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Importance of decomposition reactions for catalytic conversion of tar and light hydrocarbons: An application with an ilmenite catalyst Huong Ngoc Thuy Nguyen, Nicolas Berguerand, Georg L. Schwebel, and Henrik Thunman Ind. Eng. Chem. Res., Just Accepted Manuscript • DOI: 10.1021/acs.iecr.6b03060 • Publication Date (Web): 29 Oct 2016 Downloaded from http://pubs.acs.org on November 8, 2016

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Industrial & Engineering Chemistry Research

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Importance of decomposition reactions for catalytic

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conversion of tar and light hydrocarbons: An application

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with an ilmenite catalyst

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Huong N. T. Nguyen,* Nicolas Berguerand, Georg L. Schwebel,† Henrik Thunman

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Division of Energy Technology, Department of Energy and Environment, Chalmers University

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of Technology, SE-412 96 Gothenburg, Sweden

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*Corresponding Author: [email protected]

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ABSTRACT

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This work elucidates the contributions of different decomposition reactions, namely steam

10

reforming, hydro-cracking, dry reforming, and (thermal) cracking reactions, to the conversion of

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tar and light hydrocarbons during the catalytic cleaning of a biomass-derived raw gas. A raw gas

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that contained a high content of steam and that was produced in the Chalmers indirect biomass

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gasifier was taken as the reference. The representative reactions associated with the upgrading of

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the given raw gas were identified to investigate the individual effects, and thereafter reassembled

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to investigate the synergistic effects. Ilmenite was used as the catalyst, and the temperature range

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of 750°–900°C was in focus. For this process, it was discovered that the complete steam

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reforming, steam dealkylation, and hydro-cracking reactions are important, whereas the dry 1

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reforming reaction is not relevant. In addition, the water-gas shift reaction occurs significantly

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and can promote the hydro-cracking reaction. These results provide insights into the most

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important reactions for inclusion in kinetic models of catalytic gas cleaning.

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1. INTRODUCTION

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Biomass gasification, which is a thermo-chemical process to convert biomass into an energy-

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rich raw gas, is an attractive technology with strong potential to reduce CO2 emissions and

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dependence on fossil fuels.1,2 The raw gas mainly consists of H2, CO, CO2, and light

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hydrocarbons (HC), such as methane. However, tar, which is a mixture of condensable organic

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compounds, is also present. The condensation of tar causes operational problems in the process

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equipment integrated downstream of the gasifier, thereby imposing limitations on the direct use

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of the raw gas. Furthermore, tar may account for up to 15% of the energy content of the dry and

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ash-free biomass feedstock, which constitutes a significant impact on the cold gas efficiency of

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the gasification process.3-6 Therefore, the removal of tar is a prerequisite for process viability.

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Among the methods available for tar abatement downstream of the gasifier, catalytic gas

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cleaning is of great interest. This method enables the recovery of the energy content of the tar, as

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the tar is converted into more valuable products, such as methane, CO, and H2. Using catalysts,

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an operating temperature in the range 700°–900°C can be applied to remove efficiently the stable

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tars, such as aromatic species; this temperature is considerably lower than that employed (e.g.,

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1100°C) for inert thermal methods.2,4,7-10 Owing to the complexity of the raw gas, the catalytic

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gas cleaning involves numerous reactions. Among these, the decomposition reactions of the tar,

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herein defined as steam reforming, dry reforming, hydro-cracking, and (thermal) cracking 2


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reactions, are of particular interest. The conversion of light HC can also take place via these

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reaction pathways. However, the ways in which the different decomposition reactions contribute

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to the total conversion of tar/light HC have not been fully elucidated.11-13 Furthermore, an

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investigation of the individual reaction pathways does not provide a sufficiently comprehensive

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overview of the entire gas cleaning process. Taken together, these issues reveal the difficulty

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associated with selecting the most appropriate reactions to be incorporated into the kinetic

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modeling of the process.

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Therefore, the aim of the present work was to gain an understanding of the aggregate effects of

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the decomposition reactions on the conversion of tar/light HC, particularly in relation to the

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carbonaceous products formed during the gas cleaning process. Furthermore, the goal included a

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description of a simple method to identify the contributions of the different decomposition

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reactions to the entire process. In this context, a raw gas produced in the Chalmers indirect

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biomass gasifier operated at about 820°C was taken as the reference. The raw gas consists of

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steam at a level of about 50–60 vol%, and H2, CO, CO2, methane and ethene are the main

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components of the permanent gas, and the tar composition is dominated by aromatic species.3,14

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Given the raw gas properties, representative reactions associated with the gas cleaning process

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were identified and investigated independently, so as to examine the individual effects of the

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reactions. Furthermore, the aggregate effects that are manifested during the real process were

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investigated by combining the reactions. As the raw gas contains a high content of steam, the

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water-gas shift (WGS) and steam reforming reactions were expected to play key roles in the

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catalytic upgrading of the raw gas. Thus, the WGS, methane steam reforming, ethene steam

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reforming, and toluene steam reforming reactions were studied, both independently and in 3



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combinations. In the latter case, hydro-cracking and dry reforming reactions could also occur,

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since the reactant gases contained H2 and CO2. Toluene was chosen as a tar representative, as it

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is a major tar component of the given raw gas, i.e., it accounts for up to 12 wt% of the total

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tar.13,15,16 Furthermore, benzene, which is one of the most stable tar species, can be produced

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from toluene, which provides further details about tar evolution.13,14,17,18

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In the present investigation, ilmenite, which is an iron-titanium oxide, was chosen as the

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demonstration catalyst. The ability of ilmenite to reduce the tar content and adjust the final gas

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composition via WGS has been proven.17,19 However, the detailed activity of ilmenite towards

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the different decomposition reactions and the contributions of these reactions to the overall gas

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upgrading process remain unclear. Furthermore, in a previous investigation by the same authors,

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the kinetic model for a gas cleaning process that was conducted under excess steam conditions

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and catalyzed by ilmenite required the identification of the most important reactions.14

