Gas Cleaning and Hydrogen Sulfide Removal for COREX Coal Gas by

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Gas Cleaning and Hydrogen Sulfide Removal for COREX Coal Gas by Sorption Enhanced Catalytic Oxidation over Recyclable Activated Carbon Desulfurizer Tonghua Sun,† Yafei Shen,†,‡ and Jinping Jia*,† †

School of Environmental Science and Engineering, Shanghai Jiaotong University, 800 Dongchuan Road, Shanghai, 200240, P. R. China ‡ Department of Environmental Science and Technology, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, G5-8, 4259 Nagatsuta, Midori-ku, Yokohama, 226-8502, Japan S Supporting Information *

ABSTRACT: This paper proposes a novel self-developed JTS-01 desulfurizer and JZC-80 alkaline adsorbent for H2S removal and gas cleaning of the COREX coal gas in small-scale and commercial desulfurizing devices. JTS-01 desulfurizer was loaded with metal oxide (i.e., ferric oxides) catalysts on the surface of activated carbons (AC), and the catalyst capacity was improved dramatically by means of ultrasonically assisted impregnation. Consequently, the sulfur saturation capacity and sulfur capacity breakthrough increased by 30.3% and 27.9%, respectively. The whole desulfurizing process combined selective adsorption with catalytic oxidation. Moreover, JZC-80 adsorbent can effectively remove impurities such as HCl, HF, HCN, and ash in the COREX coal gas, stabilizing the system pressure drop. The JTS-01 desulfurizer and JZC-80 adsorbent have been successfully applied for the COREX coal gas cleaning in the commercial plant at Baosteel, Shanghai. The sulfur capacity of JTS-01 desulfurizer can reach more than 50% in industrial applications. Compared with the conventional dry desulfurization process, the modified AC desulfurizers have more merit, especially in terms of the JTS-01 desulfurizer with higher sulfur capacity and low pressure drop. Thus, this sorption enhanced catalytic desulfurization has promising prospects for H2S removal and other gas cleaning. by strict government regulation.1 To remove H2S from coalderived gas, several metal oxides have been studied for the development of general sorbents in various temperature ranges under highly reducing conditions.2−6 COREX coal gas is frequently characterized by large volume (180 000 N m3/h), low concentration of hydrogen sulfide (150 ppm in usual), and low pressure (≤17 kPa). Therefore, the key success of this project is taking measures to improve the sulfur capacity of desulfurizer, reduce the dosage of desulfurizer, and decrease the pressure drop of the facility. Activated carbons (AC) are widely used as adsorbents of gases and vapors, catalyst supports, and separation media.7,8 Their features useful for pollutant removal are large surface area, rich microporosity,9 and rapid adsorption velocity.10−13 The selective abilities on the surface of AC can adsorb the H2S rapidly from coal gas stream via physical forces or chemical reaction.14,15 The objective of this study was to present a novel self-developed JTS-01 desulfurizer and JZC-80 alkaline adsorbent for gas cleaning of the COREX coal gas in

1. INTRODUCTION As a large-scale industrial facility with the merit of environmental friendliness, COREX processing has been successively developed and applied by the iron-making factories in South Africa (Lscor, COREX-C1000, 1989), Korea (POSCO, COREX-C2000, 1995−1999), India (Jindal, COREX-C2000, 1999−2001), and China (Baosteel, COREX-C3000, 2007); it has been the largest iron-making device by the melting reduction process up to now. COREX has some advantages such as short process and low pollution, so it is creating a revolution with respect to conventional blast-furnace ironmaking technologies. This frontier technology has been attracting more and more attention by iron-making industries in current society. Figure 1 shows the COREX process flow sheet. Another merit of COREX is ability to produce abundant coal gas which comes from part melting gasifier and part top gas. Inevitably, COREX coal gas includes certain amounts of hazardous hydrogen sulfide (H2S). H2S in the COREX coal gas usually corrodes equipment and pipeline in the follow-up process; on the other hand, H2S can be oxidated into sulfur oxides (e.g., SO2 or SO3), which are known as a precursor of acid rain after combustion and whose emission into the atmosphere is limited © 2014 American Chemical Society

Received: Revised: Accepted: Published: 2263

November 6, 2013 January 16, 2014 January 23, 2014 January 23, 2014 dx.doi.org/10.1021/es4048973 | Environ. Sci. Technol. 2014, 48, 2263−2272

Environmental Science & Technology

Article

Table 1. COREX Coal Gas Physicochemical Characteristics and Technical Parameters technical Parameters (dry gas, standard state) gas processing capacity (N m3/h)

