Flue-gas condensation

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Flue gas condensation is a process, where flue gas is cooled below its water dew point and the heat released by the resulting condensation of water is recovered as low temperature heat.

Cooling of the flue gas can be performed either directly with a heat exchanger or indirectly via a condensing scrubber.

The condensation of water releases more than 2 gigajoules (560 kWh) per ton of condensed water, which can be recovered in the cooler for e.g. district heating purposes.

Excess condensed water must continuously be removed from the process.

The downstream gas is saturated with water, so even though significant amounts of water may have been removed from the cooled gas, it is likely to leave a visible stack plume of water vapor.

If the fuel contains sulfur, the flue gases will contain oxides of sulfur. If the flue gases are cooled below the acid dew-point the acid vapor (sulfuric acid, H₂SO₄) will begin to condense. Acid condensation can result in low-temperature corrosion, which can threaten the safety of plant.^[1] Appropriate corrosion resistant material selection is important.

The heat recovery potential of flue gas condensation is highest for fuels with a high moisture content (e.g. biomass and municipal waste), and where heat is useful at the lowest possible temperatures. Thus flue gas condensation is normally implemented at biomass fired boilers and waste incinerators connected district heating grids with relatively low return temperatures (below approximately 55 °C (131 °F)).