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2. THEORY ON CATALYTIC GAS CLEANING

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2.1. Decomposition reactions and nature of the carbonaceous products

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The most relevant reactions associated with the catalytic gas cleaning process are summarized

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in Table 1.12,13,18,20

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Table 1. Most relevant reactions in catalytic gas cleaning process. Reaction

Formula

Steam reforming

Cx Hy + H2 O → Cx' Hy' + CO + H2

Dry reforming

Cx Hy + CO2 → CO + H2

Hydro-cracking

Cx Hy + H2 → Cx' Hy' 4


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(Thermal) cracking

Cx Hy → Cx' Hy' + C + H2

WGS

CO + H2 O ⇄ CO2 + H2

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The conversion of tar and possibly of light HC takes place via decomposition reactions, i.e.,

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steam reforming, dry reforming, hydro-cracking, and (thermal) cracking reactions. Note that in

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Table 1, these decomposition reactions are focused on aromatic-HC tar and light HC. The steam

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reforming reaction represents either complete steam reforming to produce only H2 and CO or

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steam dealkylation in which relatively stable tar/light HC Cx' Hy' can also be produced.12,13 The

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formation of carbon via the decomposition of tar/light HC is included in the (thermal) cracking

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reaction. In addition to the decomposition reactions, the WGS reaction is important, as it can

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modify significantly the final composition of the upgraded gas. Furthermore, the changes in the

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compositions of steam, H2, and CO2 that occur due to the WGS reaction can affect the selectivity

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of the decomposition reactions.

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The progress of the decomposition reactions and their aggregate effects on the conversion of

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tar/light HC are depicted in Figure 1. Note that the figure is dedicated solely to a raw gas with a

90

high content of steam, so the formation of relatively larger tar/light HC is not included.14

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Figure 1. Conversion of tar/light HC via decomposition reactions, and formations of

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carbonaceous products.

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In Figure 1, it is assumed that tar/light HC molecules are converted into tar/light HC reactive

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intermediates that are free radicals in the gas phase or other intermediates on the catalyst surface.

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Similarly, reforming/cracking agents, i.e., steam, CO2, and H2, are assumed to be dissociated into

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the hydrogen intermediate H* and the oxygen-containing intermediates O* and OH*.14 The

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operating temperature, and more importantly, the catalyst are essential for converting the tar/light

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HC molecules and reforming/cracking agents into reactive intermediates.14,21 In particular for the

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reforming/cracking agents, the dissocation of steam and H2 is thermodynamically more favorable

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than that of CO2.22 In the subsequent steps, the tar/light HC intermediates react with the

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reforming/cracking agent-originated intermediates to produce carbonaceous products. The

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selectivity of carbonaceous products, i.e., HC products or oxidation products, depends on

105

whether the tar/light HC intermediates react with hydrogen intermediates or oxygen-containing

106

intermediates. More precisely, the reactions that occur between tar/light HC intermediates and

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hydrogen intermediates produce another tar/light HC, in this case Cx' Hy' , which indicates the

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involvement of either the hydro-cracking reaction or steam dealkylation. Oxygen-containing 6


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intermediates oxidize the tar/light HC intermediates to produce CO, and subsequently produce

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CO2 via the WGS reaction. This route reveals the effect of either the dry reforming reaction or

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the complete steam reforming reaction if no other tar/light HC are produced. The tar/light HC

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intermediates can also self-decompose to produce tar/light HC Cx' Hy' or carbon deposit, as it is

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the case for the (thermal) cracking reaction.14 Since the (thermal) cracking reaction does not

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require any reforming/cracking agents, it can occur as long as tar/light HC are converted into

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reactive intermediates. However, this reaction pathway can be ignored in relation to other

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decomposition reactions if reforming/cracking agents are available in excess within the reaction

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environment. Overall, the composition of the raw gas and operating conditions for the catalytic

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gas cleaning establish the selectivity of the decomposition reactions. The in situ formed tar/light

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HC Cx' Hy' are more stable than the parent tar/light HC Cx Hy .14,19,23 Whether or not Cx' Hy' can be

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degraded in the same way as Cx Hy depends on the process conditions. An abundance of Cx' Hy'

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in the upgraded gas indicates that the catalyst and/or other process conditions are moderate, and

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that more severe conditions must be applied if these stable Cx' Hy' species are to be eliminated.

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2.2. Using ilmenite as the catalyst

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Ilmenite possesses both oxygen transport and catalytic activities, which are largely attributed

125

to the iron content in ilmenite.14,17,24-26 The dominant activity is manifested depending on the

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redox state of the iron. The higher the oxidation state of iron towards Fe+3, the higher is the

127

oxygen transport capability of the material. However, if the iron species are present in lower

128

oxidation states, such as Fe+2 and Fe°, the material is more catalytically active.

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induce the reactivity of fresh ilmenite, an activation step is required. During activation, ilmenite 7


14,17,24-26

To


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must be exposed to alternating oxidizing and reducing conditions at a temperature of not lower

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than 800°C. This procedure enhances the porosity (i.e., the specific surface area) of the ilmenite

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particles, and triggers the migration of iron to the particle surface.27-29 Furthermore, for ilmenite

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to function efficiently as a catalyst and not an oxygen carrier, the activation process needs to

134

ensure that the iron in ilmenite is eventually in its reduced form.14

135

In the gasification context, ilmenite is known to facilitate the destruction of heteroatom-

136

containing and branched tar components, as well as light HC, such as ethene. However, the

137

material is not sufficiently active to convert methane and non-branched aromatic tars, such as

138

benzene and naphthalene.13,14 Ilmenite is also known for its activity towards the WGS

139

reaction.13,14 Furthermore, when using ilmenite catalyst under conditions with a high content of

140

steam, it was reported in literatures that carbon deposition is insignificant.19,20

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3. EXPERIMENTAL SECTION

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3.1. Experimental setup

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A schematic of the experimental setup is presented in Figure 2. The main element is a quartz

144

glass fluidized bed reactor. The reactor is 870 mm in height and has an inner diameter of 22 mm.