169 000 (design point) 181 000 (maximum value) inlet gas pressure (kPa) 14−17 gas pressure drop (kPa) ≤4 gas temperature (°C) 40 dry gas composition (bulk, %) CO2 33.17 CO 45.23 H2 17.72 CH4 1.68 N2/Ar 2.2 O2 100−500 ppm H2S concentration in desulfurizer (ppm) inlet

Figure 1. Schematic diagram of COREX process flow sheet.

outlet

the small scale and commercial desulfurizing facilities at a low pressure, and micro-oxygen in coal gas worked as the oxidant. The large Brunauer−Emmett−Teller (BET) surface area of AC was favorable for loading the catalytic metal oxides onto its surface,16−19 preparing for the JTS-01 AC desulfurizer with synergistic effects of adsorption and catalysis. First, H2S can be absorbed on AC surfaces, while it is oxidized into sulfur (S) by the superficial catalyst without additional water pollution.20 To improve the sulfur capacity of desulfurizer, the current study utilized ultrasonically assisted impregnation, contributing to the increase of the loaded dosage of catalyst and sulfur capacity. Meanwhile, the other purities in coal gas (i.e., acidic gases and dusts) can also be pretreated and removed using the largegrained AC impregnated with sodium carbonate (Na2CO3). Significantly, the big particle size of AC (diameter: 4 mm, columnar) was selected as the optimal desulfurizer support, while the desulfurizing facility employed the parallel connection with the large-diameter towers to reduce the system pressure drop. In practice, the self-developed JTS-01 desulfurizer and JZC-80 alkaline adsorbent have been successfully used in the commercial plant at Baosteel Corporation in Shanghai, China. Hence, it is noteworthy for us to publish this H2S removal technology via sorption enhanced catalytic oxidation by AC desulfurizer, which could be widely applied in dry desulfurization processes.

154 (normal value) 185 (maxmium value) ≤10 other impurity (ppm)

HCl HF NH3 HCN BTX (benzene, methylbenzene, dimethybenzene) PAH (aromatic) ash

∼1.50 ≤0.10 ∼2 ∼0.30 ∼500 ∼40 ≤5

impurity gases including HCl, HCN, NH3, H2S, and some organic gases (e.g., “polyaromatic hydrocarbons”-PAHs) can bring about environmental pollution and machine corrosion. Based on this special desulfurization apparatus, H2S removal efficiency and gas pressure drop controlled below 4 kPa are the primary studied parameters. 2.2. Preparation of JTS-01 Desulfurizer and JZC-80 Adsorbent. Preparation of JTS-01 Desulfurizer. The preparation method of JTS-01 AC desulfurizer is shown in Scheme 1. Phthalocyanine metal complexes are structurally related to porphyrin complexes, which are widely used in the active sites for catalytic oxidation.21 The method of preparation of the supported catalyst can influence the catalyst structure in terms of distribution of the active sites on the surface and their accessibility and of the state of the complex. Consequently, the appropriate choice of the method has primary importance for obtaining active and selective catalysts. In this study, the trace amount of activator (sulfonated cobalt phthalocyanine, etc.) was initially mixed into the prepared mixed solution (Fe(NO3)3:Na2CO3 = 2:1, mass ratio). Then, the prepared φ4.0 × (3−8) mm AC was added into the 10% mixed solution (proportion: 1 g/mL) for incipient wetness impregnation. Each impregnation time was 2 h. After complete impregnation, the AC was treated by ultrasonic and vacuum filtration. Subsequently, the AC would be dried in the oven at 120 °C until constant weight. At last, the dried AC desulfurizer was put into the tube furnace and heated at 450 °C until no more NOx release. During the period of temperature decrease, N2 could be continued until the furnace temperature was close to room temperature. Preparation of JZC-80 Adsorbent. The φ6.0 × (4−10) mm AC was added into 5% NaOH solution (proportion: 1 g/mL) at room temperature. After impregnation for 5 h close to

2. EXPERIMENTAL SECTION 2.1. Experimental Setup and Chemicals. Experimental Apparatus. Ultrasonic generator (KH-50B, power: 50 W, ultrasonic frequency: 40 kHz), tube furnace (SK2−1−10, Φ × L = 35 × 300 mm2, Shanghai Techeng Mechanical Equipment Ltd. Co.), drying oven (GZX-9070 MBE, digital display air-dry oven, Shanghai Boxun BSQT), vacuum pump (SHB-IIIA, Shanghai Yukang Educational Instrument Ltd. Co.). AC and Chemical Reagents. Fe(NO3)3, sulfonated cobalt phthalocyanine, NaOH, Na2CO3 (Shanghai National Chemical Reagent Co.), φ4.0 × (3−8) mm and φ6.0 × (4−10) mm activated carbons (Ningxia Baotower Activated Carbon Ltd. Co.), H2S (≥99.9%, Shanghai Baolaite Gas Ltd. Co.). COREX Coal Gas. COREX coal gas consists of carbon dioxide (CO2, 33.17%), carbon monoxide (CO, 45.23%), hydrogen (H2, 17.72%), methane (CH4, 1.68%), nitrogen (N2, 2.2%), and other trace gaseous (Table 1). Commonly, the other 2264

