Reducing the global warming potential of coal mine ventilation air by combustion in a free-piston engine

O'Flaherty, Brendan (2012). Reducing the global warming potential of coal mine ventilation air by combustion in a free-piston engine PhD Thesis, School of Mechanical and Mining Engineering, The University of Queensland.

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Author O'Flaherty, Brendan
Thesis Title Reducing the global warming potential of coal mine ventilation air by combustion in a free-piston engine
School, Centre or Institute School of Mechanical and Mining Engineering
Institution The University of Queensland
Publication date 2012-06-01
Thesis type PhD Thesis
Supervisor Peter Jacobs
Richard Morgan
Total pages 267
Total black and white pages 267
Language eng
Subjects 0913 Mechanical Engineering
Formatted abstract
The increase in the atmospheric mole fraction of greenhouse species since pre-industrial times has forced Earth's atmosphere to higher temperatures. The individual effect of these species on the atmosphere is compared using a global warming potential (GWP) index, which is the cumulative radiative forcing of a species over a given period relative to carbon dioxide (CO2). After CO2, the next highest contributing species to global warming is methane (CH4). Over a hundred-year period, the GWP of CH4 produces a twenty-five times greater heating effect than CO2 [1].

One such anthropogenic source of CH4 is that of underground coal mines. For Australia, this source constitutes 6.5% of its greenhouse gas emissions [2]. Of this, almost two-thirds are contained in mine ventilation air at mole fractions typically between 0.3 and 0.7% and flow rates between 150 and 300m3s-1. The CH4 is purposefully diluted to ensure safe operation of the mine. If the CH4 contained in the ventilation air from these mines were converted to CO2 by combustion, for example, the result would be a 73% reduction in the GWP of ventilation air. The problem that will be addressed in this thesis is: can the CH4 in ventilation air be oxidised using a self-sustaining process (and without a supplementary fuel or catalyst) in order to reduce its global warming potential?

For selection of a device to employ for this task, consideration was given to both the operating temperature and the thermal efficiency. After a review of potential devices, a reciprocating engine concept was selected for investigation, primarily since compression ignition occurs at temperatures too low for nitrogen chemistry to be significant.

Engines are by definition work producing devices, however, the purpose of this device is not principally to produce work but instead to burn ventilation air, converting it to less harmful products. The engine would be applied primarily as an emission control device. However, even without aiming to produce power, the operation of an engine on a <0.7% mole fraction of methane represents a substantial challenge. Diesel engines, for example, may run well at very low global equivalence ratios, but ignition occurred in a region surrounding vaporising droplets where the stoichiometric ratio locally passes through the optimum range for ignition. It is harder to ignite the low equivalence ratios for premixed CH4 mixtures of uniform composition. At 37MPa, for example, the ignition temperature is about 1330K -- much higher than the pre-injection temperatures of about 900K found in even large stationary diesel engines. The main objective of this thesis is to evaluate a conceptual engine design which is capable of achieving combustion of these weak mixtures.

The proposed device makes use of the "free-piston" concept, whereby an unconstrained piston is contained between two in-line, opposing combustors. Oscillation of the piston along the axis of the cylinders drives the thermodynamic cycles in both combustors. A linear electric motor provides the energy to drive the engine with the moving piston representing the driven element. The same motor can also be used to extract energy from the piston if it is available.

The problem of oxidising ventilation air in this way was approached in three parts which form the body of material for this thesis. The first investigated the thermochemical properties of ventilation air itself to find a suitable gas model and kinetic mechanism. The second investigated the modelling of the component processes including piston dynamics, cylinder exhausting, heat-transfer, friction and control models. The third combined these processes into a free-piston engine model which was then used to find the minimum mole fraction of CH4 required to sustain operation.

The free-piston engine designed for this application had a 0.16m bore, 1.1m cylinder length and 200kg piston mass. The net cycle output was found to be insufficient to overcome the associated losses with the models based on current diesel engine technology and the T4 shock tunnel. The analysis showed that low friction piston seals will be essential to make the concept work. Lower friction seals may be able to be developed since the sealing demands for this engine are not as rigorous as for a normal engine, where the sump oil has to be protected from gas leakage.

If the CH4 content increases above about 0.9%, the cycles become self sustaining and positive energy can be generated. Alternatively, if about 15 MJ.kgCH4-1 of work were added, this device could be used to reduce the GWP of ventilation air. In effect, for this application, the engine would have to be partially motored. Multiple engines would be required to process all of the ventilation air.

This thesis has established the basic principles for evaluating the viability of systems for low mole fraction CH4 conversion by combustion. It sets the groundwork for a family of similar devices and has identified critical issues which need to be addressed to enable the technology. The analysis can be refined as needed by incorporating more detailed physical models of the various processes involved, such as multidimensional flow. Useful generic tools have also been developed which may have other applications.
Keyword Global warming potential
Free-piston engine
Real gas models
Finite-rate chemistry
Unsteady heat transfer
Mixed friction

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Created: Wed, 04 Jul 2012, 19:21:41 EST by Brendan O'flaherty on behalf of Scholarly Communication and Digitisation Service