; and (D) modifying subpart VVa of part 60 of title 40, Code of Federal Regulations, to ensure that— (i) the term no detectable emissions is defined to mean an instrument reading of less than 50 parts per billion above background concentrations; and (ii) the term leak is defined to mean an instrument reading of not less than 50 parts per billion above background concentrations. (6) Natural gas-fired steam boilers Not later than 3 years after the date of enactment of this Act, the Administrator shall promulgate a final rule revising subpart Db of part 60 of title 40, Code of Federal Regulations, to ensure that boilers or heaters located at an affected covered facility regulated under that subpart may only burn gaseous fuels, not solid fuels or liquid fuels. (7) National emission standards for hazardous air pollutants implementation improvements (A) Equipment leaks of benzene Not later than 3 years after the date of enactment of this Act, the Administrator shall promulgate a final rule to strike subsection (c) of section 61.112 of title 40, Code of Federal Regulations. (B) Benzene waste operations Not later than 3 years after the date of enactment of this Act, the Administrator shall promulgate a final rule modifying subpart FF of part 61 of title 40, Code of Federal Regulations, to ensure that— (i) the term no detectable emissions is defined to mean an instrument reading of less than 50 parts per billion above background concentrations