After the campfire died out, the charcoal continued to smoulder, sending up wisps of smoke.

smoulder 是动词，指"无火焰地缓慢燃烧"或"闷烧"。这个词来自古英语，描述的是燃烧物在缺氧条件下的不完全燃烧状态，通常伴随烟雾产生。