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Urban energy mining from sewage sludge
Chemosphere 90 (2013) 1508–1513
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Technical Note
Urban energy mining from sewage sludge E.E. Kwon a,b, H. Yi b, H.H. Kwon a,⇑ a b
Department of Civil Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu,Jeonju-si, Jeollabuk-do, 561-756, South Korea Bio-Energy Team, Research Institute of Industrial Science and Technology, Pohang 540-090, South Korea
h i g h l i g h t s " Biodiesel production from sewage sludge. " Enhanced biodiesel production in CO2. " Utilizing heterogeneous catalyst with magnesium slag. " Reduction of condensable hydrocarbons (tar) in CO2.
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Article history: Received 12 March 2012 Received in revised form 25 July 2012 Accepted 26 July 2012 Available online 25 September 2012 Keywords: Sewage sludge Biodiesel Transesterification reaction Gasification Pyrolysis Syngas
a b s t r a c t This work showed that sewage sludge could be a strong candidate for biodiesel production. High lipid content (18–20%) with C16 18-carbon range was experimentally identified and measured. These lipids from sewage sludge were converted into biodiesel via the transesterification reaction with MgO–CaO/ Al2O3 derived from magnesium slag, and biodiesel conversion was 98%. The experimental work enabled explaining that temperature is the main driving force for the transesterification reaction, which can be enhanced in the presence of CO2. This also enables combination of esterification of free fatty acids and transesterification of triglycerides into a single process within 1 min in the temperature range of 350– 500 °C. Sewage sludge residue after extracting lipids was also a good feedstock for recovering energy via thermo-chemical processes. The impact of CO2 co-feed on the pyrolysis/gasification process of SS residue was also investigated in this work. The CO2 injected into the thermo-chemical process remarkably increased the generation of CO by a factor of 2. Moreover, the introduction of CO2 into the pyrolysis/gasification process enabled reducing condensable hydrocarbons (tar) by expediting cracking; thus, utilizing CO2 as chemical feedstock for the gasification process not only leads to higher thermal efficiency but also has environmental benefits. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The generation of sewage sludge (SS) from the wastewater treatment plant (WWTP) as byproducts is inevitable (Adegoroye et al., 2004; Afif et al., 2011). Approximately 4 Mt of SS are annually generated in South Korea. Most generated SS in South Korea has been disposed of in the ocean. However, disposing of SS in the ocean will be prohibited after 2011 due to the London Convention 97 protocol, which bans waste dumping into the ocean. Thus, environmentally and ecologically benign SS treatment should be implemented immediately. The conventional disposal of SS includes industrial utilization, individual combustion (Font et al., 2001, 2005; Amand et al., ⇑ Corresponding author. Address: Department of Civil Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu,Jeonju-si, Jeollabuk-do, 561-756, South Korea. Tel./fax: +82 63 270 2464. E-mail address: [email protected] (H.H. Kwon). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.07.062
2006), co-combustion with coal (Amand et al., 2006), and composting (Chiang et al., 2011) for farmland utilization as fertilizer. However, that farmland application as fertilizer is very limited due to potential contamination by metals present in SS (Albores et al., 2000; Fytili and Zabaniotou, 2008). In addition, combusting only SS needs stringent control of volatile pollutants. Thus, the Korean EPA does not allow combusting only SS. Moreover, SS co-firing with coal for power generation is regulated by the minimum heating value of co-firing feedstock, which is 3500 kJ kg 1. Thus, most SS for co-firing feedstock must be thermally treated to reduce moisture content by up to 10%. Moreover, reducing the moisture content of SS delays the decomposition of SS. Thus, investigating the environmentally benign and economically feasible treatment of SS must be considered. One of the alternatives could be the production of biodiesel from SS (Angerbauer et al., 2008). Biodiesel has been receiving more and more attention as alternative fuel due to its environmental benefits (Meng et al., 2008, 2009; Song et al., 2008; Converti
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et al., 2009; Khan et al., 2009; Lardon et al., 2009; Powell and Hill, 2009; Um and Kim, 2009; Umdu et al., 2009). Based on life cycle analysis (LCA), the replacement of petroleum-derived diesel fuel with biodiesel reduces greenhouse gas emission by up to 45% due to carbon neutrality of biodiesel feedstock (Basha et al., 2009; Lardon et al., 2009). Biodiesel consists of long chain, fatty acid methyl esters (FAMEs) produced from various lipids by the transesterification of triglyceride since some physicochemical properties of vegetable oils disable their use in diesel engines (Basha et al., 2009; da Cunha et al., 2009). In a typical biodiesel process, triglycerides with low free fatty acid (FFA) (less than 0.5%) content are transesterified with methanol (MeOH) in the presence of alkaline homogeneous catalyst such as potassium hydroxide (Meng et al., 2008, 2009; da Cunha et al., 2009; Lu et al., 2009). Biodiesel transformation is complicated if the oil contains large amounts of FFAs (more than 1%) resulting in the formation of soap with alkaline catalyst (Jagtap et al., 2008; Basha et al., 2009; Patil and Deng, 2009). The amount of FFAs is reduced through pretreatment, based on the esterification reaction using acid catalyst followed by the usual transesterification (Chongkhong et al., 2007, 2009; Lin et al., 2009). Thus, finding cheap materials for heterogeneous catalysts would be desirable since heterogeneous catalysts (Benjapornkulaphong et al., 2009; Cho et al., 2009) are independent on the amount of FFAs. In this work, utilizing slag as heterogeneous catalyst derived from a magnesium-smelting factory was investigated. In addition, lipid extraction from SS inevitably generates a substantial amount of SS residue, which would be desirable for recovering energy by means of thermo-chemical processes (pyrolysis/ gasification). Pyrolysis (Font et al., 2001; Fonts et al., 2009) has been used to convert various feedstocks into a blend of solid, liquid, and gaseous products. Gasification (Butterman and Castaldi, 2009; Nipattummakul et al., 2010a, 2010b) is also an attractive technology for the production of syngas (H2 and CO). Considerable works (Parnaudeau and Dignac, 2007; Nipattummakul et al., 2010a, 2010b; Saw et al., 2012; Vitasari et al., 2011) have been documented with respect to the pyrolysis/gasification of conventional fuel, such as coal and cellulosic biomass; however, accessible information on unconventional fuels such as SS residue is very limited. In addition, the impact of CO2 on the pyrolysis/gasification process of SS has not been investigated. Thus, another objective of this work was to investigate mechanistically and evaluate the CO2 cofeed impact on the pyrolysis/gasification process of SS.
