18USC-334

"Whoever, being a Federal Reserve Agent, or an agent or employee of such Federal Reserve Agent, or of the Board of Governors of the Federal Reserve System, issues or puts in circulation any Federal Reserve notes, without complying with or in violation of the provisions of law regulating the issuance and circulation of such Federal Reserve notes; or

Whoever, being an officer acting under the provisions of chapter 2 of Title 12, countersigns or delivers to any national banking association, or to any other company or person, any circulating notes contemplated by that chapter except in strict accordance with its provisions—

Shall be fined under this title or imprisoned not more than five years, or both."

12USC-411

"Federal reserve notes, to be issued at the discretion of the Board of Governors of the Federal Reserve System for the purpose of making advances to Federal reserve banks through the Federal reserve agents as hereinafter set forth and for no other purpose, are authorized. The said notes shall be obligations of the United States and shall be receivable by all national and member banks and Federal reserve banks and for all taxes, customs, and other public dues. They shall be redeemed in lawful money on demand at the Treasury Department of the United States, in the city of Washington, District of Columbia, or at any Federal Reserve bank."

So if Powell refuses to redeem (as the Fed currently does) he can simply be arrested.