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NRDC: Think Wood Pellets are Green? Think Again. (PDF)
NRDC issue brief
may 2015 
ib:15-05-a
Think Wood Pellets are Green? Think Again.  
biomass is often described as a clean, renewable fuel and a greener alternative 
to coal and other fossil fuels for producing electricity. but recent science shows 
that many forms of biomass—especially from forests—produce higher carbon 
emissions compared to fossil fuels. in particular, a growing body of peer-reviewed, 
scientific studies shows that burning wood from whole trees in power plants to 
produce electricity can increase carbon emissions relative to fossil fuels for many 
decades—anywhere from 35 to 100 years.1 This time period is significant: climate 
policy imperatives require dramatic short-term reductions in greenhouse gases, and 
these emissions will persist in the atmosphere well past the time when significant 
reductions are needed.
Unfortunately, the biomass wood pellet industry in the 
and branches from logging operations). Using the model, 
southeastern United States is expanding rapidly. Wood pellet 
we estimated the total amount of carbon released over time 
exports from the United States doubled from 1.6 million tons 
(cumulative CO2 emissions) for each scenario and compared 
in 2012 to 3.2 million tons in 2013, and they are expected to 
those emissions with those from coal and natural gas.
reach 5.7 million tons in 2015.2 This growth is driven largely 
by exports to Europe in response to flawed policy incentives 
on renewable resources that regard all biomass as carbon 
How we moDeleD Pellet CaRboN 
neutral.3 
emiSSioNS
Although recent science shows that many forms of forest 
biomass are high-carbon sources of fuel, under the right 
The SIG model first generates a working forest landscape—
circumstances, true wood waste could serve as a low-carbon 
including timber harvest and forest regrowth—based on 
option for producing pellets. For example, sawdust and chips 
typical forest management in the sourcing ecoregion. 
from sawmills that would otherwise quickly decompose and 
The model then estimates the emissions from removing 
release carbon anyway—can be a low-carbon source.
additional forest materials each year for wood pellet 
4 On the 
other hand, burning whole trees can produce higher carbon 
production and by burning the pellets to produce electricity. 
emissions than coal, and this elevated CO
(See the Technical Appendix for information on the SIG 
2 level with respect 
to coal can persist in the atmosphere for decades.
model and analytic methods.)
5 Therefore, 
the composition of wood pellets matters greatly: the amount 
  Our modeling assumed that the biomass feedstocks used 
of whole trees used in wood pellets can have a significant 
to produce pellets were typical of the pellet industry.8 They 
impact on the estimated carbon emissions of this fuel source.
are: 
NRDC therefore modeled the carbon impacts of burning 
n   forestry residues—tops and limbs from forestry 
wood pellets of varying composition in power plants to 
operations that are non-merchantable to other markets; 
produce electricity. We used a carbon accounting model 
n   whole trees—merchantable pulpwood, trees from 
developed by the Spatial Informatics Group (SIG) to model 
thinning operations, and non-merchantable trees; and
scenarios in which pellets sourced from bottomland 
hardwood forests in Atlantic plain6 of North Carolina and 
n   mill waste—by-products of sawmill operations such as 
South Carolina supplied a typical power plant in the United 
sawdust and chips.
Kingdom7. In our analysis, we modeled a range of scenarios 
(See the Technical Appendix for more information on 
in which pellets are made of varying amounts of whole trees, 
feedstock definitions.)
mill waste, and non-merchantable forestry residues (tops 
 
for more information, 
www.nrdc.org
please contact:
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Sami yassa 
www.twitter.com/NRDC
Senior Scientist
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We varied the amounts of each of these three feedstocks, 
Figure 1: Cumulative emmissions (mgCo
running scenarios in which the proportion of whole trees in 
2e/mw)
the wood pellets ranged from 20 percent to 70 percent.9 In 
Pellets made of 70 percent whole trees
1,000,000
each of these scenarios, we modeled the emissions resulting 
from burning the pellets in a typical power plant over a 
800,000
100-year period. We compared these estimated biomass CO2 
emissions with the CO2 that would have been emitted by 
600,000
fossil fuels to produce the same amount of electricity.
