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Characteristics of the product from steam activation of sewage sludge
Journal of Industrial and Engineering Chemistry 18 (2012) 839–847
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Characteristics of the product from steam activation of sewage sludge Young Nam Chun a,*, Mun Sup Lim a, Kunio Yoshikawa b a b
BK21 Team for Hydrogen Production, Department of Environmental Engineering, Chosun University, 375, Seosuk-dong, Dong-gu, Gwangju 501-759, Republic of Korea Frontier Research Center, Tokyo Institute of Technology G5-8, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
A R T I C L E I N F O
Article history: Received 9 August 2011 Accepted 12 November 2011 Available online 22 December 2011 Keywords: Steam activation Gasification Sewage sludge Producer gas Tar Sludge char
A B S T R A C T
Steam activation of a dried sewage sludge was studied to produce hydrogen rich gas and sludge char for converting to energy and resources. A batch-type wire mesh reactor was used to study the characteristics of the steam activation. The characteristics of activation product (i.e., producer gas, gravimetric tar, light tar, and sludge char) were identified. With the increase in the steam feed rate, the sludge char decreased but the producer gas increased, having higher gas heating value. And tar generation slightly increased when a small amount of steam was fed, but when the steam feed rate significantly increased, tar decreased because part of the tar was converted into light gas. Hydrogen and carbon monoxide increased with the increase in the steam feed rate. And carbon dioxide, methane, ethylene, and ethane reached their maximum according to different production mechanisms up to decreasing the species. The gradually increase in the steam feed rate resulted in the creation of micropores, which developed to the maximum when the steam flow rate was 14 mL/g min. When excessive steam was supplied, however, micropores sank due to the resulting sintering phenomenon, and the adsorption capacity deteriorated. The sludge char had a mean pore size of 6.229 nm, which is the size of mesopores from which condensible tar (the cause of damage on the device) is properly adsorbed and removed. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The management of sewage sludge in an economically and environmentally acceptable manner is one of the critical issues facing society today. In fact, the amount of sludge produced by wastewater treatment plants is going to increase dramatically in both industrialized and emerging countries. One of the main characteristics of sewage sludge is the presence of high amounts of inorganic ash and low carbon contents when compared with other materials, such as wood or lignocellulosic char from agriculture. As a result, sewage sludge has a relatively low-energy value, but is sufficient for some kind of waste-toenergy process to be considered as feasible. In addition, large amounts of sewage sludge are generated in every wastewater treatment plant, and as mentioned before, appropriate disposal methods need to be developed [1]. Recycling to agriculture (landspreading), incineration, or landfilling is the most common disposal routes. However, the landspreading leads to an increase in concentration of heavy metals in
* Corresponding author. Tel.: +82 62 230 7872; fax: +82 62 230 7156. E-mail address: [email protected] (Y.N. Chun).
the soils and indirect emissions into air and water. Disposal by the landfilling requires considerable space and poses a potential environmental hazard [2]. The incineration can reduce the material volume by 90% and recover the energy. However, it may cause secondary air pollutants, including dioxin, SOx, and NOx, and heavy metal in the material. Therefore, studies are conducted to obtain producer gas [3,4], liquid oil [5,6], and sludge char [7,8] via the pyrolysis and/or gasification of the sewage sludge. The producer gas includes combustible component (e.g., hydrogen, carbon monoxide, and methane) like gas fuel. The liquid oil is produced by cooling the producer gas and can be used as a liquid fuel via refinement. The sludge char is produced via carbonization and/or activation, and is used as an adsorbent, etc. [2]. However, sludge liquefaction is a more expensive process and gives rise to problems in relation to feeding of the sludge at high temperature, product separation from solvents (when used), and use of high-pressure hydrogen [9]. In addition, because of the high nitrogen and sulphur contents in the products, the sludge liquefaction requires additional processing techniques [10]. The pyrolysis is a process of producing gas and oil using the high-temperature thermal cracking due to the external heat source without supplying air or steam. Tar and refractory soot formation can damage the relevant devices [11]. The gasification technique
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.144
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uses the low-oxygen, partial-oxidation combustion heat that involves minimum air injection. The gas produced includes carbon dioxide and nitrogen, and the gases pose a problem in terms of low-heating value. It is known that steam gasification produces synthetic gas with a high hydrogen production rate from hydrocarbon fuels or waste. In particular, the surface area of char increases due to the steam injection, as steam activation, and it can be used as an adsorbent. In this case, the heavy metals in the sludge (except for mercury and cadmium) are trapped in the char and are not eluted. In this study, steam activation test in a bench scale was achieved to investigate the conversion of the sewage sludge into an energy and resources (i.e., producer gas and sludge char), including generating tar which causes clogging and corrosion in the pipeline and leads to operation trouble and damage to equipments. The physical characteristics of sludge char, the composition and heating value of producer gas, and the composition and concentration of tar were identified to show the effect of steam activation for the sewage sludge. 2. Experimental setup and procedure 2.1. Experimental equipment Fig. 1 shows the bench-scale gasification device, which consists of a wire mesh reactor, steam and gas feeding line, sampling and analysis line, and control and monitoring device. The wire mesh reactor is a cylindrical stainless tube (500 mm length and 85 mm inner diameter). To ensure uniform gas flow, a honeycomb-type distribution plate was installed at the reactor bottom. To apply heat to the reactor, an electric furnace (Model CLF-T1320, SERIN, Korea) with a controller featuring the fine temperature control within the range reaching up to 1000 8C, surrounded the reactor outside wall. A sample container with a stainless 400-mesh plate net was positioned on the honeycombtype distribution plate. The steam and gas feeding line consisted of a steam generator, syringe water pump, and N2 cylinder. The steam generator had an internal stick-type cartridge heater, which is regulated by the controller to maintain a specific temperature (350 8C). The syringe water pump (Model KDS 100, KD Scientific, USA) supplied a small amount of water and the N2 cylinder supplied the carrier gas. They passed though the Venturi mixer and entered the steam generator. In the sampling and analysis line, a gas meter (Model W-MK10-ST, Shinagawa, Japan) was installed to measure the producer
Table 1 Physicochemical characteristics of dewatered sludge. Analysis methods
Contents
Value (wt.%)
Proximate analysis
Moisture Volatile matter Fixed carbon Ash
9.6 51.1 32.9 6.4
Ultimate analysis
C H O N S
52.3 8.2 32.2 7.92 0.01
gas volume with sampling time, and the gas volume was used for showing tar concentration. The analysis device consisted of a GCFID for tar analysis (GC-14B, SHIMADZU, Japan) and a GC-TCD for gas analysis (CP-4900, Varian, Netherlands). The control and monitoring device controlled the electric furnace and steam generator. The electric furnace was heated by the reactor heat controller, and the steam generator was regulated by the heat controller to maintain its set temperature. In addition, thermocouples were installed at sludge bed and gas layer in the reactor, on the electric furnace wall and in the steam generator. A data logger (Model Hydra data logger 2625A, Fluke, USA) was used to continuously monitor the temperatures. 2.2. Experimental method The sewage sludge was dehydrated using a centrifuge in the urban sewage treatment plant at a moisture content of approximately 85%. To make the dried sludge with a moisture content of 10% or less, the dewatered sludge was dried for approximately 7 h at 105–110 8C, using an electric furnace (Model KS-35, Kwang Sung Co. Ltd., Korea). The dried sludge was crushed using a grinder; a tailor sieve (RoTap Sieve Shaker, Chunggye Ltd., Korea) was used to make the sludge particle diameters uniform at 1–1.5 mm. Table 1 shows the results of the proximate and ultimate analyses for the dried sludge. Before the main test, the sample container with 20 g dried sludge was placed on the distribution plate in the reactor. The carrier gas (N2) was fed into the reactor at a flow rate of 100 mL/ min for 30 min to sufficiently purge the reactor and sample line. And then the main test achieved sequential pyrolysis (i.e., carbonization) and steam gasification (i.e., steam activation). For the pyrolysis, the temperature was increased from room tempera-
Fig. 1. Experimental setup for the steam activation of sewage sludge.
