Steam Reforming of Tar Derived from Walnut Shell and Almond Shell

May 5, 2014 - The iron–ceria catalyst and red mud (as the iron oxide catalyst) ..... We calculated the carbon yields from off-line gas analysis data...
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Steam Reforming of Tar Derived from Walnut Shell and Almond Shell Gasification on Red Mud and Iron−Ceria Catalysts Seckiner Dulger Irdem,† Elena Parparita,‡ Cornelia Vasile,‡ Md. Azhar Uddin,§ and Jale Yanik*,† †

Faculty of Science, Department of Chemistry, Ege University, 35100 Bornova, Izmir, Turkey Physical Chemistry of Polymers Laboratory, “Petru Poni” Institute of Macromolecular Chemistry, 700487 Iasi, Romania § Department of Environmental Chemistry and Materials, Okayama University, 3-1-1 Tsushima Naka, Okayama 700-8530, Japan ‡

ABSTRACT: Catalytic steam gasification of shells (almond and walnut) was carried out in a flow-type double-bed microreactor. Temperature-programmed thermal steam gasification of shells was performed at 200−850 °C, followed by the catalytic gasification of volatiles (including tar) at different temperatures. The iron−ceria catalyst and red mud (as the iron oxide catalyst) were used as a catalytic bed material. Effects of the catalyst on tar decomposition and tar gasification were investigated. The iron−ceria catalyst enhanced both tar degradation and hydrogen production via the water−gas shift reaction. The tar conversions of 94% (for almond shell) and 100% (for walnut shell) were achieved at a very low temperature (600 °C). The red mud was found less active in tar decomposition because of the inhibition of sodium aluminum silicate hydrate to the activity of iron oxide. and H2.8 On the other hand, tar elimination temperatures decreased using lab-made nickel catalysts, which were prepared with different promoters and supports. For example, Nishikawa et al. reported that a nickel-based catalyst promoted with Pt showed high activity for the steam reforming of tar at lower temperatures.9 Similarly, the Ni/Al2O3 catalyst showed excellent performance at low temperatures (600−650 °C) in steam reforming of tar derived from biomass.10 The rapid deactivation from sulfur and high tar contents in the feed is the main drawback of Ni-based catalysts. In addition, because of their toxic property, the disposal of the spent nickel catalysts is an environmental concern, because they are classified as hazardous waste. Non-Ni metal catalysts have been developed for the degradation of tar in biomass gasification. Although these catalysts, such as Rh, Ru, Pd, and Pt, can significantly eliminate the tar, they are more expensive than nickel catalysts.11 On the other hand, iron-based catalysts are cheaper than nickel-based catalysts, and they are nontoxic. As known, iron has a catalytic activity for the water−gas shift (WGS) reaction and the Fischer−Tropsch synthesis reaction. Uddin et al. reported that the tar from gasification of cedar could be decomposed by the use of the iron oxide catalyst. In addition to the tar destruction effect, iron oxide also affected the composition of the gaseous products by catalyzing the WGS reaction.11 In another study related to steam gasification of Miscanthus × giganteus, hematite was found as an active catalyst to breakdown the tar and sensibly increase the rate of hydrogen production.12 Iron oxides also showed a catalytic effect on the production of hydrogen from pyrolysis oil because of the enhanced reforming of hydrocarbons.13 However, deactivation of the iron oxide because of the sintering of the surface was observed. This shows that the stability of the iron oxide has to be enhanced. In

1. INTRODUCTION As known, gasification is a reliable and flexible technology that converts carbon-containing materials (such as coal, biomass, and waste) into electricity and other valuable products, such as chemicals and fuels (http://www.gasification.org). The slagging of ash and formation of tar are major problems in gasification. The carbonaceous materials with a high ash content can be easily gasified if an appropriate ash removal system is installed along with low-temperature operation that keeps the temperature below the melting point of the ash. On the other hand, a low gasification temperature led to tar formation. Tar is a complex mixture of condensable hydrocarbons and causes problems in the engines and turbines used in application of the gas product besides the process equipment.1 In the case of biomass gasification, to obtain a high carbon conversion and low tar content in the gas product, a high operating temperature (above 800 °C) in the gasifier is preferred.1,2 However, high operation temperatures led to the slagging of ash because the biomass ash consisted of mainly alkali and earth alkali metals. Hence, the development of clean and economical tar-removing technologies for biomass gasification is becoming increasingly important. The use of catalysts in gasification has many advantages, such as lowering the gasification temperatures and changing production compositions. Especially, in ex situ tar steam reforming, the tar content in the product gas could be reduced extensively and, more importantly, tar could be converted into useful gas compounds, such as H2 and CO, during gasification. Extensive studies have been reported on the tar reduction in the literature. Dolomite and olivine have been widely tested because of their low cost.3−7 However, they cannot remove or decompose the tar completely. Nickel-based catalysts, which are commercial steam-reforming catalysts for methane and naphtha reforming have been found to be very effective for tar removal in biomass gasification. It was found that commercial steamreforming catalysts could completely eliminate the tar at temperatures of around 850 °C and increase the yields of CO © 2014 American Chemical Society

