Radiant section efficiency in Pyrolysis furnaces

[AndreChE]

Dear friends

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?

Thank you

AndreChE

 

[davefitz]

There is also a heat flux from the furnace exit to the convective section, which will add to the heat absorbed by the convection pass tubes but will not show up in the gas temperaure drop in the convection pass. By not accounting for this, you may be overpredicting the hedat abosrbed by the furnace.

 

[25362]

Some qualitative comments. The mechanisms of heat transfer in the radiant section are quite complex. The presence of solid matter as carbon in the flame, derived from fuel molecules' dissociation, contribute significantly to the total heat radiated from the flame.

Anyway, when speaking of the radiation mechanism, it is interesting to note that the radiating binary gases CO2 and H2O have a minor portion (~1/3) of the heat content of the combustion gases int the flame.

This portion is even smaller if the combustion is carried out with appreciable amounts of excess air. The major part of the combustion heat rests with the non-radiating diatomic (diathermic=radiation transparent) gases O2 and N2. These transmit their heat by collision with the former binary molecules and by convection to the high-emissivity refractory surfaces, all of which reradiate their heat to absorptive surfaces.

This is not to say that refractory surfaces don't receive heat by radiation, but it seems this is small when compared with the heat received by convection. Gas movement in the heater appears to be a major factor in the efficiency of the radiation zone when measured as a fraction of the total available released heat.

 

[Pyro77]

Andre,

I would have to ask first of all why are you bothering? There must be a point to it but it cannot be economics.

Secondly, how do you kbnow the threee things that you state you know. The biggest error is usually in assumptions.

Thirdly, why do you think Linde would get this calculation correct? 35-37% looks to be a low efficiency even by their standards. Fuel flows are always wrong, oxygen readings are wrong most of the time, thermocouples in pyrolysis furnace radinat sections are next to useless.

From field data it is usually necessary to correct fuel flows, run calc at "measured" excess air and then adjust iteratively until you converge flue gas and process heat and material balances. Hoever I doubt that you need this accuracy.

 

[smckennz]

Hi Andre
your numbers are not unusually high for a cold sink and a hot well mixed luminous flame. The old empirical correlations by Wilson Lobo Hottel et al will provide a backcheck on your numbers.
Re the convection; I am afraid you have to do the hard yards and work it all out.

Cheers

Steve