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Phasing out fossil coal: the bio-coal production in the steel industry

The “sustainable transition” of our economic system is one of the urgent challenges of our times. To this end, a quick exit from fossil fuels consumption towards carbon neutral solutions is needed at a global scale. One of the key most urgent objectives of the EU is to phase out fossil coal, still largely used by the EU power and steel industry. In 2022, the EU consumed 454 million tons of coal, of which 160 million tonnes of hard coal, and 294 of brown coal. According to the EU Commission, in 2021, about 52% of the hard coal and about 92 % of brown coal were used for power production. The rest was consumed by steel and other non-ferrous metal production industries.

The European steel industry represents a major environmental concern, responsible for nearly 190 million tonnes of CO2, equal to about 5 % of the EU greenhouse gas (GHG) emissions [1]. If the coal consumption for power generation can be reduced by replacing the fuel with multiple renewable energy sources (biomass, biomethane, wind power, hydroelectric and photovoltaic energy), much more difficult is to decarbonize the steel sector, where coal is used as a source of carbon. In this context, Biocoal could represent a green, sustainable source of carbon, an alternative to fossil coal for the steel sector.

The role of coal in the EU steel and ironmaking sector

Steel is produced by two routes: (1) blast furnace–basic oxygen furnace (BF–BOF) and (2) electric arc furnace (EAFs). As an estimation, 56.7 Mt/y of coal is consumed by the EU the steel sector, with the traditional BF-BOF route, is characterized by the more intensive coal consumption.

On average, based on literature and statistical values, the traditional route consumes around 725 kg of coal per ton of hot metal produced, while the EAF route consumes much less, about 2-12 kg per t of hot metal.

In the BF process system (Figure 1), four steps are included: sintering, coking, blast furnace (BF), and basic oxygen furnace (BOF).

  • The sintering process agglomerates iron ore in compacted, porous granules to be charged in the BF. Coal, or coke with very low volatile content and high calorific value is needed by the process.
  • The coking process is adopted to turn coal (or coking coal) into a less volatile, more resistant material, named coke. The process takes place in coke ovens, usually installed within the ironmaking site. Coking coal presents also high volatile content, but low ash and must present softening properties at coking temperatures.
  • In the Blast Furnaces, the sintered iron ore and the coke are charged from the top, while hot air blast and pulverised coal (PCI) are injected from the bottom. Reducing gases (CO) are produced from pulverized coal and coke, which react with iron ore to produce hot metal. Coal with relatively high volatile content can be used as PCI.
Figure 1. Coal mass flow in BF-BOF process route

The EAF (Figure 2) route is based on metal melting, with no iron ore reduction reactions to take place. The process is characterized using mainly solid materials, like scrap, or direct reduced iron (DRI), hot briquetted iron (HBI), or pig iron, which are melted in the furnace. The charged material melts at around 1600°C by a mix of electric and chemical energy. Chemical energy is supplied by coal and natural gas. Up to 12 kg of coal are used per ton of steel produced in the EAF. Coal used in EAF must present reduced volatility (e.g. < 20%), with ash composition to be monitored, in order to avoid affecting the stability and the pH of the slag. All coals, in general, must present low concentration of S (< 1%), P (e.g. < 0.2%), and alkali elements, such as K, and Na.

Figure 2. Scheme and mass flow of an Electric Arc Furnace.

Can biocoal replace fossil coal in the EU steel sector?

Biocoal can be defined as a coal-like material obtained by thermochemical conversion of any biobased feedstock. As for the fossil coals, biocoal can be classified based on its H/C and O/C content (Figure 3.) as bio-anthracite, bio-coal, bio-lignite.

Figure 3. Van Krevelen Diagram [2].

Theoretically, biocoal can replace fossil coal in most of its ironmaking application. The only process where biocoal can’t fully replace fossil coal is the coking process. The reason stays in the fact that biocoal is similar to inertinite, so it does not undergo to softening or deformation during the coking process, needed for coke formation. In this case, only partial coal replacement is feasible. In all other processes the use of biocoal (and torrefied biomass, so biolignite) as biobased reductant in Blast Furnaces is already proven and the technical feasibility of using biocoal in EAF has been already demonstrated by multiple industrial tests.

So, the question is: why biocoal is not yet fully replacing fossil coal in the steel sector?

First, the materials are different. Despite a similar physical and chemical composition biocoal usually presents specific and variable density, volatility, porosity, and grinding index, that make it different than fossil coal. This different can be a marginal issue when the coal replacement rate is reduced to small percentages, but modifications to the injection systems, to the storage system, as well as to the reactants mixture must be considered for a full coal replacement.

