Energy & Fuels 2008, 22, 2341–2345
2341
Investigation of H2O and CO2 Reforming and Partial Oxidation of
Methane: Catalytic Effects of Coal Char and Coal Ash
Hongcang Zhou,†,‡ Yan Cao,† Houyin Zhao,† Hongying Liu,† and Wei-Ping Pan*,†
Institute for Combustion Science and EnVironmental Technology, Western Kentucky UniVersity, Bowling
Green, Kentucky 42101, and School of EnVironmental Science and Engineering, Nanjing UniVersity of
Information Science and Technology, Nanjing 210044, People’s Republic of China
ReceiVed October 27, 2007. ReVised Manuscript ReceiVed April 1, 2008
Methane reforming and partial oxidation was studied to evaluate the catalytic effects of coal chars and coal
ashes on methane (CH4) conversion, sum selectivity (the sum of H2 and CO), and ratio selectivity (the ratio
of H2/CO) in an atmospheric fluidized bed. The kinetics study presented the possibility of CH4 reforming and
partial oxidation with a favorable H2/CO ratio, greater than 5. The higher H2/CO ratio in CH4 reforming and
the partial-oxidation process can reduce the consumption of CH4 needed to adjust the H2/CO ratio during
combined coal gasification and methane reforming. Coal ashes failed to be good candidates of catalysts on
CH4 reforming and partial oxidation because of their very low specific surface area available for catalytic
reactions. However, coal chars presented very promising catalytic performance on CH4 reforming and partial
oxidation because of their larger specific surface area. In this study, no other constituents in coal fly ash or
special surface properties of coal chars were correlated with the enhanced methane-conversion efficiency. It
seems that the specific surface area is only variable in controlling methane-conversion efficiency.
1. Introduction
It is well-known that many chemical products are synthesized
through syngas (H2 and CO). The production of syngas is of
great importance in the chemical industry because it is the raw
material for methanol synthesis, Fischer-Tropsch (F-T) synthesis, and dimethyl ether (DME).1–3 Natural gas is an important
resource for syngas production. With an insufficient supply and
the rising price of petroleum, great importance has been attached
to the research and development of natural gas reforming. Coal
gasification is also a promising resource for syngas production
because the carbon in coal can react with H2O to produce CO
and H2. Therefore, natural gas reforming and coal gasification
are two primary resources for the production of syngas and may
become the new source of the modern chemical industries in
the future instead of petroleum.
The downstream synthesis of different chemical products
requires syngas with different H2/CO ratios. The H2/CO ratio
of syngas usually depends upon the H/C ratio of raw materials
and the reaction routes of the syngas production. The desired
H2/CO ratios for methanol synthesis and F-T synthesis of
different chemical products are usually 1.5-2.2 Presently, syngas
is mainly produced by H2O reforming of methane. However,
syngas produced from H2O methane reforming has a H2/CO
ratio between 3 and 4 higher than what is needed for the
downstream synthesis processes and thus requires further
adjustment to be used in methanol synthesis and F-T synthesis.
Syngas produced from CO2 methane reforming and steam
* To whom correspondence should be addressed. E-mail: wei-ping.pan@
wku.edu.
† Western Kentucky University.
‡ Nanjing University of Information Science and Technology.
(1) Bai, Z. Q.; Chen, H. K.; Li, W.; Li, B. Q. Int. J. Hydrogen Energy
2006, 31, 899–905.
(2) Song, C. S.; Pan, W. Catal. Today 2004, 98, 463–484.
(3) Li, Y. B.; Xiao, R.; Jin, B. S. Chem. Eng. Technol. 2007, 30, 91–
98.
gasification of coal cannot also be directly used in methanol
synthesis or F-T synthesis because the H2/CO ratio is close to
1. Methane partial oxidation needs to be carefully controlled to
obtain an available yield of H2 and CO, although this reaction
can produce syngas with a H2/CO ratio close to 2. At the same
time, CH4 partial oxidation requires pure O2 and thus increases
the investment and operation costs of CH4 partial oxidation.
