A PROMISING NEW ON-LINE METHOD OF TAR QUANTIFICATION BY MASS SPECTROMETRY DURING
STEAM GASIFICATION OF BIOMASS
F. DEFOORT, S. THIERY, S. RAVEL, F. LABALETTE#, M. CAMPARGUE*
CEA, DRT, LITEN/DTBH/LTB, 17 rue des Martyrs; 38054 Grenoble Cedex 9; France
#
GIE-Arvalis ONIDOL Paris France * RAGT Energie SAS Albi France
Corresponding author: [email protected] – phone: 33-(0)4 38 78 46 53 – fax: 33-(0)4 38 78 52 51
ABSTRACT: This paper presents a method of measuring on line ppbv level of BTX and PAH tar for biomass
gasification applications. This method is based on the Ion Molecule Reaction (IMR) mass spectrometer principle.
Due to the low energy of ionisation of Hg, Xe or Kr almost no overlapping spectra can damage the interpretation of
the detected results. IMR-MS results of concentrated and trace tars are presented after a bubbling fluidized bed and a
tar cracker respectively. Results are compared with others tar measurement methods such as micro gas
chromatography for benzene and toluene or Tar Protocol or Solid Phase Adsorption technique for BTX and
Polycyclic Aromatic Hydrocarbon (PAH) giving good confidence to the results.
Keywords: tar, measurement, gasification, biomass, syngas
1
INTRODUCTION
Contaminants such as tar present in the gas obtained
from the biomass gasification are important to be
measured online for detecting rapidly their variations
which may induce saturation of the gas cleaning systems
or degradation of the catalysts used for fuel synthesis.
Furthermore it is necessary to find an online method able
to measure tar at ppbv level after gas cleaning systems.
On line measurements of BTX (Benzene, Toluene,
Xylene)
are
currently
made
by
micro-gas
chromatography with a thermal conductimetry detector
(µGC-TCD) above several ppmv (50 mg/Nm3) [1].
Heavier tars PAH (Polycyclic Aromatic Hydrocarbon) as
naphthalene by FTIR (Fourier Transform Infra-Red) [2]
or naphthalene, fluoranthene and pyrene by LIF (Laser
Induced Fluorescence) [3], [4], [5] can also be measured
on line directly in the gas phase but again above several
ppmv level for FTIR or present strong interferences with
other species present in the gas for LIF. Molecular beam
mass spectrometry (MBMS) is an on line method to
measure all tars from BTX to PAHs and even more [6]
using a specific gas sampling to collect condensable
species thanks to a molecular beam inlet system. But this
method suffers from the conventional high ionization
energy (70 eV) due to the electronic impact method.
Therefore the major draw-back with this instrument is the
fragmentation of molecules leading to complex mass
spectra and multiple overlapping intensities in complex
gas mixtures. To overcome this limitation several soft
ionizations have been used with photoionization
techniques such as Resonance-enhanced multiphoton
ionization (REMPI) or single-photon ionization (SPI)
with Time-of-flight mass spectrometry (TOFMS) [7],
however, only identification and no quantification are
performed. Other methods measuring online total tar
contents exist, such as the ECN Tar Dew Point [8], the
FID (Flame Ionization Detector) tar analyzer (TA 120-3)
currently used by IFK U. of Stuttgart [9] or the PID
(Photo-ionization detection) tar analyzer recently
developed by [10] but the nature of the species is not
known for these methods.
This paper presents a soft ionization mass
spectrometry method based on the Ion Molecule Reaction
(IMR) mass spectrometer (MS) principle. Such IMR-MS
method has already been used to measure online major
components (CO, CO2 etc) as well as some inorganic
(SO2, NO) or organic (benzene, acetaldehyde) pollutants
at the exhaust of incineration processes [11], [12].
However this method has never measured the
composition of a syngas from a biomass gasification
process.
2
EXPERIMENTAL
2.1 IMR-MS
Principle of the method: The IMR method is wellknown in ion physics. It is an important path of primary
ion loss in a plasma and can be represented as:
A+ + G = G+ + A
[eq.1] Ion molecule reaction
The monograph by Harrison is especially
recommended for further reading [13]. A simplified
description is given in our paper.
IMR-MS method is similar to CI-MS (Chemical
Ionization Mass Spectrometry) but one of the differences
is that the primary ion source is run with an inert gas such
as Hg, Xe, Kr instead of a chemical gas such as NH3,
CH4 or N2/H2O mixture. The main pathway in IMR-MS
to ionize a sampling gas G with a primary inert gas ion
A+ (A= Hg, Xe or Kr) is a charge transfer as in [eq.1].
For CI-MS more complex ionization pathways are
occurring as protons transfer [Eq.2]
G + [NH4]+ → [GH]+ + NH3
[eq.2]
The principle of the IMR-MS method is shown in
Figure 1.
