1 Introduction
Owing to the environmental problems caused by fossil fuel use, renewable energy sources are being promoted as alternatives [
1,
2]. In particular, biomass energy has attracted attention because of its wide distribution but limited use due to its low energy density and inconvenient storage [
3]. Gasification is a mature technology in which a solid (coal or biomass) is partially oxidized in the presence of an oxidant (steam, air, or oxygen) to produce a combustible gas containing a small amount of coke, ash, and tars [
4]. Biomass gasification is a viable substitute to fossil fuels, which can reduce greenhouse gas emissions
Tar is a complex mixture of condensable hydrocarbons including monocyclic and polycyclic aromatic compounds (including complex ones) and other oxygen-containing hydrocarbons [
5,
6]. Tar formation is currently hampering the commercialization of biomass gasification because it fouls and erodes the process equipment [
7]. Tar components provide abundant chemical information for understanding thermochemical processes. Understanding the formation and conversion of tar components should help in finding ways to reduce tar, control its formation, and improve the biomass gasification efficiency. Many studies have been conducted to improve the understanding of migration of tar components from gasification. Zhang and Pang [
8] used radiation pine as the raw material and conducted experiments using a 100-kW dual fluidized-bed gasifier. The experimental results showed that, with increasing temperature, the concentrations of heterocyclic and light aromatic compounds within tar components decreased whereas those of light/heavy polyaromatic hydrocarbon compounds increased. However, compared with biomass devolatilization, the total tar concentration and tar yield from gasification decreased. Zhou et al. [
9] used lignin and cellulose extracted from tea leaves for gasification experiments. The results indicated that with increasing temperature, the concentrations of tar increased and the composition changed from monocyclic hydrocarbons to polycyclic aromatic hydrocarbons (PAHs). The main components in tar were phenols derived from lignin and ketones and aldehydes primarily derived from the breakdown of cellulose. Phenol and cresol were produced owing to lignin and cellulose, respectively; however, their formation pathways were different. Horvat et al. [
4] studied the influence of different factors on formation of tar components during gasification by using poultry manure as the raw material. The results demonstrated that the tar yield decreased for poultry litter blended with limestone as equivalence ratio (ER) varied, and it increased for poultry litter alone over the ER range tested. The concentrations of PAHs, benzene, toluene, and heterocyclic compounds tended to decrease over the ER range tested in the presence of limestone. Kirnbauer et al. [
10] conducted gasification experiments in a dual fluidized-bed gasifier and found that as the gasification temperature decreased, the concentrations of naphthalene and PAHs decreased whereas those of phenols, other aromatic compounds, and furans increased. Feng et al. [
11] found that in the absence of biochar catalyst at 800°C, tar components mainly comprised aromatic compounds with good thermal stability, like phenanthrene, fluorene, and indene. The catalyst caused gradual dissociation of fluorene and phenanthrene as well as partial reduction of the remaining tar components.
Dolomite is a cheap, abundant, natural, and nontoxic material capable of catalyzing the decomposition of tar components. It is a calcium magnesium rock with the general chemical formula [Ca,Mg(CO
3)
2], which can be used as a catalyst directly or after pretreatment by calcination [
12], and has been studied by many researchers for its catalytic activity. Yu et al. [
13] used dolomite from different locations as a catalyst to study its effect on the cracking of gasification tar components. The generated data suggested that all dolomites except Anhui dolomite effectively decomposed tar components into gases. Anhui dolomite exhibited a low catalytic capacity to crack tar components produced at 700°C and 800°C. Tian et al. [
14] also used different catalysts to study biomass gasification and found that compared to olivine and dolomite, calcined dolomite was more effective for gas production and decomposition of tar components. Cortazar et al. [
15] found that the reforming activity of dolomite was more effective in consuming PAHs in tar components than the cracking activity of the acidic catalyst. Miccio et al. [
16] studied the influence of different bed materials on the gasification reaction in a fluidized bed by using spruce wood pellets as the raw material and found that the use of dolomite as the bed material enhanced the gasification performance, improved the hydrogen content in syngas, and promoted the tar components’ conversion in light hydrocarbons.
Previous studies focused on the tar content evolution and tar classification but hardly focused on individual tar components. The present paper examined tar formation during gasification and the conversion of tar components to provide a theoretical basis for reducing tar content and improving gasification efficiency. Besides, it also investigated the influences of running parameters including temperature, gasification agents, bed materials, and ER on the content and components of tar.
