Tar measurement in synthetic gas produced by plasma gasification by
solid phase microextraction (SPME)
M. Hlina, M. Hrabovsky, T. Kavka, M. Konrad
[email protected]
Institute of Plasma Physics, ASCR, Za Slovankou 3, Prague 8, Czech Republic
Abstract: There are many sampling methods for tar analysis. Solid phase
microextraction (SPME) is one of them. SPME method might be used as a
sampling technique for tar measurements in synthetic gas (syngas) produced in
plasma gasification process. Gasification is carried out in the reactor equipped
with hybrid gas-water stabilized DC torch. The plasma torch features relatively
low mass flow rate of plasma and very high enthalpy. The previous results
revealed low tar content that is in good accordance with high reaction
temperature and also with the possibility of using SPME sampling method.
Keywords: tar, sampling, gasification, solid phase microextraction, SPME
1. Introduction
The plasma gasification of biomass is studied very
intensively nowadays. The concentration of major
gas components such as hydrogen and carbon
monoxide are always carefully monitored; however,
further utilization of produced gas is also strongly
influenced by tar content. What is tar? Webster
dictionary gives us a general answer: “a dark brown
or black bituminous usually odorous viscous liquid
obtained by destructive distillation of organic
material (as wood, coal, or peat)” [1] that is
definitely correct, but scientific approach needs
more accurate definition. Nevertheless, surprisingly
there is not just one right definition, but more than
30 in 1999 [2]! Tar is the sum of organic compounds
with boiling point above the boiling point of
benzene (80.1 °C) accordingly to Tar Protocol [3, 4]
and many other related sources. Tar can be classified
with respect to its physical properties [5] which are
also closely related to GC measurements aspects
(Table. 1).
The lower tar content the better but some
applications assume relatively high tar concentration
and possible usage of almost “tar-free” syngas
would economically handicap them. For example for
engines the maximum tar content of 100 mg/Nm3 is
acceptable [6] but for Fischer-Tropsch synthesis tar
concentration has to be less than 0.1 mg/Nm3 [7].
Tar yield in gasification process might be also
plotted as the function of the temperature of
gasification, the higher temperature the lower tar
yield. Minimum is achieved approximately at
temperature of 1000 °C and higher [8].
2. Tar sampling & separation
Sampling and separation are due to the huge range
of the physical properties of tar a key-step at tar
measurement. While upstream pointed heated probe
with heated particle filter is an ordinary setup,
sample preparation (condensation, concentrating)
and sample modifying for further analysis gives us
wider choice of possibilities. There are two
techniques preferably used for tar sampling:
-
cold trapping
-
solid phase extraction (SPE)
Cold trapping is the most used method but it requires
a lot of manual work and is also very time
consuming. Frequently used setup is in Fig. 1 [9] syngas is forced to flow through several impinger
bottles filled with isopropanol in series connection.
Table 1. Tar classification
Tar trapped in solvent is after decanting and
concentration analyzed.
after sorption, solvent with dissolved tar
concentrated and then analyzed. It is possible to do
very rough estimation of tar content (especially of
heavier fraction) by simple weighing of unheated
filter before and after the passing of syngas.
3. Solid phase microextraction (SPME)
SPME is widely used as a sample preparation
technique in other fields but there is also possibility
to use it for tar sampling and extracting.
Figure 1. Possible setup for tar collection – impinger bottles.
Solid phase extraction (SPE) uses unheated sorbent
with high affinity to tar connected to a sampling duct
as a filter. Sorbents have to be sometimes activated
by rinsing with a proper solvent. Sorbent is washed
SPME involves the use of a fibre coated with an
extracting phase. When the fibre gets to contact with
sample the quantity of extracted analyte is
proportional to its concentration in the sample as
long as equilibrium is reached. SPME fibre is
transferred to the injection port of separating
instruments, such as a gas chromatograph, where
desorption of the analyte takes place and analysis is
carried out. The attraction of SPME is that the
extraction is fast and simple and can be done without
solvents, and detection limits can reach parts per
trillion (ppt) levels for certain compounds [10, 11].
Several polymer coatings were tested for tar
measurements
at
French
Atomic
Energy
Commission and 100 μm polydimethylsiloxane
(PDMS) fibre was the most suitable. It allows
extraction of the wide range of compounds and it
features also a good repeatability. Possible setup for
SPME tar extraction is in Fig. 2.
stabilization of arc. This torch features very high
plasma temperature together with relatively low
mass flow rate [12]. Inner volume of the reactor is
0.22 m3 and it is insulated by 400 mm of ceramics
from water-cooled steel walls. Gasified material is
supplied to the reactor by a screw feeder that enters
in the proximity of the nozzle of the torch [13].
Total input power of the torch during the
measurement of presented results was 95 and 138
kW. Saw dust from spruce with humidity of 20%
was used as a gasified material. The reactor was
operated at a wall temperature from 1100 to
1400 °C. Syngas was a gas mixture that comprises
mainly hydrogen and carbon monoxide. Tar content
was very low (Table 2) [14].
The measurements of the content of tar were based
on solid phase extraction method (SPE) using DSCNH2 phase or silica gel as an adsorbent. The
samples were analyzed by means of gas
chromatography.
Table 2. Content of polyaromatic hydrocarbons in syngas
Figure 2. Possible setup for SPME tar extraction from syngas
(sampling line).
A glass bulb is vacuumed before syngas enters.
When the glass bulb is filled with syngas (pressure
equals to reactor pressure and is roughly constant
during all experiments and calibration) and thermal
equilibrium (at a given temperature) is reached,
SPME fibres are inserted and let in the vessel for
certain time (till adsorption equilibrium is reached).
Then the fibres are desorbed in GC injector and tar
content analyzed.
Distribution coefficients between fibre and syngas in
the glass bulb can be calculated, however calibration
for concrete compounds and conditions is believed
to be more accurate.
4. Plasma gasification
Gasification was carried out in the reactor equipped
with the thermal plasma torch with gas-water
Results revealed low polyaromatic hydrocarbon
concentrations (that is in good accordance with high
reaction temperature) which might meet detection
limits of SPE method. Simple increasing of the
volume of the syngas passing the absorbent is not
usually possible due to low porosity of the absorbent
and fast sticking of it.
5. Conclusions
SPME is a new promising method for tar
measurement from this point of view apart from the
fact that SPME is much simpler, faster and no
hazardous solvents are needed.
Acknoledgement
The authors gratefully acknowledge the support by
the Grant Agency of the Czech Republic under the
project No. P 205/11/2070.
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