Kinetic Coil
This Coil can power the Heated Furnace with Lasers.
Place Lava Coils
This time you were a bit conservative on using Diamonds to upgrade something, considering just how many you’d need, but then again you were curious about it. It was clear from the start where these would be getting their source of power.

Lasers.

Diamonds are great conductors of energy after all and neither Coal nor Lava would ever generate enough heat to make Diamonds spin. After this one you’re never having a Furnace wall again.