Steel furnaces cause pollution but electric furnaces do not.
Firstly, this is not quite true. Electric furnaces do produce pollution, but only about a quarter of the amount steel furnaces do (0.9 pollution to 3.6, according to their linked wiki pages).
Electric furnaces use up 180kW of electrical energy.
Steel furnaces use 180 kW of fuel energy. However, boilers have an efficiency of only 50%. They generate 390kW of heat energy for every 780 kW of fuel the burn, and in the process produce 6 pollution. Steam engines then convert the heat energy to electrical energy - I believe this is at 100% efficiency, but I'm not sure.
So the output from one boiler can power two electrical furnaces with 30kW to spare, and combined they produce (6 + 0.9 + 0.9) = 7.8 pollution. Two steel furnaces produce only 7.2 pollution, with nothing to spare.
Scale that up so you can use the excess power output from the boiler, and you have:
- 6 boilers at 6 pollution each - 36 pollution
- 13 electric furnaces at 0.9 pollution each - 11.7 pollution
- Which makes 47.7 pollution in total
This compares with 13 steel furnaces at 3.6 pollution each, which makes 46.8 pollution in total.
Therefore: steel furnaces are greener than boiler-powered electrical furnaces.
Of course two other factors can come into play:
- Solar power, which produces no pollution and uses no fuel (set up costs aside)
- Modules. Electric furnaces can store two modules. These can be used to reduce the electricity consumed by the furnace to 20% (I think) of their standard usage. In this case 6 boilers could power 65 electric furnaces, producing 36 + (65 * 0.9)=94.5 pollution. Meanwhile 65 steel furnaces would produce 65 * 3.6 = 234 pollution - nearly two and a half times more.
Edit: As @MSalters points out, efficiency modules also reduce the pollution from the furnaces themselves, so polution is actually reduced to 36 + (65 * 0.9 * 0.2) = 47.7 pollution. That means steel furnaces pollute nearly five times more than this.