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Venky October 20, 2008 22:06

Coal combustion - volatile species
 
Hi,

I got question for combustion CFD Engineers,

How to determine the appropriate stoichiometric ratios of species? (Vol. Species -> xH2O + yCH4 + zCO + gaseous species)

How to set the reaction to go instantaneously or with some delay?

How to avoid to avoid low temperature (below 273 K) in the PC Boiler?

Is PDF a good approch for Coal combustion?

Thanks in advance Venky

Allan Walsh October 21, 2008 22:03

Re: Coal combustion - volatile species
 
I would have to believe that there is some experimental data for the fractions of the gas species evolved during devolatilization. This would be a good starting point. You will probably find there are appreciable amounts of CO2 and H2. Use whatever data you have to determine a mass fraction breakdown that at least satisfies the fuel elemental analysis and fraction of volatiles. Then, calculate the heat of reaction of volatiles and the amount of oxygen needed for combustion. Are these close to measured values? Then you probably aren't far off.

How to set the reaction? Kinetics and turbulence mixing rate.

You can set the minimum temperature. If lots of cells are dropping below 273K, something is likely wrong. The heat capacity of particle or gas? Heats of formation?

No personal experience with PDF.

Venky October 22, 2008 07:14

Re: Coal combustion - volatile species
 
Hi Allan, Thanks for your answers.

1. I dont have the experimental data for the fractions of the gas species of coal. I have proximate and ultimate analysis from i determine the molecular wt and heat of formation of volatile CxHyOz. From Measurment i have air excess =1.27 And i have no problems with stoichiometry calculation. And they are close to experimental values.

2. For Coal should I change the speed of reaction. Does it depends on the coal?

3. I have 20 cells below 273K out of 8 Million cells. So i think it should be a problem. LCV of coal 23200 KJ/Kg and heat of formation of volatiles = -417MJ/Kmol

4. I am not planing for PDF model but i will give a try while finish my thesis.

Thanks once again. Venky

Allan Walsh October 22, 2008 13:39

Re: Coal combustion - volatile species
 
Sounds reasonable.

20 cells less than 273K shouldn't be too much of an issue if it doesn't change with iteration.

I assume that you are doing pulverized coal burners? Are the low temperatures around the burners? Could you have large sensible energy sink terms due to particle heating and large particle concentrations due to discrete nature of particle tracking?

venky October 22, 2008 19:28

Re: Coal combustion - volatile species
 
yes, its a 535MWe tangentially fired pulverized coal boiler. They are near the burner zone, but my results are not yet converged. High temp (>5000 K) decreased when I change from first order scheme to second order but not low temperature cells, i have to wait and see.

1,Could you have large sensible energy sink terms due to particle heating and large particle concentrations due to discrete nature of particle tracking? *** this might increase the temperature isn't it!!!

2,What are the best coal sub models? for volatiles and char.

thanks again Allan. venky



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