Session 4 cycles and combustion

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cycles and combustion

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Session 4 cycles and combustion

  1. 1. Session 4 – Cycles and Combustion • Last session: » » » » » » » • • • • Some additional units and concepts Human energy Photosynthesis Primary energy and energy carriers Conversion efficiency Primary fuels compared Reserves and depletion Thermodynamic cycles (4) Reaction Rates Combustion CO2 Production T. Ferguson, University of Minnesota, Duluth, 2008. 1
  2. 2. Thermodynamic Cycles Efficiency is of key interest – work out vs. energy in Temperature, materials, and chemical rates are key limitations Cycles involve a working fluid – Carnot Cycle – max theoretical efficiency for a heat engine (no phase change) – Rankine – steam engine with phase change – Brayton – turbines, no boiling or evap – Combined Cycle – Brayton feeds Rankine 2
  3. 3. Carnot Cycle • After Nicolas Carnot, published 1824 • Max efficiency for heat engine • Depends only on hi-temp source and lo-temp sink T = T -T Where T1 is the low temp in deg K ηc = 1T2 is the hi temp in deg K 1 2 T2 1 T2 Low temp must be at least ambient (293 deg K or 20 deg C) Hi-temp: materials limited – up to 1000 F or 540 C ηc =1-(293/813) = 64% max 3 Assumes all friction, other losses, eliminated
  4. 4. Rankine Cycle or Steam Cycle • Working fluid changes phase • James Watt patented steam engine in 1769, but William Rankine wrote the manual • Fossil, nuclear, solar, geothermal, biomass Steam (high pressure and temp) W Generator Turbine Boiler Water Pump Q2 , T 2 High temp heat source Condenser Q1 , T 1 Waste Heat 4
  5. 5. Brayton Cycle • Turbine engines (single and combined cycle) • No boiling or evaporation • Heat generated internally – no sig heat transfer issues • Exhaust still at high temp, so combined cycle is attractive 5 Source: GE
  6. 6. Brayton Cycle – Jet Engine Source: Wikipedia 6
  7. 7. Combined Cycle Rankine Cycle Brayton Cycle Source: New York Power Authority Web Site Queen’s Plant, East River, NYC, 500 MW Commercial ops in December, 2005 “50% more electricity from same fuel input compared to simple cycle” 7
  8. 8. Chemical Rate Processes Temporal Aspect of Conversion: Rates • Fuel/Oxidant Mixing Rate – Time required to mix specified quantities • Heating the Fuel/Air Mixture – Time required to heat mixture to temp of combustion • Compression Rate – Time to adequately compress working fluid • Materials Limitations – Time required to safely transfer heat in equipment • Chemical Kinetics – How rapidly a fuel releases its chemical energy through oxidation 8
  9. 9. Combustion • Combustion characteristics: • Rate is temperature sensitive • Process is complex • Combustion of any hydrocarbon with O2 gives CO2 and water • Combustion: fuel + oxidant products + heat • With air as the oxidant, nitrogen is involved • Oxidant: • Readily picks up electrons • Oxygen and fluorine have highest electronegativity of non-Nobles • So, they have greatest ability to pick up electrons, to oxidize fuel • Oxidation (or combustion) of methane: CH4 + 2O2 CO2 + 2H2O Heat and/or light 9
  10. 10. Quick Review of Electronegativity Source: Dr. James Kimball, Harvard University, Kimball’s Biology Pages, http://biology-pages.info, Used with permission of Dr. Kimball 10
  11. 11. Carbon Dioxide Production Consider the combustion of coal (if 100% C) C + O2 → CO2 Energy released = 94 E 6 cal/kg-mole (recall that 252 cal = 1 Btu) How much carbon dioxide is generated? – In kg-moles – In kilograms 11
  12. 12. Carbon Dioxide Production Summary of Selected Oxidation Processes: Coal: C + O2 → CO2 94 E 6 cal/kg-mole Methane: CH4 + 2O2 → CO2 + 2H2O 211 E 6 cal/kg-mole Ethane: C2H6 + 3.5O2 → 2CO2 + 3H2O 368 E 6 cal/kg-mole Propane: C3H8 + 5O2 → 3CO2 + 4H2O 526 E 6 cal/kg-mole 12
  13. 13. Carbon Dioxide Production Summary of Selected Oxidation Processes: Coal: C + O2 → CO2 94 E 6 cal/kg-mole Methane: CH4 + 2O2 → CO2 + 2H2O 211 E 6 cal/kg-mole Ethane: C2H6 + 3.5O2 → 2CO2 + 3H2O 368 E 6 cal/kg-mole Propane: C3H8 + 5O2 → 3CO2 + 4H2O 526 E 6 cal/kg-mole Homework Assignment #4: Calculate the weight in kg of carbon dioxide produced by combusting each of the fuels above, when each is used to heat a typical Midwest house for one heating season (assume an annual heating load of 20 MWh) 13

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