Received: January 24, 2014 Revised: May 5, 2014 Published: May 5, 2014 3808

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a previous study, the Fe/γ-Al2O3 catalyst was used for the steam reforming of volatile products from rice husk pyrolysis.14 The result indicates that large-moleculer-weight compounds in biooil decomposed into gaseous products and, at the same time, the gases were catalytically reformed in the presence of iron. In another study, the iron-based mixed metal oxides, such as iron− alumina (Fe−Al) and iron−zirconia (Fe−Zr), were found to be highly active for naphthalene conversion.15 In that study, the addition of CuO also enhanced the activities and stability of the metal oxide catalysts. Bimetallic iron alloys, such as Ni−Fe16 and Co−Fe,17 also revealed good catalytic performance in the steam reforming of tar from cedar wood pyrolysis. Li et al.18 pointed out that the catalytic performance of alloys depends upon the compositions [Fe/Ni(Co) ratio], crystal structures, and uniformity. In general, there are three components in metal-based catalysts: an active catalyst, a promoter, and a support.11 Metals, such as Ni, Rh, Ru, Pd, Pt, etc., are being used as the active site of the catalyst. On the other hand, promoters, which are transition-metal-based catalysts, such as Mo, Mo3O, CeO2, WO3, LaCoO3, etc., increase activity and/or stability. Ceria (CeO2) has been studied by some researchers as a promoter and/or support, mainly in gasification of bio-oil derived from biomass pyrolysis, real biomass, and bio-oil model compounds. Because of its redox properties, ceria shows promising catalytic activities and selectivity in partial oxidation (2CeO2 → Ce2O3 + 1/2O2).19 It was found that CeO2 played an important role on the decrease of coke deposition in steam reforming or gasification, promoting the gasification of deposited carbon, which led to an increase in the stability of the catalyst, as well as the WGS reaction.20−27 It also promotes the metal−ceria interaction and metal dispersion.28 The redox features of ceria can be greatly enhanced by the addition of a transition metal because of the formation of the strong metal−ceria interaction.29 Iron−ceria catalysts can be considered as a tarreforming catalyst in steam gasification of real biomass. In this paper, we investigated the performance of iron-based catalysts in the steam gasification of walnut and almond shells. As far as the authors know, there is no such study that investigated the catalytic effect of red mud and Fe2O3−CeO2 in the literature. In addition, gasification of walnut and almond shells have been studied for the first time. It should be noted that biomasses tested in this study have a higher lignin content than the biomasses, such as cedar wood 11 and M. × giganteus,12 reported in the literature. The purposes of this study are (1) to test the tar decomposition activity of the Fe2O3−CeO2 catalyst, which showed good performance in steam gasification of safflower seed cake, for walnut and almond shells and (2) to compare the catalytic performances of red mud (a solid waste product of the Bayer process) and Fe2O3−CeO2 catalysts in steam gasification.

Table 1. Properties of the Shells walnut shell proximate analysis (as received, wt %) moisture 7.7 volatile matter 77.9 fixed carbon 13.5 ash 0.9 ultimate analysis (dry, wt %) C 48.24 H 5.69 N 0.11 S 0.09 component analysis (dry, wt %) cellulose 26.9 lignin 47.7 hemicellulose 22.5 extractives 2.1

