Black Carbon Sequestration As an Alternative to Bioenergy$

ARTICLE IN PRESS

Biomass and Bioenergy 31 (2007) 426–432 www.elsevier.com/locate/biombioe

Black carbon sequestration as an alternative to bioenergy$

Malcolm FowlesÃ

Department of Technology Management, The Open University, Walton Hall, Milton Keynes, MK7 6AA, UK

Received 25 November 2005; received in revised form 16 January 2007; accepted 16 January 2007 Available online 6 March 2007

Abstract

Most policy and much research concerning the application of biomass to reduce global warming gas emissions has concentrated either on increasing the Earth’s reservoir of biomass or on substituting biomass for fossil fuels, with or without CO2 sequestration. Suggested approaches entail varied risks of impermanence, delay, high costs, and unknowable side-effects. An under-researched alternative approach is to extract from biomass black (elemental) carbon, which can be permanently sequestered as mineral geomass and may be relatively advantageous in terms of those risks. This paper reviews salient features of black carbon sequestration and uses a high-level quantitative model to compare the approach with the alternative use of biomass to displace fossil fuels. Black carbon has been demonstrated to produce significant benefits when sequestered in agricultural soil, apparently without bad side-effects. Black carbon sequestration appears to be more efficient in general than energy generation, in terms of atmospheric carbon saved per unit of biomass; an exception is where biomass can efficiently displace coal-fired generation. Black carbon sequestration can reasonably be expected to be relatively quick and cheap to apply due to its short value chain and known technology. However, the model is sensitive to several input variables, whose values depend heavily on local conditions. Because characteristics of black carbon sequestration are only known from limited geographical contexts, its worldwide potential will not be known without multiple streams of research, replicated in other contexts. r 2007 Elsevier Ltd. All rights reserved.

Keywords: Carbon sequestration; Black carbon; Fossil fuel displacement

1. Introduction burning of biomass from Earth’s biological reservoirs, primarily by clearing forests and other ecosystems. The potential of global warming to destroy civilised The scale on which our activities release global warming society is now thought to be so great that it requires urgent gases is so vast, and our ability to cut these releases is so responses. This paper evaluates one of the less well-known constrained, that no one remedy is enough to address more and less researched responses. than a small fraction of the problem and our overall Mankind’s chief contribution to global warming is the response must be cumulative. Most remedies mooted to release of carbon into the Earth’s atmosphere, in the form date have focused on avoiding unnecessary releases of of the heat absorbing gases carbon dioxide (CO2) and carbon, for example by energy efﬁciency measures, and on methane (CH4). The primary release mechanism is the substituting fossil fuels with renewable energy sources, extraction and burning of fossil fuels from Earth’s including bioenergy. Relatively few have focused on permanent geological reservoirs (‘geomass’ for short). A returning carbon to the Earth’s reservoirs, or ‘carbon second but important mechanism is the decomposition or sequestration’. Sequestration in biomass has received more world attention; much debate over the Kyoto climate change treaty has been about whether and how to account for it, for example [1]. Sequestration in geomass has been $A prior version of this paper was published in Renew issue 165, Jan–Feb 2007. mostly associated with the pumping of CO2 into imperme- ÃTel.: +44 0 1908 652105; fax: +44 0 1908 653718. able rock formations, the gas being collected pre- or post- E-mail address: [email protected]. combustion of fuel, as in the working example of Norway’s

