Study On Characteristics And Flame Structure Of Biomass Derived Gas

Biomass derived gases produced via gasification, pyrolysis, and fermentation are carbonneutral alternative fuels that can be used in gas turbines, furnaces, and piston engines. To make use of these environmentally friendly but energy density low fuels the combustion characteristics of these fuels have to be fully understood. The main objects of this thesis are as follows.The structure and laminar burning velocity of biomass derived gas flames are investigated using detailed chemical kinetic simulations. The studied gaseous fuels are the air-blown gasification gas, co-firing of gasification gas with methane, pyrolysis gases, landfill gases, and syngas. The simulated burning velocities of reference fuel mixtures using GRI Mech 3.0 and the San Diego mechanism, are compared with the experimental data to explore the uncertainties and scattering of the simulation data. The different chemical kinetic mechanisms are shown to give a reasonable agreement with each other and with experimental data, with a discrepancy within 7% over most of the conditions. The results show that the structures of typical landfill gas flames and co-firing of methane/gasification gas flames share essential similarity with methane flames. The reaction zones of these flames consist of a thin inner layer and a relatively thick CO/H2 oxidation layer. In the inner layer methane is converted through chain reactions to intermediates. The structures of gasification gas flames, pyrolysis gas flames, syngas flames share similarity with the oxidization layer of the methane/air flames. Overall, the chemical reactions of all biomass derived gas flames occur in thin zones of the order of less than 1 mm. The thickness of all BDG gas flames is inversely proportional to their respective laminar burning velocity. The laminar burning velocities of landfill gases are found to increase linearly with the mole fraction of methane in the mixtures, whereas for gasification gas, syngas and pyrolysis gas where hydrogen is present, the laminar burning velocities scale linearly with the mole fraction of hydrogen.The thermodynamic and combustion characteristics of oxy-fuel combustion in gas turbines using detailed chemical kinetic and thermodynamic calculations were studied. The oxy-fuels considered are mixtures of CH4, O2, CO2 and H2O, representing natural gas combustion under nitrogen free gas turbine conditions. GRI mechanism is used in the chemical kinetic calculations. Two mixing conditions in the combustion chambers are considered; a well-stirred reactor, and a typical non-premixed flame condition. The required residence time in the well-stirred reactor for the oxidation of fuels is simulated and compared with typical gas turbine operation. The flame temperature and extinction conditions are determined for non-premixed flames under various oxidizer inlet temperature and oxidizer compositions. It is shown that most oxy-fuel combustion conditions may not be feasible if the fuel, oxygen and diluent are not supplied properly to the combustors. It appears that for oxy-fuel combustion there is a range of oxygen/diluent ratio within which the flames can be not only stable, but also with low remaining oxygen and low emission of unburned intermediates in the flue gas.The heat flux method was used in the measurement of laminar burning velocities of five biomass derived gases on a perforate flat flame burner. In addition, co-firing of the methane with industrial gasification gas is studied in two mixture ratios, 20% and 80% of the gasification gas on volume basis, respectively. Simulations using detailed mechanisms with transport properties are conducted to compare with the measured flame speed. The results of the bio-methane flame generally agree well the data in the literature and the simulation. The measured laminar burning velocity of the industrial gasification gas is higher than the calculations from GRI-Mech 3.0 mechanism but in good agreement with the predictions from GRI-Mech 2.11 for lean and moderate rich conditions. The GRI mechanisms under-predict the laminar burning velocities for rich mixtures. The maximum burning velocities of the co-firing gases/air and the gasification gases/air mixtures are in richer mixtures comparing with the bio-methane/air mixtures due to the existence of hydrogen and CO in the gasification gases. The laminar burning velocities for the bio-methane at different preheating temperatures are measured and compared with the literature data.