1. Background

Basic information on trace metals can be found in CF58. This combustion file follows on from CF206, which details the equipment needed for a trace metals and particulates sampling train.

This Combustion File (CF) deals with the preparation required for a sampling train conforming to the requirements of US EPA Method 29: Determination Of Metals Emissions From Stationary Sources. The method can also be used to sample for particulates. The use of this sampling train, selection of traverse points, and the subsequent analysis of the sample are covered in CFs 204 to 209.

Safety

This CF does not cover safety procedures. The reader should consult other safety literature and particularly local safety regulations before performing the acquisition of the sample.

2. Preparation

Please read CF206 for details on the equipment referred to in this CF before proceeding any further.

1.        For all glassware:

a)       Rinse in hot tap water

b)       Wash in hot soapy water

c)       Rinse in tap water 3 times

d)       Rinse in distilled water 3 times

e)       Soak in a 10% v/v nitric acid solution for 4 hours

f)       Rinse 3 times with distilled water

g)       Rinse once with acetone

h)       Allow to air dry

i)        Cover all glass openings from contamination, until sampling is about to begin

2.       Check filters visually against light for irregularities, flaws or pinhole leaks. Label filters on the back side near the edge using numbering machine ink (commonly available from stationary stores).

3.       Desiccate the filters at 20 ­± 5.6 ^oC and ambient pressure for at least 24 hours

a)       Weigh the filters every 6 hours until there is less than 0.5mg change.

b)       Do not expose the filter to lab atmosphere for more than 2 minutes

4.       Select the sampling site and minimum number of sample points (see CF 204, CF 205).

5.       Determine the stack pressure, temperature and range of velocity heads

6.       Perform leak check of pitot lines by:

¨        Blow through the pitot impact opening until at least 7.6 cm H₂O velocity registers on the manometer, then close off the impact opening

¨        Criterion:  the pressure must remain stable for 15 seconds

¨        Do the same for the static pressure side, except using suction to obtain a minimum of 7.6 cm H₂O

7.       Determine the moisture content and the stack gas dry molecular weight. This is outside the scope of this CF.

8.       Select a nozzle size based on the range of velocity heads, such that it is not necessary to change the nozzle size in order to maintain isokinetic sampling rates – this may be achieved by using a Nozzle Flow Range Chart (which uses flow rates and pressures to determine nozzle size).

a)       During the run, do not change the nozzle size

b)       Ensure the proper gauge selection is used for the velocity profile. The pressure gauge should be of water-column, inclined-vertical type, 254 mm (10 inches) , with 0.25 mm (0.01 inch) H₂O divisions for the first 25 mm (1 inch) of the scale, then 2.54 mm divisions for the rest of the scale. However, if a) the arithmetic mean of all pressure readings at the traverse points is less than 1.27mm, or if b) 10% or more of the pressure readings are below 1.27mm, then a manometer of greater sensitivity should be used.

9.       Select a suitable probe liner and probe length such that all traverse points can be sampled.

a)       For large stacks, consider sampling from opposite sides of the stack to reduce the required probe length

10.     Select a total sampling time greater or equal to the minimum total sampling time, which must be chosen specific to industry – consult relevant authorities. It should be such that:

a)       The sampling time for each point is not less than 2 minutes

b)       The sample volume taken is not less than the required minimum total

c)       The sampling time at each point should be the same

d)  Sometimes, it may be necessary to sample for a shorter time at the traverse points to obtain small gas sample volumes.

11.      Set up the train as described in CF 206.

a)       During all preparations, keep all openings where the contamination can occur covered until just prior to assembly or until sampling is about to begin

b)       To ensure leak-free sampling train connections, and to prevent possible sample contamination, use Teflon tape in setting up the sampling train

c)       Exercise extreme caution to prevent contamination while setting up the train. Do not let the KMnO₄ solution contact any glassware that contains sample material to be analysed for Mn.

12.     Put 100mL of HNO₃/H₂O₂ into the 2^nd and 3^rd impingers (what about the 4^th impinger)

13.     Put 100mL of the KMnO₄ solution in the 5^th and 6^th impingers

14.     Put 200-300g of silica gel in the 7^th impinger

15.     Using a tweezer or clean disposable surgical gloves, place a labelled and weighed filter in the filter holder.

a)       Be sure it is properly placed so as to prevent the sample gas stream from circumventing the filter

b)       Check the filter for tears after the assembly has been completed

16.     Install selected nozzle using a Viton A O-ring when stack temperatures are <260^oC, or a heat resistant string gasket for temperatures above this. For temperatures significantly above this, it is suggested that a water-cooled probe be considered, for example an Alcan probe (CF137), suitably modified.

17.     Perform Pre-Sample Leak Test:

a)       Criteria: If there is more than 4% leakage, or if the leak is greater than 0.00057m³/minute, then the leakage rate is unacceptable.

b)       Turn on and set the filter and probe heating systems to the desired operating temperatures. Allow time for the temperatures to stabilise.

c)       If a Viton A O-ring or other leak-free connection is used, leak check at the sampling site by plugging the nozzle and pulling a 380mm Hg vacuum. Start the pump with the bypass valve fully open and the coarse adjust valve completely closed.

d)       Partially open the coarse adjust valve, and slowly close the bypass valve until the desired vacuum is reached. If the vacuum is exceeded in the course of this procedure, then leak check at his higher vacuum. Do not reverse the direction of the bypass valve, as this will cause water to back up into the filter holder.

e)       If the leakage rate is unacceptable, then the train must be examined for leaks, and repairs/replacements should be made until there is an acceptable leakage.

f)       When the leak check is completed, first slowly remove the plug from the inlet to the probe or filter holder, and immediately turn off the vacuum pump. This prevents the water in the impingers from being forced backward into the filter holder, and silica gel from being entrained backward into the neighbouring impinger.

18.     Leak checks during a sampling run:

a)       If, during a run, a component change is necessary, a leak check shall be conducted immediately before the check is made. The leak check shall be done as for the pretest, except that it will be performed at a vacuum equal to or greater than the maximum value recorded up to that point

b)       If the leakage rate is higher than the criteria, continue, but record the leakage rate in order to correct later on. Alternatively, void the sample run.

c)       Immediately after component changes, leak checks are optional.

19.     Leak check after sampling run:

a)       Follow same procedure as for pre-sample leak check, but at a vacuum equal to or greater than the maximum value recorded during sampling

b)       If the leakage criteria is exceeded, record the value and correct during analysis or void the sample.

3. What to do next

After the above steps have been taken, the train can be used for sampling  – see CF208.

It is important to remember to perform the leak checks described in steps 17, 18 and 19.

Sources

[1] US EPA, US EPA Method 29: Determination Of Metals Emissions From Stationary Sources, US Federal Registry.

[2] US EPA, US EPA Method 2: Determination Of Stack Gas Velocity and Volumetric Flow Rate, US Federal Registry.

[3] US EPA, US EPA Method 5: Determination Of Particulate Matter Emissions From Stationary Sources, US Federal Registry.