Technical Market Support » Metallurgical Coal
This project investigated the use of Laser Desorption/Ionisation time of flight mass spectrometry (LDI-TOF-MS) as a tool for measuring molecular weight changes occurring during the development of thermoplasticity in a suite of industrial coking coals.
The study indicates that the technique provides novel and new coal characterisation options for
coking coals, whereby:
- Peak molecular weight and MALDI signal intensity in raw coal and semi-cokes was related to coal rank and fluid properties; providing a molecular basis for characterising coals;
- Solvent extractable “fluid phase” compounds are ranked on MALDI signal intensity, peak molecular weight, number of compounds of MALDI identified molecular groups and low boiling point volatiles by TGA; and
- The Plastic Layer in simulated coke oven heated samples was found to correspond with the highest MALDI signal intensity across coal/plastic layer/semi coke regions. It was found that this value did not correspond with trends in standard fluidity measurements. This indicated that fluid properties in a coke oven may differ to those determined in a Geiseler fluidity test. Differences in the ratio of Low and High range molecules (below and above 2000Da) at this maximum fluidity point and the raw coal showed no clear trend and may be a coal specific process.
It is widely acknowledged that coal pyrolysis chemistry acts as a driver for the fluid properties produced when coked. However, the challenging nature of each coals structural complexity as it decomposes has remained a barrier to more fundamental understanding and greater optimisation in blending. MALDI has been a high range molecular tool for protein identification in biotechnology and for studying polymer behaviour. The use of LDI-TOF-MS (without a matrix) has only recently been found suitable for detecting the high range molecules found in coal and formed during coking.
Five coal samples were selected based on fluidity (40-2000ddpm) and coke strength. Of this suite, two coals were considered as (i) high strength + high fluidity, two coals were considered as (ii) high strength + low fluidity and one coal was considered as (iii) low strength + medium fluidity. Each coal was heated under conditions and temperatures associated with fluid development (softening, maximum fluidity and resolidification). These “fluid range” semi-cokes were either solvent extracted or analysed directly as a solid sample. Solvent extracts ranged up to 500Da for acetone soluble material and ~2000Da for THF soluble material. Pyridine extracts typically fell between these values. A peak molecular weight was observed in the extracts at ~350Da for all extract samples. By comparison, the solid samples ranged up to 7000Da in molecular size with peak molecular weight occurring between 1000-2000Da. This peak in the solid state was found to correspond with the relative rank of the coal with higher ranked coal showing higher peak molecular weight. The intensity of the LDI-TOF-MS signal for these solid samples was found to be related to the extent of the coals fluidity.
Both the solvent extracts and solid samples showed evidence of repeating peaks across the spectrum. Below 500Da, the solvent extracts showed repeating peaks every 12-14Da, while above this molecular size, both solvent extracts and the solid samples displayed evidence of repeating 24Da peaks. These findings led to the hypothesis that they represent monomeric units (below 500Da) and oligomers (+500Da, polymer fragments) that makes up the coal structure.
A novel dimensional heating experiment was optimised to simulate heating conditions similar to a coke oven. This produced a quenched composite sample containing raw coal/plastic layer/semicoke and was analysed by LDI-TOF-MS to track molecular changes in the plastic layer. This work found that the total LDI-TOF-MS signal became significantly higher during the plastic layer for all five coals. Molecular changes across the plastic layer indicated that more material was being generated during the plastic event and was typically above 1000Da in size.
A series of blending tests were performed on each sample type (solvent extract, solid state and dimensional heating) and in each case found evidence of molecular interaction-typically above 500Da in the extracts and 1000Da in solid samples.
This study has shown that broad molecular changes occur during coke making. It has uncovered a number of previously unobserved phenomena and developed a series of potential indices to evaluate coking coals for technical marketing. The number of coal specific measurements made over the course of this work shows that coal thermoplasticity can develop across unique molecular pathways that are likely to be under-utilised. The use of LDI-TOF-MS to evaluate these molecular changes in coking coal blending provides a fundamental basis for understanding and optimising coal performance and value-in-use.
Specific areas of coking coal utilisation where LDI-TOF-MS may be of use are:
- Characterising the plastic forming component of coking coals;
- Rationalising differences between standard fluid properties and those occurring in a coke oven;
- Providing molecular understanding of blending impacts and aiding in blend design;
- Developing a greater understanding of fundamental pyrolysis chemistry of maceral constituents and development of coke strength;
- Understanding molecular changes that lead to fluidity decay over the coal supply chain from freshly mined, after washing and through the shipping process;
- As an alternative assessment of fluidity.