Combustion calculations and the required amount of air 6

[moonunits]

I'm working as an intern at a factory that produces refractory bricks, mainly doing measurements on a tunnel kiln they use for firing the bricks. The bricks are heated with several natural gas burners in a firing zone. To determine the required air for both stoichiometric and excess-air burning (?=1,1 and 1,2), I've done some basic combustion calculations we used to do back at the University. However, I'd like to check whether I've made correct assumptions and/or if I've simplified the problem too much.

The composition of the used natural gas is roughly the following (with mole-%):

                           mole-%
Methane              CH4   89,51
Ethane               C2H6  5,8
Propane              C3H8  2,25
Butane  i-C4H10 & n-C4H10  0,9
Pentane i-C5H12 & n-C5H12  0,21
Hexane              C6H14  0,06
Carbon dioxide        CO2  0,85
Nitrogen               N2  0,42

and hence M_NG=?x_i*M_i = 17,822 g/mole

I've assumed that the reactants combust completely and that both CO2 and N2 do not react. I've also assumed the following combustion reactions (is this oversimplifying things?):

1) CH4 + 2 O2 -> CO2 + 2 H2O
2) 2 C2H6 + 7 O2 -> 4 CO2 + 6 H2O
3) C3H8 + 5 O2 -> 3 CO2 + 4 H2O
4) 2 C4H10 + 13 O2 -> 10 H2O + 8 CO2
5) C5H12 + 8 O2 -> 5 CO2 + 6 H2O
6) 2 C6H14 + 19 O2 -> 14 H2O + 12 CO2

While considering combusting 1 mole of natural gas, I've then calculated (with the reactions above) the required amount of O2 (for methane n_O2=2?n_CH4, ethane n_O2=7/2?n_C2H6 etc):

        moles   O2 required (moles)
CH4    0,8951   1,7902
C2H6    0,058   0,203
C3H8   0,0225   0,1125
C4H10   0,009   0,0585
C5H12  0,0021   0,0168
C6H14  0,0006   0,0057
CO2    0,0085   0
N2     0,0042   0
       ===============
            1   2,1867

N2 in the air = 3,77 * 2,1867 = 8,243859 moles

=> tot. required air for stoichiometric combustion =
= (2,1867 + 8,243859) mole = 10,430559 mole_air/mole_{natural gas}

So then the molar stoichiometric AF ratio would be 10,43 (with ?=1,1 it would be 10,43*1,1 = 11,47 and with ?=1,2 => 12,52).

And then the stoich. AF ratio using masses would be

m_air/m_NG = 10,43 ? M_air/M_NG

Now the question is: are these calculations correct or should I approach this in some completely different way? Am I simplifying things too much / am I not taking something essential into account (that might render these calculations useless)? I know you tend to make these assumptions in classes, but how well do they transfer to practical situations in real life?

What I am trying to determine is the ? value for each burner. Apparently we've had some reducing atmosphere on one side of the kiln, and I'm trying to figure out if this could be caused by poor adjustment of a burner/some burners (you manually set the (constant) air and gas flow rates). If I measure the volumetric flow rate of natural gas to a burner (by measuring the differential pressure over an orifice plate assembly) and convert it to massflow, can I then use

m_air = 12,52 ? m_NG ? M_air/M_NG

to set the desired inlet airflow for ?=1,2? Do you have any other thoughts on my method or calculations? Thanks in advance!

 

[jmw]

The first step might be to visit

http://www.Chereources.com

and download the combustion calculations spreadsheet to get a double check.

This is the fired furnace excess air calculation speadsheet:

http://www.cheresources.com/invision/files/file/28-fired-furnace-excess-air-calculation/

And this the combustion calculations:

http://www.cheresources.com/invision/topic/4731-two-calculation-spreadsheets/

I hope this helps but there will be some expert members comments following shortly I'm sure.

JMW

http://www.ViscoAnalyser.com

 

[25362]

In a very old book I found that depending on their partial pressure CO₂ and H₂O could decompose as function of their temperature, modifying the composition of the combustion gases.

