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In the United States, Subtitle D landfills are used to dispose of municipal solid waste (MSW). Solid waste is defined by the Environmental Protection Agency as “any garbage, refuse, sludge from a wastewater treatment plant, water supply treatment plant, or air pollution control facility and other discarded material, including solid, liquid, semisolid, or contained gaseous material resulting from industrial, commercial, mining, and agricultural operations, and from community activities.” Furthermore, municipal solid waste consists of the household and commercial solid waste streams (Sharma and Reddy 2004). Generally, MSW in the United States consists primarily of organics (such as food waste), paper (such as cardboard and office paper), plastics (such as shopping bags and plastic packaging), and durable goods (such as appliances). Additionally, glass, residues, and metals constitute the remaining portion of the MSW stream.
Organics and paper alone account for more than 65% of the MSW stream, and these are the materials that are capable of producing biogas through the process of methanogenesis when degraded in the anaerobic conditions provided by a landfill setting. The three materials in the MSW stream with the highest mean methane yield are food waste, office paper, and old corrugated cardboard with yields of 300.7, 217.3, and 152.3 cubic meters of methane produced per megagram of dry refuse, respectively. These three materials represent over a quarter of the entire waste stream (Staley and Barlaz 2009).
The majority of the waste disposed of in Subtitle D landfills have methane production potential. Methane is a greenhouse gas, along with water vapor, ozone, carbon dioxide, nitrous oxide, and chlorofluorocarbons. An increase in the atmospheric concentrations of greenhouse gases trap thermal energy in the atmosphere, a phenomenon known as global warming. In terms of global warming potential, methane is 25 times more potent than carbon dioxide; therefore, it is important to measure how much methane landfills are emitting (Gardner et al. 1993).
Although only surface methane concentration is regulated at landfills, emissions are commonly modelled using a tool developed by the Environmental Protection Agency (EPA) called LandGEM (Landfill Gas Emissions Model). The model is based on a simplified first-order decay equation with input parameters that cannot change with time (Dillah et al. 2013), and its predictions have not been tested against actual field measurements; therefore, it is critical that methane emissions from landfills are quantified directly to validate the LandGEM predictions and to understand the effects landfills have on global warming.
The purpose of this paper is to highlight and compare the most common methods currently available that are capable of measuring methane emissions from a landfill. Techniques that are available to obtain point-source methane measurements are flame ionization detection (FID), photoionization detection (PID), and flux chambers. A major difference between these point-source methods is that the FID and PID measure concentration, whereas the flux chambers directly measure emissions. More advanced optical remote sensing (ORS) technologies exist that are capable of measuring concentration over a spatial path. Examples of these technologies are Fourier transform infrared (FTIR) spectroscopy, tunable diode laser (TDL) absorption spectroscopy, and cavity ring-down spectroscopy (CRDS). These ORS technologies also require emission estimation models, such as tracer gas correlation, radial plume mapping (RPM), or differential absorption light detection and ranging (DIAL), to convert the ORS methane concentration data into spatial emission data.
A flame ionization detector (FID) is a gas detector commonly used to measure methane concentrations. An air sample enters the device, and is combusted by a hydrogen flame. There are two charged electrodes in the device, a positively charged nozzle head and a negatively charged collector plate. The flame is produced at the nozzle head, and the collector plate is a tubular electrode above the nozzle head to which the ions generated from the chemical combustion reaction are attracted. When the ions hit the plate, a current is induced, measured with a high-impedance picoammeter, fed into an integrator, amplified, and converted into a digital reading of ion concentration. This ion concentration has been shown to be proportional to the concentration of the gas species (Chasteen 2009).
FID Diagram (Harvey 2017)
FIDs are relatively common due to their many advantages. The detector is low cost and extremely accurate for the measurement of methane and other hydrocarbons (Chasteen 2009). Additional benefits include that it has a wide linear range of 107 (Li et al. 2016), and it is not affected by small changes in the flow rate of the carrier gas (Golden). FIDs also have many limitations. The device is destructive to the sample (Golden), and it is a single-point measuring technique as opposed to an open-path technique that can collect emission data over a large area (such as a landfill surface) (USEPA 2011). Also, the detector requires a constant hydrogen supply for the flame (TSI 2013), and it is only capable of measuring the concentrations of non-oxygenated organic carbon compounds. This means that other gases such as hydrogen, oxygen, nitrogen, carbon monoxide, carbon dioxide, nitric oxide, nitrogen dioxide, ammonia, or water vapor are undetectable (Golden; Li et al. 2016).