dx.doi.org/10.1021/es4048973 | Environ. Sci. Technol. 2014, 48, 2263−2272


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Scheme 1. Procedure of JTS-01 AC Desulfurizer Preparation

according to the National Standard of China (GB/T7702.22− 1997), and the saturated sulfur capacity of desulfurizer was measured according to the National Standard of China (GB/ T7702.14−1997). The surface characteristics of AC desulfurizer were detected by scanning electron microscope “SEM” (FE-SEM, Sirion 200, Netherlands) and energy dispersive spectrum “EDS” (INCA, X-Act, England), respectively.

saturation, the AC alkaline adsorbent was taken out and dried at 105 °C for 2 h. Lab-Scale Test. A lab-scale adsorption facility shown in Figure 2A was used to study the H2S adsorption properties of the JTS-01 desulfurizer. The experimental gas was the synthetic model gas, including two pathways for 99.99% H2S pure gas and low-velocity air by air pump. The model H2S mixture gas passed through a designed desulfurization reactor (flow rate: 500 mL/min, room temperature). Ultimately, the off-gas was absorbed and purified by NaOH solution. 2.3. Large-Scale Desulfurization Setup. Figure 2B shows the desulfurizing apparatus with five parts including the coal gas inlet, compressed air pipeline, desulfurizing tower (Φ7.4*12.9 m), operating instrument, and coal gas outlet. The loaded sorbent and desulfurizer in each tower were the same quality and mass fraction. JZC-80 adsorbent and JTS-01 desulfurizer were placed in the bottom layer and top layer of each desulfurizing tower, respectively. One of the five towers can be reserved, and the others are used for the desulfurization process. In the inlet, the air supply device was installed. After detection of H2S and O2 concentration in coal gas, the PLC can supply air automatically to ensure the stable ratio of H2S and O2 (H2S:O2 = 1:2). 2.4. Analytical Methods. The H2S concentration of inlet and outlet were determined by the portable gas chromatograph (GCRAE 1000, PID). The industrial H2S concentration of inlet and outlet were measured by the H2S Online Analyzer purchased from Triastoria Group (TTC). The gas flow was recorded by the Annubar flowmeter, and the gas pressure was measured by the YN-series of seismic pressure gauges (Y-100BFZQ) purchased from the Fourth Factory of Automation Instruments, Shanghai. The strength test of desulfurizer was measured according to the National Standard of China (GB/T7702.3−1997), the breakthrough sulfur capacity of desulfurizer was measured

3. RESULTS AND DISCUSSION 3.1. Effect of Ultrasonic Time on the Loading Capacity of Catalysts on AC. Ultrasonic can cause mechanical vibration and speed up the mass transfer rate of internal liquid by a physical effect. The reinforcement effects of mixed liquid phases, solute dissolution, and diffusion can be significantly improved by the ultrasonically assisted method.22,23 The experimental results show that the loading capacity of catalyst precursor on the surface of AC was greatly improved by the increase in ultrasonic time. After 20 min, the increasing trend of loading capacity became slower, while no apparent improvement in the loading capacity of catalyst without ultrasonically assisted impregnation could be observed. In summary, the loading capacity of catalyst precursor with ultrasonic was greater than 6 times that without ultrasonic (Figure 3A). Furthermore, it can be concluded that ultrasonically assisted impregnation has a positive effect on catalyst loading on the surface of AC. 3.2. Effect of Ultrasonic Time on Sulfur Saturation Capacity. Obviously, ultrasonically assisted impregnation can improve the loading capacity of catalysts on the surface of AC, to improve the H2S removal efficiency of desulfurizer. It can also be observed that as the loading capacity of catalysts increased from 1.5% to 3.5%, the sulfur saturation capacity of desulfurizer improved rapidly. However, when the ultrasonic time exceeded 30 min, the sulfur capacity showed a decreasing trend (Figure 3B). It is possible that the excessive loading 2265



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Figure 2. Schematic diagrams of (A) the lab-scale setup and (B) the large-scale experiment. Vij means the gas valve; raw coal gas enters this apparatus from V1, and clean gas exits from V3.

