Greenhouse Gas Emissions from Silicon Production -Development of Carbon Footprint with Changing Energy Systems

As the world pushes for a low-carbon future, populous regions of the world develop rapidly, increasing the global demand for materials. Emissions from the primary production of metals can be split into three parts; a) direct process related emissions, b) emissions due to auxiliary processes and c) indirect emissions due the production of the electrical energy used in the production process. As production of metallurgical grade (MG) silicon is energy intensive, the indirect emissions from the electrical energy production contribute a significant part of the overall specific carbon footprint per kg of silicon produced. In this paper, the trend in the energy use for MG silicon production on a global basis from 1995 is tracked, and the development of the carbon footprint from the energy mix is estimated. Silicon production increased by 240% from 2000 to 2019, and by 456% from 1995. As most of this increase in production has occurred in Asia, and more specifically China, the energy mix for the production site determines the indirect carbon dioxide emissions. The ratio of low carbon electric power sources such as hydro power or nuclear energy is still significant in the energy mix for silicon production. It was around 57% in 2019 as compared to 67% in 2000 and 71% in 1995, while the share of coal thermal power has increased from 20% in 1995 to 40% in 2019. This increased share of coal-based power has led to an increase in the indirect energy related carbon emissions to 6.3 kg CO2e/kg Si in 2019, up from 3.3 in 1995, and is now similar to the direct process emissions that are around 4 kg CO2e/kg Si. That is indeed a development in the wrong direction for the silicon industry, facing increasing climate related pressures.