Three years ago, an article in POWER described how Cheng Power Systems, by modifying the combustors of several popular gas turbines, had used steam injection to lower the units’ NOx output to about 5 ppm—but some models had substantial CO levels without combustor modifications. Since then, the company has developed new combustor nozzles that recently proved capable of limiting a turbine’s emissions of both NOx and CO levels to even lower single-digit levels. Field test results for a naphtha-burning LM2500 are presented.
Gas turbine users have successfully used steam injection for power augmentation and NOx control for more than 30 years. A small amount of steam injected into the turbine’s combustion nozzles or compressor discharge plenum was all that was needed to meet early limits on emissions of the pollutant. A pleasant side effect was a noticeable increase in power output—assuming the turbine could handle the added mass flow.
Back in 1982, Dr. Dah Yu Cheng of Cheng Power Systems (CPS) saw the commercial possibilities of injecting steam into small and midsize turbines in industrial cogeneration plants that use a heat-recovery steam generator (HRSG) to produce process steam. The commercialization of the technology through his company—International Power Technology (San Jose, Calif.)—began with the 1984 installation of the “Cheng Cycle” on an Allison 501-KH turbine on the campus of San Jose State University in California (POWER, June 2003, p. 51). On that project, capacity rose from 3.25 MW to 5.6 MW, combustion efficiency increased from 28% to 40%, and NOx remained less than 21 ppm.
Further tests demonstrated that it was possible to significantly increase the output of a number of gas turbines of that era by increasing the nozzle steam/fuel mass ratio to 1.5. At the time, several gas turbines were being marketed with a steam injection option for boosting peaking power. Among them was the GE LM5000 steam-injected gas turbine (STIG), which promised an increase in capacity from a non-injected 33.5 MW to a fully injected 50 MW. When NOx reduction via steam injection hit a lower limit of 25 ppm, users turned to selective catalyst reduction (SCR) and CO catalysts to reach single-digit levels, which many localities were beginning to require.
Don’t confuse the Cheng Cycle with the standard STIG cycles available from original equipment manufacturers. STIG uses traditional, constant-pressure waste heat boiler technology. Operation of STIG cycles is limited to near full load, because low exhaust temperature at partial load would cause heat-transfer imbalances in the HRSG. But because the Cheng Cycle requires the use of a variable-pressure HRSG, its operating range extends from idle to full load.
How low can you go?
The technology development that has made possible even lower NOx levels—and now, CO levels as well—is the Cheng low-NOx (CLN) system. Simple to explain but difficult to implement, the CLN system precisely meters steam into the fuel gas under controlled conditions, unlike earlier systems that injected steam into the compressed air before it enters the combustor. With a properly designed set of combustor nozzles, the homogenous mixture of fuel and steam increases diffusion rates and improves the rate of combustion. The results are a lower peak flame temperature, more uniform temperature distribution, and a shorter combustion residence time—all of which combine to inhibit NOx formation.
The CLN system has been in continuous operation at the at the SRI Cheng Cycle Cogen plant for the past eight months. The NOx level has been maintained below 7 ppm. Rig tests of the Allison combustor shows the potential of reaching much lower NOx than that, if properly designed fuel nozzles and fuel manifolds can be installed. Potentially, there are eight more of the Allison 501-based Cheng Cycles in California, 15 in Australia, 18 in Europe, and hundreds in Japan that could adopt the same process. A partial Cheng Cycle (Cheng Boost) also is benefiting two GE 6B turbines in combined-heat-recovery service at a California refinery. But perhaps the most interesting installation of a partial CLN system is on a naphtha-burning LM2500 Cheng Cycle in Kauai, Hawaii, that went into service in 2002. More on that plant in a moment.
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