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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.

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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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Carnot Cycle

• After Nicolas Carnot, published 1824• Max efficiency for heat engine• Depends only on hi-temp source and lo-temp

sink

ηc = 1- T1

T2

Where T1 is the low temp in deg KT2 is the hi temp in deg K

= T2-T1

T2 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

Assumes all friction, other losses, eliminated

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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

Condenser

Boiler

Pump

Turbine Generator

Steam (high pressure and temp)

W

Q2 , T2

High temp heat source

Q1 , T1

Waste Heat

Water

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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

Source: GE

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Brayton Cycle – Jet Engine

Source: Wikipedia

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Combined Cycle

Brayton Cycle

Rankine Cycle

Source: New York Power Authority Web SiteQueen’s Plant, East River, NYC, 500 MWCommercial ops in December, 2005

“50% more electricity from same fuel inputcompared to simple cycle”

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Chemical Rate ProcessesTemporal 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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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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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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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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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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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)