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A porous quartz plate at the center of the reactor serves as a gas distributor. The reactor is placed

146

vertically in an oven, to enable high-temperature operation. The reactor temperature is measured

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below and above the porous plate using K-type thermocouples. The pressure drop over the bed is

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measured using Honeywell pressure transducers with a frequency of 10 Hz, ensuring that proper

149

fluidization is maintained.

8


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150 151

Figure 2. Experimental setup.

152

Experiments are performed in cycles that consist of the successive stages of reduction, inert,

153

and oxidation. The reduction stage is the main focus, in which an investigated reactant gas

154

mixture is introduced to the reactor. During this stage, steam is produced by a steam generator

155

(Cellkraft Precision Evaporator E-1000) and injected into the reactant gas. Moreover, toluene is

156

injected using a micro-annular gear pump (model mzr-2531). For the oxidation stage, an

157

oxidation gas of 5 vol% O2 diluted in N2 is introduced into the reactor to regenerate the catalyst

158

through the burning of carbon that may be deposited on the catalyst particles during the

159

reduction stage. The inert stage, which occurs between the reduction stage and oxidation stage,

160

applies pure N2 to flush the reactor.

161

The product gas that exits the reactor is led to a condenser to remove the steam. The

162

composition of the dry gas is analyzed by a Rosemount NGA 2000 Multi-Component gas 9



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analyzer with the data being logged a 1-second intervals. The NGA gas analyzer measures online

164

the concentrations of O2, CO2, CO, CH4, and H2, as well as the volume flow of the dry gas

165

exiting the reactor. If needed, gas bag samples are collected during the reduction stages for

166

further analysis using a micro-Gas Chromatograph (micro-GC; model Varian 4900). This is

167

mainly to measure light HC that are heavier than methane. For experiments that use toluene, the

168

product gas that exits the reactor instead enters a series of three impingers that contain 2-

169

propanol in a cold bath at -8°C, before being introduced to the condenser. The samples of 2-

170

propanol are collected and analyzed by a Gas Chromatograph-Flame Ionization Detector (GC-

171

FID). This measures the remaining toluene and other organic condensable compounds that are

172

likely to be formed during the reduction stages. Details of the micro-GC and GC-FID analyses

173

can be found elsewhere.3,13

174

3.2. Activation of fresh ilmenite

175

Fresh ilmenite concentrate with a purity of 94.3% FeTiO3 (Titania A/S, Norway) was used in

176

this work. Details of its composition are available elsewhere.25,28 The activation of the fresh

177

ilmenite was carried out as described below.

178

Table 2. Schemes for the activation of fresh ilmenite. Stage

Composition of the inlet gas

Time (s) Scheme 1

Scheme 2

Oxidation

5 vol% O2 and 95 vol% N2

300

20

Inert

Pure N2

90

90

Reduction

20 vol% syngas (CO/H2 ratio of 1:1) and 20 80 vol% N2

300

10


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179

Typically, 15 g of the fresh particles with a grain size in the range of 125–180 µm were

180

placed in the reactor at ambient temperature. To avoid agglomeration of the ilmenite, the

181

activation was initiated by heating the reactor to 800°C under an oxidizing condition. The gas

182

inlet flow rate was 900 ml/min (20°C, atmospheric pressure). The sample was maintained at

183

800°C until 5 vol% O2 was constantly recovered in the off-gas leaving the reactor, indicating

184

complete oxidation. Thereafter, five cycles of Scheme 1 (presented in Table 2) were performed,

185

which ensured that 5 vol% O2 was recovered in the off-gas throughout the oxidation stage. The

186

activation process was continued according to Scheme 2 (Table 2) until no changes in the

187

concentrations of CO and H2 were observed during the reduction stage, thereby ensuring that the

188

ilmenite was fully reduced. In total, eight cycles of Scheme 2 were conducted. To conclude the

189

activation process, the reactor was heated to 900°C during an inert period and three cycles of

190

Scheme 2 were conducted.

191

To examine whether the ilmenite was activated properly, physical characterizations were

192

performed for activated and fresh ilmenite samples. The crystalline phase was identified using

193

powder X-ray diffraction with Cu Kα radiation source performed by a Siemens D5000

194

Diffractometer, which was mainly to assess the changes in the redox state of iron. Furthermore,

195

the Brunauer-Emmett-Teller (BET) specific surface area was measured by N2-adsorption using a

196

Micromeritics TriStar 3000.

197

3.3. Experiments with activated ilmenite

198

The catalytic activity of activated ilmenite was investigated at four different temperatures, i.e.,

199

750°C, 800°C, 850°C, and 900°C. These temperatures cover the range relevant for catalytic gas

200

cleaning processes, being the basis for this work.2,11 The gas inlet flow rate for all the 11



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experiments was 900 ml/min. The used gas inlet flow rate and amount of catalyst were chosen to

202

ensure an appropriate bubbling regime and to yield a short gas-solid contact time, i.e., 0.15 s,

203

which would support the previous investigation in elucidating the reactions that occur in a short

204

time-step.14 The durations of the reduction, inert, and oxidation stages were 120 s, 90 s, and 20 s,

205

respectively. During the reduction stage, different reactant gases were used to investigate the

206

effects of the activated ilmenite on the different reactions. Not taking into account the actual

207

reactions in focus, 50 vol% of steam was used. In addition, in selected cases, toluene was

208

injected. The gas analyses were adapted to the actual components present in the gas. The

209

compositions of the reactant gases used and analyses performed in the different experiments are

210

listed in Table 3.

211

Table 3. Compositions (vol%) of reactant gases, use of toluene injection, and details of analyses

212

applied in experiments using activated ilmenite.

