biofuels: a sober look at the potential chris field carnegie institution: department of global...
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Biofuels:A sober look at the potential
Chris Field
Carnegie Institution: Department of Global EcologyStanford University, Department of Biology
www.global-ecology.org
• Bioenergy basics
• Bioenergy options
•Bioenergy potential
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Why biofuels?
• Climate protection– Offset fossil fuels– Account for fossil fuels used to produce– Account for site carbon balance– Account for other greenhouse gases
• Energy security– Local potential– Diversify sources
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How can biofuels be lower carbon?
• Photosynthesis– Light + CO2 plant + O2
• Plant combustion– Plant + O2 energy + CO2
• Net– Light + CO2 energy + CO2
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How Biomass is Used for Energy
Biomass
Burn produce electricity
Thermochemicalconversion to
syngas products
Biochemicalconversion toethanol and other fuels
Mature Semi-mature(Capital intensive
ineficient)
In development
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Carbon cycle basics
• Fossil fuel + oxygen carbon dioxide– Coal: C + O2 CO2
– Oil: C8H16 + 12O2 8CO2 + 8H2O
– Natural Gas: CH4 + 3O2 CO2 + 2H2O
• How much CO2?
– Burning 1 lb of coal produces 3.6 lb of CO2
– Burning 1 gal of gas produces 18 lb of CO2
– The average person produces 30 lb CO2/day
– The average American produces 170 lb CO2/day
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Setting the scale
• Food for 1 person for one year– ~ 250 kg corn
• = ethanol for one fill-up– ~ 80 l (20 gal)
• At 25 mpg and 10,000 miles/y– The corn required to fuel one car on
corn ethanol– Would feed 25 people
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0.5 MJspeeding
car
1 kJ 1 MJ 1 GJ 1 TJ 1 PJ 1 EJ 1 YJ 1 ZJ1 J
0 3 6 9 12 15 18 21 24
4 GJ =ton TNT6 GJ = barrel
crude oil
1017 Jbiggest
nuclear bomb1 kg matter
1011 Jcar/yr
1 MJ240 kcal
100 Jaction
4 ·1022 JWorld fossil
Fuel reserves
450 EJworld energyconsump/yr1013 J
1 gram E = Mc2
Powersof ten
Energy
12 ·1015 Jsupertanker
QUADBTU
10 MJ2400 kcal
human/day
1 ZJKT impact
5 ZJSolar energyOn Earth in
1 year
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Today, the world
consumes 20X as much
energy as in 1900!
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Future energy needs:Many times current
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Global annual plant growth
• ~57 x 109 ton C on land
• ~57 x 109 ton C in the oceans
= 2500 EJ or 5 x global primary energy
= 2500 EJ or 5 x global primary energy
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Land Type
Area (Mha)
Mean NPP(ton C/ha/y)
Total NPP(Pg C/y)
Total Energy*(EJ/y)
Global Crop 1,445 4.6 6.7 119
Pasture 3,321 3.4 11.3 200
US Crop 173 5.7 1.0 18
Pasture 226 3.5 0.8 14
Global Primary Energy = 450 EJ/y* In ½ biomass (to allow for roots), assume 45% C
Energy in ag and pastures?
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Will yields increase dramatically?
• Historical trends – a century of success– 1-2%/y for major crops
• Will this continue?– Can it accelerate?
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Ag yields – a century of successincreases of 1-2% y-1
Lobell and Field ERL 2007
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Limiting factors for global NPP
Baldocchi et al. 2004 SCOPE 62
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Potential yield
• Ag in relation to natural NPP– Ag/NPP -- Globally about 65%
• Global average crop yields unlikely to exceed natural NPP for at least the next several decades
15
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Burn or Ferment?
• If you want energy– Burn
• If you want oil independence– Liquid biofuels– Battery technology
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Net energy balance ratio(biomass energy out/fossil energy in)
• Corn ethanol ~1.2• Sugarcane ethanol ~ 8• Soy biodiesel ~ 2• Palm biodiesel ~ 9
• Cellulosic ~5(?)
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Hill et al PNAS 2006
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Is sugarcane the answer?
• High yields in warm, wet climates• Limited need for fossil energy
– Burn bagasse for processing energy
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Fargione et al. Science 2008
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Is cellulosic the answer?
Yield of 26.5 tons/acre in limited area test plots
Cou
rtes
y of
Ste
ve L
ong
et a
l
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Lignin occludes polysaccharides
LigninHemicellulose
Cellulose
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Effect of lignin content on enzymatic recovery of sugars from Miscanthus
D Vrije et al (2002) Int J Hydrogen Energy 27,1381
Lignin
Switchgrasscomposition
cellulose
Hemi cellulose
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USDA Amber Waves 2007
Biofuels and food
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Thow & Warhurst, 2007 (divide by 2.2 for break even oil price in $/bbl)
Ethanol production cost per tonCO2 equivalent emissions offset
(not accounting for land use)
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Food – the perfect storm?
• Population• Food preferences• Climate change• Biofuels
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Bioenergy – the climate protective domain
• Increase growth• Increase efficiency of conversion to
useful products• Utilize sites where C loss from
conversion is small in relation to bioenergy yield
• Utilize sites that are not needed for something else
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Field, Campbell, Lobell TREE 2008
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Field, Campbell, Lobell TREE 2008
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Land Type Area (Mha)
Mean NPP(ton C / ha / yr)
Total NPP(Pg C / yr)
Global Crop 1,445 4.6 6.7
Pasture 3,321 3.4 11.3
Abandoned 531 4.7 2.5
Potential from abandoned land
Field, Campbell, Lobell TREE 2008
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Land Type Area (Mha)
Mean NPP(ton C / ha / yr)
Total NPP(Pg C / yr)
Global Crop 1,445 4.6 6.7
Pasture 3,321 3.4 11.3
Abandoned 531 4.7 2.5
In Forest 72 6.5 0.5
In Crop 199 5.5 1.1
In Urban 18 5.0 0.1
In Other 242 3.5 0.8
From available abandoned land
0.8 Pg C x 2 g Plant/g C x 0.5 g top/g plant x 16 EJ/Pg = 13 EJ= 3% of current global energy system
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Bioenergy
• Climate impact depends on pre-existing ecosystem
• Indirect as well as direct paths to carbon loss• Natural NPP reasonable proxy for potential
yield under ag management• Available land resource limited
– Quantity and quality
• Big potential in absolute terms• But a small slice of present or future demand
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