Biogas production is all in the mixing

There were then, as now, 100 anaerobic digesters in operation in the United States, but a remarkably high percentage – 76 percent – regularly failed. He and his colleagues at Washington University, Oak Ridge National Laboratory and ultimately the Iowa Energy Center based in Ames, Iowa, studied the configuration, design, hydrodynamics and mixing parameters of reactors, first on a laboratory scale, in reactors that held less than four liters of manure.

“A systematic study had never been done before, so we wanted to get a notion of what was behind the high failure rates reported,” al-Dahhan said. “We tested by gas injection, mechanical agitation, slurry circulation and liquid circulation and at different intensities. We found that at the laboratory scale, all of the different mixing modes performed adequately.”

They then went to Oak Ridge Laboratory to a pilot plant and tested a reactor that held 100 liters.

“As size increased, we found mixing plays a very important role in successful operations,” al-Dahhan said. “Intensity of mixing also is important. We found that if intensity of mixing is reduced, failure often is a consequence.”

Anaerobic digestion of manure is opaque, which means to understand the hydrodynamics of anaerobic digestion al-Dahhan and colleague developed a unique mutiple-particle, computer-automated radioactive particle tracking system, computational fluid dynamics and gamma ray computed tomography to see where and under what conditions biochemical stagnant or dead zones occurred. They also analyzed mixing systems, hydrodynamics, shear effect and reactor configuration.

“We then used all of our knowledge to redesign the commercial digester at the Iowa Energy Center,” al-Dahhan said.

The redesigned digester is running there today.

“The research we’ve done provides the basis to scale up in the future, “ he said. “The process is complex, but we’re seeking to simplify it for use as a quick assessment and evaluation of the digester. The final goal is a simple system ready for use by farmers on site for bioenergy production and for animal and farm waste management.”