University of Central Florida Civil and Environmental Engineering A New Method for Estimating Greenhouse Gas Emissions from Landfills Veronica K. Figueroa, C. David Cooper, and Kevin R. Mackie CECE Dept., Univ. of Central Fla. Presented at AWMA Annual Conference Portland, Oregon June 24-27, 2008

Background Landfill gas emissions are produced by decomposition of waste  Gases are mainly methane (CH 4 ) and carbon dioxide (CO 2 )  But also many trace gases (odors, VOCs, etc) CH 4 is a strong greenhouse gas – radiative forcing is about 23 times that of CO 2 We would like to know  How much methane is being emitted?  From where in the landfill it is coming?

Methane Emissions in U.S. (Tg CO 2 equiv.) Source Category Landfills Natural Gas Systems Enteric Fermentation Coal Mining Manure Management Wastewater Treatment Other Total for U.S Note – 1 Tg = g

EPA regs Require Ambient Monitoring for Large Landfills 1996 EPA NSPS & Emission Guidelines for MSW Landfills - 40 CFR Part 60, Subpart WWW  Landfills with the potential to emit more than 50 Mg/year of NMVOCs must collect & combust biogas  Reduces odors, safety concerns, & methane emissions  Required quarterly surface VOC monitoring  Exceedance of 500 ppm above background requires remedial action

Background (cont.) Methane generation can be estimated from EPA landfill gas generation models. Problems – theoretical, need detailed records, cannot account for capture

Background (cont.) Methane flux can be measured with a flux chamber

Problems with flux chamber Very labor intensive Takes a lot of time to get only a few measurements Gives only point measurements Can give highly variable results

Background (cont.) Methane flux can be estimated with optical systems Source: Thoma et. al, 2005

Problems with this method Very costly Time consuming and labor intensive Depends on proper wind orientation Cannot distinguish variability within landfill

Our New Method Hundreds of ambient CH 4 measurements as receptor concentrations (C’s)  Walking survey (1-2 minutes per measurement)  Measure wind speed and direction  All done in a day Choose hundreds of point sources (Q’s)  Use a plot plan or aerial photo Invert dispersion equations to solve for Q’s

Basic Gaussian Equation Use dispersion eqn. for a point source: Modified for z=0 and H=0:

Sigmas are functions of x σ y = ax b σ z = cx d + f where: σ y, σ z = horizontal, vertical dispersion coeff’s x = downwind distance from source to receptor (so we must calculate x from each source to each receptor!)

Source-Receptor Geometry y u x S (x j,y j ) Receptor (x i,y i ) Ө x dist y dist

Trigonometry X-dist = Δx sin Ө + Δy cos Ө Y-dist = Δx cos Ө - Δy sin Ө  Where Δx, Δy = differences in the x and y coordinates of source and receptor pair

Model C from each source & Sum to get total Modeled C C i,j = f(x,y) i,j *Q j where:

Compare C i,mod. vs C i,meas. Goal is set of C i,mod to minimize: where m = number of receptors

In Matrix Notation...

A Single County Landfill  Class 1 MSW landfill, located in Central Florida, serving 1 county.  Serves over 300,000 residents in 7 cities.  Receives about 800 tons/day of waste.  The total disposal area is 232 acres, and only 127 acres currently have been used.

The Single County Landfill (SCL)

Other Facts Closed Cell has a liner, a cover, and a gas collection system. Flared LFG until just last week when new LFG-fueled combustion engines started burning the gas to generate electricity

Receptors: Surface Emission Readings (ppm as methane) Receptor Locations

Choose Source Locations Goal: Get best-fit emission rates at all locations, ug/s

Results Started with 357 receptors and 356 sources Programmed matrix equations into Matlab End result was reduced number of sources and modeled source strengths at each one Final calculated output was about 1 kg/s of methane from the SCL, most of which was coming from the active cell

Emitting Sources

ISCST model Next, used ISCST to model the event Compared modeled vs measured C’s Prepared Scatterplot

Results (cont.) Used ISCST to model all sources & receptors

Results (cont.) Emissions same order of magnitude as at least 2 other U.S. landfills Landfill Waste in Place (tons) YearEstimated Methane Emissions (g/sec) SCL, Florida 6,456, Approximately 1000 Kenosha, Wisconsin 5,720, Central Landfill, Rhode Island 5,916,

Sensitivity Studies Different source locations and different number of sources – total methane stayed about the same Case 1Case 2Case 3Case 4 Number of Original Sources Inputted by authors Number of Sources after processed by MATLAB Methane Emissions Inventory (g/sec) Percent Difference from Case 10.0%4.0%12.5%6.6%

Sensitivity (cont.) Numbers of larger emitters increased when total number of sources decreased

Conclusions Method is robust Method appears to be accurate Method appears to work better with larger number of sources and receptors

Implications Methane emissions from landfills are big contributor to GCC; accurate inventory is important Methane “hotspots” within a landfill may identify leaking gas collection system Also, methane can be used a surrogate for odor emissions – odor buffer zones can be determined

Summary This is a work in progress, but the technique offers promise If successful our method will give an easy and accurate way to  predict methane emissions from landfills  assess variability of emissions within a landfill  predict odor buffer distances

Acknowledgments The authors gratefully acknowledge the financial support of the Hinkley Center for Solid and Hazardous Waste Management

Questions How do birds know where to go when they migrate? How do birds know where to go when they migrate? What is the meaning of life? What is the meaning of life? Aww, come on... I meant questions that I have a chance of answering! Aww, come on... I meant questions that I have a chance of answering!