Short version: yes, burning gasoline in a clean-running engine does less damage to the environment than simply letting it evaporate.
Long version:
When you burn gasoline, you mostly end up with water and carbon dioxide in the atmosphere. If you burn it in a car (as opposed to in a lawnmower or burn pit), the emissions controls assure that very, very little raw hydrocarbon (HC) gets out of the tailpipe.
HC vapor, OTOH, is a precursor to photochemical smog. Carbon dioxide may warm the planet a smidge down the road, but smog sickens and kills people today (and in the end it will degrade into mostly water and carbon dioxide anyway). On Ozone Action Days, the weather is particularly conducive to smog formation and the public is advised to avoid (if possible) refueling things like cars and lawn mowers because of the HC vapor that inevitably gets put into the air.
HC vapor emissions are such a concern that for decades now cars have been fitted with evaporative emission recovery systems. The fuel tank vents to atmosphere so that it doesn’t build pressure or vacuum when the temperature fluctuates (or when fuel gets used). The vent line passes through a canister filled with activated charcoal chunks that preferentially adsorb the HC vapor, leaving only clean air to be emitted to atmosphere. When the engine runs, a valve moves and the engine sucks a bit of air through this canister, removing HC vapors from the charcoal and sending them to the engine to be burned, and cleaning the canister out.
Small engines like your lawn mower don’t have evap systems like that, but it doesn’t mean HC emissions from them aren’t harmful; it’s just that evap systems aren’t practical at that scale because they would make your lawn mower bulky, heavy, and expensive.
Bottom line: don’t leave a few gallons of gas out to simply evaporate. Either burn it in an engine, or turn it in to a household hazardous waste collection facility.
How safe is it to burn gasoline? Suppose you have a decent-sized lawn, can you just pour the gasoline on the ground, light it up and run?
I had this question when I replaced lawn mowers last summer. First I poured the gasoline from the old mower into a container, thinking I would transfer it to the new one or a car, but some dirt fell into the container, and I didn’t want to risk ruining another machine. So I was stuck with it.
I let it sit around a bit but got nervous that it was a fire hazard, and was also nervous that lighting it up as above would cause some sort of explosion or fireball. (I think I ended up just pouring it back into the old machine when I put it out at the curb, and let the guy who decided to deal with the old machine deal with that as well.)
It’s pretty insidious. Not only does it burn with great vigor, but the vapors are much heavier than air and will collect near the ground and flow for substantial distances. If you pour a gallon of gasoline on the ground and toss a match, two things are going to happen:
-the flames will travel through the pool of vapor around your legs, singing them, and
-the fireball from the main puddle will be so massive that it will give you radiant heat burns (this assumes you aren’t actually engulfed by said fireball).
Can you please give a cite as to how efficient this charcoal system is ? Something like XX% of the the hydrocarbon evaporated from the tank is adsorbed and recycled back to the engine. My guess is XX is pretty low - maybe like 70% and deteriorates rapidly as the car ages.
Household Waste Collection facilities take fuels like this and burn in cement plants or furnaces without much emissions control. This of course reduces fire and explosion hazards, but I am not sure it reduces environment impact ? Can you please present an analysis for the following :
A. Consider the environmental impact of fuel used in transporting and handling/processing/filtering the small amount of leftover gasoline and then burning that small amount of gasoline in an operation like cement making which produces HC emissions of its own.
B. Burning the small amount of fuel in your backyard .
Which is more environmentally friendly: A or B ? Understood that A is more safe.
I don’t know how effective evap canisters were in 2001, but here they’re talking about LEV II canister emission targets that are only 3-10% of the 2001 level. Even if we assume 2001 canisters only had a 70% capture rate (this seems unlikely based on the language I quoted from my first link), this means they’re talking about canisters for the LEV II standard with a 97-99% capture rate.
Fuel used in transporting your hazardous waste will be small; they’re not going to put your three gallons of gasoline on a 53’ trailer all by itself, it’s going to be shipped with plenty of other waste to make the trip worthwhile.
Cement plants and incinerators may not have as stringent emissions control as your car’s engine does, but what they do have is going to be better than what the average citizen has (i.e. nothing at all) in his backyard fire pit.
For someone disinclined to surrender gasoline to a HHW collection facility, burning it in their backyard by whatever safe means they can devise is likely to pollute less than letting it simply evaporate.
Add another voice supporting diluting it into your car and burning it off.
Here is a breakdown so far of the various disposal methods you could conceptually use.
Allow to evaporate:
[ul]
[li]Most harmful: results in smog-causing raw hydrocarbons which later break down into further greenhouse gases[/li][li]Some danger: open container yields fumes which are a fire risk[/li][li]Will not completely disappear. A thick varnish-like residue will be left in container that is still hazardous to rinse down drain.[/li][/ul]
Burn in car engine:
[ul]
[li]Least harmful. Gas burned off and largely converted to carbon dioxide, wastes processed through car emissions control devices[/li][li]Safe, easy[/li][li]Small amounts diluted into large tank will have no ill effect[/li][/ul]
Open burn:
[ul]
[li]Less harmful than evaporating, but emissions won’t be handled as in a car or industrial process.[/li][li]Dangerous: most people are not equipped to safely burn bulk liquid fuels in an open fire.[/li][li]May be illegal in your locality.[/li][/ul]
Take to hazardous waste collection:
[ul]
[li]Least harmful: Gas and other wastes will be incinerated in controlled furnaces, same effect as burning off in vehicle[/li][li]More difficult to take fuel to collection location[/li][li]May have to pay disposal fee[/li][/ul]