Dominic Woolf, June 2012
The role of biochar in a negative emissions portfolio
Dominic Woolf Cornell University
Dominic Woolf, June 2012
Biochar: An intervention in the terrestrial carbon cycle
Anthropogenic emissions: 8 Pg C yr-‐1
Terrestrial NPP: 60 Pg C yr-‐1
Heterotrophic respiraDon:
(60 – X) Pg C yr-‐1
Biochar: X Pg C yr-‐1
Dominic Woolf, June 2012
Sensitivity of Avoided Emissions to Biochar Stability ! Biochar consists of both labile (short-‐lived)
and recalcitrant (long-‐lived) components ! Biochar stability depends primarily on (1)
the labile fracDon & (2) T½ of recalcitrant fracDon
! Centennial avoided emissions insensiDve to T½ > 200 yrs
! Centennial avoided emissions are sharply reduced for T½ < 100 yrs
! Increasing soil BC stocks give rise to increasing soil CO2 emissions from BC decomposiDon
! Eventually, “peak biochar” occurs when losses ≈ addiDons
! Long-‐term biochar sequestraDon requires high biochar stability (T½ ≳ 500 yrs over 500 yr Dmescale)
(Leh
man
n et
al.,
201
0)
SequestraDon
Dominic Woolf, June 2012
Influence of Temperature on Biochar Structure
! Biomass conversion to biochar fundamentally changes the chemistry of organic ma]er ! Both decomposiDon and condensaDon reacDons occur ! Increasing temperature:
! reduces H & O content ! increases aromaDcity ! increases condensaDon of biochar (increased size of conjugated aromaDc sheets) ! increases 3D order ! increases porosity & surface area ! increases pH & CEC
Dominic Woolf, June 2012
Biochar stability increases with pyrolysis temperature
! EsDmates of biochar ½ life span 6 orders of magnitude (101 – 107 yrs)
! T½ corellates with O:C raDo ! Lower O:C => higher T½ ! O:C reduced by
! higher pyrolysis temperature ! longer reacDon Dme
! Feedstock also contributes to stability ! Manure biochar least stable ! Grasses less stable ! Wood most stable
! T ½ > 1000 yrs can be achieved by use of slow pyrolysis at >500°C
(Spokas, 2011)
Poultry-‐manure biochars
Dominic Woolf, June 2012
Global Distribution of soil BC
! BC is ubiquitous in global soils
! BC% of SOC peaks at 20-‐30° laDtude (arid zone & regions with pronounced dry season)
! High BC% generally associated with most ferDle soils (anthrosols, mollisols, chernozems)
(Krull et al., 2008)
Dominic Woolf, June 2012
! Anthropogenic Dark Earths in Amazonia, West Africa & Borneo highly ferDle relaDve to adjacent soils ! ADEs built gradually over many decades/centuries. ! Also contain raised non-‐pyrogenic SOC ! Higher pH, Ca, n, P , K, Ca and water holding capacity ! High P content possibly due to addiDon of bones or reduced
leaching ! May not be possible to replicate in short Dme scale
! Short-‐term field & pot trials have typically shown a 5-‐40% increase in yield
! Mechanisms include: ! reducing pH constraints ! lowering Al toxicity ! Increased CEC ! SorpDon of anionic nutrients (nitrate, phospate) ! Increased water retenDon ! Synergies with mycorrhizae
! Yields can also be supressed by ! raising pH on neutral/alkaline soils ! N immobilisaDon from high labile-‐content biochar ! lowering water retenDon in clayey soils
! Different biochars need to be tailored to local soil constraints
! Long-‐term field trials sDll required.
++ CEC ++ SOC ++ water retenDon -‐ -‐ nutrient leaching
Verheijen et al. (2009)
Effect on crop yields
Dominic Woolf, June 2012
Fertility effect on yield response
! Yield improvements most pronounced on degraded or inferDle (low CEC, low OM, sandy or nutrient constrained) soils
! Highly ferDle soils typically do not show any improvement in yields
! Site-‐specificity is a common feature of all types of organic amendment
! Yield generally shows a strong synergisDc posiDve interacDon with mineral & organic ferDlisers
Dominic Woolf, June 2012
Char Gas
Water Tar & VolaDles
Products of Pyrolysis
! Biochar yield falls with temperature, but... ! C yield falls less strongly ! Fixed-‐C yield has li]le temperature response for T > 400°C ! C-‐sequestraDon fairly independent of pyrolysis temp. ! Pyrolysis condiDons should be opDmised for energy producDon,
economics, and emissions.