Y.N. Chun et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 839–847
ture to 500 8C at a heating rate of 25 8C/min, and then it was maintained for 10 min (holding time). After the carbonization, the steam activation was achieved by increasing the temperature up to the final temperature of 900 8C at a heating rate of 25 8C/min. And then, the condition was maintained for 10 min at the final activation temperature. The steam flow rate for the steam activation was changed to 4–19 mL/g min to investigate the effect of the amount of steam feed. After each test was completed, N2 gas was supplied until the temperature in the reactor reached room temperature. The reactor cover was then opened, the sludge char left in the sample container was collected, and its physical characteristics were analyzed. 2.3. Analytical method 2.3.1. Tar sampling and analysis To measure the total amount of tar produced during carbonization and activation, the wet-type tar sampling method was used to determine the gravimetric tar and selected light tar (Fig. 1). The selected tars are benzene, naphthalene, anthracene, and pyrene that have 1–4 benzene rings, and benzonitrile and benzeneacetonitrile including nitrogen content in the sewage sludge [12]. The wet-type tar sampling and analysis in this study were followed to biomass technology groups (BTGs) method [13–15]. The wet-type tar sampling was achieved for testing period, and the gravimetric tar mass and light tar were analyzed. The wet-type sampling line was fabricated by installing impingers in two isothermal baths to condense and absorb tar. The temperature of the first isothermal bath was kept constant at 20 8C or below, and four impingers were installed and filled with 100 mL isopropyl alcohol. A chiller (ECS-30SS, Eyela Co., Japan) was used as the second isothermal bath, and its temperature was kept constant at 20 8C or below. Of the two impingers, one impinger was filled with isopropyl alcohol and the other was left empty. In the series of impinger bottles, the first impinger bottle acts as a moisture and particle collector in which water, tar, and soot are condensed from the process gas by absorption in isopropanol. Other impinger bottles collect tars and the empty bottle collects drop. Immediately after completing the sampling, the content of the impinger bottles were filtered through a filter paper (Model F-5B, Advantec Co., Japan). The filtered isopropanol solution was divided into two parts. The first was used to determine the gravimetric tar mass by means of solvent distillation and evaporation by evaporator (Model N-1000-SW, Eyela, Japan) in which temperature and vapor pressure were 55–57 8C and 230 hPa, respectively. The second was used to determine the concentrations of the light tar compounds using GC-FID (Model 14B, Shimadzu, Japan). Quantitative tar analysis was performed on a GC system, using a RTX-5 (RESTEK) capillary column (30 m – 0.53 mm id, 0.5 mm film thickness) and an isothermal temperature profile at 45 8C for the first 2 min. This analysis was followed by a 7 8C/min-temperature gradient to 320 8C and finally an isothermal period at 320 8C for 10 min. Helium was used as a carrier gas. The temperature of the detector and injector were maintained at 340 8C and 250 8C, respectively. 2.3.2. Producer gas sampling and analysis The producer gas was sampled at the end point of the wet-type tar sampling line as shown in Fig. 1. A set of backup VOC adsorber was installed downstream of the series of impingers to protect the column of the gas chromatography from the residual solvent or VOCs, which may have passed through the impinger train. The set of backup VOC adsorber consisted of two cotton filters and an activated carbon filter connected in a series. The GC-TCD (Model
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CP-4900, Varian, Netherland) was used for the producer gas analysis. The Molecularsieve-5A column was used for H2, CO, O2, and N2, and the PoraPlot-Q column was used for CO2, C2H4, and C2H6. 2.3.3. Sludge char analysis To analyze the pore formation characteristic of the sampled sludge char, nitrogen adsorption was conducted. The adsorption isothermal curve was measured using an analytical equipment (Model NanoPOROSITY-XQ, Mirae SI, Korea), which was formed according to the adsorption of nitrogen gas at 196 8C [1]. The specific surface area was calculated by applying the adsorption characteristic of the specimen and BET equation. To calculate the pore distribution and mean pore diameter, Barret–Joyner–Halenda (BJH) equation was used for mesopores, and Horvath–Kawazoe (HK) equation for micropores. To compare the development of pores in the sludge char, scanning electron microscopy (SEM; Model S-4800, Hitachi Co., Japan) was used. In SEM, the pores were magnified by 50,000 times. To identify the chemical properties and contents, the energy-dispersive X-ray spectroscopy (EDS; Model 7593-H, Horiba, UK) was used. The adsorption capacity of the sludge char produced was assessed from the adsorption test using benzene to identify the tar adsorption characteristic.
3. Results and discussion 3.1. Characteristics of pyrolysis and gasification in the sewage sludge The pyrolysis gasification of dried sludge consists of two steps: the primary pyrolysis (or carbonization) and secondary gasification (or activation) reaction as shown in Fig. 2. The dried sludge is converted into char, tar, and gas in the primary pyrolysis (200– 500 8C), while tar and char are converted to gas in the secondary gasification process. The sewage sludge mostly consists of cellulose; a significant portion of it is converted into tar in the primary pyrolysis and into gas in the secondary gasification [1]. The primary pyrolysis is mainly influenced by the heating rate, and the secondary gasification is influenced by the reactor temperature [16,17]. The secondary gasification reaction includes the gasification of sludge char (Eqs. (1)–(4)), which is the carbonous residue produced from the primary pyrolysis, decomposition of tar (Eqs. (5) and (6)), and light produced gas reactions (Eqs. (7)–(11)). In general, steam gasification include reactions of O2, CO2, H2, and H2O with the combustible fraction of carbon in the carbonized sludge, thereby producing gaseous products. The essential features involve chemisorptions of the reacting gas species on the carbon surface [18]. The tar decomposition takes place at pyrolysis temperatures lower than 600 8C. The generation of CH4 and C2 hydrocarbons is caused by the easy breaking of C–H bond of alkyl.
Fig. 2. Material balance for pyrolysis and gasification of dried sludge.
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A large amount of CO and CO2 is released as a result of C5 5O diminishing. At high temperatures (>600 8C), the sharp decrease of the absorbance amount of C–O and C-Haromatic demonstrates that the cracking and reforming reactions of aromatics occur and favor the production of H2 and CO [19]. Gasification reaction of sludge char Partial oxidation C þ 1=2O2 ! CO
DH ¼ 110:5 kJ=mol
(1)
Water–gas reaction C þ H2 O ! CO þ H2
DH ¼ 131:3 kJ=mol
(2)
Boudouard reaction C þ CO2 ! 2CO
DH ¼ 171:7 kJ=mol
(3)
Hydrogasification C þ 2H2 ! CH4 DH ¼ 74:9 kJ=mol
(4)
Fig. 3. Mass yield of products.
Decomposition reaction of tar Tar pyrolysis Tar ! wH2 þ xCO þ yCO2 þ zCn Hm
(5)
Tar steam gasification Tar þ vH2 O ! xCO þ yH2
(6)
Reaction of light gas Methanation reaction CO þ 3H2 $ CH4 þ H2 O
DH ¼ 206:2 kJ=mol
(7)
Water–gas shift reaction CO þ H2 O $ CO2 þ H2
DH ¼ 41:1 kJ=mol
(8)
Steam reform Cn Hm þ nH2 O ¼ nCO þ ðn þ m=2ÞH2
(9)
the primary pyrolysis, and the residual tar in the char was generated with the increase in steam during steam activation [20]. The role of steam cannot be limited only for transportation and stabilization of the volatile products such as nitrogen. The ability of steam to penetrate into solid materials and to help desorption, distillation, and efficient removal of the volatile products from it can explain the higher yield of tar [21]. When the steam feed rate was high (19 mL/g min), however, the total tar slightly decreased. This was because the relatively sufficient amount of steam promoted the tar decomposition reaction which is tar steam gasification (Eq. (6)). The amount of gas was calculated excluding the amount of sludge char and tar, which does not include fed and produced steam. With the increase in the steam feed rate, the amount of gas increased. As mentioned earlier, this was because the water–gas reaction (Eq. (2)), which is the gasification reaction of sludge char, and tar steam gasification (Eq. (6)), which is the decomposition reaction of tar, led to the reaction of fixed carbon and tar with steam into producer gas.
where CnHm is the light hydrocarbon. 3.2. Effect of the amount of steam feed The test was conducted in the fixed pyrolysis condition (heating rate: 25 8C/min; pyrolysis temperature: 500 8C; and holding time: 10 min) and the fixed steam activation condition (heating rate: 25 8C/min; final activation temperature: 900 8C; and holding time: 10 min). To know the effect of the amount of steam feed, the four steam feed rates (4 mL/g min, 9 mL/g min, 14 mL/g min, and 19 mL/ g min) were used for the test. 3.2.1. Mass yield of products Fig. 3 shows the mass yield of products according to the change in the steam feed rate. The order according to the mass yield was gas, char, and tar. With the increase in the steam feed rate, the sludge char decreased. The sludge char generally decreased according to Eqs. (1)–(4) which are the sludge char gasification reactions. Particularly, with the increase in the steam, the fixed carbon of the sludge char was converted into gas due to the water–gas reaction (Eq. (2)) and subsequently decreased. The total amount of tar increased with the increase in the steam feed rate within the low-steam, feed-rate range (less than 14 mL/ g min). This was because most of the volatile tar was generated in
3.2.2. Characteristics of producer gas From the beginning of the test, the producer gas from the pyrolysis and steam gasification was sampled and analyzed as online at a GC. Therefore, the analysis was delayed for approximately 20 min which took the initially produced gas to flow from the reactor outlet to the GC sampling port. In addition, steam was injected for steam activation after 36 min at the end of the pyrolysis. Therefore, as shown in Fig. 4, the producer gas for the steam injection was sampled from 56 min. Fig. 4 shows the concentrations of selected producer gases, such as H2, CO, CO2, CH4, C2H4, and C2H6. Tar and CO, CO2, and CH4 gases were produced from the primary pyrolysis, while tar and char were converted into light gas from the secondary steam activation. Some light gases reacted with one another under the steam atmosphere, and their production and extinction occurred. H2 was hardly generated from the initial primary pyrolysis. It rapidly increased from the secondary steam activation with the steam feeding, the increase of which had a great effect. Since H2 is produced according to the water–gas reaction (Eq. (2), wherein the fixed carbon in the sludge reacts with steam), and tar steam gasification (Eq. (6), wherein the generated tar reacts with steam), the production rate of H2 was increased at higher steam feed rate. And the light gas CO and light hydrocarbons (CnHm) that was produced from primary pyrolysis was converted into H2 via the
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Fig. 4. Effect of steam feed rate on composition of the gaseous products according to elapsed time.
