> If you're already wasting sufficient amounts of the requisite elements as waste products of some other thing you're already doing then finding a way to bond

> If you're already wasting sufficient amounts of the requisite elements as waste products of some other thing you're already doing then finding a way to bond those elements and then burn the resultant compound for rotary power may be more economically efficient. Your sentence is a bit vague, but "some other thing you're already doing" sounds like petroleum refining, which is precisely what we need to do less for use as a fuel. Also, ammonia created from petroleum isn't being wasted today. It's used for making fertilizer, and the whole process has a high cost in CO2 emissions. When the time comes that we have so much carbon-free energy that we can afford to use the excess to generate ammonia, we ought to use it to displace petroleum based ammonia for fertilizer. There may be a few niches (ocean shipping) that require a liquid fuel like ammonia, but it makes little sense to use it for ground transportation. If a liquid fuel is required, things like sustainable biodiesel make more sense.