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DPM that has been deposited in a filter must be burnt off regularly if the perform- ance of the engine is to be maintained. As Stachulak pointed out, the option of removing the filter and cleaning it manual- ly every shift is not practical, so the DEEP project has been searching for systems that can regenerate themselves automatically. This in turn presents challenges, since the burn-off process in a diesel particulate filter (DPF) requires it to operate at high temperatures for extended periods. As the CIM article earlier this year explained, "A vehicle's duty-cycle does not always allow the engine to run hot enough to reach the required burn-off temperature. If a DPF regenerates at 500°C, the engine will have to operate at or above this temperature at least 20% of the time to avoid excessive soot buildup." Potential solutions include both active and passive approaches. Active systems involve the use, for example, of fuel injec- tion into a DPF in order to maintain the correct temperature for regeneration, or to reduce the temperature needed. The pas- sive approach relies on the use of heating elements to achieve the required tempera- ture automatically on a cyclical basis. Then there is the question of fitting diesel oxidation catalyzers (DOCs), which although their main aim is to convert gaseous contaminants such as carbon monoxide and PAHs to water vapor and CO 2 , can also assist in filter regeneration by increasing the temperature. However, research has found that DOCs can also have an unwanted side-effect through the oxidation of NO x . The NO x effect Until the beginning of this year, when the Tier 4 Final emissions regulations came into force for mid-size (130–560 kW; 173–751 hp) off-road engines, engine manufacturers could achieve a balance between DPM and NO x emissions through operational adjustments. Not any more: the new requirements are for DPM emissions to remain more or less the same as for Tier 4 Interim, but with NO x emissions reduced by a half. This means emissions of less than 0.02 g/kWh of PM and 0.30 g/kWh of NO x in this class of engine, which is wide- ly used in underground mining machines. Stachulak and two co-authors, Mahe Gangal from CANMET Mining and Cheryl Allen from Vale, addressed the topic of NO oxidation in a paper presented at the 10 th International Mine Ventilation Congress. Held in Sun City in August this year, the meeting was organized by the Mine Ventilation Society of South Africa (MVSSA). In their paper, they noted that while diesel engines emit toxic NO 2 , the effect of DOCs on NO 2 production is not well under- stood: they may oxidize NO to NO 2 , thus increasing personnel exposure. The research on which their paper was based involved the evaluation of a series of DOCs fitted to a 242-kW off-highway engine under laboratory conditions, from which they found that while the DOCs were effec- tive at reducing CO and hydrocarbon emis- sions, NO 2 production increased signifi- cantly at higher exhaust-gas temperatures. By way of background, they explained that in 2012, the American Conference of Government Industrial Hygienists (ACGIH) revised its exposure threshold limit value for NO 2 from 3 ppm to 0.2 ppm over an eight-hour shift. Hence the need for a bet- ter understanding of the conditions under which NO 2 is produced during diesel- engine operation, with the aim of develop- ing effective control strategies and tech- nologies that are applicable to machines working underground. The Effects on Ventilation Design Without question, the implementation of tighter controls on the level of contami- nants allowable in the air in a working place has a direct impact on the design of ventilation systems. And while some of the controls are regulatory and mandatory, oth- ers fall into place simply because it makes technical sense for them to do so. Hence the comments in the CIM article attributed to Kevan Browne, communica- tions director at Cummins. From a decade- long position where emissions were driving all of the technology changes, he was quot- ed as saying, now that Tier 4 Final is in place the future focus for engine manufac- turers will be on performance improve- ments rather than regulatory compliance. Not all of the impacts on ventilation requirements are negative, by any means. In a paper presented at the 14 th SME Mine Ventilation Symposium in 2012, Robert Haney of Haney Environmental Consulting noted that while the traditional rule of thumb has been to allow 100 cfm of air per unit of horsepower for emissions dilution, this need not be the case now that the emissions-control technologies are in place. The change in fuel feeding from indirect to direct injection is just one factor that has helped in the battle to reduce emissions, he 86 E&MJ; • DECEMBER 2014 www.e-mj.com V E N T I L AT I O N The hard-rock mining industry in Western Australia has been at the forefront of gaining new understanding into DPM, its potential effects on personnel, and ways of mitigation. (Photo courtesy of AirEng)