Replacing Coal used in Steelmaking with Biocarbon from Forest Industry Side Streams

Steelmaking is a highly carbon-intensive process due to its extensive use of coal as both an energy source and a reductant. The CO₂ release of Finnish steel production plants exceeds 5 million tonnes/a, accounting for nearly 10% of Finland's CO₂ impact. Approximately 80% of these emissions originate from the blast furnace ironmaking process, the production of which currently exceeds 2 million tonnes/a of raw steel. Blast furnaces will remain the major steel production route in Finland for the foreseeable future.

In the last 10–15 years a remarkable research and development effort has been focused within the steel industry on improving the performance and energy efficiency of the blast furnace. While significant advances have been made, possibilities to gain further decreases in fuel or reductant consumption are limited due to the high level of maturity and efficiency of the current furnace technology. Thus, new technologies are needed to further reduce greenhouse gas (GHG) emissions associated with steel production. For this, the substitution of fossil carbon with biocarbon has been proposed. The first option is to use biocarbon as a component of the pulverized coal injection (PCI), which takes place through tuyeres in the lower part of the blast furnace.

In this research, two kinds of side streams from the forest industry were tested and evaluated as raw materials for PCI biocarbon. Softwood bark from biorefineries (e.g. in the form of black pellets from steam explosion) or residual hydrolysis lignin from sawdust-based bioethanol production were used. Both raw materials were treated with slow pyrolysis and the physical, chemical and rheological properties of the biochar thus produced were compared with those of typical injection coal. Both materials appear to be viable PCI substitutes.

Various scenarios for integrating pyrolysis to either the forest industry or steel industry site were examined and techno-economic analysis of the use of biocarbon as a substitute for fossil PCI coal was carried out for each case. It is inferred that there will be sufficient sources of both bark from biorefineries and hydrolysis lignin in Finland in the near future to replace substantial fractions of PCI coal. Use of biochar will decrease GHG emissions; however, the extent is dependent on the operational environment and the power source used to compensate the respective biomass-based electricity and district heat production. The economy of biochar usage as a PCI substitute is becoming more viable with rising CO₂ emission costs.