Benzene Conversion in a Packed Alumina Bed Continuously Fed with

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Biofuels and Biomass

Benzene conversion in a packed alumina bed continuously fed with woody char particles Mario Morgalla, Leteng Lin, and Michael Strand Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b01249 • Publication Date (Web): 07 Jun 2018 Downloaded from http://pubs.acs.org on June 7, 2018

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Benzene conversion in a packed alumina bed continuously fed with woody char particles Mario Morgalla*, Leteng Lin, Michael Strand Dept. of Built Environment and Energy Technology, Linnaeus University, 351 95 Växjö, Sweden Keywords: Biomass, Gasification, Tar, Char, Aerosol ABSTRACT This paper investigates the decomposition of benzene (as a model tar) over finely dispersed char particles continuously distributed into a packed bed. Fragmented char particles and benzene plus a gasification agent (H2O or CO2) were supplied into a ceramic reactor that was heated electrically. The supplied char particles were retained in the reactor by a bed of alumina grains. Woody char as well as iron-doped and potassium-doped woody char were used. The influence of the gasification agent, char concentration, char weight time (proportional to the instant char mass present in the bed) and bed temperature (600–1050°C) was investigated. Increasing the char concentration and char weight time increased benzene conversions for all tested chars. At similar char weight times the benzene conversion increased with temperature whereas the iron- and potassium doped char did not affect the specific conversion. At similar char concentrations, changing the gasification agent from CO2 to steam as well as using doped char led to decreased benzene conversions. This can be explained by accelerated char gasification reactions and thus a diminished char mass in the packed bed. Furthermore benzene conversion rates were enhanced in presence of CO2 compared to steam. As the temperature was increases from 950 ℃ to 1050 ℃ the benzene conversions was slightly reduced. This was interpreted as a combined effect of the enhanced benzene conversion rates and reduced char weight times. The highest benzene conversions achieved in the experiments were approximately 80% at 950-1000 ℃ using CO2 as gasification agent and supplying approx. 20–30 g Nm-3 undoped woody char.

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1. Introduction Biomass gasification is a procedure in which organic matter is converted into a combustible gas (product gas) in a sequence of thermo-chemical reactions. The product gas can be catalytically upgraded to a gas with a high content of H2 and CO, which can be used to produce a number of gaseous and liquid chemicals (e.g. dimethyl ether (DME) or methanol). Tars are an undesirable by-product formed during biomass gasification. At high temperatures tars can form coke, which is known for its ability to deactivate catalysts used to upgrade the product gas. If the product gas is cooled downstream the gasifier, tars start to condense, plugging pipes and fouling heat exchangers. In the product gas from fluidized bed gasifiers the tar concentration might be as high as 30 g Nm-3.1 In order to use the product gas for methanol synthesis, for example, tar concentrations should be below 0.1 mg m-3.2 Considerable effort has been expended to eliminate tars inside3 the gasifier (primary measures) or downstream (secondary measures). Secondary measures include the physical separation of tars and thermal or catalytic conversion of the tars. Physical separation would require cooling of the product gas, and so the conversion of tars at elevated temperatures into lighter hydrocarbons, CO and H2 is preferred since the chemical energy of the tars as well as the sensible heat of the product gas is preserved. Both coal char and biomass-derived char have been shown to have a catalytic activity comparable to dolomite, fluid catalytic cracking,- or Ni-catalysts.4 The advantage of biomass char compared to other catalytic materials is its production within the gasification process. This has been utilized in some gasification designs where unconverted biomass char is collected in the form of a stratified5, 6 or fluidized7 char bed in which nascent tars are removed. Using a hot char bed installed downstream of the gasifier as a tar pre-reformer is beneficial since lower tar concentrations might help avoiding soot formation in partial oxidation units 8 and the deactivation of tar cracking catalysts


