TABLE D-1 Economic Factors for an Annual Production of 1,000 Tons of Polysilicon in China

Items

Economic costs/benefits

Yield of polysilicon

1,000 tons per annum

Total investment

U.S.$ 0.17 billion

Production cost

U.S.$ 70–80/kg (China)

U.S.$ 25/kg (Europe and U.S.)

Power consumption

0.6 billion kWH per annum

Power produced by solar cells

200 MW per annum

Power regeneration ratio (over 20 years)

About 8

Power generation cost

7–12 times biomass power generation, 6–10 times wind power generation, 11–18 times traditional coal power generation.

By-product SiCl4

8,000 tons per annum

In the above steps, there are two pollution problems that need to be solved: the capture of by-product CO2 in the preparation of industrial silicon, and the recycling of the tail gas in the purification of SiHCl3 from the gas mixture.

Reduction of SiHCl3

High purity polysilicon is obtained from the reduction of SiHCl3. The reactions are as follows.

  1. Main reaction: SiHCl3 + H2 = Si + 3HCl,

  2. Secondary reaction: 4SiHCl3 = Si + 3SiCl4 + 2H2.

Si obtained from reactions (1) and (2) is deposited to form high purity polysilicon. In this process, about 25 percent of trichlorosilane is converted into polysilicon, and the remainder into tail gas. A large amount of tail gas containing many useful materials such as H2, HCl, SiHCl3, and SiCl4 is emitted from the reduction furnace. Moreover, the tail gas includes many erosive and toxic substances. The tail gas must be recycled, or else it will cause serious pollution and also increase costs.

Silicon tetrachloride (SiCl4) is the main component in the tail gas. It is a highly corrosive and toxic liquid. About 8,000 tons of by-product SiCl4 are produced in the production of 1,000 tons of polysilicon. Due to the expensive treatment cost of SiCl4, most Chinese companies do not have such treatment equipments, which then makes tail gas treatment become the bottleneck in the manufacture of polysilicon.



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