Gas Chromatography Study of Sewage Sludge Pyrolysis Liquids

Ind. Eng. Chem. Res. 2009, 48, 5907–5915

5907

Gas Chromatography Study of Sewage Sludge Pyrolysis Liquids Obtained at Different Operational Conditions in a Fluidized Bed I. Fonts,* M. Azuara, L. La´zaro, G. Gea, and M. B. Murillo Aragoń Institute for Engineering Research (I3A), Chemical and EnVironmental Engineering Department, Thermochemical Processes Group (GPT), UniVersity of Zaragoza, Marıá de Luna, 3, 50018 Zaragoza, Spain

Sewage sludge was pyrolyzed in a fluidized bed under different operational conditions with the aim of studying the influence of some operational conditions on the composition of the liquid product. These operational conditions were bed temperature (450-650 °C), inlet nitrogen rate (nitrogen flow per bed surface unit 0.057-0.090 m s-1), and solid feed rate per bed volumetric unit (0.169-0.338 kg s-1 m-3). The composition of the pyrolysis liquids obtained was analyzed by means of GC-MS and GC-FID. Different families of compounds appeared in the liquid samples obtained under the different conditions studied: aliphatic, aromatic, and polycyclic aromatic hydrocarbons; oxygen-containing aliphatic and aromatic compounds; nitrogencontaining aliphatic and aromatic compounds; steroids; chlorine-containing compounds; and sulfur-containing compounds. The composition of the liquids varied qualitatively with the temperature and quantitatively with the three operational parameters studied, with the temperature being the most influential variable. The liquids obtained at 450 °C contained oxygen-containing aliphatic compounds > steroids > aliphatic hydrocarbons > nitrogen-containing aliphatic compounds. The compounds found in the liquids obtained at 550 °C were quite similar to those obtained at 450 °C, although their proportions were different: steroids > aliphatic hydrocarbons > oxygen-containing aliphatic compounds > nitrogen-containing aromatic compounds. The composition of the liquids obtained at 650 °C varied considerably, and the most abundant compound groups were nitrogencontaining aromatic compounds > polycyclic aromatic hydrocarbons g aromatic hydrocarbons. Introduction Sewage sludge, in common with other organic wastes, contains a considerable number of volatile compounds. Thermal treatments are useful for obtaining both chemicals and energy from this volatile matter. Various thermal treatments such as gasification and pyrolysis are currently being studied to valorize sewage sludge. By means of pyrolysis, three products are obtained: gas, char, and liquid. Some authors have investigated the pyrolysis of wet sewage sludge in a microwave oven at high temperatures in order to obtain gas with a high heating value.1 Others have investigated the pyrolysis reaction at moderate temperatures, with a high heating rate of the sample and a short residence time of the vapors in order to maximize the liquid yield.2 In previous stages of the present work, the authors have studied the influence of pyrolysis operational conditions on product yields and properties of the liquid fraction, with the aim of maximizing the liquid yield and optimizing some of its properties with a view to its possible use as a fuel (homogeneity, high heating value, water content, and solid content).3,4 The chemical composition of pyrolysis liquid depends on the raw material and on the pyrolysis operational conditions. Several authors have investigated aspects of the composition of liquid obtained from sewage sludge pyrolysis at high and moderate temperatures.5-12 Domıńguez et al.5 investigated the differences between the composition of the liquids when the sewage sludge is heated by microwave radiation or by an electrical furnace. They analyzed the liquid composition by gas chromatographymass spectrometry (GC-MS) and claimed that the liquid contained more polycyclic aromatic hydrocarbons when the sewage sludge sample was heated by electrical furnaces. Fullana et al.6 investigated by GC-MS the presence of nitrogencontaining aromatic compounds in liquids obtained from the * To whom correspondence should be addressed. Tel.: +34976762897. E-mail: [email protected].

pyrolysis of sewage sludge at high temperatures and found that these compounds were characteristic of the liquids obtained at 850 °C. Shen et al.7 studied the pyrolysis of sewage sludge in a fluidized bed at moderate temperatures to obtain bio oil, and suggested that the structure of sewage sludge liquid was made up of clusters of aromatic compounds of up to three aromatic rings connected by long straight chains of hydrocarbons with hydroxyl groups. Doshi et al.8 investigated the improvement of the odor and the viscosity of the liquids obtained in the pyrolysis of sewage sludge. They studied the pyrolysis liquid composition by GC-MS and found mainly palmitic acid, oleic acid, stearic acid, alkenes (C10-C18), alkanes (C10-C18), cholestene (sterenes), myristic acids (C14), cholesterol (sterols), and oleic acid esters. Jindarom et al.9 classified the compounds of the sewage sludge pyrolysis liquid by their functional groups into six classes: monoaromatics and single ring heterocyclic compounds, aliphatic compounds, oxygenated compounds, nitrogenated compounds, steroids, and polycyclic aromatic compounds. Some of these authors studied the influence of the pyrolysis heating mode,5 the pretreatment of the sewage sludge,6 the atmosphere,9 the temperature,9,12 and the composition of the sewage sludge sample10,11 on the pyrolysis liquid composition. The objective of the present work is to determine the effect of certain sewage sludge pyrolysis operational conditions on the composition of the liquid product analyzed by gas chromatography. Knowing the chemical composition of the pyrolysis liquids is an important step toward understanding the sewage sludge pyrolysis process and evaluating possible applications. Materials and Methods Experimental Procedure. An anaerobically digested sewage sludge (S1) provided by a Spanish urban wastewater treatment plant was used as raw material in the pyrolysis experiments. Elemental analysis, ultimate analysis, and heating value of the