Photoionization detectors (PIDs) are another very common gas sensor. The sample enters the device which is equipped with an ultraviolet (UV) lamp emitting high energy short-wavelength UV photons. The sample absorbs the UV light, which causes the volatile organic compounds to ionize and eject electrons, thus becoming positively charged. These positively charged ions are attracted to a negative electrode within the device. Similar to FIDs, an electrical current is generated as the ions hit the electrode. This current is amplified and output to an ammeter. Also, similar to FIDs, PIDs are based on an empirical, directly-proportional relationship between the ion concentration and the concentration of the gas species (Hernandez Bennetts et al. 2012; RAE Systems 2013).
PID Diagram (EQUIPCO)
Although the PID technique is still a single-point measuring technique, it is often preferred over FID because it is non-destructive to the sample (RAE Systems 2013), it does not require a constant hydrogen supply, and it is 50 times more sensitive to aromatic compounds compared to FID (Li et al. 2016); however, these sensors are incapable of measuring compounds with less than 2 carbon atoms, such as methane (CH4), regardless of which type of UV lamp is used (TSI 2013). The only way that PIDs can be used to give an indication of methane concentration is through a process called surrogate compound measurement. The methane concentration must be assumed constant, and a surrogate concentration measurement is made based on the detectable compounds within the gas mixture (RAE Systems 2013).
Emission isolation flux chambers have been around for about 25 years; therefore, the scientific principles are well known and documented. Flux chambers are in situ point sampling instruments. The standard chamber has a volume of approximately 30 liters, encapsulates an area of 0.13 square meters (Klenbusch 1986), and has a standard geometry of a dome superimposed on a cylinder; however, the volume, encapsulated area, and geometry can depart from the standard. A variety of flux chambers of different shapes and sizes have been successfully deployed in the field. The best performing flux chambers are those which optimize the mixing of the chamber atmosphere (Eklund 1992). Some chambers enhance this mixing by using small fans installed on the inside to help with circulation. Each chamber comes with a separable collar which should be inserted one inch into the soil with the chamber installed and sealed on top. The goal is to isolate a known volume of air above the ground surface and measure the accumulation of methane. This is achieved by taking frequent concentration measurements, typically once every 1 to 5 minutes for approximately 25 minutes. Some chambers are equipped with internal sensors that can directly measure the concentration, while other methods require air samples to be taken at the prescribed interval. If samples are taken from the chamber, they can be analyzed for methane concentration using a gas chromatograph equipped with a flame ionization detector (Green et al. 2010). Methane emission flux can be determined by plotting the accumulation of the methane inside the chamber (i.e. concentration versus time).
Various Flux Chambers (Photo by Julie Bateman) (Babilotte 2011) (Almund-Hunter et al. 2015)
Some advantages to flux chambers are that they are inexpensive and relatively simple to use. A critical limitation; however, is that they only provide point data; therefore, several measurements are required to characterize the methane emissions of an entire landfill. Additionally, methane travels the path of least resistance and tends to gather and leak out of cracks or looser spots in the landfill cover. This creates methane hotspots as opposed to uniform methane emissions across the entire landfill surface. It has been shown that as little as one third of the area of a landfill can be responsible for 99% of the methane emissions due to these hotspots (Spokas et al. 2003), which is problematic for a point-source sampling method. To overcome this challenge, most flux chamber methods systematically divide the land area into a dense grid to decide where to place the devices for methane measurement. Due to the abundance of points contained in this grid, and the fact that each measurement takes at least 30 minutes, the method quickly becomes extremely time and labor intensive.