Chamber of
primary ion
generation
Reaction
chamber
Quadrupole ion
separation
Octopole ion guide
A
A+
Inert gas
A = Kr, Xe or Hg
A + e-
A+ + 2 e-
Ion detector
G+
Mass
spectra
sampled Gas G
A+ + G
G+ + A
Ion Molecule Reaction
Figure 1: IMR-MS principle (partly reported from
[14])
An inert gas (Hg, Xe or Kr) is ionized by electron
impact in the chamber of primary ion ionization as:
A + e = A+ + 2e
technical characteristics are summarized in Table 1. The
apparatus is equipped with a combined primary ion
source for Kr, Xe and Hg.
[eq.3] primary ionisation
Primary ions are conducted to the reaction chamber
through a high frequency octopole ion guide. Here the
primary ions react with the sampled gas G with the Ion
molecule reaction [eq.1]. Contrary to CI-MS, the
chamber of primary ion generation and the reaction
chamber are separated in IMR-MS. In addition, the Ion
molecule reaction happens with a high vacuum of ~10-4
mbar for IMR-MS (CI-MS about 1 mbar). The ions G+
generated are conducted to a classical quadrupole and a
downstream counter to be separated (mass separation),
detected and quantified.
The ion molecule reaction [eq.1] can only occur if the
ionisation potential of the primary ion A is higher than
the one of the gas sample G. Figure 2 shows the
ionisation potential of several molecules of interest and to
the inert gas Hg, Xe and Kr.
Figure 3: photo of the Airsense apparatus
Table 1: Technical details of the Airsense apparatus
Mass range
Resolution
Analysis time
Measuring range
Response time
Lower detection
limit
Drift concentration
Reproducibility
Accuracy
Figure 2: ionization potential of several molecules
For example the ionisation potential of mercury
(IP(Hg)= 10.43 eV) is higher than the one of BTX, PAHs
and thiophene (C4H4S). If the excess energy IP(Hg) –
IP(G) is small (~< 1 eV) no fragmentation will occur
otherwise it might be used to break the weakest bond of
the ionised molecule leaving a lower molecular weight
fragment ion. Mercury is able to ionize these species
almost without fragmentation as the Hg excess energy is
small. Figure 2 shows also that NH3, H2S and C2H4 can
be ionized by mercury. Xenon (IP(Xe)= 12.12 eV) is able
to ionize COS, CH4, C2H2, C2H6 and H2O. Kripton
(IP(Kr)= 13.99 eV) is able to ionize CO, CO2, HCl and
HCN. Finally all syngas molecules can be measured by
these three inert ionized gases. Mercury primary ionized
gas is the best case as it is not sensitive to species having
higher ionization potential than Hg. The Hg mass
spectrum is simpler than the Xe one that is itself simpler
than the Kr one. A careful study must be done with Xe
and Kr to check if there is a mass overlapping due to
isotopes and fragmentation.
Apparatus tested: The IMR mass spectrometer
apparatus tested in this work is called “Airsense” and it
has been developed by an Austrian company (V&F
analyze: www.vandf.com). It was rented for one month at
CEA grenoble. It is currently used by V&F customers
mainly for automotive, food, beverages & tobacco
applications but also for medical use [14], [15]. It has
never been used to analyse syngas. A picture of the
apparatus is shown in Figure 3. It is very compact
(59x65x73cm) and easy to carry (100kg - wheels). Its
0 – 512 amu
< 1 amu
> 1 msec
104
T90 < 50 msec
< 1 ppb (Benzene in air)
< 10 ppb (Benzene in exhaust
gas)
< ± 5% over 12 h (1ppm
Benzene)
< ± 3% (1ppm Benzene)
< ± 2% (1ppm Benzene)
To avoid condensation of water and condensable
species (tars, NH4Cl) a heated (180°C) low pressure
capillary samples the gas of the process (Figure 4). A
small filter is positioned just before the capillary to avoid
clogging by dust or deposits. A dilution with a neutral gas
is possible just entering the vacuum chamber.
Figure 4: photo of the heated capillary sampling line
The software is able to pilot the apparatus with two
modes:
1- The “scan” mode where all masses from 1 to 500 amu
are scanned for each ion source. This mode allows
isotopes and overlapping to be checked and the choice of
the correct amu to record during the process.
2- The “monitoring” mode, in which more than 20 mass
amu for each of the three ion sources are recorded versus
time. This allows the tracking of the process versus time
for the selected amu.
