Measurement and Evaluation of Tapping Gas Energy from the Silicon Furnace

Abstract

A characteristic of submerged arc furnace (SAF) silicon production is the intermediate process gas consisting of silicon monoxide (SiO) and carbon monoxide (CO). While most of this gas ascends though the charge, reacts with carbon materials and eventually combusts over the charge burden, a portion of the gas can leave through the taphole. Combustion of process gas from the taphole can be an environmental challenge and contributes to large amounts of heat. It has been proposed that very little SiO gas is consumed on its way from the crater zone of the furnace to the taphole.

To investigate the composition of taphole gassing and the mechanisms described above, a measurement campaign was conducted at Elkem Thamshavn. Over a 3-day period the gas extracted from the tapping area was analysed with Agilent Micro-GC, Protea AtmosFIR, and Testo 350 FTIR and the dust concentration in the gas was measured with a NEO LaserDust instrument. Existing temperature and flow measurements were used to calculate mass flows and an energy balance for the system. Assuming close to equilibrium conditions in the furnace crater zone, the ratio between SiO and CO in the tapping gas was used to estimate the temperature in the crater zone.

The tapping gas energy was measured to be in the range of 0.6 up to peaks of 3 MW. Energy from combustion of CO and SiO gas varied greatly with periods of almost no energy added up to periods of 1.8 MW added from combustion. A substantial amount of radiant and convective heating from warm surfaces was also found to contribute to the energy of the tapping gas. Estimations of the partial pressure of SiO in the crater zone (S-factor), based on the CO2 to SiO2 fume ratio in the tapping gas, gave an average of 0.43. Calculations of crater temperature from the S-factor was found to be on average 2046 °C with a 95 % confidence interval of 1890 to 2200 °C.

Keywords: Silicon production, Tapping gas, Silica Fume