Article pubs.acs.org/EF
Investigation of Natural and Synthetic Bed Materials for Their Utilization in Chemical Looping Reforming for Tar Elimination in Biomass-Derived Gasification Gas Martin Keller,*,† Henrik Leion,† Tobias Mattisson,‡ and Henrik Thunman‡ †
Department of Chemical and Biological Engineering, and ‡Department of Energy and Environment, Chalmers University of Technology, S-412 96 Göteborg, Sweden ABSTRACT: The removal of condensable hydrocarbons or tars from raw gas derived from biomass gasification presents an obstacle in the widespread application of biomass gasification. Hot catalytic tar cleaning as a secondary tar removal strategy is discussed as a tar cleaning technology. This can be realized in a dual-fluidized-bed reactor system, in which a catalytically active bed material is continuously regenerated. Such a process is termed chemical looping reforming (CLR). In such a process, it has been suggested that oxygen carrier particles employed for chemical looping combustion may be used, with the oxygen transfer from the particles to the gas promoting tar decomposition. Experiments were conducted in a small-scale, batch-wise fluidized-bed reactor with the aim of investigating a variety of bed materials for this process. The purpose of the present work is thus to conduct a screening study of a variety of bed materials based on the transition metals Fe, Mn, Ni, and Cu. The experiments were conducted in a batch fluidized bed, where the particles are exposed to reformer and regenerator conditions alternatingly. The conversion of ethylene from a synthetic gasification gas mixture was used as an indicator for the suitability of the materials for tar conversion. It was found that the natural material bauxite and the synthetic bed materials NiO/α-Al2O3, CuO/MgAl2O4, and La0.8Sr0.2FeO3/γ-Al2O3 exhibit high ethylene conversion rates and, thus, possess promising properties for their application in CLR.
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INTRODUCTION AND BACKGROUND
In such a process, the tar removal is envisaged to be realized in a dual-fluidized-bed reactor system. The bed material, active for tar removal, is circulated between a reformer, in which the bed material is contacted with the raw synthesis gas, and a regenerator, in which the bed material is regenerated by oxidizing coke deposits with air. When a metal oxide is used, which is able to transport some oxygen between these two reactors, the process is referred to as chemical looping reforming (CLR).10 The proposed reactor layout of a CLR system for the tar cleaning of gasification gas is depicted in Figure 1. In the CLR process, the degree of oxidation of the raw gas is determined by the amount of oxygen transported by the
Gasification of raw biomass feedstock presents a promising route to generate carbon-neutral synthesis gas (CO + H2), which can subsequently be converted to valuable gaseous and liquid fuels, such as substitute natural gas (SNG), dimethyl ether (DME), or others.1−3 However, apart from the major syngas components, the raw gasification gas exiting the gasifier generally contains unwanted contaminants, such as sulfur compounds, ammonia, and condensable hydrocarbons. The latter contaminants, often referred to as “tars”,4 are especially cumbersome because they start condensing already at rather high temperatures, which may cause operational problems, such as fouling and blocking of downstream equipment.5 Hot catalytic tar removal has been put forward as one of the process options to tackle this problem.4,6 It can be achieved by either primary measures, i.e., the conversion of tar directly inside the gasifier, or secondary measures downstream of the gasifier.4,7 Secondary measures offer the advantage of greater flexibility in process parameters, such as operating temperature and additional steam injection, because these processes are conducted in a separate downstream unit. Further, a separate cleaning step offers the potential of achieving a higher degree of tar removal.4,7 Such a catalytic cleaning step can be realized in fixed beds, fluidized beds, or monolithic reactors. However, most catalysts tested suffer from rapid deactivation mostly because of the formation of coke on the catalytically active surface.8,9 Recently, reactor configurations have been proposed that circumvent this problem by continuously regenerating the catalyst in a way similar to how it is conducted in a fluid catalytic cracking (FCC) unit.10,11 © 2014 American Chemical Society
Figure 1. Schematic of the CLR process for tar removal from biomass producer gas. Received: February 10, 2014 Revised: April 30, 2014 Published: May 1, 2014 3833
dx.doi.org/10.1021/ef500369c | Energy Fuels 2014, 28, 3833−3840
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Article
Figure 2. Schematic overview of the experimental setup.
bed material from the regenerator to the reformer. The degree of oxidation may thus be controlled by the oxygen carrying capacity of the bed material, the recirculation rate of the bed material, or providing an understoichiometric amount of oxygen to the regenerator. In the reformer of a CLR system, hydrocarbons contained in the raw gasification gas may be converted by numerous different reactions that may be catalyzed on the surface of the bed material (catalytic reactions) or that modify the oxidation state of the bed material (non-catalytic reactions). Catalytic Reactions.
full oxidation ⎛ m⎞ CnHm + ⎜2n + ⎟MexOy ⎝ 2⎠ ⎛ m⎞ m → ⎜2n + ⎟MexOy − 1 + nCO2 + H 2O ⎝ ⎠ 2 2
combustion with gas-phase O2 ⎛ m⎞ m CnHm + ⎜n + ⎟O2 → nCO2 + H 2O ⎝ ⎠ 4 2
water−gas shift
(1)
CO + H 2O ↔ CO2 + H 2
dry reforming
CO + MexOy → CO2 + MexOy − 1 (2)
(9)
H2 conversion
cracking
H 2 + MexOy → H 2O + MexOy − 1
m CnHm → nC + H 2 2
⎛ 4n − m ⎞ ⎜ ⎟H → nCH 4 ⎝ 2 ⎠ 2
(4)
Non-catalytic Reactions.
