Trace Level Halides in Coal by Metrohm Combustion IC

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Application Work AW IC US6-0167-012013

Trace Level Halides in Coal by Metrohm Combustion IC

Branch

General analytical chemistry; energy, power plants; environmental protection

Keywords

IC 881 / Metrosep A Supp 5 - 150/4.0 / MMS 5000 / 920 / Absorber Module / Combustion Module / combustion ion chromatography / CIC / Coal / matrix elimination / fluoride / chloride / bromide / branch 16; branch 1; branch 2

Summary

The application work describes the determination of trace (μg/g) levels of halides in coal samples using Metrohm Combustion IC system.

Mercury is highly volatile and a neurotoxin. Coal power stations are responsible for the highest amount of mercury emissions in Germany, the US and worldwide. They emit the poisonous metal, also known as quicksilver, into the atmosphere and it can eventually make its way into the food chain. Mercury is only found in coal in small amounts, but coal is burned around the world and in large quantities. The United Nations Environment Program estimates that internationally coal power plants emit some 500 tons of mercury a year - Germany is responsible for more than five tons, the US for almost 50 tons and China for more than 100 tons

1.

Mercury is released when coal is burned and takes three forms in the flue gas: elemental, oxidized, and particulate. Oxidized and particulate mercury can be controlled by abatement equipment. However, elemental mercury is gaseous at combustion temperatures and is difficult to capture. Therefore, chemical additives have been developed to oxidize the elemental mercury in the flue gas, converting it to a form which can be captured by current abatement equipment. Calcium bromide is effective in reducing total mercury emissions by oxidizing elemental mercury that is then captured by the abatement equipment. Brominated powdered activated carbon has been demonstrated to capture over 90 per cent of mercury emissions from coal fired plants

2,3,4, operating at high

temperatures and capturing the metal in concrete-friendly, non-leaching forms to help tackle a rising global health threat. Bromine and iodine in atmospheric particles or coal can cause environmental problems such as destruction of ozone in the atmosphere; therefore, the presence of these compounds has recently received increased attention.

Fluorine occurs in trace amounts in most coals. It is typically associated with minerals of the apatite group, principally fluorapatite and clays, and with fluorite, tourmaline, topaz, amphiboles and micas. The average fluorine content of US coal is, according to the tabulation of Swanson, er al, 74 µg/g. In the United States, the lowest average fluorine concentration of 30 µg/g is found in coals from Eastern Kentucky and the highest average value of 160 µg/g is found in coals from Wyoming and New Mexico. The concentration range of fluorine in

European coals is similar to that found in the US while the average fluorine content of Australian coals ranges from 15 to 500 µg/g.

Fluorine and chlorine is released into the atmosphere during the combustion of coal. Because of its effect upon environmental quality and health, fluorine and chlorine have been classified as elements of concern in the development of the coal resource. With the increased utilization of this energy resource, it is important to be able to reliably determine these halides content in coals and coal by-products.

In this application work trace level of fluoride, chloride and bromide are determined in various coal samples using Metrohm Combustion IC. The Repeatability and reproducibility was tested and the resulting precision is summarized.

Reagents

Sodium carbonate , CAS 497-19-8

Sodium bicarbonate, 144-55-8

Sulfuric acid, CAS 7664-93-9

Fluoride standard stock solution, β(F -) = 1 g/L; Sigma Aldrich 77365

Chloride standard stock solution, β(Cl-) = 1 g/L; Sigma Aldrich 39883

Bromide β(Br-) = 1 g/L in H2O, No. 43147

Sulfate standard stock solution, β(SO4-2) = 1 g/L; Sigma Aldrich 90071

Ultrapure water, resistivity >18 MΩ·cm (25 °C), type I grade (ASTM D1193)

Multicomponent Anion Standard in H2O -IC1- 100 µg/mL

Hydrogenperoxide, 30% H2O2, TraceCert®, from Sigma Aldrich 16911

Instruments

881 Compact IC pro – Anion – MCS 2.881.0030

IC conductivity detector 2.850.9010

Combustion Module 2.136.0700

Autosampler MMS 5000 2.136.0800

Kit for solid sampling 6.7302.000

920 Absorber Module 2.920.0010

MagIC Net TM

2.4 Compact 6.6059.241

Metrosep A Supp 5 - 150/4.0 6.1006.520



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Metrosep A Supp 4/5 Guard column 6.1006.500

Metrosep A PCC 1 HC 6.1006.310

Solutions Eluent: 3.2 mmol/L sodium carbonate + 1.0 mmol/L

sodium bicarbonate Eluent is prepared from the A Supp 5 Concentrate 20X (Metrohm USA number: REAIC1101) by the EPOD system. Alternatively eluent concentrate from Sigma-Aldrich (product number: 62414) can also be used. The 849 level sensor continuously monitors the eluent level and it triggers the eluent preparation when the level is less than full.

