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Biofuels and Biomass

Elemental migration and transformation from sewage sludge to residual products during the pyrolysis process Marius Praspaliauskas, Nerijus Pedisius, and Nerijus Striugas Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.8b00196 • Publication Date (Web): 02 Apr 2018 Downloaded from http://pubs.acs.org on April 2, 2018

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Elemental migration and transformation from sewage sludge to residual products during the pyrolysis process Marius Praspaliauskasa [email protected] Nerijus Pedišiusa [email protected] Nerijus Striūgasb [email protected] a

Laboratory of Heat-Equipment Research and Testing, Lithuanian Energy Institute, Breslaujos str. 3, LT-44403 Kaunas, Lithuania b Laboratory of Combustion Processes, Breslaujos str. 3, LT-44403 Kaunas, Lithuania

Abstract The elemental distribution and physicochemical properties of residual products—namely, char, tar and produced gas—obtained during the pyrolysis of anaerobically treated dried sewage sludge was investigated and compared with those of raw sludge. Experimental investigations were performed in a laboratory fixed-bed reactor and revealed the migration fate of heavy metals (Cd, Co, Cr, Cu, Ni, Pb, Ti and Zn), alkaline earth metals and alkali metals (Ba, Be, Ca, K, Mg and Na), intermediate metals (Fe, Mn, Al and Si) and non-metals (P, S, Cl, C, H, and N) in pyrolysis products. Primary attention was focused on heavy metal migration. The highest heavy metal recoveries were found for Cd > Co > Cr > Cu from sewage sludge into the gas-phase, with 68.17 % of the Cd eliminated. This study also showed that high-temperature pyrolysis technology is an effective and promising thermochemical treatment for sewage sludge to reduce its volume. Keywords: sewage sludge; pyrolysis; heavy metals; relative enrichment; elemental recovery INTRODUCTION Biomass and organic waste are considered the main substitutes for fossil fuels. Because of the development of modern technologies (in industry, agriculture, and households) favorable conditions have emerged to meet a growing demand for renewable and sustainable energy1. Sewage sludge is an inevitable byproduct of sewage treatment. It is necessary to develop infrastructure to use sewage sludge by improving treatment facilities and adapting them for further thermal processing of sludge. Today, sewage sludge disposal in landfills is forbidden. Incineration and composting are the most common methods of sewage sludge treatment to reduce the growing quantities of sewage waste2. Sludge storage sites are considered the simplest method for the removal of sewage sludge because they provide a relatively cheap method of treatment. However, storage sites are of a limited capacity, and to avoid hazardous substances entering the soil, the storage sites must be installed in accordance with stringent environmental requirements3. Incineration is an efficient method to reduce sewage sludge volume. Due to a favorable ratio between the relatively low emission of pollutants and the amount of energy generated, this

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method is gaining increased attention. However, due to the resulting solid particles, incineration as a method of sewage sludge disposal has a negative effect on air quality3. Low energy density and physical properties such as high ash content and high mechanical durability of sludge pellets complicate the direct burning of sewage sludge. For this reason sewage sludge is converted to syngas, char and condensates. Such use of sewage sludge could significantly contribute to the development of sustainable energy and the generation of cleaner energy4. Currently, pyrolysis a thermal process that occurs in the atmosphere of an inert gas and has become increasingly popular in the management of solid waste5. The volume of waste is reduced during this process by generating valuable byproducts to obtain chemically stable products6. The pyrolytic conversion of sewage sludge into char is a promising method to manage this waste and simultaneously take advantage of environmental benefits7. Pyrolysis is also a beneficial chemical procedure to convert organic waste into valuable oils and high-calorific value combustible syngas3. Pyrolysis is considered the main alternative for the utilization of sewage sludge. Three main fractions remain after a pyrolysis process: tars with condensate, synthetic gas with a high calorific value and char8. Sludge pyrolysis technology takes full advantage of the calorific value of the sludge, obtaining higher energy utilization efficiency at a lower cost, and realizes a harmless and reducing treatment for the sludge. For this reason, the further use of char after a pyrolysis process is more acceptable than the use of char after other thermal processes, such as incineration or gasification8,9. Pyrolysis is a relatively clean technology that quickly pays for itself10. The advantage of this technology lies in the facts that valuable products are obtained from the process and the amount of waste is reduced. Moreover, heavy metals are concentrated and pathogens are destroyed during pyrolysis11. Approximately 60 % of char consists of inorganic compounds. Char is characterized by a low calorific value that fluctuates between 5 and 10 MJ/kg. Further use of char for incineration is energetically disadvantageous. For this reason char can be used effectively for soil remediation12. However, an increased concentration of heavy metals limits the use of sewage sludge. Products of pyrolysis can also be used as a source of chemicals13 in the manufacture of absorbents13,14, fertilizers14,15, and raw materials for energy generation14,16 and in the manufacture of activated carbon and catalysts. Scientific research has revealed that the char from sewage sludge improves the quality of degraded soil11. Heavy metals may exist as various mineral salts (carbonate, sulfate, chlorate, phosphate, etc.), sulfides, hydroxides, oxides and clathrates in sewage sludge, and mineral salts and hydroxides are generally converted into oxides or sulfides with better thermostability under reductive pyrolytic conditions17. Therefore, a great proportion of the heavy metals remain in the biochar with a lower mobility7,18. Studies19 have shown that metals with a relatively low boiling point (e.g., Hg, Cd, Se) are removed from the reactor during the pyrolysis process, whereas higher concentrations of metals with high boiling points (e.g., Pb, Ni, Cu, Zn, Sr) are found in the char. During pyrolysis, heavy metals are most effectively immobilized in solid phase for this reason a high percentage of metals remain in the char after pyrolysis20. However, the mobility of the heavy metals becomes significantly lower in biochar21. The physicochemical characteristics of sewage sludge char depend on the parameters of the sewage sludge used as the raw material and on the pyrolysis process. The temperature parameters have the greatest effect on the pyrolysis process because the distribution of the product tars, gases and char and their properties depend on the temperature20,23. The higher the pyrolysis temperature, the lower amount of char that is generated. High reaction temperatures also enhance the extraction effects for heavy


