How an alternative to combined cycles never saw the light of the day

At the 2019 ASME Turbo Expo, Hans E Wettstein, formerly of BBC that was taken over by ABB then by Alstom, recounted his contributions to gas turbine development including the GT24 and GT26. He said that it was known to developers that the gas turbine may have trouble delivering on the promised performance. But the GT24 and GT26’s other features continued to be a draw: low NOx and CO emissions all the way down to 20% of baseload, as well as tolerance to high C2+ fuel gases. This made for the most flexible gas turbine at that time, he said.

It was at that time that the research group in the company that had been taken over by Alstom proposed a Semi Closed Recuperated Cycle (SCRC). This was a supercharged GT with partial exhaust gas recirculation. It had no bottoming cycle yet challenged the combined cycle in power density, efficiency, simplicity as well as high and constant part load efficiency down to very low load. The cycle was CO2 capture ready and, importantly enough, featured combustion water condensation. This meant there was no need for make-up water for the wet compression, which was developed by Alstom, and used in the SCRC.

The semi-closed recuperated cycle (SCRC) has since then seen several versions. One uses two compressors with one intercooling stage each. In another, the intercooled main compressor has been replaced by a compressor with high fogging and no intercooling anymore. It is assumed that the system and the main compressor have its design points in the middle of the intended fogging water injection range. This turns out to allow another thermal efficiency gain by 2 to 3 percent points to clearly above 60% also combined with increased specific power related to the consumed combustion air and with no bottoming cycle.

The cycle uses proven turbomachinery technologies. The thermodynamic assumptions have been derived from existing gas turbine (GT) technology and are used within already confirmed operating ranges. With the same firing temperature also the thermal efficiency level of current Combined Cycles (GTCC) can be achieved, Wettstein claims.

A special feature of the SCRC is the opportunity for inventory control of part load operation. This means that part load operation can be made by pressure reduction instead of temperature reduction as in open gas turbines. Thermal transients leading to hot part life consumption can therefore be avoided to a large extent and the combustor can operate at nearly constant temperature also at low part load with corresponding low emissions. Low part load operation achieves the same efficiency as base load. The result is more flexibility than in current GTCC technology associated with less complexity due to the needlessness of an extra bottoming cycle.

Due to its specific features the SCRC in general or with wet compression could be developed in the micro turbine power output size as well as up to above 1000MW single block size. Its inherent water condensation at elevated pressure makes an external source of make-up water obsolete.

Wettstein says that the SCRS was shot down in Alstom because it did not get the support of sales departments responsible for bottoming cycle components such as the steam turbine. He has recounted this in his paper, “80 YEARS OPEN GT DEVELOPMENT IN BADEN,” presented at the conference. As a result, the SCRC was never developed, he says.