Method and system for emissions measurement

Abstract

Disclosed herein is a novel system and method for the remote characterization of visible emissions, and more particularly, to compact, optical sensors which can remotely measure the opacity of a visible emission plume from a stationary source. Assessing visible emissions is important for compliance with environmental regulations and to support the regulatory reporting needs of Federal and State inspectors. By reducing the power consumption of the laser source and the signal processing, a compact, handheld or portable, battery-operable opacity measurement system can be realized while allowing eye-safe operation. The system and method may also be applied to non-stationary sources.

Claims

1. A compact, handheld or portable system for the remote measurement of visible emissions, the system comprising: an optical transmitter that provides single or multiple pulses of light from one or more light sources at a wavelength between 0.2 m and 11 m that are transmitted towards a target of interest and can be powered by a compact, lightweight battery; an optical receiver that collects the backscattered light and guides it onto one or more detectors which can be powered by a compact, lightweight battery; and signal processing hardware that can analyze the receiver output, store intermediate measurements, determine the opacity of the target, and can be powered by a compact, lightweight battery.

2. The compact emissions measurement system described in claim 1 where an outgoing laser pulse of the system has an energy density low enough to meet eye-safety requirements.

3. The emissions measurement system described in claim 1 where there is one laser source that operates at one wavelength within the preferred eye-safe range from 1.4 to 2.6 m, such as a 1.54 m Er-doped solid-state laser.

4. The emissions measurement system described in claim 1 where more than one light source is used to allow refinement of the opacity measurement and distinguish between different types of scattering sources.

5. The emissions measurement system described in claim 1 wherein a narrow bandpass optical filter is provided before one or more detectors to remove background light.

6. The emissions measurement system described in claim 1 where the optical receiver splits the light into separate polarization components to allow possible refinement of the opacity measurement and obtain additional information.

7. The emissions measurement system described in claim 1 where the means of detection comprises one or more photodiodes (either regular or Avalanche type).

8. The emissions measurement system described in claim 1 where the means of detection comprises one or more photomultiplier tubes.

9. The emissions measurement system described in claim 1 where the signal processing method is analog, digital, or a combination of both, has low power consumption, and can be powered by a compact, lightweight battery.

10. The emissions measurement system described in claim 1 where the signal processing consists of one or more low-power analog integrators controlled by associated timing signals to define the integration period and hold the result.

11. The emissions measurement system described in claim 1 where the signal processing consists of a low-power digitizer.

12. The emissions measurement system described in claim 1 where a microcontroller is used to provide system control and local display of results.

13. The emissions measurement system described in claim 1 where the battery can be either standard or rechargeable.

14. A method for remotely measuring the opacity of emissions sources, the method comprising: an optical transmitter that provides single or multiple pulses of light from one or more light sources at a wavelength between 0.2 m and 11 m that are transmitted towards a target of interest and can be powered by a compact, lightweight battery; an optical receiver that collects the backscattered light and guides it onto one or more detectors which can be powered by a compact, lightweight battery; and signal processing hardware that can analyze the receiver output, store intermediate measurements, determine the opacity of the target, and can be powered by a compact, lightweight battery.

15. The method of claim 14 applied to remote measurement of visible emissions from stationary sources.

16. The method of claim 15 wherein it meets the EPA requirements for Method 9, Alternate Method 1.

17. The method of claim 14 applied to remote measurement of visible emissions from non-stationary sources.

Description

BRIEF DESCRIPTION OF DRAWINGS

[0017] FIG. 1 shows a typical biaxial lidar system and the corresponding backscatter return signal.

[0018] FIG. 2 is a graph of the maximum permissible exposure (MPE) per pulse at different light wavelengths. The graph presented is a specific example for light sources with a pulse duration of 5 ns and a repetition rate of 5 Hz.

[0019] FIG. 3 displays a typical timing diagram for the present disclosure. In this example, the signal from beyond the emission plume is being averaged.

[0020] FIG. 4 graphs the power consumption of laser sources and signal processing hardware and compares prior art hardware with the present disclosure.

DESCRIPTION OF EMBODIMENT

[0021] The above, as well as other objects and advantages of this disclosure, will become readily apparent to those skilled in the art from reading the following description of an embodiment of the invention. The description and drawings illustrate exemplary embodiments of the invention and serve to enable one skilled in the art to make or use the invention and are not intended to limit the scope of the invention in any manner. With respect to the methods disclosed and illustrated, the steps presented are exemplary in nature, and thus, the order of the steps is not necessary or critical.

