Proceedings of the ASME 2011 Power Conference (POWER 2011)
and the International Conference on Power Engineering 2011 (ICOPE-11)
July 12-14, 2011, Denver, Colorado, USA

POWER2011-55377

EXPERIMENTAL STUDY ON A COMPACT METHANOL-FUELED REFORMERWITH HEAT REGENERATION USING CERAMIC HONEYCOMB(2ND REPORT: REACTION REGION DETECTION BY A POSITIVE ION CURRENT PROBE)
Yasuhiro RAI
Dept. of Mech. Eng. & Sci., Kyoto Univ.
Kyoto, Japan

Hideyuki KOGAME
Dept. of Mech. Eng. & Sci., Kyoto Univ.
Kyoto, Japan

Kazuya TATSUMI
Dept. of Mech. Eng. & Sci., Kyoto Univ.
Kyoto, Japan

Kazuyoshi NAKABE
Dept. of Mech. Eng. & Sci., Kyoto Univ.
Kyoto, Japan

ABSTRACT
A compact tubular-type fuel reformer was fabricated andoperated under fuel-rich combustion conditions of methanol,
focusing on the partial oxidation reaction (POR). Ceramichoneycomb strainer blocks were inserted in the reactor. In theauthors‟ previous study, Case-1 of only one honeycomb block
insertion showed that the reaction region formed in thedownstream of the block. This block worked as a reaction
stabilizer. The other condition, Case-2, was operated with thesecondary honeycomb block inserted in the downstream of thereaction region in addition to the first block. This geometricalstructure sandwiched the reaction region between the two
blocks, and the thermal energy possessed by the exhaust gas
could be regenerated to the reaction region by radiationexchange between these two blocks, which resulted inenhancing the preheating of the premixed gas. By this effect, themethanol-conversion and hydrogen-production in Case-2 wereenhanced by about 10% compared to Case-1.
In the present study, the reaction characteristics of the fuelreformer were investigated in detail, by detecting the location of
the reaction region. Detailed temperature profiles in thestreamwise direction were measured with traversable
thermocouples, and positive ion current distributions
corresponding to the reaction region were measured with aLangmuir probe. It was confirmed by the both measurementsthat there exists a reaction region right after the first honeycomb
block which accompanies with sharp temperature gradients. The
estimated thickness of the reaction region, however, was aswide as several millimeters to a centimeter, which is believed to
be a „mild reaction‟ stabilized by the first honeycomb block. In
Case-2, the high-temperature region became broader compared
to Case-1, which indicates that the enhancement of preheatingof premixed gas was achieved by the heat regenerated from thesecondary honeycomb block.
1. INTRODUCTION
Hydrogen is considered to be one of the most promisingfuels in the path of sustainable society. However, in thetransition period until the completion of large scale hydrogen
distribution system, reforming devices, which convert easy-tocarry hydrocarbon fuels into hydrogen, must be locallyinstalled. In this research, the goal is to establish advanced
system for heat and reaction control in small-scale fuelreformers.
By considering the social and technical trends, bio-fuels
may be the most promising hydrogen carriers. Also, in the
future, low purified fuels may be used as a reformant due to thedepletion of energy resources. Such low energy density fuels
need more advanced design with heat energy management.
When it comes to relatively small-sized compact fuel reformers,
this consideration is more important, since heat release to theout of the system will be dominant due to its large surface area
to volume ratio [1,2].
Okuyama et al. [3] reported that by applying heat
regeneration by porous media which works as a radiantconverter, it is possible to stabilize super-fuel-rich flame. Thisporous radiant converter efficiently converts the enthalpy of
exhaust gas penetrating through the porous media and realizes
energy regeneration to the upstream region of the reactor.
In the authors‟ previous study [4], the results of a prototype
tubular-typed reformer using heat regeneration by ceramichoneycomb were reported. Methanol was used as fuel. A
catalyst was not mounted in order to separate the effects of thecatalyst characteristics from the thermal effects.

