|One of the possible configurations of the proposed H2CAR process. Click to enlarge.|
Purdue University chemical engineers have proposed a modification to the conventional biomass- or coal-to-liquids Fischer-Tropsch process that could, by their calculations, produce sufficient fuel for the entire US transportation sector.
Their process, however, relies on an abundant supply of hydrogen—between 239 and 276 billion kilograms of hydrogen per year—to produce 13.8 million barrels of synthetics per day. The new approach—which the researchers call a “hybrid hydrogen-carbon process,” (H2CAR)—proposes co-feeding a gasifier with hydrogen from a carbon-free energy source, such as solar or nuclear power, and CO2 recycled from the syngas (hydrogen and carbon monoxide) conversion reactor.
The H2CAR process offers a number of advantages compared to conventional biomass- or coal-to-liquids technologies according to models the team developed:
No CO2 is released to the atmosphere or is required to be sequestered as a result of the chemical processing system. When conventional methods are used to convert biomass or coal to liquid fuels, 60% to 70% of the carbon atoms in the starting materials are lost in the process as carbon dioxide whereas no carbon atoms would be lost using H2CAR.
Approximately 40% of the amount of coal or biomass is needed to deliver the same quantity of liquid fuel. This is advantageous for prolonging the life of the known coal reserves as well as in reducing the land area needed for the bioenergy crop. The land area required to grow the biomass for H2CAR is accordingly less than 40% of that needed by other pathways that solely use biomass to support the entire transportation sector.
Current estimates suggest that an annual US biomass supply of 1.366 billion tons could produce approximately 30% of the United States transportation fuel with conventional processes. The H2CAR process shows the potential to supply the entire United States transportation sector from that quantity of biomass.
The synthesized liquid provides H2 storage in an open loop system. The addition of hydrogen atoms to carbon atoms from coal or biomass provides a high-density method for storage of massive quantities of hydrogen. On a carbon atom basis, the energy content of the liquid fuel is higher than that of coal or biomass.
The process is detailed in an open access research paper appearing online this week in the Proceedings of the National Academy of Sciences.
The researchers explain the rationale of using H2 in the H2CAR process by the significantly higher annualized average solar energy conversion efficiency for hydrogen generation versus that for biomass growth.
|Production of 13.8 Mbpd of synthetic oil by using biomass|
|Biomass land area|
billion kg/yr km2
|H2 land area|
I’m saying, treat biomass predominantly as a supplier of carbon atoms, not as an energy source.—Rakesh Agrawal, Purdue University
|Land-area and H2 requirement for conventional and process using PHEVs as a function of drivable distance traveled per single full charge H2CAR of batteries. Click to enlarge.|
Agrawal argues that the new process also would be more practical than all-electric or hydrogen-powered cars, in part because of the limited storage capacity of batteries and hydrogen storage tanks.
The tremendous convenience provided by the existing infrastructure for delivering and storing today’s fuels is a huge deterrent to introducing technologies that use only batteries or hydrogen alone. A major advantage of our process is that it would enable us to use the current infrastructure and internal combustion engine technology. It is quite attractive for hybrid electric vehicles and plug-in hybrid electric vehicles.—Rakesh Agrawal
Realizing this requires significant research in two areas: finding ways to produce cheap hydrogen from carbon-free sources and developing a new type of gasifier needed for the process.
The proposed H2CAR-based processes also have a strong impact on the future areas of research. The primary research emphasis needs to be on cost-effective H2 production from a carbon-free energy source such as solar or nuclear. In addition, efficient, low-cost, and easy-to-operate methods are needed for the conversion of biomass through reaction with H2 to a suitable hydrocarbon liquid fuel. In the short term, the same is true for the conversion of coal to liquid. The current conversion route of gasification followed by a H2-CO liquid conversion reaction is quite inefficient, and an alternative efficient hydrogenation process is highly desirable.
In the mean time, until such alternate processes are discovered, the preservation of carbon atoms in the current gasification and H2-CO liquid conversion reaction is essential. A proposed solution in this work is to co-feed H2 and recycle CO2 from the H2-CO liquid conversion reactor to the gasifier. Feasibility and development of such gasifiers especially for biomass will require extensive research.
Purdue has filed a patent for the concept. The approach is in the conceptual stages, and a plan for experimental research is in progress.
“Sustainable fuel for the transportation sector”; Rakesh Agrawal; Navneet R. Singh; Fabio H. Ribeiro; W. Nicholas Delgass; PNAS published March 14, 2007, 10.1073/pnas.0609921104