C + O2 → CO2
(11)
2MexOy − 1 + O2 → 2MexOy
(12)
Bed materials for CLR have been evaluated in a continuous unit at Chalmers University of Technology, namely, the natural ore ilmenite,10,12 synthetic Mn3O4 supported on ZrO2,13 and NiO supported on α-Al2O3,12,14 with promising results. However, the testing of bed materials in such a continuous unit is
partial oxidation m H2 2
(10)
In the regenerator, the reaction scheme is rather simple. Carbon that has been deposited on the bed material surface can be combusted with the oxygen supplied to the reactor. The bed material may also become reoxidized.
(3)
hydrocracking
CnH m + n MexOy → n MexOy − 1 + nCO +
(8)
CO conversion
⎛m⎞ CnHm + nCO2 → 2nCO + ⎜ ⎟H 2 ⎝2⎠
CnHm +
(7)
Simultaneously, the other components of the gasification gas may interact with the bed material or each other in a large number of different reactions, of which some of the most important reactions are listed below.
steam reforming ⎛ m⎞ CnHm + nH 2O → nCO + ⎜n + ⎟H 2 ⎝ 2⎠
(6)
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dx.doi.org/10.1021/ef500369c | Energy Fuels 2014, 28, 3833−3840
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Table 1. Composition and Production Parameters of Synthetic Bed Materials name
composition
CaMn0.775Mg0.10Ti0.125O3−δ CuO/MgAl2O4 CuO/ZrO2 Fe2O3/α-Al2O3 Fe2O3/MgAl2O4 La0.8Sr0.2FeO3 unsupported La0.8Sr0.2FeO3/γ-Al2O3 Mn3O4/ZrO2 NiO/α-Al2O3
100 wt % CaMn0.775Mg0.10Ti0.125O3−δ 40 wt % CuO and 60 wt % MgAl2O4 (Baikowski S30CR) 40 wt % CuO and 60 wt % ZrO2 60 wt % Fe2O3 and 40 wt % α-Al2O3 (Almatis CT3000SG) 60 wt % Fe2O3 and 40 wt % MgAl2O4 (Baikowski S30CR) 100 wt % La0.8Sr0.2FeO3 10 wt % La0.8Sr0.2FeO3 and 90 wt % γ-Al2O3 (Puralox NWa155) 37 wt % Mn3O4 and 63 wt % ZrO2 65.4 wt % NiO and 34.6 wt % α-Al2O3 (Almatis CT3000SG)
production method
reference
°C °C °C
°C
30 31 32 33 34 19 35 36
80, 120, 160, 250, 500, and 1000 s. After each exposure, the coke was oxidized and the amount of coke deposited was quantified via the CO and CO2 amounts detected in the flue gas. Gas Analysis. Downstream of the fluidized-bed reactor, the effluent gas was transported in heated lines to a Thermo-Scientific iS50 Fourier transform infrared (FTIR) analyzer. Both the heated gas lines and the FTIR heated gas cell were kept at a temperature of 120 °C to avoid condensation of the water and allow for the determination of the wet gas composition. The FTIR analyzer was calibrated for the quantification of the concentration of CO, CO2, H2O, CH4, C2H2, C2H4, and C2H6 by conducting calibration experiments with both pure gases diluted in N2 and various wet and dry gas mixtures. Subsequently, the gas was cooled, and the condensate was removed from the gas stream. The dry effluent gas was then sent to a dry gas analyzer to determine the concentration of H2 and O2 with thermal conductivity and paramagnetic gas sensors. Bed Materials. In total, 13 different bed materials were investigated for their utilization in CLR, of which 4 are naturally occurring minerals/ores and 9 are synthesized particles. Quartz sand was used as a reference material, where no or little catalytic effects are expected. All of the materials were sieved to a size range of 125−180 μm. The composition and production parameters of the synthetic particles are summarized in Table 1. All of these particles have shown promise for their utilization in either chemical looping combustion (CLC), which is a technique similar to CLR to produce a nitrogen-free flue gas from the combustion of carbonaceous fuels,17,18 or the CLR of methane.19,20 In Table 1, references are provided that contain additional information on the particle synthesis and their properties. All of the particles consist of transition metal/metal oxides supported on MgAl2O4, ZrO2, or Al2O3, with the exception of the two unsupported perovskitic materials CaMn0.775Mg0.10Ti0.125O3−δ and La0.8Sr0.2FeO3. A short description of the natural materials investigated in this work is provided in Table 2. Of these, ilmenite has been investigated most
laborious and does not allow for the detailed resolution of the reduction and oxidation properties of the materials. The purpose of the present work is thus to conduct a screening study of a variety of bed materials that may be of interest for the utilization in CLR of tar components. The basis of selection was a number of oxygen carrier materials that have shown promise in the related CLC technology and could have both non-catalytic oxygen transfer and catalytic properties. The study was conducted using a small-scale, batch-wise fluidized-bed reactor, where the bed material is exposed to reformer and regenerator conditions alternatingly. Tar consists of a complex mixture of organic non-aromatic and aromatic compounds. In this study, ethylene is used as a tar surrogate because it is an indicator for the relative ability of bed materials to convert tars.15,16 This is likely due to the π bonds of ethylene being similar to those found in aromatic tars, such as benzene,15 and the similar C−C bond length of ethylene and aromatic compounds.
