Experimental Study Examining the Evolution of ... - ACS Publications

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Energy & Fuels 2009, 23, 3236–3245

Experimental Study Examining the Evolution of Nitrogen Compounds during the Gasification of Dried Sewage Sludge Marıá Aznar,* Marta San Anselmo, Joan J. Manya`, and M. Benita Murillo Thermo-chemical Processes Group (GPT), Aragoń Institute of Engineering Research (I3A), UniVersity of Zaragoza, Maria de Luna 3, E-50018 Zaragoza, Spain ReceiVed December 18, 2008. ReVised Manuscript ReceiVed March 4, 2009

Gasification of dried sewage sludge, which contains a high percentage of nitrogen, was experimentally studied to determine the effects of some operational parameters, such as freeboard temperature and equivalence ratio, on the partitioning of the fuel nitrogen among nitrogenous species. Experiments were performed using a bench-scale fluidized-bed gasifier and according to a well-specified procedure implemented to recover and quantify the nitrogen compounds. A mixture of argon and oxygen in similar proportions to air was used as a gasification agent in order to correctly quantify the N2 produced in the gasification process in an experimental way. Important findings of this research include the following: applying the analytical procedure developed in this work, a reasonably good inventory of nitrogen in the gasification products was obtained (mass balance closures near to 100%); most of the nitrogen goes to form gaseous products, N2 being the main gaseous nitrogencontaining product obtained; concentrations of N2, NH3, and N-tar in the producer gas strongly depend on the freeboard temperature, an increase of which causes an important decrease of both NH3 and N-tar contents and a substantial increase of the N2 amount.

Introduction Nowadays the management of sewage sludge produced in wastewater treatment plants is turning into a serious problem as the generation of this waste has increased during recent years. Sewage sludge air gasification is an attractive technology to convert this solid waste to gaseous products, which can be used in power generation. However, the high content of nitrogen in sewage sludge1 (for instance, from 130 to 1.5 times higher than two common biomass feedstocks, sawdust and leucaena, respectively2) represents a drawback of this technology due to the fact that the fuel-bound nitrogen (N-fuel) reacts during the thermochemical conversion to form, among other products, ammonia (NH3), hydrogen cyanide (HCN), and organic nitrogenated compounds (N-tar).3,4 These compounds are toxic and contaminants. Besides, it is well known that NH3 and HCN are considered as NOx precursors, the emissions of which should be minimized because they are contributors to photochemical smog and acid rain, and the N2O specifically contributes to global warming and ozone layer depletion. Special attention should be focused on understanding the formation and distribution of nitrogen-containing products in order to minimize the content of NH3, HCN, and N-tar in the raw gas, preferably to reach a maximum yield of N2 from the nitrogen released. It is important to carefully analyze the evolution of all of the main nitrogen-containing products obtained from gasification, on a comprehensive basis, to check if a given decrease in the amount of one of them could produce * To whom correspondence should be addressed. Telephone: +34-976762897. Fax: +34-976-761879. E-mail: [email protected]. (1) Werther, J.; Ogada, T. Prog. Energy Combust. Sci. 1999, 25, 55– 116. (2) Zhou, J.; Masutani, S. M.; Ishimura, D. M.; Turn, S. Q.; Kinoshita, C. M. Ind. Eng. Chem. Res. 2000, 39, 626–634. (3) Yu, Q.; Brage, C.; Chen, G.; Sjo¨stro¨m, K. Fuel 2007, 86, 611–618. (4) Leppa¨lahti, J.; Koljonen, T. Fuel Process. Technol. 1995, 43, 1–45.

an increase of the above-mentioned contaminant compounds. Several researchers have studied the formation of nitrogencontaining species during biomass (and coal) gasification2-15 with only a few about the production of nitrogen-containing products from the gasification of sewage sludge.16,17 However, little information is available regarding the distribution and interactions of the nitrogen-containing compounds, taking into account in the same study the solid, liquid, and gas phases. Zhou and co-workers2 investigated the effects of operational parameters and nitrogen content of six biomass feedstocks on the partitioning of fuel-bound nitrogen among nitrogenous gas species (N2, NH3, and HCN) and the nitrogen remained in char. Other researchers studied the production of aromatic nitrogencontaining compounds included in tar from different biomass feedstocks and peat.3,18,19 Nevertheless, it is necessary to analyze (5) Tian, F.; Yu, J.; McKenzie, L. J.; Hayashi, J.; Li, C. Energy Fuels 2007, 21, 517–521. (6) Tian, F.; Yu, J.; Mckenzie, L. J.; Hayashi, J.; Chiba, T.; Li, C. Fuel 2005, 84, 371–376. (7) Kurkela, E.; Staåhlberg, P. Fuel Process. Technol. 1992, 31, 1–21. (8) Vriesman, P.; Heginuz, E.; Sjo¨stro¨m, K. Fuel 2000, 79, 1371–1378. (9) McKenzie, L. J.; Tian, F.; Li, C. EnViron. Sci. Technol. 2007, 41, 5505–5509. (10) Paterson, N.; Zhuo, Y.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels. 2002, 16, 127–135. (11) Zhuo, Y.; Paterson, N.; Avid, B.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2002, 16, 742–751. (12) Amure, O.; Hanson, S.; Cloke, M.; Patrick, J. W. Fuel 2003, 82, 2139–2143. (13) Wang, W.; Padban, N.; Ye, Z.; Andersson, A.; Bjerle, I. Ind. Eng. Chem. Res. 1999, 38, 4175–4182. (14) Xie, K.; Lin, J. Y.; Li, W. Y.; Chang, L. P.; Feng, J.; Zhao, W. Fuel 2005, 84, 271–277. (15) Norton, G. A.; Brown, R. C. Energy Fuels 2005, 19, 618–624. (16) Cousins, A.; Zhuo, Y.; George, A.; Paterson, N.; Dugwell, D. R.; Kandiyoti, R. Energy Fuels 2008, 22, 2491–2503. (17) Paterson, N.; Zhuo, Y.; Dugwell, D.; Kandiyoti, R. Energy Fuels 2005, 19, 1016–1022. (18) Kurkela, E.; Staåhlberg, P. Fuel Process. Technol. 1992, 31, 23– 32.

