Catalytic Oxidation of Biomass Tar over Platinum and Ruthenium

Jan 19, 2011 - Climate Change Technology Research Division, Korea Institute of ... ABSTRACT: The catalytic oxidation of a model biomass tar, toluene, ...
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Catalytic Oxidation of Biomass Tar over Platinum and Ruthenium Catalysts Sang Jun Yoon, Yong Ku Kim, and Jae Goo Lee* Climate Change Technology Research Division, Korea Institute of Energy Research, 71-2 Jang-dong, Yuseong-gu, Daejeon 305-343, Republic of Korea ABSTRACT: The catalytic oxidation of a model biomass tar, toluene, was studied using platinum and ruthenium on γ-alumina catalysts at various temperature, catalyst sizes, and metal contents in an environment with either the presence or absence of syngas. As the reaction temperature increased and the size of the catalyst decreased, the conversion of toluene increased. Usually, the higher content of platinum and ruthenium in the catalyst showed higher conversion of toluene. It was found that the presence or absence of syngas greatly affected the toluene conversion. The platinum catalyst showed a higher toluene conversion efficiency than the ruthenium catalyst at the same temperature in the absence of syngas, while in the presence of syngas, the ruthenium catalyst showed a better conversion efficiency than the platinum catalyst. The results indicate that a temperature of over 300 °C is required in order to oxidize tar efficiently using these catalysts.

1. INTRODUCTION The utilization of biomass as a source of renewable energy has received significant attention recently as a possible replacement for fossil energy. It can provide an alternative to the limited reserves of fossil fuels and reduce CO2 emissions. Furthermore, biomass is distributed worldwide and is recognized as an important renewable energy source.1-4 As part of the method of utilizing biomass, gasification is recognized as a promising and competitive technology for producing chemicals, hydrogen, and electricity.5-7 However, lignin, which is a primary component of woody biomass, is a substance that has a high probability of forming tar during gasification.8,9 Tar is a highly thermal stable hydrocarbon compound that remains liquid at room temperature and is a condensable mixture of diverse substances with a ring structure. Tar that is formed with syngas derived from biomass gasification causes corrosion and plugging problems in the devices of the gas purification process, heat exchangers, engines, and turbines. This decreases the efficiency of gasification.8,10,11 Therefore, to utilize the syngas from the biomass gasification effectively, this aromatic hydrocarbon mixture must be removed. A number of studies on tar removal have led to the development of a physicochemical removal method,12-14 as well as a thermochemical conversion method.15-18 Thermochemical steam reforming converts the tar derived from biomass gasification to a fuel gas or syngas and increases the process maintenance and efficiency. However, the thermal cracking and reforming of tar without a catalyst requires high reaction temperatures that exceed 900 °C.11,16 To decrease the required reforming temperatures, catalytic steam reforming with natural mineral catalysts, such as dolomite and olivine,19-23 or metal containing catalysts, such as Ni, Co, La, and Rh, have been suggested.24-30 Typically, these metal-containing catalysts are highly efficient in terms of tar conversion compared with mineral catalysts at the same temperature. However, tar reforming using most metal r 2011 American Chemical Society

containing catalysts must maintain the temperature at over 600 °C for effective decomposition.9 Nickel-based catalysts have been closely investigated owing to their high level of tar decomposition activity. Unfortunately, Ni catalysts are strongly poisoned by sulfur compounds and deactivation can occur due to coke formation.31,32 To overcome these drawbacks, an operating temperature in excess of 800 °C is required.33 However, biomass has a relatively lower heating value than that of other fossil fuels like coal, natural gas, and petroleum. Therefore, it is difficult to maintain temperatures over 600 °C in the reformer without additional heat supplies in airblown fixed bed and fluidized bed biomass gasification systems. For biomass gasification, fixed bed or fluidized bed gasifiers are usually applied due to the irregular size and shape of the biomass feedstock. In these gasification systems, the reformer is usually positioned after the gasifier or cyclone. The dust generated from the gasifier must be removed before the reformer because it leads the fouling of the catalyst surface and subsequent catalyst deactivation. Several researchers have defined the components contained in tar.4,34 Benzene and toluene constitute more than 50% of the tar that is generated in the biomass gasification process. In this regard, benzene and toluene are used as a model tar by many researchers in the relevant literature.15,16,20 To remove the volatile organic compounds containing benzene and toluene at low temperatures, catalytic oxidation has been studied by many research groups.35-38 However, most previous studies were performed under syngas-free conditions and with levels of tar content that differed from the biomass gasification producer gas. To date, there are no catalytic tar oxidation results in the literature regarding the platinum and ruthenium catalysts used Received: October 7, 2010 Accepted: December 23, 2010 Revised: December 6, 2010 Published: January 19, 2011 2445

