(134c) Replacing Coal Used in Steelmaking with Biocarbon from Hydrolysis Lignin -Techno-Economic Evaluation

Most of CO₂ emissions in steel production occur during the reduction of iron ore to hot metal through the use of coal and coke. Renewable carbon containing material from the forest sector is seen as an asset for reducing fossil emissions of steel industry. This bio-based carbon can be used as bio-reducing agents in the short term for providing rapid reduction of CO₂ emissions. New process concepts based on usage of renewables for energy and reduction can be implemented in the long run [1] [2]. However, raw biomass as such is unsuitable for applications in iron and steel making and thermochemical conversion into charcoal (biocarbon) is required. Slow pyrolysis is the most promising technology to upgrade biomass as a metallurgical reducing agent as indicated in a recent review [1].

The key focus of current study is to evaluate direct substitution of fossil based pulverized coal injection (PCI) with biocarbon from sustainable sources. One of tempting raw material is hydrolysis lignin, which is a side-product from bioethanol process (such as St1 Biofuels Cellunolix® bioethanol plant, in Kajaani in eastern Finland [3]). The amount of 2^nd generation bioethanol production utilising lignocellulosic side streams (such as saw dust, bagasse and corn stover) are increasing in the future. Accordingly, also the amounts of hydrolysis lignin will increase. In addition, the study focused on creating an industrial symbiosis between bio-based / forest industry and metals processing industry.

Methodology

A techno-economic assessment for replacing coal in steelmaking with biocarbon from hydrolysis lignin was conducted. The experimental proof and arrangements for bio-reducer production from hydrolysis lignin were carried out concurrently and are presented in detail in [4]. Thermochemical conversion process was fixed as slow pyrolysis and fossil coal was used as a reference material. Balas [5] and Wingems [6] software were used for process modelling. Techno-economic analysis is conducted in a Finnish context (location, prices, etc.).

Three optional scenarios were evaluated for biocarbon production: L1) a stand-alone mill processing moist (50%) hydrolysis lignin, L2) a stand-alone mill processing dried (10% moisture content) hydrolysis lignin, L3) an integrated process where lignin is processed in a plant integrated to a kraft pulp mill. Stand-alone production (L1 & L2) refers to the selected scenarios where the biocarbon pyrolysis unit is located on the site of a steel/iron plant on the north-west coast of Finland. Lignin is transported to the production site from eastern Finland. Pulp mill integrated biocarbon (L3) production refers to the selected scenario where the production of biocarbon takes place at the pulp and paper mill, utilizing integration possibilities. The location of this typical Nordic Kraft pulp and paper mill is on the west coast of Finland.

For all the scenarios mentioned hydrolysis lignin valuation is based on 20 €/MWh [7] and the biocarbon production volume is 44 200 tonnes per year, which would cover ca. 10 wt% of PCI coal injected to blast furnace (in Raahe SSAB steelworks).

Detailed information and data of the modelling work and techno-economic calculations can be found from [8].

Results

In the scenario (L1), lignin is pressed to 50% moisture content before transportation 200 km by rail to the biocarbon production site in close connection with the iron/steel plant. The biocarbon manufacturing process is not integrated with the steel plant itself, although the produced biocarbon is used on site to replace part of the PCI coal injected in the blast furnace. The steam (generated internally) is used in the drying of the lignin to 10% moisture content before the pyrolysis, and the excess steam is assumed to be used in electricity generation (efficiency set to 35% of the available thermal load). The pyrolysis gases are burned to provide steam. The power plant itself is not included in the scenario estimation. The estimate for biocarbon production cost is 405 €/tonne of biocarbon.

The second scenario (L2) resembles previous case but after pressing lignin is further dried to 10% moisture content before transportation to biocarbon plant. No further drying is needed at the biocarbon production site. The production cost estimate for scenario this scenario is somewhat lower, 380 €/tonne of biocarbon

