Fate of Polycyclic Aromatic Hydrocarbons during Tertiary Tar

Article Cite This: Energy Fuels XXXX, XXX, XXX−XXX

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Fate of Polycyclic Aromatic Hydrocarbons during Tertiary Tar Formation in Steam Gasification of Biomass Huong N. T. Nguyen,* Martin Seemann, and Henrik Thunman Division of Energy Technology, Department of Space, Earth and Environment, Chalmers University of Technology, SE-412 96 Gothenburg, Sweden S Supporting Information *

ABSTRACT: This work investigates the fate of polycyclic aromatic hydrocarbons (PAH) in relation to the process severity in the steam- and H2-containing reaction environment of steam gasification of biomass. The focus is on the regimen of tertiary tar formation during the gasification in a fluidized bed gasifier; the tertiary tar is tar that is predominantly aromatic compounds. The process severity reflects the following operating conditions: temperature, gas residence time in the reactor, and contact time between the gas phase and catalytic bed material. The conducted experiments employed a raw gas upgrading process downstream of the gasifier. A mature tar-containing raw gas produced in the Chalmers 2−4-MWth dual fluidized bed biomass gasifier was upgraded in a bench-scale, bubbling fluidized bed reactor, in which inert silica sand and a naturally occurring ilmenite catalyst were used as the bed materials. The obtained results show that, following the increased process severity, the growth of PAH can either enhance or suppress. For the growth of PAH being suppressed, it is required that the process severity is sufficient to convert steam and H2 into the reactive hydrogen intermediates that prevent the combination of the carbon-containing species. To ensure this, the application of silica sand as a bed material requires an operating temperature of 850 °C and a gas residence time of >11.5 s, while the use of ilmenite requires an operating temperature of 800 °C and a gas residence time of >3.4 s, together with a gas−solid contact time of about 0.7 s. In particular, the results obtained for ilmenite encourage the use of naturally occurring catalysts in fluidized bed gasifiers, despite the fact that their catalytic activities are lower than that of synthetic catalysts.

1. INTRODUCTION Tar, which is present in the raw gas produced from biomass gasification, is a significant cause of operational problems during the gasification and integrated downstream processes.1−3 To minimize the tar yield and increase the cold gas efficiency of the gasification, the gasifier is often operated at the highest operational stringency (“process severity”), e.g., at the highest possible temperature. As a consequence, the composition of the tar is dominated by aromatic compounds, and the formation of polycyclic aromatic hydrocarbon (PAH) tar can be significant.4−7 PAH tar is one of the most difficult classes of tar to eliminate chemically during the raw gas upgrading for tar removal conducted downstream of the gasifier. Therefore, understanding how the formation of PAH tar is affected by the conditions inside the gasifier, toward minimizing the formation of this tar, is important for optimization of the gasification process. During biomass gasification, the formation of PAH tar and the increase in its molecular size are initiated as the PAH precursors are produced.6,8,9 To reduce the yield of PAH tar, either the formation of the precursors or the growth of PAH tar from the precursors needs to be suppressed; in the present work, we focus on the latter strategy. The corresponding conversion stage in the gasification process is referred to as “tertiary conversion”, and the produced tar is the “tertiary tar”, which comprises mainly aromatics.8 During this tertiary conversion stage, the growth of PAH tar is attributed to the combinations of carbon-containing species (in the present work, carbon-containing species are defined as organic carboncontaining intermediates and molecules).8,10 These combina© XXXX American Chemical Society

torial events must be prevented if the growth of PAH tar during the tertiary conversion is to be limited. In petroleum refineries, the use of H2 in hydrocracking processes and the use of other hydrogen donors, such as tetralin, in the thermal cracking of residues are well-known to reduce tar formation, as compared to other cracking processes that do not use hydrogen donors.11−15 In the literature on coal pyrolysis, the effect of H2 on reducing the formation of relatively heavier tar compounds has been reported.16−18 In the literature on gasification, several studies on the cracking of individual aromatic tar compounds, such as naphthalene, have also reported that H2 suppresses the formation of products larger than the reacting tars.19,20 It has been proposed that the reactive hydrogen intermediates that originate from hydrogen donors terminate with carbon-containing intermediates. Thus, the potential is reduced for these carbon-containing intermediates to react with themselves to produce relatively larger molecules or for these carbon-containing intermediates to react with carbon-containing molecules to form larger intermediates.13 In steam gasification of biomass, steam and H2 are inherently available in the reaction environment. Steam is added to the gasification process as the gasifying agent, and consequently, H2 can be produced at a significant level from the water−gas shift (WGS) reaction, which is in addition to the steam and H2 produced from the degradation of biomass.2,7 Furthermore, in Received: November 15, 2017 Revised: January 16, 2018 Published: February 6, 2018 A

DOI: 10.1021/acs.energyfuels.7b03558 Energy Fuels XXXX, XXX, XXX−XXX

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Figure 1. Biomass gasification with the focus on the maturation of tar in relation to process severity. The term “OC” denotes oxygenated organic compounds. The permanent gas consists mainly of H2, CO, CO2, CH4, and other light hydrocarbons, such as C2H4. The reaction pathways for producing the permanent gas are represented by thicker arrows, to indicate that they are the main routes throughout the gasification. Reactions of char are not included.

fluidized bed reactor in which the upgrading was carried out. As the tar in the applied raw gas contains mainly aromatics, the reaction pathways that are active during the upgrading are comparable to those in the tertiary conversion stage of the gasification. Furthermore, employing this downstream approach, the authentic reaction environment and the entire aromatic-tar spectrum involved in the gasification process are considered; this represents an advantage over other studies in the literature that have taken into account only a limited number of reactants from the gasification process.19,20 Finally, ilmenite was used so as to reflect the selection of catalytic bed materials applied in commercial-scale fluidized bed biomass gasifiers. Naturally occurring catalytic materials (e.g., olivine, bauxite, dolomite, and feldspar, in addition to ilmenite) are preferred over synthetic materials (e.g., nickel-based materials), even though the naturally occurring catalysts have lower catalytic activities. While the synthetic materials are highly active in the catalytic sense, they are often environmentally hazardous, thereby hindering their applications to biomass gasification where the environmental issue is prioritized.25−29

addition to H2, steam acts as a hydrogen donor in reaction environments relevant to that of the gasification process.21,22 Therefore, the objective of the present work is to formulate a comprehensive explanation about the fate of PAH tar during the tertiary conversion stage of steam gasification of biomass, in which the role of the reactive hydrogen intermediates originating from steam and H2 is taken into account. The present work also aims to identify the process severity that is relevant to the authentic operation of the steam gasification process conducted in fluidized bed gasifiers and that induces the formation of the reactive hydrogen intermediates from steam and H2. Whereby, the combination of carbon-containing species is prevented by the reactive hydrogen intermediates, and thus, the growth of PAH tar is suppressed. The process severity is contributed by both thermal and catalytic effects. The thermal effect can be adjusted by changing temperature and gas residence time in the reactor (“gas residence time” is denoted as “RT”). The catalytic effect can be adjusted by changing temperature and gas−solid contact time between the gas phase and the solid catalyst (“gas−solid contact time” is denoted as “CT”). In total, the process severity is influenced by three operating parameters: temperature, RT, and CT. When silica sand is used as a bed material, catalytic effect is not present, and thus, the value of CT is defined as equal to zero.7,23,24 The experiments were conducted with a downstream approach, which involves the upgrading of a mature tarcontaining raw gas (i.e., the composition of the tar was dominated by aromatic compounds) downstream of the gasifier. The raw gas was produced in the Chalmers 2−4MWth dual fluidized bed biomass gasifier (abbreviated as the “Chalmers gasifier”), and the steam and H2 contents were approximately 60 and 10 vol % (wet basis), respectively. Silica sand and ilmenite (FeTiO3) were used as bed materials in the