Open-path Fourier transform infrared (OP-FTIR) spectroscopy is a method that takes advantage of how well methane absorbs into the infrared portion of the electromagnetic spectrum. Open-path FTIR works by first generating a beam of infrared light using an infrared source within the instrument. The light is sent to a beam splitter, where one portion of the beam is directed to a fixed mirror, while the other portion is directed to a moving mirror. These mirrors direct the beams back to the beam splitter where they are recombined either constructively or destructively, resulting in a certain amount of interference. This recombined beam is then projected over the defined open-path where some of the infrared energy is absorbed by the gases that are present. The beam is then reflected to an infrared detector where the interference pattern is detected and translated into a standard single beam infrared frequency spectrum using a Fourier transform algorithm. To obtain the data in terms of absorbance, the negative log base-10 is taken of each data point, and the concentration of a specific gas (i.e. methane) is proportional to the amount of infrared energy absorbed from the beam. The output is given in path integrated concentration (PIC), which has units of concentration per unit length (i.e. ppmm – parts per million per meter) (USEPA 2011).
Open-Path FTIR Diagram (USEPA 2011)
The advantages of OP-FTIR spectroscopy are that it is capable of simultaneously measuring large numbers of compounds in real-time and the spectra can be saved to be analyzed later. The instrument itself is durable and portable. Daily calibration of the device is unnecessary, therefore; it can support unattended sampling for up to one week. The technique can also be used to locate hotspots which are common in landfill gas applications because the gas tends to find weak spots in the landfill cover and exit via the path of least resistance (USEPA 2011).
The limitations of OP-FTIR spectroscopy are that it is not simple to run and therefore requires experienced operators. The instrumentation itself takes two people about 5-8 hours to set up, and measurements must be taken along multiple paths to calculate emissions. It also cannot measure concentrations of homonuclear diatomic gases, or any other compounds that do not absorb infrared radiation. Additionally, water vapor, carbon monoxide, and carbon dioxide may cause undesirable interference. The path length of OP-FTIR is also limited to 400-500 meters, and the infrared detector requires weekly refills of liquid nitrogen for cryogenic cooling (USEPA 2011).
Some examples of studies that have used OP-FTIR spectroscopy as an optical remote sensing technique to measure methane emissions at municipal solid waste landfills include the following: Galle et al. 2001; Börjesson et al. 2009; Mønster et al. 2014; and Mønster et al. 2015.
Open-path tunable diode laser spectroscopy (OP-TDLAS) is an infrared laser technique, like OP-FTIR, except it can only measure the concentration of one compound at a time. This technique works by directing a laser beam through a gas sample at a specific wavelength that is “tuned” to that of the target compound by adjusting the temperature and bias current. The instrument then measures how much of the infrared energy is absorbed by the target gas, and produces absorption spectra. When combined with gas temperature and pressure and path length, the spectra can be translated into concentration readings to be integrated over the beam’s path. Similar to FTIR, the output is given in path integrated concentration (PIC), which has units of concentration per unit length (i.e. ppmm – parts per million per meter). Tunable diode lasers operate in the near infrared electromagnetic range, and because they operate at a specific wavelength, there is very little interference. This lack of interference, in combination with the high intensity of the laser beam, make it possible for OP-TDLAS to operate over paths of up to 1-2 kilometers long (USEPA 2011).
Open-Path TDL Diagram (USEPA 2011)
The advantages of OP-TDLAS are that the instrument is compact, lightweight, robust, and low maintenance. The technique also returns near real-time measurements and supports continuous remote sensing. Calibration of the device is simple, and interferences due to other gases are minimized due to the narrow wavelength at which the laser emits the infrared light. The absence of interference allows the technique to achieve longer path lengths (USEPA 2011).
The limitations of OP-TDLAS are that it is only capable of measuring the concentration of one gas at a time. Also, each laser has a limited range of wavelengths; therefore, additional lasers may need to be purchased if multiple gases need to be detected. Only approximately twenty compounds are detectable by TDLAS because the target gas must absorb into the near- or mid-infrared range of the electromagnetic spectrum. Lastly, a laser beam that is blocked by either dust or objects will result in no measurements (USEPA 2011).
Some examples of studies that have used OP-TDLAS as an optical remote sensing technique to measure methane emissions at municipal solid waste landfills include the following: Babilotte et al. 2010; Green et al. 2010; Goldsmith et al. 2012; and Bateman et al. 2016.
