High-temperature steam reforming of bio-oil derived light organics and methane to hydrogen-rich gas with trace CO via rational temperature control

Article abstract of DOI:10.1039/c4ra02037e

Steam reforming of methane or bio-oil is generally performed at high temperatures (>600 °C) to maintain the efficiency of the process. One main disadvantage of steam reforming at that high temperature is the formation of a large amount of CO due to the predomination of the reverse water gas shift reaction and the reformate gas with this level of CO cannot feed fuel cells. In this study a reactor with constant and decreasing temperature zones is developed to produce hydrogen-rich gas with trace CO from bio-oil derived light organics and methane. In the constant-temperature zone, the high temperature employed effectively promotes the reforming of organics and suppresses the generation of both complex organic by-products and coke. In the decreasing temperature zone, the CO produced in the constant-temperature zone is stepwisely and efficiently reduced to a ppm level using steam. In addition, the coke distribution along the catalyst bed varied a lot in the constant-temperature zone and the decreasing temperature zone, due to the different reaction network in the different temperature ranges. Via rational reaction temperature control, the efficient reforming of methane and the bio-oil derived light organics and the simultaneous elimination of CO is successfully achieved in one step in one reactor.

Full text of DOI:10.1039/c4ra02037e

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High-temperature steam reforming of bio-oil
derived light organics and methane to hydrogen-
rich gas with trace CO via rational temperature
control
Cite this: RSC Adv., 2014, 4, 18924
b
c
a
Xun Hu,ac Lijun Zhang, Dehua Dong and Gongxuan Lu
*
Steam reforming of methane or bio-oil is generally performed at high temperatures (>600 ꢀC) to maintain
the efficiency of the process. One main disadvantage of steam reforming at that high temperature is the
formation of a large amount of CO due to the predomination of the reverse water gas shift reaction and
the reformate gas with this level of CO cannot feed fuel cells. In this study a reactor with constant and
decreasing temperature zones is developed to produce hydrogen-rich gas with trace CO from bio-oil
derived light organics and methane. In the constant-temperature zone, the high temperature employed
effectively promotes the reforming of organics and suppresses the generation of both complex organic
by-products and coke. In the decreasing temperature zone, the CO produced in the constant-
temperature zone is stepwisely and efficiently reduced to a ppm level using steam. In addition, the coke
distribution along the catalyst bed varied a lot in the constant-temperature zone and the decreasing
temperature zone, due to the different reaction network in the different temperature ranges. Via rational
reaction temperature control, the efficient reforming of methane and the bio-oil derived light organics
and the simultaneous elimination of CO is successfully achieved in one step in one reactor.
Received 9th March 2014
Accepted 10th April 2014
DOI: 10.1039/c4ra02037e
www.rsc.org/advances
Nevertheless, bio-oil is very complex and steam reforming of
bio-oil at low temperatures is difficult, due to the low conversion
Introduction
of heavy components in bio-oil and the production of a signif-
icant amount of organic by-products and unacceptable level of
coke.12 Steam reforming of bio-oil at high temperatures can
prevent the formation of organic by-products effectively and
suppress coke formation to some extent. The disadvantages of
bio-oil steam reforming in the high temperature range is the
formation of large amount of CO due to the predomination of
reverse water gas shi reaction at this condition [eqn (1)].13 CO
is not desirable in the reformate gas as it can poison the anode
catalyst and signicantly lower efficiency of fuel cells.
How to suppress CO formation in bio-oil reforming at the
temperature above 600 ꢀC is the bottleneck problem for the
application of bio-oil as the source of hydrogen. CO comes from
both decomposition of organics and the reverse water gas shi
reaction, reaction equilibriums of which are controlled by
reaction temperature. Hence, via rational reaction temperature
control, the efficient reforming of bio-oil and the simultaneous
elimination of CO may be achieved in one step or in one reactor.
