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Sci.chem FAQ - Part 6 of 7
Section - 27. Fuel Chemistry

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27.1  Where does crude oil come from?.

The generally-accepted origin of crude oil is from plant life up to 3 
billion years ago, but predominantly from 100 to 600 million years ago [1]. 
"Dead vegetarian dino dinner" is more correct than "dead dinos".
The molecular structure of the hydrocarbons and other compounds present 
in fossil fuels can be linked to the leaf waxes and other plant molecules of 
marine and terrestrial plants believed to exist during that era. There are 
various biogenic marker chemicals such as isoprenoids from terpenes, 
porphyrins and aromatics from natural pigments, pristane and phytane from 
the hydrolysis of chlorophyll, and normal alkanes from waxes, whose size 
and shape can not be explained by known geological processes [2]. The 
presence of optical activity and the carbon isotopic ratios also indicate a 
biological origin [3]. There is another hypothesis that suggests crude oil 
is derived from methane from the earth's interior. The current main 
proponent of this abiotic theory is Thomas Gold, however abiotic and
extraterrestrial origins for fossil fuels were also considered at the turn 
of the century, and were discarded then. A large amount of additional
evidence for the biological origin of crude oil has accumulated, however
Professor Gold still actively promotes his theory worldwide, even though
it does not account for the location and composition of all crude oils.  

27.2  What are CNG/LPG/gasoline/kerosine/diesel?. 

Crude oil consists mainly of hydrocarbons with carbon numbers between one and
forty. The petroleum refinery takes this product and refines it into several
fuel fractions that are optimised for their intended application. For spark
ignition engines, the very volatile and branched chain alkane hydrocarbons
have desirable combustion properties, and several fractions are produced.
          
CNG ( Compressed Natural Gas ) is usually around 70-90% methane with 10-20% 
ethane, 2-8% propanes, and decreasing quantities of the higher HCs up to 
pentane.  The major disadvantage of compressed gaseous fuels is the reduced 
range. Vehicles may have between one to three cylinders ( 25 MPa, 90-120 
litre capacity), and they usually provide about 50% of the gasoline range. 

LPG ( Liquefied Petroleum Gas ) is predominantly propane with iso-butane
and n-butane. It has one major advantage over CNG, the tanks do not have
to be high pressure, and the fuel is stored as a liquid. The fuel offers   
most of the environmental benefits of CNG, including high octane - which
means higher compression, more efficient, engines can be used. Approximately 
20-25% more fuel than gasoline is required, unless the engine is optimised 
( CR 12:1 ) for LPG, in which case there is no decrease in power or any 
significant increase in fuel consumption [4,5].

Gasoline contains over 500 hydrocarbons that may have between 3 to 12 
carbons, and gasoline used to have a boiling range from 30C to 220C at 
atmospheric pressure. The boiling range is narrowing as the initial boiling 
point is increasing, and the final boiling point is decreasing, both 
changes are for environmental reasons. A detailed description of the 
composition of gasoline, along with the properties and compositions of CNG, 
LPG, and oxygenates can be found in the Gasoline FAQ, which is posted monthly 
to rec.autos.tech.

Kerosine is a hydrocarbon fraction that typically distils between 170-270C
(narrow cut kerosine, or Jet A1) or 100-250C ( wide cut kerosine, or JP-4 ). 
It contains around 20% of aromatics, however the aromatic content will be 
reduced for high quality lighting kerosines, as the aromatics reduce the 
smoke point. The major use for kerosines today is as aviation turbine (jet) 
fuels. Special properties are required for that application, including high 
flash point for safe refuelling ( 38C for Jet A1 ), low freezing point for
high altitude flying ( -47C for Jet A1 ), and good water separation 
characteristics.  Details can be found in any petroleum refining text and
Kirk Othmer.

Diesel is used in compression ignition engines, and is a hydrocarbon fraction 
that typically distils between 250-380C. Diesel engines use the Cetane 
(n-hexadecane) rating to assess ignition delay. Normal alkanes have a high 
cetane rating, ( nC16=100 ) whereas aromatics ( alpha methylnaphthalene = 0 ) 
and iso-alkanes ( 2,2,4,4,6,8,8-hexamethylnonane = 15 ) have low ratings, 
which represent long ignition delays. Because of the size of the hydrocarbons,
the low temperature flow properties control the composition of diesel, and
additives are used to prevent filter blocking in cooler temperatures. There
are usually summer and winter grades. Environmental legislation is reducing
the amount of aromatics and sulfur permitted in diesel, and the emission of
small particulates ( diameters of <10um ) that are considered possibly
carcinogenic, and are known to cause other adverse health effects. Details 
can be found in any petroleum refining text and Kirk Othmer.

