case studies of dust ams calibration at extreme conditions jurij cretnik.pdf · case studies of...

Tehnološki park 24, SI-1000 LJUBLJANA, SLOVENIA, E-mail: [email protected] 1

CASE STUDIES OF DUST AMS CALIBRATION AT EXTREME CONDITIONS

Jurij ČRETNIK RACI d.o.o., Tehnološki park 24, SI-1000 LJUBLJANA, SLOVENIA [email protected] Matjaž ŠLIBAR RACI d.o.o., Tehnološki park 24, SI-1000 LJUBLJANA, SLOVENIA [email protected]

INTRODUCTION

Environment protection is more and more important in our society. Apart from general consciousness of people and companies, environment protection is requested by law. One of the best available techniques (BAT) to prevent, reduce and as far as possible eliminate emissions of harmful substances into the air, water and soil is monitoring. Multiple benefits of monitoring can be achieved only if the data are reliable and comparable and they have been obtained from an appropriate quality monitoring program. The examples shown in the paper are only a few excerpts from our field work, which required adjustment of commonly used measurement procedures.

LEGAL BACKGROUND

Monitoring is included in the new European Union Industrial Emissions Directive (IED) [1]. The purpose of IED is to achieve a high level of protection for the environment, taken as a whole, from harmful effects of industrial activities. The new IED is an update and merger of seven existing EU directives: IPPCD [2], LCPD [3], WID [4], VOCD [5] and three TiO2D [6, 7, 8]. It has been established that for certain activities minimum standards of environmental protection from emissions due to certain activities have to be ensured. IED requires that BAT reference documents (BREF) are the reference for setting permit conditions and that emission limit values (ELV) do not exceed the emission levels associated with the best available techniques (BAT) [1]. Monitoring is a useful investment with wide practical benefits and should also be BAT. According to IED (and national legislation), compliance monitoring of different pollutants, including particulate matter, is required. To confirm the quality of the data, in order to increase the public confidence, appropriate quality assurance (QA) procedures (standard methods of measurements, certification of instruments, certification of personnel and accredited laboratories) shall be used. For permanently installed automated measuring systems (AMS) at large pollution sources IED requires QA, i.e. that the installation and functioning of the automated monitoring equipment shall be subject to control and to annual surveillance tests. This QA shall be performed in accordance with the European standard EN 14181 [9]. An important part of continuous monitoring is quality assurance and quality control (QA/QC) of installed AMS according to EN 14181:2004 [9], performed for calibration and verification of installed AMS with parallel measurements. The requirements for AMS are very complex. The existing quality level of AMS throughout Europe and in particular countries is very different. Standard EN 14181:2004 represents a new approach to quality assurance of AMS. It specifies three so called quality assurance levels (QAL) as well as additional fourth level of regular annual surveillance test (AST): QAL1 – the AMS is suitable for the application, QAL2 – verification of the measurement quality of the individual installed AMS, QAL3 –


verification of the measurement quality during continuous operation and AST – annual check of AMS, the performance, variability and calibration function remain valid. QAL2 is performed every 5 years at LCPD plants and every 3 years at WID plants. EN 14181:2004 can according to national legislation also be used at other plants, especially IPPC [2] plants. QAL2 and AST procedures include comparison of AMS measured values with values, obtained with standard reference methods. For periodic measurements with manual or automatic reference methods (RM) or other applicable methods standard EN 15259:2007 [10] is used. It determines general principles used for emission measurements at different plant types and for different measurement purposes, procedures for representative sampling in waste gas ducts, and a procedure for determining the best sampling point for AMS for continuous measurement.

CONSTRAINTS FOR PARTICULATE MATTER MEASUREMENTS

Whether an AMS needs to be installed or periodic measurements to be performed, there are many constraints that need to be taken into account. A very important aspect is measurement site geometry (duct: dimensions, wall thickness and material, available measurement ports and working area). Particle properties (size, shape, colour, material, concentration) and ambient conditions are very important for the choice of an AMS, but less for manual methods, where weather can have some influence. Waste gas properties (temperature and pressure, gas composition, especially humidity) are important for both. Most of the influencing quantities are shown in Fig.1.

Figure 1. Dust measurement constraints.

PARTICULATE MATTER AMS

The problem of particulate matter monitoring AMS (AMS for dust) is that they don’t measure mass concentration of particulate matter itself, but rather some other physical quantity. This is actually the case with gas analysers as well, but it is not as obvious. Reference materials for most gases are readily available, but reference materials for AMS for dust, that would make possible a direct calibration of the AMS using reference materials, are not. This means that calibration needs to be performed via comparison of AMS results with standard reference method (SRM) results, which in case of particular matter is manual gravimetric method [11, 12]. Apart from that, properties of particular matter, such as size, colour and shape, tend to change with different operating conditions (including different fuels) what makes the calibration of particular matter AMS even more difficult.


