jQuery - how can I find the element with a certain id?

This

var verificaHorario = $("#tbIntervalos").find("#" + horaInicial);

will find you the td that needs to be blocked.

Actually this will also do:

var verificaHorario = $("#" + horaInicial);

Testing for the size() of the wrapped set will answer your question regarding the existence of the id.

If you're trying to find an element by id, you don't need to search the table only - it should be unique on the page, and so you should be able to use:

var verificaHorario = $('#' + horaInicial);

If you need to search only in the table for whatever reason, you can use:

var verificaHorario = $("#tbIntervalos").find("td#" + horaInicial)

I don't know if this solves your problem but instead of:

$("#tbIntervalos").find("td").attr("id", horaInicial);

you can just do:

$("#tbIntervalos td#" + horaInicial);