90% of ash from coal production is fly ash, the rest is bottom ash. 1% of fly ash makes it into the atmosphere after scrubbers and bag houses.
https://energyeducation.ca/encyclopedia/Fly_ash
Bottom ash microplastic ratios for plastic incinerators are 360 / metric ton at the low end to 102000 / metric ton of waste at the high end.
https://www.sciencedirect.com/science/article/abs/pii/S0304389420314187
So if we pretend (false) that the particle size distribution from coal and from plastic incineration is the same (it's not) and we pretend that scrubbers and baghouses would work the same (they won't) then your range of microplastic particles released is between 3240 and 918000 per metric ton of waste from the plastic incineration process. So then you have to correct for how much waste is actually produced in plastic incineration. I can't find science on this, I can only find this obviously biased source:
https://grist.org/living/whats-worse-burning-plastic-or-sending-it-to-a-landfill/
...which states that somewhere between 15% to 30% of plastics turn to ash when incinerated, which is far less efficient than coal.
So using all of those bad coal assumptions above about fly ash distribution sizing and interception, and using the biased source on plastics incineration ash waste ratios, the best case for burning 1 metric ton of plastic is 486 microplastic particles released into the atmosphere, and the worst case is 275,400 microplastic particles released into the atmosphere.
That's a wide range, from "no big deal" up to "holy shit," and I can find no study on fly ash microplastic release ratios anywhere. Does anyone know of any hard research on this topic? The only/best way to answer this question would be to sample plastic incineration plant effluent.
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Notes -
"somewhere between 15% to 30% of plastics turn to ash when incinerated, which is far less efficient than coal."
My gut check says that this makes sense. We started using coal precisely because it burned so well. It does seem likely that plastic wouldn't burn nearly as thoroughly.
Looks like that 15% for bottom ash is what's expected, though from some reading it heavily depends on the composition of the waste that's being burned.
In a modern incineration plant (https://www.icevirtuallibrary.com/doi/full/10.1680/jcien.17.00042) it looks like they expect 150 kg/t or 15% (table 4) for bottom ash. The amount of fly ash is much less, did OP transpose the composition of fly and bottom ash? They reference 15–20 kg/t in the report. So I think OP of by ~ 100x on the fly ash particle count. Then did he include the 99% efficient scrubbers?
e.g. 1 ton of waste -> 20 kg of fly ash. 20 kg x 565 (n/kg) -> 11,300 high estimate of plastic particles in the fly ash. (1 - 0.99) x 11,300 -> 113 high estimate particles per tone waste after scrubbing? (decimal points, commas for 1000's separators)
Table 2 might also be of interest, contains expected emitted "Total organic carbon" and "dust." Though in mg/m^3.
Also, a modern multi-stage scrubber with electrostatic, wet, and filtration like in Amager Bakke should be much higher efficiency than 99% Also, I think @slothlikesamwise is correct you should be able to see appreciable plastic micro particle emissions counted as PM or dust in the flue gas monitoring, as well as environmental sensors around the plant.
Edit: I see OP did calculations in terms of waste tonnage. I converted my calculation to be the 565 n/kg from OP link 2.
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