Energy metrics of fuel juxtaposed with mass yield metrics

Without oxygen, there would be no combustion. Yet the crucial role of oxygen in energy systems has been largely underrepresented. The oxygen-dependence of fuel caloricity is used to derive energy-based metrics that challenge the prime facie objective of maximising biofuel mass yields—an objective that currently dominates the biofuel industry. Application of two energy metrics, namely the change in energy quality (ΔEQ) and the energy yield (ΔEη), demonstrates that any improvement in energy quality of combustibles must accompany increases in combusted oxygen (mO2), as ΔEQ=mO2|productmO2|feed−1, and that ΔEη is a function mass yield, η, and mO2: ΔEη=η(mO2|productmO2|feed). Literature data produced mostly positive ΔEQ values: biocrude achieved the highest (126.3%), followed by bioethanol (107.7%), and catalytic pyrolysis (78.3%). Most data produced similar changes in energy yield per mass yield, ΔEη/Δmη ranging from 0.7 for biodiesel to 1.6 for bioethanol. Carbon yields and overall mass yields are demonstrated to be poor metrics for biofuel synthesis, and could undermine biofuel quality, hampering progress for conversion technologies aimed at producing biofuels. Biodiesel mass yields in particular were found to be inflated by as much as 73%. As such, ΔEQ and ΔEη are useful for cross-field comparison of biofuels.