capacity may cause a decrease of the BET surface area for desulfurizer. Meanwhile, when the loading capacity exceeded 3.5%, the desulfurizer strength apparently decreased. From the analytical results of the ultrasonically assisted impregnated desulfurizers, the strength of desulfurizer after ultrasonic for 15 min decreased by 5.6% compared with the directly impregnated desulfurizer, while the strength after ultrasonic for 40 min decreased by 22.3%. After overall consideration, the ultrasonic time was selected to be 25 min. Compared with direct impregnation without ultrasonic assist, the sulfur saturation capacity improved more than 30.3% via ultrasonic for 25 min (Figure 3B). 3.3. Effect of Ultrasonic Assist on Sulfur Breakthrough Capacity. This work also investigated the sulfur breakthrough capacities of raw AC, directly impregnated desulfurizer, and ultrasonic assist for 25 min desulfurizer. Figure 3C shows the sulfur breakthrough capacity of three materials, where C0 refers to the inlet H2S concentration and C refers to the outlet H2S concentration. When the outlet H2S concentration approached 10 × 10−6 (volume fraction), breakthrough of desulfurizer was indicated. It could be found that the raw AC breakthrough was after 20 min, while the breakthrough time of the directly impregnated desulfurizer reached about 340 min. This

indicated that the H2S removal process is a priority of chemical desulfurization, rather than adsorption only. Significantly, the breakthrough time of 25 min ultrasonic assist for desulfurizer could reach 435 min. Thus, ultrasonically assisted impregnation can improve the desulfurizer breakthrough capacity by 27.9%. The BET surface area of the desulfurizer can significantly influence the adsorption performance and sulfur capacities. From Figure 3D, it can be seen that the BET surface area of the unmodified AC was 829.639 m2/g, while it decreased to 44 m2/ g and 80 m2/g by direct static impregnation and ultrasonically assisted impregnation, respectively. This indicated that the loaded metals will block the micropores on the surface of AC. However, these abundant micropores in the AC desulfurizer can provide the space for desulfuring reactions, so the blocking of pores has a negative effect on desulfurization efficiency. In other words, the lower BET surface area means the ultrasonically assisted impregnation can improve the loading amount of metal sites. The small differences in BET surface areas accompanied by the significant differences in the sulfur saturation capacities could be observed among the unmodified AC and directly impregnated and ultrasonically assisted impregnated AC desulfurizers, indicating that the predominate desulfurization process is catalytic oxidation. In addition, the 2266



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Figure 3. Effect of ultrasonic time on (A) the concentration of metal oxides loaded on AC (impregnation time: 120 min, thermolysis at 450 °C, N2 atmosphere) and (B) saturated sulfur capacity, (C) effect of ultrasonically assisted on the breakthrough time of desulfurizer, and (D) BET surface areas of raw AC, AC desulfurizers with or without ultrasonically assisted impregnation, and industrial fresh and waste AC desulfurizers.

Table 2. Elemental Analyses of the Selected Points by SEM-EDSa fresh desulfurizer spectra 1

a

waste desulfurizer spectra 2

spectra 1

spectra 2

spectra 3

element

W

A

W

A

W

A

W

A

W

A

C O S Si Na Al Ca Fe

65.50 27.16 0.44 0.08 0.73 0.06 4.96

74.35 23.15 0.19 0.04 0.44 0.03 1.21

80.21 16.56 1.07 0.39 1.09 0.15 0.24 0.18

85.32 13.22 0.43 0.18 0.61 0.07 0.08 0.04

75.19 8.32 13.74 0.86 0.49 0.55 0.42 0.43

85.76 7.13 5.87 0.42 0.29 0.28 0.14 0.10

52.43 42.54 2.26 1.21 1.55

74.68 22.70 1.38 0.77 0.48

60.09 5.72 28.50 1.85 0.53 1.12 1.20 0.98

77.84 5.56 13.83 1.02 0.36 0.64 0.47 0.27

W − weight (%), A − atomic (%).

large-grained AC was favored for removing the organic impurities just like PAHs. JTS-01 desulfurizer (Φ4 × 3−8 mm, columnar) was supported by the large-grained AC loaded variety of metal oxide catalysts (i.e., ferric) and activator (i.e., sulfonated cobalt phthalocyanine). In the presence of oxygen, it could directly oxidize H2S into elemental sulfur (S). The characteristics of JTS-01 desulfurizer have some obvious merits, e.g., high sulfur capacity (40−60%), lower dosage (3/4 of common AC desulfurizer), and lower loading and unloading charges. Each desulfurizing tower was loaded with 9.4 × 104 kg of the JTS-01 desulfurizer. As for the COREX coal gas desulfurizing project at