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using 1 M nitric acid; the mixture was filtered to remove the insoluble materials, and 2 M of ammonia formed gel was added; the excess ammonia was washed away from the gel with deionized water until the pH was between 9 and 11; after drying, slag material and Al2O2 (Sumitomo, Japan) were soaked in ethanol (e.g., 7 g of slag material and 100 g of Al2O3), and then calcined in N2 flow at 600 °C for 6 h. The transesterification reaction at varying alcohol-to-oil ratio, temperature, and pressure conditions was tested. The pressure reactor (Parr Pressure reactor, Series 4520, USA) also was used to understand the pressure effect on the transesterification reaction. After the reaction, the mixture was allowed to settle for overnight before separating into separate bottles the glycerol layer and the top layer including the FAME fraction removed and was weighed and analyzed with GC/MS (HP-7890A/5975C MSD) and GC/FID (HP-7890A). GC/MS and GC/FID were calibrated with Supelco FAME mixture (Lot #LB-80557). The GC column, DB-WAX (J&W 127-7012) and DB-5ht (Agilent, 123-5711), was employed. The injector temperature was 350 °C. The temperature programming of the GC oven was done through a method in ChemStation software (Agilent, USA). The initial temperature was set at 50 °C and was maintained for 3 min. And then temperature rose at a rate of 10 °C min 1 to 180 °C (held 3 min), 8 °C min 1 to 260 °C (held 5 min), and 30 °C min 1 to 320 °C (held 32 min). EN-14103 method (European standard for determining FAME contents) was used for the conversion of biodiesel (FAME). A tubular reactor (TR), made of 2.54 cm od quartz tubing (Chemglass CGQ-0800T-13) and 2.54 cm Stainless Ultra-Torr Vacuum Fitting (Swagelok SS-4-UT-6-400), was used to maintain airtight conditions. The required experimental temperature was achieved using a spit-hinged furnace (Multiple Unit, Hevi Duty Electric Company, USA) over the temperature range 250–500 °C, with the temperature simultaneously compared with an S-type thermocouple reading to ensure that the target temperature had been met. An insulation collar (Duraboard high temperature insulation, USA) at end of the furnace was used to block heat transfer during operation and to secure quartz tubing. Oil feedstock and MeOH was introduced continuously to the TR using HPLC pump (Lab Alliance PN#F40SFX01). All gases used in the experiments were of ultra-high purity and obtained from Daesung Industrial Gases. All gas flow rates were set using Brooks mass flow controllers (Brooks SLA5800 Series, USA). 2.3. Thermo-chemical process
2. Material and methods 2.1. Sample preparation and characterization A total of 15 SS samples were obtained from WWTPs in Korea. The heat of combustion of SS was determined using a bomb calorimeter (Parr 6500, USA). The amount of C, H, N, and S in SS was determined using Elemental Determinator (LECO CHN-2000 and S-144DR, France). ASTM D-2361 determines the chlorine content of SS with Eschka mixture. 2.2. Biodiesel (FAME) conversion Lipid from SS was extracted with non-polar solvent (i.e. n-Hexane) (Sigma–Aldrich, St. Louis, USA). A rotary evaporator (Cole Parmer, USA) was used to remove the solvent from the extracted lipid from SS. The magnesium slag used for the experiment was derived from dolomite ore, which was obtained in powder form from a magnesium-smelting factory in South Korea. The catalyst was generated using the following procedure: magnesium slag was pretreated
A Netzsch STA 499 F1 Jupiter thermo-gravimetric analysis (TGA) unit capable of TGA and differential temperature analysis measurements was used. The TGA unit used for the experimental work enabled increasing temperature from ambient to 1200 °C at heating rate of 10–1500 °C min 1. The same TR was used for the thermo-chemical processes. However, the rate of sample loading into TR was controlled using a screw feeder. The effluent from the TGA unit and TR was sent to either l-GC (Agilent 3000A) or GC/MS (Agilent 9890/5973) for the identification and quantification of chemical species. The sampling system that includes transfer lines was maintained at almost 300 °C to minimize the condensation and/or adsorption of hydrocarbons onto its surfaces. 3. Results and discussion 3.1. Basic properties of SS The characterization of SS including proximate analysis, ultimate analysis, and heat of combustion in SS was carried out as
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Fig. 1. Representative chromatogram of extracted SS oil and fatty acid identification.
summarized in Supplementary Material (SM) Table SM-1. A total of 15 sludge samples were randomly collected from WWTPs, and the collected samples were denoted as alphabet from A to O for convenience. The moisture content of SS varied from 74% to 86%, but its average was 82%. Compared to cellulosic biomass, SS has exceptionally high ash content (25.7%) and relatively low fixed carbon content. For instance, cellulosic biomass such as Oakwood (Butterman and Castaldi, 2009) has 17% and 1.5% fixed carbon content and ash content, respectively; however, the heat of combustion is roughly similar to that of biomass. This observation implies the high amount of volatile materials in SS. Among the volatile materials in SS, the amount of lipids that can be used for biodiesel transformation has yet to be determined. Thus, lipid extraction using nhexane was performed. Various SS samples were extracted with n-hexane; their lipid contents varied from 18% to 20% of dry basis of SS. Based on the total generation of SS in Korea (4 Mt), 40% of the current biodiesel consumption (BD2 renewable standard portfolio base in South Korea) can be supported by SS. Thus, SS can be a strong candidate for biodiesel production feedstock. The identification of fatty acid profiles in the extracted SS oil was visualized with the representative chromatogram in Fig. 1. Such visualization was experimentally carried out by means of acid esterification with H2SO4 at 60 °C for 65 h. As illustrated in Fig. 1, most fatty acids in SS oil are C16 18 carbon range lipids, making up more than 96%. 3.2. Biodiesel conversion using a heterogeneous catalyst Since MgO–CaO/Al2O3 derived from magnesium slag is not a commonly used catalyst for the transesterification reaction, the reference value with respect to the optimal ratio of MeOH to oil should be established. Thus, various MeOH to SS oil ratios (10, 20, 30, 40, and 50 wt%) were tested. The experimental conditions were 150 °C for 1 h at 500 kPa. The weight percent ratio of MgO–CaO/Al2O3 to the extracted lipid from SS was 20%. To maintain the experimental pressure and investigate the CO2 co-feed impact on the transesterification reaction, the pressure reactor was filled with N2 and CO2. This idea inferred from the enhanced pyrolysis/gasification process by means of using CO2 as the reaction medium (Butterman and Castaldi, 2009). For example, the CO2 co-feed impact on the thermo-chemical process of biomass and municipal solid waste is substantial; i.e. the enhanced thermal cracking of volatile compounds, which directly leads to a reduction of condensable hydrocarbons (tar) and an increase of the generation of CO. In general, this observation at the temperature regime below 550 °C was not discernible. However, the
authors postulated that utilizing CO2 indeed expedited the rate of transesterification reaction without the thermal cracking of oil feedstock, which provided the favorable condition for the transesterification reaction by means of impeding the reversible transesterification reaction. All experimental results are shown in Fig. 2. One interesting observation is that the conversion of biodiesel indeed improved in the presence of CO2. This observation enables explaining that CO2 expedites the transesterification reaction. This observation also suggests useful methodologies for utilizing CO2 during biodiesel production. As discussed above, the CO2 co-feed impact on the transesterification would be more effective at relatively high temperature regime (i.e. the temperature regime higher than 150 °C). More detailed information on the temperature regime higher than 150 °C will be addressed later. The extracted lipid from SS had high acid value (18.74). Particularly, in the case of using conventional homogeneous catalysts, acid value higher than 1 should be pretreated by means of esterification with H2SO4 due to saponification (soap formation). However, a heterogeneous catalyst would not be a matter of high acid value, which is one of the advantages. Furthermore, utilizing heterogeneous catalyst for biodiesel conversion does not generate the effluent of wastewater. For example, wastewater is generated from the washing process to remove the used homogenous catalyst. As shown in Fig. 2, 20% MeOH to extracted lipid from SS ratio under the presence of CO2 reached the maximum achievable conversion of biodiesel with the extracted lipid from SS. The work done by Isayama and Saka (2008) reported that both temperature and pressure were the crucial factors for expediting the rate of the transesterification reaction. However, the authors postulated that temperature would be the main driving force of the transesterification reaction. Thus, the conversion of biodiesel under varying temperature and pressure was also carried out to investigate the temperature and pressure effect on the transesterification reaction. The experimental work in the presence of CO2 was performed at temperature range of 130–250 °C and pressure range of 500– 5000 kPa. The MeOH to extracted SS oil ratio was 20%. Unlike the experimental work in Fig. 2, the transesterification using MgO– CaO/Al2O3 was carried out for 30 min. One interesting observation is that the pressure effect on the transesterification reaction was not critical as compared to temperature effect. For example, the observation in Fig. 3a validates that temperature is the main driving force for transesterification reaction (i.e. the pressure effect on the temperature regime higher than 230 °C is almost negligible). Thus, the same catalyst (MgO–CaO/Al2O3) was packed into a TR in order to get more detailed information on the effect of temperature and to modify a real continuous flow system for producing biodiesel.
