Hot prospect: simple burner keeps pollution counts down.

A new type of combustion chamber reduces pollution with less complexity and a safer, more reliable design than other low-emission burners, the device's developers say.

The novel chamber, or combustor, could replace conventional chambers in water heaters, power turbines, and perhaps even jet-aircraft engines, says Ben T. Zinn of the Georgia Institute of Technology in Atlanta, who heads the invention team.

As one way to meet increasingly stringent legal limits on emissions of pollutants such as nitrogen oxides ([NO.sub.x]) and carbon monoxide, combustor makers incorporate premixers that function much like carburetors in old ear engines. In a typical premixer, internal baffles force a dilute blend of fuel in air to swirl around and mix thoroughly before being ignited. Without regions of concentrated fuel, the mixture burns coolly, which limits [NO.sub.x] production.

There are penalties to premixing, however, says Zinn. For one, the flame in the chamber can ignite the fuel-air mixture in the premixer--an event known as flashback--sometimes causing an explosion. Also, premixers add complexity and costs to combustor design and manufacturing, he says.

The Georgia Tech combustor differs dramatically from typical low-emission combustors, Zinn says. A conventional pipelike combustor takes in an air-fuel mixture at one end, burns it, and then expels exhaust out the other end. In contrast, the new device both receives its air and fuel--as separate streams--and rejects its exhaust at the same end. The other end is closed.

A nozzle injects fuel and air down the middle of the combustor cylinder as hot exhaust gases rise along the walls, explains team member Jerry M. Seitzman. Ignition of some incoming material by hot exhaust as the flows meet roils the remaining air and fuel so that they mix well. The result is a constant flame. The device can cleanly burn either gases or liquid fuels.

Because there's no premixer, flashback can't occur. Moreover, the design keeps the device simple and potentially cheap, Zinn contends. He adds that the prototypes also avoid two other problems of low-emission combustors--an easily blown-out flame and potentially damaging pressure fluctuations.

"The real breakthrough here is that they burn the nonpremixed gas or liquid fuel and get those low emission numbers," comments Robert K. Cheng of Lawrence Berkeley (Calif.) National Laboratory.

Priya Gopalakrishnan of the Georgia Tech team plans to describe the device in August at a combustor symposium in Heidelberg, Germany.