Annual Guide 2021 I SOURCE TESTING ASSOCIATION
Another of the major techniques is cross-duct analysers, which project IR or UV (ultraviolet) energy across the stack and detect the change in energy state of the gas molecules as they absorb this energy at characteristic wavelengths. Most cross-duct systems can measure multiple gases over a range of wavelengths and also have the advantage that, because the analysers don’t come into contact with the target gases, far less maintenance and operator involvement is needed.
One drawback is that cross-duct systems can be more complicated to calibrate, although this can be overcome using an automatic calibration system that can demonstrate accurate and reliable calibration checking.
Smooth operations
Extractive techniques can sample for either particulates or gases. For particulates, fl ue gas is extracted from a stack with the particulate material collected on a fi lter. Sampling for a gas involves extracting the sample and collecting the required substance in either a solution or onto a solid adsorbent.
Extractive techniques consist of two main methods. Heated extraction involves extracting the sample gas from the stack using a sample probe, heated line, gas conditioning equipment and a heated sample pump. Before the analysis, condensate is usually removed from the sample and the temperature is reduced to protect the analysers. With this technique, it is vital to ensure that the sample arrives at the inlet of the analysers in the exact same state as it was in the stack. Another consideration is to ensure that the design of the sampling system prevents any sample loss or degradation. A number of single component and multi-gas analysers can be used with this method.
The other major extractive method is dilution extraction. Although this technique also involves using a probe and sample line to transport the sample gas to the analyser, it differs in that the sample is diluted with clean, dry air to a predetermined factor. With dilution extraction, there is less need to heat the sample line. Because the sample gas is diluted, lower range “ambient” gas analysers can be used.
An alternative to extractive monitoring is in-situ “probe” analysers. In this technique, the analyser is directly connected to the probe installed at the point of measurement. One of its major advantages is that it reduces possible problems that can result from extractive monitoring, particularly heated extraction.
Most of these in-situ systems use infrared measurement techniques such as non-dispersive IR (Infrared), FTIR (Fourier-transform infrared spectroscopy) and Gas Filter Correlation. These techniques can often measure more than one constituent component of the measured emission, such as nitrous oxide and carbon monoxide. For example, ABB offers an FTIR based multi-component CEMS that includes features such as a high-resolution spectrometer (1 cm–1
) and a long-life laser (20 years) and IR source (>5 years).
However, an issue with in-situ techniques is the necessity to ensure that the gas is homogenous, uniform and well mixed, as the probe relies on being inserted into the stack to conduct measurements.
Several criteria determine the choice of CEMS for a particular monitoring application. For example, in the UK, the CEMS must be MCERTS certifi ed for the determinands specifi ed in the IED and must also be certifi ed for a measurement range that is suitable for the application.[1]
The operator must also ensure that any CEMS being considered will not have its performance degraded by any specifi c site conditions and that the intended CEMS is proven on comparable installations.
All CEMS must allow the performance of zero, span and linearity tests following installation, while new, extractive CEMs must have a facility for leak checks. Particulate monitors may be sensitive to factors such as changes in fl ow rate, as well as particle size and shape. Operators with a need for particulate monitoring should determine whether stack conditions could degrade the monitoring data.
Some substances, for example organic pollutants such as dioxins, and inorganic pollutants such as mercury, can exist in both gaseous and particulate forms simultaneously. The monitoring method must therefore be able to sample the selected phase or both phases, as needed.
As well as choosing the right technology, facilities needing to monitor air emissions must ensure the right conditions exist and that correct procedures are adhered to. A major factor is the variability of the emissions. In general, the greater the variability of the emissions, the more frequent monitoring needs to be. This is where CEMS win over more intermittent, periodic monitoring techniques – CEMS are the correct choice where emissions levels vary so signifi cantly that intermittent sampling would lead to unrepresentative results or too many samples would be needed.
An appropriate data recording system must also be in place. Instrument-based methods such as CEMS provide real-time data, which must be recorded to allow interpretation and reporting. A variety of systems can be used for onsite storage, from the most basic, such as a simple chart recorder, to automatic data loggers that can send emissions data to a remote central processing unit.
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