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Opportunity is in the Air

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World Coal,

The process of extracting minerals like coal from below the earth’s surface can be a major source of methane (CH4) emissions, but it does not have to be the major contributor to climate change that the world has come to fear. Thanks to advances in air pollution control and power generation technologies, mine operators are now looking at emission mitigation of ventilation air methane (VAM) as an energy source.

Experts estimate that 70% of the CH4 from coal mines is released as VAM; the remaining 30% is highly concentrated CH4, which is typically sent to gas engines for power or vapour combustors for destruction. Of all the recoverable CH4 from coal seams, VAM has the lowest concentration levels because of its high exposure to air that ensures miner safety. It is these relatively low CH4 concentrations (0.1 - 1.9%) that have made the effective utilisation of VAM relatively insignificant – until now. Mine operators are starting to leverage existing technologies and developing techniques for the mitigation of CH4 and utilisation of the byproducts from the emission abatement process.

Experts predict that more than 50% of all VAM is exhausted from mine ventilation systems and remains unutilised. CH4 emissions vary from mine to mine and regulations and incentives differ from country to country, but the technical feasibility of capture has advanced to the point where mines must evaluate its economic and environmental benefits, especially considering that upwards of 80% of the heat energy in VAM can be converted to useable energy.

Oxidisers are a well-established technology that have been utilised for decades to destroy volatile organic compounds (VOCs) and hazardous air pollutants (HAPs) from industrial processes and manufacturing operations. Oxidisers create an exothermic reaction using temperature, time and turbulence to break apart contaminants into carbon dioxide (CO2) and water vapour. The heat released during combustion is often used to preheat incoming emissions to the oxidiser, making it more efficient. That energy can also be recovered and converted into heated process air, plant heat or even electricity.

Some of the more traditional oxidiser applications treat off-gases from coating, packaging, painting and printing processes. While several oxidiser suppliers began pilot scale demonstrations for VAM emission mitigation in the late 1990s, the abatement technology has only achieved commercial scale in the past decade. A possible reason for this lag is lack of regulation, but mine locations, large flows and unpredictable concentrations surely play a role in adoption. Demand for oxidisers in most industries is almost always regulatory driven, but it can be economics that creates interest, especially when payback exists like it can in mines.

Payback drives demand

Very few industries use custom-designed oxidiser solutions like the mining industry and the unique project conditions warrant it. These systems must be built conservatively and with great flexibility to ensure continuous operation, as most mine operators demand that the emission control system and related hooding for collection have zero impact on the operation and employee safety.

Often referred to as a VAMTOX in this industry, the regenerative thermal oxidiser (RTO) is the preferred technology for VAM due to its extremely effective and efficient operation. The internal heat recovery can be as high as 97% and still leave enough energy in the stack for secondary heat recovery and utilisation. Emission-laden air is either pushed or pulled into the RTO inlet manifold of the oxidiser via a system fan.

Flow control or poppet valves then direct this gas into ceramic energy recovery chambers where it is preheated. The process gas and contaminants are progressively heated in the ceramic media beds as they move toward the combustion chamber. Once oxidised in the combustion chamber, the hot purified air releases thermal energy as it passes through the media bed in the outlet flow direction.

The outlet bed is heated and the gas is cooled so that the outlet gas temperature is only slightly higher than the process inlet temperature. Poppet valves alternate the airflow direction into the media beds to maximise energy recovery within the oxidiser. The high energy recovery within this type of oxidiser reduces the auxiliary fuel requirement and saves operating costs. The regenerative style oxidiser consistently achieves destruction efficiencies over 99%.

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