Sampling of Dioxins and Carbon Dioxide

The best way to manage waste is to not generate it, and the second-best option is to recycle it. However, some forms of waste cannot be easily recycled. A viable option is then to incinerate the waste and thereby at least recycle the embedded energy. Waste incineration has had a somewhat bad reputation due to emissions of pollutants to air and water, but modern incineration facilities and modern legislation controlling and capping the emissions have made this less of an issue.

Emissions from waste incineration facilities are strictly regulated in many parts of the world. By example, the European Union Industrial Emissions Directive sets emission limits and monitoring requirements for many pollutants through a “Best Available Techniques Conclusions” document for waste incineration facilities, the WI-BATC. The latest version was introduced in 2019 and its provisions are to be fully implemented in national legislation by now. Among other things, the 2019 version introduced conditional requirements on long-term sampling of dioxins.

Dioxins

Dioxins are a class of substances with many variations. The base is plain “dioxin” (C4H4O2) which is a benzene ring (C6H6) with two of its carbon atoms replaced by oxygen. Dioxin is essentially harmless but add more carbon rings and chlorine or bromine, and things quickly worsen. The result is persistent organic pollutants (POPs) which are accumulated in the food chain. These compounds may be very toxic and may result in for example cancer and developmental effects on offspring.
There is a multitude of these compounds, divided in subgroups such as “polychlorinated dibenzo-p-dioxins” (PCDD) and its polybrominated analogue (PBDD). An example of a PCDD is 2,3,7,8-tetrachlorodibenzodioxin (TCDD), known to be the most toxic of them all. For simplicity, the compounds are collectively often referred to as “dioxins” (plural), and they then often also include dioxin-like compounds of furans (PCBF, PBDF) and polychlorinated biphenyls (PCBs).
Formation of dioxins requires organic material, oxygen, chlorine or bromine, and a temperature of 400-700 °C. Unfortunately, these conditions are not unusual in industrial processes such as paper pulp production, herbicide and pesticide production, and, notably, waste incineration. This is the reason for the WI-BATC expressing dioxins emissions limits and requiring either short-term or long-term sampling of dioxins with subsequent analysis of the sample.
The sampling process is discussed below but it is not of interest for dioxins only. The same principles can be applied to sampling of carbon dioxide (CO2) so let us first take a brief look also at that topic.

Fossil and Biomass Carbon Dioxide

Carbon dioxide is one of the culprits in the ongoing climate changes. However, within the European Union, CO2 emissions are not regulated in the WI-BATC but through taxation of CO2 generated by incineration of fossil fuels. The approaches are very different, but the objectives are the same: to minimize the emissions.
CO2 originating from biomass fuels can be exempted from taxation since the net contribution to the ambient CO2 level is zero. Having an oil- or coal-fired power plant means full taxation typically based on fuel consumption while a biomass-based power plant pays no CO2 tax at all. Incineration of mixed fuels means taxation of the fossil part of the CO2 emissions.
But what if the mixing ratio is not known and perhaps varies over time? This can be the case for example at waste-to-energy facilities. In lack of exact knowledge of the fuel mix, the supervising authorities tend to make conservative assumptions with a fossil share on the high side. It means increased tax incomes but is not necessarily welcomed by the waste-to-energy facility paying the tax. However, there is a clever method to find out the true fossil CO2 emissions.
There are three natural isotopes of carbon on Earth. About 99 % of all carbon is carbon-12, and almost all the rest is carbon-13. However, then there is also a tiny fraction of the radioactive isotope carbon-14, 14C. The rates of decay and production of 14C essentially balance and give a constant 14C/12C ratio in the atmosphere. This ratio also shows in living organisms and fresh biomass. However, fossil-based fuel is old and has no radioactive 14C left due to decay.
Effectively all carbon in the CO2 emissions originates from the fuel. By running long-term sampling on the emissions, trapping a representative sample of CO2, and then analysing its 14C/12C ratio, the actual fossil CO2 emissions can be established. This can mean huge tax savings for the industry.

Long-term Sampling

So, both dioxins and CO2 emissions can be monitored with the help of long-term sampling. Further, a single sampling device can be used for both types of sampling, also if the subsequent analysis and sought-after results are quite different. But why “sampling” and not real-time measurements, and why “long-term”? It has to do with the expected levels to detect, the methods available for such detection, and getting a representative sample.
The concentrations of both dioxins and 14C are very low and there are no readily available methods to measure these levels of concentrations in real time. Instead, the dioxins and/or the CO2  need to be aggregated in a sample to reach detectable levels.
The long-term aspect is a matter of making the sample representative, covering all levels of the respective compound as they may vary over time in the emissions. In the WI-BATC, “long-term” is defined as a period of 2 to 4 weeks, and the sampling is then to be repeated every month. Accordingly, this regulation does not require truly continuous dioxins sampling but with a sampling period of 4 weeks, the sampling at least approaches continuous measurements.

The Sampling Process

There are standards for how to conduct the sampling. Dioxins sampling is described in the EN 1948-1 standard, although limited to short-term sampling only. However, a supplementary technical specification CEN/TS 1948-5 extends this to long-term sampling, and the practical differences are small. The TS document is also currently proposed to become EN 1948-5. The process for CO2 sampling is introduced in the EN ISO 13833 standard.
The main parts of a sampler are a probe inserted in the stack, a container for the sample, and a pump driving the gas flow from the probe through the container. To make the sample representative for the content of the emissions over the sampling period, the sampling rate must follow the variations of the flue gas flow in the stack. This is called isokinetic sampling. Rather accurate measurements of the flue gas flow as well as the flow through the sample is required to control the sampling pump with the prescribed precision. To achieve this, it is also necessary to measure temperature, pressure, and water content in the flue gas in the stack, as well as in the sampler.
The standards also tell which sorbents to use for the accumulation of dioxins and CO2, respectively. Dioxins are to be accumulated in a polymer resin such as naphthyl isocyanate or XAD-2. CO2 is captured in an alkali solution such as sodium hydroxide, NaOH. In either case, the sorbents and sorbent containers must also be prepared following specific routines, and the flow as well as sorbent volume must be selected to prevent saturation of the sorbent before the sampling period ends.

Sample Extraction and Analysis

The sampling is followed by extraction of the sample from the sorbent and analysis. These processes are of course just as critical to the ultimate result as the sampling but goes beyond the scope of this article. Refer to the standards EN 1948-2 and -3 (dioxins) and EN ISO 13833 (CO2) for detailed descriptions. However, with a well working sampler, the foundation is laid for trustworthy emissions reporting, whether to show compliance with WI-BATC requirements or for correct CO2 taxation.