Air Carrier Captain reported a fume event upon arrival at the aircraft. The APU was operating in spite of two active APU MELs. After troubleshooting and conferring with Maintenance; Dispatch and Chief Pilot an aircraft swap was requested.

Narrative

After deadheading into ZZZ I arrived at Gate XX where passenger boarding had already started. I had pulled up the release ahead of time (while the deadhead flight taxied in) and had noticed 3 MELs (including the APU and the APU bleed) and 1 CDL on the release. When I arrived at the cockpit I observed that the APU was already running. I questioned the Mechanic who was there at the time working on a #2 RMP write up and she knew nothing about it. The ground power AVAIL light was not illuminated and the APU bleed was off. I called Operations to request that the Ground Crew check the GPU connection and the ground power switch. We were unable to get ground power established so I requested a ground power cart. Operations replied that they would try and find one and that it 'could be awhile.' I reviewed the maintenance logbook and noted that the APU was definitely still on MEL status and was on the Logbook Deferral sheet. I also noticed that the aircraft had experienced a fume event the day prior. I called ZZZ Maintenance and Disptach to advise them of this. While on the phone with Dispatch; ground power was established and I switched over to it and shut the APU down. While talking with Dispatch; a ramp person disconnected the ground air without any notification to us and the temperature rapidly rose to 92 degrees in the cabin. I went outside and questioned why this was done. No answer was available. I requested HP air be connected so we could more efficiently cool the cabin. Ground Personnel did this and I proceeded back to the cockpit; Shortly after running HP air through the packs; we detected a chemical smell in the aircraft and I turned off the packs; The Mechanic onboard smelled it too and requested the packs be turned back on so that she could verify it and see if the Flight Attendants noticed it as well. Citing crew and passenger safety; I refused. I left the cockpit to ask the Ground Crew to reattach low pressure ground air. I noticed two men in sport coats at this point in the jetway. They later introduced themselves as ZZZ Chief Pilots and graciously offered any assistance necessary. I called Dispatch to inform them of the situation and request that the Equipment Coordinator find a new aircraft. The LP ground air was once again disconnected for some unknown reason and the cabin climbed to 96 degrees. I made the decision to deplane the passengers at this point and coordinated with the Gate agent. As we were deplaning; a ZZZ Chief Pilot informed me that he had contacted the last crew and they had used the APU on both legs because Maintenance had told them it was okay to do so.Cause - The previous crew's decision to use the APU; despite it being on MEL.1. Thoroughly investigate Smoke/Odor/Fume events before the aircraft is released back into service. It is an extremely serious safety concern and I do not feel like it is treated this way based on my experience. 2. As has been requested by crews many times; have Ramp Personnel request the removal of ground air just like the procedure to remove electrical power. Ground crews should not arbitrarily remove this without any notification to the flight deck!