Smaller, Cheaper Biofuel Reactors

Researchers at the University of Minnesota have developed a fast way to convert sawdust and waste biomass directly into a mixture of gases that can be burned to generate electricity or made into liquid fuels such as diesel. If the process can be scaled up, it could be a more energy-efficient method for making biofuels by allowing for small, fast reactors located close to biomass sources.
The researchers developed a system that makes it possible to transform solids directly into a useful mixture of gases. The process begins when small, millimeter-sized particles come into contact with a 700 to 800 degree Celsius porous surface and instantly form a mixture of gaseous compounds. These interact with a catalyst made of the precious metal rhodium that facilitates partial oxidation reactions that both keep the system hot and convert the gases to hydrogen and carbon monoxide. This mixture of gases, called syngas or synthesis gas, can then be burned in a gas turbine to make electricity, or purified and made into a number of different fuels using well-known processes.

The key to the new process is a catalyst bed with the right kind of porous structure to maintain the temperatures and movement of materials needed for the chemical reactions. The resulting system breaks down the biomass in just 70 milliseconds. That is ten times faster than other methods for making syngas, says Lanny Schmidt, professor of chemical engineering and materials science at the University of Minnesota. Ideally, that means a reactor with a given volume could make ten times the amount of syngas using the new method compared with conventional methods. Or put another way, it could allow for reactors one-tenth the size, he says.

The catalytic approach is one of a number of methods in development that could convert cheap sources of cellulosic biomass, such as sawdust, grass, and agricultural waste, into liquid fuels. It’s still not clear which of two broad categories of approaches will be more practical, thermochemical methods, such as Schmidt’s, or methods that use enzymes and organisms. Thermochemical methods are expensive but have the potential advantage of being able to use a number of different source materials, whereas biological systems will likely need to be fine-tuned for particular feed stocks.

But the ability to make smaller reactors for converting waste biomass to syngas could help solve one of the most significant challenges of producing fuels from biomass. Transporting bulky materials such as wood chips and corn waste long distances to central facilities uses a lot of energy, often in the form of fossil fuels.

Technology Review