10.1021/ef801108s CCC: $40.75  2009 American Chemical Society Published on Web 04/15/2009

Examining the EVolution of Nitrogen Compounds

Energy & Fuels, Vol. 23, 2009 3237

Table 1. Ultimate and Proximate Analysis of Dried Sewage Sludge Samples analytical standard moisture ash volatile fixed carbon

ISO-589-1981 ISO-1171-1976 ISO-5623-1974 by difference

Ultimate % by (organic weight fraction) 7.06 41.02 46.59 5.33

carbon hydrogen nitrogen sulfur

Analytical instrument Carlo Carlo Carlo Carlo

Erba Erba Erba Erba

1108 1108 1108 1108

% by weight (daf) 53.27 7.02 7.55 1.44

Table 2. ICP Analysis of the Untreated DSS for Several Elementsa element

% (by weight)

Al Ca Fe K Mg Na Si Ti

8.2 11.9 4.3 2.2 2.6 0.7 19.3 0.5

a Results are reported in weight percentage of each element within the ash.

in a simultaneous way for the nitrogen contained in the solid, liquid, and gaseous products. On the other hand, the conclusions of the above-mentioned studies cannot be extrapolated to sewage sludge feedstock, which exhibits some peculiarities in its composition (high ash and nitrogen contents). Concerning the sewage sludge gasification, some results have been reported for HCN and NH3 production in a pressurized spouted bed.16,17 In addition, all of these previous works have shown that there are many factors that influence the production and distribution of the nitrogen-containing products.4 In this sense, parameters such as temperature, equivalence ratio, reactor geometry, and chemical structure of the solid can affect the distribution of the nitrogen species. The purpose of the present work is to investigate the nitrogen evolution during the gasification of dried sewage sludge, in a bubbling fluidized bed, and study the influence of freeboard temperature on these results taking into account the nitrogen contained in the main gas, solid, and liquid nitrogen-containing products. To reach this aim, it was necessary to implement, in a first stage, a procedure to recover and analyze the nitrogenous species. In addition, some preliminary results regarding the effect of both equivalence ratio and ash content (in the sewage sludge) on the nitrogen-containing products are presented. Experimental Section Materials. The samples of dry sewage sludge (DSS) used in the present study were obtained as a dried, granulate product from an urban wastewater treatment plant (located at Madrid, Spain). The sludge was previously treated by anaerobic digestion and thermal drying. Proximate and ultimate analyses of the received sewage sludge are shown in Table 1. The lower heating value (LHV) has also been determined using a calorimeter IKA A-2000 (standard procedure ISO-1928-89). The value obtained for LHV was 10.26 MJ/kg. All analyses were performed by the “Instituto de Carboquı´mica” (CSIC, Zaragoza, Spain). The dried sewage sludge was crushed and sieved to provide a feed sample particle size ranging from 250 to 500 µm. The received dried sewage sludge was analyzed for several elements using inductively coupled plasma-optical emission spectroscopy (ICP-OES). The results concerning this analysis are shown in Table 2. (19) Leppa¨lahti, J.; Kurkela, E. Fuel 1991, 70, 491–497.

Bench-Scale Facility. All of the experiments performed in this work were carried out in a bench-scale plant operating at atmospheric pressure, continuously feeding sewage sludge and a gasification agent flow (without steam), and with a continuous ash removal system. The reactor is made of refractory steel (AISI 310), with an inner diameter of 38.1 mm and a height of 800 mm. In the experiments presented in this work, the sewage sludge flow rate was varied in the range from 3.00 to 3.77 g min-1. The gasification agent flow rate (at normal conditions) was varied from 3.26 to 4.12 L min-1. The bed height is kept at 300 mm during the experiment by means of a concentric pipe (13.1 mm i.d.) which goes through the distributor plate, enabling the bed material to overflow and be collected in an ash hopper. The plant is coupled to a gas cleaning system, which is composed by three elements connected in series: a heated filter (a glass fiber thimble located inside a stainless steel tube operating at 400 °C), two water and tar traps (cooled by ice), and a cotton filter. Figure 1 shows a detailed diagram of the experimental device. More details concerning the plant description and experimental procedure are available in previous works.20,21 Following the suggestions of Zhou et al.,2 the gasification experiments were performed using a mixture of oxygen (20% vol.) in argon rather than air. In this way, the measurement of the N2 produced during sewage sludge gasification, using a micro gas chromatograph (Agilent 3000A, a TCD system equipped with two microcolumns: Porapak N and Molecular Sieve), is feasible, and the quantified N2 content is entirely due to the release of the fuelbound nitrogen. Two additional experiments have also been performed using air as gasification agent for comparative purposes. Several operating conditions were selected based on results reported in previous studies (concerning sewage sludge gasification), which have been conducted using the same facility.21,22 In this sense, the bed height (300 mm) and bed temperature (850 °C), the gasification agent velocity (6.5 times the umf value), and the experimental run time (90 min) were kept constant during the present study. These operating conditions have been proven effective in practice to assess the thermal efficiency of the gasification process, minimizing, at the same time, the tar yield. Nevertheless, the freeboard temperature and equivalence ratio were varied for the present study. Furthermore, two different initial beds were used (one bed composed by sand and another one composed by DSS ash) in order to analyze the potential effect of sewage sludge ash on the distribution of the nitrogen-containing products. Table 3 shows the complete set of experiments conducted in the present study. Two analytical procedures were followed in order to measure the tar content in the raw gas. (1) One procedure is the gravimetric method. The tar and water condensed inside the glass traps were quantified by weight difference at the end of the experiment. This mixture was recovered using 100 g of 2-propanol. Water was determined off-line using a Karl Fischer titration, and tar was calculated by difference. This method has been used in several previous studies.20-22 (2) The second is quasi-continuous online measurement. This method is based on the comparison of the total hydrocarbon content of the hot gas and that of the gas with all tars removed (after filtration). For this purpose, analysis equipment (tar analyzer, TA 120-3) developed at the University of Stuttgart (Germany) was used. By means of this equipment, the total content of hydrocarbons of a slipstream of the gas is determined using a flame ionization detector (FID). At the same time, another gas sample pass through a special filter where all tar compounds are retained and, then, a second measurement is performed (also using the FID) to measure the amount of noncondensable hydrocarbons. The difference between these two measurements is the amount of (20) Manya`, J. J.; Sanchez, J. L.; Gonzalo, A.; Arauzo, J. Energy Fuels 2005, 19, 629–636. (21) Manya`, J. J.; Sanchez, J. L.; Abrego, J.; Gonzalo, A.; Arauzo, J. Fuel 2006, 85, 2027–2033. (22) Manya`, J. J.; Aznar, M.; Sanchez, J. L.; Arauzo, J.; Murillo, M. B. Ind. Eng. Chem. Res. 2006, 45, 7313–7320.