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metals, and thus, more of the heavy metals are distributed into bio-chars (solid phase product)21. Meanwhile, the total content of heavy metals in the biochar increases, and the degradation and transformation of organic substances in the sewage sludge is enhanced. However the resulting microstructure of the char, generated at high temperatures, has a greater effect when compared to that of the char generated at low temperatures23. If high temperatures are maintained during the process, the char loses a large portion of its carbon and other functional groups. The chemical composition, pH, surface shape, thermal stability and content of heavy metals are strongly influenced by the pyrolysis temperatures22,24. Studies25,26 also showed that wastewater sludge biochar produced at low temperatures (300 °C – 400 °C) is acidic, whereas at high temperatures (400 °C – 700 °C), the wastewater sludge biochar is alkaline in nature. If the soil intended for treatment with biochar application is acidic in nature, biochar produced at temperatures of 700 °C or higher can be used to neutralize the soil, improve the soil fertility and sequester carbon27. Unlike the organic content, the heavy metal content is not removed from the sewage sludge during a pyrolysis process. Due to high accumulation in the soil and food chain, heavy metals are potentially toxic. For this reason, their concentrations are limited28. The content of heavy metals in char after the pyrolysis of sewage sludge, animal waste and municipal waste is widely described in scientific literature23,29. Research studies have revealed that the content of metals in the char of sewage sludge and animal waste is significantly lower than that in the char of municipal waste30. The transfer characteristics of heavy metals depend upon their respective boiling points and their corresponding forms in the sludge, e.g., chlorides and sulfides. For example, chlorides easily vaporize, and sulfides poorly vaporize10,18,31. The variability in the micronutrients with temperature is due to their volatility and the effect of the pyrolysis temperature on both the composition and chemical structure of the biochar25. Biochar has a lower content of available trace nutrient elements compared to that in sewage sludge. The leaching toxicity of heavy metals in the biochar is lower than the leaching toxicity of heavy metals in sewage sludge, but the pyrolysis process intensifies the enrichment of heavy metals in the biochar32. Currently, intensive research on the pyrolysis of sewage sludge is being performed, but there are gaps in the information about the fate of the heavy metals within the reaction products. It is necessary to perform more research and accumulate more information about the distribution of heavy metals, not only in the char but also in tar, condensate and gas. As the global demand for renewable energy and organic matter increases, sewage sludge pyrolysis products could be one of the resources available for use in addressing this need33. Pyrolysis gas and tar could also be used in direct energy production, but there is a lack of data about impurities in these products. This is the reason studies on heavy metal migration are needed to avoid environmental pollution during the process of energy production from pyrolysis gas and tar. Anaerobically digested dried (10 wt. %) sewage sludge was chosen as the object of this study. The aim of the paper is to determine the elemental distribution in the pyrolysis products (char, tar, condensate and gas) and to compare the results obtained with the results of other research studies. During the investigation, a condensable tar and condensate mixture was separated into two separate factions, i.e., condensate and tar To achieve this objective, sewage sludge was pyrolyzed at a temperature of 850 °C. This temperature was selected to minimize the volumetric quantity of sludge and to avoid slagging in the reactor during the pyrolysis process. Numerous elements (Al, As, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Sb, Si, Ti, V, Zn, Ba, Be, and Se) were selected for investigation to determine how these elements are distributed individually in the pyrolysis products. Sufficient knowledge of the situation and fate


