by paragraph (1), is amended by striking alternative fuel credit or alternative fuel mixture credit and inserting alternative fuel credit, alternative fuel mixture credit, or renewable natural gas fuel credit . (C) Section 6427(e)(6) of such Code, as redesignated by paragraph (1), is amended— (i) by striking paragraph (1) or (2) and inserting paragraph (1), (2), or (3) , and (ii) by striking any mixture or alternative fuel and inserting any mixture, alternative fuel, or renewable natural gas . (d) Registration Section 4101(a) of such Code is amended— (1) by striking and before every person producing second generation biofuel , and (2) by inserting , and every person producing renewable natural gas (as defined in section 6426(k)(2)) after (as defined in section 40(b)(6)(E)) . (e) Effective date The amendments made by this section shall apply to fuel sold or used after December 31, 2023.