Abstract :

Increasingly stringent emission requirements and dwindling petroleum reserves have generated interest in expanding the role of synthesis gas (syngas) fuels in power generation applications. Syngas fuels are the product of gasifying organic-based feedstock such as coal and biomass and are composed of mainly H2 and CO. However, the use of syngas fuels in lean premixed gas turbine systems has been limited in part because the behavior of turbulent flames in these mixtures at practical gas turbine operating conditions are not well understood. This seminar presents the resultsof an investigation of the influence of fuel composition and pressure on the turbulent consumption speed, ST,GC, and the turbulent flame brush thickness, δFBT, for these mixtures. ST,GC and δFBT are global parameters which represent the average rate of conversion of reactants to products and the average heat release distribution of the turbulent flame respectively. A comprehensive database of turbulent consumption speed measurements obtained at pressures up to 20 atm and H2/CO ratios of 30/70 to 90/10 by volume is presented. There are two key findings from this database. First, mixtures of different H2/CO ratios but with the same un-stretched laminar flame speeds, SL,0, exposed to the same turbulence intensities , have different turbulent consumption speeds. Second, higher pressures augment the turbulent consumption speed when SL,0 is held constant across pressures and H2/CO ratios. These observations are attributed to the mixture stretch sensitivities, which are incorporated into a physics-based model for the turbulent consumption speed using quasi-steady leading points concepts. The derived scaling law closely resembles Damköhler’s classical turbulent flame speed scaling, except that the maximum stretched laminar flame speed, SL,max, arises as the normalizing parameter. Scaling the ST,GC data by SL,max shows good collapse of the data at fixed pressures, but systematic differences between data taken at different pressures are observed. These differences are attributed to non-quasi-steady chemistry effects, which are quantified with a Damköhler number defined as the ratio of the chemical time scale associated with SL,max and a fluid mechanic time scale. Finally, a systematic investigation of the influence of pressure and fuel composition on the flame brush thickness is presented. Several key findings are reported. First, the flame brush thickness is shown to be a weak function of the fuel composition and stoichiometry at constant , even though the corresponding turbulent consumption speed, ST,GC varies appreciably. This result is consistent with findings reported in the literature, and is shown here to persist for a much wider range of H2/CO mixtures, pressures and turbulence intensities than for which data were previously available. This observation is explained by attributing the turbulent flame brush characteristics to large-scale turbulent diffusion processes, which are independent of the instantaneous flame topology and characteristics. Second, the pressure is observed to augment the turbulent flame brush thickness at fixed . For instance, a factor of ten increase in pressure results in an enhancement of the flame brush thickness by as much as a factor of two when SL,0 is held constant across pressure.