Steam cycles

Limitations in the material of constmction make it difficult to use the high temperature potential of fuel hiUy. This restriction has led to the insertion of gas turbines into power generation steam cycles and even to the use of gas turbines in preheating air for ethylene-cracking furnaces. 

Simplified Cycle. A simplified fossil steam cycle appears in Figure 19. The water accumulates in the bottom of the condenser, called the hotweU. It goes through a feed pump to pressurize it. The pressurized water passes through one or more feedwater heaters, which raise the temperature. The water then enters the boiler where heat from the fuel converts it to steam. The steam expands through the engine, usually a turbine, which extracts work. In the middle of the turbine some of the steam is extracted to supply heat to the feedwater heater. The remainder expands through the turbine and is condensed. The rejected heat is carried away by the condenser coolant, which is usually water, but sometimes air. The condensed steam then returns to the... 

Feedwater The feedwater for a steam cycle must be purified. The degree of purity depends on the pressure of the boiler. Higher pressure boilers require higher feedwater purity. There is some trade-off between feedwater purity and boiler blowdown rate. However, increasing blowdown rate to compensate for lower feedwater purity is expensive, because blowdown water has been heated to the saturation temperature. Typical feedwater specifications for utihty boilers are given in Table 4. To some extent turbine steam purity requirements determine the feedwater purity requirements. The boiler-water siUca required to maintain adequate steam purity for higher pressure steam turbines is considerably less than the boiler could tolerate if deposition in the boiler were the only issue. 

Water Treatment. Water and steam chemistry must be rigorously controlled to prevent deposition of impurities and corrosion of the steam cycle. Deposition on boiler tubing walls reduces heat transfer and can lead to overheating, creep, and eventual failure. Additionally, corrosion can develop under the deposits and lead to failure. If steam is used for chemical processes or as a heat-transfer medium for food and pharmaceutical preparation there are limitations on the additives that may be used. Steam purity requirements set the allowable impurity concentrations for the rest of most cycles. Once contaminants enter the steam, there is no practical way to remove them. Thus all purification must be carried out in the boiler or preboiler part of the cycle. The principal exception is in the case of nuclear steam generators, which require very pure water. These tend to provide steam that is considerably lower in most impurities than the turbine requires. A variety of water treatments are summarized in Table 5. Although the subtieties of water treatment in steam systems are beyond the scope of this article, uses of various additives maybe summarized as follows ... 

Fig. 30. Schematic of nuclear steam cycle where BFPT = boiler feed pump turbine. To convert MPa to psi, multiply by 145.

G. J. Silvestri, Jr., Steam Cycle Peformance, Power Division, ASME, New York. 

There is, however, only a limited quantity of by-product power available, and for large process operations the demand for power is usually far greater than the simple steam cycle can produce. Many steam system design decisions fall back to the question of how to raise the ratio of by-product power to process heat. One simple approach is to limit the turbines that are used to extract power to large sizes, where high efficiency can be obtained. 

Most gas turbine appHcations in the chemical industry are tied to the steam cycle, but the turbines can be integrated anywhere in the process where there is a large requirement for fired fuel. An example is the use of the heat in the gas turbine exhaust as preheated air for ethylene cracking furnaces as shown in Figure 4 (8). 

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. 

Chodkiewicz, R. A Recuperated Gas Turbine Incorporating External Heat Sources in the Combined Gas-Steam Cycle, ASME Paper No. 2000-GT-0593. 

Macchi, E,. Consonni, S,. Lozza. G, and Chiesa, P. (1995), An assessment of the thermodynamic performance of mixed gas-steam cycles. Parts A and B, ASME J. Engng Gas Turbines Power 117, 489-508. 

Fig. 7.3. Open circuit gas lurbine/closed steam cycle combined plant (CCGT). No supplementary firing...

The interaction between the gas turbine plant and the steam cycle is complex, and has been the subject of much detailed work by many authors [5-8]. A detailed account of some of these parametric studies can be found in Ref. [1], and hence they are not discussed here. Instead, we first illustrate how the efficiency of the simplest CCGT plant may be calculated. Subsequently, we summarise the important features of the more complex combined cycles. 

For the steam plant, the condenser pressure, the turbine and pump efficiencies are also specified there is also a single phase of water/steam heating, with no reheating. The feed pump work term for the relatively low pressure steam cycle is ignored, so that /ij, = /i. For the HRSG two temperature differences are prescribed ... 

With the gas temperature at turbine exit known (T ), the top temperature in the steam cycle (T ) is then obtained from (a). It is assumed that this is less than the prescribed maximum steam temperature. 

Fig. 7.8. Single pressure steam cycle system with LP evaporator in a pre-heating loop, as alternative to feed...

Rufli, P.A. (1987), A systematic analysis of the combined gas-steam cycle, Proc. ASME COGEN— Turbo I, 135-146. 

C3 Rankine type double steam cycle Closed upper cycle None Hydrogen/air None High efficiency... 

Fig. 8.15 shows a simple gas turbine plant (Cycle Cl) supplied with a mixture of hydrogen and nitrogen for combustion in air a cooler is shown but a bottoming steam cycle may be added (see later, C2, C3). 

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