For example, CO₂ @ a partial pressure of 0.1 atm, would decompose (by volume), releasing CO, as follows:

% ^oC

0.5 1500
1.5 1600
2.8 1700
4.6 1800
7.6 1900
12.5 2000
35.9 2300

Water vapor decomposes, releasing hydrogen. For example, @ a partial pressure of 0.2 atm:

% ^oC

0.5 1600
0.8 1700
1.4 1800
2.4 1900
3.4 2000
10.4 2300

Lower partial pressures involve higher decomposition rates.
I wonder whether these facts could affect your "reducing atmosphere" measurements.

 

[davefitz]

The combustion calculations look OK, but explicit combustion calculations can be had in the B&W book "steam" or in ASME PTC 4 .

Regardless of the calcs , if the real problem is that you have a reducing environment, then to solve that issue then you need to ensure the burner to burner unbalance in stochiometry is less than your 20% excess air value. Measuring fuel gas is only half the job- you would also need to measure the air flow in each burner if you are going to use that direct approach. Look at the desing of modern low nox burnres for clues in how to obtain better air flow distribution and/or air flow monitoring.

 

[danw2]

calculations aside for a moment.

>(you manually set the (constant) air and gas flow rates).

What exactly does that mean?
Is there a temperature controller that drives a modulating fuel valve?
Is the fuel metered automatically by any/some means?
Is the combustion air metered automatically?
Is there some form of ratio control between gas flow and combustion air flow? (mechanical linkage, pressure pilot regulation?

Or is the fuel valve turned on manually, a burning rag is tossed in to light the burner and the air valve cranked open until
- the color of the flame is 'right'?
- the burner's flame just starts to rumble?
- when red paint marks line up?
- or some other technique?

Has a burner guy (many times a service/contractor guy) come in and stuck his oxygen probe in to get a reading of how much excess air (or lack thereof) is really running?

IF so, is there a record of what the reading is/

 

[moonunits]

Thanks for the much appreciated replies!

davefitz: I have measured both the air and fuel gas differential pressures over all the orifices. I have calculated the volumetric air flow for each burner, and I am waiting for an upgraded diagram for the natural gas orifices (they seem to be underdimensioned, the graph only shows pressure differences up to 24mbar when ours are 35-60mbar). But even though not knowing the exact volumetric fuel flow, I have noticed some rather big differences in the air flows. While most burners run on approx. 90-95 m3/h, three are running on nearly half of that at 52 m3/h! The measured natural gas pressure differences are pretty constant, so this would lead me to believe that the three burners are running with a way lower lambda than the others. Thanks for the tip on low NOx burners!

danw2: The burners are temperature regulated impulse burners, which to my knowledge fire a constant air-fuel mix set by manual valves. There are thermocouples between each pair of burners, and the reading of these determine if the burners fire or not (with the intention of maintaining a constant temperature). They were regulated years ago, by an outsider, but at the moment no one seems to know/remember how. Apparently the prevailing assumption is, that "it's workin' rather good, let's not mess with it too much" (which to me seems very odd). Some kind of fuel/gas ratio control was installed a couple years ago, with the intent of making the burners lambda controlled, but these do not seem to be operating (don't know why yet, I'll have to get back on this one). So at the moment the only way to determine the fuel and air flows is with the orifice assemblies.

A burner guy measured the excess oxygen and CO2 levels 3 years ago, and back then the levels seemed okay (~4% +-2,5% O2 and 8,5% +-1% CO2). I recon that I should recommend that the company invests in a flue gas analyzer (some interest for this has already been shown). Do you have any recommendations on what to consider when aquiring one of these? The firing temperature goes up to 1400 C, which probably puts some constraints on the choice of probe?

 

[ccfowler]

moonunits,

For simple practicality, you would not be far off if you simply assumed that the natural gas is just 100% methane, and you just use the most simplistic calculations. Available instruments for measuring fuel and air flow are not likely to sufficiently accurate to provide such levels of precision to justify worrying about subtle variations in the fuel composition.

