No person shall burn within the District any gaseous fuel containing sulfur compounds in excess of 50 grains per 100 cubic feet of gaseous fuel, calculated as hydrogen sulfide at standard conditions, or any liquid fuel or solid fuel having a sulfur content in excess of 0.5 percent by weight. The provisions of this rule shall not apply to:

  1. The incinerating of waste gases provided that the gross heating value of such gases is less than 300 British Thermal Units per cubic foot at standard conditions and the fuel used to incinerate such waste gases does not contain sulfur compounds in excess of the amount specified in this rule.
  2. The use of solid fuels in any metallurgical process.
  3. The use of fuels where the gaseous products of combustion are used as raw materials for other processes.
  4. The use of liquid, of solid fuel, to propel or test any vehicle, aircraft, missile, locomotive, boat or ship.
  5. The use of liquid fuel whenever the supply of gaseous fuel, the burning of which is permitted by this rule, is not physically available to the user due to accident, act of God, act of war, act of the public enemy, or failure of the supplier.
  6. The use of liquid fuel during a period for which the supplier of gaseous fuel, the burning of which is not prohibited by this Rule, interrupts the delivery of gaseous fuel to the user.