Article pubs.acs.org/EF
Conversion of Phenol-Based Tars over Olivine and Sand in a Biomass Gasification Atmosphere Xavier Nitsch,* Jean-Michel Commandré,* Paul Clavel, Eric Martin, Jérémy Valette, and Ghislaine Volle 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 describes tar conversion on olivine in fluidized-bed gasification conditions. A laboratory-scale reactor (Aligator) was used to characterize phenol conversion to higher tars, before adding a sand and olivine bed to investigate heterogeneous steam reforming and the cokefaction of these tars. H2 and H2O atmospheres were tested both separately and together to characterize tar conversion on olivine. Catalytic activity in steam reforming was shown to be much improved by the presence of H2. In the absence of H2O in the reactive atmosphere, olivine caused a high cokefaction of tars. With 10% H2O and 20% H2, olivine became highly active in steam reforming of tars. Carbon deposition on the catalysts was quantified by temperature-programmed oxidation (TPO), and optical photographs of olivine were taken after tar conversion.
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INTRODUCTION
Biomass gasification is considered to be one of the most efficient alternative ways to replace fossil energy, but there are still some critical issues that prevent its industrial development. One of the main problems is the high tar content of the syngas,1 especially when this syngas is used on catalysts for hydrocarbon synthesis; to prevent early deactivation, it is considered that the amount of tars should not exceed 5 mg/m3 at standard temperature and pressure (STP).2,3 To reduce tar content, gas treatments, such as filtering, absorption, chemisorption, thermal cracking, or catalytic beds, have been studied, but one of the most interesting options for fluidized beds is to reduce the tar content at its source using a catalytic bed material. This bed material should therefore be temperatureand attrition-resistant while remaining catalytically active to improve tar cracking. Several studies have been conducted to select, synthesize, and experiment catalysts in fluidized-bed conditions, and the results indicated a significant decrease in the amount of tar.4−11 One of the most promising catalyst is calcined olivine, because of its ability to convert tar, its resistance to attrition, and moderate cost.4,5,12−14 Although there have been some studies on olivine, especially on the presence and structure of iron at its surface, the mechanisms of destruction of tars using this catalyst are not well-understood. Studies have also been performed on thermal maturation and destruction of tars, especially in flames, coal pyrolysis, or aromatic conversion.15−20 Some authors focused on gasification,1,21−24 and a few proposed detailed chemical mechanisms involved in the formation of secondary tar from biomass to heavy tars.25,26 The latter indicated that phenolic compounds are the main pathways to tar formation, especially through the formation of cyclopentadiene. In this study, we investigated the mechanisms of destruction of tars over calcined olivine at the laboratory scale on a fixed bed, to focus on tar cracking over the catalyst. Temperature and gas concentrations as close as possible to gasification conditions in a fluidized bed were reproduced. © XXXX American Chemical Society
EXPERIMENTAL SECTION
Apparatus. Figure 1 shows the specially designed fixed-bed reactor named Aligator. The bed is crossed by all of the gases in the
Figure 1. Schematic diagram of the Aligator reactor. atmosphere. The flow rate of these gases is high enough (>200 mL/ min at STP) to ensure that contact time remains low and similar to times in industrial fluidized beds. The apparatus consists of an external electrically heated quartz tube (inner diameter, 30 mm; length, 420 mm) and an internal quartz tube (inner diameter, 20 mm; length, 410 mm), equipped with a filtering sheet located 150 mm from the top of the tube. The gas that passes through the catalytic bed is previously preheated in the annular space between the external and internal tubes. The gas preparation system (Figure 2) consists of three Brooks mass flow controllers that control the gas flow of N2 and H2. Tar is introduced into the gas by a bubbler feeding system. The temperature Received: May 7, 2013 Revised: July 27, 2013
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dx.doi.org/10.1021/ef400817z | Energy Fuels XXXX, XXX, XXX−XXX
Energy & Fuels
Article
Figure 2. Experimental device for determination of mechanisms of tar decomposition. is controlled by a heating/cooling fluid, and the mass of tars inside the bubbler is high enough so that variation in height as a result of transfer between gas and tar remains very low during the experiment. The theoretical amount of tar in the gas can be calculated and predicted with Antoine’s law. To check if the partial pressure of tar is correct, the bubbler is weighed before and after each test and the weight loss is compared to measurements made by the analytical system. Liquid water flow is controlled by a Coriolis flowmeter and is introduced into the gas line at a temperature of 240 °C. To prevent condensation of tar or steam, all of the lines from the tar bubbler to the end of the analytical system are heated to a temperature of 150 °C. This temperature has proven to be sufficient to avoid tar and water condensation in all of our experiments. Volumic gas concentrations were chosen to match as much as possible the conditions inside a dual fluidized bed (Table 1).
°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 more complex analyses were 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. To measure carbon deposition over the reactor and the catalyst, internal and external quartz tubes were weighed before and after each experiment. Olivine and Sand Preparation. Olivine came from Austria and was calcined by Magnolithe GmbH at 1600 °C for 3 h before being crushed and ground to a granularity of 300−800 μm. To obtain maximum free iron on the catalyst surface,27 it was calcined at 1100 °C under air for 4 h, with a heating rate of 3 °C/min. The olivine was then sieved to 400−600 μm corresponding to an industrial fluidized-bed gasifier particle size range. Sand was calcined following the same procedure, i.e., 1100 °C under air for 4 h with a heating rate of 3 °C/min and sieved to 400−600 μm. Olivine and sand were characterized after calcination by inductively coupled plasma (ICP); the mass fractions obtained are indicated in Table 2. Characterization of the Catalysts. Temperature-programmed oxidation (TPO) was performed on a Micromeritics Autochem 2920 II fitted with a TCD. A gas flow of 10% O2/He at 10 mL/min was
Table 1. Volumic 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
For experiments without a catalyst, total gas flows of 224 mL/min at STP were used, with a gas residence time of 6 s in the reactor. Experiments with sand or olivine were conducted with gas flows of 209 and 211 mL/min at STP, with a gas residence time in the reactor of 6 s before it reached the catalytic bed and a contact time of 0.2 s with the catalyst. A total of 7.449 g of sand and 8.227 g of calcined olivine were used for each experiment. A weight hourly space velocity (WHSV) of 1.36 h−1 was calculated for phenol over olivine. The catalytic bed was placed at the bottom limit of the isothermal zone to minimize tar conversion after the catalyst. The first analytical device downstream from Aligator is a PerkinElmer gas chromatography−thermal conductivity detector (GC− TCD). The amount of gas injected into the column is controlled by a loop connected to a six-way valve. The column used was a Varian CPPoraBOND Q, heated from 120 to 300 °C for 16 min to measure cyclopentadiene (CPD), benzene, toluene, and naphthalene concentrations. After gas chromatography (GC), gases were cooled to −10
Table 2. Mass Fraction of the Catalysts by ICP Analysis
Si Ca Mg O Fe Ni B
sand (%)
olivine (%)
44.5
