There shall be exempt from the tax imposed in §67-3-202 all kerosene sold or used in the state for purposes other than in any motor vehicle or equipment operated on the public highways of this state. This includes K-1 kerosene sold at retail through dispensers that have been designed and constructed to prevent delivery directly from the dispenser into a vehicle fuel supply tank, and K-1 kerosene sold at retail through non-barricaded dispensers in quantities of not more than twenty-one gallons (21 gals.) for use other than for highway purposes, under such rules as the department shall reasonably require. Quantities sold at retail stations, other than stated above, are subject to the diesel tax imposed in §67-3-202.