Sleipner West gas field [2]. When collected from combusted However, the evidence to be reviewed below suggests that it biomass, the net effect would be to remove CO2 from the is quite the opposite. atmosphere [3]. Sequestration of biomass within geomass Black carbon sequestration in soil follows a pathway has also been proposed [4], in the hope that its carbon that occurs naturally in the Earth’s carbon cycle, in which would not return to the atmosphere by the usual pathway some residue from forest and grassland fires is not fully of decay. Proposals for sequestration in geomass appear to burnt, rather it is carbonised and enters the soil. This may be subject to unresolved risks of high cost, impermanence be an appreciable global carbon sink [5]. Many soils and unknowable side-effects. around the world contain relatively high concentrations of This paper examines a further variation on carbon black carbon (some even above 20%) as a result of natural sequestration in geomass that tackles the issues of urgency, fires, with no apparent ill-effects [6]. cost, impermanence and side-effects. Elemental carbon There are also man-made soils with high concentrations (or ‘black carbon’) can be extracted from biomass and of carbon. The prototypical example occurs as large sequestered in soil. In comparison with carbon capture and pockets of permanently fertile soil known as terra preta storage after biomass fuel combustion, this likewise do Indio, which means Indian black earth, in the otherwise removes CO2 from the atmosphere, but in contrast: desperately poor (leached and acid) soil of the Amazon region. For a review, see [7]. The black earth was created it is based on relatively mature technology; long ago by the practices of generations of farmers, and in it is inherently cheaper per tonne of carbon thanks to a the hundreds of years since they and their farming culture shorter processing chain; succumbed to introduced diseases, the soil has remained it transforms the carbon itself into geomass, rather than fertile. One site has been dated at about 6850-years old [8], using geomass as a finite storage location; illustrating how stable and permanent the carbon seques- its side-effects are accessible to testing and are beneficial tration effect can be. The black earth sites taken together in some contexts. illustrate the scale on which sequestration can be imple- mented even without modern technology, if a population is suitably mobilised. Research into soil black carbon is active and growing, Artificial conversion of biomass into black carbon uses but the focus is not yet on sequestration, rather on benefits the technique of pyrolysis, which is best known for to agriculture and their physical and biochemical founda- converting wood into charcoal. Varied technological tions. Further, it has not addressed the potentially designs for pyrolysis can convert other organic materials significant influence of varying local conditions. Therefore, such as crop residues like straw, prunings and nut shells, this paper seeks to identify where sequestration researchers and domestic and sewage waste [9,10]. Pyrolysis works by might focus their efforts, and where local researchers heating the biomass in the absence of air, driving off many around the world might contribute. constituent parts such as hydrogen and oil tars, but, Black carbon sequestration from biomass has not been crucially, leaving some carbon behind in solid form rather widely considered by policy makers as a means of than releasing it in global warming gases. The parts that mitigating atmospheric carbon. For example, no mention are driven-off can be collected for use, usually as fuels or is made of it in the more obvious sources such as chemical feedstock. They are not collected in traditional parliamentary and governmental online archives. It is methods, but appear to be innocuous at that scale. The overshadowed on the sequestration side by the prospect of Kayapo people of the Xingu River still create a version of CO capture and storage from power station emissions, 2 terra preta, known as terra mulata. According to a and on the biomass side by the prospect of bioenergy with researcher who witnessed it, their method ‘‘y wasn’t negligible net emissions. This paper seeks to compare it catastrophic burning. It was that the whole landscape was with these ideas. smouldering all the time’’ [Susanna B. Hecht, UCLA, quoted in 11]. 2. Black carbon sequestration Numerous studies have observed that a soil containing black carbon can be a good agricultural soil. Among recent In order to achieve permanent sequestration, carbon reports, [6,12] between them cover the performance must be placed out of harm’s way. Black carbon in attributes in the following list: particular is at risk of burning or other chemical reactions that would release carbon within CO2 or methane. But Black carbon increases soil fertility through at least two where is out of harm’s way? Numerous locations might be mechanisms. The first improves cation exchange capa- identified such as in soil, in landfill, deep underground or at city, which in effect means that plants can take up sea. As with any form of sequestration, we would need to nutrients more easily. be sure that it is indeed permanent, and that any side- In the second mechanism, nutrients bind to the carbon effects are acceptable given the objective being pursued. In in such a way that rainfall washes less of them from the the latter respect, sequestration by adding black carbon to soil. Together with the first mechanism, this effectively soil may seem superficially like environmental pollution. increases the productivity of fertiliser. It incidentally ARTICLE IN PRESS 428 M. Fowles / Biomass and Bioenergy 31 (2007) 426–432

