Pore structure changes from wood to vitrain: a casy study from New Zealand coal

Abstract

Porosity in coal plays an important role in determining sorption capacity and diffusion behaviour of a given gas. Total porosity and pore size distribution are known to change with rank and composition. Total porosity declines whereas micropore volume increases with rank. On the other hand, the variability in composition among coals makes it difficult to track the changes due to the influence of various plant precursors. This study examines pore structure changes from the original plant structure represented by wood to vitrain across a range of ranks. Samples used in this study include a coal rank suite from brown coal to medium volatile bituminous of Cretaceous age from New Zealand. Porosity evolution during coalification was assessed using vitrain bands isolated from these coals which is derived from the genus Lagarostrobos. Previous palynological studies suggest that the precursor wood is related to the modern Huon pine (Lagarostrobos franklinii). Therefore, a wood sample of this species from Tasmania was also analysed. The methods used in this study were optical microscopy, low pressure gas adsorption using nitrogen and carbon dioxide at 77 K (-196.150C) and 273 K (00C) respectively, high pressure adsorption isotherms using carbon dioxide and methane at 320C, and small/ultra-small angle neutron scattering (SANS/USANS). Petrographic analysis shows that the vitrain suite has a vitrinite content ranging from 79 % to 100 %, telinite in particular, ranging from 13.3% to 84.7% mmf, with random vitrinite reflectance ranging from 0.39 % to 1.49%. Qualitative comparison of wood microstructure and cell diameter in the Huon pine sample and that revealed in the vitrain samples by etching, shows a similar simple cell structure (bordered pits, unicellular rays). Based on an average cell diameter of 20 µm, compaction estimates ranged from 9:1 to 4:1, except for samples that did not appear woody (i.e. they were attrital and derived from herbaceous plants), or were heat affected, and tectonically deformed. This supports the assumption that most vitrain samples in this study have a Huon pine like precursor plant. Results from low pressure adsorption, using nitrogen, shows that the percentage of cumulative pore volume of micropores (those less than 20A), from wood to vitrain increases with rank. The increase in micropore volume percentage rose significantly and peaked at sub-bituminous rank (Rr= 0.6%); thereafter it decreased exponentially with rank to medium volatile bituminous rank (Rr=0.9%). There is also an inverse correspondence between a decrease in percentage of cumulative macropores (pores larger than 500 A) volume, at sub-bituminous rank (Rr=0.6%), that increases with increasing rank. Mesopores show a similar trend to micropores with a peak exception at 0.90% Rr. The total cumulative pore volume of micro-, meso- and macropore sizes show a good relationship with telinite maceral content. The result suggests that the changes in pore structure of the vitrain suite of samples, is more affected by vitrain composition (and especially the prescence of the telinite maceral) than rank, in some cases. Total porosity calculation on a restricted range of mesopores (250 to 500 A) to macropores (pores more than 500 A) from SANS/USANS, shows that the total porosity of samples is primarily from mesopores with a small contribution from macropores. Micropores were excluded for calculation on the basis of a “peer to peer” comparison because only a few of the samples examined had micropores. Pore number density show an increasing trend with rank with JE 01 and huon pine as an outlier. Interestingly, pore size distribution (PSD) of incremental pore volume from low pressure nitrogen adsorption, on all sample, shows a similar multi-modal distribution of peaks at the same pore size with different intensity. This suggests that the pore structure in vitrain bands in this study is inherited from similar coal plant precursor (Huon pine wood). However, pore number density (similar with PSD) from small angle scattering methods, did not show a similar trend. Furthermore, the variability between samples of different rank was high and the trend with rank on micro-, meso- and macropores is different between each technique. Therefore, further study needs to be undertaken using more vitrain and whole coal samples with the addition of different methods e.g. small angle X-ray scattering, to establish the role of inheritance of pore structure from wood, to coals of a wide range of rank.