Method for quantifying fugitive methane emissions rate using surface methane concentration

Abstract

A method was invented to convert methane concentration at the surfaces emitting fugitive methane into methane emission rate. This method requires surface scan of methane concentration using handled devices such as flame ionization detector (FID) to measure the fugitive methane near-surface concentration based on which, the methane emission rate can be calculated using a correlation expressed in a mathematical form.

Claims

1. A method for estimating fugitive methane emission rate over the surface of a landfill and similar fugitive methane emission surfaces, by sole measurement of surface methane concentration and using a generalizable correlation equation between surface concentration of fugitive methane measured at the surface of different zones of fugitive methane emitting surface, and corresponding emission rates adjusted against barometric pressure, the method comprising: a. Dividing an area of interest into a plurality of zones (Z.sub.i) based on any one or any combination of the following attributes: i. The geometry of the site, ii. Type of cover, iii. Type of vegetation, iv. Status of vegetation in terms of density, health and level of stress, v. Expected emission levels in form of concentration or rate based on any previous field measurement records, vi. Expected emission levels in form of concentration or rate based on the results of previous emission rate or concentration modeling; b. Sampling of surface methane concentration (SMC) in accordance with any of existing protocols and standards for qualitative assessment of emissions from municipal landfills established by regulatory organizations; c. Adjusting SMC values (SMC.sub.a) to account for effects of barometric pressure rate of change during the sampling; d. Measuring Methane Emission Rate (MER) in each zone (Z.sub.i); e. Calculating an average SMC.sub.a (SMC.sub.a-i) for each zone (Z.sub.i); f. Calculating an average MER (MER.sub.a-i) for each zone (Z.sub.i) g. Correlating the average SMC.sub.a (SMC.sub.a-i) linearly to the average MER (MER.sub.a-i) for each zone to obtain a Correlation Factor (C.sub.f); and h. Converting SMC measurement data points into MER values for every zone over the entire methane emitting surface using the Correlation Factor (C.sub.f); Wherein for any methane emitting surface a site-specific value for C.sub.f can be calculated using steps a.-g., and the site-specific C.sub.f can then be used to calculate MER from SMC for each specific site.

Description

BRIEF DESCRIPTION OF THE FIGURES

(1) FIG. 1. Sampling path within a zone

(2) FIG. 2. BP change rate and MER multiplier correlation

(3) FIG. 3. SMC.sub.a and MER.sub.a for 12 zones

(4) FIG. 4. SMC and MER correlation

DETAILED DESCRIPTION OF THE INVENTION

(5) 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).

(6) 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.

(7) 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.sub.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.sub.a accounting for effects of barometric pressure rate of change during sampling campaign, (iv) SMC.sub.a data are integrated for each measurement zone to calculate an average SMC.sub.a for each zone denoted as SMC.sub.a-i, and (v) SMC.sub.a-i values in form of data points or measurements for each zone is correlated linearly to quantitative measurement data points of MER.sub.a-i which is the Average Methane Emission Rate for Zone Z.sub.i (vi) The correlation as shown in Equation 1, and in FIG. 4, results in a Correlation Factor C.sub.f.
MER.sub.a-i=SMC.sub.a-i×C.sub.f±ΔSMC.sub.a-i  Equation 1
Where:
MER.sub.a-i=average methane emission rate for zone Z.sub.i in g CH.sub.4/m.sup.2/d
SMC.sub.a-i=average surface methane concentration for zone Z.sub.i in ppmv CH.sub.4
C.sub.f=Correlation factor

(8) For every calculated numeric value of the SMC.sub.a for each zone, a corresponding MER.sub.a can be calculated using Equation 1 and the line shown in FIG. 4 denoted as Correlation.

(9) 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.sub.T=Σ.sub.i=1.sup.n(A.sub.i×MER.sub.a-i×3.65×10.sup.−4)  Equation 2
Where:
E.sub.T=total annual methane emission from the project boundary in tonnes/year
A.sub.i=Footprint Area of zone Z.sub.i in m.sup.2
MER.sub.a-i=average methane emission rate for zone Z.sub.i calculated from Equation 1 in g CH.sub.4/m.sup.2/d
3.65×10.sup.−4=unit conversion multiplier

(10) Note: The default correlation factor C.sub.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.sub.f.

(11) 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:

(12) 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'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.

(13) 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.

(14) Therefore, the MER values should be adjusted to the true values presented as MER.sub.a, based on the recorded ΔP/t at the time of sampling relative to the equalized condition meaning that ΔP/t equals zero.
MER.sub.a=MER×(1+1.9731×|ΔP/t|){circumflex over ( )}(ΔP/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.sub.a=SMC×(1+1.9731×|ΔP/t|){circumflex over ( )}(ΔP/t/|ΔP/t|)  Equation 4
D) Data compilation and analyses:

(15) 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.

(16) As shown in FIG. 3, plotting the SMC.sub.a data against the MER values showed a reasonable correlation between these two values.

(17) Based on this correlation:
MER=SMC×(0.32±0.034)+(1.39±0.755)  Equation 5
Where:
MER=methane emission rate in g CH.sub.4 m.sup.−2 d.sup.−1
SMC=surface methane concentration in ppmv CH.sub.4

(18) 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.

(19) 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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