Steam generator Irreversibilities

The basic idea of using TCR in a gas turbine is usually to extract more heat from the turbine exhaust gases rather than to reduce substantially the irreversibility of combustion through chemical recuperation of the fuel. One method of TCR involves an overall reaction between the fuel, say methane (CH4), and water vapour, usually produced in a heat recovery steam generator. The heat absorbed in the total process effectively increases... 

More helium heat should be coupled in the steam reformer. In that case we were able to gasify more brown coal, because more hydrogen would be produced in the steam reformer, and in parallel we would reduce the irreversibilities in the steam generator... 

In a waste heat recovery system, we might reduce the heat transfer irreversibility by designing a heat recovery steam generator with a smaller stream-to-stream temperature difference, and/or reduce friction by designing a turbine with a higher efficiency. 

Almost 70% of the total exergy loss is lost in reforming section and the associated steam generation, mainly due to irreversibilities present in the combustion. 

The exothermic reactions occur in a reactor containing tubes packed with a precious metal catalyst on a silica support. Heat is removed from the reactor by generating steam on the shell side of the tubes. Water flows to the reactor from a steam drum, to which makeup w-ater (boiler feeder water BFW) is supplied. The steam leaves the drum as saturated vapor. The reactions are irreversible and the reaction rates have an Arrhenius-type dependence on temperature. 

Second law analysis has been widely used in the last several decades by many researchers. Exergy analysis usually predicts the thermodynamic performance of an energy system and the efficiency of the system components by accurately quantifying the entropy generation of the components [1]. Furthermore, exergoeconomic analysis estimates the unit cost of products such as electricity, steam and quantifies monetary loss due to irreversibility. Also, this analysis provides a tool for the optimum design and operation of complex thermal systems [1], [2], [3]. 

The formal result (6.2.10) can be physically interpreted as increased irreversibility on the average, increasing T or T2 heat flows from even higher temperatures to the final T. Still, what kind of inference can be drawn herefrom is left to further speculations. For instance increasing the combustion temperature T, can mean smaller excess of oxygen in the combustion chamber it is not immediately clear why more exploitable energy is thus wasted in the generator per unit steam production. See the next example, after formula (6.2.12). 

The performance of actnal steam turbines includes several different types of losses and irreversibilities. The efficiency of a steam turbine-generator (TG) is defined as the output at the generator terminals divided by the available energy. Typical industrial steam turbine efficiencies are 70 to 80 percent. The actual steam rate (ASR) is defined as the TSR divided by the turbine-generator efficiency. 

The exhaust gas is led through a recuperator to heat the air before it enters the stack. A temperature of at least 500°C is necessary to avoid thermal shock that may cause possible irreversible damage to the stack. An additional heat exchanger is used to produce hot water. By positioning this heat exchanger between the two recuperators, as shown in Figure 13.5, process steam can be generated instead of hot water. 

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