Energy Policy 36 (2008) 940945

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Carbon-negative biofuels

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Keywords: Carbon-negative biofuels; Biochar; Bioenergy with carbon capture and storage

burning replaces the carbon absorbed during the growing most straightforward means of doing so is through

ARTICLE IN PRESS

www.elsevier.com/locate/enpol

E-mail address: John.Mathews@mq.edu.au1For recent reviews and overviews, see for example Ulgiati (2001),

and Storage, available at http://arch.rivm.nl/env/int/ipcc/pages_media/

SRCCS-nal/IPCCSpecialReportonCarbondioxideCaptureandStorage.htm0301-4215/$ - see front matter r 2007 Elsevier Ltd. All rights reserved.

doi:10.1016/j.enpol.2007.11.029

Puppan (2002), and for a comprehensive evaluation of energy balances,

Hill et al. (2006). For a recent cautious ofcal overview of biofuels for

transport, jointly from the IEA and OECD, see OECD (2007).

and the draft special report prepared under the auspices of the UNFCCC

by Chris Hendriks, available at http://unfccc.int/les/cooperation_and_

support/nancial_mechanism/application/pdf/hendriks.pdf.of the crop. A furious debate is currently underway overthe real degree of carbon neutrality achieved by biofuels,with some produced in temperate climates such as NorthAmerica and Europe being argued to be almost entirelycarbon positive, due to the heavy inputs of fossil-fuelledactivities and products such as fertilizers.1

production of biocharsetting aside a portion of thebiomass and, instead of converting it into fuel, reducing itto biochar through pyrolysis. It is a strategic choice howmuch of the biomass to convert into biofuel and how muchinto biochara choice that can be made by farmers andfuel producers.

Tel.: +61 2 9850 6082; fax: +61 2 9850 9942. 2On this, see the IPCC special report on Carbon Dioxide Capture1. Introduction

The world is on the threshold of an epochal transitionfrom the petroeconomy, fuelled by carbon from the past, tothe bioeconomy, fuelled by biomass created throughphotosynthesis. The petroeconomy is overwhelminglycarbon positivecontributing carbon to the load alreadyimposed, and increasing at the rate of approximately 8gigatonnes per year. The bioeconomy is widely consideredto be carbon neutral, in that the carbon emitted through

Yet there is a way to sweep away this controversy, andsettle the issue in favour of biofuels for evermoreand thatis to make them carbon negative. Biofuels that are carbonnegative are fuels that remove more carbon from theatmosphere than they put back in through burning. How isthis achieved?Any biofuel that draws carbon from the atmosphere

during the growing of the biomass (by photosynthesis) canbe rendered carbon negative by returning a portion of thebiomass to the soil in more or less permanent form.2 TheJohn A.

Macquarie Graduate School of Management, M

Received 16 November 200

Available onlin

Abstract

Current Kyoto-based approaches to reducing the earths greenh

these are palliative at best, and at worst will allow the problem to ge

that the problem can be solved. Carbon-negative biofuels represen

already technically feasible and practicable. The key to carbon ne

strategic choice, whereby farmers and fuel producers can decide ho

not only sequesters carbon but also enhances the fertility and vital

negative by denition, and, as such, they will sweep away existing

r 2007 Elsevier Ltd. All rights reserved.athews

uarie University, Sydney, NSW 2109, Australia

ccepted 21 November 2007

January 2008

se gas problem involve looking for ways to reduce emissions. But

ut of hand. It is only through sequestration of atmospheric carbon

he rst potentially huge assault on the problem, in ways that are

tivity is to see it not as technically determined but as an issue of

much carbon to return to the soil. Biochar amendment to the soil

of the soil. The time is approaching when biofuels will be carbon

bates over their contribution to the solution of global warming.