2. THEORY 2.1. Conversion of Biomass during Gasification. The biomass gasification process is described in Figure 1, with the focus on the maturation of the tar in relation to the process severity. The figure summarizes the literature on the different aspects of biomass gasification1,6−9,30−35 and the literature that relates to the growth in molecular size of aromatic compounds.10,16,36−41 In the literature on biomass gasification, the criteria used to classify tar compounds according to the degree of maturation, as well as the classification of the conversion stages of biomass gasification, are not consistent.1,42 In the present work, the definitions of “primary tar”, “secondary tar”, and “tertiary tar” follow those provided by Evans and B


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Energy & Fuels Milne.6,8 The “primary conversion”, “secondary conversion”, and “tertiary conversion” stages presented in Figure 1 relate to the types of tar produced, which should be viewed in relation to the evolution of tar itself, rather than the degradation of biomass as a whole. Indeed, cellulose, hemicellulose, and lignin, which constitute the biomass, have different chemical structures and reactivities, which means that they are converted differently during gasification. The extractives are not considered in Figure 1 due to their low concentrations in woody biomass,43,44 which is the fuel used in the present work. The tar, in general, evolves from oxygenated organic compounds (OC) that are produced during the primary conversion stage to PAH precursors in the secondary conversion stage and then to PAH tar in the tertiary conversion stage, in accordance with the increased process severity (Figure 1). Furthermore, the tar becomes more thermally stable, and oxygen is gradually removed from the tar, such that the tertiary tar eventually contains a negligible amount of oxygen. The OC have chemical composition similar to that of the biomass feedstock. Furthermore, the OC consist of nonaromatic compounds that are mainly derived from cellulose and hemicellulose, as well as aromatic compounds that are mainly derived from lignin. The PAH precursors are either nonaromatic compounds, such as cyclopentadiene, or monocyclic aromatics. The nonaromatic precursors are produced most likely from nonaromatic OC. The monocylic aromatic precursors are produced from both nonaromatic and aromatic OC, i.e., nonaromatic OC are converted into monocyclic aromatics, whereas aromatic OC are converted to less-branched and less-oxygenated aromatics.8,30,35 The monocyclic aromatic precursors are also produced via Diels−Alder reactions of light alkenes in the permanent gas, followed by dehydrogenations of the formed cyclic hydrocarbons.8,16 After the PAH precursors are formed, two main pathways are followed: (1) the formation of smaller products, e.g., benzene and/or permanent gases and (2) the formation of larger products, i.e., PAH tars. The extents to which these pathways are active depend on the reaction environment and the process severity in the gasifier. It should be noted that benzene can be produced from the monocyclic aromatic precursors that contain branches, and the produced benzene can be further converted in to permanent gases or PAH tars, see Figure 1.45−47 Furthermore, at a relatively higher level of process severity, light hydrocarbons in permanents gases can be converted into other light hydrocarbons that are smaller such as CH4 and into CO/CO2.21,48 The growth of PAH tar occurs mainly via two main mechanisms: (i) ring cross, e.g., combination of two aromatic species to produce another aromatic species with a higher number of fused rings, and (ii) consecutive additions of unsaturated light hydrocarbon molecules, most likely ethyne (C2H2) produced during the tertiary conversion, to an aromatic intermediate, which is followed by cyclization and dehydrogenation and ultimately results in an increase in the number of aromatic rings in PAH tar molecules.10,16,34,36−41 Finally, the reactions of nonaromatic precursors create aromatic compounds, e.g., the reactions of two cyclopentadiene molecules result in a single naphthalene molecule, which can be further converted into heavier PAH tar via the two above-mentioned mechanisms.16,39 2.2. Product Selectivity in the Tertiary Conversion Stage. Figure 2 shows a simplification of the tertiary conversion stage, with the focus on the selectivity of products that are formed and are thus present in the raw gas produced

Figure 2. Product selectivity in the tertiary conversion stage. HC, hydrocarbons.

from the gasification. Previous works21,23 show that the numerous reaction pathways can be simplified as reactions between three types of reactive intermediates: carboncontaining intermediates C*, hydrogen intermediates H*, and oxygen intermediates O*, in which H* and O* originate mainly from the mixture of steam and H2. Depending on whether C* react with O*, with H* or with other C*, different types of products are obtained in the raw gas, see Figure 2. With the process severity commonly applied to fluidized bed gasifiers (i.e., operating temperature in the range of 800−900 °C, together with the use of silica sand or naturally occurring catalysts as the bed material), the complete conversion of C* to CO through the oxidation of C* by O* is not possible. Indeed, for C* to be converted exclusively to CO, an operating temperature of ≥1400 °C under a noncatalytic condition is required; if the operating temperature is in the range of 800− 900 °C, the use of highly active catalysts, such as nickel-based catalysts, is necessary.2,7,19,49−51 Thus, the other two reaction routes (see Figure 2), which require a relatively lower process severity, occur during the tertiary conversion stage. Part of C* react with H* to produce tar/light hydrocarbon molecules, or C* can react with other C* to produce larger tar molecules (as compared to the molecules produced from the reactions between C* and H*). The mutual combination of C* contributes to the formation of PAH tar and a further increase in the molecular size of the PAH tar. To prevent the mutual combination of C*, a strategy is to ensure a sufficient level of H* so as to dilute the concentration of C* in the reaction environment. Thus, C* react with H*, or C* are susceptible to the decomposition at higher process severity, which eventually results in the formation of smaller products (e.g., permanent gases). Furthermore, as C* already react with H*, the probability that C* will combine with carbon-containing molecules to form larger C* as mentioned earlier, which also contributes to the growth of the PAH tar, is reduced. Overall, a sufficient level of H* in the reaction environment prevents the combination of carbon-containing species. Thus, the growth of the PAH tar is suppressed. 2.3. Relationship between the Fate of PAH Tar and Process Severity in the Tertiary Conversion Stage. Within the temperature range of 800−900 °C and with an RT of less than 5 s being applied to a noncatalytic gasification/ devolatilization/pyrolysis process, it has been reported in the literature that the yield of PAH tar and the number of aromatic rings in the produced PAH tar molecules are both increased as the process severity is increased, regardless of the fact that steam is added to the process or not.6,9,10,30,52 In particular, Fuentes-Cano et al.10 observed that the formation of PAH tar C


