7 replies to this topic

[Adriaan]

I want (need) to make a simple program to give a rough indication of flame temperature, for burning natural gas with excess preheated combustion air. I found the following formulae (Eberline and Johnson);

Tangential heat input

Ht = 0.03457 ( t – 65.6 ) + ( LHV ) ( F / A )

Ht tangential heat input, defined as net (lower) heating value of fuel plus air enthalpy
above 65.6 °C (150 °F) in Btu/scf air
t air temperature in °C
LHV Net (lower) heating value of fuel in Btu per unit quantity of fuel (oil or gas)
F quantity of fuel in same units used to express LHV
A quantity of air in scf

Theoretical tangential flame temperature

gas tF = 456 + 17.2 Ht + 0.0020 ta (Ht – 1)
non aromatic oil tF = 450 + 18 Ht + 0.0032 ta (Ht – 34)
aromatic oil (500 °C air) tF = 386 + 20 Ht

tF theoretical flame temperature in °C
ta air temperature in °C
Ht tangential heat input in Btu/scf air

Range of applicability 65 < Ht < 85
Error limits approximately ± 10 °C




Now I am a continental European (never used Btu's or cubic feet) so for Westgas I would convert the heating value and get;

40950 kJ/m³ = 1099,21 Btu/scf

650 °C air, air/gas ratio 16:1 Ht 88,9 flame temperature 2099 °C
800 °C air, air/gas ratio 16:1 Ht 94,1 flame temperature 2223 °C


These flame temperatures are "in the ballpark" so I guess this method will do for me. My problem is that the Ht value is out of the range of applicability though.

Question 1; can I use this method to calculate an approximate flame temperature (I think YES)
Question 2; how significant is it that the Ht values are out of range?

[joerd]

1100 Btu/scf looks pretty good for a rich gas (contains C2+ components). A lot of "commercial" gas is a little lower (around 1000 Btu/scf).
I have not seen that formula before, but usually calculated adiabatic flame temperature with a simulator. It strikes me that the enthalpy is in Btu while temperature is in degrees C. Are you sure the conversion from deg.F to deg.C is taken into account in the factors?

[mbeychok]

Adriaan:

I must admit that I don't understand those equation you found and I have never seen them before, so I can't answer the questions you asked.

Howver, I am quite supprised by your air preheat temperatures of 600 and 800 °C ... do you truly preheat your combustion air that much??

I have my own spreadsheet program for calculating adiabatic flame temperatures. For a natural gas with a molecular weight of 18.4, a gross heating value (i.e., caloric value) of 1093 Btu/scf at 60 °F and 1 atmosphere, combustion air at 25 °C (i.e., no preheat at all) and 60% relative humidity, and 12% excess combustion air, I get an adiabatic flame temperature of 1902 °C. So your final answers are certainly in the ball park ... but I am still quite amazed by your combustion air preheat temperatures.

[Adriaan]

Howver, I am quite supprised by your air preheat temperatures of 600 and 800 °C ... do you truly preheat your combustion air that much??

I have my own spreadsheet program for calculating adiabatic flame temperatures.

I don't suppose you could e-mail me that spreadsheet?

Yup, our normal preheated combustion air temperature is 650 (on older units) to 800 'C, reactor temperatures are well over 1000 'C (thermal cracking of heavy oil types). My problem is that the temperature is measured with a thermowell, that means we measure the wall temperature but such a measurement is VERY inprecise (the wall surface condition etcetera influencing the measurement). We have a theoretical value (in the process control system) for the temperature but its badly (NOT, basically) documented and probably not up to date (different type of natural gas now).

1100 Btu/scf looks pretty good for a rich gas (contains C2+ components). A lot of "commercial" gas is a little lower (around 1000 Btu/scf).

Are you sure the conversion from deg.F to deg.C is taken into account in the factors?

Yeah its odd that it uses Btu and 'C, but that's what it uses!

We use Westgas, a high calorific gas for industrial use (switched to it some years ago), normal natural gas is lower in energy. In the Netherlands NG for home use is somewhat higher in energy than that used in - for example - Germany too (how different the gas is is best illustrated by the combustion air required; the Dutch gas needs 10 times the gas amount, the German 9 times for complete combustion).

[mbeychok]

Adriaan:

I would be happy to email you a copy of my spreadsheet if you will let me have your email address. My email address is available on my website at www.air-dispersion.com and you can look it up there.

Now, let me tell you about my program:

(1) It has three sections. One for gas fuel, one for oil fuel, and one for solid fuel.

(2a) Each section has an entry page wherein you provide the indicated entry data. The required entry data is different for each of the three fuels. It is mostly in the customary USA units (rather than SI metric), so you will have to get familiar with them. The entry page also includes instructions on how to use the Excel Solver to obtain the adiabatic flame temperature.

(2b) Each section then calculates and displays a page with the complete material balance information for the fuel and for the combustion air (including the adiabatic flame temperature).

(2c) Each section then calculates and displays another page with the complete material balance information for the wet flue gas and for the dry flue gas.

(2d) For the solid fuel, you also have an option for providing data specifying a spray dryer (using an alkaline slurry of lime) for SO₂ removal. If you elect to provide that entry data, then the program calculates and displays yet another page with the complete material balance for the spray dryer exit wet flue gas and for the spray dryer exit dry flue gas along with the calculated spray dryer exit gas temperature and water dewpoint.

(3) There are three page of documentation:
..... (a) one page explains how the adiabatic flame temperature is calculated
..... ( one page provides the physical properties used for each of the 19 components that the fuel gas may contain
......(c ) one page provides the specific heats and enthalpies used for the flue gas components

When you receive the spreadsheet, it will have the last sets of data that I entered for all three types of fuel when I last used the program. You should not erase or delete anything ... because you might delete some of the required equations and instructions. Just write over my last sets of entry data. In fact, I would strongly advise you to IMMEDIATELY make a copy of the spreadsheet when you receive it and file it away where you cannot accidently alter it in any way. Also, save a copy of this response.

Beyond that, you are on your own and you must promise me that there will be no follow-up of questions about how to use the spreadsheet.

[engware]

Adriaan:

Here is a free MS Excel file containing combustion calculations and plots for given fuel and oxidant compositions, fuel and oxidant temperature values and stoichiometry.

The free MS Excel file is for coal/oil type of fuel and the URL is as follows:

http://engware.i-den...m/ec-p-demo.xls

Output values provide flame temperatue and both combustion gas weight and mole basis composition breakdown.

A few plots containing major output values are provided.

I do hope that this free MS EXcel file will help you get answers quickly and effectively and provide you with an idea of what it takes to get flame temperature calculations up and running.

Thanks,

Gordan

Below are a few plots related to combustion -- complete combustion of coal, oil and gas with air at standard conditions (298 [K] and 1 [atm]) with no heat loss and stoichiometric conditions.

[Govind]

Dear Mr. Beychok,
I am new to the subject as I am an Mechanical Engineer drifted into Chemical Engineering.Could you Email the spreadsheet for flame temperature at the Emai ID:project@casepl.com. with explanation please.

[Fish]

Hi,

I was reading this thread as I am also on course of calculating adiabatic flame temperature of natural gas firing. I did my stoichiometric calculations & wanted to calculate AFT further.

Can Mr.mbeychok help me by sending the excel sheet to this mail id orionvishal@gmail.com.

It would be a great help.

Thanks

Regards,
Vishal