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In this study, one of the objectives is to clarify how the heat
regeneration affects the reaction characteristic and reforming
efficiency. To analyze reactions in the reformer, an ion current
probe was introduced to the fuel reformer. For hydrocarbon
fuels, in general, it is well known that combustion reactions
always accompany an ion-rich region which corresponds to theflame region [5]. Therefore, measuring ion concentration by an
ion current probe helps analyzing the flame structure and the
kinetics of combustion [6,7]. Detailed temperature profiles werealso measured in the reformer. These measurements clearly
revealed the connection between the hydrogen production andthe reaction characteristics of the methanol fuel reformer, whichenables us to establish some guideline for designing compact
fuel reformers.
NOMENCLATURE
D : Inner diameter of the reactor,
L : Thickness of one honeycomb block,l : Thickness of the rib of the honeycomb,
lP : Pitch of the honeycomb cell,
M : Mole flow rate,
Q : Volume flow rate,r : Coordinate in radial direction,
T : Temperature,
W : Length of honeycomb block,x : Coordinate in streamwise direction,

mm
mm
mm
mm
mol/s
cm3/s
mm
K
mm
mm

Greeks : Methanol conversion ratio : Production ratio with respect to fuel supply
 : Equivalence ratio
Subscripts
air : value of supplied air
fuel : value of supplied fuelin : value at the center axis of the reactor
ex : value at the outer wall of the reactor
H2 : value of hydrogen
2. EXPERIMENTAL METHOD
2.1 Experimental apparatus
Figure 1 shows the schematic and cross-sectional views of
the present compact fuel reformer. This reformer was composedof two parts, i.e., the evaporator and reactor. The evaporator
was made of a galvanized steel pipe with inner diameter andlength of 28mm and 200mm, respectively. A fuel supply port
was located on the surface, on which a fuel injector with electric
valve was mounted. A rectangular electric signal withappropriate frequency and duty ratio was input to the injector
from a function generator (Yokogawa; WE500, WE7281) tocontrol the fuel flow rate. An air supply port was set at the
upstream end of the evaporator. Air supplied to this port wasprovided from a compressor, and the flow rate was controlled
by a rotameter with a needle valve (Kofloc; RK1250). On theFig. 1: Experimental apparatus.

(a) Baffle plate(b) Ceramic honeycomb
Fig. 2: Elements of the reformer.
both side of the fuel supply port, electric band heaters(Sakaguchi E.H Voc Corp.; BH3430) were attached, so that theevaporator could be heated up and held at a specified
temperature. Therefore, the fuel injected from the fuel supply
port was vaporized instantly after impinging on the hightemperature inner wall of the evaporator. The vaporized fueland air were mixed in the evaporator and then supplied to thereactor through a baffle plate.
As shown in Fig. 1, the baffle plate was located between
the evaporator and reactor. Multiple holes were drilled in the
12mm-thick stainless steel disk. The configuration of the holes
is shown in Fig. 2 (a). This multi-hole baffle plate was expected
to enhance the mixing of fuel and air, and to prevent backfire
from the reactor to the evaporator.
The reactor was made of a stainless steel pipe with innerdiameter D = 36.7mm and 300mm in length. Several ports wereapplied on the sidewall of the reactor at the locations shown in
Fig. 1. These ports can be used to insert measurement probes
and possible multiple reactant supplying nozzles.
Another electric band heater (Hakko; SWD1020) was
twined around the reactor at the location of 3.1 ≤ x/D ≤ 4.3.
This heater was attached for preheating the reactor before theexperiment started, so that POR starts smoothly after reactants
were supplied.
Ceramic honeycomb blocks shown in Fig. 2 (b) wereinserted in several locations in the upstream region of the2

