Measurements of methane emissions, methane oxidation

1
Measurements of methane emissions, methane
oxidation and VSCs at Swedish landfills using
FTIR spectroscopy
Jerker Samuelsson Bo Galle Chalmers University
of Technology Department of Inorganic
Environmental Chemistry S-412 96 Göteborg Sweden

Gunnar Börjesson Bo Svensson Department of water
and Environmental studies Linköping
University S-581 83 Linköping Sweden
During the year 1998/99 total methane emission
measurements have been successfully conducted on
a Swedish landfill, measuring every second month.
The method used is based on tracer gas release
from the landfill combined with time resolved
concentration measurements downwind the landfill
using FTIR absorption spectroscopy. A project has
been initiated, aiming at improving the Swedish
landfill methane budget, including studies of
methane oxidation in the top soil layer of the
landfills and presence of VSCs (Volatile Sulphur
Compounds). Below, the methodology used is
described along with obtained results and a
presentation of the initiated Swedish landfill
methane project. Introduction Methane is an
important climate gas contributing to the global
warming.The concentration of CH4 in the
atmosphere increases globally by 0.6-0.8
annually. The IPCC has estimated that more than
10 of the anthropogenic methane emissions
originate from landfills. Besides developing and
implementing new waste treatment practises such
as incineration and separation at source, it is
important to find methods to reduce the methane
emissions from existing and future landfills. The
organic carbon available in a landfill is
biogenically degraded to CH4 and CO2. As CH4 has
a GWP (Global Warming Potential) that is 20 times
stronger than for CO2, a considerable improvement
would be made if a larger fraction of the carbon
was emitted as CO2. This can be achieved either
by pumping the gas out for incineration or by
bacterial methane oxidation in the top soil layer
of the landfill. To validate the effect of
different methods there is a need to be able to
quantify both the total emissions from landfills
and the degree of methane oxidation in the cover
layer. Due to the spatial variability of the
emissions, integrating methods are needed. A
method called the TCT (Time Correlated Tracer)
method has been successfully applied at a Swedish
landfill, estimating the total methane
emission. Total methane emission
measurements During one year the total methane
emission from a landfill serving a small Swedish
town, Falköping, has been monitored using FTIR
absorption spectroscopy. Measurements were
carried out over 1-3 days every second month. The
observed average emission over the year was 37.5
kg/h, with an estimated accuracy of about 15 in
respectively measurement. In December a strong
rise in the emission, starting from a low level,
was observed, coinciding with thawing of the
ground. The methodology used, called the TCT
method (Time Correlated Tracer), is based on
tracer release at the landfill, accompanied by
time resolved concentration measurements in the
plume downwind the landfill. The emitted tracer
mixes with the leaking methane, and analysis of
the ratio between the methane and the tracer
concentrations in the landfill plume, yields the
methane emission as, If the tracer simulates the
methane emission well, indicated by the degree of
correlation between the concentrations, the total
emission from the landfill is derived. The
experimental set up used is based on a medium
resolution FTIR spectrometer coupled to an
optical White cell. Infrared light is transmitted
through the gas cell over a pathlength of total
56 meters, and an absorption spectrum of the gas
is recorded. The spectra are analysed using
multiregression techniques (CLS), fitting pre
stored, synthetic calibration spectra of known
concentrations. The system offers sensitive, real
time analysis of a number of interesting gases
(CH4, CO2, CO, N2O, H2O, NH3, hydrocarbons etc.)
simultaneously, and with high time resolution.
Mixing ratios down to a few ppb is detectable.
The Swedish landfill methane project A
project has been initiated aiming at improving
the Swedish landfill methane budget. Every second
month during the year 2000/2001 ten
representative landfills, located from the south
to the very north of Sweden, will be measured
upon. The study includes total CH4 emission
measurements, estimation of the methane oxidation
in the landfill top soil layer, and presence of
VSCs (Volatile Sulphur Compounds) such as CS2 and
CH3SH. The project also comprise studies of the
influence of VSCs on the oxidation of methane.
VSCs show an inhibitory effect on the oxidation
process. Additionally to this, measures of the
efficiency of the landfill gas extraction systems
at the different sites will also be generated.
Total flux measurements are conducted by means
of tracer release at the landfill and time
resolved concentration analysis, as described by
the TCT-method. In order to enable in situ, real
time, sensitive analysis, a temperated, medium
resolution FTIR-system with a multireflection
optical gas cell is used. The system is compact
and stable, allowing rough field conditions and
enabling quick and comfortable transporting
between different measurement locations.
Particularly attractive is the possibility to
observe the quality of the measurement on line,
enabling correction of the measurement, due to
for instance changing meteorological conditions.
Compared to the traditional, manual gas sampling
methods with subsequently laboratory analysis,
this methodology saves a lot of valuable time and
limits the tracer gas emissions. The methane
oxidation is studied by analysis of the change in
isotopic signature between the emitted methane
and CH4 originating from the anoxic zone. The
methane oxidizing organisms in the landfill top
soil cap, have a preference for lighter isotopes,
resulting in an enrichment in both D (deuterium)
and 13C for methane emitted from the soil cap. In
this project mainly the shift in the 13C ratio is
used for the oxidation quantification, but also
deuterium (DCH3) is studied. The isotopic
analysis is carried out by means of high
resolution FTIR. This set up is quite unique and
it allows online incubation studies, that can be
highly resolved in time. Incubation studies are
conducted on soil samples from the different
sites, generating the preference of 12CH4 over
13CH4 for the soil methane oxidizing bacteria,
needed to quantify the methane oxidation
potential. High resolution FTIR is also used to
study the inhibitory effect of VSCs on the
methane oxidation, and to determine the presence
of VSCs in the landfill gas at the different
sites. To enhance the spectral signal of
interest, a cryo gas sampling equipment is used.
This is based on condensation of the sampled gas
by cooling in liquid nitrogen (T77 K). Utilizing
the difference in vapour pressure between the
different gases, the oxygen and nitrogen may be
removed from the sample by decreasing the
pressure in a controlled way. In the remaining
sample a concentration enhancement of about
thousand times is thus obtained .
Figure 1. Correlating concentrations of methane
and tracer gas, N2O, from a total emission
measurement made at a Swedish landfill,
Falköping, using the TCT-method.
Figure 3. The emission of methane from the
landfill in Falköping, Sweden, during one year,
derived with the TCT-methodology.
Figure 4. Methane emission from the landfill in
Falköping during three consecutive days in
December 1998. An emission increase coinciding
with thawing of the ground, but also with an
atmospheric pressure decrease, is observed.
Figure 2. Correlation plot for determination of
the ratio between the leaking methane and the
tracer gas from the TCT measurement in figure 1,
yielding a total methane emission of 44 kg/h.
Figure 6. Absorption spectrum of methane measured
in a gas cell with 56 m optical path, shown
along with a CLS fitted, synthetic calibration
spectrum, corresponding to 2.60 ppm CH4.
Figure 5. Schematic overview of the FTIR
multireflection cell system, which may be
attached to an incubation study device.
Figure 8. Absorption spectrum of methanethiol
(CH3SH), corresponding to 100 ppm-meters.
Figure 9. Lineweaver-Burke-plot, showing the
effect of methanethiol on the methane
consumption rate in a mineral soil, at different
concentrations of methane respectively
methanetiol. A competitive inhibition by CH3SH
is observed.
Figure 7. Map indicating the location of
landfills included in the Swedish landfill
methane project, where total methane emission
measurements will be conducted every second
month during 2000/2001.