2.2 Gasification tests and tar measurement methods
Gasification tests were performed to measure tars in
concentrated as well as in trace amounts. Concentrated
tar measurements were performed at the exhaust of the
fluidized bed facility (LFHT) developed at CEAGrenoble and traces at the exhaust of the tar cracker (gas
reforming) reactor (PEGASE). Both facilities are linked
by an insulated line, heated to 800°C in order to avoid
condensation of inorganic species. Full description of
both facilities is emphasized in [16]. Steam gasification
of straw at 850°C 2 bar (S/B = 2g/g) 1kg/h has been
performed in the fluidized bed. The temperature of the tar
cracker has been set at 1400°C. Figure 5 is a schematic
view of the facilities where tars sampling are presented.
Figure 5: Schematic view of the pilot scale facilities
LFHT linked to PEGASE and gas analysis
methods allow measurements of BTX and PAHs. TP
allows measurement above ppmv and SPA above roughly
10 ppbv [1].
3
RESULTS
3.1 Mass spectra
An example of syngas mass spectra acquired on wet
gas (dilution 4:1) at the exhaust of the fluidized bed with
the mercury ions source is presented in Figure 6a and 6b
for mass 10 to 100 and 100 to 200 respectively.
Fluidized bed
a: mass 10 to 100 amu
Fluidized bed
Sampling is possible before and after the cold traps
for both facilities.
The IMR-MS measurements were performed in both
places to check the accuracy of results in dry and wet gas
and to measure steam content on wet gas. A dilution
factor of 4 with nitrogen was applied for wet sampling
before cold traps. No dilution was applied on dry gas
after cold trap. Calibration of the apparatus was done for
BTX, steam and some other components (H2S, COS,
NH3, HCN, HCl, CH4, C2H4). No calibration was done
for naphthalene, PAH and thiophene. It is known by V&F
that the sensitivity of these components is very close to
benzene. Then the same cps/ppmv was given to these
components. IMR-MS records are every 25 seconds.
Other tar measurement methods were performed in
order to compare the results between them and the IMRMS one.
BTX tars C6H6, C7H8 C8H10 are measured on line by
micro-Gas Chromatography (µGC) coupled with
Thermal Conductivity Detectors (TCD). As dry gas is
necessary for this apparatus, gas is sampled after the cold
traps. Other species are also analyzed by µGC as the
analyzer is equipped with four columns allowing the
measurement of Ar, CO2, CO, CH4, N2, H2, C2H2, C2H4,
C2H6, C3H8 and H2S and COS. Every three minutes a
spectrum is acquired. This method allows measurement
of BTX above several ppmv.
Two other tar measurements methods are also
implemented: the well known tar protocol method (TP)
[17] and the solid phase adsorption SPA method [18].
Both methods are off line and based on condensation of
tar in liquid phase (isopropanol) thanks to an impinger
train for TP and in a solid amino phase adsorbent for the
SPA. Gas is sampled before the cold trap in order to
measure condensable heavy tar such as PAHs. Liquid
samples of TP and SPA probe are analyzed later by gas
chromatography (GC) with a flame ionization detector
(FID) and after a thermal desorption for SPA. Both
b: mass 100 to 200 amu
Figure 6: Mass spectra at the exhaust of LFHT (wet gas)
It is observed that the masses corresponding to
benzene, toluene, thiophene, phenol and some PAHs
(indene, acenaphthylene, biphenyl + acenaphtene,
fluorene, phenanthrene+ antharcene) are well defined.
The other masses observed are from their isotopes.
Calculations of the isotopic ratio are in line with the
intensity ratio measured in Figure 6. Furthermore it is
observed that some others masses corresponding to some
inorganics such as H2S NH3 or hydrocarbon C2H4 are
present (Figure 6a).
3.2 Concentrated tars
Figures 7 to 11 present the concentration of benzene,
toluene, thiophene some PAHs (indene, acenaphthylene,
biphenyl + acenaphtene, fluorene, phenanthrene+
antharcene) recorded versus time at the exhaust of the
fluidized bed before the cold traps in wet gas (12:00 to
13:25) and after the cold traps in dry gas (13:25 to
14:40).
In these figures the dilution correction and the
wet/dry correction have been done to present all results as
ppmv dry with neutral gas included. Results obtained are
in agreement with expected BTX and PAH values
currently obtained in these conditions, i.e. in the 1000
ppmv level for benzene, 50 ppmv for naphthalene, 10
ppmv for toluene and 1 ppmv for PAHs [16], [19].
A very good agreement is observed for Benzene and
toluene between the four analytical methods IMRMS/µGC-TCD/TP/SPA. Results are in the 15% relative
uncertainties range.
Fluidized bed
Fluidized bed
PAH
Benzene (78, Hg)
Figure 10: PAH concentration (fluidized bed)
Wet gas
dry gas
Figure 7: Benzene concentration (Fluidized bed)
Fluidized bed
Wet gas
Toluene (92, Hg)
dry gas
Figure 11 presents the IMR-MS results for thiophene
which is between 1 and 10 ppmv. Thiophene is not
calibrated and there is no other method to compare.
