The final report of a study evaluating the use of filter systems for underground diesel equipment points the way forward to lower emissions of potentially harmful diesel particulate matter and safer working conditions for miners.
Conducted under the Diesel Emissions Evaluation Program (DEEP), a consortium of mining companies, government agencies and unions, the study concluded that filters are only a part of the solution.
An effective particulate filter system must be compatible with engine performance and vehicle function. It also has to be able to to burn off carbon deposits on the filter to avoid clogging and engine malfunction.
Authored by Bruce Conard, a former Inco vice president of environmental and health sciences, the report recommends "simple but effective dashboard signals" to inform operators about the filter's effectiveness and suggests that particulate filter systems should be "invisible" to vehicle operators, so they can go about their jobs and not have to worry about them.
Nine filters on five load-haul-dump machines and four service vehicles were tested at Inco's Stobie Mine in Sudbury between April 2000 and December 2004.
The study found that effective particulate filter systems can reduce diesel particulate matter to less than 50 micrograms per cubic metre, "a huge improvement," said Conard.
Filters for diesel equipment were first introduced to the mining industry in the early '80s, but quickly fell out of favour.
"In a few cases, the vehicles operated very well and the filters worked, but in other cases, it was disastrous," said Conard.
Engine performance
Engine performance suffered when the filters were clogged, so vehicle operators got rid of them.
"They work on a production bonus and can't be held up. If their vehicle is stalling because the manifold is plugged up, they're not going to take the time to drive back to the garage to install another filter."
Conard unwittingly paved the way for the Diesel Emissions Evaluation Program when he picked up an obscure publication of the American Conference of Government and Industrial Hygienists (AICGH) in 1996. The booklet, entitled "Notice of Intended Changes," alerted members to the possible establishment of a threshold limit value for diesel particulate matter for the first time.
"I was new to my role as vice president and I was on a steep learning curve, trying to understand all of the regulations and scientific challenges we faced throughout our operations," he said.
Most mining companies in Canada at the time were following guidelines established by a committee chaired by CANMET in 1990. It set a limit of 1.5 mg of respirable combustible dust per cubic metre.
The ACIGH was proposing a limit of .15 mg/cubic metre - 10 times less than the standard that had been agreed to in the Canadian mining industry.
"We didn't even know how to measure at that level because when we got down to levels of .3 to .5 mg/cubic metre, there was a lot of uncertainty in the analytical technique," said Conard.
The ACIGH doesn't have regulatory authority, but governments often accept its threshold limit values for exposure to toxic substances in the workplace.
Conard shared news of the intended change with officials at Ontario's Labour Ministry and representatives of other mining companies.
"'This will put us out of business. We couldn't operate diesel equipment underground,'" they told him.
The alarm led to the establishment of DEEP in 1997. Its 17 members include Inco, Falconbridge, Barrick Gold, Hudson Bay Mining & Smelting, Natural Resources Canada, two unions, and such disparate organizations as the Toronto Transit Commission and several soybean and canola growers' associations.
DEEP studies also looked at alternative fuels and engine maintenance as possible solutions, but "they only got us part way to the ACIGH levels," said Conard.
Filters
The material used for the filters tested ranged from woven glass and metal fibers to ceramics such as silicon carbide and an aluminum oxide called cordorite. Physical stability, including heat resistance, and just the right amount of porosity to allow gas to escape were essential characteristics.
All of the filters appeared to work whenfirst installed, but many plugged up rapidly, resulting in unacceptable engine backpressure.
A number of techniques can be used to regenerate filters, including the use of diesel additives and catalytic coatings on the filters themselves to lower the temperature of carbon ignition. Also effective are auxiliary heat sources such as electric heaters or diesel burners located within the exhaust manifold.
Ten years after issuing its notice of intended change relating to the establishment of a threshold limit value for diesel particulate matter, the ACIGH has still not acted on its warning. Neither has the Ontario government taken any action to limit diesel particulate emissions.
However, research undertaken by DEEP has led to a better understanding of the problem. Techniques for measuring very low concentrations of diesel particulate matter have improved and we know that the answer to filter effectiveness lies in regeneration with minimal operator intervention.
Now it's up to manufacturers and sup-pliers to deliver the solutions recommended in the report, said Conard.
