ARTICLE pubs.acs.org/EF
Gas Upgrading in a Downdraft Fixed-Bed Reactor Downstream of a Fluidized-Bed Coal Pyrolyzer Xi Zeng,† Yin Wang,*,† Jian Yu,† Shisheng Wu,‡ Jiangze Han,† Shaoping Xu,‡ and Guangwen Xu*,† †
State Key Laboratory of Multi-phase Complex Systems, Institute of Process Engineering, Chinese Academy of Sciences, Beijing 100190, People’s Republic of China ‡ State Key Laboratory of Fine Chemicals, Department of Chemical Engineering, Dalian University of Technology, Dalian 116012, People’s Republic of China ABSTRACT: A new two-stage gasification process, consisting of a fluidized-bed (FB) pyrolyzer and a downdraft fixed-bed (DFB) gasifier, has been proposed to gasify powder coal for fuel gas production with low tar generation. Aiming at developing the new technology, this paper investigated the means for coal pyrolysis gas upgrading, with the focus on tar removal by both thermal cracking with or without the presence of oxygen and catalytic cracking or reforming over a char bed in a fixed-bed reactor downstream of a FB coal pyrolyzer. The presence of oxygen and the adoption of a char bed evidently facilitated the tar removal performance and also improved the produced fuel gas quality. Analyzing the tar sample collected at the outlet of the tar removal reactor with gas chromatographymass spectrometry (GCMS) clarified that the oxidation by O2 and the char-catalyzed reforming both exhibited certain selectivity to the tar-containing chemical species. In addition, the property of char played an important role on its catalytic activity. The higher the specific surface area, the better the activity of char for removing tar. The spent char showed a much reduced specific surface area of micropores but evidently elevated the specific surface area of mesopores, even by up to 3 times. The tests with the metal oxides impregnated onto the demineralized char demonstrated that both Ca and Fe oxides enabled better catalytic activity for tar removal than Na and Mg oxides. This study clarified as well that the viable operating conditions for tar removal in a char bed were at 1100 °C with an excessive air ratio (ER) of 0.04 and gas residence time above 1.3 s.
1. INTRODUCTION Gasification is the core technology for clean and high-efficiency use of carbon-containing fuels. Through coal gasification, not only syngas but also industrial fuel gas can be produced. While the former is generally associated with chemical synthesis or integrated gasification combined cycle (IGCC), which requires large-scale, oxygenblown operation and high pressure, the latter has a wide range of variation in capacity and operates generally at atmospheric pressure using air as the gasification agent. Thus, the coal gasification for fuel gas adopts rarely high-pressure and high-temperature entrained flow gasifiers but employs air-blown fixed-bed or fluidized gasifiers. For fuel gas production by air gasifiers, their relatively lower operating temperature makes the tar contamination a problem, entailing serious consideration for suitable remedies.14 However, most of the tar removal technologies are complicated and not cost-effective for small- to medium-scale applications. As an exception, the two-stage gasification has been widely considered to be effective for low-tar gas generation and the technology allows for substantial tar removal inside the gasifier.5,6 A representative technology of this kind was developed by the Technical University of Denmark for gasifying biomass.6 The system consisted of an upstream screw pyrolyzer (externally heated) and a downstream downdraft fixed gasifier. Fuel was pyrolyzed first, and the generated pyrolysis gas and char were all forwarded to the fixedbed gasifier to allow for the tar in the gas to be cracked and reformed during its passing through the char/ash bed inside the gasifier. It was reported that the tar content in the produced fuel gas was below 50 mg N1 m3. By far, the two-stage gasification technology has been mainly applied to biomass for fuel gas production. Some r 2011 American Chemical Society
Figure 1. Conceptual diagram of the proposed new two-stage gasification process.
limitations, however, still exist for the technology using the externally heated screw pyrolyzer, including the difficulty in scale-up and the lack of flexibility in feedstock.7 To develop an effective technology for producing low-tar fuel gas from powder coal, biomass, and other carbon-containing granular fuels, the Institute of Process Engineering (IPE), Chinese Academy of Sciences (CAS), proposed a new two-stage gasification process, illustrated in Figure 1. The process consists of a fluidized-bed (FB) reactor for coal pyrolysis and a downdraft fixed-bed (DFB) reactor for char gasification and tar catalytic reforming and cracking over the char bed. The principle of Received: August 12, 2011 Revised: October 10, 2011 Published: October 17, 2011 5242
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Figure 2. Schematic diagram of the adopted experimental apparatus.