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Figure 1. Schematic diagram of the bench-scale facility. Table 3. Summary of the Operating Conditions of Experiments Conducted experiment

gasification agent

λ (%)

Tf (°C)

initial bed material

1 2 3 4 5 6 7 8 9 10

O2/Ar O2/Ar O2/Ar O2/Ar O2/Ar air air O2/Ar O2/Ar O2/Ar

30 30 30 21 30 30 30 30 30 30

600 725 850 850 850 600 850 600 850 850

sand sand sand sand ash sand sand sand sand sand

tar. More details concerning this analyzing technique are available in a paper by Moersch et al.23 Results obtained using the previously mentioned alternatives were compared for each experiment performed. A relatively good agreement (differences lower than 10%) was observed between the tar content obtained from the gravimetric method and that obtained using the tar analyzer (tar content calculated by integration of all measurement data points over the experimental run time). Recovery and Quantification of Nitrogen Compounds. The quantification of the nitrogen-containing products obtained during the gasification process in solid, liquid, and gas phases was carried out as follows. (1) For experiments performed using a mixture of O2/Ar as gasification agent, N2 was measured by means of the above-described micro gas chromatograph. (2) Nitrogen contained in char was measured using a CHNS analyzer (ThermoElectron Flash 1112) in an external laboratory (“Instituto de Carboquı´mica”, Zaragoza, Spain). (3) Nitrogen-containing species in tar were detected following a GC/MS and GC/FID analytical method especially suited for tar characterization.24 A sample of the mixture formed by tar, water, and 2-propanol was analyzed by means of a gas chromatograph. The applied procedure is based on the simultaneous use of two instruments: a gas chromatograph equipped with a flame ionization detector (GC Agilent-6890) and a GC/MS instrument (GC HP-5890). The same Agilent HP-5 capillary column (30 m × 320 µm) was used in both chromatographs. An initial oven temperature of 50 °C and a ramp rate of 4 °C min-1 were (23) Moersch, O.; Spliethoff, H.; Hein, K. R. G. Biomass Bioenergy 2000, 18, 79–86. (24) Aznar, M.; Manya`, J. J.; Garcıá, G.; Sańchez, J. L.; Murillo, M. B. Energy Fuels 2008, 22, 2840–2850.

implemented to reach a final column temperature of 250 °C. This temperature was maintained for 10 min. The carrier gas was helium at a linear flow velocity of 0.19 m s-1. The FID gas flow rates (measured under normal conditions) were 40 mL min-1 for hydrogen, 450 mL min-1 for air, and 50 mL min-1 for nitrogen as the makeup gas. The injector and detector temperatures were 250 and 275 °C, respectively. The sample was injected in splitless mode. The applied method is based on a semiquantitative analysis24 assuming the same response factor (1.0) for all species detected. This semiquantitative approach has been followed in various studies,25,26 and it must be remarked that the adopted procedure to measure N-tar is only suitable for comparative purposes. (4) Because of the high solubility of NH3 and HCN in water, it was expected that a considerable amount of these compounds could be retained in the condensed phase (located into the glass traps). For this reason, the content of both NH3 and HCN in the aqueous phase was also determined using volumetric methods taking into account the APHA standards.27 For this purpose and as a preliminary step to avoid any possible interference, it was necessary to remove the aqueous phase from the tar by distillation. For each compound (NH3 and HCN), a specific distillation process was implemented in order to ensure a complete recovery of the desired compounds. At that point, volumetric methods were applied to analyze the distillates. A specific indicator depending on the particular compound (a boric acid solution for NH3 and p-dimetilaminebenzalrodamine for HCN analysis) was added to the distillate. Then, the titration was carried out using a solution of 0.1 M H2SO4 in the case of NH3 and a solution of 0.5 M AgNO3 for HCN. (5) The NH3 and HCN contents in the gas leaving the tar cleaning system were measured by means of an offline quantification procedure, which was applied to the retained amounts of ammonia and hydrogen cyanide in absorbing solutions. The NH3 sampling train consisted of four impingers arranged in series. The second and third bubblers were filled with 100 mL of deionized water used to trap ammonia. The use of water as absorbing medium is based on previous studies.11,17 The remaining impingers were kept empty: the first one was connected (25) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Pis, J. J. Fuel Process. Technol. 2005, 86, 1007–1020. (26) Marin, N.; Collura, S.; Sharypov, V. I.; Beregovtsova, N. G.; Baryshnikov, S. V.; Kutnetzov, B. N.; Cebolla, V.; Weber, J. V. J. Anal. Appl. Pyrol. 2002, 65, 41–55. (27) Eaton, A. D.; Clescer, L. S.; Greenberg, A. E. Standard Methods for the Examination of Water and Wastewater; American Public Health Association; American Water Works; Water Environment Federation: Hanover, MD, 1995.