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concentration of H* in the reaction environment is sufficient to prevent the combination of carbon-containing species. It is worth mentioning once again that, in addition to formulating a comprehensive explanation about the fate of PAH tar during the tertiary conversion stage, the present work focuses on identifying the process severities required to suppress the growth of PAH tar during the tertiary conversion stage of the biomass steam gasification in fluidized bed gasifiers for two cases: (i) silica sand used as the bed material and (ii) ilmenite used as the bed material. 2.4. Using Ilmenite as Catalyst for Tar Removal. Ilmenite has both oxygen transport and catalytic activities, which are largely attributed to the iron content in ilmenite. The redox state of the iron determines the dominant activity. Indeed, if the iron is present in lower oxidation state such as Fe2+ and Fe0, ilmenite is more catalytically active. Contrarily, if the iron is present in higher oxidation state (i.e., Fe3+), oxygen transport capacity is dominant.47,53−56 To induce the activities of the fresh ilmenite, an activation step is required. The activation involves the exposure of the fresh ilmenite to alternating oxidizing and reducing conditions at a temperature of at least 800 °C, which results in the increase of porosity of ilmenite particles and the migration of iron to the particle surface.56,57 Ilmenite has been investigated as catalytic bed material for primary tar removal in the Chalmers dual fluidized bed gasifier.25,53,58 It has been reported that the use of ilmenite can lead to the decrease in heating value of the produced raw gas and thereby to the decrease in cold gas efficiency of the gasification process. This is because part of the raw gas is combusted by oxygen that ilmenite transports from the combustion chamber to the gasification chamber. Thus, to ensure the relatively high level of the heating value of the raw gas, this oxygen transport effect must be restricted. The catalytic activity of ilmenite is moderate, which was reported in previous investigations where ilmenite was used as catalyst for secondary tar removal downstream of the gasifier.21,47,59 Lind et al.47,59 reported that, when ilmenite was used to upgrade the raw gas produced in the Chalmers gasifier, significant reductions of phenolic, 1-ring branched, 2-ring branched, and 3-ring and larger tars were obtained. However, ilmenite was not sufficiently active to convert more-stable tars such as benzene and naphthalene. Furthermore, Lind et al.59 compared the catalytic activities of ilmenite and a manufactured nickel catalyst. The experimental data obtained by these authors59 showed that the tar decomposition at 800 °C was about 70% for the nickel catalyst while this value was only about 20% for ilmenite. In the present work, ilmenite was used as catalyst and its oxygen transport capacity was excluded. The ilmenite had previously been activated during a time-on-stream of approximately 1 day in the Chalmers 12-MWth biomass boiler, which at the time was operated at about 900 °C. Ilmenite was activated by the exposure to the alternating oxidizing and reducing conditions of the combustion process.58,60 Furthermore, ash elements (such as K, Na, and Ca) originating from the biomass fed to the boiler could deposit on the ilmenite particles, thereby the total catalytic activity of this processactivated ilmenite were attributed to both the ilmenite itself and the ash. It is noteworthy that for biomass gasification conducted in fluidized bed gasifiers, that the catalytic effect of the bed material can be enhanced due to the biomass ash deposit is well-known.25,61,62 Thus, the use of the process-activated

from the devolatilization of dried sewage sludge was almost identical for a case in which steam was added and a case in which steam was not added. These previous results imply that within the range of process severities applied in the previous studies, as the process severity increases, a part of small tar compounds combine to produce larger tar compounds, rather than these small tars undergoing decomposition to smaller products. Regardless of the available level of steam and the level of H2 possibly produced from WGS reaction, the reactive H* are not present or their concentration in the reaction environment is not sufficient to prevent the combination of carbon-containing species. Thus, the growth of PAH tar is increased. Furthermore, the trend is for products that are more stable to be formed at higher process severities, i.e., PAH tar molecules with a higher number of aromatic rings are formed at higher process severities. It should be noted that this increase in the molecular size of the PAH tar ultimately results in the formation of soot.9,49,50 In the work of Houben et al.,20 in which the conversion of naphthalene was investigated in the presence of H2 at temperature of about 500 °C, it was found that the H2 concentration in the reactant gas mixture determined the nature of the produced products. At low H2 concentration, naphthalene was polymerized into heavier tar and ultimately into soot. However, at high H2 concentration, naphthalene was cracked into lighter tar and permanent gases. This result obtained by Houben et al.20 implies that the higher the concentration of H* in the reaction environment, the smaller are the products produced. Based on the previous findings discussed here and the fact that steam and H2 are available at significant levels during the steam gasification and are a source of H*, the authors of the present work suggest a relationship between the fate of PAH tar and process severity during the tertiary conversion stage of steam gasification of biomass, which is shown in Figure 3. This

Figure 3. Hypothesized relationship between the fate of PAH tar and process severity during the tertiary conversion stage of steam gasification of biomass.

hypothesized relationship takes into account the effect of H*. The availability of H* in the reaction environment is dependent upon the process severity applied to the gasification process. There is a critical process severity beyond which the concentration of H* in the reaction environment is sufficient to prevent the combination of carbon-containing species. Thus, the growth of PAH tar is suppressed, and smaller products are produced. Within the range of process severity that is lower than the critical level, the growth of PAH tar increases following the increased process severity, which is extensively reported in the literature as discussed earlier. Thus, the relationship presented in Figure 3 complements the available literature on the fact that the growth of PAH tar can be suppressed, if the D


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C2H4, C2H6, C3H6, C3H8, N2, O2, H2, and He. The results obtained from the micro GC are used to interpret the experiment. The composition of the upgraded gas is also analyzed online every second using a Rosemount NGA 2000 multicomponent gas analyzer, which is applied solely for monitoring the experiment progress. The experimental procedure used is described briefly as follows. Initially, air was introduced into the reactor containing bed material, and the reactor was heated to the desired operating temperature. Air was fed continuously until the concentration of oxygen at the reactor outlet stabilized at about 21 vol %, thereby ensuring that combustible contaminants on the bed material were burnt off. Thereafter, the air flow was replaced by nitrogen. When no oxygen was detected at the reactor outlet, the raw gas was progressively introduced into the reactor and replaced the nitrogen flow. During stable operation with raw gas (i.e., stable permanent gas composition of the upgraded gas was obtained), samples of the tar and permanent gas of the upgraded gas were collected. For each sampling point, four tar samples and about ten permanent gas samples were taken within 20−25 min and analyzed to obtain the average value, which was to ensure that the obtained results were representative. After the tar and gas samplings were completed, the raw gas stream fed to the reactor was stopped and nitrogen was introduced into the reactor until such time that no combustible gases were detected at the outlet of the reactor. The duration of the raw gas operation was about 40−50 min in silica sand experiments and about 60−80 min in ilmenite experiments. It should be noted that the tar and permanent gases in the raw gas were also analyzed for comparison with the upgraded gas. To determine the amount of carbon deposited on the bed material during the raw gas operation, the used bed material was regenerated in air. The amount of deposited carbon was calculated from the amount of CO2 obtained during the regeneration. Detailed descriptions of the experimental setup and procedure can be found in the previous publications.23,64 It is noteworthy that in the ilmenite experiments, when air was introduced to the reactor to combust contaminants on ilmenite before the raw gas operation, ilmenite was also oxidized. Thus, during the initial stage of the raw gas operation, some of the raw gas was combusted due to the available oxygen carried on the oxidized ilmenite. After about 30−45 min depending on the experimental conditions applied, ilmenite reached its reduced state and acted as a catalyst, which was evidenced by the stable concentrations of CO and CO2 in the upgraded gas, see Figure 5. During this period, samples of the tar and permanent gas were taken from the upgraded gas, which is also indicated in Figure 5. 3.2. Operating Conditions. The main operating conditions applied in the Chalmers gasifier are summarized in Table 1. For the applied operating conditions, the raw gas produced in the Chalmers gasifier contained negligible amounts of C4−5 light hydrocarbons and tar compounds larger than 4 aromatic rings as previously reported by

ilmenite in the present investigation represented the authentic condition regarding the incorporated catalytic effect of the biomass ash. However, the individual contribution of the catalytic effect of the ash to the total catalytic effect of the process-activated ilmenite was not quantified in the present study.