reduces leaching of nitrogen into the water table, a A second value stream would come from eliminating serious problem of intensive agriculture. costs associated with waste biomass, such as transport, There may also be a third mechanism in which black landfill and processing. Black carbon can be both made carbon provides an environment for the proliferation of and sequestered locally to the source of biomass; it can soil micro-organisms. By this mechanism, it is believed even be spread on the land whose plants collected it. that terra preta may perpetuate and even regenerate Even if sequestered elsewhere it is still cheaper to itself. transport, being more compact than biomass, and it can Black carbon improves the water retention capability of be further compressed if it is to be buried. If agricultural a soil. benefits are sought, black carbon should probably be Black carbon neutralises acid soil. mixed with fertiliser or compost for best effect, in which Black carbon significantly reduces the release of CH4 case the marginal cost to spread it may be nearly nil. and N2O (nitrous oxide, another almost equally A third value stream depends on technology to capture important global warming gas) from natural decay other products of pyrolysis, such as hydrogen, tar and processes in the soil. acetic acid, for which there may be an economic use [15]. A form of this technology is already found in industrial- Observations of terra preta and in laboratories suggest scale charcoal-making equipment, for example [16], that black carbon is very stable in soil. It works its way where its roles are to recycle gases to power the process downwards, increasing the fertile depth, but little of it and thus minimise the burning of feedstock or extra- vanishes in chemical reactions. This makes it effectively a neous fuel; to reuse waste heat, notably to dry feedstock permanent carbon reservoir. In the context of global and thus to maximise the efficiency of pyrolysis; and to warming, permanence and stability are the primary limit polluting emissions. This stream carries a risk that measures of sequestration’s performance. The potential if the profit on other products is high, the process will be agricultural advantages listed above are, in comparison, sub-optimised with respect to carbon sequestration. secondary. However, they may be very desirable to farmers The remaining value streams depend on how much the and others who would need to be engaged in the process of agricultural benefits of black carbon can be realised. carbon sequestration in soil. They may also be desirable to Wherever they can be realised, there may be value from those who want to halt or reverse the loss of soil fertility. the improved productivity of fertiliser and compost, Therefore, agricultural benefits may significantly influence from reduced nitrogen leaching, from reduced water the amount of crop directed to pyrolysis rather than demand, and from the recovery of depleted land that alternative uses. In which case, research is needed into how would in turn allow more plants to be grown to the benefits accrue. sequester more carbon. This last issue has global The permanent sequestration of black carbon in soil may implications for the recovery of depleted land in the attract several value streams: developing world. There may be another value stream from global carbon markets if reduced emissions of greenhouse gases from natural soil processes can be One value stream is payment for carbon sequestration verified. under the Kyoto protocol or from other markets in carbon offsets. Even small-scale producers can benefit if The feasibility and effectiveness of the permanent they join an aggregation scheme. The value of offsets sequestration of black carbon in soil may vary significantly has varied from a few euros per tonne of carbon to 1 between locations. The agricultural benefits of black about 80 at the time of writing. For the future, it has carbon are real enough in the warm and humid Amazon been suggested that payments should reflect the social region and in laboratories, but we do not yet know how cost of climate change, which would vary by location; much they depend on their context. Local physical for example, one estimate for the UK suggests about 100 conditions vary in terms of feedstocks, types of soil, euros per tonne [13]. Whatever the scale at any one climates, processes used for pyrolysis, and so on. For moment, black carbon would probably command the instance, municipal waste seems unlikely to be a suitable top price, because it is a permanent store and its feedstock for agricultural carbon due to its contamination sequestration is easily and cheaply verified [14]. All with heavy metals. Local economic conditions vary in sequestration offsets must be verified against fraud and terms of the availability of biomass residues, the returns incompetence, for example, to prevent multiple claims associated with alternative uses such as charcoal as fuel, the and the plunder of already valuable reserves in trees and costs associated with waste processing, and so on. Research hedgerows. Research is required into suitable forms of across the world could therefore focus on the effects of verification scheme for black carbon. pyrolysis and sequestration on a large scale in local conditions. 