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, OregonJune 24-27, 2008

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?

Methane Emissions in U.S. (Tg CO2 equiv.)

Source Category 1990 2000 2001 2002 2003

Landfills 172.2 130.7 126.2 126.8 131.2

Natural Gas Systems 128.3 132.1 131.8 130.6 125.9

Enteric Fermentation 117.9 115.6 114.5 114.6 115

Coal Mining 81.9 56.2 55.6 52.4 53.8

Manure Management 31.2 38.1 38.9 39.3 39.1

Wastewater Treatment 24.8 34.3 34.7 35.8 36.8

Other 45.8 43.5 41.7 39.6 39.8

Total for U.S. 605.3 554.2 546.7 542.3 544.9

Note – 1 Tg = 1012 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 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

Basic Gaussian Equation

Use dispersion eqn. for a point source:

Modified for z=0 and H=0:

2

2

2

2

2

2

2

1exp

2

1exp

2

1exp

2 zzyzy

HzHzy

u

QC

2

2

2

1exp

yzy

y

u

QC

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!)

Source-Receptor Geometry y

u

x

S (xj,yj)

Receptor (xi,yi)

Ө

xdist

ydist

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

Ci,j = f(x,y)i,j*Qj

where:

Ci,mod Ci, jj1

n

2

2

2

1exp

1

yzy

y

uf

Compare Ci,mod. vs Ci,meas.

Goal is set of Ci,mod to minimize:

where m = number of receptors

R2 Ci,measured Ci,mod 2

i1

m

In Matrix Notation. . .

minq

F q cmeasured 2

2

q Fcmeasured

F FTF 1FT

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

Trendline:y = 1.0447xR2 = 0.651

0

100

200

300

400

500

0 100 200 300 400 500

Measured Ambient VOC Concentration (PPM as methane)

Pre

dict

ed M

etha

ne C

once

ntra

tion

(P

PM

as

met

hane

)

Results (cont.)

Emissions same order of magnitude as at least 2 other U.S. landfills

Landfill

Waste in Place (tons)

Year Estimated Methane Emissions (g/sec)

SCL, Florida

6,456,000

2007 Approximately 1000

Kenosha, Wisconsin

5,720,000

2003 899

Central Landfill, Rhode Island

5,916,449

1996 1293

Sensitivity Studies Different source locations and different number of

sources – total methane stayed about the same

Case 1 Case 2 Case 3 Case 4

Number of Original Sources Inputted by authors 356 356 356 228

Number of Sources after processed by MATLAB 328 318 335 215

Methane Emissions Inventory (g/sec) 970 930 1090 900

Percent Difference from Case 1 0.0% 4.0% 12.5% 6.6%

Sensitivity (cont.)

Numbers of larger emitters increased when total number of sources decreased

0

20

40

60

80

100

120

140

160

180

200

Number of Sources

Case 1 Case 2 Case 4

Cases

Comparing Methane Emission Ranges for Case 1, Case 2, & Case 4

Emissions < 5 g/s 5 g/s < Emissions < 20 g/s Emissions > 20 g/s

171

1012

169

10

14

155

20

18

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 How do birds know where to go when they migrate?they migrate?

What is the meaning of life?What is the meaning of life? Aww, come on. . . I meant questions Aww, come on. . . I meant questions

that I have a chance of answering!that I have a chance of answering!