400,000
200,000
ReSultS
0
In the figures below we show the results for three 
representative scenarios: pellets made of 70 percent, 40 
-200,000
Power (coal) old plants
percent, and 20 percent whole trees. The solid black line 
Power (coal) new plants
-400,000
Power (natural gas) new plants
represents the estimated cumulative carbon emissions per 
Biomass power
megawatt of power from a power plant burning wood pellets, 
-600,000
accounting for the effects of forest regrowth. The dashed 
lines represent cumulative emissions from fossil fuels per 
2015
2025
2035
2045
2055
2065
2075
2085
2095
2105
2115
megawatt of power. 
Figure 2: Cumulative emmissions (mgCo
For the first several decades of the plant’s operations, 
2e/mw)
the burning of pellets creates a pulse of emissions to the 
Pellets made of 40 percent whole trees
1,000,000
atmosphere—namely, increased carbon emissions resulting 
from combustion. This pulse occurs because wood is less 
800,000
energy-dense than fossil fuels, so burning biomass generally 
emits more carbon than fossil fuels to produce the same 
600,000
amount of energy.10 Over time, however, forest regrowth 
400,000
reduces this atmospheric carbon.11 And after many decades, 
this regrowth can recapture enough carbon to reduce the 
200,000
cumulative emissions below those of fossil fuels. These 
results are similar to the “carbon debt” outcomes found in 
0
recent biomass studies.12 
-200,000
Power (coal) old plants
Figures 1 and 2 together show the modeled emissions 
Power (coal) new plants
when the proportion of whole trees in pellets ranges from 
-400,000
Power (natural gas) new plants
Biomass power
40 percent to 70 percent. The modeling shows that it will 
-600,000
take approximately 55 years for forest regrowth to recapture 
enough carbon from the atmosphere to reduce the plant’s 
2015
2025
2035
2045
2055
2065
2075
2085
2095
2105
2115
cumulative emissions below those of coal. At levels greater 
than 40 percent, pellets emit more carbon than coal for most 
Figure 3: Cumulative emmissions (mgCo2e/mw)
of this period. In addition, as the percentage of whole trees 
Pellets made of 20 percent whole trees
increases above 70 percent (not shown in the figures), the 
1,000,000
level of carbon emissions continues to increase.13 
800,000
When whole trees make up 20 percent of the wood in 
pellets, emissions are slightly higher than natural gas and 
600,000
slightly lower than coal for a period of approximately 55 
years, as shown in Figure 3. Even when whole trees make 
400,000
up as little as 12 percent of pellets, our modeling showed 
200,000
that burning pellets still produces emissions comparable 
to natural gas trend line for approximately 50 years. (See 
0
Technical Appendix for information on additional scenarios.)
-200,000
Power (coal) old plants
Power (coal) new plants
-400,000
Power (natural gas) new plants
Biomass power
-600,000
2015
2025
2035
2045
2055
2065
2075
2085
2095
2105
2115
PaGe 2 | bioenergy model


CoNCluSioNS
In sum, our modeling shows that wood pellets made of whole 
trees from bottomland hardwoods in the Atlantic plain of 
the U.S. Southeast—even in relatively small proportions—
will emit carbon pollution comparable to or in excess of 
fossil fuels for approximately five decades. This 5-decade 
time period is significant: climate policy imperatives 
require dramatic short-term reductions in greenhouse gas 
emissions, and emissions from these pellets will persist in the 
atmosphere well past the time when significant reductions 
are needed. Moreover, several studies have concluded that 
logging residuals alone may be unable to meet bioenergy 
demands in the region we modeled, and that pulpwood trees 
may need to be used to meet the increasing demand.14
These results have significant implications for the wood 
pellet industry and its expansion. Pellet manufacturer Enviva 
LP and British utility Drax Power are at the head of this 
industry. Drax operates the United Kingdom’s largest coal-
fired power plant and is converting half of its six generating 
units to run solely on wood pellets.15 Enviva is the largest 
producer and exporter of wood pellets in the United States 
enviva facility in ahoskie, NC.
and a primary biomass supplier to Drax. The company owns 
and operates five production plants in the Southeastern U.S. 
that have a combined wood pellet production capacity of 
as the calls to curb carbon pollution grow louder, power 
approximately 1.7 million metric tons per year.