1The futures price of EU emission allowances averaged around h22.50 in October 2005 (data from EEX spot market). This equates to h82.50 per Local variations in degrees of forestation and cultivation tonne of carbon, as EUAs are priced per tonne of CO2. This in turn make calculations of the scale of the contribution of black equates to h24.75 per tonne of biomass assuming 30% carbon extraction. sequestration difficult to generalise. In the UK for example, ARTICLE IN PRESS M. Fowles / Biomass and Bioenergy 31 (2007) 426–432 429 about half of biomass residue is crop straw, other smaller become more complex. An obvious example of this is the components being forestry waste, municipal organic waste, use of biomass to generate renewable energy. and sewage waste [13]. In North America and Nordic In the context of global warming, the effective difference countries for example, forestry waste is far more significant between bioenergy and sequestration is measured by their and abundant. Most plant biomass typically contains from impacts on atmospheric carbon. In both cases, the carbon 45% to 50% carbon by weight when bone dry [19].A in biomass has been absorbed from the atmosphere, and skilful traditional charcoal maker might aspire to extract there is an assumption that subsequent growth will replace 25% from wood [20], whilst modern equipment makers what is harvested. In the case of bioenergy, the carbon is claim, perhaps optimistically, up to 35% [9,10]. Taking a released back to the air, but it substitutes for carbon in reasonable target of 30%, the example of the UK straw fossil fuel that does not then need to be released from crop of over 12 Mt in 20042 represents 3.6 Mt of geomass. These two quantities are not necessarily identical, technologically (if not economically) available carbon. however, because different fuels and different technologies Human activity in that country releases about 150 Mt of release different amounts of carbon to produce the same carbon each year from all sources, supposedly reducing to amount of energy, and because of ‘rebound’ effects in about 142 Mt by 2010 as a result of actions in progress [21]. which some biomass is treated as an extra supply rather Hence the entire country’s straw crop represents only 2.5% than a replacement. The case of sequestration is simpler: of the 2010 overall emissions forecast, or about 8.5% of the amount of carbon extracted from biomass is the emissions from electricity generation. This is not insignif- amount sequestered, as long as it is kept ‘out of harm’s icant when compared with other carbon mitigation actions, way’. In both cases, the carbon savings are reduced by a but regions with more available biomass and lower carbon quantity of carbon released by the process itself. emissions have much greater potential. To compare bioenergy and sequestration, a numerical There appear to be no examples in the developed world model was built in spreadsheet form, using assumptions of black carbon being used on the scale of even one pocket from the public domain [23]. It became clear that many of terra preta, though it does occur on a smaller scale in factors in the comparison are minor issues. Some factors traditional practices. Charcoal is well known as a soil are similar or identical on both sides; examples include the conditioner and as an additive to compost to keep it ‘sweet’ carbon released in cultivation and harvest (or their [20], and has market value as such. Equally traditional is equivalents for human wastes), in pre-processing steps the ‘smother’ that slowly consumes uncompostable garden such as the pulverisation of feedstock, and in gas cleaning waste, reminiscent of the Kayapo observation. One garden and other pollution control measures. For the same reason smother is pleasant enough alone, but if farms did the one can omit post-combustion CO2 capture as described in equivalent after a cereal harvest, without emission control [3], which could be applied to pyrolysis products. Other technology, the effect would be similar to stubble burning, factors differ but, from present knowledge, affect the and subject to safety, health and nuisance regulations [22]. results by small amounts; examples include releases from Furthermore, although the by-products of pyrolysis are transport to collect biomass (pyrolysis may release less well known [15], the best approach to managing them because its technology value chain is more suited to at- seems to be less so. These topics are prime candidates for source or near-source operations), also from transport to research into the influences of scale. dispose of end-products (both approaches dispose of ash, but of course the pyrolysis approach must in addition dispose of black carbon), and from the burning of starter 3. Carbon sequestration compared with bioenergy fuel. Excluding all these factors as constants for the moment (though they must be reintroduced for reasons Biomass residues have many potential uses. Therefore, given later) the primary comparison becomes relatively from an economic perspective, carbon sequestration from simple. biomass would have to compete with alternative uses for The comparison depends on three efficiency variables: the raw material. Sequestration may simply be delayed until after recycling, if alternative uses preserve the carbon The carbon extraction efficiency of pyrolysis, typically content. Otherwise, society is faced with a trade-off expressed as weight of output per weight of input, between sequestration and alternatives that return carbon measured as a percentage. A ‘reasonable’ figure was to the air. Our priorities in such trade-offs would in effect estimated above to be 30% from wood using modern be stated by the price of black carbon offsets on world retort technology, though more is claimed. markets. However, when the alternative uses of biomass The efficiency of energy generation in terms of conver- are likewise aimed at carbon reduction, the trade-offs sion of the energy content of the fuels concerned, also expressed as a percentage. It varies greatly with the fuel- 2 Estimated from observed sizes of 2003 and 2004 yields and planted technology mix. For electricity generation, 33% is areas [17] and observed ratios of straw to cereal [18]. This estimate, based on observations, seems safer than more optimistic figures that are regarded as impressively high for biomass combustion occasionally published, and underlines the limited biomass resource in (less than 30% is more typical) [24]; wood co-fired with densely populated regions. coal may reach 35%. Efficiencies up to 45% are ARTICLE IN PRESS 430 M. Fowles / Biomass and Bioenergy 31 (2007) 426–432

