My species have dark green blood, which comes from biliverdin. They have developed a resistance against it, but that only makes it green. The atmosphere is only 7% oxygen, which is why it's dark. I want to have fire in this world, though. Could they have a specific way of getting oxygen? Air is combustible in our atmosphere, could their air be more combustible, to the point where fuel could be replaced by air? Would they develop ways to resource oxygen like we did with fuel (wood, paper, etc) ?
Hi. I’m honestly very confused by your question, but I’ll do my best.
Fire needs an atmosphere of at least 16% oxygen to burn (ours is just under 21%). So in order to have fire, yes. You would need to figure out ways of producing more oxygen. Plants, of course, produce oxygen naturally, but if those were readily available in the supply you’d need to create that O2 for a fire, then it wouldn’t follow that the atmosphere would be so depleted of oxygen. Our oxygen comes from plants through photosynthesis (both from land plants and phytoplankton). But photosynthesis…and thus oxygen…comes from the sun, which you said it’s dark in this world, so I don’t know how you plan on having plants survive anyway.
Our air is not combustible. It supports combustion. Something is combustible when it can catch fire and burn easily. Oxygen (the only flammable element in the air) is a component of the chemical reaction that produces fire. It is the reaction of a combustible liquid/item with oxygen that creates the fire aka the combustion. And creating an atmosphere which is combustible (something that oxidizes rapidly, releasing heat), could be really difficult to pull of without making it overly complicated or too inconsistent to maintain suspension of disbelief (how can a fire light without expanding through the atmosphere and burning everything up?).
Here’s some “special situations” in which nitrogen (our air’s primary element) acts differently than normal.
The last question I specifically don’t understand exactly what you mean…but it also seems like a question only you could answer. Would they develop other means of achieving goals we did? Probably. But I can’t answer that for you.
But aside from all of that…these answers are based in the science of our world. If you’re creating a new world with new laws and a totally new make up of elements, then you could likely find a way to achieve anything you want. A different atmosphere might have less oxygen, but what does it have more of? Nitrogen? Carbon dioxide? Something else? You control your world, so you have a lot of freedom, even if you have to invent fictional elements in order to pull of what you want.