23. May 2013

Waste incineration: the last word in cleanliness?

Energy and Environment Environment

Household waste always used to end up left untreated in landfills, and the effects of this practice are well-known: these waste disposal sites were quite often ecological "death zones". With the incineration of municipal waste, there was some mitigation of this problem: despite the overall increase in quantities of waste, the areas claimed by landfill have been limited in recent decades thanks to waste recycling and incineration. Moreover, both electricity and heat for domestic and industrial use are both produced by waste incineration plants. However, waste incineration remains far from a panacea. Some combustion products that are already present in the burnt materials or that arise just during the combustion process itself are harmful to human health and the environment and some of them still find their way out of waste incineration plants and into landfill sites as their final destination.

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

The new instrument in detail

Size counts – is the chemistry right?

This new instrument should emerge by combining a particle counter and a plasma mass spectrometer. The particle counter forms the basis of an established measurement technique, and consists of two devices. In the first device- the so called particle classifier - the nanoparticles will be filtered according to their size. This is achieved by injecting the particles (after a charging step) as an aerosol into a cylindrical condenser, in which they will flow along the outer wall (which acts as a positive electrode). By applying an electrical voltage, the charged aerosol particles are accelerated along the inner wall (negative electrode). At the upper end of the inner wall there is a narrow opening (outlet slit), which only particles of a certain size can enter. The size of the particles to be filtered out depends directly on the applied voltage. To ensure that particles are only diverted to the inner wall and pass through the aperture on the basis of voltage, the condenser is filled with what is termed a sheath gas, which separates the aerosol layer on the outer wall from the inner wall. This sheath gas layer prevents the aerosol particles from simply diffusing to the inner wall. Air usually serves as the sheath gas.

In the second section, the now size-selected particles are counted. For this, the nanoparticles are mixed with a moisture-saturated, hot gas. By subsequently cooling this gas becomes supersaturated and small droplets form around the aerosol nanoparticles which here therefore act as condensation nuclei. The particles are detected using a laser whose light cone does not reach the detectors in the initial state (no droplet formation). Only the resulting droplets deflect the laser light so that it hits the sensors. Thus a droplet or condensation nucleus containing aerosol particle corresponds to every light pulse registered. In this way, particles of a particular size category are counted. To compile a complete inventory of all possible particle sizes, a new voltage is set each time, and the particles of a given size are counted.

Determination of nanoparticle chemical composition is also performed in a commercially available plasma mass spectrometer. The plasma in this instrument is needed to produce charged particles (ions). Here in fact, individual atoms are ionised in the hot argon plasma with temperatures ranging between 5000 -10000 degrees Celsius. Like several types of mass spectrometers, this operates by deflecting the ions in an electric field, so that they can be separated according to their mass to charge ratio (m/z), and therefore, information on which chemical element they are made from can be found. Plasma mass spectrometry is one of the most sensitive and reliable techniques for determining the elemental composition of an aerosol sample.

Prof. Dr. Christian Ludwig, Head of the Chemical Processes and Materials Research Group, Paul Scherrer Institute,
Telephone: +41 56 310 26 96, E-mail: christian.ludwig@psi.ch
Original Publication
Persistence of engineered nanoparticles in a municipal solid-waste incineration plant
Tobias Walser, Ludwig K. Limbach, Robert Brogioli, Esther Erismann, Luca Flamigni, Bodo Hattendorf, Markus Juchli, Frank Krumeich, Christian Ludwig, Karol Prikopsky, Michael Rossier, Dominik Saner, Alfred Sigg, Stefanie Hellweg, Detlef Günther & Wendelin J. Stark,
Nature Nanotechnology 7 (2012) 520–524
DOI: 10.1038/nnano.2012.64