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UTSR Areas of Research CombustionCombustors of future gas turbines using coal syngas and hydrogen fuels produced from syngas (SGH fuels) may operate with a variety of fuel compositions (corresponding to various gasification processes) and conditions (corresponding to different turbines). Possible fuel compositions for coal syngas could range from 15 to 40% H2, 20 to 45% CO with up to 25% H2O levels in the syngas and lower heating values from 100 to 150 Btu/ft3. Possible ranges in composition for hydrogen fuels produced from syngas could be from 50 to 65% H2 with most of the remainder N2. Combustor operating conditions could range from 700 to 950 F inlet temperatures, 2300 to 2650 F outlet temperatures, and pressures from 15 to 25 atmospheres. Although lean premixed combustion approaches have been developed to control turbine NOx emissions for conventional fuels, NOx control at IGCC plants has typically been achieved by injection of water, steam, or nitrogen diluents. NOx is decreased through reduction of flame temperatures by the diluents. Combustion instability has been a major problem (apparently now resolved) for syngas turbines that have operated with external silo combustors at European IGCC plants. Although acceptable emissions by current standards have been achieved using injection, the DOE Turbine Program goals are to reduce NOx for advanced IGCC plant to single digit levels (significantly less than 10 ppm). The primary overall goal for the university research is to provide fundamental information and data or computational tools that will enable design of syngas and hydrogen fuel turbine combustors with improved stability and emissions. Proposed research should give highest priority to addressing fuel composition and variability issues associated with use of syngas and alternate fuels in gas turbine combustors but may address conventional turbine fuels. Aero/Heat Transfer An issue for IGCC plants related to injection for NOx control is the increased mass flow through the turbine hot sections which, along with high water vapor levels in the hot section flow path, has increased heat transfer to cooled vanes and blades. Consequently, syngas turbine inlet temperatures have been reduced in order to maintain airfoil surface temperatures at design levels for acceptable part lifetimes. The reduction in turbine inlet temperature along with loss of the heat of vaporization to the cycle for water and steam injection has contributed to decreased syngas turbine and plant efficiencies compared to combined cycle gas turbine plants that operate with conventional fuels. A DOE Turbine Program goal is to increase turbine and plant efficiencies in order to improve performance and economics of future IGCC plants. Vanes and blades in syngas turbine hot sections have experienced limited deposition, erosion, and corrosion, although the extent and nature of these degradations have apparently not been reported in the open literature. Turbine airfoil passages are precisely specified, designed, and manufactured to provide high engine performance. Even small deviations in shape or surface roughness from design values can significantly reduce turbine power and efficiency. Residual ash entering turbine flow passages from syngas and alternate fuels can cause surface erosion, corrosion and deposition to degrade turbine surface contours and roughness. The end result of these degradations is higher aerodynamic losses and increased heat transfer to surfaces protected by cooling. With increasing deposit growth, heat transfer to cooled surfaces can decrease due to insulation from deposits but flow through the airfoil passages can be partially blocked. Deposits at first stator vane throats are particularly harmful to turbine performance because these throats define the minimum flow area in the turbine hot section. Thus, deposition in this region directly reduces the engine flow and power. The first stage stators are also particularly susceptible to deposition because they operate at the highest gas temperatures in the hot section that, as will be discussed later, tends to produce the highest deposition rates. Experience with aircraft turbines has shown that deposits can also block cooling holes, starving the neighboring vane surface of critical cooling air. Turbines with higher inlet temperatures compared to current IGCC turbines are being considered for the next generation coal syngas plants in order to improve turbine performance and consequently power plant economics. Limited past turbine operation experience with alternate fuels containing ash impurities has shown that deposition and corrosion can be drastically higher for increased inlet temperatures. Past rig tests with specimens exposed to combustion products from various alternate fuels have shown fuel dependent transition temperatures, below which specimen deposition and corrosion may not be excessive. However, above the transition temperature, deposition and corrosion drastically increase, and the nature of the degradation changes from characteristics at lower temperatures. For example, ash constituents other than alkalis (typically implicated in past turbine corrosion) contribute to extreme corrosion and deposits grow at much high rates and are much more tenacious than those produced at lower temperatures. Analyses of past test data has indicated that the cause of the accelerated corrosion and deposition above the transition temperature was increased levels of molten ash phases that stick upon arrival at surfaces and are more chemically reactive (corrosive) than solid phases. Materials Degraded coatings on first stage vanes and blades in the hot section of coal syngas turbines have needed replacement during maintenance inspections. Although a range of syngas constituents may contribute to the attack of turbine coatings (as indicated above), little has been reported about trace contaminant carryover into turbines at IGCC plants or the nature of the degradations that required coating replacement. However, sulfur compounds (e.g., SO2) and water vapor are known to enter turbines that operate at syngas plants and also have caused degradation of turbine coatings in other applications. Experience with aircraft turbines has shown that deposits can penetrate into thermal barrier coatings, resulting in coating loss during shutdown due to stresses produced from differential contraction between the deposits and coatings. The higher turbine inlet temperatures being considered to improve IGCC turbine and plant efficiencies and economics raise the question whether operation might be above the transition temperature for extreme and new characteristics of materials corrosion, as well as deposition. |
