measurement of low mercury concentrations in flue gases … johannes mayer.pdf · scrubber for flue...

CEM 2014 – Conference on Emission Monitoring 14th – 16

th May 2014 Istanbul, Turkey

Presenter: Dr. Johannes Mayer, E.ON New Build & Technology GmbH, Gelsenkirchen, Germany

Presentation: Measurement of low Mercury Concentrations in Flue Gases of Combustion Plants 1

Measurement of low Mercury Concentrations in Flue Gases of Combustion Plants Dr. Johannes Mayer, Zentrale Analytikdienstleistungen E.ON New Build & Technology GmbH, Gelsenkirchen / Deutschland Dipl.-Ing. Sebastian Hopf, Institut für Apparate und Umwelttechnik Otto-von-Guerike Universität Magdeburg, Merseburg / Deutschland Dr. Frans van Dijen, Laborelec / GDF SUEZ, Linkebeek / Belgium Dr. Alessio Baldini, Emission Technical Support Enel Produzione, Firenze / Italy Abstract

In the context of the implementation or tightening of mercury emission limits on a national

(German) and European level the question may be raised how mercury emissions can be

measured reliably at very low concentrations. Furthermore, especially upstream of a wet

scrubber for flue gas desulphurization, the oxidation stage of the mercury in the flue gas is

of specific importance to achieve further reduction of mercury emissions. The European

standard reference method for determination of total mercury in flue gases EN 13211 is

not adequate to control emission limits in the range of 1 - 3 µg/m3, as already proposed

and partly agreed for single new build projects of power plants in Europe. In addition, the

standard reference method delivers only numbers for total mercury but does not allow

differentiation between elemental (metallic) mercury and oxidized mercury which can be

removed in a wet scrubber much more easily.

In this project, which has been funded by VGB, alternative manual and automatic

measuring methods following the adsorption principle, as well as alternative continuous

measuring methods have been compared with each other and with the standard reference

method. In the scope of two measuring campaigns, performance data regarding the

equivalency of alternative methods and standard reference method and the uncertainty of

measurement have been determined. Conventional and newly developed continuous

monitoring systems have been included in the comparison. During this work the so-called

Dowex®/Charcoal method for differentiation of oxidized and elemental mercury in flue

gases has been validated in terms of total mercury determination. This allows a significant

simplification of flue gas measurements in the future which focus on further optimization of

mercury oxidation and removal from the flue gas.





1. Introduction

Mercury emissions from large combustion plants to air have been regulated on a

European level at the first time with enacting of the European Directive 2010/75/EU on

industrial emissions (IED) [1]. However, in this directive a mercury emission limit value

(ELV) has only been fixed for waste incineration plants at 50 µg/m3 based on a

sampling/measuring time of 0.5 to 8 hours. For large combustion plants firing coal or

lignite only the request for annual measurements of total mercury emissions has been

established in the directive.

In Germany, ELVs for mercury of large combustion plants have been assigned by law for

the first time with revision of the German large combustion plant ordinance [2] in 2004.

Limits were set to 50 and 30 µg/m3 respectively as half hourly and daily average values.

Unless specifically exempted by the authorities continuous monitoring of mercury was

mandatory. With the latest revision of the German large combustion plant ordinance in

May 2013 the existing ELVs for mercury have been confirmed and in addition an annual

average ELV of 10 µg/m3 has been introduced.

In the first draft of the LCP-BREF document [3] BAT associated emission levels for

mercury emissions to air from the combustion of anthracite and bituminous coal (> 300

MWth) are referred to as 0.2 - 2 µg/m3 for new plants and as 0.2 - 6 µg/m3 for existing

plants.

In Europe there are meanwhile a couple of legally binding operating permits for new and

recently modified coal fired power plants in place where limits of mercury emissions to air

have been fixed to values in the range of 1 – 3 µg/m3.

In the context of the above described implementation or tightening of mercury emission

limits on a national (German) and European level the question may be raised how

mercury emissions can be measured reliably in very low concentrations. Furthermore,

especially upstream of a wet scrubber for flue gas desulphurization, the oxidation stage of

mercury in the flue gas is of specific importance to achieve further reduction of mercury

emissions. For the European standard reference method for determination of total

mercury in flue gases EN 13211 [4] 2.6 µg/m3 is mentioned as the detection limit and

repeatability is described as 1.7 µg/m3. This is not sufficient to control emission limits in

the range of 1 - 3 µg/m3, as they are already effective in individual permits of coal fired

power plants in Europe. In addition, the standard reference method delivers only numbers

for total mercury but does not allow differentiation between elemental (metallic) mercury

and oxidized mercury which can be removed in a wet scrubber much more easily.