Experiment

Component (vol%)

Toluene injection (g/min)

Analysis

CO

CO2

H2

CH4

C2H4

N2

Steam

21.5

-

21.5

-

-

7

50

-

NGA

-

-

-

-

1.6

48.4

50

-

Gasification gas

21.5

7.5

11.5

7

2.5

-

50

-

NGA, micro-GC NGA

Methane steam reforming

-

-

-

7

-

43

50

-

NGA

Toluene steam reforming

-

-

-

-

-

50

50

0.1

21.5

7.5

11.5

7

2.5

-

50

0.1

WGS Ethene steam reforming

Synthetic raw gas

NGA, micro-GC, GC-FID NGA, micro-GC, GC-FID

12


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213

As evident from Table 3, six different experiments with activated ilmenite were conducted.

214

Each experiment was performed twice at each temperature level. The experiments were designed

215

to investigate the individual effects of the different reactions, as well as the effects of their

216

combinations. The WGS experiment was conducted to investigate the effect of the activated

217

ilmenite on WGS reaction and to confirm if activated ilmenite acted as a catalyst, not an oxygen

218

carrier. Ethene conversion was investigated in the ethene steam reforming and gasification gas

219

experiments. Note that in the gasification gas experiment, the concentration of ethene in the

220

product gas was determined by the element balance. This method was justified in this case, since

221

the results obtained from analyzing the gas bag samples from the ethene steam reforming

222

experiment showed negligible levels of light HC other than methane and ethene. Methane makes

223

up about 14 vol% of the dry raw gas produced in the Chalmers gasifier. Thus, methane steam

224

reforming experiment was carried out, even though, according to the literature, methane is

225

insignificantly converted in ilmenite catalysis.14,19 The toluene steam reforming and synthetic

226

raw gas experiments were performed to investigate the toluene conversion.

227

Finally, reference experiments were conducted using silica sand with the particle size 125-180

228

µm instead of ilmenite. The silica sand experiment used the same reactant gas as used for

229

gasification gas experiment, and four different temperatures: 750°C, 800°C, 850°C and 900°C

230

were investigated.

231

3.4. Data evaluation

232

3.4.1. Conversion efficiencies for methane, ethene, and toluene

13



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233

The molar flow rate at the time ti and the total amount of the gas component ‘j’ exiting the

234

reactor during the reduction stage were calculated using data obtained with the NGA analyzer,

235

following Eq. (1) and Eq. (2), respectively.

236

𝑛̇𝑗,𝑜𝑢𝑡 (𝑡𝑖 ) = 𝑥𝑗,𝑜𝑢𝑡 (𝑡𝑖 ) .

237

𝑛𝑗,𝑜𝑢𝑡 = ∫𝑡 2 𝑛̇𝑗 ,𝑜𝑢𝑡 (𝑡𝑖 ) 𝑑𝑡

𝐹̇𝑑𝑟𝑦,𝑜𝑢𝑡 (𝑡𝑖 )

Eq. (1)

̅ 𝑉

𝑡

Eq. (2)

1

238

where 𝑛̇𝑗,𝑜𝑢𝑡 (𝑡𝑖 ) is the molar flow rate of product gas component j [mol.s-1], 𝑥𝑗,𝑜𝑢𝑡 (𝑡𝑖 ) is the

239

̇ mole fraction of the product gas component j [-], 𝐹𝑑𝑟𝑦,𝑜𝑢𝑡 (𝑡𝑖 ) is the volume of dry product gas

240

exiting the reactor [L.s-1], 𝑉̅ is the mean molar volume [L.mol-1], 𝑛𝑗,𝑜𝑢𝑡 is the total amount of

241

product gas component j exiting the reactor during the reduction stage [mole], and t2–t1 is the

242

duration of the reduction stage [s].

243 244

The conversion efficiencies of methane, ethene, and toluene were estimated as the relative changes comparing the product gas and reactant gas as follows: 𝑛𝑗,𝑜𝑢𝑡

245

𝜂𝑗 = 1 −

246

where 𝑛𝑗,𝑖𝑛 is the amount of component j introduced to the reactor in the reduction stage

Eq. (3)

𝑛𝑗,𝑖𝑛

247

[mole].

248

Equation (3) was adapted in specific cases as follows:

249 250 251

(i)

The conversion efficiency of ethene (𝜂𝐶2 𝐻4 ) in the ethene steam reforming experiment was estimated using: 𝜂𝐶2 𝐻4

=1−

𝑥𝐶2𝐻4 ,𝑏𝑎𝑔 .

𝑉𝑏𝑎𝑔 ̅ 𝑉

𝑛C2 𝐻4 ,𝑖𝑛

Eq. (4)

14


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252

where 𝑥𝐶2 𝐻4 ,𝑏𝑎𝑔 is the mole fraction of ethene in the gas bag given by the micro-GC [-

253

], and 𝑉𝑏𝑎𝑔 is the volume of dry gas in the gas bag [L].

254

(ii)

The conversion efficiency of toluene in the toluene steam reforming experiment was

255

estimated by dividing the total amount of carbon in the products by the total amount of

256

carbon fed to the reactor during the reduction stage, according to Eq. (5). This was

257

because the production levels of heavier tar compounds and carbon deposits were

258

neglected.14,19 𝑉𝑏𝑎𝑔

(𝑥𝐶𝑂,𝑏𝑎𝑔 + 𝑥𝐶𝑂2 ,𝑏𝑎𝑔 + 𝑥𝐶𝐻4,𝑏𝑎𝑔 + 2.𝑥𝐶2𝐻4,𝑏𝑎𝑔 ). ̅ 𝑉 7.𝑛𝐶7 𝐻8 ,𝑖𝑛

+ 6.𝑛𝐶6 𝐻6

259

𝜂𝐶7 𝐻8 =

260

where 𝑥𝑗,𝑏𝑎𝑔 is the mole fraction of component j in the gas bag given by the micro-GC

261

[-], 𝑛𝐶6 𝐻6 is the amount of benzene formed in the reduction stage [mole], and 𝑛𝐶7 𝐻8 ,𝑖𝑛

262

is the amount of toluene injected into the reactor in the reduction stage [mole].

