Correspondence TO BURY OR TO BURN: OPTIMUM USE OF CROP RESIDUES TO REDUCE ATMOSPHERIC CO2

We argue that sequestering of carbon waste is inherently more efficient and will probably cost less than using the carbon for biomass burning. The ratio of carbon emitted per unit of primary energy released through combustion, C/E, favors sequestration for fundamental reasons of chemistry and engineering, strongly suggesting a low C/E strategy. This is a more direct way of analysis than assigning dollar estimates to technologies of which we have as yet little or no direct experience. Crop residues can be a valuable resource for carbon sequestering and/or power generation. How should we assess the relative utilities of the two conflicting uses? We define residues as organic material remains left behind on fields after harvesting, such as corn stalks and husks. Often this bulk holds more carbon than the crop itself. During a growth season, crops store carbon from air, then exhale it as the crop residues rot, giving no net change in atmospheric CO2. Rather than letting this carbon rot, one has two choices that reduce net carbon return to the atmosphere: (a) burn crop wastes to generate electrical power. This returns all the carbon content to the atmosphere (the same as in just letting it rot), but the power generated lessens the need for power generation through gas/oil/coal-fired plants, thereby effectively reducing overall carbon emissions (Keith, 2001). (b) sequester the crop residue and its carbon in such locations as the deep oceans beneath the thermocline, thereby blocking it from reentering the atmosphere for at least centuries (Metzger and Benford, 2001). Because crop residues can be fuel, Keith (2001) suggests that sequestering wastes is inherently less effective than using it for biomass burning. He assigns dollar costs to the two methods, rather than following the tradeoff between carbon and power. His argument would only be true if the carbon emitted per unit of primary energy released through combustion, (C/E), was the same for all organic fuels. Corresponding author. Climatic Change 54: 369–374, 2002. © 2002 Kluwer Academic Publishers. Printed in the Netherlands. 370 ROBERT A. METZGER ET AL. Figure 1. Carbon emitted per unit of primary energy released by combustion (C/E) increases as the H:C ratio decreases – hence the interest in ‘decarbonization’ for carbon dioxide emission reductions. The lowest (C/E) in the fossil fuel family is that of methane, which can be computed from first principles from the heats of formation of reactants and products. In fact this is not even nearly so. The high (C/E) ratio of crop residue as a biomass fuel produces about twice as much atmospheric carbon, compared to a combination of (a) crop residue sequestering, plus (b) production of an equivalent amount of electrical power using a much lower (C/E) fuel such as gas (methane) in a conventional gas-fired plant. This emerges clearly by examining the (C/E) ratios for various organic fuels. The atomic structures of typical molecules of coal, oil and gas appear in Figure 1 (Ausubel, 2000a). The H:C ratio of coal, oil and gas (methane) are 0.55, 2.2, and 4.0, respectively. The lowest (C/E) ratios are from organic fuels with the highest H:C ratios. As an example, this can be computed from first principles for methane, using the heats of formation of reactants and products in Table I (Glassman, 1977). The heat of reaction for natural gas (methane) combustion can be written: CH4 + 2O2 → CO2 + 2H2O(g) , where from Table I the energy released during combustion of methane, DE, is the difference between the heats of formation of the reactants and the products: DE = [−74.9 − 2(0)+ 394 + 2(242)] kJ/mol = 803 kJ/mol. OPTIMUM USE OF CROP RESIDUES TO REDUCE ATMOSPHERIC CO2 371