Measurement of methane emission
from biogas and biogas upgrading
plants
Recommended methods
September 2015
Measurement of methane emission from biogas and biogas upgrading plants
DGC-report
Table of Contents
1
Page
1
Preface .................................................................................................................................. 3
2
Aim and requirements for methane loss determination ........................................................ 4
3
System/Plant boundaries ....................................................................................................... 5
3.1 What part of the plant should be included? What are the system boundaries? ................... 5
3.1.1 Biogas plants, based mainly on manure .................................................................. 5
3.1.2 Biogas plants, based mainly on wastewater sludge................................................. 6
3.1.3 Biogas upgrading plants .......................................................................................... 7
3.2 Typical places where methane emission can be found ....................................................... 8
4
Detection of individual leakages ......................................................................................... 10
5
Methods for leakage detection and quantification .............................................................. 12
5.1 Remote sensing methods ................................................................................................... 12
5.2 On-site measurements ....................................................................................................... 13
5.3 Remote sensing versus on-site methods ............................................................................ 14
6
The recommended principle ............................................................................................... 15
6.1 Determination of flow ....................................................................................................... 16
6.1.1 High-volume sampler air flow measurement ........................................................ 17
6.1.2 Flow measurement at ventilation grilles in buildings ........................................... 18
6.1.3 Ventilation outlets through ducts and pipes .......................................................... 20
6.1.4 Correcting flow to standard conditions ................................................................. 20
6.1.5 Uncertainty of air flow measurements .................................................................. 21
6.2 Determination of methane concentration .......................................................................... 22
6.2.1 Measuring methods ............................................................................................... 22
6.2.2 Non-methane hydrocarbon cutter .......................................................................... 23
6.2.3 Calibration ............................................................................................................. 23
6.2.4 Uncertainty of methane concentration measurements .......................................... 24
6.3 Sample practice ................................................................................................................. 24
6.3.1 Intrusion of measurement ...................................................................................... 25
6.4 Combined uncertainty ....................................................................................................... 26
7
Operating conditions and peripheral parameters ................................................................ 27
8
Safety aspects ...................................................................................................................... 28
DGC-report
9
2
References ........................................................................................................................... 31
Appendices
Appendix A
Performance criteria of the methane measuring system according to standard
DGC-report
3
1
Preface
During the autumn 2015 the Danish Energy Agency and the Danish biogas
Association will conduct a measurement program for determination methane
loss at Danish biogas plants and biogas upgrading plants.
This report describes the methods that can be applied for detection and
quantification of possible methane emissions from biogas and biogas upgrading plants. There is freedom of choice to apply other methods, but alternative methods must be described and give equal quality in the measurement results.
DGC-report
4
2 Aim and requirements for methane loss determination
The overall aim of the measurement program is to document the sources for
methane emissions from biogas and upgrading plants1. This include

Scanning of the plant for detection of the individual leakages

Quantification of the methane loss from the plants
The scanning is required in order to know where the individual leakages are
and thus the possibility to act on the findings by repairing the found leakages.
1
Methane emission can occur from unknown leakages and/or well-known emission sources
as vents, ducts, ventilation grilles, safety valves etc.
DGC-report
5
3
System/Plant boundaries
To be able to compare different plants, it is important to use the same system/plant boundaries, so the measurements will include the same part of the
plants. The boundaries will be described below.
3.1 What part of the plant should be included? What are the
system boundaries?
In the measurement program, three types of plants are included. These are