2. Benefits of alternative measuring methods to standard reference method

The European standard reference method EN 13211 for measuring mercury emissions to

air was released already in 2001. At that time ELVs were in place only in the range of 30 –

50 µg/m3 and the lower application limit of EN 13211 was appropriate for monitoring

mercury emissions within that context. Nowadays, when ELVs are in the range of 1 – 3

µg/m3, alternative methods with lower application limits are necessary to demonstrate

reliably compliance with such low emission limits.

EN 13211 is a wet chemical measuring method with limited flue gas throughput and

limited sampling time. Furthermore it requires a lot of manual operations. Therefore

reducing the lower application limit is not so easy. Many of the existing alternative

methods are based on the adsorption of mercury on solid adsorbent traps, consisting of

activated charcoal or iodized activated charcoal. Other solid adsorbents like potassium

chloride (KCl) or the ion exchange resin Dowex® retain selectively oxidized mercury.

Handling of the traps is usually very easy and the sampling can easily be automated

which allows extension of the sampling time from hours to days and weeks. In particular,

the combination of KCl or ion exchange traps with activated charcoal traps allows

furthermore the differentiation of oxidized and elemental mercury in the flue gas duct.

The aforementioned benefits make adsorbent trap measuring methods a valuable

alternative to the standard reference method as they allow easy and cost effective

verification of mercury mitigation measures as part of permit obligations, differentiation of

mercury oxidation state and thus reduction of costs for base line and performance tests

required by authorities and finally verification of performance guarantees from catalyst

suppliers regarding mercury oxidation activity of SCR catalysts.

In this project, that has been funded by VGB and the participating power companies GDF

SUEZ, Enel and E.ON, the main focus was on the so-called Dowex®/Activated Charcoal

method, later called the Dowex/AC method, which has been applied and enhanced across

many years by E.ON New Build & Technology GmbH and the Otto-von-Guericke

University Magdeburg. Other adsorption methods based on the US EPA method 30B [5]

and continuously operating mercury monitors have been included in the project and

compared against the standard reference method.

3. The Work Programme

The practical work within this project was carried out at two different coal-fired power

plants with anticipated low mercury emissions. The initial goal was to compare the

following four different sampling and measuring techniques for the determination of





mercury concentrations in flue gases: The European standard reference method EN

13211, the Dowex/AC method for speciation of the mercury oxidation state, a semi-

automatic method according to EPA Method 30B using activated charcoal traps and finally

the continuously operating PSA mercury monitor using thermo-catalytic mercury reduction

and atomic fluorescence spectrometry for detection. In addition to these four methods

other available systems for mercury measurement in flue gases have been included in to

this comparison, e.g. the plant operators calibrated mercury CEM and temporary

installations that have been operated at the plants for test reasons. For evaluation of the

test results and for validation of alternative methods compared to the standard reference

method the European Technical Specification CEN/TS 14793 [6] was applied.

Determination of the uncertainty of emission measurement for both, the standard

reference method EN 13211 and the Dowex/AC method was performed according to VDI

Guideline 4219:2009 [7].

The first measuring campaign was performed in October 2011 at a 510 MWel hard coal-

fired power plant with a conventional flue gas cleaning system consisting of high-dust

SCR, ESP and wet FGD. Mercury concentrations in the exhausted clean gas were

expected to be on average below 5 µg/m3. For the test campaign a fuel blend of South

African coal with a mercury content of approx. 0.1 mg/kg and a chlorine content of 0.04 –

0.06 wt.% and a Columbian coal with ~ 0.05 mg/kg mercury and 0.01 wt.% chlorine was

anticipated.

The second measuring campaign was carried out in December 2012 at a 108 MWel

lignite-fired power plant with a flue gas cleaning system consisting of pulverized lignite

coke and lime injection followed by an ESP. The fuel fired during the test campaign was

mainly lignite with significant amounts of waste derived fuel and sewage sludge. Historic

mercury concentrations in the cleaned flue gas were mostly in the range below 20 µg/m3.

The goal of the project was to compare different measurement systems and thus provide

as far as possible identical flue gas to all of the tested systems. Therefore no grid

measurements were performed but instead of that it was intended to extract the flue gas

for all systems as close together as possible at the same point in the middle of the duct.

All samplings and measurements should ideally be started and terminated simultaneously

and six samplings / measurements a day, each lasting one hour, were planned for each of

the methods. Each manual or automated sampling test should be carried out in duplicate.

