problems with date and interval queries.

I am having problems adding an interval to a date in a select statement
where the interval is specified as a selected column. Is this possible?
Below is an example of the query that I'm executing. The query gives and
error at the num_min column in the outer query. It works fine if that is
replaced with the text '90 minutes'. Thank you in advance for all help
provided.

-John

select message_id,
to_char((now() + ((interval num_min) *
(2 ^ (num_fail - (num_fail - fail_count))))),'mon DD YYYY HH24:MI:SS') as
abs_send_date,
fork_count
from (
select message_id, number_reattempt_failures as num_fail, resend_count as
fail_count,
(to_char(coalesce(minutes_between_reattempt, 90), '999') || ' minutes')
as num_min, last_completion_date as last_date, fork_count
from pa_mailblast_message
where send_state = 'sent' and
number_reattempt_failures > resend_count) a