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of the elements distributed in the char, tar, condensate and gas will reduce environmental pollution. 2. Material and methods 2.1 Experimental setup The schematic diagram of the experimental setup is presented in Fig. 1. A vertical, batchtype pyrolysis reactor was designed in the laboratory and used to conduct experiments. The pyrolysis chamber had an internal diameter of 5 cm and a length of 85 cm. The process temperatures were controlled with PicoLog software using a thermocouple positioned at the center of the reactor in the pyrolysis chamber. Anaerobically digested, thermally dried sewage sludge was obtained from a wastewater treatment plant located in Šilutė (Lithuania) for use in this study. The sludge sample (Table 1) had a particle size in the 5–20 mm range and was not ground and sieved prior to feeding into the pyrolysis reactor. Dry sewage sludge (300 grams) was loaded into a laboratory-scale reactor. The feed of the material was flushed with nitrogen gas for 3 min at a constant flow rate of 4 L/min to obtain an oxygen-free atmosphere. The temperature program was set to heat at a constant heating rate of 15 °C/min to achieve the set-point temperature (850 °C), and the reactor was maintained at this temperature for approximately 90 min. The pyrolysis experiments were performed at atmospheric pressure. The condensable tar and condensate mixture was weighed directly during the pyrolysis process with a Kern balance, and data was recorded with KernBC2006 software. The gases generated by the pyrolysis reaction were first allowed to pass through a cold trap to condense the tar. Non-condensable gases—H2, CO, CO2, and CH4—were analyzed online with the VISIT 03H analyzer and Win-Data 3 software.

Fig. 1. The schematic configuration of pyrolysis experiment. 1. Nitrogen gas 2. Flowmeter 3. Temperature controller 4. Thermocouple of reactor 5.Thermocouple of sludge feed 6.Furnance 7.Balance 8. Ice bath 9. Gas analyzer 10. PC

Nitrogen gas was used as the carrier gas at a rate of 6 L/min to maintain an inert atmosphere. This flow rate was considered sufficient to prevent the accumulation of the pyrolysis gas that was generated and to have no effect on the temperature of the surface of the sediment sample. After each run, the furnace was turned off, and the reactor was allowed to cool naturally to room temperature. The cooled samples were collected, and the masses of the sludge, char, tar


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and condensate in the samples were determined. Then, the samples were subjected to elemental and heavy metal analyses. The syngas mass was calculated by finding the difference (equation 4). The experiment was repeated at least five times to ensure the reliability of the mass balance and data. The data are reported in this paper as the mean value of five replicates. To avoid repeatedly describing the analyzed subjects, the subjects were shortened as follows: sewage sludge (SS), sewage sludge char (SSCh), sewage sludge tar (SST) and sewage sludge condensate (SSC). 2.2 Proximate and ultimate analysis Laboratory samples (300 g) of SS were taken randomly from a large bag with a pipe (spear). Sewage sludge char portions, approximately half of the sample after pyrolysis, were taken from all five replicates. After pyrolysis, the condensable tar and condensate mixture were obtained. The condensable tar and condensate mixture consisted of heterogeneous compounds, such as heavy tar and water that separate into different layers according to density. The mixture was centrifuged and separated using laboratory funnels. The separated tar and condensate were prepared in portions for laboratory samples (approximately 3 ml) for further determination. From the collected sludge and char samples, the moisture content was identified according to specification CEN/TS 15414-1:2010, and the ash content in the sludge, char, tar and condensate was determined according to the standard LST EN 15403:2011 method. Proximate analysis to determine the weight percentage of volatile matter was conducted using a TGA 4000 with a simultaneous TGA/DTA analytical method. The fixed carbon content was calculated from the difference. The analysis of C, H, N and S present in the solutions was performed using a Flash 2000 analyzer. The C, H and N contents were determined according to the standard LST EN 15407:2011 method. The O content was calculated from the difference. The Cl and S contents were estimated using an ISC-5000 DC ion chromatographic system according to the standard LST EN 15408:2011 method. The high heat value (HHV) for the solution was determined using an IKA C5000 calorimeter according to the standard LST EN 15400:2011 method for automated bomb calorimeters. The surface morphology and surface area were determined using scanning electron microscopy (SEM) and Brunauer, Emmett and Teller (BET) analysis, respectively. The results of all of the determined parameters are presented in the Results and Discussion section, Table 1 and Fig. 4. 2.3 Elemental determination The obtained samples of the sewage sludge, char, tar and condensate were mineralized for determination of the selected metals. At the first mineralization step, the test samples (approximately 0.2 – 0.4 g of laboratory samples) were flooded with 3 ml of concentrated nitric acid, 3 ml of hydrofluoric acid and 1 ml of hydrochloric acid. The samples (in triplicate) were placed in a mineralizer and mineralized for 1 h and 10 min (at 800 W, 6 MPa, pRate: 50 kPa/s), with 10 min allocated for heating, 45 min for mineralization (in accordance with established parameters) and 15 min for cooling. After the second mineralization, the samples were flooded with 18 ml of boric acid (H3BO3 to avoid and eliminate fluoride toxicity) and again placed into a mineralizer for 1 h and 10 min (at 800 W, 6 MPa, pRate: 30 kPa/s). After mineralization, the solution was poured into 50 ml flasks and diluted to 50 ml using deionized water. The analysis of the solutions (including determination of Al, As, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Na, Ni, P, Pb, S, Sb, Si, Ti, V, Zn, Ba, Be, and Se) prepared from the sewage sludge char, tar and


