Conversion of Phenol-Based Tars over Biomass Char under H2 and

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Conversion of Phenol-Based Tars over Biomass Char under H2 and H2O Atmospheres Xavier Nitsch,* Jean-Michel Commandré,* Jérémy Valette, Ghislaine Volle, and Eric Martin Centre de Coopération Internationale en Recherche Agronomique pour le Développement (CIRAD), TA B-42/16, 73 rue Jean François Breton, 34398 Montpellier Cedex 5, France ABSTRACT: This study focused on the effect of a wood char on tar cracking and reforming in the context of fluidized-bed gasification. Phenol was used to create in situ a mixture representative of tars produced by pyrolysis and gasification in a dual fluidized-bed reactor. The wood char was placed in a fixed-bed reactor, and atmospheres composed of H2 and steam were tested both together and separately. Gasification of the wood char under the different atmospheres was characterized. The char was found to be highly catalytic with tars in the presence of steam. The influence of H2 on catalytic activity remained low. A comparison to the results on olivine under the same reaction conditions showed that chars were much more efficient and may have a strong influence on tar reduction in fluidized-bed biomass gasification.

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INTRODUCTION

Apparatus. The fixed-bed reactor called Aligator is described in Figure 1. The catalytic bed was crossed by all of the gases introduced into the reactor. The flow rate of these gases was high enough to ensure that the contact time remained similar to that in industrial fluidized beds. The apparatus consisted of an external electrically heated quartz tube (inner diameter of 30 mm and length of 420 mm) and an internal quartz tube (inner diameter of 20 mm and length of 410 mm) equipped with a quartz filtering sheet supporting the solid bed and located 150 mm from the top of the tube. The gas passing through the solid bed was previously preheated in the annular space between the external and internal tubes. The gas preparation system (Figure 2) consisted of two Brooks mass flowmeters that controlled the N2 and H2 supply. Phenol was introduced into the gas by a bubbler feeding system; gas passed through the liquid tar, whose temperature was controlled by a heating/cooling fluid. The mass of tars inside the bubbler was high enough for the variation in height because of transfer between gas and tar to remain very low during the experiment. The theoretical amount of phenol in the gas could be calculated and predicted by Antoine’s law: Pphenol = 10A − (B/(T + C)), where A = 7.540 98, B = 1801.28, and C = 204.687. To check wether the actual partial pressure of phenol was correct, the bubbler was weighed before and after each test and the weight loss validated the measurements made by the analytical system. Typical weight loss was about 2 g, with a difference in the values between the weight loss and the analytical system below 15%. The theoretical temperature calculated by Antoine’s law had to be increased from 73 to 82 °C to compensate for non-optimum exchanges between gas and liquid. Liquid water flow was controlled by a Coriolis flowmeter and was introduced into the gas line at a temperature of 240 °C. To prevent tar or steam condensation, all of the lines from the tar bubbler to the end of the analytical system were heated to a temperature of 150 °C. This temperature had proven to be sufficient to prevent tar and water condensation in all of our experiments. Volume gas concentrations were chosen to match as precisely as possible the conditions inside a steamfluidized bed operating with wood pellets at 850 °C at atmospheric pressure (Figure 1). For experiments without char, total gas flows of 224 mL/min at standard temperature and pressure (STP) were used, with a gas

Replacing fossil fuels by renewable and reliable sources of energy is one of the most important challenges for the future. Gasification is an efficient way of using biomass to produce syngas, which can be used for catalytic hydrocarbon synthesis or the production of electricity. However, one of the main problems of biomass gasification is the high tar content of the gas produced, 1 which leads to pipe clogging and catalyst deactivation. To limit tar concentration in syngas, it is necessary to understand the mechanisms of tar formation and destruction and the influence of the different solids present in the reactor. In fluidized beds, the catalytic effect of the fluidizing media has been studied for some time,2−4 but char has received less attention, probably because of its constant consumption and formation in the bed, which makes it difficult to study under gasification conditions. In fixed beds, char is well-known for its very efficient reduction of the tar load by adsorption and catalytic destruction.5−10 The amount of steam available during pyrolysis seems to have an important influence on the resulting char, with an increase in the specific surface area, the presence of oxygen functional groups, and the formation of large aromatic ring systems.11−13 The mechanisms of tar cracking over a char bed with a gasification atmosphere have been specifically studied by several authors.9,14−17 The presence of steam or oxygen is decisive in maintaining catalytic activity by the continuous formation of micropores in the char structure. These micropores induce coking of the tars, which can be oxidized by the presence of steam or oxygen. Gasification of the char seems to be hindered by the presence of H2 and tars.17 In this study, we chose to investigate the effect of biomass char on model tars during its gasification in a fixed bed. Phenol was used to create the tar mixture, while steam and H2 were injected into the gas stream to understand how they affected tar destruction over a biomass char. The same experimental setup and conditions described in our previous work on olivine18 were reproduced here to compare the catalytic activity of both solids. © XXXX American Chemical Society