Alternatively for eluent preparation

Pour 1 full snip (REAIC1102) of Supp 5 eluent concentrate in a mobile phase reservoir and dilute it to 1 L with ultrapure water.

Regenerant for Suppressor: 100 mmol/L sulfuric acid

5.5 mL of concentrated sulfuric acid is transferred in a 1 L reagent bottle containing 0.5 L of water and diluted up to 1 L with ultrapure water.

Samples

1. Certified Reference Material for Chloride in coal

- BCR-180

2. Secondary Coal Standards provide by customer,

tested and verified by XRF technique

a. 14 ppm BRAY Coal

b. 24 ppm BRAY Coal

c. 82 ppm BRAY Coal

Standard

Mixed anion standard of 1 mg/L is prepared from 10 mg/L of multi anion high purity standard.

Method Description

About 20 to 40 mg of coal samples are weighed accurately on an analytical balance with 0.1 mg resolution. 10 mL Dosino connected to the Absorption solution empties out the absorption tube and adds 2 mL of absorption solution. The absorption solution volume is an important variable to achieve the sensitivity. For samples containing halides less than 100 mg/L, absorption solution of 2 mL is recommended. For samples containing high concentration of halides the absorption solution volume can be proportionately increased to a maximum of 8 mL. ABD feed starts and the combustion begins. The flame sensor measures the intensity of the light and controls the feed rate of the sample boat into the oven in proportion to

it. As a result, the duration of the combustion is optimized so that, firstly, combustion is always complete (no soot formation), and, secondly, there is no need for waiting times as a safety buffer. The automated combustion process eliminates the time consuming method development steps. ABD moves the quartz boat to annealing at the end position specified. Subprograms for water and absorption solution addition are carried out simultaneously and the total volume addition is calculated by MagICNet. For coal application 0.2 mL/min water and absorption solution addition is recommended. Accurate liquid addition by Dosino and total volume calculation by MagICNet eliminates the need for Phosphate internal standard which are typically used in combustion IC method. The remaining pyrolysis gases are burned in the O2 stream. The duration is specified as “Post Combustion Time” in MagICNet method. For coal it is recommended to use 180 s, which ensures the complete combustion.

Optimized IC and Combustion parameters are summarized below.

IC Parameters

Eluent Flow 0.7 mL/min

Eluent Source EPOD

Column temperature 30°C

Absorber Solultion 100 mg/L H2O2

Sample loop 250 μL

Inj. Vol. (Standards) 5 - 200 μL

MSM Regenerent 100 mmol/L Sulfuric acid

MSM Rinsing water

Carbonate Suppressor ON

Combustion Parameter

Argon 100 mL/min

Oxygen 300 mL/min

Oven temperature 1050 °C

Time for post-combustion 180 sec

Initial volume of absorbing solution 2 mL

Water addition 0.2 mL/min

Absorbtion solution addition 0.2 mL/min

Boat Blank

It is important to bake the boats before using it for sample analysis in order to remove the ionic impurities present in it. The quartz boat is much cleaner compared to the ceramic boat and doesn’t leach out ionic impurities. For trace analysis it is essential to keep the boats and the pyrotubes clean. For trace analysis analyte concentration from the boat blank should be, ideally, less than 2 µg/L (ppb).

Sample analysis

20 mg to 40 mg of the coal sample is combusted. Depending on the expected bromine concentration about 100 μL to 200 μL of the combusted sample is preconcentrated and injected into the IC. A guide line



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correlating the analyte concentration to the sample weight and injection volume is provided in Appendix Tables.

Calculation

Automatic integration with MagIC Net 2.4 software using peak area for all analytes.

Time program

The time program was set up with sub-routines which are activated depending on the sample type (e.g. sample, standard, check standard and blank) defined in the determination series table.

Results

Linearity

IC is calibrated separately using mixed standard anions. Linearity is established by variable volume preconcentration using the 1 mg/L mixed anion standard.

