Energy Fuels 2010, 24, 3340–3345 Published on Web 02/23/2010
: DOI:10.1021/ef901504x
H2-CH4 Mixtures Produced by Carbon-Catalyzed Methane Decomposition as a Fuel for Internal Combustion Engines† M. J. L azaro,‡ J. L. Pinilla,‡ R. Utrilla,‡ I. Suelves,*,‡ R. Moliner,‡ F. Moreno,§ and M. Mu~ noz§ ‡
Instituto de Carboquı´mica, Consejo Superior de Investigaciones Cientı´ficas (CSIC), Miguel Luesma Cast an 4, 50018 Zaragoza, Spain, and §Departamento de Ingenierı´a Mec anica, Universidad de Zaragoza, 50009 Zaragoza, Spain Received December 9, 2009. Revised Manuscript Received February 15, 2010
In this work, the performance of carbon black catalysts in the catalytic decomposition of methane (CDM) was studied in a fluidized-bed reactor (FBR). Carbon black catalysts have recently gained attention for the CDM because of their relatively high stability and efficiency. The use of fluidized bed reactors may allow for the implementation of CDM on a large scale because of improvements in the economics of the process. Results showed that it was possible to obtain a CH4/H2 ratio of 10-70% in the outlet gas. A specific ratio can be achieved by simply varying the operating conditions, including the catalyst, temperature, and space velocity. This leads to the possibility of coupling the gas stream from the CDM reactor to a conventional combustion engine, yielding significant reductions in CO2 emissions.
power. Among the disordered forms of carbon studied in the literature, activated carbon (AC)6-12 and carbon black (CB)13-15 have been widely investigated. A comparative study of the behavior of carbonaceous materials16-19 revealed that AC was rapidly deactivated; in comparison to AC, CB (with an open porosity) was characterized by a lower reaction rate and a much higher catalytic sustainability.
1. Introduction The catalytic decomposition of methane (CDM) has been proposed in the past decade as an alternative process to conventional methane steam reforming for the production of hydrogen without CO2 emissions.1-3 CDM is a one-step process that produces marketable solid carbon, and the gas products are mixtures of hydrogen and unreacted methane. Among the catalysts used in CDM, carbonaceous catalysts have attracted attention because of several advantages over metallic catalysts: carbonaceous catalysts have higher fuel flexibility, are not poisoned by sulfur,4 are lower in price, and have higher operational temperatures (which can provide higher methane conversions because of a positive shift in thermodynamic equilibrium). According to Muradov,5 disordered forms of carbon are more catalytically active on a weight basis than ordered forms, such as graphite or diamond
(8) Moliner, R.; Suelves, I.; Lazaro, M. J.; Moreno, O. Thermocatalytic decomposition of methane over activated carbons: Influence of textural properties and surface chemistry. Int. J. Hydrogen Energy 2005, 30, 293–300. (9) Bai, Z.; Chen, H.; Li, B.; Li. J, W. Catalytic decomposition of methane over activated carbon. J. Anal. Appl. Pyrolysis 2005, 73, 332–341. (10) Ashok, J.; Naveen Kumar, S.; Venugopal, A.; Durga Kumari, V.; Tripathi, S.; Subrahmanyam, M. COx free hydrogen by methane decomposition over activated carbons. Catal. Commun. 2007, 9, 164– 169. (11) Krzy_zy nski, S.; Kozzowski, M. Activated carbons as catalysts for hydrogen production via methane decomposition. Int. J. Hydrogen Energy 2008, 33, 6172–6177. (12) Abbas, H. F.; Wan Daud, W. M. A. Deactivation of palm shellbased activated carbon catalyst used for hydrogen production by thermocatalytic decomposition of methane. Int. J. Hydrogen Energy 2009, 34, 6231–6241. (13) Lee, E. K.; Lee, S. Y.; Han, G. Y.; Lee, B. K.; Lee, T. J.; Jun, J. H.; Yoon, K. J. Catalytic decomposition of methane over carbon blacks for CO2-free hydrogen production. Carbon 2004, 42, 2641–2648. (14) Ryu, B. H.; Lee, S. Y.; Lee, D. H.; Han, G. Y.; Lee, T.-J.; Yoon, K. J. Catalytic characteristics of various reinforcing carbon blacks in decomposition of methane for hydrogen production. Catal. Today 2007, 123, 303–309. (15) Suelves, I.; Lazaro, M. J.; Moliner, R.; Pinilla, J. L.; Cubero, H. Hydrogen production by methane decarbonization: Carbonaceous catalysts. Int. J. Hydrogen Energy 2007, 32, 3320–3326. (16) Muradov, N.; Smith, F.; T-Raissi, A. Catalytic activity of carbons for methane decomposition reaction. Catal. Today 2005, 102-103, 225–233. (17) Pinilla, J. L.; Suelves, I.; Lazaro, M. J.; Moliner, R. Kinetic study of the thermal decomposition of methane using carbonaceous catalysts. Chem. Eng. J. 2008, 138, 301–306. (18) Suelves, I.; Pinilla, J. L.; Lazaro, M. J.; Moliner, R. Carbonaceous materials as catalysts for decomposition of methane. Chem. Eng. J. 2008, 140, 432–438. (19) Serrano, D. P.; Botas, J. A.; Guil-Lopez, R. H2 production from methane pyrolysis over commercial carbon catalysts: Kinetic and deactivation study. Int. J. Hydrogen Energy 2009, 34 (10), 4488–4494.
