Research On Laminar Burning Characteristics Of Syngas-air Premixed Flames

Given the increasing concerns on energy efficiency and environmental protection, synthesis gas (syngas) has attracted significant interests as a promising alternative and environmentally clean fuel in the field of energy and combustion. Syngas primarily consists of hydrogen, carbon monoxide, methane, nitrogen, carbon dioxide and can be derived from numerous sources, such as coal, coke, natural gas and heavy oil. Syngas can be used in many power devices, such as gas turbines for integrated gasification combined cycle systems. However, both the design of syngas-fuelled devices and the control operation in syngas combustion are very complex because of significant differences in composition. Laminar burning velocities are fundamentally important in developing and justifying the chemical kinetic mechanisms of combustion systems, as well as in predicting their performance and emissions. Hence, an in-depth understanding of the fundamental combustion characteristics of syngas is important for the further application of syngas.The experimental method employed for the present study is mainly composed of six parts:a closed combustion vessel, discharge system, ignition system, optical access system, high-speed camera and data acquisition and control system. The measurement of laminar burning velocities in the present study can be well matched with most reported data.The laminar burning velocities and Markstein length of H2/CO lean and stoichiometric premixed flames under various hydrogen fractions were measured at atmospheric pressure in a constant volume vessel by using the Schlieren system. The effects of hydrogen on the stability of syngas premixed flame are obvious for syngas premixed flames with hydrogen fractions less than 50%. However, the flame stability for syngas premixed flames with hydrogen fractions of more than 50% is insignificantly changed by increasing hydrogen fraction. The laminar burning velocity increases rapidly as the hydrogen fraction varies from 0% to 10%, and then the laminar burning velocity increases slowly as the hydrogen fraction varies from 10% to 100%. The mechanism of the nonlinear increasement of laminar burning velocity was analyzed. The adiabatic flame temperature and thermal diffusivity were studied; the adiabatic flame temperature is not the dominant factor. The laminar burning velocity has a linear relationship with the maximum value of (H+OH).For providing more insight into the understanding of the laminar burning velocity reduction of diluted syngas-premixed flames, the laminar burning velocities of diluted syngas-premixed (H2/CO/N2/air and H2/CO/CO2/air) laminar spherical flames have been systematically and deeply investigated by the experimental method via Schlieren technology. The fitting formula was proposed considering the dilution fraction and equivalence ratio. Four assumptive substances were simulated with the Chemkin software. Compared with N2 dilution, the laminar burning velocity reduction of CO2 dilution was analyzed through thermal effect, transport effect and chemical effect, in addition, the contributions of third-body and direction effect were also been studied. The effect of change in transport is negligible for the difference of laminar burning velocity between N2 and CO2 dilution, the thermal effect and chemical effect plays a more import role on the laminar burning velocity reduction of CO2 diluted compared with N2 diluted flames. Compared with the chemical effect, the thermal effect plays a more important role on laminar burning velocity reduction of CO2 diluted compared with N2 diluted flames. Compared with the third-body effect, the direct reaction effect dominates the laminar burning velocity reduction caused by the chemical effect.The effects of variations in the fuel composition on the characteristics of H2/CO/CH4 flames were studied by experiment and Chemkin simulation. The sensitivity analysis and rate of production were anaylsised. The relationship between radical concentration and laminar burning velocity was also studied, the result shows that the maximum of H and H+OH have a good linear relationship with the laminar burning velocity. However, for CH4/CO flame, the laminar burning velocity only has a good relationship with H.The intrinsic instabilities of premixed flames are caused by hydrodynamic, diffusive-thermal, and body-force instabilities were studied. The effect of initial pressure, hydrogen fraction and equivalence ratio on cellular instability was studied. With the increase in initial pressure, the flame tends to be more unstable. With the increase in equivalence ratio, the flame tends to be more stable at high hydrogen fraction; however, the flame becomes more unstable with the increase in equivalence ratio at a low hydrogen fraction. With the increase in hydrogen fraction, the flame tends to be more unstable. The effect of dilution on flame instability was also investigated. The combined effect of body-force instability and cellular instability was proposed.