Not carbon neutral: Assessing the net emissions impact of residues burned for bioenergy

Abstract

Climate mitigation requires emissions to peak then decline within two decades, but many mitigation models include 100 EJ or more of bioenergy, ignoring emissions from biomass oxidation. Treatment of bioenergy as 'low carbon' or carbon neutral often assumes fuels are agricultural or forestry residues that will decompose and emit CO2 if not burned for energy. However, for 'low carbon' assumptions about residues to be reasonable, two conditions must be met: biomass must genuinely be material left over from some other process; and cumulative net emissions, the additional CO2 emitted by burning biomass compared to its alternative fate, must be low or negligible in a timeframe meaningful for climate mitigation. This study assesses biomass use and net emissions from the US bioenergy and wood pellet manufacturing sectors. It defines the ratio of cumulative net emissions to combustion, manufacturing and transport emissions as the net emissions impact (NEI), and evaluates the NEI at year 10 and beyond for a variety of scenarios. The analysis indicates the US industrial bioenergy sector mostly burns black liquor and has an NEI of 20% at year 10, while the NEI for plants burning forest residues ranges from 41%–95%. Wood pellets have a NEI of 55%–79% at year 10, with net CO2 emissions of 14–20 tonnes for every tonne of pellets; by year 40, the NEI is 26%–54%. Net emissions may be ten times higher at year 40 if whole trees are harvested for feedstock. Projected global pellet use would generate around 1% of world bioenergy with cumulative net emissions of 2 Gt of CO2 by 2050. Using the NEI to weight biogenic CO2 for inclusion in carbon trading programs and to qualify bioenergy for renewable energy subsidies would reduce emissions more effectively than the current assumption of carbon neutrality.

Full text

Environ. Res. Lett. 13 (2018) 035001 https://doi.org/10.1088/1748-9326/aaac88
LETTER
Not carbon neutral: Assessing the net emissions impact
of residues burned for bioenergy
Mary S Booth1,2
1Partnership for Policy Integrity, Pelham, Massachusetts, United States of America
2Author to whom any correspondence should be addressed.
OPEN ACCESS
RECEIVED
4 September 2017
REVISED
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ACCEPTED FOR PUBLICATION
2 February 2018
PUBLISHED
21 February 2018
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E-mail: mbooth@pfpi.net
Keywords: bioenergy, wood pellets, climate mitigation, residues, biomass, forest residues, carbon accounting
Abstract
Climate mitigation requires emissions to peak then decline within two decades, but many mitigation
models include 100 EJ or more of bioenergy, ignoring emissions from biomass oxidation. Treatment
of bioenergy as ‘low carbon’or carbon neutral often assumes fuels are agricultural or forestry residues
that will decompose and emit CO2if not burned for energy. However, for ‘low carbon’assumptions
about residues to be reasonable, two conditions must be met: biomass must genuinely be material left
over from some other process; and cumulative net emissions, the additional CO2emitted by burning
biomass compared to its alternative fate, must be low or negligible in a timeframe meaningful for
climate mitigation. This study assesses biomass use and net emissions from the US bioenergy and
wood pellet manufacturing sectors. It defines the ratio of cumulative net emissions to combustion,
manufacturing and transport emissions as the net emissions impact (NEI), and evaluates the NEI at
year 10 and beyond for a variety of scenarios. The analysis indicates the US industrial bioenergy sector
mostly burns black liquor and has an NEI of 20% at year 10, while the NEI for plants burning forest
residues ranges from 41%–95%. Wood pellets have a NEI of 55%–79% at year 10, with net CO2
emissions of 14–20 tonnes for every tonne of pellets; by year 40, the NEI is 26%–54%. Net emissions
may be ten times higher at year 40 if whole trees are harvested for feedstock. Projected global pellet
use would generate around 1% of world bioenergy with cumulative net emissions of 2 Gt of CO2by
2050. Using the NEI to weight biogenic CO2for inclusion in carbon trading programs and to qualify
bioenergy for renewable energy subsidies would reduce emissions more effectively than the current
assumption of carbon neutrality.
Introduction
Meeting the Paris Agreement goal of limiting global
temperature increase will require fast deployment of
zero-emissions energy and greatly increased carbon
sequestration. In developing pathways to limit atmo-
spheric CO2, many climate mitigation models include
a doubling or more of bioenergy to at least 100 EJ in the
coming decades [1–3], with much of the fuel assumed
to come from forestry and agricultural residues [3].
Though oxidizing 100 EJ of biomass would emit about
9GtofCO
2each year, most mitigation models assign
bioenergy zero net emissions.
The assumption of bioenergy carbon neutrality
underpins many renewable energy investments, includ-
ing in the EU, UK and Asia where dried wood pellets ar e
imported as a replacement for coal. Such policies, and
the lucrative subsidies they provide, have driven rapid
growth in the woodpellet sector in North America, with
US exports growing from less than 0.1 Mt in 2008 [4]to
4.9 Mt in 2016 [5]. Canadian pellet exports increased
46% from 2015–2016 [6], and US pellet exports are
projected to double or triple from 2016 levels by 2025
[5,7].
Biomass power plants tend to emit more CO2
than fossil fueled plants per MWh, and as shown by
a number of studies, net emissions from bioenergy
can exceed emissions from fossil fuels for decades
[8–12]. Nevertheless, some studies conclude rapid
carbon benefits from burning wood pellets by employ-
ing various assumptions: that forest planting will
increase in response to demand for wood [13]; that
© 2018 The Author(s). Published by IOP Publishing Ltd

Environ. Res. Lett. 13 (2018) 035001
replanting occurs immediately after harvest [14]; or
that forest growth elsewhere compensates for emis-
sions from harvesting and combusting trees [15,16]
(for a review, see Ter-Mikaelian et al 2015 [17]).
Some discussions of bioenergy in mitigation modeling
include similar assumptions that burning ‘sustainable,’
‘optimal’[1,3]or‘surplus’[18] forest wood can reduce
net CO2emissions as long as forest carbon stocks are
increasing. Such assumptions often disregard the role
of the forest carbon sink, thus the controversy around
bioenergy carbon accounting continues.