3. EXPERIMENTAL SECTION 3.1. Experimental Setup and Procedure. The experimental setup is shown in Figure 4. The main component is the bench-scale,

Figure 4. Schematic of the experimental setup (thicker lines represent the main flows). bubbling fluidized bed reactor located downstream of the Chalmers gasifier. The body of the reactor is composed of the chromium-rich austenitic stainless steel RA 253 MA. The gas distributor in the reactor is a porous quartz plate. It is noteworthy that the previous study conducted by Lind et al.47 confirmed that the materials that compose the reactor body and the gas distributor are inert under the conditions applied for the raw gas upgrading process. The reactor is inserted in an oven, which enables operation of the reactor at high temperature. The pressure in the reactor is about −2 to −3 kPa (below atmospheric pressure), mainly due to the pressure level in the gasifier. The gasifier is operated at slightly below atmospheric pressure for safety reasons.7 The bed material in the reactor can be exposed to raw gas, air, and nitrogen. Operation with raw gas is the main focus of the experiment. Air and nitrogen are used to regenerate the bed material and to flush the reactor, respectively. During raw gas operation, the raw gas is mixed with a trace stream of He in the mixing chamber, and thereafter, the mixture is introduced into the reactor. The He tracer is used to derive the flow rate of the upgraded gas that exits the reactor. At the reactor outlet, there is a port for sampling the tar in the upgraded gas. For the tar sampling, the solid-phase adsorption (SPA) method is employed. The SPA method uses dual-layer, solid-phase extraction columns that contain a layer of aminopropyl-bonded silica and a layer of activated carbon (Supelclean ENVI-Carb/NH2 SPE tube; SigmaAldrich). It should be emphasized that the presence of the activated carbon layer allows efficient quantification of light tar components, such as benzene, toluene, xylene, and styrene, which are additional to the heavier components that can be captured efficiently by the aminopropyl-bonded silica layer. In total, aromatic tars ranging from benzene to coronene (in terms of boiling temperature) are efficiently captured by the SPA method used.24,63 The collected SPA tar samples are eluted, and further analyzed using a gas chromatograph with flame ionization detector (GC-FID). The detailed procedures for extracting, preserving and eluting the SPA samples, and the setup for GC-FID method for tar analysis are described elsewhere.24,63 After the conditioning step to remove steam and tar, the permanent gases in the upgraded gas are analyzed online every 3 min using a micro gas chromatograph (micro GC), which identifies CO, CO2, CH4, C2H2,

Figure 5. Concentrations of CO, CO2, CH4 and O2 (dry basis) at the reactor outlet during raw gas operation in a typical ilmenite experiment, which was given by the Rosemount NGA 2000 multicomponent gas analyzer to monitor experiment progress. E


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4. RESULTS AND DISCUSSION In Figures 6−11, from left to right, the silica sand experiments are arranged in order of increasing RT, and the ilmenite experiments are arranged in order of increasing CT. The carbon balance across the reactor, which comprises carbon from the permanent gases, tar, and the deposits on the bed material particles, is provided in the Supporting Information. 4.1. Changes in the Tar and Permanent Gases. In this section, the results for the changes in the tar and permanent gases, comparing the upgraded gas and the raw gas in the conducted experiments, are summarized. To evaluate the importance of RT and CT at the same temperature, the results are presented separately for the two investigated temperature levels and regardless of the bed materials used. 4.1.1. Tar Composition and Tar Decomposition Efficiency. Figures 6 and 7 show the tar composition and tar decomposition efficiency for the experiments conducted at 800 and 850 °C, respectively. In these figures, the pie charts are located to correspond to the illustrated scales of the RT and CT. The components included in the presented tar groups can be found in the Figures 10 and 11 below. In general, following the increases in RT and CT, increased percentages of naphthalene and benzene and decreased percentages of phenolic and oxygen-containing, 1-ring branched, and 2-ring branched compounds were observed (see Figures 6a and 7a). These changes were mainly due to the destruction of less-stable tars (e.g., phenolic and oxygen-containing, 1-ring branched, and 2-ring branched compounds) producing benzene and naphthalene, which are smaller and more stable. The similar observations have been reported in the literature.47,59,65 In I850C0.8 and I850C1.8, where the process severity was relatively higher, the degradation of naphthalene to produce, for example, benzene, became more significant, as evidenced by the decreased percentage of naphthalene and the considerable increase in the fraction of benzene (see Figure 7a). As the process severity was enhanced, the degradation of benzene could also occur to produce, e.g., permanent gases as earlier mentioned. In S800R3.3, S800R5.5, and S850R4, it is noteworthy that there was a clear increase in the percentage of 3 and 4-ring compounds, while the opposite trend was seen in the other experiments. The increase of 3- and 4-ring compounds most likely indicates that the growth of PAH tar from smaller tar compounds (i.e., polymerization of smaller tar compounds) took place, rather than that this increase was the result of the destruction of larger tar compounds. This is because the level of tars larger than 4 aromatic rings was negligible in the used raw gas as mentioned in the Experimental Section. Further details

Table 1. Main Operating Conditions for the Chalmers Gasifier bed material

silica sand

temperature (°C) fuel fuel flow rate (kg/h) steam flow rate for fluidization in the gasifier (kg/h) raw gas residence time (s)

820 wood pellets 300 160 ∼5

Israelsson et al.24,63 Thus, the tar spectrum (from benzene to coronene) measured by the SPA method applied in the present work was wider than the tar spectrum actually present in the raw gas. In total, all of the carbon-containing compounds present in the raw gas could be identified by the measurement systems used in the present work. The main operating conditions applied in raw gas upgrading experiments are shown in Table 2. Nine experiments involving raw gas upgrading were carried out in total, five of which were conducted with fresh silica sand and four with process-activated ilmenite. The particle grain sizes of the silica sand and ilmenite were 125−180 and 45−90 μm, respectively. Different process severities were investigated by varying the temperature, RT, and CT (see Table 2). The investigated temperatures were 800 and 850 °C. The temperature range of 800−850 °C was chosen, because this range is the most relevant to the operating temperature of fluidized bed gasifiers in practice, especially of dual fluidized bed gasifiers.7,24,53 The RT was calculated as the length of time that the gas resided in the empty part of the reactor (i.e., volume of the solid bed particles subtracted from the reactor volume). The CT was the length of time that the gas resided in the ilmenite bed, which was estimated following the formula previously presented by Lind et al.59 The RT values of the ilmenite experiments approximated those of the silica sand experiments, which were designed to distinguish the contributions of RT and CT to the changes in the tar and permanent gases. In Table 2, the names assigned to the silica sand experiments include the investigated temperature and RT levels, and the designations of the ilmenite experiments include the investigated temperature and CT levels. For example, the name “S800R3.3” indicates that the experiment was conducted with silica sand, at 800 °C, and with a RT of 3.3 s. In the same way, the name “I850C0.8” indicates that the experiment was conducted with ilmenite, at 850 °C, and with a CT of 0.8 s. Finally, it is noteworthy that, for presenting the results obtained from the conducted experiments, the yields of tar and permanent gases are presented per kg of dry and ash-free fuel (kgdaf fuel) fed to the gasifier, i.e., “g/kgdaf fuel” for tar and “mol/kgdaf fuel” for permanent gases. Thus, the increase in volume (dry basis) of the upgraded gas compared to the raw gas due to, for example, the WGS reaction, does not affect the comparison of the presented results. The tar decomposition efficiencies were estimated by comparing the tar yields in raw gas and in upgraded gas [((g/kgdaf fuel)raw gas − (g/kgdaf fuel)upgraded gas)/(g/ kgdaf fuel)raw gas].

