How can i get the particular Row Id in foreach loop ...?

It's not really clear what the problem is.
What's the code of your forms (post and comment)?
Maybe you just forgot to add a hidden field with the post_id to your comments form?

Like this (with the form helper):

PHP Code:

echo form_open();
echo form_hidden('post_id', $post_id);
//your other fields
echo form_submit('submit','Submit');
echo form_close();