Particulate Material Emissions
18.104.22.168 Mass Concentration
Sampling Methods: Gravimetric Method
The gravimetric method depends on the sampling of flowing, particulateladen gas from different positions across the exhaust gas duct and the determination of the mass of the particulate material. The sample is collected over a certain time period from each point. The volumetric gas flow is measured. The result is obtained by the following steps:-1
• Determine the volumetric flow of the gas, based on the measurement of the gas velocity.
• Collect the particulate material sample (on filter).
• Adjust the sample gas flow extracted (isokinetic sampling).
• Determine the volume of the sample gas.
• Weigh the particulate material.
• Calculate the concentration and the mass flow.
The method is applicable for the determination of the concentration (and emission) of the particulate material from a mixture of gas and particulate material flowing through a known cross-sectional area of a duct.
Procedure A sharp-edged nozzle is positioned in the duct, facing into the moving gas stream, and a gas sample is extracted isokinetically (see later) for a measured period of time,6 To allow for nonuniformity of particulate concentration in the duct, samples are taken at preselected positions in the duct cross-section. The particulate concentration is calculated from the weighed particulate mass and the gas sample volume. Figure 13.39 shows the measuring arrangement.
Isokinetic Sampling The sample gas partial volume flow must be extracted isokinetically to avoid aerodynamic separation effects and to ensure correct particle size distribution. Isokinetics means that the velocity and direction of the sample gas partial flow at the sample nozzle are the same as at the main gas stream.7
FIGURE 13.39 Example of a measuring-equipment arrangement with water removal upstream of the gas-metering device6
1. Entry nozzle
2. Probe tube
3. Particle separator
4. Sampling flow rate control device
6. Gas volume meter
7. Sampling flow rate measuring device
8. Water-removing device
9. Duct thermometer
10. Instrument for measuring effective static pressure in duct
11. Sensitive differential pressure instrument connected to Pitot tube
12. Gas velocity measuring device
13. Humidity measuring instrument
14. Thermometer at gas-metering device
I S. Instrument for measuring effective static pressure at gas-metering device.
For isokinetic sampling, the duct sainpling-point gas velocity has to be measured, and the corresponding sample gas flow calculated and adjusted. ^ Normally, a Pitot static tube is used for the measurement of duct gas velocity.
Particle Collection Particles in the extracted partial volume flow are retained in the collector filter. The particle mass emitted is determined by the weight difference of die filter before and after the collection. Factors crucial to the measuring precision and the smallest measuring range of particle concentration are7
• Correct configuration of the extraction and collection systems
• Correct preparation and subsequent handling of the measuring filter
• Resolution of the precision scales used
Sampling Points To obtain a representative result, the gas normally has to be sampled at more than one point in the sampling plane, depending on the sampling plane area. This plane is usually divided into equal areas at the centers at which gas is extracted.11 To determine the particulate concentration in the plane, the nozzle is moved from one sampling point to the other,
Extracting gas isokinetically at each point. Sampling periods should be equal for each sampling point, resulting in a composite sample.
The degree to which this sample represents the total gas flow depends on6
• Homogeneity of the gas velocity within the sampling plane
• Sufficient number of sampling points in the sampling plane
• True isokinetic extraction of the sample
Sampling Train The sample is extracted through a sampling train, which consists of
• sampling probe tube with entry nozzle,
• particle separator, in-stack or external,
• gas-metering system, in-stack or external, and
• suction system.
Tapered-Element Oscillating Microbalance Sampler
The tapered-element oscillating microbalance (TEOM) sensor, as described by Patashnick and Rupprecht,8 consists of an oscillating tapered tube with a filter at its free end (Fig. 13.40).9 The mass of the filter increases due to the collected aerosol and produces a shift in the oscillation frequency of the tapered tube that is directly related to mass.
The TEOM sampler draws air through a hollow tapered tube, the wide end of the tube being fixed, while the narrow end oscillates in response to an applied electric field. The narrow end of the tube contains the filter cartridge. The sampled air flows from the sampling inlet, through the filter and tube, to a flow controller. The tube-filter unit acts as a simple harmonic oscillator with10
W = (k/m)°-s
To = the angular frequency,
K = the restoring force constant, and
M — the oscillating mass.
As particles are collected on the filter, the oscillating mass changes, resulting in a change of the oscillating frequency. An electronic control system maintains the tapered tube in oscillation and continuously measures this oscillating frequency and its changes. The relationship between the changes in mass and frequency can be expressed as11
Dm = K
Dm = change in mass, K0 = spring constant,
/0 = initial frequency (Hz), and Fl ~ final frequency (Hz).
In the measurement of emission gas mass concentration at sources, a gas sample is extracted via an automatic isokinetic particulate-sampling instrument. The monitoring system generates a direct, real-time emission particulate mass
Concentration signal (mg nr-1) using a mass transducer in the stack.1- The mass transducer contains two sample inlets: the main flow is the low-flow sample that passes through the TEOM mass transducer and the manual sample flow is a high-flow sample directed through a manual filter holder mounted behind the mass transducer. The manual sample is intended as a verification tool. Mass concentration averaging times can range from a few seconds to 15 minutes.