To accomplish this concept, a reactor with constant and
decreasing temperature zone is developed in this paper. Steam
reforming of bio-oil occurs in the constant high temperature
zone, efficiently producing the gas mixtures containing mainly
H2, CO, and CO2. The stepwise and efficient removal of CO by
water gas shi reaction [eqn (2)] is achieved in a decreasing
Fuel cells have great potential to power the next generation of
vehicles due to their high efficiency and easy portability.1–5
Hydrogen can be used as the fuel for some high performance
fuel cells such as the proton-exchanged membrane fuel cell
(PEMFC). However, the use of hydrogen fuel creates problems
such as safety issues due to its inammable nature and the lack
of infrastructure for its storage. On-board hydrogen production
from liquid fuels is a potential solution to solve the safety and
storage problems associated with the use of hydrogen as a
fuel.6–10 Among the various fuels available, bio-oil, in particular,
is a very promising hydrogen source.11,12 Bio-oil can be produced
via fast pyrolysis of the renewable, wide spread and abundantly
available biomass with the yields up to 70 wt%. The use of bio-
oil as hydrogen resource almost does not create net CO2 emis-
sion as the CO2 released during steam reforming of bio-oil is
took up by the growth of biomass.
aNational Engineering Research Centre for Fine Petrochemical Intermediates, State Key
Laboratory for Oxo Synthesis and Selective Oxidation, Lanzhou Institute of Chemical
Physics, Chinese Academy of Sciences, Lanzhou 730000, P. R. China. E-mail: gxlu@lzb.
ac.cn; Tel: +86 931 4968178
bDepartment of Chemistry, Lanzhou University, Tianshui Road 222, Lanzhou 730000,
P. R. China
cFuels and Energy Technology Institute, Curtin University of Technology, Perth, WA
6845, Australia
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temperature zone. The total reactor system can efficiently
convert bio-oil to gas mixtures at high temperatures and
simultaneously reduce CO concentration to a ppm level in one
step. This reactor has the potential application in the vehicles
with fuel cell system, since it does not require extensive space
and can efficiently generate the hydrogen-rich gas with trace CO
from bio-oil or methane steam reforming in a very wide
temperature region.
H2 + CO2 / H2O + CO (DH0298 ¼ 41.1 kJ molꢁ1
)
(1)
(2)
CO + H2O / H2 + CO2 (DH0298 ¼ ꢁ41.1 kJ molꢁ1
)
Experimental section
Catalytic reactor
Fig. 2 Actual temperature distribution in the reactor with rational
Conguration of the reactor system is present in Fig. 1. An
electric furnace (i.d. 35 mm) and a quartz reactor loaded with
catalyst (i.d. 8 mm) compose the steam reformer. The output
power of the furnace is different in different position. In upper
part of the furnace (Zone I), distribution of the resistance wire is
uniform and dense, while the resistance wire is very sparse in
lower part of the reactor (Zone II), especially at end of the
furnace. In addition, there are gaps existed between the quartz
reactor and inner wall of the furnace. If the gap on top of the
reactor is sealed with insulation material while the gap in
the end keeps open, then the cooling air will be introduced to
the reactor due to the cross-ventilation. The decreasing power
output plus the cooling air will result in formation of a region
with decreasing temperatures. In converse, the temperature in
Zone I is constant and not affected much by the airow, since
the cooling air is gradually heated up by the furnace.
temperature distribution.
categorized into two zones: Zone I is from top to 22 cm of the
reactor, where the temperatures approximately equals to the
setting temperatures. Zone II locates in the range of 22–50 cm,
where the temperatures decrease almost continuously. At outlet
of the reactor, the temperatures are decreased in the region of
140–300 ꢀC.
Theoretically, the CO produced in Zone I can be stepwisely
converted to H2 and CO2 with steam in Zone II. This is because
the temperatures in Zone II decreases gradually to the mild
range, which is highly favorable for occurrence of water gas shi
reaction to remove CO. High-temperature reforming of bio-oil
and methane is performed in this reactor to investigate its
feasibility for elimination of high concentration of CO in
hydrogen-rich gas.
ꢀ
Four typical reforming temperatures (500, 600, 700, 800 C)
are set in the reactor with rational temperature distribution,
and the actual temperatures measured by a thermocouple along
with the furnace (top to end, length: 50 cm) are depicted in
Fig. 2. The temperature distribution can be approximately
Experimental procedures
Catalytic tests are performed at atmospheric pressure in a
traditional reactor and the reactor with cooling device that has
been shown in Fig. 1. A mixed solution of acetic acid, ethanol,
and acetaldehyde (molar ratio: 1 : 1 : 1) is prepared to simulate
the light components in bio-oil. It needs to be noted that the
simulated bio-oil is very different to the real bio-oil. The real bio-
oil contains hundreds of both light and heavy organics, which
includes the sugar derived light components (i.e. carboxylic
acids, aldehydes, ketones and alcohols), sugars (i.e. glucose,
levoglucosan, xylose), and phenolics (i.e. phenol and other big
aromatics).14–17
Typically, the simulated bio-oil is fed into a pre-heater by a
syringe pump with a ow rate of 0.16 ml minꢁ1 and a steam to
carbon ratio (S/C) of 7. The vaporized reaction mixture is then
fed into the reactor using N2 (40 ml minꢁ1) as a carrier gas.