reaction with steam (H2O), as in water–gas shift reaction (Eq. (8)) and steam reform (Eq. (9)), leading to the increase for hydrogen. In addition, regardless of steam feed, H2 increased with the increase in the activation temperature according to the pyrolysis (Eq. (5)) of tar generated from the primary pyrolysis. In all three cases wherein the steam feed rate was less than 14 mL/g min, the CO gradually increased with the increase for steam, showing same pattern. This is because the CO was continuously produced during the primary pyrolysis and secondary steam activation processes according to the gasification of sludge char (Eqs. (1)–(3)), tar decomposition (Eqs. (5) and (6)) and steam reform (Eq. (9)). Particularly, with the increase in the steam, the production rate of CO was higher due to the effects of water– gas reaction (Eq. (2)), tar steam gasification (Eq. (6)) and steam reform (Eq. (9)). When the steam feed rate was higher than that in the three cases above (e.g., at 19 mL/g min), a significant amount of volatile CO was produced [21]. In particular, when steam gasification atmosphere was maintained with sufficient steam at a later time, the amount of CO decreased because CO was converted into CO2 due to water–gas shift reaction (Eq. (8)). When the steam feed rate was less than 14 mL/g min, CO2 was produced in the greatest amount during the primary pyrolysis process. And then its amount decreased and was kept almost constant. However, at the steam feed rate of 14 mL/g min or higher, the amount of CO2 increased with the increase in the steam feed rate. This was because CO in the gas was converted to CO2 according to water–gas shift reaction (Eq. (8)). The effect was greater when the steam feed rate was higher.
The release of CO and CO2 at lower temperature (<450 8C) is mainly caused by the breaking of carbonyl and carboxyl functional groups in sewage sludge. The release of CO, as a major secondary product from tar cracking, was favored high temperature [19]. CH4 was produced in the greatest amount during the primary pyrolysis process and disappeared according to the steam reform (Eq. (9)). C2 hydrocarbons (C2H4 and C2H6) were produced in a smaller amount than other gases in the tar pyrolysis (Eq. (5)) which is the decomposition of tar produced in the primary pyrolysis process. After having maximum values, they disappeared by the steam reforming like the CH4 removal. The products C2H4 and C2H6 were converted into CH4 via the following decomposition reactions (Eqs. (10) and (11)) [19]. C2 H6 ! C2 H4 þ H2
(10)
C2 H4 ! CH4 þ C
(11)
When the temperature was high, the temperature had a greater effect than steam in the both reactions. That is why each value in the light hydrocarbons had almost similar according to the different steam feed rate. Fig. 5 shows the total amount of gas and higher heating value according to the steam feed rate. With the increase in the steam feed rate, the total amount of producer gas produced from the carbonization and steam activation increased. This was because more residual volatile gas containing tar was produced with the increase of steam feed for secondary activation [20]. The generated tar was converted into CO
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Fig. 5. Mass yield of product and total amount of producer gas.
and H2 gases via tar steam gasification (Eq. (6)). And the water–gas reaction (Eq. (2)), which is the gasification of sludge char, resulted in the reaction of fixed carbon with steam into the gas. Therefore, gas production increased [18,22]. The heating value increased with the increase in the steam feed rate because the total amount of combustible gases, including H2, CO, C2H4, and C2H6, increased. In particular, the steam had a great effect when the steam feed rate was 19 mL/g min, resulting in a heating value of 15,233 kJ/Nm3. 3.2.3. Characteristics of selected light tar generation Fig. 6 shows the distribution of selected light tar concentration according to the steam feed rate. When the steam feed rate was lower than 14 mL/g min, benzene (light aromatic tar) and naphthalene (light polycyclic aromatic hydrocarbon: PAH) concentrations increased with the increase in the steam feed rate. This is because the activation led to the easy penetration of steam into sludge char pores created from
Fig. 6. Light tar contribution according to the steam feed rate.
the primary pyrolysis, and the resulting desorption, distillation, and efficient removal of the volatile products increased the amount of light tar. At a steam feed rate of 19 mL/g min, however, the amount of both light tars decreased. This was because the light tars were converted into light gases and decreased according to the tar steam gasification reaction (Eq. (6)) with the increase in the steam feed rate. Benzene and naphthalene were more influenced by steam than other PAHs [16]. The other selected light PAHs, anthracene and pyrene, slightly increased with the increase in the steam feed. The two tar compounds were hardly influenced by the amount of steam and may be evolved directly from the dried sludge by pyrolysis process [16]. Benzonitrile and benzoacetonitrile (aromatic nitrile) showed the similar phenomena as with benzene and naphthalene but
Fig. 7. SEM photos for sludge chars in case of different steam feed rate: (a) 4 mL/g min; (b) 9 mL/g min; (c) 14 mL/g min; (d) 19 mL/g min.
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Fig. 8. Element compounds measured by EDX.
appeared in a relatively small amount because the nitrogen content in the sludge was small (Table 1). 3.2.4. Characteristics of sludge char Fig. 7 shows the SEM pictures to identify the pore characteristics of activated sludge char produced from the steam feed rate change test. The pore size classification in this study follows the IUPAC classification [23], that is, micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). The increase in the amount of steam feed gradually resulted in the creation of micropores and mesopores; they developed most pores when the steam feed rate was 14 mL/g min. At a steam feed rate of 19 mL/g min, however, the excessive steam supply led to the sintering and the pores sank into ashes. In the primary carbonization, the residual moisture and volatile material evaporated and pores developed. Bagreev et al. proved that water released by the dehydroxylation of inorganic material could aid pore formation and could act as an agent for creating micropores [24]. In addition, Inguanzo et al. proposed that carbonization increased the porosity through unblocking many of the pores obscured by volatile matter [25]. In the secondary steam activation, the high temperature steam injected from the outside penetrates into pores that are produced from the primary pyrolysis (carbonization) process. It leads to gas production and pore development according to the water–gas reaction (Eq. (2)) which is the on-site surface reaction.
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As mentioned above, however, the excessive water–gas reaction not only increases the gas production, but also makes the pores sink due to the loss of fixed carbon in sludge char, as in the case of 19 mL/gmin. Furthermore, the optimal adsorption capacity cannot be ensured. Therefore, a proper amount of steam must be supplied like 14 mL/g min in this study. Fig. 8 is obtained from the EDX analyzer coupled to SEM measurements. The fixed carbon decreased with the increase in the steam feed rate. This is because the fixed carbon on the sludge char pore surfaces was consumed by the water–gas reaction (Eq. (2)). Especially, at a steam feed rate of 19 mL/g min, at which pores sank, more fixed carbon content decreased than the case with lower steam feed rates. As shown in the EDX results, the inorganic atoms AAEM (alkali and alkaline earth metallic) species (Mg, K, Ca), Al, and Fe did not volatilize but remained in the sludge char [26]. It is known that these contents serve as catalysts in the pyrolysis and gasification. For example, with Al, if existing in the form of Al2O3, it would be an acid catalyst for cracking reaction [27]; or with K and Ca atoms, they were already reported as the catalyst for pyrolysis in literature [28]. To evaluate adsorption properties of the sludge chars, which were produced with different amounts of steam feed, input concentration of benzene for adsorption test was fixed at 1%; GHSV (gas hourly space velocity) at 25 8C was 6315/h (gas at 2 L/min, volume at 19 mL). Fig. 9 shows the breakthrough curve and adsorption amount for comparing adsorption characteristics of the different sludge chars (a) or a commercial activated carbon (b). C is effluent concentration, and Ci refers to input concentration. The breakthrough point was defined as the point at which the effluent concentration is 10% higher than the influent concentration. The adsorption amount refers to the amount of adsorption in which the effluent concentration reaches the saturation point which is the same as the influent concentration. As shown in Fig. 9(a), the sludge char that was produced when the steam feed rate was high had breakthrough curve shifted to longer time. The rightmost curve was obtained at a steam feed rate of 14 mL/g min, and the adsorption capacity was best. It took 15 min to reach the breakthrough point, and the adsorption amount was 174.8 mg/g. However, the breakthrough point of sludge char was at the leftmost position when the steam feed rate was 19 mL/g min because part of pores sank due to steam gasification and the adsorption capacity rapidly decreased, as expected in the results of Fig. 7.