10.1021/ie900421a CCC: $40.75  2009 American Chemical Society Published on Web 05/18/2009

5908

Ind. Eng. Chem. Res., Vol. 48, No. 12, 2009

Figure 1. GC-MS chromatogram of extractives of raw sewage sludge: 6, 1-hexadecene; 17, 1-nonadecene; 29, hexadecanoic acid; 30, 9-octadecenoic acid; 31, octadecanoic acid; 37, 1,2-benzenedicarboxylic acid diisooctyl ester; 46, cholestan-3-ol; 47, 4-cholesten-3-ol acetate; 48, cholestan-3-one; 49, cholestan3-ethoxy; 50, stigmast-7-en-3-ol; 51, hexadecanoic acid hexadecyl ester. Table 1. Proportion of the Chemical Families in the Pyrolysis Oil Obtained in the Experiments experiment number

T (°C) uN2 (m/s) u/umf Qfeed (kg/s m3) % aliphatic HCsa % aromatic HCs % polyarom HCs % O-containing aliph compds % O-containing arom compds % N-containing aliph compds % N-containing arom compds % steroids % Cl-containing compds % S-containing compds a

1

2

3

4

5

6

7

8

9

10

11

12

13

450 0.057 6.7 0.338 10.1 1.6 0.0 33.9 5.8 9.3 5.1 30.0 3.5 0.7

650 0.057 10.1 0.338 7.7 15.7 23.2 3.2 5.6 0.7 42.7 1.3 0.0 0.0

650 0.090 15.8 0.338 12.2 17.3 21.8 1.3 5.1 0.3 41.5 0.6 0.0 0.0

450 0.090 10.6 0.338 16.4 2.8 0.0 15.9 8.5 7.5 4.2 41.4 2.8 0.9

550 0.074 10.7 0.254 23.0 8.1 0.8 21.3 2.9 5.1 10.6 26.5 0.6 2.1

550 0.074 10.7 0.254 24.0 7.0 0.7 22.3 3.7 5.2 9.3 26.4 0.5 1.9

450 0.090 10.6 0.169 12.7 5.6 0.0 27.6 5.2 5.8 13.6 26.6 1.3 2.6

650 0.090 15.8 0.169 11.2 22.9 13.1 1.4 3.8 0.6 41.3 5.7 0.0 0.0

650 0.057 10.1 0.169 10.3 22.7 6.4 1.2 2.7 0.2 55.6 1.0 0.0 0.0

450 0.057 6.7 0.169 14.2 4.3 0.0 23.5 6.1 4.0 15.1 28.1 2.0 2.8

550 0.074 10.7 0.254 24.8 7.9 0.9 20.3 2.5 5.2 10.4 26.8 0.0 1.6

450 0.074 8.7 0.254 17.6 5.0 0.0 21.1 8.3 7.9 8.3 28.6 1.8 1.9

650 0.074 13 0.254 14.8 14.7 17.6 1.1 2.5 0.6 37.6 11.1 0.0 0.0

mean std dev

23.9 7.7 0.8 21.3 3.0 5.2 10.1 26.9 0.4 1.9

0.9 0.6 0.1 1.0 0.6 0.1 0.7 0.8 0.3 0.3

variability coeff

3.8 7.6 12.5 4.7 20.1 1.1 6.9 3.0 87.7 13.5

HCs, hydrocarbons.

sewage sludge sample can be found elsewhere.3 Extractives of a similar sample of sewage sludge were determined by Soxhlet extraction with dichloromethane and continuing the extraction with acetone. GC-MS analysis of the extractives (Figure 1) showed the presence of aliphatic hydrocarbons from C10 to C30 (above all linear alkanes, alkenes, and some cyclic and ramified); benzenes with side chains from C1 to C9; one naphthalene methyl derivative; some ketones, some aldehydes, and some fatty alcohols of different chain lengths; fatty acids (for example, tetradecanoic acid, hexadecanoic acid, and 9-ocatadecenoic acid); some phenols; some amines; steroids, and sterols (C27-C29, mainly derived from cholestene and cholesterol); and some fatty acids with side fatty ester chains (for example, hexadecanoic acid hexadecyl ester and octadecanoic acid octadecyl ester). Sewage sludge was analyzed by X-ray diffraction. The main minerals found in the sample were illite, kaolinite, magnetite, albite, quartz, calcite, microcline, and dolomite. These minerals contain some alkali and alkaline earth metals such as K, Na, Mg, and Ca that can act as catalysts in some reactions that take place in the pyrolysis process13 and also some other elements such as Al, Fe, and Si. The pyrolysis experiments were carried out in a fluidized bed laboratory-scale plant equipped with a continuous feed of sewage sludge and a continuous char removal system by overflow. The detailed experimental procedure followed in these experiments is described in depth elsewhere.3 Pyrolysis liquids were extracted from the condensers using isopropyl alcohol as a wash. The whole liquid was dissolved in the solvent, and samples were taken for the gas chromatographic analyses performed by GC-MS and gas chromatography with flame

