DEHUMIDIFIER FOR FLUE GASES

Abstract: Flue gases from combustion plants have water vapour from the combustion in the flue gas. With a condenser part of the vapour can be condensed and the condens heat be used to heat the return water from a heating system. By humidifying the combustion air with rest heat from the flue gas it is possible by using known technology to cool the flue gas to a temperature in the magnitude of 15 DEG C under the return water temperature and recover still more heat energy. The water content of the flue gas is high even after the condensation, especially if the return water temperature is high. It is an advantage if the flue gas temperature can be still more cooled, because forceful condensation improves the cleaning of the gas from dust particles and to avoid a vapour plume. This innovation reaches these goals by cooling the flue gas with ambient air.

Description: Dehumidifier for flue gases Flue gases from combustion plants have water vapour from the combustion in the flue gas. With a condenser part of the vapour can be condensed and the condense heat be used to heat the return water from a heating system. By humidifying the combustion air with rest heat from the flue gas it is possible by using known technology to cool the flue gas to a temperature in the magnitude of 15 C under the return water temperature and recover still more heat energy. The water content of the flue gas is high even after the condensation, especially if the return water temperature is high. It is an advantage if the flue gas temperature can still more be cooled, because: • The cleaning efficiency of dust particles from the gas improves when more condensation is arranged. Only changing their flow direction in some type of cleaning device cannot wash small particles away. The particles are so light that they follow the gas flow around the collector devices. The dust particles build in a condensing gas cores around which condense grows. The particles get consequently more weight and can be caught more easily. A better cleaning efficiency can therefore be reached with forceful condensation. It is an advantage for this reason to dehumidify the flue gas still more after the cooling to get heat recovery. • The water vapour, which is left in the flue gas, builds a disturbing vapour plume, especially at cold weather. The plume is aesthetically disturbing and dangerous for f.ex. air traffic if the plant is near an airport. Using this innovation considerably reduces the vapour content of the flue gas. As an example the vapour content of flue gas from a 6 MW boiler fired with wood chips is shown in Fig. 2. In the fig.1. it is shown as an example how the dehumidification is done in a plant where the heat recovery to the return water is used to maximum. The innovation is not only used coupled to heat recovery but can be used alone also. A humidifier 08 of the combustion air according known technology is included in the plant and sends humidified, warm combustion air to the boiler through the outlet 1 7. Humid flue gas from the boiler enters the condenser through the inlet 01. Heat is recovered into the return water in a heat exchanger 04 from a water circuit 19 where a pump 03 and a control valve 05 are installed. After the water has gone through the heat exchanger it is cooled additionally in a heat exchanger 06 with another water circuit, which humidifies the combustion air. Normally the water after passing the heat exchanger 06 would be used to cool the outgoing flue gas in a second step of the condenser. In the system according this innovation the ambient air is used to additionally cool the water to the second condenser step. One or two heat exchangers are used for the final cooling. The water is cooled either directly in a air cooler by ambient air or in a secondary circuit with freezing protection, like glycol, is used to protect the system from freezing. Two secondary circuits with a separate glycol circuit for both of them are shown in the example. The water leaving exchanger 06 through the pipe 09 is first cooled with glycol, which is chilled by ambient air in a cooler 15. A small amount of ambient air or air from the boiler room is heated in the cooler 15 and blown into the stack 22 where it is blended in an ejector with cold flue gas which has been cooled in the second condenser step. The temperature of the flue gas is now increased and its relative humidity decreased why the risk that plume is build is also decreased. The water in the pipe 09 is then cooled in the heat exchanger 11. The remaining heat is thrown away into the ambient air in an air cooler 14 and cold water goes through the pipe 20 into the top of the second step of the condenser. The valve 13 controls the flow. The flue gas is cooled in the second step of the condenser to a temperature, which is only a few degrees Centigrade higher than the water temperature is the pipe 20. The flow in the pipe 20 is controlled so that the outgoing temperature from the second step of the condenser will be the same as the water temperature in the pipe 16. The heat recovery of the system is therefore not decreased. The efficiency of the dehumidification increases by decreasing ambient temperature. The plume would normally be more pronounced when the ambient temperature decreases. This innovation decreases the plume to minimum and the dust cleaning efficiency is good in the wintertime when the boiler is run by its maximum power.

Claims: Patent claims - 1. A device for dehumidification of flue gases characterised by that water is injected in the flue gas and that the water heated by the gas is collected and pumped through a cooling system where ambient air cools the water in a heat exchanger. 2. A system according to the claim 1 characterised by that fluid with freezing protection is used in a circuit between the chilled water and the ambient air. 3. A system according to the claim 1 or 2 characterised by that heat from the heated water is first used to heat air, which is mixed with the flue gas for increasing the temperature of the flue gas, and to decrease its relative humidity why the risk for plume is reduced. 4. A system according to the claim 1 or 2 characterised by that heat from the heated water is first used to heat air from the boiler room, which is then mixed with the flue gas for increasing the temperature of the flue gas, and to decrease its relative humidity why the risk for plume is reduced.

Inventors: ERMA EERO, ERMA, EERO
Applicants: ERMA EERO, ERMA, EERO
Application year: 1998
Main classification: F23J15
IPC codes: B01D47, F23J15, F23L15