27.3  What are oxygenates?.

Oxygenates are just pre-used hydrocarbons :-). They contain oxygen, which can 
not provide energy, but their structure provides a reasonable anti-knock 
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases [6]. Most oxygenates used 
in gasolines are either alcohols ( Cx-O-H ) or ethers (Cx-O-Cy), and contain 
1 to 6 carbons. Alcohols have been used in gasolines since the 1930s, and
MTBE was first used in commercial gasolines in Italy in 1973, and was first
used in the US by ARCO in 1979. The relative advantages of aromatics and 
oxygenates as environmentally-friendly and low toxicity octane-enhancers are 
still being researched.

    Ethanol                                  C-C-O-H      C2H5OH
  
                                               C
                                               |
    Methyl tertiary butyl ether              C-C-O-C      C4H9OCH3
    (aka tertiary butyl methyl ether )         |
                                               C

They can be produced from fossil fuels eg methanol (MeOH), methyl tertiary 
butyl ether (MTBE), tertiary amyl methyl ether (TAME), or from biomass, eg 
ethanol(EtOH), ethyl tertiary butyl ether (ETBE)). MTBE is produced by 
reacting methanol ( from natural gas ) with isobutylene in the liquid phase 
over an acidic ion-exchange resin catalyst at 100C. The isobutylene was 
initially from refinery catalytic crackers or petrochemical olefin plants, 
but these days larger plants produce it from butanes.

Oxygenates have significantly different physical properties to hydrocarbons,
and the levels that can be added to gasolines are controlled by the EPA in
the US, with waivers being granted for some combinations. Initially the
oxygenates were added to hydrocarbon fractions that were slightly-modified
unleaded gasoline fractions, and these were commonly known as "oxygenated"
gasolines. In 1995, the hydrocarbon fraction was significantly modified, and
these gasolines are called "reformulated gasolines" ( RFGs ). The change to
reformulated gasoline requires oxygenates to provide octane, but also that 
the hydrocarbon composition of RFG must be significantly more modified than 
the existing oxygenated gasolines to reduce unsaturates, volatility, benzene,
and the reactivity of emissions. 

Oxygenates that are added to gasoline function in two ways. Firstly they
have high blending octane, and so can replace high octane aromatics
in the fuel. These aromatics are responsible for disproportionate amounts
of CO and HC exhaust emissions. This is called the "aromatic substitution 
effect". Oxygenates also cause engines without sophisticated engine 
management systems to move to the lean side of stoichiometry, thus reducing 
emissions of CO ( 2% oxygen can reduce CO by 16% ) and HC ( 2% oxygen can 
reduce HC by 10%)[7]. However, on vehicles with engine management systems,
the fuel volume will be increased to bring the stoichiometry back to
the preferred optimum setting. Oxygen in the fuel can not contribute 
energy, consequently the fuel has less energy content. For the same
efficiency and power output, more fuel has to be burnt, and the slight
improvements in combustion efficiency that oxygenates provide on some 
engines usually do not completely compensate for the oxygen.
 
There are huge number of chemical mechanisms involved in the pre-flame 
reactions of gasoline combustion. Although both alkyl leads and oxygenates 
are effective at suppressing knock, the chemical modes through which they 
act are entirely different. MTBE works by retarding the progress of the low 
temperature or cool-flame reactions, consuming radical species, particularly 
OH radicals and producing isobutene. The isobutene in turn consumes 
additional OH radicals and produces unreactive, resonantly stabilised 
radicals such as allyl and methyl allyl, as well as stable species such as 
allene, which resist further oxidation [8,9]. 

The major concern with oxygenates is no longer that they may not be 
effective at reducing atmospheric pollution, but that their greater water 
solubility, and very slow biodegradability, can result in groundwater
pollution that may be difficult to remove. Their toxicological and
environmental effects are also still being researched. 

27.4  What is petroleum ether?.

Petroleum ether ( aka petroleum spirits ) is a narrow alkane hydrocarbon
distillate fraction from crude oil. The names "ether" and "spirit" refer
to the very volatile nature of the solvent, and petroleum ether does not
have the ether ( Cx-O-Cy ) linkage, but solely consists of hydrocarbons. 
Petroleum ethers are defined by their boiling range, and that is typically 
20C. Typical fractions are 20-40C, 40-60C, 60-80C, 80-100C, 100-120C etc. 
up to 200C. There are specially refined grades that have any aromatic 
hydrocarbons removed, and there are specially named grades, eg pentane
fraction (30-40C), hexane fraction (60-80C, 67-70C). It is important to
note that most "hexane" fractions are mixtures of hydrocarbons, and pure
normal hexane is usually described as "n-hexane".  

