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Thermodynamically combustion of pyrolysis products does not, cannot, use more energy than it produces. If it did, plastic would not burn. Plastic burns, hence p

Thermodynamically combustion of pyrolysis products does not, cannot, use more energy than it produces. If it did, plastic would not burn. Plastic burns, hence pyrolysis is a net energy source. [1] That's not to say it makes sense. Myself, I'm in favor of a high temperature burn of plastic + scrubbers + insane taxes on halogenated polymers + insane deposits on fluorinated polymers -> to keep the flu stream clean. [1] pyrolysis itself is endothermic, clearly, since polymerization is extothermic. But the article you linked explicitly says burning pyrolysis by products which must, net, be exothernic An other way of looking at it is pyrolysis is a necessary first step for plastic combustion hence the cycle is net exo

samatman 2y ago
You don't really need the insane taxes part, just an insistence that the plants are not allowed to emit halogens in the flue gas. The halogens are completely recoverable by bubbling them through an alkali mixture, doesn't matter what earth metals are used (some combination of sodium and calcium is cheapest), this will result in salts. You want to be a bit more careful about some metals, so that you don't have to precipitate them out of the salts to render them nontoxic. There are ways to preferentially bind the metals even in the gas phase, but it's best not to burn them to begin with, which isn't that hard because metals aren't found in high quantities in plastics to begin with, especially not the bad ones like cadmium and lead. Other than that, plastics are economically useful, but burying them is just delaying their release into the environment, which is not great. So burn them, and stop pretending they're useful for anything but fuel at the end of their service life. Pretty simple.
- fuzzfactor 2y ago
  >bubbling them through an alkali mixture Which turns the liberated halogens back to salt automatically, in a very favorable reaction which is so strongly favored it is almost irreversible. Almost. But not if you carefully apply energy to the brine using an electrochemical process, especially one which has been improved for over a century after the patent expired, and you can't get much more efficient than that. Especially at the scale these operate at. These are not petrochemical plants since they were doing this before petrochemicals actually had plants. Crude oil wasn't even that popular yet. But who's going to need to do that, the spent alkali after scrubbing the halogens has no need for additional energy to be added. Unless you wanted to turn it back into full-strength alkali. Which wouldn't be cost-efective anyway, and nobody should be paying more than it's worth. Because there's planty of fresh material coming out of the caustic plants already. But you know how much energy it takes to produce that alkali originally from the virgin brine feedstock those plants use? More than it's worth. It a plain electrochemical reaction. No other raw materials needed. It's that "irreversable" reaction that's just a little too strong. So it takes a lot of energy to reverse. Kind of like trying to turn CO2 back into hydrocarbons. And if you want to try and capture CO2 from the atmosphere using alkali, fuggedaboutit. You're going to need a bigger caustic plant, and a lot more of them. There's a thing called material balance too, not just electrochemistry and thermodynamics at work.
  - fuzzfactor 2y ago
    Sorry to hurt anybody's feelings who absolutely hates electrochemistry. Or industrial-scale chemical operations. I don't blame you, this stuff can be confusing.
fuzzfactor 2y ago
>hence the cycle is net exo You mean to say net Exxon? Thermodynamics is a bitch. Combustion of the pyrolysis oil, which is the oil the plastic is turned into by the pyrolysis process, is not much different than burning any other oil to release the stored energy. The problem is that making the pyoil from the plastic as a "recycling" method takes more energy than it is worth. That's whether money is involved or not, even though the oil itself is worth way more money than the raw waste plastic (whch is about zero to negative as solid waste). Burning pyoil on its own is a gross positive energy source, but when you figure all the energy wasted actually pyrolyzing the plastic to begin with before it turns into oil, you'd be better off burning the plastic directly. Since the pyrolysis process on its own is very gross negative, consumes energy not releases it. It's just thermodynamics, you can't fool Mother Nature. And I'm talking really gross, more toxic than crude oil too and you really don't want to smell it. A spill has a lingering odor far worse than most crude oils, even though some crudes can be deadly at first if it has high H2S. Since H2S is a gas and dissipates relatively quickly once spilled. However the final pyoil product comes in the same kind of tank barges and tankcars as crude oil, and some traders can now handle it like this which is not possible for solid waste. Turns out combustion and pyrolysis are two different animals. Who knew? Too bad Google may not help with things like this any more. >pyrolysis itself is endothermic, clearly, since polymerization is extothermic. There are also classes in logic for anyone who is seriously interested. Also, most places already have plenty of taxes that are very insane already, you want to tax people's mental health even more? ;) The more you understand about thermodynmics, the more of a bitch you realize it is.

