Coal-packed methane biofilter for mitigation of green house gas emissions from coal mine ventilation air.

Hendy Limbri,Bettina Rosche,Torsten Thomas,Jason Scott,Cindy Gunawan,Andrew Smith,Stephen J Johnson

doi:10.1371/journal.pone.0094641

Abstract

Methane emitted by coal mine ventilation air (MVA) is a significant greenhouse gas. A mitigation strategy is the oxidation of methane to carbon dioxide, which is approximately twenty-one times less effective at global warming than methane on a mass-basis. The low non-combustible methane concentrations at high MVA flow rates call for a catalytic strategy of oxidation. A laboratory-scale coal-packed biofilter was designed and partially removed methane from humidified air at flow rates between 0.2 and 2.4 L min−1 at 30°C with nutrient solution added every three days. Methane oxidation was catalysed by a complex community of naturally-occurring microorganisms, with the most abundant member being identified by 16S rRNA gene sequence as belonging to the methanotrophic genus Methylocystis. Additional inoculation with a laboratory-grown culture of Methylosinus sporium, as investigated in a parallel run, only enhanced methane consumption during the initial 12 weeks. The greatest level of methane removal of 27.2±0.66 g methane m−3 empty bed h−1 was attained for the non-inoculated system, which was equivalent to removing 19.7±2.9% methane from an inlet concentration of 1% v/v at an inlet gas flow rate of 1.6 L min−1 (2.4 min empty bed residence time). These results show that low-cost coal packing holds promising potential as a suitable growth surface and contains methanotrophic microorganisms for the catalytic oxidative removal of methane.

Highlights

To prevent explosions during underground coal mining, mine shafts are continuously ventilated to dilute methane released from the coal seam to non-combustible concentrations (usually #1% v/v since the lower flammable limit of methane is 5% (v/v) in air)
At the initial flow rate of 0.2 L min21, which is equivalent to a methane inlet load (IL) of 17.3 g m23 h21, steady state was reached after two weeks of operation
The fact that the noninoculated biofilter 2 eliminated significant amounts of methane indicates that the microorganisms native to the coal at the mine site may be catalytically active for methane removal

Summary

Introduction

To prevent explosions during underground coal mining, mine shafts are continuously ventilated to dilute methane released from the coal seam to non-combustible concentrations (usually #1% v/v since the lower flammable limit of methane is 5% (v/v) in air). Methane has an approximately twenty-one times higher potential impact on global warming than carbon dioxide (on a mass-basis in a 100-year time frame) arising from its higher molar absorption coefficient for infrared radiation and a longer residence time in the atmosphere [1]. Worldwide emissions of methane from coal mining are extensive, estimated to be over 329 million tonnes (Mt) carbon dioxide-equivalent in 2005 [2] with approximately 70% of these methane emissions released as MVA [3]. Major challenges for MVA methane mitigation are the low methane concentrations limiting its usefulness as an energy source, the high flow rates (50–500 m3s21) and the considerable variability of these parameters [4]. Various thermal technologies have been considered [5,6,7] and while capable of treating MVA, they attract high capital and operating costs and require considerable safety measures

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Conclusion