Waste incineration: the last word in cleanliness?

Last year, in collaboration with the Paul Scherrer Institute PSI, a research team reported on the fate of nanoparticles emitted during waste incineration at the end of their lifecycle in the scientific journal Nature Nanotechnology. Whilst the investigated nanoparticles are not emitted into the atmosphere during combustion, they remain intact in solid or liquid wastes from incineration plants, and therefore end up in the mountains of waste. Clearly they are not being removed from the environment at all.

For this reason, PSI-scientists saw the need to act before the spread of nano-structured products reaches an irreversible level. In collaboration with colleagues at ETH Zurich and Empa, they began by asking how many nanoparticles - of which size and chemical composition - actually leave waste incineration plants. Because these two factors –size and chemistry – determine the reactivity - and therefore the possible biotoxicity - of nanoparticles.

What can be done?

To reduce risks, you have to understand them thoroughly first. And unfortunately, there is still too little known about nanoparticle emissions from combustion processes, or from other thermal waste processing plants. Scientists have now taken the first steps in this direction. They are currently building an instrument with which nanoparticle emissions from combustion process can be analysed on the basis of chemical composition and particle size. Measurement devices that shed light on either nanoparticle sizes or their elementary composition have been around for a long time. But none can do both simultaneously and in real time. “For a better understanding of the conditions under which these nanoparticles survive combustion, dynamic, time resolved measurements are essential”, said Mohamed Tarik, Post-Doctoral Student in the PSI Chemical and Materials Processing Research Group at the Bioenergy and Catalysis Laboratory. With current technologies, measurements are only possible with a time delay. In the future, however, it should be possible in experiments, for example, to adjust the operational parameters of a combustion system, and monitor the effect this has on nanoparticle emissions directly. The goal for PSI-scientists is therefore to combine these two as yet separate operations (the determination of size distribution and chemical composition) into one device (More details in the box at the bottom).

Argon is the new air

Some modifications to existing commercial instruments are however required. A commercial plasma mass spectrometer (to determine the mass spectra) needs to use argon for plasma production. Air or even oxygen gas (from a certain flow) would make the plasma unstable and/or change the plasma conditions. For the new device then, this means that the filtering and counting particles must be performed using argon instead of air as a sheath gas. Since gas flows must be precisely defined here, and argon flows in a different way to air (different viscosity), a few of the operating parameters have to be modified. Furthermore, to function correctly, the plasma mass spectrometer requires the amount of input gas to be precisely defined. The scientists have therefore placed what is termed a rotation diluter upstream, which incorporates precise doses. Another challenge is the use of argon, which is expensive. To limit costs, the scientists are working on an idea for reducing their instrument’s argon consumption. Argon brings another level of difficulty into play: the voltage you can apply without worrying about causing electrical breakdown, is lower for this noble gas than it is for air. Because higher voltages are required to filter larger particles, there is therefore a particle size maximum with argon, which is lower than with air. But the scientists want to measure particles over as wide a size range as possible, because only then will they have a complete picture of the potential danger of combustion products. They are therefore honing their tricks to bypass the limits on the maximum measurable particle size.

Christian Ludwig, Head of the Chemicals and Material Processing Group, is confident that despite these challenges, construction of a highly sensitive and accurate measuring device for complete characterization of nanoparticles will succeed in the long term. The first prototype has already been built, and recent tests have made him optimistic. The long-term goal is to build a mobile instrument, with which the measurements can be performed on site at waste processing plants.

Text: Leonid Leiva