†
This paper has been designated for the special section Carbon for Energy Storage and Environment Protection. *To whom correspondence should be addressed. E-mail: isuelves@ icb.csic.es. (1) Muradov, N. Z. CO2-free production of hydrogen by catalytic pyrolisis of hydrocarbon fuel. Energy Fuels 1998, 12, 41–48. (2) Muradov, N. Z.; Veziroglu, T. N. From hydrocarbon to hydrogencarbon to hydrogen economy. Int. J. Hydrogen Energy 2005, 30, 225–237. (3) Moliner, R.; L azaro, M. J.; Suelves, I.; Palacios, J. M.; Pinilla, J. L.; Echegoyen, Y. In Natural Gas Research Progress; David, N., Michel, T., Eds.; Nova Science Publishers, Inc.: Hauppauge, NY, 2008. (4) Pinilla, J. L.; Suelves, I.; Lazaro, M. J.; Moliner, R. Influence on hydrogen production of the minor components of natural gas during its decomposition using carbonaceous catalysts. J. Power Sources 2009, 192, 100–106. (5) Muradov, N. Catalysis of methane decomposition over elemental carbon. Catal. Commun. 2001, 2, 89–94. (6) Kim, M. H.; Lee, E. K.; Jun, J. H.; Han, G. Y.; Kong, S. J.; Lee, B. K.; Lee, T.-J.; Yoon, K. J. Hydrogen production by catalytic decomposition of methane over activated carbons: Deactivation study. Korean J. Chem. Eng. 2003, 20, 835–839. (7) Kim, M. H.; Lee, E. K.; Jun, J. H.; Kong, S. J.; Han, G. Y.; Lee, T. J.; Yoon, K. J. Hydrogen production by catalytic decomposition of methane over activated carbons: kinetic study. Int. J. Hydrogen Energy 2004, 29, 89–93. r 2010 American Chemical Society
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Table 1. Ultimate Analysis, Textural Parameters, and Surface Chemistry of the Catalysts ultimate analysis
textural properties
amount of COx generated in TPD tests
catalyst
C
H
N
S
O
SBET (m2/g)
Vp (cm3/g)
CO (cm3/g)
CO2 (cm3/g)
BP1300 BP2000
83.7 97.1
0.5 0.2
0.37 0.16
0.8 0.73
15.4 2.7
495 1337
0.92 3.06
13.82 8.23
9.84 3.75
The catalytic activity of commercially available AC and CB has been thoroughly investigated in previous works carried out by our research group in a thermobalance (acting as a differential reactor)17,18 and a fixed-bed reactor.4,8,15,20 For instance, BP2000 CB, characterized by a large pore volume, showed the highest activity for methane decomposition per mass of catalyst and became deactivated when the amount of carbon deposited was equal to 6 g/gcat. The maximum amount of carbon that carbonaceous catalysts can accommodate was found to be dependent upon the pore volume. The initial reaction rate was found to be related to the surface chemistry of carbonaceous catalysts, including the amount of COx desorbed in a temperature-programmed desorption (TPD) test. For instance, the BP1300 catalyst had a high amount of COx desorbed in a TPD test and was found to provide a greater methane decomposition rate at early stages of the reaction compared to BP2000, accounting for a lower amount of COx desorbed.17,18 From an industrial point of view, the best conceptual design for conducting CDM with carbonaceous catalysts in a continuous mode is the fluidized-bed reactor (FBR). This reactor configuration allows CDM to be conducted continuously by adding fresh catalyst and simultaneously withdrawing spent catalyst.21 Thus, the use of FBR for methane decomposition with metallic catalysts has been considered one of the best alternatives to the co-production of hydrogen and nanotubes or nanofibers.22-25 Recently, reviews on the production of carbon nanotubes in a FBR by chemical vapor deposition have been published.26,27 In addition to continuous operation and substantial cost savings, other advantages of FBR include the efficient mixing of catalyst grains in the bed, efficient mass transfer between the gaseous carbon source and solid catalyst