almond shell 8.8 72.0 17.6 1.0 48.08 6.06 0.25 0.07 24.8 36.3 36.5 1.5

Aluminum Company, Turkey, as sludge, was activated by boiling in aqueous HCl, following precipitation and then calcination steps, according to the literature.30 Activated red mud (ARM) contains mainly Fe2O3 (41.10%), Al2O3 (21.32%), and SiO2 (17.02%). It has a surface area of 158 m2/g. 2.2. Gasification Experiments. Gasification experiments were carried out in a two-stage (two fixed-bed) quartz microreactor. This type of reactor system is very convenient for the determination of the catalytic activity and gas yields. The schematic diagram of the experimental setup has been given in the previous study.11 In a typical run, 0.04 g of shell sample was put in the top section on a quartz wool bed (top bed) and 0.065 mL of the catalyst was placed on the bottom section (catalyst bed). At first, the reactor system was purged with N2 flow for 30 min, and then the catalyst bed was heated to the desired temperature (600−850 °C) at a heating rate of 3 °C/min under N2 flow. As soon as the catalyst bed temperature reached the set value, heating of the top section to the 850 °C was started with a heating rate of 3 °C/min. A mixture of 30% H2O−N2 (flow rate of 20 cm3 standard temperature and pressure (STP)/min) was used as the gasification agent. Both nitrogen and steam flows were chosen on the basis of a previous study11 in this reactor system. When the biomass bed temperature reached 200 °C, analysis of the reactor outlet gas (product gas) was started with an online gas chromatograph (GC) during heating. The online GC analyses were performed during heating at a fixed time interval of 15 min. The product gases are analyzed with two online gas chromatographs (SHIMADZU GC-8A and YANACO G2700T) equipped with a thermal conductivity detector (TCD) and three columns [GCI: column 1, Molecular Sieve 13X (carrier gas, He); column 2, Porapak QS (carrier gas, He); and GCII: column, Molecular Sieve 13X (carrier gas, Ar)]. The evolved gases were also collected in two sampling bags for off-line analysis: a tar evaluation (200−500 °C) step in one bag and a char gasification (500−850 °C) step in another bag. Because of the low amount of the biomass sample used in this study, the tar product could not be analyzed. For this reason, we evaluated the activity of the catalysts and the gas yields using the data of the gaseous products only.

3. RESULTS AND DISCUSSION 3.1. Non-catalytic Gasification of Shells: Effect of the Temperature on Gas Composition and Tar Conversion. In the dual-bed gasification system of this study, the thermal degradation of biomass in the presence of steam proceeds in two steps: the first step (at 200−500 °C) involves degradation of biomass and evolution of volatile matter, including tars, and the second step involves the steam gasification of char (in the top bed) at 500−800 °C.11 In this section, the yield and distribution of main gas products from biomass steam gasification were investigated in the absence of a catalyst. The

2. MATERIALS AND METHODS 2.1. Materials. The samples of almond shell (AS) and walnut shell (WS) were provided by a local food processing plant (Izmir, Turkey). The shells were ground to a particle size of less than 2 mm and then dried overnight at 105 °C. Some properties of shells are given in Table 1. The 10% Fe2O3/90% CeO2 catalyst was prepared by the coprecipitation method using ammonia as the precipitating agent. Briefly, Fe(NO3)3 and Ce(NO3)3 solutions was added to 7 wt % ammonia solution (10% in excess of the stoichiometric amount) and stirred vigorously. The precipitate was then filtered and dried at 110 °C and calcined at 700 °C. The red mud, which was supplied by the Seydisehir 3809

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for WS. In the temperature range of 200−500 °C, pyrolysis and tar degradation reactions are responsible for gas evolution (volatile matter). Over 500 °C, gases are generated from the gasification of pyrolysis char. It is worth mentioning that, in our experiment, online analysis of CO2 was not possible with the same GC column (Molecular Sieve 13X). The gases collected in two sets of bags were measured off-line for calculating the cumulative production of CO2, CO, CH4, and C2H4 for the overall reaction period, and the data were used to determined the total yield of carbon later. In this study, the degree of the tar decomposition (tar conversion) was evaluated on the basis of carbon yields of char and volatile matter. We calculated the carbon yields from offline gas analysis data for CO, CO2, CH4, and C2H4. Thus, for volatile matter

top bed (biomass bed) was heated from room temperature to 850 °C, while the temperature of the bottom bed was held at 850 °C, which is high enough for steam gasification of char. The distribution of main gaseous products obtained from almond and walnut shells are given in panels a and b of Figure 1, respectively. The maximum evolution rate of gases was

yield of carbon (%) = (amount of carbon in gases evolved between 200 and 500 °C)/(amount of carbon in the feed biomass) × 100

and for char gasification yield of carbon (%) = (amount of carbon in gases evolved between 500 and 850 °C)/(amount of carbon in the feed biomass) × 100

