Ignitability and combustibility of Yallourn pyrolysis char under simulated blast furnace conditions

- Brown coal char can be used as top-grade PCI coal substitute.
- Brown coal char can burn out under mild conditions inlcuding a lower temperature and larger particle size.
- A minimum 6 wt% volatile content is essential for stable ignition.
- Gas diffusion resistance is minimal for brown coal char combustion due to the large surface area and porosity.

In this paper we have examined the potential of Yallourn brown coal char (collected from an industry-scale pyrolyser) to be used as a pulverized coal injection (PCI) fuel, its size-dependent properties, ignitability and combustibility under the simulated conditions of the blowpipe-tuyere section in a blast furnace. The combustion of individual sizes for Yallourn char was tested in a lab-scale drop-tube furnace (DTF) using pre-heated hot gas with a temperature up to 1000 °C, and a particle residence time as short as 0.6 s. Computational fluid dynamics (CFD) modelling was further conducted to optimize the char combustion conditions via sensitivity analysis. Irrespective of the pyrolysis condition, Yallourn char is superior over bituminous coal for being used as a top grade PCI fuel, due to its higher calorific value (7500-8110 kcal/kg), lower ash content (< 10 wt%), high ash melting temperatures (> 1550 °C), and abundance of iron (> 40 wt% in ash). The performance of Yallourn char is also superior over bituminous coal under the simulated blast furnace conditions, for a rapid ignition and burnout even for a coarse char size of 300 μm under the stoichiometric O2/C molar ratio and using low blast temperatures of 800-1000 °C. All these are beneficial for reducing the energy consumption related to particle pulverization and the amount of oxygen for the combustion. With regard to the Yallourn char ignition and combustion in the hot gas, a minimum 6 wt% volatile content is essential for a stable and rapid ignition of the volatiles at a gas temperature of 1000 °C or below, since homogeneous ignition is predominant at low temperatures. However, once the blast temperature rises to 1200 °C, the dependence on volatile content turns insignificant due to the dominance of the heterogeneous ignition, high C-O2 reactivity for the solid char, as well as the minimized pore diffusion control due to the large porosity (52.0-63.1%) of the Yallourn char tested here.