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. Van der Drift et al.10 developed a tar pre-reformer based on a granular bed filtration

technology. The product gas of an air-blown fluidized bed gasifier, containing char particles and tar molecules passes a hot (900 ℃) granular moving bed made of inert bed material (silica sand) which captures the char particles. The tar content was reduced by 80 % downstream of the prereformer. The conversion of tar in a fixed char beds has been investigated and reviewed.11 Fuentes et al.12 sudied naphthalene decomposition activities using a hot char bed in the presence of steam (15 vol. %). They suggested a mechanism involving (1) tar adsorption on the active sites of the char, (2) polymerization reactions forming H2 and coke, and (3) coke deposition on the active sites of the char. They stated that the char stays active if the carbon consumption of the char is higher than the carbon deposition by the coke which was the case for a temperature of the char bed ≥ 950 ℃.

A novel experimental method for investigation of the tar conversion has been developed and applied. 13 In this method fine char particles are suspended and are continuously supplied to either a high-temperature filter13 or an inert Al2O3 bed.14 The use of very small char particles is advantageous as tar decomposition kinetics can be established without potential internal and external mass transport limitations. Another advantage of this experimental approach is the continuous supply of char, which closely resembles a real gasification process. The method has been used to study the decomposition of benzene at different bed temperatures (900–1100 °C), steam concentrations (0–27 vol. %), and char concentrations (5–50 g Nm–3). It was reported that

for a similar char concentration, the benzene conversion was higher at 900 ℃ than at 1100 ℃.

This was assumed to be caused by higher char gasification reactions at 1100 ℃, resulting in lower amounts of char in the packed bed. If the packed Al2O3 bed is intended to be used


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downstream of a fluidized bed gasifier as a tar pre-reformer, the retained char mass inside the packed bed will be limited by the fraction of char leaving the gasifier. Gasification processes are often optimized for maximum char conversion, that is, only a minor fraction of the char will end up in the packed bed and be available for tar conversion. It is therefore essential to not only produce a char with high tar conversion capacity but also provide conditions that minimize char conversion inside the bed. Iron and potassium have both been reported to show catalytic activity affecting tar decomposition at elevated temperatures,15, 16 but are also known to enhance char gasification reactions.17, 18 In order to study the combined effect of tar kinetics and char gasification on benzene decomposition, the influence of different gasification media (CO2 and steam), bed temperatures (600–1050 ℃), and the effect of potassium- and iron- doped chars were investigated in this

study. 2. Materials and methods 2.1 Material Three different char samples were used in the experiments. A commercially certified (EN1860-2) barbeque char (Skandivaror, Malmö, Sweden) made from broadleaf served as the original char sample (O-char). The second and third char samples were prepared from the O-char by wet impregnation with a K2CO3 solution (K-char) and a FeN3O9 solution (Fe-char): Potassium and iron solutions were prepared by mixing K2CO3 (Fisher Scientific International, Inc. [now Thermo Fisher Scientific], Waltham, USA) and FeN3O9 (Acros Organics/Thermo Fisher Scientific) with water and ethanol. The O-char was crushed to form particles < 0.5mm. The char particles were soaked in either the K- or Fe-solution. The char slurry was dried for 15 h


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at 105 ˚C. The elemental analysis of the three chars as well as the moisture and ash content are presented in Table 1. Benzene (Merck & Co., Inc, Kenilworth, USA) was used as the model tar. Table 1. Elemental analysis, moisture and ash content of the O-char., K-char and Fe-char. Content (% dw)

O-char

K-char

Fe-char

C

87.1

82.4

85.0

H

2.5

2.9

2.8

N

0.48

0.48

0.97

Cl

0.01

;g NI5J @) collected in the packed bed. The first term in Eq. 3 contains the gaseous


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carbon (CO, CO2) released during the simultaneous char, steam, and benzene supply. The CO and CO2 from the benzene conversion was then subtracted (second term, Eq. 3). %ℎ#+G> ;"@ KL

ML NO E

/0123,4 6 ∙KYZ[

PQ/R ;"@ + Q/RT ;"@U − WX

4

N\

] ^1 − X

/0123,C 6

/0123,4 6

4

]_`

&Ka

KYZ[ ∙EML

(3)

