Please tell me if I\'m understanding correctly. (because I might not be.)
UPDATE
well you are doing something wrong you do not need to renew access token even if its expired all you need is sending user facebook id & content you want to post together to your server by ajax then post it without access token Just check up here
if you have publish_stream permission you dont need access token this is documentation for publish_stream https://developers.facebook.com/docs/reference/api/permissions/
Enables your app to post content, comments, and likes to a user's stream and to the streams of the user's friends. With this permission, you can publish content to a user's feed at any time, without requiring offline_access. However, please note that Facebook recommends a user-initiated sharing model.