The first measuring campaign covered all four of the above mentioned methods and in

addition three other measurement systems were available to deliver comparable mercury

data for at least a part of the time. Unfortunately during the second campaign the PSA





mercury analyser of Enel was not available and only one other system provided

comparable data. Table 1 gives an overview of all the involved methods and systems in

both measuring campaigns including the underlying measuring principle and the type of

data provision.

Table 1: Applied methods and available instruments for delivery of mercury data during the two measuring campaigns

1. Measuring campaign in October 2011 at a hard coal-fired power plant Methods applied by the participating research institutes

Applied method Applied by Measuring principle Type of data

EN 13211 E.ON New Build

& Technology

Absorption in a KMnO4/H2SO4

sampling train, European SRM

Total mercury

Dowex/AC Otto von Geuricke

University [OvGU]

Solid adsorption traps with Dowex

and iodized charcoal

Elemental and

oxidized vapour

phase mercury

EPA 30B Laborelec /

GDF SUEZ

Solid adsorption traps with activated

charcoal

Total vapour phase

mercury

PSA CEM Enel Atomic fluorescence spectrometry

after catalytic reduction

Elemental and total

vapour phase

mercury

Methods applied by plant operator and equipment manufacturers


Sick Mercem Plant operator Photometrically after wet chemical

reduction

Total vapour phase

mercury

Amesa M,

applying EPA 30B

Environnement SA

Deutschland


charcoal

Total vapour phase

mercury

Sick Mercem300Z Sick GmbH Zeeman AAS after thermal converter Total vapour phase

mercury

2. Measuring campaign in December 2012 at a lignite-fired power plant Methods applied by the participating research institutes


EN 13211 E.ON New Build

& Technology

Absorption in a KMnO4/H2SO4

sampling train, European SRM

Total mercury

Dowex/AC Otto von Geuricke

University [OvGU]

Solid adsorption traps with Dowex

and iodized charcoal

Elemental and

oxidized vapour

phase mercury

EPA 30B Laborelec /

GDF SUEZ


charcoal

Total vapour phase

mercury

Methods applied by plant operator


SM4 Mercury Instruments Atomic absorption spectrometry

after thermo-catalytic converter

Total vapour phase

mercury

4. Results of the Mercury Measuring Campaigns

The first measuring campaign in October 2011 resulted in a set of 36 flue gas samples

from each of the three discontinuous sampling methods (3 x 6 samples, all performed in

duplicate). At the first two of the three days the PSA analyser delivered 12 comparable 1-

hour-average values. The AMESA M system was operated only on the third day of the





measuring campaign and delivered 6 duplicate samples. Fortunately, the newly developed

Mercem300Z system was also installed and operated by the manufacturer for a test run.

The instrument was only ex-works calibrated but not fully calibrated according to EN

14181.

Figure 1 summarizes all results of the seven different measuring methods. If duplicate

samples were taken the average value of the two individual results is printed in the graph.

As can be seen from this figure all seven methods delivered more or less comparable

results. Within the discontinuous manual sampling methods the EN 13211 and the

Dowex/AC method fit best together, followed by the EPA 30B method and the AMESA M

system. Within the continuously operating measuring systems the PSA monitor showed

systematic lower values than the standard reference method on the first day and

systematic higher values on the second day. The official calibrated CEM delivered the

lowest values of all and the Mercem300Z delivered also values very close to the EN

13211 values with exception of the first day. Furthermore Figure 1 shows a very similar

trend of the mercury concentrations determined with the different methods in a range

between 1.3 and 4.0 µg/m3.

Figure 1: Summarized results of seven different measuring methods during the first measuring campaign at a hard coal-fired power plant.

During the second measuring campaign in December 2012 at the lignite-fired plant only

three of the four project partners were able to participate and they applied the three





methods EN 13211, Dowex/AC and EPA 30B. Again the schedule was to take 6 duplicate

samples on each of the three successive days. Due to equipment failures some measure-

ments were missing at the end and thus the second measuring campaign resulted in only

12 fully comparable duplicate measurements for each method. In addition to the manual

sampling systems the continuously operating mercury monitor SM4 was installed by the

plant operator for a test run and delivered 18 comparable 1-hour-average values.

As Figure 2 shows the mercury concentrations in the flue gas of this plant were

temporarily significantly higher compared to the first campaign and up to almost 20 µg/m3.

The values resulting from the methods EN 13211 and Dowex/AC match perfectly for both

the lower and the higher concentrations. The values resulting from the EPA 30B method

match also very well in some sampling series but are significantly lower in other series. An

explanation for this was seen by the handler to be due to poor sealing of the adsorbent

traps in the probe causing occasional leakages and fouling of the equipment in previous

tests. Finally the SM 4 instrument delivered well-matching values for higher mercury

concentrations but did not follow quickly enough the trend towards lower values. This

observation might be due to memory effects in the sample gas line from the stack to the

analyser.