paid so far to reducing carbon emissions, through the useof alternative energy sources and renewable fuels that arelow-carbon in their effects and which lead to a low-carboneconomy. Every gigatonne of saved emissions counts. Butsaving or reducing emissions levels, through improvementsin energy efciency, switching to non-carbon pollutingsources of power generation and to carbon-neutralbiofuels, can only go so far to alleviate the problem,slowing the catastrophe but not eliminating its source.Only carbon sequestration, or direct removal of carbonfrom the atmosphere, can accomplish that. Carbonsequestration, or CCS has so far focused on such mattersas reversing land clearing (clearing releases carbon stored

ARTICLE IN PRESS

8 Gt go ineach year

31Gt is 1 billion tonnes. Emissions expressed in units of carbon can be

converted to emissions in units of CO2 by making an adjustment in terms

of molecular weights: carbon is 12, while CO2 is 44, and so the ratio

between the two sets of units is 44/12. Thus, 1Gt carbon is equivalent to

3.67Gt CO2. In volumetric terms, 1 ppm carbon is equivalent to 2.1Gt

Policy 36 (2008) 940945 941There are many other options for carbon sequestra-tionas demonstrated by the work currently underway inclean coal technology. Here the approaches associatedwith carbon capture and storage (CCS) can be applied notto fossil fuels but to biofuelsthereby again making themcarbon negative. The various geosequestration optionsbeing investigated include disposal of carbon dioxide (CO2)down mineshafts, down oil wells, and in various kinds ofgeological formations. The irony is that coal technologycan never be clean, in the sense of being carbon negative. Infact, it can never even achieve carbon neutrality, since thisis a theoretical optimum, which could never be achieved inpractice, given that fossil fuels would need to be input atvarious steps of the value chain.These approaches to carbon sequestrationboth biose-

questration and geosequestrationwhen applied to bio-fuels have the potential to draw more carbon from theatmosphere than is emitted through their use as fuel.They are technically feasible and in some cases alreadyeconomic.In this contribution I link biofuels to global carbon

accounting, to make the point that what determineswhether a biofuel is carbon negative is not technicalconsiderations so much as strategy. It is open to a producerof biofuels to ensure that the fuel as supplied to the markettakes more carbon out of the atmosphere than is returnedthrough burning the fuel. This decision can be takenstrategically, depending on prevailing policy settings. Aproportion of 90:10 might maximize the fuel productionbut fail to render it carbon negative; a proportion of 60:40might be erring on the side of caution in under-producingbiofuel and over-producing biochar; and a proportion of75:25 might be optimal, in the sense of producing a strongamount of biofuel but ensuring that it is carbon negative.Carbon negativity is dened in technical terms (in terms

of eld to wheel calculations) but it is not determinedtechnically. It is rather a strategic choice on the part of abiofuels producer to produce a fuel that is carbon neutralor carbon negative. Thus, the debate over whether biofuelsare carbon neutral, or the degree to which they may be saidto be carbon neutral, in one sense misses the point. Thedegree to which a biofuel is carbon negative is a question ofstrategic choice, pure and simple.

2. Global carbon accounting

The reason that there is so much concern over atmo-spheric carbon pollutionas opposed to carbon build-upin the earth or seais that the atmosphere is a highlysensitive layer in which all life survives. Over geologicaltime, levels of carbon in the atmosphere have uctuated,but a pre-industrial level is reckoned by the Intergovern-mental Panel on Climate Change (IPCC) as being around600 gigatonnes (Gt), or 285 ppm. Since then the level hasbeen rising remorselessly, through the burning of fossil

J.A. Mathews / Energyfuels, by transport and industry. Think of the atmosphereas a bathtub. The current carbon level is 800Gt, or380 ppm. Around 8Gt is being added each year from theburning of fossil fuels and industrial processes such asenergy and cement production, while around 2Gt is beingabsorbed by the ocean (leading to a slow acidication) and2Gt by the earth, meaning that there is a carbon ux ornet carbon addition of 4Gt per year.3 According to abusiness as usual scenario, the carbon ux is likely togrow to 10Gt per year by 2025 and perhaps to 15Gt peryear by 2050. By this time the carbon level in theatmosphere would be approaching 1000Gt, which isreckoned by the IPCC scientists as a tipping point beyondwhich climate changes would be triggered that arecatastrophic and irreversible. Al Gore and others havebeen warning that the world has to do something, andquickly, before such a tipping point is reached (Chart 1).