ionization detection (GC-FID) using a chromatographic method explained elsewhere.11 GC-FID analyses were used to find out the relative proportion of each compound in the sample by calculating the chromatographic area percentage. Calibration was not carried out due to the large number of compounds. This method, widely used by other authors,10,11,14 considers the response factors of all the compounds to be similar and therefore gives only semiquantitative results suitable for comparing relative percentages of compounds in pyrolysis liquids obtained from different experiments. Experimental Design. The study of the quantitative influence of the pyrolysis operational conditions on the liquid product composition has been carried out using an experimental design. This method is appropriate for studying the influence of the experimental variables, and also the influence of their interactions on the composition of the liquids. The operational variables studied were bed temperature (T) between 450 and 650 °C, inlet nitrogen rate (nitrogen flow per bed surface area, uN2) between 0.057 and 0.090 m s-1 (nitrogen flow rate 3.5-5.5 dm3(NTP) min-1 and u/umf 6-16), and solid feed rate per bed volumetric unit (Qfeed) between 0.169 and 0.338 kg s-1 m-3 (solid feed rate 3.0-6.0 g min-1). The experimental conditions and the u/umf ratio followed in each pyrolysis experiment are shown in Table 1. Further information about gas and solid residence times of the experiments can be found elsewhere.4 The proportions of chromatographic area of the different chemical families that appeared in the pyrolysis liquids were the response variables studied. The evolution of these chemical families within the operational conditions was studied because the large number


5909

Figure 2. GC-FID chromatogram of pyrolysis liquid obtained at 450 °C: 1, butanoic acid; 14, styrene; 90, hexadecanoic acid; 91, 9-octadecenoic acid; 92, octadecanoic acid; 93, 9-octadeceneamide; 94, 3-hexadecanol; 100, cholest-5-ene; 102, cholestan-3-ol acetate; 103, cholest-8(14)-ene-7,15-diol; 104, cholesta4,6-dien-3-ol; 105, cholest-5-en-3-ol; 106, 2-anthracenecarboxylic acid; 107, 3-ethylidenecholestane.

Figure 3. GC-FID chromatogram of pyrolysis liquid obtained at 550 °C: 1, pyridine; 13, styrene; 26, phenol; 49, cyclotetradecane; 51, 1H-indole 3-methyl; 111, cholestan-3-ol acetate; 112, cholest-8(14)-ene-7,15-diol; 115, 2-anthracenecarboxylic acid; 118, 1-norcholestan-3-one 5-ethenyl; 119, epicoprostanol; 120, dihydrocholesterol; 121, cholesterol; 122, cholestan-3-ol.

of compounds appearing in the pyrolysis liquids made it impractical to analyze them one by one. By means of ANOVA analyses, the operational conditions that significantly affected each chemical family were determined using a confidence level of 95%. Results and Discussion Influence of the Operational Conditions on the Qualitative Liquid Composition. The compounds identified in the pyrolysis liquids obtained at different temperatures were significantly different. However, the same compounds were found in the liquids from the experiments carried out at the same temperatures although at different inlet nitrogen rates and/or at different solid feed rates. That is, the temperature was the only factor affecting the qualitative liquid composition. Around 70%, 81%, and 76% of the total chromatographic areas of the compounds susceptible to be determined, following this gas chromatographic method, were identified in the liquids obtained at 450, 550, and 650 °C, respectively. The chromatograms of the pyrolysis liquids obtained at the three temperatures studied (450, 550, and 650 °C) are shown in Figures 2, 3, and 4, respectively. A large number of the compounds that appeared in the liquids obtained at 450 and 550 °C were quite similar to those found in the extractives of the raw sewage sludge. These compounds could therefore have come directly from the devolatilization of the sewage sludge sample. The chromatogram of the liquids obtained at 650 °C showed quite different compounds from those found in the

liquids obtained at 450 and 550 °C, and also from the compounds found in the extractives of the raw sewage sludge. This could be due to the increase of the pyrolysis process temperature since this would lead to the formation of different compounds in secondary reactions of the primary pyrolysis products.5,15 The compounds appearing in the different pyrolysis liquids were classified according to their functional groups as follows: aliphatic hydrocarbons, aromatic hydrocarbons, polycyclic aromatic hydrocarbons, oxygen-containing aliphatic compounds, oxygen-containing aromatic compounds, nitrogen-containing aliphatic compounds, nitrogen-containing aromatic compounds, steroids, chlorine-containing compounds, and sulfur-containing compounds. Different chains of aliphatic hydrocarbons (above all linear and some cyclic and/or ramified) were found with a number of carbons ranging between C6 and C30. These compounds could have come directly from the devolatilization of the alkanes and alkenes detected in the sewage sludge and also from the pyrolysis of different sewage sludge constituents (triglycerides,16,17 carboxylic acids,9 steroids18). The chain length (number of carbons) was not the same in the liquids obtained at the three temperatures: the chain lengths were longer in the liquids obtained at 550 °C. For example, in the liquids obtained at 450 °C there were compounds with a number of carbons ranging between C6 and C19, with one C30 (ramified). In the liquids obtained at 550 °C, the number of carbons of the aliphatic hydrocarbons ranged between C6 and C27. Lastly, in the liquids

