| Alternative energy source for electricity production is a subject of considerable value globally in view of the ever-increasing cost and diminishing supply of petroleum fuels. Polysilicon is presently one of the best materials for application in photovoltaic energy conversion and the polysilicon is the base material in95%of solar cells. The modified Siemens process is the mian process for manufacturing polysilicon. However, how to find the best methods to increase the silicon yield, increase silicon growth rate and decrease energy consumption is the challege in present. Furthermore, the recycle of the by-production SiCl4is also one of the reasons which affect the cost of polysilicon.Based on the thermodynamic data of related species, the thermodynamics of TCS synthesis in Siemens process for manufacturing polysilicon has been studied. The optimum conditions have been obtained as550K-600K,2MPa-3MPa, H/Cl>100, which is accord to that in the actual operation. Under the above conditions, the calculated TCS selectivity is81.21%-83.21%, which is a little higher than65%-75%in industrial production.The interesting results can be obtained by investigating the thermodynamics of Si-H-Cl system. As a precursor for silicon deposition in Siemens process, SiHCl3is preferred for deposition of silicon to SiCl4.By the same reasoning SiH2Cl2would be even better but it is not as stable a gas and hence it is less frequently used. The interesting results show that the mixture of SiCl4and SiHCl3as the precursor can be fed into Siemens reactor. It is worthwhile to study the implication of this in the overall system by a process simulation model in a future study. The thermodynamics, therefore, have been studied when TCS, STC, the mixture of TCS and STC and SiH2Cl2as the precursors are fed into Siemens reactor for manufacturing polysilicon. The conversion, yield and product distribution have been calculated and the optimum conditions have been obtained and compared with limited published data in the open literature. When TCS is as the precursor, the optimum conditions are1425K,0.1MPa and the ratio of H2to TCS of15, which are good agreement with that reported by experimental studies. Under these conditions, the calculated silicon yield is35.58%, which is much higher than20%that is the average silicon yield in industrial production. It is an interesting method to deal with the main by-production STC that the mixture of TCS and STC as the precursor are fed into Siemens. The optimum temperature is1400K and the best pressure in the reactor is0.1MPa. The higher silicon yield can be obtained at even higher Cl/H ratio when the higher molar fraction of TCS is fed into the reactor. The relations of silicon yield, then, have been studied. From the results, we can find the range of Cl/H ratio which should be kept to obtain reasonable silicon yield when the fixed molar fraction of TCS is fed into the reactor and can also determine the range of molar fraction of TCS fed into the reactor to obtain reasonable silicon yield when the Cl/H ratio is fixed. When the SiH2Cl2without H2is fed into the reactor, the temperature has slight influence on silicon yield. The silicon yield can be up to be45%, which is much higher than that obtained with TCS as the precursor at lower temperature. In order to increase the silicon yield, the H2can be inject into reactor together with SiH2Cl2. However, the gas in the reactor should be heated to be1400K-1450K, which cause the power increase significantly. The SiH2Cl2without H2is, therefore, the best method when the precursor is SiH2Cl2.The heat transport model of rods heated by DC and AC has been developed in Siemens reactor. The results show that the temperature inside the rods is more homogenous when the rods is heated by AC than DC and the temperature inside the rods becomes more and more homogenous with increasing the frequency of AC. The temperature in the centre of the rods decreases when the surface temperature is fixed as a constant when the temperature inside the rods becomes more homogenous. When the radius of the rods growth because of silicon deposition on the silicon rod surface, the central temperature of the rods increases significantly when the rods are heated by AC with lower frequency but become flat when the rods are heated by AC with higher frequency. The central temperature of the rods almost keeps as a constant when the radius increases to be a somewhat value. The diffrence of the temperature between the center and the surface of the rods decreases by AC heating. Therefore, the radius of the rods can grow to a bigger value. The energy consumption can reduce as a result.The kinetics-transport model of Siemens reactor has been developed. The model is employed to investigate the effect of transport rate of gas and reaction rate on the silicon surface on the silicon growth rate. The results show that the silicon growth rate firstly increases significantly and becomes flat with increasing gas flow rate. But the silicon deposition efficiency decreases and power loss for manufacturing1kg plolysilicon increases when the gas flow rate increases. With increasing fraction of hydrogen in the feed gas, both of the silicon growth rate and HCl concentration on the silicon surface increase firstly and then decrease with a maximum value while the power loss for manufacturing lkg polysilicon decreases firstly and then increases. The silicon growth rate is up to maximum (5.54μm/min) and the power loss is the lowest (21.58kWh) at the optimum molar fraction of hydrogen (89%) in the feed gas. The total energy consumption is55.9kWh, which is much lower than120kWh which is the result in the inductrial production. With increasing the surface temperature on the rods, both of the silicon growth rate and HCl concentration on the silicon surface increase significantly and power loss for manufacturing lkg polysilicon decreases. Furthermore, the higher surface temperature on the rods can increase the desorption rate from silicon surface. Both of the silicon growth rate and HCl concentration on the silicon surface increase with increasing the temperature of the feed gas in the inlet, but the change is slight. However, the power loss for manufacturing1kg polysilicon decreases. Although the higher pressure can increase the silicon growth rate significantly, the power loss for manufacturing lkg silicon increase and the difficulty of design and control of the reactor increase because of the higher pressure in the reactor. The optimum pressure in the reactor should be0.1MPa-0.2MPa.The thermodynamics in STC hydrogenation process has been studied, including direct hydrogenation (hydrogenation process at higher temperature) and hydrochlorination process (hydrogenation process at lower temperature). The optimum conditions for direct hydrogenation of STC at high temperatures are studied and found to be in the range of1323K temperature,0.3MPa pressure and a flow ratio of4for hydrogen to STC. The conversion value is around25%. The actual SiHCl3conversion ratio of less than20%is much less than25%calculated under these conditions. The optimum conditions in hydrogenation of SiCl4process at lower temperature in presence of silicon (hydrochlorination process) have been obtained as850K,1.5MPa and H/Cl ratio of10. Under these conditions, the SiCl4conversion ratio is58.89%against the actual conversion ratio of15%-30%. Compared to hydrogenation of SiCl4process at higher temperature, the hydrogenation of SiCl4process at lower temperature is operated at lower temperature but the pressure and H/Cl ratio is much higher. The power assumption for the process at lower temperature is much lower but operation is difficult and complicated.By the study of thermodynamics in zinc reduction process for manufacturing polysilicon, the optimum conditions have been obtained as1200K, about2atm and the molar ratio of Zn to STC of4, which is accord to these reported in the open literature. Under these conditions, the calculated silicon is90.3%against about67%in experimental results.The study of heterogeneous reaction mechanisms of zinc reduction of SiCl4for manufacturing polysilicon is reported in this paper. The results show that the zinc tends to combine with atomic chlorine which dissociated in dissociation absorption. And the excessive gaseous zinc is necessary in the process. When there is enough gaseous zinc in the system, the energy consumption can be reduced and polysilicon yield rate increase which can reduce the cost in actual manufacturing production. Finally, the optimum reaction mechanisms have been obtained. |
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