BET surface area of AC desulfurizer can be significantly reduced after use in industrial application. 3.4. JTS-01 Desulfurizer and JZC-80 Adsorbent Application. The JTS-01 desulfurizer and JZC-80 adsorbent have been successfully applied in the commercial plant at Baosteel. The industrial JZC-80 adsorbent was made of largegrained AC (Φ6 × 4−10 mm, columnar) impregnated with the alkaline compounds. Each desulfurizing tower was loaded with 8.6 × 103 kg of the JZC-80 adsorbent. JZC-80 adsorbent can effectively remove the other impurities of HCl, HF, HCN, H2S, and ash in the coal gas, guaranteeing device operating life and maintaining stable pressure drop in the system. Besides, the 2267



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Figure 4. Schematic of gas cleaning and H2S removal from COREX coal gas by JTS-01 desulfurizer in industrial application.

water (reaction 6). In addition, the reaction between elemental sulfur and oxygen is possible, so sulfur dioxide occurs as a coproduct (reaction 7).25 Sulfur (S0) could be reacted with H2S, so polysulfides, HSxSH, are produced (reaction 8).26

Baosteel, the operating expense of JTS-01 desulfurizer was only 0.03 RMB ($0.0049)/m3. The main constituents of waste desulfurizer were AC and sulfur (S), so JTS-01 desulfurizer was an environmentally friendly and safe desulfurizer. The physicochemical properties of JTS-01 desulfurizer and JZC-80 adsorbent were shown in Table 2. As shown in Figure 4, JTS-01 desulfurizer (B) was added into the fixed beds in the industrial desulfurizing tower (A), producing the waste desulfurizer (C); Figure4D indicated the schematic of H2S removal by adsorbent and desulfurizer. In this work, the desulfurizing process included two parts of physicochemical adsorption and catalytic oxidation. In the first part, H2S gas was physicochemically adsorbed in the abundant micropore of JTZ-01 desulfurizer. For physical adsorption, a conceptual reaction of H2S adsorption is proposed. First, the H2S is transferred from the bulk stream into the pore or surface of the AC. Second, H2S adsorbed on AC (reaction 2), and the adsorbed H2S dissolved in a water film (reaction 3). Finally, the adsorbed H2S in the water film was dissociated (reaction 4), so hydrogen sulfide ions (HS−) and hydrogen ions (H+) were produced. The presence of water on AC leads to the dissociation of H2S.24 H2S (g), H2S (ads-liq), and H2S (ads) correspond to H2S in gas, liquid, and adsorbed phases, respectively. H 2S(g) → H 2S(ads)

(2)

H 2S(ads) → H 2S(ads − liq)

(3)

−

H 2S(ads − liq) → HS (ads) + H

+

2Cf + O2 → 2C(O)

(5)

C(O) + H 2S → Cf + S + H 2O

(6)

S + O2 → SO2

(7)

S + H 2S → HSSH

(8)

where Cf refers to a free active site of carbon for chemisorption of oxygen and C(O) is an active site with chemisorbed oxygen. Consequently, the H2S molecule is oxidized into sulfur (S0) via the loaded desulfurizer and synergistic catalytic oxidation promoter, completely eliminating the secondary pollution. The chemical reactions are presented as follows (reactions 9−13). H 2S + OH− → HS− + H 2O

(9)

H 2S + 2OH− → S2 − + 2H 2O

(10)

2H 2S + O2 → 2H 2O + 2S ↓ (catalytic oxidation)

(11)

−

2HS + O2 → 2S + 2OH

−

2S2 − + O2 + 2H 2O → 2S + 4OH−

(12) (13)

27

Klein and Henning also proposed a similar mechanism for the oxidation of H2S on AC. The first step is the diffusion of the reactants H2S and O2 to the internal surface of the catalyst, then sorption of one or both reaction components onto the catalyst’s surface;28 third, the chemical interfacial reaction of the adsorbed species, and the last step is desorption of water into the void volume and adsorption of the sulfur at the internal surface leading to self-poisoning of the catalyst, thereby

(4)

Chemical adsorption of H2S on AC could be proposed as follows. First, the free active site of carbon reacts with oxygen (reaction 5). Then, oxygen is adsorbed on the surface of carbon. After that, H2S reacts with oxygen that is adsorbed on the surface of carbon to produce elemental sulfur (S0) and 2268



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Figure 5. (A) H2S concentration in the inlet of the desulfurizing tower and (B) H2S concentration in the outlet of the desulfurizing tower. The horizontal axis shows the testing time (test frequency: per day) from December 29, 2007 to June 30, 2009 (unit: Year-Month-Day), and the vertical axis shows the H2S concentration (unit: ppm).

influencing the course of the reaction. After impregnation with metal oxides (MO), the MO on the M/AC catalysts also reacted with H2S: MO + H2S → MS + H2O. For instance, the iron oxide (Fe2O3) in the JTS-01 desulfurizer may react with H2S and HS−, the mechanism of which was described as follows (eqs 14, 15). Accordingly, the waste desulfurizer presented acidity with the pH value of 3.45, while the pH value of the fresh desulfurizer was 8.98. HS− + 2Fe3 + → S0(solid) + 2Fe 2 + + H+