Volume (μL) 200 100 40 20 10 5

Conc.(μg/mL) 1 0.5 0.2 0.1 0.05 0.025

A regression line was obtained by plotting the peak area verses concentration using the least square method. The relationship between the peak response and the concentration is linear with correlation coefficient (r

2) of

>0.999 for all the ions with a response factor RSD less than 3%. Results of linearity study is shown in below Table

Ions Range

(mg/L)

Correlation

Coeffiecnt (r2)

%RSD

Fluoride 0.025 to 1 0.99964 3.08

Chloride 0.025 to 1 0.99995 1.24

Bromide 0.025 to 1 0.99970 3.12

Example chromatogram for blank, boat blank, standards and the calibration details are provided in appendix.

Sample Results

Samples are run in triplicate and the average results in µg/g are shown in table below.

Ions BRAY

14 ppm

BRAY

24 ppm

BRAY

82 ppm

Fluoride 25.1 25.2 30.5

Chloride 9.4 6.8 7.3

Bromide 14.0 23.4 81.6

Detailed precision for individual samples are provided in appendix.

Detection Limit

Detection limit with respect to the coal sample weight was arrived out by analyzing the Bray 14 ppm Coal samples seven times. From the standard deviation (s) of replicate

analysis the detection limit is calculated, as per EPA method.

DL= s X 3.14

3.143 is the Student’s t value at a 99% confidence level

Calculated detection limit for Chloride, Fluoride and Bromide respectively are, 1.25, 4.08 and 1.25 µg/g.

Precision and Accuracy

Method performance was evaluated by analyzing the BRAY 14 ppm coal sample on two different days. Overall precision for seven replicate on two different days is less than 3%.

Results obtained by CIC are in agreement with the XRF method for all three BRAY coal samples for bromide. Recovery more than 95% was achieved for Bromide with CIC method for all the BRAY coal samples. Overall, recovery study indicates that the method performs well for the trace level halides determination in coal matrix.

Comments

Iodide cannot be determined using this method. Iodide gets converted to iodate and other oxide form during combustion in presence of oxygen. This conversion also facilitated by the presence of hydrogen peroxide. An alkaline absorption solution is needed for Iodide determination.

The recovery in all the cases is more than 95% and there is no need for catalyst such as tungsten or tungsten oxide.

Date

28*01*2013

Reference

1. http://www.dw.de/a-little-bromine-cuts-coals-

mercury-emissions/a-15830213-. Author: Ralph

Heinrich Ahrens / nh.Editor: Holly Fox

2. GEOBROM White paper final 081910-White Paper

on Calcium Bromide Technology for Mercury

Emissions Reductions

3. Mercury control evaluation of Calcium Bromide

injection into a prp-fired furnace with an SCR

4. Vosteen, B. et al., “Process for Removing Mercury

from Flue Gases,” United States Patent #

6,878,358

Authors

Dr. Hari Narayanan

Market Development Manager

Metrohm USA Inc.

Appendix

Instrumentation setup, calibration curves, chromatograms with results and tables.

http://www.dw.de/a-little-bromine-cuts-coals-mercury-emissions/a-15830213-

http://www.dw.de/a-little-bromine-cuts-coals-mercury-emissions/a-15830213-



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Instrumentation setup

5 mL Dosing unit is used for:

During Combustion: Water addition through port -4 @ 0.2 mL/min

Post Combustion:

Sample aspiration and injection to the loop – via port 3

Sample Transfer from loop to Preconcentration column and matrix elimination – via port 1

10 mL Dosing unit is used for:

2 mL - Absorption solution addition via port 3 at the beginning of combustion

0.2 mL/min - Absorption Solution addition via port 3 during combustion

1.0 mL - Absorption solution addition to rinse dead volume from combustion oven outlet

to absorber tube

Final rinse the combustion tube after sample injection via port 1

Absorption solution and water addition during combustion is calculated automatically from

the combustion time and the rate of addition. Accurate addition of liquid using Dosino

eliminates the need for phosphate internal standard typically used in Combustion IC

methods.



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Appendix Calibration, Blank and Standard chromatograms



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Chromatograms:

IC- System Blank

Boat Blank

With Linear Calibration curve the Boat Blank concentration values is negative. The blank value is determined using linear thro

zero calibration curve. With respect to average sample weight of 25 mg these blank values corresponds to ~ 2 and 4 µg/g of

Fluoride and Chloride respectively.