the process was detailed in our previous study.8 As a result from integrating the advantages of FB and DFB reactors, the new twostage gasification process is not only applicable to powder feedstock (e.g., powder coal) but also allows rather large treatment capacity. Surely, the performance of the DFB reactor is critical to the tar content in the final gas product,9 and it is highly necessary to clarify how to optimize the tar suppression capability in the DFB reactor. In principle, there are three possible contributions to the tar removal inside a gasifier: thermal cracking, oxidation (combustion), and catalytic reforming. The reaction temperature is critical for all three of these contributions, while the oxidation and reforming contributions are further related to the catalyst (such as char) and the contents of oxygen and other agents in the reaction atmosphere. Literature studies showed that high temperatures and a long reaction time are needed for effective thermal cracking of tar, such as at about 1300 °C in 10 s generally.10,11 The thermal cracking has to cause a large part of tar components, which are generally aromatics, to convert to agglomerated soot particles. The tar removal by partial oxidation was also investigated in the literature. The key issue is how to maintain the trade-off between the resulting gas quality and the tar removal degree.12 An excessive O2 supply would burn out the tarry species but result in bad gas quality and, thus, a low heating value. In testing the tar removal by partial oxidation, Hoeven et al.13 have found that free radicals played an important role in breaking the ring structure of aromatic hydrocarbons. The tar catalytic reforming has received great attention as well. Although various catalysts have been tested,14,15 the use of char as a catalyst has been considered to be most promising for practical application because of its nature of low cost and good integration with the char gasification.1618 The catalytic activity of char for tar elimination is closely related to the pore sizes and specific surface area of the char and the mineral species present in the char. There are many literature studies regarding char-catalyzed tar removal, but most were tested in batch reactors19,20 and adopted model compounds, including benzene, toluene, phenol, naphthalene, etc.2123 Furthermore, the chars used in most documented studies were commercial biomass activated carbon and rarely reflected the nature of fresh hot char inside the gasifier.16,24 There was almost no study conducted to correlate the tar reformation with the activities of
metal oxides present in the char, including alkali, alkali earth, and transition metals. Moreover, the variation of the gasification activity of the spent char after tar reforming also needs to be studied. With the aim at clarifying the optimal conditions and the achievable technical performance for the proposed new two-stage gasification process conceptualized in Figure 1, this work tested the upgrading effects for the gas from a FB coal pyrolyzer in a DFB reactor under conditions with varied temperatures, oxygen contents in the reaction agent, and gas residence times. The adopted experimental facility simulated the process of Figure 1, except that the char in the downstream DFB was not directly from the upstream pyrolyzer. The upstream pyrolysis was under conditions preset according to the results of a previous study.8 As a consequence, the work hopes to provide new insights into the fundamentals of tar removal via cracking, oxidation, and char-catalyzed reforming and also to facilitate the development of the new process illustrated in Figure 1. In the literature, there were in fact very limited fundamental studies that connected the fuel pyrolyzer and gas-upgrading reactor.
2. EXPERIMENTAL SECTION 2.1. Apparatus and Test Procedure. Figure 2 shows schematically the experimental two-stage gasification apparatus adopted in this study. It was a continuous testing rig and operated at atmospheric pressure. Both its FB pyrolyzer and DFB gasifier were electrically heated and had their independent reactant feeds. A screw feeder was adopted to feed fuel continuously into the FB pyrolyzer, while the char in the DFB gasifier was loaded before experiments. The tar collection system could be switched to fit the gases from the pyrolyzer and the gasifier to measure the tar contents in these two gas streams, respectively. The exhaust gas from the gasifier was burnt off in a CH4-fuled pilot fire. The reactor and test method for the pyrolyzer, the first stage, were described in a previous publication.8 The second-stage DFB reactor, 1200 mm in length and 50 mm in inner diameter, was made of corundum alumina and heated by a siliconite electric furnace (10 kW) to make it possibly work at 1400 °C. A porous plate mounted at 700 mm from the top of the reactor was used to support the bed material loaded into the reactor. A S-type thermocouple monitored the temperature inside the reactor. This study tested a kind of sub-bituminous coal from Xinjiang 5243
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Table 1. Characterization of the Tested JMSE Coal and Its Ash proximate analysis (wt %, ada)
coal analysis
ash analysis (wt %) a
ultimate analysis (wt %, dafb)
Mc
Ad
Ve
FCf
C
H
S
O
LHV (MJ/kg)
14.5
7.6
29.2
48.7
76.9
4.2
0.3
17.6
26.33
CaO
SO3
SiO2
Al2O3
MgO
Fe2O3
Na2O
P2O5
TiO2
19.7
22.3
18.6
13.9
6.2
7.1
6.2
3.3
2.7
ad = air dried. b daf = dry and ash free. c M = moisture. d A = ash. e V = volatile. f FC = fixed carbon.