3. Emissions reduction or direct reduction?

Under the Kyoto Protocol, almost all attention has been

600 Gt (Pre-Industrial)

800 Gt (Today)

1000 Gt (Tipping point)

2 Gt Ocean 2 Gt Land (net)

Chart 1. The atmosphere as bathtub. Source: Adapted from Princeton

CMI, available at www.princeton.edu/cmi/resources/CMI_Resources_new_les/CMI_Wedge_Game_Jan_2007.pdf.carbon. For a discussion of global carbon accounting and options for

reducing carbon emissions, see Pacala and Socolow (2004).

in the soil and in organic matter such as trees) and onretaining native forests, particularly rain forests. Or CCSfocuses on direct pumping of CO2 into the earth or theocean, treating these as the ultimate sinks. So far theUNFCCC and Kyoto only recognize reversal of landclearing as well as industrial reductions in carbon emissionas well as reductions in emissions of other greenhouse gasesas sources of carbon credits suitable for trading.The time is approaching when the UNFCCC and the

Kyoto Protocol procedures will likely come to recognize amuch broader class of carbon sequestration processes,some of which are capable of storing carbon in inaccessibleand sometimes highly benecial ways for long periods oftime, and in ways quite different from the pumping of CO2down mineshafts. It is on these alternative pathways ofcarbon sequestration, with their potential to earn carboncred

4. Mbiof

are

(1)

(2)

burnt (and in practice will always be at least somewhat

4.1. Biochar

The most straightforward and attractive idea forbiosequestration is the recycling of part of the biomassproduced in the form of charcoal, or biochar. Biochar isproduced by a process of slow burning of biomass in theabsence of oxygen (slow pyrolysis). There is an alternativeof fast pyrolysis where the biomass is exposed to a hightemperature (in excess of 500 1C) for a few seconds, but thishas largely been focused on the production of gases orliquids as fuels, rather than on biochar.4 Either way, it isthe addition of black carbon to soil that provides themeans of permanently sequestering the carbon. But thisprocess turns out to have an array of benecial effects, andthat are now being discussed in a growing and lively

5

ARTICLE IN PRESSJ.A. Mathews / Energy Policy 36 (2008) 940945942(Chart 2).

Carbon Positive Carbon Neutral Carbon Negative

Chart 2. Carbon-positive, -neutral and -negative systems. Source:carbon positive depending on the fossil fuels used intheir production and transport and

(3) carbon-negative fuels absorb CO2 as they grow andrelease less than this amount into the atmosphere whenused as fuel, either through directing part of thebiomass as biochar back into the soil or through CCSAda

stran2

atmosphere;carbon-neutral fuels absorb CO2 as they grow andrelease the same carbon back into the atmosphere whenhe technical means of turning biofuels carbon negativealready available. The differences are clear:

carbon-positive fuels are drawn from fossil fueldeposits, and are burned releasing CO into theTits, that debate should be focused.

echanisms for sequestering carbon associated withuelspted from Biopact.com, available at http://biopact.com/2007/10/

ge-world-of-carbon-negative.html.Energy Act 2007. The content of the Bill can be found at http://www.

