The Combined Nomenclature (CN), v1:2018
8404 10 00: Auxiliary plant for use with boilers of heading 8402 or 8403, e.g. economizers, superheaters, soot removers and gas recoverers;
https://www.dst.dk/en/Statistik/dokumentation/nomenklaturer/kn?id=bdccfab4-a71c-4f32-8b96-d3d9ed56013c