I was reading the NY Times today-they had an article about garbage to energy plants in Denamrk. Apparently, the Danes generate a lot of electricity by burning domestic trash and garbage.
Here in the USA, we bury most of our trash in landfills-which of course, means that the carbon in the trash is kept away (sequestered).
If you burn the trash, you are releasing CO2-which is a no-no.
So, should the USA keep burying trash? Is it an efficient way to sequester carbon?
Here, let me do it for you:
From here, the US generates around 500 Million tons of solid waste each year.
From here, the US generates 5.7 Billion tons of CO2 a year.
So, even if 100% of the solid waste was CO2 (which it isn’t), we would only be sequestering 1/11 of it that way (and then only for a short time, since landfills tend to outgas).
This doesn’t really address the question asked in the OP.
Yes, landfills sequester carbon. Paper, organics, plastics, etc. are all buried. Presuming it isn’t dug back up, most of the carbon will stay under the ground, and perhaps will eventually turn back into fossil fuel.
As far as the efficiency of that sequestration, it’s difficult to understand what you mean by efficient. Do you mean energy-wise, or do you mean the amount that stays in the landfill, or what?
Finally, the decision to bury our trash is an interesting one. Plenty of people have strong feelings either way. But it looks as if our landfill technology is pretty mature, and the landfill can be converted back into usable land quickl. Pollution from well-designed landfills is very very low, and with the move to fewer, larger dumps, safety is increased. Our trash doesn’t really take up much space, either, so in the U.S. there isn’t much concern about land usage.
I doubt that landfills sequester carbon as efficiently as coal does, so if the alternative to burning garbage for electricity is burning coal, burning the garbage would probably make more sense.
You left out the part about where 72% of the weight of CO2 comes from the O2. So your figure of 500 million tons of solid carbon would actually produce 1.85 billion tons of CO2, the O2 essentially all coming from the atmosphere.
Trash contains a large amount of Oxygen & Hydrogen by weight. The actual CO2 produced would be considerably less. I don’t have a cite for how much is in it, but I would WAG that it’s somewhere on the order of half stuff that would not produce CO2 on burning.
One of the problems with landfills is that they produce methane. Since methane is far worse as a greenhouse gas than CO2, even a small amount of methane can easily counteract the carbon sequestered.
Ironically, methane (CH4) has the same amount of carbon as carbon dioxide (CO2) and as carbonate rocks like limestone (CaCO3) which, I think, should serve to remind us that the “enemy” is not carbon per se. It’s greenhouse gas and the effects that those may potentially have on the environment.
As noted, yes, landfills do in some sense sequester carbon, but,
- This is partially, or maybe even completely, offset by the methane emitted
- Coal/oil/natural gas is sequestered carbon, so it’s not like burning trash instead of a fossil fuel is worse from a greenhouse perspective.
- It’s not a huge amount compared to fossil fuel use anyway.
It’s also worth noting that, by and large, landfills sequester carbon that would have stayed out of the atmosphere anyway - paper, um, other physical stuff I can’t recall. And then they emit methane, as noted above. I think the main argument for landfills concerns land use. Places that have plenty of space should probably keep landfilling. Those with limited space can argue for burning their trash.
I will trumpet the usual warning that creating less garbage in the first place is the best solutin.
Here is a link to the EPA site on the program to capture and reuse Methane from land fill sites instead of letting it be released into the atmosphere.
Holy crap, I can’t believe I missed this thread for two weeks. I work on solid waste air emissions, including greenhouse gas (GHG) emissions. Full disclosure: I generally work with the solid waste industry, and I also consider myself an environmentalist and believe global warming is probably real and a result of human activity.
The two big effects of landfills have already been touched upon: much of the carbon in landfills decomposes into methane, and some carbon never decomposes. It’s also worth noting that the carbon also decomposes into carbon dioxide, but that’s generally considered part of the natural carbon cycle and not a net increase in GHG emissions.
Naturally emitted CO2 is basically CO2 that’s derived from part of the normal carbon cycle. Biofuels like ethanol and biodiesel are natural emissions. Anything derived from oil, coal, or natural gas is not natural. Wood derived fuels are a little tricky because that wood could be renewably grown (natural) or clearcut to make way for development (not natural).
Destroying the waste in an incinerator, usually for energy (called waste-to-energy, or WTE), also generates carbon dioxide. Most of that CO2 is considered natural. The plastics result in some CO2 emissions that are derived from oil, but the fraction is generally not large.
As linked by JoelUpchurch, Landfill gas is sometimes burnt for fuel in generators at the landfill. Again, the resulting CO2 emissions are considered natural. Federal regulations require the capture of landfill gas at most large landfills, and most of the sites destroy it in a flare. Again, these are natural emissions.
Here comes the math. This is back of the envelope and involves some unit conversions, so bear with me. I’ll be using the IPCC SAR 100-year global warming potentials, to be consistent with most reporting methods. These calculations overlook the small amounts of methane oxidized in the soil or the methane uncombusted in the flare or generator.
Based on the EPA’s GHG reporting rule, a ton of solid waste generates 0.067 metric tons of methane per wet metric ton of waste, which contributes to global warming equal to 1.28 metric tons of carbon dioxide per short wet ton. Not all of this methane is emitted. As discussed above, landfills capture the gas and either burn it for energy or in a flare. The amount of methane captured is the subject of some debate, so I’ll use a high, medium, and low capture rate for illustration. At 50% capture, the emissions are 0.63 MT of CO2e. At 75%, it’s 0.32, and at 95% it’s 0.064.
The numbers I have in front of me show 0.16 MT of CO2e stored for each short wet ton of waste landfilled. That carbon storage value means that the breakeven collection efficiency 88%, somewhere between the mid range 75% capture and the high 95% capture. In conclusion, landfills are probably net emitters, but the carbon stored mitigates some the methane emissions.
The solid waste industry would love to have the carbon storage credit show up in their inventories, but they’re hardly the only group trying to claim it. The timber industry wants inventories to assume durable wood products never emit their carbon. The anti-solid waste groups don’t want landfills to receive the credit and see it as a perverse incentive to landfill waste.
Nice post. Thanks for sharing your expertise, and giving an actual concrete answer!