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Sustainable Energy

Increasing Yield from Gasification

A new process can make more fuel from biomass.

Biomass can be converted to fuels via a process called gasification, which uses high temperatures to break feedstock down into carbon monoxide and hydrogen, which can then be made into various fuels, including hydrocarbons. But there’s a major drawback–about half of the carbon in the biomass gets converted to carbon dioxide rather than into carbon monoxide, a precursor for fuels. Now researchers in University of Minnesota and the University of Massachusetts, Amherst, have developed a method for gasifying biomass that converts all of the carbon into carbon monoxide.

Furnace fuel: Small white particles of cellulose fall onto a hot catalyst bed in the presence of carbon dioxide, oxygen, and methane. The catalysts break down cellulose and methane into hydrogen and carbon monoxide, which can be used to make liquid fuels.

In the new approach, the researchers gasify biomass in the presence of precisely controlled amounts of carbon dioxide and methane, the main component of natural gas, in a special catalytic reactor that the researchers developed. When they did this, all of the carbon in both the biomass and the methane was converted to carbon monoxide. “In the chemical industry, even a few percent improvement makes a big impact. The increase from 50 percent to 100 percent is profound,” says Dionisios Vlachos, the director of the Catalysis Center for Energy Innovation at the University of Delaware.

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To increase the yields from gasification, researchers at the University of Minnesota and UMass Amherst added carbon dioxide, which promotes a well-known reaction: the carbon dioxide combines with hydrogen to produce water and carbon monoxide. But adding carbon dioxide isn’t enough to convert all of the carbon in biomass into carbon monoxide instead of carbon dioxide. It’s also necessary to add hydrogen, which helps in part by providing the energy needed to drive the reactions. It’s long been possible to do each of these steps in separate chemical reactors. The researchers’ innovation was to find a way to combine all of these reactions in a single reactor, the key to making the process affordable.

The process could both greatly reduce greenhouse gas emissions and increase the amount of fuel that can be made from an acre of biomass using gasification. Many companies are pursuing biological approaches to converting biomass into fuel (using enzymes and yeast, for example), rather than thermochemical methods such as gasification, in part because biological approaches tend to convert more biomass into the desired fuel than thermochemical methods. But biological approaches are each designed to work with just one type of biomass. Gasification has the advantage of being more flexible. The same facility could potentially process grass, wood, and even old tires.

The researchers found that to make the process work, it was necessary to precisely balance three variables: the amount of carbon dioxide, the amount of oxygen added, and the amount of methane relative to the amount of cellulose–a material derived from biomass. The mixture is fed into a high-temperature reactor that consists of a rhodium- and cerium-based catalyst. In the reactor, particles of cellulose are quickly converted into a liquid, which spreads over the catalyst, enhancing the reactions that lead to the production of hydrogen and carbon monoxide gases.

Paul Dauenhauer, a professor of chemical engineering at UMass Amherst, and one of the researchers involved in developing the new process, says a commercial version of the process could be set up near an existing natural gas power plant, which would provide ready access to methane and carbon dioxide. But the process isn’t yet ready for commercialization. The researchers will need to demonstrate that it works with biomass, not just with cellulose derived from biomass. Biomass contains various contaminants not found in pure cellulose. Those contaminants could have a negative effect on the catalyst, and this could make it necessary to reengineer the reactor, he says. And there could be challenges scaling up the process, including ensuring that heat moves through the reactor the same way it does on a small scale.

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