Second, the economic aspect. Although biocoal contribution to the decarbonisation of steel industry is well known, many studies identified that there is a lack of economic competitiveness of using biocoal to replace coal. According to previous publications, wood-based biocoal obtained by slow pyrolysis could cost in a range of 400 €/t, thus a high CO2 price will be needed to make it competitive with fossil coal. High quality carbonised and devolatilized wood based biocoal is, today, in most countries, too expensive to compete with fossil carbon sources.

Third, the availability. The increasing consumption of wood-based biocoal by the steel sector will direct impact on woody biomass consumption and imports. Replacing 56Mt of fossil coal per year would need approximately 200 Mt of dry biomass per year, about 10 times the present EU pellet market. A so high demand could generate risks of deforestation in less monitored non-European forests.

The solution to the economic barrier is given by a key instrument: the European Emission Trading System (EU ETS) which puts a price on CO2 emissions. ETS works with allowances, with one allowance that gives the right to emit 1 ton of CO2eq (carbon dioxide equivalent).  Since 2005, the EU ETS has helped bring down emissions from power and industry plants by 37% [3]. With the EU ETS, the price per one ton of CO2 exceeding 70 €/t [4], fossil carbon sources are becoming expensive, favouring the growth of an EU “renewable carbon economy” based on the valorisation of renewable sources into carbon neutral, sustainable alternatives to fossil coal. The solution to the availability issue is found in two key innovations: The first is the gradual replacement of the traditional BF-BOF system with the combination of DRI and EAF, where the consumption of fossil coal, and thus of the biocoal to replace it, is much less. The second is the production of biocoal from agro-residues, as well as wet biowaste streams continuously produced in Europe. Materials like sewage sludge, urban biowaste, digestate from biomethane production, etc, represent a huge reserve of biobased carbon, which can be turned into biocoal suitable to replace fossil coals in most steelmaking applications.

The combination of innovative, more efficient steelmaking processes and new advanced solutions to produce biocoal from daily produced biowaste might push the EU steel sector towards its decarbonisation, even quicker than expected.

 

References

  1. European Commission, “Coal production and consumption statistics.”
  2. Somers, “Technologies to decarbonise the EU steel industry,” Luxemburg, 2022.
  3. Salimbeni, G. Lombardi, A. M. Rizzo, and D. Chiaramonti, “Techno-Economic feasibility of integrating biomass slow pyrolysis in an EAF steelmaking site: A case study,” Appl Energy, vol. 339, p. 120991, Jun. 2023, doi: 10.1016/j.apenergy.2023.120991.
  4. Reichel, “Increasing the sustainability of the steel production in the electric arc furnace by substituting fossil coal with biochar,” 4th Central European Biomass, 2014.
  5. Echterhof, “Review on the Use of Alternative Carbon Sources in EAF Steelmaking,” Metals (Basel), vol. 11, no. 2, p. 222, Jan. 2021, doi: 10.3390/met11020222.
  6. W. van Krevelen, Coal – Typology, physics, chemistry, constitution, 3rd ed. Amstedam, New York: Elsevier, 1993. Accessed: Apr. 25, 2023. [Online]. Available: https://openlibrary.org/books/OL1147163M/Coal–typology_physics_chemistry_constitution
  7. Sundqvist Ökvist and M. Lundgren, “Experiences of Bio-Coal Applications in the Blast Furnace Process—Opportunities and Limitations,” Minerals, vol. 11, no. 8, p. 863, Aug. 2021, doi: 10.3390/min11080863.
  8. Mandova et al., “Possibilities for CO2 emission reduction using biomass in European integrated steel plants,” Biomass Bioenergy, vol. 115, no. May, pp. 231–243, 2018, doi: 10.1016/j.biombioe.2018.04.021.
  9. Mousa, C. Wang, J. Riesbeck, and M. Larsson, “Biomass applications in iron and steel industry: An overview of challenges and opportunities,” Renewable and Sustainable Energy Reviews, vol. 65, pp. 1247–1266, 2016, doi: 10.1016/j.rser.2016.07.061.
  10. Suopajärvi et al., “Use of biomass in integrated steelmaking – Status quo, future needs and comparison to other low-CO2 steel production technologies,” Applied Energy, vol. 213. Elsevier Ltd, pp. 384–407, Mar. 2018. doi: 10.1016/j.apenergy.2018.01.060.
  11. EUROPEAN COMMISSION, “What is the EU ETS?,” https://climate.ec.europa.eu/eu-action/eu-emissions-trading-system-eu-ets/what-eu-ets_en#eu-ets-legislative-framework.
  12. Trasing economics, “EU Carbon Permits,” https://tradingeconomics.com/commodity/carbon.

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