Influence of Coal Blending on Coke Nanoporosity and CO2 Reactivity

Reactivity of coke is one of the most important parameters that defines coke quality. Parameters such as nanoporosity and catalytic mineral matter also control coke reactivity. If coke nanoporosity can be controlled to any extent through coking conditions and blend composition, it would give coke producers greater control over coke reactivity. The aims of this project was determine the influence of coal blending and the effect of coking conditions on the amount of open and closed nanoporosity and identify the effect of coal blending on gasification rate. Understanding how blending coals influence closed nanoporosity of cokes would improve prediction of coke reactivity and this would give coke producers greater confidence in selecting coals for blending.

This project extended the investigation into the nanoporosity (pores in the size range 1 to 1600 nm) in cokes. It investigated the effect of blending, to see if there were interactions that affected the pore size distribution and how the coke reacted with carbon dioxide. The influence of position was examined in the oven on nanopore numbers by examining samples taken from near the wall of the oven, near the centre of the oven, and in the middle of these two positions. The nanoporosity of cokes made from coals sourced from Australia with those sourced internationally was compared.

The most important finding is confirmation that cokes sourced from the Rangal and probably Moranbah measures had far more nanoporosity than cokes made from international coals of the same rank. The numbers of nanopores in cokes from samples from the German Creek and Illawarra coal measures were similar to those in cokes made from coals sourced internationally. The excess porosity in the cokes from Rangal and probably Moranbah measure coals was greatest in 20 to 80 nm-sized pores.

The report details the following findings.

Anomalous thermoplastic behaviour attributed to Rangal coals are explained.

Blends:

• Cokes made from blends of coals of similar rank, but different maceral composition, showed no evidence of any significant interaction that affected nanopore numbers, either before or after reaction;
• Cokes made from blends of coals of different rank showed some differences in nanostructure that could be caused by interactions.

Nanoporosity of coke samples taken across the oven:

• Cokes samples taken nearest the wall of the oven had more fine open pores than those from the middle, in all cokes except for the ones of highest rank. These excess fine open pores are created during coking;
• Cokes made from lower rank coals had more fine open pores at the centre of the oven than samples taken from the middle. This effect disappeared on blending even adding small amounts of higher rank coals;
• We attribute the variation in numbers of open pores near the wall and centre of the oven to the flow of volatile material across coke surfaces, which either erodes or deposits carbon on the surface and produces roughness on exposed coke surfaces on the scale of 1 to 10 nm.

Effect of reaction with CO2:

• CRI and apparent reaction rate (as measured in a fixed-bed reactor) increase with increasing numbers of 1 nm pores in the reacted coke. There is no correlation between CRI and numbers of open pores > 20 nm;
• The number of open 1nm pores as measured by SANS in reacted and unreacted cokes correlates strongly to surface area as measured by CO2 adsorption;
• Total porosity in most cases decreases after 10% reaction but increases after 25% reaction;
• The number of 1-85 nm closed pores decreases substantially after 10% reaction but did not change further when the reaction was extended to 25%;
• The number of open pores generally increases substantially after reaction. At some sizes there is a decrease in open pore numbers after 10% reaction, presumably because these pores were selectively 'eaten out' on reaction;
• The number of open pores does not correlate with inertinite content in unreacted coke but does after reaction. We attribute this behaviour to the formation of a thin skin on the inertinite during coking, which is quickly removed on reaction. This is consistent with the findings from the ACARP project C27056 in which it was found that inertinite derived maceral component becomes accessible to gas only after some reaction.

The outcomes of this project include a literature review that summarises previous ACARP project outcomes on the influence of blending on coke properties.