At 8PM CROs alerted of main baghouse fan trip via PLC PC cntrl sys-2 plnt electricians responded to check cause-in opening E. door to Main Baghouse MCC electricians observed room filled w/black smoke-notified shift super who called 911. Electrician's discharged 1 fire ext.& pulled power to the bldg-Forks Twp FD arrived on scene w/in 10 mins-fire extinguished by 9PM. Cause TBD.