The US market is currently making a double jump in its HRSG requirements. Heretofore, HRSGs were used largely in industrial size cogen applications. According to the PURPA (Public Utility Regulatory Policy Act), public utilities were required to purchase that electric power generated in excess of the steam host’s needs. Thus, HRSGs were relatively small and operated under constant conditions. Now, HRSGs are much larger (utility size) and also more complex due to the introduction of triple pressure plus reheat behind powerful heavy duty gas turbines. With the onset of deregulation and consequent merchant power, combined cycle plants are now required to supply electrical power to the grid as and when needed with consequent day/night and weekday/weekend cycling. Those merchant plants have to come on and off line with minimal notice and be run sometimes at partial loads. Even units which were originally designed for base load are all eventually forced to cycle as new more efficient power plants are built. Thus, substantial changes in basic HRSG design are needed to cope with these changes. Coincidentally, the types of service projected for USA HRSGs have been in effect in Europe for over two decades. For this reason, European HRSG manufacturers/operators have adopted cycling tolerant Vertical HRSGs based on designs which permit the tubes to expand/contract freely and independently of one another, as distinguished from the more rigid horizontal gas pass design. Thus, fatigue stresses related to load following swings are minimized. This is just an illustration of the specific features of the Vertical European HRSGs for minimizing damages due to cycling related fatigue stresses. Vertical HRSG design shall be considered not only in terms of smaller footprint, but also as a solution to cycling related problems. As generally recognized, the cycling criterion is an integral part of HRSG design. This paper presents solutions to HRSG design issues for cycling tolerant operation. It relates to published data on problems observed with cycling Horizontal HRSGs, and it describes how these problems can be overcome. Concepts, design features and calculation methods applied to cycling tolerant HRSGs are reviewed in detail. Vertical HRSGs have been criticized because of their need for circulation pumps. Interestingly, the need for such pumps was eliminated a decade ago, with the advent of natural circulation for Vertical HRSGs up to 1800 psia (124 bar A) operating pressure.
Utility Alternatives for Coal-Fired Power Plants