All chefs know that “you have to break some eggs to make an omelette”, and that includes engineers at Iowa State University in the US who are using high-frequency sound waves to break down plant materials in order to cook up a better batch of biofuel.
Research by David Grewell, associate professor of agricultural and biosystems engineering, and colleagues Melissa Montalbo-Lomboy and Priyanka Chand, has shown that “pretreating” a wide variety of plant feedstocks with ultrasound consistently enhances the chemical reactions necessary to convert the biomass into high-value fuels and chemicals.
In one example of ultrasound’s positive impact on biofuel production, the Iowa State researchers found that they could significantly increase the efficiency of removing lignin from biomass in solution.
Lignin is the chemical compound that binds cellulose and hemicellulose together in plant cell walls. Commonly, enzymes or chemicals are used to remove it from biomass and allow the freed sugars to be dissolved for further processing into biofuel.
Grewell and his colleagues found that pretreating instead with ultrasound makes lignin removal so efficient that sugar dissolution occurs in minutes rather than the hours needed with traditional mixing systems.
The Grewell team also found that hydrolysis of corn starch could be greatly accelerated using ultrasonics. In a conventional ethanol plant, ground corn is steamed with jet cookers at boiling point temperatures.
This breaks down the corn, leaving a starch mash that is then cooled and treated with enzymes in a process known as hydrolysis to release glucose for fermentation.
The Iowa State team replaced the initial steaming with ultrasound, sonically smashing the corn into tiny particles in the same way physicians use ultrasound to shatter kidney stones. The smaller corn fragments provided more surface area for enzymatic action, and therefore, resulted in fermentation yields comparable to jet cooking.
Grewell and his colleagues report a third application for ultrasound in biofuel production, showing that they can accelerate transesterification, the main chemical reaction for converting oil to biodiesel.