The CO2 mitigation potential, technical and economic viability of biomass char utilization in blast furnaces

This thesis addresses the issues of climate change mitigation and reduction of carbon emissions within the iron- and steel-making industry, which is responsible worldwide for 5 % of total greenhouse emissions. The problem in mainstream hot metal production is that the principal reductants are coke and coal, which come from fossil endowments. This generates the major portion of the carbon emissions from blast furnaces. In this respect, the re-incorporation of biomass as fuel/reductant in ironmaking may significantly help to reduce the carbon footprint of the industry. The goal of this study is to investigate such a hybrid solution to the problem of carbon emissions in coke-based blast furnaces. There are fundamental barriers to the wide diffusion of ironmaking based 100 % on charcoal replacement of coke. The most significant of these include the large land areas needed for plantation growing the biomass required, and elevated processing cost. Nevertheless, the partial injection of biomass char fines into blast furnaces as auxiliary fuel, which in this thesis I call Bio-PCI, appears to be a favorable, technically feasible and economically viable option to mitigate part of the CO2 emissions in iron-making. This hybrid solution has been used at industrial scale in small blast furnaces in Brazil for several years. Based on process simulation, results have been generated that indicate the technical and economic feasibility of Bio-PCI. In this Thesis a cost objective function was employed to evaluate the variation in production cost when biochar replaces coal as injection fuel. From the technical perspective, this Thesis presents estimates that the use of biomass chars as Bio-PCI may reduce total CO2 emissions in a coke based blast furnace by between 19 % and 40 %, according to the injection rate. Additionally, its utilization would not alter the quality of hot metal or slag composition. From the economic perspective, I report an increase in the production cost of hot metal when Bio-PCI was introduced. Accordingly, two alternative pathways were explored to improve the economic attractiveness of Bio-PCI – 1) the introduction of carbon taxes and 2) the utilization of residual biomass for charcoal-making. With respect to carbon taxes, the Thesis estimates that a tax of US$47.1 to $198.7 per tonne CO2 would be needed to enhance the attractiveness of Bio-PCI in reducing carbon emissions. With respect to the use of residual biomass, the production cost of biochar could be reduced by US$120–180 per tonne through the use of agricultural and forestry residues instead of hardwood. From the strategic perspective, the Thesis argues that the use of sustainable residual biomass, together with the introduction of carbon taxes, can create the economic conditions needed to produce carbonaceous residues at a competitive price in order to reduce CO2 emissions in coke based blast furnaces with a view to mitigating their impact on climate change.