CRJ-900 Captain reported a burning smell and smoke in the cabin during boarding. The flight crew shut down the APU and the recirculating fan and disembarked the passengers and crew.

Narrative

On Day 0; my crew and I were preparing Aircraft X ZZZ to ZZZ1. We were still attached to gate XXX in ZZZ when around XA45 the entire plane started to fill with strong fumes that smelled like an electrical fire. The APU and recirculation fan were on and the jet-bridge power was also plugged in. The smell was subtle at first in the cockpit and I thought it may have just been a bad smell that was associated with the ground air being disconnected and that it would pass momentarily. Within about 30 seconds of my First Officer (FO) and I noticing the smell in the cockpit; the aft Flight Attendant called me in the flight deck and informed me that it smelled like something was burning in the back and a deadheading pilot suggested shutting down the APU. There was no indication that anything was wrong in the cockpit with the APU or any of the electrical systems; but I now knew that this burning smell was all throughout the aircraft and strong in the cabin. I immediately took this advice and shutdown the APU and turned off the recirculation fan; I then radioed Local Maintenance to let them know of the electrical burning smell; and called Dispatch to inform them that I would be deplaning immediately in the interest of safety due to the unknown cause of the burning smell. As I stood up to make a PA announcement from the galley I sent my FO up to tell the gate agent that we were deplaning. Both of my Flight Attendants were standing in the galley and I told them we were deplaning; I then informed the passengers that due to the abnormal smell in the aircraft we were going to deplane immediately with everything they brought onboard through the main cabin door. The smell was less pronounced now with the APU and recirculation fan off; and the deplaning process was expedited in a safe controlled manor through the main cabin door. The entire process from noticing the smell; to informing Maintenance; Dispatch; and completely deplaning took 10 minutes. The plane was completely empty by XA55 and I stayed onboard until Maintenance arrived. We were assigned a new aircraft right next door at XXY and completed our flight as normal from there. Later that day I called Maintenance Operational Control to see if they had found the cause of the smell and they informed me that the recirculation fan had gone bad and caused the electrical burning smell and that is why it was present almost immediately throughout the aircraft.Later that day I called Maintenance Control to see if they had found the cause of the smell and they informed me that the recirculation fan had gone bad and caused the electrical burning smell and that is why it was present almost immediately throughout the aircraft.I felt my crew and I handled the situation well. I made the decision to deplane without hesitation and would do it again; but in the future I will start the deplaning process first; because that is what takes the longest. Getting everyone off the plane took about 8 minutes; and before I made the PA to deplane I made a 30 second call to maintenance and a 1 minute and 20 second call to dispatch to keep them in the loop. I know time is critical in any fire situation and that is why next time I would start the deplaning process first and inform dispatch and maintenance of the situation second.