HRSGs

Eig. 16. Coal gasification power plant HRSG = heat recovery steam generator. Courtesy of Black and Veatch. 

WerkstofTe u. Korrosion, J2, 33 (1981) Kommentar zu DIN 50900 in Korrosion durch Information und Normung , Hrsg. W. Fischer, Verlag 1. Kuron, Bonn 1988. 

B. H. Hanson, Titanium Progress Nr. 8, Hrsg. Imperial Metal Industries (Kynoch) Ltd., Birmingham 1969. 

Figure 1-39. Comparison of a drum type HRSG to a onoe through steam generator. (Courtesy Innovative Steam Teehnoiogies.)...

Figure 1-40. Energy transfer diagram in an HRSG of a oombined oyoie power piant.

The Brayton cycle in its ideal form consists of two isobaric processes and two isentropic processes. The two isobaric processes consist of the combustor system of the gas turbine and the gas side of the HRSG. The two isentropic processes represent the compression (Compressor) and the expansion (Turbine Expander) processes in the gas turbine. Figure 2-1 shows the Ideal Brayton Cycle. 

Figure 2-32. A typioai iarge oombined oyoie power piant HRSG.

The steam turbines in most of the large power plants are at a minimum divided into two major sections the High Pressure Section (HP) and the Low Pressure Section (LP). In some plants, the HP section is further divided into a High Pressure Section and an Intermediate Pressure Section (IP). The HRSG is also divided into sections corresponding with the steam turbine. The LP steam turbine s performance is further dictated by the condenser backpressure, which is a function of the cooling and the fouling. 

The efficiency of the steam section in many of these plants varies from 30-40%. To ensure that the steam turbine is operating in an efficient mode, the gas turbine exhaust temperature is maintained over a wide range of operating conditions. This enables the HRSG to maintain a high degree of effectiveness over this wide range of operation. 

Understanding the design characteristics of the dual or triple pressure HRSG and its corresponding steam turbine sections (HP, IP, and LP turbines) is important. Increasing pressure of any section will increase the work output of the section for the same mass flow. However, at higher... 

Figure 2-33. Energy/temperature diagram of the tripie pressure HRSG.

Injection of humidified and heated compressed air—Compressed air from a separate compressor is heated and humidified to about 60% relative humidity by the use of an HRSG and then injected into the compressor discharge. 

Steam Injection—Injection of the steam, obtained from the use of a low-pressure single stage HRSG, at the compressor discharge and/or injection in the combustor. 

Injection of Water or Steam at the Gas Turbine Compressor Exit. Steam injection or water injection has been often used to augment the power generated from the turbine as seen in Figure 2-42. Steam can be generated from the exhaust gases of the gas turbine. The HRSG for such a unit is very elementary as the pressures are low. This technique augments power and also increases the turbine efficiency. The amount of steam is limited to about... 

Injection of Steam in the Combustor of the Gas Turbines Utilizing Present Dual Fuel Nozzles. Steam injection in the combustor has been commonly used for NO control as seen in Figure 2-43. The amount of steam, which can be added, is limited due to combustion concerns. This is limited to about 2-3% of the airflow. This would provide an additional 3-5% of the rated power. The dual fuel nozzles on many of the industrial turbines could easily be retrofitted to achieve the goal of steam injection. The steam would be produced using an HRSG. Multiple turbines could also be tied into one HRSG. 

The concept of injecting humidified and heated compressed air just after the gas turbine compressor is another very interesting way to increase power and efficiency. In this system, compressed air is added to the compressed discharge air. The compressed air is about 5% of the main gas turbine air and this air after it has been compressed using an external compressor is then injected into an air saturation device where steam obtained from the HRSG unit is then injected into the device to saturate the air with water and the saturated air then is further heated in the HRSG before it is injected into the compressor discharge of the gas turbine. 

Control Systems. The eosts in all these systems have taken into aeeount modifieations of the eontrol systems. The eontrol systems in most of these eases will have to be new to take into aeeount the injeetion of steam, and the heated and humidified eompressed air, the HRSG, and all its assoeiated equipments sueh as pumps. 

Turbine back pressure. In this ease, the operator is relatively limited sinee he eannot do anything about the downstream design. If there is some obstruetion in the dueting to the HRSG that ean be removed or if the duet has eollapsed in an area the duet eould be replaeed. 

The turbine firing temperature should be computed by knowing the gas characteristics of the combustion gas. If these characteristics are known then one can use the combustion gas equations given in the ASME performance test codes 4.4 (1991) for gas turbine HRSG. Usually the gas constituents are not known so it is not a bad assumption to use the 400% theoretical air tables in the Keenan and Kaye gas tables. The following equations for... 

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