Coal

Coal is a solid fuel resource used throughout mid-game industry and medicine. It is one of the earliest reliable sources of power generation, but it is also consumed in refining and crafting recipes, so its value is not limited to burning it in a generator.

The most direct use for Coal is as fuel in the Coal Generator, where 1 kg/s Coal produces 600 W of power and 20 g/s Carbon Dioxide. This makes it a compact and dependable source of electricity before more advanced power systems are established. Because it also releases Carbon Dioxide, Coal power is often paired with gas management or carbon disposal setups.

Coal is also an important crafting ingredient. In the Kiln, it can be refined into Refined Carbon, which is later used in Steel production. The conversion rate is 125 kg Coal to 100 kg Refined Carbon. The Kiln can also combine Coal with Clay to produce Ceramic at a rate of 100 kg Clay + 25 kg Coal = 100 kg Ceramic. These recipes make Coal a support material for both metallurgy and construction.

At the Apothecary, Coal is used in medicine recipes for duplicants with the proper skill. It can be turned into Vitamin Chews, which slightly increase germ resistance for one cycle, at a rate of 1 kg Coal per 1 unit Vitamin Chews. It is also used to make Curative Tablets, which cure Food Poisoning, at 1 kg Coal + 1 kg Water per 1 unit Curative Tablet.

Coal comes from several sources. Hatches, Sage Hatches, and Stone Hatches excrete Coal after consuming various resources, while Smooth Hatches do not produce it. This makes ranching a renewable Coal source if the animals are fed renewable materials. Coal can also be harvested from space on rocket missions to renewable asteroid fields, including Carbon Asteroid Fields and Glimmering Asteroid Fields. The amount obtained depends on the field and mission duration.

A useful property of Coal is that it can be safely stored as a liquid or solid if cooled below its condensation temperature. It is also considered renewable when produced through Hatch ranching or collected from Carbon Asteroids, since both supply chains replenish themselves. In practice, this means Coal remains relevant well past the early game as long as the player maintains a ranch or space harvesting operation.

One unusual interaction is that Coal can be used with a building placed over vents or geysers in a way that exploits its high-temperature phase change. If a Coal building is placed on a vent whose output temperature is above Coal’s phase change point, the Coal can convert into a large block of Refined Carbon during an eruption, which can force the vent to overpressure and stop it from erupting.