Figure 2: Summarized results of four different measuring methods during the second measuring campaign at a lignite-fired power plant.





5. Validation of an alternative method according to CEN/TS 14793

Figure 3 gives a summary of the whole set of data resulting from the three manual

methods that have been employed in both measuring campaigns. This set of data has

been used for evaluation of the comparability of the two alternative methods according to

CEN/TS 14793:2005.

Figure 3: Summary of the whole set of data resulting from the three methods involved in both measuring campaigns.

The requirements for application of CEN/TS 14793 are:

• Up to 30 pairs of comparative values for each method

• Outlier test according to Grubbs to be applied for each set of comparative values

The conditions that need to be fulfilled for acceptance of the alternative method are:

• Repeatability of the alternative method must not be higher than the repeatability of

the standard reference method

• Linear regression must be evaluated (y = C1x + C0)

• The following conditions must be fulfilled:

R ≥ 0.97 1-Sr(Zq)/Zq ≤ C1 ≤ 1+Sr(Zq)/Zq C0 ≤ Sr(Zq)

with Zq and Sr(Zq) from standard reference method EN 13211:

Zq = 10 µg/m3, Sr(Zq) = 1.2





In the first instance the evaluation of comparability of the Dowex/AC method with

EN13211 was carried out. For this purpose a set of 30 duplicate samples were suitable for

evaluation and have been subject to a Grubbs test [8] in the first step of the evaluation. No

outliers have been detected within the duplicate samples from EN 13211. Within the

Dowex/AC samples in total 5 outliers according to Grubbs could be identified. In this case

the identified values from both methods have been eliminated leaving a set of 2 x 25

verified duplicate values for both of the methods. Figure 4 shows the evaluation matrix

according CEN/TS 14793 for the alternative method.

Figure 4: Evaluation matrix according to CEN/TS 14793 for the alternative method Dowex/AC.

The graph of the resulting regression line is given in Figure 5. The final test for

acceptance of the alternative method, as given in Table 2, shows that all three

requirements for acceptance are fulfilled. For this reason the Dowex/AC method for

measuring concentrations of total mercury in flue gases can be considered to be

equivalent to the standard reference method in the range of 1 to 20 µg/m3.





Figure 5: Regression line of the comparison between Dowex/AC method and the standard reference method EN 13211

Table 2: Results of acceptance test of alternative method Dowex/AC

Test for Acceptance of Alternative Dowex/AC Method

Condition:

Condition:

Condition:

R ≥ 0,97 1-Sr(Zq)/Zq ≤ C1 ≤ 1+Sr(Zq)/Zq C0 ≤ Sr(Zq)

1-0,12 ≤ C1 ≤ 1+0,12 C0 ≤ 1,2

R = 0,996028 C1 = 0,978187 C0 = -0,063747

yes yes yes

When applying the criteria of CEN/TC 14793 to the data of the EPA 30B method the

requirements for acceptance as an alternative method were not fulfilled. This is mainly

due to the results from the second measuring campaign where some problems with

leakages and fouling occurred. Also validation of the determination of mercury oxidation

state by use of the Dowex/AC method and the use of the PSA analyser was not possible

within this project because agreement of the results from the first campaign was not very

good and the PSA analyser was not available during the second campaign. Both issues

are subject to further investigation.





6. Determination of the uncertainty of measurement of EN 13211 and Dowex/AC methods

The determination of the measurement uncertainty of emission measurements by

repeated sampling and analysis of the same object of measurement is usually not

possible since locally and temporally varying distributions of the exhaust gas components

can occur in the measurement cross section. Sampling of reference materials as

alternative is also mostly not possible since appropriate reference materials and testing

facilities are not available for this complex exhaust gas matrix. Therefore the

determination of measurement uncertainty has been performed on the basis of the

performed paired measurements using the overall measurement method (direct approach)

according to VDI Guideline 4219. In the direct approach to determine measurement

uncertainty by means of paired measurements, the influence of all factors is investigated

in a pooled way.

