New member

Re: In stack NOx in diesel application

Pierre, for emissions/dispersion studies of combustion exhaust plumes the more significant parameters are exit temperature and momentum of the flow (no need to speciate molecules).


Quick question - I'd be interested to know what the "publications seen" are?





Cat Dealer

Re: In stack NOx in diesel application

Thanks All,


Molecular mass of NO is less than NOx therefore the bouyancy as a gas in air is greater is it not?


We have placed all the stacks together (less than 1/2 diameter seperating them) and reduced the stack diameter. This way we retain the plume temp and increase exit velocity. Reduced our stack height required by 30%, this has been accepted. We are also basing it on a 20% NO/NOx ratio, conservative but reasonable based on publications seen. Most importanly it was accepted and approved.




New member

Re: In stack NOx in diesel application

Besides the Texas values which range up to 40% NO2 in NOX for low bsfc engines, the San Joaquin Valley are also drafting guidlines that modellers must use

see their draft document.  Procedure For Determining NO2 Monitor Background Values (Design Values) For Use In Calculating NAAQS...  at their site


My understanding is that the default 10% criteria specified previously is a value intended really for boilers and now the regulators are becoming aware that reciprocatng engines can have higher NO2/NOX ratios.


Therefore I would think more regulators will require modelling with higher values than 10% NO2 in NOX and that this will create difficulties in planning.  From the Texas document lower NOX emission plant actually may have still the same amount of NO2 emissions, so in terms of the local NO2 impact as modelled there may not be much benifet in decreasing total NOX unless this also decreases the NO2 emissions.


What is needed in terms of modelling a specific engine is not just the NOX emissions but also the NO2 emissions or NO2/NOX ratio. 

Cat Dealer

Re: In stack NOx in diesel application

Any thoughts from the group regarding the NO vs. NO2 ratios that Texas has adopted per this link? This is becoming a significant issue when modeling site emissions.

Trusted Contributor

Re: In stack NOx in diesel application

Do other engine OEMs have this issue?

New member

Re: In stack NOx in diesel application

Pierre123 is correct when he says the assumption that all in-stack NOx is 100% NO2, and that this value is too conservative.  Obtaining accurate in-stack ratios is a big problem for the regulatory community. 


However, the assumption that 100% NO2 decreases the buoyancy isn't accurate.  The buoyancy is a function of the gas exit temperature (thermal buoyancy) and the gas exit velocity (mechanical buoyancy). 


So far, the only guidance the state regulatory community has received from the U.S. EPA is:


June 28, 2010

Applicability of Appendix W Modeling Guidance for the 1-hour NO2 NAAQS


"Key model inputs for both the OLM and PVMRM options are the in-stack ratios of

NO2/NOx emissions and background ozone concentrations. While the representativeness of

these key inputs is important in the context of the annual NO2 standard, they will generally take

on even greater importance for the new 1-hour NO2 standard, as explained in more detail below.

Recognizing the potential importance of the in-stack NO2/NOx ratio for hourly NO2 compliance

demonstrations, we recommend that in-stack ratios used with either the OLM or PVMRM

options be justified based on the specific application, i.e., there is no “default” in-stack NO2/NOx ratio for either OLM or PVMRM."


The in-stack ratio of NO2/NOx is highly variable, depending of the kind of emission unit, e.g., boiler, turbine, diesel engine, the age of the unit, if oxygen is supplied to the combustion process, and so on. An NO2/NOx ratio needs to be established on a case by case basis, or the use of a conservative value has to be used. 


The South Coast AWMD Modeling Guidance for AERMOD (the regulatory model for air dispersion) suggests a ratio of  0.1 for NO2/NOx.


I don't know if this is of any help, or whether of not your local regulatory agency will accept this  value.


Cat Dealer
Accepted Solution

In stack NOx in diesel application

Recenty we were asked by the local environmental authority to model ground level concentration (GLC) of NOx for  5 x 3516B's. The exhaust gas is untreated and generally GLC's and dispersion is met via stack height (since site has low background levels).


Cat publish the data as all NOx being NO2 in the exhaust tables. This creates a problem with the modelling since it decreases the bouyancy of the gas exiting the stack and therefore GLC's are higher (too conservative), this then requires taller stacks. 


Obviousy the majority of the NOx is in the form of NO is the stack and not NO2, but where to find a clear reference. Having a ratio Nox composition (which is citeable) input in the dispersion model  will reduce stack height.


My question is what is the ratio NO to NO2 in stack, and is there a Caterpillar or non-Caterpillar published reference that can be cited for this?