Fig. 9. Breakthrough curve and adsorption amount of benzene for the sludge chars and a commercial activated carbon.
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Table 2 Porous characteristics and adsorption capacity of the adsorbents from this study and other results [12]. Adsorbent a
Sludge char Commercial activated carbonb Activated carbonc Wood chipc a b c
Mean pore size (nm)
Specific surface area (m2/g)
Pore volume (cm3/g)
Adsorption amount (mg/g)
6.229 1.83 1.128 10.077
80.28 1376.6 987.1 1.072
0.135051 0.63 0.5569 0.0058
174.8 586.2 97.5 155.7
Steam-activated char produced in this study (benzene adsorption). Commercial activated carbon (benzene adsorption). Tested by Thana Phuphuakrat et al. (tar adsorption generated from Japanese cedar).
Fig. 9(b) shows the adsorption characteristics of the sludge char and commercial activated carbon when the steam feed rate, as the reference condition, is 14 mL/g min. The commercial activated carbon had a significantly better benzene adsorption capacity than the sludge char produced in this study. This was because the commercial activated carbon had not only well-developed micropores, which allows easy benzene adsorption, but also large specific surface area (see Table 2). The breakthrough point of the commercial activated carbon was 240 min, and its adsorption capacity was 586.2 mg/g. Table 2 represents the analysis and test results of the sludge char and commercial activated carbon of which benzene adsorption test were achieved in this study. In addition, two adsorbents (activated carbon and wood chip) as studied by Thana Phuphuakrat et al. are shown [13]. As for the sludge char, its mean pore size was bigger than those of commercial activated carbon and activated carbon, as studied by Thana Phuphuakrat et al., and its specific surface area and pore volume were smaller. The sludge char (6.229 nm) had a mean pore size of mesopores, like wood chips (10.077 nm), but two activated carbon materials had a mean pore size of micropores. That is, the sludge char and wood chips had the characteristic similar to adsorption of mesopores. And the sludge char had a specific surface area of 80.28 m2/g, an influential factor for adsorption amount, larger than that of wood chips (1.072 m2/g). In the case of adsorption amount, the sludge char and commercial activated carbon were of benzene test in this study. But the activated carbon and wood chips used in the study of Thana Phuphuakrat et al. were obtained from total tar adsorption the pyrolysis of Japanese cedar. Therefore, it is difficult to compare the adsorption capacity results from the two both case. However, the benzene adsorption capacity of the sludge char was relatively smaller compared to that of commercial activated carbon in this study. As mentioned above, the reason is that the pore size was larger and the specific surface area was smaller in the sludge char. Some portion of heavy tar, that is almost all of the gravimetric tar in the Fig. 3, was converted to light tar or gas via thermal cracking or steam reforming. Among the light tars, non-condensible aromatic tars (e.g., benzene, benzonitrile, benzoacetonitrile, etc.) will be no damage on devices (i.e., combustor, engine, pipe line, etc.) [13]. That is, when light aromatic tars are contained in the producer gas, they can be effectively used with increases of the heating value and thus energy efficiency. However, condensable light PAH tars such as naphthalene, anthracene, and pyrene may damage the devices and thus need to be removed. Therefore, it should be effective that adsorption of the sludge char could not be better for the non-condensible light aromatic tar to improve energy efficiency. For the sludge char produced from the steam activation, the benzene passed through the sludge char after it reached the breakthrough point in a short time. As mentioned above in the tar adsorption test, as determined by Thana Phuphuakrat et al. [13], the development of mesopores in the sludge char easily leads to the adsorption of condensable light
PAH tar, making it an efficient adsorbent for tar removal of producer gas generated from the pyrolysis and gasification.
4. Conclusions Steam activation of dried sewage sludge was studied for converting the sewage sludge from the waste treatment plant to energy and resources. It could be identified that the steam activation had good characteristics for hydrogen rich gas production and sludge char which can be used for gas energy and tar adsorbent, respectively. The mass yield of product was in the order of gas, char, and tar. With the increase in the steam feed rate, the sludge char decreased, and producer gas increased. The tar increased up to 14 mL/g min, but decreased at a higher steam feed rate. The amount of producer gas increased and the heating value also increased with the increase in the steam feed rate. The produced gases from the carbonization and activation included H2, CO, CO2, CH4, C2H4, and C2H2. The H2 was hardly produced from the primary carbonization, but rapidly increased during the secondary steam activation. The amount of increase became higher as more steam was fed. The CO and CO2 were produced in large amounts with the evaporation of volatile materials in the primary carbonization. From the secondary steam activation, CO continued to increase, while CO2 reached its maximum and then decreased. When the steam feed rate was higher, however, CO decreased and CO2 increased at a high temperature. The CH4 and C2 hydrocarbons (C2H4 and C2H6) were produced in the primary carbonization and steam activation, respectively. They subsequently reached their maximums and then decreased. For benzene (light aromatic tar) and naphthalene (light PAH), their tar concentrations increased with the increase in the amount of steam feed, when the steam feed rate was less than 14 mL/g min. At a steam feed rate of 19 mL/g min, however, the benzene and naphthalene decreased. The light PAHs, anthracene and pyrene, slightly increased with the increase in the steam feed. Benzonitrile and benzoacetonitrile (aromatic nitrile) showed the similar phenomenon as with benzene and naphthalene, but appeared in a relatively small amount. The increase in the steam feed gradually resulted in micropores and mesopores. They developed most when the vapor volume was 14 mL/g min. At a steam feed rate of 19 mL/g min, however, the excessive steam supply led to sintering and the adsorption capacity deteriorated. The inorganic atoms, AAEM species (Mg, K, Ca), Al, and Fe did not evaporate but remained in the sludge char. These materials serve as catalysts for pyrolysis or gasification. The sludge char had a mean pore size of 6.229 nm which is the size of mesopores from which condensated tar, which damages the device, is properly adsorbed and removed. The specific surface area was 80.28 m2/g, smaller than that of commercial activated carbon, but it was feasible enough. In conclusion, proper steam feeding is very important in making low-tar gas to be used gas energy and sludge char to be used tar removal adsorbent from sewage sludge using steam activation. The
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study results showed that the steam feed rate of 14 mL/g min was most efficient. Acknowledgment This study was supported by research funding from Chosun University, 2011. References [1] J. A´brego, J. Arauzo, J.L. Sa´nchez, A. Gonzalo, T. Cordero, J. Rodrı´guez-Mirasol, Ind. Eng. Chem. Res. 48 (2009) 3211. [2] T. Karayildirim, J. Yanik, M. Yuksel, H. Bockhorn, Fuel 85 (2006) 1498. [3] N. Nipattummakul, I.I. Ahmed, S. Kerdsuwan, A.K. Gupta, Int. J. Hydrogen Energy 35 (2010) 11738. [4] A. Midilli, M. Dogru, G. Akay, C.R. Howarth, Int. J. Hydrogen Energy 27 (2002) 1035. [5] H.J. Park, H.S. Heo, Y.K. Park, J.H. Yim, J.K. Jeon, J.H. Park, C.K. Ryu, S.S. Kim, Bioresour. Technol. 101 (2010) S83. [6] I. Fonts, M. Azuara, G. Gea, M.B. Murillo, J. Anal. Appl. Pyrol. 85 (2009) 184. [7] T.J. Bandosz, K. Block, Appl. Catal. B: Environ. 67 (2006) 77. [8] M.J. Martin, E. Serra, A. Ros, M.D. Balaguer, M. Rigola, Carbon 42 (2004) 1389. [9] D.T. Furness, L.A. Hoggett, S.J. Judd, J. CIWEM 14 (2000) 57. [10] I. Fonts, E. Kuoppala, A. Oasmaa, Energy Fuel 23 (2009) 4121.
