Gasoline is a mixture of hydrocarbons derived from petroleum that is used as a fuel in internal combustion engines. It has a very high calorific value, about 46 MJ/kg.
Gasoline is a colorless, highly flammable liquid, not as dense as water (relative density: 0.70 to 0.75). Gasoline is obtained from petroleum by direct distillation, between 60° and 200°C, or by cracking the heavy fractions.
From a chemical perspective, gasoline is a mixture of alkanes, cycloalkanes, and aromatic compounds with 4 to 10 carbon atoms, and occasionally alkenes. Gasoline is primarily used as fuel in internal combustion engines. For this reason, this fuel must have a high anti-knock capacity, which is measured by the octane rating (octane).
This antiknock power can be improved by varying the chemical composition, refining processes (cracking, reforming, isomerization, etc.), and the addition of antiknock agents (tetraethyllead). It is also used as a solvent in many applications, and the desired volatility is achieved by varying the distillation end point.
Chemical composition of gasoline
Gasoline can be considered a mixture of hydrocarbons, primarily octane and octane. Naphtha is generally referred to as the petroleum fraction whose boiling point is between 28 and 175°C, although this range can vary depending on the commercial needs of each refinery. This fraction is subdivided into light naphtha (with a boiling point of up to about 100°C) and heavy naphtha (the remainder).
Light naphtha, which has an octane rating close to 70, is one of the components of gasoline. Heavy naphtha, on the other hand, lacks the necessary quality for direct use in gasoline production, so it undergoes a catalytic reforming process. This chemical process not only improves the octane rating of heavy naphtha, but also produces hydrogen.
In addition to light naphtha and reformate naphtha, other components used in the formulation of commercial gasoline include FCC naphtha, light isomerized naphtha, debenzenized pyrolysis gasoline, butane, butenes, MTBE, ETBE, alkylates, and ethanol. The formulations used at each refinery can vary, even within the same company, depending on the available processing units and the season (summer or winter).
FCC naphtha, obtained through the fluid catalytic cracking (FCC) process from heavy diesel fuel, can contain up to 1,000 ppm of sulfur if unrefined. This type of naphtha is composed of approximately 40% aromatics and 20% olefins. Its octane rating (MON/RON) is around 80/93.
Light isomerized naphtha is obtained from straight-run light naphtha using a process that uses solid catalysts based on platinum/aluminum or zeolites. This component is free of sulfur, benzene, aromatics, and olefins, and its octane ratings (MON/RON) are around 87/89.
Debenzenized pyrolysis gasoline is a byproduct of ethylene production from light naphtha. It is composed primarily of 50% aromatics (such as toluene and xylenes) and 50% olefins (such as isobutene and hexene). It contains approximately 200 ppm sulfur. The benzene present in its composition is usually purified and marketed as a petrochemical feedstock. The octane ratings (MON/RON) of this gasoline range between 85/105.
Alkylate is obtained from isobutane and butenes using a process that employs acid catalysts, such as sulfuric acid or hydrofluoric acid. This component is free of sulfur, benzene, aromatics, and olefins, and its octane ratings (MON/RON) are around 94/95.
Gasoline formula
The chemical formula for gasoline is not a single, fixed formula, as it is a complex mixture of many hydrocarbons. However, gasoline is primarily composed of organic compounds with between 4 and 12 carbon atoms. The main types of hydrocarbons present in gasoline include:
- Alkanes (paraffins) : Saturated hydrocarbons, such as octane (C₈H₁₈) .
- Cycloalkanes (naphthenic) : Saturated cyclic hydrocarbons.
- Aromatics : Hydrocarbons with benzene rings, such as toluene (C₆H₅CH₃) and xylene (C₆H₄(CH₃)₂) .
- Olefins (alkenes) : Unsaturated hydrocarbons, although in smaller quantities.
The blend varies by refinery, season, and gasoline type, but some of the major components include:
- Octane (C₈H₁₈) : Important component of gasoline due to its octane rating, which is crucial for efficient combustion in engines.
- Toluene (C₆H₅CH₃) : An aromatic hydrocarbon that improves octane.
Origin and extraction
Naphtha is also obtained from the conversion of heavy petroleum fractions (vacuum gas oil) in process units called FCC (fluidized catalytic cracking) or hydrocracking.
A series of specifications must be met for the engine to function properly, as well as other environmental specifications, both of which are regulated by law in most countries. The most characteristic specification is the octane rating, which indicates its tendency to detonate.
There are different types of commercial gasoline, classified according to their octane rating. The most popular gasoline in Europe (2004) has a minimum octane rating of 85 and a minimum octane rating of 95.
Carbon dioxide generation from gasoline
Approximately 2.36 kg of carbon dioxide (CO 2 ) are generated when 1 liter of gasoline containing no ethanol is burned. 2.69 kg of CO 2 are generated from 1 liter of diesel fuel .
The U.S. EIA estimates that U.S. motor fuel and gasoline (distillate) consumption for transportation in 2015 resulted in the emission of approximately 1,105 million metric tons of CO 2 and 440 million metric tons of carbon dioxide , respectively, for a total of 1,545 million metric tons of CO 2 . This total was equivalent to 83% of total carbon dioxide emissions from the U.S. transportation sector and equivalent to 29% of total U.S. energy-related carbon dioxide emissions in 2015.
Most retail gasoline now sold in the United States contains about 10% fuel ethanol (or E10) by volume. Burning one gallon of E10 produces approximately 17.68 pounds (8.02 kg) of carbon dioxide, which is emitted from the fossil fuel content.
Considering carbon dioxide emissions from ethanol combustion, about 18.95 pounds (8.60 kg) of carbon dioxide are produced when one gallon of E10 is burned. About 12.73 pounds (5.77 kg) of carbon dioxide are produced when one gallon of pure ethanol is burned.