Formation of Nitride Oxide

[Speedy]

We have a process as follows;

Methane(Town Gas or bottled Methane) and Oxygen are mixed and ignited in a chamber at high pressure, up to 25 Bar. The ratio varies and could be twice Stoichiometric, i.e. double the amount of O2 required.

We have been told that Nitric Oxide is forming at high levels from the process.

After each cycle the gases are purged from the open chamber with air.

Am I right in assuming that it is because of this air that the NO is forming?

How dangerous is NO and can it be removed with a Catalytic Converter etc.

Any comments appreciated.

 

[mbeychok]

Speedy:

Whenever any fuel is combusted in air, part of the resultant NOx comes from any nitrogen compounds in the fuel and part of the resultant NOX comes from the nitrogen in the combustion air.

Also whenever any fuel is combusted in air, the higher is the excess combustion air (above the stoichiometric amount), the higher is the resultant amount of nitrogen oxides (NOx).

In your case, assuming that the town gas or bottled gas has no nitrogen compounds and the fact that you are using oxygen (rather than air), then the resultant NOx should be very low ... perhaps even nil.

In other words, for NOx to form, there must be some nitrogen present.

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

 

[dcasto]

As for dangerous, I was just on an 75/25 mix N2O to O2 at the dentists office. N2O2 does have an exposure limit.

Other than aphixiation from the other gases pushing out the O2, NOx will not be a health hazard. CO would be more of a concern.

NOx can be oxidized across a catalytic converter, but the temperature must be over 600 or 800F.

 

[gr2vessels]

Speedy,
The range of NOx's is a toxic byproduct in combustion, difficult to control and even more difficult to remove from the exhaust gases, particularly the ones using air (did I read correctly, are you using "pure Oxygen" for combustion of your town gas?).
The amount of NOx varies with the fuel / oxidizer ratio, the flame temperature, etc...In large combustion processes with relative high combustion temperature, like power station and the like, the amount of NOx generated could be significant and must be limited / controlled under the local EPA regulations. There are methods of controlling the formation of NOx, by lowering the combustion temperature, limiting the amount of available oxygen, controlled ammonia injection in the hot exhaust gas or quenching the tail end of combustion, many, many other ways. This is a slice of the combustion big picture.
Now, what combustion cycle have you mentioned, are you purging the combustion chamber every year, once a month, what is the frequency of purging. Would the purging air be a significant amount in your heating process, compared with the amount of Nitrogen in the fuel, particularly the town gas?
The catalitic converter is a great thing in the car exhaust system, but not feasible in a large combustion systems. It can be evaluated, however, against the process requirements, emision / polution limits, etc..
Sorry, long story, you better take your time to read more about combustion and combustion products, poluting the atmosphere..
cheers,
gr2vessels

 

[gr2vessels]

Geez, when I started my reply, no other reply was recorded, marked. I hope I didn't confuse the topic raised by speedy, I also try to type faster, to out-run other typing posters...
cheers,
gr2vessels

 

[Speedy]

Guys,

Thanks for your help, very informative.

Firstly let me explain a little more about the process.
This is not a continuous burn process but rather a cycle by cycle one as follows;

-A cylindrical chamber is used with completely open lower.
-The lower end is closed and clamped.
-Bottled Oxygen and either Town Gas or Bottled Methane is primed into the Chamber at up to 25 Bar.
- The ratio can vary from Stoichiometric up to twice O2 required.
-Ignition via Spark Plug.
-Chamber is completely reopened at lower end.
-Air is purged through the open chamber to remove any residual gases.
-End of cycle.

The process is used to treat metal parts and the Gas pressures and ratio are those required to optimise the metal treatment.

From reading your e-mails it would seem that the only possible sources of this NO is either the Nitrogen impurities in the gas or the residual air in the chamber from the purging.

There are 2 possible solutions;
- Remove air from chamber via vacuum prior to gas injection
- Delay opening of Chamber to allow the NO to fully oxidise with the excess O2.

Would this be correct?

 

[mbeychok]

Speedy:

I think it would be helpful if you let us know how harmful NOx would be to your process. Are you aware that the amount of NOx formed would be very small ... perhaps more or less a 100 ppm by volume? In fact, you might be able to confirm that quite easily by sampling and testing some of the combusted gas.

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

 

[Speedy]

mbeychok,

The measured emission level is 18 mg/m3.
The safe limits we are told is 0.2 mg / m3.

This is measured in the exhaust where the gases from the chamber are diluted with a large extraction fan.

The NO itself is not harmful to our process.
It is just an environmental concern.

Cheers,
Speedy

 

[mbeychok]

Speedy:

18 mg/m3 of NOx is 10 ppm by volume (ppmv) at 25 degrees C based on using the molecular weight of NOx as 46.01 as is commonly accepted.

The Short Term Exposure Limit (STEL) is 9.4 mg/m3 and the IDLH value is 20 ppmv.

Is this oxidation chamber of yours indoors? If so, I would think that all you need to do is to increase the ventilation rate.

If the chamber is outdoors, then you might consider routing the exhaust up a vent stack.

When you say "The safe limits we are told is 0.2 mg/m3", by whom were you told and what standard or regulation is that based upon?

Once again, if indoors use more ventilation. If outdoors, then use a vent stack. I don't think that you need anything more elaborate than that.

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