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40 Residual oil as a fuel is not possible without specialized gas turbine design features. Corrosion, plugging and fouling will occur. Higher firing temperatures in most contemporary high performance gas turbines require complex blade cooling, expensive super alloys and substantial derating. The -13D has integrally cast blade and vane cooling passages, with relatively simple geometry (versus a high performance aerofoil which normally has laser produced cooling passages) and a large flow cross section. This provides better resistance against plugging. Cooling air is extracted after the last compression stage, at the blade root. The air is routed to the first stage turbine blades below the rotor surface. The single piece welded rotor supported by two bearings is a simple, less vibration prone design. No through bolts are used: another useful maintenance feature. This design has only one silo combustor, a solid cast design. It has one large bore fuel nozzle, which helps avoid clogging and erosion. No air atomization is required, which means no compression air stream is required. The nature of the burner design means that water injection is required. At Shunde, water injection is 1.3 times the fuel flow rate (maximum 10.5litres/s). Water injection adds 9 to 10MW of power. No flow divider is required in this design, so no consequential temperature unbalance is observed. This also helps cut down on maintenance costs. The generator is driven from the cold end, which means turbine exhaust end inspections are easier. All bearings are accessible without disassembly and no elbow conduits are required. As the generator is air cooled, no hydrogen system or hazards have to be allowed for. The cooling loop is closed and maintenance free. The boiler, a vertical assisted circulation, single pressure design type, has a preheating loop. It delivers 44kg/s of 37.5 bar steam at 475 degrees C. Sodium phosphate (Na2SO4) is used for anticorrosion measures in steam treatment. Although the primary focus for this case is gas turbine system design modifications, these gas turbines are part of a combined cycle operation. (The steam turbine is a single cylinder design with a single flow low pressure section. Its gross output is 92MW. The steam turbine at Shunde runs with 472 degree C steam (480 degrees maximum) at 36 bars. The exhaust is condensed. Total gross power output then is 280 MW nominal. At Shunde 273 MW is guaranteed. The gross efficiency (LHV) at Shunde is 43% (43.8% nominal), based on a guaranteed heat rate of 8376 kJ/kWh (8221 nominal). Slow roll to running speed with the gas turbine takes 5 minutes. Getting the steam turbine running takes approximately two hours). Combustion and fuel economics are as follows. Sodium (Na), Sulfur (S), and Vanadium (V) content in the fuel are the major problems. Na is removed by mixing preheated fuel with water and demulsifier and then centrifuging. Potassium (K) impurities are removed in the same manner and at the same time as the sodium down to 0.5 ppm total (for both the Na and K). The sulphur left in the fuel becomes SOx upon combustion. The 120 meter stack at Shunde provides dispersal for the SOx. In areas where legislated SOx limits are tighter, flue gas desulphurisation or other methods can be used. Magnesium additives combine with the vanadium to form salts that deposit onto the blade surfaces. When the turbine is shut down, the salt levels fall off with the drop in temperature. Remaining salts are washed off with plain water. In Shunde, the wash is done every 100 operating hours for heavy oil. If gas or diesel fuel (back up fuel) is used, no wash is required. For inspection of the hot gas path, the inspector visually inspects the tiles on the inside of the combustor, the transition piece, and the first stage vanes. He uses a mirror to check the first stage blades. The other turbine and compressor stages can be observed by borescope. For major inspections every 16 to 24,000 hours, the burner is lifted off in one piece. The limit for magnesium addition is 1105 degrees C, as at 1120 degrees C, MgO (magnesium oxide) solidifies to the extent it can only be chiseled off, and V2O5 (vanadium oxide) with its low melting point corrodes. (Both MgO and V2O5 are formed from the safe additive compound after 1120 degrees C). The turbine inlet temperature of the Shunde units is maintained at 990 degrees C. When starting the gas turbines, diesel fuel is used until synchronous speed and then heavy fuel is used. This helps prevent clogging. The turbines are run for 5 minutes on diesel when shutting down. Again this prevents clogged nozzles and ignition problems. The - 11N2 can also handle the same rough fuel as the -13D. Peak metal temperatures, internal metallurgy and fuel treatment requirements are all quite similar. The single burner design for this model can get NOx down to 42 ppm with water injection. An EV silo combustor (several fuel nozzles) option is available if the end user has gas or diesel fuel. NOx can then be reduced to 15 ppm when at base load on natural gas. A gas turbine inlet filtration system is also necessary in this location. This particular inlet filtration system has three stages. In the first stage the air flow direction is changed. The second stage consists of mats. The third stage is for fine filtration. The gas turbine compressors are still washed off-line every 300 to 400 operating hours. Cheap fuel more than offsets the capital expenditure required for fuel treatment and additives, washing the fuel and other costs. This cost savings increases with the power capacity of a plant. Using a difference in residual oil and diesel prices of $50 per ton, a 300 MWPDF Image | GAS TURBINES IN SIMPLE CYCLE COMBINED CYCLE APPLICATIONS
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