June/July 2014

A cleaner burn

Modern diesel engines keep soot and nitrogen oxides under control

By Eavan Moore

On January 1, 2015, the final stage of a 20-year journey toward near-zero diesel emissions will commence. By that date, every new diesel engine sold in the United States will reduce the diesel particulate matter and nitrogen oxide in its exhaust stream by more than 90 per cent of their pre-1996 levels, in accordance with the Tier 4 Final regulations phased in by the U.S. Environmental Protection Agency (EPA).

“The regulations have preoccupied every engine manufacturer,” says Kevan Browne, communications director at Cummins Engine Company. “What are the emissions levels? How do we meet them? And when do we meet them?”

Diesel combustion produces a soot composed of carbon, hydrocarbons, and solid sulfur compounds and generates toxic gases and vapours like carbon monoxide, polyaromatic hydrocarbons, sulfur dioxide, and nitrogen oxides (NOx). The World Health Organization says that without adequate controls, such as exhaust after-treatment, these diesel byproducts can cause a range of possible health problems, including respiratory illnesses and cancer for workers. Although the degree of risk is still being debated, latest industry standards are reducing diesel emissions to near-zero levels, and the mining industry is rapidly moving ahead to introduce cleaner diesel-powered equipment.

High levels of soot – otherwise known as particulate matter (PM) – and NOx have forced mine operators, governments, and suppliers to work hard on finding solutions to reduce diesel emissions and worker exposure. While a holistic approach is recommended – from cleaner fuel to vigourous maintenance practices to better ventilation in underground mines – the greatest technological change has occurred in engine combustion and exhaust after-treatment.

After-treatment retrofits

With an older engine, the burden of dealing with PM falls to a diesel particulate filter (DPF). The basic concept has existed since the 1970s: a honeycomb-shaped ceramic, metal, cordierite or knitted glass fibre filter traps soot from the exhaust, and the accumulated soot is periodically burned off to regenerate the filter.

Al Hovda, global manager of the exhaust and emissions business at Minneapolis-based filtration company Donaldson, says soot buildup problems have driven improvements in DPF design. Any burn-off during regeneration will result in some ash production, and newer generations of DPFs have larger inlet channels to collect more ash and extend service intervals.

But excessive soot buildup, which can cause the DPF to require service, persists because 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 per cent of the time to avoid excessive soot buildup. A test of new DPFs, conducted from 2000 to 2004 at Vale’s underground Stobie mine, as part of the mining industry-led Diesel Emission Evaluation Program, showed that even heavily used vehicles needed assistance to get their engines to that temperature that often.

Strategies for raising exhaust temperature can either be active, involving some intervention such as injecting extra fuel or starting up a heater, or passive, which can mean either raising the exhaust temperature automatically or lowering the threshold temperature for filter regeneration.

One filter Vale plans to install on selected light-duty vehicles across its Sudbury operations combines both active and passive strategies. The Stratus SMF-AR, a sintered metal filter made by HJS Emission Technology and sold in North America by T.F. Hudgins, uses heating elements encircling the filter to help it to ignite, while a ferrocene fuel additive lowers the reaction temperature. Bob Clayton, manager of international sales at T.F. Hudgins, says the filter can drop the ignition temperature by 200 C while cutting PM by up to 99.7 per cent. According to Cheryl Allen, principal engineer of ventilation at Vale, the filter reduced PM emissions by more than 99 per cent on the two engines Vale tested.

Diesel oxidation catalysts (DOCs) can also help reach and sustain burn-off temperatures. DOCs serve many functions, but their most common use – converting carbon monoxide and hydrocarbons into water vapour with a precious-metal catalyst, often platinum and/or palladium – also produces heat that boosts regeneration in the DPF.

The downside to platinum and palladium is that they convert nitrogen oxide to NO2, a respiratory irritant. But Clayton says that is another benefit of the Stratus SMF-AR: it can reduce NO2 depending on the engine’s operating condition. Johnson Matthey, which uses NO2 to oxidize soot in its CRT filters, has a new “Low-NO2” version that Vale is currently testing on heavy-duty vehicles.

