Fossil Fuels are the molecular mummies of photosynthesizers. The primary periods of fossil fuel formation are mass extinction events, particularly the late Paleozoic ( ~ 248 myear ago) and late Mesozoic (~ 65 myear ago). The Paleozoic mass extinction was the worst in the fossil record, with over 90% of marine fossils disappearing; the Mesozoic was much milder.
Mass extinction provides the local surge in dead organic material necessary for the eventual formation of oil-containing sediments. An additional requirement is that the organic material has to be rendered unavailable for metabolism, otherwise the surviving creatures would just eat all the dead ones and there’d be nothing left for us. In a review of mass extinction events leading to oil formation, a rapid rise in sea level seems to be the culprit.
The current favored cause for the sea-level rise are massive volcanic eruptions that deposited gigatons of carbon dioxide into the atmosphere. A rapid rise in sea level converts the light-filled shallow waters of continental shelves into dark depths. Without light, the photosynthesizes die, and without the oxygen they release, aerobic conversion of hydrocarbons into carbon dioxide and water ceases. The flooding of the littoral (tidal) zone kills most of the algae that thrive there. Drowning of previously dry areas and increased precipitation near the new coastline washes millions of tons of organic detritus into the former continental shelf. Decay proceeds extremely slowly in the endless frigid night of this mass grave, and the ongoing volcanism causes rapid burial. Over millions of years the dead matter, now compressed into oil-rich rock, experiences varying amounts of temperature and pressure. This squeezes out the oil that can be motivated to move, and it collects in big pools of what we know of as crude oil.
Crude oil is a liquid- but just barely. It consists of a complex mixture of hundreds or thousands of different carbon-rich molecules that vary from one carbon to hundreds of carbons. The thickest bits are tar and pitch; mixed with stones it becomes asphalt. These giant molecules are classed as asphaltene, and are made up of hundreds of carbon atoms. Slightly further down, you get waxes; paraffin candles are the iconic product. Smaller still, and you’ve got hydrocarbons that have roughly a lard-like consistency, and it gets made into products like vaseline. The thinner stuff makes for fuel oil, largely used for heating. Thinner yet and you get lubricating oil; finally you get the hydrocarbons used in diesel, then gasoline, and finally naphthalene, kerosene and so on down to methane.
From Crude to Gas
Because gasoline is the end-product of crude oil with the highest demand, oil companies have long been modifying what they get from the well to make them into the 7-11 carbon molecules that make up gasoline. The two techniques used are cracking and reforming; the first involves breaking the large hydrocarbons that predominate in crude oil into smaller pieces using high temperatures and pressures in the presence of a catalyst.
Knock Knock… Octane who?
|Lead vs. MTBE vs. Ethanol|
|Alkyl lead was the first anti-knock agent used by the oil industry. After the health consequences of lead were fully realized, they switched to methyl-tertyl-butyl-ether. This is toxic as well, and so a gradual switch to ethanol is underway. The oil industry resisted this switch for many years despite ethanol’s superiority to MTBE as an anti-knock agent because the hydrophilic nature of ethanol made ethanol enriched gasoline difficult to send through pipelines.|
Reforming is a curious process because it mostly involves shuffling the carbons around a molecule while keeping the molecule the same size. This is necessary for gasoline engines because the spark-ignited explosion creates a wave of pressure and heat that cause the straight-chain hydrocarbons to break apart in chunks before they start burning. Each piece burns at both ends, or breaks into smaller chunks creating even more ends, and the resulting extremely rapid combustion causes the characteristic “knocking” sound. To combat this, the oil industry looked for ways to change their hydrocarbons and for an anti-knock agent. They found their anti-knock agent in the form of alkyl lead (see sidebar), and reforming proved to be the best way to change hydrocarbons to reduce knock. By changing straight-chain hydrocarbons into fuzzy hydrocarbons with many branches, the bonds between the carbons and hydrogens was strengthened, reducing the pre-ignition reactions that cause knock.