Figure 2 shows the carbon yields for volatile matter and char gasification as a function of the bottom bed temperature. Because the temperature of the top bed in all experiments was kept the same (850 °C), the yields of carbon from the chars (ca. 36%) were almost the same for each run of experiments. In a previous study carried out with cedar sawdust in this reactor system, a similar result was obtained.11 In our study, we found that the yields of carbon from AS and WS char gasification were 54 and 59%, respectively. If all of the volatile matter (including tars) content of the biomasses are decomposed into gaseous products, then the calculated maximum possible yield of carbon from the volatile matter (including tars) should be about 46 and 41% for AS and WS, respectively, by assuming that the total carbon yield of steam gasification is 100%. Tar conversion was calculated as follows:

Figure 1. Production rate of gases from thermal steam gasification of almond and walnut shells.

observed in two temperature areas. The formation of mainly CO and some H2, CH4, and C2H4 (not shown) took place in the first area (with a peak at around 300 °C). The second peaks appeared at around 800 °C of primarily H2 evaluated from the biomass char gasification. In the case of AS, the maximum formation rate of hydrogen was observed at 750 °C, whereas the temperature for the maximum formation rate was 850 °C

Figure 2. Effect of the bottom bed temperature on the total yield of carbon for volatile matter and char gasification products: (a) AS and (b) WS. 3810

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CO increased, because the high temperatures improve the steam reforming and thermal cracking of tar. These results are in agreement with the results reported by Gusta et al.31 that tar conversion increased with an increasing temperature because of the reaction between the steam and coke formed from thermal cracking of tar. 3.2. Catalytic Gasification of Tar Derived from Shells. In the non-catalytic steam gasification, we obtained a tar conversion of 100% at the high bottom bed temperature; however, a high temperature is unfavorable in terms of the energy efficiency. On the other hand, tar reduction using a catalyst is a favorite method to convert tar into light gases at low temperatures. As mentioned before, the various types of catalyst have been tested to reform tar and its model compounds. In this study, tar conversion was performed on 50% Fe2O3/50% CeO2 (Fe−Ce) and red mud (in the bottom bed) by steam reforming. The effects of the catalyst type and catalyst bed temperature on the steam reforming of tar were investigated in terms of gas composition and tar conversion. The effect of catalysts on the tar reforming was investigated in a reaction system consisting of steam gasification of AS and WS at 200−850 °C (with temperature-programmed heating), followed by the catalytic gasification of tar (reforming of tar) in the catalyst bed at the fixed temperatures of 600 and 700 °C. After temperature-programmed heating, the final temperature of the top bed was maintained at 850 °C in all experiments. Figure 3 shows the yield of the main gaseous products (CO2, CO, CH4, and H2) from catalytic gasification of almond shell using the Fe−Ce catalyst and red mud at two different bottom bed temperatures. Remarkable changes in the cumulative production of gases were observed because of the use of the Ce−Fe catalyst. H2 and CO2 production increased, whereas CO production decreased, with the use of catalyst. For the catalytic bed temperature of 700 °C, H2 production increased from 933 cm3/g biomass (in the thermal case) to 1604 cm3/g biomass, whereas CO production decreased from 257 to 34 cm3/g biomass. In addition, CO2 production increased from 403 to 742 cm3/g biomass. Generally, the reduction in the CO content is desirable when the maximum hydrogen production is aimed. A similar effect of the Fe−Ce catalyst was also observed at the catalytic bed temperature of 600 °C. From these results, it is evident that the Fe−Ce catalyst enhances the WGS reaction (CO + H2O → CO2 + H2). Similar results of increasing hydrogen formation by the WGS reaction in the presence of ceria-based catalysts were also reported by others.20−25 In contrast to the Fe−Ce catalyst, the presence of red mud led to no significant change in the cumulative production of gases. For the catalytic bed temperature of 700 °C, using red mud, the productions of H2 and CO were 1021 and 292 cm3/g biomass, respectively. The X-ray diffraction (XRD) pattern of red mud (not shown here) displayed existence of hematite and sodium aluminum silicate hydrate. A previous study by Xie et al.32 demonstrated that Fe2O3 enhanced reforming reactions of tar and also displayed a role in improving the WGS reaction in the steam gasification of rice straw at 900 °C. Similar results of increasing hydrogen formation by the WGS reaction in the presence of iron oxide were also reported by others.13,14 On the other hand, Matsuoka et al.33 investigated the steam reforming of tar in the presence of Fe2O3 and reported that steam reforming of the tar to form H2 on the iron oxide was not significant at a low temperature. In contrast to our results, the iron oxide catalyst prepared by a precipitation method using

tar conversion (%) = (measured yield of carbon from volatile matter evolved from 200 to 500 °C)/(calculated maximum possible yield of carbon from volatile matter) × 100

It is clearly seen that the bottom bed temperature has a significant effect on the carbon yield from volatile matter and, thereby, tar conversion. When the temperature of the bottom bed was increased from 600 to 850 °C, the carbon yield increased from 27 to 46% for AS and from 27 to 42% for WS, indicating that a higher temperature promotes thermal cracking of tar. An increase in the total gas yield with the temperature of the bottom bed supports this conclusion (Figures 3 and 4). For

Figure 3. Overall gaseous product distribution during thermal and catalytic gasification of almond shell at different catalytic bed temperatures and over different catalysts.

