LOS GATOS RESEARCH CASE STUDY
Fast Industrial Process Monitoring:
Trace H2S Measurements in Reformed JP-8 Fuel
Authors: Subir Roychoudhury and Christian Junaedi, Precision Combustion, Inc., North Haven, CT,
and Feng Dong and Manish Gupta, LGR, Inc.
Introduction
Precision Combustion, Inc. (PCI) has evaluated LGR’s H2S
portable electronics applications because of the weight and
Analyzer for monitoring trace levels of hydrogen sulfide
lifetime limitations of current battery technologies. Fuel
(H2S) in real-time in reformed military logistics fuel (JP-8)
cells are an attractive potential source of electrical power
processed in an autothermal fuel reformer. Measurements
for these applications because they produce less noise
were made before and after the reformed JP-8 fuel was
and zero emissions; this makes them harder to detect in
passed through a desulfurizing bed downstream of the
covert operations and they are demonstrably greener than
reformer. This is an important “proof of principle” application
traditional power sources.
since the high hydrogen content in reformed liquid fuels
and other hydrocarbons (e.g., Syngas) is widely viewed as
a convenient feedstock to power fuel cells with efficiencies
as high as 60%.1 However, even trace (ppm) levels of sulfur
compounds in reformate gas can quickly deactivate the
cells by poisoning their anodes.2 Moreover, existing sulfur
monitoring technologies have significant limitations for this
application. This is also a challenging application for any
gas analyzer since the gas reformate is a complex mixture
containing a wide range of concentrations of other species.
This makes high selectivity, i.e., immunity to interference, a
mission-critical requirement.
Reformed Fuels and Desulfurization
Since their early use in the NASA space program, the use of
fuel cells has been pioneered in numerous applications as
an alternative to grid power and/or battery storage. Systems
up to hundreds of kilowatts have been demonstrated for
stationary back-up (e.g., hospital) applications. The US
military and others are considering replacing battery packs
with solid oxide fuel cells (SOFCs) in certain mobile and
There are several types of fuel cells; however, they all produce
electricity by oxidizing hydrogen (or sometimes a simple
hydrocarbon, such as methanol) into water. This is a relatively
cold process that occurs catalytically between an anode and
a cathode separated by some type of electrolyte. Suitable
quantities of hydrogen can be created by “reforming”
petrochemical fuels (e.g., diesel, kerosene, military logistic
fuels, JP-8, etc.) or by gasification of coal in a Syngas
generator. However, all these fossil fuel-derived sources of
hydrogen contain high levels of sulfur compounds, such as
mercaptans. These can poison the catalysts used in early
type fuel reformers, but sulfur-tolerant alternatives have
been successfully developed where the reductive reforming
conditions also convert the miscellaneous sulfur compounds
into hydrogen sulfide (H2S). However, this H2S has to be
scrubbed from the reformed fuel mix because even trace
(ppm) levels of H2S or other sulfur compounds (e.g., COS)
will quickly poison the anodes used in SOFCs. This sulfur
scrubbing is accomplished using a sulfur sorbing “bed”
located between the reformer and the fuel cell.
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Each sulfur removal bed has a finite capacity to absorb
sulfur compounds (principally in the form of H2S) before it
must be replaced. For example, military specifications using
reformed JP-8 are typically targeted to handle 100 ppm (by
weight) sulfur for 1000 hours. The finite lifetime, and the
potential of catastrophic, sulfur-induced damage to SOFCs,
has thus created a clear need for a method to detect trace
H2S in gas streams of varying complexity and composition.
This requirement covers developers, manufacturers and
users of fuel reformers, sulfur removal beds, and SOFCs.
Unfortunately, both types of established sulfur sensors are
not suitable for this application. Specifically, optical sensors
based on UV absorption are susceptible to interference from
UV absorption by the high levels of CO2 in reformed fuels.
On the other hand, solid-state sensors based on doped
semiconductor films exhibit interference with water vapor
and carbon monoxide (CO), which are both present in high
Figure 1. This data shows the linearity of the LGR analyzer over a
very wide dynamic range. The sample points were obtained using
1054 ppm H2S in N2 that was precision diluted in dry air using
mass flow controllers.
concentrations in reformed fuels. Also, their response is
non-linear, restricting their absolute accuracy to a very small
dynamic range (e.g., 0-10 ppm H2S).3
Combining OA-ICOS with Chemometric Analysis
LGR analyzers monitor trace gas concentrations and/or stable
isotope ratios by means of a sensitive optical absorption
measurement using an integrated tunable laser that delivers
multiple decades of linear dynamic range – see figure 1.
But, unlike older optical techniques, such as conventional
cavity ringdown spectroscopy (CRDS), LGR analyzers are based
on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS),
which delivers a detailed scan of an extended spectral region
with every measurement. This is a critical advantage in this
reformed fuel application because of the presence of potential
interference from absorptions by numerous other gases in
the flow. As a result, several gases may be simultaneously
quantified from the measured high resolution absorption
spectra using standard chemometric techniques.