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condensate samples were performed using an ICP-OES according to the standards LST EN 15410:2011 and LST EN 15411:2011. 2.4 Processing of the results This study compares the obtained results with the results of other authors. To better identify the results obtained, data calculations were performed using the formulas below. First, the absolute amount (1) of the determined element in the product was calculated. The data were used to calculate the element recovery (2) from the char, tar and condensate. To compare our results with those of other studies, the results obtained were recalculated to determine the relative enrichment factor (3) and to evaluate the element recovery from the gas (4). The calculations were performed by taking the difference between the calculated element recoveries in the char, tar and condensate. In these calculations, the concentrations of the elements in the SS were equated to 100 %. The calculations were performed using the formulas below.

(1) where AAp is the absolute amount in product of element (mg), cp is the element concentration in char, tar and condensate (mg/kg), and mp is the product mass after pyrolysis (kg). (2) where ER is the element recovery in char, tar and condensate (%), AAp is the absolute amount in the product (mg), and AAss is the absolute amount in SS. (3) where RE is the relative enrichment (dimensionless parameter) and css is the element concentration in sewage sludge (mg/kg). And element recovery from the gas by difference: ER(gas) = 100 – ER(char) – ER(tar) – ER(condensate)

(4)

3. Results and discussion 3.1 The yield of pyrolysis products Fig. 2 shows the percentage distributions of the pyrolysis products. The yield of the char, tar, condensate and gas is presented as a mass ratio product relative to the initial sludge sample.


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Fig. 2. Percentage distribution of char, tar, condensate and gas

The measurements from the experimental investigation showed that the highest yield of char was obtained as 41.8 wt. %. ± 0.07 %. The condensable tar and condensate mixture formed 30.70 wt. % ± 0.66 % of the total pyrolysis products. After separation, the condensate was 18.47 wt. % ± 0.40 % of the total products, and the tar yield was 12.23 wt. % ± 0.26 %. The water content obtained in this work was relatively low compared to the range of 20 to 70 wt. % reported in the literature34 because the reference sewage sludge moisture content was quite low at 9.84 wt. %. The gas yield was calculated by the difference and found to be 27.50 wt. % ± 0.65 %. The composition of the gas and the formation of the condensable tar and condensate mixture during the pyrolysis process are presented in Fig. 3. First, diagram (A) shows the typical gas output during the pyrolysis process described in equations 5-12. The second diagram (B) shows the condensable tar and condensate mixture formation during the pyrolysis process with different masses of added sewage sludge in the pyrolysis reactor.


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Fig. 3. Typical gas composition (A), condensable tar and condensate mixture yield (B) during pyrolysis process in laboratory fixed bed reactor.

The dried sludge conversion of main products: char, condensable vapors, and gas, during pyrolysis process generally is described by equation 5. The condensable vapors are described as a mixture of tar and condensate (water). This equation consists of many reactions (6-13) that step by step describe pyrolysis products generation9. These equations also describe the process and succession of producing gases in different stages of the pyrolysis process. The typical gas formation is illustrated in Fig. 3-A. Main reaction of sewage sludge pyrolysis CxHyOz + heat →char + CO + CO2 +H2O + condensable vapors Char gasification reaction - Partial oxidation: C + 1/2O2 → CO ∆H = -110.5 kJ/mol - Water gas reaction: C + H2O → CO + H2 ∆H = 131.3 kJ/mol - Boudouard reaction: 2CO → C + CO2 ∆H = 171.7 kJ/mol - Hydrogasification: C + 2H2 → CH4 ∆H = -74.9 kJ/mol Tar decomposition reaction - Tar pyrolysis: Tar → wH2 + xCO + yCO2 + zCnHm - Tar steam gasification: Tar + vH2O → xCO + yH2 Light gas reaction - Methanation reaction: CO + 3H2 → CH4 + H2O ∆H = -206.2 kJ/mol - Water gas shift reaction: CO + H2O → CO2 + H2 ∆H = -41.1 kJ/mol