2 Methods
2.1 Materials
Compressed corn straw was obtained from Jilin province, China. The raw material particles were cylindrical, with diameters of ~8 mm and lengths of 10 mm. The samples were dried naturally for 5 days before being crushed and sieved; the resulting average particle size was 6 mm. Table 1 lists the results of the elemental, proximate and three components analyses. Notably, the main components of lignocellulosic biomass are hemicellulose, cellulose, and lignin with small amounts of proteins, fats, extractives, and ash. Therefore, the sum of the total content of the three components cannot reach 100% in Table 1.
2.2 Experimental apparatus
The corn straw gasification experiments were conducted in an internal circulating fluidized-bed gasifier (ICFBG), which was designed and built using an actual fluidized bed gasifier (20 MW) at Datang Changshan Thermal Power Plant Co., Ltd. (China) as the prototype. The ICFBG with high thermal efficiency, simple operation, low air pollution, and compact structure is highly suitable for agricultural waste thermal treatment because agricultural waste, such as the corn straw used in this study, has a low energy density but a high transportation cost. Thus, the waste must be treated using an on-site method. Figure 1 shows the schematic of the system used for this purpose, which mainly comprises of a feeding system, a ventilation system, a reaction system, a sample collection system, and a data acquisition system. The height of the gasifier was 2900 mm. Nine measurement points for temperature and pressure were set along the height, and these parameters were tested using K-type armored thermocouples and U tube manometers, respectively. The feeding rate of the corn straw was controlled between 10 to 40 kg/h, and the water vapor required for the experiments was supplied by a water vapor generator with a maximum evaporation rate of 32 kg/h. Further details of the experimental system can be referred to in Ref. [
6]. A gas bag was used for collecting the gaseous products, and a portable infrared gas analyzer (Photon, Madur, Austria) was used to measure the compositions.
2.3 Collection and measurement of tar
The tar generated during the experiment was captured by five consecutive gas washing bottles. The first gas washing bottle was liquid-free; the next three contained dichloromethane; and the last one contained silicone for removing moisture. The solvent in the bottles enhanced heat transfer but, more importantly, facilitated the immediate dissolution of the tar components, thereby minimizing frit in the traps [
17]. The first four gas washing bottles were placed in an ice-water bath (0°C) for cooling. The solution in the fourth trap was used to wash the other traps after the experiments. The recovered tar solutions from the four bottles were mixed well and placed in a drying oven at 105°C for 4 h for removing dichloromethane, and their constituents were analyzed using a gas chromatograph-mass spectrograph (GC-MS, 6890N/5975, Agilent, USA) [
6].
The GC-MS contained an HP-5 quartz capillary column with helium carrier gas flowing at a constant rate of 0.9 mL/min. The initial column temperature was set at 60°C and maintained isothermal for 3 min; then, it was increased to 280°C at 10°C/min. Approximately 10 μL of tar samples were injected into the gas chromatograph without a shunt at 280°C, and the gas valve was turned on after 1 min. The temperature of the ion source was 230°C whereas that of the transfer line was 325°C. The data were collected in the full scan mode with mass-to-charge ratios (m/z) of 10–400. After the GC-MS tests, the chromatographic peaks were recognized using the National Institute of Standards and Technology (NIST) library mass spectrometry database to identify the components in the tar, and the relative content of the components within the tar was calculated based on their proportions accounted for in the total absolute peak area. In addition, a special tuning solution of perfluorobutylamine was used to adjust the molecular weight as well as the high, medium, and low ion abundance every week to ensure the sensitivity and accuracy of the equipment.
To better understand the general transformation of corn straw gasified tar, tar components were grouped according to the classification system reported in Refs. [
8,
11]. They were classified into five classes based on the number of aromatic rings and molecular weight. Class 1 refers to tar components that cannot be detected by GC. Class 2 refers to tar components having a heterocyclic structure and a high water solubility, such as pyrrole and pyridine. Class 3 refers to light hydrocarbons with a single ring, like benzene and toluene. Class 4 refers to light PAHs with 2–3 rings that condense at a low temperature even at very low concentration, such as acenaphthylene, anthracene, and fluorene. Class 5 refers to heavy PAHs with more than three rings, like pyrene and benzopyrene [
11]. Table 2 tabulates the tar components identified from the GC-MS analyses.