Cavity ring-down spectroscopy (CRDS) uses a tunable laser source, like TDLAS; however, the laser can emit light ranging from the far-infrared to the ultraviolet portions of the electromagnetic spectrum. This laser is cast through a gas sample which is bounded on each side by concave high reflectivity mirrors. The light beam continuously reflects off these mirrors while a photodetector is measuring the rate of light intensity decay. This rate of decay is a function of the length of the cavity, the reflectance of the concave optical mirrors, and the absorptivity of the sample. Because the cavity length and mirror reflectance are constant, the rate of decay is dependent only on the absorptivity of the sample. The amount of time that is elapsed until the intensity of the light beam decays to 1/e of its original value is called the cavity ring-down time (RDT). The difference between the RDT curve of the sample and the RDT curve of the empty cavity is proportional to the concentration of the target gas within the sample (USEPA 2011).
CRDS Diagram (USEPA 2011)
The advantages of CRDS are that the instrumentation is compact, user-friendly, easy to install, low maintenance, and does not require a large power source. It also delivers real-time results and frequent calibration is unnecessary. Because the technique measures time as opposed to absorbance, it is resistant to environmental fluctuations such as temperature and humidity, and it has a wider linear dynamic range. Due to the high reflectivity of the mirrors, CRDS can also achieve longer path lengths than some other techniques (USEPA 2011).
The limitations of CRDS are that it is extremely difficult to detect multiple species due to the small wavelength range of the high reflectivity mirrors. Sample filtering may also be necessary to avoid interferences. The limitations of CRDS are that it is extremely difficult to detect multiple species due to the small wavelength range of the high reflectivity mirrors. Sample filtering may also be necessary to avoid interferences. Lastly, the instrumentation for the CRDS is more expensive due to the high-quality requirements for the laser and the mirrors (USEPA 2011).
Some examples of studies that have used CRDS as an optical remote sensing technique to measure methane emissions at municipal solid waste landfills include the following: Green et al. 2010; Mønster et al. 2014; Mønster et al. 2015; and Cambaliza et al. 2017
The tracer gas correlation method is used to measure the total emissions of a pollutant, such as methane, over a large area by introducing a tracer gas with a known emission rate. First, a tracer gas is selected that is chemically stable, is expected to fully mix with the pollutant, and is assumed to behave similarly to the pollutant under expected meteorological conditions. Next, the tracer gas is released at a known emission rate at or upwind from the source. Concentration measurements are taken downwind of the source, far enough to ensure adequate mixing of the gases. Optical remote sensing technologies, most commonly FTIR or CRDS, are used to measure the ratio of the pollutant concentration in exceedance of background levels to the tracer gas concentration in exceedance of background levels. This ratio, multiplied by the known tracer gas emission rate yields the estimated emission rate of the pollutant (USEPA 2011). The following figures illustrate the orientation of the pollutant source and measurement location with respect to the wind direction.
Tracer Gas Correlation Setup: (Monster et al. 2014)
The advantages of the tracer gas correlation method are that its field units are lightweight, rugged, and easy to operate. Also, the method takes weather conditions and other meteorological effects into consideration, yielding relatively high precision (USEPA 2011). The limitations of the method are that the polluting source must be strong enough to be detected at significant distances downwind in order to allow for adequate mixing of the gases (Czepiel et al. 1996). Also, tracer gas cylinders can be expensive, and the method requires that there is a road nearby the site that runs perpendicular to the wind direction from with to take the downwind measurements (USEPA 2011). Lastly, some tracer gases that are typically used to quantify methane emissions, such as sulfur hexafluoride, are potent greenhouse gases; therefore, releasing these gases into the atmosphere is undesirable (Galle et al. 2001).