Overlay of standard chromatograms from level 1 to level 6



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Standard Overlay

Coal Reference Standard BCR-180

Theoretical Value = 593±29 µg/g



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Coal Chloride Standard: Coal BCR 180

Theoretical Value = 593±29 µg/g

Coal Reference Standard BCR-180-Reburn the same boat

Concentration of chloride is 5 µg/g, close to the Blank Boat burn, indicates the completeness of the sample burn.



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BRAY 14 ppm Coal Sample



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BRAY 14 ppm Coal Sample: Precision Study I

Seven Replicates to calculate the Detection Limits.

Detection limit for Bromide on Mass basis = DL= s X 3.14

= 0.4*3.14

= 1.256 mg/kg or 1.256 μg/g



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Detection Limit for Fluoride

Detection limit for Fluoride on Mass basis = DL= s X 3.14

= 0.3*3.14

= 4.08 mg/kg or 4.08 μg/g



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Detection Limit for Chloride

Detection limit for Chloride on Mass basis = DL= s X 3.14

= 0.4*3.14

= 1.256 mg/kg or 1.256 μg/g



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BRAY 14 ppm Coal Sample: Precision Study II

Seven Replicates to calculate the Detection Limits

Detection limit for Bromide on Mass basis = DL= s X 3.14

= 0.4*3.14

= 1.256 mg/kg or 1.256 μg/g



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BRAY 24 ppm Coal Sample: Seven Replicates Statistical Data for Bromide.



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BRAY 82 ppm Coal Sample: Three Replicates Statistical Data for Bromide.

Tables

Calibration

Injection volume, concentration and Mass Relation

Volume (µL)

Concentration (µg/mL)

Mass (ng)

5 0.025 25

10 0.05 50

20 0.1 100

40 0.2 200

100 0.5 500

200 1 1000

Choose the appropriate stock standard to establish calibration in lower or higher concentration range, covering the sample

concentration range.



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Blank Contribution:

It is important that all the reagents and solvents used in CIC are of trace select grade. The pyrotubes and boats must be clean

for trace analysis. The following table gives an idea about the Blank contribution from the CIC system and its equivalent

concentration with respect to the different sample weights. For Trace level halide determination the CIC Blank contribution

should be less than 0.005 µg/mL (5 ppb).

Blank Concentration

(µg/mL)

Sample Weight

(mg) Total Volume

(mL)

Final Concentration With Respect to Sample Weight

(µg/g)

0.001 25 4 0.16

0.005 25 4 0.8

0.01 25 4 1.6

0.02 25 4 3.2

0.025 25 4 4

0.001 25 5 0.2

0.005 25 5 1

0.01 25 5 2

0.02 25 5 4

0.025 25 5 5

0.001 50 5 0.1

0.005 50 5 0.5

0.01 50 5 1

0.02 50 5 2

0.025 50 5 2.5

Final volume in absorption tube can be increased or decreased by adjusting the initial absorption solution volume, rate of water

and absorption solution addition during combustion. The combustion time is a variable, which is automatically monitored and

used for calculating the total volume in absorption tube.

To reduce the Blank contribution the sample weight can be increased proportionately. However, higher sample size will reduce

the pyrotubes life on long term. Sample weight higher than 100 mg is not recommended for coal application.



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Guide Line for Trace Level Halides Determination

Guideline to choose the injection volume based on Analyte concentration, sample weight and Absorption solution volume after

combustion. It is recommended to choose the sample weight and volume so that the sample area falls well within the

established calibration curve.

Concentration (mg/g)

Sample Weight

(mg)

Total Volume

(mL) Injection

volume (µL)

<10* 25 4 500-1000

<10 50 4 500-1000

<10 75 4 500-1000

<10 100 4 500-1000

10 25 4 200

10 50 4 200

10 75 4 200

10 100 4 200

25 25 4 100

25 50 4 200

25 75 4 100

25 100 4 100

75 25 4 100

75 50 4 100

75 75 4 50

75 100 4 50

100-200 25 4 50

100-200 50 4 25

100-200 75 4 10

100-200 100 4 10

*To achieve detection limit less than 5 μg/g, the Boat Blank should be as low as 0.001 µg/mL (1 ppb)

Samples with high concentration, the initial absorption solution volume can be increase as high as 8 mL and the injection

volume can be reduced as low as 5 μL.

Alternatively, it is recommended to pre-dilute the samples in compatible solvents and combust reduced volume of the diluted

samples.

Trace Level Halides in Coal by Metrohm Combustion IC

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