How can we get a desired syngas to meet the demand of the
modern chemical industry? Combined methane reforming and
coal gasification is expected to easily produce syngas with the
desired H2/CO ratio of 1.5-2 by changing feed composition.4–6
With the use of H2O and CO2, methane can be reformed to
produce H2 and CO according to the following reactions shown
in eqs 1 and 2. These two reactions are so-called methane
reforming. With a supply of lower stoichiometric coefficients
of oxygen, methane can be partially oxidized to produce H2
and CO according to the following reaction, which is shown in
eq 3. Carbon monoxide can further react with an excessive
supply of H2O to produce more H2. This reaction is called the
water-gas shift reaction, as shown in eq 4. Carbon deposit is
one of major problems during methane reforming and partial
gasification. The possible carbon deposit reaction is shown in
eq 5.
CH4 + H2O S CO + 3H2
+205.9 kJ/mol
(1)
CH4 + CO2 S 2CO + 2H2
+247.1 kJ/mol
(2)
1
CH4 + O2 S CO + 2H2
2
-35.9 kJ/mol
(3)
(4) Wu, J. H.; Fang, Y. T.; Wang, Y.; Zhang, D. K. Energy Fuels 2005,
19, 512–516.
(5) Haghighi, M.; Sun, Z. Q.; Wu, J. H.; Bromly, J.; Ng, E.; Wee, H. L.;
Wang, Y.; Zhang, D. K. Proc. Combust. Inst. 2007, 31, 1983–1990.
(6) Li, Y. B.; Jin, B. S.; Xiao, R. Korean J. Chem. Eng. 2007, 24, 688–
692.
10.1021/ef700638p CCC: $40.75  2008 American Chemical Society
Published on Web 06/04/2008
2342 Energy & Fuels, Vol. 22, No. 4, 2008
H2O + CO S CO2 + H2
CH4 S C + 2H2
-44.0 kJ/mol
-74.9 kJ/mol
Zhou et al.
(4)
(5)
During coal gasification in the presence of H2O and/or air,
the reactants in the gasifier consist of CO, H2, O2, H2O, CH4,
and CO2.7 Once natural gas is introduced into the gasifier during
coal gasification, CH4 in natural gas will react with H2O, CO2,
and O2. That is to say, combined methane reforming and coal
gasification involve CH4 reforming of H2O and CO2 and partial
oxidation by O2. Presently, many references mainly focus on
the research of CH4 reforming of H2O and CO2 and partial
oxidation by O2, especially in the presence of catalysts, such
as noble metals and their oxides supported on the carriers
(normally metal or nonmetal oxides).8–14 Do coal char and coal
ash have obvious effects on H2O and CO2 reforming and partial
oxidation of CH4 during combined CH4 reforming and coal
gasification? Very few papers in the literature have dealt with
this topic until now.1,4,5,15,16
In this study, steam and CO2 reforming and partial oxidation
of CH4 in the presence of coal chars and coal ashes were
performed in a fluidized bed reactor. The catalytic effects of
coal chars and coal ashes on methane reforming and partial
oxidation were evaluated.
2. Experimental Section
2.1. Experimental Apparatus. Figure 1 shows a schematic
diagram of the fluidized bed reactor for methane reforming and
partial oxidation in this study. The methane reforming and partial
oxidation experiment system consists of four parts: electric heating
furnace, fluidized bed reactor, steam generator, and control unit.
Figure 1. Schematic diagram of the test facility.
The fluidized bed reactor is made of quartz and is 600 mm long
with a porous quartz plate of 20 mm in diameter placed 300 mm
from the bottom. The temperature was measured 30 mm above the
porous quartz plate. The steam generator is composed of a syringe
pump, a syringe, and a heating tube. The heating tube includes a
stainless-steel tube, heating tape, glass bead, and septum. The
septum has a sealed function for water and gas. The glass beads
can promote the conversion of water into steam because they can
provide surface area for vaporization nucleation and prevent
superheating and bumping for water. The heating tape was used to
heat the whole steam-generator system. This steam generator can
provide the desired flow rate of steam for the experiment. The flow
rates of methane, air, carbon dioxide, and nitrogen are controlled
by the mass flow controller (MFC).
