Fuel mas flowrate claculation 1

Hello everybody:

Boiler efficiency (%)= ((Heat exported by fluid (steam))x 100)/ Heat provided by the fuel.

Independently of the accuracy of the values you have posted, by using the method 1, it is solved the above equation.

okay it goes like this:
U are calculaing something already calculated and you are trying to calculate it agaian, and the first time it may be calculated wrong. is this boiler working with a stio. ratio of 1 or more? because if you get these reading in a practical way then the boiler may be working too lean.

There seems to be a mismatch. Speak to the supplier.

You should consider HHV if yours is a condensing type boiler, otherwise, go with LHV.

omarbakr,

"U are calculaing something already calculated and you are trying to calculate it agaian..."

What are you speaking of?
If you are speaking about the burner power input, my problem is that i can't contect the supplier, therefore i need to calculate this value.

What is "stio. ratio"?

Thank you very much!

Hi there:

Here is a free MS Excel spreadsheet that can help you with some of the calculations regarding fuel HHV, flame temperature, combustion gas composition etc.

Thanks,

G. Feric, PE

Hi,

I've already saw this sheet, and i have one like this made by myself, but this doesn't help me on my problem.
this sheet doesn't claculate the fuel mass flowrate.

thanks anyway...

dianad:

What you have put out there so far is correct.

However, you need to know the stream values too in order to close your problem properly -- boiler water vapor outlet and water liquid inlet mass flow rates and state conditions and combustions products flame temperature (either oxidant to fuel ratio and/or stoichiometry determine the flame temperature as well as the oxidant and fuel compositions) and outlet conditions in order to find out exactly how much fuel and oxidant is required to meet the boiler heat requirements.

It is good to work with HHV values, but it can be misleading -- you skip dealing with flame temperature and the combustion products composition on weight and mole basis. As a result, you are faced with a more complex problem which is time consuming to do properly and becomes a challenge by itself when going through such an engineering exercise -- lots of calculations and information is required -- physical properties etc.

There are two ways of handling the stated problem: a simplified and a more detailed where one needs to work with streams and ends up doing a mass and heat balance for the boiler.

In the end, one needs to get the calculations done by using some proven and robust software -- it just does not work by hand ...

I do hope that you will find my input useful to some degree ...

Thanks,

G. Feric, PE

Feri,

Thanks for your kindly attention!

I didn't undersant 2 things:
1) What is the influence of the flame temperature in this kind of calculations?
2)Do you think i need to use HHV or LHV? Why?

Thanks!

dianad:

I was trying to indicate that one thing is to work with either HHV and/or LHV. But when working with such values, one needs to remember that they are valid only for the reference temperature which is usually 298 [K] -- maximum amout of heat that unit weight amount of fuel can generate.

Here are a few plots to help me make my point:










In a real engineering application, the combustion products do not leave the boiler with a temperature of 298 [K], it is always something higher due to other issues -- that is the reason why HHV needs to be adjusted.

However, for the purpose of conceptual design study type of work and first estimate, you can go with HHV and or something less than HHV, but be careful what you are doing in your calculations.

Again, I do hope that my input is useful to you ...

Thanks,

G. Feric, PE

Feric,

thank you for your attention!

I understood perfectly what you just said.
As i told you before, i have a big excel sheet that makes combustion calculations: adiabatic flame temperature, specific heat of the flue gas, densities, air/fuel rations, flue gas dewpoints, and LHV of the fuel, calculated by a formula using "enthalpies of formation" and fuel molar composition.
Following what you told, me i went to see this (that i almost forget) and saw this. And i verified that the LHV calculated by this complex formula, it gave me: 45164,5 kJ/kg that is lower than 58000 kJ/kg that i saw in a book.

But also this way, i don't use the flame temperature.

One more question: how can i calculate the combution products (in m3) by kg of fuel? I've used the formula of ideal gases(Pv=nRT), but i think that isn't correct. And this rises another question: what is the flue gas pressure?

Thank you soo much for your patience!

dianad:

I am glad to hear that you are making progress and getting something useful coming out from your numbers.

It is good that you can compare your numbers to some other reference values and examples ...

In my opinion, from the oxidant to fuel ratio and/or stoichiometry, you will have the mass flow rates. By working with the molecular weight of the species involved, one can switch to mole and volume based flow rates. Again, let me make a reference to my free MS Excel spreadsheet where one can have both weight and mole (volume) mass flow rates:


The ideal gas state equation should hold too. I would go with this one pv = RT and/or pV = mRT -- make sure that the units are satisfied -- [kJ/kg], [kJ] and/or [kJ/kmol] depending upon what you pick ...