samatman 2y ago

You don't really need the insane taxes part, just an insistence that the plants are not allowed to emit halogens in the flue gas. The halogens are completely recoverable by bubbling them through an alkali mixture, doesn't matter what earth metals are used (some combination of sodium and calcium is cheapest), this will result in salts. You want to be a bit more careful about some metals, so that you don't have to precipitate them out of the salts to render them nontoxic. There are ways to preferentially bind the metals even in the gas phase, but it's best not to burn them to begin with, which isn't that hard because metals aren't found in high quantities in plastics to begin with, especially not the bad ones like cadmium and lead. Other than that, plastics are economically useful, but burying them is just delaying their release into the environment, which is not great. So burn them, and stop pretending they're useful for anything but fuel at the end of their service life. Pretty simple.

fuzzfactor 2y ago
>bubbling them through an alkali mixture Which turns the liberated halogens back to salt automatically, in a very favorable reaction which is so strongly favored it is almost irreversible. Almost. But not if you carefully apply energy to the brine using an electrochemical process, especially one which has been improved for over a century after the patent expired, and you can't get much more efficient than that. Especially at the scale these operate at. These are not petrochemical plants since they were doing this before petrochemicals actually had plants. Crude oil wasn't even that popular yet. But who's going to need to do that, the spent alkali after scrubbing the halogens has no need for additional energy to be added. Unless you wanted to turn it back into full-strength alkali. Which wouldn't be cost-efective anyway, and nobody should be paying more than it's worth. Because there's planty of fresh material coming out of the caustic plants already. But you know how much energy it takes to produce that alkali originally from the virgin brine feedstock those plants use? More than it's worth. It a plain electrochemical reaction. No other raw materials needed. It's that "irreversable" reaction that's just a little too strong. So it takes a lot of energy to reverse. Kind of like trying to turn CO2 back into hydrocarbons. And if you want to try and capture CO2 from the atmosphere using alkali, fuggedaboutit. You're going to need a bigger caustic plant, and a lot more of them. There's a thing called material balance too, not just electrochemistry and thermodynamics at work.
- fuzzfactor 2y ago
  Sorry to hurt anybody's feelings who absolutely hates electrochemistry. Or industrial-scale chemical operations. I don't blame you, this stuff can be confusing.

fuzzfactor 2y ago

>hence the cycle is net exo You mean to say net Exxon? Thermodynamics is a bitch. Combustion of the pyrolysis oil, which is the oil the plastic is turned into by the pyrolysis process, is not much different than burning any other oil to release the stored energy. The problem is that making the pyoil from the plastic as a "recycling" method takes more energy than it is worth. That's whether money is involved or not, even though the oil itself is worth way more money than the raw waste plastic (whch is about zero to negative as solid waste). Burning pyoil on its own is a gross positive energy source, but when you figure all the energy wasted actually pyrolyzing the plastic to begin with before it turns into oil, you'd be better off burning the plastic directly. Since the pyrolysis process on its own is very gross negative, consumes energy not releases it. It's just thermodynamics, you can't fool Mother Nature. And I'm talking really gross, more toxic than crude oil too and you really don't want to smell it. A spill has a lingering odor far worse than most crude oils, even though some crudes can be deadly at first if it has high H2S. Since H2S is a gas and dissipates relatively quickly once spilled. However the final pyoil product comes in the same kind of tank barges and tankcars as crude oil, and some traders can now handle it like this which is not possible for solid waste. Turns out combustion and pyrolysis are two different animals. Who knew? Too bad Google may not help with things like this any more. >pyrolysis itself is endothermic, clearly, since polymerization is extothermic. There are also classes in logic for anyone who is seriously interested. Also, most places already have plenty of taxes that are very insane already, you want to tax people's mental health even more? ;) The more you understand about thermodynmics, the more of a bitch you realize it is.