[0022] The present disclosure is built upon a typical lidar system which includes an optical transmitter, an optical receiver, and associated signal processing and control electronics. In FIG. 1, a representative basic lidar system 10 is shown with a transmitter 11 and a receiver 12. A light pulse is transmitted along path 13 through the atmosphere towards a target of interest and a small fraction of that transmitted light is backscattered along path 14 to the lidar receiver from atmospheric constituents or objects within light path 13. In FIG. 1, the target is an emission plume 18 from an exhaust stack 17. The intensity of the backscattered light from aerosol particles and molecules is many orders of magnitude smaller than the source intensity, thus the optical transmitter is typically a high-energy pulsed laser and the optical receiver is highly sensitive. There are two types of lidar arrangements: coaxial and biaxial. In a coaxial system the light is transmitted and received along the same optical axis and in a biaxial system the optical axes are spatially separated. For both types of lidar systems, measurements typically start from a convergence distance 15 where the transmitter field-of-view and the receiver field-of-view fully overlap. The upper part of FIG. 1 displays this overlap for an example biaxial lidar system. Many alternative overlap arrangements are possible and are known to those skilled in the art. This disclosure applies to both coaxial and biaxial lidar systems.

[0023] As the light pulse is transmitted into the atmosphere towards the target, a backscatter signal is returned. The temporal response of this signal corresponds to a distance from the lidar system, since light must travel from the transmitter to a distance or range R and back to the receiver. This correspondence is given by R=c t/2, where c is the speed of light and t is the round-trip transient time from source to the plume. The lower part of FIG. 1 displays an example backscatter return signal 20 with amplitude 21 vs. range or time 22. The signal level 20 starts at zero, rises rapidly until it reaches a peak near the convergence distance, and then decreases in amplitude as 1/R.sup.2 due to atmospheric backscatter. The spike represents the backscatter signal due to an emission plume. The amplitude of the spike is much greater than that of the atmospheric return because the particulate density is far greater in the emission plume than in the surrounding air. The level of the atmospheric return after the spike compared to the level of the atmospheric return if there were no spike gives the opacity of the emission plume.

[0024] Laser wavelengths used in an opacity measurement system can range from 0.2 m to 11 m where certain wavelengths are preferable over others. The wavelength of the laser source used for an opacity measurement system needs to be non-resonant with the various molecular constituents of the atmosphere, otherwise the attenuation will not be dominated by the atmospheric aerosol content. When dealing with open or public areas where people may intersect the path of the laser beam, the beam must meet the laser safety standards ANSI Z136.1 and IEC 60825-1 for eye and skin exposure. For pulsed laser sources, these standards specify the maximum permissible exposure (MPE) in units of energy per unit area. In FIG. 2, the MPE per pulse for a laser source with a pulse duration of 5 ns and a pulse repetition rate (PRF) of 5 Hz is shown for wavelengths from 0.2 m to 11 m. Similar figures can be developed for any combination of pulse duration and PRF. The figure shows that higher energy densities are permissible away from visible wavelengths (0.4-0.75 m) at either ultraviolet wavelengths below 0.4 m or infrared wavelengths greater than 1.4 m. The table below presents numerical values taken from FIG. 2 for some common pulsed laser wavelengths. They show that going from any visible wavelength, i.e. 0.532 m, to a wavelength of 1.064 m allows a greater than threefold increase in energy density; however, going to a wavelength of 1.54 m allows an increase in energy density of 40,000. Thus, the selected laser wavelength can have a significant impact on the permissible energy density for eye safety.

TABLE-US-00002 Wavelength 0.532 m 1.064 m 1.54 m MPE/pulse 0.5 J/cm.sup.2 1.8 J/cm.sup.2 0.02 J/cm.sup.2 Beam diameter for 100 113 mm 59.5 mm 0.56 mm J pulse energy

[0025] To keep the system eye-safe, the energy density must be less than the MPE rating. For a given laser source, this means the transmitter area must be large enough to keep the energy density at any location below the MPE/pulse. The table above shows the minimum beam diameters for a laser pulse with a Gaussian intensity profile, pulse energy of 100 J, pulse duration of 5 ns, and a pulse repetition rate (PRF) of 5 Hz. This table indicates that visible and near-infrared wavelength laser sources need their diameters significantly expanded to be considered eye-safe. It should also be noted that while the MPE rating applies to one single pulse, the rating does depend on the number of pulses per second (the PRF).

[0026] The optical transmitter for the opacity measurement system in this disclosure is a high-energy pulsed laser with low average power. For this application, the pulse duration of the pulsed laser source is typically short, usually less than 50 nanoseconds. The power consumption of a pulsed laser system is determined from the product of its pulse energy, pulse repetition frequency (PRF), and laser efficiency. Since the efficiency typically depends on the type, or wavelength, of the laser source, the pulse energy and PRF become the adjustable parameters. To maintain the same relative sensitivity, every doubling of the measurement range quadruples the necessary pulse energy, leading to high pulse energies for large measurement ranges. Since high pulse energies are typically desired this means the PRF must be kept low. A method to keep low power consumption of the laser system is an integral part of this patent disclosure. Current state-of-the-art high pulse energy laser sources that can meet these requirements include diode lasers, fiber lasers, and diode pumped solid state lasers.

