Autoignition of H₂/CO at elevated pressures in a rapid compression machine

Abstract

Autoignition of H 2 /O 2 and H 2 /CO/O 2 mixtures has been studied in a rapid compression machine at pressures from 15 to 50 bar, temperatures from 950 to 1100 K, and equivalence ratios from 0.36 to 1.6. In addition, the effect of change in relative concentrations of H 2 and CO is investigated by replacing H 2 with CO, while keeping the total fuel mole fraction of the combined H 2 /CO fuel constant. Under the experimental conditions of intermediate temperature and high pressure, reactions involving formation and consumption of HO 2 and H 2 O 2 are important. Results show that for H 2 autoignition, the mechanism of O'Conaire et al. agrees well with the experimental data, although some improvements can still be made. At all the pressures investigated, replacement of some amount of H 2 by CO in a reacting mixture leads to an increase in ignition delay, even with small amounts of CO addition. The inhibition effect of CO addition is also observed to be much more pronounced with increasing pressure. For H 2 /CO mixtures, the existing mechanisms of Li et al., Davis et al., and GRI-Mech 3.0 fail to describe the experimental trend of ignition delay. At a pressure of 50 bar, ignition delays are seen to decrease monotonically with O 2 addition, although such an effect is rather weak. Kinetic analysis further demonstrates that under the present experimental conditions, CO + HO 2 = CO 2 + OH is the primary reaction responsible for the mismatch of experimental and calculated ignition delays. Significant improvements in the ignition delay predictions can be made by reducing this reaction rate.

Comparison of the performance of several recent hydrogen combustion mechanisms

“…This gives an indication that in some of the mechanisms, third body collision coefficients might not be chosen ideally in order to match experimental data. Among the nine best overall mechanisms (full columns), only Konnov-2008 andHong-2011 perform better for RCM data measured in N 2 or N 2 /H 2 O [32,33], all others perform better for the measurements using Ar/N 2 [9]. Figure 5 shows the performance of the mechanisms compared to ignition delay measurements in shock tubes and RCMs according to interval ranges of temperature, pressure, equivalence ratio and diluent concentration, respectively.…”

Comprehensive H2/O2 kinetic model for high‐pressure combustion

“…Comparisons of these experimental data and model predictions using recently published kinetic models [12][13][14][15][16][17][18] reveal noteworthy disagreement, particularly for high-pressure and/or dilute flames [6][7][8][9]. Since the publication of many of these studies [4][5][6][7][8][9][10][11], Hong et al [19] published an updated H 2 /O 2 model on the basis of their recent shock tube measurements to determine improved rate constants for several reactions. The model of Hong et al [19] shows significant improvements against homogeneous targets, particularly for recent shock tube speciation and ignition delay time data.…”

“…The higher pressures, lower flame temperatures, and high collision efficiencies of common syngas diluents such as CO 2 and H 2 O produce a kinetic regime that is largely controlled by HO 2 and H 2 O 2 pathways, which are considerably less characterized than the branching reactions that dominate many of the systems previously used as validation targets for H 2 mechanisms. A number of studies (e.g., [4][5][6][7][8][9][10][11]) have recently emerged that present experimental data at high-pressure and/or low-temperature conditions. Comparisons of these experimental data and model predictions using recently published kinetic models [12][13][14][15][16][17][18] reveal noteworthy disagreement, particularly for high-pressure and/or dilute flames [6][7][8][9].…”

The rate constant for the CO+H2O2 reaction

“…Calculations of the potential energy surface for CO+HO 2 revealed barrier heights of 17.9 and 18.9 kcal/mol for initial trans-and cis-adduct formation respectively. You et al estimated a value of k 3 = 1.6 × 10 for the range 300-2500 K, with an estimated uncertainty factor decreasing from a value of 8 at 300 K to below a factor of two at above 1000 K. Most previous determinations of k 3 fall outside the prediction range of You et al, but their value is supported by data from autoignition experiments [17]. Following Rasmussen et al [9], we have adopted the value of k 3 from You et al for the present analysis.…”

“…This expression has been widely accepted as the best way to estimate k 3 [15,16]. However, recently the reaction has received considerable attention [4,12,[17][18][19]. Even though some scatter is observed among experimental and theoretical determinations, there are strong indications that reaction (R6) is considerably slower than indicated by the analysis of Baldwin et al The most reliable value for the rate constant presumably comes from the theoretical work by You et al [12].…”