Biogas plants, based mainly on manure, see Figure 1

Biogas plants, based mainly on wastewater sludge, see Figure 2

Biogas upgrading plants, see Figure 3
For each of these plants a system boundary is set.
3.1.1
Biogas plants, based mainly on manure
For biogas plants, primarily based on manure, measurements should be
made on all parts and components in contact with the biogas and the substrate on the plants location where gas can be found. This also includes
pickup and delivery locations. The system boundary is therefore set between
where the substrate is received and picked up again. As regards the biogas,
measurements should be made at all components and installations until the
gas is sent out of the plant or consumed locally, e.g. in a gas boiler or engine. A diagram is shown in Figure 1.
DGC-report
6
Figure 1 Example of biogas plant showing system boundaries
3.1.2
Biogas plants, based mainly on wastewater sludge
On a biogas plant based mainly on wastewater sludge, the parts and components that have to be included in the measurements are those concerning the
biogas plant. Roughly, it is the same parts as on a biogas plant based on manure. This means components from where the sludge is received to where it
is picked up. For the gas components it is all components on the plants until
the biogas is sent out of the plant or consumed locally, e.g. in a boiler or
engine. Components regarding the waste water cleaning process are not to
be included in the measurements. On the other hand, components regarding
pre-treatment of the sludge before the biogas plant should be included. A
diagram is shown in Figure 2.
DGC-report
7
Gas grid/Upgrading/Engine
Gas booster
Torch
Gas storage
Dewatering
Digester
Digester tank
Digestate storage
Pickup
Pre-treatment
Waste water plant
Figure 2 Example of biogas plant based on wastewater sludge showing
system boundaries
3.1.3
Biogas upgrading plants
A biogas upgrading plants must be treated as a separate unit, and should not
be included in the measurement done at a biogas plants. This is due to the
fact that an upgrading plant can be placed at a biogas plant, but also at a
separate location. Also, there can be cases where the upgrading plant receives gas from more than one source.
The boundary for the upgrading plant is set where the gas is delivered to the
plant, roughly speaking is “extracted from the ground”, to where the gas is
sent out of the plant. All components, except for compressor to increase the
gas pressure to above distribution level, should therefore be included. A
diagram is shown in Figure 3.
DGC-report
8
Upgrading unit
H2S removal
Gas drying
Gas blower
Odorising
High pressure
compression
Figure 3 Sketch of biogas upgrading plant showing system boundaries
3.2
Typical places where methane emission can be found
Previous biogas leak measuring programs have shown where leaks typically
can be found. An indicative list is shown below. It is important to note that
the list is only indicative and does not cover all possible leak types. A complete scan of the plant, also including other possible leak sources, should be
made in order to determine all leaks.
Some sources emitting methane from biogas and upgrading plants are wellknown emission points due to the nature of the process layout. Examples are
off gas flow from upgrading plants, venting of plant instruments, breather
tubes etc. These sources are not leakages by definition, but must be included
in the measurement program.
DGC-report
9
Joint between concrete
wall and membrane
Leaking flap valve
Holes flange on gas booster
Gooseneck output
Ventilation exit
Stirrer casing
Ventilation tube
Flange
Hole in concrete roof
Manhole flange
and bolt / nut
assembly
Hole in biogas reactor
Dry siphon
feed-through of cables
in the slurry tank
Leaking stuffing box at stirrer
Leaking safety valve
Figure 4 Examples of leakages on biogas plants
Hul forsøgt lappet
DGC-report
10
4
Detection of individual leakages
There are several methods for detection of leaking gas flows. The detection
methods comprise:

Smell, sound or vision

Leak detection spray (foam)

Portable leak detection instrument
o Methane laser
o Semiconductor sensor
o Catalytic sensor
o Infrared sensor, illustrated in Figure 5

IR camera, illustrated in Figure 6
The latter allows for quick scans of large areas. Portable leak detection instruments are cheaper and can be used for systematic maintenance activities,
but are more time-consuming when covering large areas.
Figure 5 Leak detection instruments
DGC-report
11
Figure 6 FLIR GF320 infrared camera that can be used for detection leakages at biogas plants
DGC-report
12
5
Methods for leakage detection and quantification
Two fundamentally different kinds of methods exist for measuring and
quantifying methane emissions/losses/leakages.
5.1