There are many researchers that have used tracer gas correlation methods to estimate methane emissions from a municipal solid waste landfill. The following studies used tracer gas correlation with extractive FTIR as the optical remote sensing technique to measure the gas concentrations: Galle et al. 2001; Börjesson et al. 2009; and Scheutz et al. 2011 while the following used CRDS to measure the gas concentrations: Green et al. 2010; Mønster et al. 2014; and Taylor et al. 2016. Additionally, Mønster et al. 2015 used both FTIR and CRDS optical remote sensing techniques for concentration measurements. Of these studies, Galle et al. 2001; Scheutz et al. 2011; and Mønster et al. 2015 used nitrous oxide as a tracer gas, Green et al. 2010; Mønster et al. 2014; Mønster et al. 2015; and Taylor et al. 2016 used acetylene as a tracer gas, Czepiel et al. 1996 used sulfur hexafluoride as a tracer gas, and Scheutz et al. 2011 used carbon monoxide as a tracer gas.
Radial plume mapping (RPM) is a technique that utilizes optical remote sensing instrumentation, retroreflectors, and algorithms to characterize a gas concentration profile over a horizontal or vertical plane (HRPM and VRPM). Optical remote sensing instrumentation, typically an OP-TDL (Sect. 3.2), is used to obtain path integrated concentration (PIC) data for the gas (i.e. methane). To use this method for measurement of emission flux from a MSW landfill, a monostatic configuration is required. A monostatic configuration is when the detector, transmitting optics, receiving optics, and OP-TDL are placed at the same location. In a monostatic configuration, retroreflectors are strategically placed to reflect the beam back at the instrumentation (USEPA 2011).
Monostatic Configurations (USEPA 2011)
To obtain emission flux data, the RPM method must be deployed in a vertical configuration and combined with wind data. VRPM requires that the optical remote sensing (ORS) instrumentation be directed towards a vertical structure with retroreflectors fitted along its length. The ORS instrumentation scans the optical path to each fitted retroreflector generating PIC data for each path. Wind speed and vector data are then acquired at the base and top of the structure (Green et al. 2010). The VRPM algorithm, developed by ARCADIS, uses the collected PIC data, wind speed, and wind direction to calculate an average methane emission flux, and ARCADIS software is readily available to perform these computations (Babilotte 2011; Green et al. 2010).
VRPM Configuration (Babilotte 2011)
While VRPM is used to estimate emission flux, HRPM is a useful method for locating methane hotspots across a surface. For HRPM, the ORS instrumentation typically scans close to the surface with the survey area divided into a cartesian grid of rectangular cells (USEPA 2011). Retroreflectors are placed in each cell and the ORS instrumentation scans and dwells at each path for 15 to 30 seconds (Green et al. 2010). These scans collect PIC data for each path and are input into the HRPM algorithm to develop a concentration contour map of the surface (Babilotte et al. 2010).
Wind conditions will play a vital role in the integrity of RPM data. While low wind speeds will not significantly affect the HRPM algorithm, they will affect the VRPM methodology. Very low wind speeds may prevent the source plume from moving through the vertical plane, making it difficult to accurately estimate methane emission flux. High wind speeds will also significantly alter the data for both vertical and horizontal RPM by vibrating the ORS instrumentation and disrupting the optical alignment, thus reducing the quality of the PIC data. High wind speeds may also displace any high emissions identified via HRPM (USEPA 2011). Due to these complications, the EPA Handbook for Optical Remote Sensing for Measurement and Monitoring of Emission Flux recommends the following wind speed ranges for RPM.
Recommended Wind Speeds for RPM
RPM Configuration | Wind Speed (m/s) |
Horizontal | 0 – 5 |
Vertical | 1 – 8 |
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An advantage to using radial plume mapping is that ORS instrumentation needed for RPM, such as the OP-TDL, has become compact, robust and economical due to technological advances in the fiber optics communication field (USEPA 2011). Additionally, VRPM and HRPM data complement each other very well. HRPM can provide concentration information at the surface level, thus revealing hotspots and potential leaks across the landfill, while VRPM can actually estimate methane emission flux. Alternatively, some limitations of RPM are that it can be complicated by difficult terrain such as side slopes, and the integrity of the data is strongly dependent on wind speed; therefore, is limited to days with optimal weather conditions (USEPA 2011).
There are many researchers that have used radial plume mapping to estimate methane emissions from a municipal solid waste landfill. The following studies used RPM methods with TDL as the optical remote sensing technique to measure the gas concentrations: Green et al. 2010; Babilotte et al. 2010; Goldsmith et al. 2012; and Cambaliza et al. 2017.
