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

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

How can biofuels be lower carbon?

• Photosynthesis– Light + CO2 plant + O2

• Plant combustion– Plant + O2 energy + CO2

• Net– Light + CO2 energy + CO2

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

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

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

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

Today, the world

consumes 20X as much

energy as in 1900!

Future energy needs:Many times current

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

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?

Will yields increase dramatically?

• Historical trends – a century of success– 1-2%/y for major crops

• Will this continue?– Can it accelerate?

Ag yields – a century of successincreases of 1-2% y-1

Lobell and Field ERL 2007

Limiting factors for global NPP

Baldocchi et al. 2004 SCOPE 62

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

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Burn or Ferment?

• If you want energy– Burn

• If you want oil independence– Liquid biofuels– Battery technology

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(?)

Hill et al PNAS 2006

Is sugarcane the answer?

• High yields in warm, wet climates• Limited need for fossil energy

– Burn bagasse for processing energy

Fargione et al. Science 2008

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

Lignin occludes polysaccharides

LigninHemicellulose

Cellulose

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

USDA Amber Waves 2007

Biofuels and food

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)

Food – the perfect storm?

• Population• Food preferences• Climate change• Biofuels

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

Field, Campbell, Lobell TREE 2008

Field, Campbell, Lobell TREE 2008

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

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

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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