Table 2. Main Operating Conditions Used in the Raw Gas Upgrading Experiments experiment

bed inventory

bed amount (g)

temperature (°C)

minimum fluidization velocity, umf (cm/s)

fluidization velocity, uo (cm/s)

RT (s)

CT (s)

S800R3.3 S800R5.5 S850R4 S850R6.9 S850R11.5 I800C0.7 I800C0.9 I850C0.8 I850C1.8

silica sand silica sand silica sand silica sand silica sand ilmenite ilmenite ilmenite ilmenite

200 200 200 200 200 300 300 300 300

800 800 850 850 850 800 800 850 850

0.89 0.89 0.86 0.86 0.86 0.25 0.25 0.24 0.24

10.42 6.32 8.77 5.06 3.01 10.27 7.22 8.90 3.07

3.3 5.5 4 6.9 11.5 3.4 4.8 3.8 11.2

0 0 0 0 0 0.7 0.9 0.8 1.8

F


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Figure 6. (a−b) Tar composition (%) and tar decomposition efficiency (%) for the experiments conducted at 800 °C. Detailed data for the tar composition are provided in the Supporting Information.

Figure 7. (a−b) Tar composition (%) and tar decomposition efficiency (%) for the experiments conducted at 850 °C. Detailed data for the tar composition are provided in the Supporting Information.

Figure 8. Molar yields of permanent gases for the experiments conducted at 800 °C.

Figure 9. Molar yields of permanent gases for the experiments conducted at 850 °C.

about the tar components present in the raw gas and upgraded gas and the observed increase of 3- and 4-ring compounds are discussed in section 4.2. The results for tar removal efficiency in Figures 6b and 7b reveal that in the ilmenite experiments only the CT contributed to the tar decomposition or its contribution was several orders

of magnitude higher than that of the RT. This was revealed by comparing the ilmenite experiments and the silica sand experiments that had the same RT values, i.e., comparing I800C0.7 and S800R3.3, I800C0.9 and S800R5.5, I850C0.8 and S850R4, and I850C1.8 and S850R11.5. Furthermore, even at the lowest CT value, considerably higher decomposition G


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Energy & Fuels efficiency was observed than for cases in which only the RT affected the tar decomposition, even with the highest RT level (i.e., comparing I800C0.7 and S800R5.5 and I850C0.8 and S850R11.5). The negative values of the decomposition efficiency in S800R3.3 and S850R4 could be due to, e.g., measurement error related to the tar yield, or that unsaturated light hydrocarbons in the raw gas participated in reactions with tar as earlier discussed, resulting in the increase of tar yield in the upgraded gas. 4.1.2. Changes in the Permanent Gases. Figures 8 and 9 compare the molar yields of permanent gases in the upgraded gas and raw gas for the experiments conducted at 800 and 850 °C, respectively. Reduced levels of C2−3 light hydrocarbons are clearly seen. C2H2 and C3H6 were converted almost completely in all experiments. For C2H4 and C2H6, their conversions were enhanced following the increases in the RT and CT. Furthermore, it is evident from the level of C2H4 that in the ilmenite experiments, CT contributed more significantly to the conversion of C2H4 than did RT. The decreased CH4 level was only observed in the ilmenite experiments conducted at 850 °C. The changes in levels of H2, CO, and CO2 comparing the upgraded gas to the raw gas were clearly evident, which was most likely due to the decomposition of tar and light hydrocarbons and the WGS reaction. Overall, the obtained results reveal that both the thermal effect (represented by RT) and catalytic effect (represented by CT) contribute to the changes in the tar and permanent gases. However, the catalytic effect predominates. 4.2. Fate of the PAH Tar. The increase in the fraction of 3and 4-ring compounds was observed in the silica sand experiments conducted at both temperature levels but not in the ilmenite experiments, as mentioned in section 4.1. Therefore, in this section, the results are presented separately for silica sand and for ilmenite, regardless of the investigated temperature levels. Figures 10 and 11 show the relative changes in the contents of the different tar components, comparing the upgraded gas to the raw gas for the silica sand experiments and ilmenite experiments, respectively. The presented tar components are those that were present at noticeable levels in the raw gas and in the upgraded gas. For reference, the yields in the raw gas of these tar components are provided in the Supporting Information. In the silica sand experiments, increases in 3- and 4-ring compounds, i.e., acenaphthylene, phenanthrene, anthracene, fluoranthene, pyrene, and chrysene, were observed, in which the increase in acenaphthylene was less significant (see Figure 10). Considerable increases in dibenzofuran and biphenyl were also seen. The more likely scenario is that the increases in dibenzofuran and biphenyl were also due to the combination of smaller tars. The results in Figure 10 imply that, when the reactor temperature was changed from 800 to 850 °C, there was a shift from dibenzofuran and biphenyl, which are less stable, to phenanthrene, anthracene, fluoranthene, pyrene, and chrysene, which are more stable. The increases in phenanthrene, anthracene, fluoranthene, pyrene, and chrysene were highest in S850R4. Following the increase in RT at 850 °C, the increases in the levels of these tar compounds were less prominent. Thus, as process severity was increased, the growth of PAH tar increased to a certain level and then decreased. All carbon at the reactor inlet was quantified as earlier mentioned. The results obtained from GC-FID analysis for the tar in the upgraded gas showed that tar compounds larger than those in

Figure 10. Relative changes in the contents of the different tar components, comparing the upgraded gas to the raw gas [(g/kgdaf fuel)/ (g/kgdaf fuel)] for the silica sand experiments.

