GAS TURBINE HRSGs
Gas turbine-based combined cycle and cogeneration plants are springing up throughout the world. The advantages of gas turbine plants are discussed in Chapter 1. Though gas turbine exhaust is used to heat industrial heat transfer fluids and gases, the emphasis here will be on steam generation. Gas turbine exhaust is clean; therefore water tube boilers with extended surfaces are the natural choice for heat recovery applications. It is also relevant here to mention briefly a few peculiar aspects of gas turbine exhaust gases in order to understand the design features of HRSGs better.
As discussed in Chapter 1, gas turbine combustor temperature is limited to about 2400-2500°F for metallurgical reasons. Therefore a large amount of compressed air is used to cool the flame, which in turn increases the exhaust gas flow from the turbine. After expansion in the turbine, the gas exits at about 1000°F and at a few inches of water column above atmospheric pressure. The exhaust gas contains about 6-10% by volume (vol%) of water vapor and about 14 vol% of oxygen. Gas turbines that are heavily injected with steam have a different exhaust gas analysis, which is discussed later. The large amount of oxygen in the exhaust gases enables fuel to be fired in the exhaust gases without the addition of air; the higher gas inlet temperature to the HRSG in turn generates more steam in the HRSG. Because of these large ratio of gas to steam flow compared to steam generators, HRSGs are huge in comparison. For example, the cross section of an unfired HRSG generating, say, 100,000 lb/h of steam will be about 6 times as large as that of a packaged boiler generating the same amount of steam.
Another important aspect of gas turbine HRSGs is that the exhaust gas flow remains nearly constant, and increasing the gas inlet temperature through auxiliary fuel firing increases the steam generation. Unlike in a conventional steam generator, the ratio of gas to steam flow in an HRSG varies significantly with steam generation. This in turn affects the gas and steam temperature profiles in the HRSG.
A water-steam mixture boils at a constant temperature at a given steam pressure; hence the gas temperature distribution across the HRSG surfaces is influenced by the saturation temperature of steam. Generally, the lower the gas inlet temperature to the HRSG, the lower will be the steam generation and the higher the exit gas temperature. This is due to fact that the heat sink in the form of an economizer does not have the ability to bring the exhaust gas stream to a lower temperature. In order to cool the gas stream to a reasonably low temperature, on the order of 250-300°F, multiple-pressure steam generation is usually required.
Heat recovery stream generators are generally of the water tube type with extended surfaces. This makes their design compact. Because of the large duty and low log-mean temperature differences at the various heating surfaces, plain tubes cannot serve the purpose effectively. The resulting HRSG design would be huge and uneconomical; the gas pressure drop also would be very high. One exception is the furnace-fired HRSG, which is very close in design to a conventional steam generator operating at much higher log-mean temperature differences; bare tubes may be used in this case. Fire tube boilers are rare in gas turbine heat recovery applications because they use plain tubes, which makes them large and unwieldy. They are sometimes used behind small gas turbines, often less than 3 MW in size, for generating low pressure saturated steam for use in chillers.