A modified method for production of hydrogen ... - Wiley Online Library

INTERNATIONAL JOURNAL OF ENERGY RESEARCH Int. J. Energy Res. 2012; 36:1133–1138 Published online 27 April 2011 in Wiley Online Library (wileyonlinelibrary.com). DOI: 10.1002/er.1854

TECHNICAL NOTE

A modified method for production of hydrogen from methane Sushant Kumar,y, Surendra K. Saxena and Vadym Drozd Center for the Study of Matter at Extreme Conditions, College of Engineering and Computing, Florida International University, Miami, FL 33199, U.S.A.

SUMMARY The steam–methane-reformation (SMR) reaction has been modified by including sodium hydroxide in the reaction. It is found that the reaction: 2NaOH1CH41H2O 5 Na2CO314H2 takes place at much lower temperatures (300–6001C) than the SMR reaction (800–12001C). The reaction rate is enhanced with a nickel catalyst. We have studied the effect of variously ball-milled nickel on the reaction rate and determined the optimum particle size of the catalyst. Best results were achieved by grinding the catalyst for 2 h. Prolonged ball milling caused the nickel platelets to coalesce and grow in size decreasing the reaction rate. Copyright r 2011 John Wiley & Sons, Ltd. KEY WORDS SMR reaction; hydrogen production; carbon sequestration; catalyst Correspondence *Sushant Kumar, Center for the Study of Matter at Extreme Conditions, College of Engineering and Computing, Florida International University, Miami, FL 33199, U.S.A. y E-mail: [email protected] Received 12 March 2011; Accepted 14 March 2011

1. INTRODUCTION Most hydrogen for industrial use is produced by steam–methane reforming technique (SMR) [1]. This technique results in a large carbon dioxide emission. This problem can be regarded as one of the reasons that has downgraded hydrogen as an energy alternative in the near future. Although there are several possible biological alternatives on small scale that may be considered as carbon neutral, electrolysis of water is the only carbon emission free alternative to generate hydrogen on an industrial scale. But this method is very energy intensive and that energy cannot come from fossil fuel. There are several proposed methods currently under study to solve this problem [2–6]. The purpose of this paper is to show that it is possible to modify the SMR technique in such a way that all carbon emission is sequestered during the production of hydrogen. Once this problem is solved, the technology already exists which can use hydrogen as a fuel-carrier in many applications. It may be further noted that other than using alternate energies for electrolysis of water or the biological methods, there are no techniques to produce carbon emission free hydrogen from fossil fuels. We have eliminated the Copyright r 2011 John Wiley & Sons, Ltd.

problem of carbon emission by using a hydroxide as a reactant with methane and water, producing hydrogen with zero carbon emission. We note that if NaOH is to be produced for the purpose of carbonation, the method cannot be considered as carbon emission free because sodium hydroxide is produced from electrolysis of brine. Such a process is very energy intensive. However, if we use the sodium hydroxide that is produced as a byproduct from existing chlor-alkali plants, we can make use of it and turn it into useful soda and produce hydrogen. This study follows our earlier studies on the use of coal for hydrogen production [7] and of methane [8]. With concurrent carbonation, the reaction chemistry changes favorably to produce hydrogen at temperatures lower than those used by the industry. In this paper, we present results of modifying the SMR reaction by introducing sodium hydroxide, which will be demonstrated to lower the reaction temperature significantly. We check further the possibility of further reducing the temperature by catalysis. We use variously sized nickel powder to demonstrate the effect on temperature but the study is not about catalysis. This paper deals with the SMR technique, which is currently the most popular technique to produce hydrogen on an industrial scale. There are other 1133

S. Kumar, S. K. Saxena and V. Drozd

Modified method for production of hydrogen

proposals to treat organic materials and fuels that use the carbonation reaction. One such method is proposed by Reichman et al. [9], which uses sodium hydroxide in the same way as we do. They used a closed system as opposed to our open system reaction which is industrially more appropriate.

1.1. THE MODIFIED STEAM– METHANE REFORMATION The cheapest and most common method of producing commercial bulk hydrogen as well as the hydrogen used in the industrial synthesis of ammonia is considered to be the one involving the reactions at high temperatures (973–1373 K) and in the presence of a metal-based catalyst (nickel). Steam reacts with methane to yield carbon monoxide and hydrogen: CH4 1H2 O ! CO13H2 ðendothermicÞ Additional hydrogen can be recovered by a lowertemperature gas-shift reaction with the carbon monoxide produced in the reaction above. The reaction is CO1H2 O ! CO2 1H2 ðexothermicÞ The United States produces 9 million ton of hydrogen per year, mostly with steam reforming of natural gas. The modified reaction is as follows: CH4 12NaOH1H2 O ¼ Na2 CO3 14H2 The modified reaction proceeds at lower temperatures and the metal-based catalysis may not be required. The total energy requirement for the combined SMR and gas-shift reactions for the complete reaction (maximum hydrogen production) is 430 kJ mol1 of methane (at 1200 K) versus 244 kJ (at 700 K) for the modified reaction. Note that the carbonation reaction is combined with the SMR reaction and is not a scrubbing process which is a totally different chemistry.

