If there had actually been such a high concentration of flammable gasses in the sewers to spontaneously combust with just a match-flame, the inspectors could not have remained conscious; an atmosphere with that much fumes would not support human life.
Earl is trying to strike matches on the front of the pack where there is no striker-strip.
When setting off the explosive charges with his "old school" seven-post contact-nails array, Scott sparks the fourth detonation-nail twice, rather that moving on to the #5 nail; he also skips Nail #6 altogether and goes directly to #7 for the "grand finale" explosion.