MC (g mol-1) represents the molar mass of carbon, Vm (l mol-1) the molar volume of an ideal

gas at NTP (20˚C, 1bar), xc the mass fraction of carbon in the char, %, the benzene 4

concentration at NTP, Q/R and Q/RT the gas concentrations (ppm) of CO and CO2, respectively. When CO2 was used as gasification agent, Eq. 3 could not be used to calculate the char concentration. In this case the char concentration was assumed to be similar to the one established when the gasification medium was switched to steam during the same experiment. 2.3.4

Weight time (τ) determination

The weight time τ was used as measure of the amount of char in the packed bed (Eq.4). The weight time is the ratio of the deposited char and the gas volume flow rate. The char mass (Ibcde ) was established by measuring the total CO and CO2 released through char gasification after the char feeding had been stopped. f [hi ℎ I5J ] -

kLlmn o

Dv Ka ∙o q ∙ ras ;D@trasT ;D@∆D

- E ∑ E

ML NO o

(4)

In Eq.4 I/cde [hi] is the mass of char in the packed bed, w x [IJ ℎ5E ] the gas volume flow rate at room temperature, w [IJ ℎ5E ] the gas volume flow at the bed temperature yz=D . ∆" [ℎ] and

"{ [ℎ] represent the time interval between two data points and the time applied for char gasification, respectively. τ was determined under steady-state, as well as non-steady-state conditions.


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Another way to calculate the char mass in the packed bed is to integrate the difference of the steady-state char concentration (%ℎ#+G> ) and the char gasification rate based on the released CO and CO2 (%ℎ#+{d| ;"@) during the char build-up phase:

Ibcde ;"@ - ∑D%ℎ#+G> − %ℎ#+{d| ;"@w∆"

The steady-state char mass Ibcde Ibcde

|D=d}r

|D=d}r

in the packed bed is reached for " ≥ "|D=d}r

- ∑~m %ℎ#+G> − %ℎ#+{d| ;"@w∆" D

(5)

(6)

where "|D=d}r is the time needed to reach steady-state conditions. %ℎ#+{d| ;"@ is calculated in accordance with Eq. 3 but before reaching steady-state conditions. 2.3.5

Tar and char kinetics

The benzene conversion during the experiments was assumed to follow a pseudo first-order reaction with respect to the benzene concentration (assuming plug flow conditions and negligible influence of the steam or CO2 concentration (high excess)). The reaction rate constant,

h [IJ hi5E ℎ5E ] was calculated according to Eq. 7 k-

5;E5@

(7)

where X represents the benzene conversion as calculated in Eq. 2. This approach is widely accepted

4, 12, 15

and allows comparison of catalyst activities in tar elimination based on the

apparent first-order rate constant k. Rearranging Eq. 7 for the benzene conversion X [%] gives X - [1 − exp;−k ∙ τ@] ∙ 100

(8)

Since the weight time at steady-state conditions is decided by the combined effect of char feeding rate (Char ) and the char gasification rate Eq. 8 was modified in order to express the

benzene conversion as a function of the char concentration %ℎ#+G> , X - 1 − exp P

5∙2 ∙q E

U ∙ 100

(9)


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where t x [ℎ] represents a characteristic contact time between benzene and char. The char accumulation in the bed was described by a first-order differential equation: }kLlmn }D

- %ℎ#+G> ∙ w x − +/cde ∙ Ibcde

Solving the differential equation with the condition that Ibcde ;0@ - 0 gives Ibcde ;"@ -

/cdeZ ∙o q 5/cdeZ ∙oq ∙= nalmn ∙ ealmn

( 10 )

( 11 )

where +/cde represent the char reactivity (s-1). 3. Results and discussion 3.1 Char build-up tests

Figure 3 shows the experimentally determined char masses in the packed bed based on Eq. 5, as well as the char mass as calculated according to Eq. 11 when steam was used as the gasification medium. Char reactivities from Eq. 11 were fitted to match the experimental char masses and are reported in Table 3.