(14)

H 2S + 2Fe3 + → S0(solid) + 2Fe 2 + + 2H+

(15)

(4) The adsorbed oxygen is consumed to oxidize the sulfur in H2S into sulfur S0 which will be incorporated into the polysulfide chain on the site C-M-S. The polysulfide chain will act as an intermediate toward sulfur Sx formation. (5) Desorption of Sx occurs when the polysulfide chain C-MSx becomes too long. The Sx removes from the site and desorbs on the surface of sorbent. The Sx can be further oxidized into SO2 through the reaction S + O2 → SO2: this secondary reaction is the reason for the formation of SO2. However, the Claus reaction may also take place: 2H2S + SO2 → 3S + 2H2O, which is favorable for the removal of H2S. In the temperature range of 150−250 °C, the Claus reaction on the M/AC needs to be further confirmed. 3.5. Evaluation of JTS-01 Desulfurizer. 3.5.1. Theoretical Evaluation. The desulfurization performance of desulfurizer could be evaluated by eq 1, where Mp refers to the practical amount of H2S removal (kg), Vp refers to the bulk volume of coal gas (m3), C0 refers to the inlet H2S concentration (ppm), and C refers to the outlet H2S concentration (ppm).

However, the amount of H2S that reacted with metal oxides only accounted for 3% of the captured sulfur for the MO/AC sorbent. Considering the bad performance of AC for the catalytic oxidation of H2S, there is a synergistic effect on the catalytic oxidation of H2S between AC and MO. In general, the mechanism of the selective oxidation may be as follows (1)− (5): (1) The MO form active site on the surface of AC MO + C → C − M − O

M T = QT × 365 × 24 × (C0 − C1)

(1)

Substituting the data in Table 1 (QT = 169 000 N m3/h, C0 = 154 × 106 kg/m3, C1 = 10 × 106 kg/m3), the theoretical value of H2S removal is about 213 × 103 kg. According to eq 1, the elemental sulfur (S) yield can be estimated at 200 × 103 kg per year. That is to say, the sulfur capacity of selective desulfurizer was greater than 42.3% (200/473).

(2) Chemisorption of both H2S and O2 on the sorbent surface (3) The reaction between the adsorbed H2S and C-M-O to form C-M-S29,30 H 2S + C − M − O → C − M − S + H 2O 2269



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3.5.2. Practical Evaluation. The practical amount of H2S removal was 252 × 103 kg (S - 237 × 103 kg per year). This concurs with the conclusion that the sulfur capacity of desulfurizer was 50.1% (237/472) higher than the theoretical value (42.3%). Figure 5 presented the inlet and outlet H2S concentration from December 2007 to June 2009. After August 2008, the inlet H2S concentration of COREX coal gas became stable in the range 300−500 ppm. With tower A as an example, H2S was almost removed after treatment from December 2007 to October 2008. However, the desulfurization efficiency decreased during the period from November 2008 to March 2009. The reason was that the desulfurizer in tower A was close to its sulfur breakthrough capacity. From March to April 2009, tower A began to renew the desulfurizer. On April 2009, it could be observed that desulfurization efficiency improved, and H2S was removed completely from April to June 2009. Moreover, the result indicated that service life of desulfurizer approached one year. 3.6. System Pressure Drop. Pressure drop was one of the significant parameters in the industrial process. Since this desulfurizing apparatus was strict to the requirements of pressure drop, the desulfurizing tower should employ a novel structure with larger diameter tower and shorter bed. Meanwhile, it was necessary to ensure the homogeneous coal gas distribution and sufficient residence time. 3.6.1. Theoretical System Pressure Drop. Pressure Drop of Desulfurizing Tower (ΔP1). In the industrial process, the pressure drop of the desulfurizing tower can be calculated by the Ergun eq 2, where ΔP1 refers to the pressure drop (kPa) due to the desulfurizing tower, and L refers to the length of reaction bed (m). The related parameters are shown in SI Table 1. Substituting the parameter values, the pressure drop (ΔP1) of the desulfurizing tower was 2.04 kPa. If the conventional desulfurizers (e.g., iron oxides, AC) were utilized in this apparatus, the usage amount would be more than twice as much as this novel JTS-01 desulfurizer. Consequently, even if the tower diameter (Φ5.5 m) and all ten desulfurizing towers (parallel connection) were adopted, it cannot meet the requirements of system pressure drop; on the other hand, with the tower diameter of Φ5.5 m and only adding the bed height (more than 3 m), the pressure drop would not meet with the requirement. Under consideration of the tower diameter and bed height causing the pressure drop, the JTS01 desulfurizer was quite appropriate for this desulfurization apparatus. ΔP1 (1 − ε)2 ρ·u2 μ·u 1−ε = 150 × 2 + 1.75 3 × 2 L dP ε ε dP

the desulfurizing tower, the increase of later pressure drop, and so forth. The total system pressure drop as the aforementioned design (desulfurizing tower, desulfurizer) conformed to the requirements of the technical parameters in the COREX coal gas desulfurizing apparatus. n