biochar-international.org/images/S.1884_Salazar_Harvesting_Energy_Act_literature. It turns out that biochar increases the fertility ofthe soil, not in the form of organic carbon, but in the waythat a coral reef increases the nutrients available to biota inthe sea. Microorganisms that x nitrogen, for example, areencouraged by the addition of biochar, and it has quite aspectacular impact on reducing the release of othergreenhouse gases such as nitrous oxide.6 Thus, soils thatare being impoverished by conventional fertilizer-drivenagriculture have the chance to be regenerated throughproduction of biofuels combined with biochar amendmentto the soils.In terms of atmospheric carbon sequestration, Lehmann

and others believe that gigatonnes of carbon can beremovedup to 4Gt per year, or as much as the carbonux currently created through burning of all fossil fuels.There is already a legislative initiative in the US Congressto channel federal support towards biochar initiatives.7

4.2. Bioenergy with CCS: BECS

Another straightforward option is to apply the range ofCCS techniques developed in relation to clean coaltechnology and apply them to the case of biofuels usedto produce electric power. This has been explored under

4See for example the review by Bridgwater et al. (1999). The

ArgentinianCanadian company Dynamotive is currently building fast

pyrolysis plants, produced for it by the engineering services company

Tecna.5The Cornell group led by Johannes Lehmann is prominent in this

regard; see for example Lehmann (2007a ,b) as well as Lehmann et al.

(2006). For a popular and accurate introduction to the topic, see Renner

(2007).6The dramatic reduction in emissions of other greenhouse gases

achieved in soils amended by biochar is one line of response to the

ndings reported by Nobel Prize winner Paul Crutzen and colleagues

(Crutzen et al., 2007) regarding increases in nitrogen oxide emissions

associated with intensive fertilizer inputs.7The Bill introduced by Rep. Salazar is the Salazar Harvesting

of_2007.pdf.For a commentary on its biochar provisions, see http://www.biochar-

international.org/policyintheus.html.

the rubric of BECS by scholars associated with theAbrupt Climate Change group, amongst others.8

Geosequestration is not as kind to the earth asbiosequestration, and many of the technologies promotedunder the label of CCS (and hence of BECS)such aspumping CO2 into the deep oceansare frankly incredible.Geosequestration represents the hard path towardscarbon removal, while biosequestration represents what isbest described as the soft paththe forgiving, exible andbenign option.9

4.3. Algae and other photosynthetic approaches

Yet another avenue involves the use of photosynthesis togrow algae on a large scale, where a portion of the biomass

then the fossil fuel inputs to the process are negligible andthe carbon negativity is beyond contradiction.

ARTICLE IN PRESSJ.A. Mathews / Energy Policy 36 (2008) 940945 943yield is pyrolized to bio-oils and a part is set aside tocapture carbon credits. This is a future-oriented approach,not to my knowledge practised anywhere on a large scale.10

But the theoretical existence of such an option points toanother option nearer home, namely current advancedproduction methods for sugarcane, where traditionalpractices such as burning off are dispensed with, andinstead the green tops of the cane are returned to the soilas the cane is harvested. This practice in itself creates anoption on carbon negativity, since only a part of the canesbiomass is being harvested and fermented to ethanol; thebalance is allowed to stay in the eld as organic carbon. Ifthese new practices are combined with organic approachesto cultivation, including dispensing with agriculturalchemicals in favour of biological control, and with powerfor the biorenery being provided through cogeneration,then the carbon negativity of the resulting ethanol wouldbe assured.11 This is current best practice in Brazil; evenwithout the addition of biochar these practices would, ifproperly computed, be revealed as carbon negative whenthe carbon absorbed by the total biomass grown (perhectare) is contrasted with the carbon released as the fuelproduced (per hectare) is burnt. If soil amendment withbiochar is added, as well as cogeneration using bagasse,

8See for example Read and Lermit (2005) for an overview, as well as

Mollersten et al. (2003) and Azar et al. (2006). Authors such as Kraxner

et al. (2003) discuss the concept of BECS in the case of biomass

production by semi-natural temperate forests, while Tilman et al. (2006)

discuss it in relation to prairie grasses. For an overview of CCS generally,

in addition to IPCC and UNFCCC sources cited above, see Riahi et al.

