Negative pressure dependence of mass burning rates of H 2/CO/O 2/diluent flames at low flame temperatures

• Published in: Combustion and flame Vol. 157; no. 4; pp. 618 - 631
• Main Authors: Burke, Michael P., Chaos, Marcos, Dryer, Frederick L., Ju, Yiguang
• Format: Journal Article
• Language: English
• Published: Elsevier Inc 01.04.2010
• Subjects: Burning velocity; Flame speed; High pressure; Hydrogen; Kinetic mechanism; Low flame temperature; Negative reaction order; Syngas; Kinetic mechanism; Hydrogen; Negative reaction order; Flame speed; Low flame temperature; Syngas; Burning velocity; High pressure
• ISSN: 0010-2180; 1556-2921
• DOI: 10.1016/j.combustflame.2009.08.009

Experimental measurements of burning rates, analysis of the key reactions and kinetic pathways, and modeling studies were performed for H 2/CO/O 2/diluent flames spanning a wide range of conditions: equivalence ratios from 0.85 to 2.5, flame temperatures from 1500 to 1800 K, pressures from 1 to 25 atm, CO fuel fractions from 0 to 0.9, and dilution concentrations of He up to 0.8, Ar up to 0.6, and CO 2 up to 0.4. The experimental data show negative pressure dependence of burning rate at high pressure, low flame temperature conditions for all equivalence ratios and CO fractions as high as 0.5. Dilution with CO 2 was observed to strengthen the pressure and temperature dependence compared to Ar-diluted flames of the same flame temperature. Simulations were performed to extend the experimentally studied conditions to conditions typical of gas turbine combustion in Integrated Gasification Combined Cycle processes, including preheated mixtures and other diluents such as N 2 and H 2O. Substantial differences are observed between literature model predictions and the experimental data as well as among model predictions themselves – up to a factor of three at high pressures. The present findings suggest the need for several rate constant modifications of reactions in the current hydrogen models and raise questions about the sufficiency of the set of hydrogen reactions in most recent hydrogen models to predict high pressure flame conditions relevant to controlling NO x emissions in gas turbine combustion. For example, the reaction O + OH + M = HO 2 + M is not included in most hydrogen models but is demonstrated here to significantly impact predictions of lean high pressure flames using rates within its uncertainty limits. Further studies are required to reduce uncertainties in third body collision efficiencies for and fall-off behavior of H + O 2(+M) = HO 2(+M) in both pure and mixed bath gases, in rate constants for HO 2 reactions with other radical species at higher temperatures, and in rate constants for reactions such as O + OH + M that become important under the present conditions in order to properly characterize the kinetics and predict global behavior of high-pressure H 2 or H 2/CO flames.