figure
S4 available at stacks.iop.org/ERL/9/024007/mmedia). These
trips remained the same across different power plant capacities.
We found that an owner of the wood pellet plant procured
required quantities of feedstocks only from those harvested
tracts which were located in the vicinity of the wood pellet
plant starting from the nearest harvest tract. The distance of the
last harvested tract from the wood pellet plant is shown in figure
S5 (available at stacks.iop.org/ERL/9/024007/mmedia). Har-
vested tracts were arranged in a circular shape around the wood
pellet plant. The radius of the procurement area was inversely
proportional to the average annual feedstock availability per
unit forestland and directly proportional to the quantities
6

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Figure 3. Total GHG emissions related with transatlantic wood pellet trade. Emissions from any extra distance covered to transport excess
feedstock from the last harvested tract are not considered. The same is true for GHG emissions related to any excess forestland area present
in the last harvested tract. LR: logging residues; PW: pulpwood; intensive: intensive forest management; and non-intensive: non-intensive
forest management.
Table 4. GHG emissions of steps present in the supply chain of transatlantic wood pellet trade. Reported numbers are based steps explained
in the methods section starting from equation (3) onwards.
Power plant capacity (MW) →20 40 60 80 100
Steps present in the supply chain (1000 Mg CO2e) ↓
Manufacturing of wood pellets 15.3 26.5 36.9 46.8 56.4
Transportation of wood pellets using railroads in United States 0.3 0.6 0.8 1.1 1.3
Transatlantic shipment of wood pellets 9.7 16.8 23.4 29.7 35.8
Transportation of wood pellets using railroads in United Kingdom 0.2 0.4 0.6 0.7 0.9
Burning of wood pellets at Selby, United Kingdom 3.4 5.8 8.1 10.3 12.4
of wood pellets manufactured (figure S5 available at stack
s.iop.org/ERL/9/024007/mmedia). Total distance covered to
transport feedstocks to a wood pellet plant increased with an
increase in the power plant capacity (figure 2). Total distance
traveled to source sufficient feedstock was directly dependent
on feedstock availability per unit forestland area mediated by
the required number of trips. Total distance traveled to source
required wood was inherently variable in nature and much
larger than previously published estimates [5–7].
Total GHG emissions increased with an increase in power
plant capacity (figure 3). Total GHG emissions were lowest
when both pulpwood and logging residues were used as a feed-
stock for wood pellet production for a given power plant capac-
ity because of higher feedstock availability per unit forestland
area relative to a situation when only pulpwood or only logging
residues were used for wood pellet production. Similarly,
total GHG emissions under non-intensive forest management
choice were typically lower than intensive forest management
choice. This was mostly because of higher allocation of GHG
emissions at the time of wood production to feedstocks under
intensive than non-intensive forest management. This higher
allocation compensated any reduction in GHG emissions due
to a decrease in number of harvested tracts needed to source
sufficient wood or total distance covered to source sufficient
quantities of feedstock for wood pellet production.
7

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Figure 4. Relative contribution (percentage) of different steps present within the supply chain of transatlantic wood pellet trade towards total
GHG emission.
Contribution of five out of seven steps present within
the supply chain of transatlantic wood pellet trade towards
total GHG emission remained constant independent of harvest
age, forest management choice, and feedstock type considered
for wood pellet production for a given power plant capacity
(table 4). However, GHG emissions related with steps wood
production and transportation of wood to a wood pellet plant
were dependent on rotation age, forest management choice,
and feedstock type used for wood pellet production, and
therefore were primarily responsible for any variability in
total GHG emissions for a given power plant capacity (fig-
ure 4). GHG emissions arising from wood pellet production
contributed most significantly towards total emissions (about
48%), followed by transatlantic transportation of wood pellets
(about 31%) and burning of wood pellets (about 10%). The
contribution of GHG emissions related to transportation of
required feedstock from harvested tracts to a wood pellet plant
was only about 1%–3%. GHG emissions related to feedstock
production were at least three times higher than GHG emis-
sions related to transportation of required feedstocks to a wood
pellet plant. Additionally, GHG emissions related to feedstock
production decreased smoothly with an increase in harvest
age contrary to GHG emissions related to transportation of
feedstocks to a wood pellet plant. Therefore, distribution
of total GHG emission was relatively smooth without any
noticeable variability between any two consecutive harvest
ages.
