Patent Publication Number: US-11378563-B2

Title: Method for quantifying fugitive methane emissions rate using surface methane concentration

Description:
BACKGROUND OF INVENTION 
     1. Field of Invention 
     This invention relates generally to methods for measurement and estimation of fugitive methane emission rates from the surface of a landfill or any other source of fugitive methane emission specifically using concentration of fugitive methane near the surface of the emitting source. 
     2. Description of the Related Art 
     Landfill gas (LFG) is a by-product of natural decomposition of organic materials in landfills that can create unsafe air quality, health issues, unpleasant odours, and contribute to global climate change. LFG predominantly consists of methane (CH 4 ) and carbon dioxide (CO 2 ); both being potent greenhouse gases (GHG). While CO 2  produced in the waste sector (e.g. municipal landfills, wastewater treatment plants, and burning of non-fossil fuel waste) is not accounted for as a GHG due to its biogenic origin, the fugitive emission of CH 4  from landfills is of significant concern (IPCC, 2006) in terms of global warming potential (GWP). Methane is a naturally occurring GHG with a GWP 28 to 34 times greater than carbon dioxide over a 100-year timeframe (IPCC, 2013). In Canada, about 3% of the 2010 national GHG emissions were reported to be from the waste sector, of which about 91% was attributed to fugitive methane emissions from landfills (Environment Canada, 2012). 
     Methane is an important GHG with a much shorter atmospheric lifetime, approximately 10 years, in comparison with other greenhouse gases (Bogner and Matthews, 2003). Accordingly, changes made to CH 4  emission sources can affect the atmospheric concentrations on relatively shorter timescales. 
     Attention to methane emissions from landfills has grown significantly due to the fact that emission reduction from landfills is amongst the most feasible and cost-effective measures to reduce greenhouse gas emissions (Kormi et al, 2018). However, quantifying landfill fugitive methane emissions is challenging due to the high temporal variability and spatial heterogeneity. 
     Thus, development of reliable and cost-effective methods for measurements of landfill methane emissions is critically important. 
     Various methods have been attempted by scientists and practitioners over the past few decades. The most widely method seemingly more favorable for the purpose of regulatory compliance assessment is the use of flux chamber which directly measures methane emission flux from the surface of landfills. In addition to chamber methods, other methods including but not limited to eddy covariance and co-advected prow tracer plume measurements and methods relying on remote sensing and plume mapping have been used (Gardiner et al., 2017; Delre et al., 2018; Kormi et al., 2017; Goldsmith et al., 2012; Gollapalli et al., 2018; Monster et al., 2014: Innocenti et al., 2017; Delkash et al., 2016; Allen et al., 2018). Chamber-based measurements are relatively easy to conduct as emissions can be estimated from the rate of change of CH 4  concentration in a chamber, the footprint area of the chamber and volume of the chamber. However, the chamber method suffers practical drawback due to the typically heterogeneous nature of the landfill resulting in high spatial variability of emissions (Riddick et al., 2018). 
     Eddy covariance (EC) methods have also been studied for methane emission estimation from landfills over longer periods of time, Xu et al. (2014). Eddy covariance which calculates a gas flux from the covariance between vertical wind speed and gas concentration at a high sampling rate has the main advantages of providing mean flux estimates over a larger area and being automated. The drawbacks however are that the emission in the fetch needs to be homogeneous and that the measurement needs to be carried out on a topographically flat surface to obtain meaningful results (Riddick et al., 2018). 
     Using acetylene as the tracer gas is the current state of the art tracer gas dispersion measurements for determining methane emissions from landfills. Measurements of the tracer gas and methane concentrations are made downwind of the source (Mønster et al. 2015). The tracer gas dispersion technique relies on the assumption that full mixing between the tracer and landfill plume has occurred at the point of monitoring (Rees-White et al., 2018). A key logistical limitation of the tracer release method is that it requires a mobile measurement team to coordinate with the person releasing the gas and then traverse an accessible road perpendicular to the landfill plume in the time it takes for the plume to travel from the release site. Furthermore, it should be ensured that the tracer gas is well mixed with the landfill methane as insufficiently mixed plumes can invalidate the co-advection assumption, result in large uncertainties in the emission estimate (Riddick et al., 2018). 
     Additionally, the relationship between the emission rate and the gas concentration at a given location is dependent on the meteorological conditions and local topography, preventing accurate quantification of the emission rate. 
     Remote sensing techniques represent a more integrated approach for quantification of methane flux. These techniques have gained popularity in recent years. One of these techniques is the Radial Plume Mapping (RPM) methodology recognized by the US-EPA as “other test method 10 (OTM-10)” since July 2006 (USEPA, 2006). This technique uses optical remote sensing (ORS) instrumentation to characterize gas emissions from non-point sources. Some of these ORS instruments include; (i) Open-Path Fourier Transform Infrared (OP-FTIR) spectroscopy. (ii) Ultraviolet Differential Absorption Spectroscopy (UV-DOAS), and (iii) Open-Path Tunable Diode Laser Absorption Spectroscopy (OP-TDLAS) (USEPA, 2007). 
     The RPM techniques carry many advantages over the “close range measurement” methodologies, such as the flux chamber technique. However, the relatively high cost of the RPM method, as well as the uncertainties associated with the possible effect of the methane plume buoyancy on the results, made the flux chamber methodology a more suitable option for the present invention. 
     Prior to the applicant&#39;s invention, no methods were known to utilize a relationship between surface concentration of fugitive methane from a landfill surface and the emission rate of methane typically or as an accepted standard method measured using flux chamber. More specifically, prior to this invention, a correlation between the concentration of fugitive surface concentration of methane and methane flux or emission rate that can be generalized to other typically similar landfills, only through adjustment of barometric pressure has not been developed. 
     Several embodiments of the present invention relate to a methodology using which fugitive methane emission can be characterized and quantified for further applications such as reporting methane emissions for regulatory purposes, evaluating performance of landfill bio-covers and identifying hotspots in terms of methane emission. 
