ER 100:
Energy Toolkit I: Combustion
Dan Kammen
September 4, 2003
Overview
• What do we do with fossil fuels: burn ‘em
– Combustion: impacts
• Fuels
• Balancing combustion chemical equations
• Combustion products
• Equivalence ratios
• Energy content and temperature
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Importance of fossil fuels
• The major contributors to Energy Use in the US and in the
world
Energy Supply 1995 World U.S.
Total Energy use (Quads) 420 91
Coal (%) 33 38
Natural Gas(%) 20 24
Biomass fuels (%) 13 3
Hydropower (%) 6 4
Nuclear (%) 6 8
Solar, wind, geothermal (%) <0.5 0.4
Electricity Supply 1995 World U.S.
Net generation (TWh) 12,500 3,400
Fossil fuel (%) 62 68
Hydropower (%) 19 9
Nuclear (%) 17 20
Biomass and other (%) 1 3
Energy and the Environment
Global and Regional Environmental Impacts
• Electricity use accounts for 36% of America’s greenhouse
gas emissions
• Power plants burning coal, oil, and natural gas produce
64% of the U. S. SO2 emissions, 33% of Hg, and 26% of
NOx. [This also highlights a key issue about our energy
‘budget’ and its wider environmental impacts]
• The coal, oil, and natural gas that we can economically
recover today may lead to a doubling of pre-industrial
carbon concentrations in the atmosphere in the 21st
century, and a quadrupling over several centuries.
2
Energy and the (Social) Environment
Health and Social Welfare
• Several studies correlate air pollution to increased mortality rates,
hospital stays, and emergency room visits. Particulate pollution kills
64,000 Americans/year, through heart and lung disease. Inhabitants of
polluted cities and areas -- in both developed and developing nations --
run substantially higher risks.
• Families with annual incomes below $10,000 suffer over twice the
asthma incidence of families with incomes over $35,000.
• The poverty rate of communities located within one mile of coal-fired
power plants is almost double that of the general population.
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Energy and the Environment
Impacts on Land and Water
• While overall tonnage of major air pollutants has fallen since 1970, NOx
emissions have risen 11%; NOx from coal plants has risen 44%.
• Coal mining accounts for about 95% of acidic mine drainage, a potent
threat to streams, wildlife, and vegetation.
• Despite reductions in SO2 emissions, acidity continues to trouble large
regions of North America - NOx rising emissions may be equally to
blame.
• Coal mines supplying American power plants disturb 1.7 million acres of
land.
• Power plants deposit 11 - 15% of the nitrogen in the Chesapeake Bay,
thought to have contributed to the rapid growth of toxic organisms in
recent years.
What happens in combustion?
• Fuel + oxidizer -> Products + light + heat
• Combustion, in its simplest form, e.g: methane
CH4 + 2O2 → CO2 + 2H20
A clean reaction, except for the issue of carbon
dioxide and the global climate
This idealized reaction takes place in an
‘atmosphere’ (oxygen) free of impurities
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How much CO2 is produced when 1 ton of wood (C6H12O6) is burned?
Balance the equation:
C6H12O6 + O2 → CO2 + H2O
First the carbon:
C6H12O6 + O2 → 6CO2 + H2O
Then the hydrogen:
C6H12O6 + O2 → 6CO2 + 6H2O
Last, the oxygen (because you can change the oxygen without altering other elements):
C6H12O6 + O2 → 6CO2 + 6H2O,
Oxygen: 6 + x Oxygen: 12 + 6 =18
So, 6 + x = 18 → x = 12, or 6 O2The balanced equation is:
C6H12O6 + 6O2 → 6CO2 + 6H2O
What about the grams per mole of wood (molecular weight):
C6H12O6 = (6 x 12) + (12 x 1) + (6 x16) = 180 grams/mole
How much CO2 is produced when 1 metric ton of wood is burned,
continued ...
From the equation we can find out how much CO2 we get per
metric ton of wood.
So, from C6H12O6 → 6CO2 + 6H2O, we see that:
180g C 6H12O 6 10 6 g C 6H12O 6
=
264g CO 2 X gCO 2
(12+(2x16)) = 44 grams per mole of CO2
264g x 106 g
x= = 1.47 x 106 g = 1.47 tons of CO 2
180g
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Simple combustion equation,
but put it in air
• Example: methane reacts with air
CH4 + (O2 + 3. 76N2) -> CO2 + H2O + N2
(this is termed the ‘unbalanced’ version)
CH4 + 2(O2 + 3. 76N2) -> CO2 + 2H2O + 7.52N2
(balanced, stoichiometric: exactly the correct amount
of oxidizer to convert all C to CO2, and all H to H2O)
– Note: Air is 78% nitrogen, 21% oxygen, + other stuff
Real Combustion
• If combustion occurs without complete oxidation
instead, we get:
CH4 + O2 + N2 → mostly (CO2 + 2H20 +N2)
+ traces (CO + HC + NO...)