A comparison of GHG intensities of electricity generated
from imported wood pellets (figure 5) and the United King-
dom’s current mix of fossil fuel-based grid electricity revealed
that relative savings in GHG emissions increased with a rise
in power plant capacity (figure 6). This contradicts a general
belief that high capacity wood pellet plants should not be
promoted in the United Kingdom and elsewhere as they do
not provide any GHG benefits [11]. Figure 6 also shows that
relative savings in GHG emissions start to flatten out with a rise
in the capacity of a power plant responding to the behavior of
conversion efficiency which initially increases at an increasing
rate but then flattens out with a further rise in the power plant
capacity [23].
8

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Figure 5. GHG intensity of a unit of generated electricity at Selby, United Kingdom. LR: logging residues; PW: pulpwood; intensive:
intensive forest management; and non-intensive: non-intensive forest management.
4. Discussions and conclusions
This study extends our understanding about the impact of
transatlantic wood pellet trade on the United Kingdom’s
electricity-related GHG emissions by combining a simulation-
based approach with life-cycle assessment under realistic
assumptions at landscape level. The GHG intensity of a unit
of electricity generated using imported wood pellets in the
United Kingdom is at least 50% lower than the GHG intensity
of grid electricity derived from fossil fuels. Therefore, use of
imported wood pellets from the southern United States for
electricity generation could help in reducing GHG emissions
in the United Kingdom.
Relative savings in GHG emissions were only higher by
up to 2% for wood pellets manufactured using feedstocks
obtained from non-intensive than intensive forest management
choice especially when the age of non-intensively managed
plantations was greater than 12 years. This implies that feed-
stock obtained from both intensive and non-intensively man-
aged forest plantations can be used for manufacturing wood
pellets to achieve reductions in GHG emissions without any
significant drop in relative savings of GHG emissions. Addi-
tionally, relative savings in GHG emissions were almost sim-
ilar irrespective of type of feedstock used for manufacturing
of wood pellets. This implies that the use of logging residues
along with other feedstocks for manufacturing of wood pellets
in the southern United States and subsequent utilization of
manufactured wood pellets for electricity generation in the
United Kingdom could save a significant amount of GHG
emissions.
Logging residues and pulpwood derived from mature
plantations should be used as a feedstock to ensure highest
savings in GHG emissions for any power plant capacity.
However, relative savings in GHG emissions were at least
50% even at lower rotation ages. These results contradict
a general belief that the use of wood pellets, manufactured
from feedstocks (mostly pulpwood and logging residues)
obtained from 10 to 15 year old pine plantations in the
southern United States do not provide any GHG savings over
electricity generated from fossil fuels in the United Kingdom
[13]. Relative savings in GHG emissions increased with a
rise in the capacity of power plant mostly because of higher
conversion efficiencies of high capacity power plants. This
further suggests that high capacity power plants will be much
better for reducing GHG emissions than low capacity power
plants.
The approach adopted in this study for determining total
distance covered to source sufficient feedstock for wood pellet
9

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
Figure 6. Relative savings in GHG emissions with respect to grid electricity derived from fossil fuels. LR: logging residues; PW: pulpwood;
intensive: intensive forest management; and non-intensive: non-intensive forest management.
production assumes that forestland owners are willing to sell
logging residues and pulpwood to an owner of wood pellet
plant only. Additionally, this study assumes that all harvested
tracts are similar in terms of planted species, plantation age,
and located contiguously. Dynamics of sourcing required
wood for manufacturing of wood pellets or any other wood-
based product is much more complex at the landscape level.
This study acknowledges this complexity and suggests that
results of this study should be considered as a best case
only. However, relative contribution of GHG emission related
with the transportation of feedstocks is relatively small (<3%)
towards overall GHG emissions. Therefore, it is very unlikely
that relative percentage savings in GHG emissions will change
a lot even after relaxing these assumptions.