     BRIEF SUMMARY OF THE INVENTION 
     In accordance with several embodiments of the invention, a method wherein the fugitive methane emission rate is calculated based on surface concentration of fugitive methane measured using surface scanning. 
     In one embodiment of the method in accordance with this invention, this method is based upon a strong correlation developed between landfill surface methane concentration (SMC, part per million volume (ppmv) CH 4 ) and methane emission rate (MER, g CH 4 /m 2 /d). 
     In one embodiment, landfill is a confined or semi-confined space wherein different types of waste materials are disposed of following a standard procedure prescribed by local, regional, national or international authorities and regulatory bodies. 
     In a preferred embodiment, landfill as defined above, receives municipal solid waste that includes organic materials which can decompose and generate methane. 
     In one embodiment, the landfill is equipped with a cover or a cap through which fugitive methane generated as a result of anaerobic decomposition of organic material inside the landfill is emitted to the atmosphere. 
     In one embodiment, the landfill is equipped with landfill gas collection system through which all gases including methane generated within the body of landfill excluding the portion emitted fugitively into the atmosphere are collected for further processing. 
     In one embodiment, the cap over the landfill will be made of layers of different earthen or synthetic materials aimed at reducing fugitive methane emission. 
     In preferred embodiment, quantification of fugitive emission is critically important to quantify the landfill contribution to overall greenhouse gas emission and climate change. 
     In one embodiment, the landfill is divided into several zones. In preferred embodiment, the zones are selected based on geometry, type of cover, type or status of vegetation including but not limited to healthy and stressed, other visual observation, expected emission levels and pre-sampling results if such results are available. 
     In preferred embodiment, measurement of surface methane concentration (SMC) is completed through a modified version of an existing protocol for qualitative assessment of emissions from municipal landfills established by the United States Environmental Protection Agency (US EPA) (Title 40 CFR Pat 60, Standards of Performance for Municipal Solid Waste Landfills). 
     In preferred embodiment, SMC values are adjusted accounting for effects of barometric pressure rate of change during sampling campaign. 
     In one embodiment, adjusted SMC data are integrated for each measurement zone to calculate an average adjusted SMC for each zone. 
     In preferred embodiment, the adjusted average SMC in each zone is correlated with the average methane flux measured within each zone based on standard protocols available including but not limited to the Flux Chamber method. 
     In preferred embodiment, the resulting correlation in form of an equation can be used under a variety of methane emitting surfaces such as landfills, to calculate methane emission rate solely based on surface methane concentration requiring none, minimum or limited number of emission rate measurements through methods such as flux chamber. 
     In preferred embodiment, the method presented in this invention can be used in a variety of landfills where methane emission rate is to be reported. 
     The preferred embodiment relies solely on significantly cost effective and less time-consuming measurement of surface methane concentration as an advantage to the existing methods such as using flux chambers for methane emission rate measurement. 
     The one embodiment, the equation can be recalibrated and/or validated for a new methane emitting site or a methane emitting site with significantly different surficial features through additional yet limited number of flux chamber measurement data points. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURES 
         FIG. 1 . Sampling path within a zone 
         FIG. 2 . BP change rate and MER multiplier correlation 
         FIG. 3 . SMC a  and MER a  for 12 zones 
         FIG. 4 . SMC and MER correlation 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     A unique approach was developed under this research allowing for quantification of the fugitive methane emissions rate (MER) from the entirety of a given landfill surface area at a considerably lower cost in comparison with the conventional methods. The core of the proposed method is to use measured surface methane concentration (SMC) data obtained through surface scan by handheld devices such as a portable flame ionization detector (FID). 
     The correlation between SMC data and MER values was developed based on representative emission rate values measured using flux chamber technique and adjustments made in terms of barometric pressure fluctuations during the fieldwork. The resulting equation can be used to simply predict the methane flux through the surface of any given landfill (active, cover soil, or biocover), using SMC data which are obtained through a less costly method. 
     In this invention, this method may be accomplished through the following steps:
         (i) Area of interest or the project boundary such as landfill footprint is divided into several zones denoted as Z i , based on one or more of the following attributes of a given site; landfill or otherwise:
           a. The geometry of the site,   b. Type of cover including but not limited to earthen covers, geosynthetics, biocover and a combination of different types of covers   c. Type of vegetation,   d. Status of vegetation in terms of density, health and level of stress   e. Other visual observations,   f. Expected emission levels in terms of concentration or rate based on any previous field measurement records   g. Expected or anticipated emission levels in terms of concentration or rate based on the results of previous emission rate or concentration modeling   
           (ii) Measurement of SMC is completed through any of existing or future protocols and standards for qualitative assessment of emissions from municipal landfills established by regulatory or otherwise organizations such as the US EPA, Title 40 CFR Pat 60, Standards of Performance for Municipal Solid Waste Landfills,   (iii) SMC values are adjusted to SMC a  accounting for effects of barometric pressure rate of change during sampling campaign,   (iv) SMC a  data are integrated for each measurement zone to calculate an average SMC a  for each zone denoted as SMC a-i , and   (v) SMC a-i  values in form of data points or measurements for each zone is correlated linearly to quantitative measurement data points of MER a-i  which is the Average Methane Emission Rate for Zone Z i      (vi) The correlation as shown in Equation 1, and in  FIG. 4 , results in a Correlation Factor C f .
 
