While turbine OEMs continue to push the NO_x envelope, a variety of smaller equipment manufacturing firms and engineering consultants have joined the race to design and implement retrofit solutions to help existing turbine operators reach NO_x levels below 5 ppm, reports Suzanne Shelley in an article in Turbomachinery International. Power Systems Mfg, LLC, for instance, offers the LEC-III, an advanced dry low NO_x combustion system, that guarantees NO_x levels below 5 ppm. A dozen commercial installations are in operation. The LEC-III can be retrofit to GE Frames 6B, 7B-E, 7E/EA and 9E and the Siemens and Mitsubishi W501D5 and the company is pursuing a lean-premix option for F-class turbines. Power Systems says it has devoted a lot of attention to improving the uniformity of the air-fuel mixture. While designing the system, Power Systems made 15 to 20 geometrical and other changes to the fuel nozzles and combustion chamber, including a multitude of effusion holes.

Alzeta Corp. has patented nanoSTAR injectors, whose porous, metal-fiber mat surface promotes the formation of precisely controlled high-flow and low-flow flame zones. While low-NO_x combustors using the nanoSTAR injectors are not yet commercially available, the technology has been tested on combustor rig tests, where NO_x levels below 2-3 ppm were achieved. Alzeta is working with Solar Turbines to test and demonstrate the technology, but the nanoSTAR technology will work with any turbine.

John Zink offers a low-NO_x duct burner, which also incorporates staged combustion using proprietary injectors to delay fuel-air mixing within the existing burners. As of May/June 2006, John Zink had installed ten commercial-scale systems. With the addition of John Zink’s low NO_x duct burners, plus some additional turbine modifications, a Texas polypropylene facility was able to reduce NO_x emissions on its GE Frame 7EA system to below 10 ppm (corrected to 15 percent O₂) without the addition of SCR.

Precision Combustion (PCI) is developing catalytic combustion injectors that target 2 ppm NO_x (at 15 percent O₂) across a wide turndown range without the use of SCR. PCI’s catalytic combustor is a two-stage system. The first stage burns a portion of the fuel in a fuel-rich catalyst, and uses the remainder of the compressor air to cool the catalyst. In the second stage, the two streams (heated cooling air and partially oxidized fuel) merge to form a hot, lean mixture that can be burned more stably, with lower NO_x and lower combustion dynamics. The company plans to bring its first product to engine trial in 2007.

Power Tech Associates has designed ultra-low NO_x solutions for turbines equipped with conventional diffusion-flame combustors. Power Tech Assoc.’s approach has been to modify the combustor design itself (by altering air distribution within the combustor and refining fuel nozzle design), and add upstream water or steam injection and downstream SCR. To date, the company has modified the existing diffusion-flame combustion systems in about 30 different turbines.