Hi
  what happens if we increase Air pressure in combustion reaction?
Is it safe to have Natural gas at high pressure (70 Bars) in combustion reactor?
Why do we have to control fuel flow rate but not oxygen?
 

First of all, let me start by saying that you have some good questions.

When considering oxidant (air and/or oxygen enriched air) as an ideal gas, its enthalpy value is a function of temperature only.  Therefore, there is no pressure dependency when doing an energy balance and trying to calculate the combustion products flame temperature where the enthalpy value of reactants (fuel + oxidant) is equal to the enthalpy value of combustion products for given stoichiometry value (oxidant to fuel ratio).  

However, when doing more time consuming and more computing complex calculations for the purpose of determining the combustion products flame temperature based upon the minimization of the Gibbs free energy, then the pressure becomes one of the input variables.  In general, the higher the pressure value, the higher the combustion products flame temperature value.  Again, such calculations cannot be done by hand and require good software and computer tools and resources.  

In my opinion, combustion pressure of 70 [bar] and/or 70 [atm] is too high.  Gas turbine combustion takes place around 15 [atm].  With high pressure, there are many issues that need to be considered and addressed for reliable and safe operation of such combustion equipment.

In atmosphere, one can always find standard air, which is on mole (volume) basis 79 [%] nitrogen and 21 [%] oxygen while on weight basis 77 [%] nitrogen and 23 [%] oxygen.  Molecular weight for nitrogen is 28 [kg/kmol] and for oxygen is 32 [kg/kmol] resulting that air molecular weight is 29 [kg/kmol].

Therefore, it is easier to change the oxidant to fuel ratio by changing the amount of fuel getting to the combustor -- for the gas turbine operation, the amount of air is pretty much steady, while by changing the amount of fuel one controls the combustion products flame temperature.      

As far as I know, when using methane as the fuel and standard air as the oxidant at stoichiometric combustion conditions, the combustion products flame temperature is ~ 2,300 [K].  Gas turbine operation requires that the operating conditions at the inlet of the gas turbine are: ~ 15 [atm] and 1,500 [K] -- meaning that the oxidant to fuel ratio is much higher than what is required for the stoichiometric combustion conditions.

One can change the oxidant composition, but an air separation unit is required to have oxygen enriched air as the oxidant -- compression is required ...

Here are a few combustion plots for stoichiometric conditions:
















 
Here is a URL providing some engineering/technical background information related to the combustion process for stoichiometric conditions:
http://www.engineering-4e.com/components.htm

Here is a URL for an online calculator for stoichiometric combustion of various fuel and oxidant compositions providing what the combustions products composition on both weight and mole (volume) basis is:
http://www.engineering-4e.com/calc4.htm

I do hope that my input will be of some help to you.

Good luck!

http://www.engineering-4e.com

FOUR provides a lot of good detailed information.

Just to add my two cents concerning industrial furnaces.  Usually the excess O2 is controlled.  Minimizing excess O2 improves your efficiency.  This is true for standard atmospheric air or pure oxygen feed.

Gas burners are designed for some operating pressure.  Exceeding this design pressure can lead to dangerous conditions such as lifting the flame off the burner and impinging tubes or worse blowing the flame out allowing gas rich environment.  I don't know what your burner design is but 70 atm seems extremely high.

I've "seen" combustion pressures that high, but they were in a research rocket engine combustion chamber for NASA.  Not with air, but with LOX.  AFAIK, there were no more than usual safety concerns for high pressure methane/nat. gas.  Coking of fuel coolant passages in the combustion chamber/nozzle throat were a concern as with any other hydrocarbon, which don't occur with pure hydrogen as fuel...

Here is another URL that provides a spreadsheet for engineering combustion calculations -- so that one can understand basic combustion trends in no time:
http://www.engineering-4e.com/eng-tips/combustionwithplots.xls

The questions asked by "chem101" are good and need to be addressed accordingly.

I have tried my best by focusing on the theoretical aspects.

I would like to thank both "ash9144" and "btrueblood" by providing hands on approach.

I do believe that a few additional posts from the Eng-Tips.com members will be beneficial regarding the subject matter.

http://www.engineering-4e.com