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reactor, which were expected to enhance heat transfer to thereactant gas and also to utilize heat regeneration from theexhaust gas by using radiative heat transfer. This honeycomb
was made of cordierite ceramic, and has mesh of cell number
300. Each block was 8mm thick, so that several blocks were put
together by fireproof cement for desired thickness.
Gas sampling for gas component analysis was conductedby inserting a sampling probe into one of the ports on thereactor. The sampling probe was made of stainless steel tube
with 3mm in outer diameter. The position of the tip end was set
at the reactor centerline, and the gas was collected byconnecting the probe outlet to a vacuum-collecting chamber. On
the way to the vacuum bottle, a 0.2mm-hole was placed in orderto freeze the gaseous reactions. The collected gas was thensupplied to a gas chromatograph (Shimadzu; GC-8A) through afiltering chamber packed with silica gel, by which water andunburned methanol were removed from the gas. A gas
component detector on the basis of TCD (Thermal Conductivity
Detection) method was applied to the gas chromatograph. The
column (Shinwa chem.; Shincarbon ST) mounted in the gas
chromatograph oven was calibrated for H2, N2, O2, CO, andCO2. Argon was used as the carrier gas.
2.2 Temperature measurement
Two temperature measurement probes were fabricated:radial-type and transversal-type ones. The radial-type probe wascomposed of 0.1mm K-type thermocouple wires and two-holeceramic tube with 3mm in outer diameter. The thermocouple
wires were led into each hole and the both ends were welded at
the ceramic tube tip. This probe was radially inserted into thereactor at each port located on the reactor side wall so that thegas temperatures at five locations could be measured
simultaneously during experiments. The transversal-type probe,on the other hand, was inserted from the uppermost port of theevaporator and stretched along the whole reformer inside with
0.1mm K-type thermocouple wires. By using this type of probe,
it was possible to transversally measure gas temperature
distributions along with the central axis of the reactor.
The welded junction of thermocouple wires was coatedwith silica particles by which the probe was prevented fromboth deteriorating and working as a catalyst for the reaction.
Signals from either type of probe were recorded by a personal
computer through a digital multi-thermometer (Keyence; NR1000). The sampling rate and accuracy of the temperature
measurement was 10Hz and ±1K, respectively.
2.3 Ion current measurement
It is well known that there exists positive ion in the limited
narrow region corresponding to the region where oxidation
reaction of hydrocarbon fuel is taking place. There are manystudies on the correlation between positive ion distribution andkinetics of flame propagation or turbulent flame by using ion
current probe method or mass-spectrometry. Ion current probe
method is widely used in combustion analyses due to its
simplicity and inexpensiveness. In this study, an ion current

Fig. 3: Langmuir probe and its configuration.
Table 1: Flow rate conditions (Qfuel = 0.092cm3/s).
3.0
3.5
4.0
4.5

Qair [L/min]
7.65
6.56
5.74
5.10
probe was fabricated and inserted to the reactor in order todetect the POR of methanol.
Figure 3 shows the ion current probe, so-called Langmuirprobe, and its configuration including amplifying circuit. A
0.26mm-in-diameter alumel wire was used as an ion current
sensor and extended for 2mm in length from the tip of quartz
glass tube (O.D. 2.1mm). The quartz glass tube was surroundedwith a stainless tube with water cooling system, so that theprobe was not excessively heated which causes thermal noise.
Negative electric potential, -18V, relative to the groundedreactor wall was imposed on the ion current probe. Positive ion
current flowing into the probe was observed through anamplifier with a current-voltage converter (Hamamatsu;
C7319). The gain of the amplifier was 106 V/A. Amplified
voltage signal was obtained by an A/D converter (Keyence; NR1000) with sampling rate 10Hz and analyzed by PC.
3. EXPERIMENTAL CONDITIONSIn this study, we investigate how the ceramic honeycomb
blocks affect reforming characteristics and reaction efficiencies.
First as Case-1, one honeycomb block (called Honeycomb A)
with thickness of 40mm was inserted in the region of thereactor, 1.8 < x/D < 2.9. Case-1 is the experiment where basic
reformer characteristics were investigated without using
positive heat regeneration; Honeycomb A was inserted tomaintain a stable reaction under fuel-rich operation conditions
by increasing the heat transfer to the reactant gas. If honeycomb
block was not inserted at all in the reactor, POR did not occur
for any fuel-rich equivalence ratio conditions.
Second as Case-2, another honeycomb block with the same
thickness (called Honeycomb B) was inserted downstream of
Honeycomb A. Honeycomb B was expected to play the role ofpositive heat regeneration, i.e., the enthalpy of exhaust gas wasabsorbed by Honeycomb B when the gas flow through it, and a
portion of the enthalpy was regenerated to the upstream reaction
region by radiative and conductive heat transfer. By using
energy regeneration from the exhaust gas, the reforming
efficiency could be increased. Honeycomb B was inserted
downstream of Honeycomb A with streamwise space of 1.6D,
which works as a reaction region of POR. It was confirmed that