Examining the EVolution of Nitrogen Compounds in reverse orientation for preventing reverse gas flow, and the last one was placed in an ice bath in order to remove any remaining moisture. The arrangement of the HCN sampling train was exactly the same as that described above for NH3. In this case, the HCN was retained in two impingers filled with 200 and 100 mL of 0.1 M NaOH, respectively. The two sampling trains were connected in parallel as shown in Figure 1. A vacuum pump was used to force gas to pass through the sampling trains. Samples were taken for intervals of 10 min (for each train). The retained liquid samples were analyzed using the above-described volumetric method. The total content of ammonia and hydrogen cyanide was determined as the sum of the measurements in the aqueous and gas phases. (6) In preliminary experiments, a small slipstream of the producer gas was directed into a UV spectrophotometer (ABB Limas 11), downstream of the impingers, to detect and quantify NO.

Results and Discussion In this section, the effect of freeboard temperature (Tf) and some additional parameters, such as the equivalence ratio (λ) and the initial bed material (silica sand or DSS ash), on the formation and distribution of the nitrogen-containing compounds is discussed. Before presenting and discussing the experimental results obtained to achieve this purpose, details concerning the nitrogen evolution (during the different gasification stages) and validation of the methodology followed to quantify the nitrogen compounds are reported. Nitrogen Evolution during Gasification. As a previous step to analyze the dependence on operating conditions of the nitrogen compounds distribution, it is interesting to investigate the evolution of nitrogen contained in the solid waste and its chemical nature during the different process stages (pyrolysis and gasification). To achieve this aim, the results provided by elemental analysis (performed at the “Instituto de Carboquı´mica” using the above-mentioned CHNS analyzer) of solid particles corresponding to three different steps of the process were analyzed: the received sewage sludge, the pyrolyzed char (for both slow and fast heating rates), and the final char obtained after gasification. The char obtained from gasification was produced in the bench-scale reactor described above, operating at a bed temperature of 850 °C. The slow pyrolysis char was obtained using a thermobalance (CAHN TG131; final temperature, 850 °C; heating rate, 30 °C min-1; initial mass, 25.8 mg; atmosphere, N2), whereas the pyrolyzed char corresponding to a high heating rate was formed inside a laboratory-scale fast pyrolysis reactor (average heating rate, 200 °C min-1; final temperature, 850 °C; initial mass, 3.43 g; atmosphere, N2). The fast pyrolysis reactor used is cylindrical (and made in refractory steel, AISI 310) with 150 mm length and a diameter of 90 mm. Sewage sludge pyrolysis takes place into a steel basket hanging from the upper part of the reactor. The sample basket is moved inside the reaction zone once the desired temperature is reached. Thus, heating of samples takes place at high heating rates (around 200 °C min-1). Nitrogen is used as inertizing and carrier gas, entering the reactor at the top with a flow rate of 100 mL min-1. A more detailed description of this experimental device can be found elsewhere.28 (28) Abrego, J.; Garcıá, G.; Gonzalo, A.; Cordero, T.; Rodrı´guezMirasol, J. Influence of temperature and heating rate in the fixed bed pyrolysis of two types of sewage sludge. In Proceedings of the 15th European Biomass Conference; Maniatis, K., Grimm, H. P., Helm, P.; Grassi, A., Eds.; ETA Florence: Florence, Italy, 2007; pp 1156-1160. (29) Mansuy, L.; Bourezgui, Y.; Garnier-Zarli, E.; Jarde´, E.; Re´veille´, V. Org. Geochem. 2001, 32, 223–231. (30) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Pis, J. J. Afinidad 2004, 61, 280–285. (31) Grube, M.; Lin, J. G.; Lee, P. H.; Kokorevicha, S. Geoderma 2006, 3-4, 324–333.


Figure 2. Evolution of the nitrogen loss in solid products during DSS gasification.

Regarding the elemental nitrogen contained in the different char samples obtained after pyrolysis and gasification steps, Figure 2 shows the nitrogen percentage contained in sewage sludge, in the solid obtained after pyrolysis, and in the solid obtained after gasification. As can be deduced from Figure 2, a high percentage of nitrogen was released during the devolatilization stage independent of the heating rate used during the pyrolysys step. In order to obtain more information about the changes in the nitrogen contained in the sewage sludge during the pyrolysis and gasification steps, the FTIR spectra of the sewage sludge, the char obtained after pyrolysis, and the solid obtained after gasification were analyzed (using a Brooker Vertex 70 instrument). Figure 3 shows the corresponding FTIR spectra. In Table 4, the principal IR absorption bands, due to nitrogen contained in the solids, are shown. In Figure 3a, which corresponds to the FTIR spectrum of the untreated feedstock, a broad band located at 3100-3600 cm-1 was observed. This band can be assigned to H-bonded OH and NH groups.29,30 Two other main bands that can be assigned to nitrogen-containing groups and indicate the presence of proteins structures in sewage sludge are the bands located at 1660 and 1540 cm-1 (amide I and amide II, respectively).29-31 Additional bands attributed to secondary amides have been observed at 1241 cm-1.30 (32) Ros, A.; Lillo-Ro´denas, M. A.; Fuente, E.; Montes-Morań, M. A.; Martıń, M. J.; Linares-Solano, A. Chemosphere 2006, 65, 132–140. (33) Li, C.-Z.; Tan, L.-L. Fuel 2000, 79, 1899–1906. (34) Tsubouchi, N.; Ohshima, Y.; Xu, C.; Ohtsuka, Y. Energy Fuels 2001, 15, 158–162. (35) Kilpinen, P.; Hupa, M.; Leppa¨lahti, J. Report 91-14, Abo-Akademi, 1991. (36) Tian, F.; Yu, J.; McKenzie, L. J.; Hayashi, J.; Li, C. Energy Fuels 2007, 21, 517–521. (37) Ledesma, E. B.; Li, C.; Nelson, P. F.; Mackie, J. C. Energy Fuels 1998, 3, 536–541. (38) Scha¨fer, S.; Bonn, B. Fuel 2000, 79, 1239–1246. (39) Chang, L.; Xie, K.; Li, C. Fuel Process. Technol. 2004, 8-10, 1053–1063. (40) Norman, J. S.; Pourkashanian, M.; Williams, A. The formation of ammonia in IGCC gasifiers and its control. In Proceedings of the Instituteof-Energy 2nd International Conference on Combustion and Emissions Control, London England, 1995; pp 109-118. (41) Narvaéz, I.; Orıó, A.; Aznar, M. P.; Corella, J. Ind. Eng. Chem. Res. 1996, 35, 2110–2120. (42) Manya`, J. J.; Aznar, M.; Sańchez, J. L.; Arauzo, J.; Murillo, M. B. The catalytic role of the ashes of sewage sludge during their gasification with air in a fluidised bed. In Proceedings of the 15th European Biomass Conference; Maniatis, K., Grimm, H. P., Helm, P., Grassi, A., Eds.; ETA Florence: Florence, Italy, 2007; pp 920-923. (43) Piskorz, J.; Scott, D. S.; Wsterberg, I. B. Ind. Eng. Chem. Proc. Des. DeV. 1986, 25, 265–270. (44) Simell, P. Kurkela, E.; Ståhlberg. P. Formation and Catalytic Decomposition of Tars from Fluidized Bed Gasification. In AdVances in Thermochemical Biomass ConVersion; Bridgwater, A.V., Ed.; Blackie Academic Press: London, 1994; pp 265-279. (45) Leppalahti, J.; Koljonen, T.; Hupa, M.; Kilpinen, P. Energy Fuels 1997, 1, 30–38. (46) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Pı´s, J. J. Bioresour. Technol. 2006, 97, 1185–1193.