In this study, the average annual feedstock yield remains
constant. However, yields may change over time, which creates
a need to analyze the impact of future changes in feedstock
yields on overall GHG savings. Additionally, much insight
would be gained from integrating the model developed in
this study with market equilibrium models [34] to analyze
the consequences of an increase in demand for feedstocks for
wood pellet production on the rotation age. This is especially
true as a change in rotation age could affect carbon benefits
of transatlantic wood pellet trade [35]. Furthermore, it will
be interesting to explore the potential of other technologies
like combined heat and power [36] to determine GHG savings
of wood pellets not only in Europe but in the United States
as well. We have analyzed only two scenarios of biomass
production in this study. Forestland owners practice multiple
ways to manage their plantations. Thus, future research should
consider the impact of multiple forest management practices
on the relative savings in GHG emissions associated with
transatlantic wood pellet trade. Finally, a better understanding
of soil carbon behavior especially for short rotation cycles is
also needed. We hope that findings of this study will help in
guiding the debate on sustainability of wood-based bioenergy
products at local, regional, national, and global levels. We
also hope that this study will be able to guide future research
appropriately.
Acknowledgments
Authors are thankful for the funding provided by Energy
Biosciences Institute @ University of Illinois at Urbana-
Champaign/University of California, Berkeley. Authors are
thankful to Shawn Baker at Warnell School of Forestry and
Natural Resources, University of Georgia for his help with
data on log-trucks.
10

Environ. Res. Lett. 9(2014) 024007 P Dwivedi et al
References
[1] Goetzl A 2012 Pellet Power—Global Trade in Wood Pellets
(Internet) (Washington, DC: United States International
Trade Commission) Available from: www.usitc.gov/publica
tions/332/executive briefings/EBOT Wood Pellets Final.p
df
[2] DECC 2011 UK Renewable Energy Roadmap (Internet)
(London: Department of Energy & Climate Change)
Available from: www.gov.uk/government/uploads/system/u
ploads/attachment data/file/48128/2167-uk-renewable-ener
gy-roadmap.pdf
[3] Lovell J 2013 Europe’s 2nd-biggest coal-fired power plant will
turn to wood from North America ClimateWire (Internet)
Available from: www.eenews.net/stories/1059979005
[4] Ekstrom H 2012 The US Surpassed Canada as the Largest
Wood Pellet Exporter in the World in the First Half of 2012
(Bothell, WA: Wood Resources International LLC)
Available from: www.pr.com/press-release/447430
[5] Dwivedi P, Bailis R, Bush T G and Marinescu M 2011
Quantifying GWI of wood pellet production in the southern
United States BioEnergy Res. 4180–92
[6] Magelli F, Boucher K, Bi H T, Melin S and Bonoli A 2009
An environmental impact assessment of exported wood
pellets from Canada to Europe Biomass Bioenergy
33 434–41
[7] Damen K and Faaij A 2006 A greenhouse gas balance of two
existing international biomass import chains Mitig. Adapt.
Strateg. Glob. Change 11 1023–50
[8] Smith W, Miles P, Perry C and Pugh S 2009 Forest Resources
of the United States, 2007: A Technical Document
Supporting the Forest Service 2010 RPA Assessment
(Washington, DC: United States Department of Agriculture
Forest Service (Available from http://www.nrs.fs.fed.us/pub
s/rn/rn nrs38.pdf))
[9] Butler B 2008 Family Forest Owners of the United States,
2006 (Newton Square, PA: United States Department of
Agriculture Forest Service)
[10] Homer C, Huang C, Yang L, Wylie B K and Coan M 2004
Development of a 2001 national land-cover database for the
United States Photogram. Eng. Remote Sens. 70 829–40
[11] Harrabin R 2013 Biomass fuel subsidies to be capped says
energy secretary BBC News (London) Available from
http://www.bbc.co.uk/news/business-23334466
[12] ISO 2006 Environmental Management—Life Cycle
Assessment—Principles and Framework (Geneva:
International Organization for Standardization)
[13] ISO 2006 Environmental Management—Life Cycle
Assessment—Requirements and Guidelines (Geneva:
International Organization for Standardization)
