Chapter 9
Semi-Closed-Cycle Gas Turbines


In closed-cycle gas turbines, the heater for gas, liquid or solid fuel accounts for a large portion (around 40%) of the plant costs. Also, it is of an enormous volume and requires a great deal of space. However, closed, charged gas turbines systems have the advantage of increased heat transfer, which means the other heat exchanging units — the pre coolers, intercoolers and recuperator — can be of a compact design. The turbo set can also be markedly reduced in size due to the increased density of the working medium.

Therefore, right from the start of gas turbine construction, engineers aimed to develop turbocharged systems with internal combustion. Sulzer Winterthur of Switzerland built a plant of this type with an output of 20 MW in the 1950s for an NOK power plant in Weinfelden. Unfortunately, there were major problems during operation, which were caused by the use of heavy oil as fuel. The units became dirty and the flow machines suffered excessive wear. Furthermore, there was corrosion due to the vanadium contained in the fuel, a problem, which was unknown at the time and could not have been foreseen. Figure 136 is a cycle diagram of this plant, which was nevertheless remarkable. Sulzer later replaced it with two 10 MW open cycle gas turbines [35]. (NOK owned two other gas turbines in Beznau, although these were of an open design, which in spite of a complex, double shaft construction with inter cooling for the compressor, intermediate combustion and recuperation, operated very well. They were built by Brown Bovery & Cie in Baden and operated from 1948 to the end of the century. Their electrical power was 13 and 27 MW respectively).

It is, however, quite possible that semi-closed gas turbine systems can be feasible if a clean fuel such as natural gas is used. Thus, studies have recently been performed for carbon dioxide gas turbine cycles with internal firing of natural gas and oxygen as an oxidizer. The idea behind this was to find a way of extracting the carbon dioxide generated by combustion in liquid form so that it could be disposed of. In a cycle system such as this, the carbon dioxide would not be at low partial pressure, as in the flue gases of conventional plants, but practically under total pressure. Therefore, it could be liquefied using a heat sink at ambient temperature.

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