A closer look at the revolutionary coal+biomass-to-liquids with carbon storage project

Earlier we reported on an feasibility study released by the U.S. Department of Energy’s National Energy Technology Laboratory (DOE/NETL) and the U.S. Air Force (USAF) which focused on a highly advanced generation of fuels made from combining the liquefaction of both coal and biomass, and then coupling the system to carbon sequestration technologies. It's a mouthful, but the radical concept comes down to: coal+biomass-to-liquids (CBTL) + carbon capture and storage (CCS), or CBTL+CCS. The CBTL process consists of the production of so-called synthetic fuels, obtained from the gasification of feedstock, with the gas then liquefied via Fischer-Tropsch synthesis into an ultra-clean synthetic fuel (schematic, click to enlarge). During the process, carbon dioxide is captured and then stored in geological formations such as depleted oil and gas fields or saline aquifers.

The CBTL+CCS project is a first step towards the transition to what researchers call 'Bio-energy with Carbon Storage' (BECS) systems. Such BECS systems result in radically carbon-negative fuels and energy, which take carbon emissions from the past out of the atmosphere (earlier post and references there). More familiar renewables like wind, solar or hydro deliver carbon-neutral energy at best; that is, they do not contribute emissions to the future. Carbon-negative biofuels on the contrary go much further, they effectively clean up the past.

Scientists working on the CBTL+CCS project include Robert Williams, a senior research scientist at the Princeton Environmental Institute and Professor of Mechanical and Aerospace Engineering Fred Dryer, also of Princeton. They clarify some of the challenges of the research. The project focuses on the production of future jet fuels, but the concept as such applies to all transport fuels.

Alternative energy sources, if designed appropriately, could significantly reduce the amount of greenhouse gasses released in creating and burning jet fuel. According to the U.S. Department of Transportation, aviation is responsible for around 10 percent of the greenhouse gas emissions from transportation in America, or roughly 2.7 percent of the country's total greenhouse gas emissions.

Two alternative fuel sources that are the subject of much investigation in the aviation field - coal and biomass - present a major quandary to researchers attempting to develop low-emissions jet fuels. Coal, a relatively cheap and readily available source of energy, has an emissions profile at least as harmful as petroleum. Biofuel - fuel made from plants - presents an attractive alternative because the carbon dioxide emitted from burning biomaterials will be removed from the atmosphere by a new generation of plants during photosynthesis. But the potential for large-scale biomass production in the U.S. is limited.

To take advantage of the positive characteristics of each of these sources, the Princeton researchers will center their efforts on the synthesis of jet fuels from a combination of coal and biomass. A key component of their solution is isolating and storing the carbon dioxide produced during the production of the so-called synfuels. This technique, called carbon capture and sequestration, is a promising strategy being investigated intensively by Princeton's Carbon Mitigation Initiative, among other research programs.

An especially attractive feature of processing coal and biomass together to make synfuels is that it requires only half the amount of biomaterial as pure biofuel production, while still making fuels with near-zero greenhouse gas emissions. If the fraction of biomass is increased, the ultra-clean fuels become carbon-negative.

The ultimate success of the research efforts will depend on how well the synfuels compare with traditional fuel sources, in terms of fuel characteristics, costs and environmental and safety issues:
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"There is no doubt that developing feasible alternatives to petroleum for the aviation industry will be a long and expensive process," Dryer said. "And success, in the form of an enduring solution, will be priceless."

The CBTL+CCS research project provides the opportunity to make substantial progress toward the launching of green technologies not only for corporate jets, but also for commercial aviation and transportation in general, according to Williams. At Princeton, the team also includes Ju and Eric Larson, a research engineer at the Princeton Environmental Institute. In addition, the work will involve collaboration with researchers at the Institute of Transportation Studies at the University of California-Davis.


Dryer is also working on a major project funded by the U.S. Air Force, focused on developing computational and kinetic models that accurately simulate the burning of jet fuel, a complex and poorly characterized mix of chemicals.

"In order to make alternative jet fuel sources feasible, they need to be compatible with petroleum and produce similar combustion performance," Dryer said. "This will only be possible if we fully understand how both petroleum and alternative fuels burn and design engines based on this fundamental knowledge."

The Air Force program is one of the Defense Department's highly competitive Multi-disciplinary University Research Initiative (MURI) grants. One of only ten such projects supported by the Air Force this year, the collaboration involves researchers from four institutions - Princeton, Case Western Reserve University, Pennsylvania State University and the University of Illinois-Chicago. The award, with an overall value of up to $7.5 million, will provide support for three years with the option of a two-year extension. Research began in July and the kick-off meeting for the project will be held Sept. 17 in Princeton.

Dryer and his MURI collaborators, including Princeton Associate Professor of Mechanical and Aerospace Engineering Yiguang Ju, will develop methods to predict and evaluate how jet fuels will behave in actual engines and characterize the emissions they will produce. While current guidelines specify some overall properties of jet fuels, they do not spell out the actual chemical composition. Depending on the source and processing method, jet fuel typically consists of hundreds to thousands of molecular structures that behave in a variety of ways.

The models developed by the team will represent and characterize the behavior of this broad range of jet fuel species using only a few types of molecular structures as surrogates for the larger whole. Dryer previously developed similar "surrogate fuel" models to represent gasoline, which are now being used for engine design by the automotive industry.

"The composition of fuels changes with the geographic source, the refining process and even with the season," Dryer noted. "Since we have an energy security problem, we need to be sure that alternative fuel sources are going to work and, in order to do that, we need to understand exactly how petroleum-based fuels work alone and in combination with alternative fuels."

References:
Princeton University: Green skies: Engineer's work may reduce jet travel's role in global warming - September 13, 2007.

Biopact: NETL and USAF release feasibility study for conceptual Coal+Biomass-to-Liquids facility - August 30, 2007