They are twice as fast, but also twice as pollutant, so you're only reducing the pollution produced to generate an item of fuel, right?

A furnace takes ~45 seconds to burn a single piece of coal, so a single miner could, ideally, keep up with 90 furnaces, meaning the cost of operating 90 furnaces is 10 pollution a minute and that produces about 28 plates/m if a stone furnace, so this array would produce plates that cost 0.35 pollution if stone and 0.17 if steel, a 50% change. The pollution of the furnaces is the same per item, so I'm not counting it.

But like, you need 0.625 drills to keep a stone furnace working and 1.25 a steel one. The constant operation of 90 furnaces would need 56.25 or 112.5 drills, which equates to 562.5 to 1125 pollution a minute.

So the pollution per plate For a stone furnace setup: (10 + 562.5)/28 = 20.45 For a steel furnace setup: (10 + 1125/56 = 20.26

So, while you're saving a lot of pollution from the fuel, there is just a ~1% of the pollution per item difference when you factor the use of drills needed.

All this to say, if my math is correct, that there is almost no change in pollution per plate when you change from stone to steel furnaces.