(2004).9The hard and soft paths recall the distinction made by Amory

Lovins in his 1976 article in Foreign Affairs, Energy strategiesthe road

not taken (Lovins, 1976).10Under the rubric of algaculture these ideas have been pursued,

for example under the Aquatic Species Program of the National

Renewable Energies Laboratory. For a comprehensive report on the

achievements of the ASP, see Biodiesel from Algae: A Look Back at the

Aquatic Species Program, available at http://www1.eere.energy.gov/

biomass/pdfs/biodiesel_from_algae.pdf.11These practices can be found in advanced sugarcane sugar/ethanol

businesses in Sao Paulo, Brazilsuch as the organic sugar/ethanolbusiness run by the Balbo family, at Usinas SantAntonio. See the

description at http://www.commondreams.org/headlines06/0409-07.htm.5. Certication of carbon negativity

Clearly carbon negativity needs to be certied in someway. How is this to be accomplished? Sets of technicalspecications are starting to be produced, reecting publicconcerns over such matters as deforestation associated withbiodiesel production from palm oil. The most comprehen-sive of these sets of specications are those produced by theCramer Commission in The Netherlands, the worlds rstcriteria to be given the imprimatur of a government.12 Suchcriteria only become of practical signicance if they areincorporated into standards or specications. This is wherethe UNFCC and in particular its Executive Committeeestablished to implement the Clean Development Mechan-ism can play a role. Another avenue would be throughcommodity exchanges eventually implementing such criter-ia in their biofuel futures contracts.How could carbon negativity be reconciled with the

common practice of blending of fuels from different origin,engaged in by the global trading rms that deal in ethanol,biodiesel and increasingly in biofuels more generally?Without changes to the way in which biofuels are sold,no such reconciliation would be possible. But if biofuels aresold through commodity exchanges with a certicationattached as to their point of origin, then fuels fromdifferent origins could be blended and the commoditiestrader would be enabled, and required, to keep track of thesources of the blended fuel.Obviously what will drive the widespread adoption of

carbon-negative biofuels will be the carbon credits theypotentially attract.

6. The role of carbon credits

The global carbon credit economy will eventually (andsooner rather than later) come to mirror the economy ofphysical transformations. Each carbon credit will represent1 tonne of carbon either removed from the atmosphere(through sequestration) or saved in the sense that it is notreleased, i.e. a tonne of carbon avoided. Since around 8Gtof carbon are being added to the atmosphere each yearcurrently through the burning of fossil fuels, this representsa possible 8 billion carbon credits being created each year,rising under business as usual to as much as 10 billion peryear. At a price of US$10 per carbon credit, this representsa total pool of $100 billion; if the price rises to US$100 percarbon credit (predicted by many) then the total wouldrepresent a pool of $1000 billion, or $1 trilliona very

12The Commission was established by Dutch Environment Minister,

Jacqueline Cramer, and released its rst report in July 2006 (in Dutch):

http://www.snm.nl/pdf/1000_060714biomassarapportciecramerjuli2006.pdf.

For a news report on the Commission, see http://www.ctv.ca/servlet/ArticleNews/story/CTVNews/20070428/dutch_biofuel_070428?s_name=

&no_ads.

large pool of funds. At this point, the carbon crediteconomy would be comparable in size to the current fossil

tionsor by speculators, betting that the price of the

7. Carbon negativity of biofuels: from strategic choice todening characteristic

I have argued so far that producers of biofuels can make

ARTICLE IN PRESSJ.A. Mathews / Energy Policy 36 (2008) 940945944credits will rise.

13On this topic, see for example the discussion with Laurens Rade-

makers of Biopact at Mongabay.com, Carbon-negative bioenergy to cutfuel economy.