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in the presence of syngas. The evaluation of the catalytic tar oxidation performance in the presence of syngas has only been investigated with catalysts containing zirconia.33 In the present work, two metal catalysts, platinum and ruthenium, with different loading conditions were investigated in the conversion of the pollutant tar produced from the biomass gasification. As tar is difficult to analyze, making the experimental results complicated to understand, the experiments were conducted using a model biomass tar, toluene, which is a major component of tar. A lab-scale fixed-bed tube reactor was used to identify the oxidation characteristics according to the type and size of the catalyst under atmospheric pressure in a range of temperatures.

2. EXPERIMENTAL SECTION Pt and Ru on γ-alumina catalysts with different metal loadings of 0.5 wt.% and 0.7 wt.% supplied by ECOPRO Co. were used. These catalysts were prepared by the incipient wetness impregnation method and calcined at 550 °C for 4 h. The chemical and morphological properties of the catalysts are shown in Table 1. Figure 1 shows a schematic diagram of the lab-scale catalytic tar oxidation system with a capacity of 12 L/h. The diameter of the reaction area was 30 mm and the total length was 520 mm. After the target catalyst was crushed to sizes of 0.045-1 mm, 0.5 g of catalyst was used to fill the quartz reactor. Then, the reactor was placed in the furnace and the temperature increased to the desired level. As the temperature increased with a heating Table 1. Chemical Composition and Morphological Properties of the Catalysts active metal metal loading (wt.%)

Pt 0.5

Pt 0.7

Ru 0.5

Ru 0.7

rate of 2 °C/min, 1000 g/Nm3 of toluene, more than ten times the amount of tar emitted from a biomass gasification,39,40 was supplied into the reactor using a piston pump. The temperature of the injection line of toluene was raised to the phase change temperature using a band heater. For the experiment under syngas-free conditions, air served as the oxidant and the carrier gas of toluene at a flow rate of 40 mL/min and was controlled by a mass flow controller. For the experiment with syngas, oxygen was supplied at 8 mL/min and the model biomass syngas at 32 mL/min via MFC to maintain a constant space time in the experiments of 0.21 kg 3 h/m3. The temperature and pressure of the gases surrounding the reactor were measured using a K-type thermocouple and pressure gauge with a precision of (0.1 °C and (0.1 bar, respectively. The temperature of the reactor was measured using a K-type thermocouple located on a quartz filter supporting the catalyst. Gases and residual toluene that were produced after the catalytic oxidation in the reactor were introduced into a GC (HP 6890), and then analyzed with a TCD (Carbosphere 80/100 Packed column, Alltech) and a FID detector (BP5 Capillary column, SGE). The analysis results from the GC were stored in real time, and the gases emitted from the GC were directed into a hood via a cold trap. The conversion, Xc, of toluene according to the experimental conditions was determined by measuring the amount of toluene before and after the reactor using a GC. The measurement was calculated as follows: Xc ð%Þ ¼

Ctar, in - Ctar, out  100 Ctar, in

ð1Þ

where Ctar,in and Ctar,out are the tar concentrations at the inlet and outlet of the reactor, respectively.

