If pollution had a mascot, it would be the smokestack. Do a Google image search for "pollution." What do you see? A bunch of smokestacks with ominous gray clouds billowing out the top.
It is a reasonable association to make. Smokestack emissions contain nasty chemicals, including mercury, sulfur dioxide and nitrogen oxides — not to mention carbon dioxide, the biggest threat to our environment over the long term.
But there have been some interesting breakthroughs in the past couple of years in the management of power plant emissions.
One strategy is to capture the energy that remains embedded in smokestack gas. Even after combustion of fossil fuels, there is a significant amount of energy bound up in the carbon dioxide itself. Pumping that energy into the atmosphere is a missed opportunity. If we can recapture it instead, the amount of fossil fuel we would have to burn — and smokestack gas we would need to release — would drop noticeably.
The idea behind smokestack gas extraction grew out of a technology called blue energy, which is based on the principle of osmosis: particles move spontaneously from areas of high concentration to areas of low concentration.
"If you take a lump of sugar and put in tea, the sugar dissolves and spreads through the tea in a spontaneous process," said Bert Hamelers of Wetsus, a sustainable-water technology company. "That's the energy we're using."
Experimental blue energy facilities are built where saltwater and fresh water meet. A membrane that allows water — but not salt — to pass through is placed between the two types of water. Nature wants to eliminate the difference in salt concentration, so fresh water floods across the membrane to dilute the salt. That movement can power a turbine and generate electricity.
The smokestack technology applies this principle to a carbon dioxide concentration gradient. Smokestack emissions are between 5 and 20 percent carbon dioxide. Ordinary air is less than 0.04 percent carbon dioxide.
As with blue energy, water would be involved. The system would be based around two containers of water — one with dissolved air and the other with dissolved smokestack gas — separated by a membrane that allows only particles with a certain electrical charge to pass through.
Ions from the dissolved air would cross the barrier to dilute the carbon dioxide molecules on one side of the membrane. Just as the movement of water powers the turbine in a blue energy system, this movement of ions would power an electrical cell in the smokestack gas system.
So how far off is real-world use of this technology?
"Our development of blue energy technology has taken about 10 years, and the first pilot plant will open in October or November in the Netherlands," Hamelers said. "We have similar technological development issues to work out in the carbon dioxide system, but once that's done, it could happen quickly."
If it works, the results could be significant. Depending on how efficient the extraction process becomes, installing such systems on power plants as well as the world's home and industrial heaters could produce 400 times the energy generated by the Hoover Dam, according to Hamelers' estimates.
Scientists are developing a more biologically based strategy closer to home. Kathryn Coyne and Jennifer Stewart of the University of Delaware are researching the capacity of algae to process smokestack gas.
The idea of feeding power plant emissions to algae has been around for years. Like other plant life, algae breathe in carbon dioxide. The challenge, however, has been the high levels of nitric oxide in smokestack gas. Most algal species can't process the compound, and they die in the presence of high concentrations. Coyne and Stewart recently stumbled upon a special species.
"The algal species H. akashiwo has the unique capability to convert nitric oxide into nitrate," Coyne said. "It then converts nitrate into nitrite and then into ammonium. Ammonium is a building block for amino acids the algae needs."
The algae process would also happen in water. The smokestack emissions would be bubbled through a pond next to the power plant. The algae in the pond would use the nitrate and carbon dioxide to multiply.
At the end, the power plant would have an enormous algal bloom. In a bay or estuary, an algal bloom is a terrible problem. In a power plant, however, an algal bloom can be a beautiful thing. Algae contain lipids, a kind of fat packed with useful energy. Engineers can extract the lipids and use them as biodiesel, thereby squeezing extra energy out of the power plant's fossil fuels.
When will we see little ponds of algae next to our power plants?
"It's hard to say," Coyne said. "We're currently experimenting on small samples, and algae can be difficult to scale up. On the other hand, this species is capable of living in high densities in marine environments, so it may be easier than usual."