Biochar for Carbon Sequestration, Reduction of Greenhouse Gas Emissions and Enhancement of Soil Fertility; A Review of the Materials Science

Abstract

When biomass is pyrolysed it produces a syngas, bio-oils and charcoal. When this charcoal (biochar) is applied to soils, research has shown that it can increase soil health, crop yields, reduce leaching of organic and inorganic fertilisers, and some evidence exists that it can reduce soil emissions of N 2 O and CH 4 ... Biochars have been found to be very stable in many soil environments, when compared to uncharred organic matter, and have considerable potential to sequester carbon from the atmosphere. Progress has been made in understanding the material properties of these chars that result in their efficacy. In this paper an overview of the literature will be presented along with recent results of research work carried out at the University of NSW and Cornell University. Areas where further work is required will be outlined.

Full text

Proceedings of the Australian Combustion Symposium
December 9-11, 2007, University of Sydney
Biochar for Carbon Sequestration, Reduction of Greenhouse Gas
Emissions and Enhancement of Soil Fertility; A Review of the
Materials Science
S D Joseph1, *, A Downie, P Munroe, A Crosky, J Lehmann,2
1School of Material Science and Engineering, University of NSW, NSW 2052 Australia
2 Department of Crop and Soil Sciences, College of Agriculture and Life Sciences, Cornell University, Ithaca, NY
14853, USA;
Abstract
When biomass is pyrolysed it produces a syngas, bio-oils and charcoal. When this charcoal (biochar) is applied to soils,
research has shown that it can increase soil health, crop yields, reduce leaching of organic and inorganic fertilisers, and
some evidence exists that it can reduce soil emissions of N2O and CH4... Biochars have been found to be very stable in
many soil environments, when compared to uncharred organic matter, and have considerable potential to sequester
carbon from the atmosphere. Progress has been made in understanding the material properties of these chars that result
in their efficacy. In this paper an overview of the literature will be presented along with recent results of research work
carried out at the University of NSW and Cornell University. Areas where further work is required will be outlined.
Keywords: Biochar, Biosequestration, Pyrolysis, Greenhouse gas emissions
1. Introduction
An intensification of agricultural production on a
global scale is necessary in order to secure the food
supply for an increasing world population. As a result,
more and more organic carbon is being removed from
the soil leading to dramatic soil degradation. Many of
Australia’s agricultural soils are already reaching
critically low levels of soil organic carbon. Intensive
agriculture and horticulture in Australia have depleted
organic carbon levels in soil from an estimated 3% to
less than 1% [1]. Organic carbon in the soil enables soil
biota to flourish, assisting the processes of nutrient
flow, increases cation exchange capacity, water and
nutrient retention.
One method of increasing soil health is through the
application of carbonized materials (biochars) that have
long term stability [2]. These materials, added as
pyrolysed organic matter, can be produced from
purpose grown biomass (wood and grasses) and
biomass waste streams such as paper sludge, manure or
greenwaste. Moreover, these chars appear to be able to
reduce the emissions of N2O and CH4.. [3] from soil.
Over the past 3 years considerable research has been
undertaken to measure the physical and chemical
properties of the biochars and relate these to their long
term stability, effect on soil health and improved crop
yields and reduction in emissions. An overview of the
available literature will be undertaken of chars
produced at temperatures between 400°C and 600°C,
along with a summary of the research carried out at
UNSW. This paper will not review the extensive
research carried out on activated carbon.
2. Characterisation of Feedstocks
Biochar has been produced from a wide range of
biomass feedstocks. For agronomic use these are
characterized by the relative proportion of lignin,
hemicellulose, cellulose, mineral content, protein,
carbohydrate and other extractives and their internal
structure. The relative proportion of these constituents
will influence the final chemical composition and
structure of the char (for a given set of process
conditions).