• This can occur when:
– temperature too low,
– insufficient O,
– combustion too rapid,
– poor mixing of fuel and air, etc. ...
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Real, Real (fully nasty), Combustion
• At higher temperatures, N reacts with O:
air(N2 +O2) + heat → NOx (thermal)
• So much for pure fuels, now add impurities:
enter N, S, metals and ash (non-combustibles)
What we really get:
• Fuel (C, H, N, S, ash) + air (N2 +O2) →
(CO2, H2O, CO, NOx, SOx, VOCS, particulates) +
ash
– Volatile Organic Compounds: VOCs
Historical trend in the average efficiency of electricity generation in
central-station thermal power plants in the U.S.
(efficiency in percent, higher heating value (HHV) basis)
Higher heating value (gross heat): includes heat released when water condenses.
Lower heating value (net heat): assumes water stays in the vapor state.
Note: the most efficient furnaces (> 90%) by
causing combustion gases to cool enough to
condense the water vapor before it leaves the
Stack (condensing furnaces).
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Heat Needed to Convert 1 kg of ice to steam:
2.1 kJ/°C to heat ice 4.18 kJ/°C to heat water 2257 kJ/kg needed to vaporize
333 kJ/kg needed to melt ice 2.0 kJ/°C to heat the vapor
HHV versus LHV
Higher heating value: includes energy of state changes
Lower heating value: without state changes included.
The energy released by burning methane (CH4) is 890 kJ/mol,
including the energy of condensation of the water vapor.
The energy is 810 kJ/mol without the condensation, or the LHV
Calculate the same way :
Balance the equation
CH 4 + 2O2 → CO 2 + 2 H2 O
Since 1 mol of CO has 12 g of carbon, the HHV intensity is :
2
12gC
HHV carbon intensity = = 0.0135gC / kJ = 13.5gC / MJ
890 kJ
12gC
LHV carbon intensity = = 0.015gC / kJ = 15gC / MJ
810 kJ
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Carbon Emissions Intensities
Fuel LHV C Int. (gC/MJ)
C EmissiosHHV
(GtC/yr)
C Int. (gC/MJ
Nat. Gas 15.3 1.1 13.8
Oil 20 2.6 19.7
Coal (bit.) 25.8 2.3 24.2
Renewables 0 0 0
National Ambient Air Quality Standards
PollutantMetric Fed. Std. CA Std.Effects
3
CO 8 hr 10 mg/m 9 ppm Angina, pre
asthma
NO
2 Ann. mean
40 mg/m3 20 ppm Respiratory
PM10 50µg/m3
Ann. Mean 30 mg/mResp. dise
PM 2.5 Ann. Mean
15 mg/m3 in reviewResp. dise
SO
2 80µg/m3 in reviewwheez, pla
Ann. Mean
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Aerosols
http://earthobservatory.nasa.gov/Library/Aerosols/
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Aerosols:
from power plants & cars
http://earthobservatory.nasa.gov/Library/Aerosols/
Aerosols: from Ships Cloud formation
Normal, large particulates
http://earthobservatory.nasa.gov/Study/Pollution/pollution_2.html
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Summary: Combustion Products
• Air, N2, O2, Ar
• Products of complete combustion: CO2, H2O
• Products of incomplete combustion: trace
hydrocarbons, unburned hydrocarbons, CO, H2,
aldehydes, soot
• Fuel impurities: SO2, SO, metals, metal oxides, ash
(silica, sand)
• Nitrogen compounds: N source is the air or
the fuel, e.g.
NO, NO2, N2O, HONO, NH2
Emissions
• Mole fraction (%, ppm, ppb)
• mass/energy in (pollutant/MMBtu, Kg/KJ)
• mass/distance (g/mile) [vehicle standards]
• mass/volume = µg/m3
• reference emissions to corrected oxygen content in
exhaust to prevent apparent emission reductions
by dilution
– Remember, the solution to pollution is … dilution.
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Fossil fuel combustion (chemistry)
• Coal = Carbon (C) + impurities (e.g., sulfur)
• Oil = Mixture of hydrocarbons (CxHy) + imp.
• Natural Gas = methane (CH4) + carbon dioxide (CO2) + imp.
