Modeling And Experimental Study Of Gasification Process Of Solid Fuels And Piloted Ignition In Counter-Flow Of Pyrolysis Volatiles

It is proposed that piloted ignition of flames near solid surfaces can be effectively investigated by separating the gaseous reaction from the energy and mass balance at the surface. Piloted ignition in gas phase above solid surfaces is investigated by simulating the transport and chemical reaction in a counter flow arrangement where a known fuel (methane) is supplied through a porous burner and the power and the location of the igniter are varied. The porous burner arrangement simulates a pyrolysing solid fuel at constant temperature by separating the gaseous phase from the solid conduction and pyrolysis phenomena. An Arrhenius one-step global reaction and a simplified transport model were used in the simulation. Only quasi-steady conditions are considered for the gaseous phase in this work because the response time for the solid phenomena is, in general, much larger than the response diffusion time for the gaseous phenomena. The relation of piloted ignition to extinction is also investigated. The effect of Damkohler number on ignition and extinction (without igniter) and the effect of the igniter on ignition are presented through a characteristic S curve obtained by plotting the evolving maximum temperature as a function of fuel mass flux. Based on the S-shaped curve, the relationship between the piloted ignition and extinction turning points and mass fluxes has been demonstrated in this paper. The piloted ignition turning point gradually approaches the extinction turning point with increasing Damkohler number and also with increasing power of the igniter. The ignition mass flux is found to depend basically on three parameters, Damkohler number, the location of the igniter and the power of the igniter all expressed in dimensionless forms.In order to verify the piloted ignition model of solid fuels through experiments, the comprehensive model of pyrolysis process in solid fuels and study the ambient pressure effects on the gasification in solid fuels, the ignition mass flux of PMMA was experimentally determined for locations of the igniter between6to70mm above the solid surface, and the powers of igniter between30-240W, under two external heat fluxes of21.2and25.4kW/m2. In addition, in another series of experiments the ignition mass flux for elm wood decreases by a factor0.6at reduced pressure0.67(Tibet0.67atm) compared to the ignition mass flux at normal pressure (Hefei,1.0atm). The results of this work are explained well by a numerical piloted ignition model which also explains recent observations on the ignition mass flux at reduced pressures in a forced-flow ignition and flame spread apparatus. Comparison with experimental data shows the simulated mass loss rate versus time is in good agreement with experiments as well. The results indicate that a lower atmospheric pressure can result in higher mass loss rates under the same external heat flux, and the ignition time of wood in Tibetan Plateau is much earlier than that in Hefei.In addition, for the gasification process in solid phase, an extended comprehensive model considering atmospheric pressure and unsteady gas phase processes was developed to predict the pyrolysis of wood. The effects of ambeint pressure and external heat flux on the mass loss rate of wood were investigated for0.6-1.0atm and18-50kW/m2. The mass loss rate of wood decreases with a decrease of ambient pressure and external heat flux. The predicted results have a good agreement with the expremental data.