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[11] C.S. Ku, S.P. Mun, J. Ind. Eng. Chem. 12 (6) (2006) 853. [12] A. Fullana, J.A. Conesa, R. Font, I. Martı´n-Gullo´n, J. Anal. Appl. Pyrol. 68–69 (2003) 561. [13] T. Phuphuakrat, T. Namioka, K. Yoshikawa, Appl. Energy 87 (2010) 2203. [14] J.P.A. Neeft, Rationale for Setup of Impinge Train, SenterNovem (Netherlands), (2005), p. 1. [15] Y.I. Son, M. Sato, T. Namioka, K. Yoshikawa, J. Environ. Eng. 4 (2009) 12. [16] K. Umeki, Ph.D. Thesis, Tokyo Institute of Technology, Japan, 2009. [17] O. Masˇek, M. Konno, S. Hosokai, N. Sonoyama, K. Norinaga, J. Hayashi, Biomass Bioenergy 32 (2008) 78. [18] S.T. Chaudhari, A.K. Dalai, N.N. Bakhshi, Energy Fuel 17 (2003) 1062. [19] B. Zhang, S. Xiong, B. Xiao, D. Yu, X. Jia, Int. J. Hydrogen Energy 36 (2011) 355. [20] E. Pu¨tu¨n, F. Ates, A.E. Pu¨tu¨n, Fuel 87 (2008) 815. [21] M.F. Yardim, E. Ekinci, V. Minkova, M. Razvigorova, T. Budinova, N. Petrov, M. Goranova, Fuel 82 (2003) 459. [22] N. Nipattummakul, I. Ahmed, S. Kerdsuwan, A.K. Gupta, Appl. Energy 87 (2010) 3729. [23] G.Q. Lu, Environ. Technol. 16 (1995) 495. [24] A. Bagreev, T.J. Bandosz, D.C. Locke, Carbon 39 (2001) 1971. [25] M. Inguanzo, J.A. Mene´ndez, E. Fuente, J.J. Pis, J. Anal. Appl. Pyrol. 58–59 (2001) 943. [26] D.M. Keown, J.I. Hayashi, C.Z. Li, Fuel 87 (2008) 1127. [27] J.H. Sinfelt, J.C. Rohrer, J. Phys. Chem. 66 (8) (1962) 1559. [28] S. Yaman, Energy Convers. Manag. 45 (2004) 651.
Contents lists available at SciVerse ScienceDirect
Journal of Industrial and Engineering Chemistry journal homepage: www.elsevier.com/locate/jiec
Characteristics of the product from steam activation of sewage sludge Young Nam Chun a,*, Mun Sup Lim a, Kunio Yoshikawa b a b
BK21 Team for Hydrogen Production, Department of Environmental Engineering, Chosun University, 375, Seosuk-dong, Dong-gu, Gwangju 501-759, Republic of Korea Frontier Research Center, Tokyo Institute of Technology G5-8, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan
A R T I C L E I N F O
Article history: Received 9 August 2011 Accepted 12 November 2011 Available online 22 December 2011 Keywords: Steam activation Gasification Sewage sludge Producer gas Tar Sludge char
A B S T R A C T
Steam activation of a dried sewage sludge was studied to produce hydrogen rich gas and sludge char for converting to energy and resources. A batch-type wire mesh reactor was used to study the characteristics of the steam activation. The characteristics of activation product (i.e., producer gas, gravimetric tar, light tar, and sludge char) were identified. With the increase in the steam feed rate, the sludge char decreased but the producer gas increased, having higher gas heating value. And tar generation slightly increased when a small amount of steam was fed, but when the steam feed rate significantly increased, tar decreased because part of the tar was converted into light gas. Hydrogen and carbon monoxide increased with the increase in the steam feed rate. And carbon dioxide, methane, ethylene, and ethane reached their maximum according to different production mechanisms up to decreasing the species. The gradually increase in the steam feed rate resulted in the creation of micropores, which developed to the maximum when the steam flow rate was 14 mL/g min. When excessive steam was supplied, however, micropores sank due to the resulting sintering phenomenon, and the adsorption capacity deteriorated. The sludge char had a mean pore size of 6.229 nm, which is the size of mesopores from which condensible tar (the cause of damage on the device) is properly adsorbed and removed. ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved.
1. Introduction The management of sewage sludge in an economically and environmentally acceptable manner is one of the critical issues facing society today. In fact, the amount of sludge produced by wastewater treatment plants is going to increase dramatically in both industrialized and emerging countries. One of the main characteristics of sewage sludge is the presence of high amounts of inorganic ash and low carbon contents when compared with other materials, such as wood or lignocellulosic char from agriculture. As a result, sewage sludge has a relatively low-energy value, but is sufficient for some kind of waste-toenergy process to be considered as feasible. In addition, large amounts of sewage sludge are generated in every wastewater treatment plant, and as mentioned before, appropriate disposal methods need to be developed [1]. Recycling to agriculture (landspreading), incineration, or landfilling is the most common disposal routes. However, the landspreading leads to an increase in concentration of heavy metals in
* Corresponding author. Tel.: +82 62 230 7872; fax: +82 62 230 7156. E-mail address: [email protected] (Y.N. Chun).
the soils and indirect emissions into air and water. Disposal by the landfilling requires considerable space and poses a potential environmental hazard [2]. The incineration can reduce the material volume by 90% and recover the energy. However, it may cause secondary air pollutants, including dioxin, SOx, and NOx, and heavy metal in the material. Therefore, studies are conducted to obtain producer gas [3,4], liquid oil [5,6], and sludge char [7,8] via the pyrolysis and/or gasification of the sewage sludge. The producer gas includes combustible component (e.g., hydrogen, carbon monoxide, and methane) like gas fuel. The liquid oil is produced by cooling the producer gas and can be used as a liquid fuel via refinement. The sludge char is produced via carbonization and/or activation, and is used as an adsorbent, etc. [2]. However, sludge liquefaction is a more expensive process and gives rise to problems in relation to feeding of the sludge at high temperature, product separation from solvents (when used), and use of high-pressure hydrogen [9]. In addition, because of the high nitrogen and sulphur contents in the products, the sludge liquefaction requires additional processing techniques [10]. The pyrolysis is a process of producing gas and oil using the high-temperature thermal cracking due to the external heat source without supplying air or steam. Tar and refractory soot formation can damage the relevant devices [11]. The gasification technique
1226-086X/$ – see front matter ß 2011 The Korean Society of Industrial and Engineering Chemistry. Published by Elsevier B.V. All rights reserved. doi:10.1016/j.jiec.2011.11.144
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uses the low-oxygen, partial-oxidation combustion heat that involves minimum air injection. The gas produced includes carbon dioxide and nitrogen, and the gases pose a problem in terms of low-heating value. It is known that steam gasification produces synthetic gas with a high hydrogen production rate from hydrocarbon fuels or waste. In particular, the surface area of char increases due to the steam injection, as steam activation, and it can be used as an adsorbent. In this case, the heavy metals in the sludge (except for mercury and cadmium) are trapped in the char and are not eluted. In this study, steam activation test in a bench scale was achieved to investigate the conversion of the sewage sludge into an energy and resources (i.e., producer gas and sludge char), including generating tar which causes clogging and corrosion in the pipeline and leads to operation trouble and damage to equipments. The physical characteristics of sludge char, the composition and heating value of producer gas, and the composition and concentration of tar were identified to show the effect of steam activation for the sewage sludge. 2. Experimental setup and procedure 2.1. Experimental equipment Fig. 1 shows the bench-scale gasification device, which consists of a wire mesh reactor, steam and gas feeding line, sampling and analysis line, and control and monitoring device. The wire mesh reactor is a cylindrical stainless tube (500 mm length and 85 mm inner diameter). To ensure uniform gas flow, a honeycomb-type distribution plate was installed at the reactor bottom. To apply heat to the reactor, an electric furnace (Model CLF-T1320, SERIN, Korea) with a controller featuring the fine temperature control within the range reaching up to 1000 8C, surrounded the reactor outside wall. A sample container with a stainless 400-mesh plate net was positioned on the honeycombtype distribution plate. The steam and gas feeding line consisted of a steam generator, syringe water pump, and N2 cylinder. The steam generator had an internal stick-type cartridge heater, which is regulated by the controller to maintain a specific temperature (350 8C). The syringe water pump (Model KDS 100, KD Scientific, USA) supplied a small amount of water and the N2 cylinder supplied the carrier gas. They passed though the Venturi mixer and entered the steam generator. In the sampling and analysis line, a gas meter (Model W-MK10-ST, Shinagawa, Japan) was installed to measure the producer
Table 1 Physicochemical characteristics of dewatered sludge. Analysis methods
Contents
Value (wt.%)
Proximate analysis
Moisture Volatile matter Fixed carbon Ash
9.6 51.1 32.9 6.4
Ultimate analysis
C H O N S
52.3 8.2 32.2 7.92 0.01
gas volume with sampling time, and the gas volume was used for showing tar concentration. The analysis device consisted of a GCFID for tar analysis (GC-14B, SHIMADZU, Japan) and a GC-TCD for gas analysis (CP-4900, Varian, Netherlands). The control and monitoring device controlled the electric furnace and steam generator. The electric furnace was heated by the reactor heat controller, and the steam generator was regulated by the heat controller to maintain its set temperature. In addition, thermocouples were installed at sludge bed and gas layer in the reactor, on the electric furnace wall and in the steam generator. A data logger (Model Hydra data logger 2625A, Fluke, USA) was used to continuously monitor the temperatures. 2.2. Experimental method The sewage sludge was dehydrated using a centrifuge in the urban sewage treatment plant at a moisture content of approximately 85%. To make the dried sludge with a moisture content of 10% or less, the dewatered sludge was dried for approximately 7 h at 105–110 8C, using an electric furnace (Model KS-35, Kwang Sung Co. Ltd., Korea). The dried sludge was crushed using a grinder; a tailor sieve (RoTap Sieve Shaker, Chunggye Ltd., Korea) was used to make the sludge particle diameters uniform at 1–1.5 mm. Table 1 shows the results of the proximate and ultimate analyses for the dried sludge. Before the main test, the sample container with 20 g dried sludge was placed on the distribution plate in the reactor. The carrier gas (N2) was fed into the reactor at a flow rate of 100 mL/ min for 30 min to sufficiently purge the reactor and sample line. And then the main test achieved sequential pyrolysis (i.e., carbonization) and steam gasification (i.e., steam activation). For the pyrolysis, the temperature was increased from room tempera-
Fig. 1. Experimental setup for the steam activation of sewage sludge.