Mineral matter in plants forms different functions
and occurs in different forms [4]. Potassium and
chlorine ions are highly mobile and will start to
vaporize at relatively low temperatures during thermal
decomposition. Calcium is mainly located in the cell
walls and the ions are bound to organic acids. Silicon
is bound in the cell walls as silica or as opal phytoliths.
Both are released during pyrolysis at much higher
temperatures than potassium. Magnesium is both
ionically and covalently bonded with organic molecules
and only vaporizes at high temperatures. Phosphorous
and sulphur are associated with complex organic
compounds within the cell and are relatively stable at
low pyrolysis temperatures. Nitrogen is associated with
a number of different organic molecules and can be
__________________________
* Corresponding author:
Phone: (+61) 0408415477
Email: s.joseph@unsw.edu.au - 130 -

released at relatively low pyrolysis temperatures. Other
elements such as iron or manganese exist in a number
of organic and inorganic forms in the biomass and are
largely retained during pyrolysis.
Wood and related plants (e.g. bamboo, coconut
shells) are characterized by a very low ash composition
(<3%) and a very open porous structure. When wood is
pyrolysed it maintains its internal cellular structure
(Fig. 1). Animal manures and some other high mineral
ash residues are not very porous but increase their
porosity when pyrolysed. Husks (e.g. rice) can have
both high ash content and high porosity. They will
maintain their structure when pyrolysed.
2. Effect of Process Conditions on the
Properties of Biochars
A range of temperatures, heating rates, time at final
reaction temperature, pressures (1 to 10bar gauge) and
partial pressures of oxygen/steam are used to produce
biochars. The reactions conditions are one of the main
determinants of the yield of biochar and its physical
and chemical composition. Much research is still
required to fully characterize the affect of these
parameters but the following is a summary of findings
to date.
2.1 Properties between 400°C and 600°C
At temperatures near 400°C a cross-linked
amorphous aromatic structure is formed from the
reaction of volatile gases with the unreacted biomass.
This structure is porous and could contain randomly
curved sheets [5]. At the surfaces of the biochar there
are a range of functional groups that include pyranone,
phenolic, carboxylic, lactone and amine groups. Asada
[6] noted that the formation of free radicals in biochar
that is produced at a temperature below 600°C. These
radicals could be associated with aromatic and aliphatic
compounds, especially those that contain nitrogen and
sulphur.
Metals and non metals can be ionically or
covalently bonded with the aromatic compounds.
Mobile atoms such as potassium could be intercalated
in the carbon aromatic structure [7]. In high ash
biochars crystalline and amorphous mineral phases
exist between these aromatic structures. The
distribution of mineral phases is complex and varied
and depends on the structure of the original biomass.
As the temperature increases it is probable that the
composition and the structure of these metallic phases
will change [7]. Further work is required to fully
characterize these changes.
As the temperature increases above 450°C
microcrystalline graphene sheet formation occurs.
Kercher [8] has hypothesized that as the temperature
increases the disordered carbon decomposes and
becomes incorporated into the graphene sheets of the
turbostratic carbon. The rigid covalent structure of the
disordered carbon cannot rearrange during
decomposition to allow the non-uniform growth of
turbostratic crystallites. Instead, some graphene sheets
grow extensively, and other sheets become terminated
and pinned by structural defects [8]. The conversion of
low-density disordered carbon into high-density
graphene sheets causes the volumetric contraction
observed during carbonization. Kercher [8] found that
the graphene sheets had there maximum thickness
around 450°C. It is probable that the micro-crystal
structure and the defect structure of the biochars are
affected by the type, quantity and structure of the
mineral matter. For low ash biochars the electrical
conductivity of the biochar increases as the volume of
graphene sheets increases [9]. Higher mineral ash
biochars probably have higher conductivity especially
those that have high potassium ion contents due to the
mobility of the potassium ions. Further research is
required to determine the affect of mineral content on
the internal structure and electrical conductivity.
Final process temperature determines the average pore
size, the number of micro pores/unit mass and the total
surface area. Sousa [10] found for pinus elliotti biochar
that maximum concentration of pores and minimum
pore diameter occurred around 450°C (around 2µm).
