Can interval take a value from a field?

Sometimes I think the hurdle to learning more advanced sql is figuring
out how to phrase the search question.  Well, I'm not having any luck
there so here I come again to the list, hat in hand :-)

I have a table with an id key and an expiration value in years

   trgexpd_trg_id | expiration_value
----------------+------------------
             240 | 1 year
             749 | 3 years
            4917 | 2 years

and so on.

I'm trying to use the expiration value as the value for an interval in a
query that looks like:

select
         ts_date as "Transcript Date",
         ts_expiration_date as "Current Expiration Date",
         expiration_value as "Expiration Interval"
         from transcript, training_expiration_value where
         ts_training_id = trgexpd_trg_id and
         ts_training_id in (select cda_training_number from cdas) and
         ts_expiration_date != ts_date + interval 'expiration_value';

and I'm getting the following error:

ERROR:  invalid input syntax for type interval: "expiration_value"

Is there a way to use the value in expiration_value for the interval?

Thanks,

Jeff Ross