Differential absorption LiDAR (DIAL) is an application of light detection and ranging (LiDAR) technology that was originally developed in the 1960s (USEPA 2011). In the DIAL method, lasers are directed into the atmosphere at two different wavelengths. One wavelength is specific to the absorption characteristics of the target gas (i.e. methane), and is termed the “on-resonant wavelength”. The second wavelength, or “off-resonant wavelength”, is carefully set so it will not be significantly absorbed by the target gas or any other atmospheric constituents (Babilotte et al. 2010; USEPA 2011). The wavelengths can be generated by two different lasers or by one laser coupled with a wavelength switching unit that alternates between the two. DIAL systems utilize a calibration cell, which is a small cell filled with a known concentration of calibration target gas. A small portion of the laser output is directed at the calibration cell while the rest is expanded and directed into the atmosphere by use of mirrors and optics. The “off-resonant wavelength” scatters elastically due to atmospheric particles while the “on-resonant wavelength” is absorbed by the target gas present in the atmosphere, thus scattering much less. The light scatters in various directions and a portion is scattered back to a detector that gathers the information and converts it into a digital signal (USEPA 2011).
Componenets of a DIAL System (USEPA 2011)
The concentration of methane can be calculated as a function of the difference of backscatter intensities between the “on-resonant” and “off-resonant” wavelengths. DIAL systems can measure methane concentrations from 50 m to 1000 m away with a minimum detection limit of 76 ppb (USEPA 2011). By analyzing the backscatter as a function of time, the location of the concentrations may be estimated. With this information, the DIAL system can yield spatially resolved concentration maps in the 2-D scan plane. Scans take about 15 minutes to complete. Similar to radial plume mapping, horizontal scans can be used to locate methane hotspots along the landfill surface, while vertical scans, combined with wind speed measurements, can be used to determine emission flux. Wind speeds taken at two or three different elevations will provide enough information to determine emission flux through the plane. Typical vertical scans are 600m by 600m with a range resolution of 25m. DIAL systems can be fixed but are often mobilized for field deployment (Babilotte et al. 2010; USEPA 2011).
DIAL Scan Plane (Babilotte 2011)
There are many advantages to using the differential absorption LiDAR method. To begin, three-dimensional mapping of concentration measurements is made possible by DIAL. To achieve this, multiple scans must be taken with short distances between the planes. Additionally, the alternate emission estimation methods discussed in this paper average concentration over a specified path, resulting in PIC data, whereas DIAL’s capability of spatially resolving concentration data is a very powerful advantage (USEPA 2011). DIAL systems are also able to detect backscattered light without the use of retroreflectors, giving it further advantage over the other methods. DIAL systems are highly mobile and have great range. For these reasons DIAL systems work well for monitoring methane emissions from municipal solid waste landfills; however, the equipment is expensive, sparse, and requires skilled technicians for operation. There are only a handful of vendors that offer the technology, and most North American measurements rely on equipment imported from the United Kingdom. DIAL systems, like RPM methods, are also heavily affected by the wind, as changing wind directions and speeds can skew the measurements; therefore, DIAL systems must be properly positioned to minimize adverse wind effects (USEPA 2011).
There has been some research that has used differential absorption LiDAR to estimate methane emissions from a municipal solid waste landfill. One study is Babilotte et al. 2010, which compared results from all three emission estimation methods covered in this paper. Another study, Innocenti 2017, discusses an investigation commissioned by the UK Department for Environment Food & Rural Affairs (Defra) which used DIAL to measure methane emissions at nine landfills.
The purpose of this paper was to highlight and compare the most common methods currently available that are capable of measuring methane emissions from a landfill. It is important to measure the methane emissions from landfills because methane is a harmful greenhouse gas that is 25 times more potent than carbon dioxide. Physically measuring how much methane is truly emitted into the atmosphere is important for global warming research, and this knowledge can lend itself to mitigation methods such as enhanced landfill gas collection systems, landfill cover systems with lower gas permeability, and more waste-to-energy landfill facilities that optimize and take advantage of the methane produced in landfills (i.e. bioreactor landfills).