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efficiency of biodiesel is shown in Fig. 3b. This methodology also enables combination of esterification of FFAs and transesterification of triglycerides into a single process, and leads to a 98% conversion efficiency of FAME within 1 min in the temperature range of 350–500 °C. Thus, this observation validates the temperature is the main driving force of biodiesel production. 3.3. Thermo-chemical processes (pyrolysis/gasification) with SS residue
Fig. 2. FAME conversion in N2 and CO2 atmosphere using MgO–CaO/Al2O3 (150 °C and 500 kPa).
A series of thermo-gravimetric analysis (TGA) were carried out with dried SS (i.e. without extracting lipid from SS) and SS residue. The SS residue after extracting lipid in Section 3.1 was used. TGA measurements were carried out at a heating rate of 800 °C min 1 over a temperature range of ambient 1000 °C in N2 and CO2 to characterize the thermal degradation and to determine the impact of CO2 co-feed on the pyrolysis of SS. The thermograms in Fig. 4 show that more mass conversion was achieved with dried SS. Indeed, SS residue has a substantial amount of volatile materials. However, the mass change at the temperature range of 200–370 °C seems to be attributed to the non-polar solvent (n-hexane), since the lipid content (18–20%) of SS in Section 3.1. cannot be explained with the final mass conversion in Fig. 4. One interesting observation in Fig. 4 is the thermal degradation rate. For example, the thermal degradation of dried SS and SS residue in N2 was initiated earlier than that in CO2. However, the thermal degradation rate shown as the slope in Fig. 4 became faster at the temperature range of 400–600 °C. Two scenarios can explain this observation. For example, the impact of CO2 co-feed impact is only effective on a certain temperature regime. CO2 can modify topographic properties that enhance volatilization. Note, however, that the expected effect of the Boudouard reaction (C + CO2 ? 2CO) was not observed in Fig. 4. In general, more achievable mass conversion through the Boudouard reaction is desirable because CO2 can be used as chemical feedstock to achieve high thermal efficiency; thus directly leading to an environmental benefit. The reaction between volatile matter and CO2 is not filly explained in Fig. 4. Thus, the effluent from the TGA unit was analyzed with micro-GC. The observed and identified gaseous chemical species (H2, C1 4-hydrocarbons, and CO) were mostly identical. However, their concentrations were much different in the presence of CO2. For example, more CO by a factor of 2 was generated in the presence of CO2. Thus, the authors hypothesized that the amount of condensable hydrocarbons (tar) could be reduced in the presence of CO2.
Fig. 3. (a) FAME conversion under varying temperature and pressure, and (b) FAME conversion under varying temperature and 100 kPa.
MeOH and the extracted lipid from SS were continuously fed into the TR using HPLC pumps. The initial feeding ratio of the extracted lipid from SS to MeOH was 10:2 at the temperature range from 250 to 500 °C under the ambient pressure. The experimental temperature regime higher than 500 °C was not considered due to the thermal cracking of the extracted lipid from SS. For example, the thermal cracking evidenced by the detection of hydrogen was identified at the temperature regime higher than 530 °C via the TGA analysis. The feeding rate of the extracted lipid from SS and MeOH was 10 and 2 mL min 1, respectively. Moreover, carbon dioxide was also used as the reaction medium to increase the conversion efficiency of biodiesel as discussed in Fig. 2. The conversion
Fig. 4. Representative thermogram with dried SS and SS residue in N2 and CO2.
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Fig. 5. Representative chromatograms of the pyrolytic oil derived from SS residue in N2 and CO2.
To validate the authors’ hypothesis, a scale-up experiment was carried out, which resolved the uncertainty arising from the heterogeneous matrix of SS and small amount of sample used for the TGA experiments. Thus, the experimental work was performed using a TR at heating rate of 950 °C min 1 and final temperature of 1000 °C. The pyrolytic oil was analyzed using GC/MS, and its chromatogram is shown in Fig. 5. The most abundant identified chemical species, based on peak areas, in the pyrolytic oil were labeled in Fig. 5; the most abundant chemical species were fatty acids, aromatic hydrocarbons, or hetero-aromatic hydrocarbon compounds such as pyridine. The observation of fatty acids reflects the incomplete lipid extraction. While aromatic hydrocarbons and hetero-aromatic hydrocarbons may not be the best starting point of fuel oil, this process may be better
suited for manufacturing chemical feedstock. Recovering energy from SS residue via the pyrolysis process would not be the best choice due to the high nitrogen content of SS residue as a form of protein. Thus, recovering energy via the gasification process would be a better choice since syngas is mainly H2 and CO. In addition, the purification and separation of gaseous products are relatively easy. One interesting observation in Fig. 5 is that utilizing CO2 in pyrolysis served to mitigate the formation of tar (50%) based on total peak area in Fig. 5. For example, a significant amount of ring structure breakdown occurred in the presence of CO2 during the pyrolysis process of SS residue, which suggests that pyrolytic products can be tailored for any purpose. In addition, this observation validates the authors’ hypothesis as discussed above. Pyrolysis is the first step in the gasification process, which can then be followed by the partial oxidation of the primary product. Thus, the identified impact of the CO2 co-feed on pyrolysis will enhance the gasification process. To verify this, the steam gasification of SS residue was done using the TR. The concentration profiles of major chemical species (H2, CO, CH4, and C2H4) are shown in Fig. 6. An interesting observation in Fig. 6 is the enhanced generation of CO by a factor of 2 in the presence of CO2. Moreover, the collected tar in the presence of CO2 was substantially decreased (50%), which directly led to the enhanced generation of CO. For example, CO2 expedites the thermal cracking of condensable hydrocarbons, which was consistent with the discussion in Fig. 5. Another interesting observation in Fig. 6 is the concentration of CO at the high-temperature regime (800–1000 °C), which decreased substantially as the concentration of H2 increased. This could be explained by the water gas shift (WGS) reaction (H2O + CO ? H2 + CO2). For example, this WGS reaction would be expedited under CO2-rich condition.
4. Conclusions
Fig. 6. Concentration profiles of major chemical species.
