In the first part of the sentence, the word 煙 is read as けむり and is a noun. The reason there is no Hiragana at the end is that you can choose to omit it - for a Japanese reader it would be self-evident so it is not necessary. In the second part, the adjective 煙い (けむい) is transformed into a noun by replacing the Hiragana い with the Hiragana さ.
It both means smoke, but for stilistic reasons (probably to avoid repetition), two different approaches were chosen to express the same thing. Or a different explanation could be that 煙を吐く(けむりをはく) is a phrase that means "to puff smoke" and it is predetermined that it be used like that. But in the later part, "smoke" is used on its own, so the author could decide more freely how to express theirself.
I hope that I could be of help. Have fun studying Japanese!