Estimation Of The Residual Gas Content In Mined Coal And Coal Waste

This report presents the results of a project on the behaviour of residual gas in mined coal and waste components after mining. The objective of the project is to estimate the residual gas content in product and waste coal at the point of disposal as this may represent an important factor in the calculation of a mine's fugitive gas emissions.

Coal in the sub-surface often naturally contains methane mostly as an adsorbed gas. The quantity of methane stored in the coal is a function of the pore pressure. Once mined the coal and other carbon rich geologic waste material, such as low grade coals and carbonaceous shales, will involve a broad range of fragment sizes within which will be residual gas. The mechanisms that could drive migration of the residual from the mined material will be a combination of the pore and partial pressure gradients between the gas within the fragments, the void space between the fragments and then the surrounding atmosphere. When initially mined, gas contents could be expected to be relatively high and thus provide a pore pressure gradient to drive gas flow into the void space between the coal fragments. Over time the pore pressure will become equilibrated with atmospheric pressure and gas will diffuse out of the coal driven by the partial pressure or concentration differences, such as the difference between a coal rich in methane and air where almost no methane is present.

Existing measurements of the gas content of mined coal are extremely limited. Only one study was available to the project; measurements made by Day et al. (2007) for ACARP C15077, where samples of product coal were collected from a number of mines shortly after processing. In these samples there was a distinct contrast between the low gas contents found with open cut mines and the high methane contents with samples from gassy underground mines. However this is a complex problem influenced by a wide range of operating conditions and coal properties, including storage times and processing practices.

The analyses presented in this report focus on the diffusion limited stage after pore pressure has equilibrated with atmospheric pressure and used published information on product coal particle size distribution but also examined the sensitivity of the predictions of the residual to the coal fragment particle size. The physical arrangement of the product coal is also an important aspect of this problem as it determines the diffusion lengths that gas must travel and thus the rates of gas emission. In the analyses presented in this report two scenarios were considered; product coal within a hypothetical stockpile and within a rail transport hopper.

The predictions were performed by deriving a set of differential equations to describe the gas migration process (based on accepted approaches to describing gas migration in coal) and then implementing these in the computational multi-physics package, COMSOL. This provided an efficient approach to investigating the various physical processes and arrangements operating. The physical properties were estimated from the literature, in particular, ACARP reports. However an important step which was not possible within the scope of the project was testing of the predictions against observations of the residual gas content with time. As a result these predictions should be treated as rough guides as to the behaviour of the residual gas content with time.

It is assumed here that during and shortly after processing the run of mine coal into smaller fragments the pore pressure within the coal fragments will rapidly equilibrate to atmospheric pressure. This means that the initial residual gas content is the gas content at atmospheric pressure calculated directly from the adsorption isotherm; thus any gas within the coal above this is assumed to become fugitive during processing. This important point should be noted in calculating the fugitive emissions.

As expected the large size of the stockpile, involving longer gas migration pathways, meant that the rate at which the residual gas content decreased with time was much slower than for coal within the much smaller rail hopper. In addition, while the particle size of the product coal was important for gas migration from the coal hopper, it was much less important for coal within the stockpile, since gas diffusion within the coal void space played a more important role.