Gas flow regulator

2. EXPERIMENTAL SECTION The reaction 2NaOH ðsÞ1CH4 ðgÞ1H2 OðgÞ ! Na2 CO3 ðsÞ14H2 ðgÞ was studied experimentally in a previous study without the use of a catalyst [8]. NaOH pellets (98% purity) were supplied by Sigma Aldrich. A minimal amount of distilled water was added to an approximate amount of 0.1 g NaOH in an alumina boat. The equipment used for this reaction has been illustrated in Figure 1. The tubular furnace is comprised of alumina tube in which this alumina boat was positioned at exactly the hot spot. Initially the tube was flushed with nitrogen gas along with steam. Once the required temperature was achieved, the flow of nitrogen gas was ceased and the methane gas was allowed to flow at 25ml min1 for the specified time. The rate of formation of sodium carbonate was studied at different temperatures over different times. The product analysis was performed by the method of titration using 0.1009 N volumetric standard solution of nitric acid (Aldrich). Phenolphthalein and Methyl Orange were used as indicators. Nickel powder (2–5 mm) with a purity of 99.99% was obtained from Aldrich and used as a catalyst for this reaction. This catalytic behavior was also studied for different sizes of nickel powder produced by ball milling. Nickel is of a low cost, high activity and widely employed catalyst for the industrial application [10]. Various studies [11–14] have been carried out so far nickel supported on alumina as a catalyst, which resulted in better performance of the SMR reaction. Methane dissociates on catalytic surface due to increased reaction rate. The nickel powder is homogenously mixed with NaOH, which is spread in a thin layer. On melting, the powder will spread with nickel distributed evenly. The products of reaction and the catalysts were analyzed by powder X-ray diffraction using the Bruker GADDS/D8 X-ray system with the Apex Smart CCD

Gas flow meter Sample in alumina boat Furnace with thermocontroller Alumina tube

N 2 carrier gas H2 CH 4 gas

Steam generator Figure 1. Experimental setup for studying the reaction kinetics of the modified SMR reaction.

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Int. J. Energy Res. 2012; 36:1133–1138 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er


Detector and Mo Ka-radiation. Ball milling of a catalyst was performed using the planetary Retsch PM100 ball mill.

3. RESULTS AND DISCUSSION 3.1. XRD Analysis In 1918, P. Sherrer explained that when parallel monochromatic radiation falls on a random-oriented mass of crystals, the diffracted beam is broadened when the particle size is small. The two major informations that can be gathered from peak width analysis are crystallite size and lattice strain. Here, we are mainly focused to determine the crystallite size of the catalysts. Crystallite size represents the size of coherently diffracting domain. Owing to the presence of polycrystalline aggregates, crystallite size is generally not the same as of the particle size.


The diffraction patterns from the different ballmilled catalysts have been observed based on Bragg’s law nl 5 2d sin y, where n is an integer, l is the wavelength of Mo Ka1 radiation, d is the inter planar spacing and y is the diffraction angle. The output from the X-ray analysis of these catalysts yields various plots of intensity vs angle of diffraction. The evaluation of XRD peak broadening inherently adds error due to the instrument interference. Therefore, it is necessary to eliminate this problem. To do so, it is customary to collect a diffraction pattern from the line broadening of a standard material such as LaB6 to determine the instrumental broadening. The Scherrer equation explains the relationship between mean crystallite size and diffraction line breadth. The equation that was employed here is D ¼ 0:9l=ðb b0 Þ cos y where l is the wavelength of X-ray radiation (Mo Ka1 5 0.7093171), b the breadth at half maximum

Figure 2. Particle size distribution: (a) Raw Ni; (b) Ball milled for 2 h; par;c) Ball milled for 3 h; and (d) Ball milled for 4 h. Int. J. Energy Res. 2012; 36:1133–1138 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

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essentially complete at 6001C without using any metal catalyst. According to X-ray diffraction study the product of complete reaction is decahydrated sodium carbonate, Na2CO3 10H2O. While the result in Figure 4 shows substantial improvement over the unmodified SMR reaction, we decided to study the effect of catalysis on the reaction using variously ball-milled Ni catalyst. Nickel was used in the amount of 3 wt% of initial amount of sodium hydroxide. Different sizes of nickel were considered. Figure 5 shows a comparison of the ball-milled results with the data when no catalyst was used. As can be seen from the figure that there is a substantial effect of the ball-milled catalyst on the carbonation reaction. This effect is most pronounced at lower temperature, where the conversion increases by close to 30% (Figure 6). But the effect decreases with temperature such that the conversion in 30 min is 100% complete between 500 and 5501C for the catalyzed sample, whereas it takes only 6001C for the sample without catalysis (Figure 6).

