Experimental Investigation on Hydrogen Production by Anthracene

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Experimental Investigation on Hydrogen Production by Anthracene Gasification in Supercritical Water Hui Jin,* Shanke Liu, Wenwen Wei, Deming Zhang, Zening Cheng, and Liejin Guo State Key Laboratory of Multiphase Flow in Power Engineering, Xi’an Jiaotong University, Xi’an 710049, China ABSTRACT: Char conversion is the rate-determining step for coal gasification in supercritical water, because the main components of char are polycyclic aromatic hydrocarbons (PAHs) with chemical stability. Anthracene was selected as the model PAH compound to explore the gasification mechanism of coal in supercritical water. The experimental parameters investigated with autoclaves were within the following ranges: temperature, 600−750 °C; residence time, 5−30 min; and pressure, 23−25 MPa. The main gas products were H2 and CO2. The liquid intermediates were analyzed by gas chromatography−mass-selective detection (GC-MSD) for qualitative analysis and quantitative analysis, respectively. Benzene and naphthalene were found to be relatively stable compounds in the liquid products. Effective catalyst and appropriate temperature were obtained to enhance the gasification process. view to know about the reaction characteristics,10,28,29 such as desulfuration, denitration, and pyrolysis. Anthracene is the simplest polycyclic aromatic hydrocarbon (PAH) of formula C14H10, consisting of three fused benzene rings.30,31 It appears commonly in the coal chemical industry and the petrochemical industry, especially in the incomplete gasification process of coal gasification.32,33 Therefore, anthracene was selected as a model compound to explore the complete gasification method. The product distribution was investigated as the residence time increased, and stable intermediates were obtained. The influences of the catalyst and temperature were obtained.

1. INTRODUCTION China is abundant in coal resource and suffering from the heavy pollution caused by coal burning in the air. Coal gasification for hydrogen production is much cleaner for the environment than direct burning, because most the emissions are collected in the gasification process.1−3 Supercritical water gasification is a clean and effective method in converting coal with no nitrogen oxide or sulfur oxide emission. Supercritical water provides a homogeneous and rapid reaction environments for coal gasification, because of the following unique properties:4−6 (1) High solubility: The high solvent solubility of supercritical water can greatly reduce mass-transfer limitations and extract the coke precursor to prevent coking reaction by intermediates.7−9 (2) High reactivity: It is proved from the molecular dynamics point of view that the water clusters in supercritical water weaken the C−C bonds in aromatic rings to decrease the C(ring)−C(ring) bond cracking energy. Therefore, coal gasification reaction can conduct in relatively low temperature in supercritical water.10−12 (3) High dif f usivity: Coal is a complex polymeric material with a complicated porous structure,13−16 and supercritical water has near-zero surface tension17,18 and get easier access inside the porous structure of coal. Moreover, high density makes the intermediates diffused more efficiently to prevent unwanted polymerization reaction.19−21 Coal gasification is a two-step process: pyrolysis and char gasification. At the usual temperature of gasification process, the char gasification reactions are slower than the pyrolysis reactions.22−24 Then, the decomposition of the porous polycyclic aromatic structure is the rate-determining step for complete gasification and the gasification rate is relatively slow, because of the relative chemical stability of the polycyclic aromatic matter.25−27 Therefore, the elements affecting char gasification reactions are considered. Coal consists of thousands of organic compounds (mainly hydroxyl- and alkyl-substituted aromatic and heterocyclic rings) that are connected by methylene and ether linkage,2,15 and different model compound were selected for different point of © 2015 American Chemical Society