5910


Figure 4. GC-FID chromatogram of the pyrolysis liquid obtained at 650 °C: 1, pyridine; 2, 1H-pyrrole; 4, pyridine 2-methyl; 9, styrene; 17, benzene 1-ethyl-2-methyl; 28, 1H-indene; 31, phenol 4-methyl; 36, naphthalene; 41, naphthalene 1-methyl; 44, 1-tetradecene; 45, 1H-indole 3-methyl; 46, naphthalene 1,7-dimethyl; 49, acenaphthylene; 56, anthracene; 59, naphthalene 2-phenyl; 60, hexadecanoic acid; 63, chrysene; 64, cholest-3-ene. Table 2. Empirical Coded Models That Fit the Evolution of the Chemical Families Inside the Ranges Studied as a Function of Significant Terms

aliphatic HCs aromatic HCs polycyclic aromatic HCs O-containing aliphatic compds O-containing aromatic compds N-containing aliphatic compds N-containing aromatic compds steroids Cl-containing compds S-containing compds

intercept

T

uN2

Qfeed

TuN2

TQfeed

uN2Qfeed

T2

uN22

TuN2Qfeed

T(uN22)

R2

23.91 7.60 0.75 21.36 2.98 5.20 10.16 26.83 0.38 1.83

-1.49 5.17 8.76 -11.40 -2.90 -3.63 14.71 -8.75 -1.14 -0.89

1.27 0.50 0.56 -1.97 0.41 -1.766 × 10-3 -2.22 1.62 -0.19 0

-0.26 -2.31 3.77 0.072 0.35 0.90 -4.02 1.62 0.37 -0.48

0 0 0.75 1.52 0 0 -1.64 0 0.19 0

0 -0.86 2.63 0 0.73 -0.85 0.83 -2.81 0 0.48

1.42 0 -0.51 -3.03 0 -0.55 1.72 1.05 -0.37 0

-7.71 1.91 8.01 -8.32 2.45 -0.96 12.86 -6.02 0.76 -0.94

-4.35 2.04 0.47 0 -1.28 -0.70 4.37 -3.09 0 0

0 0 -1.53 2.52 0 0.35 1.55 -2.41 0 0

0 2.94 -1.87 0 3.02 0.52 3.17 -5.82 0 0

0.99 0.99 1 0.99 0.94 0.99 0.99 0.99 0.94 0.99

obtained at 650 °C the number was between C6 and C16 (above all linear). This fact could be a result of the effect of the temperature, considering that 450 °C was not high enough to devolatilize long straight-chain aliphatic hydrocarbons from the sewage sludge. On the other hand, the absence of aliphatic hydrocarbons with a number of carbons larger than C16 in the liquids obtained at 650 °C could be due to the cracking of the long chains of these compounds into shorter ones or to the formation of other kinds of compounds such as aromatic hydrocarbons.9,12,15,17 The aromatic hydrocarbons identified in the liquids at each temperature had different structures. In the liquids at 450 and 550 °C, the few aromatic hydrocarbons that were found had similar structures. They were characterized by having a benzyl radical group attached to aliphatic side straight chains (C1-C6). The aromatic hydrocarbons that appeared in the liquids obtained at 650 °C were methyl- and ethylbenzene derivatives. The liquids obtained at the three temperatures contained styrene, which can originate from the pyrolysis of phenylanilinecontaining proteins and peptides of the sewage sludge.19 The proportion of polycyclic aromatic hydrocarbons was only important in the liquids obtained at 650 °C. The compounds that appeared in larger quantities were naphthalene methyl derivatives and anthracene. The variety of these compounds increased when the temperature increased possibly due to their formation via the HACA mechanism.20 The main oxygen-containing aliphatic compound groups determined were fatty acids, some fatty alcohols, some esters, some aldehydes, and some ketones. The most abundant group was the fatty acids, above all C16, C14, and C18. Fatty acids have also been found in large quantities in sewage sludge pyrolysis liquids by other authors.5,8-12 The fatty acids, fatty alcohols, aldehydes, and ketones that were determined in the pyrolysis

liquids obtained at low temperatures were also present in the extractives of the raw sewage sludge. It is therefore thought that they could come directly from the devolatilization of the sewage sludge. Moreover, fatty acids could come also from the pyrolysis of triglycerides.17 The typical oxygen-containing aromatic compounds in these liquids were phenol and its methyl and phenyl derivatives. Phenolic compounds may originate from the pyrolysis of polysaccharides and proteins.21 The main nitrogen-containing aliphatic compounds were nitriles and amides. Nitrogen-containing compounds (aliphatic and also aromatic) contained in the liquid product came from the pyrolysis of the protein fraction of the sewage sludge.16 Some amines were determined, but only in the liquids obtained at 450 °C. This low presence of amines in spite of the importance of this group in sewage sludge constituents is consistent with the results found by other authors.6 Nitrogencontaining aliphatic compounds were found above all in the liquids obtained at 450 and 550 °C. Some of the nitriles and amides appeared as radicals in short chains, C5 and C6, but most were linked to long straight chains (C15-C19). In the pyrolysis liquids obtained at 650 °C, only hexadecanenitrile was found. It should be noted that the nitriles found in the liquids obtained at 550 °C appeared linked to longer chains (C15-C19) than those of the liquids obtained at 450 °C (C15). As occurred with the aliphatic hydrocarbons, nitriles of longer straight chains were determined in the liquids produced at 550 °C. The most important nitrogen-containing aromatic compounds were pyridine, pyrrole, and indole and their respective methyl derivatives, and benzonitrile. Moreover, in the liquids obtained at 650 °C compounds such as quinoline and indolizine were also found.


