With diesel-engine vehicles ubiquitous in underground mining, and Tier 4 Final requirements now in place, research is focusing on how best to operate ventilation systems to handle residual emissions

By Simon Walker, European Editor


Dirty exhaust streams away from a truck in a small French uranium mine 25 years ago. (Photo: Simon Walker)

The introduction of diesel-engine vehicles into underground mining in the late 1920s brought with it a whole new set of challenges, not all of which have been addressed completely yet.

Locomotives led the way, with the subsequent move toward trackless haulage and personnel transport in both coal and hard-rock mines adding further complexity to the situation.

National research organizations were, in some respects, well ahead of the game; for example, the U.S. Bureau of Mines began its work on the effects of diesel emissions underground as early as the 1930s. This program continues, under the auspices of National Institute for Occupational Safety and Health (NIOSH), and was reviewed in detail in the Society for Mining, Metallurgy and Exploration (SME) 2010 reference volume, Extracting the Science: A Century of Mining Research.

Anyone who has had the experience of working with diesel engines in confined spaces will readily acknowledge that emissions can be singularly unpleasant from a number of perspectives. Smoke, smell and eye irritation are the most obvious, and were the first to be tackled in terms of post-combustion treatment. Early solutions focused on technologies such as water-traps, through which the engine exhaust was bubbled, the aim being to grab as much of the visible soot and unburnt fuel as possible.

Aside from direct impacts on the individual, diesel-engine vehicles brought with them wider implications for the design and operation of ventilation systems. Not only did they need additional air in order to run efficiently, but ventilation volumes needed to be increased in order to handle the new source of heat generation and to dilute emissions. And while these issues could be addressed relatively easily in main haulages where airflows were strong, the same could not be said for blind development ends or drawpoints where the atmosphere could quickly degenerate as exhaust fumes built up. Auxiliary fan systems provided an answer, with the recent development of ventilation on demand (VOD) bringing a new level of control to managing air supplies on an as-needed basis while cutting overall ventilation costs.

The last time that E&MJ looked at ventilation (December 2013, pp.60–69), mention was made of the increased concerns within the hard-rock mining industry of the effects of diesel particulate matter (DPM) on workers’ health. This is, of course, by no means confined to the mining industry, with widespread recognition of the damaging potential of DPM within society as a whole: one has only to look at requirements for trucks and buses to be equipped with exhaust filters in day-to-day life for that to be apparent.

The need for diesel engines to perform better on a number of fronts, including minimizing their emissions, has led to long-term efforts by legislators and manufacturers alike to achieve specific, staged targets. As the Canadian Institute of Mining (CIM) pointed out in an article in the June/July edition of its magazine earlier this year, achieving Tier 4 Final compliance has been a 20-year task, and while the mining industry has in some respects lagged behind the adoption of each stage on the journey, the manufacturers that supply engines for LHDs and other underground vehicles are working hard to transfer the new standards to the much heavier-duty mining environment.

As the CIM article also noted, from January 2015 every new diesel engine sold in the U.S. will have to achieve a 90% reduction in DPM and nitrogen oxide (NOx) emissions compared to pre-1996 levels. And while meeting targets like this is hard enough in the open air, the enclosed environment of an underground mine drift is an even tougher place to achieve compliance.

AUSTRALIAN AND CANADIAN PERSPECTIVES
The fourth Annual Hard Rock Mine Ventilation conference, which was organized by IQPC and took place at the end of April in Perth, Western Australia, included several workshops addressing specific areas of ventilation—including DPM and VOD. In her workshop notes the facilitator for the VOD event, Dr. Allison Golsby, CEO of Consult-Mine, provided an overview of the current position in Australia in relation to DPM.

“The Coal Services (NSW), Simtars (Qld) and the WA Mining Department are monitoring and raising awareness of the DPM and dust health issues,” she said. “The industry, to their credit, is seeing less of the dust and DPM diseases, while less contamination in the ventilation will improve production, health and safety, while reducing downtime and maintenance over time.

“DPM history as an emission has not changed,” she went on. “Diesel and petrol [gasoline] engines all produce emissions and heat. These issues are magnified underground as the emissions cannot escape from where they are produced, so they need to be managed and removed. The management of these issues is developed by ventilation officers and engineers, in consultation with mine management and the workforce.”

Meanwhile, the properties of DPM (which is basically all the exhaust particle emissions) are changing. “A Tier 1 particle (about 200–300 micron (µm) in size) could usually be removed from the airways using normal human functions. However, since the DPM size is reducing as a result of the engineering solutions to remove the visible smog from engines, the smaller particles are harder to remove naturally from our lungs.

“The industry is now aware of the issues and new legislation is going to drive the maximum exposure to lower levels. The other trend is that the industry is aware that there are long-term health problems from not managing these hazards, so mines, operators and regulators are developing new technologies as well as using monitoring, maintenance and proactive management systems to reduce the heat, fumes, DPM, gas, humidity and dust levels in mine ventilation systems,” Golsby explained.