through large exchange surfaces, and a nearly uniform temperature in the reaction zone.28 The use of CDM in a FBR with carbonaceous catalysts (CB or AC) has recently been studied. For instance, Lee et al.29 tested several types of ACs to examine activity in a fluidized bed with a quartz reactor with a diameter of 0.055 m. The results were compared to those obtained in a fixed-bed reactor and revealed similar reaction rates and deactivation patterns. Dunker et al.30 studied the behavior of various types of CBs in methane decomposition in a quartz FBR with a diameter of 0.042 m. The hydrogen content of the exhaust gas was above 40% for an extended time, up to 1600 min on stream when the BP2000 catalyst was used. Muradov also studied the scaling up31 of CDM in a FBR and proposed a system composed of a circulating fluidized bed of carbonaceous particles formed by a catalytic decomposition reactor and a catalyst regeneration reactor. The aforementioned process was modeled with a hydrogen production capacity of 50 Tm/day, and the results predicted reactor dimensions comparable to those already operating in fluidized catalytic cracking (FCC) plants. In this work, CDM with various CB catalysts has been studied in a FBR to establish operating conditions that produce different ratios of H2/CH4 and has been achieved by varying the catalyst and other parameters. This process may be a suitable method for producing hydrogen-methane mixtures on demand, and they could be used directly as fuel in an internal combustion engine (ICE). Advantages of this configuration are thoroughly explained. 2. Experimental Section 2.1. Carbonaceous Catalyst. Two commercial CB catalysts, BP1300 and BP2000 (supplied by Cabot), were purchased in pellet form and used as catalysts for CDM. Table 1 shows textural parameters, including surface area and pore volume, obtained from N2 adsorption at 77 K, the surface chemistry expressed as cm3/g of CO and CO2 released from the catalyst in a TPD test, and ultimate analysis of carbonaceous catalysts. BP1300 showed significant surface chemistry, including a high oxygen content and a large amount of groups desorbed as CO and CO2. BP1300 also displayed moderate textural development, including a specific surface area and pore volume of 495 m2/g and 0.96 cm3/g, respectively. The BP2000 catalyst displayed a high degree of texture, with a surface area of 1337 m2/g and a pore volume of 3.06 cm3/g, and poor surface chemistry, including low oxygen content and a low number of groups desorbed as CO/CO2 in TPD. The majority of the catalyst surface was open and relatively accessible to methane. 2.2. Experimental Setup. The FBR was composed of Kanthal and possessed an inner diameter of 0.065 m and a height of
(20) Pinilla, J. L.; Suelves, I.; Utrilla, R.; Galvez, M. E.; Lazaro, M. J.; Moliner, R. Hydrogen production by thermo-catalytic decomposition of methane: Regeneration of active carbons using CO2. J. Power Sources 2007, 169, 103–109. (21) Muradov, N. Hydrogen via methane decomposition: An application for decarbonization of fossil fuels. Int. J. Hydrogen Energy 2001, 26, 1165–1175. (22) De Jong, K. P.; Geus, J. W. Carbon nanofibers: Catalytic synthesis and applications. Catal. Rev.-Sci. Eng. 2000, 42, 481–510. (23) Qian, W.; Liu, T.; Wang, Z.; Wei, F.; Li, Z.; Luo, G.; Li, Y. Production of hydrogen and carbon nano tubes from methane decomposition in a two-stage fluidized bed reactor. Appl. Catal., A 2004, 260, 223–228. (24) Shah, N.; Ma, S.; Wang, Y.; Huffman, G. P. Semi-continuous hydrogen production from catalytic methane decomposition using a fluidized-bed reactor. Int. J. Hydrogen Energy 2007, 32, 3315–3319. (25) Pinilla, J. L.; Moliner, R.; Suelves, I.; Lazaro, M. J.; Echegoyen, Y.; Palacios, J. M. Production of hydrogen and carbon nanofibers by thermal decomposition of methane using metal catalysts in a fluidized bed reactor. Int. J. Hydrogen Energy 2007, 321, 4821–4829. (26) See, C. H.; Harris, A. T. A review of carbon nanotube synthesis via fluidized-bed chemical vapour deposition. Ind. Eng. Chem. Res. 2007, 46, 997–1012. (27) Philippe, R.; Moranc-ais, A.; Corrias, M.; Caussat, B.; Kihn, Y.; Kalck, P.; Plee, D.; Gaillard, P.; Bernard, D.; Serp, P. Catalytic production of carbon nanotubes by fluidized bed CVD. Chem. Vap. Deposition 2007, 13, 447–457. (28) Vahlas, C.; Caussat, B.; Serp, P.; Angelopoulus, G. N. Principles and applications of CVD powder technology. Mater. Sci. Eng., R 2006, 53, 1–72.
