Heat recovery can take many different forms. In general terms, it can be classified as direct recovery, indirect recovery and secondary recovery. Direct recovery refers to the use of flue gas, which is a combination of carbon dioxide, nitrogen and oxygen generated by the combustion process in an incinerator or boiler, to preheat or dry products directly. Indirect recovery takes place when flue gas is used to preheat combustion air or fuel. Secondary recovery utilises the waste heat to preheat an external medium or to generate power.