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Figure 3. FTIR spectra of dried sewage sludge: (a) untreated, (b) after slow pyrolysis, (c) after fast pyrolysis, and (d) after air gasification. More information about FTIR spectra is given in Table 4. Table 4. Assignment of the Principal IR Absorption Bands Attributed to Nitrogen in the Spectra of Dried Sewage Sludge wave number (cm-1)

assignment

3100-3600 1650 1540 1383 1241

O-H, N-H amide I amide II (NO3)amide III

refs 29, 29, 29, 39, 30,

30, 30, 30, 40 31,

46 46, 47 31, 46 46

no. in Figure 2 1 2 3 4 5

From analyzing the rest of the spectra, it seems clear that after the pyrolysis and gasification steps, the spectra become simpler. The intensity of the broad band in the 3100-3600 cm-1 region is reduced, probably due to the removal of N-H bonds. The observed absorbance could be due to the remaining amount of some O-H groups linked to the inorganic matter and water.32 The peaks corresponding to amide I, II, and II are not present in the obtained chars. Instead of the corresponding peaks to amides, a broad band is observed after the pyrolysis at approximately 1590 cm-1. This band is characteristic of carbon materials.32 On the other hand and as can be observed in Figure 3b-d, the band assigned to nitrate has been detected for all solid samples obtained after heat treatment. As can be seen by comparing Figure 3b and 3c, the detected bands of the char obtained from both slow and fast pyrolysis are very similar. This fact, coupled to the observed similarity (47) Barth, A. Biochim. Biophys. Acta, Bioenerg. 2007, 1767, 1073– 1101.

Table 5. N-Atom Balance Closure for Experiment 1 mass percentage of N-fuel N-gas

N-char N-tar N-atom balance closure

N2 NH3 HCN NO total

44.3 23.8 0.7 not detectable 68.8 5.4 19.2 93.4

among the percentages of nitrogen retained in the solid shown in Figure 2, seems to indicate that the pyrolysis heating rate does not affect the nitrogen release evolution in a relevant way. Distribution of the Fuel-Bound Nitrogen into the Gasification Products. To check the applicability of the methodology followed to quantify the nitrogen-containing compounds, the results corresponding to a given gasification test (experiment 1) were carefully analyzed. The N-atom mass balance closure can be a useful criterion to inspect how reliable the experimental data are. In this sense, a balance closure near to 100% could represent a good indicator. Experimental results corresponding to experiment 1 (performed at Tf ) 850 °C, λ ) 30%, and using sand as initial bed) are reported in Table 5. A relatively good N-atom mass balance closure (93.4%) was obtained. This fact would seem to confirm the usefulness of the analytical procedures implemented in the present work. Similar results concerning the nitrogen inventory



Table 6. List of the Main N-Containing Compounds Present in the Tar

were obtained for the rest of experiments reported here (the average nitrogen balance closure was 92.3% with a standard deviation of (5.7%). From results reported in Table 5, it can be observed that most of the fraction of nitrogen fed is released into the gas phase. This behavior should be considered as expected, and it confirms the results reported in earlier studies involving biomass gasification.2-4,11 Nevertheless, the relatively high production of N-tar could be interpreted as characteristic of sewage sludge because of the high nitrogen content of this material, and consequently, its behavior cannot be compared directly with other solid fuels. The nitrogen-containing products present in the tar exhibit an aromatic character. Nitrogen is retained in tar as heterocyclic compounds, nitrile, or amine functional groups bonded to aromatic rings. Table 6 shows a list of the main nitrogen tar compounds detected in the analyzed samples. On the other hand, the NO content in the producer gas was negligible during the course of preliminary tests, which were conducted to adjust the experimental setup. For this reason, this compound has not been included in the overall results. Effect of Freeboard Temperature. Table 7 reports the experimental results regarding the nitrogen distribution to different products obtained during gasification for experiments 1-3, which were performed at a constant equivalence ratio (λ ) 30%) and varying freeboard temperature. As can be deduced from values shown in the table, freeboard temperature strongly affects the N-compounds distribution, especially for NH3, N2, and N-tar production. A clearly observable effect is the substantial decrease of the N-tar content with freeboard temperature (the standard deviation for N-tar was (1.9, calculated for the three experiments performed at 850 °C, λ ) 30%, using an initial bed of sand and a mixture Ar/O2 as gasification agent). This effect was examined in previous work,24 where a lower percentage of nitrogen-containing compounds in the tar produced from gasification of sewage sludge was obtained at a freeboard