Saying no to NOx

NOx emissions increase in the same high-temperature combustion conditions that reduce PM and it is tougher to treat through retrofits. Under looser emissions rules, it was possible to strike a balance between PM and NOx using precise engine controls. But starting with Tier 3 engines, “the requirements to reduce both PM and NOx were such that the only way they could do that is not play one against the other, but come up with after-treatment technologies that reduce both,” says Michel Grenier, regional director of research programs with CanmetMINING.

That is reflected in the technology used to meet Tier 4 Final standards, which began to apply to mid-size off-road engines – between 173 and 751 horsepower (hp) – as of January 1, 2014. These machines power most underground mining equipment and must now emit no more than 0.01 grams of PM and 0.30 grams of NOx per horsepower-hour. That is the same PM content as the Tier 4 Interim levels, but only half the previously allowable NOx. “We call it near-zero levels because it’s so low it’s virtually impossible to get any lower,” says Cummins’s Browne.

NOx-specific after-treatment, called selective catalytic reduction (SCR), is now standard across Tier 4 Final engines. A solution of water and urea, called diesel exhaust fluid (DEF), is sprayed into the exhaust stream, where the heat converts it into ammonia. The ammonia reacts with NOx to form nitrogen and water with the help of a catalyst, reducing exhaust-borne NOx by up to 90 per cent. A second slip catalyst eliminates any excess ammonia.

SCR systems can reduce an engine’s operating cost as well as its emissions. Manufacturers say such systems improve fuel efficiency by up to 5 per cent because the engine operation can be optimized for criteria other than nitrogen reduction. With a typical DEF requirement of two to three gallons per 100 gallons fuel burned, the total cost of fluid and diesel is lower.

This also reduces the burden on PM filters, according to Doug Mihelick, commercial manager at Caterpillar’s engine division. “The SCR system allows us to tune the engine to produce a little higher engine-out NOx and lower particulates,” he explains. For Caterpillar, that means that some Tier 4 Final machine models accomplish enough soot removal through passive regeneration that they never need supplemental active regeneration. Some of its engines do not require a DPF at all.

Cummins’s Browne remarks that when the less severe Tier 4 Final kicks in for engines of 751 hp and above – like those of large surface mining equipment – Cummins will be meeting the requirements with SCR alone.

That is possible in part because in-cylinder controls are steadily improving. The more thoroughly fuel burns, the fewer waste products it sends into the exhaust stream. The Tier 4 engines use high-pressure fuel injection systems controlled electronically; the more precisely timed the injection and the greater the surface area of fuel droplets are exposed to oxygen, the better combustion will occur.

A typical mid-size engine also uses exhaust gas recirculation (EGR) to reduce the production of NOx by cooling down the combustion chamber with a portion of the inert gas from the exhaust stream. “Since 2011, most of our off-highway engines below 751 horsepower have had EGR on them,” Browne says. “You’ll find them on almost every engine supplier now.”

A new track

While technology users will be ironing out kinks for years to come, the engine manufacturers have dusted off their hands and declared “job done” on emissions. Browne says Cummins is changing track now. “From Cummins’ perspective, emissions have driven all of the technology changes for the last 10 years,” he comments. “We’ve had dates which are fairly aggressive to meet from the EPA and the European Union.” Now, he says, the focus will be on performance improvements rather than regulatory compliance.

That means back-to-basics reliability, but it also means the conversion catalysts are likely going to continue to become more efficient and the after-treatment devices smaller. Alternative fuels and electric hybrid installations may receive more attention.

Meanwhile, Canmet is conducting further research into new after-treatment technologies. The American Conference of Government and Industrial Hygienists (ACGIH), which proposes health-motivated workplace guidelines, has offered it a challenge by suggesting that the threshold limit value for NO2 over an eight-hour shift be reduced from 3.0 parts per million to 0.2 parts per million. Though the ACGIH does not itself set regulations, a number of provinces tend to adopt its guidelines.

“This is causing a lot of concern with mine operators across Canada,” says Grenier. “So whatever technology we are looking at right now must not only take into account the need to reduce diesel particulate matter, but also to have an impact on NO2, specifically.”

As well, says Grenier, “We’re working on alternative energies for powering underground equipment. In other words, whether it’s fuel cell-powered vehicles or fully electric battery-driven vehicles, our aim here is eventually to go to the zero-emission underground mine, thereby affecting workers’ safety and health in a positive way.”

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