CHAT Technology: An Alternative Approach to Achieve Advanced Turbine Systems Efficiencies With Present Combustion Turbine Technology
Authors: Nakhamkin Michael , Swensen Eric C. , Touchton George , Cohn Arthur , Polsky Michael 
Published: December 2014

An approach to improving the thermal cycle efficiency of combustion turbine (CT) based power plants is to develop thermal cycles with interceding, reheat, recuperation and humidification. Until recently, this was viewed by combustion turbine manufacturers as cost prohibitive and involving new operating and maintenance challenges. Also, early attempts by some manufacturers to develop sophisticated thermal cycles resulted in the realization that significant funds, personnel, and time are required. This investment could not be justified, particularly considering the availability of efficient and economical combined cycle (CC) plants.

Therefore, increased efficiency for both simple and CC power plants has been achieved by raising the firing temperature and pressure of the basic Brayton cycle. However, every increase in the CT firing temperature required progressively higher development cost and increased NOx control challenges, which has re-awakened interest in advanced cycles.

The Department of Energy’s Federal Energy Technology Center (FETC), in cooperation with combustion turbine manufacturers, is working on an Advanced Turbine Systems Program. The program goal is to develop technologies to provide a significant increase in natural gas-fired CC power generation plant efficiency with thermal efficiency target values in excess of 60%. Materials published in the program show that participating large CT original equipment manufacturers (OEM), are relying heavily on an increase in the CT’s firing temperature to approximately 2600 F (1700 K), with associated advancement in materials and cooling techniques, to achieve the target efficiency. Also employed are some improvements in component efficiency and various methods of utilization of heat in the bottoming cycle. The development of the necessary sophisticated materials and cooling techniques requires very significant development costs and is based on long duration and expensive experimental investigations and field demonstrations. Increasing the bottoming cycle efficiency primarily depends on the practicality of engineering solutions and capital vs operating cost trade-offs, and not on technology advancements.

The current Cascaded Humidified Advanced Turbine (CHAT) technology, which utilizes existing, commercially proven combustion turbine and industrial hardware integrated in sophisticated thermal cycles, offers an achievable, practical and cost-effective alternative to a current CC plant. Current CHAT plants require relatively minimal engineering developments associated primarily with a) modification of the power shaft CT’s compressor discharge and turbine inlet plenums — for interfacing the HP shaft and other thermal cycle components — a very important engineering task, but not comparable in complexity to the development associated with further increase of the CT inlet temperature; and b) engineering of an HP expander with an inlet temperature of 1600 F (1145 K), essentially integrating steam turbine and industrial expander technologies.

As it was shown collectively in the previously published references, in addition to an efficiency equal to that for CC plants (based on the same CTs), CHAT plants have significantly lower (10–20%) specific capital costs and have important operating advantages (higher than CC efficiency at part-load operation, with excellent load following and dynamic benefits, including rapid start capability). Those features reduce both the CHAT plant cost of electricity and offer a method to improve improve the economics of power generation systems due to the operational flexibility added by a CHAT plant.

One of the most effective ways to increase the CHAT plant efficiency is to increase the HP expander inlet temperature from the current level of 1600 F (1145 K), which represents the level of the combustion turbine technology of the late 1960’s – early 1970’s. EPRI and ESPC have identified that a CHAT plant, based on the current combustion turbine technology (with turbine inlet temperature (TIT) of 2550 F (1670 K)), could achieve the ATS Program target efficiency of 60% with an HP expander inlet temperature of approximately 2000 F (1365 K). HP expanders with this relatively low turbine inlet temperature, as shown later, will require cooling of only the first stage nozzles and stage blades, if the newest single crystal alloys are used. However, the increase of the HP expander inlet temperature will be complicated by the relatively-high inlet pressure. This presents a significant engineering challenge, particularly if one would like to preserve the excellent start-up characteristics and other dynamic benefits of the CHAT plant.

EPRI and ESPC are co-sponsoring the development of a high pressure expander with a target inlet temperature of 2000 F (1365 K). It will be shown that this HP expander, when integrated with a power shaft based on W 501 FA modified for the CHAT application, results in a power plant that will achieve the ATS Program target efficiency of 60%.

This paper presents a) current CHAT plant’s performance and cost characteristics, b) findings of the project for the development of the HP expander with target temperature of 2000 F (1365 K), and c) a comparison of the advanced CHAT concept’s performance and development costs to those of the ATS program. This paper also shows how, in the future, the new ATS technology can be incorporated into even more efficient, cost-effective and reliable CHAT power plants.