In Canada, meanwhile, the Mining Diesel Emissions Council (MDEC) program has now been running in various guises since the 1970s, with its 20th annual conference having taken place in early October in Toronto, Ontario. From its early days as a joint research project between the Canadian, U.S. and Ontario governments into the effects of diesel engine emissions and potential control systems, the MDEC has since become an internationally recognized forum for government organizations, mines, engine manufacturers and emission control-systems suppliers to discuss how best to progress technological developments in this field.

By way of illustration, the agenda for this year’s conference included presentations on the use of biodiesel, fuel additives to reduce emissions, health aspects, DPM measurement systems, exhaust filter technology and alternatives to the use of diesel engines (such as fuel cells), as well as a workshop on OEMs’ integration of Tier 4 engines into mining machines.

Equipment suppliers have approached the challenge of reducing diesel particulate content in mine air by employing active and passive methods. Shown here, Caterpillar’s DPF is designed to work with its Ventilation Reduction (VR) Package on Cat LHDs and trucks. The DPF, says Cat, will not plug with particulates and ash and requires no additional service intervals for cleaning.Equipment suppliers have approached the challenge of reducing diesel particulate content in mine air by employing active and passive methods. Shown here, Caterpillar’s DPF is designed to work with its Ventilation Reduction (VR) Package on Cat LHDs and trucks. The DPF, says Cat, will not plug with particulates and ash and requires no additional service intervals for cleaning.

DPM: THE CHALLENGES
In a presentation to the 2013 Hard Rock Mine Ventilation conference, also organized by IQPC, Dr. Patrick Glynn of the Australian research organization, CSIRO, expanded on some of the issues relating to DPM. “With the aim of reducing DPM mass, engine manufacturers have improved the combustion efficiency of diesel engines by the introduction of common rail and turbocharging,” he noted, before pointing out that: “An unwanted outcome of the improved diesel engine efficiency has been an increase in the number of diesel particulates with a more than 50% reduction in average diesel particulate size.

“This reduction in DPM size is of particular concern as larger DPM (less than 2.5 µm) coated with polyaromatic hydrocarbons (PAH, a known carcinogen) will affect a minority of the population, whereas the smaller (less than 100 nanometer) DPM can cross the lung membrane barrier into the bloodstream. This has the potential for health effects on 100% of the population,” Glynn cautioned.

Cutting DPM emissions by whatever means is most appropriate is seen as being a better, more cost-effective way of addressing the problem than merely increasing ventilation flows to compensate for them. In Canada, the Diesel Emissions Evaluation Program (DEEP) project has been running for nearly 15 years, with Vale’s operations in the Sudbury district having been one of the research centers. In 2010, the company’s project leader, Dr. Joe Stachulak, noted in an interview that the main challenge had been that previous exhaust-treatment systems required extensive human intervention.

DPM that has been deposited in a filter must be burnt off regularly if the performance of the engine is to be maintained. As Stachulak pointed out, the option of removing the filter and cleaning it manually 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 injection into a DPF in order to maintain the correct temperature for regeneration, or to reduce the temperature needed. The passive approach relies on the use of heating elements to achieve the required temperature 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 CO2, 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 NOx.

THE NOX 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 NOx 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 NOx emissions reduced by a half. This means emissions of less than 0.02 g/kWh of PM and 0.30 g/kWh of NOx in this class of engine, which is widely 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 10th 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 NO2, the effect of DOCs on NO2 production is not well understood: they may oxidize NO to NO2, 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 effective at reducing CO and hydrocarbon emissions, NO2 production increased significantly 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 NO2 from 3 ppm to 0.2 ppm over an eight-hour shift. Hence the need for a better understanding of the conditions under which NO2 is produced during diesel-engine operation, with the aim of developing effective control strategies and technologies that are applicable to machines working underground.

THE EFFECTS ON VENTILATION DESIGN
Without question, the implementation of tighter controls on the level of contaminants 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, others 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, communications director at Cummins. From a decade-long position where emissions were driving all of the technology changes, he was quoted as saying, now that Tier 4 Final is in place the future focus for engine manufacturers will be on performance improvements rather than regulatory compliance.

Not all of the impacts on ventilation requirements are negative, by any means. In a paper presented at the 14th 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 said, before showing that air requirements for satisfactory engine operation could be as low as 39 cfm/hp for Tier 4 Final engines. Much depends on the level of maintenance, unwanted fuel constituents such as sulphur, and the specific duty cycle for an individual machine, he added, while pointing out that older engines still in use, such as those designed to Tier 2 standards, may actually require higher airflows in order to dilute exhaust NO2.

“If airflows are reduced by using filtered Tier 2/3 or Tier 4 engines, the airflow needed for dust control or heat and humidity control may govern ventilation requirements,” he concluded.
As noted in last year’s E&MJ article, the Australian authorities have been pushing hard for the adoption of new, lower DPM limits, such that in Western Australia the eight-hour threshold for exposure to elemental carbon is now 0.1 mg/m3. Speaking at the MVSSA congress, Dr. Michael Tuck from Federation University in Australia looked at the impact of these new guidelines on dilution airflow rates, and whether dilution ventilation is the best way of achieving compliance. The baseline used for the study was the previous requirement for a minimum dilution airflow of 5 m3/s per 100 kW of rated diesel power, which equates to around 80 cfm/hp.