γ-alumina γ-alumina γ-alumina γ-alumina

3. RESULTS AND DISCUSSION

surface area (BET, m /g)

196.6

195.1

216.6

203.2

mesopore volume

0.47

0.49

0.50

0.49

(BJH desorption, cm3/g) average pore diameter (nm) 7.22

7.52

7.40

7.45

3.1. Tar Oxidation without Syngas. To compare the oxidation characteristics of toluene according to the catalyst with 0.5 wt.% platinum supported on γ-alumina and by reaction temperature, a test was conducted while the air and toluene injection

support 2

Figure 1. Schematic diagram of a lab-scale tar oxidation system. 2446

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Figure 2. Effect of temperature and catalyst size on toluene conversion with 0.5 wt.% Pt catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

Figure 3. Effect of temperature and catalyst size on toluene conversion with 0.5 wt.% Ru catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

conditions were consistent; the results are depicted in Figure 2. As the reaction temperature rose, the toluene conversion increased, and as the catalyst size decreased, the specific surface area increased; consequently, the toluene conversion increased at the same reaction temperature. When this catalyst was used, the catalytic oxidation of toluene began at approximately 170 °C; the rapid conversion started at an early stage according to the temperature; finally at approximately 370 °C, toluene conversion of 100% through oxidation took place. In the same conversion and removal conditions, the largest temperature difference with other tests was 30 °C which was a result of the catalyst size. The toluene conversion through oxidation using the 0.5 wt.% ruthenium catalyst on the γ-alumina was examined and the results are illustrated in Figure 3. As the temperature increased, the conversion due to the oxidation of toluene increased; furthermore, as the size of the catalyst decreased, the toluene conversion increased. At approximately 180 °C, the oxidation of toluene due to the catalyst began, and at approximately 390 °C, the toluene was completely oxidized, converted, and removed. It was confirmed that to obtain the same conversion of toluene, the

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Figure 4. Effect of temperature and catalyst size on toluene conversion with 0.7 wt.% Pt catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

temperature difference according to the catalyst size was as large as 40 °C. One unusual point was that, depending on the catalyst size, the temperature at which oxidation began varied slightly by approximately 8 °C, whereas the temperature at which the 100% conversion was reached was almost identical. Figure 4 shows that the toluene conversion depending on the catalyst size and the reaction temperature with 0.7 wt.% platinum supported on γ-alumina at a constant concentration of toluene and flow rate of air. As the reaction temperature increased, the toluene conversion increased, and in general, as the catalyst size decreased, the conversion increased. Unlike the aforementioned results, however, the influence of the catalyst size is not great, particularly for catalysts smaller than 45 um and between 45 and 75 um. The temperature at which the toluene oxidation began for catalysts smaller than 45 um was approximately 10 °C lower, but afterward the conversion characteristics were almost identical. Generally, the oxidation of toluene using this catalyst begins at about 150 °C and 100% conversion and removal occurs near 280 °C. The catalytic oxidation began at a lower temperature than the catalysts with 0.5 wt.% platinum and ruthenium, and at a temperature about 100 °C lower than the results in Figures 2 and 3, the 100% conversion of toluene occurred. To examine the oxidation characteristics of toluene according to the reaction temperature and the catalyst size with 0.7 wt.% ruthenium supported on γ-alumina, the toluene conversion before and after the reaction were measured and the result is shown in Figure 5. As the reaction temperature increased or the size of the catalyst decreased, the conversion of toluene increased. Compared with the previous result, the toluene oxidation began at a lower temperature of around 130 °C, and for the catalysts smaller than 45 um, a 100% conversion was shown at approximately 270 °C. Notably, between the initial oxidation start temperature and the reaction temperature of 200 °C, the influence of the catalyst size on the conversion was minimal. However, between 200 and 260 °C, the conversion differed by up to 16% depending on the size of the catalyst. Furthermore, the temperature at which a 100% conversion was reached varied by about 10 °C depending on the catalyst size. To compare the conversion efficiency of the four catalysts used in this work, the toluene conversion for catalysts sized between 45 and 75 um are illustrated in Figure 6 as a function 2447

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Figure 5. Effect of temperature and catalyst size on toluene conversion with 0.7 wt.% Ru catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