Dominic Woolf, June 2012
Coproduction of Energy with Biochar
! Per unit feedstock, increased biochar producDon implies reduced energy producDon
! IniDal biomass (dry, ash-‐free) has ~ 19 GJ Mg-‐1
! approx 7 – 8 GJ remains in biochar (slow pyrolysis)
! Once losses accounted for, up to ~7 GJ energy available
! Several pathways to produce liquid & gaseous fuels or electricity
Dominic Woolf, June 2012
Sustainable Global Potential
Cropland soil (top 15cm)
begins to saturate
Cropland soil (top 15cm) saturated
Sustainable Biomass availability scenarios (Pg C yr-‐1)
Moderate 1.0 Pg C
Extreme 2.3 Pg C
AmbiDous 1.6 Pg C
! Biochar can be produced sustainably or unsustainably ! Feedstock source of prime importance (as with all
biomass technologies) ! Pyrolysis emissions and energy producDon also
important ! Sustainability criteria:
! Agricultural & forestry residues procured at a rate that does not cause soil erosion or degradaDon or reduce food security
! Li]le C debt from land-‐use change or long-‐lived feedstocks
! No loss of habitat or biodiversity from direct or indirect land conversion
! No contaminated wastes used ! Low-‐emissions conversion technology ! Energy co-‐producDon
Dominic Woolf, June 2012
Avoided emissions attributions
! Main contribuDon due to sequestered C & fossil-‐fuel offsets
! Significant avoided CH4 from paddy rice producDon
! Avoided N2O accounts for 9% of miDgaDon impact
! Main negaDve impacts are BC decomposiDon and SOC loss
! Tillage & transport losses negligible ! Avoided emissions from bioenergy
slightly larger than C-‐sequestraDon effect alone of biochar: greater benefit of biochar requires coproducDon of energy
Dominic Woolf, June 2012
Biochar vs Bioenergy
! RelaDve miDgaDon potenDal of biochar and bioenergy (combusDon) depends strongly on fossil fuel that is offset and local soil ferDlity
! ⨂ indicates baseline C-‐intensity and global-‐mean cropland soil-‐ferDlity
! Least ferDle soils yield greater benefit from biochar than bioenergy
! RelaDve benefit of biochar increases as C intensity decreases
! Contours steepest for biomass crops ! Highest relaDve benefit (>80%) for
poorest soils growing biomass crops offseung low C-‐intensity fuels
! Lowest relaDve benefit (-‐19%) for most ferDle soil growing biomass crops and offseung coal
! RelaDve benefits of biochar and bioenergy depend highly on local condiDons!
! So far, comparison has only been on a per unit biomass basis. But…
⨂
⨂
⨂
⨂
Dominic Woolf, June 2012
SOC Priming
! Biochar affects (primes) turnover rates of non-‐pyrogenic SOC ! Both increased iniDal
respiraDon rates (+ve priming) and increased stabilisaDon (-‐ve priming) have been observed
! In the long term, stabilisaDon effects dominate ! biochar may significantly
increase npSOC
! SOC depleDon is the limiDng factor in sustainable biomass residue harvesDng
Dominic Woolf, June 2012
1 ha of corn Total crop residues per ha
Maximum sustainably harvestable crop residues
144 GJ (8 Mg)
30% conver
sion
efficiency
108 GJ (6 Mg)
Biochar 45 GJ (1.4 Mg)
Ethanol 32 GJ
80% conversion efficiency
Biochar returned to soil maintains / improves soil funcDon and builds soil carbon
Liquid biofuel 29 GJ
36 GJ (2 Mg)
Biochar / bioenergy conversion
Dominic Woolf, June 2012
Economic effects of C Credits & fertility
! Payback period of 25 years or more ! Net profitability of biochar depends heavily
on crop value enhancements ! Most cost effecDve on moderate ferDlity
soils where total yield responses are highest
! Least ferDle soils that benefit most from biochar are constrained by lower economic return and by lower feedstock availability
! No payback on highly ferDle soil
! C credits have essenDally no impact on relaDve profitability of biochar and bioenergy
JE Amone]e 22Jan2012
Without C credits Solid lines show effect of $200/Mg C credits
Figures courtesy of J. Amone]e, Pacific Northwest NaDonal Laboratory
Dominic Woolf, June 2012
! Cost of biochar producDon varies considerably with feedstock (-‐£200 to +£390 Mg-‐1 for large-‐scale biochar systems in the UK) ! Biochar from waste products for which ‘Dpping fees’ paid may have
negaDve producDon cost
! Feedstock cost is largest component of producDon cost
! In UK,
6x106 Mg CO2 yr-‐1 abatement potenDal
for < £20 Mg-‐1
(Shackley et al. 2010)
Dominic Woolf, June 2012
Global cost curve for GHG abatement
Source: Enkvist e
t al.