This study showed that SS could be a strong candidate for biodiesel. High lipid content was experimentally identified and measured. These lipids from SS were converted into biodiesel via the transesterification reaction with MgO–CaO/Al2O3 derived from magnesium slag, and biodiesel conversion was 98%. The experimental work enabled explaining that temperature is the main driving force for the transesterification reaction, which can be enhanced in the presence of CO2. SS residue after extracting lipids was also a good feedstock for energy recovery via thermo-chemical processes. The CO2 injected into the thermo-chemical process remarkably increased the generation of CO by a factor of 2. Moreover, the introduction of CO2 into the thermo-chemical (pyrolysis/ gasification) process enabled reducing condensable hydrocarbons
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(tar) by expediting cracking; thus, utilizing CO2 as chemical feedstock for the pyrolysis/gasification process not only leads to higher thermal efficiency but also has environmental benefits. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2012.07.062. References Adegoroye, A., Paterson, N., Li, X., Morgan, T., Herod, A.A., Dugwell, D.R., Kandiyoti, R., 2004. The characterisation of tars produced during the gasification of sewage sludge in a spouted bed reactor. Fuel 83, 1949–1960. Afif, E., Azadi, P., Farnood, R., 2011. Catalytic hydrothermal gasification of activated sludge. Appl. Catal. B-Environ. 105, 136–143. Albores, A.F., Cid, B.P., Gomez, E.F., Lopez, E.F., 2000. Comparison between sequential extraction procedures and single extractions for metal partitioning in sewage sludge samples. Analyst 125, 1353–1357. Amand, L.-E., Leckner, B., Eskilsson, D., Tullin, C., 2006. Deposits on heat transfer tubes during co-combustion of biofuels and sewage sludge. Fuel 85, 1313–1322. Angerbauer, C., Siebenhofer, M., Mittelbach, M., Guebitz, G.M., 2008. Conversion of sewage sludge into lipids by Lipomyces starkeyi for biodiesel production. Bioresource Technol. 99, 3051–3056. Basha, S.A., Gopal, K.R., Jebaraj, S., 2009. A review on biodiesel production, combustion, emissions and performance. Renew. Sust. Energy Rev. 13, 1628– 1634. Benjapornkulaphong, S., Ngamcharussrivichai, C., Bunyakiat, K., 2009. Al2O3supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil. Chem. Eng. J. 145, 468–474. Butterman, H.C., Castaldi, M.J., 2009. Syngas production via CO2 enhanced gasification of biomass fuels. Environ. Eng. Sci. 26, 703–713. Chiang, K.-Y., Lu, C.-H., Chien, K.-L., 2011. Enhanced energy efficiency in gasification of paper-reject sludge by a mineral catalyst. Int. J. Hydrogen Energy 36, 14186– 14194. Cho, Y.B., Seo, G., Chang, D.R., 2009. Transesterification of tributyrin with methanol over calcium oxide catalysts prepared from various precursors. Fuel Process. Technol. 90, 1252–1258. Chongkhong, S., Tongurai, C., Chetpattananondh, P., 2009. Continuous esterification for biodiesel production from palm fatty acid distillate using economical process. Renew. Energy 34, 1059–1063. Chongkhong, S., Tongurai, C., Chetpattananondh, P., Bunyakan, C., 2007. Biodiesel production by esterification of palm fatty acid distillate. Biomass Bioenergy 31, 563–568. Converti, A., Casazza, A.A., Ortiz, E.Y., Perego, P., Del Borghi, M., 2009. Effect of temperature and nitrogen concentration on the growth and lipid content of Nannochloropsis oculata and Chlorella vulgaris for biodiesel production. Chem. Eng. Process. 48, 1146–1151. da Cunha, M.E., Krause, L.C., Moraes, M.S.A., Faccini, C.S., Jacques, R.g.A., Almeida, S.R., Rodrigues, M.R.A., Caram, E.B., 2009. Beef tallow biodiesel produced in a pilot scale. Fuel Process. Technol. 90, 570–575.
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Font, R., Fullana, A., Conesa, J., 2005. Kinetic models for the pyrolysis and combustion of two types of sewage sludge. J Anal. Appl. Pyrol. 74, 429–438. Font, R., Fullana, A., Conesa, J.A., Llavador, F., 2001. Analysis of the pyrolysis and combustion of different sewage sludges by TG. J. Anal. Appl. Pyrol. 58–59, 927– 941. Fonts, I., Azuara, M., Gea, G., Murillo, M.B., 2009. Study of the pyrolysis liquids obtained from different sewage sludge. J. Anal. Appl. Pyrol. 85, 184–191. Fytili, D., Zabaniotou, A., 2008. Utilization of sewage sludge in EU application of old and new methods: review. Renew. Sust. Energy Rev. 12, 116–140. Isayama, Y., Saka, S., 2008. Biodiesel production by supercritical process with crude bio-methanol prepared by wood gasification. Bioresource Technol. 99, 4775– 4779. Jagtap, S.R., Patil, Y.P., Fujita, S.-I., Arai, M., Bhanage, B.M., 2008. Heterogeneous base catalyzed synthesis of 2-oxazolidinones/2-imidiazolidinones via transesterification of ethylene carbonate with b-aminoalcohols/1,2-diamines. Appl. Catal. A-Gen. 341, 133–138. Khan, S.A., Rashmi, Hussain, M.Z., Prasad, S., Banerjee, U.C., 2009. Prospects of biodiesel production from microalgae in India. Renew. Sust. Energy Rev. 13, 2361–2372. Lardon, L., Helias, A., Sialve, B., Steyer, J.-P., Bernard, O., 2009. Life-cycle assessment of biodiesel production from microalgae. Environ. Sci. Technol. 43, 6475–6481. Lin, L., Ying, D., Chaitep, S., Vittayapadung, S., 2009. Biodiesel production from crude rice bran oil and properties as fuel. Appl. Energy 86, 681–688. Lu, H., Liu, Y., Zhou, H., Yang, Y., Chen, M., Liang, B., 2009. Production of biodiesel from Jatropha curcas L. oil. Comput. Chem. Eng. 33, 1091–1096. Meng, X., Chen, G., Wang, Y., 2008. Biodiesel production from waste cooking oil via alkali catalyst and its engine test. Fuel Process. Technol. 89, 851–857. Meng, X., Yang, J., Xu, X., Zhang, L., Nie, Q., Xian, M., 2009. Biodiesel production from oleaginous microorganisms. Renew. Energy 34, 1–5. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K., 2010a. High temperature steam gasification of wastewater sludge. Appl. Energy 87, 3729–3734. Nipattummakul, N., Ahmed, I.I., Kerdsuwan, S., Gupta, A.K., 2010b. Hydrogen and syngas production from sewage sludge via steam gasification. Int. J. Hydrogen Energy 35, 11738–11745. Parnaudeau, V., Dignac, M.-F., 2007. The organic matter composition of various wastewater sludges and their neutral detergent fractions as revealed by pyrolysis-GC/MS. J. Anal. Appl. Pyrol. 78, 140–152. Patil, P.D., Deng, S., 2009. Optimization of biodiesel production from edible and nonedible vegetable oils. Fuel 88, 1302–1306. Powell, E.E., Hill, G.A., 2009. Economic assessment of an integrated bioethanol– biodiesel–microbial fuel cell facility utilizing yeast and photosynthetic algae. Chem. Eng. Res. Des. 87, 1340–1348. Saw, W., McKinnon, H., Gilmour, I., Pang, S., 2012. Production of hydrogen-rich syngas from steam gasification of blend of biosolids and wood using a dual fluidised bed gasifier. Fuel 93, 473–478. Song, D., Fu, J., Shi, D., 2008. Exploitation of oil-bearing microalgae for biodiesel. Chin. J. Biotechnol. 24, 341–348. Um, B.-H., Kim, Y.-S., 2009. Review: a chance for Korea to advance algal-biodiesel technology. J. Ind. Eng. Chem. 15, 1–7. Umdu, E.S., Tuncer, M., Seker, E., 2009. Transesterification of Nannochloropsis oculata microalga’s lipid to biodiesel on Al2O3 supported CaO and MgO catalysts. Bioresource Technol. 100, 2828–2831. Vitasari, C.R., Jurascik, M., Ptasinski, K.J., 2011. Exergy analysis of biomass-tosynthetic natural gas (SNG) process via indirect gasification of various biomass feedstock. Energy 36, 3825–3837.
