UL group finds additives could lead to increase in pollution

 

Scientists at the University of Limerick are trying to reduce pollution by developing new filters to clean up vehicle exhausts, writes Anna Nolan

Some car and truck manufacturers use additives to improve the efficiency of the ceramic filters used to clean up the emissions from diesel fuel.

These same additives however might also break down the ceramic, leading in time to a reduction in filter performance and more, not less, pollution.

A team at the University of Limerick recently completed a study looking at the chemical interaction between the additives and different ceramic filters at a range of temperatures.

The group found that the most commonly used additives in fact do not break down two particular ceramic filter types, making them very suitable as a way to clean up vehicle exhausts.

The work was done by Professor Stuart Hampshire, director of the Materials Ireland Research Centre (MIRC) and a member of the Materials and Surface Science Institute (MSSI), both at UL, by Dr David O'Sullivan, a senior R&D consultant in MIRC, and by Professor Michael Pomeroy of MSSI.

They worked for the past two years on a diesel particulate filter project funded by the US giant, Corning Incorporated.

They investigated two ceramics, cordierite - a mineral containing aluminium, silicon and magnesium - and silicon carbide.

They found that cordierite, which has been in use for many years, and silicon carbide, which is a newer material, were not degraded by the additives.

Cordierite is used in filters because it expands very little during heating to high temperatures, and therefore is not prone to cracking.

Silicon carbide is a very hard material that is difficult to melt or fuse, and therefore also has advantages for use at the high temperatures occurring inside filters, which can heat to more than 1,000 degrees Celsius.

The filters, which are cylinders six inches long and almost six inches in diameter, are made up of many long, thin channels.

Each channel is open at one end only and they are stacked alternately, either open or blocked.

This means that when the gases enter and travel along a channel, they cannot go out at the end, but are forced to go through the pores of the ceramic into the neighbouring channels, the walls trapping the pollutants.

The problem is that over time there is a build-up of particulates on the walls of the filter, and so the filtering systems are designed to burn them off given the high working temperatures. Usually this takes place at between 550 and 650 degrees Celsius, but this can be reduced by up to 250 degrees Celsius with the use of catalytic fuel additives.

European car manufacturers tend to use a cerium-based additive or else ferrocene, an iron oxide.

"They are both viewed as good candidate additive species for diesel because of their ability to reduce soot build-up in the filter and to lower the "light-off temperature of the soot", explained Hampshire.

The ignition of the trapped pollutants releases a lot of heat, and if this is not controlled, temperatures can increase to well above 1,000 degrees Celsius inside the filter.

"After repeated burn-offs, a residual ash comprising the contaminant oxides from the lubricating oil, iron from engine abrasion and oxides from catalytic fuel additives will remain on and in the wall of the filter," says Dr O'Sullivan.

At temperatures above 1,100 degrees Celsius this ash may react with the ceramic structure, and cause premature failure through the formation of liquids that eat into the ceramics. Crystalline or glassy substances can also form that expand at different rates to the ceramic and cause fractures.

The UL team, working with Dr Martin Murtagh of Corning in New York, studied various chemical reactions between ashes and ceramics.

"We made up equivalent ashes containing oxides of phosphorus, and oxides and sulphates of calcium and zinc, and reacted those mixtures with the ceramics," explains O'Sullivan.

"We studied these reactions without additives to establish a baseline, and then with the reaction products of ferrocene and the cerium-based additive, at a wide range of temperatures."

The team found that, at temperatures up to 1,100 degrees Celsius, neither cordierite nor silicon carbide was degraded by either of the additives.