October 14, 2008
More flexible method floated to produce biofuels, electricityWEST LAFAYETTE, Ind. - Researchers are proposing a new "flexible" approach to producing alternative fuels, hydrogen and electricity from municipal solid wastes, agricultural wastes, forest residues and sewage sludge that could supply up to 20 percent of transportation fuels in the United States annually.
The method offers a potential solution to problems that might be created by increasing production of ethanol with conventional methods, which use corn grain as a feedstock. Boosting ethanol production with conventional methods would require additional crops and heavy fertilizer use, increasing runoff into waterways and threatening ecosystems.
The new concept, however, which Purdue researchers call a flexible carbon-to-liquid fuel process, would require no additional crops and use primarily wastes as the feedstock, said Fu Zhao, a Purdue assistant professor of mechanical engineering.
"This technique is more flexible than conventional methods because we can process a wider range of very different feedstocks and, at the same time, we can generate a wider range of end products - not just gasoline and diesel but ethanol and hydrogen. Or we could generate electricity directly from the gas produced," he said.
The method also would be immune to the market fluctuations of corn and other crops and less affected by disturbances such as feedstock supply shocks and market demand changes. The method also could reduce greenhouse gas emissions by more than 50 percent compared with petroleum-derived gasoline.
Findings were detailed in a paper presented on Sept. 29 during the 6th Global Conference on Sustainable Product Development and Life Cycle Engineering in Busan, Korea. The preliminary analysis was written by Zhao; Purdue doctoral student Dongyan Mu; P. Suresh Rao, the Lee A. Rieth Distinguished Professor of Civil Engineering and Agronomy; and Thomas Seager, an associate professor in the Golisano Institute for Sustainability at the Rochester Institute of Technology.
The system first requires processing carbon-containing waste, such as paper, wood, plastic and rubber, into small pieces with a diameter of a few millimeters, or thousandths of a meter. The pieces would then be fed into a "gasifier," where the materials would be turned into a gas containing hydrogen, carbon monoxide, carbon dioxide, methane and other hydrocarbons.
This gas would be further processed to get rid of everything but the hydrogen and carbon monoxide, referred to as synthesis gas or syngas. This gas could then be used to directly run a turbine to generate electricity, or it could be converted into gasoline and diesel fuel for transportation using a process called Fischer-Tropsch synthesis. The technique could be used to produce ethanol, jet fuel and other biofuels from the solid wastes.
Data indicate enough wastes are generated to support large production facilities using the system. A report prepared by the U.S. Department of Agriculture and Department of Energy found that an estimated 1.3 billion tons of biomass - including agricultural and municipal wastes - are generated annually in the United States. Coal could be used to supplement the waste feedstocks if needed, Zhao said.
The analysis suggests that it is possible to replace 15 percent to 20 percent of transportation fuels consumed daily in the United States with liquids derived from this flexible process. These estimates are based on the present consumption level, which is about 390 million gallons per day, he said.
The researchers estimate the method would be economically competitive with petroleum-based fuels and plan to develop "an integrated process simulation model" to test the technique with a variety of feedstocks, including waste plastics. Raw data for the model will be generated with an experimental gasifier being built at Purdue.
The research has been supported by the Energy Center and the Center for the Environment, both in Purdue's Discovery Park. The work also is supported by the School of Mechanical Engineering and the School of Civil Engineering.
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Technical, Economic, and Environmental Feasibility of Flexible Carbon to Liquid Process: A Preliminary Analysis
Fu Zhao1, Dongyan Mu2, P. Suresh Rao2,
1School of Mechanical Engineering,
2School of Civil Engineering, Purdue University
3Golisano Institute for Sustainability,
Currently the U.S. transportation sector relies almost exclusively on liquid hydrocarbons derived from petroleum oil as the energy source, which leads to significant concerns on environmental degradation (especially global warming and climate change) and energy security. As alternatives to petroleum oils, biofuels are attracting increasing interest. However, large scale biofuel production raises environmental issues such as landscape change, soil erosion, eutrophication, pesticides and herbicides release, water resource depletion, and biodiversity loss. In this paper, a so-called flexible carbon to liquid fuel (FCTL) process is proposed. This FCTL process converts domestic biomass wastes/residues (e.g. forestry residues, agricultural residues, municipal solid waste, and sewage sludge) to liquid fuels through gasification and Fischer-Tropsch (FT) type synthesis, with hydrogen, chemicals, and electricity as co-products or alternative products. The FCTL process has the potential to replace 15%-20% of transportation fuels consumed in U.S., and thus could significantly reduce foreign oil dependency. At the same time, it could be more environmentally sustainable since few changes will be needed in agricultural and forestry practices, and may be more resilient against external disturbances such as feedstock supply shocks and market demand changes. A preliminary analysis on the technical, economic, and environmental feasibility of the FCTL process is performed by dividing the entire FCTL process into two sub-processes i.e. biomass gasification and FT type synthesis on which extensive research has been conducted. The analysis suggests that by selecting fluidized bed gasifier a variety of biomass waste/residue can be used in FCTL plants and the plant availability can be further improved by using low rank coal as the backup feedstock. Over the entire life cycle, fuels produced by FCTL process have significantly lower environmental impacts within all the major categories considered i.e. greenhouse gas emissions, water consumption, and land use, if compared with corn ethanol and soybean biodiesel. Moreover, the cost of the derived fuels is estimated to be in the range of $1.50-$2.00/gallon gasoline equivalent, which is competitive with other biofuels under production or consideration. This paper calls for the development an integrated process simulation model of a FCTL plant and a complete life cycle assessment in order to achieve a better picture of the economic and environmental performance of the proposed FCTL process.
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