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Topic: Gas flow balances in a furnace.  (Read 7083 times)

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Offline ytszazu

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Gas flow balances in a furnace.
« on: August 02, 2006, 11:01:41 PM »
Dear all,

I am designing a furnace for aluminium smelting in my company. The furnace is a single hearth, reveberatory furnace.

I have calculated the thermal balance for the furnace and I have obtained this numbers:

(at 45% thermal efficiency)

Total input energy needed to melt per hour = 34 million BTU per hour

We are using oxygen-fuel burners for the furnace,

And I calculated that we need:

912 standard meter cubes of natural gas per hour and
1824 standard meter cubes of oxygen per hour.

both of this data are volume inputs in to the furnace at ambient temperature....

The question is:

What are the volume calculations when they are mixed and burned in the furnace?
How do we calculate it?
I remember we have to consider Rault's law in heat and volume calculations. Not to mention we need to consider the stochiometry of the combusted gasses volume as well.
Assume the temperature of the combustion runs at 1000K, so how do I count the volume for the gasses running in the furnace?

Regards,

Yong Tze Shoong,

tsyong@yechiu.com.my




Offline mbeychok

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Re: Gas flow balances in a furnace.
« Reply #1 on: August 03, 2006, 01:13:28 AM »
Yong Tze Shoong:

You really should provide more and better detail:

-- What is the heating value (i.e., caloric value) of your natural gas?

-- What percent excess combustion oxygen (if any) will you be using?

-- What are your reference temperature and pressure for your "standard meter cubed" of gas? Are you using 0 °C and 1 atmosphere, or 0 °C and 1 bar, or 25 °C and 1 atmosphere, or 25 °C and 1 bar? Please be be more explicit, because each of those are used by one organization or another.

-- Why are you assuming 45 percent thermal effciency? Is that typical for a single hearth, reverbatory furnaces? Many other types of industrial furnaces have much higher thermal efficiencies (anywhere from 60 to 90 percent).

If I assume that your natural gas has a gross heating value of 1000 Btu/ft3 at 60 °F and 1 atmosphere (i.e., 39.35 MJ/m3 at 0 °C and 1 atmosphere) and I further assume that you will not use any excess combustion oxygen, then I calculate you will need 911 m3 of natural gas and 1822 m3 of oxygen ( both at 0 °C and 1 atmosphere) to provide your 35×106 Btu/hour which is a very good check of your calculations.

CH4 + 2O2 ==> CO2 + 2H2O

Since the number of mols of combustion product gas are the same as the number of mols of fuel gas and oxygen (namely, 3 mols), then the volume of combustion product gas will equal the volume of the fuel gas and oxygen. In other words, the volume of combustion product gases will be:
911 + 1822 = 2733 m3 at 0 °C and 1 atmosphere.
 
At a temperature of 1000 K and at 1 atmosphere pressure, the volume of combustion product gases in the combustion zone will be:
(1000/273.15)×2733 = 10,005 m3 .

Does this help?
« Last Edit: August 04, 2006, 01:16:36 PM by mbeychok »
Milton Beychok
(Visit me at www.air-dispersion.com)

Offline ytszazu

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Re: Gas flow balances in a furnace.
« Reply #2 on: August 06, 2006, 10:00:17 PM »
Thanks for your help.

It seems that a standard PV = nRT calculation is enough for the volume calculations? Do we need to do correlation calculations?

This are the further details:

- Calorific value of natural gas: 9530 Kcal/standard meter cube (sm3)

- We don't have much sources on excess air that should be using , but isn't oxy-fuel burner running in less than stochiometric calculations? Stochiometric values for our natural gas is 2.132 (not including oxygen used to burn impurities from the melt)
-273k at 1 atm.

- Reveberatory furnaces are "traditional" funaces. The concept of the furnace is something like a bowl of soup with flames over it's surface. The efficiencies typically ranged from 5 percent to 28 percent. However, we made improvements to the furnace and expects to have at least 26 percent.

I have one more question to ask. For a typical aluminium melt bath where shredded aluminium scrap is charged into the furnace. How much oxygen is taken for the combustion of impurities off the scrap?

Regards,

Yong Tze Shoong,

tsyong@yechiu.com.my


Offline mbeychok

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Re: Gas flow balances in a furnace.
« Reply #3 on: August 06, 2006, 11:15:38 PM »
YTong Tze Shoong:

Your fuel gas caloric value of 9530 kcal/m3 converts to 39.89 MJ/m3 which is within 1.5 % of my estimated value of 39.35 MJ/m3 , so we are essentially in agreement.

The gas law PV=nRT is not required at all.  All you need to convert a given volume at one temperture to the equivalent volume at another temperature is Charles's Law, which is what I used:

V2/V1 = T2/T1

AS for your question about how much oxygen is required to combust the impurities in shredded aluminum scrap, I simply don't know.

I do want to repeat what I said before.  You must define what temperature and pressure you are using when you say "standard meter cubed" because there are many different definitions of standard conditions. I strongly suggest that you read http://en.wikipedia.org/wiki/Standard_conditions_for_temperature_and_pressure

Regards,
Milton Beychok
(Visit me at www.air-dispersion.com)

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