Nanoporosity in cokes: their origin, control and influence on CO2 reactivity

This project using the outcomes of previous project C24060, examined how the total, closed and open porosity of cokes is affected by partial gasification of the coke and how this varies with pore size. Because maceral composition is believed to be important in predicting coke reactivity, we also investigated cokes made from maceral concentrates.

The coals used for coke preparation were bituminous coals from the Bowen Basin and the Illawarra Measures. Maceral concentrates were prepared from three Bowen Basin coals and one Illawarra Measures coal. The maceral concentrates along with the original coals were carbonised in a 70 g oven. A set of six coals of similar sources were carbonised in a pilot coking oven (300 kg). The cokes prepared in different ovens enabled us to investigate the influence of coking conditions on coke structure and its reactivity. The reactivity tests on the cokes were performed in a fixed-bed reactor under chemically controlled conditions, to different burn-outs 25%, 50% and 75%. Small Angle Neutron Scattering and Ultra Small Angle Neutron Scattering techniques were used for analysis of closed and open porosity in cokes and coals. Three cokes were annealed to 1400°C and 1600°C to determine how porosity changes at high temperature.

It was confirmed that nanoporosity in cokes is largely inherited from the coals. If there were no connections between nanoporosity of coals and nanoporosity of the product cokes, then predictions of coke reactivity from coal rank and maceral composition could never be improved, since there would be no fundamental connections between the organic part of the coal and coke properties, only empirical ones.

Using neutron scattering to characterise coke structure, some of the new findings were unprecedented and shed new light on what happens to coke after reaction with CO₂. These include:

• On gasification closed pores were opened and new pores were formed. The amount to which each occurred at a fixed burn-out level varied with rank, maceral and mineralogical composition of the original coal.
• In three of the inertinite derived cokes nearly all of the pores - even those of 1nm size - became accessible at 25% burnout, but in vitrinite-derived cokes much of the remaining nanoporosity remained inaccessible even after extensive burn-out.
• New pores were produced during burnout: this means that even on accessible surfaces, the reaction of coke with carbon dioxide does not occur at a uniform rate. In the cokes made from the two lower rank vitrinite-rich components, there appeared to be selective attack on open 20-80 nm sized pores.
• Gasification increased dramatically the number of finer open pores (1-30 nm radius) in all cokes compared to those of larger pores (30-100 nm radius). Coal rank, maceral type and region appear to have no influence on this. However, catalytic minerals appear to promote the production of larger (40-100 nm radius) open porosity.
• Annealing has a dramatic effect on coke structure from 1 nm to 2000 nm radius. It increases both closed and open porosity, and the effect increases with increasing temperature. The largest difference in coke porosity occurred at about 10 nm radius.
• The effect of temperature on structure differs between cokes. Thus we would expect that NSC tests results of cokes after annealing at high temperature will be different to those of the unannealed cokes.

The results show that the nanostructure of cokes is modified during reaction with CO₂ and also how the modification is affected by the maceral composition, rank and mineralogical composition of the original coal. By determining how coke nanostructure changes on reaction with CO₂, it becomes feasible to better predict what happens to a coke when it reacts with CO₂ and how fast the reaction happens.