Table 7. Distribution of Fuel-Nitrogen Results (in normalized weight percentages) in the Producer Gas for Experiments Performed experiment gasification agent λ (%) Tf (°C) initial bed material

1

2

3

4

5

6

7

O2/Ar 30 600 sand

O2/Ar 30 725 sand

O2/Ar 30 850 sand

O2/Ar 21 850 sand

O2/Ar 30 850 ash

air 30 600 sand

air 30 850 sand

nitrogen distribution on the N as N2 47.4 N as NH3 25.5 N as HCN 0.7 N-gas 68.8 N-tar 20.6 N-char 5.8

obtained products (%, g N/g N 58.0 74.7 73.8 61.0 19.6 13.2 13.2 19.8 1.6 0.9 0.8 1.2 79.2 88.8 87.8 82.0 18.0 5.9 6.7 9.5 2.8 5.3 5.6 8.5

of DSS fed) 48.8 70.5 28.1 18.5 1.1 0.8 77.9 89.7 17.7 8.9 4.4 1.4

temperature of 850 °C. Nevertheless, the effect of freeboard temperature was not the same for different nitrogen-containing compounds present in the tar phase. In line with this, Figure 4 shows the fluctuating trend in the percentage (in a chromatographic area basis) of some nitrogenated organic compounds against freeboard temperature. All the compounds shown in Figure 4 were significantly affected by this parameter: the percentage of some of them decreases with increasing freeboard temperature, such as 1H-indole and benzenacetonitrile, while other species, like 2-naphthalenecarbonitrile and quinoline, reach their maximum percentage at 850 °C. Li and Tan33 suggested that the stability of the nitrogen compounds contained in tar could influence the production of HCN and NH3: the most stable compounds produce NH3, while the more reactive yields HCN. In the present work, the percentage of many of the most stable compounds (those containing two aromatic rings from the obtained compounds) generally increases with temperature. In this sense, Yu et al.3 mentioned that a tar containing a higher concentration of

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Aznar et al.

Figure 4. Nitrogen-containing aromatic tar compounds affected by the freeboard temperature (average values from experiments 1-3 and 8-10): (9) 1H indole; (O) quinoline; (2) pyridine; (×) 2-naphthalencarbonitrile; (]) benzenacetonitrile.

Figure 5. Evolution of the N2 content in the producer gas as a function of freeboard temperature: (O) N2; (×) average values.

nitrogen-containing aromatic compounds with two rings is expected to yield a larger amount of NH3. Gaseous products are also affected by freeboard temperature. As can be deduced from Figure 5 (experiments 1-3), a significant increase in the N2 content (and a decrease of ammonia production) is observed when Tf increased. In accordance with Leppalahti and Koljonen,4 this positive finding could be explained by the fact that an increase of the freeboard temperature enhances the ammonia decomposition taking into account the exothermic behavior of the reaction given below 0 -1 N2 + 3H2 T 2NH3 (∆rH 900 °C ) -112 kJ mol )

(1)

Table 8 shows the results for NH3, HCN, and N2 expressed in units of concentration (ppmV in the producer gas) for experiments 1-3 and 8-10. In all cases, N2 presents the higher concentration in the gas, and both N2 and NH3 are much higher than HCN content in the obtained gas. Table 9 shows the comparison between the N2, NH3, and HCN contents in gas obtained in the present work with those obtained by Zhou and co-workers2 (for a biomass feedstock, leucaena leucocephala, which has a percentage of nitrogen much higher than the average value for a biomass feedstock) under similar gasification conditions as in the present work. In Table 9 the results obtained by Paterson et al.17 from the pressurized gasification of sewage sludge samples can also be seen. A very interesting behavior can be deduced from this comparison: the ammonia concentration obtained here for dried sewage sludge is lower than that reported by Zhou and co-workers for the

Figure 6. Temporal evolution of both (a) N2 and (b) H2 contents in the producer gas: (4) experiment 3, sand bed; (9) experiment 5, ash bed).

leucaena gasification at 850 °C. It was expected that larger nitrogen content in the feedstock could give a larger NH3 production.4,15 Nevertheless, the reported results for leucaena and sewage sludge samples are in contradiction to this prediction. Besides, the N2 production measured in the present work is higher than that obtained from leucaena gasification in similar conditions. Otherwise, the HCN contents are considerably high for both studies involving sewage sludge. It is possible that the higher content of mineral matter for the sewage sludge samples could play an important role in the fuel-nitrogen distribution. According to previous results obtained by Tsubouchi and coworkers34 for pyrolysis tests of several coal samples, the high content of alkali metals in the DSS ash (see Table 2 for details) can catalyze conversion to N2. From Table 9 it can also be observed that the sewage sludge used by Paterson et al.17 contains less nitrogen than the sewage sludge used in the present work but results in a higher NH3 production. This fact could be due to the effect of pressure, which improves NH3 formation.4 In any case, the experimental NH3 content into the raw gas differs greatly from the theoretical concentration at equilibrium. Kilpinen et al.35 studied the equilibrium concentrations for the gasification products at different temperatures (500-1000 °C), equivalence ratios (0-100%), using different coals, with nitrogen contents between 0.3% and 1.9%. They found that, in general, the sum of NH3 and HCN at equilibrium was less than 100 ppmV.



Table 8. Concentrations of NH3, HCN, and N2 in the Producer Gas for Experiments Performed experiment 1

2

3

4

5

gasification agent λ (%) Tf (°C) initial bed material

O2/Ar 30 600 sand

O2/Ar 30 725 sand

O2/Ar 30 850 sand

O2/Ar 21 850 sand

O2/Ar 30 850 ash

N2 NH3 HCN

10558 11368 316

12 842 8678 705

concentration on the producer gas (ppmV) 16 389 18 546 11 550 11 987 5068 6579 7477 13794 379 420 464 525

a

6

7

8

9

10

air 30 600 sand

air 30 850 sand

O2/Ar 30 600 sand

O2/Ar 30 850 sand

O2/Ar 30 850 sand

18 638 9769 408

9484 naa na

15 793 na na

15 076 na na

na: not analyzed.