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Figure 3. Calculated (curves) and experimental (markers) char mass in the packed bed supplying 8 g Nm-3 O-char, using 13.5 % steam. Table 3. Experimentally determined char reactivities rchar (s-1) for the O-char using 13.5 % steam as used in Eq. 11. Temperature [℃]

+bcde [, 5E ]

950

1050

0.0051

0.0143

The reported char reactivities using steam reported in this study may be lower than the values found by others since in a previous study char reaction rates were reported to be slowed down in presence of benzene.13 Further deviations might be caused as char reactivities might vary with time. The calculated and the experimentally determined steady-state char masses were both lower at higher temperatures due to enhanced char reactivities as shown in Table 3. When CO2 was used as gasification medium, it was not possible to determine the char gasification rate %ℎ#+{d| ;"@ using Eq. 3 and therefore to establish the experimental char mass in the packed bed

with Eq. 5. In literature CO2 gasification rates were generally reported lower compared to those |D=d}r

for steam.19, 20 Therefore, in this study the steady-state char mass (Ibcde ) in the packed bed was estimated higher when using CO2 than when using steam.

3.2 Influence of the gasification medium and temperature Figure 4 shows a typical experimental set of data used to evaluate the influence of the gasification medium. The CO2 and benzene concentration at the required temperature (the

example shown here is for 1050 ℃) was first stabilized. Thereafter char was continuously fed (t

= 0 min) and the steady-state benzene conversions and corresponding gas concentrations before


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and after (t = 17 min) the gasification medium was switched from CO2 to steam were recorded. The benzene conversion was lower when steam was used as a gasification medium compared to |D=d}r

CO2. This can be explained by the establishment of a smaller steady-state char mass (Ibcde ) in the presence of steam compared to the mass with CO2, as explained in the previous section.

Changing the gasification medium from steam to CO2 (or CO2 to steam) at 950 ℃ also led to a

higher (or lower) benzene conversion, respectively ;results not presented here@. The lower supplied benzene concentrations using CO2 compared to steam (see Figure 2) were not assumed to have a major effect on the benzene conversions as shown in a previous study.13

Figure 4. Benzene conversion and gas concentrations at 1050℃ supplying approx. 8 g Nm-3 Ochar, 13.5 vol. % steam or 13.5 vol. % CO2 and benzene. Figure 5a and b show the steady-state benzene conversions and gas concentrations including the

initial experimental phase (char build-up phase) supplying O-char, steam and benzene at 950 ℃

and 1050 ℃, respectively. The gray shaded area in Figure 5a and b can be related to the ACS Paragon Plus Environment

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experimentally determined char mass in the packed bed calculated using Eq. 6 and presented in Figure 3. The time needed to reach steady-state ("|D=d}r ) at 950 ℃ and 1050 ℃ as shown in

Figure 3 corresponds to the time the benzene conversion stabilizes as shown in Figure 5a and b, respectively. This shows the correlation between accumulated char mass in the packed bed and

benzene conversion. The steady-state benzene conversion at 950 ℃ is slightly higher than at

1050 ℃. This is explained by the combined effect of the char reactivity and reaction rate of the tar conversion. A higher temperature on the one hand enhances the char reactivity and reduces

the steady-state char mass (Figure 3); on the other hand, it increases the reaction rate of the tar

conversion. That means that at 1050 ℃ the combined effect is a diminished benzene conversion

compared to 950 ℃.


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Figure 5. Steady-state experiments including the char build-up phase at a) 950 ℃ and b) 1050 ℃

using 13.5 vol. % steam, benzene and approx. 8 g Nm-3 O-char.

Since the results (Figure 5) indicated that lowering the temperature from 1050 ℃ to 950 ℃ is beneficial in terms of the benzene conversion, it was intended to perform steady-state experiments at temperatures below 950 ℃ as well. However, the packed bed was blocked before the steady-state benzene conversions could be established due to the time needed to reach steady-

state conditions ("|D=d}r @, as well as the char mass increase with decreasing temperature. The benzene conversions were therefore reported as a function of the weight time only (section 3.3), for temperatures < 950℃.


