I think the sentence:
- La ciudad se renueva constantemente.
can be parsed either as a pure reflexive or as a "se"-passive sentence, and this is because the noun phrase "la ciudad" can be understood as a human structure capable of evolving by itself (in which case the reflexive interpretation is more feasible), or as a product of human construction (in which case the passive interpretation will prevail).
Another similarly ambiguous example is:
- La esperanza se renueva todos los días.
If we think of hope as a human virtue that has the power of renewing itself, the meaning is reflexive: hope renews itself every day; if we think of it as a human creation, then the meaning is passive: hope is renewed every day.
With other subjects and the same verb, we can tell the difference more easily, for example:
Los planes de desarrollo urbano se renuevan constantemente. (Urban development plans are constantly renewed: it is clear that plans cannot renew themselves.)
Los ciudadanos se renuevan constantemente. (Citizens are constantly renewed / Citizens are subject to permanent renewal: though animate, here "ciudadanos" refers to changes in population, due to emigration and immigration, so the meaning is also passive.)
Algunas personas buscan renovarse a través de cirugías. (Some people seek their own renewal through surgery: here reference is made to the intention of the subject to renew themselves, so the meaning is reflexive.)
