Method and Apparatus for Monitoring Particulate Mass Concentration of Emissions From Stationary Sources

Particulate emissions from stationary sources are regulated by visible emission (opacity) and particulate mass concentration standards. Available white-light in-stack transmissometers are a reliable and relatively inexpensive means for monitoring opacity. The opacity-to-mass concentration relationship is strongly dependent on the size distribution of the scattering particles; therefore, opacity readings can be calibrated to provide reliable mass concentration readings only for sources where the particle size distribution is relatively constant. No similarly acceptable technique using transmissometry is currently available for monitoring mass concentration of particulate emissions.

Transmission data in the infrared energy range are shown in an article in APPLIED OPTICS, Vol. 16, No. 6 June 1977 entitled, "Infrared extinction spectra of some common liquid aerosols" by Hugh R. Carlon et al. In the article independency of the ratio of extinction coefficient to water content of fogs as a function of particle size for a particular wavelength used (12.5 .mu.m) is noted. The present invention, however, distinguishes over the disclosure of the Carlon et al article in the use of a transmissometer wavelength at which the extinction coefficient-to-mass concentration ratio for typical pollution aerosols is substantially independent of the particle size. Pollution particles differ from fog particles in that they are normally smaller, of higher refractive index (real part) and may be irregular in shape.

An object of this invention is the provision of improved method and means for monitoring particulate mass concentration of emissions from a stationary source which are substantially independent of the particle size distribution of the emissions.

An object of this invention is the provision of an improved transmissometer and method of operating the same at a single wavelength selected to provide a measure of the mass concentration of the emitted particulate material substantially independent of the particle size distribution.

The above and other objects and advantages of this invention are achieved by use of a transmissometer comprising an energy source operating at the selected single wavelength, such as for a Helium-Neon laser having an output at 3.39 .mu.m. The laser beam is directed through the pollutant aerosol, such as fly ash, along a path of predetermined length therein, and thence impinges upon an infrared detector filtered for the wavelength employed. The wavelength employed is selected such that the ratio of the extinction coefficient to the mass concentration of the particles is substantially independent of the particle size. As a result, the detector output provides a measure of the mass concentration independent of variation in the particle size distribution of the aerosol.

The invention as well as the above and other objects and advantages thereof, will become apparent from the following detailed description when considered with the accompanying drawings.

In the drawings:

FIG. 1 are curves computed from scattering theory based on measured particle size distributions showing the dependence of the extinction coefficient-to-mass concentration ratio of fly ash aerosols on particle size and wavelength of the energy source;

FIG. 2 is a block diagram showing of a transmissometer for monitoring the mass concentration of fly ash and other aerosol pollutants emitted from a source thereof; and

FIG. 3 are curves showing experimental extinction-to-volume concentration ratios as a function of particle size for visible and infrared wavelengths .

Reference first is made to FIG. 1 wherein a series of curves of extinction coefficient-to-mass concentration ratio (.sigma./M) for different size distributions of fly ash particles as a function of wavelength are shown. As shown on the curves, four size fractions of fly ash were employed in the development thereof, including the size fractions represented by the diameter ranges of 0-2.5, 2.5-5, 5-10 and 0-10 .mu.m. In FIG. 1 computational results for the extinction coefficient-to-mass concentration ratio (.sigma./M) as a function of wavelength for three of the four fly-ash particulate size fractions (0-2.5, 2.5-5 and 5-10 .mu.m) are shown, together with experimentally determined white-light .sigma./M values for all four fractions. As seen in FIG. 1, the experimentally determined white-light .sigma./M values are in good agreement with the .sigma./M values computed at 0.7 .mu.m. (Four observed values of .sigma./M at a wavelength of 3.39 .mu.m also are shown in FIG. 1, which values are discussed in greater detail below.)

The data presented in FIG. 1 show that .sigma./M generally becomes smaller at longer wavelengths, with the greatest decrease occuring with the smaller size fraction. At a wavelength of 10 .mu.m, the dependence of .sigma./M on particle size is just reversed from that in the visible wavelength region, with .sigma./M increasing with increasing particle size. It is apparent that a wavelength region exists where .sigma./M is substantially independent of particle size. For the illustrated fly ash fractions, this region exists between wavelengths of 3 to 4 .mu.m. Among the four fractions, .sigma./M values vary by a factor of 4.4 at the visible wavelength, whereas the variability is within a factor of approximately 1.6 at the infrared region of 3 to 4 .mu.m. As used, here and in the claims, operation of the radiant energy source at a wavelength at which the extinction coefficient-to-mass concentration ratio is "substantially insensitive to" of "substantially independent of" particle size, or variations in particle size, is intended to include factors between, say, 1 to 2.

