Process and apparatus for heating feedwater in a heat recovery steam generator

Abstract

A feedwater heater (14) in a heat recovery steam generator (A,B) lies within a flow of hot exhaust gas. The feedwater heater (14) converts subcooled feedwater into saturated feedwater water, the temperature of which is only lightly above the acid dew point temperature of the exhaust gas so that corrosive acids do not condense on coils (18) of the feedwater heater (14). Yet the temperature of the saturated feedwater lies significantly below the temperature of the exhaust gas at the coils (18), so that the coils (18) operate efficiently and require minimal surface area. Pumps (26, 28, 30) elevate the pressure of the saturated feedwater and direct it into an economizer (64, 90) where, owing to the increase in pressure, the water is again subcooled. The economizer (64, 90) elevates the temperature still further and delivers the higher pressure feedwater to evaporators (34, 70, 78) that convert it into saturated steam that flows on to the superheaters (50, 78, 84). Higher pressure pegging stem admitted to the feedwater heater (14) controls the pressure—and temperature—of saturated steam and water in the feedwater heater (14).

Description

RELATED APPLICATION[0001]This application derives priority from and otherwise claims the benefit of U.S. provisional application 61 / 300,222 filed 1 Feb. 2010, which application is incorporated herein by reference.TECHNICAL FIELD[0002]The invention relates to boilers and more particularly to a heat recovery steam generators having improved feedwater heating.BACKGROUND ART[0003]Boilers designed to convert liquid water into steam by extracting energy from hot gases have become more efficient over the years, and much of this efficiency derives from extracting heat from the gases at lower temperatures—temperatures at which the gases might otherwise be exhausted to the atmosphere. But this increased efficiency has created its own problems that if left unaddressed can result in corrosion of the low temperature surfaces of the boilers.[0004]Heat recovery steam generators (HRSGs) represent an important class of high efficiency boilers. The typical HRSG operates in a system that includes a ga...

AI Technical Summary

Problems solved by technology

But this increased efficiency has created its own problems that if left unaddressed can result in corrosion of the low temperature surfaces of the boilers.

Those compounds will combine with water to produce sulfuric acid which is highly corrosive.

But if any surface drops to a temperature below the acid dew point temperature, sulfuric acid will condense on that surface and corrode it, and the vulnerable surfaces exist on the feedwater heater.

The bypass water, when employed, does not achieve the benefit of an initial temperature rise in the feedwater heater and decreases the efficiency of the HRSG.

That too decreases efficiency.

Moreover, the temperature of the exhaust gas as it passes into the feedwater heater is often not much greater than the temperature of the water leaving the feedwater heater, and as a consequence the feedwater heater must contain a rather large and expensive grouping of coils.

The large feedwater heater coupled with the bypass results in a significant pressure drop across them, and this imposes a substantial load on the feedwater pump.

However, when dew point temperature is higher, such as at 230° F. for exhaust gas derived from some fuel oils, a large temperature differential is not available at the feedwater heater (it becomes very tight—FIG. 1A).

As a consequence, the feedwater heater requires a large bundle of coils, making a conventional feedwater heater expensive in its own right and also requiring a significant head from the condensate pump just to force water through it and into the economizers and low pressure evaporator beyond it.

Images