4.5 What are oxygenates?
Description
This article is from the Gasoline FAQ, by Bruce Hamilton with numerous contributions by others.
4.5 What are oxygenates?
Oxygenates are just preused hydrocarbons :-). They contain oxygen, which can
not provide energy, but their structure provides a reasonable antiknock
value, thus they are good substitutes for aromatics, and they may also reduce
the smog-forming tendencies of the exhaust gases [15]. 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. MTBE production has
increased at the rate of 10 to 20% per year, and the spot market price in
June 1993 was around $270/tonne [15]. The "ether" starting fluids for
vehicles are usually diethyl ether (liquid) or dimethyl ether (aerosol).
Note that " petroleum ethers " are volatile alkane hydrocarbon fractions,
they are not a Cx-O-Cy compound.
Oxygenates are added to gasolines to reduce the reactivity of emissions,
but they are only effective if the hydrocarbon fractions are carefully
modified to utilise the octane and volatility properties of the oxygenates.
If the hydrocarbon fraction is not correctly modified, oxygenates can
increase the undesirable smog-forming and toxic emissions. Oxygenates do not
necessarily reduce all exhaust toxins, nor are they intended to.
Oxygenates have significantly different physical properties to
hydrocarbons, and the levels that can be added to gasolines are
controlled by the 1977 Clean Air Act amendments in the US, with the
laws prohibiting the increase or introduction of a fuel or fuel
additive that is not substantially similar to any fuel or fuel
additive used to certify 1975 or subsequent years vehicles. Waivers
can granted if the product does not cause or contribute to emission
device failures, and if the EPA does not specifically decline the
application after 180 days, it is taken as granted. In 1978 the EPA
granted 10% by volume of ethanol a waiver, and have subsequently
issued waivers for <10 vol% ethanol (1982), 7 vol% tertiary butyl
alcohol (1979), 5.5 vol% 1:1 MeOH/TBA (1979), 3.5 mass% oxygen derived
from 1:1 MeOH/TBA = ~9.5 vol% of the alcohols (1981), 3.7 mass% oxygen
derived from methanol and cosolvents = 5 vol% max MeOH and 2.5 vol%
min cosolvent - with some cosolvents requiring additional corrosion
inhibitor (1985,1988), 7.0 vol% MTBE (1979), and 15.0 vol% MTBE
(1988). Only the ethanol waiver was exempted from the requirement to
still meet ASTM volatility requirements [16].
In 1981 the EPA ruled that fuels could contain aliphatic alcohols ( except
MeOH ) and/or ethers at concentrations until the oxygen content is 2.0
mass%. It also permitted 5.5 vol% of 1:1 MeOH/TBA. In 1991 the maximum
oxygen content was increased to 2.7 mass%. To ensure sufficient gasoline
base was available for ethanol blending, the EPA also ruled that gasoline
containing up to 2 vol% of MTBE could subsequently be blended with 10 vol%
of ethanol [16].
Initially, the oxygenates were added to hydrocarbon fractions that were
slightly-modified unleaded gasoline fractions, and these were known as
"oxygenated" gasolines. In 1995, the hydrocarbon fraction was significantly
modified, and these gasolines are called "reformulated gasolines" ( RFGs ),
and there are differing specifications for California ( Phase 2 ) and Federal
( simple model ) RFGs, however both require oxygenates to provide Octane.
The California RFG requires the hydrocarbon composition of the RFG to be
significantly more modified than the existing oxygenated gasolines to reduce
unsaturates, volatility, benzene, and the reactivity of emissions. Federal
regulations only reduce vapour pressure and benzene directly, however
aromatics are also reduced to meet emissions criteria [16].
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%) [17], and other researchers have observed similar
reductions also occur when oxygenates are added to reformulated gasolines
on older and newer vehicles, but have also shown that NOx levels may
increase, as also may some regulated toxins [18,19,20].
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 [21,22].
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