Energy consumption and environmental protection issues in the production of polysilicon using the modified Siemens method

Thread Content

Production of polycrystalline silicon using the modified Siemens method: I have found some relevant information that everyone can share. Topics include energy consumption and environmental protection issues associated with the production of polycrystalline silicon using this method. Recently, there has been significant interest in polycrystalline silicon on the part of society and the industry, which has turned what was once a little-known product into a topic of public discussion. In my opinion, there are two reasons for this: first, the rapid development of the photovoltaic industry has led to excessive demand for polysilicon products, causing their market prices to deviate significantly from their actual value. Some polysilicon manufacturers suddenly became strikingly wealthy newcomers overnight, and the attractive profit margins attracted many investors who were willing to invest heavily in building polysilicon production plants. The large number of new projects, the huge amount of capital invested, and the scale of construction are unprecedented in China and rare on a global scale as well. Second is the issue of energy consumption and pollution in polysilicon production, which has drawn a lot of criticism. Some even argue that it involves using China’s energy to produce clean energy for other countries ； Keep the pollution at home and sell clean energy to others. I would like to take this opportunity to share a few thoughts on \"the energy consumption and pollution associated with polysilicon.\" Characteristics of polysilicon production: 1. The production process of polysilicon is a process of purifying elemental silicon; it involves purifying industrial silicon with 2 \"9\" levels of purity into high-purity silicon with 7–11 \"9\" levels of purity ； 2. The method for purifying silicon involves converting elemental silicon into compounds that are easy to produce, pure, and reduce (such as SiHCl3, SiCl4, SiH4, etc.), with the purification of these silicon compounds serving as the main approach to purify elemental silicon ； 3. The production of polysilicon is essentially a chemical process, with the main chemical techniques including: the synthesis of silicon compounds, the separation of various silicon compounds, the purification of silicon compounds, and the vapor-solid phase deposition of polysilicon via hydrogen reduction of silicon compounds ； 4. Polysilicon production is an energy-intensive industry, with the average electricity consumption per ton of polysilicon exceeding 200,000 kWh ； 5. The polysilicon production process is a complex and integrated process that combines elements such as chemical engineering, gas purification, electrical control, and physical and chemical analysis; it requires close cooperation among various types of specialized professionals to be successful ； 6. Polysilicon production is a \"three-intensity\" industry that requires substantial capital, advanced technology, and skilled personnel; on average, the investment per ton of polysilicon amounts to 700,000 to 1,000,000 RMB ； 7. The construction of a modern polysilicon plant takes 24 to 28 months, and most of the equipment used is non-standard, requiring considerable time for design, processing, installation, and commissioning ； 8. Polysilicon production is a process of purifying elemental silicon to a very high degree of purity; any leakage in this production system, minor contamination, as well as the raw materials, reagents, and protective atmospheres introduced into the system can all have a significant impact on the quality of the product. Strict control and effective management of the cleaning, drying, and installation of various equipment, pipelines, valves, fittings, etc. in the production system are one of the keys to producing high-quality products that meet standards. Several issues to consider when building a polysilicon project 1. Process selection and product design 1) The industrial production of polysilicon worldwide began in the late 1950s and early 1960s; after half a century of research, two main processes have been developed to date: one is the SiHCl3 hydrogen reduction method (also known as the Siemens process) ； The second is the SiH4 thermal decomposition method. Currently, about 77% of the world’s polysilicon produced by the Siemens process accounts for the total output. Well-known companies that use this process include Hemlok in the United States, Wacker in Germany, and several companies in Japan. The only companies that produce polysilicon using the thermal decomposition of silanes are Asimi in Norway, belonging to REC, and the Pasadena plant of MEMC in the United States; the former produces rod-shaped polysilicon while the latter produces granular polysilicon. Their combined production accounts for about 23% of the global total. 2) Comparison of the two processes mentioned above: In the 1990s, industry professionals in the U.S. silicon sector evaluated and scored the methods of producing polycrystalline silicon using four raw materials—SiH4, SiH2Cl2, SiHCl3, and SiCl4—based on 12 different criteria. The results were as follows:

| Criterion | SiH4 | SiH2Cl2 | SiHCl3 | SiCl4 |
|--------------------|------|---------|--------|-------|
| 1. Raw material purification | 8 | 8 | 7 | 10 |
| 2. Cost of raw materials | 4 | 6 | 7 | 7 |
| 3. Safety of raw materials | 4 | 3 | 9 | 10 |
| 4. Availability of raw materials | 0 | 0 | 10 | 10 |
| 5. Transportation of raw materials | 2 | 2 | 8 | 9 |
| 6. Storage of raw materials | 4 | 2 | 9 | 9 |
| 7. Recovery of by-products | 2 | 3 | 9 | 10 |