(29) Lee, K. K.; Han, G. Y.; Yoon, K. J.; Lee, B. K. Thermocatalytic hydrogen production from the methane in a fluidized bed with activated carbon catalyst. Catal. Today 2004, 93-95, 81–86. (30) Dunker, A. M.; Kumar, S; Mulawa, P. Production of hydrogen by thermal decomposition of methane in a fluidized-bed reactor— Effects of catalyst, temperature, and residence time. Int. J. Hydrogen Energy 2006, 31, 473–484. (31) Muradov, N.; Chen, Z.; Smith, F. Fossil hydrogen with reduced CO2 emission: Modelling thermocatalytic decomposition of methane in a fluidized bed of carbon particles. Int. J. Hydrogen Energy 2005, 30, 1149–1158.
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Table 2. Methane Flow Rate, Minimum Fluidization Velocity, and Physical Properties of the Catalysts catalyst
apparent density (g/cm3)
aggregate particle size (mm)
umf, N2 (cm/s)
Qmf, CH4 (h-1)
BP1300 BP2000
0.369 0.155
0.5-1 0.5-1
0.55 0.52
90 85
0.8 m. A perforated horizontal plate with holes with a diameter of 3 mm was used to divide the reactor into two chambers. All variables affecting the process, including pressure, temperature, and gas flow rate, were recorded continuously by an online personal computer (PC). The gas entering the stainless-steel reaction zone was preheated by an electric furnace at 550 °C. Type K thermocouples (Thermocoax) were used to monitor the temperature in the preheater and reactor chambers. Hydrogen, methane, and nitrogen flow rates in the feed gas were controlled by mass flow controllers (Bronkhorst). The combined pressure drop across the distributor and fluidized bed was measured with a differential pressure transducer. 2.3. Activity Tests. All experiments were conducted at atmospheric pressure. Catalysts were pretreated under nitrogen at 850 °C for 3 h to achieve stability, cooled, and weighted before tests were conducted. The catalyst was placed in the reactor, and nitrogen flowed, while the temperature was raised to the reaction temperature. Next, pure methane (99.99%) was fed into the reactor for methane decomposition at the selected temperature. The desired weight hourly space velocity (WHSV) was achieved with flow rates of pure methane between 80 and 240 L N h-1. The composition of the outlet gas was determined by micro gas chromatography (GC). The carbon deposited during each run (Cdep) was determined by direct weight differences. 2.4. Calculation of the Minimum Fluidization Velocity. The minimum fluidization velocity of the CB catalysts was determined experimentally with nitrogen gas at the reaction temperature (850 °C). To avoid undesired reactions and agglomeration, an inert gas was used during the determination of umf. The reactor was charged with 80 g of fresh carbonaceous catalyst. In these experiments, the pressure drop across the bed increased with an increase in the fluidizing gas velocity until a constant value was achieved. A plot of Δp versus gas velocity was extrapolated to a value corresponding to the maximum theoretical pressure drop (Δpmax = W/S) and yielded the minimum fluidization velocity (umf). To account for any effects of using methane as the fluidizing gas, the minimum fluidization velocity with methane was calculated using the theoretical minimum fluidization velocity under the reaction conditions according to the method proposed by Wen and You,32 using as constants those proposed by Adanez et al.33 for carbonaceous materials. The ratio between the minimum fluidization velocity for nitrogen and methane was found to be 1.4. From the reaction-temperature fluidization velocities obtained experimentally for nitrogen, the minimum fluidization velocity for CH4 was calculated with the appropriate correction factor to account for the use of CH4. 2.5. Characterization Techniques. The textural properties of carbonaceous catalysts were measured by N2 adsorption at 77 K in a Micromeritics ASAP2020 apparatus. The specific surface areas and pore volumes were calculated by applying the Brunauer-Emmett-Teller (BET) method to the respective N2 adsorption isotherms. The amount of CO and CO2 released was determined by TPD. Bulk density, Fb, was determined by measuring the weight of a known volume of catalyst.