MER a-i =SMC a-i   ×C   f ±ΔSMC a-i   Equation 1
 
Where:
 
MER a-i =average methane emission rate for zone Z i  in g CH 4 /m 2 /d
 
SMC a-i =average surface methane concentration for zone Z i  in ppmv CH 4  
 
C f =Correlation factor
       

     For every calculated numeric value of the SMC a  for each zone, a corresponding MER a  can be calculated using Equation 1 and the line shown in  FIG. 4  denoted as Correlation. 
     Values for the correlation factor and variation are developed under this work and provided through the correlation and the total methane emission from the project boundary abbreviated as E r can be then calculated as:
 
 E   T =Σ i=1   n ( A   i ×MER a-i ×3.65×10 −4 )  Equation 2
 
Where:
 
E T =total annual methane emission from the project boundary in tonnes/year
 
A i =Footprint Area of zone Z i  in m 2  
 
MER a-i =average methane emission rate for zone Z i  calculated from Equation 1 in g CH 4 /m 2 /d
 
3.65×10 −4 =unit conversion multiplier
 
     Note: The default correlation factor C f  is developed by completing a quantitative field measurement of MER using the US EPA flux chamber methodology for various zones with different emission levels, qualitative assessment of SMC for the same areas, and plotting the SMC data against the MER values. Similar exercise can be repeated, when possible and desired, to calculate a site-specific value for C f . 
     A) Zoning: Zoning of a site is necessary only if different areas of the site are expected to have significantly different methane emission rates. This is done prior to completing the field measurements. The area of interest is divided into different zones based on expected levels of methane emission rates such as landfill crest, side slopes, type of cover, type of vegetation, etc. While the end results in form of the estimated total methane emission from the site will remain the same, zoning of the site will help identifying the areas with higher emission rates.
 
