tackling climate change
Biochar
Can it get us back down below 350 ppm atmospheric CO2?
by CHUCK KUTSCHER
chuck Kutscher is a
principal engineer
and manager of the
thermal Systems group
at the national Renew-
able energy Laboratory.
he is a past aSeS chair
and was chair of the
SoLaR 2006 confer-
ence, which resulted in
the aSeS report, “tack-
ling climate change in
the u.S.” (Free down-
load at ases.org/
climatechange.)
The opinions expressed
here are solely those of
the author.
For an explanation of
the biochar process,
see solartoday.org/
biochar.
Alandmark 2008 paper by NASA’s James Hansen et al., the latest measurements of polar ice and new studies of the negative effect of CO2 on coral lead
to the conclusion that atmospheric carbon dioxide must be
stabilized below 350 ppm. As the level is now at about 390
ppm and rising rapidly, more attention is being paid not
only to reducing carbon emissions but also to removing car-
bon from the atmosphere and sequestering it in the earth.
As I have pointed out previously in this column (SOLAR
TODAY, Nov./Dec. 2009, p. 20, http://bit.ly/92HNdg),
biomass is a renewable energy resource that, if applied
properly, can potentially be carbon-negative, as opposed
to just carbon-neutral. Reducing deforestation and pro-
moting reforestation are key biological ways to help reduce
atmospheric carbon dioxide. Another approach gaining in
popularity is the concept of biochar.
Biochar is the charcoal that results from subjecting bio-
mass to temperatures of 350°–600°C (662°–1,112°F) in the
absence of oxygen. In the process of “slow pyrolysis,” the
biomass feedstock is converted over a period of hours to a
combination of perhaps 25 percent to 35 percent biochar
and the rest biofuel, equivalent to about a 50-50 split on a
carbon basis. In fast pyrolysis, up to 80 percent biofuel is
created in a matter of seconds. The advantage of the slow
approach is that while the carbon-neutral biofuel can be
used to displace carbon-emitting fossil fuels (preferably
coal), the carbon-negative biochar can be used as a renew-
able fertilizer, thus sequestering in the soil carbon that was
removed from the atmosphere by the plant growth. This
all assumes that the overall carbon impacts of land use are
properly considered.
Proponents of biochar point to the fertile “terra preta”
(Portuguese for “black earth”) soils found in the Ama-
zon Basin. Researchers believe pre-Colombian farmers
produced biochar by smoldering agricultural waste. How
long biochar carbon will stay sequestered in different soils
(expressed in terms of its half-life, or how long it takes for
half of the original biochar to be lost from the soil) can
vary over a considerable range. In one interesting exchange
of letters in Science, Swedish biologist David Wardle stat-
ed that, in his experiments, the introduction of biochar
increased bacterial growth, which led to more carbon being
released. The carbon half-life and crop productivity effects
of biochar are dependent on the feedstock used for the bio-
char, how the biochar is produced, amount of rainfall, soil
temperature, specific fungal and microbial colonies present
and other soil properties.
There has been a wide range of opinions regarding the
20 January/February 2011 SOLAR TODAY solartoday.org
Copyright © 2011 by the American Solar Energy Society Inc. All rights reserved.
total impact extensive production of biochar might have on
world carbon emissions. The most thorough analysis I have
seen to date was presented in an August paper in Nature
Communications by Dominic Woolf et al. ( http://bit.ly/
bcOK1r). Cognizant of scientific papers that have criticized
the negative land-use (and resulting carbon) impacts asso-
ciated with some biofuels production, the authors placed
a number of practical constraints on biochar production,
such as excluding the use of rainforests and the conversion
of land being used for food crops. They then developed
three scenarios for biochar production, the most ambitious
being an estimate of the maximum possible amount that
could be produced sustainably. Their conclusion was that
the production of biochar could theoretically reduce total
world greenhouse gas emissions by an amount equal to
(at most) about 12 percent of today’s emissions. That is
substantial, although it is a lot lower than some biochar
enthusiasts might claim. One should keep in mind that this
was based on the authors’ most aggressive scenario and,
despite the constraints they placed on land use, some envi-
ronmental groups have criticized the paper for espousing
what they call a huge land grab from indigenous peoples
and peasant farmers.