Table 2. Product Distribution and Pyrolysis Gas Composition of the First Stage under the Identified Optimal Conditions for Pyrolysis yielda (wt %)
conditions
a
gas composition (vol %)
T (°C)
ER
S/C
char
gas
tar
H2
CO
CO2
CH4
C2H4
Ctarb (g N1 m3)
850
0.15
0.15
49.3
67.6
1.50
10.6
8.67
6.11
1.21
0.40
1.86
Yield = mass ratio of the product (char, gas, and tar) to dry-base coal. b Ctar = tar content in gas.
Jimusaer (JMSE) of China. Table 1 summarized the results of proximate and ultimate analyses for the coal and the X-ray fluorescence (XRF) analysis for its ash. The coal sample used in the tests was in size of 0.51.0 mm. Experiment was started with raising the empty-reactor temperature (no particles in and no gas feeding to the two reactors). When their preset values were reached, a gas mixture of N2, O2, and steam was fed into the pyrolyzer (first stage) to form the pyrolysis circumstance, while N2 was introduced into the downstream DFB to purge out any residual air and keep its atmosphere inert. Fuel pyrolysis was in turn performed by continuously feeding coal into the FB reactor. About 15 min was needed for reaching the steady state, and then a sufficient amount of char, taken out by overflow, was produced to form the char bed in the DFB. In this pyrolysis period, the gaseous product was directly exhausted through bypassing the DFB gasifier. In succession, if needed, a specified amount of the produced coal char was loaded through a hopper-like feeder into the second-stage DFB to form a char bed there (the first-stage was kept at its pyrolysis conditions). The two-stage gasification test was then implemented by letting the pyrolysis gas from the first stage, including noncondensable gas and tar species, enter the second-stage reactor. Dependent upon experimental design, the N2 flow into the DFB gasifier was turned off and/or switched into an oxygen flow. To prevent steam and tar from condensation, all pipelines between the two reactors were heated to 350 °C by electric ribbon heaters. After reaching their steady states in both the reactors, a sucked stream of the gas product was passed through a cooling system to collect tar and cool the gas. The noncondensable pyrolysis gas was further metered and passed through a silica gel column to remove its in-taking moisture and impurities, so that the cleaned gas can be analyzed in a gas chromatograph (Agilent Micro GC 3000A) for its composition. For this analysis, the gas was sampled with gas bags at a 1 min interval. The tar-containing liquid collected from condensation and washing the pipelines via acetone was treated through, in succession, dehydration in MgSO4 and acetone evaporation at 30 °C to quantify the generated tar amount and also to prepare the tar sample for gas chromatographymass spectrometry (GCMS) analysis. 2.2. Methods. The pyrolysis gas yield was estimated on the basis of the GC-analyzed gas composition by taking a well-controlled nitrogen stream as the tracer gas. The tar composition feature was analyzed using GCMS (HP 6890), for which a high-purity helium gas at 1.0 mL/min was adopted as the carrier gas. The typical operating conditions for GCMS were by an electronic impact ionization at 70 eV and a scan
per second over an electron (m/z) range of 40600 amu with an injection volume of 1.0 μL. Scanning electron microscopy (SEM; JEOL JSM-840) was used to observe the morphology of the unreacted and spent chars. The specific surface area and pore volume of the char were determined via N2 adsorption at 77 K in an automatic volumetric sorption analyzer (Quantachrome Autosorb-1, NOVA1200). Via uncatalyzed CO2 gasification at 1000 °C in a thermogravimetric analyzer (TGA; Nano S II 6300), the char gasification reactivity was evaluated following a literature-reported approach.25 The realized carbon conversion x and gasification activity R were calculated with eqs 1 and 2, respectively, where w0, wi, and wash are the initial, actual, and ash masses of the sample. x¼
w0 wi w0 wash
R ¼
ð1Þ
1 dwi dx ¼ w0 dt dt
ð2Þ