The surface area of wood biochar at 400°C is
approximately 50-100m2/g and at 500°C 300-500m2/g
[11]. The cation exchange capacity (CEC) and pH of
fresh biochar from wood increases as the temperature
of pyrolysis rises [12].
There is a wide range of highly oxygenated volatile
compounds (e.g. levoglucosan,
hydroxyacetaldehyde, furfurals, and methoxyphenols)
that are retained on the surface of the pores of the
biochar at temperatures below 500°C. Some of these
compounds are water soluble. The quantity and
composition of the volatile organic compounds on the
pore surfaces change. At approximately 500°C phenolic
ethers, alkyl phenolics and heterocyclic ethers may be
deposited on the biochar surface [13]. As the
processing temperature increase the percentage of the
different functional groups in the carbon matrix
changes with the carboxylic and carboxylic anhydrides,
and lactones groups decomposing to CO2. In high ash
biochars amine functional groups are converted to
pyridine groups at higher temperatures [14].
Figure 1: Images of char particles under the SEM a)
wood-based char, b) manure-based char.
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The proportion of different functional groups, the
surface porosity and the form of mineral matter changes
when biochars are reacted with oxygen, and/or steam
and/or CO2 at their final pyrolysis temperature. The
reactions of low mineral ash chars to oxidizing
environments should be different to high mineral ash
chars. Koutcheiko [22] measured an increase in the
pyridinic-N and quaternary-N and a decrease in the
amine functional groups and pyrrolic N when chicken
manure biochar was reacted with CO2.
2.2 Effects of Pressure, Particle Size, Time
and Atmosphere
There is very little consistent data on the effects of
the other process variables on the structure and
composition of biochars produced at high heating rates
(>50°C/minute) and/or higher pressure (>2
atmospheres). At high pressures (and in the presence of
steam) porosity can be high and density low due to the
rapid breakdown of the biopolymers [15]. The structure
of the amorphous biochar is different to that found in
pyrolysis carried out atmospheric temperatures in that
there is less cross linkage and a great proportion of
dimers and monomers [15]. Antal [16] notes that the
functional groups are similar for wood biochars made at
both high and low pressures under slower heating rates.
For most particulate wood biochars it appears that high
heating rates result in biochars that are amorphous,
have lower alkaline metal content [17] and that have
low porosity due to the formation of a melt phase [18].
3. Reaction of Biochars with Soils,
Microbes and Plants
There are complex interactions between water
within the pores and on the surface of the biochar, the
soil, plants, and micro-organisms. Sugimoto [23]
suggest that water has an ice like structure in the
nanopores of biochar produced above 400°C. Water is
probably in the supercooled state when adsorbed into
biochars produced above 450°C [24]. Turov [23] noted
that the interactions of water with carbon can be
considered as interactions of clusters or droplets
localized near oxygen containing groups. In those
entities water molecules interact with each other
stronger than with hydrophobic patches of the carbon
surface. Thus the kinetics and the energetics of
reactions that take place at the interface between
biochar and the water will be influenced by the charring
temperature and the distribution of functional groups.
Most biochar has been produced in pit, beehive or
vertical shaft kilns that have residence times ranging
from 24 hours up to 1 month. More recently reactors
have been developed that utilise residues that have a
relatively small particle size (<15mm) and have
reaction times that vary from seconds (for fast
pyrolysis) to 1 hour for slow pyrolysis. Very little
systematic research has been undertaken on the effect
of time and particle size on the composition and
structure of different biochars. Mathematical modeling
indicates that, for a given particle size, the shorter the
process time the greater is the concentration gradient of
carbon and the more volatile minerals across the
particle [19]. It is probable that the microcrystalline
graphene sheets will grow when the biochar is held for
long times at the final reaction temperature [8].
However, this needs to be verified experimentally.