2.2. Experimental Materials. CH4, CO2, and N2 used in this
study are high pure gases. The air used in this study is general,
compressed air. Coal carbonization and char activation were carried
out in the fluidized-bed reactor, as shown schematically in Figure
1. The carbonization temperature was 450 °C. After 10 g of coal
was added to the fluidized bed, the reactor was connected to a N2
supply at a flow rate of 800 mL/min. After 30 min of purging at
room temperature, the reactor was heated to the desired temperature.
After 30 min of carbonization time, the electric furnace power was
shut off. During the purge, carbonization, and cooling stages, N2
flow was constant to prevent char oxidation. The prepared char
samples were activated by steam following the carbonization
procedure in the same reactor at 800 °C for 30 min. Water injection
was controlled by a syringe pump, steamed in a preheater, and then
carried in a flow of N2 at 320 mL/min. The coal ashes used in the
experiment were sampled from the coal-fired plant. Two commercial
gasification chars were derived from commercial integrated gasification combined cycle (IGCC) processes.
Catalytic Effects of Coal Char and Coal Ash
Energy & Fuels, Vol. 22, No. 4, 2008 2343
2.3. Experimental Procedure. During the experiment, N2 was
introduced into the fluidized-bed reactor for 1 h to avoid the
interference of oxygen during methane reforming and partial
oxidation. After the activated char (or fly ash) was added into the
reactor, it was preheated to 200 °C and the heating tube was
preheated to 150 °C. Simultaneously, methane and steam generated
from the steam-generating system (or carbon dioxide or air) were
introduced into the fluidized-bed reactor. The flow rate of methane
(or carbon dioxide or air) can be adjusted by the mass-flow
controller, while the flow rate of steam can be adjusted by the
syringe pump. When the fluidized-bed reactor was heated to 700
°C and stabilized for half an hour, the fuel gas sample was collected.
Then, the fluidized-bed reactor was heated to 800, 900, and 950
°C in turn. After the above run, the fuel gas was sampled at different
temperatures. After the experiment, the fluidized-bed reactor must
be cooled to room temperature. Therefore, the power of the system
was shut off.
2.4. Method of Analysis. The compositions of fuel gas samples
were analyzed by a gas chromatograph (Shimadzu Model GC-8A)
with a thermal conductivity detector (TCD) and an injector
connected to a Carboxen-1000 column 60/80 (mesh range) of 15
ft × 1/8 in. stainless steel (2.1 mm inner diameter). Chromatography
calibration was performed with standard gas mixtures of H2, CO,
O2, N2, and CO2, and the standard deviation curve of the typical
component was drawn. Argon was used as the carrier gas at a flow
rate of 40 mL/min. The temperature of the chromatography column
was 70 °C, and the temperature of TCD was 110 °C. The porous
properties including Brunauer-Emmett-Teller (BET) surface area,
pore volume and average pore diameter of fly ashes, pyrolysis chars,
and activated char samples were measured by nitrogen adsorption/
desorption isotherms with a Micrometritics instrument ASAP 2020.
2.5. Methods of Data Processing. CH4 conversion in steam
reforming and partial oxidation of CH4 was calculated as described
below:
XCH4 (%) )
CCO + CCO2
CCH4 + CCO + CCO2
× 100
Figure 2. CH4 conversion of methane steam reforming by different
coal chars and coal ashes.
(6)
where XCH4 is CH4 conversion and CCO, CCO2, and CCH4 are the
contents of CO, CO2, and CH4 in fuel gas, respectively.
The selectivity in methane reforming and partial oxidation was
described by two modes. One called the sum selectivity is the
content sum of H2 and CO in fuel gas, and the other called the
ratio selectivity is the ratio of H2/CO in fuel gas during methane
reforming and partial oxidation.
