Biochar is a name for charcoal when it is used for particular purposes, especially as a soil
amendment. Like all charcoal, biochar is
created by pyrolysis of biomass. Biochar is under investigation as an approach to carbon sequestration
to produce negative carbon dioxide emissions. Biochar thus has the potential to help mitigate climate
change, via carbon sequestration. Independently, biochar can increase soil fertility, increase
agricultural productivity, and provide protection against some foliar and
soil-borne diseases. Furthermore, biochar reduces pressure on forests. Biochar is a stable solid, rich in carbon and can endure in soil for thousands of years.
History
Pre-Columbian Amazonians
are believed to have used biochar to enhance soil productivity. They produced
it by smoldering
agricultural waste (i.e., covering burning biomass with soil)
in pits or trenches.
European settlers called it terra preta de Indio.
Following observations and experiments, a research team working in French Guiana
hypothesized that the Amazonian earthworm
Pontoscolex corethrurus was the main agent of fine powdering and incorporation of
charcoal debris to the mineral soil.
The term “biochar” was coined by
Peter Read to describe charcoal used as a soil improvement.
Production
Pyrolysis produces biochar, liquids, and gases from biomass by
heating the biomass in a low/no oxygen environment. The absence of oxygen
prevents combustion. The relative yield of products from pyrolysis varies with
temperature. Temperatures of 400–500 °C (752–932 °F) produce more char, while temperatures above
700 °C (1,292 °F) favor the yield of liquid and gas fuel components.
Pyrolysis occurs more quickly at the higher temperatures, typically requiring
seconds instead of hours. High temperature pyrolysis is also known as gasification,
and produces primarily syngas.
Typical yields are 60% bio-oil, 20% biochar, and 20% syngas. By comparison, slow pyrolysis
can produce substantially more char (~50%). Once initialized, both processes
produce net energy. For typical inputs, the energy required to run a “fast”
pyrolyzer is approximately 15% of the energy that it outputs.
Modern pyrolysis plants can use the syngas created by the pyrolysis process and
output 3–9 times the amount of energy required to run.
The Amazonian pit/trench method
harvests neither bio-oil nor syngas, and releases a large amount of CO
2, black carbon, and other greenhouse gases (GHG)s (and potentially, toxins) into the air. Commercial-scale systems process agricultural waste, paper byproducts, and even municipal waste and typically eliminate these side effects by capturing and using the liquid and gas products.
2, black carbon, and other greenhouse gases (GHG)s (and potentially, toxins) into the air. Commercial-scale systems process agricultural waste, paper byproducts, and even municipal waste and typically eliminate these side effects by capturing and using the liquid and gas products.
Centralized,
decentralized, and mobile systems
In a centralized system, all biomass
in a region is brought to a central plant for processing. Alternatively, each
farmer or group of farmers can operate a lower-tech kiln. Finally, a truck equipped with a
pyrolyzer can move from place to place to pyrolyze biomass. Vehicle power comes
from the syngas stream, while the biochar remains on the farm. The biofuel is
sent to a refinery or storage site. Factors that influence the choice of system
type include the cost of transportation of the liquid and solid byproducts, the
amount of material to be processed, and the ability to feed directly into the
power grid.
For crops that are not exclusively
for biochar production, the residue-to-product ratio (RPR) and the collection
factor (CF) the percent of the residue not used for other things, measure the
approximate amount of feedstock that can be obtained for pyrolysis after
harvesting the primary product. For instance, Brazil harvests approximately 460 million
tons (MT) of sugarcane annually,
with an RPR of 0.30, and a CF of 0.70 for the sugarcane tops, which normally
are burned in the field.
This translates into approximately 100 MT of residue annually which could be
pyrolyzed to create energy and soil additives. Adding in the bagasse
(sugarcane waste) (RPR=0.29 CF=1.0) which is otherwise burned (inefficiently)
in boilers, raises the total to 230 MT of pyrolysis feedstock. Some plant
residue, however, must remain on the soil to avoid increased costs and
emissions from nitrogen fertilizers.
Pyrolysis technologies for
processing loose and leafy biomass produce both biochar and syngas.
Thermo-catalytic
depolymerization
Alternatively, thermo-catalytic
depolymerization using microwaves has recently been used to efficiently convert organic
matter to biochar on an industrial scale, producing ~50% char.
Uses
Carbon
sink
The burning and natural
decomposition of biomass and in particular agricultural waste adds large amounts
of CO
2 to the atmosphere. Biochar that is stable, fixed, and 'recalcitrant' carbon can store large amounts of greenhouse gases in the ground for centuries, potentially reducing or stalling the growth in atmospheric greenhouse gas levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.