6.1. Carbon credit multipliers

It is usually assumed that carbon credits will be createdin proportion 1:1 in relation to tonnes of carbon saved orremoved from the atmosphere through sequestration. Butthere is nothing to stop us from attaching priority tosources of carbon credits, through the use of carbon creditmultipliers. Let us assume that it is widely agreed that it ismore important to preserve rainforests, as an ultimatecarbon sink (not to mention a precious source of biologicaldiversity and of medicinals), rather than to allow people todestroy the rainforest and establish biomass plantations, inorder to benet from the carbon credits so created. So letus assume that the CDM Committee of the Kyoto Protocolsets a multiplier of 2 (or even 3) on the carbon creditscreated by preserving rainforests. In this way the gainsfrom preserving the forest intact would outweigh any gainspossible through growing plantations and producingbioenergy from them.13 The use of carbon credit multi-pliers would thus represent a means of imposing socialpriorities on the carbon credit economy. The carbon creditscould be exchanged as suchbut their creation could be inproportion to the degree of social signicance their sourcesare deemed to have. Societies already impose such socialchoices on the economy in the form of differential tax rates.What works for the old economy can be made to work forthe new.Indeed one of the principal functions of global arbiters like

the CDM executive committee set up under the KyotoProtocol will come to be seen as their capacity to expresspublic priorities in the way they set carbon credit multipliers.These multipliers will represent the common, global view ofwhat kinds of fuels are acceptable, and what kinds are not.There can be no retreat from the principle that carbon creditshave to be allocated strictly in terms of demonstrated carbonsaved or removed, but the allocation of multipliers is aseparate matter best left to properly constituted authoritiessuch as the CDM executive committee.The attraction of the idea of carbon credits is that they

can be created de novo, as an act of policy. Forests can bepreserved by an agreed process of allocating carbon creditsto their preservation. No money need change hands. Thevery act of creating the carbon credit thereby creates aninstrument of exchange; the credit so created can then besold on a carbon exchange, and bought by any party thatneeds credits to offset statutory carbon reduction obliga-global warming could drive deforestation (6 November 2007), available at

http://news.mongabay.com/2007/1106-carbon-negative_becs.html.a strategic choice as to how much of the biomass they raiseinto production of fuel and how much into carbonsequestrationsuch as through production of biocharand its permanent sequestration of carbon in the soil.14

While carbon negativity remains a strategic choice for abiofuels producer today, it is likely that in the relativelynear future (say, in 23 years) carbon negativity will bemade a condition of being accepted and defined as abiofuel. That is to say, for a fuel to be imported or tradedand sold in a market and recognized as a biofuel, it willneed to meet certain specications, of which carbonnegativity would come to be the most important.The imposition of carbon negativity as a condition for

labelling a fuel as a biofuel would be the simplest andmost transparent means of setting standards for sustain-ability and responsible production. It would dispense withall the fruitless arguments engaged in by NGOs and othershostile to biofuels over the degree to which they deviatefrom carbon neutrality.15 If fuels are carbon negative, bycontrast, then they will have to be seen as a contributionto a solution to global warming, not a contributor to theproblem.The overwhelming advantage of biofuels over petrofuels

or fossil fuels is the possibility of making them carbonnegative. Only carbon-negative fuels represent a solution tothe build-up of carbon in the atmosphere and its green-house effect. All other fuels are carbon positive, or at best,carbon neutral. In a world where carbon negativity is seento be realistic, practicable and economic, it will becomeoverwhelmingly desirable, and no doubt mandated by lawto become the norm. Under such circumstances, to burn acarbon-positive fuel and contribute carbon to the atmo-spheric load will come to be seen as socially and humanlyunacceptable.

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ARTICLE IN PRESSJ.A. Mathews / Energy Policy 36 (2008) 940945 945

Carbon-negative biofuelsIntroductionGlobal carbon accountingEmissions reduction or direct reduction?Mechanisms for sequestering carbon associated with biofuelsBiocharBioenergy with CCS: BECSAlgae and other photosynthetic approaches

Certification of carbon negativityThe role of carbon creditsCarbon credit multipliers

Carbon negativity of biofuels: from strategic choice to defining characteristicReferences