3.1. Determination of the Minimum Fluidization Velocity. Table 2 shows the apparent density and mean aggregate particle size of carbonaceous catalysts. The carbon particles belong to group A of Geldart’s classification. These solids fluidize easily and display smooth fluidization at low gas velocities and controlled bubbling with small bubbles at high gas velocities. A FCC catalyst is representative of these types of solids.34 Figure 1 shows the experimental values of Δp measured by increasing (9) and decreasing (b) the superficial gas velocity in the presence of BP2000. The nitrogen umf was found to be 0.52 cm/s and was obtained through the extrapolation of a plot of Δp versus superficial gas velocity at standard temperature and pressure (STP) to the maximum theoretical pressure. Similarly, the determination of umf for BP1300 was conducted using the same procedure (results not shown) and resulted in a umf of 0.55 cm/s. The umf values of the BP1300 and BP2000 catalysts with methane as a fluidizing gas have been calculated by applying the aforementioned correction factor. Table 2 shows the methane flow rate necessary to operate at the minimum fluidization velocity in the experimental setup. 3.2. Catalyst Stability in Long-Term FBR-CDM Tests. Previous work by our research group in a fixed-bed reactor showed that BP2000 had a high catalytic sustainability.35 For instance, extended reaction times revealed that catalyst activity was maintained during long times on stream and was deactivated after 1200 min at 950 °C. Figure 2 shows the hydrogen concentration measured in the gas stream from the reactor as a function of time in a test conducted with BP2000 at 850 °C, a WHSV of 2.25 L gcat-1 h-1, a bed height of 8 cm, and a uo/umf of 1 for 20 h. The BP2000 catalyst showed a high catalytic stability after a slight initial decay in the hydrogen concentration from 36 to 30% (balanced with methane) and a quasi-steady state
(32) Wen, C.; Yu, Y. A generalized method for predicting minimum fluidization velocity. AIChE J. 1966, 12, 610–612. (33) Ad anez, J.; Abadanes, J. C. Minimum fluidization velocities of fluidized-bed coal-combustion solids. Powder Technol. 1991, 67, 113– 119.
(34) Kunii, D.; Levenspiel, O. Fluidization Engineering, 2nd ed.; Butterworth-Heinemann: Oxford, U.K., 1991; pp 77-78. (35) Lazaro, M. J.; Pinilla, J. L.; Suelves, I.; Moliner, R. Study of the deactivation mechanism of carbon blacks used in methane decomposition. Int. J. Hydrogen Energy 2008, 33, 4104–4111.
Figure 1. Pressure drop (Δp) across the reactor bed versus superficial fluidization velocity. Fluidization gas, nitrogen; temperature, 850 °C; and catalyst, BP2000. (9 and b) Data points obtained at increasing and decreasing gas velocities, respectively.
3. Results
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Figure 4. Influence of the temperature on the change in the hydrogen concentration at different bed heights. uo/umf = 1.
Figure 2. Change in the hydrogen content for the BP2000 catalyst in an extended test. Temperature, 850 °C; WHSV, 2.25 L gcat-1 h-1; bed height, 8 cm; and uo/umf, 1. The test duration was 20 h.
