The RBAEF project, which was launched in 2003, is the most comprehensive study of the performance and cost of mature technologies for producing energy from biomass to date. It seeks to identify and evaluate paths by which biomass can make a large contribution to energy services in the USA and determine how we can accelerate biomass energy use. In addressing these issues, the study has focused on future, mature technologies rather than today’s technology.

—Professor Lee Lynd, Dartmouth College

The RBAEF involves experts from 12 institutions, and is jointly led by Dartmouth College and the Natural Resources Defense Council and sponsored by the US Department of Energy, the Energy Foundation and the National Commission on Energy Policy.

Professor Lynd, from Dartmouth College’s Thayer School of Engineering, and a co-founder of Mascoma Corp., a company commercializing a cellulosic ethanol production process, is co-author of five of the eight papers in the special issue. Three of these papers are open access, including a paper in which Mark Laser and his colleagues carry out the comparative analysis.

The biomass refining scenarios considered include both biological and thermochemical processing with production of fuels, power, and/or animal feed protein. The emissions analysis does not account for carbon sinks (e.g., soil carbon sequestration) or sources (e.g., forest conversion) resulting from land-use considerations. The scenarios evaluated are:

1. Ethanol + Rankine power
2. Ethanol + gas turbine with combined cycle (GTCC) power
3. Ethanol + F-T fuels + GTCC power
4. Ethanol + F-T fuels (w/once-through syngas) + CH₄
5. Ethanol + F-T fuels (w/recycle syngas) + CH₄
6. Ethanol + H₂
7. Ethanol + protein + Rankine power
8. Ethanol + protein + GTCC power
9. Ethanol + protein + F-T fuels
10. F-T fuels + GTCC power
11. Dimethyl ether + GTCC power
12. H₂ + GTCC power
13. Rankine power
14. GTCC power

The analysis compares the technologies in their mature context—i.e., a state of advancement such that additional R&D effort would offer only incremental improvement in cost reduction or benefit realization.

...performance parameters were selected consistent with the above operational definition according to a knowledgeable optimist’s most likely estimate. This is neither the optimist’s best-case estimate, nor the average, most likely estimate of experts spanning the optimist–pessimist spectrum. Estimates were made by members of the project team with some consultation with experts not part of the project, but without a systematic survey of such experts.

—Laser et al.

Overall findings include:

• The thermochemical scenario producing only power achieves a process efficiency of 49% (energy out as power as a percentage of feedstock energy in); 1,359 kg CO₂ equivalent avoided GHG emissions per Mg feedstock (current power mix basis); and a cost of $0.0575/kWh ($16/GJ) at a scale of 4,535 dry Mg feedstock/day, 12% internal rate of return, 35% debt fraction, and 7% loan rate.

• Thermochemical scenarios producing fuels and power realize efficiencies between 55 and 64%, avoided GHG emissions between 1,000 and 1,179 kg/dry Mg, and costs between $0.36 and $0.57 per liter gasoline equivalent ($1.37 – $2.16 per gallon) at the same scale and financial structure.

• Scenarios involving biological production of ethanol with thermochemical production of fuels and/or power result in efficiencies ranging from 61 to 80%, avoided GHG emissions from 965 to 1,258 kg/dry Mg, and costs from $0.25 to $0.33 per liter gasoline equivalent ($0.96 to $1.24/gallon).

Field-to-wheel CO2 emissions and petroleum use for mature biorefinery scenarios relative to a conventional gasoline base case. Vehicle fuel consumption is 12 L/100km (19.6 miles/gallon); base case emissions are 349 g CO2/km; base case petroleum use is 4.31 MJ/km. Laser et al. (2009) Click to enlarge.

Most of the biofuel scenarios offer comparable, if not lower, costs and much reduced GHG emissions (>90%) compared to petroleum-derived fuels. Scenarios producing biofuels result in GHG displacements that are comparable to those dedicated to power production (e.g., >825 kg CO₂ equivalent/dry Mg biomass), especially when a future power mix less dependent upon fossil fuel is assumed.

Scenarios integrating biological and thermochemical processing enable waste heat from the thermochemical process to power the biological process, resulted in higher overall process efficiencies on par with petroleum-based fuels in several cases.

We conclude that mature biomass refining is highly competitive with the fuels currently available, based on all the factors considered. The most promising class of processes we analysed combined the biological fermentation of carbohydrates to fuels with advanced technologies that thermochemically convert process residues to electrical power and, or, additional liquid fuels. One of our important findings, which contradicts conventional wisdom, is that similar greenhouse gas emission reductions on a per ton biomass basis are anticipated for the production of liquid fuels and electricity via mature technology.

—Lee Lynd

The researchers also found that the mature cellulosic biofuel technologies analysed:

• Have the potential to realize efficiencies on par with petroleum-based fuels.

• Require modest volumes of process water.

• Achieve production costs consistent with gasoline when oil prices are at about $30 a barrel.

The RBAEF project has examined many potential biorefinery scenarios, but there are still aspects that we did not examine. For example, a more extensive field-to-wheels life cycle assessment that incorporates the RBAEF process design results—including a comparison of alternative feedstocks—would be useful, as would an evaluation of chemicals co-production. Also, the papers in this special issue do not directly address the issue of gracefully reconciling large-scale biofuel production with competing land use and this clearly needs more study. Finally, it would be of great value to look at how we could find ways to accelerate progress towards the sustainable, large-scale production of cellulosic biofuels.

—Lee Lynd

Resources

• Biofpr Special Issue: The Role of Biomass in America’s Energy Future. Volume 3 Issue 2 (March/April 2009)

• Lee R. Lynd et al. (2009) The role of biomass in America’s energy future: framing the analysis. Biofuels, Bioprod. Bioref. 3:113–123 doi: 10.1002/bbb.134

• Mark Laser et al. (2009) Comparative analysis of efficiency, environmental impact, and process economics for mature biomass refining scenarios. Biofuels, Bioprod. Bioref. 3:247–270 doi: 10.1002/bbb.136