3. Results and Discussion
3.1. Steam Methane Reforming. Commercially, the steam
methane reforming needs a catalyst to promote the reaction
kinetics. The most popular commercial catalyst for steam
methane reforming is NiO with a large specific surface area. In
this study, the kinetics of methane reforming and partial
oxidation was evaluated in a fluidized bed reactor. Catalytic
(7) Zhou, H. C.; Jin, B. S.; Zhong, Z. P.; Huang, Y. J.; Xiao, R. Energy
Fuels 2005, 19, 1619–1623.
(8) Matsumura, Y.; Nakamori, T. Appl. Catal., A 2004, 258, 107–114.
(9) Hou, K. H.; Hughes, R. Chem. Eng. J. 2001, 82, 311–328.
(10) Mo, L. Y.; Zheng, X. M.; Jing, Q. S.; Lou, H.; Fei, J. H. Energy
Fuels 2005, 19, 49–53.
(11) Rice, S. F.; McDaniel, A. H.; Hecht, E. S.; Hardy, A. J. J. Ind.
Eng. Chem. Res. 2007, 46, 1114–1119.
(12) Ruckenstein, E.; Hu, Y. H. Ind. Eng. Chem. Res. 1998, 37, 1744–
1747.
(13) Pistonesi, C.; Juan, A.; Irigoyen, B.; Amadeo, N. Appl. Surf. Sci.
2007, 253, 4427–4437.
(14) El-Bousiffi, M. A.; Gunn, D. J. Int. J. Heat Mass Transfer 2007,
50, 723–733.
(15) Chen, W. J.; Sheu, F. R.; Savage, R. L. Fuel Process. Technol.
1987, 16, 279–288.
(16) Sun, Z. Q.; Wu, J. H.; Haghighi, M.; Bromly, J.; Ng, E.; Wee,
H. L.; Wang, Y.; Zhang, D. K. Energy Fuels 2007, 21, 1601–1605.
Figure 3. Sum selectivity (H2 plus CO) of methane steam reforming
by different coal chars and coal ashes.
effects of coal chars and coal ashes from gasification and
combustion processing on steam methane reforming were
evaluated. Methane conversion efficiency, sum selectivity, and
ratio selectivity (H2/CO) of steam methane reforming by
different coal chars and coal ashes are shown in Figures 2–4,
respectively. A blank test without any catalysts was conducted
to compare the catalytic effects of different coal chars and coal
ashes on steam methane reforming. Powder River Basin (PRB)
ACC and Lignite ACC are the gasification chars derived from
low-rank coals, for which the specific surface areas are higher
at 649.3 and 359.9 m2/g, respectively. Kentucky (KY) bit-3 SCC
is a carbonization char from the pyrolysis process with a lower
specific surface area at 3.03 m2/g. Two commercial gasification
chars come from commercial IGCC processes. Their specific
surface areas are lower because they become slag after higher
temperature treatment in the gasifier.
3.1.1. Effect of Coal Chars and Fly Ashes on Methane
ConVersion Efficiency. As indicated in Figure 2, the temperature
is a major factor in CH4 conversion efficiency. The increase of
CH4 conversion efficiency is nearly 25% when there is a
temperature increase from 700 to 900 °C for PRB ACC.
Methane conversion efficiencies by coal chars are all greater
than that in the blank test, which confirms the occurrence of
catalytic effects by coal chars. It seems that coal chars with a
2344 Energy & Fuels, Vol. 22, No. 4, 2008
Zhou et al.
Figure 5. Effects of RSM on CH4 conversion efficiency, sum selectivity
(H2 plus CO), and ratio selectivity (H2/CO) under steam methane
reforming by KY bit-3 chars.
Figure 4. Ratio selectivity (H2/CO) of steam methane reforming by
different coal chars and coal ashes.
higher specific surface area, such as PRB ACC and Lignite
ACC, result in higher CH4 conversion efficiencies and carbonization char and commercial chars result in lower CH4 conversion efficiencies, which are comparable to that in the blank test.
