Summary of the development of open-cycle gas turbine-steam cycles

Combined-cycle plants employing gas turbine cycles superimposed on conventional steam plants are well developed. Nearly 200 units are operating in the US on clean fuels (natural gas or distillate fuel oils) and giving overall thermal efficiencies as high as 42%. Future plants will have to use coal or coal-derived fuels, and this presents problems because gas turbines are very sensitive to particulates and contaminants in the fuel such as sulfur, potassium, lead, etc. If clean liquid or high-Btu gaseous fuels are made from coal, it appears that the conversion efficiency will be no more than 67%. Thus, the overall efficiency of utilization of coal would be less than if it were burned in a conventional steam plant unless the permissible gas turbine inlet temperature can be increased to approx. 1500/sup 0/C (2732/sup 0/F). Coupling a combined-cycle power plant directly to a low-Btu coal gasifier increases the fuel conversion efficiency and permits salvaging waste heat from the gasifier for feedwater heating in the steam cycle. By using a gas turbine inlet temperature of 1315/sup 0/C (2400/sup 0/F), well above the current maximum of approx. 1040/sup 0/C (1904/sup 0/F), an overall efficiency of approx. 40% has been estimated for the integrated plant. However, as discussed in companion reports, it is doubtful that operation with gas turbine inlet temperatures above 1100/sup 0/C (2012/sup 0/F) will prove practicable in base-load plants.