2. ENVIRONMENTAL ChEMISTRy & TEChNOLOGy 2.1. Lectures

Chem. Listy, 102, s265–s1311 (2008) Environmental Chemistry & Technology
P73 POSSIbILITy OF ObjECTIVE CONTROL OF
NATuRAL GAS ODORISATION
DAnIEL TEnKRáT, OnDřEJ PROKEŠ and JAn
BERánEK
Institute of Chemical Technology Prague, Department of
Gas, Coke and Air Protection, Technická 5, 166 28 Prague,
Daniel.Tenkrat@vscht.cz
Introduction
natural gas is one of the most important energy carriers
in Europe (in the Czech Republic as well). Total consumption
of natural gas (nG) in the Czech Republic in 2006 was
9,269 × 10 6 m 3 . Within the distribution to end users and also
during natural gas utilization the most important requirement
is the safety of customers.
natural gas odorisation means operations involving the
addition of odorant to gas to ensure characteristic odour of
nG so that a person can judge the odour to be distinctive and
unpleasant so that the presence of gas in air (in concentrations
below lower explosive limit – LEL) is readily detectable.
By the odorant addition any physical or chemical property
(except the smell) of nG cannot be changed. As odorants
organic sulfur compounds are often use (mercaptans and sulfides).
nowadays a new type of sulfur free odorant is being
introduced to the nG market.
The nG odorisation in fact does not have any technological
purpose; its main sense consist in evoking psychological
effect, because the odour of nG must be alarming and
incommutable with any common smell.
In the Czech Republic the odorisation process is specified
by the technical regulation TPG 918 01. This regulation
lists as odorants just organic sulfur compounds. However, it
does not mean that sulfur-free odorants can not be used.
Experimental
The main task of natural gas odorisation is to ensure
such operating condition when natural gas in every part of
the distribution grid fulfils the requirement of a “warning
odour level”. In case of a gas leakage the warning odour level
(see Table I.) must be reached until the 20 % of lower explosive
limit (LEL; Ld ) is reached. Odorisation level can be
verified by:
• The odorisation level control – which can be done by
olfactometry in selected points on distribution grid or by
means of questionnaires at selected representative sample
of customers. In both cases indirect indicators are
taken into account so that both forms are considered to
be subjective methods.
• Odorant concentration measurement – in natural gas can
be estimated continuously or discontinuously in selected
points on grid. In this case particular concentration of
odorant in nG is measured. This is so called objective
method.
s487
The aim of this work was a critical comparison of the
subjective odorisation control (according to TPG 91801)
with the objective odorisation control. It means direct measurement
of an odorant concentration in nG using modern analytical
techniques and comparison with results from olfactometry
measurement.
A n a l y t i c a l E q u i p m e n t
The accurate odorant concentration in nG was estimated
by gas chromatograph HP 6890 equipped with mass
detector MSD 7393 (Hewlett-Packard). For the analysis Supelco
24158 SPB-1 Sulfur (30 m × 320 μm × 4 μm) column
was used. Starting temperature was 30 °C with heating rate
15 °C min –1 . The analysis terminated at 110 °C.
The odorisation level control was estimated by dynamic
olfactometry using olfactometer Ecoma TO 8-8. The procedure
is based on step-by-step evaluation of olfactory perception
of at least four (max. eight) observers. A sample of odorised
natural gas is diluted by synthetic air in ratios between
1 : 131,072 and 1 : 8 (nG:air). As soon as the observer indicates
olfactory perception to be odour treshold (or warning
odour level) the actual dilution of the sample is recorded.
T e r m i n o l o g y
Minimal odorant concentration represents the odorant
content in nG [mg m –3 ] which fulfill the requirement for creating
warning odour level – grade 3 (see Table I).
Estimation of the minimal odorant concentration is
determined by:
K value [mg m –3 •
] which represents the minimal concentration
of an odorant in natural gas-air mixture which
reliably ensures the warning odour level,
• lower explosive limit (Ld ) –expressed by % vol. of natural
gas in air,
•
and from the requirement to evoke the warning odour
level before one fifth (i.e. 20 %) of LEL of natural gas
in air is reached.
Minimal odorant concentration c n can estimated according
to the following formula:
c
n
100 ⋅ K
= [mg m
0.
2 ⋅ L
–3 ]. (1)
d
Typical K values of commonly used odorants are
0.08 for tetrahydrothiophene, 0.03 for mercaptans and
0.07 mg m –3 for the GASODOR S-free odorant.
Odour intensity is the extent of odour perception which
is by the odour evoked. Commonly the odour intensity is evaluated
as an odorisation level. List of odorisation levels can
be found in the Table I.
Results
Two samples of a real odorised “Russian” natural gas
sampled directly from natural gas pipeline into tedlar sam-