However, on one aspect of bioenergy carbon
accounting there is wide agreement: that when biomass
is sourced from residues from forestry, wood products
manufacturing, or agriculture, net carbon emissions are
properly assessed as the difference between emissions
from their use as fuel (which can include emissions
from fuel manufacturing and transport), and emissions
from an alternative fate, such as leaving material on-
site to decompose or burning it without energy recovery
[8–10,12 19–23].
Studies using this approach generally conclude net
bioenergy emissions are not zero over varying peri-
ods of time. Nonetheless, many policies still treat
bioenergy as having zero or negligible emissions.
European Commission guidance for the EU carbon
trading program explains bioenergy emissions should
be ‘taken to be zero,’and that wood pellets consist
of ‘processing residues from forest based industries’
[24]. The IPCC acknowledges harvesting trees for fuel
can increase cumulative emissions for years to cen-
turies, but concludes that ‘agricultural and forestry
residues can provide low-carbon and low-cost feed-
stock for bioenergy’[3]. The IPCC renewable energy
report identifies potential for 100 EJ of bioe nergy specif-
ically from residues [18] and does not discuss potential
emissions.
For the assumption that residues have negligible
net emissions to be reasonable, at least two conditions
must be met. First, biomass classified as residues must
actually be residues—that is, materials generated by
some other process, where the alternative fate is decom-
position or burning without energy recovery. Second,
net emissions from bioenergy, that is, the cumulative
additional CO2emitted from processing and burning
biomass versus from an alternative fate, must be low,
if not negligible, within a timeframe meaningful for
climate mitigation.
What should ‘low net emissions in a meaningful
timeframe’mean? Most scenarios for climate change
mitigation that constrain temperature rise consistent
with Paris Agreement goals require emissions to peak
between 2020 and 2030 and decline to less than half
2010 levels by 2050 [25], with negativeemissionsshortly
thereafter. Actions that reduce or end emissions in the
next ten years are thus essential, given that elevated CO2
is already driving essentially irreversible polar ice loss,
permafrost melting, and ocean acidification, along with
thermal sea-level rise, which has been shown to r espond
to temperature changes from short-lived climate pol-
lutants in a ten-year timeframe [26].
Here, ‘low net emissions’from bioenergy implies a
comparison to gross or ‘direct’emissions from manu-
facturing and burning biomass. This study uses a simple
model to calculate a new metric, the ‘net emissions
impact’(NEI), which is the ratio of cumulative net
emissions to direct emissions from burning residues
for energy. The NEI expresses the proportion of direct
CO2emissions that contributes an additional warm-
ing effect over a fifty-year period. Fuel and feedstock
use, net emissions and the NEI are calculated for three
main case studies: the existing US bioenergy sector, new
wood-burning plants using chipped wood, and wood
pellets that are exported to the EU to be burned as a
replacement for coal.
Approach
Built in Excel, the model calculates cumulative net
emissions as cumulative direct emissions (CO2from
combustion for energy plus CO2from harvesting, pro-
ducing, and transporting biomass, or ‘HPT emissions’),
minus cumulative counterfactual emissions (what
emissions would be if the biomass were left in the field
to decompose or were burned without energy recov-
ery). The net emissions impact (NEI) is the ratio of
cumulative net emissions to cumulative direct emis-
sions.
HPT emissions are calculated as explained below.
Direct combustion emissions are calculated as joules of
heat input for each fuel multiplied by fuel-specific CO2
emission factors [27] (non-CO2greenhouse gas emis-
sions are not included in this version of the model). The
spreadsheet sums cumulative counterfactual emissions
from biomass collected in each year in columns, then
sums across columns to calculate cumulative emissions
by each year from all biomass collections up to and
including that year.
Counterfactual carbon emissions (with conversion
to CO2at the last step) are calculated as:
PE′(𝑡)=1−(𝑒−𝑘′𝑡)(1)
cE′(𝑡)=BC
′∗PE
′(𝑡)(2)
CE(𝑡)=
𝑡
∑
1
cE′(𝑡)(3)
where
PE′(t) = proportion of carbon from biomass collected
in a given year that has been emitted by year t
k′= rate-constant for decomposition of biomass col-
lected in a given year
cE′(t) = carbon from biomass collected in a given year
that has been emitted by year t
BC′= carbon content of biomass collected in a given
year
CE(t) = carbon emitted by year tfrom biomass collected
in all years
2

Environ. Res. Lett. 13 (2018) 035001
Table 1. Model inputs for biomass burned for energy in the US. Heat input is average summed value per year for the industrial and
non-industrial sectors, 2001–2016. See text for details.
Fuel GJ yr−1 CO2EF HPT factor Alternative fate k
Agricultural biomass 31.7 0.101 7.5% Decomposition 0.65
This constitutes a small percentage of biomass burned in the US, but can represent a large variety of materials, including crop stover, nut
hulls, and sugarcane bagasse. The k-constant produces a half-life for residues of one year.
Black liquor 800.5 0.087 — Burn w/o ER —
This is a high moisture content material left residue of pulp- and paper-making. The model assumes no net emissions from burning it for
energy.
Other biomass solids 18.3 0.101 4% Burn w/o ER —
EIA does not specify what these materials are. While there are likely processing costs, the model assumes a minimal 4% HPT emissions to
be conservative.
Sludge waste 6.3 0.072 — Burn w/o ER —
Sludge waste is another residue of pulp- and paper-making.
Wood liquor 11.3 0.072 — Burn w/o ER —
This material is related to black liquor.
Wood solids 548.8 0.081 4% Decomposition 0.083
This includes forestry wood, mill residues, urban tree trimmings, and construction and demolition wood. For consistency with wood pellet
scenarios below, the k-constant is 0.083.
To evaluate emissions from the US bioenergy
industry, the model uses bioenergy data from the
Energy Information Administration (EIA) for 2001–
2016 [28]andCO
2combustion emission factors used
by the US Environmental Protection Agency (EPA)
for power sector modeling (original units short tons
mmbtu−1; converted here to metric tonnes GJ−1)
[27]. Alternative fate emissions are calculated using
k-constants particular to each fuel (table 1).
