Kiln cold.

Lava hot.

Put kiln on lava, now kiln hot!

But seriously, does it make sense to extract heat from lava rather than burning the other products we have around? We have perfectly good heat at home.

When I was first designing the planet, I'd wanted heat to be like a resource you extract from the environment. And in fact I was going to prevent heating towers from being allowed on the surface. But when the production chains were coming together, I realized the planet needed a way to get rid of extra items and fluids from the mixed recipes, so I added lava to the surface. And in playtesting, to keep production from getting backed up on heavy oil, I was turning heavy oil into solid fuel and belting it past my heat-generators to throw it away in the lava. Then it hit me that I was overcomplicating things.

When I cut the special heating entity and embraced heating towers, the mod finally started to click. There was enough new complexity on the planet that it actually needed some elements that were familiar. (The planet has coal in part because I just couldn't bring myself to add yet another alt recipe for plastic). And I ended up keeping the lava on the surface, partly for utility (I like to void sulfur when I'm backed up on water), but mostly because the landscape was too boring without it.

Part of me does still really like the idea of getting heat from the lava or a resource or something. But I think I'd only add it to Crucible if another mod added the mechanic as a standalone feature and it could be made compatible easily.

Totally fair. I'd been staring at the heating towers for a while (and liquid heating towers, which I also have) trying to sus out the best way to create heat at this point when I walked over to a lava pool and the question hit me. I'm honestly surprised that, given the like 10 planet mods I have, no one has added a recipe for lava to heat and it doesn't work in the liquid heat towers. It may trivialize (or overcomplicate, depending on what the player does) some other areas, but it'd just make sense to have as an option. I do agree that the lava adds to the surface properties of the planet - both visually and as a build complication, which I think is important to have.

I ultimately landed on heavy -> light -> petroleum -> liquid heating tower but IDK if that'll be a permanent solution. I have no idea how you're getting backed up on water - I seem to be having the opposite problem, but I'm sure I'll get there.

As an aside, I suspect there may be an issue with the coal generation getting crowded out by skarn. This is the only coal in all of the area I've explored.

And as a [i]complete[/i] aside, what is the purpose of the Crucible Approach? It definitely gave me an ominous feeling hitting that and going on to the planet, but I don't think that feeling really paid off as the planet itself is tranquil, space is relatively hazard-free, etc.

I'm almost never backed up on water, especially when producing hydrogen. But occasionally I must just be making tons of iron and hydraulic fluid or something. Yeah I'm also surprised that there's not been a lava->heat mechanic on a planet yet (AFAIK). Ooh, I just had an idea: a lava/ice planet; it freezes like Aquilo, heat is produced in lava-consuming towers (with a large range), and heat pipes aren't allowed. Although I guess Cerys is like 75% of that. Anyway...

Coal is supposed to be pretty rare, so you're probably just getting unlucky. You don't need that much.

Have you tried to return to Crucible approach from Crucible yet? It exists because Crucible's gravity pull on platforms is very high, and people's platforms that were just transiting past ended up getting stuck. The approach lets Crucible be a dead-end, so that mechanic can still exist without messing with anything else. And it's a bit of a hint to double check thay you're ready.