Experimental and numerical study of the reduction of silica in a thermal plasma reactor

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High purity silicon production is impeding the expansion of solar energy industry due to high cost. Using high purity SiO2 and carbon, it is possible to economically produce solar grade silicon through two-step process with SiC as an intermediate product. This work investigates the reduction behavior of SiO2 by natural gas in a thermal plasma reactor from both experimental and numerical approaches. Effects of CH4/SiO2 and plasma power input were studied by conducting experiments. Products were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). The composition of each phase, including both crystalline and amorphous phases, were quantified using partial or no known crystal structure (PONKCS) and internal standard methods based on whole pattern Rietveld refinement. SiC is the major product in this study. Higher power input and higher CH4/SiO2 ratio gives higher SiC yield. Maximum SiC yield of 69% was achieved at 20kW with CH4/SiO2 = 7.5. Reaction kinetics model was developed based on the reaction mechanism. Activation energy is 184.81 kJ/mol. With X represent the reduction degree and a_C represent the activity of carbon, the overall kinetic rate expression is: dX/dt=(8.43×〖10〗^5)∙exp⁡((-184812)/RT) ∙(a_C )^2∙(3×(1-X)^(2/3)) A 3D comprehensive computational fluid dynamics (CFD) model was developed based on experimental set-up. A new plasma nozzle boundary conditions determination method was developed based on empirical expressions and experimental conditions. The model was validated with experimental temperature data. Temperature and velocity profiles in the reactor was developed. Based on CFD simulation results, the SiO2 particle interaction with plasma gas stream and the reduction behavior was studied using Lagrangian method. An algorithm was developed to optimize the kinetics parameters based on CFD results. The optimized activation energy is 217 kJ/mol. The optimized kinetic rate expression is: dX/dt=(2.70×〖10〗^6)∙exp⁡((-217000)/RT) ∙(a_C )^2∙(3×(1-X)^(2/3)) To summarize, the reduction of SiO2 by methane in our thermal plasma reactor is successful in producing SiC. Yield of SiC in the thermal plasma reactor is comparable to literature data. A 3D comprehensive CFD model was developed and verified. The optimized kinetic rate expression obtained in this study can be used to predict the SiC production process using CFD simulations.

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