The model includes HPT emissions for forestry
residues and other wood as equivalent to 4% of the
carbon content of green chips, based on Domke et al
(2012) and reviews of other studies [9,29]. The model
assumes the alternative fate for agricultural residues is
decomposition, as crop burning occurs on less than
1% of agricultural acres in the US [30]. Selecting an
HPT factor for agricultural residues is not straightfor-
ward, as emissions from harvest, transport, shredding,
baling, and sometimes pelletizing can be significant.
Depending on how system boundaries are drawn,
emissions from crop cultivation, including N2Ofrom
fertilizers, can be ascribed to residues [31]. Storage
also imposes lifecycle emissions because agricultural
materials can only be collected at fixed intervals. Most
importantly, removing agricultural residues can deplete
soil carbon [32]; some estimates of total HPT emis-
sions including soil C loss sum to more than 100%
of fuel carbon content [31]. This model used an HPT
factor of 7.5% for agricultural residues based solely
on harvest and transport estimates for corn stover
in Whitman et al (2011) and did not include soil car-
bon impacts because this factor was not included for
forestry residues. As agricultural residues provided a
small percentage of total fuels, the choice of HPT fac-
tor had only a trace effect, but any study of large-scale
use of agricultural residues should include soil carbon
effects.
Data on wood use by the US pellet manufacturing
sector was obtained from the forest-industry track-
ing company Forisk [33]. Five pellet scenarios were
modeled to examine how the k-constant and chang-
ing use through time affect net emissions (details
in table 2). Scenarios 1–4 estimated HPT emissions
(which include harvesting, transport to plant, debark-
ing, chipping, pulverization, pellet extrusion, drying,
and oversea transport) as 322 kg CO2per tonne of pel-
lets, following Jonker et al [15], similar to an estimate
by Dwivedi et al [14]. Also following Jonker et al the
model assumed that pellet drying consumes 0.51 green
tonnes of residues per tonne of pellets, an estimate con-
firmed by checking the dryer fuel to pellet production
ratio in permits for two industrial-scale plants in the
US [34,35]. The model assumed residues burned to
dry pellets would decompose with a k-constant of 0.15
if not burned for energy. Facilities that use fossil fuels
to dry pellets have much higher HPT emissions [36],
butthiseffectwasnotincluded.
Results and discussion
Sources of biomass burned for heat and power in the
US
The US bioenergy sector can be divided into industrial
plants, which mostly burn black liquor and wood to
generate onsite heat and power for paper and wood
products manufacturing, and non-industrial plants,
which mostly burn wood to generate power for the
electrical grid (figure 1). The industrial sector mostly
utilizes biomass for heat; on average, just 21% of fuel
energy was used for electricity generation from 2001–
2016, while the smaller non-industrial sector allocated
87% of fuel energy to electricity generation [28].
Combined, industrial and non-industrial facili-
ties burning biomass generated less than 1% of total
electricity in the US in 2016 [37]. However, average
annual generation over a three-year period in the non-
industrial sector increased 62% from 2001–2003 to
2014–2016, while electricity generation stayed relat ively
constant in the industrial sector.
3

Environ. Res. Lett. 13 (2018) 035001
Table 2. Five scenarios for wood pellet manufacturing and use.
10 years (2020) 25 years (2035) 40 years (2050)
Scenario Direct
CO2(t)
Net
CO2(t)
NEI Direct
CO2(t)
Net
CO2(t)
NEI Direct
CO2(t)
Net
CO2(t)
NEI
1: 1 tonne pellets yr−1 ,k=0.15 25 14 55% 62 21 34% 99 26 26%
2: 1 tonne pellets yr−1 ,k=0.03 25 20 79% 62 40 64% 99 54 54%
Direct
CO2(Gt)
Net
CO2(Gt)
NEI Direct
CO2(Gt)
Net
CO2(Gt)
NEI Direct
CO2(Gt)
Net
CO2(Gt)
NEI
3: Actual US exports to year 7
(2017); modeled 15% yr−1 increase
to 12.8 Mt yr−1 at year 15 (2025);
continue at that level; k= 0.083
0.09 0.07 73% 0.53 0.28 53% 1.01 0.40 39%
4: Like Scenario 4, but cease use at
year 20 (2030); k= 0.083
0.09 0.07 73% 0.37 0.16 43% 0.37 0.08 21%
5:Actualglobaldemandof13Mt
tonne pellets yr−1,increasingto
28.2 Mt at y r 6 (2016); modeled
increase to 66.4 Mt yr−1 at year 15
(2025); continue at that level;
k= 0.083
0.63 0.44 71% 2.92 1.48 51% 5.35 1.99 32%
Figure 1. Fuels burned by the US industrial and non-industrial bioenergy sectors, 2001–2016 [28]. Wood burning for electricity
generation in the non-industrial sector increased by about60% from the beginning of the period.
The dominance of black liquor as fuel for industrial
bioenergy means that many facilities at least partially
meet the first of the low carbon conditions—that fuels
genuinely be residues of some other process. How-
ever, the provenance is less clear for wood burned by
the industrial and non-industrial sectors, which totaled
45 Mt (green) in 2016 [28]. Forisk estimates wood
use by US biomass facilities at 35 Mt (green), a figure
that omits certain large industrial users reported by
the EIA, and reported as of late 2016 that operating
and under-construction plants were burning pulp-
wood (7.4%); ‘dirty chips/forest residues’(49.6%);
urban wood (19.4%); and mill residues (23%) [33].
Residues appear to provide the most fuel for US
bioenergy sector, but since there is no set definition
for ‘residues,’it is not possible to know if this wood
is truly the product of some other process. Conserva-
tive definitions for forest residues are found in Domke
et al:the‘tip, portion of the stem above the mer-
chantable bole, and all branches, and excluding foliage’
4

Environ. Res. Lett. 13 (2018) 035001
Figure 2.Trucks lined up waiting to deliver pellet feedstock to North Carolina plant owned by En viva,t helargest US pellet manufacturer.
Much of the wood is tree trunks, not tops and limbs (Photo: Dogwood Alliance).