Figure 11. Relative changes in the contents of the different tar components, comparing the upgraded gas to the raw gas [(g/kgdaf fuel)/(g/kgdaf fuel)] for the ilmenite experiments.

the raw gas were not formed. Furthermore, the carbon balances of the silica sand experiments reached almost 100% (see the Supporting Information). These together indicate that all of the carbon-containing products in the upgraded gas were quantified using the applied measurement techniques. In addition to that, the suppressed levels of phenanthrene, anthracene, fluoranthene, pyrene, and chrysene were not due to these tars being converted into larger products but more likely reflected that they were formed to a lesser extent at the higher RT. In contrast to the silica sand experiments, only slightly increases in H


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Energy & Fuels

Figure 12. Illustration of the qualitative relationship between the content of 3- and 4-ring PAH tar (i.e., phenanthrene, anthracene, fluoranthene, pyrene, and chrysene) in the upgraded gas and the process severity, as observed in the conducted experiments. That ilmenite experiments are located at the right of the “process severity” axis is mainly to indicate that the growth of 3- and 4-ring PAH tar was suppressed to a greater extent in ilmenite experiments than in silica sand experiments.

result in problems related to agglomeration of the silica sand bed, as well as a low rate of conversion of biomass due to lowlevel steam fluidization for a high RT value.7,25 If ilmenite is used instead of silica sand, a lower temperature (800 °C) and a shorter RT (3.4 s) are efficient, applied together with a CT value of about 0.7 s. Furthermore, as discussed in section 4.1, significantly higher levels of tar and permanent gas conversions are obtained when ilmenite is used due to its catalytic effect. The challenge with ilmenite is that the oxygen transport effect must be restricted to preserve the relatively high level of the heating value of the produced raw gas if this material is used in dual fluidized bed gasifiers as earlier mentioned. Nevertheless, a finding related to the results obtained for ilmenite is that the use of catalysts, even those with moderate catalytic activities, is an efficient way to reduce the growth of PAH tar during the tertiary conversion stage of steam gasification of biomass. Thus, using a naturally occurring catalyst as the bed material in commercial-scale fluidized bed biomass gasifiers is encouraged, despite that these materials possess lower catalytic activity than the synthetic catalysts. Particularly for dual fluidized bed gasifiers, the selection of bed material should consider the oxygen transport capacity of the material. If the oxygen transport effect is expected to be limited, using the naturally occurring catalysts with low oxygen transport capacity such as olivine appears promising.25

and suppressed growth of 3- and 4-ring PAH tar in the conducted experiments as a function of the applied process severity is qualitatively incorporated. The results obtained in the silica sand experiments alone are sufficient to prove the suggested relationship about the fate of PAH tar and the process severity. As earlier discussed in the Theory section, the increased growth of PAH tar observed in the conducted experiments could be explained by the fact that the applied process severity was not sufficient to convert steam and H2 into hydrogen reactive intermediates H*, and thereby, the combination of carbon-containing species was enhanced. As the process severity was sufficient to induce a sufficient level of H* in the reaction environment, the combination of carboncontaining species was prevented. Thus, the growth of PAH tar was suppressed. Using ilmenite, the formation of H* was favorable due to the presence of catalytic effect provided by ilmenite,21 which contributed significantly to suppressing the growth of PAH tar, even at lower temperature level (i.e., 800 °C), see Figure 12. 4.3. General Discussion and Application of the Obtained Results. Figure 12 outlines the principle for operating the gasifier with the goal of minimizing the formation of PAH tar for both silica sand bed and ilmenite bed. The process severities, which only trigger the reactions of carboncontaining species and are not sufficient to favor the production of H* from steam and H2, can induce the growth of PAH tar. For a given process, the critical level of process severity at which there is a sufficient level of H* to reduce the growth of PAH tar needs to be identified. Operating the gasifier at a process severity that is higher than the critical level is necessary to minimize the yield of PAH tar. For the reaction environment in which steam and H2 contents similar to those in the present work, to suppress the growth of PAH tar, an operating temperature of 850 °C and a RT of >11.5 s are required, if silica sand is used as the bed material in the gasifier. These operating conditions can easily

5. CONCLUSIONS The fate of PAH tar during the tertiary conversion stage of steam gasification of biomass in fluidized bed gasifiers was investigated in relation to process severity, which was influenced by the following operating conditions: temperature, gas residence time in the reactor, and the contact time between the gas phase and catalytic bed material. In the conducted experiments, a mature tar-containing raw gas produced in the Chalmers 2−4-MWth dual fluidized bed biomass gasifier was fed into a bench-scale, bubbling fluidized-bed reactor located downstream of the gasifier. Silica sand and ilmenite were used as the bed materials in this downstream reactor, and the process severity was varied to assess its effect on the changes in the tar and permanent gases. The employed approach ensured that the investigation represents the authentic environment of the steam gasification and the entire aromatic-tar spectrum of the raw gas is taken into account. From the obtained results, the following conclusions are drawn: • The comprehensive relationship between the fate of PAH tar and the process severity is elucidated. The growth of PAH tar is favored, if the process severity is sufficiently high to induce the interactions of carboncontaining species but is not sufficient to activate steam and H2. To limit the growth of PAH tar, it is essential that the process severity induces the conversion of steam and H2 into the reactive hydrogen intermediates that prevent combination of the carbon-containing species. • For operating the gasification process toward minimizing the yield of PAH tar, an operating temperature of 850 °C and a gas residence time of >11.5 s are required if the bed material in the gasifier is silica sand. If ilmenite is used instead, less-intense operating conditions (i.e., temperature of 800 °C, gas residence time of 3.4 s, and gas− solids contact time of 0.7 s) are efficient. In this respect, using naturally occurring catalysts as the bed material is

biphenyl and phenanthrene were noted in I800C0.7 and I800C0.9 (see Figure 11). Figure 3 in the Theory section is modified corresponding to the outcomes of the conducted experiments to formulate Figure 12. In Figure 12, the observation about the increased

