The porous structure of coal is analyzed to understand the complexities involved in the process of coal bio-conversion to methane. Computed X-ray tomography (CT) imaging is used to obtain the three-dimensional porous microstructure of coal in a non destructive manner. Novel image processing techniques combined with stochastic analysis are used to analyze the 3D images. A mercury intrusion porosimetry study is done to analyze the pore sizes present in the coal structure. Finally, permeability measurements are done using a conventional core experiment. The porosimetry studies show that coal has a bimodal porous structure with a primary mode in the range of 5-10 nm and a secondary mode in the range of 2-10 mu m. The imaging analysis showed that almost half of the pores in 2-10 mu m range are percolating. However the porous volume accessible to microbes and nutrients is quite small due to low porosity. The permeability analysis at core scales shows that coal is an extremely low permeability medium due to its low porosity and small pore sizes. Overall, the study shows that one must improve the permeability and porosity of the coal matrix to make the coal bio-conversion process faster and economically viable. (C) 2015 Elsevier Ltd. All rights reserved.