EXPERIMENTAL SECTION

Received: April 30, 2014 Revised: September 28, 2014

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dx.doi.org/10.1021/ef500980g | Energy Fuels XXXX, XXX, XXX−XXX

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min under an inert atmosphere. Then, the temperature was lowered to 850 °C for 15 min before the start of the experiment. The aim of this procedure was to avoid any trace of tar produced from the char. A test showed that this procedure induced a mass loss of 9.7%, and traces of H2O, CO, and CO2 were measured. This mass loss was taken into account in the global carbon balance of each experiment. Char Characterization. The elementary composition of the char was analyzed by an Elementary Variomacro Cube CHN; the results are presented in Table 2. Humidity (6.2%) and ash content (3.4%) were measured as prescribed by standard NF EN 1860-2. The mass of char was weighed before and after each experiment to calculate its mass loss and include it in the total carbon balance. Temperature-programmed oxidations (TPOs) were performed on a Micromeritics Autochem 2920 II fitted with a thermal conductivity detector (TCD). A gas flow of 10% O2/He at 10 mL/min at STP was used for oxidation, with a temperature ramp of 0.5 °C/min between 200 and 700 °C. Method. The methodology to produce phenol-based tars before the solid bed has already been described in a previous work,18 but it has been reproduced here for a better understanding and clarity. The overall approach is divided into three sets of experiments, where temperature, gas concentrations, and residence time in the homogeneous zone of the reactor remain unchanged: (1) Conversion of phenol at 850 °C for 6 s is studied without any solid. Because phenol is largely converted at this temperature, the reactions in the homogeneous zone create a mixture of tars and gases, which are analyzed and quantified. (2) A bed of char is added at the end of the isothermal zone. The reactivity of the char is measured under the same conditions (presence of steam and H2) but without any tar. (3) A new bed of char is added at the end of the isothermal zone. The phenol injected into the reactor is converted for 6 s in the homogeneous zone before reaching the bed. Therefore, the mixture of tars reaching the catalyst is considered to be the same as that analyzed and quantified in set 1. The effect of the bed is then measured and compared to the previous experiments to quantify a possible catalytic effect of the solid. Before each experiment, gas mixtures (Table 1) were prepared and analyzed with GC−TCD and micro gas chromatography (μGC) to ensure that gas concentrations were correct. An amount of 1.5 vol % of phenol was used to simulate the high concentration of secondary tars inside the reactor. A system of two three-way valves was used to precisely start and stop the gas flow in the reactor. Calculations. Molar concentrations measured by the analytical system (Ci) were used to calculate the carbon molar flow of the different products

Figure 1. Schematic diagram of the Aligator reactor. residence time of 6 s in the reactor. Experiments with char were conducted with gas flows of 193 mL/min at STP, with a gas residence time in the reactor of 6 s before it reached the solid bed and a contact time of 0.32 s with the char. A total of 2.705 g of pyrolyzed char was used for each experiment. A weight hourly space velocity (WHSV) of 4.55 h−1 was calculated for phenol over char. The solid bed was placed at the bottom limit of the isothermal zone to ensure rapid cooling of gases. The first analytical device downstream from Aligator was PerkinElmer gas chromatography−thermal conductivity detector (GC−TCD). The amount of gas injected into the column was controlled by a loop connected to a six-way valve. The column used was a Varian CP-PoraBOND Q, heated from 120 to 300 °C for 16 min to measure cyclopentadiene (CPD), benzene, toluene, and naphthalene concentrations. After GC, gases were cooled to −10 °C to condense water and heavy tars. Afterward, the incondensable species were transferred to a Varian micro-GC 4900. Three columns were calibrated for the analysis and quantification of N2, H2, CO, CO2, O2, CH4, C2H2, C2H4, C2H6, cyclopentadiene, benzene, and toluene. When a more complex analysis was required, a valve upstream from the micro-GC took the gas to a condenser at −10 °C filled with acetone for analysis on calibrated gas chromatography−mass spectrometry (GC−MS). Species were recovered for identification and quantification on an Agilent GC 6890 fitted with a 5975 mass spectrometer system. Samples were diluted in acetone, and two injection modes were used to analyze light and heavy tars. The column was a J&W DB1701, heated from 40 to 270 °C. Detection was by electron impact at 70 eV. Char Preparation. The char used in these experiments was chosen for its very low tar content. Beech wood was pyrolyzed in a staged reactor19 at a temperature of 750 °C for 1 h under an inert gas. The resulting char was ground and sieved to a particle size of 710−1000 μm. Before each experiment, the loaded char was heated to 900 °C for 10

