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Study: Carbon dioxide in biomass gasification increases efficiency

By Lisa Gibson
Posted November 18, 2009, at 10:42 a.m. CST

When carbon dioxide is used in biomass gasification, it increases the conversion efficiency and offers a solution for processing carbon dioxide on a global scale, according to a recent study by Columbia University researchers.

The study, "CO2 as a Carbon Neutral Source via Enhanced Biomass Gasification," is featured on the Web site of the Journal of Environmental Science and Technology and can be seen at pubs.acs.org/journal/esthag. The researchers processed 50 different kinds of biomass including beach grass, pine needles, poplar wood and municipal solid waste, along with coal, at temperatures of 25 to 1,000 degrees Celsius (77 to 1,832 degrees Fahrenheit) at rates of 1-100 degrees Celsius per minute in pure carbon dioxide and in a mixture of steam nitrogen gas and carbon dioxide, according to the university. They found that a carbon dioxide-steam mixture significantly increased the conversion of biomass to volatile products at lower temperatures.

The team used about 30 percent carbon dioxide by volume of biomass and typically between 20 micrograms and two grams per second of biomass, said researcher Marco Castaldi, assistant professor of Earth and Environmental Engineering. The only difference in the syngas produced is the ratio of hydrogen to carbon, which is adjustable, he added.

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 carry out the water-gas shift exothermic reaction, liberating some energy, according to the university. The researchers found that by replacing 30 percent of the steam with carbon dioxide, the overall process is more efficient because the carbon dioxide is more reactive than steam and can readily access the carbon char for conversion into syngas. "It seems pure carbon dioxide is best," Castaldi said.

The use of carbon dioxide in biomass conversion on a global scale has the potential to process tens to hundreds of megatons of carbon dioxide per year, the researchers believe. According to their calculations, using carbon dioxide during gasification of biomass fuel results in better emissions reduction than just the use of biofuels alone. For example, the incorporation of carbon dioxide in the low-temperature gasification of beach grass on a global scale could create a beneficial use for 437 million metric tons (482 million tons) of carbon dioxide, based on estimated transportation needs in 2008, according to the university. That's the equivalent of taking about 308 million typical vehicles producing 6 metric tons of carbon dioxide or more per year off the road.

Carbon dioxide for gasification 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 manufacturers that emit carbon dioxide as a byproduct, researchers said. The use of industrial carbon dioxide will lead to further emissions reductions.

The next step in the research is to further develop and understand the process and look at different waste streams, such as agricultural waste, to determine where catalysts can be incorporated to refine the resulting biofuel, according to the university. The researchers would like to commercialize their process and are searching for the necessary funding, Castaldi said.
 

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