2. EXPERIMENTAL SECTION 2.1. Apparatus and Procedure. The supercritical water gasification reaction conducted in a high-throughput batch reactor system.34 The system contained six channels with the parameter of each reactor can be controlled individually. It can be also ensured that six experiments were conducted, as much as possible, under the same reaction conditions to avoid the influence of other parameters when the influence of a certain parameter was investigated. The reactor employed in this paper was Hastelloy C276 with a capacity of 10 mL. The designed temperature and pressure of each reactor are 750 °C and 30 MPa, respectively. A thermocouple and a pressure transducer monitored and recorded the temperature and pressure inside the reactor, respectively. The anthracene was measured and then loaded at the bottom of the reactor. Deionized water was injected into the reactor with a syringe. The reactor then was purged three times to ensure that the air was replaced by argon. The reactor then was filled with argon with an initial pressure of 5 MPa and placed into the electric furnace rapidly with the required temperature. The average heating rate of the fluid inside the reactor was ∼70 °C/min.34 The residence time was recorded as the period from the time when the fluid inside the autoclave reached the desired temperature to the time when the autoclave was removed from the oven to shut the reaction down. During the residence time, the fluid in the autoclave was kept at the desired temperature. After the reactor temperature was cooled to a Received: July 9, 2015 Revised: August 25, 2015 Published: September 2, 2015 6342

DOI: 10.1021/acs.energyfuels.5b01558 Energy Fuels 2015, 29, 6342−6346

Article

Energy & Fuels temperature below 50 °C, at an average rate of ∼200 °C/min,34 the gas was sampled and measured with a wet gas flow meter. The liquid product in the autoclave was first poured into a sample bottle. The autoclave wall then was washed five times with 1 mL of ethyl acetate, and each milliliter of the used ethyl acetate solution was poured into the sample bottle. An injection needle was used to pump 10 μL of the supernatant solution of the sample bottle to the GC/MS system. 2.2. Materials and Analytical Methods. An Agilent Model 6890GC-5973MSD system that was equipped with HP INNOWax chromatographic column was used to analyze the liquid intermediates qualitatively. The quantitatively analysis was conducted using the following settings: The initial temperature was 40 °C and hold 3 min, followed by a 10 °C/min ramp to 210 °C and held for 7 min, then increased to 250 °C and held for 5 min. High-purity (≥99.999%) helium was used as a carrier gas with a flow rate of 1.5 mL/min and a pressure of 10.72 psi. The injection port temperature was 250 °C. The injection method was splitless injection, and the injection amount was 2 μm. The conditions of the mass spectrometry analysis were as follows: EI ion source; ionization energy, 70 eV; ion source temperature, 230 °C. The temperature of the quadrupole was 150 °C. The scanning range was 15−500 amu. Three types of solvent were used to analyze the intermediates quantitatively; ethyl acetate has better extracting power for the typical intermediates than carbon tetrachloride and dichloromethane, so ethyl acetate was selected as the solvent in this paper. The typical liquid product analysis result is shown in Figure 1, and the main liquid products were determined to

3. RESULTS AND DISCUSSION 3.1. Effect of Residence Time. Figure 2a shows that, as the residence time increased from 5 min to 30 min, the main

Figure 2. Influence of residence time upon the gas product: (a) gas fraction and (b) carbon gasification efficiency and hydrogen yield. (Conditions: temperature, 700 °C; pressure, 23−25 MPa; concentration, 10 wt %.)

gas fractions remained constant. The main noncatalytic gas products were H2, CO, and CO2, and the fractions remained within the ranges of 46%−49%, 17%−21%, and 28%−33% respectively. Figure 2b shows that the carbon gasification efficiency and hydrogen yield increased almost linearly with residence time, and it proved that the gasification process proceeded gradually. The influence of residence time on the liquid product can be seen in Figure 3. It was obviously observed that the reactant

Figure 1. Typical liquid product analysis result by GC/MS. Legend: (1) benzene, (2) naphthalene, (3) 2-methyl-naphthalene, (4) 3phenyltoluene, (5) fluorine, (6) 9,10-dihydro-anthracene, and (7) anthracene. (Conditions: temperature, 700 °C; pressure, 25 MPa; concentration, 10 wt %; and residence time, 10 min.)

