I've tried loading kra in photoshop and irfanview but none of them work

No... Adobe wouldn't want to support Krita, of course, and IrfanView's developers have some weird ideas about how .kra works. But there are two different ways of achieving your goal:

* use Krita on the commandline to convert the image with the convert option
* unzip the .kra file to get at the mergedimage.png file and use that.