Bengt Löfstedt discusses emissions monitoring through long-term sampling
Monitoring of flue gas emissions to the ambient air for environmental protection is often a matter of continuous monitoring. However, there is also periodic monitoring, which can be divided into sampling during a short period of time, and long-term sampling, which may approach continuous monitoring.
Emissions monitoring is often carried out by continuously operating instruments providing measurement results in real time, or at least in close-to real time. The data can then be averaged and compared to emission limits. Using this kind of instruments is not only a possibility but often a legislative requirement. This is the case for many of the “usual pollutants” such as NOX, SO2, CO, NH3, HCI, and dust, and for supporting parameters such as flow, temperature, and pressure.
Continuous monitoring versus sampling
However, there are situations where continuous, real-time monitoring is not required. An example of this is when the pollutant levels are expected to be steady and/or low, and the legislators have ruled that it is sufficient to verify this only with certain intervals. Then short-term, long-interval sampling may be applied.
“Sampling” means that a stream of flue gas is led through a filter or a sorbent material inside a container, leading to accumulation of pollutants of interest. After a prescribed sampling period, perhaps as short as an hour or so, the container is removed from the stream and its content of pollutants can be analysed. In some cases, the analysis can be done using locally available analysis methods. In other cases, it may involve an off-site laboratory hosting advanced instrumentation operated by specialists. As a trade-off between ability to catch deviations from the expected and operating costs, the interval between samplings can often be in the range
of months or even a year.
An example where short-term, long-interval sampling can be applied is the monitoring of emissions of certain metals such as As, Cd, Co and Cr from waste incineration facilities within the European Union. Three samples are to be taken for at least 30 minutes each – the “short term”. It is enough to do this once every six months – the “long interval”. Each sample is collected on a filter and the metal content accumulated on the filter is typically determined at a laboratory by means of X-ray fluorescence (XRF).
Long-term sampling
So far so good, but what if the concentration of a pollutant varies notably over time, but there are no instruments available to measure the concentration continuously, in real time? Short-term, long-interval sampling would not do as the results probably would not be representative for the actual emissions. Short-term, short-interval sampling may seem like a solution, but the costs and other practical aspects make it a less attractive approach.
What remains is long-term sampling, with equally long intervals between the sample analyses. Special attention must be paid to the sampling rate. The sampling speed at the nozzle should ideally be the same as the speed of the surrounding flue gas. This is called isokinetic sampling. Only at equal speeds will the amount of pollutant accumulated in the sorbent be representative for the average pollutant concentration in the flue gas.
After the sampling period, the sorbent container is sent to a laboratory for analysis. Here, the total weight of the pollutant accumulated in the sorbent can be determined. Dividing this weight by the total volume passing the sorbent container during the sampling period yields a pollutant concentration. Thanks to the isokinetic sampling, this concentration is also equal to the pollutant concentration in the sampled flue gas, averaged over the sampling period.
The long-term sampling becomes continuous by inserting a new sorbent container in the sample flow just after the exposed one is removed. Subsequent results from the sample analyses then give a continuous series of concentration data, although with low time resolution.
An example where long-term sampling is applied can be found in EU regulations on waste incineration. Dioxins and furans (to be more precise “polybrominated and polychlorinated dibenzo-p-dioxins and -furans”) can result from incineration of plastics and hazardous waste. These substances are very toxic and bad for the environment even in very low concentrations, and the emissions should therefore be monitored. Further, the concentrations in the flue gas can vary widely depending on fuel composition and incineration temperature, so continuous or semi-continuous monitoring may be required. In lack of other options, long-term sampling is prescribed. The sampling time is two to four weeks. Sample analyses are then carried out at laboratories using GC-MS (gas chromatography-mass spectrometry) instruments. The long sampling time allows accumulation of low-concentration dioxins and furans to match the detection ability of the instruments. Sampling is to be repeated at least once every month, which effectively means at least close-to continuous sampling and monitoring.
Opsis is a manufacturer of isokinetic long-term samplers compliant with EN standards and other requirements, very well suited for emissions monitoring of dioxins, furans and other complex compounds. The company offers valuable guidance on long-term sampling as well as on systems for continuous emissions monitoring and process control.
Bengt Löfstedt is with Opsis