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Biochar for Carbon Sequestration

The
anthropogenic emissions of Carbon dioxide have risen by more than 3% annually
since 2012(Raupach et al.)(Woolf, Amonette, Alayne Street-Perrott, et al.). The earth’s ecosystem is moving towards rapid
climate change which not only dangerous but also irreversible(Solomon et al.). An immediate attention is required to bring
about a timely and ambitious program to mitigate climate change. According to
several studies, to stabilize the mean surface temperature of earth, the cumulative
anthropogenic greenhouse-gas (GHG) emissions must be maintained under a maximum
upper limit (Woolf, Amonette, Street-Perrott, et al.). This can be achieved only if the future net
anthropogenic emissions approach zero(Solomon et al.)(Broecker)(Matthews and Caldeira)(Allen et al.)(Meinshausen et al.)(Woolf, Amonette, Alayne Street-Perrott, et al.). Upon exceeding this threshold, it will not
be possible to bring the climate back to safe limits(Woolf, Amonette, Alayne Street-Perrott, et al.). This limit may have, in fact, already been
exceeded(Hansen et al.). Under these circumstances, strategies that
can draw down excess atmospheric CO2 concentrations would be of prime
importance(Woolf, Amonette, Alayne Street-Perrott, et al.). Use of biochar in carbon sequestration is
one such technique.

 

Biochar is a fine-grained
charcoal formed from pyrolysis (“Biochar Introduction | US Biochar Initiative”). Biochar use has been recognized as a useful
method of improving soil conditions and has been used for over 2500 years now(“Biochar Introduction | US Biochar Initiative”). Biochar’s potential to sequester carbon has
also been recognized, which stems from its highly recalcitrant nature(Schmidt and Noack)(Kuzyakov et al., “Black Carbon Decomposition and
Incorporation into Soil Microbial Biomass Estimated by 14C Labeling”)(Cheng et al.)(Woolf, Amonette, Alayne Street-Perrott, et al.). This paper aims to discuss the possible use
of biochar as a carbon capture technique through a few literature reviews of published
scientific studies.

 

History and Background:

Biochar is
found naturally in some places because of vegetation fires(“Biochar Then & Now | US Biochar Initiative”). However, due to its benefits on soil
properties, it is also intentionally produced by burning biomass. Biochar has
been used in agricultural practices in the Amazon Basin of South America for as
long as 2500 years(“Biochar Then & Now | US Biochar Initiative”). The traditional method of biochar
production is to pile the biomass in an earthen pit and cover it to burn it
under low oxygen conditions(“Biochar Then & Now | US Biochar Initiative”). This method emits half the carbon dioxide present
in the original biomass in the atmosphere. The heat energy produced also goes
to waste. This method is still practiced in developing countries(“How Is Biochar Made?”).  However,
the method of producing biochar by pyrolysis in specially designed furnaces is
very environmentally friendly. It can capture all the emissions from burning. These
emissions which include volatile gases and hydrocarbons are also known as
synthetic gas(syngas). Syngas can be used as natural gas. The liquid captured,
known as biooils can also be used as a source of energy. In addition, the heat
energy released can also be captured and used to electricity generation. The
carbon-rich biochar that is left behind can then be applied to soil(“How Is Biochar Made?”).

 

The world
production of biochar in 2005 was more than 44 million tons (“Countries Compared by Energy > Charcoal
> Production from Charcoal Plants. International Statistics at
NationMaster.com”)(“Methods for Producing Biochar and Advanced Biofuels
in Washington State Part 1: Li Eactors Terature Review of Pyrolysis R”). The feedstock used for biochar are waste
products like excess manure, wood debris, waste from food processing etc(“How Is Biochar Made?”). While biochar has the potential to resolve
many environmental, social and technological issues(“Methods for Producing Biochar and Advanced Biofuels
in Washington State Part 1: Li Eactors Terature Review of Pyrolysis R”), the focus of this paper will be to discuss its
application in carbon sequestration.

 

Soil contains
3.3 times more carbon than the atmosphere and 4.5 times more than plants and
animals making it an important source of carbon dioxide and other greenhouse
gases. However, with the right management, it can also be a potential sink(“Methods for Producing Biochar and Advanced Biofuels
in Washington State Part 1: Li Eactors Terature Review of Pyrolysis R”). Pyrolysis of biomass transforms the carbon
in the biomass into stable carbon structures in biochar which is recalcitrant
against decomposition. The biochar can remain sequestered in the soil from a hundred
to thousands of years. Thus, biochar application can make soil a carbon sink
while also improving soil quality and productivity(“Climate Change and Biochar | International Biochar
Initiative”)(“Biochar Carbon Sequestration – Introduction”).