[20], and Lagani`
ere et al:‘all woody debris generated
in harvest operations for traditional wood products
(e.g. branches, tree tops, bark), excluding stumps and
downed nonmerchantable trees’[12]. However, prac-
tices on the ground vary. For example, Dominion
Energy Resources in Virginia, which re-fired three coal
plants with wood and has a total bioenergy capacity
of over 250 MW, wrote to the EPA that waste wood
‘to us’means ‘forest materials including residues (tree
tops, non-merchantablesections of stem, branches,and
bark), small trees and other low value materials’[38].
Some facilities clearly burn whole trees for fuel, like a
new 70 MW plant in Berlin, New Hampshire that burns
113 tonnes of ‘clean wood chips’per hour, including
‘whole tree chips’[39] (Forisk lists this plant as burn-
ing 408 000 green tonnes of hardwood pulpwood and
408 000 green tonnes of residues per year [33]). The
evidence for use of whole trees as fuel suggests that
many facilities do not burn materials that meet conser-
vative definitions of residues (i.e. branches, tree-tops,
and bark left over from other harvesting).
Sources of wood utilized by the US pellet manufac-
turing industry
As of late 2016, annual production capacity at operat-
ing and under-construction wood pellet manufacturing
facilities in the US was 13.2 Mt, requiring about 28.6 Mt
of green wood as feedstock [33], though not all plants
produced at capacity. Pellet companies emphasize use
of residues, downplaying the use of roundwood as
feedstock [40]. However, exported wood pellets must
meet specifications including restrictions on bark con-
tent [41], thus there is a limit on the amount of
low-diameter branches and tops that can be used.
Accordingly, industry data indicate about 56% of pellet
feedstock is supplied from pulpwood (41% from soft-
wood, 14% from hardwood); 42% from mill residues,
1% from urban wood; and just 1% from logging
residues [33]. Investigations of pellet feedstock at some
large US mills have confirmed that a significant por-
tion of feedstock is bolewood (figure 2). The feedstock
supply from small pellet producers in the Northeast-
ern US also appears to avoid residues; a study of nine
pellet mills in Maine found only 2% came from tops
and limbs, with the remainder classified as pulpwood
or small diameter trees [42].
Similarly, company-supplied data on sources of
wood pellets burned in the UK indicate that the major-
ity of US pellets burned by the coal- and wood-fired
Drax power station in 2015 was sourced from logs that
‘formed part of the trunk of a tree which grew for at least
ten years’from harvesting that was a ‘mix of clearfell
and thinning’[43]. While mill residues currently con-
stitute a proportion of pellet feedstock in the US, they
will not supply a meaningful amount for future capac-
ity, because supplies are limited [44]. The dominance
of pulpwood and the documented use of bolewood
thus indicate that pellets often fail to meet the first
condition for low carbon residues, that feedstocks gen-
uinely be residues that if not collected would otherwise
decompose or be burned without energy recovery.
Emissions from the US biomass industry
Emissions modeling examined the industrial and non-
industrial bioenergy sectors separately. The model
estimated cumulative direct CO2emissions from
industrial facilities at 1135 Mt at year 10. However,
because black liquor and other wastes provide a
large proportion of industrial sector biomass, and the
assumed alternate fate for these materials is combus-
tion without energy recovery, cumulative net emissions
are 224 Mt, for an NEI of 20% (figure 3(a)). Cumu-
lative direct emissions for the smaller non-industrial
sector are 208 Mt at year 10, but because the majority
of fuel for this sector is wood and the weighted k-
constant is lower, cumulative net emissions are 120 Mt
and the NEI is 58% at year 10 (figure 3(b)). Both sectors
show cumulative net emissions still increasing in the
40–50 year period, though less steeply than in the initial
decades.
This analysis calculates emissions at the sector level
as if all units initiated operation at the same time,
while ideally, sector-level accounting would consider
how long each facility has been operating. While inad-
equate data render this impractical, in general, the
industrial sector has shrunk since the 1980s [45]and
the present day NEI should be shifted toward the right
(lower), as facilities are on average older. In contrast,
5

Environ. Res. Lett. 13 (2018) 035001
Figure 3. Cumulativenet emissions and the NEI for the industrial (a) and non-industrial (b) bioenergy sectors over a 50 year timeframe,
estimated using the average fuel mix for 2001–2016. High use of black liquor as fuel reduces the NEI for the industrial sector, while
greater reliance on wood increases it for the non-industrial sector.
new construction of wood burning power plants and
coal-to-wood conversions in the non-industrial sector
since the early 2000s (figure 1)[28]meanstheaverage
age of the sector is younger, shifting the NEI to the left
(higher).
Refined emissions estimates for new wood-burning
power plants
The industry-level analysis assumed an average
k-constant for wood of 0.083, but plotting the NEI
against the full range of k-constants (figure 4)demon-
strates that the NEI for facilities burning forest residues
exceeds 70% at year 10 for all decomposition con-
stants lower than 0.07, and exceeds 40% at year 10
for the full range of decomposition constants for North
American forests [46]. This conclusion is likely valid
even if the alternative fate for wood is not being left
in the forest to decompose, but disposal in a landfill.
Conversion of carbon in landfilled wood to land-
fill gas (carbon dioxide and methane) is generally
less than 3% after landfilling [47], thus even tak-
ing methane’s global warming potential into account,
the NEI from burning wood that would otherwise be
landfilled is greater than 40% at year 10.
On a practical level, estimating stack emissions
from a biomass power plant is easy, but estimating
net emissions can be difficult, because the k-constant
for wood can vary [48,49]. One solution is to ‘bracket’
likely net emissions using a range of decomposition
constants to estimate the NEI. Since burning a tonne
of green wood emits about one tonne of CO2,yearly
direct emissions assuming a 4% HPT adjustment are
about 1.04 tonnes per tonne of fuel, and cumulative
direct emissions at year 10 are 10.4 tonnes. Taking
a biomass facility located in the US southeast as an
example, average decomposition constants for south-
eastern hardwoods (0.082) and softwoods (0.057) [46]
translate to values of 67% and 75% on the ten year
NEI curve; multiplying these NEI values by direct
emissions gives cumulative net emissions of 6.97–7.80
tonnes of CO2at year 10 for each tonne of wood
burned at such a facility. This approach to brack-
eting emissions could have policy applications. For
instance, using the NEI to estimate net biogenic emis-
sions could help integrate biomass power plants into
carbon trading and carbon tax programs, as well as
qualify bioenergy for renewable energy subsidies.
