By Samantha Tanzer [1] and Kornelis Blok

If the world wants to keep global temperature increase below 1.5 to 2 °C compared to pre-industrial levels, we need not only to rapidly reduce emissions but also  remove a lot of CO2 from the atmosphere in the course of this century. This is probably one of the biggest climate action challenges we are facing. Carbon dioxide removal is often indicated with the term negative emissions. Negative emissions can be achieved in many different ways, e.g. through reforestation, or changes in agricultural practices that lead to enhanced storage of carbon in the soil.

However, there is wide agreement that on top of increased carbon storage in the biosphere, we also need technological solutions. The option that has received most attention so far is bio-energy with carbon capture and storage (BECCS). The principle is straightforward: when trees or other crops grow, they remove carbon dioxide from the atmosphere. In energy conversion processes, the carbon turns to CO2 again. This CO2 is captured and stored in underground reservoirs. The overall effect in a well-designed BECCS system is a net removal of CO2 from the atmosphere. Most attention so far has been paid to to CO2 removal in the power sector. In a recent article, we examined how carbon dioxide removal could look for industrial process.

In a new article [2], we looked at one of the most important sectors: the iron and steel industry, responsible for over 5% of global CO2 emissions. We looked at a variety of steelmaking processes, starting from the currently most common blast furnace process, but also including alternative processes: smelt reduction and direct reduction. Without the use of bio-energy and carbon capture and storage, CO2 emissions would be in the range of 1.3 – 2.5 tonne per tonne of steel. Can the emissions turn negative? The answer is: yes. The application of bio-energy or CCS can each substantially reduce emissions, but the combination can lead to net-zero or net-negative emissions:

Emission from steel production with different combinations of bio-energy and CCS.  Blue = emissions, green = removals. The diamonds indicate net CO2 emissions: the balance of emissions and removals.

BBF = blast furnace, TGR = blast furnace with top gas recovery, HIS = smelt reduction (Hisarna), MID and ULC are direct reduction processes (Midrex and Ulcored).

In the assessment, we assumed current average European carbon intensity of power production, approximately 400g CO2 per kWh. If the power sector would be fully decarbonized, the emissions would drop about another 0.5 tonne CO2 per tonne of steel, so all production routes can then turn clearly negative. Having said that, possible doesn’t mean easy: it would require a complete makeover of the steel industry. Most important is the use of charcoal. Charcoal is already used for steel making in Brazil, but that charcoal is produced in a relatively inefficient process. The more efficient Missouri process is at least a factor two more efficient – using that process would reduce emissions in the production chain by 0.2 tonne CO2 per tonne of steel. Upscaling an efficient charcoal production chain based on sustainable biomass production is probably the no. 1 challenge in the conversion to carbon negative steel.

Talking about sustainable biomass – what about the carbon debt? We also analysed that: taking that into account would eat up the equivalent of about 0.5 tonne CO2 per tonne of steel. Overall, the conclusion remains: with zero-carbon electricity production and efficient charcoal production, carbon negative steel is possible, even when taking into account the carbon debt.

But it will not be easy, and it will likely also not be cheap, but for the latter more investigation is necessary.

[1] Samantha Tanzer studied Industrial Ecology in Delft/Leiden and is now PhD researcher at Delft University of Technology.

[2] S.E. Tanzer, K. Blok, A. Ramírez: Can bioenergy with carbon capture and storage result in carbon negative steel?, International Journal of Greenhouse Gas Control, 100(2020)103104.