| 8. Reuse of by-products | 5 | 5 | 8 | 9 |
| 9. Deposition rate | 3 | 5 | 7 | 4 |
| 10. Requirements for buildings | 5 | 5 | 9 | 10 |
| 11. Selection of reaction furnaces | 6 | 3 | 9 | 9 |
| 12. Electricity consumption | 6 | 5 | 4 | 1 |

Total score: 49, 47, 96, 98

3) The modified Siemens process is the preferred choice for producing polycrystalline silicon. The reason for this is that it is a well-established process with relatively high safety levels, and the purity of the product produced meets the requirements of the microelectronics industry ； Nearly 80% of the polysilicon produced globally is made using the modified Siemens process ； New polysilicon projects around the world also choose the Siemens process ； All the polysilicon projects under construction or newly built in our country use the modified Siemens process ； 4) Product solution: It is advisable to include three specifications in the product solution selection: namely, FZ monocrystalline silicon material, accounting for approximately 1% of the total output ； CZ monocrystalline silicon materials account for about 20% of the total ； Solar-grade polysilicon accounts for 79% of the total. This is considered both from the perspective of the factory’s construction standards and from the standpoint of risk resistance. 2. Construction period and investment estimate 1) Construction period: Based on experience, it takes approximately 24–28 months for a large-scale polysilicon production project to go from the initial planning stage to operation. Of this time: (1) The project planning phase – which includes assembling a team, preparing feasibility studies, selecting a design firm and carrying out design discussions, choosing a site, ensuring the necessary construction conditions are in place, selecting a construction contractor, and starting the process of obtaining quotes for equipment – takes about 4 months ； (2) Project construction period: (civil engineering construction for each sub-project) 10–12 months ； (3) Equipment installation period: (installation of process pipelines and equipment) 6–8 months ； (4) Process commissioning period: 4 months. 2) Investment estimate: Based on the actual experience of polysilicon projects that have been completed or are under construction in China, the average investment per ton of polysilicon is approximately between 700,000 and 1,000,000 yuan. 3. Comprehensive utilization and environmental protection 1) Comprehensive utilization: Comprehensive utilization refers to the processing of by-products generated in the manufacturing process, such as HCl, SiCl4, and SiHCl3 that is no longer useful in the process, into other industrial products. The goal is to reduce material consumption, save resources, and contribute to environmental protection. To this end, many manufacturers at home and abroad have made strenuous efforts over two to three decades, developing a set of effective methods. They classify the recycling of recovered exhaust gases into three types: The first type involves polysilicon manufacturers using purchased TCS as raw material to produce polysilicon; the TCS contained in the exhaust gases is retained for internal use, while HCl and STC are sold externally. This is suitable for polysilicon plants with an annual production of around 200 tons, as the export volume of HCl and STC is very limited. The second type: Polysilicon manufacturers use purchased TCS as raw material to produce polysilicon. They retain the TCS and STC recovered from the exhaust gases for internal use, exporting only HCl. The remaining STC must be converted back into TCS through hydrogenation before it can be used in polysilicon production. This is suitable for medium-sized polysilicon plants, namely those with a production capacity of less than 1,000 tons. The third type is a closed-loop production system: the polysilicon plant uses purchased MG-Si and HCl as raw materials to produce TCS for its own use on-site; the recovered STC is converted back into TCS, and the recovered HCl is used again together with MG-Si to synthesize TCS. This is suitable for building large polysilicon plants with an annual production capacity of over 3,000 tons. This process is a production method that is widely adopted today; it fully meets the requirements of reducing material consumption and costs, conserving resources, and being environmentally friendly. However, establishing such a production process requires substantial capital as well as advanced technology. There are not many manufacturers that possess this technology at present, which is one of the reasons why the scale of polysilicon production cannot be expanded rapidly. Regardless of the type of plant construction model we adopt, even in a closed-loop system, there will be high and low boiling points substances with high impurity levels as well as a small amount of exhaust gas that need to be removed; thus, the issue of comprehensive utilization still exists. For example, we recycle the waste HCl gas to produce primary hydrochloric acid for use internally or for export, to be used in cleaning equipment or facilities. We use discarded TCS and STC with high impurity content to produce silicone, silicone resins, silicone rubbers, and similar products. The comprehensive utilization of by-products from polysilicon plants is a complex issue that requires further in-depth research. Any polysilicon plant with an annual production capacity of 5,000 tons or more should establish corresponding research institutions dedicated to studying the comprehensive utilization of these by-products. This is not only necessary for enterprises to save energy, reduce consumption, and cut costs, but it also represents a responsibility of such enterprises toward society and environmental protection. 