Hydrogen injection could boost biofuel production

The "hybrid hydrogen-carbon process," or H2CAR has been proposed by engineers from Purdue University for the production of iquid fuels from biomass or coal. The process adds hydrogen from a "carbon-free" energy source, such as solar, wind or nuclear power, during gasification of the biomass, supressing the formation of carbon dioxide and increasing the efficiency of the process, making it possibe to produce three times the volume of biofuels rom the same quantity of same quantity of biomass. However, the new method hinges on having a cheap source of hydrogen – something which is not yet readily available.

When conventional methods are used to convert biomass or coal to liquid fuels, 60 percent to 70 percent of the carbon atoms in the starting materials are lost in the process as carbon dioxide, a greenhouse gas, whereas no carbon atoms would be lost using H2CAR, said Rakesh Agrawal, Purdue's Winthrop E. Stone Distinguished Professor of Chemical Engineering.

"This waste is due to the fact that you are using energy contained in the biomass to drive the entire process," he said. "I'm saying, treat biomass predominantly as a supplier of carbon atoms, not as an energy source."

The process, which would make possible the dawning of a "hydrogen-carbon economy," is detailed in a research paper appearing online in the March 6 issue of Proceedings of the National Academy of Sciences.

Other researchers have estimated that the United States has a sustainable supply of about 1.4 billion tons of biomass each year that could be used specifically for the production of liquid fuels. With conventional methods, that quantity of biomass would provide 30 percent of the fuel required for the nation's annual transportation needs. But the same quantity of biomass would provide enough fuel to meet all transportation needs using the new H2CAR method, Agrawal said.

To grow enough biomass for the entire nation's transportation needs using the conventional method for producing biofuels would require a land area 25 percent to 55 percent the size of the United States, compared with about 6 percent to 10 percent for the H2CAR process.

A major reason less land would be needed is because of the overall higher efficiency of generating hydrogen by splitting water molecules using solar energy to drive the electrolysis. Usually, the hydrogen in liquid fuels made from biomass comes from the plant matter itself. But it typically takes more than 10 times the solar energy to grow crops than it does to produce the equivalent quantity of hydrogen possessing the same energy content by using the solar-power electrolysis method, he said.

"So providing hydrogen derived from water through solar electrolysis reduces the amount of biomass needed," Agrawal said. "The average energy efficiency of growing crops is typically less than 1 percent, whereas the energy efficiency of photovoltaic cells to split water into hydrogen and oxygen is about 8-10 percent. I am getting hydrogen at a higher efficiency than I get biomass, meaning I need less land."

Advantages cited in the paper for the process are:

1. The estimated land areas for both the conventional processes are too large. Even with the anticipated advancements, the land area for the conventional–II case is 27.5% of the total United States land area. This land area is greater than the current United States cropland area.
2. The land area requirements for the proposed processes, especially for the H2CAR–II case, are substantially lower and have a potential to be manageable.
3. The carbon efficiency for the conventional biomass process is quite low. Nearly two-thirds of the carbon contained in the biomass is lost as CO2.
4. The addition of H2 in the H2CAR process improved the overall efficiency of the process.
5. Another associated benefit of H2CAR process is that diversity of crops can be maintained because any type of biomass can be gasified. It has been shown that plant diversity enhances the biomass yield by 180% over monocultures. Also, a diverse biomass growth has a better chance of survival in droughts.
6. The ability to use diverse biomass also provides an additional degree of freedom to tailor biomass growth for the maximization of carbon pickup from the atmosphere without the constraints of relative quantities of lignin, cellulose, hemicellulose, starch, oil, sugar, etc. in a plant.
7. Land area radius decreases to support a given size of plant.
8. Less space is required for storage of biomass.
9. Less fertilizers and pesticides would be required for the same quantity of liquid fuel production, if any.
10. There would be less wear and tear to the land.
11. Less biomass demand to produce same quantity of transportation fuel implies less energy and water input to grow the required amount of biomass.

The researchers suggest in the paper the chemical processing steps needed to make the new approach practical. But making the concept economically competitive with gasoline and diesel fuel would require 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.

"Having said that, this is the first concept for creating a sustainable system that derives all of our transportation fuels from biomass," Agrawal said.

The process, which would make possible the dawning of a "hydrogen-carbon economy," is detailed in a research paper appearing online this week in the Proceedings of the National Academy of Sciences.