27.3 What are oxygenates?
Description
This article is from the Chemistry FAQ, by Bruce Hamilton B.Hamilton@irl.cri.nz with numerous contributions by others.
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.
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