A Carbon Life Cycle Analysis of Wood Pellets
Summary
A joint research project by The Alliance for Green Heat and VU University
Amsterdam shows that heating with wood pellets can emit about one tenth the
carbon as heating with oil and one sixth the carbon if heating with natural gas.
A life cycle analysis of wood pellets showed that pellets emit as little as 6.04
grams of carbon per mega joule of energy (6.04 g CO2/MJ, or 14.4 lbs/million
BTU). Heating oil and natural gas emit 100.1 grams of carbon per measure
(232.8 lbs/million BTU) for oil and 62.8 g CO2/MJ (146.1 lbs/million BTU) for
gas. As a result, switching from fossil fuel heating to a wood pellet stove or
furnace could lead to a 60 to 90% reduction of carbon emissions.
The most compelling conclusions are that wood pellets can be a very low carbon
source of heat, if certain conditions are satisfied. These conditions concern
rigorous sustainable harvesting, distance of shipping, and mode of production.
Transatlantic journeys will increase the carbon footprint of pellets substantially
and to maintain a balance between carbon absorbed by forests and emitted by
pellets appliances, rigorous sustainable harvesting practices should be practiced.
Introduction
Until the 1940s, more than 20% of the American population used wood as their
predominant source of domestic heat. During the 1950s, 60s and 70s, when fossil
fuels became more subsidized, cheaper and more available for the consumer,
wood heat was dropped by
millions of households to a
low of about 2% of the
American public, where it
remains today. In the face
of a warming planet there is
renewed interest in all
renewable forms of energy
and biomass for heat is
proven to be the highest
efficiency use of limited and
complex resource.
The
underlying equation for
biomass lies in the balance
between carbon dioxide
absorbed
from
the
atmosphere by trees and
the carbon emitted by wood
burning appliances.
The emergence of the pellet industry in
the
1980s
and
1990s
offered
dramatically lower emissions and a level
of automation that had not been
achieved with wood. For this reason
some countries, particularly in Europe,
latched onto pellet appliances as the way
to bring a high efficiency renewable
energy source to scale.
Since the late 1990s, the usage of wood
pellets
has
become
increasingly
common. Pellets are chunks of densified
sawdust, usually derived from sawmills.
Pellets are used, rather than chips or
logs, as they have a homogeneous
composition and low moisture content.
This results in a very continuous
combustion and low emissions of byproducts. Pellet fueled appliances are
often regarded as carbon neutral along
with other uses of biomass, but all
sources of energy, including solar and
wind, have carbon footprints.
This Life Cycle Assessment addresses the
carbon neutrality of the use of wood
pellets by examining aspects of tree
growth,
transportation
and
manufacturing and combustion of
pellets. This provides an overview of
where the biggest inefficiencies and
losses in the cycle are.
Carbon Sequestration by
Trees
Trees have the ability to capture carbon
dioxide (CO2) from the atmosphere
through photosynthesis, which occurs in
the leaves,. This is allocated over
different parts of the tree and gets stored
in both aboveground and underground
parts of the ecosystem. Generally, the
aboveground biomass is harvestable and
therefore usable for heating purposes.
When considering harvesting, one must
be sure that this happens in a
sustainable way. In other words: if a
certain amount of wood is taken away
from the forest, the forest must be left in such a shape that it can re-grow to, or
even outgrow the original volume. It is beyond the scope of this research to
examine sustainable harvesting in-depth, but the acknowledgement is made that
this is an important part of the life cycle of wood pellets.
From harvesting… to retailing
In this study, data is analyzed from a research paper by Italian scholars1. The
scholars surveyed all energy and Carbon contributions in every part of the life
cycle of wood pellets. The harvesting and the production of wood residues, that
eventually become pellets, consumes 0.53 GJ/ton pellets of energy. Then, the
transportation of these pellets to a pellet manufacturing plant takes 0.07 GJ/ton,
but the bulk of the energy in being consumed there where the pellets are being
produced.
In order to produce pellets, the wood residues have to be dried. To do that, two
common methods are usually used; drying through the use of natural gas as a
heating fuel, or by using sawdust/woodchips or stick wood as a heating fuel. The
amount of energy used when the sawdust is dried with biomass rather than
natural gas turns out to be higher: 3.78 GJ/ton vs. 2.97 GJ/ton pellets for natural
gas. Nonetheless, the Carbon emission of drying through biomass will be lower,
as biomass is a low carbon, assuming it is being sustainably harvested
For pellets exported to Europe, they are transported by train to a harbor. In
British Columbia, where this is done in great volume, the trip from the pellet mill
to the harbor in Vancouver, contributes to the consumption of another 0.26
GJ/ton pellets. From Vancouver, the pellets are shipped to northern Europe. It
makes sense to examine this route, as 60% of the wood pellets produced in
Canada are shipped to harbors in Europe that are at about the same distance as
Stockholm, Sweden. This trip by vessel adds another 2.6 GJ/ton pellets to the
equation.
All these losses of energy correspond to different carbon emissions. In this study,
four different pathways have been identified: the two different means of drying
the sawdust have been distinguished and both domestic and international
transport have been considered. As sawdust is considered carbon neutral for
these calculations, the least carbon-emitting pathway appears to be the one of
pellets consumed locally and dried with sawdust. The carbon emission of these
pellets is as low as 6.01 grams per Mega Joule.
After the examination of all the logistic processes, emissions have been examined
for two different pellet appliances: a 7 kW pellet stove and a 20 kW pellet boiler.
These appliances can be very clean and have hardly any hazardous byproducts.
1 Magelli, F., Boucher, K., Bib, H.T., Melin, S., Bonoli, A. (2009) An environmental impact assessment of exported wood
pellets from Canada to Europe, Biomass and Bioenergy, 33, pp. 434–441
Considerations for other forms of biomass
This study focused on pellets, which are most often used in very high efficiency
heating appliances. However wood chips are also transported domestically and
internationally for energy. Strong parallels apply for wood chips and logs. Also
for these types of wood based fuels, sustainable harvesting guidelines have to be
obeyed in order to reach low emissions and emissions during transportation and
processing will occur. The emissions of byproducts during combustion will be
somewhat higher for chip and log appliances than for wood pellets, due to higher
moisture content and less homogenous composition. Also, the emissions during
transportation will be higher, as less mass will fit on a vessel or truck, due to the
lower density of these products.
Key questions for this study included,

