Influence of bio-coal properties as a substitute to fossil coal in carbon composite agglomerates and in coke

Abstract: The iron-ore-based blast furnace (BF) process is still the most dominant method for producing metallic iron units for steelmaking, and the BF is also the main contributor to the 7-9% of global CO2 emissions which, according to World Steel Association, originate from the steel industry.  The steel industry is aiming to reduce CO2 emissions by different means. In the short term, replacing fossil coal with renewable carbonaceous material like bio-coal (pre-treated biomass) is possible and, in the longer term, by using hydrogen. The use of bio-coal as a part of top-charged self-reducing composites containing iron oxide (bio-agglomerates) or as part of coking coal blend producing bio-coke are potential ways to introduce bio-coal into the BF. The aim of this study is to understand the impact of bio-coal properties i.e., volatile matter, carbon structure and ash content and composition on the self-reduction of composites as well as on cokemaking and the quality of produced coke. In order to select a suitable bio-coal to be contained in agglomerates with iron oxide, the devolatilization behavior of different types of bio-coals was studied in thermogravimetric analyser (TGA) connected to a quadrupole mass spectrometer to monitor the weight loss and components in off-gases. The devolatilization was conducted at diffetrent heating rates: 5, 10 and 15°C/min in an inert atmosphere up to 1200°C. The obtained data were evaluated using the Kissinger-Akahira-Sonuse (KAS) iso-conversional model and the activation energy was determined as a function of conversion degree. The main finding is that bio-coal pretreated at low or high temperatures produces reducing gases that can contribute to the reduction of iron oxide in bio-agglomerates. Torrefied bio-coal containing a higher content of ash and therefore higher content of catalytic oxide as e.g., alkali and alkaline earth metal oxides, releases the volatile matter at a lower temperature, when it cannot fully contribute to the reduction. The self-reduction behavior of composites was studied in a TGA in argon atmosphere using a BF-simulated temperature profile. To investigate the effect of added bio-coals in the reduction interrupted tests using similar temperature profile as in TGA were conducted in nitrogen atmosphere in a vertical tube furnace up to temperatures selected based on TGA test results. The contents of volatile matter, fixed carbon and composition of ash in the bio-coals influenced the self-reduction. XRD analysis of composites collected after interrupted tests shows that the self-reduction of bio-coal-containing composites started at 500°C, while it started at 740°C with coke as the only carbon source. The hematite was successfully reduced to metallic iron at 850°C with bio-coal present as a reducing agent, but not until 1100°C when coke was used. Bio-coal containing a high content of volatile matter, but with a low content of catalytic oxide, enhanced the reduction mostly and wusite was detected by XRD in the sample interrupted at 680°C.The possibility to introduce bio-coal into cokemaking was investigated by carbonization of coking coal blends with addition of various types of bio-coals in the lab and on technical scale. To understand the impact of bio-coal properties (ash composition, volatile matter and bio-coal structure) and addition in cokemaking, the thermal behavior of bio-coal was investigated under carbonization conditions in thermogravimetry and tests in an optical dilatometer were conducted to evaluate the impact on plasticity. The effect from bio-coal addition on coke reactivity was studied in TGA up to 1100°C in carbon monoxide atmosphere, and for technical-scale coke by using a standard test for coke reactivity index (CRI). The optical dilatometer results show that plasticity was lowered more with higher bio-coal addition, but pyrolyzed bio-coal had a less negative effect on plasticity compared to torrefied bio-coal with a high content of oxygen. Bio-coke has higher reactivity than reference coke and the bio-cokes containing bio-coal with higher content of ash with higher content of catalytic oxides had higher reactivity. Aiming to reduce the negative effect from bio-coal on coke reactivity related to e.g., bio-coal ash and reactive carbon, possible methods for countermeasures as removal of catalyzing ash oxides by water and acetic acid washing, binding alkaline oxides by kaolin coating, agglomeration to reduce reaction surface and use of a high fluidity coal in the coking coal blend to improve the coke quality were investigated. The coking coal blend containing washed bio-coal had lower dilatation than blends containing original bio-coal, but the bio-coke reactivity was lowered by washing for bio-coke containing bio-coal with higher content of ash and catalytic oxides and lowered more with acetic acid than water washing. The hydrolysis of bio-coal structures during washing increases the surface area and introduces oxygen, having negative effects on thermoplastic properties. The addition of bio-coal with 5% kaolin coating or bio-coal ash addition lowers the dilatation moderately relative to the reference coking coal blend, but the bio-coke reactivity is higher compared to bio-coke with original bio-coal, due to potassium oxide content in kaolin. The bio-cokes containing bio-coal ash have a higher temperature for start of gasification in comparison to introduction of the reactive carbon as present in the bio-coals. Coke containing high fluidity coal has lower reactivity than other reference cokes, and bio-coke containing high fluidity coal with agglomerated bio-coal has lower reactivity when compared with bio-coke produced from another base blend with a similar added amount of bio-coal. The reactivity of coke produced in technical scale measured in CRI/CSR tests shows a similar trend regarding reactivity as measured by thermogravimetric analysis on coke produced in lab scale. Bio-coke containing agglomerated bio-coal and coking coal blend with high fluidity had the lowest reactivity.  It is possible that a bio-coal product suitable for bio-coke production can be produced by combining washing of the raw biomass before torrefaction or pyrolysis with agglomeration before or after thermal treatment. The catalytic compounds in the ash and introduced oxygen during washing are thereby removed, and also the surface area for reaction with CO2 and high porosity for diffusion of reaction gases and products are blocked by compaction.