Origins of Fossil Fuels petroleum and natural gas of marine origin photosynthetic bacteria part of carbon cycle small part of plant/animal matter (0.01%) settles to bottom with clays and sand (removed from carbon cycle) anaerobic bacteria digest (deoxygenate) organic matter leading to saturated hydrocarbons (petroleum) which accumulate in porous layers of rock deeply buried sediments subjected to extreme pressure and heat - disproportionation to oxygen and natural gas (CH4); trapped in impermeable rock
Fossil Fuels
fossil fuel reserves accumulated over millions of years
Cracking and Alkylation
Petroleum (Oil) Refining petroleum complex mixture of hydrocarbons, with small amounts of S, O, N and trace metals mostly saturated hydrocarbons CnH2n+2 n = 1…90 fractions separated by distillation fraction
n
B.Pt range
gas
1-4
gasoline
5-12
-126-0°C 0-204
kerosene
10-16
180-274
gas oil
15-22
260-371
lube oil
19-35
338-468
residue
36-90
468+
overall yield of most valuable fractions (gasoline) increased by cracking larger hydrocarbons into two smaller ones e.g. C18H38 → C8H18 (octane) + C10H20 (decene) requires high temperatures and a catalyst can build up molecules in gasoline fraction from short chain hydrocarbons (alkylation) e.g. + +
C4H10
H
yield of gasoline fractions raised from 20% to 40-50% by these processes
Combustion oxidation of gasoline a free radical process air-fuel mixture ignited by a spark; fragments hydrocarbons into free radicals chain reaction (explosion) initiated with O2; provides compression of piston (mechanical energy) heat and pressure generated during compression can cause premature spontaneous ignition (knocking) incomplete combustion leads to CO emission
Oxygenates incomplete combustion of hydrocarbons limited by fuel additives various ones tried from 1920s Et4Pb, Me4Pb added to fuel • now being phased out due to toxicity of Pb benzene, toluene, xylene (high octane rating) • benzene known to be carcinogenic oxygenates: MeOH, EtOH, t-BuOMe (MTBE) and t-BuOEt (ETBE) now most popular additives
Coal coal of terrestrial origin; derived from decomposition of plant matter (cellulose and lignin) CH2OH O HO
COOH
CH2OH O
O
HO HO
O
CH2OH O
O
HO HO
HO HO
cellulose
O
-3H2
MTBE synthesised on massive scale worldwide; major use for methanol isobutene obtained from natural gas sources; LPG mixture (with propane) +
MeOH
H+
OMe
high degree of branching; high octane rating as proportion of oxygen already enhanced, complete combustion (to CO2) assisted EtOH also a competitive fuel additive (Gasohol) able to be produced agriculturally (fermentation of sugar cane); MTBE produced by petroleum companies
cellulose broken down by aerobic bacteria (⇒ CO2); returned to carbon cycle lignin not digested (lost from carbon cycle), sinks to bottom of swamps; compacted as peat pressure and heat within Earth’s crust deoxygenates peat to give coal (carbon-rich) OH
H3CO
OCH3
HO
-H2
OH
COOH
CH2OH O
O
branched hydrocarbons more resistant to radical formation (methyl C-H bonds stronger than methylene C-H bonds); high octane rating high octane fuels more energy efficient, cleaner straight-chain molecules may be catalytically cyclised (dehydrogenated) to aromatics (reformation reaction)
O
O
O OCH3
H3CO
O
lignin polymer
bituminous coal
structure related to graphite graphite
Bond Dissociation Energies (average values)
Fuel Energy
Bond
all chemical energy derived from fuels released during oxidation (by O2) energy estimated by considering bond energies of reactants and products e.g. oxidation of natural gas CH4 + 2O2 → CO2 + 2H2O
→ CO2 + 2H2O CH4 + 2O2 4 × C-H 1640 2 × C=O 1598 2 × O=O 988 2 × 2 × O-H 1840 -1 2628 kJ mol 3438 kJ mol-1 ∆H = -810 kJ mol-1 assuming CH4 limiting reagent, can calculate energy content by mass of fuel 1 g CH4 ⇒ 50.6 kJ energy (per gram)
Comparative Combustion Energies ∆H kJ g-1
fuel
reaction
natural gas
CH4 + 2O2 → CO2 + 2H2O
-50.6
petroleum
(-CH2-) + 1.5 O2 → CO2 + H2O
-43.6
coal
(-CH-) + 1.25O2 → CO2 + 0.5H2O
-39.3
ethanol
C2H5OH + 3O2 → 2CO2 +3H2O
-27.3
cellulose
(-CHOH-) + O2 → CO2 +H2O
-14.9
hydrogen
2H2 + O2 → 2H2O
-120
Comparisons of Fossil Fuels greatest energy content with maximum degree of hydrogenation (natural gas) other issues need to be considered exploration costs recovery refinement transportation ease of use emission
Petroleum - Advantages liquid, easily stored and transported (road and sea) refinement straightforward advanced technology relatively clean • S and metal residues from distillation not released to environment
Petroleum - Disadvantages oil spills natural seepage tanker accidents discharge of tank washing water emissions high amount of CO2 produced/kJ relative to natural gas (greenhouse gas) automotive combustion engines produce NOx, SOx gases (also CO)
Gas - Advantages
Gas - Disadvantages
requires little processing CO2 emission rate lowest of all fossil fuels less free radical chemistry (leading to smog) than petroleum combustion
volatile difficult to transport safely and economically • liquified petroleum gas (LPG) required (compressors/refrigerators) CH4 also a greenhouse gas; leaks offset favourable CO2 emission levels less convenient, large tanks required in automobiles
Coal - Advantages
Coal - Disadvantages
large resource base easily mined easily and safely transportable (sea, rail, road) important fuel in electricity generating stations efficient, relatively clean combustion
inconvenient to carry around superceded by diesel in trains combustion in heaters/furnaces major air pollution source (soot) respiratory problems SO2, NO (acid rain) large proportion of CO2 produced
Coal-Derived Fuels conversion of coal to ‘cleaner’ fuels possible object to raise H/C ratio several different reactions C + 2H2 → CH4 ∆H -74.9 kJ • hydrogasification, requires very high temperature nCO + (2n+1)H2 → CnH2n+2 + nH2O • Fischer-Tropsch reaction (metal catalysed) processes not energy efficient