[0027] The optical receiver of the opacity measurement system has the flexibility to be implemented with one or more optical detectors with a telescope. To reduce the contribution of the atmospheric return from non-laser wavelengths, an interference filter, which transmits light in a passband around the laser wavelength and suppresses light outside the passband, is placed in the light path before a detector. More than one detector would be needed when implementing polarization separation or when using both low- and high-sensitivity detection schemes. Polarization separation, although not absolutely necessary, allows possible refinement of the opacity measurement and obtains additional information. To acquire the different polarization components of the received backscattered light, a polarization splitter could be used to separate the components and direct each toward separate detectors. Detectors with different sensitivities allows for extended range operation or automated target range detection. When using both low- and high-sensitivity detection, a beam splitter could be used to separate the incoming light into two light paths and direct each toward separate detectors. The separation of a light beam into multiple components, as described above, is known to those skilled in the art. Detectors are used to convert a collected optical signal into an electronic signal. Signal detection is typically realized with photomultiplier tubes or photodiodes (either regular or Avalanche types). For weak backscatter signals, a detector may be operated in Geiger mode where individual photons are counted, but this also results in significant power consumption in the subsequent signal processing hardware. For strong backscatter signals, a detector may be operated in analog mode, and this mode is preferred in this disclosure.

[0028] The second aspect of keeping the system power consumption low for a compact system is low-power signal processing method. Many different signal processing methods may be implemented, which can be classified as analog approaches, digital approaches, or a combination of both. In each case, the detected signal may be optionally range-corrected to remove the 1/R.sup.2 nature of the response. For a digital approach the range-correction may be implemented either in analog before digitization or digitally after digitization, but for an analog approach the range-correction must use an analog implementation. Analog range-correction has been performed in prior art, see A. W. Dybdahl, 1981, and digital range-correction is straightforward, so they will not be described further. The preferred low-power approach described herein, is a combination approach that consists of analog integration over a portion of the backscatter signal with subsequent sampling by a digitizer. This digitized signal level is then stored for later processing. A purely digital approach would consist of digitizing and storing the backscatter signal and then numerically integrating the desired portion. For the above or any other signal processing approach, the value obtained from a single signal acquisition may be used directly or averaged with values from other signal acquisitions. Averaging of multiple signal acquisitions is typically used to improve the accuracy of the result. Typically, an analog processing approach consumes less power than a digital processing approach.

[0029] FIG. 3 illustrates an example timing diagram for accomplishing the signal processing in an opacity measurement system. The system is synchronized by a trigger signal 50 that is produced when the laser emits its light pulse. The timing, and thus ranging, of the backscatter return signal 51 is dependent on this trigger signal. The characteristics of the backscatter return signal 51 are the same as described for signal 20 of FIG. 1. The signal 51 is integrated over a finite period 56 after a delay 55. When using an analog integrator, pulse waveform 52 is used to turn the integrator on during period 56 and off otherwise. For a purely digital approach, period 56 would be the sampling period over which the numerical integration occurs. The delay 55 and integration period 56 are adjustable parameters of pulse waveform 52. Another pulse waveform 53 is generated to provide the period 57 over which the integrator output is reset and maintained at zero. The reset period 57 is another adjustable parameter. The resulting integrator output signal 54 remains at zero until the start of integration period 56, shows a rise 58 during the integration period 56, has the integrator output maintained during the hold period 59, and is then reset to zero before the next integration cycle. It is during the hold period 59 that a subsequent digitizer must sample the integrator output. The circuitry needed to create the pulse waveforms and perform the analog integration is known to those skilled in the art. By changing the delay 55, integration period 56, and reset period 57, any practical region of the backscatter signal 51 can be integrated.

[0030] The ultimate goal of this disclosure is the realization of a compact, handheld or portable, battery-operable system for the remote measurement of opacity. Each of the three components of the system, the transmitter, the receiver, and the signal processing hardware, must have low power consumption. Typical receiver systems are low power and are battery-operable. The transmitter and signal processing hardware usually consumed significant power in prior art. FIG. 4 graphs the power consumption 70 of laser sources and signal processing hardware and compares prior art hardware with the present disclosure. Prior art laser sources 72 typically consumed significant power due to their combination of high pulse energy, high-PRF, and poor power conversion efficiency. New generation laser sources 74 can now be obtained with high pulse energy, low PRF, and improved power conversion efficiency. Similarly, prior art signal processing hardware 76 consumed significant power since they used high-speed sampling rates, typically greater than 100 MHz. The new signal processing hardware 78 in this disclosure consumes significantly less power since it uses either analog integration or low-power digitizers with low sampling speeds, under 10 MHz. The combination of low power for the transmitter 74 and low power for the signal processing hardware 78 allows the realization of a compact, handheld or portable, battery-operable system for the remote measurement of opacity.

[0031] This written description uses examples to disclose the invention and also enables any person skilled in the art to practice the invention, including making and using any devices or systems and performing any incorporated methods. The claims define the patentable scope of the invention, and may include other examples that occur to those skilled in the art. Such other examples are intended to be within the scope of the claims if they include structural elements that do not differ from the literal language of the claims, or if they include equivalent structural elements with insubstantial differences from the literal languages of the claims.