(5)

(6) (7) (8) (9)

(10) (11)

(12) (13)

The CO2 gas maximum peak was detected first at 408 °C and was 15.06 wt. %. This gas formation might be described by three reactions: Bouduard (8), tar decomposition (10) and water


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gas shift (13) reactions. The second maximum peak was CH4 gas. The formation of this gas might be the hydrogasification reaction (9) and tar decomposition reaction (10), which can lead to various hydrocarbons formation including and CH4. This gas formation during experiment was detected at 643 °C. At this temperature, the CH4 gas output was 24.42 wt. %. The CO and H2 gas outputs were lower compared to those of the CO2 and CH4 gases. These gas formations occurred in two steps. First, peaks from the formation of this gas were slight and had a wide range. The maximum peak of CO gas was detected at 416 °C and was 3.21 wt. %. At this stage, the CO formation was based on partial oxidation (6) and water-gas reactions (7). The second maximum peak of CO gas was detected at 749 °C and was 7.34 wt. %. At this stage, the CO gas formed during the tar pyrolysis (10) and tar steam gasification (11) reactions. Hydrogen formation starts at about 300 °C and still progresses at a fairly high rate at the final temperature of the examination (till 850 °C). The first hydrogen peak formation was also based on the watergas reaction (7). At this stage, the maximum yield of H2 gas was generated at 559 °C and was 4.68 wt. %. The second peak formation of the H2 gas was observed at 682 °C and was 5.63 wt. %. At this stage, the H2 gas formed during the tar decomposition reactions and light gas reactions. In summary among the non-condensable gases H2, CH4, CO and CO2 formation, it was found that CH4 and CO2 are the dominant gas products while CO and H2 were the minor products during the pyrolysis. Fig. 3-B shows the tendency of the condensable tar and condensate mixture formation. The intense formation of the condensable tar and condensate mixture was observed in a temperature range between 60 °C and 550 °C. This temperature range is known to be the volatilization point for volatile compounds in the sewage sludge, and this is a rough judgement boundary for the main decomposition components in sewage sludge35. The beginning mass for the condensable tar and condensate mixture formation was the same in all replicates, but the ending temperatures were different. The major weight increase was observed at around 700 °C for all the replicates. At the highest point of the condensable tar and condensate mixture, the weight reached the highest point, but after a few minutes, the mass of the condensable tar and condensate mixture decreased to the range of 18 g – 20 g. The mass became stable and varied by approximately 285 g/kg. These mass losses can be explained by the evaporation processes of water and volatile compounds because the temperature of the outgoing gases was high. 3.2 Chemical composition of the sewage sludge, char, tar and condensate The sludge was characterized to relate the initial feedstock composition to the final product composition. The proximate and ultimate analyses of the anaerobically digested sewage sludge char, tar and condensate are presented in Table 1. Table 1. Proximate and ultimate analysis of sewage sludge, sewage sludge char, tar and condensate. parameter C, % H, % N, % Sulphur, % O (by difference), % ash, % volatile matter, % fixed carbon, % ash, %

sewage sludge sewage sludge char ultimate analysis (wt. %) 32.3 ± 1.26 25.97 ± 1.05 5.04 ± 0.13 0.4 ± 0.04 4.23 ± 0.42 1.3 ± 0.04 1.42 ± 0.035 0.08 ± 0.03 22.43 34.57±0.04 71.41 ± 0.56 proximate analysis (wt. %) 39.55 ± 2.13 22.98 24.25 34.57 ± 0.04 71.41 ± 0.56


tar

condensate

42.67 ± 1.44 5.36 ± 0.19 4.43 ± 0.05 0.65 ± 0.03 48.83 0.064 ± 0.004

7.88 ± 2.74 1.05 ± 0.02 2.36 ±0.08 0.47 ± 0.003 88.19 0.046 ± 0.008

85.75 ± 1.40 0.064 ± 0.004

5.90 ±0.43 0.046 ± 0.008

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moisture, % H/C O/C N/C HHV, MJ/kg Cl, %

9.84 ± 0.02 0.16 0.83 0.13 13.5±0.049 0.16 ± 0.05

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4.34 ± 0.02 0.02 0.05 9.58 ± 2.82 0.27 ± 0.14