Y.N. Chun et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 839–847
ture to 500 8C at a heating rate of 25 8C/min, and then it was maintained for 10 min (holding time). After the carbonization, the steam activation was achieved by increasing the temperature up to the final temperature of 900 8C at a heating rate of 25 8C/min. And then, the condition was maintained for 10 min at the final activation temperature. The steam flow rate for the steam activation was changed to 4–19 mL/g min to investigate the effect of the amount of steam feed. After each test was completed, N2 gas was supplied until the temperature in the reactor reached room temperature. The reactor cover was then opened, the sludge char left in the sample container was collected, and its physical characteristics were analyzed. 2.3. Analytical method 2.3.1. Tar sampling and analysis To measure the total amount of tar produced during carbonization and activation, the wet-type tar sampling method was used to determine the gravimetric tar and selected light tar (Fig. 1). The selected tars are benzene, naphthalene, anthracene, and pyrene that have 1–4 benzene rings, and benzonitrile and benzeneacetonitrile including nitrogen content in the sewage sludge [12]. The wet-type tar sampling and analysis in this study were followed to biomass technology groups (BTGs) method [13–15]. The wet-type tar sampling was achieved for testing period, and the gravimetric tar mass and light tar were analyzed. The wet-type sampling line was fabricated by installing impingers in two isothermal baths to condense and absorb tar. The temperature of the first isothermal bath was kept constant at 20 8C or below, and four impingers were installed and filled with 100 mL isopropyl alcohol. A chiller (ECS-30SS, Eyela Co., Japan) was used as the second isothermal bath, and its temperature was kept constant at 20 8C or below. Of the two impingers, one impinger was filled with isopropyl alcohol and the other was left empty. In the series of impinger bottles, the first impinger bottle acts as a moisture and particle collector in which water, tar, and soot are condensed from the process gas by absorption in isopropanol. Other impinger bottles collect tars and the empty bottle collects drop. Immediately after completing the sampling, the content of the impinger bottles were filtered through a filter paper (Model F-5B, Advantec Co., Japan). The filtered isopropanol solution was divided into two parts. The first was used to determine the gravimetric tar mass by means of solvent distillation and evaporation by evaporator (Model N-1000-SW, Eyela, Japan) in which temperature and vapor pressure were 55–57 8C and 230 hPa, respectively. The second was used to determine the concentrations of the light tar compounds using GC-FID (Model 14B, Shimadzu, Japan). Quantitative tar analysis was performed on a GC system, using a RTX-5 (RESTEK) capillary column (30 m – 0.53 mm id, 0.5 mm film thickness) and an isothermal temperature profile at 45 8C for the first 2 min. This analysis was followed by a 7 8C/min-temperature gradient to 320 8C and finally an isothermal period at 320 8C for 10 min. Helium was used as a carrier gas. The temperature of the detector and injector were maintained at 340 8C and 250 8C, respectively. 2.3.2. Producer gas sampling and analysis The producer gas was sampled at the end point of the wet-type tar sampling line as shown in Fig. 1. A set of backup VOC adsorber was installed downstream of the series of impingers to protect the column of the gas chromatography from the residual solvent or VOCs, which may have passed through the impinger train. The set of backup VOC adsorber consisted of two cotton filters and an activated carbon filter connected in a series. The GC-TCD (Model
841
CP-4900, Varian, Netherland) was used for the producer gas analysis. The Molecularsieve-5A column was used for H2, CO, O2, and N2, and the PoraPlot-Q column was used for CO2, C2H4, and C2H6. 2.3.3. Sludge char analysis To analyze the pore formation characteristic of the sampled sludge char, nitrogen adsorption was conducted. The adsorption isothermal curve was measured using an analytical equipment (Model NanoPOROSITY-XQ, Mirae SI, Korea), which was formed according to the adsorption of nitrogen gas at 196 8C [1]. The specific surface area was calculated by applying the adsorption characteristic of the specimen and BET equation. To calculate the pore distribution and mean pore diameter, Barret–Joyner–Halenda (BJH) equation was used for mesopores, and Horvath–Kawazoe (HK) equation for micropores. To compare the development of pores in the sludge char, scanning electron microscopy (SEM; Model S-4800, Hitachi Co., Japan) was used. In SEM, the pores were magnified by 50,000 times. To identify the chemical properties and contents, the energy-dispersive X-ray spectroscopy (EDS; Model 7593-H, Horiba, UK) was used. The adsorption capacity of the sludge char produced was assessed from the adsorption test using benzene to identify the tar adsorption characteristic.
3. Results and discussion 3.1. Characteristics of pyrolysis and gasification in the sewage sludge The pyrolysis gasification of dried sludge consists of two steps: the primary pyrolysis (or carbonization) and secondary gasification (or activation) reaction as shown in Fig. 2. The dried sludge is converted into char, tar, and gas in the primary pyrolysis (200– 500 8C), while tar and char are converted to gas in the secondary gasification process. The sewage sludge mostly consists of cellulose; a significant portion of it is converted into tar in the primary pyrolysis and into gas in the secondary gasification [1]. The primary pyrolysis is mainly influenced by the heating rate, and the secondary gasification is influenced by the reactor temperature [16,17]. The secondary gasification reaction includes the gasification of sludge char (Eqs. (1)–(4)), which is the carbonous residue produced from the primary pyrolysis, decomposition of tar (Eqs. (5) and (6)), and light produced gas reactions (Eqs. (7)–(11)). In general, steam gasification include reactions of O2, CO2, H2, and H2O with the combustible fraction of carbon in the carbonized sludge, thereby producing gaseous products. The essential features involve chemisorptions of the reacting gas species on the carbon surface [18]. The tar decomposition takes place at pyrolysis temperatures lower than 600 8C. The generation of CH4 and C2 hydrocarbons is caused by the easy breaking of C–H bond of alkyl.
Fig. 2. Material balance for pyrolysis and gasification of dried sludge.
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842
A large amount of CO and CO2 is released as a result of C5 5O diminishing. At high temperatures (>600 8C), the sharp decrease of the absorbance amount of C–O and C-Haromatic demonstrates that the cracking and reforming reactions of aromatics occur and favor the production of H2 and CO [19]. Gasification reaction of sludge char Partial oxidation C þ 1=2O2 ! CO
DH ¼ 110:5 kJ=mol
(1)
Water–gas reaction C þ H2 O ! CO þ H2
DH ¼ 131:3 kJ=mol
(2)
Boudouard reaction C þ CO2 ! 2CO
DH ¼ 171:7 kJ=mol
(3)
Hydrogasification C þ 2H2 ! CH4 DH ¼ 74:9 kJ=mol
(4)
Fig. 3. Mass yield of products.