Remote sensing methods

On-site or point measuring techniques
Remote sensing methods
The remote sensing methods are characterised by measuring the total loss of
all sources covered by the path of measurement. The individual source is not
quantified as the method detects the component in the plume. Up- and
downwind measurements ensure correct handling of background concentration.
Open Path systems (FTIR or laser) are used with one or more meteorological stations, and the emission level can be determined by use of dispersion
models. The mobile tracer method combines a controlled release of tracer
gas from the facility examined (a biogas plants). Concentration measurements of tracer gas and the investigated component are performed by using
a mobile high-resolution analytical instrument.
Figure 7, Figure 8 and Figure 9 show examples of remote sensing methods.
Figure 7 Principle of the LIDAR/DIAL method (Source: U. S. Environmental Protection Agency)
DGC-report
13
Figure 8 Quantification with TDLAS and inverse dispersion modelling
(Source: DBFZ)
Figure 9 Quantifying methane emission from fugitive sources by combining
tracer release and downwind measurements (tracer dispersion
method) (Source: Technical University of Denmark, DTU Environment)
5.2
On-site measurements
Measuring methane emissions at biogas plants using on-site methods implies locating the emission spot and subsequently quantifying the emission
rate. Locating unknown sources is referred to as “leak detection” and can be
performed in several ways, see section 4.
Quantification of leaking biogas and other methane emitting sources is done
by measuring coherent values of volume flow and methane concentration.
The volume flow is a biogas/air mixture generated by a high-volume sampler or a direct measurement of the actual methane containing gas flow.
The methane concentration is preferably measured by an FID analyser although other measuring principles are allowed.
DGC-report
14
5.3
Remote sensing versus on-site methods
The remote sensing methods have some obvious advantages like determination of total emission rate, not influenced by plant design, no need for leakage search and encapsulation, and the time effort is partially independent of
plant size. However, the method depends on weather conditions and
topography near the plant. And a very important issue is that the method
does not detect individual leaks, which is needed for subsequent repair.
The on-site measurements technique is flexible, portable and suited for
quantifying individual leakages. This will give the plant owner the possibility of repairing the individual leakages and even make a prioritized selection
of the most severe leaks for instant repairs.
Therefore, the on-site method is recommended by the Danish Energy
Agency and the Danish Biogas Association for quantification of leakages.
The remote sensing methods are, however, accepted by the Danish Energy
Agency and the Danish Biogas Association for the pilot project provided it
is supplemented by a scanning of the plant in order to locate the individual
leakages.
DGC-report
15
6
The recommended principle
When a biogas leakage is detected, e.g. by using an IR camera or a portable
leak detection instrument the amount of leaking gas can be determined. This
can be done by collecting the leaking gas using a blower and a sampling
device that partly covers the location of the leakage.
The sampling device is partly open to the surroundings thus preventing the
pressure inside the sampling device from being affected by the blower, see
the sketch in Figure 10. The blower must deliver a flow that is sufficient to
prevent leaking gas from escaping to the surroundings.
Figure 10 Sketch of the method applied for quantification of a leakage. To
the right is a picture of the first version of the “high-volume sampler”.
The amount of leaking gas is subsequently determined by measuring the
flow of gas, which is a mixture of the leaking biogas and dilution air, and
the concentration of the methane in gas, see Figure 10.
The flow of methane is calculated as
𝑄𝐶𝐻4 = 𝑄𝑔𝑎𝑠 ∙ (𝐶𝐶𝐻4 − 𝐶𝐶𝐻4 𝑠𝑢𝑟 )
Eq. 1
Where
𝑄𝐶𝐻4
is the flow of leaking methane
𝑄𝑔𝑎𝑠
is total flow air and biogas removed by the blower
DGC-report
16
𝐶𝐶𝐻4
is the methane concentration in the air/biogas mixture
𝐶𝐶𝐻4 𝑠𝑢𝑟 is the methane concentration in the surroundings
(background concentration)
The entire system shown in Figure 10 is referred to as a “high-volume sampler” and should consist of:

Sampling hood/sampling device

Antistatic ventilation hose

ATEX approved blower

Control box including safety circuit in case of too high concentration of
methane in the sample gas

Flow measurement

FID instrument equipped with a non-methane hydrocarbon cutter
Use of the high volume sampler is not limited to the gas leakages, but can
be applied at already known emission sources.
6.1
Determination of flow
The purpose of the measuring program is to determine both unknown leakages and the anticipated methane emission from building vents, breather
valves and tubes etc. A major part of these known sources of biogas emission can be quantified by using the high-volume sampler described in section 6.
In some cases, however, the high-volume sampler technique cannot be used
and other methods must be applied:

Buildings with forced ventilation where exit air emits through wallmounted grilles