To clarify the catalytic activity of major inorganic matters in char for tar reforming, a kind of selected metal oxide was loaded on the demineralized char (charLM) with the impregnation method.26 The char free of ash was prepared by acid washing,27 and the impregnation was performed by dosing the demineralized char into an aqueous solution of the specified metal nitrate containing 5 wt % metal oxide [e.g., NaNO3, Mg(NO3)2, Ca(NO3)2, and Fe(NO3)3]. The resulting slurry was vibrated continuously by an ultrasonator at 30 °C for 12 h to enhance the dispersion of the metal species. Subsequently, the mixture was dried at 80 °C overnight and calcined at 700 °C to produce the metal-loaded char. The loaded metal oxide on the char was characterized with an X-ray diffraction (XRD) analyzer at 40 kV and 20 mA with a scanning rate of 4°/min. The XRD analyzer had a Philips PW 1710 X-ray generator and was fitted with a copper radiation source (λ1, 1.540 60 Å; λ2, 1.544 39 Å).
3. RESULTS AND DISCUSSION 3.1. Tar Formation in the First Stage. For the new two-stage gasification process, the performance of tar cracking and char gasification in the second stage significantly depends upon the properties of the tar and char formed in the first stage. Our previous study clarified the effects of the temperature, excessive air ratio (ER), and steam/coal mass ratio (S/C) on the properties 5244
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Figure 3. Tar content and gas composition under different operating conditions for the second stage (a) at different temperatures, (b) at 1100 °C with different ERs, and (c) at 1100 °C and ER = 0.04 with different gas residence times.
Table 3. Comparison of Pore and Surface Properties between the Fresh and Spent Chars Used for Tar Reforming Experiments preparation conditions
a
S/Ca
Atotal (m2/g)
Amicro (m2/g)
number
T (°C)
1
850
1c
850
2
850
0.15
285.0
2c
850
0.15
158.4
3
850
0.15
0.15
338.3
303.4
3c
850
0.15
0.15
222.3
120.2
ER
Ameso (m2/g)
5.593
5.521
19.02 249.9 98.80
Vtotal (mL/g)
Dab (nm)
0.0112
7.991
19.02
0.0335
2.659
35.10
0.1589
2.231
62.59
0.0898
2.269
34.90
0.1852
2.190
0.1284
2.311
102.1
S/C = mass ratio of steam to carbon. b Da = average diameter of pores. c Indicating the spent chars.
of the produced tar and char in the first-stage FB reactor.8 The optimal operating conditions were found to be at 850 °C, ER of 0.15, and S/C of 0.15. Under these conditions, the produced char had a large surface area and high gasification reactivity, while its corresponding tar was at a relatively low yield but highly reactive for cracking and reforming. For reference, Table 2 summarized the typical product distribution, pyrolysis gas, and tar compositions realized in the first stage under the identified optimal conditions. 3.2. Tar Removal in the Second Stage. Experiments were conducted in the second-stage reactor (gasifier) to examine the tar removal and produced-gas composition features via thermal cracking, partial oxidation, and char-catalytic reforming of the pyrolysis gas from the first stage. The pyrolysis of coal in the first stage was under the conditions specified in section 3.1. Figure 3a shows that, with increasing the thermal cracking temperature in the DFB reactor from 1000 to 1200 °C, the tar content in the produced gas quickly decreased from 0.76 to 0.51 g N1 m3. The corresponding gas composition was featured with increasing the H2 and CO fractions but decreasing the CH4 and C2H4 fractions. The content of CO2 tended to first increase and then remain steady at above 1100 °C. The main source of H2, CO, and CO2 was the thermal cracking reactions of tarry species, including alkanes and oxygen-containing heterocycles, and cyclodehydrogenation and aromatization of other aromatic compounds.28,29 Also, the watergas and Boudouard reactions would occur to increase the production of H2 and CO to some extent. The light hydrocarbon gases, such as CH4 and
C2H4, mainly came from the decompositions of methoxyl groups and aliphatic side chains at relatively low temperatures ( charLFe > charLNa > charLMg. Here, the charLM means the char loaded with the oxide of metal M. As for the produced gas, the effects for different metal oxides were not so distinctively different but there were generally higher fractions of CO and H2 corresponding to the higher tar removal, as exemplified by the gases corresponding to the chars loaded with the oxides of Ca and Fe. The action of calcium oxide may come from two aspects. As a divalent metal, Ca has been found to be a cross-linking agent, which makes the char more rigid and compact, thus, restricts the escape of large tar molecules, and enhances the polymerization reactions to lead to soot formation.41,42 The polarity of the active site in calcium
Figure 10. XRD spectra of different fresh and reacted chars: (a) fresh charLFe, (b) spent charLFe, (c) fresh charLMg, and (d) spent charLMg.