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3.3 Influence of the weight time and char concentration In this section results from a large number of experiments are compiled and the benzene conversion and corresponding correlation to the weight time (τ) are presented. The results include results for the Fe- and K- doped chars. Steady-state benzene conversion tests using ironand potassium-doped chars were performed in a similar way to the experiments in the previous sections using the O-char (Figure 4 and Figure 5). In addition to the steady-state experiments, data from experiments where a steady-state was not reached are included and compiled to establish a general model for benzene conversion in the experimental system used. Figure 6 presents the benzene conversions using the O-char, Fe-char, and K-char as a function of τ at 600–1050 °C using steam and CO2. The benzene conversions were generally slightly higher using CO2 as gasification medium compared to steam. The relative difference of the benzene

conversion between CO2 and steam became more obvious at temperatures exceeding 850 ℃.

This is in line with findings by Simell et al.21 who showed that dry reforming reactions of toluene became thermodynamically more favorable than steam reforming reactions at temperatures above 830 °C.


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Figure 6. Benzene conversion as a function of the weight time using the O-char, K-char and Fechar with a) CO2 and b) steam. The graphs (kin) represent the fitted benzene conversions as calculated in accordance with Eq. 8. The Fe-char did not show an improved benzene conversion activity compared to the O-char at any temperature. One possible explanation is that the iron was inactive, that is, present as iron oxide. According to Nordgreen et al.22 it needs to be reduced to elemental iron in order to generate the actual catalytically active metallic iron. Another possible explanation is the relatively low iron content of the char compared to that used in other studies. The K-char too did not show an effect on the benzene conversion. One explanation might be that the potassium concentration of the char is too low to show a pronounced catalytic effect. Another explanation is that enhanced char sintering due to the increased alkali metal content of the char, might have


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destroyed the pore structure of the char.16 Potential side reactions of potassium and the crushed Al2O3 balls might have also lowered the catalytic effect of the K-char. An alternative explanation of why neither Fe nor K improved the benzene conversion might be that the benzene is not converted catalytically, but mainly via coke formation. When using larger particles e.g, in a fixed char bed, catalytic effects might help keeping the char structure open from blockage by coke formation i.e. by more rapid coke and char gasification. However, in the experiments used here the particles are very small and transport limitation caused by coke formation is probably less critical. As described in section 2.3.5 a first-order kinetic approach (Eq. 8) was applied to describe the reaction rate of benzene over the three chars employed. Table 4 shows the apparent rate constants k for CO2 and steam that best fit the experimental data for the benzene conversions. Figure 6 shows the experimentally determined benzene conversions and the benzene conversions as calculated with the rate constants in Table 4. Since the specific benzene conversion of the potassium- and iron-doped chars did not change, the same rate constant was used in Eq. 8 for all three chars. The experimentally determined benzene conversion increased with increasing weight time at all investigated temperatures and for all three chars. The fitted graph at 850 °C using CO2 indicates that more than 95% of the benzene might be converted at weight times exceeding 4.5 × 10–3 kg h m–3. Graphs of the benzene conversion at 600 ℃ and 750 ℃ are not shown here since the experimentally determined benzene conversion were too low to allow for such kind of estimation.

Table 4 Estimates of the Apparent Rate Constant, k, for the O-char, Fe-char and K-char


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Gasification medium

Steam

Steam

Steam

CO2

CO2

CO2

Temperature [°C]

850

950

1050

850

950

1050

k [m3 kg–1 h–1]