Separation and quantication of the products are attained by
two on-line chromatographs equipped with thermal-conduc-
tivity detector (TCD) and ame ionization detector (FID). A CO
sensor (sensitivity: 50 ppm) is also employed to detect the low
Fig. 1 Scheme of the reactor with cooling device.
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concentration of CO. Conversion of the light organics is calcu-
lated by dividing the amount the three organic compounds
consumed with the total amount in feed. Composition of the
gaseous product (dry basis) is calculated by their molar ratios in
the reformate gas. Their yields are calculated by dividing the
amount produced with the theoretical amount produced.
Conversion and product composition in methane steam
reforming are dened in a similar way.
Catalyst preparation
The conguration of the reactor makes it necessary to have a
long catalyst bed. Thus, an impregnation–reduction method is
employed to prepare Ni/Al2O3 catalyst in situ. Briey, the reactor
(from top to end) is lled up with the support g-Al2O3 before-
hand, and a saturated solution of Ni(NO3)2 is introduced into
the reactor to impregnate the alumina support by a syringe
pump with a ow rate of 0.26 ml minꢁ1 for 60 min. Aer that,
the impregnated catalyst was heated to 600 ꢀC for 3 h in
hydrogen atmosphere, during which the nickel nitrate decom-
posed to nickel oxide or simultaneously was reduced to metallic
nickel species. Nickel loading is 16 wt% to Al2O3, which was
determined by atomic absorption aer digestion of the catalysts
with sulfuric acid. Ni–K/Al2O3 catalyst was prepared via the
similar way and the loading of potassium is 5 wt%. Via this
impregnation–reduction method the catalyst can be prepared in
situ, which is very simple and saves a lot of time when compared
with the conventional impregnation method that includes a
number of steps such as impregnation, drying, calcinations,
and reduction. The amount of coke on catalyst is analyzed by
thermo-gravimetric analysis in a PerkinElmer TG/DTA appa-
ratus. The catalyst is heated at 10 ꢀC minꢁ1 under synthetic air
ow and the mass loss is recorded (experimental error: ꢂ0.5%).
Fig. 3 Conversion of the light organics derived from bio-oil and
product distributions in steam reforming: S/C ¼ 7, bio-oil solution flow
rate ¼ 0.16 ml minꢁ1, P ¼ 1 atm; (a) in TR; (b) in RRTD; the concen-
tration of CO can be further decreased to 119 ppm at S/C of 10 and
600 ꢀC. (c) effects of the addition of K to Ni/Al2O3 catalyst on product
distribution in RRTD. Xbio-oil in figures stands for the overall conversion
of acetic acid, acetaldehyde and ethanol.
In RRTD, the results are very different. CO amounts decrease
to trace level in the whole temperature region investigated. For
example, the CO concentration decreases from 8.2% in TR to
ꢀ
412 ppm in RRTD at 600 C, while it decreases from 38.9% to
1936 ppm at 800 ꢀC. The stepwise decreasing temperature
effectively catalyzes the water gas shi reaction to remove CO,
due to the high equilibrium constant of this reaction at the mild
temperatures. Nevertheless, although the production of CO is
substantially suppressed, the production of CH4 is remarkably
promoted, which similarly leads to low H2 yield.
Results and discussion
Steam reforming of the light organics in the reactor with
rational temperature distribution
Steam reforming of the light organics is performed over the
In general, CH4 can be efficiently steam reformed at high
catalyst prepared by impregnation–reduction method in temperatures, while its amount always reaches a high level at
the traditional reactor (TR) with constant temperature and the the mild temperatures due to the domination of methanation
reactor with rational temperature distribution (RRTD), respec- reaction at this stage.19 The mild temperature region in Zone II
tively. Conversions of the reactants and gas compositions versus of RRTD should be responsible for generation of the large
reaction temperature are depicted in Fig. 3. In the high amount of CH4. In our previous study,20 we found that modi-
temperature region (>600 ꢀC), the reactant is effectively cation of reforming catalyst with potassium can effectively
reformed to gaseous products and the production of complex inhibit the production of CH4, we therefore introduce KNO3
organic by-products is suppressed, indicating that the high- solution on catalyst surface using a syringe pump. Concentra-
temperature reforming is necessary for effective gasication of tion of KNO3 solution is regulated to achieve a K loading of 5 wt
the organics. Although reactant conversions in TR and RRTD %. The catalytic results over Ni–K/Al2O3 catalyst are presented
are comparable, the product distributions are practically in Fig. 3c. The production of CH4 is substantially reduced at
distinct. Gas compositions in TR are mainly controlled by either 500 or 800 ꢀC. Only H2 and CO2 are produced as the main
thermodynamic equilibrium. Production of CH4 decreases products in the whole temperature region. Furthermore, the
remarkably at high reaction temperatures. In converse, CO addition of K also remarkably promotes the catalytic activity.