 

In 2010, Don
Hofstrand, Professor Emeritus at Iowa Stata University discussed Biochar
Systems for carbon sequestration in an Agricultural Marketing Resource
Center(AgMRC) Renewable Energy Newsletter. Hofstrand began by saying that
biochar is carbon negative. He further discusses the underlying assumptions of
the analysis of the potential size of biochar systems industry. This analysis
was conducted by the International Biochar Initiative(“Using Biochar Systems to Sequester Carbon | Agricultural
Marketing Resource Center”). As biochar is derived from biomass, its
availability becomes critical to the size of biochar system industry. The
analysis used just crops and forest residues as biomass sources which is a very
small percentage of the world’s Net Primary Production (NPP). Examples have
been given for three different scenarios: conservative, moderate and
optimistic. Under the conservative scenario, the use of 27% of the existing
crops and forests biomass residues in a biochar system will comprise only 1.2%
of the world’s NPP. Similarly, use of 50% for moderate approach and 80% for
optimistic approach will comprise of 2.1% and 3.2% of the NPP(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”). Thus, adequate biomass availability has
been assumed. Next assumption is that adequate land area is available for
storing biochar for sequestration. Biochar is added to the land from which the
biomass was taken(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”). Thus, any loss in soil organic matter by
the removal of crop residues for biochar production is restored by returning
biochar to the land which in fact will improve the productivity(Woolf, Amonette, Alayne Street-Perrott, et al.)(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”). The pyrolysis system used is assumed to be
a slow-pyrolysis (fast pyrolysis produces more biooil and less biochar than
slow pyrolysis(Woolf, Amonette, Alayne Street-Perrott, et al.)) modern high-yielding technology with a 40%
carbonization rate(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”). The system is also assumed to produce
renewable energy, which can replace fossil fuel energy, thus reducing carbon
emissions. Biochar is assumed to be stable and remain sequestered in the soils
for more than a hundred years. The estimates of the carbon sequestered for
conservative, moderate and optimistic scenarios were presented in a graph as
follows. Wedge represents one gigaton of carbon sequestered per year. From the
graph we can see that the biochar takes a few years to reach its full
potential, after which, it sequesters carbon in a stable range of 0.2 to 0.8 billion
tons(gigaton) of carbon per year. The Optimistic Plus scenario in the graph
includes the reduction in emission of nitrous oxide gas along with carbon
dioxide. The nitrous oxide emissions are 
assumed to be reduced by half(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”).

Fig
1: Carbon Sequestration
or Offset in different scenarios over time. Reprinted from: “Using Biochar Systems
to Sequester Carbon | Agricultural Marketing Resource Center.” N.p., n.d. Web.
2 Dec. 2017.

 

Bioenergy
emits carbon that is originally present as atmospheric carbon and thus is
carbon neutral whereas fossil fuels emit carbon that is originally sequestered
in the soil and is carbon positive. Thus, carbon offset is generated through
the substitution of fossil energy by the carbon neutral bioenergy generated in
the process of biochar production. In addition, the carbon dioxide generated
during biochar production can be captured and sequestered. The International
Biochar Initiative took into the account the collective effect of carbon
sequestration through biochar, substitution of fossil energy with bioenergy and
capture and sequestration of excess carbon during biochar production. The net
effect is presented in the following graph(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”).

 

Fig 2:
Combined Carbon Sequestration or Offset in different scenarios over time.
Reprinted from: “Using
Biochar Systems to Sequester Carbon | Agricultural Marketing Resource Center.”
N.p., n.d. Web. 2 Dec. 2017.

 

 

The
conservative scenario results in net reduction of more than 0.5 gigatons of
carbon per year. The net reduction in moderate and optimistic scenarios are
around 1.25 gigatons and almost 2 gigatons of carbon per year respectively.
Under the optimistic plus scenario, the reduction is about 2.2 gigatons per
year by 2050(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”). The author further discusses the role that
biochar systems can play in reducing greenhouse emissions levels. The following
table has been presented in the article to show the contribution of biochar
systems in GHG emission offset. Biochar alone can reduce the emissions by 10%
in average. When the collective effect of all three offsets is considered, the
biochar system can contribute from 20% to 30% of the reduction(“Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center”).

