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Numerical Simulation of Biomass Direct Reburning

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Tutor: WuShaoHua
School: Harbin Institute of Technology
Course: Thermal Power Engineering
Keywords: Numerical simulation,Biomass,Combustion,Reburning,Direct reburning
CLC: TK16
Type: Master's thesis
Year:  2008
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Abstract:
In this paper a mathematical model of biomass combustion and reburning was developed based on the experimental results of biomass pyrolysis. Numerical simulation of biomass direct reburning based on the built model was performed in CFD program Fluent6.3 by taking a one dimensional drop-tube furnace (DTF) as research object. Combustion and reburning characteristics of biomass rice husk and straw were studied on different stoichiometric ratio and reburning fuel percent, and NO reductions by biomass volatile and char were investigated.The biomass combustion model includes sub-model of moisture evaporation, devolatilization, volatile combustion and char combustion. Diffusion process was used for calculating evaporation, one-step pyrolysis model was used for biomass devolatilization, diffusion-kinetic control model was used for volatile combustion, and multiple-surface reaction model was used for char combustion. Voatile nitrogen and thermal nitrogen produce NO, and volatile and char remove NO were considered.Biomass combustion and NO reduction model were verified experimentally by comparing predicted and experimental results of temperature profile, main volatile compositions profile and NO profile. The compared results show that the developed models can reconstruct biomass combustion and reburning process.Later the effect of reburning zone stoichiometric ratio and reburning fuel percent on biomass combustion and reburning were studied. The results show that burning-out efficiency increases with the increase of reburning zone stoichiometric ratio while NO reduction efficiency decreases when reburning fuel rate is kept constant. The temperature and CO, CO2, CH4 and H2 content along the axial of the furnace increases with the increase of reburning fuel percent when stoichiometric ratio is kept constant. And the same trend happens to NO reduction efficiency.NO removal by volatile is more sensitive to oxygen level, whereas NO removal by carbon on the surface of char is less sensitive to oxygen availability. And NO reduction by homogeneous reaction predominates.The comparisons of reburning characteristics between rice husk and straw indicate that NO reduction efficiency by straw is higher than that by rice husk. Contributors of straw char to the total NO reduction increase when reburning zone stoichiometric ratio increases, while that of straw volatile decrease. When stoichiometric ratio reaches 0.9, contributor of straw char reaches to 46% of the total NO reduction. However the opposite trend happens to rice husk.The studies of characteristics of biomass direct combustion and reburning performed in this paper can provide some suggestions and guides on further explore of biomass clean and efficient combustion and further development of biomass use patterns.
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