1. Sustainability Metrics for Highly-Integrated Biofuel Production Facilities
H. Dennis Spriggs, Benjamin J. Brant, Daniel L. Rudnick
Byogy Renewables, Inc. 150 Almaden Blvd, Suite 700, San Jose, California 95113
Kenneth R. Hall and Mahmoud M. El-Halwagi
Artie McFerrin Department of Chemical Engineering
Texas A&M University
College Station, Texas
Introduction
Impact of Expanding Bio-fuel Industry
First generation bio-fuels may have increased food costs and caused a net increase in greenhouse gas (GHG) emissions.
Meeting the renewable fuels standards in the Energy Independence and Security Act of 2007 by 2022 will change land use indirectly, with new lands opened to replace corn diverted to fuel uses.
Continued concern over the degree of energy dependency on foreign sources of petroleum.
Motivation
Measuring Sustainability of Bio-fuels
No standard exists to measure sustainability of bio-fuels impacting “triple bottom line” (People, Planet, Profit).
Difficult to compare bio-fuel processes.
How to measure performance in terms of energy efficiency, carbon footprint, water use?
How large a circle is necessary to assess impact or performance?
Metrics should include the complete “value chain” of developing bio-fuels from “Crop to Wheel”.
Figure 1 – Bio-fuels Cycle
Examples of tools: 1) Life Cycle Assessment (LCA); 2) Exergy; Mass intensity, 3) Ecological input/output analysis, 4) Energy Efficiency Ratio
Enhancing Energy Efficiency
To improve the energy efficiency metric of the process, energy and mass integration techniques can:
Induce heat integration to minimize heating and cooling utilities
Employ cogeneration to optimize the heat and power aspects of the process
Recycle byproducts and waste streams to reduce energy needs
Use process modification and mass integration to reduce energy requirements
Use stream re-allocation to enhance energy efficiency
Use process products instead of external energy inputs
Impact of Closing the Energy Loop
Fig. 2. Closing the Energy Loop via Process Integration
Higher levels of integration can eliminate external energy:
If the metric becomes infinity has sustainability improved? Not necessarily! Closing the energy loop can worsen the impact upon the environment.
Incremental Return on Sustainability
Inspired by Incremental Return on Investment
Can assess the impact of integration by comparing the change in environmental impact from a nominal case to the integrated case by changing the net energy use.
Acceptable energy reduction must meet a minimum IROS that guarantees some level of performance.
Besides energy integration, IROS has other applications: (1) alternative pathways and (2) extent of processing. When biomass can have other process pathways (biochemical, thermal, etc.) or variations of one technology, IROS can compare the most energy-demanding alternative to others based upon the difference in energy and sustainability metrics.
Another category is extent of processing that decides if additional process steps lead to better energy outputs that are sustainabile.
CONCLUSIONS
No standard method assesses sustainability of bio-fuel plants
Sustainability metrics need a “big-picture” approach
Integration may drive energy efficiency to infinity without improving sustainability
We introduce IROS as a metric that can handle
Integrated processes without net energy input
Comparing various feed-stocks and pathways
Determining optimal extent of processing