amount increases drastically versus the increasing tempera- Nevertheless, the introducing K on catalyst surface does not
tures, leading to low H2 and CO2 yields. The high temperatures show remarkable inhibition effect on the production of CO, but
employed highly favor occurrence of the reverse water gas shi CO can be further decreased via increasing ratio of steam to
reaction to produce CO.18
reactants. For example, the concentration of CO can be
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ꢀ
decreased to 119 ppm at S/C of 10 and 600 C. The hydrogen-
rich gas with this level of CO may be even directly fed the fuel
cells with higher tolerance of CO.21 The low concentration of CO
in reformate gas is related to the high efficiency of the RRTD for
CO elimination.
Catalytic activity of the TR and RRTD for water gas shi
reaction
To verify the efficiency of RRTD for water gas shi reaction, the
catalytic activities of Ni–K/Al2O3 catalyst in TR and RRTD for
water gas shi reaction are further investigated. CO conversions
and product compositions as functions of reaction temperature
are present in Fig. 4. In TR, the catalyst is highly active at the
Fig. 5 Stability test of bio-oil steam reforming: T ¼ 700 ꢀC, S/C ¼ 7,
bio-oil solution flow rate ¼ 0.16 ml minꢁ1, catalyst: Ni–K/Al2O3, P ¼ 1
atm; (a) in TR; (b) in RRTD. Xbio-oil in figures stands for the overall
conversion of acetic acid, acetaldehyde and ethanol.
ꢀ
initial temperature of 500 C, CO is completely converted, and
CH4 is produced as the only by-product. However, CO conversion
decreases signicantly versus the increasing reaction tempera-
ture, which is caused by limitation of the thermodynamic
favored under the reforming conditions employed.22 Further-
more, the high concentration of CO in reformate gas is also
equilibrium of water gas shi reaction. In converse, the catalytic unfavorable for the gasication of coke with CO2. Conversely,
activity of Ni–K/Al2O3 catalyst in RRTD is almost independent of employing the RRTD can remarkably strengthen the catalytic
the reaction temperature. CO conversion maintains 100% stability of Ni–K/Al2O3 catalyst. The catalyst shows very stable
whether at 500 or 800 ꢀC. Actually, CO conversion also decreases catalytic behaviors and no deactivation sign is observed in the
gradually with the increasing temperature in Zone I of RRTD,
but the unconverted CO is converted continuously and step-
wisely with steam at the stepwise decreasing temperatures in
Zone II, and eventually it is completely converted. Therefore,
whole time investigated. H2 and CO2 are produced as the main
products and their molar ratio nears the stoichiometric ratio.
Amounts of the two carbon precursors, CO and CH4, are in trace
level. Hence, the coke deposition in RRTD is probably mild and
RRTD is a highly efficient reactor for elimination of CO with does not lead to catalyst deactivation.
steam in a wide temperature region.
