The Program anticipates that biomass gasification could be deployed in the mid-term time frame.
What Is Biomass?
Biomass, a renewable organic resource, includes agriculture crop residues, such as corn stover or wheat straw; forest residues; special crops grown specifically for energy use, such as switchgrass or willow trees; organic municipal solid waste; and animal wastes.
How Does Biomass Gasification Work?
Biomass is converted into a gaseous mixture of hydrogen, carbon monoxide, carbon dioxide, and other compounds by applying heat under pressure in the presence of steam and a controlled amount of oxygen (in a unit called a gasifier). The biomass is broken apart chemically by the gasifier's heat, steam, and oxygen, setting into motion chemical reactions that produce a synthesis gas, or "syngas"—a mixture of primarily hydrogen, carbon monoxide, and carbon dioxide. The carbon monoxide then reacts with water to form carbon dioxide and more hydrogen (water-gas shift reaction). Adsorbers or special membranes can separate the hydrogen from this gas stream.
Simplified Example Reaction
C6H12O6 + O2 + H2O → CO + CO2 + H2 + other species
Note: The above reaction uses glucose as a surrogate for cellulose. Actual biomass has highly variable composition and complexity with cellulose as one major component.
Water-Gas Shift Reaction
CO + H2O → CO2 + H2 (+small amount of heat)
Pyrolysis is the gasification of biomass in the absence of oxygen. In general, biomass does not gasify as easily as coal, and it produces other hydrocarbon compounds in the gas mixture exiting the gasifier; this is especially true when no oxygen is used. As a result, typically, an extra step must be taken to reform these hydrocarbons with a catalyst to yield a clean syngas mixture of hydrogen, carbon monoxide, and carbon dioxide. Then, just as in the gasification process for hydrogen production, a shift reaction step (with steam) converts the carbon monoxide to carbon dioxide. The hydrogen produced is then separated and purified.
Biomass gasification technology is most appropriate for large-scale, centralized hydrogen production, due to the nature of handling large amounts of biomass and the required economy of scale for this type of process.
Why Is This Technology Being Considered?
Biomass is an abundant domestic resource.
The United States has more biomass available than is required to meet its food and animal feed needs. Two recent reports 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 2050.1,2 This amount equates to 21 quadrillion Btu/year of primary energy. Current energy use in the United States is 98 quadrillion Btu/yr, 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.
Growing biomass removes carbon dioxide from the atmosphere.
The biomass resources used in biomass gasification consume carbon dioxide in the atmosphere as part of their natural growth process, which means that biomass gasification results in a near-zero net release of greenhouse gases.
Research Focuses On Overcoming Challenges
Key challenges to hydrogen production via biomass gasification involve reducing costs associated with capital equipment and biomass feedstocks.
Research to lower capital costs:
If oxygen is used in the gasifier, capital costs can be reduced by replacing the cryogenic process currently used to separate oxygen from air with new membrane technology.
New membrane technologies are needed to separate and purify hydrogen from the gas stream produced (similar to coal gasification).
Intensifying the process (combining steps into fewer operations).
Research to lower biomass feedstock costs:
Improved agricultural practices and breeding efforts should result in low and stable feedstock costs.
DOE and USDA support improved practices for greater use of biomass-based energy.
1Growing Energy—How Biofuels Can Help End America's Oil Dependence" (PDF 750 KB) [report of preliminary results published by the National Resources Defense Council] Download Adobe Reader
2Biomass as Feedstock for a Bioenergy and Bioproducts Industry: The Technical Feasibility of a Billion-Ton Annual Supply", USDA, ORNL, (www.osti.gov/bridge/)
3U.S. Department of Energy, Energy Information Administration, Annual Energy Review 2003, DOE/EIA-0384
4U.S. Department of Energy, Energy Information Administration, Annual Energy Outlook 2005 with Projections to 2025, DOE/EIA-0383