27.5  What is naphtha?.

Naphtha is a refined light distillate fraction, usually boiling below 250C,
but often with a fairly wide boiling range. Gasoline and kerosine are the 
most well-known, but there are a whole range of special-purpose hydrocarbon 
fractions that can be described as naphtha. The petroleum refining industry
calls the 0-100C fraction from the distillation of crude oil "light virgin
naphtha" and the 100-200C fraction " heavy virgin naphtha". The product
stream from the fluid catalytic cracker is often split into three fractions,
<105C = "light FCC naphtha", 105-160C = "intermediate FCC naphtha" and 
160-200C "heavy FCC naphtha".  

27.6  What are white spirits?.

White spirits are petroleum fractions that boil between 150-220C. They can 
have aromatics contents between 0-100%, and Shell lists eight grades with 
aromatics contents below 50%, and six grades with aromatics contents above
50%. The two common "white spirits" are defined by British Standard 245, 
which states Type A should have aromatics content of less that 25% v/v and 
Type B should have an aromatics content of 25-50% v/v. The most common 
" white spirit" is type A, and it typically has an aromatics content of
20%, boils between 150-200C, and has an aniline point of 58C, and is 
sometimes known as Low Aromatic White Spirits. The next most common is 
Mineral Turpentine (aka High Aromatic White Spirits ), which typically has
an aromatics content of 50%, boils between 150-200C and has an aniline
point of 25C. For safety reasons, most White Spirits have Flash Points
above ambient, and usually above 35C. Note that "white gas" is not white 
spirits, but is a volatile gasoline fraction that has a flash point below 
0C, which is also known by several other names. Do not confuse the two 
when purchasing fuel for camping stoves and lamps, ensure you purchase the
correct fuel.   

27.7  What are biofuels?.

Biofuels are produced from biomass ( land and aquatic vegetation, animal
wastes, and photosynthetic organisms ), and are thus considered renewable
within relatively short time-frames. Examples of biofuels include wood,
dried animal dung, methyl esters from triglyceride oils, and methane from
land-fills. The renewable aspect of most biofuels is essentially the use
of solar energy to grow crops that can be converted to energy. There is
a large monograph "Fuels from Biomass" in Kirk Othmer, and the subject
is frequently discussed in alt.energy.renewable, sci.energy, and 
sci.energy.hydrogen.  

27.8  How can I convert cooking oil into diesel fuel?. 

Diesel engines can run on plant and animal triglycerides such as tallow
and seed oils, however most trials have resulted in reduced engine life, or
increased service costs. The solution is to transesterify the triglycerides
into esters, taking care to avoid the formation of monoacylglycerides
that will precipitate out at low temperatures or when diesel is encountered.
There are several plants in Austria that produce Rapeseed Oil Methyl Esters
as fuels for diesel engines. The economics of the process are very 
dependant on the price of diesel and the market for the glycerol byproduct.

The common catalysts used to transesterify triglycerides are sodium 
hydroxide, sodium methoxide and potassium carbonate. If the esters are to 
be blended with diesel fuel, then a two stage reaction is usually required 
to ensure that monoacylglycerides are kept below 0.05%. Usually this 
involves using 22g of methanol ( containing 0.6g of sodium hydroxide ) and 
100g of tallow refluxed for 30 minutes. The mixture is cooled, the glycerol 
layer removed, and a further 0.2g of sodium hydroxide is reacted for 5 
minutes at 35C in a stirred reactor. The glycerol phase is allowed to 
separate, and the ester phase is washed with water to remove residual 
catalyst, glycerol and methanol. Note that sodium hydroxide is the most 
cost-effective catalyst, but also has the worst tendency to form soaps. 
The catalyst and methanol can be of industrial grade without further 
purification required, however care should be taken to prevent additional 
water entering the reaction [10]. 

The fuel can be converted into other esters, such as ethyl and butyl, but
it really depends on the availability of cheap alcohol along with the
desired properties of the fuels. The effect of catalysts, reagent ratio, 
temperature, and moisture on the production of methyl, ethyl, and butyl
esters from some common oils has been investigated [11]. The New Zealand 
government investigated a wide range of techniques for turning various 
vegetable and animal triglycerides into esters for diesel, and the reports 
cover many aspects of the kinetics and efficiencies [12]. There is a general 
overview of the current processes and technology available in Inform [13]. 
A specific technique for analysing the monoglycerides has been published 
[14], however I have found that acetylation followed by narrow bore 
( 0.1mm ID ) capillary chromatography is faster and cheaper.  

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Last Update March 27 2014 @ 02:12 PM