Figure 6. Conversion of toluene with different catalysts and temperatures in the absence of syngas (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

of temperature. The results showed that, for catalysts with 0.7 wt.% ruthenium, the oxidation of toluene began at the lowest temperature, approximately 130 °C. However, catalysts with 0.7 wt.% platinum were fully converted first. Comparing this result with the metal content, the higher metal loaded catalyst showed a higher conversion efficiency at a constant temperature. Usually, the platinum-based catalyst showed a higher toluene conversion at the same reaction temperature than that of the ruthenium that was used. If there were no catalyst, the toluene conversion begins at the temperature of 430 °C, with 100% conversion being reached at 540 °C. From the results, it was found that the 100% conversion can be reached at a temperature more than 240 °C lower, in case a catalyst is used. These results suggest that, even in cases where the temperature of the syngas produced by the gasifier is low due to a process problem, it is possible to effectively convert and remove the tar in the syngas without supplying additional heat. 3.2. Tar Oxidation with Syngas. Under the simulated syngas conditions of biomass gasification, the characteristics of the catalytic toluene oxidation were measured. The experiment for

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Figure 7. Effect of temperature on toluene conversion and syngas composition with 0.5 wt.% Pt catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

the concentrations of toluene and oxygen in a gas phase was conducted under the same conditions as those without syngas; catalysts sized between 45 and 75 um were used. Figure 7 shows that the toluene conversion over 0.5 wt.% platinum catalyst according to the reaction temperature existed with syngas. From these results, the initial composition of the model syngas was maintained until approximately 100 °C. At about 145 °C, the CO and H2 in the syngas began to be oxidized, and then the amount of CO2 rapidly increased. As the CO rapidly decreased, it was completely oxidized at approximately 250 °C, and the amount of CO2 increased proportionally. As the toluene in the syngas began oxidation at about 250 °C, the amount of oxidized CO contained in the syngas decreased and was maintained constant at about 2%, and the amount of oxidized H2 declined to 3%. For the toluene oxidation with syngas, the 100% conversion of toluene was achieved at 520 °C. The increase and decrease of CO2 appeared to be closely related to the change in CO; even when the toluene oxidation increased, the composition of CO2 did not change greatly. This is because the volume of the CO in the gas injected into the reactor was much greater than the concentration of toluene (1000 ppm). In other words, if the relatively higher composition of CO is burned and converted to CO2, then its influence is greater than if it is oxidized by a small content of toluene and the content of CO2 is increased. The CH4 in the product gas tends to increase slightly when the CO and H2 content decreases and the conversion of toluene increases. It is thought that the CH4 is generated due to the methanation and splitting of the methyl group in the toluene as a result of pyrolysis. From when the toluene conversion is completed, the contents of the combustible gases in the syngas (i.e., H2, CO, and CH4) tend to increase. This suggests that at a high temperature, tar can be removed so that the loss of combustible syngas is minimal. Under conditions where the oxidant, toluene concentration, and catalyst size were the same, the catalyst with 0.5 wt.% ruthenium supported on γ-alumina were used, the toluene conversion when simulated syngas is present and the influence of the changing composition of the syngas depending on the temperature were studied. The results are shown in Figure 8. When the reaction temperature reached approximately 150 °C, the CO and H2 in the model syngas began to oxidize, and the amount of CO2 began to increase. At this time, at about 180 °C, the toluene oxidization began and the amount of CO2 tended 2448

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Figure 8. Effect of temperature on toluene conversion and syngas composition with 0.5 wt.% Ru catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

Figure 9. Effect of temperature on toluene conversion and syngas composition with 0.7 wt.% Pt catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

to increase rapidly. If this catalyst was used, under the conditions that there was syngas and the reaction temperature was 530 °C, then the toluene in the gas phase was converted and completely removed. The composition of CH4 tended to increase slightly until 350 °C and decreased thereafter as the toluene conversion increased and the CO and H2 composition varied. As previously mentioned, this is a result of the methanation and pyrolysis of the toluene. From the time when the CO and toluene in the injected gas began to oxidize, the amount of generated CO2 increased to about 12% and continued to increase until the temperature reached 400 °C. However, when the reaction temperature exceeds 400 °C, the toluene conversion increased, but the amount of burned CO and H2 decreased; furthermore, the content of CO2 and CH4 decreased as well. As mentioned above, because the amount of CO is greater than the toluene concentration in the supplied model syngas, the increase and decrease of CO2 appears to be more influenced by the degree of the CO oxidation. From the results, the activation of the catalytic toluene oxidation increased proportionately as the reaction temperature increased. However, the degree of oxidation of CO and H2 in the mixed gas can be complexly affected by the reaction conditions. The catalysts used in this experiment contained metals, and steam can

