5 ms·

Hey, I've always had a question which perhaps you can answer. In essence, we're still burning lots and lots of carbon just to extract energy. Thermodynamics sa

Hey, I've always had a question which perhaps you can answer. In essence, we're still burning lots and lots of carbon just to extract energy. Thermodynamics says to reverse the process and get carbon from CO2 we need to put all that energy back, with a hefty bonus due to inefficiencies. So how can it be feasible to afford enough energy to "unburn" a century's worth of burnt carbon if we can't even get enough energy to avoid burning new carbon in the first place? Like, our energy generation might double in a few decades, but this "unburning" would require so much spare energy that it doesn't seem likely to achieve as soon as the second half of this century.

tombert 5y ago
I could be wrong, but I think what you're referring to would be to convert it back to raw carbon. I believe that "carbon capture" would be literally taking out the raw CO2 and leaving it as-is, presumably holding it underground or something, which I don't think would take nearly that much energy.
- PeterisP 5y ago
  The physics of storing equivalent weights of gas versus solids - the volume and/or pressure containment required - make it quite impractical to store meaningful amounts (on the scale of billions of tons) of it as gas; the fact that CO2 is just 28% carbon and 72% oxygen is comparably a lesser issue but doesn't help as well. Perhaps you can avoid "unburning" it by some other chemical process (which is why I was asking this) but simply pumping the CO2 somewhere does not seem a reasonable option, the only place on Earth that can easily hold that much of CO2 gas is the general atmosphere where it already resides.
  - epistasis 5y ago
    I agree that sequestering co2 gas is a losing game, and that's why I think all the fossil fuel companies' plans for CCS are pure bunk. Climeworks' proposal to pump it into basalt caverns, where it chemically reacts and becomes solid, is one way around that. Carbon Engineering is doing gas to liquids, and claims that they need ~2.25 kWh of electricity to convert atmospheric CO2 to 1 kWh worth of liquid hydrocarbons. We will see. We will need gigatons/year of sequestration in 2050. It's going to require a ton of innovation to get there.
newt_slowly 5y ago
> So how can it be feasible to afford enough energy to "unburn" a century's worth of burnt carbon if we can't even get enough energy to avoid burning new carbon in the first place? It isn't, and it won't be. Carbon capture is a pleasing myth we tell ourselves to avoid the massive and immediate actions that would be necessary to avert catastrophe. We're addicted to fossil fuels, telling ourselves that when we eventually sober up we can undo the damage we've done to ourselves.
- imtringued 5y ago
  Carbon capture will never lead to negative emissions. What it will do is merely capture CO2 at central locations for processes that have no alternative.
  - epistasis 5y ago
    I sincerely hope you are wrong, because if you are not, we will never keep to less than 1.5C of warming. Every single pathway for that requires carbon capture above and beyond what we can do with terrestrial processes. There are several startups trying to come up with ways of sequestering atmospheric carbon in ways that will last for thousands of years. We need something like that to work, and to come up with ways to fund massive deployment in the second half of this century.
    - PeterisP 5y ago
      Well, we probably will not keep to less than 1.5C of warming, e.g. in the IPCC report even the most optimistic (i.e. unrealistic) scenario has a most likely estimate of 1.6C of warming. IMHO when estimating future, starting with the desired outcome (or even considering it much) is harmful, as it causes you to accept unrealistic assumptions and estimates for the key items driving your predictions, just so the predictions will come out as "acceptable" to you. You need to consider the impact factors as they are, and have realistic expectations of the effect and likelihood of interventions (especially considering the specific motivation factors of the narrow groups who can make each of those interventions, how that aligns with their interests), and see to what prognosis they lead you. If we "need to come up with ways" then it means that it's plausible that we won't, and we (in this case, not we-as-the-world but we-as-individuals and we-as-smaller-communities) also need to plan on how to protect our interests in that (quite likely) scenario. An aching desire for something to work does not imply that it's possible in the required timeframe. Especially because there's no single "we"; those that need a solution to climate change and those that could fund it are largely separate groups; you can't look at it as a primarily technical problem when the largest impact factors are political.
abecedarius 5y ago
Gasoline is high-energy-density using a high-power-density portable quickly-starting/stopping engine. These advantages can overcome the minuses in some applications. (Fewer after the recent advances in batteries, etc.)
- PeterisP 5y ago
  My question was more about the fact that we're still burning extreme amounts of carbon purely for energy, not for density of energy or density of power or portability of engines - we're burning coal for electricity, we're burning gas to heat homes. We'd need to pay back all that energy, but in the coming decades we can't even afford the energy cost of the relatively much simpler solution of "simply" not burning bulk carbon for heat.
  - abecedarius 5y ago
    I don't think new coal plants are rational at all. Re gas heating, I sure could've used some just this past winter during the Texas snowpocalypse. My apartment uses electric power for heating/cooking, so electricity was a single point of failure. Resiliency through diversity is a point I missed above.
epistasis 5y ago
I like the other answers you got, but will add one observation. Once we get energy fro, non-carbon emitting sources, it’s possible to put energy into capture without emitting more CO2 at the same time. So whether it takes 0.5 kWh or 10 kWh to capture the amount of CO2 that produced 1kWh for us originally, as long as it came from renewable resources, we can largely do it.
- PeterisP 5y ago
  My question is more about expectations of scale. If currently renewable energy (not just electricity - heating etc matters a lot) is something like 10% or 12% of total energy; and we need to go to something of 300% or even much more (to cover energy spent not on current needs but solely to cover the "unburning" the excess of previous century, "repaying the debt" much quicker than we accumulated it), then that extreme growth of renewable energy generation doesn't seem plausible to achieve in the timeframe you suggest.
  - epistasis 5y ago
    It's been a while since I ran through the napkin math, and I'm on mobile and can't pull up the backing links, unfortunately, but I believe we are just barely on track to replace all energy use with renewables in 15-20 years. After which we should have spare production capacity, assuming there is wind/solar close enough to the carbon sequestration points. The gist is that if you look at the exponential curves of growth of both wind and solar deployments, and assume that we don't back off from those, you get X amount of TWh/year in 2040. Combine that with conversion of heating and transport to electrification, which provides huge efficiency boosts and requires only 1/3 to 1/4 the energy (most fossil fuel energy is just wasted). Then add in all the parts of the developing world which will increase their energy use to EU/China standards, and we are just barely there. However I think we are likely to see big gains in production capacity as the developing world ups their game. They will be consuming more electricity, but also be immensely more productive.
  - jacobr1 5y ago
    Consider that we didn't spend 100 years using electricity at current rates. There was was an s-curve growth that only recently flattened in developed countries.
philipkglass 5y ago
Thermodynamics says to reverse the process and get carbon from CO2 we need to put all that energy back, with a hefty bonus due to inefficiencies. The key insight is that you don't need to turn carbon dioxide back into carbon. You just need to store those carbon atoms it in a stable form that keeps them out of the atmosphere. The most plausible way of doing that at large scale is to accelerate the reaction of naturally occurring silicates from rocks with atmospheric CO2. "From a thermodynamic point of view, inorganic carbonates represent a lower energy state than CO2; hence the carbonation reaction is exothermic..." That's from chapter 7 of the IPCC Special Report on Carbon Dioxide Capture and Storage, which I recommend reading for more details.
- throwaway2037 5y ago
  PkG: This is one of the best references I have seen in ages on HN! Chapter 7 PDF: https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapter7-1.pdf https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapte... Page 321 has the executive summary with your quote. Parent page for full book: https://www.ipcc.ch/report/carbon-dioxide-capture-and-storage/ https://www.ipcc.ch/report/carbon-dioxide-capture-and-storag...
pfdietz 5y ago
Removing CO2 from the atmosphere doesn't mean unburning it. The CO2 either gets dissolved into the deep ocean, stored underground in deep aquifers, or reacted with silicates to make carbonates.

