Will this work on the ZD-915?

That I don't know for sure. I have the ZD985 base unit.

What is the heater wattage

The 88-552A is a hand-piece that connects to a base unit [ZD-985]. The wattage is set at the base unit and can go to 850 degrees F.

I recently broke my zd-915 desoldering gun and since then i've been looking for a replacement. will this model work on zd-915?

I bought mine for a ZD-985 but just looking around the two units seem very similar and the 88-552A replacement gun might work. I would call Memotronics to see if they can help. The company is moving into a larger facility now so their web page is down for now. They posted that they will try to assist in the interim so a call may be worth trying. The two units do look very similar though. It might be worth a try. The Memotronics web site is https://www.memotronics.com/.

Dies this have to be used with the soldering station? Can't it plug into 110vac and operate alone? Thanks

The 88-552A Desoldering gun has a proprietary connector that only works with the soldering station. I does not plug directly into the 120 VAC wall outlet.