Underground » Health and Safety
Control of diesel particulate matter (DPM) in the underground coal mine environment is most often achieved by use of diesel particulate filters (DPFs). Although they are generally effective at filtering DPM, DPFs have been plagued by poor service life, resulting in costly and ineffective control of DPM. The MTI-Monash University team has been working on short-term and medium-term solutions to this problem through two ACARP projects as part of an overall DPM reduction roadmap for underground coal mines.
A previous project, ACARP C21017, identified water carry-over from the water scrubber component of the diesel exhaust system (DES) as the dominant cause of poor filter life. Results from the investigation showed that filters from Freudenberg Filtration Technologies (FFT) could be dried and re-used without compromising filter performance or integrity. Although this solution is inexpensive and simple to implement, a far better solution would be to achieve an extension of filter life without the need for removal and drying. There are two complementary approaches to achieve this increase in filter life: (i) modifying the DES to reduce the water carry-over, and (ii) modifying DPFs so that they are less affected by water. Both these approaches were investigated in the current project.
Water transport from the scrubber could be in the form of liquid droplets due to aerodynamic processes or water vapour through an evaporative process, requiring correspondingly different ways to mitigate the problem. Results from experimental investigations and theoretical modelling of the DES show the following:
· The main reason for excessive water transport from the scrubber is water evaporation, not droplets due to aerodynamic processes;
· Water retention in the filter canister is due to a temperature drop across the canister resulting in water condensation.
· Water condensation in the filter canister could be resolved by modifying the temperature distribution of the exhaust as it passes through the DES;
· Some modifications to modify the temperature distribution of the DES were trialled in this project and showed a significant reduction in water transport from the scrubber;
· Further work will be required to redesign the DES to achieve this outcome in a practical and safe manner;
· A diagnostic method using the ion current signal could indicate excessive DPM emissions.
There have been recent developments in new filter models available to mine sites since the initial MTI-Monash ACARP project, including the use of fibreglass filters supplied by Global Mining Supplies (GMS) that were anecdotally less affected by water, and Cosway C100a filters supplied by UnderGround Mining (UGM). The main drawbacks of the former were the relatively poor filtration efficiency for the greening-in period, and also concerns about the possibility of fibreglass particles being ejected from the medium. The filtration efficiency of the latter filters was not investigated in previous projects. In addition to these filters, Freudenberg Filtration Technologies (FFT) have also recently released improved long-life filters.
The MTI-Monash University team has developed a comprehensive roadmap for DPM reduction in underground coal mines. In the immediate short-term, results from ACARP projects C21017 and the current investigation have already offered savings to mine operators by increasing DPF life. As a result of the previous ACARP project, C21017, mine sites that operate FFT filters can re-use them up to 5 times by intermittent drying without compromising filter efficiency or integrity. The current project has also provided an assessment of the filtration performance of experimental long-life FFT filters, data that is crucial to the real-world reduction in ambient DPM from use of these filters. For mine sites where GMS filters are used, the current project has presented a detailed set of test results for filtration efficiency and service life, as well as an investigation of the likelihood of ejection of fibreglass particles from the filter medium. Mine sites that use Cosway C100a filters will be benefited by the experimental results of initial filtration efficiency.
Long term improvements can be achieved through a redesign of the DES, or through better diagnostic tools for measurement of DPM. Both these options are discussed in this report and are proposed as future work.
The three significant outcomes of this project are:
· Water vapour from evaporation in the scrubber has been identified as the dominant mechanism of water transport into the filter canister. Condensation of the water vapour occurs in the filter canister due to heat loss;
· A modification to prevent water condensation in the filter canister was trialled and has the potential to greatly increase filter service life; and
· A modified fibreglass diesel particulate filter with a high initial filtration efficiency and a long term filtration efficiency comparable to the polypropylene filters commonly used by mine sites was developed.