Contents lists available at SciVerse ScienceDirect
Chemosphere journal homepage: www.elsevier.com/locate/chemosphere
Technical Note
Urban energy mining from sewage sludge E.E. Kwon a,b, H. Yi b, H.H. Kwon a,⇑ a b
Department of Civil Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu,Jeonju-si, Jeollabuk-do, 561-756, South Korea Bio-Energy Team, Research Institute of Industrial Science and Technology, Pohang 540-090, South Korea
h i g h l i g h t s " Biodiesel production from sewage sludge. " Enhanced biodiesel production in CO2. " Utilizing heterogeneous catalyst with magnesium slag. " Reduction of condensable hydrocarbons (tar) in CO2.
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Article history: Received 12 March 2012 Received in revised form 25 July 2012 Accepted 26 July 2012 Available online 25 September 2012 Keywords: Sewage sludge Biodiesel Transesterification reaction Gasification Pyrolysis Syngas
a b s t r a c t This work showed that sewage sludge could be a strong candidate for biodiesel production. High lipid content (18–20%) with C16 18-carbon range was experimentally identified and measured. These lipids from sewage sludge were converted into biodiesel via the transesterification reaction with MgO–CaO/ Al2O3 derived from magnesium slag, and biodiesel conversion was 98%. The experimental work enabled explaining that temperature is the main driving force for the transesterification reaction, which can be enhanced in the presence of CO2. This also enables combination of esterification of free fatty acids and transesterification of triglycerides into a single process within 1 min in the temperature range of 350– 500 °C. Sewage sludge residue after extracting lipids was also a good feedstock for recovering energy via thermo-chemical processes. The impact of CO2 co-feed on the pyrolysis/gasification process of SS residue was also investigated in this work. The CO2 injected into the thermo-chemical process remarkably increased the generation of CO by a factor of 2. Moreover, the introduction of CO2 into the pyrolysis/gasification process enabled reducing condensable hydrocarbons (tar) by expediting cracking; thus, utilizing CO2 as chemical feedstock for the gasification process not only leads to higher thermal efficiency but also has environmental benefits. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction The generation of sewage sludge (SS) from the wastewater treatment plant (WWTP) as byproducts is inevitable (Adegoroye et al., 2004; Afif et al., 2011). Approximately 4 Mt of SS are annually generated in South Korea. Most generated SS in South Korea has been disposed of in the ocean. However, disposing of SS in the ocean will be prohibited after 2011 due to the London Convention 97 protocol, which bans waste dumping into the ocean. Thus, environmentally and ecologically benign SS treatment should be implemented immediately. The conventional disposal of SS includes industrial utilization, individual combustion (Font et al., 2001, 2005; Amand et al., ⇑ Corresponding author. Address: Department of Civil Engineering, Chonbuk National University, 567 Baekje-daero, Deokjin-gu,Jeonju-si, Jeollabuk-do, 561-756, South Korea. Tel./fax: +82 63 270 2464. E-mail address: [email protected] (H.H. Kwon). 0045-6535/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.chemosphere.2012.07.062
2006), co-combustion with coal (Amand et al., 2006), and composting (Chiang et al., 2011) for farmland utilization as fertilizer. However, that farmland application as fertilizer is very limited due to potential contamination by metals present in SS (Albores et al., 2000; Fytili and Zabaniotou, 2008). In addition, combusting only SS needs stringent control of volatile pollutants. Thus, the Korean EPA does not allow combusting only SS. Moreover, SS co-firing with coal for power generation is regulated by the minimum heating value of co-firing feedstock, which is 3500 kJ kg 1. Thus, most SS for co-firing feedstock must be thermally treated to reduce moisture content by up to 10%. Moreover, reducing the moisture content of SS delays the decomposition of SS. Thus, investigating the environmentally benign and economically feasible treatment of SS must be considered. One of the alternatives could be the production of biodiesel from SS (Angerbauer et al., 2008). Biodiesel has been receiving more and more attention as alternative fuel due to its environmental benefits (Meng et al., 2008, 2009; Song et al., 2008; Converti
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et al., 2009; Khan et al., 2009; Lardon et al., 2009; Powell and Hill, 2009; Um and Kim, 2009; Umdu et al., 2009). Based on life cycle analysis (LCA), the replacement of petroleum-derived diesel fuel with biodiesel reduces greenhouse gas emission by up to 45% due to carbon neutrality of biodiesel feedstock (Basha et al., 2009; Lardon et al., 2009). Biodiesel consists of long chain, fatty acid methyl esters (FAMEs) produced from various lipids by the transesterification of triglyceride since some physicochemical properties of vegetable oils disable their use in diesel engines (Basha et al., 2009; da Cunha et al., 2009). In a typical biodiesel process, triglycerides with low free fatty acid (FFA) (less than 0.5%) content are transesterified with methanol (MeOH) in the presence of alkaline homogeneous catalyst such as potassium hydroxide (Meng et al., 2008, 2009; da Cunha et al., 2009; Lu et al., 2009). Biodiesel transformation is complicated if the oil contains large amounts of FFAs (more than 1%) resulting in the formation of soap with alkaline catalyst (Jagtap et al., 2008; Basha et al., 2009; Patil and Deng, 2009). The amount of FFAs is reduced through pretreatment, based on the esterification reaction using acid catalyst followed by the usual transesterification (Chongkhong et al., 2007, 2009; Lin et al., 2009). Thus, finding cheap materials for heterogeneous catalysts would be desirable since heterogeneous catalysts (Benjapornkulaphong et al., 2009; Cho et al., 2009) are independent on the amount of FFAs. In this work, utilizing slag as heterogeneous catalyst derived from a magnesium-smelting factory was investigated. In addition, lipid extraction from SS inevitably generates a substantial amount of SS residue, which would be desirable for recovering energy by means of thermo-chemical processes (pyrolysis/ gasification). Pyrolysis (Font et al., 2001; Fonts et al., 2009) has been used to convert various feedstocks into a blend of solid, liquid, and gaseous products. Gasification (Butterman and Castaldi, 2009; Nipattummakul et al., 2010a, 2010b) is also an attractive technology for the production of syngas (H2 and CO). Considerable works (Parnaudeau and Dignac, 2007; Nipattummakul et al., 2010a, 2010b; Saw et al., 2012; Vitasari et al., 2011) have been documented with respect to the pyrolysis/gasification of conventional fuel, such as coal and cellulosic biomass; however, accessible information on unconventional fuels such as SS residue is very limited. In addition, the impact of CO2 on the pyrolysis/gasification process of SS has not been investigated. Thus, another objective of this work was to investigate mechanistically and evaluate the CO2 cofeed impact on the pyrolysis/gasification process of SS.