Figure 3. Influence of residence time upon the liquid product. Legend: (1) benzene, (2) naphthalene, (3) 2-methyl-naphthalene, (4) 3phenyltoluene, (5) fluorine, (6) 9,10-dihydro-anthracene, and (7) anthracene. (Conditions: temperature, 700 °C; pressure, 23−25 MPa; and concentration, 10 wt %.)

be benzene, naphthalene, 2-methyl-naphthalene, 3-phenyltoluene, fluorene, 9,10-dihydro-anthracene, and anthracene. Trace amounts of pyrene and fluoranthene were also detected in certain cases. The composition of the gaseous phase was analyzed using an Agilent Model 7890A gas chromatograph, which was equipped with a thermal conductivity detector and capillary column (Model C-2000, Lanzhou Institute of Chemical Physics, China).35 High-purity argon was used as a carrier gas with a flow rate of 30 mL/min. 2.3. Data Interpretation. CE and gas yield were selected to evaluate the gasification characteristics and defined as follows:36 CE (%) =

anthracene decreased as the residence time increased. The results indicated that 9,10-dihydro-anthracene appeared to be primary decomposition product, because the amount of 9,10dihydro-anthracene decreased as the residence time increased. The amount of 2-methyl-naphthalene and 3-phenyltoluene remained almost constant; the reason for this might be that the production rate was almost the same as the consumption rate. The amount of benzene and naphthalene increased with residence time and it was proved that the benzene and naphthalene were relatively stable intermediates and the production rate was higher than the consumption rate.37,38 Therefore, it can be concluded that the anthracene can decompose into substances with small molecules containing benzene rings effectively in supercritical water; however, as the residence time increased, benzene and naphthalene began to accumulate in the reactor and the decomposition of benzene

total carbon mass in the gaseous products × 100 total carbon mass in anthracene

gas yield (mol/kg) molar amount of a certain component of the gaseous products = mass of anthracene 6343

DOI: 10.1021/acs.energyfuels.5b01558 Energy Fuels 2015, 29, 6342−6346

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Energy & Fuels

Figure 4. Influence of catalyst upon the gas product: (a) gas fraction and (b) carbon gasification efficiency and hydrogen yield. (Conditions: pressure, 23−25 MPa; temperature, 700 °C; concentration, 10 wt %; potassium carbonate:anthracene mass ratio, 1:1; residence time, 10 min.)

the amounts of benzene and naphthalene were reduced by 25% and 15%, respectively. 3.3. Effect of Reaction Temperature. The effect of temperature upon anthracene gasification can be seen in Figure 6. When the temperature increased from 600 °C to 750 °C, the hydrogen yield increased from 14.70 mol/kg to 49.04 mol/kg, and the carbon gasification efficiency increased from 14.69% to 59.54%. The effect of temperature on the liquid product distribution are shown in Figure 7, and attention is paid to the appropriate temperature for gasification of the stable benzene and naphthalene. It can be seen that the yield of reactant anthracene and most of the main intermediates decreased with temperature, because high temperature favored the gasification reaction. However, the yield of naphthalene and benzene first increased with temperature, to reach a maxima, and then decreased. Because when the temperature was comparatively low, the production reaction rate was slow, compared to the decomposition rate, so the yield began to accumulate and became the bottleneck for complete gasification.45 As the temperature increased further, the decomposition rate began to become higher than the production rate; therefore, the yield decreased as the temperature increased. The appropriate temperatures for yield maxima of different intermediates were different. The appropriate temperature for benzene gasification in supercritical water was 700 °C, and the temperature for naphthalene was 650 °C. Besides the liquid intermediates, the carbon gasification efficiency is also an obvious judge of the gasification process. The reaction conditions and the carbon gasification efficiency are listed in Table 1. It can be deduced that long residence times and temperatures obviously enhanced the conversion of anthracene to gas product.

