Eric Kaufman, a researcher at the Bioprocessing Research and Development Center in ORNL's Chemical Technology Division, and P. T. Selvaraj, a postdoctoral scientist at the center, have shown that certain bacteria can convert sulfur dioxide and other sulfur oxide products into hydrogen sulfide, which can be chemically or biologically converted to elemental sulfur. They have found that these bacteria can be maintained economically because they can live off sewage.
Sulfur, a natural component of coal that is released to the air as sulfur dioxide when coal is burned, is the largest selling element in the United States. It is used to make the largest selling commodity chemical-sulfuric acid, which is essential to the manufacture of plastics, fertilizers, and other products.
"A 500-megawatt coal-fired power plant that burns coal that is 3.5 percent sulfur with no sulfur dioxide control," Kaufman says, "has the potential of releasing up to 360 metric tons of sulfuric acid per day." Sulfuric acid, a component of acid rain, is formed in the atmosphere when sulfur dioxide reacts with moisture there.
American utilities must comply with increasingly stringent requirements to reduce emissions of sulfur dioxide to the atmosphere. These requirements are being phased in under the Clean Air Act Amendments of 1990 to protect the public from this air pollutant, which can cause potentially fatal respiratory illnesses, and to reduce the acidity of precipitation, which can slow the growth of forests and fish populations.
To conduct their bioprocessing experiments, Kaufman and Selvaraj introduced into a vertical glass cylinder, or bioreactor, two types of bacteria-sulfate-reducing bacteria (SRBs), which cannot tolerate oxygen, and heterotrophs, which remove oxygen to help the SRBs survive. The bacteria are fed a sewage digest that provides them with carbon, their main food source.
"Use of pretreated sewage as the source of carbon makes this process economical," Kaufman says. "Without this approach, we'd have to buy expensive organic acids to serve as the food source for the bacteria."
In the bioreactor, gelatin-like beads containing SRBs are suspended in sewage media through which sulfur dioxide flows. The SRBs convert the sulfur dioxide into hydrogen sulfide.
"We got the idea that SRBs can also break down the sulfate in calcium sulfate, which is gypsum," Kaufman says. "Gypsum is accumulating at many coal-fired steam plants as an end product of flue gas desulfurization, a process of removing sulfur dioxide from plant emissions by reacting it with calcium in limestone. By dissolving the gypsum in water or sewage media, we can make a slurry that can be passed through our bioreactor, where it is converted to hydrogen sulfide."
To reduce the amount of gypsum accumulating at coal plants, Kaufman and Selvaraj propose a process to reclaim sulfur from calcium sulfate and regenerate the limestone used to produce gypsum from sulfur dioxide.
"While the sulfur from the waste gypsum is reclaimed as elemental sulfur, calcium can be precipitated to form the original calcium carbonate, or limestone, using waste carbon dioxide from the coal plant," Kaufman says. "Thus, by making the process more efficient, we can at the same time reduce the coal plant's emissions of carbon dioxide, which can increase the greenhouse effect and possibly alter the climate."
Selvaraj is developing another bioreactor process for converting the hydrogen sulfide to sulfur. This process will use sulfur-oxidizing bacteria, which add oxygen to hydrogen sulfide, producing water and sulfur.
ORNL, one of DOE's multiprogram national research and development facilities, is managed by Martin Marietta Energy Systems, a Lockheed Martin company, which also manages the Oak Ridge K-25 Site and the Oak Ridge Y-12 Plant.