COMMUNICATION
www.rsc.org/chemcomm | ChemComm
Plasma reforming of glycerol for synthesis gas production
Xinli Zhu, Trung Hoang, Lance L. Lobban and Richard G. Mallinson*
Received (in College Park, MD, USA) 5th January 2009, Accepted 19th March 2009
First published as an Advance Article on the web 6th April 2009
DOI: 10.1039/b823410h
Glycerol can be effectively converted to synthesis gas (selectivity
higher than 80%) with small amounts of water or no water using
plasmas at low temperature and atmospheric pressure, without
external heating.
Renewable and CO2 neutral biomass derived fuels have been
recognized as a potential alternative source of fuels. As a
major byproduct of biodiesel production, a huge increase of
glycerol in the market makes it a good platform molecule for
production of valuable chemicals and fuels.1 Synthesis gas
production from glycerol, followed by Fischer–Tropsch synthesis to produce green diesel2 or followed by water gas shift
to produce hydrogen,3 provides attractive approaches for
alternative energy production. Glycerol reforming does not
require external oxygen (as water or oxygen) because of its
1 : 1 oxygen to carbon ratio.
C3H8O3 - 4H2 + 3CO; DH = 350 kJ mol
1
(1)
Even though glycerol can be directly decomposed to H2 and
CO, it is, however, necessary to add large amounts of water
(steam or aqueous reforming)3–6 or oxygen (partial oxidation
or autothermal reforming)7 in the catalytic processes in order
to inhibit carbon formation that eventually leads to deactivation of the catalyst. Thermodynamic analysis shows that a
H2O to C3H8O3 ratio of 9 is needed to avoid coke formation for steam reforming.8 This high H2O content in
steam/autothermal reforming carried out at high temperature
makes this process energy intensive at high capital cost,
particularly as the scale of operation becomes smaller.
Aqueous phase reforming can also be successfully performed
at relatively low temperature.3 However, this process usually
can only process a very low concentration of glycerol (B5%)
at high pressure, thus lowering the throughput of the reactor.
Furthermore, impurities (for example, NaCl) in the crude
glycerol lead to deactivation of the catalyst.9 Thus, it is still
a significant challenge to effectively utilize crude glycerol
(B80%).
In this work, it is shown that concentrated glycerol and even
‘‘pure’’ glycerol (minimum assay 99.5%; EMD CX0185-6)
can be effectively converted to synthesis gas using a nonequilibrium plasma at low temperature and atmospheric pressure without external heating. In a non-equilibrium plasma,
high energy electrons are generated and activate the reactants,
initiating radical reactions, while the bulk gas phase temperature remains low. The advantages of low temperature, quick
Center for Biomass Refining, School of Chemical, Biological, and
Materials Engineering, The University of Oklahoma, Norman,
OK 73019, USA. E-mail: [email protected]; Fax: +1 405 325 5813;
Tel: +1 405 325 4378
2908 | Chem. Commun., 2009, 2908–2910
start-up, and freedom from catalyst deactivation (due to coke
formation and/or impurities, for example, sulfur10) of this type
of plasma lead to increasing attention for utilization in fuels
conversions.10–19
The plasma configuration used in this work is a modified
point-plate configuration17–19 with a discharge gap of 15 mm,
as shown in Fig. 1. The anode is made of stainless steel tubes,
with a 1/16 inch od tube inside of a 1/8 inch od tube (these
simply act as reducers). A hollow needle with an inner
diameter of 0.26 mm is put inside the 1/8 inch tube close to
the tip of the tube electrode. The feed was fed through the
anode with a calibrated HPLC pump. The cathode is made of
a 3/8 inch od stainless steel tube with a 1/2 inch od stainless
steel sintered filter cap (B90 mm) on the top. The porous
structure of the filter cap distributes the plasma homogeneously on the cathode and allows the gases to pass through
the filter, exiting the plasma zone. If desired, a catalyst may be
put under the filter cap to selectively enhance a specified
reaction; for example, the catalytic water gas shift reaction
has been successfully tested in this configuration. The reactor
vessel is a 1 inch od quartz tube. Infrared temperature
measurement from outside of the reactor showed that the tip
of the anode is the hottest point (B290 1C) of the plasma
reactor, followed by the plasma zone (B260 1C) and the plate
electrode (220–250 1C). Due to the low IR transparency of
quartz, the true temperatures could be somewhat higher than
these values. Also, previous measurements with an IR camera
using an IR transparent sapphire reactor tube confirm similar
temperatures.19 This design allows the heat produced by the
plasma on the anode to be quickly transferred to the liquid
feed when it flows through the needle, thereby vaporizing the
Fig. 1 Schematic representation of the plasma reactor.
This journal is
c
The Royal Society of Chemistry 2009
Fig. 2 Effect of feed flow rate on the glycerol conversion and
selectivities to gas phase products, (a) 92 wt% glycerol; (b) pure
glycerol.
feed before entering the plasma zone. No diluents are used.