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using 1 M nitric acid; the mixture was filtered to remove the insoluble materials, and 2 M of ammonia formed gel was added; the excess ammonia was washed away from the gel with deionized water until the pH was between 9 and 11; after drying, slag material and Al2O2 (Sumitomo, Japan) were soaked in ethanol (e.g., 7 g of slag material and 100 g of Al2O3), and then calcined in N2 flow at 600 °C for 6 h. The transesterification reaction at varying alcohol-to-oil ratio, temperature, and pressure conditions was tested. The pressure reactor (Parr Pressure reactor, Series 4520, USA) also was used to understand the pressure effect on the transesterification reaction. After the reaction, the mixture was allowed to settle for overnight before separating into separate bottles the glycerol layer and the top layer including the FAME fraction removed and was weighed and analyzed with GC/MS (HP-7890A/5975C MSD) and GC/FID (HP-7890A). GC/MS and GC/FID were calibrated with Supelco FAME mixture (Lot #LB-80557). The GC column, DB-WAX (J&W 127-7012) and DB-5ht (Agilent, 123-5711), was employed. The injector temperature was 350 °C. The temperature programming of the GC oven was done through a method in ChemStation software (Agilent, USA). The initial temperature was set at 50 °C and was maintained for 3 min. And then temperature rose at a rate of 10 °C min 1 to 180 °C (held 3 min), 8 °C min 1 to 260 °C (held 5 min), and 30 °C min 1 to 320 °C (held 32 min). EN-14103 method (European standard for determining FAME contents) was used for the conversion of biodiesel (FAME). A tubular reactor (TR), made of 2.54 cm od quartz tubing (Chemglass CGQ-0800T-13) and 2.54 cm Stainless Ultra-Torr Vacuum Fitting (Swagelok SS-4-UT-6-400), was used to maintain airtight conditions. The required experimental temperature was achieved using a spit-hinged furnace (Multiple Unit, Hevi Duty Electric Company, USA) over the temperature range 250–500 °C, with the temperature simultaneously compared with an S-type thermocouple reading to ensure that the target temperature had been met. An insulation collar (Duraboard high temperature insulation, USA) at end of the furnace was used to block heat transfer during operation and to secure quartz tubing. Oil feedstock and MeOH was introduced continuously to the TR using HPLC pump (Lab Alliance PN#F40SFX01). All gases used in the experiments were of ultra-high purity and obtained from Daesung Industrial Gases. All gas flow rates were set using Brooks mass flow controllers (Brooks SLA5800 Series, USA). 2.3. Thermo-chemical process
2. Material and methods 2.1. Sample preparation and characterization A total of 15 SS samples were obtained from WWTPs in Korea. The heat of combustion of SS was determined using a bomb calorimeter (Parr 6500, USA). The amount of C, H, N, and S in SS was determined using Elemental Determinator (LECO CHN-2000 and S-144DR, France). ASTM D-2361 determines the chlorine content of SS with Eschka mixture. 2.2. Biodiesel (FAME) conversion Lipid from SS was extracted with non-polar solvent (i.e. n-Hexane) (Sigma–Aldrich, St. Louis, USA). A rotary evaporator (Cole Parmer, USA) was used to remove the solvent from the extracted lipid from SS. The magnesium slag used for the experiment was derived from dolomite ore, which was obtained in powder form from a magnesium-smelting factory in South Korea. The catalyst was generated using the following procedure: magnesium slag was pretreated
A Netzsch STA 499 F1 Jupiter thermo-gravimetric analysis (TGA) unit capable of TGA and differential temperature analysis measurements was used. The TGA unit used for the experimental work enabled increasing temperature from ambient to 1200 °C at heating rate of 10–1500 °C min 1. The same TR was used for the thermo-chemical processes. However, the rate of sample loading into TR was controlled using a screw feeder. The effluent from the TGA unit and TR was sent to either l-GC (Agilent 3000A) or GC/MS (Agilent 9890/5973) for the identification and quantification of chemical species. The sampling system that includes transfer lines was maintained at almost 300 °C to minimize the condensation and/or adsorption of hydrocarbons onto its surfaces. 3. Results and discussion 3.1. Basic properties of SS The characterization of SS including proximate analysis, ultimate analysis, and heat of combustion in SS was carried out as
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Fig. 1. Representative chromatogram of extracted SS oil and fatty acid identification.
summarized in Supplementary Material (SM) Table SM-1. A total of 15 sludge samples were randomly collected from WWTPs, and the collected samples were denoted as alphabet from A to O for convenience. The moisture content of SS varied from 74% to 86%, but its average was 82%. Compared to cellulosic biomass, SS has exceptionally high ash content (25.7%) and relatively low fixed carbon content. For instance, cellulosic biomass such as Oakwood (Butterman and Castaldi, 2009) has 17% and 1.5% fixed carbon content and ash content, respectively; however, the heat of combustion is roughly similar to that of biomass. This observation implies the high amount of volatile materials in SS. Among the volatile materials in SS, the amount of lipids that can be used for biodiesel transformation has yet to be determined. Thus, lipid extraction using nhexane was performed. Various SS samples were extracted with n-hexane; their lipid contents varied from 18% to 20% of dry basis of SS. Based on the total generation of SS in Korea (4 Mt), 40% of the current biodiesel consumption (BD2 renewable standard portfolio base in South Korea) can be supported by SS. Thus, SS can be a strong candidate for biodiesel production feedstock. The identification of fatty acid profiles in the extracted SS oil was visualized with the representative chromatogram in Fig. 1. Such visualization was experimentally carried out by means of acid esterification with H2SO4 at 60 °C for 65 h. As illustrated in Fig. 1, most fatty acids in SS oil are C16 18 carbon range lipids, making up more than 96%. 3.2. Biodiesel conversion using a heterogeneous catalyst Since MgO–CaO/Al2O3 derived from magnesium slag is not a commonly used catalyst for the transesterification reaction, the reference value with respect to the optimal ratio of MeOH to oil should be established. Thus, various MeOH to SS oil ratios (10, 20, 30, 40, and 50 wt%) were tested. The experimental conditions were 150 °C for 1 h at 500 kPa. The weight percent ratio of MgO–CaO/Al2O3 to the extracted lipid from SS was 20%. To maintain the experimental pressure and investigate the CO2 co-feed impact on the transesterification reaction, the pressure reactor was filled with N2 and CO2. This idea inferred from the enhanced pyrolysis/gasification process by means of using CO2 as the reaction medium (Butterman and Castaldi, 2009). For example, the CO2 co-feed impact on the thermo-chemical process of biomass and municipal solid waste is substantial; i.e. the enhanced thermal cracking of volatile compounds, which directly leads to a reduction of condensable hydrocarbons (tar) and an increase of the generation of CO. In general, this observation at the temperature regime below 550 °C was not discernible. However, the
authors postulated that utilizing CO2 indeed expedited the rate of transesterification reaction without the thermal cracking of oil feedstock, which provided the favorable condition for the transesterification reaction by means of impeding the reversible transesterification reaction. All experimental results are shown in Fig. 2. One interesting observation is that the conversion of biodiesel indeed improved in the presence of CO2. This observation enables explaining that CO2 expedites the transesterification reaction. This observation also suggests useful methodologies for utilizing CO2 during biodiesel production. As discussed above, the CO2 co-feed impact on the transesterification would be more effective at relatively high temperature regime (i.e. the temperature regime higher than 150 °C). More detailed information on the temperature regime higher than 150 °C will be addressed later. The extracted lipid from SS had high acid value (18.74). Particularly, in the case of using conventional homogeneous catalysts, acid value higher than 1 should be pretreated by means of esterification with H2SO4 due to saponification (soap formation). However, a heterogeneous catalyst would not be a matter of high acid value, which is one of the advantages. Furthermore, utilizing heterogeneous catalyst for biodiesel conversion does not generate the effluent of wastewater. For example, wastewater is generated from the washing process to remove the used homogenous catalyst. As shown in Fig. 2, 20% MeOH to extracted lipid from SS ratio under the presence of CO2 reached the maximum achievable conversion of biodiesel with the extracted lipid from SS. The work done by Isayama and Saka (2008) reported that both temperature and pressure were the crucial factors for expediting the rate of the transesterification reaction. However, the authors postulated that temperature would be the main driving force of the transesterification reaction. Thus, the conversion of biodiesel under varying temperature and pressure was also carried out to investigate the temperature and pressure effect on the transesterification reaction. The experimental work in the presence of CO2 was performed at temperature range of 130–250 °C and pressure range of 500– 5000 kPa. The MeOH to extracted SS oil ratio was 20%. Unlike the experimental work in Fig. 2, the transesterification using MgO– CaO/Al2O3 was carried out for 30 min. One interesting observation is that the pressure effect on the transesterification reaction was not critical as compared to temperature effect. For example, the observation in Fig. 3a validates that temperature is the main driving force for transesterification reaction (i.e. the pressure effect on the temperature regime higher than 230 °C is almost negligible). Thus, the same catalyst (MgO–CaO/Al2O3) was packed into a TR in order to get more detailed information on the effect of temperature and to modify a real continuous flow system for producing biodiesel.
