1/38 21 – landfill gas 21 landfill gas 1. 2/38 21 – landfill gas “landfill gas is an explosive...

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21 – Landfill gas

21 Landfill gas

1

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21 – Landfill gas

“Landfill gas is an explosive topic” (J.Jacobs, 2006)

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21 Landfill gas Overview

21.1 Landfil gas production

21.2 Prognosis of landfill gas production

21.3 Extraction systems

21.4 Production monitoring

21.5 Flaring, electricity and heat production

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21.1 LFG production

Waste contains a certain amount of degradable organic

matter

Microbiological processes result in the formation of

landfill gas (LFG)

21.1 – Landfill gas

Biological processes

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21 – Landfill gas

21.1 Landfill gas production

Degradation follows 4 biological stages during waste

disposal:

1. Hydrolosis

2. Acidogenesis

3. Acetogenesis

4. Methanogenesis

21.1 – Landfill gas production

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Biological processes


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Hydrolosis

In the first stage complex solid organic material is

solubilized by enzymes excreted by hydrolytic

micro-organisms


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Acidogenesis

In the second stage soluble organic components

including the products of hydrolysis are converted

into organic acids and alcohols


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Acetogenesis

In the third stage the products of acidogenesis

are converted into acetic acid, hydrogen and carbon

dioxide


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Methanogenesis

In the fourth and final stage methane is formed

mainly from acetic acid or from hydrogen and

carbon dioxide. Methane can also be formed

directly from products of the acidogenesis such as

formic acid and methanol


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Composition

LFG primarily consists out of methane (CH4) and

carbondioxide (CO2)

Ratio CH4:CO2 = 50:50 (theoretically)

CH3COOH (Acetic acid) --> CH4 + CO2

CO2 is more soluble in water, ratio shifts to 60:40


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Composition (cont’d)

CH4 is a green house gas (GHG) with a GHG potential of

21 times that of CO2

1 ton of CH4 has an equivalent of 21 ton of CO2

LFG can contain traces of hydrocarbons, halogenated

hydrocarbons and sulphur compounds


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21.2 Prognosis of LFG production

Modelling with LFG production models

Only two models are validated

First order model

Multi phase first order model

21.2 – Prognosis of landfill gas production

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First order model

tkt ekAC 1

1087.1

Where: t = landfill gas production at a given time [m3LFG.y-1] = dissimilation factor 0.58 [-] 1.87 = conversion factor [m3LFG.kgCdegraded-1] A = amount of waste in place [Mg] Co = amount of organic carbon in waste [kg C.Mg waste-1] k1 = degradation rate constant 0.094 [y-1] t = time elapsed since depositing [y]


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21 – Landfill gas

First order model – organic content

Waste category Organic carbon content [kgC.Mg-1] Contaminated soil 11 Construction & demolition waste 11 Shredder waste 130 Street cleansing waste 90 Sewage sludge & compost 90 Coarse household waste 130 Commercial waste 111 Household waste 130


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Multi phase first order

3

1,1,0

,1

i

tkiit

iekcAC

Where: t = landfill gas production at a given time [m3LFG.yr-1] = dissimilation factor [-]* i = waste fraction with degradation rate k1,i [kgi.kgwaste

-1]** c = conversion factor [m3LFG.kgOMdegraded

-1]*** A = amount of waste in place [Mg] Co = amount of organic matter in waste [kgOM.Mg waste-1] k1,i = degradation rate constant of fraction i [y-1]* t = time elapsed since depositing [y]


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Multi phase first order – organic content

Waste category Minimum organic matter content [kgOM.Mg-1]

Maximum organic matter content [kgOM.Mg-1]

Rap Mod Slow Total Rap Mod Slow Total

CS: contaminated soil 0 2 6 40 0 3 8 42

C&D: construction & demolition 0 6 12 44 0 8 16 46

SW: shredder waste 0 6 18 60 0 11 25 70

SCW: street cleansing waste 9 18 27 90 12 22 40 100

S&C: sewage sludge & compost 8 38 45 150 11 45 48 160

cHW: coarse household waste 13 39 104 260 19 49 108 270

CW: commercial waste 13 52 104 260 19 54 108 270

HW: household waste 60 75 45 300 70 90 48 320


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Organic matter and Organic C

)/132()/48()/72/180(

33 246126

molgmolgmolgCmolgOM

COCHOHC

Therefore:

Methane and LFG production per kgOM degraded: LFGmCHm 3

43 75.0373.0

714180

48

Methane and LFG production per kgC degraded: LFGmCHm 3

43 87.1933.0

71472

48


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Prognosis of LFG production

-

50

100

150

200

250

300

19

65

19

70

19

75

19

80

19

85

19

90

19

95

20

00

20

05

20

10

20

15

20

20

20

25

20

30

20

35

(m

³LF

G/h

)

k=0,094 k=0,04


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Prognosis of LFG production

Only two models are validated

Uncertainty depending on:

1. Waste composition

2. Degree of pre-treatment

3. Type and degree of compaction, method of operation, type and

thickness of cover material

4. Quantity of refuse, geometry and hydrogeolocic properties of landfill

5. Climatic conditions


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21.3 Extraction systems

Lay out of an extraction system

Various types of extraction systems

Recovery efficiency

21.3 – Extraction systems

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Lay out

flaring and/orutilisation

production

emission

oxidation

extraction


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Various types of extraction systems

1.Vertical

2.Horizontal

3.Slope

4.Point


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Vertical

Conventional Cell wise

Build up Build up + early extractionGRAVEL

HDPE piping (perforated)

HDPE piping (non -perforated)


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Horizontal


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Slope


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Point


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Gas well connection

RULE OF THUMB: 4 GAS WELLS PER HA


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Gas well connection


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Recovery efficiency

Disposal phase Recover efficiency

Straight after disposal PHASE 0 - (=0%)

During start up period exploitation phase PHASE 1 10% - 25% Exploitation phase PHASE 2 25% - 50% After the First year of the exploitation phase untill final top cover

PHASE 3 50% - 60%

Temporary cover and covering of slopes PHASE 4 60% - 75% After ending exploitation phase, landfill fully covered

PHASE 5 70% - 80%

Final situation after completion, liner system in tact and in operation

PHASE 6 90% - 100%


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21.4 Production monitoring

Monitoring equipment

Anemo meter Digital mano meter

Gas analyser

21.4 – Production monitoring

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Gas analyser can measure:

1. CH4

2. CO2

3. O2

4. N2 (= 100% - CH4% - CO2% - O2%)


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If CH4 and O2 concentration respectively in a gas

well is:

> 50% and < 1% then more extraction

< 50% and > 1% then less extraction


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If N2:O2 ratio is:

> 5 indicator suction of ambient air

< 4 indicator leakage piping extraction system


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21.5 Flaring, electricity and heat production

A suitable nearby use for LFG must be identified

The landfill must have a depth of at least 5 m. and preferably

10 m. of unsaturated biodegradable waste

Amount of waste deposited: 0.5 Million tonnes

Waste should not be too old (between 5 – 10 years, less if

mainly food and vegetable wastes)

Leachate level should be at least 5 m. below the landfill surface

21.5 – Flaring, electricity and heat production

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Flaring

Open flare

Enclosed flares

Passive flare


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Heat production

LFG fired boilers

Heat exchangers WWTP


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Electricity production


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