ABSTRACT
Heat recovery boilers, also known as waste heat recovery boilers or heat recovery
steam generators (HRSGs), form an inevitable part of chemical plants, refineries,
power plants, and process systems. They are classified in several ways, as can be
seen in Fig. 2.1, according to the application, the type of boiler used, whether the
flue gas is used for process or mainly for energy recovery, cleanliness of the gas,
and boiler configuration, to mention a few. The main classification is based on
whether the boiler is used for process purposes or for energy recovery. Process
waste heat boilers are used to cool waste gas streams from a given inlet
temperature to a desired exit temperature for further processing purposes. An
example can be found in the chemical industry in a sulfuric acid or hydrogen
plant where the gas stream is cooled to a particular gas temperature and then
taken to a reactor for further processing. The exit gas temperature from the boiler
is an important parameter affecting the downstream process reactions and hence
is controlled by using a gas bypass system. Steam generation is of secondary
importance in such plants. In energy recovery applications, on the other hand, the
gas is cooled as much as possible while avoiding low temperature corrosion.
Examples can be found in gas turbine exhaust heat recovery or flue gas heat
recovery from incinerators, furnaces, and kilns. The objective here is to maximize
energy recovery.
