Carbon flows in bioenergy systems

There has been plenty of debate lately on the carbon neutrality of biofuels systems. This is a critical issue, since large-scale biofuel use is being touted as one of the ways by which the world can cut carbon dioxide emissions — now currently generated at the rate of about 25 billion tons per year — and thus help mitigate climate change. Even for a country such as the Philippines, which contributes a mere fraction of one percent of this global total, biofuels are a major part of efforts to increase energy security and independence. On one hand, articles that appeared in the journal Science last year (one by Fargione and coworkers, and another by Searchinger et al.) reported that biofuel production systems, if improperly managed, can actually increase carbon dioxide emissions; on the other hand, other papers have noted that under different conditions, biofuel systems can potentially give negative carbon dioxide emissions (see, for instance, the work of Tilman et al., also published in Science in 2006, and the paper by Matthews in Energy Policy earlier this year). Which is it, then? Do biofuels really reduce carbon dioxide emissions or not?

The short answer is that this question is as moot as asking if businesses lose or make money; either scenario can occur, depending specifically on how a business is managed. I think what is essential is a clear understanding of the underlying processes and issues, and from there, a logical progression to the proper evaluation of specific ways in which effective biofuel systems can be implemented. The first essential element of biofuel systems is photosynthesis, during which solar energy is converted into chemical energy in conjunction with the conversion of carbon dioxide into carbohydrates. Thus, photosynthesis by itself is a process that fixes carbon or removes it from the atmosphere. Biofuel systems take advantage of this phenomenon by deriving fuels (such as ethanol, charcoal or wood) from the products of photosynthesis. In general the useful energy is tapped by burning the biofuel. This results in two things: first, the carbon that was originally taken from the atmosphere during photosynthesis is returned to the air in the form of combustion products; and, second, the chemical energy that was derived from sunlight is released in the form of heat, which is then utilized directly or subsequently converted to generate electricity or do mechanical work. Thus, any useful energy derived from biofuel is, in a very real sense, derived from sunlight via a series of conversion steps.    

The issue of carbon neutrality thus revolves primarily around the manner in which the photosynthetic and combustion halves of the biofuel system balance out (just as the issue of whether or not a business makes a profit depends on the balance of its revenues and costs). And, as the saying goes, the devil is in the details. In principle, a biofuel system is inherently carbon-neutral if it is at thermodynamic steady state; that is, if plants are grown at exactly the rate needed to supply the production of the final biofuel product. Arguments against the carbon neutrality of such systems (whether on the positive or negative side) are based on a deviation from this steady-state assumption. For example, the groups of Fargione and Searchinger, whose papers I previously mentioned, point to how an increase in biofuel production capacity incurs a “carbon debt” when natural ecosystems are converted into farms to grow energy crops. Note that the carbon debt is an artifact of the increase of biofuel production rates. Such a debt occurs, for instance, when carbon fixed in soil and biomass is released into the atmosphere when large tracts of rainforests are cut down to grow additional biofuel feedstocks. However, it is worth mentioning that carbon debts are also incurred in other renewable energy systems; for example, building new hydroelectric dams or wind turbine farms also generates carbon emissions even before these systems become operational. On the other hand, the papers of Tilman et al. and Matthews describe conditions in which biofuel systems achieve additional carbon sequestration into the soil during the growth of biomass, which actually results in a net removal of carbon dioxide from the atmosphere.   

The point I am trying to make here is that, as with the business analogy that I have been using throughout this column, the actual extent of the benefits (or costs) that result from the use of biomass as fuel is highly dependent on the operational details. Thus it is not particularly useful, nor informative, to say that, in general, biofuels increase (or decrease) carbon emissions, unless the statement is qualified by a description of how the specific biofuel system operates. What is essential is to resist any reflexive reactions about the biofuel issue, but to use the scientific information to make sound judgments about how actual practices in the field can be guided toward increased sustainability.

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Dr. Raymond R. Tan is a full professor of chemical engineering at De La Salle University-Manila. He is the recipient of multiple awards from the National Academy of Science and Technology (NAST) and the National Research Council of the Philippines (NRCP) for his work on life cycle analysis and process integration at the Center for Engineering and Sustainable Development Research (CESDR). He has recently published a series of papers on carbon pinch analysis in the journals Applied Energy and Energy. Dr. Tan is also a member of the editorial board of Clean Technologies and Environmental Policy. For his contact details, research profile and publication list, visit www.geocities.com/natdnomyar/web.