This paper has covered point-source methods that are used to measure methane at landfills, such as flame ionization detection (FID), photoionization detection (PID), and flux chambers. These methods are generally simple and cost effective, however, also inefficient because they can only provide data for one point on the landfill surface at a time. Additionally, the FID and PID can only provide concentration data as opposed to the flux chambers which directly measure emissions. Scientific advances have allowed for spatial measurements of gas concentrations through technologies such as Fourier transform infrared (FTIR) spectroscopy, tunable diode laser (TDL) absorption spectroscopy, and cavity ring-down spectroscopy (CRDS). The spatial concentration data from these optical remote sensing technologies must then be converted to emission data by the use of emission estimation methods such as tracer gas correlation, radial plume mapping (RPM), and differential absorption LiDAR (DIAL). The emission estimation methods have an obvious advantage over the point-source methods because they use data from optical remote sensing technologies that are gathered spatially over an area; however, they come at a cost. The table below summarizes the methods discussed in this paper and lists some of their most critical advantages and limitations. More detailed reviews of each method and technology can be found in each of the preceding sections.
Regarding the point-source methods, FID is preferable to PID for landfill gas applications because PID cannot directly measure methane. Additionally, flux chambers are preferable to FID because FID requires a constant hydrogen gas supply, although the flux chamber method can take a very long time. For optical remote sensing technologies, FTIR can detect multiple gases simultaneously, but this is often unnecessary for landfill gas applications unless using the tracer gas correlation method, where it is convenient to simultaneously measure the concentration of the methane and the tracer gas. For landfills, TDL is preferable to CRDS because CRDS is much more expensive, and the extended wavelength capabilities are not needed to measure methane. The choice of optical remote sensing technology will also be heavily reliant on the emission estimation method selected. Selecting an emission estimation method is highly dependent on the accuracy and data quality requirements. As DIAL provides spatially resolved concentrations, methane emissions, and hotspot detection, it clearly provides the highest quality data; however, the data quality may not be worth the price depending on the project. Also, for projects with difficult winds and terrain, the tracer gas method would be preferable.
Category | Technology / Method | Advantages | Limitations | Detection Limit |
Point-Source Methods | FID | Simple Easy to operate Cost effective | Requires constant H2 supply Only measures concentration | 0.5 - 50,000 ppm |
PID | Doesn’t directly measure CH4 Only measures concentration | 1 - 10 ppb | ||
Flux Chambers | Measurements take time | Dependent on sensors used | ||
Optical Remote Sensing Technologies | FTIR | Detects multiple gases | Need experienced operator | 1 - 10 ppb |
TDL | Low interference | Can only detect one gas in a narrow infrared range | 0.1 - 1,000 ppm*m | |
CRDS | Supports a wider λ range | Much more expensive | ppt - ppb | |
Spatial Emissions Estimation Methods | Tracer Gas | Accounts for | Gas cylinders can be expensive Provides averaged PIC data | Dependent of technology used |
RPM | Can detect hotspots and flux | Wind & terrain problematic Provides averaged PIC data | Dependent on technology used | |
DIAL | Provides spatially resolved data | Expensive and complex Adversely affected by wind | 0.06 - 153 ppb |
3 COMMENTS
Eva*
Dec, 06, 2017 Liked that the pros and cons of each technology are mentioned below each part, and that you mentioned examples of studies that have used each method. The graphics you chose and of course the table at the 'Comparison and Conclusion' section are very enlightening.Ronald*
Dec, 14, 2017 Cassandra and Gabriel, thanks for the interesting review of all these techniques. Could you comment on which techniques are most commonly used for landfill compliance?casscham*
Dec, 19, 2017 Eva and Ronald, thank you very much for taking the time to read our project. We are pleased that you found it interesting and enlightening.In regard to landfill compliance, the EPA only regulates methane concentration at landfills, not emissions. The EPA requires that surface methane concentrations at landfills remain below 500 ppm. According to the 2015 textbook "Sustainable Practices for Landfill Design and Operation", FID and PID technologies are most often utilized for these measurements due to their low cost.
For clarity, we have edited paragraph 4 of the introduction to briefly address the question of compliance.
Thank you, Cassandra and Gabriel