ABSTRACT:Anthropogenic emissions of non-CO 2 greenhouse gases such as fugitive methane contribute significantly to global warming. A review of fugitive methane combustion mitigation and utilisation technologies, which are primarily aimed at methane emissions from coal mining activities, with a focus on modelling and simulation of ultra-lean methane oxidation/combustion is presented. The challenges associated with ultra-lean methane oxidation are on the ignition of the ultra-lean mixture and sustainability of the combustion process. There is a lack of fundamental studies on chemical kinetics of ultra-lean methane combustion and reliable kinetic schemes that can be used together with computational fluid dynamics studies to design and develop advanced mitigation systems. Mitigation of methane as a greenhouse gas calls for more efforts on understanding ultra-lean combustion. Recuperative combustion provides a promising means for mitigating ultra-lean methane emissions. Progress is needed on effective methods to ignite and to recuperate and retain heat for oxidation/combustion of the ultra-lean mixtures. Catalysts can be very effective in reducing the temperatures required for oxidation while plasmas may be utilised to assist the ignition, but thermodynamic/aerodynamic limits of burning ultra-lean methane remain unexplored. Further technological developments may be focussed on developing innovative capturing technology as well as technological innovations to achieve effective ignition and sustainable oxidation/combustion.

Large-eddy simulation of the reacting flow field in a combustion-based mitigation system to reduce the emissions of methane contained in ventilation air methane is presented. The application is based on the preheating and combustion of ventilation air methane. Effects of preheating and methane concentration are examined in five computational cases. The results indicate that the oxidation of the ventilation air methane can take place in a co-annular jet configuration provided that the preheating temperature is as high as 500 K for mixtures containing a low methane concentration of 0.5%. It is found that the oxidation process that eventually leads to reaction and combustion is controlled by the methane concentration and the level of preheating.

The greenhouse gas (GHG) radiative forcing factor of methane is often quoted as 23 times as potent as carbon dioxide on a 100-year time horizon; thus, any reduction in atmospheric methane would be globally beneficial. The capture or use of ventilation air methane (VAM) is challenging because it is a high volume low concentration methane source. This results in the routine discharge of methane into the atmosphere.A review of VAM mitigation technologies is provided and the main disadvantages of the existing technologies are discussed. In the proposed VamTurBurner© system, the heat from the combustion chamber is transferred to the preheating zone either by a heat exchanger or by redirecting the combustion products to mix with the ventilation air stream from a coalmine. Gas turbines (GT) are used to produce electricity with the exhaust gases directed to mix with the incoming ventilation airflow. The turbulence introduced by the GT exhaust assists with mixing of the incoming ventilation airflow and the return flow of combustion products from the combustion chamber. The combustion products are a source of heat, which increases the temperature of the incoming ventilation air to a value high enough for the methane to undergo flameless combustion upon encountering the igniters.The high temperature combustion products enter a multi-generation system. The multi-generation system is based on mature engineering technology such as heat exchangers and steam turbines. The residual heat provides additional heat based products such as industrial scale drying, chilling by an absorption chiller or simply hot water.The VamTurBurner© uses the energy from the GT, igniters and VAM to provide clean efficient energy while mitigating the atmospheric emissions of methane. The opportunity to collect carbon credits may improve the economics. Since the VAM is a free energy source, the output of the system is greater than the purchased energy.