According to VDI 4219 the paired measurements shall be independently of each other, in

order to reveal random and systematic errors during uncertainty determination. Therefore,

the measurements shall also be performed with separate sampling and analytical

systems. In this case separate sampling systems have been applied and the joint use of

analytical systems was accepted because their contribution to the total variance was

assumed to be less than 5 % (see DIN EN ISO 20988). For both methods, the EN 13211

and the Dowex/AC method, the 25 simultaneous paired measurements of section 5, used

for equivalency validation, have been used to evaluate the uncertainty of measurement of

each method. The following equations have been applied according to VDI 4219:

The variance of the measurement results y from paired measurements is calculated as:

where: y (1, j) jth measured value of the first measuring system

y (2, j) jth measured value of the second measuring system

N number of evaluated paired measurements

A significant bias uB is given if uB2/var(y) ≥ 0.5, where the bias is calculated as:





The standard uncertainty u(y) of a measurement result y that is determined by the given

measurement method is calculated as:

The expanded uncertainty Up(y) of a measurement result y for a confidence level p is

calculated by multiplying the standard uncertainty u(y) by the coverage factor k, which

corresponds to the confidence level p and the number of degrees of freedom ν, according

to the following equation:

For a confidence level of p = 95 % and a number of degrees of freedom of 25, the factor

k = 2.060 was used.

The results of uncertainty evaluation are given in Table 3. This table shows that the

expanded uncertainties of both methods with values of 0.3 and 0.4 µg/m3 are very close

together and on a low level. In the standard reference method EN 13211 a repeatability of

1.2 µg/m3 and a reproducibility of 1.7 µg/m3 is reported.

Table 3: Results of determination of uncertainty for the methods EN 13211 and

Dowex/AC

Statistical parameter

Method EN 13211

Method Dowex/AC

Unit Explanation

ymin = 1.9 1.6 µg/m3 lower limit of measuring range

ymax = 19.6 19.4 µg/m3 upper limit of measuring range

N = 25 25 number of pairs (degrees of freedom)

k = 2.060 2.060 coverage factor for 95 % confidence

uB2/var(y) = 0.8 0.5 bias is significant if ≥ 0.5

u(y) = 0.1 0.2 µg/m3 standard uncertainty

U0,95 = k u(y) = 0.3 0.4 µg/m3 expanded uncertainty





7. Summary and conclusions

Within two measuring campaigns at two different power plants several discontinuous

manual and automated measuring methods as well as different continuously operating

monitors for determination of mercury concentrations in flue gases have been compared

in a mercury concentration range of 1 to 20 µg/m3. From these trials it finally can be

concluded that all of the applied measuring methods gave more or less comparable

results as long as they are applied properly. The various adsorption tube methods

including the Dowex/AC method, the EPA 30B method and the AMESA M system didn’t

show systematic deviations from the standard reference method (SRM) EN 13211. In

contrast to that, the involved continuously measuring systems (CEMs) sometimes showed

higher deviations from the SRM. The Dowex/AC method was the only method that could

be successfully validated according to CEN/TS 14793 as an alternative method to the

SRM in the concentration range of 1 to 20 µg/Nm3. According to literature, the Ontario

Hydro Method and EPA Method 30B are suitable as well [9]. Uncertainties of

measurement of the two methods EN 13211 and Dowex/AC in the covered concentration

range have been determined as 0.3 and 0.4 µg/m3 respectively. Unfortunately, due to a

lack of comparable data, a validation of the differentiation between elemental mercury and

oxidized mercury with the methods Dowex/AC and PSA was not possible.

8. References

[1] Directive 2010/75/EU of the European Parliament and of the Council of 24

November 2010 on industrial emissions (integrated pollution prevention and control);

Official Journal of the European Union, L 334/17, 17.12.2010

[2] Thirteenth Ordinance on the Implementation of the German Federal Ambient Air

Control Act (Ordinance on Large Combustion Plants and Gas Turbine Plants – 13.

BImSchV) of 20 July 2004; Federal Law Gazette I p. 1717

[3] Best Available Techniques (BAT) Reference Document for the Large Combustion

Plants, Draft 1 (June 2013); Joint Research Centre, Institute for Prospective

Technological Studies, Sustainable Production and Consumption Unit, European

IPPC Bureau

[4] European Standard EN 13211 - “Air Quality – Stationary source emissions – Manual

method of determination of the concentration of total mercury”, June 2001

[5] EPA Method; Method 30B - “Determination of Total Vapor Phase Mercury

Emissions from Coal-fired Combustion Sources using Carbon Sorbent Traps”,

November 2007





[6] European Technical Specification CEN/TS 14793 – “Stationary source emission –

Intralaboratory validation procedure for an alternative method compared to a

reference method”, June 2005

[7] VDI Guideline 4219 – “Determination of the uncertainty of emission measurements

by use of discontinuous measurement methods”; August 2009

[8] F. Grubbs; “Procedures for Detecting Outlying Observations in Samples”;

Technometrics, Vol. 11 (1969) 1 – 21

[9] D. L. Laudal; “Evaluation of alternatives to the Ontario Hydro Method as a reference

method for CAMR”; 2007-EERC-02-04

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