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(10) Fuentes-Cano, D.; Gómez-Barea, A.; Nilsson, S.; Ollero, P. The Influence of Temperature and Steam on the Yields of Tar and Light Hydrocarbon Compounds during Devolatilization of Dried Sewage Sludge in a Fluidized Bed. Fuel 2013, 108, 341−350. (11) Speight, J. G. In The Chemistry and Technology of Petroleum, 3 ed.; CRC Press: New York, 1999; Chapter 12, pp 499−535. (12) Speight, J. G. In The Chemistry and Technology of Petroleum, 3 ed.; CRC Press: New York, 1999; Chapter 14, pp 565−584. (13) Speight, J. G. In The Chemistry and Technology of Petroleum, 3 ed.; CRC Press: New York, 1999; Chapter 16, pp 604−640. (14) Fahim, M. A.; Alsahhaf, T. A.; Elkilani, A. In Fundamentals of Petroleum Refining; Elsevier: Amsterdam, 2010; Chapter 7, pp 153− 198. (15) Fahim, M. A.; Alsahhaf, T. A.; Elkilani, A. In Fundamentals of Petroleum Refining; Elsevier: Amsterdam, 2010; Chapter 6, pp 123− 152. (16) Cypres, R. Aromatic Hydrocarbons Formation during Coal Pyrolysis. Fuel Process. Technol. 1987, 15, 1−15. (17) Nelson, P. F.; Hüttinger, K. J. The Effect of Hydrogen Pressure and Aromatic Structure on Methane Yields from the Hydropyrolysis of Aromatics. Fuel 1986, 65, 354−361. (18) Wanzl, W. Chemical Reactions in Thermal Decomposition of Coal. Fuel Process. Technol. 1988, 20, 317−336. (19) Jess, A. Mechanisms and Kinetics of Thermal Reactions of Aromatic Hydrocarbons from Pyrolysis of Solid Fuels. Fuel 1996, 75, 1441−1448. (20) Houben, M. P.; de Lange, H. C.; van Steenhoven, A. A. Tar Reduction through Partial Combustion of Fuel Gas. Fuel 2005, 84, 817−824. (21) Nguyen, H. N. T.; Berguerand, N.; Schwebel, G. L.; Thunman, H. Importance of Decomposition Reactions for Catalytic Conversion of Tar and Light Hydrocarbons: An Application with an Ilmenite Catalyst. Ind. Eng. Chem. Res. 2016, 55, 11900−11909. (22) Garcia, X. A.; Hüttinger, K. J. Steam Gasification of Naphthalene as a Model Reaction of Homogeneous Gas/Gas Reactions during Coal Gasification. Fuel 1989, 68, 1300−1310. (23) Nguyen, H. N. T.; Berguerand, N.; Thunman, H. Mechanism and Kinetic Modeling of Catalytic Upgrading of a Biomass-Derived Raw Gas: An Application with Ilmenite as Catalyst. Ind. Eng. Chem. Res. 2016, 55, 5843−5853. (24) Israelsson, M.; Berdugo Vilches, T.; Thunman, H. Conversion of Condensable Hydrocarbons in a Dual Fluidized Bed Biomass Gasifier. Energy Fuels 2015, 29, 6465−6475. (25) Berdugo Vilches, T.; Marinkovic, J.; Seemann, M.; Thunman, H. Comparing Active Bed Materials in a Dual Fluidized Bed Biomass Gasifier: Olivine, Bauxite, Quartz-Sand, and Ilmenite. Energy Fuels 2016, 30, 4848−4857. (26) Berguerand, N.; Berdugo Vilches, T. Alkali-Feldspar as a Catalyst for Biomass Gasification in a 2-MW Indirect Gasifier. Energy Fuels 2017, 31, 1583−1592. (27) Shen, Y.; Yoshikawa, K. Recent Progresses in Catalytic Tar Elimination during Biomass Gasification or Pyrolysis-A review. Renewable Sustainable Energy Rev. 2013, 21, 371−392. (28) Chan, F. L.; Tanksale, A. Review of Recent Developments in Nibased Catalysts for Biomass Gasification. Renewable Sustainable Energy Rev. 2014, 38, 428−438. (29) Corella, J.; Herguido, J.; Gonzalez-Saiz, J.; Alday, F. J.; Rodriguez-Trujillo, J. L. Fluidized Bed Steam Gasification of Biomass with Dolomite and with a Commercial FCC Catalyst. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Springer: Dordrecht, The Netherlands, 1988; pp 754−765. (30) Kinoshita, C. M.; Wang, Y.; Zhou, J. Tar Formation under Different Biomass Gasification Conditions. J. Anal. Appl. Pyrolysis 1994, 29, 169−181. (31) Stiles, H. N.; Kandiyoti, R. Secondary Reactions of Flash Pyrolysis Tars Measured in a Fluidized Bed Pyrolysis Reactor with Some Novel Design Features. Fuel 1989, 68, 275−282. (32) Churin, E.; Maggi, R.; Grange, P.; Delmon, B. Characterization and Upgrading of a Bio-Oil Produced by Pyrolysis of Biomass. In

encouraged, despite the fact that they are less catalytically active than synthetic catalysts.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.energyfuels.7b03558. Tar compositions (%) in the experiments conducted at 800 °C (data according to Figure 6a; Table S1); tar compositions (%) in the experiments conducted at 850 °C (data according to Figure 7a; Table S2); average yields of the different tar components in the raw gases used in the experiments (Figure S1); total carbon levels across the reactor (Figure S2). (PDF)

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AUTHOR INFORMATION

Corresponding Author

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

Huong N. T. Nguyen: 0000-0003-3081-7612 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was made possible by financial support from E. On and the Swedish Gasification Centre (SFC). Operation of the gasifier was supported by Göteborg Energi, Metso, Akademiska Hus, and the Swedish Energy Agency (Energimyndigheten). The authors thank research engineers Rustan Hvitt, Jessica Bohwalli, and Johannes Ö hlin for their valuable help with the experimental equipment. The authors also thank Måns Collin, Teresa Berdugo Vilches, and Jelena Maric for their valuable inputs to the paper.

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REFERENCES

(1) Milne, T. A.; Evans, R. J.; Abatzoglou, N. Biomass Gasifier “Tars”: Their Nature, Formation, and Conversion; National Renewable Energy Laboratory: Golden, CO, 1998. (2) E4Tech Review of Technologies for Gasification of Biomass and Wastes; NNFCC project 09/008; National Non-Food Crops Centre: United Kingdom, 2009. (3) Bridgwater, A. V. Renewable fuels and chemicals by thermal processing of biomass. Chem. Eng. J. 2003, 91, 87−102. (4) Li, C.; Suzuki, K. Tar Property, Analysis, Reforming Mechanism and Model for Biomass Gasification-An Overview. Renewable Sustainable Energy Rev. 2009, 13, 594−604. (5) Devi, L.; Ptasinski, K. J.; Janssen, F. J. J. G. A Review of the Primary Measures for Tar Elimination in Biomass Gasification Processes. Biomass Bioenergy 2003, 24, 125−140. (6) Evans, R. J.; Milne, T. A. Chemistry of Tar Formation and Maturation in the Thermochemical Conversion of Biomass. In Developments in Thermochemical Biomass Conversion: Vol. 1/Vol. 2; Bridgwater, A. V., Boocock, D. G. B., Eds.; Springer Netherlands: Dordrecht, The Netherlands, 1997; pp 803−816. (7) Larsson, A.; Seemann, M.; Neves, D.; Thunman, H. Evaluation of Performance of Industrial-Scale Dual Fluidized Bed Gasifiers Using the Chalmers 2−4-MWth Gasifier. Energy Fuels 2013, 27, 6665−6680. (8) Evans, R. J.; Milne, T. A. Molecular Characterization of the Pyrolysis of Biomass. 1. Fundamentals. Energy Fuels 1987, 1, 123−137. (9) Morf, P.; Hasler, P.; Nussbaumer, T. Mechanisms and Kinetics of Homogeneous Secondary Reactions of Tar from Continuous Pyrolysis of Wood Chips. Fuel 2002, 81, 843−853. J