When biochar is applied to wet soil there will be an
almost immediate change in the pH of the soil around
the biochar, increase in the soil porosity, dissolution of
organic and inorganic compounds, an exchange of
cations (and possibly anions) between the clay and silt
particles and the biochar and an adsorption of gases,
metals and other organic compounds on the biochar
surface.
Leaching and dissolution experiments ([25,26] indicate
that high ash biochars produced at temperatures below
550°C release potassium, sodium, phosphorous,
sulphur, sulphates inorganic carbonates and organic
compounds into moisture surrounding the biochar
particle. This dissolution may take place within the
first week of placing the biochar in the soil and there
may be an exchange of cations, anions and organic
compounds with the surrounding soil, microbes and
plants. Over a much longer period of time calcium,
magnesium, iron will dissolve out of the biochar. Much
lower concentrations of cations and anions are released
from low ash biochars (especially wood) than high ash
biochar (chicken manure).
There is some evidence to indicate that the residual
bio-oils on char produced from the very slow pyrolysis
of logs and branches have a much greater percentage of
low molecular weight compounds (this was the
conventional method of manufacturing wood alcohol)
[20]. Schnitzer [21] has carried out a detailed analysis
of the residual bio-oils on biochars derived from the
fast pyrolysis of chicken manure. They found that the
individual compounds identified were grouped into the
following six compound classes: (a) N-heterocyclics;
(b) substituted furans; (c) phenol and substituted
phenols; (d) benzene and substituted benzenes; (e)
carbocyclics; and (f) aliphatics. Prominent N-
heterocyclics in bio-oil were methyl-and ethyl-
substituted pyrroles, pyridines, pyrimidine, pyrazines,
and pteridine. The alkanes and alkenes ranged from n-
C7 to n-C18 and C7:1 to C18:1, respectively, and those in
the biochar from n-C7 to n-C19 and C7:1 to C19:1,
respectively.
Many of the residual bio-oils on the biochar surface are
unstable and can react with water, air and with each
other to form new compounds [28]. Thus organic acids
may react with alcohols to form esters, organic acids
with olefins to form esters, aldehydes and water to form
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hydrates (referred to as glycols), aldehydes and
alcohols to form hemiacetals, aldehydes and proteins to
form dimers of the proteins.
Biochar, when placed in soil, can also adsorb heavy
metals and other dissolved organic compounds [13].
The adsorption processes appear to be complex and
depend on the concentration and distribution of mineral
phases, functional groups, radicals and defects on the
internal pore and external biochar surfaces.
Swiatkowski [14] have postulated that metals can be
absorbed on biochar that has been oxidized via a series
of acid base reactions.
Micro-organisms can start to grow on the surfaces
and in the pores on biochar within the first month of
being applied to soils containing composting material
[29]. The root hairs also penetrate the macropores of
the biochar and take in moisture and nutrients. Biochar
is also ingested by worms and are excreted with
coatings of organic compounds which provide food for
other micro-organisms. Examination of black carbons
in soil has shown that there is considerable interaction
between the clay minerals and the biochar [30]. This
organo-clay layer may reduce weathering of the
biochar. It is possible that binding can take place
through interaction between the clay minerals and the
functional groups on the surface.
4. Conclusions
Biochars produced from biomass are very complex.
Very little systematic analysis of the amorphous and
microcrystalline structure and chemical composition of
biochars produced from different biomass under a
range of process conditions have been undertaken.
There is only a basic understanding of how biochar
structure and composition improve soil fertility, crop
yields, GHG emissions and long term stability in soils.
Over the next 2 years extensive characterisation of
biochars made from high and low mineral ash biomass
under different process conditions will be undertaken
using a range of analytical techniques at UNSW and
Cornell University.
5. Acknowledgement
Funding for this research has been provided by the
Australian Research Council and the Department of
Environment, BESTEnergies Australia Pty Ltd and
Climate Change of the N.S.W. Government and
BESTEnergies Pty.
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