Exhaust Gas Boiler

By Lars Josefsson

Exhaust Gas Boilers also known as Exhaust Gas Economizers, with extended heating surface, used for waste heat recovery in motor ships.

A diesel engine loses approximately 30% of the energy to the exhaust gases. A turbocharger recover some of it, but there is still heat losses that can be recovered by means of an Exhaust Gas Boiler, EGB. Utilization of the energy in the exhaust gas is restricted due to risk of low temperature corrosion on the gas side of the tubes. The sulphuric acid dew point temperature is assumed to be at 130-140°C. A temperature difference of 40°C must be considered in order to obtain a certain margin when the gas temperature decreases due to low load on the main engine. This means that Exhaust Gas Boilers shall not be designed to reduce the gas temperature at the EGB outlet below 170-180°C at Continuous Service Rating. Besides, at gas temperatures below 170-180°C, the vaporized unburned oil particles in the gas condense to an adhesive mixture of soot and oil resulting in troublesome deposits on the tubes and on the heating surface of the Exhaust Gas Boilers.
The reasonable optimal steam production of Exhaust Gas Boilers where no economizers are incorporated are consequently reached at a steam pressure of 3-4 bar (saturated temperature 143-151°C). In practice it is found that the pinch temperature shall be kept at minimum 25°C as a lower temperature difference will substantially increase the heating surface and the price of the Exhaust Gas Boilers.

Exhaust Gas Boilers are normally built to regenerate heat from comparatively large flue gas quantities at a not very high temperature and with a limited pressure loss. To fulfill these demands, the boilers ought to be provided with tubes with extended surfaces on the gas-side. This can be achieved by means of pinned tubes, as in this example, or finned tubes.

An Exhaust Gas Boiler requires a steam dome and very often an oil fired boiler is used as steam drum for this kind of boiler. Have a look at this example of an Exhaust Gas Boiler installation. Please note the flow direction through the tubes of the Exhaust Gas Boiler.

Soot deposits will decrease the exhaust gas boiler steam production considerably and may if left unattended cause a soot fire. Therefore it would be wise to include an automatic timer controlled soot blowing equipment.