When arriving into Barcelona airport at terminal 2,
you need to go outside, where you will see a bridge, where the buses are. Board a green shuttle bus, to Terminal 1.
That will drop you off at Arrivals.
Go down to the bus stops 7 - 12. This is where the Hife bus will be.
Fare from B'clona airport to Tortosa is 21.95 each way.
If doing regular trips, there are special deals.