Improvements in Gas Turbines
In order to handle the high firing temperatures, in the range of 2500-2600°F, gas turbine suppliers are doing research and development work on turbine blades for protection against corrosion and thermal stresses. Thermal barrier coatings have been used on turbine blades for several years. The base high alloy material ensures the mechanical integrity, while the coatings protect against oxidation and corrosion as well as reducing the blade surface temperature. The rotating blades are manufactured by using single-crystal casting technology, which allows the chemical composition of the alloys to be modified to improve their resistance to fatigue and creep. Thermal barrier coatings comprise two layers: the outer ceramic layer, which prevents flow of heat into the turbine blade, and a metallic bond coating, which is a nickel — or cobalt-based material.
General Electric uses closed loop steam cooling for the blades in its quest for higher firing temperatures. This unique cooling system allows the turbine to fire a higher temperature, around 2600°F, for higher performance. Earlier designs were cooled by compressor discharge air, which causes a large temperature drop in the first-stage nozzle. Cooling with steam systems has been found to be more effective because it picks up heat for use in the steam turbine, transforming what was waste heat to usable heat. In conventional gas turbines, compressor air is also used to cool rotational and stationary components downstream of the stage 1 nozzle. This is called chargeable air because it reduces performance. In advanced systems, this air is replaced by steam, which enhances performance by 2% and increases the gas turbine output because all the compressor air can be channeled through the turbine path to do useful work in the turbine as well as in the HRSG . The high pressure steam from the HRSG is expanded through the steam turbine’s high pressure section. The exhaust steam from this turbine section is then split. One part is returned to the HRSG while the other is combined with the intermediate pressure steam and used for cooling in the gas turbine. Steam is used to cool the stationary and rotational parts of the turbine. In turn, the heat transferred from the gas turbine increases the steam temperature to approximately reheat temperature. The gas turbine cooling steam is mixed with the reheat steam from the HRSG and introduced into the intermediate pressure steam turbine section .