The Distinction Between the Cheng and STIG Cycles

Abstract

This paper identifies distinct features of the Cheng Cycle as compared to the steam injected gas turbine, STIG. Development started on the Cheng Cycle in 1974. After eight years of research and testing, the Cheng Cycle was commercialized in 1982. The commercial opportunity came by winning one of the State of California’s Energy Commission sponsored bids at the San Jose State University campus. The first Cheng Cycle power plant was built around the Allison 501KB gas turbine. The project was won on the merit of excellent thermal efficiency with maximum flexibility. It is also the most economical system because it can follow fluctuating electrical and steam loads independently. Financing, licensing and all appropriate permits were completed within one year. It took less than a year to construct and was on line by the end of 1984. Immediately, several distinct features were noticed: (1) the Cheng Cycle boosts power by 70% and efficiency by 40% over the simple cycle, (2) it can follow the electric and steam loads independently, (3) it demonstrated low emission and established 25 ppm NOx as BACT for the San Francisco Bay Area Air Quality District. In 1987, GE introduced their Steam Injected Gas Turbine, STIG, using the LM 2500 and LM 5000, and in the 1990’s GE also introduced the LM1600 version of STIG. The high pressure ratio of those engines resulted in low exhaust temperature. That is not efficient enough to power a steam cycle. Unfortunately, STIG confused some users into thinking that every steam injected gas turbine was a Cheng Cycle. STIG uses the traditional constant pressure waste heat boiler technology. Operation is limited to near full load because low exhaust temperature at partial load would cause dysfunctional heat imbalance in the heat recovery steam generator (HRSG). The Cheng Cycle, in comparison, adopted a variable pressure HRSG so its operating range extends from idle to full load. This variable pressure HRSG allows full heat recovery, whereas STIG has to limit its operating range to maintain heat transfer balance. This unique HRSG design means that the Cheng Cycle is a thermal feedback cycle. As in any feedback system it could oscillate, in this case the oscillations are between fuel-flow and steam-flow. The Cheng Cycle utilizes digital control technology to the system. The integrated system provides the user with smooth operation and rapid start-up and load change capability.