Because of the catalytic stability of CB catalysts, a series of experiments were conducted with the BP2000 catalyst, where the bed height was fixed between 8 and 16 cm and a uo/umf ratio of 1 was used. The temperature was varied randomly, and the percent of H2 in the output was measured after stabilization of the catalytic bed. The results are shown in Figure 4. The H2 concentration in the gas stream leaving the reactor increased linearly with an increasing reaction temperature. Moreover, as discussed above, an increase in the bed height (decrease in space velocity) yielded higher concentrations of H2. Thus, it can be concluded that increasing the operating temperature increases the reaction kinetics and provides a higher hydrogen concentration. Additionally, the average methane conversion and the net hydrogen production rate (expressed as liters of hydrogen produced per gram of catalyst and hour) increased with an increasing reaction temperature, as observed in Table 3. However, increasing the space velocity decreased the contact time of the solid and reactant gas, which reduced the average methane conversion and the average hydrogen production rate (Table 3). 3.4. Production of H2-CH4 Mixtures by FBR-CDM. In these experiments, the effect of operating parameters (catalyst, temperature, space velocity, and uo/umf ratio) on the concentration of H2 in the gas stream leaving the reactor was investigated. In Table 4, the results of all experiments under varying operating parameters are listed. Because of the catalyst stability under varying experimental conditions, the catalysts were further tested by varying the temperature between 850 and 950 °C at an identical space velocity and measuring the H2 concentration in the flue gas after a steady state was obtained. As shown in Figure 5, the hydrogen concentration (methane balanced) with carbonaceous catalysts under varying operating conditions in FBR are shown in a bar chart. Thus, the H2/CH4 ratio can be modified by changing the operating temperature or space velocity. For instance, the H2 concentration can be tuned between 30 and 75% by conducting CDM with BP2000 at an operating temperature of 900 °C and varying the space velocity from 0.75 to 2.25 L gcat-1 h-1. Similarly, the H2 concentration ranged from 35 to 65% with a constant space velocity of 1.13 L gcat-1 h-1 and a change in the operating temperature from 850 to 950 °C.
Figure 3. Influence of the bed height on the change in the hydrogen concentration. T, 900 °C; uo/umf, 1.
that lasted for approximately 1200 min on stream. At the end of the test, the amount of catalyst deposited was 4.2 g/gcat. In a previous work,17 it was shown that the maximum carbon accumulation capacity of BP2000 was 6 g/gcat. The catalytic stability of the BP1300 catalyst in an extended test conducted at 900 °C with a bed height of 8 cm, a uo/umf of 1.33, and a WHSV of 1.25 L gcat-1 h-1 was also remarkable and achieved a constant hydrogen concentration of 30% after 800 min on stream (results not shown). 3.3. Effect of the Operating Conditions. Figure 3 shows the effect of the bed height on the change in the H2 concentration of the gas stream leaving the reactor for experiments conducted at 900 °C with a BP2000 catalyst. The methane flow rate was set at 90 L N h-1, which corresponded to the calculated minimum fluidization flow rate. Therefore, the ratio of uo/umf in the experiment was 1. The bed height was varied from 8 to 24 cm, corresponding to a WHSV from 2.25 to 0.75 L gcat-1 h-1, respectively The BP2000 catalyst provided stable methane conversion throughout the experiments. The H2 concentration increased with an increasing bed height from 30% at a height of 8 cm to 75% at a bed height of 24 cm. 3343
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In previous studies, we stated that the reaction temperature and space velocity did not have an effect on the amount of carbon that the catalyst could accommodate before deactivation occurred.17,18 Thus, the operating conditions did not determine the final amount of carbon deposited onto the catalysts or the net hydrogen production during the lifetime of a carbonaceous catalyst. On the contrary, when a metallic catalyst was used, deactivation was dependent upon the reaction conditions used in CDM. For instance, it was determined that the temperature and space velocity had a significant role in the deactivation kinetics of a nickel catalyst in CDM. Thus, we envisaged carbonaceous catalyzed methane decomposition as a viable method for the production of different H2-CH4 mixtures on demand, achieved by varying operating conditions without detrimental effects on the catalyst performance when compared to other catalysts.