Fic = CiFtotalNi

(1)

where is the carbon molar flow of i, Ftotal is the total molar flow of the experiment, and Ni is the number of carbon atoms of i. Because the gasification of char produces carbonaceous products (CO, CO2, and CH4), the carbon molar flow of tars was determined by subtracting the experiment with char only from the corresponding experiment with char and tars. Fci

Fic(tars) = Fic(char + tars) − Fic(char)

(2)

Carbon balances shown in Figures 3 and 7 were calculated from the carbon molar flow of each species divided by the carbon molar flow input of phenol.

carbon balance =

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Fic(tars) c Fphenol(input)

(3)

RESULTS AND DISCUSSION Phenol Conversion without a Catalyst. In this study, we chose to use phenol to create a reactive atmosphere with a mixture of tars representative of biomass gasification in a fluidized bed. To determine the precise nature of these tars, we studied the decomposition of phenol in the empty reactor, and the results are summarized in Figure 3. First, it can be noted that, B


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Figure 2. Complete scheme of the experimental facility.

Table 1. Volume Gas Concentrations at the Entrance of the Reactor atmosphere

H2 (%)

H2O (%)

N2 (%)

phenol (%)

H2 and H2O H2 only H2O only N2 only

20 20 0 0

40 0 40 0

38.5 78.5 58.5 98.5

1.5 1.5 1.5 1.5

Table 2. Mass Elementary Composition of the Dry Char dry char a

N (%)

C (%)

H (%)

O (%)a

ash (%)

0.48

91.6

1.9

2.6

3.4

O (%) is calculated by difference.

for each atmosphere, about 10% of the phenol did not react and that 60−70% of the total products were aromatic tars. The main two tars produced were benzene and naphthalene, and these amounted to about 50% of the total products. H2O appeared to have very little influence on the amount of tar, which remained the same for all of the experiments. The presence of CO and CO2 was observed for each atmosphere. These gases are usually produced by steam reforming of the carbonaceous species; their presence can be explained here by the decarbonylation of phenol into cyclopentadiene.20 We can therefore conclude that steam reforming of phenol and phenol-based products is very low at 850 °C in the absence of a catalyst. The presence of steam seemed to have little influence on tar nature and concentration. However, H2 had a very strong influence on the distribution of the products. The benzene yield increased; the naphthalene yield decreased; and the amount of solid carbon deposited on the reactor was considerably reduced. Indeed, there are two main pathways toward carbon formation: hazardous air pollutant

Figure 3. Carbon distribution of the products of thermal conversion of phenol at 850 °C with a reaction time of 6 s under the chosen atmospheres.

(HAP) recombination and condensation,21−23 and hydrogen abstraction carbon addition (HACA mechanism) from acetylene.24,25 In both cases, hydrogen is a byproduct of the reaction and a strong partial pressure of H2 has a detrimental effect on heavy HAPs and solid carbon formation. Reactivity of the Char. The reactivity of the char without tar was precisely measured, and the results are summarized in Figures 4 and 5. Without any reactive gas (nitrogen only) or with 20% H2, the char showed a consumption below 10%. With 40% H2O, char gasification was intense and only 45% of the initial amount of char was left after 1 h of experiment. It can be noted that the presence of 20% H2 seemed to inhibit gasification. C


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Figure 4. Remaining mass fraction of the char depending upon the atmosphere at 850 °C after 1 h of experiment with and without tar.

Figure 6. Measured concentration of CO and CO2 during an experiment with H2, H2O, and tars.