3 Results and discussion
3.1 Analysis of gasification characteristics of corn stalk
Table 3 summarizes the operating conditions and results of fluidized-bed corn straw gasification. The CO
2, CO, H
2, and CH
4 concentrations increase as ER changes from 0.21 to 0.31. However, at ER= 0.34, the CO
2 content increases whereas the CO, H
2, and CH
4 contents decrease. The overall tar content decreases as ER increases, which is consistent with Ref. [
18]. As the steam/biomass (S/B) ratio increases, the CO
2 and H
2 concentrations gradually increase whereas the CO and CH
4 concentrations as well as the tar content decrease, which is consistent with Tian’s findings [
14]. Further, a high dolomite mixing proportion (DMP) promotes corn straw gasification, which also agrees with Tian’s findings [
14].
3.2 Effect of temperature on tar components
Figure 2 demonstrates the variation of tar components with temperature at ER= 0.21. As the temperature increases, the contents of indene, phenanthrene, naphthalene, benzene, and pyrene increase, but the 1-methylnaphthalene and 2-methylnaphthalene contents are found to be inversely related to temperature. Further, the toluene and phenol contents decrease with increasing temperature, possibly because increasing temperature promotes radical reactions. Cortazar et al. [
19] demonstrated that radical reactions are vital in thermal cracking. As the temperature increases, the concentration of naphthalene increases whereas those of 1-methylnaphthalene and 2-methylnaphthalene decrease. Further, the concentrations of tricyclic and higher-ring aromatics (e.g., fluorene, phenanthrene, pyrene) increase with temperature because they have higher thermal stabilities than monocyclic aromatics. However, with increasing temperature, the toluene, phenol, 1-methylnaphthalene, and 2-methylnaphthalene concentrations decrease. This means that as the gasification temperature increases, the tar components shift from phenolic compounds and alkyl-substituted PAHs to nonsubstituted PAHs (i.e., more stable substances), as reported in Ref. [
20]. The benzene content increases with temperature, indicating the higher thermal stability of benzene at 900°C.
3.3 Effect of ER on tar components
Notably, the temperature also increases with the increase in ER (see Table 3). The reason for this is that the temperature of the gasifier is mainly controlled by ER and fuel feed rates [
21]. Therefore, the effects of ER are the co-synergetic result of the temperature and air volumes. The effects of ER on the gasification reaction were experimentally studied. ER is the stoichiometric ratio between the real air volume inside the reactor and the theoretic air volume when the fuel is entirely combusted [
6,
18]. As exhibited in Fig. 3, as ER increases from 0.21 to 0.34, the indene and phenanthrene concentrations increase from 0.148% and 0.087% to 0.232% and 0.223%, respectively. The likely reason for this is that as ER increases, the O2 content in the reactor also increases, which enhances the reaction, resulting in the breaking of C–O and C–H bonds [
6]. The occurrence of free H· and O· radicals [
7] promotes radical reactions through the formation of new chemical bonds. The phenol concentrations tend to increase as ER increases from 0.21 to 0.26 and then decrease as ER increases further to 0.34. The toluene concentration has a similar tendency to increase and then decrease. The reason for this is that, at ER= 0.26, the gasification degree increases, thereby increasing the toluene and phenol concentrations. Notably, although the O2 content increases with increasing ER, it mainly contributes to the formation of combustible gases, such as CO. There is no extra O2 for the depredation of tar components, and therefore, it does not lead to the decomposition of toluene and phenol. However, as ER increases from 0.26 to 0.34, the O2 content further increases and the shift from gasification to combustion in the boiler causes the dissociation of the branched chains of toluene and phenol [
22,
23], thereby producing –OH and –CH3 and decreasing the toluene and phenol concentrations.
3.4 Influence of S/B ratio on tar components
Figure 4 displays the variations of tar components with S/B ratio at ER= 0.21 and 700°C. As S/B ratio increases, the concentrations of class 4 tar components, including indene, phenanthrene, naphthalene, acenaphthylene, and fluorene, increase whereas those of 1-methylnaphthalene and 2-methylnaphthalene decrease (but less obviously). Further, as S/B ratio increases from 0 to 0.6, the pyrene content increases from 0.024% to 0.049%, respectively. The probable reason for this is that steam injection enhances the steam reforming reaction of tar [
6], thus leading to increased free radical concentrations and polymerization reactions between unsaturated hydrocarbons [
17,
24] as well as the H
2-extraction-C
2H
2-addition sequence reaction [
8,
24]. At S/B ratios of 0.4–0.6, the increase in pyrene concentration gradually reduces, probably owing to the formation of active H
2 intermediates promoted by the penetration of steam. This inhibits the bonding of carbon-containing components, thus impeding the formation of heavy PAHs [
6,
24].
