They are boilers that produce;
■  Steam
■  Hot Water, Superheated Water
■  Hot Air
using the heat of the exhaust gas from diesel and gas engines, various melting furnaces, and industrial boilers, without the fuel cost.
The distinguishing feature of these boilers from other boiler models is that they use the exhaust gas as a fuel.
They are preferred by companies that want to keep operating costs at a minimum level or to reduce them.
It is also possible to recover heat and reduce fuel consumption using the flue gas heat of existing steam boilers and steam generators. These devices are called flue gas economizers.