Assuming that the field devices have a typical spec, like 12-32Vdc for power, then yes, you can power field transmitter(s) with a set of three 9V batteries (27Vdc) in series. I have used a set like that for powering field transmitters when there were power supply issues.
I used a plug/jack combo (jack on the batteries side, plug on a lead with alligator clips, so that battery pack would not accidentally short when the plug/alligator clips were disconnected. A short across a 9V battery can supply enough current to start a fire.
As a test, I ran a 2-wire loop powered radar unit at 18mA for about 4 -5 days straight before the battery pack went flat.