5911

Figure 5. Interaction plots: aliphatic hydrocarbons vs T for the lowest and highest uN2 values. (a) Qfeed ) 0.169 kg s-1 m-3; (b) Qfeed ) 0.254 kg s-1 m-3.

Steroids and sterols from C27 to C29 were also found in significant amounts in the pyrolysis liquids. The most important were cholestene, cholestadiene, and cholesterol. These compounds were also present in the extractives of the raw sewage sludge. This compound family has also been found by numerous other authors who studied the pyrolysis of sewage sludge.5,8-12 Lastly, few compounds that contained chlorine or sulfur were found. These compounds only appeared in the liquids obtained at 450 and 550 °C, and were also determined in the extractives of the raw sewage sludge. Thiophene and some of its alkyl derivatives were determined in these liquids. These sulfurcontaining compounds have also been found by other authors.9 Influence of the Operational Conditions on the Quantitative Liquid Composition. The proportions of chromatographic area of each chemical family in the pyrolysis liquids obtained from the different experiments are shown in Table 1. The average, the standard deviation, and the variability coefficient of the experimental data obtained in the central point replicate experiments (550 °C, 0.074 m s-1, and 0.254 kg s-1 m-3) are also shown in Table 1. As the variability coefficients indicate, the experimental procedure gave quite repetitive results for all the chemical families (variability coefficient < 20%), except for the chlorine-containing compounds. The effect of the temperature could be assessed from the data shown in Table 1. The liquids obtained at 450 °C contained steroids > oxygen-containing aliphatic compounds > aliphatic hydrocarbons > nitrogen-containing aliphatic compounds. The most important chemical families in the liquids obtained at 550 °C were steroids > aliphatic hydrocarbons > oxygen-containing aliphatic compounds > nitrogen-containing aromatic compounds. The composition varied considerably in the liquids obtained at 650 °C; the most abundant chemical families were nitrogencontaining aromatic hydrocarbons > polycyclic aromatic compounds g aromatic hydrocarbons. These data were analyzed statistically by means of analyses of variance (ANOVA) with a confidence level of 95%, with the aim of quantifying the influence of the temperature and clarifying the influence of the other two operational variables studied. The significant influences of the operational conditions on the proportion of each chemical family in the pyrolysis liquids are shown by means of empirical coded models in Table 2, and the most significant ones are explained next by means of interaction plots. In Figure 5 it can be seen that there is a maximum in the percentage of aliphatic hydrocarbons with the temperature

Figure 6. Interaction plot: aromatic hydrocarbons vs T for the lowest and highest Qfeed values at uN2 ) 0.074 m s-1.

around 550 °C. As commented before, this fact could be explained because at 550 °C there are more kinds of aliphatic hydrocarbons (longer straight chains) able to devolatilize from the sewage sludge than at 450 °C. However, at 650 °C the proportion of aliphatic hydrocarbons decreased because at high temperatures they disappear and probably form aromatic hydrocarbons.20 The inlet nitrogen rate influences the aliphatic hydrocarbon percentage only at the highest solid feed rate studied (Figure 5b), with the highest proportion being obtained at the highest inlet nitrogen rate (lowest gas residence time). Figure 6 shows that the proportion of aromatic compounds increased with the temperature. As can be observed, this increase is higher between 550 and 650 °C than between 450 and 550 °C. This trend has also been found by other authors.12 This phenomenon occurs because at higher temperatures the aromatic compounds increase rapidly due to their formation via the Diels-Alder reaction and other mechanisms.5,12,17,20,22 The proportion of aromatic hydrocarbons was higher for the lowest solid feed rate. At low solid feed rates the bed is renewed more slowly than at high solid feed rates, since a char removal system by overflow is used to keep the bed height constant. The char that remains for more time in the bed could lose the catalytic activity of its ash due to coke deposition or other mechanisms. For this reason, at high solid feed rates (greater catalytic activity) aromatic hydrocarbons could transform into polycyclic aromatic compounds, which are more stable.20 As can be observed, Figure 7 shows that the proportion of polycyclic aromatic hydrocarbons increased rapidly with the

5912


Figure 7. Interaction plot: polycyclic aromatic hydrocarbons vs T for the lowest and highest Qfeed values at uN2 ) 0.074 m s-1. The negative values resulted from the model adjustment.

