1) Ensure you're using MYSQL 5.5 only then will you be able to change the 
collation to utf8mb4_something

2) Ensure table columns that are going to receive emoji have their collation 
set to utf8mb4_something

3) Edit your database.php config file

  $db['default']['char_set'] = 'utf8mb4';
  $db['default']['dbcollat'] = 'utf8mb4_unicode_ci';