companies face increased pressure to find cleaner sources 
16
Enviva has claimed that its wood pellets are a clean source 
of energy. Many have turned to woody biomass for fuel, 
of fuel for electricity production and that they do not increase 
much of which comes from forests. The wood is chipped 
carbon emissions.
or turned into pellets—small, compressed cylinders of 
17 Yet the company has not publicly 
disclosed the composition of its wood pellets. If Enviva’s 
woody material. These pellets are burned in power plants 
pellets were comprised of whole trees from the modeled 
just like coal. Most suppliers are operating under the 
Southeast bottomland hardwoods—even in relatively small 
false assumption that, since trees can grow back and 
proportions—they would emit carbon pollution comparable 
resequester carbon, then they are a carbon-neutral fuel 
to fossil fuels for decades. The company has the responsibility 
when burned. However, mounting scientific evidence 
to come clean with the public, investors, and regulators by 
shows that it could take many decades for forest regrowth 
disclosing the makeup of its fuel, and to take steps to ensure 
to offset stack emissions from power plants. 
that its pellets do not increase carbon emissions.
 
 
PaGe 3 | bioenergy model

teCHNiCal aPPeNDiX: How tHe SPatial 
landscape over time (except for 19 percent of the acreage, 
iNFoRmatiCS GRouP’S (SiG) CalCulatoR 
which was put off-limits due to operational constraints and 
woRKS
protected status). Within the woodsheds, they prescribed 
more intensive silviculture to produce additional biomass 
This analysis is based on a Greenhouse Gas Calculator 
material to meet the existing facility’s demand—typically 
developed for NRDC by the Spatial Informatics Group 
an extra thinning cycle and removal of boles smaller than 
(SIG). The Spatial Informatics Group’s (SIG) Greenhouse 
4 inches dbh (hereinafter “biomass harvest”).20 This mix of 
Gas Calculator estimates the carbon emissions from woody 
default silviculture and biomass harvest across the 17-facility 
biomass energy sourced from forests in the southeastern 
landscape constitutes the business-as-usual scenario.
United States. The user first specifies a power-generating 
Colnes et al. then modeled additional timber harvesting 
facility type, plant efficiency, mix of feedstock types, sourcing 
above the business-as-usual associated with new demand in 
ecoregion, and forest type to generate a biomass power 
additional 50-mile-radius woodsheds, which had the more 
scenario. For this analysis, NRDC set parameters consistent 
intensive “biomass harvest” prescriptions described above. 
with wood pellets sourced from a mix of mill waste, forestry 
The modeling in Colnes et al. meets this increased demand 
residues, and whole tree boles from bottomland hardwoods 
first with residues and non-merchantable boles, then with 
in the Atlantic Coastal Flatwoods ecoregion. 
pulp sized boles and tree tops. The difference between 
The calculator models the greenhouse gas emissions 
emissions from new demand and emissions from the 
and sequestration over time in the chosen region under 
business-as-usual scenario produces net emissions factors on 
the specified scenario. The calculator then compares these 
a per-output-energy (per-MWh) basis. 
against a business-as-usual scenario—namely, the emissions 
The SIG Calculator builds on Colnes et al.’s results in two 
and sequestration that would occur in the absence of the 
main ways. First, it allows the user to simulate the sourcing 
specified biomass sourcing and combustion. The difference 
location and feedstock type by disaggregating emission 
between the two trajectories represents the net cumulative 
factors associated with a single ecoregion and forest type 
greenhouse gas emissions on a carbon equivalent per-
inside the Colnes et al. study area. Second, it scales the 
output-energy (per-MWh) basis attributable to the user-
net, per-MWh emission factors by the size of the power-
specified facility’s sourcing and power generation operations.
generating facility specified in a biomass power scenario. For 
The calculator relies entirely on data and results from 
the purposes of this analysis, NRDC assumed that the new 
the study Biomass Supply and Carbon Accounting for 
demand was generated in the Atlantic Coastal Flatwoods 
Southeastern Forests (Colnes et al.) to generate both the 
(Forest Service Ecoregion 232) of North Carolina.
