Converting Coal to Methane Through Biogasification

If coal is surrounded by suitable microorganisms, it can certainly be converted to methane. This process of coal biogasification has been reported in many different parts of the world. However, in underground coal seams where nutrients (nitrogen, phosphorous, etc.) are notoriously deficient, biogenic methane formation can be a slow process. To enhance methane release from coal, two approaches, bioaugmentation and biostimulation can be employed. The former generally is not necessary since most indigenous microbes are capable of gasifying coal. The latter has been practiced in coal-bed methane wells. This approach involves supplementing suitable nutrient solutions to the in situ microbial communities to improve methane production rate.

 

To identify the best nutrient recipes (low cost, highly efficient) for different basins, we have taken the most comprehensive pathway necessitating six steps. First, we sequenced the in situ microbial communities in different basins. Second, we studied the extracellular proteins released by the communities. Third, we screened different nutrient recipes to narrow down the list. Fourth, for those recipes that had high stimulating effects on methane production, we screened other environmental parameters to determine statistically significant factors. Fifth, for those critical factors, we determined the optimal value for each and established mathematical models to predict methane yield from coal under different conditions. Finally, the predicted maximal yields were confirmed by experimental studies (Fig. 1 and 2). As a result, we have developed recipes that are less than 10% of the initial cost, but enhanced methane production 200-500 fold (Table 1). Currently, we are investigating the stimulating effects of these nutrient solutions at in situ conditions.

 

The enhanced CBM has great commercial potential. In the US, among 6 trillion tons of estimated coal resources, 90% is currently unmineable. If a methane content of 200 ft³/ton could be achieved, then the total methane from unminerable coal would be 1,080 trillion cubic feet (Tcf). This would be 48.5% of 2,226 Tcf of gas potentially recoverable from traditional reservoirs, such as conventional, tight sands and carbonates and shales. Assuming $3/thousand ft³, the potential market in US is $3.24 trillion. In addition, recovery of methane from charged coalbeds should be relatively inexpensive compared to that from shale and existing CBM wells eliminate the need for drilling new ones.

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Fig. 1: Methane yield and content from coal in the IL basin adopting optimal conditions

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Fig. 2: Methane yield from coal in the SJ basin adopting the optimal conditions and in comparison with different controls

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Table 1: Features of the two nutrient recipes developed for the IL and SJ basin, respectively

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Collaborators: Dr. Satya Harpalani (SIUC), Drs. Derek Elsworth and Shimin Liu (Penn State)
Funding Source: Illinois State, DOE NETL

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