and naphthalene might be the rate-determining step of coal complete gasification.39 It was reported that phenols are important intermediates in the process of supercritical water gasification, because they may cause cross-link reactions to produce polycyclic aromatic hydrocarbon in the process of biomass/coal gasification in supercritical water.40,41 The method of ring opening may be the focus of further investigation. 3.2. Effect of Potassium Carbonate. Potassium carbonate was selected as a catalyst to discuss the effect of catalyst upon the distribution of the gas and liquid products. Potassium carbonate was proved to be an effective catalyst for coal gasification in supercritical water,34−36 Kruse conducted phenol conversion with potassium hydroxide as a catalyst,42 and it was also reported that potassium carbonate seems to force the formation of phenols by complex formation.43 Chen44 showed that the presence of the K atom reduced the net charge of the bridging C atom and enhanced the gasification process. Figure 4a shows that H2 and CO2 were also the main gas product, and their fractions were within the ranges of 49%−52% and 27%−29%, respectively. However, when the catalyst was present, the fraction of CO decreased from 20.42% to 3.03% and the CH4 fraction increased from 2.17% to 18.05%. The potassium carbonate apparently catalyzed the water−gas shift reaction and the methanation reaction. Figure 4b shows that the presence of potassium carbonate increased the hydrogen yield from 26.29 mol/kg to 31.25 mol/ kg. Figure 5 shows that the reactant anthracene and all the intermediates decreased with the addition of potassium carbonate. This observation proves that potassium carbonate could enhance the gasification process to a certain extent, and

4. CONCLUSIONS Polycyclic aromatic hydrocarbons (PAHs) are believed to be the bottleneck for complete gasification of coal and are the typical coal chemical pollutant. As the simplest PAH, anthracene was selected as the model compound to investigate the influences of the operating parameters upon the gasification results. The main conclusions obtained are as follows: (1) Analysis revealed that the main intermediates in the supercritical water gasification of PAH were benzene, naphthalene, 2-methyl-naphthalene, 3-phenyltoluene, fluorene, 9,10-dihydro-anthracene, and anthracene. The amount of benzene and naphthalene increased as the residence time increased, and they appeared to be the intermediates with the most chemical stability. (2) The method for the conversion enhancement of benzene and naphthalene was analyzed. The presence of potassium carbonate reduced the amount of benzene and naphthalene by

Figure 5. Influence of catalyst upon the liquid product. Legend: (1) benzene, (2) naphthalene, (3) 2-methyl-naphthalene, (4) 3-phenyltoluene, (5) fluorine, (6) 9,10-dihydro-anthracene, and (7) anthracene. (Conditions: pressure, 23−25 MPa;, temperature, 700 °C; concentration, 10 wt %; potassium carbonate:anthracene mass ratio, 1:1, residence time, 10 min.) 6344

DOI: 10.1021/acs.energyfuels.5b01558 Energy Fuels 2015, 29, 6342−6346

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Energy & Fuels

Figure 6. Influence of temperature upon the gas product: (a) gas fraction and (b) carbon gasification efficiency and hydrogen yield. Legend: (1) benzene, (2) naphthalene, (3) 2-methyl-naphthalene, (4) 3-phenyltoluene, (5) fluorine, (6) 9,10-dihydro-anthracene, and (7) anthracene. (Conditions: pressure, 23−25 MPa; concentration, 10 wt %; residence time, 10 min; potassium carbonate:anthracene mass ratio, 1:1.)

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Figure 7. Influence of temperature upon the liquid product. (Conditions: pressure, 23−25 MPa; concentration, 10 wt %; residence time, 10 min; and potassium carbonate:anthracene mass ratio, 1:1.)

Table 1. List of Reaction Conditions and the Carbon Gasification Efficiency (Liquid Product Concentration = 10 wt %, Pressure = 23−25 MPa) No.

residence time (min)

temperature (°C)

catalyst

carbon gasification efficiency (%)

1 2 3 4 5 6 7 8

5 10 20 30 10 10 10 10

700 700 700 700 700 600 650 750

none none none none K2CO3 K2CO3 K2CO3 K2CO3

20.63 35.51 44.44 55.83 37.76 14.69 25.56 59.54

25% and 15%, respectively. The appropriate temperature for benzene gasification in supercritical water was 700 °C, and the temperature for naphthalene was 650 °C.

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REFERENCES

AUTHOR INFORMATION

Corresponding Author

*Tel.: +86-29-82660876. Fax: +86-29-82669033. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was financially supported by the National Natural Science Foundation of China (Contract Nos. 51306145 and 51236007) and the National Basic Research Program of China (Contract No. 2012CB215303). 6345

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DOI: 10.1021/acs.energyfuels.5b01558 Energy Fuels 2015, 29, 6342−6346