To understand the behavior of an ICE fueled with hydrogen, the physicochemical properties of the fuel must be analyzed. For example, the hydrogen shows a wide flammability range. This property provides a complete combustion under lean operating conditions, besides allowing for the engine operation in low-load conditions without the throttle valve closed.36 The auto-ignition temperature of hydrogen is higher than gasoline, and that indicates that a higher compression ratio could be used in an ICE fueled with hydrogen. For this reason, the improvements are better in spark-ignition engines (SIEs) than in compression ignition engines (CIEs). Moreover, this effect improves the indicated thermal efficiency of an ICE. On the other hand, while the hydrogen combustion process occurs, the flame propagation is carried out at a very high velocity. This effect approaches the real process to the ideal process (heat investment at constant volume), which provides the maximal indicated thermal efficiency.36 Likewise, the high diffusion coefficient of hydrogen involves a very quick and homogeneous air-fuel mixture. However, hydrogen used as fuel in a SIE generates significant problems, such as backfire and an increase in knock and NOx emissions.37-40 Hydrogen combustion produces NOx as a result of the dissociation of nitrogen and occurs in high-temperature regions of the engine.41 Nevertheless, hydrogen permits complete combustion under very lean operating conditions. This property involves a significant reduction of the combustion temperature and, in consequence, the NOx production.36,42
4. Discussion In a typical CDM test, the outlet gas is a mixture of H2 and unconverted methane. A considerable amount of attention has been devoted to the study of H2-CH4 mixtures as fuel for ICEs. While hydrogen fuel cells achieve enough maturity, hydrogen could be used in ICEs to contribute to a smooth transition to the hydrogen economy, which is approaching as an energetic alternative economy. Good hydrogen properties and easy modifications of the machinery with low costs permit us to convert an ICE into a hydrogen-fueled engine. This strategy may play a relevant role in this energetic economy transition. Therefore, it is worth studying in depth the behavior of an ICE fueled with pure hydrogen or hydrogen blended with other fuels. Table 3. Average Methane Conversion and Average H2 Production Rate at Different Operating Conditions T (°C)
WHSV (L gcat-1 h-1)
CH4 conversion
H2 production (L gcat-1 h-1)
850 850 850 850 850 875 900 925 950 850 875 900 925 950
0.75 1.13 1.5 2.25 2.25 2.25 2.25 2.25 2.25 1.12 1.12 1.12 1.12 1.12
62.00 41.20 26.30 18.20 4.82 8.16 11.88 16.31 23.64 19.58 25.18 32.50 38.88 48.26
2.79 1.23 0.29 0.27 0.22 0.37 0.53 0.73 1.06 0.44 0.56 0.73 0.87 1.08
Figure 5. Bar chart showing the hydrogen concentration (methane balanced) obtained in FBR as a function of operational variables, including catalyst, temperature, space velocity, and uo/umf ratio.
Table 4. List of Experiments and Operation Parameters catalyst
T (°C)
h (cm)
w (g)
Q (h-1)
WHSV (L gcat-1 h-1)
uo/umf
run time (min)
BP2000 BP2000 BP2000 BP2000 BP2000 BP2000 BP2000 BP2000 BP2000 BP1300 BP1300 BP1300 BP1300
850 850 850 850 850-950 850-950 900 900 850 900 900 900 850
8 12 16 24 8 16 4 12 8 4 8 12 8
40 60 80 120 40 80 20 60 40 46.6 95 140.1 95
90 90 90 90 90 90 120 120 120 120 120 120 120
2.25 1.50 1.13 0.75 2.25 1.13 6.00 2.00 3.00 2.58 1.26 0.86 1.26
1.00 1.00 1.00 1.00 1.00 1.00 1.41 1.41 1.41 1.33 1.33 1.33 1.33
1200 220 150 150 15 15 320 420 420 420 700 420 420
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On the other hand, the use of natural gas (NG) as fuel in a SIE presents several positive aspects. The NG as fuel provides high thermal efficiency, low knock probability, and reduced NOx emissions. However, some problems occur, such as low flame velocity and rise of cyclical variability. One way to avoid these problems is the addiction of hydrogen to NG.43 Moderated concentrations of hydrogen do not affect the excellent resistance of knock properties of NG. Nevertheless, when the hydrogen ratio increases, the knock effect is shown earlier.43 On the other hand, the indicated thermal efficiency is increased if the ratio of hydrogen rises,43,44 mainly under lean operating conditions and ignition timing lightly retarded regarding the optimal. The maximum indicated thermal efficiency and maximum power are achieved with blends that contain around 20-25% of hydrogen in volume, and the knock effect still does not appear. According to other investigations, the optimal ratio of hydrogen to obtain the maximum indicated thermal efficiency is even higher (3045 or 50%46), and the minimal fuel consumption is achieved with a hydrogen ratio of 60%.44 The cyclical variability is also
reduced, increasing the ratio of hydrogen. This influence is more important under lean operating conditions47 and ignition timing lightly retarded regarding the optimal.48 Under an environmental point of view, hydrogen and methane blends reduce CO, CO2, and CH4 emissions43,44,46 because the content of carbon is smaller compared to gasoline. However, NOx emissions increase with ratio of hydrogen.43,44,46 Hydrogen ratios between 28 and 36% and a fuel-air equivalence ratio of 0.6 reduce NOx emissions up to extremely low levels. Under these operating conditions, the unburned hydrocarbon level is acceptable.48 According to these experiences, blends of 20% hydrogen do not provide enough reduction of NOx emissions to be the only solution. In conclusion, the use of hydrogen in an ICE as either pure fuel or an additive of NG, gasoline, etc. provides indisputable advantages and, at the same time, some significant problems; therefore, it is a matter that should be thoroughly investigated. Thus, it will be possible to find out the operation limits of this fuel and the modifications to carry out in ICEs to keep a reasonable performance and low pollutant emissions. The modification of a commercial ICE fed with a range of hydrogen-methane mixtures produced by the catalytic decomposition of methane is in progress. The results of the coupling of these two novel technologies are currently being evaluated.