263

(iii)

Eq. (5)

Finally, the conversion efficiency of toluene in the synthetic raw gas experiment was

264

calculated based on the benzene yield and the increasing amount of carbon in the

265

permanent product gas compared to the permanent reactant gas.

266

3.4.2. Predictions for the product gas

267

Predicting the changes of gas component j that compared the product gas to the reactant gas was

268

performed for the gasification gas and synthetic raw gas experiments. The estimated data were

269

then compared to the experimental data obtained in these two experiments. This was to

270

investigate the changes in product nature in the ethene and toluene decompositions comparing

271

two cases: (i) ethene and toluene were decomposed mainly via steam reforming reactions; and 15



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Page 16 of 38

272

(ii) in addition to steam reforming, ethene and toluene could be decomposed via hydro-cracking

273

and dry reforming reactions, which might be exacerbated due to the effect of the WGS reaction.

274

In general, the predictions were made by taking the amount of component j potentially produced

275

from the destruction of component k deducting the amount of component j potentially

276

decomposed during the experiments. Furthermore, the predictions used the product distribution

277

𝐴𝑗/𝑘 obtained in ethene steam reforming experiment and toluene steam reforming experiment.

278 279

For the gasification gas experiment, the predictions were made for CO plus CO2, and for CH4 as follows:

280

𝑝𝐶𝑂+𝐶𝑂2 = 𝜂𝐶2 𝐻4 . 𝑛𝐶2 𝐻4 ,𝑖𝑛 . 𝐴𝐶𝑂+𝐶𝑂2 /𝐶2 𝐻4 + 𝜂𝐶𝐻4 . 𝑛𝐶𝐻4 ,𝑖𝑛

Eq. (6)

281

𝑝𝐶𝐻4 = 𝜂𝐶2 𝐻4 . 𝑛𝐶2 𝐻4 ,𝑖𝑛 . 𝐴𝐶𝐻4 /𝐶2 𝐻4 − 𝜂𝐶𝐻4 . 𝑛𝐶𝐻4 ,𝑖𝑛

Eq. (7)

282

where 𝑝𝐶𝑂+𝐶𝑂2 and 𝑝𝐶𝐻4 are respectively the predicted changes of CO plus CO2, and of CH4

283

comparing the product gas to the reactant gas [mole], 𝜂𝐶2 𝐻4 is the conversion efficiency of C2H4

284

in the gasification gas experiment [-], 𝜂𝐶𝐻4 is conversion efficiency of CH4 in methane steam

285

reforming experiment [-], 𝑛𝐶2 𝐻4 ,𝑖𝑛 and 𝑛𝐶𝐻4 ,𝑖𝑛 are the amounts of C2H4 and CH4 in the reactant

286

gas used in the gasification gas experiment, respectively [mole], and 𝐴𝐶𝑂+𝐶𝑂2 /𝐶2 𝐻4 and

287

𝐴𝐶𝐻4 /𝐶2 𝐻4 are the ratios of the amounts of CO plus CO2, and of CH4 produced to the amount of

288

C2H4 converted in the ethene steam reforming experiment [-].

289 290 291

For the synthetic raw gas experiment, the predictions were made for CO plus CO2, CH4, and C2H4 as follows: 𝑝𝑗 = 𝜂𝐶7𝐻8 . 𝑛𝐶7𝐻8,𝑖𝑛 . 𝐴𝑗/𝐶7𝐻8 + 𝑛𝑗,𝑔𝑎𝑠𝑖𝑓

Eq. (8)

16


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292

Where 𝑝𝑗 is the predicted change in the level of component j (CO plus CO2, CH4, or C2H4)

293

[mole], 𝜂𝐶7𝐻8 is the conversion efficiency of toluene in the synthetic raw gas experiment [-],

294

𝑛𝐶7𝐻8,𝑖𝑛 is the amount of C7H8 introduced into the reactor during the reduction stage of the

295

synthetic raw gas experiment [mole], 𝐴𝑗/𝐶7𝐻8 is the ratio of the amount of component j produced

296

to the amount of C7H8 converted in the toluene steam reforming experiment [-], and 𝑛𝑗,𝑔𝑎𝑠𝑖𝑓 is

297

the absolute change in the level of component j comparing the product gas to the reactant gas in

298

the gasification gas experiment [mole].

299

4. RESULTS AND DISCUSSION

300

4.1. Characterization of activated ilmenite and reference experiment with silica sand

301

To confirm that ilmenite was appropriately activated and to support the experimental results

302

presented in the following sections about using activated ilmenite, results from characterizations

303

of activated ilmenite (i.e., results of physical characterization and of WGS experiment), and

304

results from the silica sand experiment are provided here.

305

The BET results show that the specific surface area of the ilmenite increased from 0.175 to

306

0.766 m2/g after the activation. XRD pattern for activated ilmenite in comparison with that of

307

fresh ilmenite are shown in Figure 3. There was a clear phase evolution in ilmenite after

308

activation. The main phases of the fresh ilmenite were pure ilmenite FeTiO3 and hematite Fe2O3.

309

However, for the activated sample, the hematite phase disappeared and iron species were

310

presented only at the reduced forms Fe+2 and Fe° in pure ilmenite, ulvöspinel Fe2TiO4, and

311

metallic iron Fe.

17



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312 313

Figure 3. XRD patterns (2θ scale) of fresh ilmenite (above) and activated ilmenite (below), with

314

pure ilmenite FeTiO3 (⨉), hematite Fe2O3 (∗), ulvöspinel Fe2TiO4 (o), and metallic iron Fe (■).

315 316 18


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317

Figure 4. WGS experiment. H2/CO and CO2/CO ratios in reactant gas and in product gases at

318

different reactor temperatures; and H2(p)/CO2(p) ratio that compares amount of H2 produced

319

[here termed H2(p)] to amount of CO2 produced [here termed CO2(p)].