Figure 5. Volume percentage of CO, CO2, and CH4 in the output gas during gasification of the char without tar for the selected atmospheres. Figure 7. Total carbon products of tar conversion depending upon the reactive atmosphere. A reference without char is given on the left.

Concentrations measured at the output of the reactor (Figure 5) were in good agreement with the expected reactions. The presence of 40% H2O induced gasification of the char and, thus, large amounts of CO and CO2. The presence of 20% H2 had two effects: a slight inhibition of gasification and an increased formation of methane. Tar Cracking over Char. Consumption of the solid bed by gasification of the char during the experiments had a slight effect on the measured concentration of the products over time. Figure 6 shows that the concentration of CO decreased by about 20% and that the concentration of CO2 remained almost constant during a 1 h experiment. The results of tar cracking over char are summarized in Figures 4 and 7. The overall carbon balance of tar + char + products remained between 98 and 103% for all of the experiments. In Figure 4, the presence of tars in the gas led to an increased mass of char after each experiment, probably because of coking at its surface. In Figure 7, the carbon quantification in the output gas was divided by the amount of carbon in the input gas and compared to a reference without solid for H2 and H2O atmospheres. This comparison showed a strong difference between experiments with and without steam. Experiments without steam result in a large amount of tars in the gas stream. Part of the carbon was not recovered in the gas and might be explained by coking of tars on the char. The presence of steam in the feeding gas had a very strong effect on tars: all of the hydrocarbons, except CH4 and a very small amount of benzene, were eliminated from the output

gas. The very large amounts of CO and CO2 detected and the total amount, over 200 and 175% of products in the gas, could be explained by the high gasification of the char, especially for the experiment without H2. Obviously, it was not possible to distinguish between the products of the gasification of char and the products of steam reforming of the tars. These results are consistent with the recent work of different authors,9,14,14 who also found that the presence of steam was necessary to maintain the catalytic activity of their catalysts (charcoals, coconut char, and pyrolyzed sludge). In our experiments, the presence of steam was necessary to obtain a catalytic activity in tar cracking or reforming over our wood biomass chars. This catalytic activity was very high, and the presence of H2 did not have any major effect on tar reforming. TPOs were conducted to try to differentiate between chars and coked tars at their surface. Dependent upon the graphitization of tars on the char and the structure of the char itself, different carbon structures can be oxidized at different temperatures. The original char was compared to the char used in the experiment with nitrogen only and with the char used with H2 and H2O atmospheres. The results (Figure 8) did not show any significant differences, even at very low heating rates. This lack of difference between chars in TPO analysis suggests that the tars were either completely reformed to CO and CO2 or coked up to a structure close to the carbon of the char. However, the increased amount D


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Agency (ADEME), and the GAYA Project conducted by GDFSUEZ.

