Amount of greenhouse gases released

[ChrisCell]

To whom it may concern,

I wrote this due to a confusing chemistry concept that I need closure with. Here is the question:

When you burn something, for example trash (plastic waste), will the amount of greenhouse gas emissions be equal to the weight of the trash?

I'm looking forward for the answer, thank you for your consideration.

[mbdowell]

Christian, you have the right idea but I'm going to re-state your question so that the answer will be easier to understand: When you burn something, will the amount of greenhouse gas emissions be equal to the weight of the trash plus the weight of oxygen needed to burn it?

As you know, the weight of all the products in a chemical reaction equals the weight of all the reactants-- we don't gain or lose any mass when we do reactions, we can add it all up and "mass is conserved". Let's burn polyethylene; it's easy trash. The formula is (C2H4)n where n is the number of C2H4 units in the molecule, maybe 10,000 or so for some kinds. But for any C2H4 unit the reaction is

               C2H4 + 3 O2 = 2 CO2 + 2 H2O

If you started with 1 mole of C2H4 weighing 28g and added 3 moles of O2 weighing 3x32g the reactants weighed 124g. The products are 2 CO2 weighing 2 x 44g and 2 H2O weighing 2 x 18g so the products also weigh 124 g and the mass is conserved (the reaction "balances"). Now here is my first question for you:

Are you considering H2O as a greenhouse gas?  It does warm the atmosphere, just as CO2 does. If it didn't warm the atmosphere, Earth would be a very cold place. So yes it is a greenhouse gas and in this example, the weight of greenhouse gas emissions is equal to the weight of the trash (polyethylene) plus the oxygen needed to burn it. The nice thing about water vapor, though, is that we store it in oceans and lakes and it doesn't build up much in the atmosphere; it keeps he temperature steady. When people say that CO2 is a greenhouse gas, they usually mean that adding more CO2 to the atmosphere than the oceans can absorb is harmful because the extra CO2 that stays in the atmosphere absorbs more sunlight than before and makes our environment hotter than before, as though we were living in a greenhouse.  If you allow for that, he amount of harmful greenhouse gas emissions-- emissions like CO2-- is almost always less than the weight of the trash plus the weight of the oxygen.

Polyethylene is a nice example of trash to discuss your question about weight of greenhouse gases. But household trash doesn't all burn up; there is usually a lot of ash left over. The stuff that reacted to make ashes usually does not contribute very much to harmful greenhouse gases like CO2.

[ChrisCell]

Michael, I thank you for your response and assisting to have closure with this detail. To clarify with you, I'm referring to the general perspective of greenhouse gases such as Nitrogen, Methane, and the like; however, I just needed that clarification. Can I please source your comment in my research paper in order to validate this data?

[mbdowell]

Sure, Christian. And 2 tips, one on greenhouse gases and one on steel choices.

Greenhouse gases: to qualify, a gas has to be a molecule that absorbs infrared radiation; atoms like argon can't do that. A molecule can absorb IR if it has dissimilar atoms in it; CO2, CO and H2O absorb IR and are greenhouses but O2 and N2 have the same atoms and do not. If you are going to incinerate household trash, you are going to make CO2. But you may still have a worthwhile project because if trash goes straight to landfill it makes methane (CH4) and that's a harmful greenhouse gas that stays in the atmosphere for quite a while.

Steel choices: if you burn trash in a metal container, it's hard on the container. It will wear out either from corrosion or from erosion (think a tilted rotating kiln where the un-combusted stuff scrapes the walls and falls out the exit), so you have to plan to replace containers periodically and do maintenance every day on all the other piping. It can all work, but a lot of people have lost money. I assume you have looked up trash incineration and municipal trash incineration to see how people design equipment and do operations. Another search worth doing is to go to the Patent Office (www.uspto.gov) and search recent patents using keywords like "apparatus" or "steel" and "waste incineration". The "Prior Art" section of each patent tells what problem needed solving (the Invention), and you get drawings of the invention.  This will also tell you what kind of steel works with what kind of incineration process. Then you can talk directly to steel apparatus manufacturers. Go to www.thomasnet.net to find some, look at their websites, call up a likely manufacturer and ask them to quote a price.

[ChrisCell]

That helps a lot Sir Michael. Thanks a lot!

[jschulte]

I wanted to clarify Michael Dowell's comment: "the extra CO2 that stays in the atmosphere absorbs more sunlight than before".  CO2 is essentially transparent to sunlight (i.e., it doesn't absorb most visible light).

Sunlight heats the earth's surface.  The warm surface reemits infrared light to space as blackbody radiation.  CO2 absorbs some of this infrared radiation (that would escape to space in CO2's absence) warming the atmosphere.  The more CO2 in the atmosphere, the more infrared radiation absorbed, the more warming.

[Heirtzler]

I wished to add to the discussion the following article which I just read today on the BBC internet service. Of course this is popular news report. However, it explains the premise in a straightforward fashion for the non-chemist: plastic, in particular polyethylene (HDPE), itself emits a gas which contribute to the greenhouse effect. This gas is methane.

https://www.bbc.com/news/science-environment-45043989

HDPE is the material from which disposable plastic bottles for beverages are manufactured. That much is clear.

Chemically speaking, what is the process by which the methane is produced? The answer here is not yet clear, but it seems that sunlight decomposes the bottles, which somehow produces methane. My guess is that it must involve hard UV radiation.

What does the HDPE industry think of this finding? That is less speculative.