biochar: carbon sequestration potential€¦ · talk outline • introduction – carbon storage...
TRANSCRIPT
Biochar: carbonsequestration potential
Dr Saran Paul Sohi & Simon [email protected]
UK Biochar Research Centrewww.biochar.org.uk
Biochar Application to SoilsCopenhagen 7th December 2009
Talk outline
• Introduction– Carbon storage and land-based options– Efficacy of carbon storage in soil organic matter
• Biochar (the concept)– Carbon sequestration– Bio-energy versus “carbon negative” energy– Definitions of biochar
• Net carbon benefits
‘Land based carbon storage’
Rationale for carbon storage strategies:reducing use of energy will not happen soon or fast enoughto decrease atmospheric CO2
– and for storing carbon in the biosphere:– capture undertaken by natural processes, so no
“energy penalty” (c.f. flue gas capture)– natural carbon flows large relative to fossil fuel flux,
and a significant proportion already human-modified – auxiliary benefits
Biospheric carbon cycle
Carbon is the main constituent of organic material: 45 % of plant matter,
60% of organic matter in soil
107
Soil (1500)
1000 million tonnes (Gt) / yr
(stocks) 1000 million tonnes (Gt)
flows
Atmosphere (720)
0.1
0.40.40.4
120
7
Burial
Vegetation (560)
2
60 60
60
Ocean (38000)
107 105
Biospheric carbon cycle - recap
Land-based carbon storage – plants
• Increasing ‘standing carbon’ (forests, plantation)– trend forest cover is downward– annual accrual (growth) slow and finite– planted trees vulnerable to later fire or clearance
Land-based carbon storage – soil
– returning more crop residues, manure, compost, other organic material / waste
• depends on availability of organic resource
– decreasing soil disturbance (reducing decomposition rate)
• balance of evidence suggests a small effect
Long term experiment at Rothamsted Research
Soil quality driven by labile carbon
Photo: Chris Watts
Soil carbon storage – limitations
• Stock is only the balance between the input of organic matter to soil and its decomposition– stored rather than sequestered so harder to account– developing and maintaining elevated stock requires
large ongoing increase in annual input of organic material (with, increasingly, other value)
– decomposition rate may increase with climate change making soil carbon stores vulnerable to ‘feedback’
• Only a small proportion of added organic matter much stabilised, accumulation rate diminishes– inefficient use of organic resource after equilibration
Soil C
cumulative CO2 emission
from soil organic matter
cumulative additional CO 2
from straw added to soilEmission avoided by
NOT adding straw
carbon in the soilTime period (yrs)
Acc
umul
atio
n ra
te
(tC/ h
a / y
r)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0-10 10-20 20-50 50-100 100-200
Simulated decline* in rate of carbon accumulation after increasing organic inputs Example: New return of 2 tC/yr (cereal straw), silty-loam soil, southern UK(*Roth C model)
<20% of C added
<2% of C added
Soil carbon storage – limitations
• about 0.2 Mt C yr-1 in UK, or ~0.1 % of annual CO2 emissions
Max
imum
UK
soi
l car
bon
accu
mul
atio
n ra
te (M
tC/ y
r) 4.03.5
2.5
1.5
0.5
3.0
2.0
1.0
Smith et al., 2000Soil Use and Management
0.35 tC /ha/yr on 3 Mha
2% annual UK CO2 emissions from fossil fuel
Even ‘unconstrained’ potential is small – UK case study
Soil carbon storage – limitations
107
Soil (1500)
1000 million tonnes (Gt) / yr
(stocks) 1000 million tonnes (Gt)
flows
Atmosphere (720)
0.1
0.40.40.4
120
7
Burial
Vegetation (560)
2
60 60
60
Ocean (38000)
107 105
Biospheric carbon cycle - recap
A “new” biospheric option - biochar
• Convert up to 1 GtC annual net primary productivity (NPP) into chemically stable forms– organised conversion is a clean process where heat
and combustible gases are captured and used
• Storage in soil– matches diffuse feedstock and diffuse storage– does not depend on physical stabilisation– opportunity to generate feedstock through positive
feedback (increasing harvestable NPP)
A natural analogue: the “black carbon cycle”(natural fire and charcoal) processes >0.1 GtC / yr
• International definition (includes charcoal)– pyrogenic carbon from biomass– intended for application to soil
• Enhanced definition (analogy to charcoal)– zero-oxygen conversion– high rate of carbon conservation (e.g. >30%)– dominant (chemical) configuration ‘aromatic’– structured or amorphous (physically)– capture of synthesis gases (H2,CH4,CO, etc.)
• as energy co-products– active matching of characteristics to situation
Biochar definition
Global flux 60 Gt/yr
Principles of biochar sequestration
fossil carbon
Pyrolysis(energy)
soil carbon biochar
CO2 in air
fossil carbon
Pyrolysis(energy)
soil carbon biochar
CO2 in air
plant biomass
Biochar strategy – attributes
• Proportion of current organic resource converted to stabilised form, providing certain functions typical of soil organic matter, but not diminished– maintenance not dependent on maintenance of inputs– incremental enhancement– annual augmentation is optional or opportunistic– no obvious limit to storage capacity– secure w.r.t. future change in climate, land-use, etc.
