Carbon dioxide can increase gasification efficiency

Nov. 13, 2009, New York – Researchers at Columbia University in New York City have
discovered a beneficial use for carbon dioxide in the conversion of organic
materials such as grass and bark into fuel. Their findings show that if used on
a broad scale, their technique could help significantly reduce overall carbon
emissions, both from the use of carbon dioxide in biofuel production and the
creation of a more energy-efficient production process. The study appears this
week on the website of the Journal of Environmental Science and Technology.

Current
approaches for turning biomass into fuel involve a considerable amount of
energy and water to form the steam needed to convert the raw, organic
materials. In addition, the conversion of such fuels typically leads to the
emission of some atmospheric carbon dioxide. To solve this challenge, Marco Castaldi, assistant professor, and Heidi Butterman, postdoctoral researcher,
in the department of earth and environmental engineering at Columbia’s Fu Foundation School of Engineering and Applied Science, have found that by using carbon
dioxide in the actual conversion of biomass, the process becomes more energy
efficient and reduces carbon dioxide emissions.

Gasification
is a process that converts biomass into volatile products—mostly hydrogen and
carbon monoxide—by reacting the raw material at high temperatures with a
controlled amount of oxygen and/or steam. The resulting mixture is a fuel called
synthesis gas or syngas.

In their
study, Castaldi, Butterman, and postdoctoral researcher Eilhann Kwon processed
50 different kinds of biomass, including beach grass, pine needles, poplar
wood, municipal solid waste, and coal, from 25 to 1000°C at rates of 1 to
100°C/minute in pure carbon dioxide and in a mixture of steam, nitrogen gas,
and carbon dioxide. They found that a carbon dioxide-steam mixture
significantly increased the conversion of biomass to volatile products at lower
temperatures.

When carbon
dioxide and steam are present in gasification, the carbon dioxide reacts first
to convert the solid fuel to syngas, leaving the steam to react with some of
the syngas in a reaction called water-gas shift, which liberates some energy.
The researchers found that by replacing 30% of the steam with carbon dioxide,
the overall process is more efficient because the carbon dioxide is more
reactive than steam and can more readily access the unprocessed biomass for
conversion into syngas.

According
to the researchers’ calculations, using carbon dioxide during biomass gasification
results in additional emissions reductions than just using biomass alone. For
low-temperature gasification of beach grass, for example, the incorporation of
carbon dioxide could create a beneficial use for 437 million tonnes of carbon
dioxide, based on estimated transportation fuel needs for 2008. For a typical
automobile producing 6 tonnes/year of carbon dioxide, this would be equivalent
to removing 308 million vehicles from the road.

Carbon dioxide used
in a mass-scale gasification process can be diverted from a variety of
industrial sources, including power plant exhaust, future power plants that use
syngas and compressed carbon dioxide, or from food and beverage manufactures
that emit carbon dioxide as a by-product. Using industrial carbon dioxide would
lead to a further reduction of emissions.