• Combustion = oxidation, exothermic
CxHy + O2 ⇒ CO2 + H2O + ENERGY + (CO+C)
N2 + O2 ⇒ NOx
S + O2 ⇒ SOx
• Ratio of x:y determines ratio of CO2:H2O
• CH4 has lowest x:y and thus lowest CO2 per energy
• Carbon has the highest ratio
Solid fuels
• Peat
• Coal (moisture, volatiles, fixed carbon, ash)
(CH0.8)
• Wood (moisture, volatiles, fixed carbon, ash)
• Charcoal (devolatilized wood)
• Coke (devolatilized coal or petroleum)
• Key difference among fuels: the quantity of CO2
formed per unit of energy released. Natural gas
releases ~ 42% less CO2 than coal
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Chemical Structure of Coal
Gas and Liquid Fuels
• Natural gas: CH4, C2H6, N2, CO2
• Propane(C3), Butane (C4), LPG (mixture)
• Synthetic gases (from biomass, coal products)
• Petroleum derived fuels (~CH2);
– Gasoline (C4 to C10, avg: C8)
– Diesel (C12)
– Turbine fuels, kerosene (C10)
– Heavy fuel oils
• Shale oil derived liquids
• Alcohols, ethers (have oxygen in the fuel)
• Hydrogen
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Products
• Depend on fuel, mixing, and temperature
• At temperatures < 1250 K
– fuel + oxidizer --> O2 + CO2 + H2O + N2
• At combustion temperatures (~1400-2200 K)
– many stable species dissociate
– molecular events and elementary reactions important
• CO2 <=>CO+ 1/2 O2
• H20<=> H2 + 1/2 O2
– Actual product concentrations depend on balance
among reactions that lead to formation & consumption
of intermediate species (and the kinetics)
Reaction mechanisms
contain several hundred elementary reactions
plus thermodynamic data that allow one to
calculate the major products and all the
trace products for a given mixture of
methane and oxidizer (O2 or air) at specified
conditions of temperature and pressure
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All reactions accompanied by
absorption or release of energy
• Ideal system: adiabatic (ideal) combustion
– no heat loss or energy transfer out of the system
– constant pressure process, steady state, exothermic
– Energy expressed as enthalpy
Energy = E = U + KE + PE
But for gases changes of state require another term, too:
Enthalpy = H = U + pv
internal energy + pressure volume work
Hreactants = Hproducts for an adiabatic system
∆H for constant pressure reaction = heat absorbed
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Hreactants = Hproducts
• H = hf + CpdT
= heat of formation + (specific heat) x (temp)
Other terms
• Mass fuel/mass air
• Mass air/ mass fuel = AFR (air fuel ratio
– typically about 14, 15 for HC
• Percent excess air = 100 {(1/φ) -1}
0% excess air = stoichiometric
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Extra Analytic Material
Reaction mechanisms
• Types of reactions: initiation, propagation,
termination
• Preflame zone
– CH4 -> CH3 + H (decomposition, initiation, )
– CO + OH --> CO2 + H (abstraction, initiation)
• Luminous zone: branching reactions, propagation
– H + O2 -->OH + O
– CO + OH --> CO2 + H
– O +N2 --> NO + O
• Postcombustion zone: termination
– O + O --> O2 (recombination, termination)
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Characterization of global reactions
Fuel/air ratio
Fs = stoichiometric molar fuel/air ratio
Fa = actual molar fuel/air ratio
Nfuel = number of moles of fuel
Nair = number of moles of air
Fs = [Nfuel/Nair]stoichiometric
Fa = [Nfuel/Nair]actual
Characterization of global reactions
Equivalence ratio
φ = [Fa/Fs] = [Nfuel/Nair]actual / [Nfuel/Nair]stoich
φ = 1, stoichiometric
φ > 1, rich combustion (excess fuel)
φ < 1, lean combustion (excess air)
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Combustion … to a physicist
• Generates a high enthalpy gas from which
we can extract energy
• Fuel type, and time, temperature and
turbulence (the three Ts) determine energy
release, flame properties, and products (i.e.
pollutant distribution)
• Why do we care … determines which way
the reaction runs (forward or backward)
Global reactions
Overall combustion reaction
• Ideal: CxHy + (x + y/4) (O2 + 3.76N2) -->
x CO2 + y/2H2O + (x + y/4) 3.76 N2
• Real: CxHy + (φ) (O2 + 3.76N2) -->
aCO2 + bH2O + cN2 +dO2 + eCO + fHC +gNO
+hNO2
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Adiabatic flame temperature
• For an adiabatic reaction, the temperature of
the product mixture will be the adiabatic
flame temperature - all the net energy
released is retained in the products.
• For Hydrocarbon burning in air, φ = 1, P = 1
atm,
Tadiabatic flame ~ 2200 K
Pollution & Emissions refs.
• Smokestack & environmental behavior
• http://www.rpi.edu/dept/chem-eng/Biotech-
Environ/Environmental/Air/plumes/relation.html
• NAAQs and the Clean Air Act
• http://www.epa.gov/airs/criteria.html
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