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enough space for reaction was needed to stabilize POR in thisreformer. This is because the reaction speed of POR is
considerably slower than the complete combustion reaction due
to fuel-rich POR conditions.
As described in the previous section, experiments werecarried out with regard to POR of methanol. Table 1 shows theflow rate conditions of reactants. Fuel and air were bothsupplied to the reactor from the evaporator through the baffle
plate. Qair and Qfuel are the volume flow rates of the air and fuelfed to the evaporator, respectively. Note that Qfuel represents thevolume flow rate of the fuel in liquid form.  is the equivalence
ratio based on the complete combustion reaction. The
stoichiometric equivalence ratio of the POR, therefore, is = 3.
As shown in Table 1, Qfuel was fixed to one value in the present
experiments, and Qair was varied in the range of equivalence
ratios, 3.0 ≤  ≤ 4.5.
The procedure applied in the reaction experiment is
described as follows. The evaporator and reactor pipes werefirst heated by the electric band heaters so that the temperatures
measured at the locations of x/D = −3.9, upstream of the baffle
plate rear surface, and x/D = 3.0 reached 470K and 820K,
respectively. Then, the heaters attached to the reactor wereturned off and the fuel and air were supplied to the reformer.
Note that the heaters of the evaporator continued to be powered
during the experiment and the temperature inside the evaporator
was kept at about 420K.
4. RESULTS AND DISCUSSION
4.1 Parameters for performance evaluation
In the present study, the quantity of the consumed methanol
cannot be measured directly due to the absorption of water andunreacted methanol in the silica gel chamber. Therefore, thesequantities were estimated from the concentrations of the othergases included in the exhaust gas. Since the summation of theanalyzed concentrations of H2, N2, O2, CO, and CO2 in theexhaust gas was 100±2%, the major components of the gas was
expected to be these five species plus water and unreacted
methanol that were collected in the silica gel chamber. When
paying attention to carbon atoms, the components possessing
carbon atoms among the products are CO, CO2 and unreacted
methanol. Thus, the quantity of the consumed methanol can becalculated using the concentrations of CO and CO2 in the exhaustgas. The methanol conversion ratio, , is defined as follows:

M CH3OH, consumed M CO  M CO2

M CH3OH, supplied M CH3OH, supplied(1)

YCO  YCO2
M N2

YN2
M CH3OH, supplied

where MX is the molar flow rate of the X component in theexhaust gas, and YX is the X‟s concentration.
To evaluate the production efficiency of each component, theproduction rate of component X against 1mol of methanol
supplied, X, is defined as follows:(a) Case-1(b) Case-2
Fig. 4: Temperature distributions (measured by radial-type probes).

(a) Case-1(b) Case-2
Fig. 5: Temperature distributions (measured by two types ofprobes,  = 3.5).

X 

MX
M CH3OH,supplied(YX / YN2 ) M N2
M CH3OH,supplied(2)

4.2 Temperature profiles
The present fuel reformer was possible to operate under aswide conditions as of equivalence ratio, , between 3.0 and 4.5.
Figure 4 shows the streamwise temperature distributions
measured by the redial-type probes for the cases  = 3.0 and 3.5
in Case-1 and -2. The ordinate indicates the temperature
measured, and the abscissa indicates the dimensionlessstreamwise coordinate x/D from the downstream surface of thebaffle plate. Hatched region in the graph corresponds to thelocation of Honeycomb A or B.
For Case-1, temperature distributions show similar trend
regardless of  condition. Distributions of gas temperature on
the centerline of the reactor, Tin, plotted by solid symbols andlines, take their maximum values at x/D = 4.4 and thetemperature decreases downstream. It is inferred from thesetemperature distributions that the main exothermic reaction took
place in the region of 3.0 ≤ x/D ≤ 4.4. In x/D ≥ 4.4, thetemperature decreases as x/D increases, which shows that an
exothermic reaction did not take place in the downstream
region. The surface temperature of the reactor side wall, Tex,
which is plotted by open symbols and broken lines, does notexperience a sudden temperature increase as seen in Tin but
corresponds moderately to Tin. The most upstream surface
temperature, Tex, monitored at x/D = 1.6, was slightly higher
than the corresponding gas temperature, Tin, which was causedby heat conduction through the side wall from the downstream