tombert 5y ago

I could be wrong, but I think what you're referring to would be to convert it back to raw carbon. I believe that "carbon capture" would be literally taking out the raw CO2 and leaving it as-is, presumably holding it underground or something, which I don't think would take nearly that much energy.

PeterisP 5y ago
The physics of storing equivalent weights of gas versus solids - the volume and/or pressure containment required - make it quite impractical to store meaningful amounts (on the scale of billions of tons) of it as gas; the fact that CO2 is just 28% carbon and 72% oxygen is comparably a lesser issue but doesn't help as well. Perhaps you can avoid "unburning" it by some other chemical process (which is why I was asking this) but simply pumping the CO2 somewhere does not seem a reasonable option, the only place on Earth that can easily hold that much of CO2 gas is the general atmosphere where it already resides.
- epistasis 5y ago
  I agree that sequestering co2 gas is a losing game, and that's why I think all the fossil fuel companies' plans for CCS are pure bunk. Climeworks' proposal to pump it into basalt caverns, where it chemically reacts and becomes solid, is one way around that. Carbon Engineering is doing gas to liquids, and claims that they need ~2.25 kWh of electricity to convert atmospheric CO2 to 1 kWh worth of liquid hydrocarbons. We will see. We will need gigatons/year of sequestration in 2050. It's going to require a ton of innovation to get there.

newt_slowly 5y ago

> So how can it be feasible to afford enough energy to "unburn" a century's worth of burnt carbon if we can't even get enough energy to avoid burning new carbon in the first place? It isn't, and it won't be. Carbon capture is a pleasing myth we tell ourselves to avoid the massive and immediate actions that would be necessary to avert catastrophe. We're addicted to fossil fuels, telling ourselves that when we eventually sober up we can undo the damage we've done to ourselves.

