When the German chemist Dr Rudolph Diesel first introduced his diesel engine to the world at the end of the 19th century, it was considered a major advance to the industry. The engine’s high thermal efficiency meant it burnt less fuel than a gasoline engine and, because of the diesel engine’s more efficient lubrication abilities, the engines would last longer.

But, as we have learned over the years, emissions from carbon-based products such as the diesel engine also have significant negative effects on human health.

More and more industries, governments and communities are becoming aware of the dangers of particulate material suspended in the air, notably the risk of lung cancer. Carbon soot particles from diesel engines allow to form on their surfaces other metals and toxic substances produced by diesel engines such as cancer-causing aldehydes (such as formaldehyde) and polycyclic aromatic hydrocarbons.

According to a report released last year by the Australian Coal Association Research Program (ACARP), the significant introduction of diesel engines in underground mining plant since the 1960s has created a hazard in terms of suspended particulates.

“The extent of this hazard is indicated by National Institute for Occupational Health & Safety (NIOSH) in the US where exposure levels of diesel emissions were shown to be significantly higher for underground miners than for other occupations,” the report said.

Cutting emissions

This report covers the ACARP C15021 research project into using acoustic agglomeration techniques to reduce diesel particulates from coal mine vehicle exhaust by 92%. CSIRO project leader exploration and mining division Dr Patrick Glynn says that although fuel improvements have been made, emissions control technology is still needed. “Despite the expected improvements in emission performance by new diesel fuel, end-of pipe emission control may still be required to fully satisfy OHS&E expectations and future regulation,” Glynn says.

Glynn says the goal of the ACARP project was to achieve a high removal of 2.5 μm (PM 2.5) particulate material from the exhaust stream. “These particles have a propensity to be held in suspension and will only settle by inertial (gravity) means very slowly.

“The ultrasonic agglomeration method of removal increases the mass of the diesel particulate particles using ultrasonic agglomeration.”

“The ultrasonic agglomeration method of removal increases the mass of the diesel particulate particles using ultrasonic agglomeration where a sonic probe-generated sound wave is tuned to increase the energy in a small particle such that it is attracted to other diesel particles. This increases the overall mass of the agglomerated particle, allowing it to be removed by a ‘cyclone’ using exhaust stream velocity.”

This cyclonic filtration method is similar to the one used in modern vacuum cleaners: the dust-laden air in the outer walls of the cyclone swirls downwards and at the base of the vortex begins to swirl upwards, up the inside of the vortex. The vast majority of the debris separates from the air stream as air reaches the bottom of the swirl, and is deposited in the dirt container. Only a small fraction remains in the air and can be removed by a secondary, cartridge-type filter.

Testing positive

The tests were conducted at the Control Technologies International laboratory at Archerfield, Queensland, on a water brake dynamometer and were split into four stages including bench-testing a diesel engine to measure diesel particulate emissions over the revolutions per minute spectrum, and building an electrostatic diesel exhaust filter and two prototype diesel exhaust scrubber ultrasonic filters which were fitted to a mine vehicle for a three-month test period.

The tests also involved correlating the fundamental mass-over-time measurement used in this ACARP project with those from a NIOSH diesel particulate exposure-measuring instrument. This was done to measure diesel particulate loading in real time and the testing was to verify the effectiveness of this instrument against gravimetric (or fundamental) measurement.

Good results

The results have been of significant value to scientists working to improve occupational health and safety for mine workers. “The reduction of 92% in diesel exhaust particulate achieved by this project is remarkable because there is very little back pressure on the exhaust system that would, using normal micron filters, cause reduced engine output,” said Dr Glynn.

“The outcome of the project at 92% particulate reduction came close to the 95% particulate reduction aimed for at the beginning of the project.”

Since last year’s report was submitted the research team has been awarded a second grant for a triple-chamber acoustic agglomerator that will remove up to 99% of diesel particulates. This project is now underway.

Front-end approach

The ACARP report also recommended a front-end approach to reducing particulates in diesel engine emissions such as fuel modification which can have significant implications for the underground mining industry.

The report said: “The limiting of sulphur in diesel to 50ppm is an excellent step, as is the reduction in the aromatic content of diesel to 15% but more can be done to change the very nature of diesel.”

It also said that in situations where existing engines cannot readily be adapted to fuel changes (for example, older diesel engines that were built with seals that require high aromatic content in the fuel), their early retirement from sensitive uses such as underground mining applications should be considered.

“Two quantum changes are in the offing for changing the nature of diesel. The first is the advent of bio-diesel and the second is F-T diesel. In both cases the content of aromatics is zero or near zero, whilst sulphur is virtually eliminated.

“The F-T diesel has no mineral matter, and has been shown to have superior ignition characteristics, and produces a more complete combustion than ‘natural’ diesel, especially during start-up and acceleration (deceleration),” the report said. Three F-T projects are presently in the advanced planning stage for Australia and F-T fuel is expected to come on-stream within 18-36 months.

This article first appeared in CSIRO’s magazine earthmatters.