3.5 Effect of catalyst on tar components
The addition of a catalyst promotes the catalytic degradation of tar. Figure 5 illustrates the evolution of measured tar components with DMP at ER= 0.21 and 700°C. As DMP increases, the concentrations of indene, phenanthrene, naphthalene, acenaphthylene, fluorene, and pyrene increase while those of benzene, phenol, 1-methylnaphthalene, and 2-methylnaphthalene decrease. This suggests that when dolomite was added as a catalyst, hydrodynamics improved its connection with tar components, thereby promoting the loss of the side or branched chains of the tar components (like classes 2 and 3) and further promoting polymerization. In contrast, CaO–MgO composites were formed with a better purity after dolomite calcining, thus increasing the number of active sites on the surface and enhancing the catalytic action of the dolomite [
6]. This promoted the breaking of C–C bonds as well as aromatic side chain, polymerization, alkylation, cyclization, and aromatization reactions [
25] to variable extents. When DMP increases from 0 to 50 wt%, the increase in pyrene concentration gradually reduced. The reason for this is that the formation of CaO–MgO compounds after dolomite calcining inhibited polymerization reactions between the tar components or promoted the bond breaking reaction of heavy PAHs (4–7 rings).
3.6 Formation mechanism of tar components
As an end product, tar components underwent cracking and polymerizing processes in a cracking cycle. Owing to the limitation of the GC-MS technique, Fig. 6 shows only part of the formation procedures of tar components of corn straw gasification in air-steam as the temperature increases. Functional groups like –OH and –CH
3 and benzene free radicals formed after bond breaking in monocyclic aromatic hydrocarbons (e.g., phenol and toluene) promoted polymerization to PAHs (e.g., naphthalene and biphenyl), thereby releasing gases including CO, CH
4, and H
2. For naphthalene, the steam reforming reaction produces an indene precursor, with some indene precursors undergoing bond breaking and hydrogenation to form indene and others polymerizing with pentacyclic compounds to form phenanthrene (see path (1) in Fig. 6). Biphenyls undergo a series of H
2-extraction-C
2H
2-addition sequence reactions to form pyrene (see path (2) in Fig. 6). Further, benzene and benzene radicals promote interaction and polymerization to form biphenyl, whereas methylnaphthalene produces naphthalene through dealkylation. Heavy tar components (class 5) are mainly formed by H
2-extraction-C
2H
2-addition sequence reactions and the steam reforming reaction, indicating that hydrogen or the hydrogen radical is the controlling factor in their formation. Vreugdenhil et al. [
18] found that during hydrogasification, H
2 reduces the decomposition rate tar components by reacting with the radicals produced, thereby forming a steady tar molecule and a H· radical and increasing the concentrations of stable PAHs. Two-ringed aromatic compounds with sidechains also significantly affect the formation of heavy tar components.
This is the first study that experimentally investigated the tar issue during corn straw gasification in an independently built ICFBG for the first time. The variation of tar components was probed, and the conversion paths (see Fig. 6) between the components were obtained.
4 Conclusions
This paper investigated the effects of running parameters like temperature, ER, catalysts, and S/B ratio on the development of gasification tar and the conversion processes of components using a bench-scale ICFBG with compressed corn straw. The results indicated that various operating parameters have unique effects on the variation of tar components. Overall, as the temperature, ER, DMP, and S/B ratio increased, the concentrations of benzene, indene, phenanthrene, naphthalene, acenaphthylene, fluorene, and pyrene tended to increase whereas those of toluene, phenol, 1-methylnaphthalene, and 2-methylnaphthalene decreased. Finally, a part of the mechanism of the formation and transformation of tar components during corn straw gasification was obtained.
Although this study has a great practical significance because it could enable reducing the equipment damage due to tar by providing a better understanding of the conversion process of tar components and their chemical properties in the gasification process, more comprehensive reaction pathways should be probed in the future.
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