This configuration of the plasma reactor has several advantages: (1) no external heating is needed; (2) easily combined
with catalysts; (3) relatively small diameter of the cathode
avoids discharge with the reactor wall, thus increasing the
stability of the plasma; (4) all reactants are forced to pass
through the discharge zone without bypassing; (5) erosion of
the anode is reduced due to liquid cooling. The plasma applied
in this work is a DC pulse discharge plasma (corona discharge
plasma). The DC signal was generated by a pulse generator
(HP 8011A), and magnified by a high voltage amplifier (Trek,
20/20C). The discharge was monitored by an oscilloscope
(Lecroy, wave surfer 422). The discharge current was 7 mA,
and pulse rate was 10k pulse per second (pps). The discharge
voltage was 5.5–6.0 kV, depending on feed flow rate. Many
alternatives to this waveform and geometry have also been
found to provide satisfactory results. The gas phase products
were analysed by an online GC (Carle AGC 400) after passing
through an ice–water trap. The condensed liquid was analysed
using a GC-MS (Shimadzu). The major component of the
liquid is unreacted glycerol. Glycerol conversion to gas phase
products (%) is defined as 100 (total carbon in gas phase
products)/(total carbon in feed). Gaseous carbonaceous
species (or H2) selectivities (%) are defined as 100 (carbon
in species i (or H in H2))/(total carbon (or H) in converted
Table 1
glycerol to gas phase). It should be noted that the H2
selectivity could be higher than 100% if the water gas shift
reaction contributes significantly with this definition. Material
balances are generally 495%.
The gas phase products of plasma reforming of glycerol
are H2, CO, CO2, CH4, C2H2, and C2H4. No C2H6 or C3
hydrocarbons were detected. The results of the effect of feed
flow rate on glycerol conversion and selectivities to gas phase
products with a 92 wt% glycerol (H2O : C3H8O = 0.44)
(which is a similar water content to that found in crude
glycerol from biodiesel production) are shown in Fig. 2a.
The glycerol conversion to gas phase products decreases from
79.6% to 53.9%, with the feed flow rate increasing from 0.068
to 0.135 mL min 1. However, the feed flow rate has little effect
on selectivities. H2 and CO have a similar high selectivity
(B85%), which declines slightly with increasing feed flow rate,
indicating that the major reaction route is glycerol decomposition. Note that the H2 selectivity in this work is comparable to
catalytic steam reforming with a H2O to C3H8O ratio of
6 performed at high temperature,6 and optimized autothermal reforming.20 The selectivities for other products: C2
hydrocarbons, CO2 and CH4 are B10%, B4% and B1%,
indicating that some water gas shift reaction, glycerol conversion to C2 and CH4, as well as CH4 coupling reaction occur to
a limited extent. Selectivity to C2 increases slightly with
increasing feed flow rate. CO2 and CH4 selectivities are
constant irrespective of feed flow rate.
The effect of feed flow rate on glycerol conversion and
selectivities using ‘‘pure’’ glycerol is shown in Fig. 2b. Due
to the high viscosity of pure glycerol, the feed flow rate was
tested only up to 0.078 mL min 1. The trends for glycerol
conversion and selectivities for gas phase products are similar
to those observed with 92% glycerol feed. Compared to 92%
glycerol, the selectivities to H2 and CO are lower by B4% for
pure glycerol. The selectivity to C2 increases by B5% under
the same conditions. These results indicate that a small
amount of water helps inhibit C2 formation and improves
the glycerol decomposition to H2 and CO. Note that the H2
and CO selectivity is still very high even for pure glycerol.
Table 1 compares selected results for different glycerol
concentration and feed flow rate on H2 + CO production
rate, gas phase composition and energy efficiency. When
glycerol with higher water content was used as the feed, the
water gas shift reaction took place to a small extent. However,
the energy efficiency is lower due to loss of energy absorbed by
the excess water as well as the reduced glycerol throughput.
Reducing the water content increases the synthesis gas
Effect of glycerol concentration and feed flow rate on plasma reforming of glycerol into synthesis gasa
Glycerol
concentration (wt%)
35
92
100
Gas phase composition (mol%)
Feed flow rate
(mL min 1)
H2 + CO production
rate (mmol min 1)
H2
CO2
C2H4
C2H2
CH4
CO
H2/CO
Power
input (W)
Energy
efficiency (%)
0.100
0.068
0.101
0.065
0.078
2.24
3.82
4.43
4.20
4.51
57.6
54.1
53.7
54.0
54.0
14.9
2.1
2.1
1.3
1.4
0.3
0.6
0.8
1.0
1.1
0.2
1.6
1.7
2.9
3.2
0.3
0.5
0.5
0.5
0.6
26.7
40.8
41.3
40.1
39.7
2.16
1.33
1.30
1.35
1.36
22.8
19.4
19.6
20.0
20.2
30.0
47.4
43.7
53.1
52.8
a
Reaction conditions: 15 mm discharge gap, 10k pps. Energy efficiency is defined as 100 (chemical energy output)/(chemical + electrical energy
input), the chemical energy is based on lower heating value.
