Scoping Study: Design of Cokes from Biomass-Coal Blends for Sustainable Blast Furnace Ironmaking

There is an increasing focus on improving the environmental sustainability of the cokemaking and ironmaking processes and therefore, one possible approach to enable this is through the addition of biomass from sustainable sources as a partial replacement of coal in the cokemaking process. However, biomass in its natural form presents issues which limit its addition and these include high volume, variability, and high volatile contents. Therefore, the addition in the form of biochar is considered a more practical approach.

In this project biochar additions were conducted in concentrations of 2.5, 5.0, and 10 wt% to a standard base coal; initially the biochars were sieved to a size of <2 mm prior to addition. This resulted in significant degradation of the microstructural feature, strength and performance indices of the resultant cokes, particularly at addition levels of ≥ 5 wt. However, with the use of milling processes, new coke charges were designed with particle sizes of <0.5 and <1.0 mm. The addition of these finer sizes was seen to significantly minimise the degradation, even at high addition ratios of 10 wt%. This led to an increase in the mechanical strengths of these cokes at room temperature for both the feed and gasified samples and importantly, an improvement in the performance indices.

This improvement in the coke quality and performance was clearly attributed to improved integration of the inert biochar particles into the coke matrix during the cokemaking process and this was verified by determination of the permeability and dilation values during the cokemaking process. Additionally, fractographic analysis showed that there was a weakening of the RMDC components at high biochar additions and this could have been responsible for the observed lowering of performance at the highest biochar addition level (10 wt%). High temperature strength measurements revealed higher values than those seen at room temperature and improved graphitisation of the samples with no effect seen from biochar addition levels. Moreover, crystallite sizes decreased with increasing biochar addition and anisotropy decreased, which as expected owing to the isotropic nature of biochar.

Outcomes of this project clearly establish the importance of biochar particle sizes on enhancing the extent of integration into the coke microstructure/microtexture without causing any adverse effects. In order to enhance the addition levels to values >15-20 wt% it is essential to use biochar in low particle sizes while also tailoring the plasticity behaviour of the base coal to maximise integration and enhance the resulting performance of these biocokes for use in ironmaking applications. In this regard, costs of production and potential sources of these biomass and their variabilities are important considerations to ensure consistent feedback to create high-performance biocokes economically and in a viable manner.