theorised for electricity generation by biomass gasifica- coal specifically at above-average generation efficiencies tion [25], but these projections have yet to be confirmed would be better than sequestration. by reliably operating power stations. In contrast, the However, the model is also sensitive to other variables. energy conversion efficiency (from either biomass or First, carbon savings decrease as the energy conversion fossil fuel) for heat or combined heat and power is efficiency for a displaced fuel increases. In Fig. 1, the upper typically rated at 80% or more by equipment suppliers. line for natural gas is based on the efficiency of electricity The relative ‘carbon penalties’ of the bioenergy fuel and generation in CCGT power stations, about 55%, while the of the fuel that it displaces. The penalty is the net lower line is based on the efficiency of gas heating or CHP amount of carbon released in delivering the energy systems, about 80%. In the latter case, the carbon saved by content of the fuel, expressed as a weight of carbon per displacement is well below the sequestration assumption, unit of energy, for example as tC GJÀ1. The carbon suggesting that it would be wasteful for biomass to displace penalties of fossil fuels include 0.024 tC GJÀ1 for coal, gas for heating. Second, carbon savings decrease as the 0.019 for burning oil, 0.016 for LPG, and 0.014 for associated carbon penalty falls. The penalty of the natural gas. The carbon penalty of biomass fuel is electricity background mix is presently falling with usually assumed to be zero because carbon release is the introduction of wind energy, and would fall faster with balanced by prior absorption. A small allowance ought other renewables and new nuclear build. Third, carbon to be made for process-related emissions, but it would be savings from bioenergy decrease as the calorific value of the balanced by a similar one for sequestration. biomass fuel decreases. Fig. 1 is based on the calorific value of wood, whereas savings from straw would be about one- ninth less, and from municipal solid waste over a half less. Fig. 1 compares estimated carbon savings. The horizon- The relative merits of black carbon sequestration may be tal line at a 30% saving represents the sequestration from understated in the comparison above because of additional biomass that should be achievable with modern technol- effects, including: ogy, as discussed above. Each sloping line represents savings due to bioenergy displacing a particular fuel or fuel Savings from the energy in the products of pyrolysis that mix, calculated by varying the efficiencies discussed above. remain after powering the process. For example, excess At face value, the model suggests that it would be better gas and waste heat may be used locally for space from a global warming perspective to sequester carbon heating. Excluded because technology- and situation- than to generate electricity from biomass, assuming the dependant. current and projected efficiency of generation (30–45%). Rebound effects that reduce bioenergy savings by On the other hand, the model also suggests that to displace generating additional demand. Rebound effects are common wherever undeveloped energy resources become available. In general, they are estimated to be low Sequestration to moderate [26] but even these levels are significant in Coal the present model and they may vary substantially Electricity background mix between locations. Excluded because situation-depen- Natural gas (55% efficient) dant. Natural gas (80% efficient) Savings from reduced CH4 and N2O emissions due to 60% black carbon in soil. Fifty percent reductions are typical of South American field trials, and although in 1 year they would be much smaller than the one-off sequestra- 50% tion saving, they should recur annually. Excluded because not demonstrated worldwide. Savings from other agricultural benefits of black carbon. 40% Excluded because not demonstrated worldwide.

