Water quenching for flue exhaust

[wilcox]

Here is my case:

Exhaust flue (assume its properties are similar to air):
Temp.in = 500 deg.C
Vol. flow = 8,000 m3/hr

Cooling water(spray to the hot air):
Temp. in = 20 deg.C
Temp. out, = 43 deg.C (this is the statutory requirement of my place for waste water discharging temperature)

Questions:

1. What is the required rate of water for 100% water discharge?
2. Can I figure out the evaporation rate (if any) by any means?
3. If the water are allowed to fully evaporate by recirculating the cooling water, the water consumption should be substantially reduced. Yet, the final exhaust air temperature (with steam at 100deg.C) is required to further suppress to 60 deg.C since a temperature sensitive device for smoke removal is installed at the very end of the discharge side. Any good idea, other than air dilution?

Zillion thanks in advance.

 

You can try to use a dry type flue gas cooler. This would use ambient air. A gas to air heat exchanger would be installed in the flue gas stream. Flue gases would be cooled indirectly from 500 Deg C to the desired, even ( even upto 60 Deg C), and ambient air would be preheated. This preheated ambient air, being clean, can be released directly to the atmosphere, or can be used for a variety of uses.

It is quiet a lot like a Recuperator, only in a recuperator combustion air is preheated while here another stream is being preheated.

Dilution with ambient air has the disadvantage that it increases the mixed gas volume due to which capacity of post dilution equipment like flue suction blower, pollution control blower etc., increases. This increases initial investment and more importantly operation / running cost.

Saturation with water causes water pollution and problems of water discharge.

What is the application, and the flue used for generating the heat ?

This is an interesting case. You can send me more details at sanghi@vsnl.com and I can work out more details like air volume required etc. Our company makes recuperaors and air preheaters. Such flue gas coolers have been recently commissioned for Lead smelting furnaces before the bag filters, and we are doing more research to find the limits of cooling. Your case has the lowest cooled gas temperature so far.

Best regards,

VIKAS AGRAWAL.