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1      Introduction

In this report, permanent gas compounds H2, O2, N2, CO2, and CH4 in landfill gas have been identified. The identification was done by separating the gas mixtures into individual components by means of Gas Chromatography coupled with a Thermal Conductivity Detector (GC TCD).

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2      Theory

Landfill gas is created by the action of microorganisms within a landfill. It contains a mixture of gases that majorly include methane and the remaining is carbon dioxide. Less than 1% of the landfill contains trace amounts of other volatile organic compounds. Landfill gas can be generated by the following processes:

·         Chemical reactions between waste components

·         Transfer of dissolved or adsorbed liquid or solid compounds into the gas phase

·         The biological decay of biomass under anaerobic conditions.

Figure 1 shows a typical landfill gas composition and long-term evolution.

 

Figure 1: LFG composition during specific landfill phases and long-term evolution 4

 

In order to monitor the landfill gases, gas chromatography coupled with thermal conductivity detector is used. Gas chromatography is a chromatographic technique that splits the individual components without decomposing them. A typical gas chromatographic system consists of a mobile phase and a stationary phase through which the sample passes; it also includes detectors and data systems. Figure 2 shows the set up of a Gas Chromatograph. The mobile phase is a carrier gas which is inert and does not react with the sample. In this experiment, helium is used as a carrier gas. The stationary phase is present in the separating column. The column can either be a capillary column where the stationary phase is coated on the inside of the capillary or a packed column in which the stationary phase is bound to a carrier material which acts as a molecular sieve. The particles get separated base on their molecular size. The smaller molecules get eluted first and the larger molecules are eluted later. 4 5

 

Figure 2: Schematic of a typical gas chromatograph 5

 

The separated components are analyzed using a thermal conductivity detector (TCD). The TCD measures the thermal conductivity of the analyte in carrier gas with reference to the thermal conductivity of the pure carrier gas. The TCD contains an electrically heated filament. Under normal conditions, there is a stable heat flow from the filament to the detector. When an analyte elutes, there is a change in thermal conductivity. This change in thermal conductivity is recorded and transferred into a signal which is displayed as a peak in the chromatograph.  The measuring cell consists of two channels. By means of a switch, the gas flow can be measured first in the left and later in the right half of the cell. The half which is not supplied with the analyte acts as a reference. 4 6

 

Figure 3: TCD measuring cell

 

3      Materials and Methods

3.1    Materials

3.1.1  Chemicals

·         Test gas          :           Hydrogen 3.0              4.93% (v/v)

Carbon dioxide 4.5      35.30% (v/v)

Nitrogen 5.0                 9.99% (v/v)

Methane 3.5                 49.78% (v/v)

·         Air                  :           Oxygen                       21.88 % (v/v)

Nitrogen                       78.09% (v/v)

 

3.1.2  Devices

·         Gas Chromatograph               :           HP 6890

Injector, injection volume      :           Sample loop, 250 ?L

Temperature valve                  :           180° C

GC columns                           :           Porapack Q, 2 m, 1/8″, 80/100 mesh

                                                           Molecular Sieve 5A, 2 m, 1/8″, 60/80 mesh

Carrier gas, flux                      :           Helium 4.6, 43.9 mL/min

Back pressure                         :           300kPa

Detector, temperature            :           WLD, 210° C

Oven temperature                   :           60° C isothermic

·         GC Syringe

·         Glass gas collector

 

3.1.3  Samples

Sample 1         :           HS 105 – 06.11.17
Sample 2         :           R1-1 – 14.11.17

 

3.2    Procedure

The experiment was carried out with the gas chromatograph HP 6890. The system was calibrated by injecting the test gas. 1 mL of the gas was injected in order to fill the sample loop completely. Care was taken to rinse the syringe with the test gas before injecting it into the gas chromatograph. The test gas was run for 8 minutes at 60° C. The chromatogram was obtained and the baseline of the chromatogram was adjusted depending on the start and end point of each peak so as to obtain precise data. The correction factor for each component was calculated using the formula (1). The same procedure was followed for air and the correction factor of oxygen was calculated using the formula (2). Samples 1 and 2 were subjected to the same procedure and based on the determined correction factors the volume percentage of each component was calculated.

 

3.3    Calculations

The correction factor for each substance was calculated using the formula 

 

                                         Correction factor of substance

         Theoretical volume fraction of substance  in test gas

                              Percentage peak area of substance

 

The correction factor for oxygen was calculated based on the correction factor of nitrogen

                                Peak area of substance

                                Peak area of substance

 

The volume fraction of each substance was calculated using the formula

                                Volume fraction of substance  

                                   Peak area of substance

                                        Correction factor of substance

                                  Peak area of substance

 

4      Results

At next, all measured and obtained data for the experiments will be presented. As outlined in the methods part the correction factors   for all occurred permanent gases had to be determined.

The measurements obtained by gas chromatography of the reference sample, the resulting percentage of the area as well as the theoretical percentage of the volume are shown in the following table.