Article

Energy & Fuels Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Springer Netherlands: Dordrecht, The Netherlands, 1988; pp 896−909. (33) Essig, M.; Lowary, T.; Richards, G. N.; Schenck, E. Influences of “Neutral” Salts on Thermochemical Conversion of Cellulose and of Sucrose. In Research in Thermochemical Biomass Conversion; Bridgwater, A. V., Kuester, J. L., Eds.; Springer Netherlands: Dordrecht, The Netherlands, 1988; pp 143−154. (34) Ledesma, E. B.; Marsh, N. D.; Sandrowitz, A. K.; Wornat, M. J. An Experimental Study on The Thermal Decomposition of Catechol. Proc. Combust. Inst. 2002, 29, 2299−2306. (35) Font Palma, C. Model for Biomass Gasification Including Tar Formation and Evolution. Energy Fuels 2013, 27, 2693−2702. (36) Bruinsma, O. S. L.; Moulijn, J. A. The Pyrolytic Formation of Polycyclic Aromatic Hydrocarbons from Benzene, Toluene, Ethylbenzene, Styrene, Phenylacetylene and N-decane in Relation to Fossil Fuels Utilization. Fuel Process. Technol. 1988, 18, 213−236. (37) Zanetti, J. E.; Egloff, G. The Thermal Decomposition of Benzene. J. Ind. Eng. Chem. 1917, 9, 350−356. (38) Badger, G. M.; Novotny, J. The Formation of Aromatic Hydrocarbons at High Temperatures. Part XII. The Pyrolysis of Benzene. J. Chem. Soc. 1961, 3400−3402. (39) Scheer, A. M.; Mukarakate, C.; Robichaud, D. J.; Ellison, G. B.; Nimlos, M. R. Radical Chemistry in the Thermal Decomposition of Anisole and Deuterated Anisoles: An Investigation of Aromatic Growth. J. Phys. Chem. A 2010, 114, 9043−9056. (40) Gräber, W.-D.; Hüttinger, K. J. Chemistry of Methane Formation in Hydrogasification of Aromatics. 1. Non-substituted Aromatics. Fuel 1982, 61, 499−504. (41) Gräber, W.-D.; Hüttinger, K. J. Chemistry of Methane Formation in Hydrogasification of Aromatics. 3. Aromatics with Heteroatoms. Fuel 1982, 61, 510−515. (42) Vreugdenhil, B. J.; Zwart, R. W. R. Tar formation in pyrolysis and gasification; Energy Research Centre of the Netherlands: The Netherlands, 2009. (43) Sikarwar, V. S.; Zhao, M.; Clough, P.; Yao, J.; Zhong, X.; Memon, M. Z.; Shah, N.; Anthony, E. J.; Fennell, P. S. An Overview of Advances in Biomass Gasification. Energy Environ. Sci. 2016, 9, 2939− 2977. (44) Debiagi, P. E. A.; Pecchi, C.; Gentile, G.; Frassoldati, A.; Cuoci, A.; Faravelli, T.; Ranzi, E. Extractives Extend the Applicability of Multistep Kinetic Scheme of Biomass Pyrolysis. Energy Fuels 2015, 29, 6544−6555. (45) Simell, P. A.; Hirvensalo, E. K.; Smolander, V. T.; Krause, A. O. I. Steam Reforming of Gasification Gas Tar over Dolomite with Benzene as a Model Compound. Ind. Eng. Chem. Res. 1999, 38, 1250− 1257. (46) Tamhankar, S. S.; Tsuchiya, K.; Riggs, J. B. Catalytic cracking of benzene on iron oxide-silica: catalyst activity and reaction mechanism. Appl. Catal. 1985, 16, 103−121. (47) Lind, F.; Seemann, M.; Thunman, H. Continuous Catalytic Tar Reforming of Biomass Derived Raw Gas with Simultaneous Catalyst Regeneration. Ind. Eng. Chem. Res. 2011, 50, 11553−11562. (48) Keller, M.; Leion, H.; Mattisson, T.; Thunman, H. Investigation of Natural and Synthetic Bed Materials for Their Utilization in Chemical Looping Reforming for Tar Elimination in Biomass-Derived Gasification Gas. Energy Fuels 2014, 28, 3833−3840. (49) Zhang, Y.; Kajitani, S.; Ashizawa, M.; Oki, Y. Tar Destruction and Coke Formation during Rapid Pyrolysis and Gasification of Biomass in a Drop-Tube Furnace. Fuel 2010, 89, 302−309. (50) Umeki, K.; Häggström, G.; Bach-Oller, A.; Kirtania, K.; Furusjö, E. Reduction of Tar and Soot Formation from Entrained-Flow Gasification of Woody Biomass by Alkali Impregnation. Energy Fuels 2017, 31, 5104−5110. (51) Dayton, D. A Review of the Literature on Catalytic Biomass Tar Destruction; National Renewable Energy Laboratory: Golden, CO, 2002.

(52) van Paasen, S. V. B.; Kiel, J. H. A. Tar Formation in a FluidisedBed Gasifiers: Impact of Fuel Properties and Operating Conditions; Energy Research Centre of the Netherlands: The Netherlands, 2004. (53) Larsson, A.; Israelsson, M.; Lind, F.; Seemann, M.; Thunman, H. Using Ilmenite to Reduce the Tar Yield in a Dual Fluidized Bed Gasification System. Energy Fuels 2014, 28, 2632−2644. (54) Leion, H.; Lyngfelt, A.; Johansson, M.; Jerndal, E.; Mattisson, T. The use of ilmenite as an oxygen carrier in chemical-looping combustion. Chem. Eng. Res. Des. 2008, 86, 1017−1026. (55) Min, Z.; Asadullah, M.; Yimsiri, P.; Zhang, S.; Wu, H.; Li, C.-Z. Catalytic Reforming of Tar during Gasification. Part I. Steam Reforming of Biomass Tar using Ilmenite as a Catalyst. Fuel 2011, 90, 1847−1854. (56) Schwebel, G. L.; Leion, H.; Krumm, W. Comparison of Natural Ilmenites as Oxygen Carriers in Chemical-Looping Combustion and Influence of Water Gas Shift Reaction on Gas Composition. Chem. Eng. Res. Des. 2012, 90, 1351−1360. (57) Adánez, J.; Cuadrat, A.; Abad, A.; Gayán, P.; Diego, L. F. D.; García-Labiano, F. Ilmenite activation during consecutive redox cycles in chemical-looping combustion. Energy Fuels 2010, 24, 1402−1413. (58) Berdugo Vilches, T.; Thunman, H. Experimental Investigation of Volatiles-Bed Contact in a 2−4 MWth Bubbling Bed Reactor of a Dual Fluidized Bed Gasifier. Energy Fuels 2015, 29, 6456−6464. (59) Lind, F.; Berguerand, N.; Seemann, M.; Thunman, H. Ilmenite and Nickel as Catalysts for Upgrading of Raw Gas Derived from Biomass Gasification. Energy Fuels 2013, 27, 997−1007. (60) Thunman, H.; Lind, F.; Breitholtz, C.; Berguerand, N.; Seemann, M. Using an oxygen-carrier as bed material for combustion of biomass in a 12-MWth circulating fluidized-bed boiler. Fuel 2013, 113, 300−309. (61) Corcoran, A.; Marinkovic, J.; Lind, F.; Thunman, H.; Knutsson, P.; Seemann, M. Ash Properties of Ilmenite Used as Bed Material for Combustion of Biomass in a Circulating Fluidized Bed Boiler. Energy Fuels 2014, 28, 7672−7679. (62) Marinkovic, J.; Seemann, M.; Schwebel, G. L.; Thunman, H. Impact of Biomass Ash-Bauxite Bed Interactions on an Indirect Biomass Gasifier. Energy Fuels 2016, 30, 4044−4052. (63) Israelsson, M.; Seemann, M.; Thunman, H. Assessment of the Solid-Phase Adsorption Method for Sampling Biomass-Derived Tar in Industrial Environments. Energy Fuels 2013, 27, 7569−7578. (64) Berguerand, N.; Marinkovic, J.; Berdugo Vilches, T.; Thunman, H. Use of Alkali-Feldspar as Bed Material for Upgrading a BiomassDerived Producer Gas from a Gasifier. Chem. Eng. J. 2016, 295, 80−91. (65) Berguerand, N.; Lind, F.; Seemann, M.; Thunman, H. Producer Gas Cleaning in a Dual Fluidized Bed Reformer-A Comparative Study of Performance with Ilmenite and a Manganese Oxide as Catalysts. Biomass Convers. Biorefin. 2012, 2, 245−252.

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