There are many different particulate matter monitoring AMS available on the market using several measuring principles. It is very important to know them well to be able to choose the correct AMS for the intended application. The currently used measuring principles for particulate matter monitoring AMS are: • opacity, • dynamic opacity (scintillation), • light scattering (back, side, forward), • β-attenuation, • tribo-electric, • electro-dynamic. Different techniques have different pro’s and con’s, e.g. one can be used for high concentrations and in large ducts, but has a high detection limit; another one has a very low detection limit, but is sensitive to structure and composition of particles, yet another cannot be used for applications with electrostatic precipitator upstream.

STANDARD REFERENCE METHODS FOR PARTICULATE MATTER

For every AMS for dust a calibration with standard reference method (SRM) is necessary to get a mass concentration output. In case of particulate matter SRM is a manual gravimetric method, determined by two standards, EN 13284-1:2001 [11] for low concentrations up to 50 mg/m3 and ISO 9096:2003 [12] for high concentrations in the range from 20 mg/m3 to 1000 mg/m3. Dust concentration ranges and measurement methods are shown in Fig.2.

Figure 2. Dust concentration ranges and measurement methods. The requirements of these two standards are very similar. Conditioning and weighing of filters before and after sampling shall be performed in air-conditioned weighing rooms, filtering materials shall have a low enough porosity. The equipment used shall be of appropriate accuracy and traceable to SI units through calibration. Nevertheless, there are some differences. One of the options, left to the user, is position (in-stack or out-stack) of particle collecting filter [11, 12]. They are in theory equivalent, but in practice the in-stack method, which is also the more common one, is more accurate. When using out-stack sampling, all parts of the sampling train upstream of the filter shall be heated and all parts in contact with measured gas shall be rinsed after sampling, the rinsing solution later conditioned and weighed similarly to the filters. This brings additional possibilities for contamination and additional uncertainties to the result. Therefore out stack sampling is used mainly in cases of high measured gas humidity (gas temperature close to or at dew point). There are also different configurations possible, depending on concentration of dust to be measured. EN 13284-1:2001 requires a plane filter to be used. This design is proven for concentration up to 50 mg/m3. In case of ISO 9096:2003, there are several possibilities. Typically, plane filters of diameters from 25 mm to 50 mm are used. Apart from use of a plane filter with low volume sampling, there are several options to increase filter capacity. One of them is to use a large diameter (over 100 mm) filter. A more common one is an


extended filter head, where filter thimbles or filtering bush stuffed with filtering material can be used. These filtering devices are appropriate for concentrations up to (and in favourable conditions beyond) 1000 mg/m3. There exist also sampling designs where plane filter is fixed to the sampling nozzle and cone. This combination functions as a container for particulate matter. This is a combination of the two mentioned standard designs and is suitable for measurements of particulate matter concentrations between the above mentioned limits. These configurations are applicable for measurements of emission of particulate matter, but in some cases (e.g. process measurements) the measuring objective exceeds the limits set in the standards [11, 12]. One of such cases is measurement of particulate matter at the entrance of filters (cyclones, electrostatic precipitators, bag filters, etc.). Concentrations of particulate matter in these cases can be up to several hundred g/m3. For these applications an additional pre-filtering device is necessary. A cyclone (Fig 3.) is a technique that has proven itself as appropriate and it can be used both in-stack or out-stack. It can be used as the only filtering device, but more accurate results can be obtained with filter thimbles, bushes or plane filters downstream of the cyclone.

CALIBRATION OF PARTICULATE MATTER MONITORING AMS IN EXTREME

CONDITIONS

When performing calibration of AMS for dust, there are several issues to be observed. Lack of reference materials, which could be used for calibration of the AMS’, is the most important one. This means, that the calibration for all AMS dust can only be performed by parallel measurements with SRM. The mass concentration of particulate matter is measured indirectly, so any change in particulate matter characteristics can influence the results. Therefore the calibration function is only valid for those operating conditions/particulate matter characteristics at which it was determined. In case of different operating conditions several calibration curves can be determined, or the calibrated curve may be non-linear. Also a square calibration curve may be used for particulate matter monitoring AMS according to EN 13284-2 [14]. This is mostly the case with high concentration ranges of particulate matter. A measurement section with

Figure 3. A sampling cyclone with modelled trajectories [13] (left) and a cyclone, set for in-stack sampling (right).


extreme dimensions is shown in Fig. 4. Due to varying dust concentrations a non-linear calibration curve was determined (Fig. 5).

Figure 5. Example of varying dust concentrations during a QAL2 cycle (non-linear calibration curve in case of high concentrations).