oxide can also affect the stability of the π-electronic cloud of the condensed aromatic compounds, thus accelerating the cracking of the condensed aromatics to lower the tar formation.43 It was reported that CaO well-accelerated the cracking of phenolic and other oxygen-containing compounds,44 which thus caused the higher fraction of CO. As a transition-metal catalyst, the oxide of Fe exhibited good catalytic activity for tar removal, but the XRD analysis of the Fe-loaded char (Figure 10) identified Fe3O4 in the spent char. The latter perhaps inhibited the catalyst activity of the iron oxide.45 It was suggested that Fe2O3 was easier to be reduced into Fe3O4 by volatile gas and syngas.4 The catalytic activity for Mg oxide was not so effective as the others, and this should be due to its lower activity to catalyze both partial oxidation and dehydrogenation reactions. Figure 10 reveals that the peak of MgO in the spent char was significantly high, suggesting some extent of sintering that perhaps occurred to lower its catalytic activity. Nonetheless, more systematic work is required to understand rather clearly the mechanism for the activity difference identified in Figure 9.
4. CONCLUSION To develop a novel two-stage gasification process conceptualized in Figure 1, this paper investigated the coal pyrolysis gas 5248
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Energy & Fuels upgrading, with its focus on tar removal in a DFB reactor downstream of a coal pyrolyzer by means of pure thermal cracking, partial oxidation, and char-catalyzed reforming. It was shown that both partial oxidation and char-catalyzed reforming were more effective for tar removal and gas upgrading in comparison to the thermal treatment only. Nonetheless, the oxidation and catalytic reforming exhibited certain selectivity in the removed tarry chemical species, while the partial oxidation greatly inhibited the formation of soot. The property of char, including the specific surface area, pore volume, and containing metal oxides, significantly affected its activity for tar removal. Comparatively, characterizing the fresh and spent chars clarified that both micro- and mesopores contributed greatly to the performance of the char for tar removal. The higher the specific surface area, the lower the tar content in the gas after catalytic reforming. The spent char had a much decreased specific surface area and pore volume of micropores, whereas the surface area of the mesopore increased even up to 3 times. Loading the oxides of Na, Mg, Ca, and Fe onto the demineralized char all increased the activity of the char for tar removal, but the realized activities followed an order of CaO > Fe2O3 > Na2O > MgO. This reveals essentially that the catalytic tar removal by char was due to not only the facilitated adsorption of tar on the char surface but also the catalysis effects from the inherent metal oxides inside the char. From the viewpoint of adapting to the new two-stage gasification process illustrated in Figure 1, the preceding results suggested that the viable operating conditions for maximizing the tar removal in the second-stage DFB gasifier are at a reaction temperature of about 1100 °C, an ER of about 0.04, and a gas residence time longer than 1.3 s.
’ AUTHOR INFORMATION Corresponding Author
*Telephone/Fax: +86-10-8254-4886. E-mail: wangyin@home. ipe.ac.cn (Y.W.);
[email protected] (G.W.X.).
’ ACKNOWLEDGMENT The authors are grateful for the financial support of The National Key Technology Development Programs (2009BAC64B05 and 2010BAC66B01), the National Basic Research Program of China (2011CB201304), the National High-Tech Research and Development Program of China (2009AA02Z209), the Foundation of State Key Laboratory of Coal Combustion (No. FSKLCC0910), the National Nature Science Foundation of China (21076217), and the National Nature Science Foundation of China (21006110 and 21006114). ’ REFERENCES
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