Is it possible for a competitive firm to earn supernormal profit in the long run and why?

No.

There are two basic reasons.

The first is market correction: if Alpha Bolt Company is making 30 percent profit and Bravo Bolt, Charlie Bolt and Delta Bolt are all making 20 percent because all their prices are lower than Alpha's, people will start to buy fewer Alpha Bolts, which will lead to Alpha's profits falling in line with the rest of the industry.

The other is, if Alpha Bolt is making 30 percent profit at the same price point as its competitors, and they got there through increased efficiency, at some point this 30 percent profit margin will be the "new normal."