Celery configuration for Flask application
It's because the parameter is BROKER_URL, not CELERY_BROKER_URL. Here's a complete list of possible settings: http://docs.celeryproject.org/en/latest/userguide/configuration.html#new-lowercase-settings
Note: that table is a conversion table for the new lower case settings in Celery 4.0, but as stated there: "Celery will still be able to read old configuration files, so there’s no rush in moving to the new settings format"