10.84 ± 0.05 0.13 1.19 0.11 33.05 ± 0.07 0.08 ± 0.01

93.87 ± 0.08 0.39 9.32 0.34 0.004 ± 0.03

The sludge and char displayed very distinct chemical compositions. The high ash content of the char indicates that (1) very little elemental content is transferred into the gas phase and liquid phase, and the ash forming elements such as: Si, Al, Fe, Ca, Mg, Mn, Na, K, P, S, and Cl36 remain in the solid phase (Table 1. and Table 2); and (2) the carbonaceous materials transform into hydrocarbon compounds, e.g., gases, aromatic hydrocarbons and tar10. Variations in the C, H, O, and N contents and the H/C, O/C, and N/C ratios demonstrate that the organic matter decreases during the pyrolysis process. The carbon contents in the sewage sludge and its products differ. In the SSCh, the carbon content after pyrolysis was slightly decreased. The C content decreased by approximately 6 %, which was confirmed by the mineral fraction being a dominant fraction in the initial SS and SSCh29. The ash content of the SSCh increased by twofold after pyrolysis (Table 1). A relatively high concentration of carbon was determined in the tar, and this influenced the high calorific value. The results showed that some carbonaceous products were transferred into the condensate. The content of H, N and O decreased significantly after pyrolysis of the sewage sludge compared to that of the initial sewage sludge. The decreases in these elements did not follow the same tendency. The ratio of H/C, which indicates the degree of carbonization in the sewage sludge, was always lower than 0.5, which suggested that biochar with a strong carbonization and high aromaticity can resist decomposition. These changes in the H/C and O/C ratios also illustrated that dehydrogenative polymerization and dehydrative polycondensation occurred during pyrolysis, producing a significant loss of oxygen and aliphatic hydrogen37. The N/C ratio had the same tendency as the H/C and O/C ratios. Compared with the ratios in the raw SS, the H/C and N/C ratios in the char and tar decreased, but the ratios increase in the condensate. The O/C ratio of the tar and condensate compared with that of SS increases from 1.19 in the tar to 9.32 in the condensate. After the pyrolysis process, a BET analysis was performed, and the surface area of the char was determined to be 54.40 m²/g. Fig. 4 shows the SEM photographs of the SS and SS char at 50 and 4000 times magnification. Pores of different sizes and different shapes could be observed. Comparing the surface images shown in the SEM images (Fig. 4), we can see that after pyrolysis, the texture changes continued. The biochar produced at 850° C was left with more sags and crests on its surface, while the sewage sludge was flatter on the surface. More sags and crests suggests that good contact with soil particles could be ensured with this biochar, which is beneficial for nutrient exchange 23. Sewage sludge

Sewage sludge char


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Fig. 4. Surface morphology of sewage sludge and sewage sludge char.

Many research studies have revealed and described that the surface area of the char increases when the temperature of the pyrolysis increases26,38. The chemical structure of the sewage sludge changes during a pyrolysis process. As a result, the surface area and ratio of micro and macro pores change. The surface area properties are also negatively influenced by the ash content of sewage sludge. The surface morphology is influenced not only by the temperature and properties of the sludge but also by the process of sewage sludge processing in sewage treatment plants, i.e., the temperature elevation speed during pyrolysis and the retention time in the reactor. Changes in these parameters result in changes in the surface area of the produced char7. 3.3 Elemental distribution The distributions of selected elements in the sewage sludge, char, tar and condensate are presented in Table 2, and the percentage distributions are shown in Fig. 5. After investigation As, Sb, V and Se elements were not found in sewage sludge and the further analysis in pyrolysis products was not made. The concentrations of most of the elements were higher in the char than in the sewage sludge sample. The exceptions were Cd, which was not detected, and Co and S, which had lower concentrations in the char than in the sludge. The increase in the concentrations of the other elements during sewage sludge pyrolysis is typically caused by the increased concentration of elements in the biochar samples due to the gradual loss of C, H and O. The concentrations of the elements in the tar and condensate were determined to be lower. In the tar, Ca, Cr, Ni, Ti, Ba and Si were below the detection limit. In the condensate, Cr, Ni, Ti, Ba and Si were below the detection limit. Table2. Element concentrations (mg/kg) in sewage sludge, sewage sludge char, tar and condensate after pyrolysis. The ranges in literature show the wide variation in sewage sludge of elements in general. element