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(1) Milne, T.; Evans, R. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; National Renewable Energy Laboratory (NREL): Golden, CO, 1998; NREL/TP-570-25357. (2) Sutton, D.; Kelleher, B.; Ross, J. R. H. Fuel Process. Technol. 2001, 73, 155−173. (3) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. Biomass Bioenergy 2003, 24, 125−140. (4) Abu El-Rub, Z.; Bramer, E. A.; Brem, G. Ind. Eng. Chem. Res. 2004, 43, 6911−6919. (5) Chembukulam, S. K.; Dandge, A. S.; Rao, N. L. K.; Seshagiri, K.; Vaidyeswaran, R. Ind. Eng. Chem. Prod. Res. Dev. 1981, 20, 714−719. (6) Stoeckli, H. Carbon 1990, 28, 1−6. (7) Dufour, A. Optimisation de la production d’hydrogéne par conversion du méthane dans les procédés de pyrolyse/gazéification de la biomasse. Ph.D. Thesis, Institut National Polytechnique de Lorraine, Nancy, France, 2007. (8) El-Rub, Z. A. Biomass char as in-situ catalyst for tar removal in gasification systems. Ph.D. Thesis, University of Twente, Enschede, Netherlands, 2008. (9) Hosokai, S.; Kumabe, K.; Ohshita, M.; Norinaga, K.; Li, C.-Z.; Hayashi, J.-i. Fuel 2008, 87, 2914−2922. (10) Dufour, A.; Celzard, A.; Ouartassi, B.; Broust, F.; Fierro, V.; Zoulalian, A. Appl. Catal., A 2009, 360, 120−125. (11) Savova, D.; Apak, E.; Ekinci, E.; Yardim, F.; Petrov, N.; Budinova, T.; Razvigorova, M.; Minkova, V. Biomass Bioenergy 2001, 21, 133−142. (12) Minkova, V.; Razvigorova, M.; Bjornbom, E.; Zanzi, R.; Budinova, T.; Petrov, N. Fuel Process. Technol. 2001, 70, 53−61. (13) Keown, D. M.; Hayashi, J.-i.; Li, C.-Z. Fuel 2008, 87, 1127−1132. (14) Fuentes-Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P. Chem. Eng. J. 2013, 228, 1223−1233. (15) Hosokai, S.; Norinaga, K.; Kimura, T.; Nakano, M.; Li, C.-Z.; Hayashi, J.-i. Energy Fuels 2011, 25, 5387−5393. (16) Juneja, A.; Mani, S.; Kastner, J. Proceedings of the 2010 American Society of Agricultural and Biological Engineers (ASABE) Annual International Meeting; Pittsburgh, PA, June 20−23, 2010; Paper 1009863. (17) Bayarsaikhan, B.; Sonoyama, N.; Hosokai, S.; Shimada, T.; Hayashi, J.-i.; Li, C.-Z.; Chiba, T. Fuel 2006, 85, 340−349. (18) Nitsch, X.; Commandré, J.-M.; Clavel, P.; Martin, E.; Valette, J.; Volle, G. Energy Fuels 2013, 27, 5459−5465. (19) Fassinou, W. F.; de Steene, L. V.; Toure, S.; Volle, G.; Girard, P. Fuel Process. Technol. 2009, 90, 75−90. (20) Cypres, R. Fuel Process. Technol. 1987, 15, 1−15. (21) Ahrens, J.; Bachmann, M.; Baum, T.; Griesheimer, J.; Kovacs, R.; Weilmünster, P.; Homann, K.-H. Int. J. Mass Spectrom. Ion Processes 1994, 138, 133−148. (22) Lu, M.; Mulholland, J. A. Chemosphere 2001, 42, 625−633. (23) Norinaga, K.; Sakurai, Y.; Sato, R.; Hayashi, J.-i. Chem. Eng. J. 2011, 178, 282−290. (24) Bockhorn, H.; Fetting, F.; Wenz, W. Ber. Bunsen-Ges. Phys. Chem. 1983, 87, 1067−1073. (25) Richter, H.; Howard, J. B. Prog. Energy Combust. Sci. 2000, 26, 565−608.

Figure 8. TPO of different chars: 0.5 °C/min between 200 and 700 °C with 10 mL/min of 10% O2/He.

of solid at the end of the experiments with tars suggested that the tars had been coked on the char. Consequently, the mechanism proposed by Hosokai et al.9 for tar cracking and reforming over charcoal seems to apply also for wood biomass char: the tars were adsorbed and reacted very efficiently over the micropores of the chars, but in the absence of steam, these micropores were coked and deactivated at a very fast rate, resulting in very low catalytic activity. Comparison between Olivine and Char. In our previous work,18 we studied the catalytic activity of olivine under exactly the same conditions. It was shown that a large amount of steam in the gas stream, as is usual in biomass gasification, is not ideal for olivine catalytic activity, with only a third of the tars converted. However, in the present work, the large amount of steam made the char much more efficient. Even though the mass of biomass chars in dual fluidized beds is considered to be around 3% of the fluidizing media, its low density makes it more than 20% of the volume. Consequently, the very high catalytic activity of char should not be disregarded, as current studies often focus on the catalytic activity of the fluidizing media.

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CONCLUSION This study focused on the catalytic activity of wood chars in the context of biomass gasification in fluidized beds. The char showed a very strong catalytic activity in tar removal in the presence of a large amount of steam, and the results were in good agreement with the literature on charcoals and other biomasses. The unparalleled comparison to the catalytic activity of olivine under the same conditions suggests that the presence of char in the fluidized bed may greatly influence tar cracking and reforming. Experiments over char and olivine on a small-scale fluidized bed would give more precise information on the respective catalytic effects of olivine and char in fluidized-bed biomass gasification.

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REFERENCES

AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors sincerely acknowledge the Languedoc-Roussillon Region, the French Environment and Energy Management E


Conversion of Phenol-Based Tars over Biomass Char under H2 and

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