Biogas/methane emissions venting through ducts, pipes or chimneys
where the air flow is larger than the high-volume sampler flow capacity
or where access is difficult/impossible.
DGC-report
17
6.1.1
High-volume sampler air flow measurement
The flow of the air/biogas mixture evacuated from the leakage is measured
by a built-in orifice2 located downstream the blower. The orifice can be custom-made for a specific flow and pressure range or purchased as a commercially available item at a specific flow range.
The volumetric flow is calculated by:
Q  CA2
2 p1  p2 
 1   A2 / A1 2


Eq. 2
where
Q
C
A1
A2
p1
p2
volumetric flow rate (at any cross-section), m³/s
orifice flow coefficient, dimensionless
cross-sectional area of the pipe, m²
cross-sectional area of the orifice hole, m²
fluid upstream pressure, Pa
fluid downstream pressure, Pa
ρ
fluid density, kg/m³
Figure 11 An example of a flow orifice
The response from the orifice should be controlled or calibrated using a
standard reference method, i.e. a pitot tube, see example in Figure 12.
2
Other flow measuring techniques like pitot tubes, annubars etc. may be used.
DGC-report
18
Figure 12 Example of calibration of an orifice using an L-type pitot tube
6.1.2
Flow measurement at ventilation grilles in buildings
In some cases, methane emissions originate from forced or natural venting
of buildings, e.g. compressor houses, filter houses and other buildings housing technical installations at the biogas plant.
If natural ventilation occurs, the high-volume sampler can be used by
mounting the sampling device to completely cover a ventilation exit grille.
The high-volume sampler then forces an air flow through the building from
the intake ventilation grilles to the exit grille and further through the sampler's orifice for flow determination as described in 6.1.1. The higher the
volume flow, the shorter the stabilization time for achieving the necessary
constant methane concentration.
In situations with forced ventilation of buildings, the obvious way of determining the methane emission is to measure the volumetric air flow and methane concentration quite close to the ventilation grille exit.
DGC-report
19
Figure 13 Example of forced ventilation of a building (sample points
across the grille is indicated to the right)
Flow determination can be done by using a vane or hot-wire anemometer
traversing the entire area of the grille, see Figure 14. This method can be
prone to large uncertainty and, if possible, the forced ventilation should be
turned off and the high-volume sampler should be used instead. If anemometers are used it is recommended to measure in a very detailed grid using
extended number of measuring points. Methane concentration can be measured in a single point if it is proven that no stratification occurs across the
grille surface area.
Figure 14 Example of vane and hot-wire anemometer (Source: TSI)
Another option for measuring the flow from a vent grille is to remove the
part of the duct with the orifice from the high-volume sampler and attach a
flow hood large enough to cover the ventilation exit grille. Mounting the
flow hood/orifice system on the building wall covering the grille makes it
possible to measure the air flow with the same accuracy as using the highvolume sampler.
DGC-report
20
The principle of using flow hoods is well known in the HVAC field where
this technique is used for commissioning, clean-room certification, troubleshooting, testing and balancing HVAC systems. See Figure 15.
Figure 15 Example of flow hoods for measuring air volume flow at diffusers and grilles (Source: Swema and TSI)
6.1.3
Ventilation outlets through ducts and pipes
Quantification of methane emission arising from ducts and pipes can be
difficult using a sample device as sketched in Figure 10. If there is no access
to the outlet of the pipe it may be impossible to place the sample device to
cover the duct outlet. Another obstacle is when the air flow from the duct
outlet is larger than the capacity of the high-volume sampler blower.
In such situations, the air volume flow should be determined by the primary
manual method for measuring volume flow in ducts and channels. This is
done by use of pitot tube and a differential pressure gauge. The method is
described in the standard “DS/EN ISO 16911-1 Stationary source emissions
– Manual and automatic determination of velocity and volume flow rate in
ducts – Part 1: Manual reference method”. Furthermore, the methane concentration of the must be determined. A sample can be taken at the same
location as where the flow measurement is conducted.
6.1.4
Correcting flow to standard conditions
When measuring air volume flow it is important to use correct units to reflect the physical condition of the measurand. Calculating volume flow using Eq. 2 in section 6.1.1 will give an air volume flow result at actual conditions at the place of the orifice with respect to pressure, temperature and
humidity of the air.
DGC-report
21
The mass flow of methane is calculated as the product of the methane concentration in the sample flow and the sample air volume flow. The concentration and volume flow must be converted to the same physical property,
usually standard condition (dry gas, 101325 Pa and 0 °C), before calculating
the mass flow.
Air volume flow measured by the orifice in the high-volume sampler is
converted to standard conditions by:
Qstd  Qact
p atm 273,15  Ts 100  x' ' '
1013 273,15
100
Eq. 3
where
Qstd
Qact
patm
Ts
x’’’
volumetric flow rate at standard conditions, m³/s
volumetric flow rate at actual conditions, m³/s
atmospheric pressure, hPa
sample gas temperature at the orifice, °C
sample gas humidity, % volume
Taking Eq. 3 into consideration, measuring the peripherical parameters of
atmospheric pressure, sample gas temperature and humidity becomes mandatory for further calculations.
6.1.5
Uncertainty of air flow measurements
The uncertainty associated with a measurement of air volume flow depends
on the method used and the conditions on the measuring location. One of
the advantages of the high-volume sampler is the nature of the volume flow
measurement, which is performed under optimal and constant conditions at
every measurement.
According to DS/EN ISO 16911-1, an uncertainty of 2.2-5 % is achievable
depending on type of pitot tube.
Measurement uncertainty using a vane anemometer or a hot-wire anemometer depends to a very large degree on the surrounding conditions. Especially
wind conditions and flow area will affect the accuracy of the measurement.
DGC-report
22
It is estimated that the measurement uncertainty associated to a result obtained by a vane anemometer amounts to approx. 15-40 % although lower
values may be obtained under good conditions.
6.2
Determination of methane concentration
6.2.1
Measuring methods
Methane concentration in a sample gas flow or a vent air stream can be
measured by analysers with several detection principles:

FID (Flame Ionization Detector)

GC (micro GC)

Photo-acoustic infrared spectroscopy

NDIR (Non-Dispersive InfraRed sensor)

FTIR (Fourier Transform InfraRed)
Each principle has advantages and drawbacks regarding selectivity, continuous or semi-continuous operation, precision and accuracy, calibration,
cross sensitivity, ruggedness for field use etc.
The most widely used instruments in Denmark for methane measurements
are probably FID analysers, as this type is recommended by the Danish EPA
for measuring TOC (Total Organic Carbons, which include methane) in
gasses emitted from stationary sources.
A method using the FID analyser in conjunction with a non-methane hydrocarbon cutter is described in the standard DS/EN ISO 25140:2010 Stationary source emissions – Automatic method for the determination of the methane concentration using flame ionization detection (FID). For details of
the standard, please refer to Appendix A.
However it should be noted that the quantification method described neither
relies on, nor is bound to one single method of determining the methane
concentration. There is a freedom of choice as long as the chosen method
complies with performance characteristics which ensure reliable results.
DGC-report
23
6.2.2
Non-methane hydrocarbon cutter
A non-methane hydrocarbon cutter is a catalytic device consisting of a catalyst in a reactor designed by type of catalyst and operating temperature to
combust all non-methane hydrocarbons and to leave only methane.
Operation of a methane cutter is based upon different combustion temperatures of methane compared to other non-methane hydrocarbon compounds.
The methane cutter uses an oxidizing catalyst that is maintained at a temperature specific for the catalyst used, to selectively combust the non-methane
hydrocarbons in the sample stream, while not reacting with the methane
content of the sample.
As the sample stream passes through the cutter, non-methane hydrocarbons
oxidize to CO2 and H2O, which are not detected by the FID. The FID only
measures the unreacted methane in the sample stream. By taking the difference between the methane-only measurement and the total hydrocarbon
measurement, the non-methane hydrocarbon content can be determined.
DS/EN ISO 25140:2010 describes how to check the converter efficiency
with ethane test gas.
6.2.3
Calibration
In general, the analyser used must be calibrated and maintained according to
the manufacturer’s instructions. Specific attention should be drawn to interfering substances (cross sensitivity) and field calibration suited for handling
large ranges of methane concentration.
At field measurements it is good practice to calibrate the FID analyser before and after execution of the measurements with zero gas and span gas in a
suitable concentration. Sometimes it is advisable to perform calibration/control between measurements. This is done during very long measuring periods (> 10 h), or if the analyser is moved between different measuring points. A new calibration is a must if the power has been switched off
during relocation of the analyser.
The methane concentration can take very different values from close to zero
and beyond 10,000 ppm. Therefore, it is recommended to operate with at
least three span gases with different concentrations of methane. One gas
DGC-report
24
should be of low concentration close to zero, e.g. 50 ppm. The other two
could be 2.000 ppm and 9.000 ppm methane. Please note that the balance
gas must be synthetic air in order to avoid oxygen cross sensitivity issues.
Suitable span gas concentration depends on the selected analyser measuring
range and the flow capacity of the sample blower. High methane concentrations from large biogas leaks can be avoided if the sample system is capable
of operating with large air flows.
6.2.4
Uncertainty of methane concentration measurements
The uncertainty of the methane concentration measurement depends on the
analyser specification, the chosen measuring range, zero and span calibration, the surrounding conditions (temperature etc.), interfering substances in
the gas matrix etc.
Chapter 6 in DS/EN ISO 25140 specifies performance criteria and performance characteristics to be evaluated in order to obtain reliable measurement results and to minimize the measurements uncertainty. Table 1 of Appendix 9 in the standard shows the above mentioned criteria from which the
overall uncertainty can be established by setting up an uncertainty budget.