[14] Lundgren K and Morales A 2012 Biggest English Polluter
Spends $1 Billion to Burn Wood. www.bloomberg.com
(Internet) (London) Available from: www.bloomberg.com/n
ews/2012-09-25/biggest-english-polluter-spends-1-billion-t
o-burn-wood-energy.html
[15] Simet A 2013 German Pellets plans second giant wood pellet
plant in US (internet). Biomass Mag. (cited 25 July
2013). Available from: http://biomassmagazine.com/articles
/8904/german-pellets-plans-second-giant-wood-pellet-plant
-in-u-s/
[16] Kinney S 2011 US industrial pellet market prepares for
Europe’s 20 ×2020 renewable energy mandate (internet)
www.forest2market.com (cited 25 July 2013). Available
from: www.forest2market.com/blog/US-Industrial-Pellet-M
arket-Prepares-for-Europes-20-x-2020-Renewable-Energy
[17] Spelter H and Toth D 2009 North America’s Wood Pellet
Sector (Madison, WI: United States Department of
Agriculture Forest Service)
[18] Yin R, Pienaar L and Aronow M 1998 The productivity and
profitability of fiber farming J. Forest 96 13–8
[19] Jenkins J, Chojnacky D, Heath L and Birdsey R 2003 National
scale biomass estimators for United States tree species
Forest Sci. 49 12–35
[20] Wang M 2001 Development and Use of GREET 1.6
Fuel-Cycle Model for Transportation Fuels and Vehicle
Technologies (Argonne, IL: Argonne National Laboratory)
[21] ORNL 2011 US Billion-Ton Update: Biomass Supply for a
Bioenergy and Bioproducts Industry (Oak Ridge, TN: Oak
Ridge National Laboratory) p 227
[22] DECC 2013 Digest of UK Energy Statistics’ (DUKES)
(Internet) (London: Department of Energy & Climate
Change). Available from: www.gov.uk/government/organis
ations/department-of-energy-climate-change/series/digest-o
f-uk-energy-statistics-dukes
[23] Bridgwater A V, Toft A J and Brammer J G 2002 A
techno-economic comparison of power production by
biomass fast pyrolysis with gasification and combustion
Renew. Sustain. Energy Rev. 6181–246
[24] Dwivedi P, Bailis R and Khanna M 2013 Is use of both
pulpwood and logging residues instead of only logging
residues for bioenergy development a viable carbon
mitigation strategy? BioEnergy Res. Available from:
http://link.springer.com/10.1007/s12155-013-9362-z
[25] Gibson M D, McMillin C W and Shoulders E 1986 Moisture
content and specific gravity of the four major southern pines
under the same age and site conditions Wood Fiber Sci.
18 428–35
[26] Miles P and Smith W 2009 Specific Gravity and Other
Properties of Wood and Bark for 156 Tree Species Found in
Noth America (Newton Square, PA: United States
Department of Agriculture Forest Service) Available from
http://www.nrs.fs.fed.us/pubs/rn/rn nrs38.pdf
[27] Baker S, Greene D and Harris T 2012 Impact of timber sale
characteristics on harvesting costs. Proc. South. For. Econ.
Work (Charlotte, NC: Mississippi State University)
pp 94–105
[28] Ravula P P 2007 Design, simulation, analysis and optimization
of transportation system for a biomass to ethanol conversion
plant PhD Thesis Virginia Polytechnic Institute and State
University
[29] EIA 2011 Fuel Emission Coefficients (Internet) Volunt.
Report. Greenh. Gases Progr. (Washington, DC: Energy
Information Administration, United States Department of
Energy) Available from: www.eia.gov/oiaf/1605/coefficient
s.html#tbl2
[30] WDNR 2010 Forest Biomass and Air Emissions (Olympia,
WA: Washington Department of Natural Resources)
[31] PR´
e-Consultants 2013 US LCI Database Simapro LCA
Software (Amersfoort: PR´
e-Consultants)
[32] Anonymous 2013 Sea route and distance (Internet).
www.ports.com (cited 15 June 2013) Available from:
http://ports.com/sea-route/
[33] PR´
e-Consultants 2013 Ecoinvent Database Simapro LCA
Software (Amersfoort: PR´
e-Consultants)
[34] Galik C, Abt R and Wu Y 2009 Forest biomass supply in the
southeastern United States: implications for industrial
roundwood and bioenergy production J. Forest
107 69–77
[35] Mitchell S R, Harmon M E and O’Connell K E B 2012
Carbon debt and carbon sequestration parity in forest
bioenergy production GCB Bioenergy 4818–27
[36] Bernotat K and berg T 2004 Biomass fired small-scale CHP in
Sweden and the Baltic States: a case study on the potential
of clustered dwellings Biomass Bioenergy 27 521–30
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