Table 9. Comparison of Measured Concentrations Obtained during Gasification of Several Materials present work feedstock

a

sewage sludge

Zhou et al.2

Paterson et al.17

leucaena

sewage sludge

nitrogen content (daf) ash content

7.5 41.0

2.7 7.2

4.6 26.7

operational parameters reactor used gasification agent pressure (MPa) gasification temperature (°C) freeboard temperature (°C) λ (%)

fluidized bed Ar/O2 0.1 850 850 30

fluidized bed Ar/O2 0.1 850 not specified 32

Air/N2 0.32 820 not specified not specified

spouted bed Air/N2/steam 0.26 850 not specified not specified

N2 (ppmV) NH3 (ppmV) HCN (ppmV)

16 389 5068 379

not specified not specified 2000

not specified 8120 not specified

concentrations ∼12 500a ∼11 000 ∼8

Results obtained from a reported plot.

The high NH3 concentrations in gas compared to the expected value at equilibrium is a common result for experimental gasification works. This fact emphasizes the importance of kinetics in the evolution of nitrogen compounds. In this sense, the high release of NH3 experimentally obtained in gasification, compared to the expected concentration at equilibrium, may be related to a very low rate of the reaction corresponding to the ammonia decomposition.35 Regarding the evolution of the HCN concentration as a function of freeboard temperature, no clear trend can be deduced from the results shown in Table 8. According to Tian et al.,36 the nitrogen released with volatile matter is the main source for HCN formation. Taking into account the large nitrogen percentage released from the sewage sludge during the pyrolysis step (see Figure 2), it was expected to obtain a large HCN production. Nevertheless, as can be seen from Table 8, NH3 and N2 contents are much higher than that corresponding to HCN. This finding could be explained by the fact that NH3 is formed from HCN, especially in the presence of char.17 In addition, the conditions of gasification in fluidized bed reactors could promote the conversion of HCN to NH3.4 This generated ammonia is also converted to N2 by reaction 1 at the same time that additional H2 can be produced by cracking of the volatile matter.16 For all of these reasons, the evolution of NH3, HCN, N2, and N-tar is closely interrelated.4,12,33,37,38 On the other hand, the nitrogen contained in the char must not be affected by the freeboard temperature in a relevant way. The observed decrease in Table 7 at a freeboard temperature of 725 °C could be due to a systematic error related to the sample handling process. The char is recovered from the bed and from the ash hopper as a mixture of char and sand. Taking into account that the particle size is very similar for both materials, it was not possible to separate them by a suitable means. For this reason, elemental analysis was performed on this mixture. It is possible that the analyzed sample corresponding to

experiments performed at Tf ) 725 °C contained more sand than any of the other experiments. From the overall results concerning the influence of the freeboard temperature, it can be concluded that a higher value of this parameter can promote N2 formation and decrease both NH3 and N-tar production. Besides the temperature, the equivalence ratio can also affect to the nitrogen distribution in the nitrogen-containing compounds. In gasification, the presence of oxygen plays an important role in the formation of the nitrogen-containing compounds: O2 breaks the heteroaromatic rings of volatile released during the pyrolysis step, improving NH3 and HCN production.39 Furthermore, it could also oxidize the NH3 and HCN to NOx.8 On the other hand, O2 could react with the nitrogen retained in char to form NO and N2O. These compounds can be reduced in a later stage (in the upper region of the bed) to form NH3, HCN, or N2.40 For this reason, an additional test (experiment 4) was performed at a lower equivalence ratio value (λ ) 21%) in order to examine in a preliminary way how the equivalence ratio affects the Ncompounds distribution. Unlike the freeboard temperature, the equivalence ratio does not apparently impact the NH3 and N2 formation in terms of mass distribution. This observed behavior is in agreement with that reported by Zhou et al.2 for several biomass feedstocks. However, a certain influence of λ on concentrations of nitrogenous species in gasification producer gas (values in ppmV) can be deduced from Table 8 (experiments 3 and 4). A change in the λ value from 30% to 21% involves an increment of 29.8% in NH3 content and slight increments of 13.2% in N2 content and 10.8% in HCN content. These preliminary findings suggest that operating at low λ values is not an interesting way to reduce the concentration of NOx precursors in the producer gas. In addition, it must be taken into account that the selection of a

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very low equivalence ratio could imply negative effects on tar production, obtaining a higher tar yield.21,41 Effect of the Initial Bed Material. In an earlier study,42 the use of DSS ash as the initial bed material was proven effective in reducing the tar yield. For this reason, it becomes interesting to examine the effect of using an initial bed of sludge ash on the nitrogen-containing compounds. To achieve this aim, experiment 5 was conducted using DSS ash as the initial bed material. This ash was obtained by burning with air (in the fluidized bed reactor) 100 g of a DSS char, which was previously obtained by fast pyrolysis at 530 °C in the same fluidized bed reactor. By comparison between the results of this experiment and those obtained for experiment 3 (performed at the same operating conditions but using sand as the initial bed material) it can be deduced that NH3 conversion to N2 seems to be considerably lower for the experiment performed using an initial bed of ash. According to conclusions presented in previous work,16,22 this finding could be related to the simultaneous increase of the H2 production, which improves the NH3 formation by the equilibrium reaction in eq 1. In the present work, this effect is especially noticeable during the transition period, when H2 production is higher than when an initial bed of sand is used. In line with this, Figure 6 displays the interesting evolution of both N2 and H2 species during the nonstationary period. For experiment 5 (performed with an initial bed of ash), it is observed that the N2 content decreases with time, perhaps because the relatively high H2 content promotes reaction 1 toward ammonia formation. On the other hand, Figure 6b shows that the H2 content in the raw gas (for experiment 7) remains practically constant over time in spite of its possible consumption to NH3. As mentioned before, metals present in sewage sludge (specially calcium, sodium, iron, and potassium) are excellent promoters of gasification reactions,20,22,36 such as steam and CO2 reforming, shift reaction, and tar cracking,43,44 all of them aimed to increase the H2 production, which is offset by the consumption to form NH3. The observed behavior of NH3 production when the initial ash bed is used is opposite to the previous results obtained by Tsubouchi et al.34 for pyrolysis tests of several coal samples. These authors reported the capability of the alkali metals (present in the DSS ash) to catalyze conversion to N2. Nevertheless, it must be kept in mind that in an earlier gasification study (in which a Victorian brown coal was gasified in a fluidized-bed reactor at 800 °C with a mixture of air and steam) McKenzie and co-workers9 suggested that the effect of coal ash on the ammonia decomposition did not seem to be important when steam was present in the gas phase. On the other hand, Leppa¨lahti and co-workers45 provided evidence that the NH3 formation from NO (probably present at the bottom of the bed, in more oxidizing conditions) and H2 could be catalyzed by iron, calcium, and magnesium oxides. In light of both previous experimental results and the abovementioned considerations, it seems clear that the use of an initial bed composed by DSS ash is not an effective way to increase the N2 production by reducing the formation of both NH3 and HCN. Further studies focusing on the catalytic activity of the sludge ash and char are planned in the near future. Effect of Using Air Rather than a Mixture of O2/Ar. The present section shows the results obtained from two additional experiments, which have been performed using air rather than a mixture of oxygen in argon (experiments 6 and 7). The aim of these experiments was to check whether the previously observed trends (for an initial bed composed by sand) for the

