Underground » Maintenance
Diesel particulate matter (DPM) control at underground coal mines has been an ongoing concern for some time, but mine operators have recently begun to take proactive approaches to improve the air quality in the mine atmosphere. Although there are other methods of reducing particulate emissions from diesel engines, by far the most effective approach is to use diesel exhaust particulate filters. Filters that are currently available have poor service life.
The main objectives of the project were to investigate the accelerated degradation of diesel particulate filters and understand the root causes so that strategies can be developed to improve filter life and reduce overall filter cost. Investigations were undertaken into following aspects in order to achieve these objectives:
The causes and mechanisms responsible for the DPM filter degradation;
Strategies that can be employed to reduce the degradation processes;
Safe methods of filter life extension without compromising filter efficiency and its primary function; and
Improved strategies for overall exhaust treatment to maximise filter life.
Filter life is governed by the backpressure imposed on the engine by the exhaust conditioning system that includes the filter. For many operational conditions, high filter backpressure has been shown to result from high water carry-over from the exhaust conditioning system, rather than due to accumulated DPM loading.
Water carry-over is a consequence of the exhaust system layout and engine duty cycle. Diesel engines operating underground make use of a water scrubber to eliminate exhaust sparking and reduce exhaust gas temperatures. Based on in-service testing, it is very likely that filters are changed prematurely due to high water retention, before they have reached their maximum DPM loading potential. Therefore, their life can be extended by regenerating the filter by removal of the excess water.
The project investigated the safety and performance implications of re-use of regenerated filters, both in terms of filter medium integrity and DPM filtration efficiency. Based on the current investigation, where water retention is the primary cause of poor filter life, filter life can be extended by re-using these regenerated filters up to 5 times without compromising filter medium integrity or DPM filtration efficiency. Therefore, filter life can potentially be increased five folds by implementation of practical regeneration methods. It is best if specific procedures for regeneration be developed in collaboration with filter manufacturers and vehicle OEMs rather than leaving that responsibility to individual mines. This aspect should be the subject of a separate investigation.
The wet and dry performance of three different brands of filters was investigated, along with their basic characteristics, using specifically designed laboratory tests and the Monash University Diesel Engine Facility. Due to availability issues of appropriate sizes of filters, one filter type was comprehensively tested but only a limited number of tests could be performed on the other two types. The information generated indicates that there is potential for improvement in filter designs to optimise the DPM capture while reducing the cost of filter usage.
The laboratory tests showed that the amount of water carried over from the scrubber can vary depending on different operating conditions and the control of the scrubber water levels. It is necessary to improve the performance of the wet scrubber systems to reduce the amounts of water released downstream into the filter canister. This aspect should also be a subject for a separate detailed investigation.