320

In the WGS experiment with activated ilmenite, substantial levels of H2 and CO2 production

321

and of CO consumption were observed as changes in the gas compositions (comparing the dry

322

reactant gas and product gas), i.e., the H2/CO and CO2/CO ratios in Figure 4. The forward WGS

323

reaction was the main inducer of changes in the levels of CO, CO2, and H2. The oxidizing

324

reactions of CO and H2 with oxygen (possibly carried by ilmenite particles) to produce CO2 and

325

H2O, respectively, could be ignored, as the production levels of H2 and CO2 were almost equal

326

for all the studied temperatures (see the H2(p)/CO2(p) ratios in Figure 4). The results of the WGS

327

experiment and of physical characterization confirmed that the activation procedure was

328

appropriate for specifically inducing the catalytic activity of ilmenite.

329 330

Figure 5. Silica sand experiment. Compositions (dry basis) of reactant gas and product gases for

331

different reactor temperatures. 19



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Page 20 of 38

332

Figure 5 shows the compositions of the reactant gas and of the product gases for different

333

reactor temperatures in silica sand experiment. The results showed minor changes in

334

compositions of ethene C2H4, which corresponded to the conversions of 8, 10, 13, and 19% at the

335

reactor temperature of 750, 800, 850, and 900°C, respectively. Thus, the thermal effect can

336

contribute to the conversion of tar and light HC.

337

4.2. Ethene conversion

338

In the ethene steam reforming experiment, H2, CO, CO2, and CH4 were detected in the product

339

gas with the amounts shown in Figure 6. There was also C2H6 trace in the product gas, however,

340

it was at very low level thus was not presented here. The carbonaceous products CO, CO2, and

341

CH4 were produced, in which the total amounts of CO and CO2 were considerably higher than

342

those of CH4. This indicates that both the complete steam reforming reaction and steam

343

dealkylation took place.

344

20


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345

Figure 6. Ethene steam reforming experiment. Amounts of C2H4 fed to the reactor, and amounts

346

of products achieved at different reactor temperatures.

347 348

Figure 7. Gasification gas experiment. Compositions (dry basis) of reactant gas and product

349

gases for different reactor temperatures.

350

Figure 7 shows the compositions of the reactant gas and product gases for different reactor

351

temperatures in the gasification gas experiment. It is clear that the WGS reaction adjusted

352

significantly the concentrations of CO, CO2, and H2, as already observed in the WGS experiment.

353

Decreases in the concentrations of ethene were also noticed, and these were more prominent than

354

the changes in the concentrations of methane.

355

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Page 22 of 38

356 357

Figure 8. Conversion efficiencies of ethene in: (1) ethene steam reforming experiment, and (2)

358

gasification gas experiment; and conversion efficiencies of methane in: (1) methane steam

359

reforming experiment, and (2) gasification gas experiment for different reactor temperatures.

360

The conversion efficiencies of ethene in the ethene steam reforming and gasification gas

361

experiments are presented in Figure 8. The conversion efficiencies increased in line with the

362

increase in temperature; they were rather similar for all the investigated temperatures in these

363

two experiments. The conversion efficiencies of methane in the methane steam reforming and

364

gasification gas experiments are also presented in Figure 8. As expected, the conversion

365

efficiencies in the methane steam reforming experiment were low at all the studied temperatures.

366

It is worth noting that CO, CO2, and H2 were the products obtained in this experiment. In the

367

gasification gas experiment, the conversion efficiencies were negative for all the examined

22


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368

cases. This was due to the conversion of ethene produced methane, as observed earlier (see

369

Figure 6).

370 371

Figure 9. Gasification gas experiment. Experimental (Exp.) data and predicted (Predict.) data for

372

increases in levels of CO plus CO2, and of CH4 for different reactor temperatures.

373

In gasification gas experiment, increases in the levels of CO plus CO2, and of CH4 comparing

374

the product gas to the reactant gas were predicted. The calculated data and the corresponding

375

experimental data are featured in Figure 9. The actual increases in the levels of CO and CO2

376

were lower than the predicted values, and this discrepancy was more pronounced as the

377

temperature of the reactor was increased. However, the opposite trend was observed for CH4.

378

Thus, the distribution of carbon-containing products from ethene conversion shifted towards

379

more HC products and fewer oxidation products as H2 was present in the reactant gas. This can

380

be attributed to the fact that ilmenite facilitates the dissociation of H2 and the concentration of

381

hydrogen intermediates increases, which in turn alters the nature of the carbonaceous products.

382

Thus, the hydro-cracking reaction took place and was enhanced at higher temperatures. Note that 23



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Page 24 of 38

383

the equilibrium calculation for the methane steam reforming reaction confirmed that the reverse

384

direction (towards methanation) was not favored over the methane steam reforming direction

385

under the studied conditions, even though CO and H2 were present in the reactant gas. Therefore,

386

the predictions were made assuming that the methane conversion efficiencies were equal to those

387

obtained in the methane steam reforming experiment. However, the contribution of methane

388

destruction to the predicted data was negligible owing to the extremely low methane conversion

389

efficiencies (see Figure 8). Although CO2 was present in the reactant gas, the production of

390

oxidation products did not increase. The dry reforming reaction, therefore, was not significant,

391

relative to the steam reforming and hydro-cracking reactions. It must be emphasized that in the

392

gasification gas experiment, the WGS reaction occurred significantly (see Figure 7), which

393

modulated the H2 and CO2 concentrations and thus could promote hydro-cracking and dry

394

reforming reactions. However, only the hydro-cracking reaction occurred to a significant extent.

395

4.3. Toluene conversion

24


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396 397

Figure 10. Toluene steam reforming experiment. Amounts of C7H8 fed to the reactor, and

398

amounts of products achieved at different reactor temperatures.

399

Figure 10 presents the amounts of products obtained in the toluene steam reforming

400

experiment. Carbonaceous products CO, CO2, CH4, C2H4, and benzene were obtained and the

401

total yields of CO and CO2 were higher compared to those of other carbon-containing products.