biomass power scenarios and business-as-usual scenarios.18 
Besides the analysis of forest growth, the SIG Calculator 
These researchers assumed: (1) typical and customary 
folds in greenhouse gas emissions from feedstock 
silvicultural systems implemented in each ecoregion and 
processing, transportation, and energy conversion at the 
forest type, and (2) typical markets and end uses for the 
power plant. It does not include greenhouse gas emissions 
varying size classes of wood products harvested (pulpwood 
from plant construction or plantation management. SIG 
4–10 inches in diameter at breast height [dbh] and sawlogs 
added parameters regarding the additional emissions of 
greater than 10 inches dbh). Materials less than 4 inches in 
transporting the biomass fuel from the U.S. Southeast to 
diameter were considered non-merchantable and left to 
the United Kingdom, along with analogous mining and 
decay in forests in the business-as-usual scenario.19 
transportation emissions for the displaced coal.
Silvicultural scenarios were modeled by Colnes et al. 
The SIG Calculator estimated emissions from harvesting 
using 2010 U.S. Forest Service Forest Inventory and Analysis 
equipment using a factor of 0.015 ton of CO2 per bone dry ton 
(FIA) data for each ecoregion. The U.S. Forest Service Forest 
(BDT) of harvested material. Truck transportation emissions 
Vegetation Simulator Southern Variant was used to account 
were estimated using a factor of 0.000134 ton of CO2 per BDT-
for changes in on-site forest carbon pools. The forest carbon 
mile, which assumed 12.5 tons per truck, 6 miles per gallon, 
accounting included above- and belowground live trees, 
and 22.2 lbs. CO2 per gallon of diesel fuel (see sources cited in 
standing deadwood, belowground deadwood, and down 
Colnes et al., pg. 84). 
deadwood. Carbon stored in wood products in use and in 
The SIG Calculator assumes that mill residues are carbon 
landfill pools was simulated separately in the SIG Calculator.
neutral—meaning neither biogenic nor fossil fuel (e.g., 
To establish the business-as-usual scenario, Colnes et 
transport and processing-related) carbon emissions are 
al. modeled a landscape including woodsheds around 17 
associated with their use. 
existing biomass facilities in the region as of 2010. Each 
The SIG Calculator further assumes that logging residues 
woodshed was specified by a 50 mile-radius around each 
constitute 32 percent of harvest volume for bottomland 
known existing biomass facility. The researchers first 
hardwoods in the Atlantic Coastal Flatwoods. In order to 
prescribed default silvicultural practices across the entire 
generate representative ratios of logging residues versus 
PaGe 4 | bioenergy model

roundwood for fuelsheds in the Atlantic Coastal Flatwoods 
Products Output Database.21 In 2009, the most recent year for 
(Forest Service Ecoregion 232) of North Carolina, we retrieved  which reports were available, logging residues constituted an 
reported volumes of logging residue and roundwood by 
average of 32 percent of the harvest volume in hardwoods by 
weight for six counties in the ecoregion from the Timber 
weight.
endnotes
1  Colnes, a., et al., Biomass Supply and Carbon Accounting for Southeastern Forests, The biomass energy resource Center, forest Guild, and spatial 
informatics Group, february 2012, www.biomasscenter.org/images/stories/se_Carbon_study_fiNaL_2-6-12.pdf. Harmon, M., Impacts of Thinning on 
Carbon Stores in the PNW: A Plot Level Analysis
, Oregon state university, May 2011. Mitchell, s., M. Harmon, and K. O’Connell, “Carbon Debt and 
Carbon sequestration Parity in forest bioenergy Production,” GCB Bioenergy 4, no. 6 (November 2012): 818-827. repo, a., et al., “sustainability of 
forest bioenergy in europe: Land-use-related Carbon Dioxide emissions of forest Harvest residues,” GCB Bioenergy, published online March 2014. 
stephenson, a. L., and D. MacKay, Life Cycle Impacts of Biomass Electricity in 2020: Scenarios for Assessing the Greenhouse Gas Impacts and Energy 
Input Requirements of Using North American Woody Biomass for Electricity Generation in the UK
, u.K. Department of energy and Climate Change,  
July 2014, www.gov.uk/government/uploads/system/uploads/attachment_data/file/349024/beaC_report_290814.pdf. Ter-Mikaelian, M., et al.,  
“Carbon Debt repayment or Carbon sequestration Parity? Lessons from a forest bioenergy Case study in Ontario, Canada,” GCB Bioenergy,  
published online May 2014. Walker, T., et al., Biomass Sustainability and Carbon Policy Study, Manomet Center for Conservation sciences, June 2010, 
www.mass.gov/eea/docs/doer/renewables/biomass/manomet-biomass-report-full-hirez.pdf.