The next problem will probably not occur any more in modern installations, but in some parts of Europe there are out-of-date plants still in operation. In case of stacks with up to 16.75 m internal diameter with 0.5 m and more wall thickness, extremely long sampling probes need to be used. In such cases one has two options. A stiff probe would be preferred, but that would lead to heavy probes difficult to handle. It would at least necessitate a wide platform as well (according to EN 15259:2007 platform width should be approximately 8 m, which is not realistic!). Measurement plane for ducts of such diameter can be up to 100 m above ground level. All of that taken into account a less stiff, but lighter and easier to handle probe is

Figure 4. Measurement section with extreme dimensions: Platform at 67 m, ID 16.75 m, OD 17.75 m.


probably the better option. But in certain cases (extreme positions) it will bend as much to take slightly incorrect velocity measurements due to the angle of the attached Pitot tube. An example of a velocity profile in a large stack with deviations due to probe distortion is shown in Fig. 6.

Figure 6. Velocity profile in a large diameter stack in m/s. A situation which will occur more and more often as emissions decrease is a so called “low level cluster”. In the draft prEN14181:2012 [15] this is called calibration method C (as addition to methods A and B in the existing standard). In this case there is only a group of measured values close to zero. Close to zero means that the highest value, measured with the SRM, is below the 95 % confidence interval, required for the measurand, which in case of particulate matter is below 15 % of the daily ELV [15]. Low mass concentrations of particulate matter also carry a much higher relative measurement uncertainty. To reduce the uncertainty at low levels, it is allowed to reduce the number of samples and extend the sampling time for each sample. As long as the measured values from SRM parallel measurement are above the SRM quantification limit, they can be used to determine a calibration curve. When the data from SRM are not sufficient, then the AMS may be “calibrated” based on experience: the SRM data confirm that the emissions are low, gain of the analyser is set to be able to detect small changes, and the measurement itself becomes more indicative (qualitative) than quantitative. An example of a measurement section is shown in Fig 7. A calibration diagram of low level cluster, where the SRM measurement was “just” above the limit of quantification of SRM is shown in Fig. 8. It can be seen that with a low level cluster the calculated calibration function is not really important, as the measured values are well below the ELV (shown in the upper left corner of Fig. 8).


Figure 8. Example of a low level cluster (daily ELV of the plant is 5 mg/m3).

CONCLUSION

Measurement of particulate matter is a complex task, whether it is performed manually or automatically. To be able to perform this task, good knowledge of the process is necessary. This includes, among others, measurement section, operating conditions, process gas conditions and particulate matter properties.

Figure 7. Measurement section with ID 4 m and low level cluster at 0.1 mg/m3-0.2 mg/m3.


Manual measurement of particulate matter, be it either as a single measurement or as a set of parallel measurements for calibration of AMS, shall be performed by an experienced competent laboratory. Even accreditation to EN ISO/IEC 17025:2005 [16] does not always ensure the laboratory is able to perform a certain task adequately. Since not all aspects of testing and AMS inspection (including calibration) are covered in the standards, laboratories shall be able to design and use their own written procedures supplementing the standards as necessary [17]. Particularly problematic are particulate matter measurements in extreme conditions, whether these are size, gas composition, dust properties or other. In each of such cases individual adjustments need to be made in order to carry out the measurement task appropriately. All that taken into account, no matter how much the knowledge of particulate matter monitoring has grown over the past, there is still room for experience to be gained both in the field of automated and manual measurements.

LIST OF REFERENCES

1. EU Directive 2010/75/EU On industrial emissions. 2. EU Directive 2008/1/EC concerning integrated pollution prevention and control (a

codified version of EU Directive 1996/61/EC). 3. EU Directive 2001/80/EC on the limitation of emissions of certain pollutants into the air

from large combustion plants. 4. EU Directive 2000/76/EC on the incineration of waste. 5. EU Directive 1999/13/EC on the limitation of emissions of volatile organic compounds

due to the use of organic solvents in certain activities and installations. 6. EU Directive 78/176/EEC on waste from the titanium dioxide industry. 7. EU Directive 82/883/EEC on procedures for the surveillance and monitoring of

environments concerned by waste from the titanium dioxide industry. 8. EU Directive 92/112/EEC on procedures for harmonizing the programmes for the

reduction and eventual elimination of pollution caused by waste from the titanium dioxide industry

9. EN 14181:2004: Stationary source emissions – Quality assurance of automated measuring systems.

10. EN 15259:2007: Air quality – Measurement of stationary source emissions – Requirements for measurement sections and sites and for the measurement objective, plan and report.

11. EN 13284-1:2001 Stationary source emissions – Determination of low range mass concentration of dust – Part 1: Manual gravimetric method.

12. ISO 9096:2003 Stationary source emissions – Manual determination of mass concentration of particulate matter.

13. Product info 14.0: Cyclone, Paul Gothe GmbH, Bochum. 14. EN 13284-2:2004: Stationary source emissions – Determination of low range mass

concentration of dust - Part 2: Automated measuring system. 15. Draft prEN 14181:2012: Stationary source emissions – Quality assurance of automated

measuring systems. 16. EN ISO/IEC 17025:2005 General requirements for the competence of testing and

calibration laboratories (ISO/IEC 17025:2005). 17. CEN/TS 15675:2007 Air quality – Measurement of stationary source emissions –

Application of EN ISO/IEC 17025:2005 to periodic measurements.

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