(36) White, C. M.; Steeper, R. R.; Lutz, A. E. The hydrogen-fueled internal combustion engine: A technical review. Int. J. Hydrogen Energy 2006, 31, 1292–1305. (37) Verhelst, S.; Sierens, R.; Verstraeten, S. A critical review of experimental research on hydrogen fueled SI engines. SAE Tech. Pap. 2006-01-0430, 2006. (38) Kim, J. M.; Kim, Y. T.; Lee, J. T.; Lee, S. Y. Performance characteristics of hydrogen fueled engine with the direct and spark ignition system. SAE Tech. Pap. 952488, 1995; pp 162-175. (39) Szwaja, S.; Bhandary, K. R.; Naber, J. D. Comparisons of hydrogen and gasoline combustion knock in a spark ignition engine. Int. J. Hydrogen Energy 2007, 32, 5076–5087. (40) Maher, A. R.; Al-Baghdadi, S. Effect of compression ratio, equivalence ratio and engine speed on the performance and emission characteristics of a spark ignition engine using hydrogen as a fuel. Renewable Energy 2004, 29, 2245–2260. (41) Subramanian, V.; Mallikarjuna, J. M.; Ramesh, A. Intake charge dilution effects on control of nitric oxide emission in a hydrogen fueled SI engine. Int. J. Hydrogen Energy 2007, 32, 2043–2056. (42) Kiesgen, G.; Kl€ uting, M.; Bock, C.; Fischer, H. The new 12cylinder hydrogen engine in the 7 series: The H2 ICE age has begun. SAE Tech. Pap. 2006-01-0431, 2006. (43) Karim, G. A.; Wierzba, I.; Al-Alousi, Y. Methane-hydrogen mixtures as fuels. Int. J. Hydrogen Energy 1996, 21 (7), 625–631. (44) Bauer, C. G.; Forest, T. W. Effect of hydrogen addition on the performance of methane-fueled vehicles. Part I: Effect on SI engine performance. Int. J. Hydrogen Energy 2001, 26, 55–70. (45) Kahraman, N.; C-eper, B.; Orhan Akansu, S.; Aydin, K. Investigation of combustion characteristics and emissions in a spark ignition engine fuelled with natural gas-hydrogen blends. Int. J. Hydrogen Energy 2009, 34, 1026–1034. (46) Fanhua, M.; Yu, W.; Haiquan, L.; Yong, L.; Junjun, W.; Shuli, Z. Experimental study on thermal efficiency and emission characteristics of a lean burn hydrogen enriched natural gas engine. Int. J. Hydrogen Energy 2007, 32, 5067–5075.
5. Conclusions Tests conducted in a FBR indicated that CB catalysts are highly stable in CDM. A wide range of H2-CH4 mixtures was achieved, from 10:90 to 70:30 (vol %), depending upon operating conditions, such as the type of CB catalyst, temperature, space velocity, and ratio of gas flow velocity, uo, to minimum fluidization velocity, umf (uo/umf). Acknowledgment. This work was achieved with financial support from the Plan Nacional de Energı´ a-FEDER: Project ENE2005-03801 (MEC) and Project ENE2008-06516 (MICINN). Note Added after ASAP Publication. Table 3 contained incorrect data in the version of this paper published ASAP February 23, 2010, and March 1, 2010; the corrected version published ASAP March 4, 2010. (47) Jinhua, W.; Hao, C.; Bing, L.; Zuohua, H. Study of cycle-bycycle variations of a spark ignition engine fueled with natural gas-hydrogen blends. Int. J. Hydrogen Energy 2008, 33, 4876–4883. (48) Hoekstra, R. L.; Collier, K.; Mulligan, N.; Chew, L. Experimental study of a clean burning vehicle fuel. Int. J. Hydrogen Energy 1995, 20 (9), 737–745.
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