Steam injection has been employed in gas turbines for over twenty years. Initially the emphasis was on injection for small amounts of power augmentation and NOx reduction in the turbine exhaust gas. More recently it has been used for massive power increases (more than 50% on some gas turbines) and efficiency improvements (more than 20%). Equipment selection, operation and economics are essential ingredients in producing the high-purity steam required in a steam-injected gas turbine cycle.
The most common means of producing steam for the steam-injection cycle is by means of a waste heat boiler operating in the turbine exhaust gas stream. Steam generated in this boiler may then be injected into the compressor discharge, combustor or turbine sections of the gas turbine to improve performance.
Manufacturers require extremely high purity steam for injection into their gas turbines; less than 30 parts per billion (PPB) of some contaminants is not an unusual requirement. If this steam quality is not obtained, serious damage can occur, particularly in the turbine hot section. To meet these stringent steam quality requirements without excessive amounts of boiler blowdown, it is necessary to provide highly demineralized makeup water to the boiler, i.e. less than 1 PPM TDS (Total Dissolved Solids). Low silica concentrations are particularly important since silica can vaporize at higher boiler pressures, pass through the moisture separators and deposit on turbine components. The selection of equipment required to produce high quality makeup water from various grades of raw water is critical to the successful operation of the steam injection plant. Because the steam cannot be recovered effectively, it is necessary to install a large water treatment system to provide the quantities of makeup required for steam injection. Equally critical to the cycle is the type of drum moisture separation used in achieving manufacturers’ recommended steam quality.
Just as the steam injection cycle has a dramatic impact on the economics of a gas turbine power plant, so too do the operation and selection of steam purification equipment influence the overall economics of the steam injection cycle.