Propagation And Extinction Studies Of Laminar Lean Premixed Syngas/Air Flames

The combustion of low heating value industrial syngas attracts growing interests. The knowledge of the combustion properties of syngas is important for directing its safe and clean usage in industry. A systematical study has been conducted experimentally and numerically on three aspects of combustion properties of lean premixed syngas/air flames, including the laminar flame speed, extinction limit and flammability limit. The experiments were conducted using the counter-flow flame configuration. Numerical simulations were conducted using the Chemkin II-based PREMIX and OPPDIF codes. The main conclusions are as follows:The laminar flame speed and mass burning rate of lean premixed H2/CO/air mixtures non-linearly increases with H2 content. When H2 content is small(≤15 % by volumn), hydrogen-containing species has a significant kinetic enhancement effect on CO combustion so that the mass burning rate rapidly increases with H2 content. When H2 content is large(> 15 % by volumn), H2/O2 chemistry dominates the combustion process so that the increase rate of mass burning rate becomes more gradual. The mass burning rate of laminar lean premixed syngas/air flames linearly varies with the maximum value of the summary of H and OH concentrations in the flame. Unlike previous studies, a new mixing model for the laminar flame speed of lean premixed H2/CO/air mixtures was derived through asymptotic analysis with the kinetic coupling effect considered. The new model predicts well against the experimental data over wide ranges of equivalence ratio(0.4-1.0), fuel composition(1 %-100 % H2 in the fuel blend), and unburned gas temperature(ambient to 600 K), and its predictions also agrees well with the detailed simulation results using Li’s mechanism over a wide range of pressure(ambient to 20 atm).The extinction limits of lean premixed H2/CO/air flames increases with the increasing H2 mole fraction in the fuel and equivalence ratio. From the chemical kinetic aspect, the extinction is determined by the competition between chain branching and terminating. Thus, an “extinction exponent” is introduced to describe the relative importance between chain branching and terminating reactions. It is a constant at near-limit condition when the fuel concentration is fixed, no matter the variations of the stretch rate and equivalence ratio. The “critical extinction exponent” can be the kinetic criterion for the extinction phenomenon of lean premixed syngas/air flames. The analyses based on the experimental results at micro-gravity showed that the extinction of weakly-stretched near-limit premixed syngas/air flame are more sensitive to mass diffusion rather than chemical kinetics. The preferential diffusion of H2 and H has an inverse effect on the extinction limit.The flammability limit of industrial syngases linearly decreases with the increasing unburned gas temperature, and monotonically decreases with the increasing combustible component in the syngas. The flammability limits predicted by the Le Chatelier’s rule agrees well with the experimental data. However, with the presence of large amount of H2 or inert species in the fuel, the prediction by Le Chatelier’s rule has remarkable discrepancy against the experimental data. Le Chatelier’s rule needs to be improved if large amount of H2 or inert species exists in the fuel.The dilution effect of CO2 is more profound than that of N2. The thermal effect of the dilution dominates the overall dilution effect in the reduction of laminar flame speed and extinction strain rate. The chemical effect of CO2 dilution has minor effect in reducing laminar flame speed and extinction limit. The chemical effect of N2 dilution and the diffusivity change effect are negligible in the propagation and extinction of laminar lean premixed H2/CO/air flames.