550

2400

4000

650

3000

6000

Figure 7 shows the benzene conversions and the corresponding char concentrations (%ℎ#+G> ) at 950, 1000 and 1050 ℃. Only the data from steady-state conditions are included, and consequently results for temperatures between 600 ℃ and 850 ℃ are not presented. In general all

three chars showed slightly higher benzene conversions at 950 ℃ compared to 1050 ℃ at similar char concentrations. As explained in the previous section, this can be explained by the combined effect of the char reactivity and reaction rate of the tar conversion. Using CO2 compared to steam

led to increased benzene conversions at similar char concentrations. As discussed in section 3.1, this can be explained by the comparatively slower char gasification rate of the char using CO2 and thus the higher char mass in the packed bed. Furthermore the benzene conversion rate seemed enhanced in the presence of CO2 compared to steam as shown in Table 4. The benzene conversions of the Fe- as well as the K-char were lower than the O-char at similar char concentrations. Since iron as well as potassium are known to increase char reactivity17,

23

the

lower benzene conversions of the Fe- and K-char were related to their reduced mass in the packed bed.


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Figure 7. Benzene conversion as a function of the char feeding rate at different temperatures using a) 13.5 vol. % steam and b) 13.5 vol. % CO2. The experimentally determined benzene conversions and corresponding char concentrations were used to fit Eq. 9. Values of " x for the O-char using steam and CO2 at 950 and 1050 ℃ are reported in Table 5. Since the results for 950 ˚C and 1000 ˚C are very close, a common " x was

used for both temperatures. The " x value for the K- and Fe-char could not be established due to

the limited dataset. However, initial data indicate lower " x values, due to the enhanced char

gasification rates for both chars. Note that the " x value might depend on the experimental setup. Table 5. t' values fitted to Eq. 9 for the O-char using steam and CO2. Gasification medium Temperature [°C]

CO2 950

1050

H2O 950

1050


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"′ [h] 4

0.077

0.011

0.029

Page 24 of 26

0.006

Conclusion and outlook

In this study the benzene conversion inside a packed Al2O3 bed continuously fed with suspended char particles was investigated. The tar conversion efficiencies of a woody char as well as an iron-doped and a potassium-doped woody char were tested in the temperature range of 600– 1050°C in the presence of steam or CO2 using char concentrations up to 50 g Nm–3. Increasing the char concentration increased the benzene conversion. Doping the woody char with approximately 0.7 wt. % iron or 2 wt. % potassium did not improve the specific benzene conversion of the char. The doping increased however the char gasification rate, leaving less char in the packed bed, thus leading to overall lower benzene conversions. For the same reasons the benzene conversion decreased when the gasification medium was switched from CO2 to steam. The tar conversion rate was higher in the presence of CO2 compared to steam. Increasing the temperature not only caused a higher specific tar conversion rate but enhanced the char gasification rate as well. The combined effect of char gasification and tar conversion rates, however, led to increased overall benzene conversions at 950 ℃ compared to those at 1050 ℃.

In a continuous gasification process only a limited mass of char is entrained from the gasifier and accumulated in a potential packed bed (the type investigated in this study) installed downstream of the gasifier. This study emphasizes the joint effect of the tar conversion and char gasification rates on the overall tar conversion in these applications. AUTHOR INFORMATION Corresponding Author *email address: [email protected]


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Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The financial support provided via the Swedish Energy Agency and the Swedish Gasification Centre is gratefully acknowledged. 5