Emissions from wood pellets manufactured from
residues
Emissions estimates for pellets calculated by the model
certainly underestimate actual CO2impacts because
tree boles constitute a large proportion of pellet feed-
stock, and it is unlikely that the true alternative fate for
these materials is to be left onsite to decompose. How-
ever, calculating emissions as if claims about use of
residues [40] were fully accurate can establish one type
of ‘best case’scenario for pellet emissions. Modeled
6

Environ. Res. Lett. 13 (2018) 035001
Figure 4. The NEI as a function of the k-constant at different points in time, assuming a 4% HPT adjustment. The NEI at year 10 is
always greater than 40%, even when decomposition is assumed to be rapid.
Figure 5. Cumulative net emissions for wood pellet scenarios 1–4 (details provided in table 2).
scenarios 1 and 2 estimate emissions from producing
and burning one tonne of pellets per year from round-
wood that is assumed to otherwise decompose onsite
(figure 5and table 2), illustrating the importance of
the k-constant for net emissions. Even assuming very
rapid decomposition (k= 0.15) as the counterfactual,
the NEI for Scenario 1 is 55% at year 10. Scenario 2
employs a low k-constant (0.03) representing slower
decomposition in a cool climate [11] such as Canada,
and has an NEI of 79% at year 10; by year 25, cumu-
lative net emissions are 40 tonnes CO2per tonne of
pellet capacity, nearly double those of Scenario 1. Thus
for both scenarios, simply counting cumulative direct
emissions at year 10 would provide a closer representa-
tion of the emissions impact than characterizing pellets
as ‘carbon neutral,’as is current practice.
Scenarios 3 and 4 estimate net emissions from
actual US pellet exports 2010–2016 [4,5,50,51]fol-
lowed by an increase to 12.8 Mt in 2024, commensurate
with US exports meeting half of predicted short-term
global demand for utility and industrial-use wood pel-
lets by 2025 [5,7].Thesescenariosuseak-constant
of 0.083, following a UK government-commissioned
study on lifecycle impacts of wood pellets [11]
that used values from a study of forest wood decay
in North Carolina [52]. Accelerating use in the
first part of scenario 3 elevates the NEI because
decomposition emissions do not come into equilib-
rium with combustion emissions while pellet use keeps
increasing. The NEI is 73% at year 10, 39% at year 40
and 34% at year 50. Scenario 4, where pellet use is ter-
minated at year 20 (2030), has an NEI of 43% at year
7

Environ. Res. Lett. 13 (2018) 035001
25, because cumulative decomposition emissions of the
counterfactual still have not caught up to combustion
emissions from the early years of the scenario. This
scenario shows a carbon benefit in terms of reduced
emissions over time, but it requires actually stopping
use of the fuel for this to occur.
Scenario 5 (shown in table 2but not figure 5)uses
actual data on global pellet use for 2010–2016, then
projects growth to 66.4 Mt in 2025 [53], after which
the model assumes demand is flat. It assumes not
all pellets are manufactured in North America, thus
HPT emissions are reduced by 15% to reflect shorter
transport distances. With a pellet energy content of
17.5 MJ kg−1 [54], peak use of 66.4 Mt yr−1 repre-
sents 1.16 EJ annually, or just over 1% of the 100 EJ
of new bioenergy projected to play a role in some
mitigation models [1–3]. By year 40 (2050), cumu-
lative net emissions are 1.99 Gt and are growing at
30 Mt per year.
The pellet scenarios demonstrate that fossil HPT
emissions increase continuously with pellet use and
represent a substantial ‘non-vanishing’[25] fraction of
net emissions. In reality, the model probably under-
counts HPT emissions because it does not include
releases of nitrous oxide emissions from fertilizer
used on tree plantations [55] or methane emis-
sions from wood chip piles [56] and finished pellets
[36,57]. Buildup of methane and other hazardous
gases during transport and storage [58]isofcon-
cern to the wood pellet industry [59], and has the
potential to add significantly to lifecycle greenhouse
gas emissions [36]. The model also omits combus-
tion emissions of black carbon, a significant climate
forcer [60].
Most importantly, calculating net emissions from
wood pellets as if feedstocks are derived from forest
residues underestimates emissions because a large pro-
portion of pellets are made from trees, not residues [41
61]. For instance, Stephenson and McCay (2014) [11]
found net emissions were 10–12 times higher at year
40 when native hardwood trees are harvested for fuel, a
practice that has been well documented in the US south
[33,41,62,63].
Conclusions
For bioenergy to offer genuine climate mitigation,
it is essential to move beyond the assumption of
instantaneous carbon neutrality. The NEI approach
provides a simple means to estimate net bioenergy
emissions over time, albeit one that tends to under-
estimate actual impacts. The model finds that for
plants burning locally sourced wood residues, from
41% (extremely rapid decomposition) to 95% (very
slow decomposition) of cumulative direct emissions
should be counted as contributing to atmospheric car-
bon loading by year 10. Even by year 50 and beyond,
the model shows that net emissions are a significant
proportion of direct emissions for many fuels. Sim-
ilarly, the model concludes that for wood pellets
manufactured from residues in in the US and shipped
overseas, even a rapid decomposition counterfactual
produces an NEI of 55% at year 10, while a slow decom-
position counterfactual produces an NEI of 79%. By
year 40, net emissions still represent 25% to more than
50% of direct emissions. Scenarios that increase the
amount of biomass burned each year, as is currently
occurring in the EU, have even larger net emissions
impacts.
Models like this have their critics. The IPCC
warns that using a ‘simple sum of the net CO2fluxes
over time’to highlight the ‘skewed time distribution
between sources and sinks,’is probably insufficient
to understand the climate implications of bioenergy,
which instead requires models that include tempera-
ture effects and climate consequences [3]. Bioenergy
advocates have seized on models that emphasize the
importance of cumulative emissions for warming,
pointing out that bioenergy can reduce carbon impacts
over time compared to fossil fuels [23], though they say
little about carbon impacts when bioenergy displaces
zero-emissions technologies.
