A New Method for Estimating Greenhouse Gas Emissions from Landfills

1. 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 University of Central Florida Civil and Environmental Engineering

2. Background • Landfill gas emissions are produced by decomposition of waste • Gases are mainly methane (CH4) and carbon dioxide (CO2) • But also many trace gases (odors, VOCs, etc) • CH4 is a strong greenhouse gas – radiative forcing is about 23 times that of CO2 • We would like to know • How much methane is being emitted? • From where in the landfill it is coming?

3. Methane Emissions in U.S. (Tg CO2 equiv.) Note – 1 Tg = 1012 g

4. 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

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

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

7. 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

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

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

10. Our New Method • Hundreds of ambient CH4 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

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

12. Sigmas are functions of x σy = axb σz = cxd + f where: σy, σz = horizontal, vertical dispersion coeff’s x = downwind distance from source to receptor (so we must calculate x from each source to each receptor!)

13. Source-Receptor Geometry y u Ө S (xj,yj) x Receptor (xi,yi) xdist ydist

14. 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

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

16. Compare Ci,mod. vs Ci,meas. • Goal is set of Ci,mod to minimize: where m = number of receptors

17. In Matrix Notation. . .

18. 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.

19. The Single County Landfill (SCL)

20. 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

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

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

23. 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

24. Emitting Sources

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

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

27. Results (cont.) • Emissions same order of magnitude as at least 2 other U.S. landfills

28. Sensitivity Studies • Different source locations and different number of sources – total methane stayed about the same

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

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

31. 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

32. 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

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

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