An emerging technology could offer a more efficient way of reducing air pollution from power plants. A Seattle-based company called ClearSign Combustion has demonstrated how to manipulate flames using high-voltage electric fields to prevent pollution from forming in the first place.
Much of the air pollution produced by today’s fossil-fuel power plants is the result of imperfect combustion. Hot spots in a flame increase the reactions between fuel and air molecules and lead to formation of common air pollutants like nitrogen oxides (or NOx, a precursor to smog), carbon monoxide (CO) and particulate matter. Pollutants are commonly scrubbed after combustion and before exhaust gases are released into the atmosphere. Exhaust can be recirculated back into the combustion chamber to burn up any remaining fuel or passed through a chemical process that strips out unwanted compounds (using ammonia and a catalyst)—consuming large quantities of energy in the process. "We need energy. It's a serious business”, says Michael Frenklach, professor of mechanical engineering at the University of California, Berkeley. “Scientists who are doing combustion research are basically trying to answer the question: How do you do it more effectively?"
Combustion can be improved by manipulating electrically charged particles (ions) in a flame to prevent hot spots—and therefore pollutants—from forming. Earlier this year, engineers at ClearSign Combustion demonstrated how a high-voltage electric field can control the shape and intensity of a flame while using only a small amount of power: less than one tenth of a percent of a furnace’s total power output.
Early test results suggest that controlling combustion with electric fields can lead to improved efficiency, compared with conventional methods. A more uniform flame shape means less fuel is wasted and exhausted as soot whereas more heat is transferred to surrounding surfaces (such as a boiler or heat exchanger for a crude oil treatment process). The biggest energy savings, however, could come from reducing or eliminating the need for conventional postcombustion pollution systems (like ammonia-based catalysts). The company claims its technology can lead to system-wide efficiency improvements up to 30 percent with comparable emissions reductions.
The tests used a boiler typical for a small refinery (one to two million British thermal units per hour, or MMBTU/h), and achieved simultaneous reductions of NOx, CO and soot—usually a difficult task. "Usually, a reduction in one comes at the expense of another," Frenklach says. "If you minimize soot, NOx goes up. If NOx goes down, soot goes up. We always have to fight this dichotomy.”
In the trials NOx emissions were reduced to two parts per million (ppm)—exceeding the strictest smog standards in the country. In southern California's South Coast Air Quality Management District industrial boilers between 20 and 50 MMBTU/h must reduce their emissions of smog-forming NOx to less than nine ppm by July 2014 and less than five ppm by January 2016. Industry groups anticipate that these limits will soon be required in other areas of the country, with national standards to follow.
The increased air and fuel mixing in the combustion chamber also reduces fine particulate matter, or soot, which is a product of incomplete combustion; fewer soot particles form if more fuel is burned. Soot particles that do form using this system are generally large enough to be collected and removed with inexpensive technology like cyclone separators. Particles less than 10 microns in diameter, however, pose a health concern because they can be inhaled into and accumulate in the respiratory system. Fine, airborne particulate matter smaller than 2.5 microns (about one seventh the width of a human hair) is considered dangerous because it is small enough to enter the passages of the human lungs.
The technology could also be well suited in markets like China where urban air pollution frequently reaches dangerous levels. Earlier this month, smog paralyzed Shanghai where the pollution index was "23 times and 31 times” the levels recommended by international health officials. But challenges remain as ClearSign works to scale-up current technology to larger furnaces and boilers as it maintains effective air–fuel mixing and low power usage. The company plans to test combustion chambers in the range of five to 10 MMBTU/h like those used for small power plants. “We began with flames not much bigger than my little finger”, says Joe Colannino, CTO at ClearSign. “If we can get the same kind of results that we're seeing at one million Btus per hour at five [million] and 10 million Btus per hour, we'll be good to go.”