Cooling Air and Effects on Boiler Performance

Q: Does the FAC^TM need fans/blowers to introduce the cooling air?

A: No. The FAC^TM system is applied in balanced draft boilers and uses the negative pressure inside the boiler. The FAC^TM extractor is in direct communication with the combustion chamber, so a negative pressure is obtained inside the FAC^TM system also. This negative pressure provides the motive force for drafting cooling air from the ambient into the FAC^TM extractor through special air inlet valves on the casing.

Q: Is the cooling air flow to the FAC^TM system controlled?

A: Yes. Depending on design requirements, the amount of air flowing into the FAC^TM system can either be kept constant or regulated. If the cooling air quantity has to be kept constant, the air inlet valves are fixed and the quantity of cooling air is equal to the design value (i.e. the maximum quantity that may be required). If the cooling air inflow is regulated, a control system is applied with dampers on the main air inlet valves, which are regulated as a function of the required cooling capacity (which depends mainly on the ash rate and temperature). In this way, the quantity of air flowing into the FAC^TM system is minimized continuously.

Q: Will the combustion in the furnace not be disturbed by the cooling air flowing from the FAC^TM into the boiler?

A: No. It has been amply verified with tests, that the cooling air coming from the FAC^TM does not affect the formation of NOx, the flue gas composition or the unburned carbon content of the fly ash. Firstly, this can be explained by the simple fact that the amount of cooling air required in the FAC^TM system is very low: it does not exceed 1.5% of the total combustion air flow to the boiler. Furthermore, the cooling air gets heated while cooling the ash in the FAC^TM system, and once it flows up into the boiler, it has reached a temperature of 300-400 °C [570-750 °F].Therefore, it acts exactly as combustion air for the furnace.

Q: Does the rating of the ID fans have to be increased because of the cooling air introduced into the boiler through the FAC^TM?

A: No. Cooling air needed for the FAC^TM system is not additional air, but part of the main combustion air. So, the amount of combustion air to the burners can actually be decreased. For example, if 1 % of the combustion air is the cooling air needed for the FAC^TM system, air to the burners can be reduced to 99 % of the total combustion air.

Q: If a part of the combustion air flows into the boiler through the FAC^TM system, and thus a lower quantity of air will flow through the air heater, will there be any negative effect on the boiler efficiency due to an increase of the flue gas temperature at the air heater outlet?

A: This applies only to the case of retrofit of a FAC^TM system in a boiler without any air heater bypass for flue gas temperature control. For new boilers, any flue gas temperature increase due to the FAC^TM system can be avoided by adequate air heater design. In the case of retrofit of a FAC^TM system in a boiler equipped with air heater bypass control, the flow rate through the bypass can be appropriately adjusted to ensure that there is no increase in flue gas temperature.

Even in the case of a slight increase of flue gas temperature after retrofitting a FAC^TM system, there will be a net boiler efficiency increase with the FAC^TM system, because the decrease in efficiency due to a higher flue gas temperature is more than offset by the recovery of the larger part of the energy contained in the bottom ash that is normally completely lost with a wet bottom ash system. In all cases, the balance is in favor of the FAC^TM system. For a more detailed explanation, see the paper "DRY BOTTOM ASH REMOVAL - ASH COOLING VS. BOILER EFFICIENCY EFFECTS".