 

Table 1

Potential
GHG emission offsets by 2050 under various scenarios of biochar systems

Source: “Using Biochar Systems to Sequester Carbon |
Agricultural Marketing Resource Center.” N.p., n.d. Web. 2 Dec. 2017.

 

Another study
titled “Sustainable biochar to mitigate global climate change”(Woolf, Amonette, Alayne Street-Perrott, et al.) in 2010 studied the maximum potential of
biochar to mitigate climate change with a sustainable approach(Woolf, Amonette, Alayne Street-Perrott, et al.). This theoretical upper limit of the climate
change potential of biochar under current conditions is termed as maximum
sustainable technical potential (MSTP). The author describes MSTP as “what can
be achieved when the portion of the global biomass resource that can be
harvested sustainably (that is, without endangering food security, habitat or
soil conservation) is converted to biochar by modern high-yield, low-emission,
pyrolysis methods”(Woolf, Amonette, Alayne Street-Perrott, et al.). The paper doesn’t take into account any
socio-economic and cultural barriers. However, it assumes a maximum rate of
capital investment which is in consistence with that estimated to be required
for mitigation of climate-change. The paper describes the sustainable biochar
concept with the aid of the following diagram. The paper discusses the
dependence of the climate-change mitigation potential of biochar and bioenergy
on the fertility of the soil being amended, the carbon intensity of the fuel
being offset, and the feedstock used for deriving them (type of biomass). Locations
with high soil fertility and where coal is the fuel being offset is more
suitable for bioenergy production. For all other situations, biochar has a
higher climate-change mitigation potential than bioenergy (Woolf, Amonette, Alayne Street-Perrott, et al.).

 

 

Fig 3:  Sustainable Biochar Concept Overview. Reprinted
from: Woolf,
Dominic et al. “Sustainable Biochar to Mitigate Global Climate Change.” (2010):
n. pag. Web. 3 Dec. 2017

 

Sustainable biomass-feedstock
availability is crucial to any biochar system. Thus, the study uses a strict
set of criteria to assess the availability of feedstock for biochar to ensure
the approach is sustainable. Land conversion to generate biomass is of primary
importance as it can have adverse effects on the ecosystem. In addition, it can
lead to release of carbon stored in the soils and biomass(Woolf, Amonette, Alayne Street-Perrott, et al.). This results in unacceptable limits of carbon
pay-back times before any net reduction of atmospheric carbon dioxide is
achieved. (Fargione et al.)(Woolf, Amonette, Alayne Street-Perrott, et al.). For example, clearance of rainforest for
land conversion to produce biomass-crop can results in carbon payback times more
than 50 years. Similarly, the conversion of peatland rainforest for production
of biomass-crop can result in carbon-payback times in the order of 325 years(Woolf, Amonette, Alayne Street-Perrott, et al.). Therefore, in this study, it has been
assumed that no land clearance was used to provide feedstock. Due to adverse
consequences on food security, it is also assumed that no food production land
was converted for biomass production. The analysis is restricted to systems
which use modern, high-yield, low emission pyrolysis technology. In
consideration with these constraints the paper presents a biomass-availability
scenario for their estimate of MSTP. They also present two additional
scenarios, Alpha and Beta (represent lower demands). In the Alpha scenario, the
biomass availability is restricted to moderate amount of agroforestry and
biomass cropping together with residues and waste available with the use of
current practices. All scenarios are ambitious(Woolf, Amonette, Alayne Street-Perrott, et al.).

Table 1

Annual
Sustainable Biomass feedstock available globally

 

Source: Woolf, Dominic et al.
“Sustainable Biochar to Mitigate Global Climate Change.” (2010): n. pag. Web. 3
Dec. 2017

 

 

Fig 4:  Net GHG Emissions avoided. Reprinted from: Woolf,
Dominic et al. “Sustainable Biochar to Mitigate Global Climate Change.” (2010):
n. pag. Web. 3 Dec. 2017

 