2  Wood resources international LLC, “Global Timber and Wood Products Market update,” news brief, October 11, 2012.
3  2009 renewable energy Directive, which sets an overall eu target of a 20 percent share of renewable energy by 2020. ec.europa.eu/clima/policies/
package/index_en.htm. 

4  stephenson, a. L., and D. MacKay, Life Cycle Impacts of Biomass Electricity.
5  Pingoud, K., T. ekholm, and i. savolainen, “Global Warming Potential factors and Warming Payback Time as Climate indicators of forest biomass 
use,” Mitigation and Adaptation Strategies for Global Change 17, no. 4 (January 2012): 369-386. schulze, e. D., et al., “Large-scale bioenergy from 
additional Harvest of forest biomass is Neither sustainable Nor Greenhouse Gas Neutral,” GCB Bioenergy 4, no. 6 (November 2012): 611-616. Walker, 
T., et al., Biomass Sustainability.
6  The sourcing region is the atlantic Coastal flatwoods (forest service ecoregion 232)
7  We assumed a capacity factor of 85 percent.
8  suz-anne Kinney, “Dispelling the Whole Tree Myth: How a Harvested Tree is used,” forest2Market (f2M), December 20, 2013,  
www.forest2market.com/blog/dispelling-the-whole-tree-myth-how-a-harvested-tree-is-used. The author states: “While whole trees are certainly being 
harvested, any whole tree that ends up in the wood yard of a pellet facility is either defective in some way (unmerchantable), a pulpwood-sized tree that 
took 10–15 years to grow or the upper section of a larger tree…. it is impossible to tell which of the latter two categories any single pulpwood-sized log 
falls into.”
9  for each scenario, we held the amount of mill waste constant at 20 percent.
10 Walker, T., et al., Biomass Sustainability.
11 in the model, this sequestration also includes avoided decay (decay that would have otherwise occurred).
12 Ter-Mikaelian, et al., “Carbon Debt repayment.” Colnes, a., et al., biomass supply and Carbon accounting . Walker, T., et al., biomass sustainability.
13 This is the case all the way to 100 percent whole trees— not illustrated in figures.
14 http://naldc.nal.usda.gov/download/46157/PDf, https://nicholasinstitute.duke.edu/sites/default/files/publications/forest-biomass-supply-in-the-
southeastern-united-states-implications-for-industrial-roundwood-and-bioenergy-production-paper.pdf.
15 Livermore, M., “The Contradictions of biomass,” scientific alliance, March 10, 2014, www.cambridgenetwork.co.uk/news/the-contradictions-of-
biomass.

16 http://247wallst.com/energy-business/2015/04/29/enviva-ipo-generating-heat-and-light.
17 enviva LP, Inherent Sustainability & Carbon Benefits of the US Wood Pellet Industry, white paper, 2012, www.envivabiomass.com/wp-content/
uploads/inherent-sustainability-carbon-benefits-20121005.pdf.

18  Colnes, a., et al., Biomass Supply and Carbon Accounting for Southeastern Forests, southern environmental Law Center, 2012,  
www.biomasscenter.org/images/stories/se_Carbon_study_fiNaL_2-6-12.pdf.
19 Materials less than 4 inches in diameter were collected in certain biomass power scenarios only. 
20 The “biomass harvest” generates additional timber harvest volume in the forms of (1) non-merchantable tops and limbs, (2) non-merchantable boles 
less than 4 inches dbh, and (3) pulpwood boles 4–10 inches dbh.
21 u.s. Department of agriculture forest service, southern research station, Timber Product Output (TPO) reports, Table C10: “Volume of timber 
removals by state/County, species group, removals class and source,” srsfia2.fs.fed.us/php/tpo_2009/tpo_rpa_int1.php, accessed November 24, 2014. 
The model partitions logging residues from new harvest to logging residues from existing harvest at a ratio of approximately 4:1.
PaGe 5 | bioenergy model