For each relevant comment, jot down the comment link, author name and email address (look at their HN profile or links therein) and write a personal response, then and there, referencing their comment (give the link), and why they may be interested in your app, be specific, and also point out where your app differs from what they were talking about (it probably will). Use "Re: Your HN comment" as the subject.
When you launch, you will have a whole lot of personal, relevant emails to send to people who have already expressed interest indirectly. Don't send them all at once. Send them one by one, day by day. You may learn something as you go that you will then be able to incorporate into subsequent emails.