烃 is a chemistry-coined character:
火 (fire) radical for combustibles +
巠 phonetic. The word reflects its composition -
碳 (carbon) +
氢 (hydrogen), with the pronunciation borrowing from the second character's onset. Compounds:
烷 (alkane),
烯 (alkene),
炔 (alkyne) all use the
火 radical for the same reason.