In the third scenario (L3), biocarbon production is integrated into with a pulp and paper mill producing 800 000 air-dried tonnes of chemical pulp annually. The bioethanol plant is located next to the pulp mill. The lignin at 50 wt% moisture is available at site. Lignin is further dried to 10 wt% before pyrolysis. Excess hot pyrolysis gases are fed to the lime kiln of the pulp mill, thus replacing the original lime kiln fuel. In drying, it is assumed that 50% of the energy need is provided by excess hot water, assumed to be available at the pulp mill. Remaining 50% is provided by process steam; mainly from the pulp mill and partly from internal heat recovery. No excess steam from the biocarbon production is available. The biocarbon product is transported 160 km by rail to the iron/steel plant. The effect of biocarbon production on the energy balances of an integrated pulp and paper mill are: i) annual net electricity production of the integrate is reduced by 3 GWh, and ii) the amount of available heat for district heating is reduced by ~50 GWh per year. These reductions are due to the increased demand for process steam within the biocarbon production processes. To compensate these losses, it is assumed that additional biomass can be burned in the multi-fuel boiler. The production cost estimate for scenario L3 is 275 €/tonne of biocarbon, taking also into account the use of net pyrolysis gases as a fuel for the lime kiln (fuel savings) and losses due to the increased steam demand from pulp and paper mill (losses in electricity and district heat generation).

The data from the pulp and paper mill considered is not based on exact operational data.

More detailed evaluation of the results can be found from [8].

Summary

Raw material costs (hydrolysis lignin) dominate the production costs. Effect of total energy (electricity, district heating, process electricity consumption, fuel savings, and sold electricity) is positive (reducing the costs) on total costs for three scenarios. The scenario with pulp mill integration benefits due to savings in original fuels at the lime kiln. Logistic costs create variation due to the different transport distances and weights of the transported material (as raw material or as biocarbon). Variations in capital expenditure estimates are small.

The economic feasibility will depend on both emission trading and integration of the excess energy from pyrolysis and possibly available low-grade heat at the biocarbon production site. The techno-economic analysis on the conceptual level indicates that biocarbon made of hydrolysis lignin at an integrated pulp and paper mill site will provide the potential option for bio-based carbon production within the forest to metal value chain. Hydrolysis lignin could provide a significant biocarbon source for PCI injection, in addition to other bio-based side-streams (e.g. bark, wood/forest chips and sawdust). The cost analysis coupled with the CO₂ emission trading scheme indicates that with emissions prices reaching 25 €/tonne (EUA price met in the year 2018 [9]), biocarbon replacement based on lignin is approaching to be economically viable if the PCI coal price exceeds 200 €/tonne. Further, if the lignin price is reduced to 15 €/MWh, the viability limit would decrease to the level of 150 €/tonne of PCI.

References

[1] Suopajärvi, H., Umeki, K., Mousa, E., Hedayati, A., Romar, H., Kemppainen, A., Wang, C., Phounglamcheik, A., Tuomikoski, S., Norberg, N., Andefors, A., Öhman, M., Lassi, U. and Fabritius, T. (2018) ‘Use of biomass in integrated steelmaking – Status quo, future needs and comparison to other low-CO2 steel production technologies’ Applied Energy, 213, pp. 384–407. doi: 10.1016/j.apenergy.2018.01.060.

[2] Ng, K. W., Giroux, L. and Todoschuk, T. (2018) ‘Value-in-use of biocarbon fuel for direct injection in blast furnace ironmaking’, Ironmaking and Steelmaking. doi: 10.1080/03019233.2018.1457837.

[3] Yamamoto, M. (2018) ‘St1 Cellunolix® process – Lignocellulosic bioethanol production and value chain upgrading’, Bio4Fuels Days, October 12th 2018, Oslo

[4] Toloue Farrokh, N., Sulasalmi, P. and Fabritius T. (2019) 'Added Value for Forest Industry for Metals Producing and Processing Integrates (FOR&MET) – Project Report of University of Oulu'. Available at: http://jultika.oulu.fi/Record/isbn978-952-62-2207-3

[5] VTT (2013) 'Balas® Process Simulation Software'. Available at: http://balas.vtt.fi/

[6] Valmet (2016) ‘Wingems’. Available at: http://www.valmet.com/products/automation/solutions-for-pulp-and-paper/a...

[7] Statistics Finland (2018b) ‘001 -- Kotimaisten polttoaineiden käyttäjähinnat energiantuotannossa (ei sis. alv:a)’. In English: National fuel prices for energy generation purposes (VAT excluded). Available only in Finnish. StatFin Database. Updated September 12th, 2018. Available at: https://www.stat.fi/tup/statfin/index.html

[8] Hakala, J., Kangas, P., Penttilä, K., Alarotu, M., Björnström, M., & Koukkari, P. (2019). 'Replacing Coal used in Steelmaking with Biocarbon from Forest Industry Side Streams'. VTT Technology No. 351. VTT Technical Research Centre of Finland. https://doi.org/10.32040/2242-122X.2019.T351

[9] Sandbag (2018) ‘Carbon price viewer’. Raw data from ICE via Quandl. Available at: https://sandbag.org.uk/carbon-price-viewer/