Technical Market Support » Metallurgical Coal
This project aimed to study the chemical interaction between volatiles evolved from vitrinite and inertinite during heating. The project hypothesised that volatile components from vitrinite may act as plasticising agents for portions of the inertinites, thereby providing a mechanism to explain why some inertinite rich coals can improve coke strength indices in some cases and not others. By purposefully packing vitrinite concentrates in front of inertinite concentrates, volatiles from one maceral concentrate could be forced to pass through inertinite-rich particles packed downstream.
Three coals were used in this project. Vitrinite and inertinite concentrates were prepared by reflux classifier. Thermo-swelling was investigated on Computer Aided Thermal Analysis (CATA) and volatile release was measured by Dynamic Elemental Thermal Analysis (DETA). Attenuated total reflectance-Fourier-transform infrared spectroscopy (ATA-FTIR) was used to study the variation of chemical bonding during chemical interaction and Thermogravimetric analysis (TGA) for boiling point distribution of collected coal tar. Fused carbon analysis was performed externally by Pearson Coal Petrography.
At a heating rate of 5 °C/min, CATA tests indicated that vitrinite concentrates from the three coals could swell from 150% to 400%, with a dramatic decrease in permeability between 400 and 600 °C, while inertinite concentrates showed little swelling but thermal expansion with temperature, and only a slight decrease in permeability. When the same mass of vitrinite concentrates was packed in front of inertinite particles (1:1 by mass), the permeability dramatically decreased, to a lower point than that from vitrinite concentrates only, indicating the alteration of fluidity. This is strong evidence of chemical interactions between in-situ vitrinite volatiles and inertinite particles.
FTIR analysis indicated that vitrinite concentrates showed higher aliphatic CH, CH2 and CH3 peaks at 2800-2945 cm-1 and aromatic CH peak at 2950-3090 cm-1 than inertinite concentrates. From literature, these chemical bonds are related to the development of fluidity of heating coal. When vitrinite macerals were packed in front of inertinite-rich concentrate, the quenched inertinite char from 450°C showed strong evidence of chemical interaction between the upstream in-situ vitrinite volatiles and the inertinite-rich particles. In particular, the variation of aliphatic CH, CH2 and CH3 peaks at 2800-2945 cm-1 and aromatic CH peak at 2950-3090 cm-1. The interaction may vary from coal to coal, and the extent varied with the amount of the upstream in-situ vitrinite volatiles passing through the inertinite concentrates. For concentrates from coals 1 and 3, when the same mass of vitrinite particles was packed in front of inertinite particles, the strongest interaction was observed, while this was observed when a higher ratio of vitrinite was packed in front of inertinite for macerals from coal 2, at 3:1 by mass.
To study the cracking reactions of the in-situ vitrinite volatiles themselves on the surface of the inertinite-rich particles, the inertinite was oxidised at 150°C for one week. FTIR analysis indicated that the oxidised inertinite concentrates showed little aliphatic CH, CH2 and CH3 peaks and aromatic CH peak after heating to 450°C. This suggests that the inertinite component contains reactive bonds that are conducive to be plasticised. It further suggests that the inertinite structure is also susceptible to oxidation reactions that prevent further interactions with volatiles. When the same amount of in-situ vitrinite volatiles passed through the oxidised inertinite particles, larger aliphatic CH, CH2 and CH3 peaks at 2800-2945 cm-1 were observed that that from the unoxidized inertinite char 1, while smaller aliphatic CH, CH2 and CH3 peaks from unoxidized inertinite char 2, and the similar small CH, CH2 and CH3 peaks for inertinite char 3 were observed. This is strong evidence of chemical interaction between in-situ vitrinite volatiles and inertinite, and different cracking reactions of vitrinite volatiles from coal to coal. TGA also indicated that the boiling points from condensable vitrinite tars collected at 450°C varied from coal to coal.
DETA analysis indicated that vitrinite volatiles contain higher H/C than inertinite volatiles, particularly at the early stage of volatile evolution, such as below 450°C. The interaction between in-situ vitrinite volatiles and inertinite was dependent on maceral properties. For the high swelling vitrinite 1, the interaction between in-situ vitrinite volatiles and inertinite resulted in a higher C and H release than predicted, while a lower release for macerals from coals 2 and 3 that showed relatively mild swelling. The blends of the condensable vitrinite tars and inertinite showed higher C and/or H release than the inertinite concentrates themselves beyond the boiling points of the tars (450°C), indicating the reactivity of compounds in condensable tars and the potential of secondary reactions to impact coking.
Macerals from coal 1 were selected for preparing coke buttons at 1000 °C for fused carbon analysis. Pearson Petrography analysis indicated that the total fused carbon in the inertinite-rich cokes varied between 61.62-70.28%. The measured fused carbon for the inertinite-rich concentrates was 67.57 % and this was improved with the highest volatile flow (75% vitrinite). However, lower volatile flows (50% vitrinite) appeared to reduce total fused carbon down to 61.62%. This suggests that the “blending ratios” can have both positive and negative impacts; and we suggest that this impact is worthy of future study.
In summary, the in-situ vitrinite volatiles and condensable tar could chemically interact with inertinite-rich particles, resulting in the increase or decrease of volatiles, and alter the chemical bonds (aliphatic and aromatic) of the quenched inertinite char, which altered the permeability (fluidity) of heating coal pellets. The degree of interaction may vary with the ratio of volatile against inertinite and the property of coal. However, how the chemical interaction as a function of temperature affects the chemical bonds (such as aliphatic and aromatic) relating to the fused carbon of final coke remains unknown. Therefore, the impact of the reactive components such as the radicals' concentrations from vitrinite, either from the volatile or the un-vaporised compositions, on the fusibility of inertinite at higher temperature such as 450-600°C relating to the property of final coke needs to be studied in the future.