ΔP =

∑ ΔPi i=1

= ΔP1 + ΔP2 + ΔP3 + ΔP4 = 2.04 + 0.62 + 0.80 + 0.30 = 3.76 kPa

(3)

3.6.2. Practical System Pressure Drop. In the practical conditions, some measures should be considered to meet the requirement of system pressure drop ≤4 kPa as follows: (1) This technology employed JTS-01 desulfurizer, whose single-path sulfur capacity approached 45−60%. It can realize long-term (change cycle more than 365 days) operation of the desulfurizing apparatus without regeneration. Therefore, it reduced the amount of desulfurizer and the bed height of desulfurizer, which can effectively reduce the system pressure drop. (2) The large-diameter (7400 mm) desulfurizing tower was employed, so it could reduce the superficial gas velocity (ug) and the system pressure drop. (3) Increasing the particle diameter of the JTS-01 desulfurizer [φ4 mm × (3−8)] can also reduce the system pressure drop. (4) In the aspect of gas distribution, the combination of cross and ring structure was employed to ensure the gas distributed uniformity. By adopting the above measures, the system pressure drop increased with the slight increase of operating time from 2.10 to 3.60 kPa. SI Figure 1 showed the changes of system pressure drop during the period of November 2007 to June 2009. The inset figure indicated the pressure drop of desulfurizing device A from January to December 2008. In the practical operation process, the system pressure drop increased with the increase of operating time from November 2007 to March 2008; the system pressure drop subsequently remained stable. However, the system pressure drop decreased in May 2009, when all desulfurizing apparatus renewed JTS-01 desulfurizer and JZC80 adsorbent. 3.7. Secondary Analysis of H2S Concentration. The outlet H2S concentration can indicate the breakthrough or saturation sulfur capacity of desulfurizer in each desulfurizing tower. Herein, the secondary analysis for the outlet H2S concentration was measured to confirm the renewal time of desulfurizer. The analytical results showed that renewing the desulfurizer without delay could increase the desulfurization efficiency and economize the desulfurizer. SI Figure 2 shows the analytical results of H2S outlet concentrations on May 4, 6, and 8 in 2008. In this period, tower C was just under the replacement, tower A had been renewed for two months, and tower B had been renewed for one month. During three days, the mean values of H2S outlet concentration at tower A, B, D, and E were 38 ppb, 17 ppb, 500, and 300 ppb, respectively. This indicated that tower C had reached the accumulated sulfur saturation capacity. Besides, this can estimate the H2S removal efficiency (E) according to eq 4. The maximum and minimum values of desulfurization efficiency could reach about 100% and 99.7%, respectively.

(2)

Pressure Drop of Operating Instrument (ΔP2). In order to reduce the pressure drop caused by the operating instrument, the Annubar flowmeters were only installed at the inlet or outlet of the desulfurizing apparatus. With the professional technology software, the pressure drop of the operating instrument was calculated to be 0.62 kPa (ΔP2). In addition, the total pressure drop of processing pipe, valve, and pipe fitting was 0.80 kPa (ΔP3). System Overall Pressure Drop (ΔP). System pressure drop was caused by a variety of factors, such as the desulfurizing tower, operating instrument, and valve. Herein, the system overall pressure drop was the total division pressure drop, which could be calculated by eq 3, where ΔP4 means the other pressure drop including the gas distributor, the grille board of 2270



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Table 3. Characteristics of Commonly Used Desulfurizers for Dry Desulfurization parameters mechanism

iron oxide desulfurizer oxidation desulfurization 3H2S + Fe2O3 → 2FeS + S + 3H2O single path: 10−20%, Total: 40−60% slow

sulfur capacity desulfurization rate BET surface area 100−200 m2/g, low porosity strength low strength, severe pulverization with water pressure drop

waste desulfurizer disposal

common AC desulfurizer physicochemical adsorption single path: 10−20%, Total: 40−60% slow

adsorption enhanced oxidation desulfurization H2S + O2 → 2S + H2O 3H2S + Fe2O3 → 2FeS + S + 3H2O single path: 20−30% (40−60%) Total: 100−150% (200−300%) fast