temperature from 550 °C. The formation of these hydrocarbons from secondary reactions, in which primary pyrolysis products such as aliphatic hydrocarbons,20 aromatic hydrocarbons,20 or steroids18 are transformed into polycyclic aromatic compounds, is favored at high temperatures. These reactions corroborate the experimental data, since the proportion of aliphatic hydrocarbons and steroids decreased with the temperature from 550 °C. The proportion of aromatic hydrocarbons did not decrease with the temperature because their formation is also favored at high temperatures. Other authors5,6,9,12 studying the pyrolysis of sewage sludge have also found significant quantities of polycyclic aromatic hydrocarbons in the liquids produced at high temperatures. In addition, the proportion of polycyclic aromatic compounds was higher in the liquids obtained from the experiments carried out with the highest solid feed rate. This influence is opposite that of the solid feed rate on the aromatic hydrocarbons. Therefore, it is thought that the higher proportion of polycyclic aromatic hydrocarbons at the highest solid feed rate studied could be caused by their formation from the aromatic hydrocarbons. This influence could be related to the catalytic effect of the ash of the char, since it is known that the formation of polycyclic aromatic compounds is favored when there is a greater presence of ash in the bed.11 As commented before, at low solid feed rates the bed is renewed more slowly than at high solid feed rates, and the char remaining for longer periods in the bed could lose the catalytic activity of its ash. Therefore, the formation of polycyclic aromatic compounds would be enhanced at the highest solid feed rates, when the bed is rapidly renewed and the char does not lose the catalytic activity of its ash. In Figure 8, it can be observed that the proportion of oxygencontaining aliphatic compounds was more or less constant in the liquids produced from 450 to 550 °C. However, they diminished significantly with the temperature from 550 to 650 °C. The data presented by Jindarom et al.,9 who studied the pyrolysis of sewage sludge, also showed that the oxygencontaining compounds of the pyrolysis liquids (fatty acids, esters, aldehydes, and ketones) decreased with the pyrolysis temperature from 550 to 650 °C. The effect of the inlet nitrogen rate was only significant up to 550 °C, obtaining a larger proportion of oxygen-containing aliphatic compounds with the lowest inlet nitrogen rate studied (highest gas residence time). Figure 9 shows the effect of the inlet nitrogen rate at 450 and 650 °C on the oxygen-containing aromatic compounds. As can be observed, the inlet nitrogen rate had no effect at 650 °C, but had a quadratic effect at 450 °C. For this reason, a maximum proportion of oxygen-containing aliphatic compounds

Figure 8. Interaction plot: oxygen-containing aliphatic compounds vs T for the lowest and highest uN2 values at Qfeed ) 0.254 kg s-1 m-3.

Figure 9. Interaction plot: oxygen-containing aromatic compounds vs uN2 for the lowest and highest T values at Qfeed ) 0.254 kg s-1 m-3.

Figure 10. Interaction plot: nitrogen-containing aliphatic compounds vs T for the lowest and highest Qfeed values at uN2 ) 0.074 m s-1.

was obtained for those liquids obtained at around 450 °C and at the average inlet nitrogen rate studied (uN2 ) 0.073 m s-1). As can be observed in Figure 10, the proportion of nitrogencontaining aliphatic compounds diminished with the temperature, due to the formation of nitrogen-containing aromatic compounds at high temperatures, which follow the same reaction pathways as polycyclic aromatic compounds.6 At around the lowest temperatures studied (450 °C), the proportion of nitrogencontaining aliphatic compounds in the pyrolysis liquids was higher for the highest solid feed rate studied. However, the effect of the solid feed rate disappeared when the temperature increased. The effect of the solid feed rate could also be due to the catalytic activity of the ash present in the char that would


5913

Figure 11. Interaction plot: nitrogen-containing aromatic compounds vs T for the lowest and highest uN2 values. (a) Qfeed ) 0.254 kg s-1 m-3; (b) Qfeed ) 0.338 kg s-1 m-3.

Figure 12. Interaction plot: steroids vs T for the lowest and highest uN2 values. (a) Qfeed ) 0.169 kg s-1 m-3; (b) Qfeed ) 0.338 kg s-1 m-3.

enhance the formation of nitrogen-containing aliphatic compounds at low temperatures and higher solid feed rates. The proportion of nitrogen-containing aromatic compounds was the most abundant in the sewage sludge pyrolysis liquids obtained at 650 °C. As can be observed in Figure 11, their proportion remained more or less constant in the oils produced between 450 and 550 °C, but increased exponentially with the temperature from 550 to 650 °C. The high presence of nitrogencontaining aromatic compounds obtained at 650 °C is in agreement with the findings of other authors who have claimed that this compound family is characteristic of sewage sludge pyrolysis liquids obtained at high temperatures.6 On the other hand, the inlet nitrogen rate had an effect only at the lowest solid feed rates studied (Figure 11a), with a higher proportion of nitrogen-containing aromatic compounds being obtained with the lowest inlet nitrogen rate. The effect of the inlet nitrogen rate could be related to the gas residence time. At lower inlet nitrogen rates (higher gas residence times, tr,gas > 2.1 s), the proportion of nitrogen-containing aromatic compounds could increase because these compounds have more time to be produced in secondary gas-phase reactions. Figure 12 shows that the proportion of steroids decreased with the temperature. The presence of steroids was predominant in the liquids obtained at 450 and 550 °C, since they come directly from the devolatilization of the raw sewage sludge. At 650 °C, the liquids contained few steroids due to their disappearance at high temperatures in secondary reactions to form aliphatic, aromatic, and polycyclic aromatic compounds.18 In Figure 12a it can be observed that the proportion of steroids is higher at around the highest inlet nitrogen rate (lowest gas

residence time), probably because these disappearance reactions had less time to take place. As the effect of the inlet nitrogen rate was different in Figure 12a and 12b, it was thought that this could be due to the effect of the solid feed rate that would depend on the bed temperature. If Figure 12a and 12b are compared at low temperatures, it can be observed that the effect of the inlet nitrogen rate is higher for the highest solid feed rate studied. This could be explained by the fact that at high solid feed rates the volume of gases devolatilized from the sewage sludge was higher and for this reason the gas residence time would be lower and the steroids would have less time to disappear. However, at high temperatures the effect of the solid feed rate could be related to the catalytic power of the ash, which would be higher at high solid feed rates, as commented before. It would cause the effect of the inlet nitrogen rate (gas residence time) to decrease when the catalytic power increased (high solid feed rates), since the disappearance reactions would be faster. Chlorine-containing compounds were influenced only by the temperature and the solid feed rate. The proportion decreased from the liquids produced at 450 °C to those obtained at 550 °C. No chlorinated compounds were found in the pyrolysis liquids obtained at 650 °C. At the lowest temperatures studied the proportion of chlorinated compounds was slightly higher for the highest solid feed rates studied. Sulfur-containing compounds, like the chlorine-containing compounds, were only influenced by the temperature and the solid feed rate. The proportion of this compound family was more or less similar in the liquids obtained at 450 and 550 °C. However, in the liquids produced above 550 °C the proportion decreased until they disappeared. At temperatures lower than