Deeper mining projects necessitate that ground control in high stress environments, ventilation and heat stress management be extended to the limit of current technologies; thus represent substantial research and development efforts to realise safe and profitable deep mining. In this paper, a concept is presented that serves to advance the potential for providing services and infrastructure for deep mining at potentially reduced costs. Liquid air (LA) is an energy storage vector that provides the opportunity for energy cost arbitrage through peak shaving or by smoothing the operation of wind turbines or solar power systems. By producing LA during operation of the wind turbines or during daylight hours and releasing the energy during low wind periods or evenings, the energy can be distributed more economically over time The infrastructure required for LA facilities is a reliable mature technology and competitive with competing energy storage systems. As the mine depth increases, a LA energy storage system can deliver more than just energy storage, because a LA system may reduce the amount of some critical financial variables. For example, LA can deliver: chilling to offset auto-compression, improvements to ventilation on demand, rapid rock skin temperature conditioning, heat absorption at depth, arbitrage of electrical energy cost shifting, compressed air supply and operation of large vehicles. The Dearman engine uses LA as a fuel, consequently when it is employed in trains, scoops and other equipment the exhaust of the large underground equipment becomes a source of cool air, which further reduces the ventilation required because dilution of diesel and removal of heat generated by equipment is no longer part of the ventilation calculation. LA can provide compressed air, routinely used to operate equipment or provide emergency ventilation. The losses in compressed air systems from long distance delivery and leaks en route, a common complaint of mine managers, are offset by using underground LA expansion; further absorbing heat and the proximal storage of the liquid air source near the compressed air module reduces line losses. Electricity can be generated using modular gensets so the cost of electricity substations and high voltage trunk lines to underground are replaced or supplemented by using modular underground LA electricity generation. Stirling engines are capable of operating efficiently between the LA and ambient temperatures at high efficiencies and can be used for heavy duty pumping, milling or electricity generation. The efficiency of Stirling engines can be further increased by using a solar collector with a high temperature heat storage system, such as oil or nitrate salts, to increase the efficiency. All uses of LA benefit from a higher rock temperature as the depth increases because the Carnot efficiency of a heat engine increases as the temperature difference increases. Numerical modelling of the interaction between the LA introduced to a ventilation flow and the rate of heat absorption from the un...

Deep mining presents a challenging environment for materials handling, but geotechnical and thermodynamic aspects are among the most problematic. In this paper, the development of cryogenic chilling and the impact of ancillary cryogenic technologies is discussed. Cryogenic chilling is a straight forward system based on technologies that have benefited from over a century of engineering; thus, are reliable and easily purchased from numerous suppliers. As the depth increases, the simple extension of the delivery piping and installation of local storage and vaporiser systems is all that is required. There is no return circuit required as the liquid exits the vaporiser to become part of the ventilation airflow. This paper will provide an understanding of the physics of cryogenic chilling and preliminary designs of the technologies required to deliver the chilling. The concept of chilling on demand is discussed in terms of providing consistent temperatures given varying heat loads. The introduction of electric vehicles to deep mining has the impact of potentially reducing the ventilation flow by 40 to 50% of that required by legislation when diesel equipment is in use; however, this leads to a high susceptibility to larger temperature changes for lesser amounts of heat introduced. This can be problematic if the flow cannot be increased to carry the heat away, say, just after a blast or in an area that has several electric vehicles in operation, which could produce temperatures beyond the allowable working limits rather quickly. Cryogenic chilling is an on demand system, able to respond rapidly by simply increasing the liquid flow. Not only does the liquid air provide chilling, it replaces some of the air that would be drawn from the surface, which can significantly reduce the main fan power. The concept of cryogenic chilling provides an opportunity for a chilling option that has additional uses, which can be implemented to offset the capital expenditure due to economies of scale. The scaling factor for a liquid air plant is about 0.45, so doubling the capacity from 2,000 to 4,000 tpd requires only 36% more capital expenditure and allows for the option to take advantage of ancillary markets such as the sale of oxygen and argon to the industrial market. Since mines are often remote, energy is expensive, but a cryogenic energy storage system in conjunction with wind or solar power provides for a greater energy penetration. A brief discussion of the energy storage technology is provided in the introduction. Availability of liquid air also provides the opportunity for compressed air on demand systems that deliver chilling simultaneously. An emerging cryogenic vehicle technology from the Dearman Engine Company does the exact opposite of diesel engines; for a 100 kW Dearman engine, about 200 kW of cooling is concurrently delivered in situ. In the near future a techno-economic analysis for the total cost of ownership comparison between a Dearman engine versus a diesel engine will be provided.