sessions usage in codeigniter and wordpress

I’m working on a project in which my codeigniter is placed inside the wordpress main folder,following is structure of my project.

    //codeigniter_folder
    //wp-admin
    //wp-content
    //wp-includes
    //other wordpress files

codeigniter_folder have codeigniter files in it.
As I’m using session in codeigniter and I want to access that session in wordpress.But when I echo session_id() in codeigniter it gives me different value and when I echo session_id() in wordpress it gives me different session id.I’m not understanding why different sessions are being used in one main directory(project directory),how can I use codeigniter’s session inside the wordpress(only front end,not for wp-admin).