Operators from Elkem Thamshavn are shown tapping liquid silicon from the furnace. Photo: Vegar Andersen

Recirculating off-gas contributes to carbon capture

Producing silicon results in large carbon dioxide emissions, but recirculating it can remove contaminants more efficiently.

Every tonne of silicon we produce leads to emissions of around 5 tonnes of CO₂.

Carbon capture offers a possible solution to eliminate this emission, but the CO₂ concentration in the off-gas from the furnaces is often too low for CO₂ capture to be cost-effective.

One possible solution to reduce the cost is to increase the CO₂ concentration by recirculating the off-gas. The silicon manufacturer Elkem has collaborated with NTNU and SINTEF to test this idea in a pilot project.

Illustration showing how recirculating off-gas can be done for the silicon process.

Illustration showing how recirculating off-gas can be done in the processing of silicon. Illustration modified and taken from A. Schei, J. K. Tuset, and H. Tveit, Production of high silicon alloys. Trondheim: TAPIR, 1998

Ambitions for carbon neutral production

Silicon production is an important industry for Norway. Silicon is used in electronics, solar cells, the polymer silicone and as an alloying element in cast alloys. Silicon is produced using what is called carbothermal reduction of quartz. This process involves relatively large CO₂ emissions.

Elkem would like attain carbon-neutral metal production and is working on several initiatives to reduce their climate footprint. Energy recovery, increased use of biocarbon materials and improved material yield are all part of this effort.

Finding solutions related to the capture of CO₂ from the smelters is another challenge that researchers are tackling.

The CO₂ concentration in the off-gas is low, typically only a few per cent.

Elkem has conducted an initial carbon capture study supported by the CLIMIT programme. The company studied two different capture technologies in collaboration with Aker Carbon Capture and Saipem, respectively. Norsk Energi has contributed to assessing optimal solutions for energy supply and recovery.

Process gas burns above the raw materials in contact with air and recirculated off-gas.

A glimpse into the process. Process gas burns above the raw materials in contact with air and recirculated off-gas. Photo: Vegar Andersen

Testing known technology in a new field

One of the challenges with carbon capture from smelters is that the CO₂ concentration in the off-gas is low, typically only a few per cent, which makes establishing an integrated carbon capture plant more expensive and technically challenging.

As a possible measure to increase the CO₂ concentration in the off-gas, Elkem and researchers at NTNU and SINTEF are investigating off-gas recirculating as a possible way to improve the process.

The CO₂ concentration in the off-gas is increased by replacing some of the fresh air normally supplied to the furnace with recirculated off-gas that is cleaned of dust and cooled.

This is a well-known technology for NOx reduction in incinerators, but the method has not been tested for silicon melting furnaces.

Pilot experiment a broad collaboration

In order to assess the effects of recirculating CO2 on the furnace process, a pilot experiment to recirculate the off-gas was carried out in NTNU and SINTEF’s pilot laboratories at Gløshaugen in Trondheim.

A unique recirculating plant was built here around an existing 160 kW single-phase smelter. The furnace was set up with a number of instruments and analysis equipment to obtain as much information as possible on the effect of the recirculating experiment and to extract knowledge related to the impact on other products and emissions from the furnace.

The pilot experiment was carried out through a collaboration between Elkem projects Elkem CCS, Elkem Sinoco2 and the two NTNU/SINTEF-led centres FME HighEff and the NTNU Centre for Research Based Innovation, SFI Metal Production.

Different recirculating rates were tested and mapped through an 80-hour test run.

Elkem provided skilled operators from Elkem Thamshavn as well as raw materials, and SINTEF and NTNU oversaw control of the furnace and the measurement of off-gases.

A laboratory

Both the off-gas and the combustion air were analysed using two gas chromatographs, FTIR measurements and LaserGas II instruments from NEO Monitors AS. Photo: Vegar Andersen

Sharply increased CO₂ concentration

By increasing the proportion of recirculated gas, we achieved CO₂ concentrations of over 20 per cent and also observed lower NOx production.

A lot of data analysis remains to interpret the results, but the experiments show that gas recirculation has great potential for reducing NOx emissions and making CO₂ capture easier for the silicon production process.

The unique infrastructure that has been built up in Trondheim will also prove valuable for future trials and projects with the purpose of contributing to sustainable metal production in Norway and globally.

This blog was first published as #SINTEFblogg on 26 May 2021 (in Norwegian).