Figure 4. Overall gaseous product distribution during thermal and catalytic gasification of walnut shell at different catalytic bed temperatures and over different catalysts.

both biomasses, a tar conversion of 100% was achieved at the bottom bed temperature of 850 °C (Figure 5). With an increasing bottom bed temperature, the yields of both H2 and

Figure 5. Effect of the bottom bed temperature on tar conversion in thermal steam gasification of shells. 3811

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iron salt and ammonia showed activity at a low temperature (600 °C) for tar decomposition during the steam gasification of sawdust.9 By comparing our results to the above results in the literature, we concluded that sodium aluminum silicate hydrate might inhibit the activity of iron oxide in the WGS reaction. As in the case of almond shell, the Fe−Ce catalyst also showed high activity for the H2 and CO2 production from walnut shell (Figure 4) at both bottom bed temperatures of 600 and 700 °C. For the catalytic bed temperature of 700 °C, H2 production increased from 957 cm3/g biomass (in the thermal case) to 1628 cm3/g biomass, whereas CO production decreased from 295 to 104 cm3/g biomass. In addition, CO2 production increased from 458 to 705 cm3/g biomass. Red mud showed no activity in the WGS reaction in steam gasification of walnut. Tar conversions for catalytic steam gasification of almond shell are given in Figure 6. In comparison to the thermal runs at

Figure 7. Effect of the cyclic use of the Fe−Ce catalyst on the gasification of walnut shell: (a) gas yields and (b) tar conversion.

4. CONCLUSION In this work, two types of catalyst (Fe−Ce and red mud) were investigated in a dual-bed microreactor for their activities for degradation of tar from steam gasification of shells. Use of the Fe−Ce catalyst enhanced both tar degradation and, subsequently, the WGS reaction to produce more H2. For both shells, a tar removal efficiency of 95−100% was obtained using Fe−Ce at a low temperature (600 °C). On the other hand, red mud showed no activity for the WGS reaction, but it was active in some extending for the cracking of tar at a low temperature. The Fe−Ce catalyst showed stable activity for tar degradation, whereas its activity in the WGS reaction decreased with the repeated uses.

Figure 6. Effect of the catalyst and catalytic bed temperature on tar conversion in steam gasification of shells.

600 °C, catalytic gasification significantly increased the tar degradation. At a catalyst bed temperature of 600 °C, the tar conversions increased from 58.6% (in the case of thermal decomposition) to 94.8% (with Fe−Ce) and 68.3% (with red mud). At 700 °C, a tar conversion of 100% was achieved. On the other hand, the value of tar conversions was similar for thermal and catalytic runs with red mud. As in the case of almond shell, Ce−Fe exhibited higher tar conversion capacity in the gasification of walnut shell. It should be noted that 100% tar conversion was obtained with Fe−Ce, even at the lowest catalyst bed temperature of 600 °C (Figure 6). 3.3. Cyclic Use of the Fe−Ce Catalyst. In this study, the stability of the Fe−Ce catalyst was tested by repeatedly reusing the same catalyst in the gasification of walnut shell without regeneration. These series of experiments were carried out at the catalyst bed temperature of 600 °C. Figure 7 shows the tar conversion (a) and cumulative production of gaseous products (b). After the first cycle, tar conversion slightly decreased to 90% and remained almost constant in the next cycle (cycle 3). However, after cycle 3, tar conversion decreased significantly (to 63%). On the other hand, the production of H2 and CO2 decreased constantly with the increase of the cycle number, whereas the production of CO increased. From these results, it can be concluded that the catalytic activity of Fe−Ce for the reforming of tar was more stable than its activity for the WGS reaction. On other words, the active sites for the WGS reaction are different from those for tar cracking.11

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AUTHOR INFORMATION

Corresponding Author

*Telephone: +90-232-3112386. Fax: +90-232-3888264. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from Ege University under Contract 2013 FEN 036 and FP7 Marie-Curie IRSES-Project 247550 is gratefully acknowledged.

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REFERENCES

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