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efficiency of biodiesel is shown in Fig. 3b. This methodology also enables combination of esterification of FFAs and transesterification of triglycerides into a single process, and leads to a 98% conversion efficiency of FAME within 1 min in the temperature range of 350–500 °C. Thus, this observation validates the temperature is the main driving force of biodiesel production. 3.3. Thermo-chemical processes (pyrolysis/gasification) with SS residue
Fig. 2. FAME conversion in N2 and CO2 atmosphere using MgO–CaO/Al2O3 (150 °C and 500 kPa).
A series of thermo-gravimetric analysis (TGA) were carried out with dried SS (i.e. without extracting lipid from SS) and SS residue. The SS residue after extracting lipid in Section 3.1 was used. TGA measurements were carried out at a heating rate of 800 °C min 1 over a temperature range of ambient 1000 °C in N2 and CO2 to characterize the thermal degradation and to determine the impact of CO2 co-feed on the pyrolysis of SS. The thermograms in Fig. 4 show that more mass conversion was achieved with dried SS. Indeed, SS residue has a substantial amount of volatile materials. However, the mass change at the temperature range of 200–370 °C seems to be attributed to the non-polar solvent (n-hexane), since the lipid content (18–20%) of SS in Section 3.1. cannot be explained with the final mass conversion in Fig. 4. One interesting observation in Fig. 4 is the thermal degradation rate. For example, the thermal degradation of dried SS and SS residue in N2 was initiated earlier than that in CO2. However, the thermal degradation rate shown as the slope in Fig. 4 became faster at the temperature range of 400–600 °C. Two scenarios can explain this observation. For example, the impact of CO2 co-feed impact is only effective on a certain temperature regime. CO2 can modify topographic properties that enhance volatilization. Note, however, that the expected effect of the Boudouard reaction (C + CO2 ? 2CO) was not observed in Fig. 4. In general, more achievable mass conversion through the Boudouard reaction is desirable because CO2 can be used as chemical feedstock to achieve high thermal efficiency; thus directly leading to an environmental benefit. The reaction between volatile matter and CO2 is not filly explained in Fig. 4. Thus, the effluent from the TGA unit was analyzed with micro-GC. The observed and identified gaseous chemical species (H2, C1 4-hydrocarbons, and CO) were mostly identical. However, their concentrations were much different in the presence of CO2. For example, more CO by a factor of 2 was generated in the presence of CO2. Thus, the authors hypothesized that the amount of condensable hydrocarbons (tar) could be reduced in the presence of CO2.
Fig. 3. (a) FAME conversion under varying temperature and pressure, and (b) FAME conversion under varying temperature and 100 kPa.
MeOH and the extracted lipid from SS were continuously fed into the TR using HPLC pumps. The initial feeding ratio of the extracted lipid from SS to MeOH was 10:2 at the temperature range from 250 to 500 °C under the ambient pressure. The experimental temperature regime higher than 500 °C was not considered due to the thermal cracking of the extracted lipid from SS. For example, the thermal cracking evidenced by the detection of hydrogen was identified at the temperature regime higher than 530 °C via the TGA analysis. The feeding rate of the extracted lipid from SS and MeOH was 10 and 2 mL min 1, respectively. Moreover, carbon dioxide was also used as the reaction medium to increase the conversion efficiency of biodiesel as discussed in Fig. 2. The conversion
Fig. 4. Representative thermogram with dried SS and SS residue in N2 and CO2.
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Fig. 5. Representative chromatograms of the pyrolytic oil derived from SS residue in N2 and CO2.
To validate the authors’ hypothesis, a scale-up experiment was carried out, which resolved the uncertainty arising from the heterogeneous matrix of SS and small amount of sample used for the TGA experiments. Thus, the experimental work was performed using a TR at heating rate of 950 °C min 1 and final temperature of 1000 °C. The pyrolytic oil was analyzed using GC/MS, and its chromatogram is shown in Fig. 5. The most abundant identified chemical species, based on peak areas, in the pyrolytic oil were labeled in Fig. 5; the most abundant chemical species were fatty acids, aromatic hydrocarbons, or hetero-aromatic hydrocarbon compounds such as pyridine. The observation of fatty acids reflects the incomplete lipid extraction. While aromatic hydrocarbons and hetero-aromatic hydrocarbons may not be the best starting point of fuel oil, this process may be better
suited for manufacturing chemical feedstock. Recovering energy from SS residue via the pyrolysis process would not be the best choice due to the high nitrogen content of SS residue as a form of protein. Thus, recovering energy via the gasification process would be a better choice since syngas is mainly H2 and CO. In addition, the purification and separation of gaseous products are relatively easy. One interesting observation in Fig. 5 is that utilizing CO2 in pyrolysis served to mitigate the formation of tar (50%) based on total peak area in Fig. 5. For example, a significant amount of ring structure breakdown occurred in the presence of CO2 during the pyrolysis process of SS residue, which suggests that pyrolytic products can be tailored for any purpose. In addition, this observation validates the authors’ hypothesis as discussed above. Pyrolysis is the first step in the gasification process, which can then be followed by the partial oxidation of the primary product. Thus, the identified impact of the CO2 co-feed on pyrolysis will enhance the gasification process. To verify this, the steam gasification of SS residue was done using the TR. The concentration profiles of major chemical species (H2, CO, CH4, and C2H4) are shown in Fig. 6. An interesting observation in Fig. 6 is the enhanced generation of CO by a factor of 2 in the presence of CO2. Moreover, the collected tar in the presence of CO2 was substantially decreased (50%), which directly led to the enhanced generation of CO. For example, CO2 expedites the thermal cracking of condensable hydrocarbons, which was consistent with the discussion in Fig. 5. Another interesting observation in Fig. 6 is the concentration of CO at the high-temperature regime (800–1000 °C), which decreased substantially as the concentration of H2 increased. This could be explained by the water gas shift (WGS) reaction (H2O + CO ? H2 + CO2). For example, this WGS reaction would be expedited under CO2-rich condition.
4. Conclusions
Fig. 6. Concentration profiles of major chemical species.
