Mine Site Greenhouse Gas Mitigation » Mine Site Greenhouse Gas Mitigation
The possibilities of oxidizing mine ventilation air (MVA) containing up to 1% methane over a catalyst have been reviewed. It appears possible to catalytically combust methane in varying concentrations up to 4% using a palladium on alumina catalyst. It will be necessary to mount the catalyst on a metallic monolith with separate heating such as a small flame upstream, fuelled from a separate source.
Three types of catalyst for the catalytic oxidation of methane have been found to be of interest. Palladium based catalysts are most efficient at lower temperatures, with light off occurring at about 300?C. No significant advantage could be achieved by the use of ceria or zircornia supports, and Pd/alumina is a satisfactory catalyst.
Metal oxide catalysts have also been widely used, but they are somewhat less active than Pd based systems and tend to be more sensitive to sulphur poisoning. Their best attributes are their stability at higher temperatures, making them highly suitable for catalytic gas turbines. Manganese and lanthanum doped perovskites are among the most active catalysts, with a 50% light off temperature of ca 400?C.
The second groups of mixed oxides - the hexaluminates - are of interest mainly because of their thermal stability. Light off temperatures (50%) as high as 720?C have been reported. These high temperature stabilities are not necessarily significant with MVA.
It seems unlikely that significant improvements in catalyst performance can be achieved in the context of the total removal of methane from MVA. This results from the fact the best catalysts (primarily Pd/alumina) light off at ca 300oC and need to be heated to higher temperatures (ca 500oC) to achieve 100% conversion. Despite very extensive research covering a wide range of materials, all catalysts so far examined do not improve on this performance.
If further research is to be commissioned to study catalyst performance, attention should be focused on minimising light off temperatures and minimising the temperature at which 100% conversion is achieved. The possibility of catalyst poisons and long term catalyst stability should also be examined. It seems likely that most common poisoning may be minimised by increasing operating temperatures - to values that will depend on the actual poison. The most promising systems would seem to be based on perovskites, but significant improvements, in the opinion of the author, should not be expected.
The volumes of MVA are such that catalyst should be suspended on a monolith. In order to maintain even temperature distribution, a metallic monolith such as Fecralloy is recommended. The monolith requires a coating of wash coat, support and catalyst.
Given that the methane concentration is low and variant, particular attention must be paid to temperature control of the monolith and separate heating of the catalyst or gas stream to about 500oC is recommended. This may be done with an upstream flame, fuelled from a separate source, or by electrical heating. Heat exchange from the exit gas to the inlet gas is recommended in order to save heating costs. After heat exchange, the exit gases should be suitable for drying etc, with an exit temperature expected to be of the order of 200 - 250?C.
Concentrations of hydrocarbon ranging from 10 - 4 to 4 - 6% have been reported to be oxidised catalytically with 99% efficiency. Although methane is the most difficult hydrocarbon to oxidise, similar performances can be expected with MVA.