2 to the atmosphere. Biochar that is stable, fixed, and 'recalcitrant' carbon can store large amounts of greenhouse gases in the ground for centuries, potentially reducing or stalling the growth in atmospheric greenhouse gas levels; at the same time its presence in the earth can improve water quality, increase soil fertility, raise agricultural productivity, and reduce pressure on old-growth forests.
Biochar can sequester carbon in the
soil for hundreds to thousands of years, like coal.Such a carbon-negative technology would lead to a net withdrawal of CO2
from the atmosphere, while producing and consuming energy
Researchers have estimated that
sustainable use of biocharring could reduce the global net emissions of carbon
dioxide (CO
2), methane, and nitrous oxide by up to 1.8 Pg CO
2-C equivalent (CO
2-Ce) per year (12% of current anthropogenic CO
2-Ce emissions; 1 Pg=1 Gt), and total net emissions over the course of the next century by 130 Pg CO
2-Ce, without endangering food security, habitat, or soil conservation.
2), methane, and nitrous oxide by up to 1.8 Pg CO
2-C equivalent (CO
2-Ce) per year (12% of current anthropogenic CO
2-Ce emissions; 1 Pg=1 Gt), and total net emissions over the course of the next century by 130 Pg CO
2-Ce, without endangering food security, habitat, or soil conservation.
Biochar is a high-carbon,
fine-grained residue which today is produced through modern pyrolysis
processes. Pyrolysis is the direct thermal decomposition of biomass in the absence of oxygen to obtain an array of
solid (biochar), liquid (bio-oil), and gas (syngas) products. The specific
yield from the pyrolysis is dependent on process conditions, and can be
optimized to produce either energy or biochar.
Soil
amendment
For plants that require high potash and elevated pH,
biochar can be used as a soil amendment
to improve yield. Biochar can improve water quality, reduce soil emissions of greenhouse gases, reduce nutrient leaching, reduce soil acidity,
and reduce irrigation and fertilizer requirements.
Biochar was also found under certain circumstances to induce plant systemic
responses to foliar fungal diseases and to improve plant responses to diseases
caused by soilborne pathogens.
The various impacts of biochar can
be dependent on the properties of the biochar,
as well as the amount applied,
and there is still a lack of knowledge about the important mechanisms and
properties.
Biochar impact may depend on regional conditions including soil type, soil
condition (depleted or healthy), temperature, and humidity.
Modest additions of biochar to soil reduce nitrous oxide
N
2O emissions by up to 80% and eliminate methane emissions, which are both more potent greenhouse gases than CO
2.
2O emissions by up to 80% and eliminate methane emissions, which are both more potent greenhouse gases than CO
2.
Pollutants such as metals and pesticides
seep into soil and contaminate food supplies, reducing the amount of land
suitable for agricultural production. Studies have reported positive effects
from biochar on crop production in degraded and nutrient–poor soils.
Biochar can be designed with specific qualities to target distinct properties
of soils.
Biochar reduces leaching of critical nutrients, creates a higher crop uptake of
nutrients, and provides greater soil availability of nutrients.
At 10% levels biochar reduced contaminant levels in plants by up to 80%, while
reducing total chlordane and DDX content in the plants by 68 and 79%, respectively.
On the other hand, because of its high adsorption capacity, biochar may reduce
the efficacy of soil applied pesticides that are needed for weed and pest
control.
High surface area biochars may be particularly problematic in this regard; more
research into the long term effects of biochar addition to soil is needed.
Slash
and char
Switching from slash and burn to slash and char farming techniques in Brazil can decrease both
deforestation of the Amazon basin and carbon dioxide emission, as well as increase crop
yields. Slash and burn leaves only 3% of the carbon from the organic material
in the soil.
Slash and char can keep up to 50% of the carbon in a highly stable form.
Returning the biochar into the soil rather than removing it all for energy
production reduces the need for nitrogen fertilizers, thereby reducing cost and
emissions from fertilizer production and transport.
Additionally, by improving the soil's ability to be tilled, fertility, and
productivity, biochar–enhanced soils can indefinitely sustain agricultural
production, whereas non-enriched soils quickly become depleted of nutrients,
forcing farmers to abandon the fields, producing a continuous slash and burn
cycle and the continued loss of tropical rainforest. Using pyrolysis to produce bio-energy also has the added
benefit of not requiring infrastructure changes the way processing biomass for cellulosic ethanol does. Additionally, the biochar produced can be applied by
the currently used machinery for tilling the soil or equipment used to apply
fertilizer.
Water
retention
Biochar is a desirable soil material
in many locations due to its ability to attract and retain water. This is
possible because of its porous structure and high surface area.
