ARTICLE IN PRESS Biomass and Bioenergy 31 (2007) 426–432 www.elsevier.com/locate/biombioe Black carbon sequestration as an alternative to bioenergy$ Malcolm Fowlesà 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 $A prior version of this paper was published in Renew issue 165, Jan–Feb 2007. mostly associated with the pumping of CO2 into imperme- ÃTel.: +44 0 1908 652105; fax: +44 0 1908 653718. able rock formations, the gas being collected pre- or post- E-mail address: [email protected]. combustion of fuel, as in the working example of Norway’s 0961-9534/$ - see front matter r 2007 Elsevier Ltd. All rights reserved. doi:10.1016/j.biombioe.2007.01.012 ARTICLE IN PRESS M. Fowles / Biomass and Bioenergy 31 (2007) 426–432 427 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.