What is the net CO2 emission of pellet and wood appliances?

What is the carbon absorption capacity of the trees used?

What are the emissions of the pellets or wood used during combustion?

Where are the biggest inefficiencies in the cycle?
o
Where in the cycle does the bulk of Carbon emissions occur?

What is driving these Carbon outputs? and

How can these inefficiencies be improved?

What is the influence of transportation of pellets or wood on the cumulative Carbon emissions
across the cycle?
o
In Domestic transportation?
o
In International transportation?
Conclusions
Wood pellets can be a very low-carbon heating fuel, if three conditions are
satisfied. First, sawdust should be used in pellet manufacturing plants to dry the
wet biomass. This makes sense in two ways: the carbon emissions are
considerably lower than when natural gas is used and the sawdust needed is
transported there regardless, whereas natural gas will have to be brought there
separately.
Second, a great deal of the carbon benefits is lost in the trip from Western
Canada to Central Europe. Therefore, it makes sense not to transport wood
pellets over long distances but to use it domestically.
The third, and probably the most important condition is that this low carbon
emission should be seen in perspective of a delicate balance. It can only be
achieved if, and only if the forests where the used wood has been taken from, are
managed sustainable. This means that no more wood is taken out of the forest
than will possibly grow back, and that selected parts of the forest are taken, so
that harvesting will not damage the ecosystem as a whole.
For a copy of the full research paper, please contact us at info@forgreenheat.org.
Sources
University of Wisconsin – Green Bay
www.pelletheat.org/3/2007SummerConf/Final%20PFI%20study.pdf
North America’s Wood Pellet Sector
www.fpl.fs.fed.us/documnts/fplrp/fpl_rp656.pdf
An environmental impact assessment of exported wood pellets from Canada to
Europe
http://cat.inist.fr/?aModele=afficheN&cpsidt=21295876
Carbon sequestration potential of marginal agricultural land in the Midwestern
US
http://cat.inist.fr/?aModele=afficheN&cpsidt=17543167
Carbon sequestration in the trees, products and soils of forest plantations – an
analysis using UK examples
http://www.ncbi.nlm.nih.gov/pubmed/14969967
Forests have a great potential to absorb carbon dioxide, one of the biggest causes
of the Greenhouse Effect. In the leafs, where photosynthesis takes place, carbon
dioxide is absorbed, from where it is being transported and allocated over
different parts of the tree; the aboveground biomass, the soils, the roots and the
forest floor litter. A typical forest sequesters between 0.7 and 5.0 ton Carbon per
hectare per year, depending on tree species, climate and fertility of the soil.
Wood contains about 50% Carbon, so this uptake yields to a wood productivity
of about 1.4 – 10 ton per hectare per year. Not all this carbon, however, is
converted into usable, sustainable wood, so the productivity of a forest is
somewhat lower. To ensure sustainable management, certain guidelines have to
be obeyed. It is beyond the scope of this study to set these guidelines, but they
form a vital part of the carbon balance of pellets. After all, the wood being
consumed for pellets must be grown back somehow.