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spray drying, calcined for 4 h at 1300 °C spray drying, calcined for 4 h at 1030 °C freeze granulation, calcined for 6 h at 1100 freeze granulation, calcined for 6 h at 1100 freeze granulation, calcined for 6 h at 1100 spray drying, calcined for 10 h at 900 °C impregnation, calcined for 2 h at 1100 °C freeze granulation, calcined for 6 h at 1100 spray drying, calcined for 4 h at 1400 °C
EXPERIMENTAL SECTION
Experimental Setup. The experiments were conducted in a fluidized-bed reactor made of quartz glass with an inner diameter of 22 mm immersed in an electric oven. An overview of the experimental setup is provided in Figure 2. The gas was distributed via a porous plate on which 10 g of bed material was placed. The temperature in the fluidized bed was measured with a K-type thermocouple enclosed in a quartz shell. Honeywell pressure transducers with a frequency of 20 Hz were used to measure the pressure drop over the reactor. To emulate the circulation of the bed material between the regenerator and reformer of a continuous CLR unit, as described in Figure 1, the bed material was alternatingly exposed to reducing and oxidizing conditions. In between these exposures, the reactor was flushed with N2 for 120 s. This change in inlet flow of gases was achieved with automatic magnetic valves, as indicated in Figure 2. All of the gases used were supplied by gas bottles, and their flow rates are controlled by mass flow controllers. To emulate the reducing conditions in the reformer, the bed material was exposed to a gas flow of 1 LN/min for 1000 s, consisting of a mixture of 40% dry synthetic bottled gasification gas (consisting of 43% CO, 14.9% CO2, 14% CH4, 4.98% C2H4, and 23.13% H2), 40% steam (generated by a Bronkhorst CEM liquid delivery system), and 20% N2. Subsequently, the bed material was exposed to a flow of 0.6 LN/min synthetic air (consisting of 20.9% O2 in N2) for 360−480 s to emulate the oxidizing conditions in the regenerator and achieve full reoxidation of the bed material. The system was operated at atmospheric pressure, and the temperature was varied between 750, 800, and 850 °C for each of the bed materials investigated. Experiments were repeated 3 times to ensure reproducibility. Repeatability of the experiments was high, and the standard error of the C2H4 and CH4 conversion measurement was low (around 0.5−2% absolute). In addition to these reactivity experiments, further experiments with CuO/MgAl2O4 were performed to determine the onset of coke formation and the rate of coke formation. To obtain these rates, the bed material was exposed to the gasification gas for 40,
Table 2. Overview of Natural Materials Used as Bed Materials in This Work name quartz sand bauxite ilmenite LD stone
description used as a reference raw bauxite, calcined for 9 h at 850 °C activated rock ilmenite, provided by Titania A/S LD slag is a waste product from the steel-making process, provided by SS AB
extensively as a potential bed material in CLC21−23 and to some extent as a bed material in the CLR of tars.12 Bauxite and LD stone have been investigated for utilization in CLC as well but to a much lesser extent.24 The elemental composition of the latter two materials is given in Table 3. Characterization of Bed Materials. The Brunauer−Emmett− Teller (BET) surface area of the bed materials was determined by N2 adsorption (Micromeritics, TriStar 3000) before and after the experiments. The major crystalline phases of the bed materials were 3835
dx.doi.org/10.1021/ef500369c | Energy Fuels 2014, 28, 3833−3840
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Table 3. Elemental Composition of LD Stone and Bauxite Calculated as Simple Oxides composition (wt %) FeO
Fe2O3
CaO
SiO2
MgO
MnO
TiO2
Al2O3
P2O5
Na2O
K2O
Cr2O3
ZrO2
SO3
22 0
0 18.5
43 0.02
10 10.4
9 0.11
3 0.06
1.3 3.87
1.1 66.7
0.4 0.09
0.06
