=chemistry =chemical safety =air
Air pollution is one of the biggest public health problems, so let's consider how it could be reduced.
On an individual level, the main things you can do to mitigate air pollution are:
- get a
standalone air filter for your home
- get furnace filters
- don't
smoke
- try to avoid breathing car exhaust and smoke from fires
- use
a fan over your stove when cooking
But this post is about air pollution on a societal level, so let's continue to that.
I divide air pollution into 3 basic
categories: particulates, oxidants, and toxics.
Particulates are
particles small enough to get deep inside lungs and stable enough that they
accumulate because the body can't break them down. Notable examples are
carbon particles and fine silica.
Oxidants are chemicals that can
oxidize something in the body, especially something on the surface of the
lungs. Notable examples are ozone, nitrogen oxides, and chlorine.
Toxics are hazardous volatile organic chemicals and toxic elements. Notable
examples include benzene, lead, and
benzopyrene. Some
toxics are mainly in particles and are sometimes considered "particulates",
but I prefer to consider them separately.
particulates
The main type of hazardous carbon
particulates is clusters of large
polycyclic aromatic hydrocarbons (PAHs). The main source of PAHs is
burning things - more specifically, incompletely burning things.
Historically, the main sources of exposure to PAHs were indoor cooking and
forest fires. Those are still major issues, but have been surpassed by
particulates from piston engines, which are mostly motor vehicle engines.
Regarding forest fires, the solution is to
remove flammable stuff and plant
less-flammable stuff. The average amount of air pollution from fires in
unmanaged forests is unacceptable.
Incomplete combustion happens
either when there's not enough oxygen, or when burning material is cooled
before it can finish. Diesel engines add fuel to hot air, so it starts
burning before it can mix, which produces small areas with more fuel than
air. That's why they produce more particulates than spark-ignition engines.
All piston engines have cooler areas near the piston and its cylinder,
because heat is transferred to the metal. Those cooler areas near the edges
of combustion are responsible for most particulate production in
spark-ignition engines with excess air. Spark-ignition engines obviously
produce far more particulates if their mixture has excess fuel.
Chemically, large PAH formation
generally goes through ethene -> acetylene or through benzene with 4+ methyl
groups. If you want to change the fuel to prevent particulate formation, you
want to use either a fuel with no carbon-carbon bonds, benzene, or toluene.
Benzene is quite toxic, and toluene is also fairly toxic. If you want no
carbon-carbon bonds, your options are methane, methanol, dimethyl ether,
ammonia, and methylamines.
In the shorter term, reducing the amount
of benzene with 3+ methyl groups in gasoline should reduce PAH formation,
but that would also probably decrease octane ratings.
Methane can't
be liquefied at ambient temperatures, which is inconvenient, but compressed
natural gas (CNG) is used as a fuel for some cars today. It has a very high
octane rating, so it's good for spark-ignition engines but not diesel ones.
Current CNG cars are generally standard models with a CNG tank crammed in
somewhere, which means they usually have less useful space. Also, CNG tanks
are somewhat expensive - but cheaper than batteries, of course. Finally,
most gas stations don't have CNG refueling equipment. Those are probably the
main issues with CNG vehicles, and in theory, they're not so bad.
Considering the reduced pollution, lower cost (at least in North America),
and high octane rating of natural gas, CNG cars seem viable in the USA in
theory, but getting there would probably require a sizeable program from the
federal government.
Methanol is a liquid, but it has low energy
density, and it's a solvent that tends to dissolve rubbers. Still, China is
blending some methanol with gasoline on a large scale. Methanol is made from
natural gas, and is sometimes economically competitive with gasoline.
Dimethyl ether has a high cetane number, so it's good for diesel
engines. Conversion of methanol to dimethyl ether is easy. It can be
liquefied at high pressure. Running diesel vehicles on dimethyl ether makes
a lot of sense to me, but the infrastructure for that doesn't currently
exist.
Methylamines haven't really been considered as fuels; maybe
there's some reason for that I'm unaware of. Currently, there's not enough
ammonia production so prices are high, but in theory they could be made from
natural gas at a reasonable cost.
Those would all give very low
particulate levels, but do have some issues. Another option is to use fuel
that contains at least 1 oxygen per 3 carbon atoms. That leads to non-PAH
products dominating even when there's excess fuel. Acetone is an obvious
choice here: 1 oxygen, 3 carbon, high octane rating, and low toxicity. It's
a good solvent and can dissolve some rubbers; that can be dealt with but
retrofitting existing vehicles would be expensive. It's also currently too
expensive to use for gasoline, but I designed a cheap enough route from
natural gas to acetone to use it as fuel in North America. Another obvious
possibility is methyl pentanoate, which could theoretically be made from
cellulose via levulinic acid, but that's currently much too expensive.
Vehicles also produce particulates from brake pads and tires wearing. For complicated reasons, rubber needs small particles in it, and that's usually carbon black, which is why tires are black. The solution to pollution from tires is to use calcium carbonate instead, but it needs to be precipitated in very particular ways to get the right shapes, and with no looming threat of regulation, companies aren't interested in pursuing that. As for brakes, the particles from sintered MgO shouldn't be a significant health problem.
oxidants
Nitrogen
oxides can be made by oxygen and nitrogen reacting at high temperature. The
usual mitigations are:
- use enough
fuel that there's little oxygen left (in piston engines)
- use a low
enough flame temperature that nitrogen-oxygen reaction is low (in gas
turbines)
- use a catalytic converter (on cars) to decompose nitrogen
oxides
Many volatile organics react with air in sunlight to make photochemical smog. This is the main reason for VOC regulations, but those have some issues.
toxics
There are many toxic
chemicals, and it's not practical to list them, but the most notable toxic
air pollutant has been lead from leaded gasoline. Lead is still used in
gasoline (avgas) for piston-engine aircraft, which is
unnecessary. Also, FAA approval being slow and difficult prevented
more-modern engines that don't need lead from being used. Some modern
motorcycle engines are better than current piston aircraft engines in every
way, including weight and reliability.
Once, I called the relevant
local government organization about getting a nearby airport to offer
unleaded ethanol-free gasoline as well as leaded gasoline. They responded
that the avgas supply was contracted to a company and it didn't consider
dealing with 2 types of avgas to be economically worthwhile.
Benzene
is probably the most significant toxic from spark-ignition engine exhaust
today. It mostly comes from the gasoline and passes through without being
burnt when it's near the edge of the flame. There are now regulations
limiting benzene in gasoline to <1%, which is somewhat expensive to do:
refiners now generally use selective solvent extraction of the C6
distillate, then hydrogenation of residual benzene to cyclohexane. The
reason benzene is toxic is interesting: it's oxidized to benzene oxide by a
cytochrome P450 oxidase - an extremely reactive molecule, made by a reaction
chemists are unable to duplicate.