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intensity of a peak, b0 the breadth at half maximum intensity of highly crystalline material (LaB6) and y the Bragg angle, half of the diffractometer angle. The crystallite size of different catalysts is calculated using this method. Raw nickel (304.8 A˚) when ball milled for 2, 3 or 4 h, the sizes reduced to 265.7013, 239.9057 and 184.7036 A˚, respectively. The result shows that prolonged ball-milling time has generated much lower crystallite size and thus increased the specific surface area for the catalysts. Four hours ballmilled nickel has the minimum crystallite size and therefore maximum specific surface area. However, its catalytic activity is the lowest. The reason could be the agglomeration of particles that happened at prolonged ball milling. This causes the reduction in the effective surface area exposed for gas adsorption onto the catalytic material. Figure 2 shows the distribution of particle size for different catalysts. The Gaussian fitted graph to the chart clarifies that the average particle size increases from 3–4 mm (for raw nickel) to 33–38 mm (for 4 h ballmilled nickel). SEM study of the catalysts shows that the platelets at 2 h ball-milling time grew larger as the ball-milling time progressed. XRD study of the different sized nickel revealed the absence of any oxide peaks of nickel. In Figure 3, the peak width broadened as time of milling was increased. So, the observation of smaller crystallite size at higher ball-milling time seemed to be in congruency with the X-ray results.


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2NaOH1CH4 1H2 O ! Na2 CO3 14H2 serves the dual goal of sequestering carbon and generating hydrogen gas. Figure 4 shows the % amount of soda formed from the initial sodium hydroxide at temperatures of 500, 600 and 8001C. The estimated error is around 5% as may be judged from the repeated results at 5001C. In 30 min the reaction is

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Figure 4. The data on % conversion of NaOH to Na2CO3 as a function of temperature and time. An estimate of error is provided by the repeated experiment at 5001C. The curves are smoothed spline fit to show the trends. No catalyst was used.

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Figure 3. XRD patterns for different time ball-milled nickel catalysts.

Figure 5. The experimental data on a sample with catalysis and without for 30 min between 300 and 6001C.

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In the experiments as described above, we used ballmilled nickel for 2 h. The effect of the duration of ball milling on the catalyst and resulting conversion is shown in Figure 7. Four cases are compared, one with no catalyst, second with un-ball-milled catalyst, third and fourth with the catalyst ball-milled for 2 and 4 h, respectively. It appears that ball milling beyond 2 h leads to a decatalytic effect as may be noted from the data on the sample with 4 h ball-milled nickel. To understand this phenomenon, we studied the ball-milled samples with SEM. The morphology of as-received Ni particles is uneven and particle sizes are ranging from 2 to 5 mm. Ball milled for 2-h Ni has a platelet like structure with small thickness and therefore has the highest ratio of surface area to volume. However, this ratio drastically decreases as prolonged ball milling was performed. The globular particles of the catalyst become platelets after milling for 2 h. After a continued grinding for 4 h, the platelets instead of decreasing in size become larger. The increased size would cause a decrease in the specific surface area

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leading to the reverse effect. Such decrease in a specific surface area of Ni powder with increasing ball-milling time might be related to the absence of any milling media or dispersant.

4. CONCLUSIONS The modification of the SMR reaction by inclusion of sodium hydroxide leads to carbonation yielding hydrogen that can be considered as produced without any carbon emission. The effect of sodium hydroxide, besides absorbing CO2, is also to lower the reaction temperature from a high of 1000 to 6001C. The temperature may further be lowered by using catalysis involving nickel. The particle size of the catalyst has an important role in enhancing the reaction kinetics. An optimum size is achieved with a nickel catalyst ball milled for 2 h. Longer milling causes a decatalyzation. The catalytic effect on the conversion of NaOH to Na2CO3 is most pronounced at low temperatures (3001C), but the effect decreases as temperatures increase to 6001C.

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REFERENCES

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Figure 6. The effect of using the Ni catalyst. The catalyst was ball milled for 2 h. The arrows show the significant change in the conversion amount for a given time.

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Figure 7. The effect of variously ball-milled catalyst on the carbonation reaction at 3001C. Int. J. Energy Res. 2012; 36:1133–1138 © 2011 John Wiley & Sons, Ltd. DOI: 10.1002/er

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