NOx Sensitised Oxidation of Methane 

Experiments and Modelling



Anders Broe Bendtsena, Peter Glarborga, Kim Dam-Johansena, Per Gravers Kristensenb & Bent Karllb
a) CHEC research group, Dept. of Chemical Engineering, Technical University of Denmark, DK-2800 Lyngby
b) Danish Gas Technology Centre,Dr.Neergaardsvej 5A, DK-2970 Hørsholm


Poster presented at 27th International Symposium on Combustion, Boulder, CO, USA, Aug. 3-7th 1998

Previously NO sensitised oxidation of methane has been studied by Bromly et al 1 who published experiments with residence times of 1-3 seconds. Their experiments showed that ignition of methane in the presence of NO took place at lower temperatures than in the absence of NO. They also suggested a reaction mechanism which predicted their observations well. 

As the initiation processes of methane oxidation take place significantly faster we have made experiments with shorter residence times. Furthermore we have investigated the effect of respectively NO and NO2. Based on these experiments, we have modified an existing mechanism to cover the experimental range. 



Experimental results
We have made several experiments in the form of a temperature scan of various chemical compositions of the inlet to the plug flow reactor used in several studies from our laboratory 2. Three characteristic observations were made:

1) NO and NO2 reduces the ignition temperature as shown by Bromly et al 1;

2) At short residence times (<150 ms) NO alone does not show this enhancing effect

3) In some experiments partial oxidation was seen at low temperatures and full oxidation was observed at high temperatures; but in an intermediate temperature range no oxidation was seen.

Oxidation of methane in the absence of NOx  (200 ms)

Oxidation of methane in the presence of 200 ppm NO  (200 ms)

Oxidation of methane in the presence of 200 ppm NO2  (200 ms)

Oxidation of methane in the presence of  200 ppm NO  (140 ms)

Experimental conditions: NO and NO2 concentrations as specified 
CH4 1500/2300 ppm 
H2O and O2 in 2.5-6% 
Residence times are specified at 1200K (t=t0*1200/T) 
Pressure: 1.03 atm 
Isothermal flow reactor (i.e. each point correspond to one experiment).



Mechanistic studies 
An existing reaction mechanism3 was expanded to cover the experiments. Reaction followed three main paths: Above 1200K and in the absence of NOx the direct green path dominates reactions, while the blue path with branching regeneration of NO2 dominates at low temperatures in the presence of NO2. In the presence of NO alone initiation is through the red path which produces H-radicals and NO2 which activates the blue path. The inactive intermediate temperature range in the presence of NO is caused by a slow initial generation of radicals and NO2 due to competition between the direct green path and the red initiation path. Therefore, this phenomenon is only observed at short residence times. 
- NO and NO2 enhances methane oxidation 
- On-set of oxidation is lowered from 1200K to 950K in the presence of NOx 
- At residence times below 150 ms an intermediate inactive temperature regime may exist if NO is the only NOx species present 
- CO is a stable end-product at temperatures below 1100K 
- Possible reaction paths have been identified 
- CH3O2 is an important intermediate in the absence of NO2. 

1. J. H. Bromly, F. J. Barnes, S. Muris, X. You, B. S. Haynes. Comb. Sci. Tech.
(1996), 115, 259-296. 
2. P.G. Kristensen, P. Glarborg, & K. Dam- Johansen. Comb. & Flame (1996), 107, 211- 222 
3. P. Glarborg, M. U.Alzueta, K. Dam- Johansen, J. A. Miller. Comb. & Flame (1998), 115,1-27 

This work was supported by the Danish Ministry of Energy, the Danish Gas Technology Centre and the CHEC Research Programme. The Danish Gas Technology Centre is sponsored by the national and regional Danish gas companies. The CHEC (Combustion and Harmful Emission Control) Research Programme is cofunded by a.o. the Danish Technical Research Council and the Danish energy consortia Elsam and Elkraft. 


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Last updated: February18th 2001