(538c) Bioenergy and Sustainability: An Engineer's Unit Operations/Mass Balance Approach

Biomass-based hydrocarbon fuels lack a quantitative, transparent, and flexible test if they can be produced and used sustainably. A rigorous and indisputable requirement for sustainability is that the mass balance around our planet is essentially closed. Therefore, we postulate that mass balance closure of sub-systems (say, the system including biomass production, conversion, and end use) is a good first measure of sustainability.

We generalize here for the first time the rigorous chemical engineering approach of unit operations interconnected by mass flows to integrate the ?upstream? (resource), conversion (processing), and ?downstream? (end use) sections of biomass-derived fuels through capture by an ASPEN® model. This approach brings the intellectual and technical rigor of the exceptionally successful chemical engineering unit operation/mass balance approach to bear on the entire biofuel production and utilization chain by defining unit operations such as "AIR", "SOIL", and "CROP". The example of carbon mass flows for ethanol or butanol production demonstrates that our approach allows recycle streams that are not part of life "cycle" analysis, such as quantitatively exploring if CO2 emitted can be sustainably converted in the intricate soil/air/biomass/water system. The level of sophistication of the new unit operations is essentially infinitely expandable as knowledge is or becomes available from the sciences and environmental research. This is possible through custom modeling within robust commercial simulation software. Our ability to exquisitely model chemical engineering systems such as entire refineries even at non-steady state demonstrates the promise of the approach.

The intricacies involving natural resources, processing, utilization, and public policies are then captured by combining the results of the enhanced unit operations approach with dynamic socio-economic modeling. This combination of the engineering unit operation/mass balance approach with socio economic modeling reaching from resource to end use is a completely new interdisciplinary approach to evaluate and improve sustainability.

Our approach puts sustainability on a quantitative and interdisciplinary footing. The unit operations/mass balances approach is easily expanded and refined to include any new knowledge of the physical world. Our approach will aid in breaking down the boundaries between resource management, processing, and end use by asking the same questions of the chemical engineer designing a process, the biologist improving a feedstock plant, an ecosystem researcher, and an engine designer: ?What are your inputs, what are your outputs? What can you say about how they are related??. Dynamic socio-economic simulation based on the mass balance/unit operations treatment of these questions then allows decision-making based on clear and unambiguous assumptions.