Decomposition reaction of tar Tar pyrolysis Tar ! wH2 þ xCO þ yCO2 þ zCn Hm
(5)
Tar steam gasification Tar þ vH2 O ! xCO þ yH2
(6)
Reaction of light gas Methanation reaction CO þ 3H2 $ CH4 þ H2 O
DH ¼ 206:2 kJ=mol
(7)
Water–gas shift reaction CO þ H2 O $ CO2 þ H2
DH ¼ 41:1 kJ=mol
(8)
Steam reform Cn Hm þ nH2 O ¼ nCO þ ðn þ m=2ÞH2
(9)
the primary pyrolysis, and the residual tar in the char was generated with the increase in steam during steam activation [20]. The role of steam cannot be limited only for transportation and stabilization of the volatile products such as nitrogen. The ability of steam to penetrate into solid materials and to help desorption, distillation, and efficient removal of the volatile products from it can explain the higher yield of tar [21]. When the steam feed rate was high (19 mL/g min), however, the total tar slightly decreased. This was because the relatively sufficient amount of steam promoted the tar decomposition reaction which is tar steam gasification (Eq. (6)). The amount of gas was calculated excluding the amount of sludge char and tar, which does not include fed and produced steam. With the increase in the steam feed rate, the amount of gas increased. As mentioned earlier, this was because the water–gas reaction (Eq. (2)), which is the gasification reaction of sludge char, and tar steam gasification (Eq. (6)), which is the decomposition reaction of tar, led to the reaction of fixed carbon and tar with steam into producer gas.
where CnHm is the light hydrocarbon. 3.2. Effect of the amount of steam feed The test was conducted in the fixed pyrolysis condition (heating rate: 25 8C/min; pyrolysis temperature: 500 8C; and holding time: 10 min) and the fixed steam activation condition (heating rate: 25 8C/min; final activation temperature: 900 8C; and holding time: 10 min). To know the effect of the amount of steam feed, the four steam feed rates (4 mL/g min, 9 mL/g min, 14 mL/g min, and 19 mL/ g min) were used for the test. 3.2.1. Mass yield of products Fig. 3 shows the mass yield of products according to the change in the steam feed rate. The order according to the mass yield was gas, char, and tar. With the increase in the steam feed rate, the sludge char decreased. The sludge char generally decreased according to Eqs. (1)–(4) which are the sludge char gasification reactions. Particularly, with the increase in the steam, the fixed carbon of the sludge char was converted into gas due to the water–gas reaction (Eq. (2)) and subsequently decreased. The total amount of tar increased with the increase in the steam feed rate within the low-steam, feed-rate range (less than 14 mL/ g min). This was because most of the volatile tar was generated in
3.2.2. Characteristics of producer gas From the beginning of the test, the producer gas from the pyrolysis and steam gasification was sampled and analyzed as online at a GC. Therefore, the analysis was delayed for approximately 20 min which took the initially produced gas to flow from the reactor outlet to the GC sampling port. In addition, steam was injected for steam activation after 36 min at the end of the pyrolysis. Therefore, as shown in Fig. 4, the producer gas for the steam injection was sampled from 56 min. Fig. 4 shows the concentrations of selected producer gases, such as H2, CO, CO2, CH4, C2H4, and C2H6. Tar and CO, CO2, and CH4 gases were produced from the primary pyrolysis, while tar and char were converted into light gas from the secondary steam activation. Some light gases reacted with one another under the steam atmosphere, and their production and extinction occurred. H2 was hardly generated from the initial primary pyrolysis. It rapidly increased from the secondary steam activation with the steam feeding, the increase of which had a great effect. Since H2 is produced according to the water–gas reaction (Eq. (2), wherein the fixed carbon in the sludge reacts with steam), and tar steam gasification (Eq. (6), wherein the generated tar reacts with steam), the production rate of H2 was increased at higher steam feed rate. And the light gas CO and light hydrocarbons (CnHm) that was produced from primary pyrolysis was converted into H2 via the
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843
Fig. 4. Effect of steam feed rate on composition of the gaseous products according to elapsed time.
reaction with steam (H2O), as in water–gas shift reaction (Eq. (8)) and steam reform (Eq. (9)), leading to the increase for hydrogen. In addition, regardless of steam feed, H2 increased with the increase in the activation temperature according to the pyrolysis (Eq. (5)) of tar generated from the primary pyrolysis. In all three cases wherein the steam feed rate was less than 14 mL/g min, the CO gradually increased with the increase for steam, showing same pattern. This is because the CO was continuously produced during the primary pyrolysis and secondary steam activation processes according to the gasification of sludge char (Eqs. (1)–(3)), tar decomposition (Eqs. (5) and (6)) and steam reform (Eq. (9)). Particularly, with the increase in the steam, the production rate of CO was higher due to the effects of water– gas reaction (Eq. (2)), tar steam gasification (Eq. (6)) and steam reform (Eq. (9)). When the steam feed rate was higher than that in the three cases above (e.g., at 19 mL/g min), a significant amount of volatile CO was produced [21]. In particular, when steam gasification atmosphere was maintained with sufficient steam at a later time, the amount of CO decreased because CO was converted into CO2 due to water–gas shift reaction (Eq. (8)). When the steam feed rate was less than 14 mL/g min, CO2 was produced in the greatest amount during the primary pyrolysis process. And then its amount decreased and was kept almost constant. However, at the steam feed rate of 14 mL/g min or higher, the amount of CO2 increased with the increase in the steam feed rate. This was because CO in the gas was converted to CO2 according to water–gas shift reaction (Eq. (8)). The effect was greater when the steam feed rate was higher.
The release of CO and CO2 at lower temperature (<450 8C) is mainly caused by the breaking of carbonyl and carboxyl functional groups in sewage sludge. The release of CO, as a major secondary product from tar cracking, was favored high temperature [19]. CH4 was produced in the greatest amount during the primary pyrolysis process and disappeared according to the steam reform (Eq. (9)). C2 hydrocarbons (C2H4 and C2H6) were produced in a smaller amount than other gases in the tar pyrolysis (Eq. (5)) which is the decomposition of tar produced in the primary pyrolysis process. After having maximum values, they disappeared by the steam reforming like the CH4 removal. The products C2H4 and C2H6 were converted into CH4 via the following decomposition reactions (Eqs. (10) and (11)) [19]. C2 H6 ! C2 H4 þ H2
(10)
C2 H4 ! CH4 þ C
(11)
When the temperature was high, the temperature had a greater effect than steam in the both reactions. That is why each value in the light hydrocarbons had almost similar according to the different steam feed rate. Fig. 5 shows the total amount of gas and higher heating value according to the steam feed rate. With the increase in the steam feed rate, the total amount of producer gas produced from the carbonization and steam activation increased. This was because more residual volatile gas containing tar was produced with the increase of steam feed for secondary activation [20]. The generated tar was converted into CO
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Fig. 5. Mass yield of product and total amount of producer gas.
and H2 gases via tar steam gasification (Eq. (6)). And the water–gas reaction (Eq. (2)), which is the gasification of sludge char, resulted in the reaction of fixed carbon with steam into the gas. Therefore, gas production increased [18,22]. The heating value increased with the increase in the steam feed rate because the total amount of combustible gases, including H2, CO, C2H4, and C2H6, increased. In particular, the steam had a great effect when the steam feed rate was 19 mL/g min, resulting in a heating value of 15,233 kJ/Nm3. 3.2.3. Characteristics of selected light tar generation Fig. 6 shows the distribution of selected light tar concentration according to the steam feed rate. When the steam feed rate was lower than 14 mL/g min, benzene (light aromatic tar) and naphthalene (light polycyclic aromatic hydrocarbon: PAH) concentrations increased with the increase in the steam feed rate. This is because the activation led to the easy penetration of steam into sludge char pores created from
Fig. 6. Light tar contribution according to the steam feed rate.
the primary pyrolysis, and the resulting desorption, distillation, and efficient removal of the volatile products increased the amount of light tar. At a steam feed rate of 19 mL/g min, however, the amount of both light tars decreased. This was because the light tars were converted into light gases and decreased according to the tar steam gasification reaction (Eq. (6)) with the increase in the steam feed rate. Benzene and naphthalene were more influenced by steam than other PAHs [16]. The other selected light PAHs, anthracene and pyrene, slightly increased with the increase in the steam feed. The two tar compounds were hardly influenced by the amount of steam and may be evolved directly from the dried sludge by pyrolysis process [16]. Benzonitrile and benzoacetonitrile (aromatic nitrile) showed the similar phenomena as with benzene and naphthalene but
Fig. 7. SEM photos for sludge chars in case of different steam feed rate: (a) 4 mL/g min; (b) 9 mL/g min; (c) 14 mL/g min; (d) 19 mL/g min.
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Fig. 8. Element compounds measured by EDX.
appeared in a relatively small amount because the nitrogen content in the sludge was small (Table 1). 3.2.4. Characteristics of sludge char Fig. 7 shows the SEM pictures to identify the pore characteristics of activated sludge char produced from the steam feed rate change test. The pore size classification in this study follows the IUPAC classification [23], that is, micropores (<2 nm), mesopores (2–50 nm), and macropores (>50 nm). The increase in the amount of steam feed gradually resulted in the creation of micropores and mesopores; they developed most pores when the steam feed rate was 14 mL/g min. At a steam feed rate of 19 mL/g min, however, the excessive steam supply led to the sintering and the pores sank into ashes. In the primary carbonization, the residual moisture and volatile material evaporated and pores developed. Bagreev et al. proved that water released by the dehydroxylation of inorganic material could aid pore formation and could act as an agent for creating micropores [24]. In addition, Inguanzo et al. proposed that carbonization increased the porosity through unblocking many of the pores obscured by volatile matter [25]. In the secondary steam activation, the high temperature steam injected from the outside penetrates into pores that are produced from the primary pyrolysis (carbonization) process. It leads to gas production and pore development according to the water–gas reaction (Eq. (2)) which is the on-site surface reaction.
