How Do You Render Out Keyed Footage In Adobe After Effects?

How Do You render out keyed footage in Adobe after effects? I need to render it out so that I can use it for future projects but I also want to just be able to put it into Final Cut Pro to put over footage without having to run the footage through Adobe After Effects. I know that it is possible because I have many packages of keyed out stuff from different websites like keyed out fire, gun shot flashes, explosions, ext. So I know it's possible I just don't know how to do it. Rendering it as it is know just makes it have a black background.