This study showed that SS could be a strong candidate for biodiesel. High lipid content was experimentally identified and measured. These lipids from SS were converted into biodiesel via the transesterification reaction with MgO–CaO/Al2O3 derived from magnesium slag, and biodiesel conversion was 98%. The experimental work enabled explaining that temperature is the main driving force for the transesterification reaction, which can be enhanced in the presence of CO2. SS residue after extracting lipids was also a good feedstock for energy recovery via thermo-chemical processes. The CO2 injected into the thermo-chemical process remarkably increased the generation of CO by a factor of 2. Moreover, the introduction of CO2 into the thermo-chemical (pyrolysis/ gasification) process enabled reducing condensable hydrocarbons
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(tar) by expediting cracking; thus, utilizing CO2 as chemical feedstock for the pyrolysis/gasification process not only leads to higher thermal efficiency but also has environmental benefits. Appendix A. Supplementary material Supplementary data associated with this article can be found, in the online version, at http://dx.doi.org/10.1016/j.chemosphere. 2012.07.062. References Adegoroye, A., Paterson, N., Li, X., Morgan, T., Herod, A.A., Dugwell, D.R., Kandiyoti, R., 2004. The characterisation of tars produced during the gasification of sewage sludge in a spouted bed reactor. Fuel 83, 1949–1960. Afif, E., Azadi, P., Farnood, R., 2011. Catalytic hydrothermal gasification of activated sludge. Appl. Catal. B-Environ. 105, 136–143. Albores, A.F., Cid, B.P., Gomez, E.F., Lopez, E.F., 2000. Comparison between sequential extraction procedures and single extractions for metal partitioning in sewage sludge samples. Analyst 125, 1353–1357. Amand, L.-E., Leckner, B., Eskilsson, D., Tullin, C., 2006. Deposits on heat transfer tubes during co-combustion of biofuels and sewage sludge. Fuel 85, 1313–1322. Angerbauer, C., Siebenhofer, M., Mittelbach, M., Guebitz, G.M., 2008. Conversion of sewage sludge into lipids by Lipomyces starkeyi for biodiesel production. Bioresource Technol. 99, 3051–3056. Basha, S.A., Gopal, K.R., Jebaraj, S., 2009. A review on biodiesel production, combustion, emissions and performance. Renew. Sust. Energy Rev. 13, 1628– 1634. Benjapornkulaphong, S., Ngamcharussrivichai, C., Bunyakiat, K., 2009. Al2O3supported alkali and alkali earth metal oxides for transesterification of palm kernel oil and coconut oil. Chem. Eng. J. 145, 468–474. Butterman, H.C., Castaldi, M.J., 2009. Syngas production via CO2 enhanced gasification of biomass fuels. Environ. Eng. Sci. 26, 703–713. Chiang, K.-Y., Lu, C.-H., Chien, K.-L., 2011. Enhanced energy efficiency in gasification of paper-reject sludge by a mineral catalyst. Int. J. Hydrogen Energy 36, 14186– 14194. Cho, Y.B., Seo, G., Chang, D.R., 2009. Transesterification of tributyrin with methanol over calcium oxide catalysts prepared from various precursors. Fuel Process. Technol. 90, 1252–1258. Chongkhong, S., Tongurai, C., Chetpattananondh, P., 2009. Continuous esterification for biodiesel production from palm fatty acid distillate using economical process. Renew. Energy 34, 1059–1063. Chongkhong, S., Tongurai, C., Chetpattananondh, P., Bunyakan, C., 2007. Biodiesel production by esterification of palm fatty acid distillate. Biomass Bioenergy 31, 563–568. Converti, A., Casazza, A.A., Ortiz, E.Y., Perego, P., Del Borghi, M., 2009. Effect of temperature and nitrogen concentration on the growth and lipid content of Nannochloropsis oculata and Chlorella vulgaris for biodiesel production. Chem. Eng. Process. 48, 1146–1151. da Cunha, M.E., Krause, L.C., Moraes, M.S.A., Faccini, C.S., Jacques, R.g.A., Almeida, S.R., Rodrigues, M.R.A., Caram, E.B., 2009. Beef tallow biodiesel produced in a pilot scale. Fuel Process. Technol. 90, 570–575.
1513
Font, R., Fullana, A., Conesa, J., 2005. Kinetic models for the pyrolysis and combustion of two types of sewage sludge. J Anal. Appl. Pyrol. 74, 429–438. Font, R., Fullana, A., Conesa, J.A., Llavador, F., 2001. Analysis of the pyrolysis and combustion of different sewage sludges by TG. J. Anal. Appl. Pyrol. 58–59, 927– 941. Fonts, I., Azuara, M., Gea, G., Murillo, M.B., 2009. Study of the pyrolysis liquids obtained from different sewage sludge. J. Anal. Appl. Pyrol. 85, 184–191. Fytili, D., Zabaniotou, A., 2008. Utilization of sewage sludge in EU application of old and new methods: review. Renew. Sust. Energy Rev. 12, 116–140. Isayama, Y., Saka, S., 2008. Biodiesel production by supercritical process with crude bio-methanol prepared by wood gasification. Bioresource Technol. 99, 4775– 4779. Jagtap, S.R., Patil, Y.P., Fujita, S.-I., Arai, M., Bhanage, B.M., 2008. Heterogeneous base catalyzed synthesis of 2-oxazolidinones/2-imidiazolidinones via transesterification of ethylene carbonate with b-aminoalcohols/1,2-diamines. Appl. Catal. A-Gen. 341, 133–138. Khan, S.A., Rashmi, Hussain, M.Z., Prasad, S., Banerjee, U.C., 2009. Prospects of biodiesel production from microalgae in India. Renew. Sust. Energy Rev. 13, 2361–2372. Lardon, L., Helias, A., Sialve, B., Steyer, J.-P., Bernard, O., 2009. Life-cycle assessment of biodiesel production from microalgae. Environ. Sci. Technol. 43, 6475–6481. Lin, L., Ying, D., Chaitep, S., Vittayapadung, S., 2009. Biodiesel production from crude rice bran oil and properties as fuel. Appl. Energy 86, 681–688. Lu, H., Liu, Y., Zhou, H., Yang, Y., Chen, M., Liang, B., 2009. Production of biodiesel from Jatropha curcas L. oil. Comput. Chem. Eng. 33, 1091–1096. Meng, X., Chen, G., Wang, Y., 2008. Biodiesel production from waste cooking oil via alkali catalyst and its engine test. Fuel Process. Technol. 89, 851–857. Meng, X., Yang, J., Xu, X., Zhang, L., Nie, Q., Xian, M., 2009. Biodiesel production from oleaginous microorganisms. Renew. Energy 34, 1–5. Nipattummakul, N., Ahmed, I., Kerdsuwan, S., Gupta, A.K., 2010a. High temperature steam gasification of wastewater sludge. Appl. Energy 87, 3729–3734. Nipattummakul, N., Ahmed, I.I., Kerdsuwan, S., Gupta, A.K., 2010b. Hydrogen and syngas production from sewage sludge via steam gasification. Int. J. Hydrogen Energy 35, 11738–11745. Parnaudeau, V., Dignac, M.-F., 2007. The organic matter composition of various wastewater sludges and their neutral detergent fractions as revealed by pyrolysis-GC/MS. J. Anal. Appl. Pyrol. 78, 140–152. Patil, P.D., Deng, S., 2009. Optimization of biodiesel production from edible and nonedible vegetable oils. Fuel 88, 1302–1306. Powell, E.E., Hill, G.A., 2009. Economic assessment of an integrated bioethanol– biodiesel–microbial fuel cell facility utilizing yeast and photosynthetic algae. Chem. Eng. Res. Des. 87, 1340–1348. Saw, W., McKinnon, H., Gilmour, I., Pang, S., 2012. Production of hydrogen-rich syngas from steam gasification of blend of biosolids and wood using a dual fluidised bed gasifier. Fuel 93, 473–478. Song, D., Fu, J., Shi, D., 2008. Exploitation of oil-bearing microalgae for biodiesel. Chin. J. Biotechnol. 24, 341–348. Um, B.-H., Kim, Y.-S., 2009. Review: a chance for Korea to advance algal-biodiesel technology. J. Ind. Eng. Chem. 15, 1–7. Umdu, E.S., Tuncer, M., Seker, E., 2009. Transesterification of Nannochloropsis oculata microalga’s lipid to biodiesel on Al2O3 supported CaO and MgO catalysts. Bioresource Technol. 100, 2828–2831. Vitasari, C.R., Jurascik, M., Ptasinski, K.J., 2011. Exergy analysis of biomass-tosynthetic natural gas (SNG) process via indirect gasification of various biomass feedstock. Energy 36, 3825–3837.