Towards a kinetic understanding of the NO promoting-effect on ignition of coalbed methane: A case study of methane/nitrogen dioxide mixtures

Abstract

9Nitrogen dioxide (NO 2 ) is an important impurity in coal-bed methane (CBM) and a dominant 10 component of NO x pollution in practical engines. Its promoting effect on methane ignition has been 11 studied in the current experimental and kinetic study. Ignition delay times of NO 2 /CH 4 /O 2 /Ar 12 mixtures, with blending ratios of NO 2 :CH 4 of 30:70, 50:50 and 70:30 for stoichiometric methane 13 mixtures were measured in a shock tube. Experiments cover a range of pressures (1.2 -10.0 atm) 14 and temperatures (933 -1961 K). Under all tested pressures, NO 2 addition promotes the reactivity 15 of methane and reduces the global activation energy at all pressures, and these effects are most 16 significant for the mixtures with highest NO 2 concentrations, at the highest pressures and at the 17 lowest temperatures. To simulate the experimental measurements, five literature NO x sub-18 mechanisms were integrated with AramcoMech 1.3. The simulations demonstrate that, for the 19 mixtures with low levels of NO x concentrations , the five models agree well with the experimental 20 ignition delay times. For the mixtures with high NO x content, however, all five models are unable to 21 reproduce the measured data, and the level of disagreement increases with increasing NO 2 22concentration. An updated mechanism is proposed, based on modifications made as a result of 23 2 sensitivity and reaction flux analyses performed to quantitatively determine the chemical reasons 24 for NO 2 promoting methane ignition. The results indicate that, NO 2 addition perturbs the branching 25 ratio of key reaction pathways by affecting the structure of the free radical pool at the initial ignition 26 stage of methane oxidation. A new reaction cycle via the following sequence of reactions ĊH 3 + 27 NO 2 <=> CH 3 Ȯ + NO, CH 3 Ȯ + M <=> CH 2 O + Ḣ + M, NO 2 + Ḣ <=> NO + ȮH, and CH 4 + ȮH 28 <=> ĊH 3 + H 2 O is proposed to explain the observed effect of NO 2 addition on the promotion of 29 methane ignition. 30 31

Shock tube studies on ignition delay and combustion characteristics of oxygenated fuels under high temperature

“…The temperature calculation has an uncertainty of ±15 K, and the largest uncertainty of ignition delay measurements is 10%, which is calculated according to the standard square root (RSS) method. 41,42 DME, DEE and ethanol with similar molecular formula or structure were selected as the experiment fuels. Meanwhile, E92 ethanol gasoline was studied, which contained ethanol with volume fraction of 11.6%.…”

“…The ignition delay time is the interval between the moment when the shock wave arrives PCB3 again and the ignition mark moment. The temperature calculation has an uncertainty of ±15 K, and the largest uncertainty of ignition delay measurements is 10%, which is calculated according to the standard square root (RSS) method 41,42 …”

Assessing the Predictions of A NOx Kinetic Mechanism on Recent Hydrogen and Syngas Experimental Data

“…In contrast, NO2 was observed to be more efficient in scavenging reactive radicals. However, in the shock tube measurements, the experimental observations showed the opposite effect of NO2 with either hydrogen [8] or a hydrocarbon fuel [45], because of the higher temperatures (> 1400 K) relative to the flow reactor experiments (600 -1400 K).…”

Experimental and kinetic study on ignition delay times of lean n -butane/hydrogen/argon mixtures at elevated pressures

“…In this study, the largest uncertainty of the reflected temperature is estimated to be 20 K, which will lead to an uncertainty up to 20% in the measurement of ignition delay times. Detailed uncertainty analysis can be found in our previous study by Deng [27].…”