Thanks,

G. Feric, PE

Feric,

I think that's all for now! Thank you once more!

I will ask you to take a look to my post that's related with my last question about the flue gas pressure, that i would appretiate that you could give you opinion about this issue that is far from being completed!!!

Thanks once more!

dianad:

If the boiler air is at standard conditions, then the flue gas pressure is at atmospheric conditions ~ 1 [atm].

Once it is known that the flue gas pressure is ~ 1 [atm], then by knowing the combustion products mole composition, one can estimate the partial pressure of combustion products species -- this is very important for H2O when determining the dew point temperature.

Usually, due to the safety reasons, the boiler operation is at a low vacuum conditions -- so the air gets sucked in.

dianad, I am glad to be of some help to you and Eng-Tips Forums members.

Also, other Eng-Tips Forums members are welcome to review my input and provide their operational experience and theoretical knowledge on the subject matter.

Thanks,

G. Feric, PE

Why not just stop trying to calculate it, and put a flow meter on the gas line?

Look dianad,
You said that
Boiler power (P): 6500 kW
Boiler efficiency (E): 87,8%
Natural gas HHV: 64606 kJ/kg
Air flowrate (Q(Air)): 4265,4 kg/h
Air fuel ratio (AF): 18 kgAir/kgFuel

these are known values right.
This means that air flow rate is already a known value and you are trying to calculate gas flow rate from it.
Also power is a known value and you are trying to calculate gas flow rate from it.
The 412.5 kg/h will give you the 6500 kw you want as power output.
but the 4265.4 kg/h air flow rate can burn only 237 kg/h fuel. so the air flow rate does not match the power output in the first place.
This all has to do with the Stoichiometric of the burners if these numbers are taken from a real working burner.

thank you all!

1-Feric,
what you just said is correct, but we have to account with static and dynamic pressures. As you can see, in the attached file, i plotted some results, and we can see the influence of the pressure!

2-trashcanman,
I'm far away from the installation, so i must work with this given values...

3-omarbakr,
What you exposed, is correct and i agree with you! That's why my confusion with this. Can you specify what you think i should use in my calculations? 412.5 or 23
kg/h?
Now, i'm using the 412.5, because this is the full capacity, and probably, the 237 kg/h was measured instantly and the modulating burner was in lower capacity.
Please, comment this!

thank you all for your patience!

dianad:

Can you specify the water vapor and water liquid conditions -- pressure and temperature and maybe steam quality unless it is superheated steam that we are dealing with -- and mass flow rates?

Once I can find out what is coming in and going out on the water and steam end, I would like to see what is going on with the combustion products by making some simple assumptions ...

Again, a simplified mass and heat balance needs to be done.

dianad, yes you are right that there are static and dynamic pressure values/quantities, but since the gas velocity is low, it can be assumed that the dynamic gas pressure can be neglected ...

The problem that you are dealing with does get complicated and the analysis needs to be kept easy so that some useful numbers can come out. Otherwise, it gets out of control ...

It would be nice to get a hold of the operational data, but sometimes that is not quite possible. Therefore, there is always a need for some type of quick engineering analysis and/or study to be carried out so the boiler performance and operation can be analyzed.

Thanks,

G. Feric, PE

Well dianad in any operation you should first consider your power then calculate the fuel neeeded for this power so you should use the 412.5 kg/h gas flow rate.
From the gas flow rate calculate the required air to make your Stoichiometric ratio for the boiler which the manufacturer specified and gives you your required emission level. don't worry if the boiler's smoke is alittle black, it's not always clear for the required Stoichiometric ratio for best efficeny.
Godd luck

Feric,

I've already made that heat balance and all the calculations. I've attached some images of my excel worksheets for you to see. So, thanks for the help offer for the calculations, but...i've they're already done!
Still there's 2 more questions for me to clarify everything and finish this post:
1) I used the LHV to calculate de mass flowrate. Regarding your previous posts, is this assumption correct?
2) So, you say that in normal conditions the dynamic pressure is zero. Therefore we stay only with the static head that's related with the chimney height? But, if there's the necessity of applying an air fan, then we have do consider the dynamic pressure, correct? Do you agree that the worst point of the chimney to occur acid precipitation is the top of the chimney, because of the lower pressure at this point and therefore we can assume the air pressure of 1 bar to calculate the dewpoint, as you just said?

thanks for all


omarbakr,

thanks for your reply. It was good to know that my procedures are correct, just as you said!
Thanks