Renewable Liquid Fuels Reforming
The Program anticipates that distributed reforming of biomass-derived liquid fuels could be commercial during the transition to hydrogen and used in the mid- and long-term time frames.
How Does It Work?
Biomass resources can be converted to ethanol, bio-oils, or other liquid fuels that can be transported at relatively low cost to a refueling station or other point of use and reformed to produce hydrogen.
Reforming renewable liquids to hydrogen is very similar to reforming natural gas.
The liquid fuel is reacted with steam at high temperatures in the presence of a catalyst to produce a reformate gas composed mostly of hydrogen and carbon monoxide.
Additional hydrogen and carbon dioxide are produced by reacting the carbon monoxide (created in the first step) with high-temperature steam in the "water-gas shift reaction."
Finally, the hydrogen is separated out and purified.
Steam Reforming Reaction (Ethanol)
C2H5OH + H2O (+heat) → 2CO + 4H2
Water-Gas Shift Reaction
CO + H2O → CO2 + H2 (+small amount of heat)
Biomass-derived liquids, such as ethanol and bio-oils, can be produced at large, central facilities located near the biomass source to take advantage of economies of scale and reduce the cost of transporting the solid biomass feedstock. The liquids have a high energy density and can be transported with minimal new delivery infrastructure and at relatively low cost to distributed refueling stations or stationary power sites for reforming to hydrogen.
Making Liquid Fuels from Biomass
Ethanol can be made by converting the starch in corn into sugars and fermenting the sugar to produce ethanol. Currently more than 3.5 billion gallons of ethanol are produced and utilized as a fuel additive for gasoline in the U.S. Sugars can also be extracted from other biomass resources, such as crop or forest residues or energy crops, through a series of steps involving mild acid or steam and enzyme digestion. These sugars are then fermented, producing ethanol.
Bio-oils are an oil-like liquid similar to diesel fuel. Biomass can be treated with heat, in the absence of oxygen, to break it down to produce bio-oil in a process very similar to the first step of biomass gasification.
Why Is This Technology Being Considered?
Biomass is an abundant domestic resource.
In the United States, there is more biomass available than is required for food and animal feed needs. Two recent reports5,6 project that with anticipated improvements in agricultural practices and plant breeding, 1.2 billion dry tons of biomass could be available for energy use by 2050. This equates to 21 quadrillion Btu/year of primary energy. Current U.S. energy use is 98 quadrillion Btu/yr,7,8 and it is expected to grow to 135 quadrillion Btu/yr.3,4 Current energy use for personal vehicles is about 16 quadrillion Btu/yr.4 Biomass cannot meet all of our energy needs, but it can provide a major contribution.
Biomass "recycles" carbon dioxide
Because biomass resources consume carbon dioxide in the atmosphere as part of their natural growth process, producing hydrogen through biomass gasification releases near-zero net greenhouse gases.
Research Focuses on Overcoming Challenges
Reforming renewable liquid fuels is a process very similar to reforming natural gas (a relatively mature technology). Renewable liquid fuels, however, are composed of larger molecules with more carbon atoms, so they are more difficult to reform than natural gas. Research is needed to identify better catalysts to improve yields and selectivity. Other challenges include:
Reducing the cost of ethanol and/or other biomass-derived liquid fuels (research conducted by DOE's Office of Energy Efficiency and Renewable Energy Biomass Program)
Reducing capital equipment costs, as well as operation and maintenance costs, and improving process efficiency (similar to challenges of natural gas reforming).
5"Growing Energy—How Biofuels Can Help End America's Oil Dependence" [report of preliminary results published by the National Resources Defense Council]
6"Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply," USDA, ORNL, (http://www.osti.gov/bridge/)
7U.S. Department of Energy, Energy Information Administration, Annual Energy Review 2003, DOE/EIA-0384
8U.S. Department of Energy, Energy Information Administration, Annual Energy Outlook 2005 with Projections to 2025, DOE/EIA-0383