Tuck took as his example the equipment used in a small underground gold mine, including two LHDs, some mine trucks, a charging unit, a wheel loader and a grader. He used the same approach as Haney had for his SME paper to derive airflow volumes for both the ventilation rate and particulate index (which is the airflow requirement needed to dilute DPM to 1 mg/m3), comparing the results for both older and newer engines.

The study results suggested that, “from a gaseous emissions perspective, the new guideline has little impact on airflow requirements.” However, Tuck added, meeting the new WA guideline for DPM dilution will result in higher airflow requirements. In addition, he said, there is another variable. “The WA guideline is based on 0.1 mg/m3 of elemental carbon, whilst the MSHA calculations [used by Haney] are based on the total carbon content. The relationship between total carbon and elemental carbon depends on a wide range of factors, including the type of diesel fuel, engine and filtration maintenance, machine operation and a number of other factors,” Tuck pointed out.

Looking at a larger underground operation, Newmont’s Troy Terrillion and Sandeep Arya reported to the 2012 SME symposium on the DPM control measures that have been put in place at the Leeville gold mine in Nevada. The much larger machine fleet there has a nameplate engine capacity of some 30,000 hp.

Measures adopted at the mine opening in 2006 included the use of ultra low-sulphur diesel, although most of the equipment in use then was only fitted with standard engines and did not have closed cabs. Subsequent improvements have involved replacing engines with more modern, low-emission units, changing the fuel firstly to a B10 then to a B50 (50%) biodiesel blend, fitting environmental cabs to production machines, and equipping all the units underground with DPM filters. A program of raise development also helped, allowing ventilation to flow through the workings, and the mine introduced an engine emissions-based program for its equipment maintenance.

The topic of the relative benefits of biodiesel and other fuels in relation to conventional diesel in respect to reducing emissions is outside the focus of this article, and will be addressed in E&MJ more specifically at a future date. However, suffice it to say for now that research reported at the MVSSA congress by Mark Wattrus and Roger Smith showed that using low-sulphur (10 ppm) diesel produced by the Sasol synthesis plant in South Africa cut DPM emissions by more than 20% when compared to the normal, 500 ppm diesel available in the country, as well as reducing SO2 and CO emissions. Ventilation costs were also cut because of the lower dilution volume needed.

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)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)

VENTILATION ON DEMAND?
On the face of it, a ventilation system that supplies a sufficient air volume for safety then supplements this on an as-needed basis when vehicles are at work, or to clear blasting fumes, has a lot going for it. Last year’s E&MJ article described such a system in use at Boliden’s Kristineberg mine in Sweden, claimed to have cut ventilation energy costs by some 30%.

As MineConsult’s Allison Golsby pointed out in her notes for the VOD workshop held in conjunction with this year’s conference in Perth, since ventilation costs can be a considerable proportion of mine’s energy costs, such savings can make a marked difference to the bottom line.

“Other benefits from implementing VOD systems relate to more effective dust, fume, DPM, heat and gas management, reduced pressure differentials, and thus lowered leakage,” she said, quoting Jared Haube of IQPC. “Fans can be on standby and maintained without reducing production.”

VOD has many other advantages, Golsby went on. An improved understanding of the mine ventilation system can help with troubleshooting, return airway monitoring to check the effectiveness of the ventilation design, detecting regulator failures or incorrect settings, detecting stopping failures or airway blockages, and in providing an overall assessment of fan performance. In terms of energy and cost savings, VOD can help redirect air from non/low-production zones, allow for scheduled periods for maintenance, holidays, shift changes and other downtime, reduce air requirements when diesel-powered machines are not working, reduce fan power consumption, eliminate air waste and maximize the airflow in high-production areas, and cut clearance times after blasting.

How VOD is organized is a matter for the individual operation, of course. At the MVSSA congress, Enrique Acuña and Roberto Álvarez from Codelco and Dr. Stephen Hardcastle from CANMET Mining compared three theoretical strategies for ventilating blind headings: leaving fans running all the time; running fans only when a diesel vehicle was working in the end (event-led); or fans operating only when contaminants reached certain levels (quality-led).

Both VOD strategies outperformed continuous fan operation in terms of energy usage. When it came to differentiating between event-led and quality-led strategies, the outcome was less clear, the authors said, noting that the benefits of a quality-led approach increased as contamination generation reduced through the use of modern emission-control technologies.

In addition, they said, not enough is known about the lifespan impact of turning a fan on and off repeatedly. While an event-led strategy might be effective, it could end up being more costly if the fan needed more maintenance, so fan-speed control could offer another option instead of “on” or “off.”

The last word goes to Allison Golsby. “Ventilation increases production time by reducing downtime, especially using ventilation on demand. VOD also manages day-to-day measurement and monitoring, allowing air to be moved to where it is needed.”

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