Proof-of-Concept Photocatalytic Destruction of Methane for Coal Mining Fugitive Emissions Abatement

Australia's fugitive emissions in 2015 were 41 Mt CO2-e (representing 8% of Australia's total greenhouse gas emissions), of which 27 Mt CO2-e were from coal mining. Over 60% of total coal mining fugitive emissions are from underground mines. The majority of underground mining fugitive emissions emit with the ventilation air (VA). Abatement of ventilation air methane (VAM) emissions is technically challenging due to its high volumetric flow rate and low methane concentration normally less than 1 vol%. A number of technologies of VAM capture, mitigation and utilisation are currently available for removal of methane from VA. However, no technology has been widely accepted. Particularly, the existing VAM mitigation technologies are not effective in treating very low concentration VAM below 0.3 vol% and even at hundreds ppm levels. Open cut coal mines are the second largest source of fugitive emissions accounting for over 30% of coal mining fugitive emissions. There is no technology currently available for abatement of fugitive emissions at open cut mines.

CSIRO has proposed a novel concept for abating fugitive emissions from coal mining using photocatalytic oxidation of methane into carbon dioxide under ambient temperature and pressure conditions. Photocatalysis is well known as an emerging technology that has gained much attention in air and water pollution control. It possesses inherent beneficial features in terms of dealing with dilute or trace reactants, which makes the photocatalysis technology particularly promising in treating very low concentration VAM and other very low concentration methane emissions such as from open cut mines.

The main objective of the project was to evaluate the feasibility of photocatalytic oxidation of VAM for coal mining fugitive emissions abatement. This proof-of-concept study was based on laboratory tests of photocatalytic oxidation of the simulated VAM. The initial scope was to carry out the laboratory tests of VAM photocatalytic oxidation in a batch mode reactor with commercial and synthesised titanium dioxide (TiO2) photocatalysts to assess the efficiency of VAM photocatalytic oxidation and understand the materials characteristics of TiO2 photocatalysts for high-efficiency VAM oxidation. The photocatalytic test results at the early stage of the project showed that the simulated VAM can be oxidised at ambient temperature by photocatalytic reaction with commercial TiO2; however, the reaction rate was slow. After consulting with ACARP monitors, the variation of project scope was made to focus on lifting the reaction rate by various ways including exploring other potential photocatalyst materials, modifying the existing batch mode reactor to improve reaction kinetics, and developing a new flow through photocatalytic reactor to test VAM photocatalytic oxidation in the continuous flow mode, which is more close to real applications in VAM emissions abatement.

Tremendous efforts were devoted to explore new photocatalyst materials and evaluate a variety of promising photocatalysts in the continuous flow mode tests under various reaction conditions. Particularly, significant amount of additional work was carried out to develop and test a new range of high-activity photocatalyst materials, which we refer to as VAMO, exhibiting the best photocatalytic efficiency for VAM oxidation among all the photocatalysts tested in the project.

Photocatalyst materials hold the key for boosting the efficiency of photocatalytic reactions. The project investigated and identified beneficial materials characteristics of photocatalysts for high-efficiency VAM photocatalytic oxidation. Engineering new photocatalysts for VAM oxidation should aim to enhance the light absorption, improve charge separation and transfer, and increase the surface activity. For TiO2 photocatalysts, the high surface area associated with the small particle size and the anatase crystal phase are the preferred structural characteristics for high photocatalytic activity in VAM oxidation. Moreover, loading co-catalysts and enhancing light absorption properties significantly benefit VAM photocatalytic oxidation. There is a very minimal effect of the reaction temperature on the CH4 conversion rate particularly at low VAM flow rates. However, at the high flow rate, the increased reaction temperature is more significant in lifting the reaction rate. The test results under varied humidity conditions has indicated that for practical applications to VAM mitigation, the reaction rate of VAM photocatalytic oxidation would be not affected by the water molecules contained in the actual VA to be treated, which is generally at an ambient temperature and with saturated humidity. Another important outcome of the project is the developed test rigs for VAM photocatalytic oxidation tests including the batch mode and flow through photocatalytic reactors. The know-how on the photocatalytic tests gained through the project is useful for future research of emerging photocatalysis technologies for addressing coal mining environmental and health issues.

Since photocatalytic oxidation of VAM (or dilute methane in the air) is of a new idea and emerging technology for fugitive emissions abatement, there are limited understandings of reaction mechanisms of photocatalytic oxidation of CH4. Insights into the mechanism of CH4 photocatalytic oxidation is needed, which is important for the development of efficient photocatalysts and photocatalytic system for fugitive emissions abatement.

With the Xenon lamp as a light source in the tests producing the light closely mimicking natural sunlight, the obtained results in the current project also reveal the great potential of utilising photocatalytic oxidation for destructing methane in the atmosphere by using free energy from the sunlight. Photocatalytic oxidation of atmospheric methane is less time-restricted and a substantially high reaction rate is less critical to the application in atmospheric methane compared with for use in on-site mitigation units. Nevertheless, further research efforts would be required to evaluate and develop this new pathway for destruction of atmospheric methane for off-setting coal mining fugitive emissions.