It was also found that there is a greater catalytic effect on steam
methane reforming by coal chars than by fly ashes, which were
derived from the same coals. Similarly, the specific surface areas
of coal chars are generally higher than those of coal ashes, which
possibly explains the difference between catalytic effects by coal
chars and coal ashes.
3.1.2. Effect of Coal Chars and Fly Ashes on the Sum
SelectiVity (H2 Plus CO) of Syngas. The sum selectivity (H2
plus CO) of methane steam reforming is shown in Figure 3.
Similarly, the temperature is a major factor on the sum
selectivity of steam methane reforming. The increase of sum
selectivity of steam methane reforming is nearly 35% by the
temperature increase from 700 to 900 °C for PRB ACC. The
sum selectivity also increases with the increase of the specific
surface area of coal chars. However, the variation of sum
selectivity is not greater by variation of the specific surface area
than that by the temperature variation. The sum selectivity of
coal chars is also greater than that of fly ashes, possibly because
of the same reasons as that of CH4 conversion efficiency. This
trend is apparent in higher temperatures (900 °C) than in lower
temperatures (700 °C).
3.1.3. Effect of Coal Chars and Fly Ashes on the Ratio
SelectiVity (H2/CO) of Syngas. The ratios selectivity (H2/CO)
produced from steam methane reforming by different coal chars
and fly ashes are shown in Figure 4. The temperature seems to
be negatively correlated to the ratios selectivity (H2/CO) for
both coal chars and coal ashes. Two reactions may impact the
ratio selectivity (H2/CO) during steam methane reforming. Under
the lower temperature range, the ratio selectivity (H2/CO)
increases with generating H2 and consumption of CO by the
water-gas shift reaction. Under the higher temperature range,
the methane decomposition reaction, as indicated in eq 5, could
increase the concentration of H2 in the produced syngas despite
the restriction of the water-gas shift reaction. Because of the
interference of the methane decomposition reaction, the ratio
selectivity (H2/CO) is generally larger than that of the stoichiometric factor of eq 1. Although the higher ratio selectivity (H2/
CO) is expected for the cogasification process, the formation
of soot is not expected because it is difficult to burn out.
Figure 6. Effects of GHSV on CH4 conversion efficiency, sum
selectivity (H2 plus CO), and ratio selectivity (H2/CO) under steam
methane reforming by KY bit-3 chars.
3.1.4. Effect of the Ratio of Steam/Methane on Steam
Methane Reforming. As indicated in Figure 5, steam supplied
with the ratio of steam/methane (RSM) at 2 and 3, which are
higher than the stoichiometric factor, does not help in the
abatement of soot formation at 900 °C because the ratio
selectivity (H2/CO) is still higher than the stoichiometric factor
of the syngas product in eq 1. As expected, the increase of RSM
does increase the CH4 conversion efficiency and sum selectivity
in this study because steam methane reforming is a process with
the kinetics control. Higher partial pressure of steam will
increase process kinetics. However, this impact is limited.
Because of energy penalties, we do not suggest a higher steam
ratio, which is applied in the steam methane reforming process.
3.1.5. Effect of Gas Hourly Surface Velocity (GHSV) on
Steam Methane Reforming. Figure 6 shows the impact of
variations of the GHSV on CH4 conversion efficiency, sum
selectivity (H2 plus CO), and ratio selectivity (H2/CO) during
the methane reforming process. The increase of the GHSV
results in the decrease of the CH4 conversion efficiency. The
trend of sum selectivity is similar. The main reason may be
that the residence time of reactants in the fluidized bed reactor
is shortened by the increase of GHSV. At the same time, the
reaction load on coal char increases with GHSV, which will
decrease the catalytic effect of coal char. The ratio selectivity
(H2/CO) also decreases with the increase of the temperature.
The reason has been described in the above text. At above 900
°C, the variation of GHSV does not impact the ratio selectivity
(H2/CO).