402

These observations imply that both the complete steam reforming reaction and steam

403

dealkylation occurred. As seen from Figure 10, ethene C2H4 was present at insignificant levels

404

for all studied cases, as compared to other carbonaceous products. It is also worth noting that tar

405

compounds that are heavier than toluene such as xylene, styrene, indene, biphenyl, naphthalene,

406

and acenaphthylene were detected in the collected 2-propanol samples at the amounts of

407

approximately 1–4% of total carbon fed to the reactor. Furthermore, carbon deposit was

25



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Page 26 of 38

408

observed at the amounts of 0.7 and 0.25% of the total inlet carbon, respectively at 750°C and

409

800°C. Carbon deposition was not seen in any other experiments.

410 411

Figure 11. Conversion efficiencies of toluene in toluene steam reforming experiment and

412

synthetic raw gas experiment for different reactor temperatures.

413

Figure 11 presents the conversion efficiencies of toluene. The value at 750°C in the synthetic

414

raw gas experiment could not be calculated due to an error in the measurement of the product

415

gas. The efficiencies followed the increases in temperature. Moreover, comparing the toluene

416

steam reforming experiment to the synthetic raw gas experiment, toluene conversion efficiencies

417

were lower at 800°C and 850°C, and rather similar at 900°C. The difference in toluene

418

conversion efficiencies in the two experiments can be explained by the difference in mechanisms

419

where toluene molecules were initially converted into reactive intermediates.14 In the synthetic

420

raw gas experiment, in addition to the molecule self-dissociation mechanism, toluene molecules 26


Page 27 of 38

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421

can be favorably destructed into reactive intermediates by interaction with other reactive

422

intermediates, as the reactant gas contained more components than that in toluene steam

423

reforming experiment. However, the ways in which different mechanisms operate at different

424

temperatures remain unclear.

425 426

Figure 12. Amounts of C7H8 fed to the reactor, and amounts of benzene and of total carbon

427

increasing in permanent product gas obtained in: (1) toluene steam reforming experiment, and

428

(2) synthetic raw gas experiment for different reactor temperatures.

429

The amounts of benzene and the amounts of carbon in the permanent product gases obtained in

430

the toluene steam reforming experiment at 800°–900°C are presented again in Figure 12 in

431

comparison with those obtained in the synthetic raw gas experiment. It can be seen from Figure

432

12 that the amounts of benzene obtained in the synthetic raw gas experiment were higher than

433

those obtained in the toluene steam reforming experiment. However, the amounts of carbon

434

increasing in the permanent product gases were lower. 27



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Page 28 of 38

435 436

Figure 13. Synthetic raw gas experiment. Experimental (Exp.) data and predicted (Predict.) data

437

for increases in levels of CO plus CO2 and of CH4, and for decreases in levels of C2H4 for

438

different reactor temperatures.

439

Figure 13 shows the experimental and predicted data for the levels of CO plus CO2 and of CH4

440

increased, and the levels of C2H4 decreased between the reactant gas and the product gas in the

441

synthetic raw gas experiment. The most noticeable discrepancies between actual and predicted

442

results were observed for CO plus CO2 with the actual values being lower than the predicted

443

ones, and this difference was enhanced as the temperature increased. This observation together

444

with the observed levels of benzene (see Figure 12) reveal that the production of carbonaceous

445

products during toluene conversion most likely shifted towards more HC products and fewer

446

oxidation products. Note that in the synthetic raw gas experiment, H2 was present in the reactant

447

gas and a significant amount of H2 was produced by the WGS reaction. Thus, the hydro-cracking

448

reaction occurred and was enhanced at higher temperatures. In contrast, the presence of CO2 in 28


Page 29 of 38

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449

the reactant gas and the CO2 produced by the WGS reaction did not exhibit any significant

450

effect. The results obtained here are in line with the results obtained for ethene conversion, as

451

discussed earlier, which show that the steam reforming and hydro-cracking reactions are

452

important and that the dry reforming reaction is not important.

453

4.4. Application of the obtained results

454

This work demonstrated a simple method to identify the importance of the different

455

decomposition reactions in the raw gas cleaning process. The conducted experiments were

456

designed using as the reference a raw gas that is produced in the Chalmers indirect biomass

457

gasifier. The steam reforming reactions were expected to predominate over the hydro-cracking

458

and dry reforming reactions, as steam represented about 50 vol% of the used reactant gas.

459

However, the presence of H2 in the reactant gas and of H2 produced by the WGS reaction

460

directed the resulting gas composition towards more HC products and fewer oxidation products;

461

an effect that was accelerated by the increases in temperature. Thus, the hydro-cracking reaction

462

occurred to a significant extent and was favored at higher temperature. In contrast, the presence

463

of CO2 or the dry reforming reaction did not show any noticeable effect. The formation of more

464

stable products, i.e., methane and benzene, was observed in the ethene steam reforming and

465

toluene steam reforming experiments, respectively, even though steam was present in

466

sufficiently excess levels for the complete steam reforming reactions of ethene and toluene to

467

occur exclusively. This shows the effect of steam dealkylation and reveals that the catalytic

468

ability of ilmenite under the employed conditions was not sufficient to eliminate methane and

469

benzene. Overall, to identify the roles of the different decomposition pathways in a given gas

29



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Page 30 of 38

470

cleaning process, one needs to define the representative reactions and approaches to combine

471

them, and investigate the synergistic effects accordingly.

472

The obtained results provide valuable insights regarding the most important decomposition

473

reactions to be accounted for in the kinetic modeling for catalytic raw gas cleaning process.14

474

The kinetic modeling is essential to follow the upgraded gas quality. However, one of the

475

difficulties is to handle the tar and light HC that are formed in situ. By understanding the

476

contributions of the different decomposition reactions of tar/light HC to the total process, the

477

selectivity of the tar/light HC formed in situ and their quantities relative to the levels of oxidation

478

products can be elucidated, which facilitates the kinetic modeling.