It here will be noted that the infrared computations presented in FIG. 1 assumed that the particulate refractive index was invariant over the wavelength region and that the scattering particles were spherical shaped and nonabsorbing. An infrared transmissometer operating at a wavelength of 3.39 .mu.m has been constructed and based upon transmittance readings of generated fly ash aerosols obtained using the same, .sigma./M ratios for the four fractions were calculated and are plotted in the curves of FIG. 1. These `observed` infrared .sigma./M values are in good agreement with the theoretically computed values.

A system for monitoring particulate mass concentration of emissions from a stationary source embodying the present invention is shown in FIG. 2, to which figure reference now is made. The gaseous medium containing the particulate material 10 flows in the direction of arrow 11 through a conduit, such as a stack, 12, a fragmentary portion of which is shown in FIG. 2. Windows 14 and 16 are provided in the stack walls for transmission of a beam 18 of electromagnetic radiation from an infrared laser 20 to an infrared detector 22 of a receiver through the stream of particle-containing gas along a path of length x through the particulate stream. In the illustrated arrangement a helium-neon laser is shown which includes an output at the wavelength of 3.39 .mu.m. Filter 26 passes the beam 18 but substantially prevents other wavelength energy from entering the detector 22. Any suitable infrared detector 22 may be used including, for example, a pyroelectric having a substantially linear dynamic response.

An amplifier 30 responsive to the detector 22 output amplifies the signal from the detector. Due to the low signal-to-noise ratio of the detector output a tuned lock-in type amplifier preferably is employed. For such use, a rotating disk 32, having alternate transparent and opaque sectors is positioned in the path of the beam for chopping the same as the disk is rotated by a motor 34. The speed of the motor is controlled by speed control unit 36. A synchronous signal for the amplifier for synchronizing the operation thereof with the detector output is provided by a light source 38 and photodiode 40 positioned at opposite sides of the disk. The photodiode output is amplified by amplifier 42, and the amplifier output is supplied as a synchronous signal to the tuned lock-in amplifier 30.

As seen in FIG. 2, Io represents the intensity of the beam 18 entering the conduit 12, I represents the intensity thereof after passing through the path length x of particulate material flowing through the conduit. Transmittance, T, is defined as the ratio of I/Io. That is,

Also, by definition,

wherein:

.sigma.=the extinction coefficient (1/meters)

Equation (2) may be rewritten as follows:

wherein:

M=mass concentration of the particulate material.

If the extinction coefficient-to-mass concentration ratio, .sigma./M, is constant for all particles observed, equation (4) may be rewritten as follows:

wherein:

K=a constant dependent upon the constants .sigma./M and x. Therefore:

In FIG. 2, the amplified signal from the tuned lock-in amplifier is supplied to a logarithmic amplifier 44 having an output related to ln 1/T which, as seen in equation (6), is directly related to mass concentration of particulate material 10 flowing through the conduit 12. The amplifier 44 output may be supplied to a meter, recorder, or other such utilization device 46, as desired.

The invention is not limited to mass concentration measurements of fly ash. Experiment indicates that the mass concentration of other pollutant aerosols, such as iron oxide and silica (silicon dioxide) may be monitored using the present method and apparatus. In addition to the above-mentioned fly ash aerosols, two-wave-length transmission observations have been made using one size fraction of ironoxide and three size fractions of silica. The silica fractions included 5, 10 and 15 .mu.m maximum particle diameters. These experiments involved the use of transmissometers that employ helium-neon laser 20 operating at 3.39 .mu.m, and visible light. Extinction-to-mass measurements also depend upon the specific gravity of the aerosol. Therefore, to compare results of materials of different specific gravity, the volume concentration (mass concentration weighted for the specific gravity) has been employed.

FIG. 3, to which reference now is made, is a plot of the experimentally derived extinction-to-volume concentration .sigma./v ratios of the particulate material (the four fly ash fractions, the three silica fractions and the one iron oxide fraction) as a function of Sauter diameter. The visible wavelength .sigma./v values clearly show a strong dependence on particle size, while the infrared wave-length values are substantially independent of particle size. The plot shows therefore, that the infrared transmission is a good indicator of aerosol volume concentration, regardless of particle type or size. The observed particle types were of different shapes, and probably of different absorption coefficients. Therefore, the data indicate that if the particulate emission is of a given chemical nature (e.g. specific gravity), but with varying particle sizes, then measurements with the transmissometer of FIG. 2, operating at a wavelength of 3.39 .mu.m, are good indications of mass concentration.

The invention having been described in detail in accordance with the requirements of the Patent Statutes, various changes and modifications will suggest themselves to those skilled in this art and it is intended that such changes and modifications shall fall within the spirit and scope of the invention recited in the appended claims.