Radiant section efficiency

[AndreChE]

Dear all

I have a question regarding pyrolysis furnace efficiencies.

I am calculating the overall thermal efficiency of all furnaces in my plant. But I want to calculate the efficiency of both radiant and convection section.

This is what I am doing:

- Heat input from fuel known
- Heat absorbed in total known
- Stack and bridge wall temperatures known

Since I calculate the fluegas mass flow, cp and the deltaT from bridge wall and stack, by Q=m.cp.DT, I have the convection heat absorbed. By difference from the heat absorbed I should have radiant heat absorbed.

Defining radiant efficiency as QRadiant/Qinput I achieve values of 40-45%, higher than the ones given by Linde of about 35-37%. Can I calculate the convective heat transfer this way?

Thanks

AndreChE

 

[davefitz]

There is also a contribution of heat tot eh convection section by radiant heat transfer from the furnace. If you know the area of the aperture from furnace to the convection section and are able to estimate the fluegas emissivity , then you can estimate the radiant heat emitted from the furnace to the convective pass.

The furnace efficiency will be a function of several variables, including the following:
-furnace gas emmissivity, which is a function of excess air and fuel fired
- furnace adiabatic flame temperature, again a function of excess air and fuel fired and gas recirculation
-fuel heat input divided by (furnace surface area plus furnace exit area), or use the Boltzman number as defined in the Russian Normative Method by Blokh
- wall effectiveness, which is usually a function of wall construction ( membrane waterwall vs loose tubes vs refractory )but is also affected by the slag or ash coating.
- burner patern centerline ( furnace geometry and burner location)

 

[davefitz]

Don't forget that the stack gas loss should also be factored into the overal global energy balance. That can easily exceed 15% of the heat input.

 

[25362]

In addition to davefitz's comments, please note that the heat transferred in the radiant section of fired heaters is not just a radiation effect.

Hot diatomic gases O₂ and N₂ are considered transparent (diathermic) to radiation and they indeed transfer heat by convection in the radiant section .

Robert D. Reed in Furnace Operations, Gulf Publishing Company, shows the radiant capability of CO2 and H2O when burning methane with 25% xs air is just about 30% of the total liberated heat. When the xs air % increases, the capability of radiation drops even more than expected from the cooling effect of higher xs air %.