Impact of Ethane, Propane, and Diluent Content in Natural Gas on the NO_x emissions of a Commercial Microturbine Generator

摘要

Fluid dynamics and chemical kinetics play a significant role in the formation and emission of NO_x, particularly in lean premixed gas turbine combustors, where both phenomena must be considered. However, experimental tests conducted on gas turbines generally evaluate the output variables (e.g., the stack emissions) and those results are used to infer, to the extent possible, the physical and chemical processes taking place in the combustor. This approach only gives an external point of view, which can be complimented with the use of simulation tools such as the chemical kinetics models and computational fluid dynamics. In this paper, a reactor network analysis (RNA) of a 60 kW micro gas turbine combustor fueled with mixtures of natural gas-ethane-propane; and natural gas diluted with carbon dioxide are presented. A chemical reactor network (CRN) is developed for the combustor as operated on natural gas, with the purpose of "tuning" it to match measured exhaust emission levels. Then the CRN is used to analyze the effect of fuel composition on the NO_x emissions. The development of the CRN model is guided by reacting flow computational fluid dynamics (CFD). The CFD results define some of the important variables for the CRN, such as the residence time, volume of the reactors, temperature profiles, recirculation and mixing patterns. This strategy also gives insight to the complex phenomena occurring in the combustion chamber otherwise beyond the experiments. The CRN gives a detailed description of the emissions formation pathways while taking into account the mixing patterns obtained with the CFD. Experimental results are available from a previous studies conducted at the UCI Combustion Laboratory. The results obtained with the CRN indicate that the most important mechanism leading the formation of NO_x under the operating conditions of this engine is the N_2O pathway. This is the dominant pathway regardless the fuel composition. The CRN model predicts similar trends in overall emissions levels as those observed experimentally. The results also show that fuel dilution with CO_2 hinders all the NO_x routes but has a more significant effect on the thermal and N_2O routes. The addition of heavier alkanes (propane and ethane) promotes the formation of NO_x since the production of O and H radicals are stimulated. Those radicals are involved in the N_2O mechanism.