On the structure and dynamics of lean-burning of natural gas

Lean premixed combustion of natural gas is receiving increased attention recently for its potential to reduce NO_x emissions. However, to maintain good combustion stability in lean premixed combustors is a challenging technical issue. Among the solutions that have been proposed one of the most interesting ones is the CRGT cycle, which involves recouping the waste heat by the endothermic steam reforming process of methane. The effect of methane reforming through the use of the CRGT cycle, on the cycle efficiency and flame stability was investigated experimentally and numerically. The methane reforming process results in the production of H₂, CO, H₂O, and CO₂. The effect of those gases on flame stability, flame structure, and pollutant emissions was studied by considering first each gas separately. Subsequently, the synergistic effect of all gases was assessed, Results reveled that the simultaneous presence of these four gases enhances flame stability. The effect of separately adding H₂/CO/H₂O/CO₂ in lean premixed methane/air flames on NO_x reduction was also studied. Adding the overall reformate mixture to CH₄ was found to result in lower NO_x emissions. The numerical predictions compare favorably with the experimental data, and help delineate the mechanistic reasons responsible for the observed behavior. In addition to methane reforming, the effect Oxygen Enhanced Combustion (OEC) on flame stability and pollutant emissions was assessed by considering the effect of excess O₂ on the dynamics and structure of lean CH₄/air premixed flames both computationally and experimentally. Results on flame propagation revealed that O₂ enrichment of the mixture could enhance combustion stability. The analysis of the NO_x structures revealed that O₂ enrichment can be of value, as it results in reduced NO_x production for the same flame temperature. The effect of downstream O₂ enrichment on the combustion characteristics of fuel-lean and fuel-rich CH₄/air flames was also investigated. Comparing the rate of NO_x production for flames with constant maximum flame temperature indicates that increasing downstream O₂ mole fraction has a beneficial effect in reducing NO_x production especially under fuel-rich conditions.