The system can be used for continuous measurement of the mass concentration at a single point for up to 12 hours, for traverse measurements of stack particulate mass concentrations using sample probe extensions, with the mass transducer up to 6 m in the stack, or for intermittendy measuring particulate mass concentrations of emission gases for long-term readings (e. g., 30-sec samples every 60 minutes).12
A beta attenuation sampler uses a 30-mCi Krypton-85 source (with energy of
0. 74 MeV) and detector to determinate the attenuation caused by deposited aerosols on a moving filter.9 To improve the stability over time, a reference reading is periodically made of a foil with attenuation similar to that of the filter and collected aerosol.
Quasi-continuous particulate monitors, based on the use of the isotope l4C as the beta radiation source of low activity (12.5 to 100 (iCi), are used for the emission control of different processes. Their minimum sampling cycle is approximately five minutes. They use Geiger-Miiller counter tubes as detectors. The isotope 14C emits electrons at a low energy (0.156 MeV), making the dependence of the absorption on the chemical composition of the particles lower.15,14 It reduces the requirements for site-specific reference calibration. The gas sample is extracted isokinetically.
The main parts of these monitors are
• Sample probe, heated to constant temperature
• Measurement unit
• A heated sample line through which the sample is directed onto the filter tape in a gas-tight holder
• A step motor and tape reels to accomplish filter tape feed
• Pump and mass flow controller
• Beta-ray emission source and Geiger-Miiller counter
• Microprocessor-based electronics and functions control
The concentrations monitored by these instruments range from a few milligrams to 4000 mg/m3. A flow diagram of a beta gauge particulate monitor is shown in Fig. 13.41.
Elemental Analysis of Particulate Material
Interest in the elemental composition of aerosol particles arises from concerns about health effects and the value of these elements to trace the sources of suspended particles.9 The following physical analysis methods have been applied for the elemental measurements of aerosol samples. A schematic drawing of an x-ray fluorescence system is presented in Fig. 13.42.
• Instrumental neutron activation analysis (INAA)
• Photon-induced x-ray fluorescence (XRF)
• Particle-induced x-ray emission (PIXE)
• Atomic absorption spectrophotometry (AAS)
Filter advance stepping motor
Take-up reel Venturi nozzle—Dilution flow
-Tape-filter printer (o)
/ Counter tube C-14 source
—- 4-20 mA
Total flow vacuum pump Exhaust control with bypass controller valve
Sample cooler with Mags flow control, er total flow
Sample probe, nozzle
Cover foil (o)
Principle of an impactor is that the particle is carried through an orifice with a gas stream directed against a plate (Fig. 13.43a). This is the impaction or collection plate. It changes the flow into an abrupt 90° turn. Particles, which are too slow, are not capable of following the change in the direction and collide with the plate. The cross-section of the impactor is shown in Fig. 13.43a.
Cascade Impactor For particles to be classified into various sizes, individual impactors are connected together in series to form a cascade impactor (Figure 13.43b). The individual impactors, or stages of the cascade are arranged in order, with the largest cut diameter (cut diameter is the smallest aerodynamic diameter retained by the stage) being first and the smallest last. The cut diameter is reduced h* stages by changing the orifice diameter or the number of orifices and the distance ut the impaction plate from the orifice plate. The impaction plates are demountable, si > that the collected particles can be weighed and analyzed. After the last stage, there is usually a filter to collect particles smaller than the last-stage cut diameter.
Ft is assumed that at each stage of a cascade impactor all the particles larger than its cut diameter are collected. A practical impactor collection efficiency curve is shown in Figure 13.43c). The cumulative mass of the particles is normally plotted on the particle size distribution graph as a function of the upper limit of the particle size range corresponding to each stage. Using a cascade impactor, emission gas particle samples can be classified into 12 fractions (aerodynamic diameter ranging from 0.15 to over 7 (im).
Electrical Low-Pressure Impactor
The electrical low-pressure impactor (ELPI) has been developed, using the Berner-type multijet low-pressure impactor stages.16 The cut sizes of the seven channel system range from 0.030 to 1.0 (xm. Real-time measurements can be achieved due to the instrument’s fast time response.17 The schematic representation of the impactor construction is shown in Fig 13.44.
The electrical low-pressure impactor was used to measure the number concentrations of diesel exhaust particles. The particle size distribution ranges from 30 nm upward wrere then determined using the aerodynamic diameter as the characteristic dimension.17
Instrument Based on Fraunhofer Diffraction of Laser Light
The particle size distribution is determined from the diffraction pattern. For a simplified case of monosized spherical particles, for instance, the radius R0 of the smallest dark ring is
R0 = 1.22 A F/d
A is the laser wavelength,
/ is the focal length of the lens, and
D is the particle diameter.
The wavelength of 632.8 nm of the He-Ne laser is generally used in laser particle-size analyzers.
The particle size analyzer, based on laser light diffraction, consists of a laser source, beam expander, collector lens, and detector (Fig. 1 3.45). The detector contains light diodes arranged to form a radial diode-array detector. The particle sample to be measured can be blown across the laser beam (dry sample), or it can be circulated via a measurement cell in a liquid suspension. In the latter case, the beam is directed through the transparent cell.
A laser diffraction spectrometer can measure particles as small as 0.2 jim and up to about 1000 (Jim. Some instruments allow the operation of the analyzer for
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