Since orifices are being used to measure both air and fuel flow, the first thing that I would want to do is verify the EXACT sizes of each orifice and the EXACT geometry of all of the significant elements affecting the accuracy of each orifice. The exact condition of the orifice bore (ROUND HOLE!!!, good square, sharp edge!!!!!, concentric with pipe ID!!) is essential to everything else being worth the bother. ("Odd" orifice geometries are sometimes necessary for specific operating needs, but their accuracy always suffers when compared to "standard" calculations being used to take advantage of all of the "standard" orifice meter documentation.)

All manner of excellent documentation of orifice performance characteristics are available (ASME has excellent publications on the subject), and when all necessary information is available, including proper adjustments for pressures, temperatures, secondary instrument calibration (dp cells, manometers, thermocouples, RTD's, ...), truly excellent accuracy can be achieved even with turn-down ratios exceeding 3 or 4.

In my experience, orifice meters are too commonly taken for granted when, in fact, they require reasonably frequent care and attention. They are usually quite rugged, but it is wise to remember that almost everything that can go wrong with an orifice meter due to lack of attention to the primary element itself will result in understating the actual flow rate.

I've seen process trains being mis-operated for years at significantly excessive costs because the orifice plates used to measure the flow rate had been installed backward leading to actual flow rates being at least 25% greater than indicated. Since the multiple trains operated in parallel feeding a common receiver, there was no practical means for verifying the true performance of the individual trains. A feedback measurement of product quality at each train kept everything working in a tolerable fashion.

There is a direct parallel to your situation. Measurement of the oxygen content of the exhaust flow can serve to guide your system toward "less improper" air/fuel mixtures at the burners, but it wll not provide good indications as to how each burner is being operated.

Usually, the secondary instruments will get reasonably decent attention and calibration largely because they are usually fairly easy to service. Unfortunately, the all-important primary elements usually don't get nearly enough attention because they are usually difficult and expensive to service. All too commonly, there is a tendency to just pretend that they are simple, rugged, and reliable, so they don't REALLY NEED all that time, trouble, and expense. (I've seen worse than 50% errors from worn orifice plates that weren"just fine" until the troubles were uncovered. "Problem system performance" was invariably and wrongly blamed on nearly anything and everything else.)

Previous comments relating to the distribution of the air and fuel mixtures should be given full consideration. In multiple-burner situations, it is very easy to have very poor control at individual burners be masked by the overall flow rates and the measurement of oxygen concentration too far downstream to show the local firing problems. "Good-looking" flames can still suffer from enough variation from the intended mixtures to unfavorably affect operating costs and compromise emissions.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.

 

[moonunits]

Thank you ccfowler for taking the time to write such an insightful and informative reply!

I had not thought about checking the physical properties of the orifices. This will definitely be on my checklist once the kiln is run down for maintenance. The possibility exists, that the weird values I got on a couple of the burners might be caused by something in the orifices!

I got my hands on a couple of ASME reports, and as you said, they seem very good. A lot of detailed information, just have to wrap my head around the calculations. So far I've made a spreadsheet in excel, which converts the measured values to values in the orifice supplier's diagram (and thus enabling to read the volumetric flow rates for air and gas) and then calculates the lambda value for each burner. However, this method seems prone to inaccuracy, since it depends much on the person reading the diagrams. I'd like to calculate the flows on my own without any supplier diags.

Another problem is that the temperature of the natural gas entering the burners has to be estimated. I know the temp at the inlet to our factory, but I don't know how much it rises when travelling along the hot kiln to the burners. I've assumed it rises from 16°C to 20°C (just a "hunch" based on, um, nothing really). We'd probably need thermometers along the NG lines.

Thanks for the pointers concerning measurement of oxygen!

 

[dgau008]

hi, i am doing an project on heat exchangers. i need to find the flow rate of gas and pressure drop in heat exchanger. this heat exchanger is using in fireplace where hot gas coming out of place and cooler getting in the fire place. somebody please suggest me right way to measure these two parameters. i tried pitete tube and anemometre but both are not accurate...........

 

[ccfowler]

dgau008,

The posting of your question should be the start of a new thread for discussion, and while you are re-posting, you would help everyone, including yourself, to provide much more complete information.

Valuable advice from a professor many years ago: First, design for graceful failure. Everything we build will eventually fail, so we must strive to avoid injuries or secondary damage when that failure occurs. Only then can practicality and economics be properly considered.