The analysis
in this paper states that a maximum of 12% of current anthropogenic CO2-C
equivalent emissions, which is 1.8Pg CO2-Ce (per year) of the 15.4Pg CO2-Ce
emitted annually, can be potentially offset by the sustainable global
implementation of biochar(Woolf, Amonette, Alayne Street-Perrott, et al.). The writer further says that the total net
offset from biochar implementation over a time of a century would 130Pg CO2-Ce(Woolf, Amonette, Alayne Street-Perrott, et al.). Half the amount of avoided emissions occurs
due to net carbon sequestration by biochar however the remaining 30% and 20%
are due to replacement of fossil fuel by bioenergy and avoided methane and
nitrous oxide emissions respectively. The paper also states that a maximum
offset of 10% of anthropogenic CO2-Ce emissions can be achieved on maximizing
bioenergy production rather than biochar from sustainably obtained biomass. A
detailed breakdown, shows that carbon stored as biochar is the soil (43-49 Pg
CO2-Ce) and fossil fuel offsets (18-39 Pg CO2-Ce) are the highest contributors
to reduced emissions. Among the negative feedbacks, largest was found to be
biochar decomposition (8-17 Pg CO2-Ce). The uncertainty of the models was
estimated with Sensitivity and Monte Carlo analyses. The sensitivity is
strongest to the half-life of the recalcitrant fraction of biochar. The net
avoided GHG emissions can vary by -22% to +4% from the value obtained with 300
years as the baseline assumption. However, most of this variation occurs for
half-life100 years, which is a more realistic
scenario, sensitivity is low as biochar production can be more rapid than
biochar decay(Woolf, Amonette, Alayne Street-Perrott, et al.). Current data available in literature show
that the half-life of recalcitrant fraction of biochar in soil is in the
millennial range (Sombroek, Nachtergaele, and Hebel)(Kuzyakov et al., “Black Carbon Decomposition and
Incorporation into Soil Microbial Biomass Estimated by 14C Labeling”)(Kuzyakov et al., “Black Carbon Decomposition and
Incorporation into Soil Microbial Biomass Estimated by 14C Labeling”)(Cheng et al.) (Woolf, Amonette, Alayne Street-Perrott, et al.).

 

Fig 5:  Detailed breakdown showing cumulative avoided
GHG. Reprinted from: Woolf,
Dominic et al. “Sustainable Biochar to Mitigate Global Climate Change.” (2010):
n. pag. Web. 3 Dec. 2017

 

An article
titled “Biochar as a viable carbon sequestration option: Global and Canadian perspective”
discusses the availability of sources for biochar application the amount of
carbon that can be sequestered.  Industrial
scale techniques of carbon capture from air by the closed-cycle sodium
hydroxide absorption is being contemplated at a cost of $500/tC and at half the
cost when combined with carbon capture by biomass(Keith, Ha-Duong, and Stolaroff). Significant cost is associated with the
compression and pumping of carbon dioxide into ground. These costs are however
not incurred in biochar production and application for soil amendment. The
paper doesn’t study the interactions of biochar with soil. Of the total
anthropogenic emissions of carbon from fossil fuel and cement production), 4.1
GtC/yr remains in the atmosphere. The paper assumes that the biomass available
for conversion to biochar is 10% of the net primary production (NPP) whose estimation
has been taken as 60.6 GtC/yr, which was the estimated value at the time the study
was done(Amonette, Lehmann, and Joseph). The resultant biochar production assuming
conversion of 50% of biomass carbon to biochar was 3 GtC/yr. The carbon offset achieved
through combustible products (60% of the 50% biomass) was 1.8 GtC/year. The
remaining 40% (1.3 GtC/yr) is used for pyrolysis). This shows that 10% of NPP
biomass would be enough to offset the annual increase in carbon dioxide in the
atmosphere. Further the paper discusses the distribution of the biochar
produced. Assuming the addition of 3% biochar (percentage by mass) into the top
30cm of the total agricultural area which is around 45 mil. Km2 (Ghazi et al.) worldwide,
the total capacity worldwide would be 600 GtC of biochar. The author says that
at the rate of 3 GtC/yr, this potential reservoir for biochar would be available
for two centuries. From Canadian perspective, the author says that Canada has
the largest reserves for biomass to produce biochar. From the collective
contribution of biomass from forest harvesting, forest fire reduction, mountain
pine beetle infestation, agricultural residues and fast rotation silviculture,
amounts of biomass 5 times larger than the annual requirements would be
achieved. While this could fully offset total carbon emissions, the land
available for storing biochar is limited(Matovic).

 

It is crucial
to understand that the

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