Just after the impostor Robin takes the wheel for Riddler, the Bat-mobile is shown driving down the road with Robin in the passenger seat, even though he is not in the car with Batman.
If the laughing gas was flammable, then the first time the Riddler fired his gun as a signal would have set it off. Then Batman would have set it off with his flash-bang just before Riddler supposedly set it off.
Just past the 9:00 mark, Batman emerges from the Batmobile, fire extinguisher in hand, ready to put out a fire. Good thing he didn't need to; seconds later the pressure gauge of the extinguisher is clearly visible, showing it to be completely discharged.