Gas Cleaning System - Electrostatic Precipitators - Performance Testing
December 12, 2008

Introduction Sampling Apparatus Sampling Rate Testing Analysis SO2 Removal Washing of Filters Titration for Sulphuric Acid Calculations

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Introduction

The determination of WESP performance is sometimes required during plant performance tests and during operation to help diagnose problems.  A qualitative measurement of WESP performance can be done by viewing the exit gas through a set sight glasses.  If the gas exiting the WESP is optically clear, the gas contains approximately 30 mg/Nm³ of acid mist.  This method is fine for acid mist but does not provide indication of WESP performance for other gaseous contaminants.  As well, there is not indication of what the inlet loading is to the WESP’s.

To determine WESP performance quantitatively requires isokinetic sampling of both the inlet and outlet gas streams.  Acid mist, dust and submicron particles are captured in a series of filters.  Subsequent analysis of the filter contents will give the amount of acid mist, dust, etc. captured which is then used to determine concentration in the gas and eventually WESP collection efficiencies.

Sampling Apparatus

The apparatus required for sampling the inlet and outlet gases are:

• Dry gas meters
• Aspirator or other apparatus for withdrawing gas
• Thermometers
• Pitot tube
• Gooch funnels
• Glass water condensers
• Asbestos (long fibre, chemically neutral)
• Assorted glassware

Two complete sample trains are required to enable simultaneous measurement of the inlet and outlet gases.

Preparation of the sample funnels is required to ensure capture of all the particles in the gas.  The asbestos should be packed firmly into the funnel so that the pressure drop across the sample train is not greater than 2 in. Hg at a gas flow of 0.3 to 0.5 ft³/min.

The entire sample train needs to be maintained at a high enough temperature to prevent condensation of water in the apparatus.  Typically, the sample train is housed in an insulated box which is heated by lamps or glycol type heater.  The temperature in the box is maintained a few degrees above the dewpoint of the gas.

Sampling Rate

Gas sampling should be done isokinetically, meaning the velocity of the gas through the sample tube nozzle should be approximately the same as the velocity in the gas duct at the sample point.  The following information is required in order to calculate the sample rate:

• Velocity and temperature of gas in the duct
• Area of sampling nozzle
• Temperature of the gas as metered

Testing

The inlet and outlet samples should be run at the same time if possible.  Generally, the sample duration for the inlet sample will be shorter than the outlet because the concentration at the inlet is higher than the outlet.   Longer samples are required for the outlet in order to obtain enough sample for titration and analysis.  Five to ten cubic feet of gas at inlet conditions are usually enough for sample while fifteen to twenty cubic feet will be required for the outlet sample.

The gas rates should be checked immediately after the start of sampling.  Readings of gas meter temperature, pressure and volume should be taken at five minute intervals.  The average of these readings are taken for computation of the gas volume metered at standard conditions.

Analysis

After the required volume of gas has been sampled, the entire sample train should be removed to the laboratory.  It is important when removing the train from the duct to be especially careful so that no condensed acid drips from the sample tube.  The outside of the apparatus should be wiped carefully.

SO₂ Removal

The SO₂ absorbed in the sampling train is removed by blowing out with air.  The air is first dried by passing through a wash bottle filled with 98% H₂SO₄.  Any acid entrained in the air is removed as it passes through an empty wash bottle followed by a wash bottle filled with wads of cotton.  The flow rate of air is set at 0.3 ft³/min.  All SO₂ is removed when the air is passes through a test tube full of N/100 iodine with starch as an indicator shows no discolouration in fifteen minutes.  This part of the procedure can take several hours before all the SO₂ is removed.

Washing of Filters

The sample filters are washed thoroughly in a filter flask which is set up with suction.  No. 1 filter funnel is placed in the top of the filter flask and wash five times with distilled water.   Care should be taken not to apply too much suction to the filter flask.  No. 2 funnel is then placed on top of No. 1 filter and washed five times.  The same procedure is followed for No. 3 filter and then No. 4 filter.  The result is that No. 1 filter is washed a total of 20 times, No. 2 filter is washed 15 times, No. 3 filter 10 times and No. 4 filter 5 times.   After washing all the filters the total volume of wash water should not exceed 1 litre. 

Titration for Sulphuric Acid

The wash solution should be made up to a know volume and an aliquot part should be taken for titration.  The size of the aliquot will be determined by the amount of acid present.

A tenth (1/10) normal caustic solution can be used for the titration with methyl red as an indicator.

Calculations

The gas volume is taken as the average of the five minute readings and then converted to standard conditions.

The weight in grams of H₂SO₄ represented by this titration is equivalent to the number of cubic centimetres of NaOH multiplied by the product of the normality factor and the H₂SO₄ equivalent (0.049).

With the gas volume and the weight of H₂SO₄, the concentration of H₂SO₄ in the gas can be calculated as follows:

W = W^o x 15.43
               V

W = Grains of H₂SO₄ per cubic foot at standard conditions

W^o = Grains of H₂SO₄ collected

V = Gas volume in cubic feet at standard conditions