2011+combined

2011+combined - Waste Heat Recovery From Gas Turbines Two...

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1 Waste Heat Recovery From Gas Turbines Two basic exhaust heat recovery arrangements can be used to improve cycle efficiency: – Recuperation Recovered heat is used in the same gas turbine cycle – Bottoming cycles Exhaust is used as a heat source for an essentially independent power cycle.
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2 Intro to Combined Cycles – Consists of three major components Gas turbine(s) Steam Turbine(s) and Heat Recovery Steam Generator (HRSG) – FIRST HRSG IN 1950’s. Also called heat recovery boiler – Matching of steam, gas and heating loads is important Supplemental heating in HRSG common
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3 Intro to Combined Cycles – Normally several gas turbines support a smaller number of steam turbines – Early designs could operate gas turbines separately from steam Current designs require steam in order to lower gas temperatures – In general, all of the components are smaller then historical steam-only plants, making construction times shorter
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4 Combine Cycle Efficiencies Sample Efficiencies For Various Systems System Efficiency (%) Simple gas turbine 32 Gas turbine plus unfired single pressure steam cycle 42 Advanced Gas turbine plus unfired multiple pressure steam system 48 Gas turbine plus dual pressure steam system plus process steam use (co- generation) 61
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5 Bottoming (Combined) Cycles Gas turbines have higher temperature heat input than steam cycles. – But, their exit temperatures are very high ~ 1000 °F Steam cycles have low exit temperatures – as close to atmospheric temperatures are you are willing to pay in terms of heat exchangers Combining the two provides significant improvements
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6 Typical Combined Cycle Setup
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7 Typical Combined Cycle Setup
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8 Typical Combined Cycle Setup
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9
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10 Typical Combined Cycle Setup
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11 Typical Combined Cycle Setup
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12 Typical Combined Cycle Schematic
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13 Typical Combined Cycle Schematic
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14 Typical Combined Cycle Schematic
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15 HRSG Schematic
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16
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17 HRSG DESIGN CONSIDERATIONS Key parameters for HRSG design – pinch point, superheater and economizer approach temperature In practice: – Pinch point = 20 to 50 °F – Superheater approach = 40 to 60 °F – Economizer approach = 10 to 30 °F – stack outlet temperature Lower is better need to avoid condensation
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18 HRSG DESIGN CONSIDERATIONS Key parameters for HRSG design – allowable back pressure controlled by cross-sectional area of HRSG smaller is cheaper back pressure typically 10 to 15 in wg. in wg stands for inch water gauge , another name for the inch water column or just inch of water – steam pressure and temperature high pressure limits total heat recovery in single pressure HRSG
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19 Cyclepad example – “library design” Air temp in/out = 886 °F / 212 °F Water/steam temp in/out = 132 °F /752 °F (Tsat = 563 °F ) Assume c p is constant for the air – Energy transfer from air is linear in T Determine energy “costs” for heating water (using enthalpy) – Superheating (1349 – 1186) = 163 (13%) – Boiling (1186 – 556) = 630 (51%) – Feedwater Heating (556 – 103) = 453 (36%)
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2011+combined - Waste Heat Recovery From Gas Turbines Two...

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