Cd Co Cr Cu Ni Pb Ti Zn

sewage sludge

sewage sludge char

avg., mg/kg

std., %

avg., mg/kg

std., %

6.17 20.09 52.07 124.37 17.39 73.77 919.07 2610

12.00 16.70 2.30 11.50 17.10 13.70 9.21 11.00

Cu > Pb > Cr > Co > Ni > Be > Cd. Almost the same variation in the measured elements was determined in the SSCh. The variation could be influenced by the degradation and transformation of organic substances in the SS. The concentrations in the SSCh increased, but the variation was almost the same as that in the SS. The element concentrations in the tar followed the order of: S > K > Zn > Fe > Na > P > Cu > Mg > Al > Pb > Co > Cd > Be > Mn. In the condensate, the order was: S > K > Na > Zn > P > Pb > Al > Mg > Ca > Fe > Cu > Co > Cd > Be > Mn. Estimating the elemental distribution by percentages, the largest portion of SS and SSCh was composed of ash-forming elements, such as Si (37 %), Ca (20 %), P (13 %) and Fe (10 %). The percentage distribution of the main heavy metals, Co, Cr, Cu, Ni and Pb, in the overall mass balance was less than 1 %, and the Cd distribution in the overall mass balance was less than 0.01 %. The elemental variation in the liquid products was different from the elemental variation found in the case of SS or SSCh. The largest portion of the SST composition consisted of three elements, S (39 %), K (23 %) and Zn (21 %), and in the SSC, the composition consisted of S (54 %), K (13 %) and Na (10 %). The evaluation of the elements, especially the heavy metal distribution, showed that most of the metal remained in the sewage sludge char.


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Fig. 5. Elemental distribution in sewage sludge, char, tar and condensate. The results in tables near the diagrams presented the values in range between 0.01 % and 1 %.

Relative enrichment (RE) factors help to identify the degree of enrichment of elements in the SSCh and reveal the volatility of trace elements. RE factors in SSCh greater than 1 indicate a larger enrichment of the trace element in the SSCh, and RE factors less than 1 indicate that the elements exhibit volatilization25. In this case, the relative enrichment factors were determined for Cd, Cr, Cu, Ni, Pb and Zn trace elements. These elements were chosen because their concentrations in sewage sludge are regulated by ES (86/278 EEC) requirements, and member states have transposed the European limits for sludge use into agriculture in their own regulations45. Our results were compared with those of other studies to show how the temperature influences the different element enrichment in SSCh. The comparison of the RE factors for our results and the results obtained by other authors is presented in Fig. 6. A fixed-bed pyrolysis was performed in all studies, but the rectors were different types, including infrared furnace, microwave oven and horizontal quartz and ceramic reactors. The temperature increase rate, gas flow and selected temperatures were different. The influence of the temperature on changes in the RE factor was further analyzed.


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Mustafa K. Hossain et al., 2011, X.D. Song et al., 2014, Y.D. He et al.,2010, Haoran Yuan et al., 2013, Mustafa K. Hossain et al., 2009(sample B), Mustafa K. Hossain et al., 2009(sample C), Mustafa K. Hossain et al., 2009(sample M), Qinglong Xie et al., 2014, Chen Tan et al., 2014, Haoran Yuan et al., 2015, F. Chen et al., 2015, Our study

Fig. 6. Relative enrichment in different temperatures of Cd, Cr, Cu, Ni, Pb and Zn in sewage sludge chars after pyrolysis.

An obvious difference between the increased RE factors of the elements in the range from 450 °C to 550 °C can be seen in the works of Song et al. (2014), Xie et al. (2014) and Hossain, Strezov, and Nelson (2009) (sample M). A relative enrichment of the heavy metals stands out in the general trend, and a significant increase in the RE values of Cr, Cu, Pb and Zn elements can be seen. Such a wide range of RE factors can be explained by the unequal conditions of the pyrolysis processes and the different types of SS. The assessment of the relative enrichment of Cd showed the following trend. With increasing pyrolysis temperature, the RE factor also increased. The Cd concentration during our study was below the set limit of 0.01 39.51 66.39 80.15 95.29 84.76 93.82 82.11

8.23 6.40 4.62 3.82 7.84 3.00 6.20

Ba Be Ca K Mg Na

98.03 63.93 84.68 88.52 92.53 97.41

1.46 7.47 11.36 2.40 6.72 2.44

Fe

92.10

5.73

tar

condensate

std., % avg., % std., % heavy metals 14.77 2.37 13.32 1.37 7.03 3.15 5.23 2.34 >0.01 >0.01 2.24 0.90 1.39 0.51 >0.01 >0.01 3.35 0.50 3.12 0.40 >0.01 >0.01 1.66 0.81 0.45 0.19 alkaline earth and alkali metals >0.01 >0.01 11.31 4.35 9.86 3.38 >0.01 >0.01 0.003 0.42 0.08 0.21 0.05 0.03 0.02 0.04 0.02 0.26 0.04 0.23 0.03 other metals 0.08 0.05 0.01 0.004

sum avg., %

sum avg. + std.