2) Environmental protection: As discussed above, the production process of polysilicon is essentially a chemical process. In addition to raw materials such as liquid chlorine and industrial silicon, intermediate products include TCS and STC. Moreover, acids such as nitric acid, hydrofluoric acid, and sulfuric acid are used during the corrosion cleaning process. Additionally, when breaking down and processing silicon materials, silicon slag and silicon powder are generated. From an environmental protection perspective, we cannot allow any waste to cause harm to our surrounding environment. To this end, environmental protection in polysilicon plants is extremely important. The quality of environmental protection facilities determines the success or failure of a project. Generally speaking, the investment in environmental protection facilities accounts for about 15% of the total investment. 4. Thermal balance and material balance 1) Regarding thermal balance and the utilization of waste heat, polysilicon production is an energy-intensive industry, and the vast majority of the electrical energy consumed is converted into heat energy. In particular, the electrical energy consumed by the reduction furnace accounts for about 60% of the total electricity consumption; therefore, when designing and constructing polysilicon plants, it is essential to carefully consider issues related to heat generation, transfer, and reuse. The heat transfer oil cooling technology for reduction furnaces, which has been successfully developed in China, effectively addresses the issue of utilizing waste heat during the reduction process, and is worth promoting. Based on actual operational experience, 90% of the waste heat from the reduction process is reused, which significantly reduces power consumption per unit of output; it also reduces the overall water usage in the plant by approximately 60%. 2) Material balance: When designing and constructing polysilicon plants, it is crucial to carefully calculate the amount of materials used and consumed in each process; this is essential to ensure the plant operates at low costs and with high efficiency. Whether it is thermal balance or material balance, both are strengths for any chemical engineering design firm. I emphasize its importance here to remind designers that they must conduct careful calculations and must not be careless. 5. Closed-loop process: Whether considering energy savings and cost reduction, or environmental protection, the closed-loop polysilicon production process is the necessary approach for building large-scale polysilicon plants. The closed-loop production method described by American scientist William.C.O’Mara in his 1990 book \"Handbook of Semiconductor Silicon Technology\" is one in which by-products are vertically integrated, meaning that by-products such as H2 and Cl2 are recycled and reused through a closed loop. This design involves reusing the second stage to integrate STC for the production of new TCS, while also using the by-product HCl along with MG-Si in an FBR furnace to produce both TCS and STC. This additional step is clearly intended to reduce the manufacturing costs of large-scale polysilicon plants, by recycling the by-products of H2 and Cl2 in a closed-loop system to produce polysilicon. In their vision, such a factory would purchase MG-Si and liquid chlorine, while the only product it would sell is polysilicon. This achieves a true closed-loop cycle. The aforementioned dry recovery technology for reduced exhaust gases and the SiCl4 hydrogenation technology provide an important foundation for achieving a true closed-loop system. We know that the production of polysilicon starts with MG-Si of very low purity, while the final product is high-purity silicon with a high degree of purity. Although hydrogen and chlorine are introduced for purification purposes, these gases are not removed from the final product. If hydrogen and chlorine are kept in continuous circulation within the manufacturing process, this clearly helps to reduce raw material consumption and costs, while also minimizing environmental pollution. Although many polysilicon manufacturers at home and abroad have not yet fully adopted this production process, it is the path that all similar factories will inevitably have to take in the future. Energy consumption and environmental issues in polysilicon production 1. Regarding energy consumption: The industrial production of polysilicon products has a history of over 50 years. Initially, due to small scale and outdated manufacturing processes, the electricity consumption per kilogram of polysilicon produced in China in the last century was above 500 kWh/kg. Before 2000, the specific electricity consumption for polysilicon reported abroad was approximately 240 kWh/kg. Currently, there are reports that the best efficiency in foreign countries for direct power consumption in this process has reached below 100 kWh/kg, while in China the corresponding level is around 200 kWh/kg. With the expansion of scale, technological advancements, and improvements in manufacturing processes, it is entirely possible for the specific electricity consumption per kilogram of polysilicon to reach 150 kWh/kg. It should be noted that: (1) The so-called fact that polysilicon is an energy-intensive industry means that it represents the stage in the photovoltaic industry chain that consumes the most energy. ⑵The claim that polysilicon is highly energy-intensive was put forward by the project developers in order to obtain preferential electricity rates from ** or the local authorities where