Case Study: Condensing economizer in boiler

Area of Improvement:
Smoke tube boiler of 10TPH, operating at 17.5 barg was used for generating steam for process requirement. The fuel used for boiler operation was Natural gas.


Observation:
Boiler was in operation with conventional economizer. De-aerator with 105 Deg C of feed water was fed to economizer and flue gas of 250 Deg C enters Economizer. Post economizer flue gas temperature drops to 150-160 deg C and this flue gas vented to atmosphere.
On analysis it was observed that being Natural gas (majorly Methane) as fuel there was huge amount of water formation while combustion

CH4 + 2O2 = CO2 + 2H20


One mole of Methane produces 2 mole of water. Means every Kg of methane burning will generate 2.25 kg of water in the flue gases. This water in flue gas carries higher amount of latent heat to atmosphere along with flue gas sensible heat. Typically heat loss though flue gas in natural gas fired boiler is in range of 11-15%, considering both heat being carried out though water vapour and flue gas exit temperature.


If Flue gas is cooled to below condensation temperature, both latent and sensible heat present in flue gas can be recovered.

Energy Conservation Measure:
To recover waste heat from flue gases, New Condensing economizer was designed for reducing temperature of flue gas to below its condensing temperature.

There shall challenge in metallurgy selection of heat exchanger because of impurities present in the natural gas supply like small amount of sulphur even in PPM level when brought to condensing, it may reduce life of Heat exchanger hence suitable MOC needs to be selected. MOC of Heat exchanger should be selected based on complete flue gas analysis. In presented case MOC of flue gas was selected as SS304.


While operation there shall be generation of condensate in flue gas which need to be drained and treated in Waste treatment plant. The condensate coming out from condensing economizer shall be having lower pH and needs treatment before discharge.

Schematic of condensing economizer installation

The flue gas after condensing economizer will have property of water separation while travelling though chimney. The condensate water with lower pH in chimney will reduce life of chimney, Therefore separate chimney of suitable material to be considered or suitable inside coating / lining of chimney may be planned.

Being lower temperature flue gas chimney height can be reduce drastically.

Payback for condensing economizer depends upon cost of natural gas. Typically for 10TPH boiler and above boiler size simple payback shall be lower than 1year.

Limitation:
Condensing economizer helps us to recover both latent and sensible heat of flue gases. If water contain in flue gas is high there shall be higher scope of condensing economizer, Natural gas is most favoured fuel for condensing economizer.
Condensing economizer can also be planned with other fuel also. MOC of economizer and chimney needs to be checked along with economics for other fuel.
Condensate generated at economizer needs to be treated before disposal or reusing in any process.
As the flue gas heat is recovered at lower temperature, low heat sink is required, typically we can raise ambient temperature of water ( approx 25 Deg C) to 60 deg C.

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