479

5. CONCLUSIONS

480

In this work, the aggregate effects of the decomposition reactions on the total conversion of tar

481

and light hydrocarbons in the catalytic cleaning of a biomass-derived raw gas are elucidated. As

482

part of this, a simple method to identify the contributions of the different decomposition

483

reactions to the overall cleaning process is formulated. Taking as reference a raw gas produced in

484

the Chalmers indirect biomass gasifier, representative reactions associated with the gas cleaning

485

process were are identified as: WGS reaction; methane steam reforming; ethene steam

486

reforming; and toluene steam reforming. These reactions were investigated independently, as

487

well as in combination and together with possible hydro-cracking and dry reforming reactions.

488

Ilmenite was used as the demonstration catalyst. From the obtained results, the following

489

conclusions were drawn:

490 491



The steam reforming reactions occurred to a great extent, as steam was present at 50 vol% in the used reactant gases. The presence of H2 in the reactant gases promoted 30


Page 31 of 38

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492

more hydrocarbon products and lower levels of CO and CO2, which was enhanced as

493

temperature increased and possibly favored by the water-gas shift reaction. However,

494

the presence of CO2 did not show considerable effect.

495



Generally for ilmenite catalysis, the steam reforming and hydro-cracking reactions are

496

the most important, and the dry reforming reaction can be ignored. Within the steam

497

reforming routes, both the complete steam reforming and steam dealkylation occur.

498

AUTHOR INFORMATION

499

Corresponding Author

500

*E-mail: [email protected]. Tel: + 46 (0) 31 772 14 45

501

Present Address

502

†Omnical Industrieservice GmbH-Viessmann Group, D-35684 Dillenburg, Germany

503

ACKNOWLEDGMENTS

504

This work was made possible by financial support from E.ON and the Swedish Gasification

505

Centre (SFC). The authors thank research engineer Jessica Bohwalli for valuable assistance with

506

the experimental equipment.

31



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507

Page 32 of 38

Supporting Information

508 509

Figure S1. Calculated K and equilibrium constant Keq for WGS experiment (left) and gasification

510

gas experiment (right) at different reactor temperatures, K was calculated using the equation:

511

2 K= [CO][H

512

4.33).

513

Table S1. 𝐴𝑗/𝑘 ratio (mole/mole) in ethene steam reforming experiment used for predictions.

514

Table S2. 𝐴𝑗/𝑘 ratio (mole/mole) in toluene steam reforming experiment used for predictions.

515

This information is available free of charge via the Internet at http: //pubs.acs.org.

[CO ][H2 ]

4577.8

, Keq was calculated using the equation derived by Moe (1962): Keq = exp (

2 O]

T

−

516

REFERENCES

517

(1) E4Tech. Review of Technologies for Gasification of Biomass and Wastes. NNFCC project

518 519 520

09/008; National Non-Food Crops Centre: United Kingdom, 2009. (2) Dayton, D. A Review of the Literature on Catalytic Biomass Tar Destruction. National Renewable Energy Laboratory: Golden, Colorado, 2002.

521

(3) Larsson, A.; Seemann, M.; Neves, D.; Thunman, H. Evaluation of Performance of

522

Industrial-Scale Dual Fluidized Bed Gasifiers Using the Chalmers 2–4-MWth Gasifier. Energy

523

Fuels 2013, 27, 6665.

32


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524 525 526 527 528 529 530 531


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(15) Berguerand, N.; Marinkovic, J.; Berdugo Vilches, T.; Thunman, H. Use of Alkali-

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Chem. Eng. J. 2016, 295, 80.

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Method for Sampling Biomass-Derived Tar in Industrial Environments. Energy Fuels 2013, 27,

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Composition. Chem. Eng. Res. Des. 2012, 90, 1351.

589 590

N

591

LIST OF FIGURE CAPTIONS

592

Figure 1. Conversion of tar/light HC via decomposition reactions, and formations of

593 594

carbonaceous products. Figure 2. Experimental setup.

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595

Figure 3. XRD patterns (2θ scale) of fresh ilmenite (above) and activated ilmenite (below),

596

with pure ilmenite FeTiO3 (⨉), hematite Fe2O3 (∗), ulvöspinel Fe2TiO4 (o), and metallic iron Fe

597

(■).

598

Figure 4. WGS experiment. H2/CO and CO2/CO ratios in reactant gas and in product gases

599

at different reactor temperatures; and H2(p)/CO2(p) ratio that compares amount of H2 produced

600

[here termed H2(p)] to amount of CO2 produced [here termed CO2(p)].

601 602 603 604 605 606

Figure 5. Silica sand experiment. Compositions (dry basis) of reactant gas and product gases for different reactor temperatures. Figure 6. Ethene steam reforming experiment. Amounts of C2H4 fed to the reactor, and amounts of products achieved at different reactor temperatures. Figure 7. Gasification gas experiment. Compositions (dry basis) of reactant gas and product gases for different reactor temperatures.

607

Figure 8. Conversion efficiencies of ethene in: (1) ethene steam reforming experiment, and

608

(2) gasification gas experiment; and conversion efficiencies of methane in: (1) methane steam

609

reforming experiment, and (2) gasification gas experiment for different reactor temperatures.

610 611 612 613

Figure 9. Gasification gas experiment. Experimental (Exp.) data and predicted (Predict.) data for increases in levels of CO plus CO2, and of CH4 for different reactor temperatures. Figure 10. Toluene steam reforming experiment. Amounts of C7H8 fed to the reactor, and amounts of products achieved at different reactor temperatures. 37



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614 615

Page 38 of 38

Figure 11. Conversion efficiencies of toluene in toluene steam reforming experiment and synthetic raw gas experiment for different reactor temperatures.

616

Figure 12. Amounts of C7H8 fed to the reactor, and amounts of benzene and of total carbon

617

increasing in permanent product gas obtained in: (1) toluene steam reforming experiment, and

618

(2) synthetic raw gas experiment for different reactor temperatures.

619

Figure 13. Synthetic raw gas experiment. Experimental (Exp.) data and predicted (Predict.)

620

data for increases in levels of CO plus CO2 and of CH4, and for decreases in levels of C2H4 for

621

different reactor temperatures.

622 623

624 625

For Table of Contents Only

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