cycles and combustion

1. Session 4 – Cycles and
Combustion
• Last session:
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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.
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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
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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
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Assumes all friction, other losses, eliminated

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
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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
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Source: GE

6. Brayton Cycle – Jet Engine
Source: Wikipedia
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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”
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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
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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
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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
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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
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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
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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)
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