This journal is
c
The Royal Society of Chemistry 2009
Chem. Commun., 2009, 2908–2910 | 2909
addition, this system has been scaled up several times
using multiple electrodes and tested successfully on the
laboratory scale.
In conclusion, it has been shown that concentrated glycerol
and even pure glycerol can be effectively converted to synthesis
gas with selectivity higher than 80% using a DC pulse plasma
at low temperature and atmospheric pressure without external
heating. The synthesis gas concentration is higher than 94%.
The authors thank SEMGREEN, LP for financial support
of this work.
Notes and references
Fig. 3 Effect of time on stream on glycerol conversion to gas phase
products and gas phase composition for pure glycerol feed with a feed
flow rate of 0.065 mL min 1.
production rate and improves the energy efficiency. The total
hydrocarbon concentration in the gas is lower than 5%,
even for pure glycerol. The H2 to CO ratio is B1.33 for the
higher glycerol concentrations, again indicating that glycerol
decomposition is the main plasma reaction. The energy
efficiency is higher than 50% for pure glycerol, and may be
further improved with optimizing power supply and geometry
designs and by combining with heat recovery.
The plasma is very stable even when pure glycerol was used
as the feed, as shown in Fig. 3. Both glycerol conversion and
the gas phase composition are stable with time on stream.
Little coke was formed on the wall of the reactor with
prolonged reaction time. Other reforming experiments under
similar conditions have shown stable operation up to the
maximum tested, B25 hours.
Considering the high synthesis gas selectivity, the plasma
process described here may have potential for small scale H2
rich gas (increasing efficiency for internal combustion engines)
or H2 production (combined with water gas shift reaction)
for mobile, portable applications. The throughput of the
reactor has been improved with increasing feed flow rates
and adjustment of discharge current and waveform. In
2910 | Chem. Commun., 2009, 2908–2910
1 A. Behr, J. Eilting, K. Irawadi, J. Leschinski and F. Lindner, Green
Chem., 2008, 10, 13, and references therein.
2 R. R. Soares, D. A. Simonetti and J. A. Dumesic, Angew. Chem.,
Int. Ed., 2006, 45, 3982.
3 R. D. Cortright, R. R. Davda and J. A. Dumesic, Nature, 2002,
418, 964.
4 T. Hirai, N. Ikenaga, T. Miyake and T. Suzuki, Energy Fuels, 2005,
19, 1761.
5 S. Adhikari, S. D. Fernando, S. D. F. To, R. M. Bricka,
P. H. Steele and A. Haryanto, Energy Fuels, 2008, 22, 1220.
6 A. Iriondo, V. L. Barrio, J. F. Cambra, P. L. Arias, M. B. Guemez,
R. M. Navarro, M. C. Sanchez-Sanchez and J. L. G. Fierro, Top.
Catal., 2008, 49, 46.
7 P. J. Dauenhauer, J. R. Salge and L. D. Schmidt, J. Catal., 2006,
244, 238.
8 S. Adhihari, S. Fernando, S. R. Gwaltney, S. D. F. To,
R. M. Brika, P. H. Steele and A. Haryanto, Int. J. Hydrogen
Energy, 2007, 32, 1875.
9 K. Lehnert and P. Claus, Catal. Commun., 2008, 9, 2543.
10 Y. Sekine, J. Yamadera, S. Kado, M. Matsukata and E. Kikuchi,
Energy Fuels, 2008, 22, 693.
11 G. Petitpas, J. D. Dollier, A. Darmon, J. Gonzalez-Aguilar,
R. Metkemeijer and L. Fulcheri, Int. J. Hydrogen Energy, 2007,
32, 2848, and references therein.
12 M. Kraus, B. Eliasson, U. Kogelschatz and A. Wokaun, Phys.
Chem. Chem. Phys., 2001, 3, 294.
13 Y. Sekine, K. Urasaki, S. Asai, M. Matsukata, E. Kikuchi and
S. Kado, Chem. Commun., 2005, 78.
14 S. Kado, Y. Sekine and K. Fujimoto, Chem. Commun., 1999, 2485.
15 S. L. Yao, A. Nakayama and E. Suzuki, AIChE J., 2001, 47, 419.
16 Y. Wang, C. J. Liu and Y. P. Zhang, Energy Fuels, 2005, 19, 877.
17 C. J. Liu, A. Marafee, B. Hill, G. H. Xu, R. G. Mallinson and
L. L. Lobban, Ind. Eng. Chem. Res., 1996, 35, 3295.
18 C. J. Liu, R. Mallinson and L. Lobban, J. Catal., 1998, 179, 326.
19 H. Le, L. L. Lobban and R. G. Mallinson, Catal. Today, 2004, 89,
15.
20 D. C. Rennard, J. S. Kruger and L. D. Schmidt, ChemSusChem,
2009, 2, 89.
This journal is
c
The Royal Society of Chemistry 2009