Designs and configurations for HRSGs and steam turbines depend on the exhaust gas characteristics, steam requirements, and expected power plant operations. Because the exhaust gases from a gas turbine can reach 600ºC, HRSGs for GTs may produce steam at multiple pressure levels to optimize energy recovery; thus they often have three sets of heat exchanger modules – one for high pressure (HP) steam, one for intermediate pressure (IP) steam, and one for low pressure (LP) steam. The high pressure steam in a large CCGT plant can reach 40 – 110 bar. With a multiple-pressure HRSG, the steam turbine will typically have multiple steam admission points. In a three-stage steam turbine, HP, IP and LP steam produced by the HRSG is fed into the turbine at different points.
The HRSGs present operational constraints on the CCGT power plant. As the HRSGs are located directly downstream of the gas turbines, changes in temperature and pressure of the exhaust gases cause thermal and mechanical stress. When CCGT power plants are used for load-following operation, characterized by frequent starts and stops or operating at part-load to meet fluctuating electric demand, this cycling can cause thermal stress and eventual damage in some components of the HRSG. The HP steam drum and superheater headers are more prone to reduced mechanical life because they are subjected to the highest exhaust gas temperatures. Important design and operating considerations are the gas and steam temperatures that the module materials can withstand; mechanical stability for turbulent exhaust flow; corrosion of HRSG tubes; and steam pressures that may necessitate thicker-walled drums. To control the rate of pressure and temperature increase in HRSG components, bypass systems can be used to divert some of the GT exhaust gases from entering the HRSG during startup.
The HRSG takes longer to warm up from cold conditions than from hot conditions. As a result, the amount of time elapsed since last shutdown influences startup time. When gas turbines are ramped to load quickly, the temperature and flow in the HRSG may not yet have achieved conditions to produce steam, which causes metal overheating since there is no cooling steam flow. In 1x1 configurations, the operation of the steam turbine is directly coupled to the GT/HRSG operation, limiting the rate at which the power plant can be ramped to load. Steam conditions acceptable for the steam turbine are dictated by thermal limits of the rotor, blade, and casing design.
Control equipment for nitrogen oxides (NOx) and carbon monoxide (CO) emissions are integrated into the HRSG. As these systems operate efficiently over a narrow range of gas temperatures, they are often installed between evaporator modules.