Aznar et al.

mixture O2/Ar are reproduced in the case of using air as gasification agent. Regarding the nitrogen conversion to N2, very similar results have been obtained for both types of gasification agent, the differences always being lower than 5% (the standard deviation for the nitrogen released as N2 was (5.8, calculated for the three experiments performed at 850 °C, λ ) 30%, using an initial bed of sand, and a mixture Ar/O2 as gasification agent). This finding is very interesting because the N2 production for the air experiments was determined by difference, subtracting the sum of the nitrogen contained in NH3, HCN, solid residue, and tar from the nitrogen contained in the sewage sludge fed to the gasifier (due to the inability to analytically distinguish the generated N2 from the total N2). For this reason, the analytical and experimental procedures followed in the present study could also be applied to common air gasification experiments without serious lack of performance. Furthermore, the results suggest that the N2 production (in terms of N distribution as in Table 7, gN/gN of DSS fed) does not seem to be seriously affected by the presence or not of N2 in the gasification agent, although more experimentals should be performed to confirm this finding. On the other hand, the main trends related to the freeboard temperature effect on both NH3 and N-tar contents are also observed for the experiments performed using air. In addition to this, it must be mentioned that the composition of N-tar was very similar in both cases (pyrrole, 1H-indole, and benzonitrile being always the main components). Finally, the unexpected result obtained for the nitrogen contained in N-char in experiment 7 (Table 7) could be due to an unusual sampling/analytical error. A possible explanation of this error could be that the analyzed sample contained more sand, resulting in apparently lower nitrogen content. Nevertheless, the nitrogen balance closure is not appreciably affected by relatively small fluctuations in N-char contents because these amounts represent a small fraction of the nitrogen contained in the fuel. Conclusions Experiments were performed to study the influence of selected operation conditions on the formation and distribution of the nitrogen compounds during the gasification (with a mixture of O2/Ar) of dried sewage sludge in a fluidized bed. The main findings of the performed investigation include the following. (1) A method able to quantify the nitrogenous species obtained in the gasification process was implemented. Applying this method, a reliable inventory of the nitrogen contained in the gasification products was obtained (the average nitrogen balance closure was 92.3% with a standard deviation of (5.7%). (2) Most of the fuel-bound nitrogen goes to form gaseous products. N2 is the most abundant nitrogenous compound present in the gas phase. NH3 and HCN contents were relatively low, taking into account the high nitrogen content of the sewage sludge. The observed high nitrogen conversion to N2 could be explained, among other factors, by the catalytic role of the mineral matter present in the fuel. In spite of this, the use of an initial bed composed by DSS ash is not an effective way to reduce the formation of both NH3 and HCN, the concentrations of which in the producer gas were higher than those obtained for an initial bed of sand. This unexpected finding suggests that the positive effect related to the catalytic activity of some metals present in the sludge is conditioned by multiple factors. In this sense, further experiments focusing on the catalytic activity of DSS char would be interesting. (3) Concentrations of N2, NH3, and N-tar strongly depend on the freeboard temperature, an increase


of which causes an important decrease of both NH3 and N-tar contents and a substantial increase of the N2 amount. No clear trend was deduced regarding the evolution of the HCN concentration as a function of freeboard temperature. Acknowledgment. The authors thank the Spanish Ministry of Science and Innovation (MICINN) for providing frame support for this work (project CTQ2007-66885). M. A. acknowledges the predoctoral grant received from the Aragon Government (Departamento de Ciencia, Tecnologıá y Universidad-Gobierno de Aragoń). JJM acknowledges the financial support provided by the European Social Found (“Juan de la Cierva” contract).

Nomenclature N-char ) nitrogen contained in the char fraction (mass percentage of N-fuel) N-fuel ) fuel-bound nitrogen (mass fed during a gasification test) N-gas ) nitrogen contained in the raw gas (mass percentage of N-fuel) N-tar ) nitrogen contained in the tar (mass percentage of N-fuel)

Energy & Fuels, Vol. 23, 2009 3245 Tf ) freeboard temperature (°C) umf ) minimum fluidization gas velocity at the gasifier bed conditions (cm s-1) Greek Symbols λ ) equivalence ratio, defined as ratio between the experimental flow rate of the gasification agent and the stoichiometric flow rate required for the complete fuel combustion (%) Acronyms APHA ) American Public Health Association daf ) dry ash free basis DSS ) dried sewage sludge FTIR ) Fourier transform infrared spectroscopy GC ) gas chromatography LHV ) low heating value of the produced gas, dry basis MS ) mass spectrometry EF801108S

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