the project is built (in fact, most of the materials required for processing raw materials from mines into pure substances are highly energy-intensive). ⑶Polysilicon is a fundamental raw material for the electronics and photovoltaic industries; it can be considered a strategic material as well. Its utility, power consumption efficiency, and the multiplier effect it has on subsequent products give it advantages over other materials (it takes 23,000 kWh of electricity to produce aluminum from alumina through electrolytic processes). ⑷Based on the production of 1 MW of batteries using 10 tons of polysilicon, the energy payback period for silicon-based photovoltaic cells is within 2.5 years, and the energy recovery ratio under normal conditions is above 10; therefore, it cannot be said that such cells are not cost-effective in terms of energy consumption. ⑸In the long term, solar energy will undoubtedly play a key role as the energy source of the future. The development of the solar photovoltaic industry represents an irreversible trend toward solar energy occupying a dominant position in the future energy landscape. 2. Regarding environmental protection issues, we know that a large amount of waste liquid and waste gas are generated during polysilicon production. The main waste liquid is silicon tetrachloride (SiCl4), while the main waste gas is hydrogen chloride (HCl). For these two substances, there are no issues either in terms of recycling or disposal; the key point is that sufficient attention must be paid to them. 1) In terms of recycling, the dry recycling technology developed by CDI is capable of recovering all the exhaust gases generated during reduction and other processes and separating them individually, which creates highly favorable conditions for their subsequent separate utilization ； 2) In terms of its behavior, SiCl4 decomposes very easily when it comes into contact with water to form SIO2 and HCl; it is essentially an acidic aqueous solution of SIO2. It becomes harmless as long as it is neutralized with an alkaline substance such as lime milk. 3) From the perspective of comprehensive utilization, SICl4 already has methods and technologies for growing synthetic quartz, silica aerogel, and being hydrogenated to SIHCl3 for reuse. In summary, all waste gases and liquids generated in polysilicon production can be recycled and effectively treated; there are no technical or practical issues whatsoever. The key is that we must take this seriously. Some personal views: 1) Polysilicon is the most important and fundamental functional material in the electronics industry and the photovoltaic sector; it can also be considered an **important strategic material that is essential rather than dispensable ； 2) The development of polysilicon cannot focus solely on the needs of the photovoltaic industry; at present, most of the high-quality polysilicon required by China’s microelectronics and power electronics industries is imported. This is indeed a sad situation for a country with an industrial history of over 40 years. 3) Silicon-based photovoltaic cells will not be replaced by other photovoltaic materials in the next 30 to 50 years, so the development of the polysilicon industry will enjoy a substantial market opportunity for a long time ； 4) The claim that it is not cost-effective to use polysilicon to manufacture photovoltaic cells is unfounded ； 5) The idea of using our electrical energy to produce photovoltaic products in order to provide clean energy to other countries is rather narrow-minded; if you don’t use clean energy, does that mean others aren’t allowed to use it either? Moreover, it is all done to protect our shared Earth; the photovoltaic industry is a remarkable sector that benefits all of humanity ； 6) In recent years, the polysilicon industry in our country has developed rapidly, which is a fortunate development. But in the next three years, our total volume still won’t be able to exceed that of developed countries** ； It is normal even if we surpass developed countries; our steel industry can exceed theirs, as can our non-ferrous metals industry, our home appliance industry, and our photovoltaic products industry. So why can’t our polysilicon industry do the same? 7) In recent years, there have been many new polysilicon projects launched in our country, and **comprehensive consideration is indeed necessary. There are indeed issues of high energy consumption and environmental pollution in polysilicon production, but these are not insurmountable problems. I personally believe that within the next 2 to 3 years, our polysilicon production level will surely approach or reach international standards. Problems that arise while moving forward should be solved as we move forward; there should be no constant blaming, nor should restrictions be imposed. I think no one would stop train services or close airports just because there have been safety incidents involving trains or planes ； 8) Existing and upcoming polysilicon manufacturers must invest the necessary funds and human resources in technological advancement, innovation, and environmental protection, and pay sufficient attention to safety and environmental issues in their daily operations. For many years before 2000, polysilicon manufacturers operated with minimal profits or even lost money; now that they have more resources, they are willing to invest in the development of new technologies and in environmental protection efforts. Let’s soar together with global and Chinese photovoltaic enterprises! The above is a piece of information I came across; I hope it will be useful to everyone... This post was last edited by LHY8771 on 2009-4-1 at 15:16.]