Cool idea for a game! I enjoyed the simple loop and adding complexity with extra bombs. I was a bit confused by the timings of the bombs for a while and feel it didn't explain that bombs of the same time could explode next to each other. Also there were a few spots on the grid, particularly at the top, where I couldn't place any bombs.
I wonder if setting the bomb timers to longer and having them tick down as soon as they spawn onto the grid would be fun. Then it'd be a bit more frantic and the countdown would be a more integral part of the game.
Once again though, cool game!