Growing Solutions

Rocks are not the only place to store carbon. All green plants are carbon-based, so they are natural carbon sinks. Old-growth forests are well-known repositories of carbon, but smaller organisms can do the job, too—perhaps even better. Some ASU researchers are now exploring the microscopic world, looking at algae as a way to sequester CO₂.

“Algae are much more productive than higher plants,” explains Milton Sommerfeld, a professor in the School of Life Sciences. “They can produce more biomass over a short time.”

Sommerfeld is working with Qiang Hu to develop large-scale bioreactors that will remove CO₂ from smokestacks and use it in growing algae. Hu is a research assistant professor and bioreactor expert.

In effect, the bioreactors would be algal farms producing a crop that could be used for a variety of purposes. Algae have nutritional value that makes them useful as fertilizer, animal feed, fine chemicals, and even human food supplements.

The first task the researchers faced was finding a suitable species to use for sequestration. They needed to find algae that tolerates high levels of CO₂, has a high growth rate, and serves as a useful product once grown.

The researchers isolated many species of algae from natural water environments. During laboratory testing they found several good candidates for the job. The next step will be to set up an actual bioreactor in a power plant as a pilot study.

“It’s been a small project at this point. The next level of scale will be bigger. Right now we use a synthetic gas mix. In the pilot study we will use real flue gas,” says Sommerfeld.

The flue gas will agitate the mix as well as provide CO₂. It also will provide heat to maintain a suitable culture temperature, which will help the algae grow outdoors all year round.

The pilot study will help scientists learn how the algae thrive in a real-life situation compared to a highly controlled laboratory. Sommerfeld says that using multiple species is an important strategy. The productiveness of each type of algae may vary depending on external factors such as temperature, solar light intensity, or nutrient availability.

If it proves effective, this technique could be an excellent and inexpensive way to reduce point-source CO₂ emissions. Hu says that the process is environmentally friendly and sustainable over the long term.

“The process is driven by solar energy, not electricity. This generates more CO₂. And our by-product is renewable,” he adds.

Algal biomass generated from the process can make it commercially profitable, as well. The researchers hope the bioreactors will eventually cover their own operating costs. There certainly will be no shortage of product to sell. According to Hu, the CO₂ from a single power plant could help generate about 10 million tons of algal biomass per year.—Diane Boudreau