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Figure 10. Effect of temperature on toluene conversion and syngas composition with 0.7 wt.% Ru catalyst (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

Figure 11. Conversion of toluene with different catalysts and temperatures in the presence of syngas (total flow rate: 40 mL/min; concentration of toluene: 1000 ppm; space time: 0.21 kg 3 h/m3).

be generated by the combustion of hydrogen. Accordingly, under the temperature conditions of this work, the water gas shift (WGS) reaction or steam reforming (SR) reaction can occur. As the composition of H2 and CO tended to increase or decrease almost identically, unlike the WGS reaction, the SR reaction of the toluene occurred the same time. As the toluene oxidation decreases in relative terms, it can be predicted that the composition of CO2 will decrease as well. When there is simulated syngas, the conversion of toluene using the catalysts with 0.7 wt.% platinum supported on γ-alumina was measured according to the reaction temperature, and the results are shown in Figure 9. The result shows that when the reaction temperature was approximately 195 °C, the toluene conversion began, the composition of CO decreased, and the amount of generated CO2 tended to increase. The content of H2 tended to decrease slightly from this temperature, and the composition decreased considerably beyond 300 °C. As the temperature increased, the toluene conversion increased and the CH4 content increased slightly, and as the CO composition increased, the CH4 content decreased. When the temperature was higher than 350 °C, the toluene conversion increased sharply, whereas the amount of 2449

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Industrial & Engineering Chemistry Research CO in the produced gas began to slightly increase again, and the composition of CO2 remained almost the same. This trend is similar to the result with 0.5 wt.% platinum catalyst. Figure 10 shows the changes in the toluene conversion and the change in the syngas composition according to the reaction temperature when the oxygen, toluene, and model syngas are supplied in the reactor at a constant ratio, and when catalysts sized between 45 and 75 um with 0.7 wt.% ruthenium were used. The toluene conversion began at 150 °C, and when the reaction temperature reached 390 °C, the conversion was complete. From 150 °C, not only did toluene begin to oxidize, but CO and H2 also began to oxidize, and the concentration of CO2 in the product gas increased rapidly from 5% to 12%. At this time, the content of CH4 increased slightly. Notably, as the temperature increased, the toluene conversion increased, while the H2 and CO contents in the produced gas increased at about 335 °C, and the composition of CO2 and CH4 decreased. At a temperature similar to that of the 0.5 wt.% ruthenium catalyst, the same increase and decrease patterns of syngas composition were observed due to the properties of the ruthenium catalyst, until the temperature reaches 350 °C, the characteristics of catalytic oxidation prevailed. Thus, it is believed that the oxidation, methanation, and pyrolysis of CO, H2, and toluene increase the composition of CO2 and CH4, and when the temperature is higher than 350 °C, the catalytic reforming of toluene and CH4 occurs, increasing the contents of H2 and CO, and decreasing the amount of CO2 generated. As both the oxidation and reforming reaction convert toluene into CO2 or H2 and CO, the toluene conversion increases. In Figure 11, a comparison of the toluene conversion when syngas is present through the four catalysts used in this work sized between 45 and 75 um is shown. In the same manner as that without syngas, when the 0.7 wt.% ruthenium catalyst was used, conversion began at the lowest temperature of 150 °C, and a 100% conversion was reached most quickly. Regardless of the presence or absence of syngas, the 0.7 wt.% ruthenium catalyst could oxidize toluene most effectively at low temperatures. In both cases, the toluene conversion began at a similar temperature of 150 °C. However, in the presence of syngas, the completion of the toluene conversion occurs at a temperature more than 100 °C higher than that without syngas. The 0.5 wt.% ruthenium catalyst showed an almost identical tendency. For the platinum catalyst, when syngas is present, the toluene conversion began at a temperature approximately 40 °C higher than that without syngas and the conversion was complete at a temperature more than 150 °C higher compared with that without syngas. Also, to compare the platinum and ruthenium catalyst in the absence of syngas, the platinum catalyst usually showed a higher toluene conversion efficiency at the same temperature and metal content, while in the presence of syngas, the ruthenium catalyst showed a better conversion efficiency than the platinum catalyst. Thus, it is confirmed that the presence or absence of syngas greatly affects the toluene conversion; furthermore, if it is used in connection with the biomass gasifier, then the use of the ruthenium catalyst is more effective than the platinum catalyst. To oxidize and convert tar efficiently using these catalysts in the presence of syngas, the results indicate that a temperature of more than 300 °C is required.

