More discussion from GasFAQ ... this time on solubility of MTBE
(bad, bad MTBE) and ethanol (good, good ethanol) ... regulation
... renewability ... suitability of oxygenated (that's us) gas
for existing vehicles (it is) ...
Note that this was written in 1995, I believe, when MTBE (bad, bad)
was being used to replace lead (bad, bad, BAD lead). Nowadays MTBE
has become the evil villian ...
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5.9 Why are only some gasoline compounds restricted?
The less volatile hydrocarbons in gasoline are not released in significant
quantities during normal use, and the more volatile alkanes are considerably
less toxic than many other chemicals encountered daily. The newer gasoline
additives also have potentially undesirable properties before they are even
combusted. Most hydrocarbons are very insoluble in water, with the lower
aromatics being the most soluble, however the addition of oxygen to
hydrocarbons significantly increases the mutual solubility with water.
Compound in Water Water in Compound
% mass/mass @ C % mass/mass @ C
normal decane 0.0000052 25 0.0072 25
iso-octane 0.00024 25 0.0055 20
normal hexane 0.00125 25 0.0111 20
cyclohexane 0.0055 25 0.010 20
1-hexene 0.00697 25 0.0477 30
toluene 0.0515 25 0.0334 25
benzene 0.1791 25 0.0635 25
methanol complete 25 complete 25
ethanol complete 25 complete 25
MTBE 4.8 20 1.4 20
TAME - 0.6 20
The concentrations and ratios of benzene, toluene, ethyl benzene, and xylenes
( BTEX ) in water are often used to monitor groundwater contamination from
gasoline storage tanks or pipelines. The oxygenates and other new additives
may increase the extent of water and soil pollution by acting as co-solvents
for HCs.
Various government bodies ( EPA, OSHA, NIOSH ) are charged with ensuring
people are not exposed to unacceptable chemical hazards, and maintain
ongoing research into the toxicity of liquid gasoline contact, water and soil
pollution, evaporative emissions, and tailpipe emissions [87]. As toxicity
is found, the quantities in gasoline of the specific chemical ( benzene ),
or family of chemicals ( alkyl leads, aromatics, olefins ) are regulated.
The recent dramatic changes caused by the need to reduce alkyl leads,
halogens, olefins, and aromatics has resulted in whole new families of
compounds ( ethers, alcohols ) being introduced into fuels without prior
detailed toxicity studies being completed. If adverse results appear, these
compounds are also likely to be regulated to protect people and the
environment.
Also, as the chemistry of emissions is unravelled, the chemical precursors
to toxic tailpipe emissions ( such as higher aromatics that produce benzene
emissions ) are also controlled, even if they are not themselves toxic.
5.10 What does "renewable" fuel or oxygenate mean?
The general definition of "renewable" is that the carbon originates from
recent biomass, and thus does not contribute to the increased CO2 emissions.
A truly "long-term" view could claim that fossil fuels are "renewable" on a
100 million year timescale :-). There was a major battle between the
ethanol/ETBE lobby ( agricultural, corn growing ), and the methanol/MTBE
lobby ( oil company, petrochemical ) over an EPA mandate demanding that a
specific percentage of the oxygenates in gasoline are produced from
"renewable" sources [88]. On 28 April 1995 a Federal appeals court
permanently voided the EPA ruling requiring "renewable" oxygenates, thus
fossil-fuel derived oxygenates such as MTBE are acceptable oxygenates [89].
Unfortunately, "renewable" ethanol is not cost competitive when crude oil
is $18/bbl, so a federal subsidy ( $0.54/US Gallon ) and additional state
subsidies ( 11 states - from $0.08(Michigan) to $0.66(Tenn.)/US Gal.) are
provided. Ethanol, and ETBE derived from ethanol, are still likely to be
used in states where subsidies make them competitive with other oxygenates.
5.11 Will oxygenated gasoline damage my vehicle?
The following comments assume that your vehicle was designed to operate on
unleaded, if not, then damage such as exhaust valve seat recession may occur.
Damage should not occur if the gasoline is correctly formulated, and you
select the appropriate octane, but oxygenated gasoline will hurt your pocket.
In the first year of mandated oxygenates, it appears some refiners did not
carefully formulate their oxygenated gasoline, and driveability and emissions
problems occurred. Most reputable brands are now carefully formulated.
Some older activated carbon canisters may not function efficiently with
oxygenated gasolines, but this is a function of the type of carbon used.
How your vehicle responds to oxygenated gasoline depends on the engine
management system and state of tune. A modern system will automatically
compensate for all of the currently-permitted oxygenate levels, thus your
fuel consumption will increase. Older, poorly-maintained, engines may
require a tune up to maintain acceptable driveability.
Be prepared to try several different brands of oxygenated or reformulated
gasolines to identify the most suitable brand for your vehicle, and be
prepared to change again with the seasons. This is because the refiners can
choose the oxygenate they use to meet the regulations, and may choose to set
some fuel properties, such as volatility, differently to their competitors.
Most stories of corrosion etc, are derived from anhydrous methanol corrosion
of light metals (aluminum, magnesium), however the addition of either 0.5%
water to pure methanol, or corrosion inhibitors to methanol-gasoline blends
will prevent this. If you observe corrosion, talk to your gasoline supplier.
Oxygenated fuels may either swell or shrink some elastomers on older cars,
depending on the aromatic and olefin content of the fuels. Cars later than
1990 should not experience compatibility problems, and cars later than 1994
should not experience driveability problems, but they will experience
increased fuel consumption, depending on the state of tune and engine
management system.
5.12 What does "reactivity" of emissions mean?
The traditional method of exhaust regulations was to specify the actual HC,
CO, NOx, and particulate contents. With the introduction of oxygenates and
reformulated gasolines, the volatile organic carbon (VOC) species in the
exhaust also changed. The "reactivity" refers to the ozone-forming potential
of the VOC emissions when they react with NOx, and is being introduced as a
regulatory means of ensuring that automobile emissions do actually reduce
smog formation. The ozone-forming potential of chemicals is defined as the
number of molecules of ozone formed per VOC carbon atom, and this is called
the Incremental Reactivity. Typical values ( big is bad :-) ) are [74]:
Maximum Incremental Reactivities as mg Ozone / mg VOC
carbon monoxide 0.054
alkanes methane 0.0148
ethane 0.25
propane 0.48
n-butane 1.02
olefins ethylene 7.29
propylene 9.40
1,3 butadiene 10.89
aromatics benzene 0.42
toluene 2.73
meta-xylene 8.15
1,3,5-trimethyl benzene 10.12
oxygenates methanol 0.56
ethanol 1.34
MTBE 0.62
ETBE 1.98 |