Figure 2. This figure shows fully resolved absorption spectra
(denoted “convoluted spectra”) recorded by the LGR Analyzer, as
well as absorption spectroscopic basis sets of individual species
(denoted for H2S, CO2, H2O, CH4 and C2H4) in the wavelength
region near 1.59 microns. These detailed spectral scans, along
with a proprietary chemometric fitting routine, allow simultaneous
measurements of multiple constituent gases in complex flows.
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Chemometric software packages are widely used in near-IR
spectroscopy and other fields. First, in the “learning phase,”
the instrument is used to record data spanning the full extent
of expected conditions – in this case, multiple different H2S
trace concentrations accompanied by deliberate variations
in the host gas composition. The software then fits the
known H2S (and CH4, C2H4 and CO2) concentrations to the
observed spectral data using multivariate analysis. This yields
an algorithm that the instrument’s processor uses to convert
future data into absolute concentrations of all these gases.
Overview of Results
After confirming the linearity of the LGR analyzer in
unmixed samples (see figure 1), we then conducted
extensive tests of the LGR analyzer’s precision and
Figure 3. Typical data from in situ measurements of the product
stream from a running JP-8 fuel reformer
accuracy using precision diluted and mixed realistic
1%, consistent with the indoor absolute humidity). Upstream
samples to mimic the mixtures typically found in reforming
of the desulfurizer bed, the H2S level approached 27.00 ±
JP-8. Analysis of these tests showed that the analyzer
0.42 ppm (1σ). The H2S value is consistent with the expected
provides rapid (2 s) and precise (±0.1 ppm) measurements
value in the gas phase after dilution and molar expansion,
of H2S in reformate gases over a wide dynamic range (0-
based upon the 360 ppmw sulfur in the JP-8 feedstock.
1000 ppm) with a low detection limit (3σ = 0.09 ppm in
1 s) and minimal cross-interferences from other present
species in these complex flows. Although the chemometric
fit was optimized for H2S,4 it also simultaneously quantified
other species in these samples to reasonable precision –
CO2 (±0.2%), CH4 (±150 ppm), C2H4 (±30 ppm), and H2O
(±300 ppm) – value-added data that should enable better
characterization of the fuel reforming process.
Beyond precision and accuracy, a dynamic application such
as this also needs fast instrument response time. The LGR
analyzer’s time response is mainly limited, in this case,
by the flow rate at which gas is transferred through the
analyzer’s measurement cell (i.e., pump speed). For the gas
cell (effective volume at operating pressure ~ 80 cm3) and
pump (11 SLPM) used in these studies, the 1/e time response
of the analyzer was determined to be 2.1 s by rapidly (<0.1
To demonstrate the proof-of-concept unit, we then
s) switching the inlet from a 4.2-ppm H2S (in air) standard
performed tests under real conditions using PCI’s 5 kW
to dry “zero” air. In the in situ JP-8 fuel reformer studies
autothermal reformer followed by a custom desulfurizing
typified by figure 3, a smaller pump (5 SLPM) was employed
bed. Typical measured data from these tests are shown
to avoid disrupting the reformer gas flow, increasing
in Figure 3. In this run, the measurement point was
the instrument response time to 4 seconds. In other H2S
repeatedly switched between ambient air, and upstream
applications, where this disruption is not an issue, a large
and downstream of the desulfurizer bed. As expected, the
scroll vacuum pump (600 SLPM) could alternatively be used
ambient air measurements had very low concentrations of all
to decrease the response time to less than 0.1 seconds for
reformate species (e.g., H2S <0.1 ppm) except water vapor (~
rapid industrial process control requirements, if desired.
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Summary
There are several optical methods that can measure gas
References
under controlled sampling conditions. Many real-world
For a more detailed discussion of this study, see F. Dong, C
Junaedi, S. Roychoudhury, and M.Gupta, Anal. Chem., 83 (11),
4132 (2011).
applications, however, are just not that simple and involve
1. C. Song, Catal. Today, vol. 77, 7 (2002).
flowing samples with multiple gases. The H2S monitoring
application evaluated here includes numerous reformed
2. J.P. Trembly, J.P. Marquez, A.I. Ohrn, and D.J.J. Bayless,
Power Sources, 158, 263 (2006).
constituent species, not all of which are known, nor
3. J. Gong, Sens. Acuators, B, 114, 32 (2006).
their spectra catalogued, a priori. Nonetheless, this study
4. L.D. Le, J.D. Tate, M.B. Seasholtz, M.B. Gupta, T. Owano,
D.S. Baer, T. Knittel, A. Cowie, and J. Zhu, J. Appl.
Spectrosc., 62, 59 (2008).
species diluted in a pure host gas (e.g., N2) to ppb levels
demonstrates that LGR’s OA-ICOS measurement strategy,
which combines measurements of fully resolved absorption
spectra and standard chemometric analyses, delivers trace
gas measurements with the requisite precision, accuracy
and response time needed for practical applications.
Acknowledgement
LGR gratefully acknowledges the Army SBIR Program for
funding this work through grant no. W911QX-C-0034.
Los Gatos Research, Inc.
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