• Can be deployed in conjunction with conventional strategies to build soil organic matter
Soil C
cumulative CO2 emission
from soil organic matter
cumulative additional CO 2
from straw added to soilEmission avoided by
NOT adding straw
carbon in the soilTime period (yrs)
Acc
umul
atio
n ra
te
(tone
s C
per
ha
per y
r)
0.00
0.05
0.10
0.15
0.20
0.25
0.30
0.35
0.40
0-10 10-20 20-50 50-100 100-200
Scenario: diversion of 2 tC/yr (cereal straw) to biochar one year in four (returned to soil, 90% stable), and three years in four direct to the soil – assumes no interactions
Biochar-enhanced carbon storage
• Stability of biochar and efficiency of energy use determine net gain
• Can assume a ‘carbon stability factor’ that accounts for some short-term loss
• Sensitivity of strategy minimal for average stability exceeding 100 yr
• Extensive laboratory evidence for high stability of charcoal, millennial-scale in the field…
Stability of biochar carbon
• By analogy to charcoal in natural systems– Assumptions on frequency of standing
biomass, burn frequency, and conversion– Mean residence time of 1300-2600 yrs
Stability of biochar carbon
Composite nature of biochar carbon
H2O, CO2
DEHYDRATION DEPOLYMERIZATION
Furans, phenols, BTX, ketones, olefins, CO, CO2, H2, H2O
Fuel
coke
soot soot
PAH, CO2, H2, CO, H2O, CH4
CHAR
CHARRING
PRIMARY SECONDARY TERTIARY
Primary volatiles (oxygenates)
Ondrej Masek, PhD thesis
• Less research on stability of components of carbon from pyrolysis rather than natural charcoal
• To concentrate (and stabilise) carbon, hydrogen and oxygen have to be released (and some trace elements in proportion)
• Gases from slow pyrolysis may also contain about 2/3 of initial carbon
• Burning gases from biomass pyrolysis constitutes bioenergy
• Gas capture prevents polluting emissions associated with traditional pyrolysis (charcoal)
• Technologies to retain more carbon during stabilisation may emerge
The energy angle
energy : CO2 ratios in biomass conversion
Ener
gy y
ield
GJ
per t
combustion slow pyrolysisgasification fast pyrolysis
Carbon em
ission factor (t CO
2 / GJ
CARBON EMISSION FACTOR
decreases more rapidly with C retention
than energy yield (more energy per unit of CO2)
Energy yield
CO2 emitted
Carbon retention 50%20%0%
Global flux 60 Gt/yr
“Carbon negative” bioenergy
fossil carbon
soil carbon biochar
CO2 in air
biomass
fossil carbon
soil carbon
CO2 in air
plant biomass
Naturalcycle
Global flux 60 Gt/yr
“Carbon negative” bioenergy
fossil carbon
soil carbon biochar
CO2 in air
biomass
fossil carbon
soil carbon
CO2 in air
plant biomass
Combustion(bio-energy)
Global flux 60 Gt/yr
“Carbon negative” bioenergy
fossil carbon
Pyrolysis
soil carbon biochar
CO2 in air
biomass
fossil carbon
soil carbon
CO2 in air
plant biomass
(bio-energy)
Life cycle analysis – estimates for overall gain
0 200 400 600 800 1000 1200 1400 1600
Wheat straw (small)
Barley straw (small)
Oil seed rape and other straw (small)
Short rotation coppice (small)
Arboricultural arisings (small)
Sawmill residues (large)
Forestry residue chips (large)
Miscanthus (large)
Short rotation coppice (large)
Short rotation forestry (large)
Canadian forestry residue chips (large)
Wheat straw (large)
Barley straw (large)
Oil seed rape and other straw (large)
kgCO₂eq t-1feedstock0.25
C abatement (tonnes) per tonne C in pyrolysis feedstock
0.50 0.75 1.00
Hammond et al., forthcoming
-20% 0% 20% 40% 60% 80% 100%
Wheat straw (small)Barley straw (small)
Oil seed rape and other straw (small) Short rotation coppice (small)Arboricultural arisings (small)
Sawmill residues (large)Forestry residue chips (large)
Miscanthus (large) Short rotation coppice (large)Short rotation forestry (large)
Canadian forestry residue chips (large)Wheat straw (large)Barley straw (large)
Oil seed rape and other straw (large)
Provision of biomass feedstock Transport and spreading Electricity generation & offsetHeat generation and offset Soil effects Soil sequestration
Life cycle analysis – sources of gain
Hammond et al., forthcoming
Conclusions
• Biochar has the potential to sequester (rather than simply store) carbon into the biosphere
• Sequestration could be at the Gt scale using currently available feedstock, given suitable policy and economic instruments
• Pyrolysis offers bio-energy co-products with the potential to exceed the carbon gain (abatement) from combustion
Acknowledgements
University of Edinburgh– Prof. Stuart Haszeldine (GeoScience)– EPSRC (Science and Innovation award)
– Simon Shackley (UKBRC)– Ondrej Masek (UKBRC)
– Peter Brownsort (UKBRC)– Rodrigo Ibarrola– Jim Hammond
UKBRC is a partnership between University of Edinburgh, Rothamsted Research and Newcastle University to undertake strategic research into the integration of biochar into the energy–agriculture system, for the purpose of climate change mitigation and auxiliary benefits to soil performance and food production. Core EPSRC funding at Edinburgh is backed by public sector grants, commercial sponsors and international Fellowships
www.biochar.org.uk
