"I am working right now."
正在 isn't actually a "time". It's an adverb modifying the action (工作). Putting 在 before the verb makes it a conyinuous action, like "-ing" in English. 正在 is even stronger. I believe 正在 can only indicate presently ongoing actions (hence it indicates something happening right now.
Shouldn't I be able to add 现在 to imply the "right now" part? From what I understand, 正在 and 在 are synonymous for meaning "(subject) + am + (verb) + ing". But they don't emphasize the "right now" part.
A previous comment made the point that 正在 is actually more about "right now" than 现在 as it means that you are actually in the middle of doing something whereas 现在 can just mean you are about to do something now.