Efficiency exhaust heated combined cycles

Dearation can be either vacuum or over pressure dearation. Most systems use vacuum dearation because all the feedwater heating can be done in the feedwater tank and there is no need for additional heat exchangers. The heating steam in the vacuum dearation process is a lower quality steam thus leaving the steam in the steam cycle for expansion work through the steam turbine. This increases the output of the steam turbine and therefore the efficiency of the combined cycle. In the case of the overpressure dearation, the gases can be exhausted directly to the atmosphere independently of the condenser evacuation system. 

Because fuel costs are high, the search is on for processes with higher thermal efficiency and for ways to improve efficiencies of existing processes. One process being emphasized for its high efficiency is the gas turbine combined cycle. The gas turbine exhaust heat makes steam in a waste heat boiler. The steam drives turbines, often used as lielper turbines. References 1, 2, and 3 treat this subject and mention alternate equipment hookups, some in conjunction with coal gasification plants. 

Dual pressure For comparison, a combined cycle scheme with dual pressure is shown in Figure 15.13. In this case, the waste heat recovery boiler also incorporates a low-pressure steam generator, with evaporator and superheater. The LP steam is fed to the turbine at an intermediate stage. As the LP steam boils at a lower temperature than the HP steam, there exists two pinch points between the exhaust gas and the saturated steam temperatures. The addition of the LP circuit gives much higher combined cycle efficiencies with typically 15 per cent more steam turbine output than the single pressure for the same gas turbine. 

A Rankine/Rankine combined cycle is shown in Fig. 5.16. The exhaust from the top steam turbine (TURl) is hot enough to generate freon vapor in a waste-heat boiler. The freon vapor generated can power a freon turbine, thus increasing the total work produced. The Rankine/Rankine combined cycle has a thermal efficiency greater than either a steam or freon cycle may have by itself. The power plant occupies less area, and the fuel requirements are less. 

In conclusion, it would seem that either recuperated case would always be preferred. However, the cost of a recuperator can be significant. Also, the analyses performed fixed the turbine inlet temperatures at fairly low values. Typically with larger heavy-duty turbines the temperature is pushed to material limits and heat recuperation from the turbine exhaust is done in a combined cycle (steam bottoming cycle). Such a system can achieve close to 60% LHV efficiency on natural gas. Section 8.4 will consider whether a steam bottoming cycle is potentially appropriate for a hybrid system. A steam cycle is generally not economical for smaller systems. 

The wave of the future in power generation is the combined cycle, in which gas turbines are combined with steam turbines, with the hot exhaust from the gas turbine used to generate steam. The combined cycle is a cascade of heat engines operating over temperatures from 1200-1300°C to about 30°C. This broad temperature range renders the combined cycle efficient. 

The main competition to supercritical system is from new gas turbine combined-cycle plants which are now expedited to achieve an overall efficiency of 60%, making a huge difference in generating and life-cycle costs. However, the new gas turbines will release exhaust into waste heat recovery steam generator at temperatures above 600°C (1110°F), thus necessitating the use of the high-chromium steel and nickel alloys as used in the supercritical coal-fired plants. 

The pressurized bed was developed in the late 1980s to further improve the efficiency levels in coal-fired plants. In this concept, the conventional combustion chamber of the gas turbine is replaced by a PFBC. The products of combustion pass through a hot gas cleaning system before entering the turbine. The heat of the exhaust gas from the gas turbine is utilized in the downstream steam turbine. This technology is called PFBC combined cycle (Figure 22.8). 

Gas turbines provide an alternative to steam turbines. They must be constructed of materials suitable to the high temperatures associated with combustion gases. Simple gas turbines produce efficiencies comparable to those of simple steam turbines. A combined-cycle unit that also recovers thermal energy benefits from the high temperature of the exhaust gas. Recovery of a major part of this energy, which is possible because there is not a huge latent-heat penalty, can raise the overall efficiency to as high as 80%. 

A relatively new technology for electric power generation is a combined cycle plant. These use gas turbines driving electric generators for the majority of their power production. However, gas turbines alone have relatively low efficiencies (about 35 percent), because the exhaust gas contains a lot of unused heat energy. In a combined cycle plant, this hot exhaust gas is used to produce steam, and the steam drives a turbine that powers another electric generator. The efficiency of combined cycle plants is very attractive (50 percent or more). 

The fact that thermodynamically, some of the heat generated in the electrochemical reaction can be reinvested into the chemical fuel conversion explains that SOFC have the potential to reach a net electrical system efficiency above 60% with natural gas fuel [14]. In a combined cycle system, where the exhaust gas from the high-temperature fuel cell is used to drive a gas turbine, the overall system electrical efficiency may even reach over 70% [15]. The efficiencies of gas turbines and diesel engines decrease at part load, whereas the efficiency of the fuel-cell-based systems will be almost independent of part load up to very high turndown ratios [14]. 

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