As a result, nutrients, phosphorus, and agrochemicals are retained for the
plants benefit. Plants therefore, are healthier and fertilizers leach less into
surface or groundwater.
Energy
production: bio-oil and syngas
Bio-oil can be used as a replacement
for numerous applications where fuel oil is used, including fueling space heaters,
furnaces,
and boilers.
Additionally, these biofuels can be used to fuel some combustion turbines and
reciprocating engines, and as a source to create several chemicals.
If bio-oil is used without modification, care must be taken to prevent
emissions of black carbon and other particulates. Syngas and bio-oil can also
be “upgraded” to transportation fuels such as biodiesel
and gasoline substitutes.
If biochar is used for the production of energy rather than as a soil
amendment, it can be directly substituted for any application that uses coal.
Pyrolysis also may be the most cost-effective way of producing electrical energy
from biomaterial.
Syngas can be burned directly, used as a fuel for gas engines and gas turbines,
converted to clean diesel fuel through the Fischer–Tropsch process, or potentially used in the production of methanol
and hydrogen.
Bio-oil has a much higher energy
density than the raw biomass material.
Mobile pyrolysis units can be used to lower the costs of transportation of the
biomass if the biochar is returned to the soil and the syngas stream is used to
power the process.
Bio-oil contains organic acids that are corrosive to steel containers, has a
high water vapor content that is detrimental to ignition, and, unless carefully
cleaned, contains some biochar particles which can block injectors.
The greatest potential for bio-oil seems to be its use in a bio-refinery, where
compounds that are valuable chemicals, pesticides, pharmaceuticals, or food
additives are first extracted, and the remainder is either upgraded to fuel or
reformed to syngas.
Direct and indirect benefits
- The pyrolysis of forest- or agriculture-derived biomass residue generates a biofuel without competition with crop production.
- Biochar is a pyrolysis byproduct that may be ploughed into soils in crop fields to enhance their fertility and stability, and for medium- to long-term carbon sequestration in these soils.
- Biochar enhances the natural process: the biosphere
captures CO
2, especially through plant production, but only a small portion is stably sequestered for a relatively long time (soil, wood, etc.). - Biomass production to obtain biofuels and biochar for
carbon sequestration in the soil is a carbon-negative process, i.e. more
CO
2 is removed from the atmosphere than released, thus enabling long-term sequestration.
Research
Intensive research into manifold
aspects involving the pyrolysis/biochar platform is underway around the world.
From 2005-2012, there were 1,038 articles that included the word “biochar” or
“bio-char” in the topic that had been indexed in the ISI Web of Science.
Emerging commercial sector
Calculations suggest that emissions
reductions can be 12–84% greater if biochar is put back into the soil instead
of being burned to offset fossil-fuel use. Thus Biochar sequestration offers
the chance to turn bioenergy into a carbon-negative industry.
Johannes Lehmann, of Cornell
University, estimates that pyrolysis can be cost-effective for a combination of
sequestration and energy production when the cost of a CO
2 ton reaches $37. As of mid-February 2010, CO
2 is trading at $16.82/ton on the European Climate Exchange (ECX), so using pyrolysis for bioenergy production may be feasible even if it is more expensive than fossil fuel.
2 ton reaches $37. As of mid-February 2010, CO
2 is trading at $16.82/ton on the European Climate Exchange (ECX), so using pyrolysis for bioenergy production may be feasible even if it is more expensive than fossil fuel.
Current biochar projects make no
significant impact on the overall global carbon budget, although expansion of
this technique has been advocated as a geoengineering
approach.
In May 2009, the Biochar Fund received a grant from the Congo Basin Forest Fund
for a project in Central Africa to simultaneously slow down deforestation,
increase the food security of rural communities, provide renewable energy and sequester carbon.[citation needed]
Application rates of 2.5–20 tonnes per
hectare (1.0–8.1 t/acre) appear to be required to produce significant
improvements in plant yields. Biochar costs in developed countries vary from
$300–7000/tonne, generally too high for the farmer/horticulturalist and
prohibitive for low-input field crops. In developing countries, constraints on
agricultural biochar relate more to biomass availability and production time.
An alternative is to use small amounts of biochar in lower cost
biochar-fertilizer complexes.
Various companies in North America,
Australia,
and England
sell biochar or biochar production units.[citation needed]
At the 2009 International Biochar
Conference a mobile pyrolysis unit with a specified intake of 1,000 pounds
(450 kg) was introduced. for agricultural applications. The unit had a
length of 12 feet and height of 7 feet (3.6 m by 2.1m).
A production unit in Dunlap, Tennessee by Mantria Corporation
opened in August 2009 after testing and an initial run, was later shut down as
part of a Ponzi scheme investigation.
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