fuel for a pocket warmer
Refers to the solid fuel, typically charcoal briquettes or similar, used inside a traditional Japanese pocket warmer (懐炉).
懐炉灰を新しいものに取り替える。
Replace the fuel for the pocket warmer with a new one.
Compound of 懐炉 (pocket warmer) and 灰 (ash, here referring to the fuel that turns to ash).