However, since the IPCC’s call for more com-
plex bioenergy modeling was published in 2014, the
intensity of the climate crisis has deepened; in the
US, legislation has been enacted that compels the EPA
to treat bioenergy as carbon neutral [64]; and com-
bustion of forest wood by the EU, UK and Asia is
increasing each year, unmitigated by the ca rbon capture
and storage that some climate models say is required
[65]. Also while the climate modeling community pon-
ders, governments are making practical decisions about
renewable energy funding, as in the UK, where the
government provided £809 m (about $1.2 b) [66 67]
in subsidies to biomass electricity in 2015, the same
year it announced it was terminating subsidies for off-
shore wind earlier than planned [68]. Since residues
would eventually release carbon to the atmosphere
whether through burning or decomposition, any puta-
tive reduction in CO2emissions actually depends on
residues-fueled bioenergy displacing fossil fuels, but in
the UK, it appears bioenergy may instead be displacing
zero emissions technologies, while prolonging the life
of coal plants that partially switch to subsidized wood
burning.
There is no time like the present to reduce
emissions. Given the anticipated role for bioenergy
in climate mitigation, climate-related policies should
be reformed immediately to account for bioenergy
impacts. Using the NEI to weight biogenic CO2for
inclusion in US and EU carbon trading programs
and to qualify bioenergy for renewable energy sub-
sidies would reduce emissions more effectively than
continuing with the current assumption of zero emis-
sions, though for wood pellets sourced from bolewood,
counting direct emissions is a more protective and
accurate approach.
8

Environ. Res. Lett. 13 (2018) 035001
Acknowledgments
This work was supported by funding from the Packard
Foundation and the Tortuga Foundation.
ORCID iDs
Mary S Booth https://orcid.org/0000-0001-8181-
0053
References
[1] Creutzig F et al 2015 Bioenergy and climate change
mitigation: an assessment GCB Bioenerg. 7916–44
[2] Birol F et al 2016 World Energy Outlook 2015 (Paris:
International Energy Agency) (http://www.iea.org/
publications/freepublications/publication/WEO2015.pdf)
[3] Smith P et al 2014 Agriculture, forestry and other land use
(AFOLU) Climate Change 2014: Mitigation of Climate Change
Contribution of Working Group III to the Fifth Assessment
Report of the Intergovernmental Panel on Climate Change ed O
Edenhofer et al (Cambridge: Cambridge University Press)
[4] Ekstr¨
om H 2011 North American Wood Fiber Review
(Bothell, WA: Wood Resources International LLC)
(https://woodprices.com/wp-content/uploads/2015/
01/NAWFR-SAMPLE.pdf)
[5] Flach B, Lieberz S and Rossetti A 2017 EU Biofuels Annual
2017: GAIN report number LL7015 (Washington, DC: USDA
Foreign Agricultural Service US Department of Agriculture)
[6] Statistics Canada 2017 Canadian International Merchandise
Trade Database: 440131 Wood Pellet Exports 2015–2016
(http://bit.ly/2AKrroQ)
[7] Forisk Consulting LLC 2017 Global Industrial Wood Pellet
Demand Forecast and US Wood Bioenergy Update: Q3 2017
(http://forisk.com/blog/2017/08/08/global-industrial-wood-
pellet-demand-forecast-u-s-wood-bioenergy-update-q3-
2017/)
[8] McKechnie J, Colombo S, Chen J, Mabee W and MacLean H L
2011 Forest bioenergy or forest carbon? Assessing trade-offs in
greenhouse gas mitigation with wood-based fuels Environ. Sci.
Technol. 45 789–95
[9] Bernier P and Par´
e D 2013 Using ecosystem CO2
measurements to estimate thetiming and magnitude of
greenhouse gas mitigation potential of forest bioenergy GCB
Bioenergy 567–72
[10] Walker T, Cardellichio P, Gunn J S, Saah D S and Hagan J M
2013 Carbon accounting for woody biomass from
massachusetts (USA) managed forests: a framework for
determining the temporal impacts of wood biomass energy on
atmospheric greenhouse gas levels J. Sust. Forest 32 130–58
[11] Stephenson A L and MacKay D J C 2014 Life Cycle Impacts of
Biomass Electricity in 2020 (London: UK Department of
Energy and Climate Change) (https://www.gov.uk/
government/uploads/system/uploads/attachment_data/file/
349024/BEAC_Report_290814.pdf)
[12] Lagani`
ere J, Par´
e D, Thiffault E and Bernier P Y 2017 Range
and uncertainties in estimating delays in greenhouse gas
mitigation potential of forest bioenergy sourced from
Canadian forests GCB Bioenerg. 9358–69
[13] Galik C S and Abt R C 2015 Sustainability guidelines and
forest market response: an assessment of European Union
pellet demand in the southeastern United States GCB
Bioenergy 8658–69
[14] Dwivedi P, Khanna M, Bailis R and Ghilardi A 2014 Potential
greenhouse gas benefits of transatlantic wood pellet trade
Environ. Res. Lett. 9024007
[15] Jonker J G G, Junginger M and Faaij A 2014 Carbon payback
period and carbon offset parity point of wood pellet
production in the south-eastern United States GCB Bioenergy