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of the reactor. However, Tex shows its maximum value at x/D =4.4 which is the same trend as Tin.
Comparing the temperature distributions in Fig. 4 withregard to equivalence ratio, , temperature level decreases as 
increases. This is due to the fact that when  becomes small, theoxygen supply rate to methanol increases, which results inpromoting the exothermic oxidative reaction and increasing thetemperature of the reforming gas.
Figure 4 (b) shows the temperature distributions of Case-2,
which is the case with heat regeneration by the second
honeycomb block applied. By comparison with Case-1,
temperature rise between Honeycombs A and B (3.0 ≤ x/D ≤4.4) and temperature drop downstream of Honeycomb B (x/D ≥5.7) were observed for both  conditions.
Figure 5 shows the temperature profiles of  = 3.5 forCase-1 and -2 measured by both transversal-type and radialtype probes. By using the former probes, temperature
information between the latter probes could be obtained. The
values of temperature disagree between the former (solid
symbol and line) and the latter (open symbol) at hightemperature region; higher temperature values were obtained by
the former. The differences between these values are about
100K, which is believed to be caused by radiative effect.
As described before, the latter probe was composed of
3mm ceramic tube (with two holes), and thermocouple wires
were welded together at the tip of the tube. Since thetemperature sensing junction of thermocouple has contact withthe ceramic tube tip, the temperature value obtained is affectedby temperatures of both surrounded gas and ceramic tube solid.
Ceramic solid surface has larger property of emissivity compare
to the other metallic materials, so irradiation from the ceramic
surface together with the radial conduction cools down theceramic tube itself, which causes the temperature value by thelatter radial-type probe to be lower than that by the former
transversal-type probe. For the transversal-type probe, on thecontrary, a higher temperature can be monitored due to the axial
conduction through the thermocouple wire from both the higher
temperature downstream region and the lower temperature
upstream region. In spite of these thermal effects, thesetemperature values themselves can still be used for qualitative
discussion.
In Fig. 5 (a), temperature takes almost constant value 470K
within Honeycomb A, and very steep temperature gradientexists in the downstream region. This sudden temperature
increase corresponds to the region where exothermic oxidative
reaction takes place. The maximum temperature value 1230K
was observed at x/D = 3.4 which is 20mm downstream from theoutlet of Honeycomb A. In the post-reaction region x/D > 3.4,
temperature decreases downstream. From this temperature
profile, POR begins in the downstream of Honeycomb A.
Figure 5 (b) depicts temperature profiles of Case-2.
Temperature within Honeycomb A shows different
characteristics from Case-1, i.e., temperature rise begins fromthe inside of Honeycomb A. The temperature at the outlet of

Fig. 6: Methanol conversion
ratio, , (Cases-1&2).

Fig. 7: Hydrogen production
ratio, H2 (Cases-1&2).

