OPTIMUM OPERATION FOR DIFFERENT DESIGN CONDITIONS OF A SINGLE PRESSURE HEAT RECOVERY STEAM GENERATOR (HRSG) FOR MINIMUM ENTROPY GENERATION
Heat Recovery Steam Generator (HRSG) is the critical link between the “topping” gas and “bottoming” steam power cycle of a combined power plant. Depending on the mass flow and property of exhaust flue gas from the gas cycle, optimum design and operation, for minimum entropy generation in a single pressure HRSG generating superheated steam is reported in this paper. It shows that an optimum pressure of steam generation exists for minimum entropy generation. The interdependence between the physical size (and hence cost) of different heat transfer components of HRSG with the thermodynamic optimum operation for minimum entropy generation is presented. Optimum pressure of steam generation increases with greater size of super heater, economizer and evaporator up to certain limit, beyond which the increase in optimum saturation pressure is marginal with increase in size of the heat transfer units. However, increasing evaporator size and degree of superheat have the adverse effect of lesser steam generation for a given economizer and super heater for minimum entropy generation.
Heat-recovery steam generators: Understand the basics
Gas turbines with heat-recovery steam generators (HRSGs) can be found in virtually every chemical process industries (CPI) plant. They can be operated in either the cogeneration mode or the combined-cycle mode. In the cogeneration mode, steam produced from the HRSG is mainly used for process applications, whereas in the combined-cycle mode, power is generated via a steam turbine generator. Recent trends in HRSG design include multiple-pressure units for maximum energy recovery, the use of high-temperature superheaters or reheaters in combined-cycle plants, and auxiliary firing for efficient steam generation. In addition, furnace firing is often employed in small capacity units when the exhaust gas is raised to temperatures of 2,400--3,000 F to maximize steam generation and thus improve fuel utilization. This article highlights some of the basic facts about gas turbine HRSGs. This information can help plant engineers, consultants, and those planning cogeneration projects make important decisions about the system and performance related aspects.
Effect of stratification on energy and exergy capacities in thermal storage systems
The increase in exergy storage capacity that is attained in thermal storages through stratification is assessed. A design‐oriented temperature‐distribution model for vertically stratified thermal storages that facilitates the evaluation of storage energy and exergy contents is utilized. The paper is directed towards demonstrating the thermodynamic benefits achieved through stratification, and increasing the utilization of exergy‐based performance measures for stratified thermal storages. A wide range of realistic storage‐fluid temperature profiles is considered, and for each the relative increase in exergy content of the stratified storage compared to the same storage when it is fully mixed is evaluated. The results indicate that, for all temperature profiles considered, the exergy storage capacity of a thermal storage increases when it is stratified, and increases as the degree of stratification, as represented through greater and sharper spatial temperature variations, increases. Furthermore, the percentage increase in exergy capacity is greatest for storages at temperatures near to the environment temperature, and decreases as the mean storage temperature diverges from the environment temperature (to either higher or lower temperatures). It is concluded that (i) the use of stratification in thermal storage designs should be considered as it increases the exergy storage capacity of a thermal storage and (ii) exergy analysis should be applied in the analysis and comparison of stratified thermal storage systems. Copyright © 2004 John Wiley & Sons, Ltd.