Catalytic Effects of Coal Char and Coal Ash
Energy & Fuels, Vol. 22, No. 4, 2008 2345
Figure 9. Effects of ROM on methane partial oxidation.
Figure 7. Effects of RCM on selectivity (H2 plus CO) and ratio
selectivity (H2/CO) under CO2 methane reforming by KY bit-3 char.
Figure 8. Effects of GHSV on methane partial oxidation.
3.2. CO2 Methane Reforming. The investigation of CO2
methane reforming by varying of RCM (ratio of CO2/methane)
is shown in Figure 7. As indicated in Figure 7, the sum
selectivity (H2 plus CO) increases by increasing RCM. It seems
that higher RCM increases the kinetics of CO2 methane
reforming. However, this impact is limited and can not be
compared to the impact of the temperature. The temperature
should be the most significant positive impact factor on CO2
methane reforming. From Figure 7, it can also be seen that the
concentration of CO is higher than that of H2 at high RCM and
bed temperatures. The reverse of the water-gas shift reaction
is also an endothermic reaction, which is favored at high
temperatures. The outcome of this reaction causes the concentration of CO to increase in the syngas and the concentration
of H2 to decrease. The increase of RCM means more carbon
dioxide entering into the reactor to participate in the reaction,
which can increase the partial pressure of CO2 and thus make
reactions faster to generate the concentration of CO and decrease
the concentration of H2.
3.3. Methane Partial Oxidation. Figures 8 and 9 show the
effects of temperature, GHSV, and the ratio of oxygen/methane
(ROM) on three parameters [CH4 conversion efficiency, sum
selectivity, and ratio selectivity (H2/CO)] during methane partial
oxidation. At the invariable GHSV and ROM, all parameters
increase when the bed temperature increases from 700 to 900
°C. However, the ratio selectivity (H2/CO) is always less than
1, as shown in Figure 8. At the invariable bed temperature and
ROM, all parameters just slightly increase with the GHSV
increase. This may indicate fast kinetics of methane or CH4
partial oxidation. At the invariable temperature and GHSV, the
decrease of ROM from 1/2 to 1/3 leads to a small increase of the
sum selectivity (H2 plus CO) in produced gas during methane
partial oxidation. The methane conversion increases with the
increase of ROM, while the ratio selectivity (H2/CO) and the
sum selectivity (H2 plus CO) have a reverse change rule above
900 °C. The high ROM means more oxygen will participate in
the reaction of the direct or partial oxidation of methane and
more methane will be consumed during the direct or partial
oxidation of methane. The increase of oxygen may burn CO
and H2 into CO2 and H2O, which will reduce the concentration
of CO and H2 in produced gas. Simultaneously, the high ROM
means more nitrogen is available in syngas, which dilutes the
concentrations of H2 and CO. Because the adjustability of the
partial oxidation of methane by oxygen is not ideal and there is
a possibility that it could consume H2 and CO, injecting CH4
in the cogasification process should select a reaction zone where
oxygen is not available for CH4 burnout or partial oxidation.
4. Conclusions
The methane reforming and partial oxidation experiments
have been performed in a fluidized bed reactor. The experimental results show the possibility of CH4 reforming and the
partial oxidation with a favorable H2/CO ratio, which is greater
than 5. The higher H2/CO ratio in CH4 reforming and partial
oxidation process means less CH4 needed to adjust the H2/CO
ratio during combined coal gasification and methane reforming.
Coal ashes failed to be a good candidate for a catalyst on CH4
reforming and partial oxidation because of their very low specific
surface area, while coal chars present very promising catalytic
performance on CH4 reforming and partial oxidation because
of their larger specific surface area. In this study, no other
constituents in coal fly ash or special surface properties of coal
chars were correlated with the enhanced CH4 conversion
efficiency. It seems that the specific surface area is the only
variable in controlling methane conversion efficiency.
Acknowledgment. This work was supported by the Kentucky
Governor’s Office Energy Policy (KGOEP) (S-06014932).
EF700638P