B) Field measurement abbreviated as SMC: Surface methane concentration of each zone is measured by continuous and instantaneous sampling of air using a portable device with minimum detection limit of 0.0001% methane or 1 part per million or ppm. One of the devices that can be used to measure SMC with this accuracy is portable Flame Ionization Detector denoted as FID to measure the concentration of total organic compounds measured as methane at the landfill surface. SMC measurement is completed following the protocols similar to US EPA protocol for qualitative surface methane emission monitoring under Title 40 CFR Part 60, Subpart WWW. Method includes instantaneous sampling of air at 2.5 to 10 cm above landfill surface and on paths of approximately 30 m. Reducing the distance between the measurement paths which is recommended to be 10 m or less, will increase number of samples and accuracy of the results. The SMC measurement field work can be completed only when the landfill cover soil, bio-cover, or other form of covers is not saturated, and wind speed is less than 16 km/hr equal to about 4.5 m/sec. The SMC readings are recorded at minimum every 5 to 10 seconds at sampling points approximately every 1 to 2 m along the sampling route or path. These readings are separately collected for each zone along with GPS records, time of sampling, climate conditions, ambient temperature and barometric pressure.  FIG. 1  shows an example of how a site boundary is divided into different zones and how sampling path should be in an example zone.
 
C) Other field readings and field data adjustments:
 
     Variations in the weather conditions, and in particular the barometric pressure abbreviated as BP, have an impact on rate of methane fugitive emissions from landfill&#39;s surface. Higher emission rates at landfills are reported to occur at lower ambient pressures. In general, variations in atmospheric pressure happen due to several factors including;
         Auto oscillation of air which is reported to have an insignificant effect,   Daily warming and cooling of air caused by solarization causing diurnal variations, and   Passage of atmospheric pressure lows and highs leading to long term variations.       

     Therefore, short term daily and long term seasonal variations in atmospheric pressure should be considered when conducting methane fugitive emission measurements at a landfill site. The present methodology includes development of an equation for adjusting the calculated MER values for effect of barometric pressure fluctuations at time of sampling. The true value of MER at the landfill could be measured when the atmospheric pressure remained constant, causing an equilibrium condition between landfill and the surrounding environment. The following equation was developed through finding a good correlation between change in MER values and rate of change in barometric pressure as shown in  FIG. 2 . 
     Therefore, the MER values should be adjusted to the true values presented as MER a , based on the recorded ΔP/t at the time of sampling relative to the equalized condition meaning that ΔP/t equals zero.
 
MER a =MER×(1+1.9731 ×|ΔP/t |){circumflex over ( )}(Δ P/t/|ΔP/t |)  Equation 3
 
Where:
 
ΔP/t=change in barometric pressure over time during sampling
 
ΔP/t/|ΔP/t| would be equal to −1 or +1, represent the sign of the ΔP/t. This adjustment for effect of BP shall be made once on either measured SMC data or calculated MER at the end as suggest in Equation 3. If field data is intended to be adjusted before calculation of MER. Equation 4 below can be used to find adjusted SMC, based on which true value of MER can be calculated.
 
SMC a =SMC×(1+1.9731 ×|ΔP/t |){circumflex over ( )}(Δ P/t/|ΔP/t| )  Equation 4
 
D) Data compilation and analyses:
 
     This invention is based on the correlation that was found between SMC and MER. This correlation, illustrated in  FIGS. 3 and 4  below, was developed through extensive field measurement on 12-hectare area consisting of 12 different measurement zones. 
     As shown in  FIG. 3 , plotting the SMC a  data against the MER values showed a reasonable correlation between these two values. 
     Based on this correlation:
 
MER=SMC×(0.32±0.034)+(1.39±0.755)  Equation 5
 
Where:
 
MER=methane emission rate in g CH 4  m −2  d −1  
 
SMC=surface methane concentration in ppmv CH 4  
 
     The development of this invention can be practically very important in the LFG management industry, saving time and money when full scale fugitive methane emission measurements are required. Another very important application of this methodology is performance review and/or quantification of methane emission from surfaces with very low methane emissions such as bio-cover systems, bio-filters, and bio-window systems. 
     Main objective of these systems is to minimize methane emission to the atmosphere, making it almost impossible to use conventional methods, such as flux chamber technique, for quantification of the remaining methane emission through these systems. 
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