The coke formation rates at different positions of the two
reactors are presented in Fig. 6. Position determines coke
deposition whether in TR or RRTD. At upper part of the reactors
(at 10 cm), the coke deposition is relatively serious. Decompo-
sition of the reactants may contribute to the large portion of
coke at this position, since some organic molecules are unstable
at the elevated temperatures. Nevertheless, the coke deposition
tendencies in the two reactors are very different. The coke
formation in TR reach a maximum at 20 cm and then decrease
slightly along with the prolonged catalyst bed, which should be
induced by CO disproportion and CH4 decomposition. In
converse, the coke deposition in RRTD is much mild and
Stability tests of bio-oil steam reforming in RRTD
Stability is a vital factor for successful application of a reforming
catalyst in industry. Endurance tests of steam reforming of the
light organics over Ni–K/Al2O3 catalyst in TR and RRTD are
ꢀ
performed at 700 C with an S/C of 7 for 60 h. Catalytic perfor-
mances versus reaction time are present in Fig. 5. In TR,
remarkable deactivation signs are observed aer 44 h of time-on-
stream. Signicant amount of CO is produced and the amount
increases remarkably at end of the test. Moreover, the amounts
of CH4 also increase to a high level versus the prolonged reaction
time. Production of the signicant amounts of CO and CH4 may
lead to serious coke deposition and then catalyst deactivation,
since both CO disproportion and CH4 decomposition are
Fig. 4 CO conversion and product distributions in water gas shift Fig. 6 Coke formation rates at different positions of the two reactors
reaction: nH O/nCO ¼ 3, gas hourly space velocity (GHSV) ¼ 5000 hꢁ1
,
in the endurance tests: T ¼ 700 ꢀC, S/C ¼ 7, bio-oil solution flow rate ¼
2
P ¼ 1 atm; (a) in TR; (b) in RRTD.
0.16 ml minꢁ1, catalyst: Ni–K/Al2O3, P ¼ 1 atm.
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decreases almost linearly with the prolonged catalyst bed. In 49.8%). However, CH4 cannot be completely converted in the
Zone II, the coke formation is negligible. Although the mild whole temperature region investigated in RRTD. A mild
temperatures in Zone II are highly favorable for the occurrence temperature region exists in Zone II of RRTD, which favors the
of CO disproportion, the amount of CO in reformate gas is in a occurrence of methanation reaction and consequently leads to
trace level, and therefore the coke deposition is negligible. the formation of CH4. Therefore, it seems that CH4 cannot be
Based on the catalytic results presented above, it is clearly that completely converted at any temperatures. Inhibition of
RRTD is a very selective and stable steam reformer. Its combi- methanation reaction through modication of Ni/Al2O3 catalyst
nation with Ni–K/Al2O3 catalyst can catalyze steam reforming of with potassium is expected to solve this problem. The catalytic
light organics to generate the hydrogen-rich gas with only trace results of CH4 steam reforming over Ni–K/Al2O3 catalyst in
CO and CH4 in a wide temperature region. Furthermore, RRTD RRTD are presents in Fig. 7. The presence of K species on
is more coke-resistive than TR in steam reforming.
catalyst surface drastically enhances the catalytic activity. CH4 is
converted at even lower temperatures than that in TR, and CO
concentration still maintains a trace level.
Steam reforming of methane in RRTD
In addition to the light organics, CH4 is another important
source of hydrogen. Steam reforming of CH4 is a mature tech-
nology and has been widely applied for producing large-scale
hydrogen for industrial application for many years. Generally,
steam reforming of CH4 is always performed at high tempera-
tures (e.g. 800 ꢀC). The reformate gas produced at this high
temperature mainly contains H2 and CO, which cannot feed to
PEMFC. Nevertheless, steam reforming of CH4 may signicantly
reduce the level of CO produced, due to its high efficiency for
CO elimination. Steam reforming of CH4 is performed in the TR
and RRTD in a high temperature region, respectively. CH4 and
product compositions are present in Fig. 7. In TR, steam
reforming of CH4 at 500 ꢀC mainly generates H2 and CO2, while
CO is produced as the main carbon-containing product at
800 ꢀC, which leads to remarkable decrease of H2 yield. CH4
steam reforming over Ni/Al2O3 catalyst mainly produces H2 and
CO2 in RRTD. Although the concentration of CO increases with
the increasing temperature, it does not exceed 1800 ppm, even
at 800 ꢀC, which is far less than the concentration in TR (800 ꢀC,
Conclusions
To summarize,
a reactor with constant and decreasing
temperature zones is developed to produce the hydrogen-rich
gas with trace CO from light organics derived from bio-oil and
CH4. In the constant-temperature zone, the high temperature
employed effectively promotes the reforming of organics and
suppresses the generation of both complex organic by-product
and coke. In the decreasing temperature zone, the CO produced
in the constant-temperature zone is stepwisely but efficiently
reduced to a ppm level with steam. This reactor solves the
problem of the generation of large amount of CO in high-
temperature reforming reactions. In addition, the reactor also
has potential application in the large-scale methane steam
reformers to produce the reformate gas with trace CO for
industrial application. Furthermore, it is also possible to
control the product distribution via rational temperature
distribution in other catalytic reactions.
Acknowledgements
We acknowledge nancial support of the NSFC of China (no.
21373245) and the 863 Project of China (no. 2012AA051501).
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