However, it is seen in the literature that thiophene is
measured in the 1 to 10 ppmv range in fluidized bed
gasification [20].
Fluidized bed
Wet gas
Thiophene (84, Hg)
dry gas
Figure 8: Toluene concentration (Fluidized bed)
Figure 11: Thiophene concentration (Fluidized bed)
Naphthalene IMR-MS results shown in Figure 9 are
lower than TP and SPA (factor 2). Taking into account
that naphthalene is not calibrated but is linked to the
benzene response, the agreement is acceptable.
Fluidized
bed
Naphthalene (128, Hg)
Wet gas
dry gas
Figure 9: Naphthalene concentration (Fluidized bed)
PAH results, shown in Figure 10, indicate that IMRMS results are in between TP and SPA results. As
naphthalene, PAHs are not calibrated but are linked to
benzene response.
Accidents observed in the IMR-MS (and in the µGCTCD) records which are in the 10 to 20% relative
uncertainties are not significant to process events.
However, the accidents above these uncertainties are
significant. As an example, it is observed that the
apparatus is disconnected to the syngas flux during the
TP. A strong decrease of all components is thus observed
25 seconds later. In Figure 11 strong variations of the
IMR-MS signal between 10:55 and 11:02 which are
related to process event (over pressure in the reactor) are
observed . This shows the capability of and the interest in
having an on line method even if the example cited is
also observed with pressure sensors. A study should be
made with compositional change (biomass composition
in homogeneity or saturation of a gas cleaning unit).
3.3 Trace tars
The Figures 12 and 13 show the concentration of
benzene and some PAHs. The other tars were
undetectable.
In these figures it is observed that results are more
consistent in dry gas rather than in wet gas because there
is no correction (dilution and dry gas i.e. factor 8) to
apply to the dry measurements as it is necessary for the
wet one. Indeed, before correction, measurements in wet
gas are in the ppb level which is close to the sensitivity
limit of the apparatus. Results obtained are in agreement
with expected BTX and PAHs values, i.e. hundred and
tenth of ppbv respectively, which have already been
obtained in this very high temperature condition [16],
[21].
Due to the low concentration measured (< 1 ppmv),
there are only SPA results available giving acceptable
results with IMR-MS.
Tar cracker
Tar cracker
CH4 (16, Xe)
Benzene (78, Hg)
Wet gas
dry gas
Figure 15: CH4 concentration (Tar cracker)
Wet gas
dry gas
Fluidized bed
H2O (20, Xe)
Figure 12: Benzene concentration (Tar cracker)
Tar cracker
PAH
Figure 16: H4O concentration (Fluidized bed)
Wet gas
dry gas
Tar cracker
H2S (34, Hg)
Figure 13: PAH concentration (Tar cracker)
3.4 Other results
Good results are also obtained for other components
present in the syngas as C2H4, CH4, H2O, H2S, COS
(Figure 14 to 18). These results are always in some
agreement with another method giving confidence in this
IMR-MS method for such syngas species. However some
improvements must be done for some species such as
COS, NH3, HCN, and HCl. For example for COS
(Figure 18), it is observed that it is correctly measured in
mass 60 with the Xenon primary ion in dry gas. But in
wet gas, there is a strong overlap which must be
identified and then taken into account in the software.
Fluidized bed
Wet gas
dry gas
Figure 17: H2S concentration (Tar cracker)
Fluidized bed
COS (60, Xe)
C2H4 (28,Hg)
dry gas
Wet gas
Figure 18: COS concentration (Fluidized bed)
Wet gas
dry gas
Figure 14: C2H4 concentration (Fluidized bed)
7
CONCLUSIONS
The IMR-MS method has measured concentrated and
traces tar on line successfully for the first time in a real
syngas at the exhaust of the high temperature fluidized
bed reactor (LFHT) at the CEA Grenoble during steam
gasification of straw and at the exhaust of the high
temperature reactor tar cracker (PEGASE).
Comparison with other tar measurements methods
such as µGCD-TCD, the Tar Protocol and SPA accords
having high confidence in this new promising method.
Some improvements should be made, such as the
calibration of the IMR-MS apparatus with PAHs and
thiophene to have higher confidence in the measurement.
A major interest of the method is its capability to
measure major gas, tar and inorganic pollutants in one
apparatus. To achieve this objective, an analysis of the
overlaps due to fragmentation at higher ionisation
potential than Hg should be done. To help users to
interpret their mass spectra it would be very useful to
build a fragmentation database coupled to an isotope
database at such low ionization potential.
This method has a potential for process control
monitoring as well as research and development
applications on tar behavior.
8
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9
ACKNOWLEDGEMENTS
This work was performed within the AMAZON projects,
partially funded by the French National Research Agency
(ANR).