Can I use 15V 2A transformer with this circuit?
A transformer that rated to supply more current is not usually going to be a problem. The one recommended is 1A and you have a 2A transformer - 2A means it is capable of supplying 2A if the load demands 2A.

As an extra bit of advice, are you aware that the charge current is probably limited to a few tens of mA. Also, the circuit diagram and parts list in the link do not totally match - this always makes me nervous and I would sueest some form of simulation first.