5914


Figure 13. Evolution of chemical families with temperature in the range under study at uN2 ) 0.074 m s-1 and Qfeed ) 0.254 kg s-1 m-3.

550 °C, the quantity of sulfur-containing compounds decreased when the solid feed rate increased. Empirical Models. As commented before, from the ANOVA analyses of the experimental data (Table 1) some empirical coded models were formulated as functions of all the terms that significantly affected the proportions of the chemical families (Table 2). These models were codified because the limit values of each operational condition take the values -1 and 1. For this reason, the most influential factor on the chemical families was the term with the highest coefficient. Moreover, the sign of each term indicated how this term influences the chemical family. The most influential operational condition on the composition of the sewage sludge pyrolysis liquids was the temperature, and for this reason the biggest coefficients of the coded models were always those belonging to the terms related to the temperature. Moreover, these models can be used to predict the evolution of the composition with the operational variables within the ranges studied. However, it must be taken into account that these models were not validated with further experiments because the sewage sludge sample became exhausted and it is known that the effect of the sewage sludge sample on the pyrolysis liquid composition is significant.11 Figure 13 shows the evolution of the chemical families with the temperature at 0.074 m s-1 inlet nitrogen rate and at 0.254 kg s-1 m-3 feed rate of solid predicted using the empirical models. This figure summarizes the effect of the temperature on the proportions of the different chemical families explained in the previous sections. Due to the interactions, the effect of the temperature on some chemical families could change slightly with different values of the inlet nitrogen rate and feed rate of solids. Conclusions The compounds identified by GC-MS in the sewage sludge pyrolysis liquids belong to different chemical families: aliphatic, aromatic, and polycyclic aromatic hydrocarbons; oxygencontaining aliphatic and aromatic compounds; nitrogen-containing aliphatic and aromatic compounds; steroids; chlorinecontaining compounds; and sulfur-containing compounds. The composition of the liquids was affected qualitatively by the temperature but not by the inlet nitrogen rate nor by the solid feed rate. The liquids obtained at 450 and 550 °C presented some similarities. Moreover, these liquids contained numerous compounds that were present in the extractives of the raw sewage sludge and therefore were assumed to come directly from the devolatilization of sewage sludge. However, the composition of the liquids obtained at 650 °C changed signifi-

cantly with numerous compounds being obtained from secondary reactions of the primary pyrolysis products. The proportion of aliphatic hydrocarbons in the pyrolysis liquids took on a maximum value at a bed temperature of around 550 °C. The proportion of aromatic hydrocarbons, polycyclic aromatic hydrocarbons, and heterocyclic nitrogen-containing aromatic compounds in the liquid product increased significantly with the bed temperature. The proportion of oxygen-containing aliphatic compounds, nitrogen-containing aliphatic compounds, steroids, chlorine-containing compounds, and sulfur-containing compounds in the pyrolysis liquids decreased with the bed temperature. The proportion of the chemical families in the sewage sludge pyrolysis liquids was also affected by the other two operational conditions studied (inlet nitrogen rate and solid feed rate). Some of the compounds that disappeared (such as the aliphatic hydrocarbons and steroids) or appeared (the nitrogen-containing aromatic compounds) in secondary reactions were affected by the inlet nitrogen rate. The proportions of aliphatic hydrocarbons and steroids was higher for the highest inlet nitrogen rate (lowest gas residence time), since they had less time to disappear in secondary reactions. However, the proportion of nitrogencontaining aromatic compounds was higher for the lowest inlet nitrogen rate (highest gas residence time) since these compounds have more time to appear in secondary reactions. The effect of the solid feed rate on the composition of the pyrolysis liquids could be related to the catalytic role of the ash contained in the bed material. When the bed material is renewed more quickly (higher solid feed rates), the ash of the char is assumed to have a more important catalytic activity. This catalytic activity affects some compounds involved in secondary reactions (aromatic hydrocarbons, polycyclic aromatic hydrocarbons, nitrogen-containing aliphatic compounds, steroids, chlorine-containing compounds, and sulfur-containing compounds). As a result of this catalytic activity, the presence of the polycyclic aromatic hydrocarbons tends to be favored in the liquids obtained at the highest solid feed rates. However, the presence of compounds resulting in the formation of polycyclic aromatic hydrocarbons, such as aromatic hydrocarbons, increased in the liquids obtained at the lowest solid feed rates studied. Acknowledgment The authors express their gratitude to the Spanish Ministry of Science and Technology for financial support (Research Project CTQ2004-05528-PPQ and Research Project CTQ200766885-PPQ) and for the doctoral grants awarded to I.F. and M.A.