imtringued 5y ago
Carbon capture will never lead to negative emissions. What it will do is merely capture CO2 at central locations for processes that have no alternative.
- epistasis 5y ago
  I sincerely hope you are wrong, because if you are not, we will never keep to less than 1.5C of warming. Every single pathway for that requires carbon capture above and beyond what we can do with terrestrial processes. There are several startups trying to come up with ways of sequestering atmospheric carbon in ways that will last for thousands of years. We need something like that to work, and to come up with ways to fund massive deployment in the second half of this century.
  - PeterisP 5y ago
    Well, we probably will not keep to less than 1.5C of warming, e.g. in the IPCC report even the most optimistic (i.e. unrealistic) scenario has a most likely estimate of 1.6C of warming. IMHO when estimating future, starting with the desired outcome (or even considering it much) is harmful, as it causes you to accept unrealistic assumptions and estimates for the key items driving your predictions, just so the predictions will come out as "acceptable" to you. You need to consider the impact factors as they are, and have realistic expectations of the effect and likelihood of interventions (especially considering the specific motivation factors of the narrow groups who can make each of those interventions, how that aligns with their interests), and see to what prognosis they lead you. If we "need to come up with ways" then it means that it's plausible that we won't, and we (in this case, not we-as-the-world but we-as-individuals and we-as-smaller-communities) also need to plan on how to protect our interests in that (quite likely) scenario. An aching desire for something to work does not imply that it's possible in the required timeframe. Especially because there's no single "we"; those that need a solution to climate change and those that could fund it are largely separate groups; you can't look at it as a primarily technical problem when the largest impact factors are political.

abecedarius 5y ago

Gasoline is high-energy-density using a high-power-density portable quickly-starting/stopping engine. These advantages can overcome the minuses in some applications. (Fewer after the recent advances in batteries, etc.)

PeterisP 5y ago
My question was more about the fact that we're still burning extreme amounts of carbon purely for energy, not for density of energy or density of power or portability of engines - we're burning coal for electricity, we're burning gas to heat homes. We'd need to pay back all that energy, but in the coming decades we can't even afford the energy cost of the relatively much simpler solution of "simply" not burning bulk carbon for heat.
- abecedarius 5y ago
  I don't think new coal plants are rational at all. Re gas heating, I sure could've used some just this past winter during the Texas snowpocalypse. My apartment uses electric power for heating/cooking, so electricity was a single point of failure. Resiliency through diversity is a point I missed above.

epistasis 5y ago

I like the other answers you got, but will add one observation. Once we get energy fro, non-carbon emitting sources, it’s possible to put energy into capture without emitting more CO2 at the same time. So whether it takes 0.5 kWh or 10 kWh to capture the amount of CO2 that produced 1kWh for us originally, as long as it came from renewable resources, we can largely do it.

PeterisP 5y ago
My question is more about expectations of scale. If currently renewable energy (not just electricity - heating etc matters a lot) is something like 10% or 12% of total energy; and we need to go to something of 300% or even much more (to cover energy spent not on current needs but solely to cover the "unburning" the excess of previous century, "repaying the debt" much quicker than we accumulated it), then that extreme growth of renewable energy generation doesn't seem plausible to achieve in the timeframe you suggest.
- epistasis 5y ago
  It's been a while since I ran through the napkin math, and I'm on mobile and can't pull up the backing links, unfortunately, but I believe we are just barely on track to replace all energy use with renewables in 15-20 years. After which we should have spare production capacity, assuming there is wind/solar close enough to the carbon sequestration points. The gist is that if you look at the exponential curves of growth of both wind and solar deployments, and assume that we don't back off from those, you get X amount of TWh/year in 2040. Combine that with conversion of heating and transport to electrification, which provides huge efficiency boosts and requires only 1/3 to 1/4 the energy (most fossil fuel energy is just wasted). Then add in all the parts of the developing world which will increase their energy use to EU/China standards, and we are just barely there. However I think we are likely to see big gains in production capacity as the developing world ups their game. They will be consuming more electricity, but also be immensely more productive.
- jacobr1 5y ago
  Consider that we didn't spend 100 years using electricity at current rates. There was was an s-curve growth that only recently flattened in developed countries.

philipkglass 5y ago

Thermodynamics says to reverse the process and get carbon from CO2 we need to put all that energy back, with a hefty bonus due to inefficiencies. The key insight is that you don't need to turn carbon dioxide back into carbon. You just need to store those carbon atoms it in a stable form that keeps them out of the atmosphere. The most plausible way of doing that at large scale is to accelerate the reaction of naturally occurring silicates from rocks with atmospheric CO2. "From a thermodynamic point of view, inorganic carbonates represent a lower energy state than CO2; hence the carbonation reaction is exothermic..." That's from chapter 7 of the IPCC Special Report on Carbon Dioxide Capture and Storage, which I recommend reading for more details.

throwaway2037 5y ago
PkG: This is one of the best references I have seen in ages on HN! Chapter 7 PDF: https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapter7-1.pdf https://www.ipcc.ch/site/assets/uploads/2018/03/srccs_chapte... Page 321 has the executive summary with your quote. Parent page for full book: https://www.ipcc.ch/report/carbon-dioxide-capture-and-storage/ https://www.ipcc.ch/report/carbon-dioxide-capture-and-storag...

pfdietz 5y ago

Removing CO2 from the atmosphere doesn't mean unburning it. The CO2 either gets dissolved into the deep ocean, stored underground in deep aquifers, or reacted with silicates to make carbonates.