 

Table 1: Relevant values obtained by gas chromatography of the reference sample “Standard 1”. 4

Peak

Gas

Time min

Area 25 uV*s

Area-% –

Vol.-% (theo.) –

1

H2

0.871

29.90753

9.9773E-02

4.93

2

CO2

1.141

1.41103E+04

47.0715

35.30

3

N2

4.229

3259.76855

10.8747

9.99

4

CH4

5.351

1.25762E+04

41.9540

4.78

 

For the calculation of the correction factor of oxygen   a gas chromatography of an air sample was needed. The obtained data are given in Table 2.

 

Table 2: Relevant values obtained by gas chromatography of the air sample “Standard 2”. 4

Peak

Gas

Time min

Area 25 uV*s

Vol-% (theo.) –

1

O2

3.416

5496.5542

21.88

2

N2

4.174

2.26114E+04

78.09

 

By using the Equations (1) and (2), the following correction factors for all permanent gas compounds were obtained.

Table 3: Determined correction factors.

Gas

T –

H2

49.4122

CO2

0.7499

N2

0.9186

CH4

1.1865

O2

1.0588

 

Analogous to the gas chromatography of both the test samples, Table 4 represents the relevant gas chromatographic data resulting from the first landfill sample “HS 105”, while Table 5 gives a summary of the second landfill sample “R1-1”.

 

Table 4: Relevant values obtained by gas chromatography of the landfill sample 1 “HS 105”.

Peak

Gas

Time min

Area 25 uV*s

1

CO2

1.468

1758.79126

2

O2

3.414

4120.69922

3

N2

4.168

2.29004E+04

 

Table 5: Relevant values obtained by gas chromatography of the landfill sample 2 “R1-1”.

Peak

Gas

Time min

Area 25 uV*s

1

CO2

1.433

1.54205E+04

2

O2

3.426

233.06793

3

N2

4.208

6388.08984

4

CH4

5.345

1.05139E+04

 

By applying the Equation (3) for the sample 1 and 2 respectively, the following volume fractions of all permanent gas compounds resulted.

Table 6: Determined volume fractions of all permanent gas compounds contained in sample 1 and 2.

Gas

Sample 1 Vol.-%

Sample 2 Vol.-%

CO2

4.94

38.35

O2

16.33

0.82

N2

78.73

19.46

CH4

41.37

 

Furthermore, by consulting the diagram shown in Figure 1, the obtained volume fractions listed in Table 6 are giving conclusions about the specific landfill phase in which the samples were taken. Thus, for the sample 1 “HS 105” is an aerobic phase because the composition is almost like air. Just the oxygen is slowly formed into carbon dioxide and methane is not contained. Since the proportion of methane as well as carbon dioxide has reached nearly 40 Vol.-% and nitrogen decrease strongly, the second sample “R1-1” can be categorized between phases VI-VII.

 

5      Discussion

In the following, the influences and consequences of the gas composition, presented in the results, will be discussed. In this context, manly the risks and hazardous substances with regard to the analyzed samples will be discussed.

Considering the gas compound of the first sample, the landfill represented by this sample can be assigned to the aerobic phase. Therefore, the production of carbon dioxide has just begun and the content of oxygen goes down. The higher concentration of carbon dioxide in the gas phase in comparison to the ambient air leads to an environmental degradation. Also, a high concentration of carbon dioxide may contain a risk of suffocation. The maximum permissible concentration of carbon dioxide at the workplace is 0.5 Vol.-% 1. Other hazardous substances in the first sample weren’t determined regarding this experiment.

The gas compound of the second sample can’t be allocated to a specific phase presented in Figure 1. In the second sample, a high amount of methane and carbon dioxide is noticeable. A landfill with such a gas composition should be handled with care, as a methane concentration of 40 Vol.-% in conjunction with ambient air could build a flammable or even an explosive atmosphere 2. Also, the measured concentration of carbon dioxide leads to a higher environmental degradation compared to the first sample. The greenhouse effect of methane can even have a higher environmental impact than the carbon dioxide 3. A landfill with such a gas compound needs a treatment with the appropriate methods.

Other hazardous trace substances weren’t measured in conjunction with this experiment, but in a real landfill, such substances should also be taken into account.

 

6      Error analysis

A possible source of error could be made by sampling the gas. During the sample collection from the sample bottle, a dense injection should be ensured. To avoid a gas leakage out of or in the syringe the plunger should be tested to sit tight in the syringe. To avoid errors and wrong measuring results caused by other gases in the syringe, for example from previous measurements with the same syringe, it was purged with the measured gas. Nevertheless, the contamination of the syringe with other gases is a possible source of error.

Another source of error is related to the analysis and calculation of the measured data with the computer program. The calculation of the peak area for every gas peak was related to a manual correction of the baseline for each peak. Independent of the selected start- and endpoint of the baseline, a variation in the peak area could be noticed. To minimize the calculation error for the peak area, the selection of the start- and endpoint of the baseline should be executed very precisely. The higher the measured peak and the greater the peak area, the less the value of the calculated peak area is influenced by the manual adjustment of the baseline. 

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