800−1000 m2/g, high porosity 1000−1200 m2/g (1064 m2/g), high porosity (60−70%) high strength, after steam activation at high strength, after steam activation at 900 °C, it can resist 900 °C, it can resist pulverization with pulverization with water water pressure drop increases with the increase during the service cycles, pressure drop during the service cycles, pressure drop can remain stable of use time, and the strength decreases can remain stable after regeneration resulting in the increase of pressure drop burn or landfill; the byproduct of FeS can regenerated and activated to be used as directly fabricated into the mercury remover or sulfurizing reagent; in addition, the sulfur (S0) and waste carbon can be recycled. The be self-ignited in the air, producing SO2, low-end activated carbon. During the which contaminates the environment. disposal, the produced H2S and SO2 can disposal is safe and environmentally friendly. (The new JTS-01 cause environmental pollution. The disposal is difficult and has desulfurizer can be regenerated, while sulfur can be recycled by potential safety hazard. thermal regeneration)

unequal catalytic desulfurization activity, depending on the elemental content in this point. In summary, the saturated sulfur capacity of JTS-01 desulfurizer can reach about 50.1%. In the engineering application, several measures were taken for controlling the system pressure drop below 4 kPa, to ensure year-round operation of the facility. Table 3 presents the characteristics of the commonly used desulfurizers for dry desulfurization.8,9,32−35 Compared with the other desulfurizers, the modified AC desulfurizers have more advantages, especially in terms of the JTS-01 desulfurizer with higher sulfur capacity and low pressure drop. Therefore, this sorption enhanced catalytic desulfurization has promising prospects for H2S removal and other gas cleaning. Besides, the waste desulfurizer could be easily regenerated and modified into an alternative AC adsorbent and desulfurizer with high sulfur-capacity via chemical and ultrasonically assisted approaches, hydrothermal or thermochemical activation.31 The alternative AC could also be fabricated from other biomass waste (e.g., rice husk, wood bark, sewage sludge) to cut down the costs.

E (removal efficiency, %) =

modified AC desulfurizer (JTS-01 desulfurizer)

C I (inlet conc., ppm) − CO (outlet conc., ppm) C I (inlet conc., ppm) × 100%

(4)

3.8. SEM and EDS Analyses. SI Figure 3 presents SEM micrographs of the JTS-01 AC desulfurizer in the process of COREX coal gas purification. By means of scanning electron microscopy, some features of the solid samples were enlarged 2000 times (a), 4000 times (b), and 10 000 times (c), respectively. Meanwhile, the feature point, enlarged 8000 times, was elemental analyzed by the EDS. The characteristic particle surface of the original desulfurizer, which was enlarged 2000 times, appeared concave and convex shaped, showing an evident cellular structure in partial. Moreover, the cellular structure became obvious, widely distributed into the deep micropore. By 10 000× amplification, the AC surface obviously revealed the concave−convex shape with deep holes in parts, and the cluster structure was more obvious. Meanwhile, combination of the cluster structure and the concave−convex surface could enhance the surface area and the internal holes of AC. However, there was little change in the surface morphology of waste desulfurizer after 10 000× enlargement. The surface pores of waste desulfurizer were apparently darker and larger in b and c diagrams. The pore surround and other positions of waste desulfurizer were quite smooth, which may be attributed to the internal accumulation of desulfurizing products in the desulfurizer.31 After amplifying the samples 8000 times by SEM with the same matched energy dispersive spectrum (EDS), three representative points located in the light and shaded areas were selected and analyzed for element determination. The selected points were shown in SI Figure 3, and the element content was presented in Table 2. The results indicated that all three of the selected points contained high-level carbon, sulfur, and oxygen elements, corresponding to the previous conclusion that the desulfurizing reaction products accumulated in the internal saturated desulfurizer, resulting in the pore surround and other positions being quite smooth. Meanwhile, every featureed point contained some common metallic elements such as Fe, Na, Al, Ca, and so forth. Although the sulfur contents of three featured points were different, it can be inferred that every point in the surface of the desulfurizer had

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ASSOCIATED CONTENT

S Supporting Information *

Supplementary Figure 1, Figure 2, Figure 3, and Table 1 as described in the text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*Phone: 86 (0)21 54742817; e-mail: [email protected]. Author Contributions

Tonghua Sun and Yafei Shen contributed equally to this work. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS We would like to thank the National High Technology Research & Development Program of China (“863” program, Grant NO. 2009AA062603) for financial support of this study. The author Yafei Shen was grateful of the Chinese Scholarship Council (CSC) for financial support under Grant No. 201206230168. And the thoughtful comments by editor and reviewers are greatly appreciated as well. 2271



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Article

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Gas Cleaning and Hydrogen Sulfide Removal for COREX Coal Gas by

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