4. CONCLUSIONS In this work, in conditions that exceeded the concentration of the tar emitted from a woody biomass gasifier by 10-fold, the

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catalytic oxidation characteristics of the model biomass tar, toluene, were identified according to temperature, and the catalyst species and size. As the temperature increased and the catalyst size decreased, the toluene conversion increased. Catalyst with higher metal loadings showed greater conversion efficiency under identical temperature conditions. Notably, with the platinum-based and the ruthenium-based catalysts, 100% toluene conversion was achieved, even at low temperatures of 280 °C. In addition, all applied catalysts produced 100% conversion below 530 °C in the presence or absence of syngas. It was found that the temperature required to obtain the same conversion of tar under constant conditions is affected by the presence or absence of syngas. In the presence of syngas, to obtain the same tar conversion of 100% using the same catalyst and conditions used in the absence of syngas, a temperature increase in excess of 100 °C is required. The platinum-based catalyst showed a better conversion efficiency at the same operating temperature and catalyst size under the simulated syngas absence condition. However, with the model syngas, the ruthenium-based catalyst showed a higher conversion of toluene under identical conditions. Therefore, to oxidize and remove tar in gas produced from biomass gasification, the ruthenium catalyst is more suitable than the platinum catalyst. Compared to tar reforming, the catalytic tar oxidation makes it possible to eliminate tar from syngas at a relatively low temperature. However, it oxidizes flammable gas components in the syngas such as H2 and CO. Also, the reforming reaction is endothermic, whereas the oxidation reaction is exothermic. Heat recovery from the tar oxidation reactor and syngas emitted by the oxidation reactor are very important factors with respect to improving the process efficiency under catalytic tar oxidation technology application.

’ AUTHOR INFORMATION Corresponding Author

*Phone: þ82428603353. Fax: þ82428603134. E-mail: jaegoo@ kier.re.kr.

’ ACKNOWLEDGMENT This research was supported by an Environmental Technique Development Project of the Korea Environmental Industry & Technology Institute and the Research Cooperating Program for Agricultural Science & Technology Development, Rural Development Administration (RDA) of the Republic of South Korea. ’ REFERENCES (1) Kirubakaran, V.; Sivaramakrishnan, V.; Nalini, R.; Sekar, T.; Premalatha, M.; Subramanian, P. A Review on Gasification of Biomass. Renew. Sust. Energ. Rev. 2009, 13, 179. (2) Kimura, T.; Miyazawa, T.; Nishikawa, J.; Kado, S.; Okumura, K.; Miyao, T.; Naito, S.; Kunimori, K.; Tomishige, K. Development of Ni Catalysts for Tar Removal by Steam Gasification of Biomass. Appl. Catal. B-Environ. 2006, 68, 160. (3) Li, J.; Liu, J.; Liao, S.; Yan, R. Hydrogen-rich Gas Production by Air-Steam Gasification of Rice Husk using Supported Nano-NiO/ γ-Al2O3 Catalyst. Int. J. Hydrogen Energy 2010, 35, 7399. (4) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Pretreated Olivine as Tar Removal Catalyst for Biomass Gasifiers: Investigation using Naphthalene as Model Biomass Tar. Fuel Process. Technol. 2005, 86, 707. (5) Zhao, Y.; Sun, S.; Tian, H.; Qian, J.; Su, F.; Ling, F. Characteristics of Rice Husk Gasification in an Entrained Flow Reactor. Bioresour. Technol. 2009, 100, 6040. 2450

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