The Effects of Blending of Australian Coals on the Chemistry of the Plastic Layers

Coke quality is virtually determined by the chemistry of the plastic layers that are formed during coking of coals. Most of the previous research in the open literature on the underlying chemistry of the thermoplastic behaviours of single coals and blends have been carried out under impractical coking conditions, in terms of the sample size, the dimensions of the coal bed and in particular the heating conditions. Extended from project C24054 on the development of the 4kg laboratory-scale dual-heated-wall coke oven testing rig (4kg coke oven), this project investigated the effects of blending on the chemistry of plastic layers formed during the heating of blends of Australian coking coals.

The oven has the capacity of simulating practical coking conditions and is featured with plastic layer sampling and in-situ measurements of temperature profiles and internal gas pressures (IGPs) simultaneously at five locations in the coal charge. The plastic layer samples obtained from the coke oven tests in this project were analysed by a variety of advanced analytical techniques available in various institutions. This project focused on investigating the interactions between Australian coking coals in coal blends under practical coking conditions using the University of Newcastle's 4kg lab-scale coke oven combined with various advanced analytical techniques.

The main objectives were to:

• Investigate the extent of the interactions between vitrinite-rich and inertinite-rich coking coals in blends with different blend compositions during the formation of the plastic layer;
• Explore the impacts of the interactions of the blends on the physical and chemical properties of the plastic layers formed from the blends by using Synchrotron micro-CT, IR and Solid-state 13C NMR analysis; and to
• Identify the influences of the blending on the estimated thicknesses of the plastic layers and the internal gas pressures measured in a laboratory-scale coke oven.

Highlights of the research are given below:

• Two coals with different vitrinite contents were blended using different blend compositions (50%-50% and 25%-75%) to investigate the effects of the interactions between the blend components on the chemistry of the plastic layers under practical coking conditions.
• The addition of the high inertinite coal into the coal blends reduced the Gieseler maximum fluidity and thermoplastic ranges of the coal blends. This influenced the physical and chemical changes cross the plastic layers of the blends during coking.
• It seems that the changes in pore structures in the plastic layers of the C2-C3 blend (25:75%) and the C2-C3 blend (50:50%) showed different trends, i.e. the changes in the plastic layer for the 25:75 blend and 50:50 blend followed those of the C3 and C2 single coals respectively. This essentially implies that lower fluidity coal has a more significant impact on the plastic layer chemistry.
• The vitrinite rich particles for the C3 coal in the 25:75 blend affected the pore structure changes in the plastic layer in that way that the intergranular and intragranular voids of inertinite rich C2 coal particles with limited capacity to generate fluidity in the blends (25%) were filled up by the thermoplastic mass that were generated by the C3 coal particles. This coincided with the significantly lower maximum porosity in the plastic layer from the 25:75 coal blend compared to that from the C3 single coal. This phenomena are also verified by the plastic layer from the 50:50 blend which had lower maximum porosity compared to that of the plastic layer from the C2 coal.
• The SSNMR results indicated that the chemical structure of the plastic layer from C2 single coal consisted of a higher content of aliphatic structures and less aliphatic bridge bonds. The high inertinite content in C2 coal seems to have led to a higher proportion of Ar-O (Oxygen-bonded aromatic carbon) that contributed to the earlier evolution of tar by consuming the transferable hydrogen at the earlier stage during heating.
• The addition of the C2 coal into the blends seemed to have caused the absorption of the transferable hydrogen during the early stage during the formation of the plastic layers, accompanying the early release of the tar. This may have promoted earlier cross-linking reactions and aromatic ring condensation, leading to reduced overall fluidity of the plastic layers. This may have increased the number of isolated pores.

This project:

• Provides a new methodology using advanced analytical techniques for measuring the physical and chemical interactions between the components of blends during the formation of plastic layers.
• Achieved a better understanding of the influences of the maceral compositions of blends on their coking behaviours which will play a significant future role in the cokemaking process.
• Produced a new enhanced capability for Australian technical marketers in the prediction of the coking performances of Australian coking coal blends with different vitrinite contents.