6.3
Sample practice
Methane emissions from biogas come from a number of different sources,
such as safety valves and feedthrough for stirrer of substrate in the reactor.
For information of the different types of leakages please refer to Figure 4.
Different types of leakages require different sampling devices. Examples are
shown in Figure 16.
DGC-report
25
Safety valve
Crevice on curved surface
Figure 16 Examples of use of different sample devices for different types of
leakages
Good sample practice must be maintained throughout the measurements in
order to minimize the impact of the measuring equipment on the leakage
during sampling.
6.3.1
Intrusion of measurement
The quantification method implies that the entire amount of leaking gas
must be collected during sampling. This means that it is necessary that the
blower creates a sufficient flow to prevent biogas from escaping the sampling device. To achieve this, the pressure in the sampling device must be at
least slightly lower than the ambient pressure.
When that is the case the flow of leaking gas is affected by the sampling.
For a well-defined orifice it is possible to calculate the flow based on a
measured pressure difference. Bernoulli’s principle says that the flow rate
across an opening is proportional to the square root of pressure difference
across the opening. This can be described mathematically as
𝑄 ∝ √𝑝2 − 𝑝1
Eq. 4
Where
Q is the flowrate (mass or volume based)
𝑝1 and 𝑝2 are pressures on each side of the opening.
DGC-report
26
This implies that the sampling pressure should be low compared to the pressure drop across the leakages.
In order to be able to assess the conducted measurement it is required that
the sample pressure is measured. Furthermore, the pressure on the biogas
side of the leakage should be stated.
In this way, it is possible to assess the effect of the measurement on the result of the measurement.
6.4
Combined uncertainty
The determined value of the methane loss from a leakage is associated with
a certain uncertainty. Different factors contribute to this certainty. The most
important ones are

Methane concentration measurement

Gas flow measurement,

Sample conditions, like the weather.
On top of that, there is the contribution due to intrusion as described in
6.3.1. The effect of this factor depends heavily on the individual leakages. If
the leakage occurs from a safety valve at a compressor, and the pressure is
high, e.g. 5 bar, the leakage is not affected by the sampling. But if it occurs
at a location with a pressure close to atmospheric pressure, e.g. the biogas
reactor, the leakage is sensitive to the sampling pressure.
The above mentioned uncertainties are all related to quantification of the
loss from the individual leakages. There is also an uncertainty related to
whether all leakages are found. It is hard to estimate this. This uncertainty is
very dependent on weather conditions.
DGC-report
27
7
Operating conditions and peripheral parameters
Measurements should be conducted at normal and typical operating conditions at the biogas plant or upgrading plant. For biogas plants this includes
substrate flow, biogas production, biogas composition, utilization of the
biogas, flaring, systems pressures etc. For upgrading plants the most important operating conditions is system pressures and gas flow.
Prior to the screening and quantification measurements the measuring program and schedule must be discussed with the plant operators in order to
ensure that the plant operates at normal condition and all available and necessary operating parameters can be recorded/accessed during measurements.
Operating parameters may be electronically recorded or read from instrument displays.
Peripheral parameters are secondary measurands needed for converting for
example flow measurements to standard conditions etc. i.e. humidity, temperature, duct pressure, barometric pressure etc. Peripheral parameters have
an influence on the overall accuracy of the measuring results and should be
measured and recorded using well maintained and calibrated instruments.
DGC-report
28
8
Safety aspects
There are a number of safety aspects that must be taken into consideration
when conducting this kind of measurements.
Biogas and air mixtures are flammable if the ratio between the two is in a
certain range. In order to eliminate the risk of explosions and fire different
precautions should be made. Examples of these are:

The methane concentration in the measured flow should not exceed 25
% of the lower explosion limit corresponding to 10,000 ppm.

If a plastic tube is used between the sampling point and the blower this
may cause static electricity. Therefore, the velocity of the gas should be
higher than the flame speed of the gas. This means that if the gas is ignited in the tube, it will be carried to the blower instead of being moved
towards the reactor.

An ATEX approved blower should be applied.

Biogas contains H2S, which is very toxic, see Table 1. Therefore, it is
important to ensure that the collected gas is carried away from the work
space of the persons conducting the measurement.

As some of the leakages can be at the top of reactors, work is partly to
be conducted at elevated heights, see Figure 17, and adequate precautions must be taken.

If a leakage is detected and it is assessed that is not possible to quantify
the loss applying reasonable precaution the quantification can be omitted. However, the found leakage must be mentioned in the reporting and
an argument for omitting the quantification must be stated.
DGC-report
29
Table 1
Effects of H2S exposure at different concentrations. From
www.osha.gov.
Concentration
(ppm)
Symptoms/Effects
0.00011-0.00033
Typical background concentrations
0.01-1.5
Odor threshold (when rotten egg smell is first noticeable to
some). Odor becomes more offensive at 3-5 ppm. Above 30
ppm, odor described as sweet or sickeningly sweet.
2-5
Prolonged exposure may cause nausea, tearing of the eyes,
headaches or loss of sleep. Airway problems (bronchial
constriction) in some asthma patients.
20
Possible fatigue, loss of appetite, headache, irritability, poor
memory, dizziness.
50-100
Slight conjunctivitis ("gas eye") and respiratory tract irritation
after 1 hour. May cause digestive upset and loss of appetite.
100
Coughing, eye irritation, loss of smell after 2-15 minutes
(olfactory fatigue). Altered breathing, drowsiness after 15-30
minutes. Throat irritation after 1 hour. Gradual increase in
severity of symptoms over several hours. Death may occur
after 48 hours.
100-150
Loss of smell (olfactory fatigue or paralysis).
200-300
Marked conjunctivitis and respiratory tract irritation after 1
hour. Pulmonary edema may occur from prolonged exposure.
500-700
Staggering, collapse in 5 minutes. Serious damage to the eyes
in 30 minutes. Death after 30-60 minutes.
700-1000
Rapid unconsciousness, "knockdown" or immediate collapse
within 1 to 2 breaths, breathing stops, death within minutes.
1000-2000
Nearly instant death
DGC-report
30
Figure 17 Sampling of leaking gas at the top of a biogas reactor
DGC-report
31
9
References
/1/
DS/EN ISO 25140:2010 Stationary source emissions – Automatic method for the determination of the methane concentration
using flame ionization detection (FID)
/2/
DS/EN 12619:2013 Stationary source emissions – Determination
of the mass concentration of total gaseous organic carbon - Continuous flame ionisation detection method
DGC-report
32
Appendix A
Performance criteria of the methane measuring system
according to standard
The table below is copied from the standard DS/EN ISO 25140 Stationary
source emissions - Automatic method for the determination of the methane
concentration using flame ionisation detection (FID) and indicates the performance criteria of the analyser and measuring system used for determination of methane according to the standard.
Table 1 Relevant performance criteria of the analyser and the measuring system
to be evaluated during the general performance test and by means of
ongoing QA/QC in the laboratory and during field operation