6371–89
[16] HanssenSV,DudenAS,JungingerM,DaleVHandvander
Hilst F 2017 Wood pellets, what else? Greenhouse gas parity
times of European electricity from wood pellets produced in
the south-eastern United States using different softwood
feedstocks GCB Bioenergy 91406–22
[17] Ter-Mikaelian M T, Colombo S J and Chen J 2015 The
burning question: does forest bioenergy reduce carbon
emissions? A review of common misconceptions about forest
carbon accounting J. Forest 113 57–68
[18] Chum H et al 2011 Bioenergy IPCC Special Report on
Renewable Energy Sources and Climate Change Mitigation ed
OEdenhofer et al (Cambridge: Cambridge University Press)
[19] Repo A, Tuomi M and Liski J 2011 Indirect carbon dioxide
emissions from producing bioenergy from forest harvest
residues GCB Bioenergy 3107–15
[20] Domke G M, Becker D R, D’Amato A W, Ek A R and Woodall
C W 2012 Carbon emissions associated with the procurement
and utilization of forest harvest residues for energy, northern
Minnesota, USA Biomass Bioenergy 36 141–50
[21] Zanchi G, Pena N and Bird N 2012 Is woody bioenergy carbon
neutral? a comparative assessment of emissions from
consumption of woody bioenergy and fossil fuel GCB
Bioenergy 4761–72
[22] Lamers P and Junginger M 2013 The ‘debt’is in the detail: a
synthesis of recent temporal forest carbon analyses on woody
biomass for energy Biofuels, Bioprod. Biorefin. 7373–85
[23] Miner R A, Abt R C, Bowyer J L, Buford M A, Malmsheimer R
W, O’Laughlin J, Oneil E E, Sedjo R A and Skog K E 2014
Forest carbon accounting considerations in US bioenergy
policy J. Forest 112 591–606 (https://www.fpl.fs.fed.us/
documnts/pdf2014/fpl_2014_miner001.pdf)
[24] European Commission 2010 Report from the Commission to
the Council and the European Parliament on Sustainabilty
Requirements for the Use of Solid and Gaseous Biomass Sources
in Electricity, Heating, and Cooling COM(2010)11 Final
(Brussels) (http://eur-lex.europa.eu/legal-content/EN/TXT/
PDF/?uri=CELEX:52010DC0011&from=en)
[25] Bruckner T et al 2014 Energy systems Climate Change 2014:
Mitigation of Climate Change Contribution of Working Group
III to the Fifth Assessment Report of the Intergovernmental
Panel on Climate Change ed O Edenhofer et al (Cambridge:
Cambridge University Press)
[26] Zickfeld K, Solomon S and Gilford D M 2017 Centuries of
thermal sea-level rise due to anthropogenic emissions of
short-lived greenhouse gases Proc. Natl Acad. Sci. 114
657–62
[27] Abt Associates 2017 Technical Support Document for eGRID
with Year 2014 Data (Washington, DC: US Environmental
Protection Agency Office of Atmospheric Programs Clean Air
Markets Division) (https://www.epa.gov/sites/production/
files/2015-10/documents/egrid2012_technicalsupport
document.pdf)
[28] US Department of Energy 2001–2016 EIA-923 Monthly
Generation and Fuel Consumption Time Series Files Sources:
EIA-923 and EIA-860 Reports (Washington, DC: The Energy
Information Administration US Department of Energy)
(https://www.eia.gov/electricity/data/eia923/)
[29] Kadiyala A, Kommalapati R and Huque Z 2016 Evaluation of
the life cycle greenhouse gas emissions from different biomass
feedstock electricity generation systems Sustainability 81181
[30] Pouliot G, McCarty J, Soja A and Torian A 2012 Development
of a Crop Residue Burning Emission Inventory for Air Quality
Modeling (Washington, DC: US Environmental Protection
Agency) (https://www3.epa.gov/ttnchie1/conference/
ei20/session1/gpouliot.pdf)
[31] Whitman T, Yanni S and Whalen J 2011 Life cycle assessment
of corn stover production for cellulosic ethanol in Quebec
Can. J. Soil. Sci. 91 997–1012
[32] Liska A J, Yang H, Milner M, Goddard S, Blanco-Canqui H,
Pelton M P, Fang X X, Zhu H and Suyker A E 2014 Biofuels
from crop residue can reduce soil carbon and increase CO2
emissions Nat. Clim. Change 4398–401
[33] Forisk Consulting 2016 Wood Bioenergy US Database, Q4
(Athens, GA: Forisk LLC)
9

Environ. Res. Lett. 13 (2018) 035001
[34] Georgia E P D 2013 Part 70 Operating Permit for Georgia
Biomass Permit Number 2499-299-0053-V-02-0 (Atlanta, GA:
Georgia Environmental Protection Division)
[35] Florida D E P 2013 Final Permit No 0630058-013-AV
Revision to Title V Air Operation Permit No. 0630058-005-AV
for Green Circle Bio Energy Inc. Pensacola, FL (Tallahassee:
Florida Department of Environmental Protection)
[36] R¨
oder M, Whittaker C and Thornley P 2015 How certain are
greenhouse gas reductions from bioenergy? Life cycle
assessment and uncertainty analysis of wood
pellet-to-electricity supply chains from forest residues
Biomass Bioenergy 79 50–63
[37] US EIA 2017 Monthly Generation Data by State, Producer
Sector and Energy Source; Months through December 2016
Sources: EIA-923 Report (Washington, DC: The Energy
Information Administration US Department of Energy)
(https://www.eia.gov/electricity/data/eia923/)
[38] Faggart P F 2012 Dominion Resources Services, Inc.
