Energy Content of Flue gas

[boole]

Hi,

I want to calculate the heat content of the flue gas from an incinerator that is burning animal carcasses with diesel as an auxiliary fuel so I can size a suitable heat exchanger. It has an average flue temp of 900 degrees C. Has anybody got any guidance?

I haven't been able to work out the enthalpy as I don't know exactly the composition of the flue gas. Is a full energy balance the only other option?

diesel heat value + waste heat value - latent heat for water vapourisation - heat loss to the surroundings ?

is there alot of heat loss to the environment?

I would really appreciate any help.

 

[dcasto]

You only need to know the mass of the flue gas and its temperature. Either measure the flue velocity with anometers, pitto tubes, or something. You can assume the composition to be pure nitrogen for a start. You can do samples of the flue gas to get the exact composition.

If you can't measure the velocity, then assume an air fuel ratio, 15 sounds good. for every 15 lbs of fuel, you need 1 lb air. The lbs burned will be lbs carcus-lb ash + lbs fuel. Take that number multiply by 16 to get total lbs of flue gas.

The lowest temperature you want to cool the flue gas is about 250 to 300 F, any lower and you will condense out all kinds of acids. So Q = (900F - 300F) * lbs * heat Capcity for N2 of .24.

 

[rmw]

Shouldn't the moisture content of the fuel be considered as a constituent of the flue gas mass?

rmw

 

[quark]

Why 900⁰C flue gas straight from the chimney (I presume)? You must be having a scrubber to remove suspended particles and the flue temperature dropping there significantly.

We maintain about 80⁰C at the entry to chimney (or stack or whatever you may call) and it is lined. If you do caustic washing, the flue gas condensation problem is minimised.

This website has flue gas property calculator

http://www.hrsgdesign.com/design0.htm

Go to Calculators on the left side menu.

 

[25362]

To give you a ballpark idea, enthalpies of typical flue gases at this temperature level are in the vicinity of 320 kcal per normal (O^oC and 1 atm) cubic meter.

 

[boole]

Thanks for the advise. Its very useful.

 

[Feric]

Hi there:

Here is an MS Excel spreadsheet that deals with combustion calculations for oil and coal type fuels and/or any combination of such fuels.

http://engware.i-dentity.com/eng-tips/combustion.xls

Thanks,

http://engware.i-dentity.com

 

[21121956]

Hello everybody:

dcasto: If you assume an air-fuel ratio of 15, it means that for every 1 lb of fuel, you need 15 lbs of air. Or in SI units, for 1 kg of fuel, it is needed 15 kgs of air.

 

[dcasto]

Yep, got the numbers backwards.