Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation
One of the key challenges facing the Australian coal industry is to improve the sustainability of its operations and products. Previous research has demonstrated that significant gains can be made by mitigating the methane contained in mine ventilation air (MVA). Converting this methane into carbon dioxide will lead to an immediate reduction of 87% in global warming potential from coal mines in Australia, defined as the difference between emitting methane and carbon dioxide to atmosphere.
MVA emissions are characterised by very low methane concentrations (typically less than 1%) and high flow rates, with significant fluctuations in both. These attributes constitute the major barriers to the oxidation of the methane in MVA. Consequently, a cost effective mitigation technique has yet to be proven.
Porous burners are a possible lean-burn mitigation technology with the potential to overcome these challenges. They operate on the principle that the presence of a porous solid in the combustion chamber of the burner serves as a means of recirculating heat from the hot combustion products back to the incoming methane/air mixture; this preheating of the incoming mixture enables the burner to operate in 'ultralean' mode on gases with a methane concentration below the lean flammability limit for a free flame (5 vol% methane in air).
The project had two objectives: The first was to investigate the applicability of porous burners for the combustion of the methane component of MVA. This involved determining the lowest combustion limit able to be achieved, as well as operating and control methodologies for mine site implementation. The second was to investigate the underlying scientific principles of lean methane combustion, including the influence of firing rate and heat recovery. This research is relevant to all MVA mitigation technologies that use combustion, including the Megtec VocsidizerTM and CSIRO VamcatTM systems.
To meet both objectives, a novel porous burner with a design flowrate of 129 kg/hr and duty of 40 kW was constructed. The burner comprised a 90 cm high, 26 cm diameter cylindrical combustion chamber filled with a porous bed of 60% alumina, ½" ceramic saddles, combined with an arrangement of heat exchanger tubes for preheating the incoming methane/air mixture. In designing the burner a computational fluid dynamics model of gas flow into the combustion chamber was developed in order to verify the suitability of the design as regards providing an even flow distribution. The model was used to evaluate two alternative designs-one in which the methane and air feeds were separate, and the other in which they were premixed-the burner was ultimately constructed in the premixed configuration.
A series of experiments was undertaken in which burner operating conditions were systematically varied and the temperature profile in the porous bed measured to determine if, and where in the bed, the flame stabilized. The exhaust gas was monitored for carbon monoxide and unburned hydrocarbons to confirm that combustion was complete. In respect of the first project objective, results illustrating transient combustion at a feed methane concentration of 0.5% are presented, along with a novel control strategy for continuous operation of a porous burner system at an operating mine site that does not require a flow reversal system. In respect of the second objective, results illustrating the burner's full operating range, behaviour and performance are presented.
Stable (i.e. with a stable flame front) ultra-lean combustion was demonstrated at methane concentrations of > 3 vol% methane at firing rates between 50 and 300 kW/m2, with transient combustion (i.e. a moving flame front) observed for more dilute mixtures (0.5 vol%). The combustion process was very stable to perturbations in the methane concentration and flow rate, typically taking several hours to react to any changes. When operating in the transient combustion regime, the depth of the bed allowed for ~ 45 minutes of combustion before a spike of higher concentration gas was required to return to flame front to the bottom of the bed. This process could be repeated indefinitely.
The collective results from the project suggest that porous burners are a suitable technology for MVA methane mitigation; they are simple and low cost with similar performance limits to existing MVA combustion technologies.
Further research is recommended into the use of stable, high temperature combustion catalysts supported on novel porous materials that will improve operating performance (i.e. lower the methane concentration limit and reduce the bed pressure drop) and consequently decrease operating costs. The completion of an engineering study is also recommended; such a study should incorporate an assessment of the commercialisation potential of the technology, including capital and operating costs, mine vent integration and options for remote operation.