I'm not a chemist, but I'm interested in it. What I am trying to understand is why its so hard to get energy from fossil fuels without being able to sequester the carbon released in some kind of liquid form. I understand how piston engines use the expansion of gases (through combustion) to work, but I recent learned about these Stirling engines: https://en.wikipedia.org/wiki/Stirling_engine This engine, as I understand it, simply uses a difference in heat between two sources two work. This means that any exothermic reaction, even if it produces no gases, could drive it. What I am wondering is if the exothermic reactions from fossil fuels could output only liquid waste (containing the carbon), or must always produce gaseous CO2... And if the latter is true, why? Thanks for any responses.