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As mentioned above, however, the excessive water–gas reaction not only increases the gas production, but also makes the pores sink due to the loss of fixed carbon in sludge char, as in the case of 19 mL/gmin. Furthermore, the optimal adsorption capacity cannot be ensured. Therefore, a proper amount of steam must be supplied like 14 mL/g min in this study. Fig. 8 is obtained from the EDX analyzer coupled to SEM measurements. The fixed carbon decreased with the increase in the steam feed rate. This is because the fixed carbon on the sludge char pore surfaces was consumed by the water–gas reaction (Eq. (2)). Especially, at a steam feed rate of 19 mL/g min, at which pores sank, more fixed carbon content decreased than the case with lower steam feed rates. As shown in the EDX results, the inorganic atoms AAEM (alkali and alkaline earth metallic) species (Mg, K, Ca), Al, and Fe did not volatilize but remained in the sludge char [26]. It is known that these contents serve as catalysts in the pyrolysis and gasification. For example, with Al, if existing in the form of Al2O3, it would be an acid catalyst for cracking reaction [27]; or with K and Ca atoms, they were already reported as the catalyst for pyrolysis in literature [28]. To evaluate adsorption properties of the sludge chars, which were produced with different amounts of steam feed, input concentration of benzene for adsorption test was fixed at 1%; GHSV (gas hourly space velocity) at 25 8C was 6315/h (gas at 2 L/min, volume at 19 mL). Fig. 9 shows the breakthrough curve and adsorption amount for comparing adsorption characteristics of the different sludge chars (a) or a commercial activated carbon (b). C is effluent concentration, and Ci refers to input concentration. The breakthrough point was defined as the point at which the effluent concentration is 10% higher than the influent concentration. The adsorption amount refers to the amount of adsorption in which the effluent concentration reaches the saturation point which is the same as the influent concentration. As shown in Fig. 9(a), the sludge char that was produced when the steam feed rate was high had breakthrough curve shifted to longer time. The rightmost curve was obtained at a steam feed rate of 14 mL/g min, and the adsorption capacity was best. It took 15 min to reach the breakthrough point, and the adsorption amount was 174.8 mg/g. However, the breakthrough point of sludge char was at the leftmost position when the steam feed rate was 19 mL/g min because part of pores sank due to steam gasification and the adsorption capacity rapidly decreased, as expected in the results of Fig. 7.
Fig. 9. Breakthrough curve and adsorption amount of benzene for the sludge chars and a commercial activated carbon.
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Table 2 Porous characteristics and adsorption capacity of the adsorbents from this study and other results [12]. Adsorbent a
Sludge char Commercial activated carbonb Activated carbonc Wood chipc a b c
Mean pore size (nm)
Specific surface area (m2/g)
Pore volume (cm3/g)
Adsorption amount (mg/g)
6.229 1.83 1.128 10.077
80.28 1376.6 987.1 1.072
0.135051 0.63 0.5569 0.0058
174.8 586.2 97.5 155.7
Steam-activated char produced in this study (benzene adsorption). Commercial activated carbon (benzene adsorption). Tested by Thana Phuphuakrat et al. (tar adsorption generated from Japanese cedar).
Fig. 9(b) shows the adsorption characteristics of the sludge char and commercial activated carbon when the steam feed rate, as the reference condition, is 14 mL/g min. The commercial activated carbon had a significantly better benzene adsorption capacity than the sludge char produced in this study. This was because the commercial activated carbon had not only well-developed micropores, which allows easy benzene adsorption, but also large specific surface area (see Table 2). The breakthrough point of the commercial activated carbon was 240 min, and its adsorption capacity was 586.2 mg/g. Table 2 represents the analysis and test results of the sludge char and commercial activated carbon of which benzene adsorption test were achieved in this study. In addition, two adsorbents (activated carbon and wood chip) as studied by Thana Phuphuakrat et al. are shown [13]. As for the sludge char, its mean pore size was bigger than those of commercial activated carbon and activated carbon, as studied by Thana Phuphuakrat et al., and its specific surface area and pore volume were smaller. The sludge char (6.229 nm) had a mean pore size of mesopores, like wood chips (10.077 nm), but two activated carbon materials had a mean pore size of micropores. That is, the sludge char and wood chips had the characteristic similar to adsorption of mesopores. And the sludge char had a specific surface area of 80.28 m2/g, an influential factor for adsorption amount, larger than that of wood chips (1.072 m2/g). In the case of adsorption amount, the sludge char and commercial activated carbon were of benzene test in this study. But the activated carbon and wood chips used in the study of Thana Phuphuakrat et al. were obtained from total tar adsorption the pyrolysis of Japanese cedar. Therefore, it is difficult to compare the adsorption capacity results from the two both case. However, the benzene adsorption capacity of the sludge char was relatively smaller compared to that of commercial activated carbon in this study. As mentioned above, the reason is that the pore size was larger and the specific surface area was smaller in the sludge char. Some portion of heavy tar, that is almost all of the gravimetric tar in the Fig. 3, was converted to light tar or gas via thermal cracking or steam reforming. Among the light tars, non-condensible aromatic tars (e.g., benzene, benzonitrile, benzoacetonitrile, etc.) will be no damage on devices (i.e., combustor, engine, pipe line, etc.) [13]. That is, when light aromatic tars are contained in the producer gas, they can be effectively used with increases of the heating value and thus energy efficiency. However, condensable light PAH tars such as naphthalene, anthracene, and pyrene may damage the devices and thus need to be removed. Therefore, it should be effective that adsorption of the sludge char could not be better for the non-condensible light aromatic tar to improve energy efficiency. For the sludge char produced from the steam activation, the benzene passed through the sludge char after it reached the breakthrough point in a short time. As mentioned above in the tar adsorption test, as determined by Thana Phuphuakrat et al. [13], the development of mesopores in the sludge char easily leads to the adsorption of condensable light
PAH tar, making it an efficient adsorbent for tar removal of producer gas generated from the pyrolysis and gasification.
4. Conclusions Steam activation of dried sewage sludge was studied for converting the sewage sludge from the waste treatment plant to energy and resources. It could be identified that the steam activation had good characteristics for hydrogen rich gas production and sludge char which can be used for gas energy and tar adsorbent, respectively. The mass yield of product was in the order of gas, char, and tar. With the increase in the steam feed rate, the sludge char decreased, and producer gas increased. The tar increased up to 14 mL/g min, but decreased at a higher steam feed rate. The amount of producer gas increased and the heating value also increased with the increase in the steam feed rate. The produced gases from the carbonization and activation included H2, CO, CO2, CH4, C2H4, and C2H2. The H2 was hardly produced from the primary carbonization, but rapidly increased during the secondary steam activation. The amount of increase became higher as more steam was fed. The CO and CO2 were produced in large amounts with the evaporation of volatile materials in the primary carbonization. From the secondary steam activation, CO continued to increase, while CO2 reached its maximum and then decreased. When the steam feed rate was higher, however, CO decreased and CO2 increased at a high temperature. The CH4 and C2 hydrocarbons (C2H4 and C2H6) were produced in the primary carbonization and steam activation, respectively. They subsequently reached their maximums and then decreased. For benzene (light aromatic tar) and naphthalene (light PAH), their tar concentrations increased with the increase in the amount of steam feed, when the steam feed rate was less than 14 mL/g min. At a steam feed rate of 19 mL/g min, however, the benzene and naphthalene decreased. The light PAHs, anthracene and pyrene, slightly increased with the increase in the steam feed. Benzonitrile and benzoacetonitrile (aromatic nitrile) showed the similar phenomenon as with benzene and naphthalene, but appeared in a relatively small amount. The increase in the steam feed gradually resulted in micropores and mesopores. They developed most when the vapor volume was 14 mL/g min. At a steam feed rate of 19 mL/g min, however, the excessive steam supply led to sintering and the adsorption capacity deteriorated. The inorganic atoms, AAEM species (Mg, K, Ca), Al, and Fe did not evaporate but remained in the sludge char. These materials serve as catalysts for pyrolysis or gasification. The sludge char had a mean pore size of 6.229 nm which is the size of mesopores from which condensated tar, which damages the device, is properly adsorbed and removed. The specific surface area was 80.28 m2/g, smaller than that of commercial activated carbon, but it was feasible enough. In conclusion, proper steam feeding is very important in making low-tar gas to be used gas energy and sludge char to be used tar removal adsorbent from sewage sludge using steam activation. The
Y.N. Chun et al. / Journal of Industrial and Engineering Chemistry 18 (2012) 839–847
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