References

1. Graham, R. G.; Bain, R.; group, I. b. g. w. Biomass gasification hot-gas clean-up; International Energy Agency Biomass Gasification Working Group from Ensyn Technologies and the National Renewable Energy Laboratory: Golden, CO, 1993. 2. Woolcock, P. J.; Brown, R. C., A review of cleaning technologies for biomass-derived syngas. Biomass and Bioenergy 2013, 52, (0), 54-84. 3. Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G., A review of the primary measures for tar elimination in biomass gasification processes. Biomass and Bioenergy 2003, 24, (2), 125-140. 4. Abu El-Rub, Z.; Bramer, E. A.; Brem, G., Experimental comparison of biomass chars with other catalysts for tar reduction. Fuel 2008, 87, (10–11), 2243-2252. 5. Brandt, P.; Larsen, E.; Henriksen, U. B., High tar reduction in a two-stage gasifier. Energy & Fuels 2000, 14, (4), 816-819. 6. Henriksen, U.; Ahrenfeldt, J.; Jensen, T. K.; Gøbel, B.; Bentzen, J. D.; Hindsgaul, C.; Sørensen, L. H., The design, construction and operation of a 75 kW two-stage gasifier. Energy 2006, 31, (10–11), 1542-1553. 7. Nilsson, S.; Gómez-Barea, A.; Fuentes-Cano, D.; Ollero, P., Gasification of biomass and waste in a staged fluidized bed gasifier: Modeling and comparison with one-stage units. Fuel 2012, 97, 730-740. 8. Svensson, H.; Tunå, P.; Hulteberg, C.; Brandin, J., Modeling of soot formation during partial oxidation of producer gas. Fuel 2013, 106, 271-278. 9. Bain, R. L.; Dayton, D. C.; Carpenter, D. L.; Czernik, S. R.; Feik, C. J.; French, R. J.; Magrini-Bair, K. A.; Phillips, S. D., Evaluation of Catalyst Deactivation during Catalytic Steam Reforming of Biomass-Derived Syngas. Industrial & Engineering Chemistry Research 2005, 44, (21), 7945-7956. 10. van der Drift, A.; Carbo, M. C.; van der Mejden, C. M., The TREC-module: integration of tar reduction and high-temperature filtration. In 14th European Biomass Conference & Exhibition, Paris, France, 2005. 11. Shen, Y., Chars as carbonaceous adsorbents/catalysts for tar elimination during biomass pyrolysis or gasification. Renewable and Sustainable Energy Reviews 2015, 43, (0), 281-295. 12. Fuentes-Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P., Decomposition kinetics of model tar compounds over chars with different internal structure to model hot tar removal in biomass gasification. Chemical Engineering Journal 2013, 228, (0), 1223-1233.


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Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 26 of 26

13. Morgalla, M.; Lin, L.; Strand, M., Decomposition of benzene using char aerosol particles dispersed in a high-temperature filter. Energy 2017, 118, 1345-1352. 14. Morgalla, M.; Lin, L.; Strand, M., Benzene Conversion in a Packed Bed Loaded with Biomass Char Particles. Energy & Fuels 2017, 32, (1), 554-560. 15. Kastner, J. R.; Mani, S.; Juneja, A., Catalytic decomposition of tar using iron supported biochar. Fuel Processing Technology 2015, 130, (0), 31-37. 16. Guo, F.; Liu, Y.; Liu, Y.; Guo, C., Catalytic reforming of tar using corncob char and char-supported potassium catalysts. Journal of Thermal Analysis and Calorimetry 2017, 1-10. 17. Furimsky, E.; Sears, P.; Suzuki, T., Iron-catalyzed gasification of char in carbon dioxide. Energy & Fuels 1988, 2, (5), 634-639. 18. Nzihou, A.; Stanmore, B.; Sharrock, P., A review of catalysts for the gasification of biomass char, with some reference to coal. Energy 2013, 58, (Supplement C), 305-317. 19. Guizani, C.; Escudero Sanz, F. J.; Salvador, S., The gasification reactivity of highheating-rate chars in single and mixed atmospheres of H2O and CO2. Fuel 2013, 108, (Supplement C), 812-823. 20. Di Blasi, C., Combustion and gasification rates of lignocellulosic chars. Progress in Energy and Combustion Science 2009, 35, (2), 121-140. 21. Simell, P. A.; Hepola, J. O.; Krause, A. O. I., Effects of gasification gas components on tar and ammonia decomposition over hot gas cleanup catalysts. Fuel 1997, 76, (12), 1117-1127. 22. Nordgreen, T.; Liliedahl, T.; Sjöström, K., Metallic iron as a tar breakdown catalyst related to atmospheric, fluidised bed gasification of biomass. Fuel 2006, 85, (5), 689-694. 23. Hawley, M. C.; Boyd, M.; Anderson, C.; DeVera, A., Gasification of wood char and effects of intraparticle transport. Fuel 1983, 62, (2), 213-216.


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Benzene Conversion in a Packed Alumina Bed Continuously Fed with

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