Honeycomb A was about 670K, which is 200K higher thanCase-1. While the maximum temperature was observed at x/D =3.1 for Case-2, which is about 10mm upstream-side of Case-1.
From these results, the reaction of Case-2 starts from more
upstream region than Case-1, since Honeycomb B worked as anenergy regenerator and increased the temperature of
Honeycomb A by radiative energy exchange.
A large temperature decrease within Honeycomb B was
observed in Case-2. This temperature drop was caused byabsorption of the exhaust gas enthalpy by Honeycomb B. The
temperature difference between the inlet and outlet of
Honeycomb B is about 180K, monitored by either probe. Thistemperature drop proves that Honeycomb B absorbs a portionof enthalpy possessed by the exhaust gas and works as anenergy regenerator.
4.3 Reforming efficiency
Figure 6 shows the relation between methanol conversion
ratio, , and . For both Case-1 and -2,  takes its maximum
value at = 3.5 and decreases as  increases. Much larger
equivalence ratio represents excessive fuel-rich condition,
which means the lack of oxygen supply and causes smaller
methanol conversion ratio compared to the smaller conditions. For all  conditions, Case-2 has larger  values
compared to Case-1. This is because energy regeneration
caused by Honeycomb B increased the temperature between
Honeycombs A and B, which resulted in higher decomposition
ratio of methanol.
The hydrogen production ratio, H2, is shown in Fig. 7. H2
takes its maximum value at , and decreases in the region of
> 3.5. This trend is similar to the methanol conversion ratio, .
This result indicates that the production efficiency of hydrogen is
greatly affected by the methanol conversion ratio. Thus, if it ispossible to increase  close more to unity, it may result in largerhydrogen production. However,  is already more than 96%.
Other effective reaction paths from methanol to hydrogen should
be searched for the improvement of hydrogen production
efficiency.
4.4 Positive ion detection
From the results of temperature distributions discussed in
Section 4.2, the reaction region was formed at the outlet of
Honeycomb A. Positive ion current detection by ion current

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probe was conducted to precisely ensure the location of thereaction region.
Ion current was observed only at x/D = 3.0. At postreaction region like x/D = 4.4, soot deposition disturbed the ion
current detection. Operated under fuel-rich conditions, thecooled ion current probe promoted soot formation on thesurface of the ion current probe in the post-reaction region
which caused break-down of the electrical insulation of theprobe.
Figure 8 shows the radial distributions of ion current for
both Case-1 and -2 at x/D = 3.0, which is 6mm downstream ofthe outlet of Honeycomb A. For Case-1, regardless of 
conditions, ion current was within almost negligible level. This
indicates that reactions involving positive ions were still not
activated at x/D = 3.0. Although Fig. 5 (a) indicates that there
exists large temperature gradient at x/D = 3.0, ion current was
not detected here, which means that this region is so-called preheat region, where thermal pyrolysis reactions are taking place.
However, for Case-2 at the same x/D location, large ion
current was observed for both  = 3.0 and 3.5. This is due to thefact that heat regeneration from Honeycomb B made reaction
region moving upstream. In this case, pre-heat region moves
inside Honeycomb A, which enhances the pyrolysis reaction
since thermal energy for pyrolysis is supplied not only from thereaction region by thermal conduction through the reactant gas
but also from Honeycomb B by means of radiative energy
exchange.
Comparing Fig. 8 (a) and (b),  =3.5 experiences higher
ion current for Case-2. The result implies that the combustion
reaction region can be shifted a bit upstream or the active
formation of soot might be detected here. Calcote et al. [8]reported that the minimum ion concentration of hydrocarbon
fuel is reached in a slightly sooting flame. Larger  cases withfurther fuel enrichment have the possibility of the growth of ion
current related to soot formation which involves much large
molecules.
5. CONCLUSIONSIn the present article, major conclusions obtained are listed in
the following:
A methanol fuel reformer was fabricated and operated under
the conditions of 3.0    4.5. A better performance wasobtained under the conditions of = 3.5.
Temperature distribution measured by the transversal-typeprobe showed about 100K higher than that by radial-typeone.
For the single honeycomb insertion (Case-1), POR begins
about 10mm downstream of Honeycomb A, while ion
current was not observed at x/D = 3.0 which was located in
the pre-heat region of the reaction.
At the outlet of Honeycomb A for the double honeycomb
insertion (Case-2), much higher temperature (200K higher)
was observed than for Case-1.
Reaction in Case-2 began about 10mm upstream compared
to Case-1 and ion current was detected at x/D = 3.0, which is(a)  = 3.0(b)  = 3.5
Fig. 8: Radial distributions of ion current at x/D = 3.0.

caused by the energy regeneration effect by the downstream
honeycomb block.
By increasing the equivalence ratio from  = 3.0 to 3.5, ion
current level was increased possibly because of the upstream
shift of the reactions or active formation of soot.

ACKNOWLEDGMENTS
The authors express their thanks to Kyocera Corporation,
Toyota Central R&D Labs., Inc., and Mitsubishi Electric
Corporation for the supply of some experimental device parts.
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