Comments to the Science Advisory Board Biogenic Carbon
Emissions Panel on Its Draft Advisory Report Regarding EPA’s
Accounting Framework for Biogenic CO2Emissions from
Stationary Sources (Washington, DC: US Environmental
Protection Agency)
[39] New Hampshire D E S 2010 Air Permit Laidlaw Berlin
BioPower, LLC (Concord New Hampshire: New Hampshire
Department of Environmental Services)
[40] Booth M 2016 Carbon Emissions and Climate Change
Disclosure by the Wood Pellet Industry—A Report to the SEC
on Enviva Partners LP (Pelham, MA: Partnership for Policy
Integrity) (http://www.pfpi.net/wp-content/uploads/
2016/03/Report-to-SEC-on-Enviva-March-14-2016.pdf)
[41] Goetzl A 2015 Developme nts in the global trade of wood pellets
(Washington, DC: Office of Industries US International Trade
Commission) (https://www.usitc.gov/publications/
332/wood_pellets_id-039_final.pdf)
[42] Buchholz T, Gunn J S and Saah D S 2017 Greenhouse gas
emissions of local wood pellet heat from northeastern US
forests Energy 141 483–91
[43] OFGEM 2017 Biomass Sustainability Dataset 2015-16
(London: Office of Gas and Electricity Markets)
(https://www.ofgem.gov.uk/environmental-
programmes/ro/applicants/biomass-sustainability)
[44] Oswalt S N, Smith W B, Miles P D and Pugh S A 2014 Forest
resources of the United States, 2012: a technical document
supporting the forest service update of the 2010 RPA
assessment general technical report WO-91: table 42 Weight of
Bark and Wood Residue from Primary Wood-using Mills by
Type of Material, Species Group, Region, Subregion, and Type
of Use, 2011 (Washington, DC: US Department of Agriculture,
Forest Service) (https://www.srs.fs.usda.gov/pubs/47322)
[45] US EIA 1989–1998 Nonutility Power Producer Data
(Washington, DC: The Energy In formation Administration
US Department of Energy) (https://www.eia.gov/
electricity/data/eia923/)
[46] US EPA 2014 Framework for Assessing Biogenic CO2Emissions
from Stationary Sources, appendix L: Illustrative Forestry and
Agriculture Case Studies Using a Future Anticipated Baseline,
table L-15, FASOM-GHG Annual Coarse Woody Debris
Decomposition Rates (Washington, DC: US Environmental
Protection Agency, Office of Air and Radiation, Office of
Atmospheric Programs, Climate Change Division)
[47] Micales J A and Skog K E 1997 The decomposition of forest
products in landfills Int. Biodeterior. Biodegradation. 39
145–58
[48] Harmon M E et al 1986 Ecology of coarse woody debris in
temperate ecosystems advances Ecol. Res. 15 133–302
[49] Zeng H, Chamb ers J Q, Negr ´
on-Ju´
arezRI,HurttGC,Baker
D B and Powell M D 2009 Impacts of tropical cyclones on US
forest tree mortality and carbon flux from 1851–2000 Proc.
Natl Acad. Sci. 106 7888–92 (http://www.pnas.org/
content/106/19/7888)
[50] Copeley A 2017 Forisk Blog: Wood Bioenergy Update and
Wood Pellet Exports: Q1 2017 (http://forisk.com/blog/
2017/02/17/wood-bioenergy-update-wood-pellet-
exports-q1-2017/)
[51] Copeley A 2017 Forisk Blog: Wood Bioenergy Update and
Wood Pellet Exports: Q2 2017 (http://forisk.com/blog/
2017/05/25/wood-bioenergy-update-wood-pellet-
exports-q2-2017/)
[52] Mattson K G 1987 Decomposition of woody debris in a
regenerating, clearcut forest in the Southern Appalachians
Can. J. Forest Res. 17 712–21
[53] Strauss W 2017 Overview of global pellet markets and
micro-scale pellet-fueled combined heat and power: a new
distributed power solution for the smart grid of the future The
New Forest Economy—Biobased Power, Products and Fuels:
E2Tech Forum 24 March 2017 (Portland Maine: E2Tech)
(http://growsmartmaine.org/wp-content/uploads/2017/02/
Strauss_FutureMetrics_ForestEconomy_3-24-17.pdf)
[54] Enviva Biomass 2015 Wood Pellet Manufacturing in the
Southeast United States (Bethesda, MD: Enviva Biomass)
[55] Castro M S, Peterjohn W T, Melillo J M, Steudler P A, Gholz H
L and Lewis D 1994 Effects of nitrogen fertilization on the
fluxes of N2O, CH4,andCO
2from soils in a Florida slash pine
plantation Can. J. Forest Res. 24 9–13
[56] BTG Biomass Technology Group BV 2002 Methane and
Nitrous Oxide Emissions from Biomass Waste
Stockpiles—PCFplus Report 12 (Washington, DC: PCFplus)
(https://wbcarbonfinance.org/docs/CH4_emissions_from_
woodwaste_stockpiles.pdf)
[57] Kuang X, Tumuluru J S, Bi X, Sokhansanj S, Lim J and Melin S
2008 Characterization and kinetics study of off-gas emissions
from stored wood pellets Ann. Occup. Hyg. 52 675–83
(https://www.ncbi.nlm.nih.gov/pubmed/18714087)
[58] Svedberg U, Samuelsson J and Melin S 2008 Hazardous
off-gassing of carbon monoxide and oxygen depletion during
ocean transportation of wood pellets Ann. Occup. Hyg. 52
259–66 (https://www.ncbi.nlm.nih.gov/pmc/articles/
PMC2413103/)
[59] Melin S 2008 Safety in handling wood pellets Summary of the
Proceedings of BioEnergy Conference and Exhbition 2008
(Prince George BC: Canadian Bioeconomy Conference and
Exhibition) (http://bioeconomyconference.com/wp-content/
uploads/2015/10/2008_Proceedings.pdf)
[60] Jacobson M Z 2001 Strong radiative heating due to the mixing
state of black carbon in atmospheric aerosols Nature 409 695–7
[61] Kittler B, Olesen A S, Price W, Aguilar A and Bager S L 2015
Report to the European commission Environmental
Implications of Increased Reliance of the EU on Biomass from
the South East US (Denmark: COWI)
[62] Abt K L, Abt R C, Galik C S and Skog K E 2014 Effect of
Policies on Pellet Production and Forests in the US South: A
Technical Document Supporting the Forest Service Update of
the 2010 RPA Assessment General Technical Report SRS-202
(Asheville, NC: United States Forest Service Southern
Research Station) (https://www.srs.fs.usda.gov/pubs/47281)
[63] Dogwood Alliance, Southern Environmental Law Center,
Natural Resources Defense Council 2017 European Imports of
Wood Pellets for ‘Green Energy’Devastating US Forests
(www.southernenvironment.org/uploads/words_docs/
Wood_Pellets_Report.pdf?cachebuster:66)
[64] 115th Congress of the United States 2017 H.R. 244
Consolidated Appropriations Act, 2017 (www.congress.
gov/115/plaws/publ31/PLAW-115publ31.pdf)
[65] Fuss S et al 2014 Betting on negative emissions Nat. Clim.
Change 4850–3
[66] EPowerAuctions 2017 e-ROC track record 2017: ROC auction
prices for 2015 (epowerauctions.co.uk/erocrecord.htm)
[67] Renewable Energy Foundation 2017 Grouped totals for
Renewable Generation: 2002–2017 (ref.org.uk/)
[68] Wintour P and Vaughan A 2016 Tories to end onshore
windfarm subsidies in 2016 The Guardian 18 June 2015
10