possibly in gas, %

28.10 51.77 66.39 83.77 95.29 91.23 93.82 84.22

31.83 65.49 72.78 89.80 99.11 99.97 96.82 91.43

68.17 34.51 27.22 10.20 0.89 0,03 3.18 8.57

98.03 85.09 84.69 89.15 92.61 97.90

99.49 100.29 96.05 91.68 99.37 100.41

0.51 -0.29 3.95 8.32 0.63 -0.41

92.20

97.97

2.03

avg., %


Energy & Fuels 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Mn Al Si

92.63 99.12 99.85

7.15 1.48 0.35

0.03 0.01 >0.01

C Cl H N P S

34.86 85.98 3.63 16.30 95.59 5.75

3.14 1.91 0.09 3.94 3.15 0.73

11.67 4.34 8.22 7.48 0.02 2.36

0.02 0.02 0.01 0.02 >0.01 non-metals 0.65 4.84 0.80 0.51 0.73 3.59 1.47 11.12 0.01 0.02 0.19 2.14

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0.01 0.02 -

92.68 99.14 99.85

99.86 100.65 100.20

0.14 -0.65 -0.20

0.24 0.003 0.98 0.97 0.00 0.30

51.36 90.83 15.44 34.90 95.63 10.25

55.40 93.55 17.25 41.29 98.78 11.47

44.60 6.45 82.75 58.71 1.22 88.53

During the sludge pyrolysis at 850 °C, most of the heavy metals (Co, Cr, Cu, Ni, Pb, Zn, and Ti) remained in the char (elemental recoveries varied from 39.51 to 95.24 %). The same tendency was observed with alkaline earth and alkali metals (Be, Ca, Mg, Ba, K, and Na) (63.93 – 98.03 %). The variations in the non-metals, P, S, Cl, C, H, and N, in the char were very different, ranging from 3.63 % for hydrogen to 95.59 % for phosphorus. The intermediate metals, such as Fe, Mn, Al and Si, in the char showed the greatest recovery, up to 90 %, which showed their good stability during the pyrolysis process. The elemental recoveries in the tar and condensate were low. The heavy metal recoveries in the tar vary from 14.77 % (Cd) to 1.66 % (Zn) and from 13.32 % (Cd) to 0.45 % (Zn) in the condensate. Cr, Ni and Ti were below the detection limit. Alkaline earth and alkali metal recoveries were quite low compared with the heavy metal recoveries. The values of K, Mg and Na were lower than 1 %, with the exception of Be, for which the recoveries in the tar and condensate were 11.31 % and 9.86 %, respectively. The Ca in tar and the Ba in the tar and condensate were determined to be below the detection limit. In general, the non-metal recovery variation was the most abundant. The highest values of the non-metal recovery were C > H > N > Cl > S in the tar. In the condensate, the highest non-metal recoveries were N > C > H > S. The phosphorus recovery was lower than 1 % for both the tar and condensate. One of the factors which could make influence to incomplete recovery of metallic elements in pyrolysis products might be mass balance obtained for the samples19. From the calculations performed during this experiment, a high recovery was determined for Cd – 68.17 %, Co – 34.51 % and Cr – 27.22 %. Our findings show that the metal recovery from the sludge depends on the corresponding form of the metal in the sludge (mostly chlorides)46 and on specific characteristics of the elements. These elements are presented as volatile (Cd) or semivolatile (Co, Cr) heavy metals. The calculations showed that the Cu and Zn recoveries were also quite high, at 10.20 % and 8.57 %, respectively. Conclusions During the pyrolysis process, valuable products are produced that can later be used as a raw material in other directions. One of the products is sewage sludge char which is characterized by large surface area and a high amount of sags and crests on its surface. This type of char has a good contact with soil particles, which can be beneficial for nutrient exchange process. Exhaust gases, as an alternative energy source, can be used for energy production. Liquid products have high calorific value and after treatment can be used for oil and fuel production. The usage of pyrolysis technology and its formed products could potentially reduce environmental pollution. After the process, most of the elements and heavy metals were retained by solid residue, but the total amount compared to the sewage sludge remains unchanged. The elemental


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Energy & Fuels

recovery varies up to 39.51 % of the heavy metals, 63.93 % of the alkaline earth and alkali metals and from 3.63 % to 95.59 % of the non-metal in the sewage sludge char after pyrolysis. This study showed a promising perspective on immobilization of heavy metals from sewage sludge to char. Also the study showed that high temperature pyrolysis favours the transformation effect on heavy metal Cd, Co, Cr, Cu and Zn, migration from sewage sludge to synthetic pyrolysis gas. References

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