Literature Cited (1) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Pis, J. J. Productions of bio-fuels by high temperature pyrolysis of sewage sludge using conventional and microwave heating. Bioresour. Technol. 2006, 97, 1185– 1193. (2) Bridle, T. R.; Pritchard, D. Energy and nutrient recovery from sewage sludge via pyrolysis. Water Sci. Technol. 2004, 50, 169–175. (3) Fonts, I.; Juan, A.; Gea, G.; Murillo, M. B.; Sańchez, J. L. Sewage sludge pyrolysis in fluidized bed: optimization of the operational conditions to obtain oil. Ind. Eng. Chem. Res. 2008, 47, 5376–5385. (4) Fonts, I.; Juan, A.; Gea, G.; Murillo, M. B.; Arauzo, J. Sewage Sludge Pyrolysis in a Fluidized Bed, II: Influence of Operational Conditions on some Physicochemical Properties of the Liquid Product. Ind. Eng. Chem. Res. 2009, 48, 2179–2187. (5) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Pis, J. J. Investigation into the characteristics of oils produced from microwave pyrolysis of sewage sludge. Fuel Process. Technol. 2005, 86, 10071020. (6) Fullana, A.; Conesa, J. A.; Font, R.; Martıń-Gulloń, I. Pyrolysis of sewage sludge: nitrogenated compounds and pretreatment effects. J. Anal. Appl. Pyrolysis 2003, 68-69, 561–575. (7) Shen, L.; Zhang, D. An experimental study of oil recovery from sewage sludge by low-temperature pyrolysis in a fluidised bed. Fuel 2003, 82, 465–472. (8) Doshi, V. A.; Vuthaluru, H. B.; Bastow, T. Investigations into the control of odour and viscosity of biomass oil derived from pyrolysis of sewage sludge. Fuel Process. Technol. 2005, 86, 885–897. (9) Jindarom, C.; Meeyoo, V.; Rirksomboon, T.; Rirksomboon, T.; Rangsunvugit, P. Thermochemical decomposition of sewage sludge CO2 and N2 atmosphere. Chemosphere 2007, 67, 1477–1484. (10) Domıńguez, A.; Meneńdez, J. A.; Inguanzo, M.; Bernard, P. L.; Pis, J. J. Gas chromatographic-mass spectrometric study of the oil fractions produced by microwave-assisted pyrolysis of different sewage sludges. J. Chromatogr., A 2003, 1012, 193–206. (11) Fonts, I.; Azuara, M.; Gea, G.; Murillo, M. B. Study of the pyrolysis liquids obtained from different sewage sludge. J. Anal. Appl. Pyrolysis 2009, 85, 184-191.

5915

(12) Park, E.; Kang, B.; Kim, J. Recovery of Oils With High Caloric Value and Low Contaminant Content By Pyrolysis of Digested and Dried Sewage Sludge Containing Polymer Flocculants. Energy Fuels 2008, 22, 1335–1340. (13) Mohan, D.; Pittman, C. U.; Steele, P. H. Pyrolysis of Wood/Biomass for Bio-oil: A Critical Review. Energy Fuels 2006, 20, 848–889. (14) Gonzalves, A. R.; Meier, D.; Faix, O. Pyrolysis-gas chromatography of the macromolecular fractions of oxidized organocell lignins. J. Anal. Appl. Pyrolysis 1997, 40-41, 543–551. (15) Rodrı´guez, I.; Laresgoiti, M. F.; Cabrero, M.; Torres, A.; Chomoń, M. J.; Caballero, B. Pyrolysis of scrap tyres. Fuel Process. Technol. 2001, 72, 9–22. (16) Boocock, D. G. B.; Konar, S. K.; Mackay, A.; Cheung, P. T. C.; Liu, J. Fuels and chemicals from sewage sludge 2. The production of alkanes and alkenes by the pyrolysis of triglycerides over activated alumina. Fuel 1992, 71, 1291–1297. (17) Maher, K. D.; Bressler, D. C. Pyrolysis of triglyceride materials for the production of renewable fuels and chemicals. Bioresour. Technol. 2007, 98, 2351–2368. (18) Britt, P. F.; Buchanan, A. C.; Kidder, M. K.; Owens, C., Jr. Influence of steroid structure on the pyrolytic formation of polycyclic aromatic hydrocarbons. J. Anal. Appl. Pyrolysis 2003, 66, 71–95. (19) Tsuge, S.; Matsubara, H. High-resolution pyrolysis-gas chromatography of proteins and related materials. J. Anal. Appl. Pyrolysis 1985, 8, 49–64. (20) Ritcher, H.; Howard, J. B. Formation of polycyclic aromatic hydrocarbons and their growth to sootsa review of chemical reaction pathways. Prog. Energy Combust. Sci. 2000, 26, 565–608. (21) Parnaudeau, V.; Dignac, M. F. The organic matter composition of various wastewater sludges and their neutral detergent fractions as revealed by pyrolysis-GC/MS. J. Anal. Appl. Pyrolysis 2007, 78, 140–152. (22) Cunliffe, M.; Williams, P. T. Composition of oils derived from the batch pyrolysis of tyres. J. Anal. Appl. Pyrolysis 1998, 44, 131–152.

ReceiVed for reView March 13, 2009 ReVised manuscript receiVed April 30, 2009 Accepted May 1, 2009 IE900421A

Gas Chromatography Study of Sewage Sludge Pyrolysis Liquids

Recommend Documents