(200
7)
€21-30 per tCO2e (from McCarl 2009)
3.7-‐6.6 GtCO2e/yr abatement (Woolf et al 2010)
Biochar
Dominic Woolf, June 2012
Summary & Conclusions
! Biochar can be engineered to be sufficiently stable to sequester C for several centuries
! Short term field & pot trials typically show improved yields in poor soils
! FerDle soils typically show no improvement in yield (although water and ferDliser inputs and runoff may be reduced)
! No long-‐term field-‐trial data are available, although BC-‐rich soils o~en have high ferDlity
! Climate miDgaDon potenDal of biochar is greater than bioenergy except where ferDle soils coexist with high C-‐intensity energy supply
! Short-‐term economics favour bioenergy over biochar; long-‐term favours biochar
! Payback Dmes before biochar is more economic than bioenergy range from 25-‐70 yrs (shortest on soils with moderate ferDlity constraints)
! If applied equally, C credits have li]le impact on relaDve economics of biochar and bioenergy
! A more comprehensive comparaDve-‐analysis of the uses of biomass for GHG-‐miDgaDon is required, looking at a wide range of opDons (co-‐firing, AD, burial, biochar, biofuels, electricity, BECCS...) for an array of potenDal feedstocks and geographic locaDons
! Comparisons between uses of biomass must consider not just economics, energy and GHGs, but also wider issues including soil conservaDon, biodiversity, hydrology & nutrient cycling.
Dominic Woolf, June 2012
References
! Antal M, Grønli M (2003) The Art, Science, and Technology of Charcoal ProducDon†. Ind Eng Chem Res 42:1619–1640. ! Enkvist P-‐A, Naucler T, Rosander J (2007) A Global Cost Curve for Greenhouse Gas ReducDon. The McKinsey Quarterly 1
! Krull et al. (2008) Grasslands: Ecology, Management & RestoraDon, Nova Science Publ. ! Lehman et al. (2011) Role of biochar in miDgaDon of climate change, Imperial College Press ! McCarl B, Peacocke C, Chrisman R, et al. (2009) Chapter 19: Economics of biochar producDon, uDlisaDon and emissions.
Biochar for environmental management: science and technology, Lehmann, J. & Joseph, S. (eds) ! Neves D, Thunman H, Matos A, et al. (2011) CharacterizaDon and predicDon of biomass pyrolysis products. Progress in
Energy and CombusDon Science
! Shackley S, Hammond J, Gaunt J, Ibarrola R (2011) The feasibility and costs of biochar deployment in the UK. Carbon 2:335–356.
! Spokas K (2010) Review of the stability of biochar in soils: predictability of O:C molar raDos. Carbon Manage 1:289–303.
! Verheijen F, Jeffery S, Bastos AC, et al. (2009) Biochar ApplicaDon to Soils: A CriDcal ScienDfic Review of Effects on Soil ProperDes, Processes and FuncDons. Joint Research Centre. InsDtute for Environment and Sustainability, Ispra, Italy
! Woolf D, Amone]e JE, Street-‐Perro] FA, et al. (2010) Sustainable biochar to miDgate global climate change. Nature CommunicaDons 1:1–9.
Dominic Woolf, June 2012
Economic Model Assumptions
! Crop Value—Maize: $300/Mg
! Yield increase builds with addiDonal biochar amendments ! Biomass Amount: 1 Mg C/yr; biochar from this applied to 1 ha
annually.
! Fossil Fuel Carbon Intensity: 17.5 KgC/GJ ! Energy Value: $3.00/GJ ! Cost of ProducDon/TransportaDon:
! $70/ MgC slow pyrolysis
! $50/ MgC combusDon
! C Credits: $0-‐$200/Mg C
! Soil FerDlity Response Factor: 0-‐1 ! Time: 0-‐100 years
Dominic Woolf, June 2012
C intensity of fuel offsets
! On average, biochar has higher miDgaDon potenDal than bioenergy except when in most C-‐intense economies (e.g. where coal is only fuel)
! SensiDvity to C intensity is lower than bioenergy
! In low carbon-‐intensity locaDon or future, biochar maintains significant GHG reducDons
Amb.
Ext.
Mod.
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