The above discussion of soil black carbon addressed 30% issues of permanence and side-effects of sequestration. Two other issues were raised earlier, one being urgency, the 20% other being cost. Carbon saved per weight input With regard to urgency, the carbon market exists now, and technology exists for industrial-scale operations, at 10% least for wood biomass. Efficient technology for smaller 30% 40% 50% 60% 70% 80% scale operations is available but is not in widespread Efficiency of bioenergy generation production, and this matters because for urgent action the Fig. 1. Carbon savings from sequestration and from fuels displaced by population would have to be mobilised: farmers, foresters bioenergy. and communities would need to own and profit from local ARTICLE IN PRESS M. Fowles / Biomass and Bioenergy 31 (2007) 426–432 431 equipment, perhaps after the example of Denmark’s now permanent sequestration of black carbon in soil. Their world-leading wind energy industry [27]. The logical general thrust is to discover the feasibility and effectiveness conclusion for agricultural technology, on-field pyrolysis of such sequestration in local conditions. They include: and deposition, does not exist, but appears feasible because existing static equipment would fit easily within the volume The extent to which atmospheric carbon reduction and and power supply of a combine harvester. The one key agricultural benefits of sequestering black carbon in soil component that appears to be missing is an infrastructure can be realised in local conditions without unacceptable for the verification and aggregation of black carbon offsets side-effects. This must include knowledge of the lifecycle from highly distributed sources. greenhouse gas emissions of the possible variants of the With regard to cost, no quantitative analysis has been black carbon sequestration process. attempted. Qualitatively and intuitively, however, black The economics and organisation of a reward scheme, carbon sequestration in soil has advantages. It can be done probably based on existing carbon offset markets, to as locally as need be, wherever its feedstock is produced, at encourage permanent carbon sequestration activity. whatever scale, with relatively simple technology and (most The integration of permanent carbon sequestration into important) a short supply chain. It can avoid the problems local biomass residue processing. of seasonality and scattered geographical distribution that Technology for the safe and efficient local production of make the collection, transport and storage operations of a black carbon and other desirable by-products suitable bioenergy supply chain complex and expensive [25].Itis for use on a national scale. vastly cheaper than CO2 capture and storage [3], although the latter is not a competing alternative, rather an optional The paper has also compared the potential carbon addition to pyrolysis and most other thermochemical reductions due to permanent carbon sequestration from processing of biomass. Black carbon sequestration other biomass with those due to energy generation from biomass. than in soil, for example, burial in hazardous waste sites It suggests that sequestration has more carbon saving when the source biomass is contaminated, has the potential than electricity generation from biomass, because advantage of compactness. the latter is not efficient enough, but that the displacement The main economic doubt is not on the cost side, but on of coal in particular by biomass has more carbon saving the revenue side. It is not clear that the market price for potential than sequestration. The sensitivities in this carbon offsets is sufficient alone to divert resources. For comparison are such that in different conditions the point instance, the retail price of charcoal is much higher, which at which sequestration is preferable to energy generation would not be wasteful from a global warming perspective if could vary either way, sometimes quite significantly. Thus buyers use charcoal fuel to displace coal or burning oil, but even variables that are minor overall may be significant in this is not how charcoal is normally applied. Therefore, local contexts. Therefore, to analyse specific cases would black carbon sequestration’s advantages may fit it initially require the model presented here to be replaced with a to applications in which one or more of the additional more detailed methodology that embraces the variables value streams identified above can supplement the value that can be omitted from high-level comparison. from carbon offsets, especially where the biomass source is The renewable energy field mixes carbon reduction with otherwise uneconomic to use. These applications would a number of other policy objectives from land use to energy serve as niches to develop the technology and infrastruc- security, job creation to political ascendancy. Such ture. competing policy objectives are argued to have crippled biomass energy development [28]. Lest we make the same 4. Conclusions mistake with sequestration, let us not forget that it is atmospheric carbon that threatens social destruction, in This paper has described an under-researched strategy comparison with which our energy supply, waste and other for reducing global levels of atmospheric carbon, namely problems are mere nuisances. Policy makers need to be the permanent sequestration of carbon from biomass, and aware that energy supply is not the only way, nor perhaps in particular the sequestration of black carbon in soil. The the quickest or the most effective way, to deploy limited essence of the strategy is to use the Earth’s natural biomass resources to attack atmospheric carbon. Farmers, processes for absorbing carbon from the air, coupled with foresters and communities need to be aware that they may relatively simple conversion technology to move it into get a better return by processing their unused biomass permanent reservoirs in and under the Earth’s surface. resources themselves, rather than by depending on a There are encouraging indications from both scientific and downstream bioenergy supply chain. Hence to the above traditional knowledge that moving black carbon into the list of topics for research, we should perhaps add: soil reservoir has several additional benefits, especially for agriculture and for the recovery of depleted land, enhan- The complex interaction of social, technological, eco- cing rather than threatening the processes of life. nomic and policy interests that bear upon the success of The paper has identified several areas where local research permanent carbon sequestration from biomass as a would contribute useful knowledge towards a strategy of weapon against global warming. ARTICLE IN PRESS 432 M. Fowles / Biomass and Bioenergy 31 (2007) 426–432

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