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Nitrogen transformation during sewage sludge pyrolysis Lihong WEI, Lina WEN, Tianhua YANG, Na ZHANG (College of Energy and Environmental, Shenyang Aerospace University, Shenyang110136, China)
The influences of different temperatures and sewage treatment processes on the pyrolysis of sludge obtained from three municipal waste water plants in Shenyang, China were studied in a fixed bed reactor. To clarify nitrogen transformation mechanisms, the functional forms of sewage sludge nitrogen(SS-N) were evaluated through X-ray photoelectron spectroscopy; the NOX precursor was identified with spectrophotometric method. The results show that the nitrogen present in sludge in forms of protein-N(P-N), pyridine-N(N-6), pyrrole-N(N-5), quaternary-N, and nitrogen oxides(N-X), P-N and N-6 account for approximately 80% of the total nitrogen in raw sludge samples(SS-Raw). NH3 is the main product of P-N conversion during sludge pyrolysis; the majority of N-6 tends to be converted into HCN at 400°C–600°C. N-6 removal rate is closely linked to the type and origin of municipal sludge that originates from different sewage treatment processes, as well as to the various stages of the microbial growth curve. The anaerobic process facilitates the removal of N-6 from chars and its conversion to HCN. SS-Raw does not contain N-X; an increase in temperature increases the amount of bound N-X in the SS-N fraction. This N-X mainly originates from the conversion of heterocyclic-N (N-6 and/or N-5).The transformation routes of fuel-nitrogen during sludge pyrolysis are discussed as well.
1. Introduction NOX emission reduction is a key task that should be performed to meet increasingly stringent
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environmental standards in the development of the thermal disposal process for sewage sludge (SS). SS nitrogen (SS-N) content reaches a maximum of 9 wt.%1–4 because of its origin and its chemical composition. This concentration is considerably higher than that of coal (< 2.5 wt.%).5 The most effective thermal technology established is incineration, which requires extensive gas cleaning and a safe ash disposal method.3, 6 Pyrolysis is an attractive approach as a clear alternative to incineration and an advanced thermal technology. In addition, this technique has good economic and environmental prospects because the process conditions can be optimized to maximize the production of gases, oils, or chars.7 Several studies have been conducted on coal pyrolysis to understand coal nitrogen conversion.5, 8–14
X-ray photoelectron spectroscopy (XPS) analysis results imply that pyridine-N (N-6) and
pyrrole-N (N-5) are the dominant nitrogen functionalities in coal5, 8; N-5 is converted into N-6, and the conversion of char-N into HCN is attributed to these functionalities. Quaternary-N (N-Q) is mainly converted into NH3with an increase in pyrolysis temperature.8, 9 Nelson et al.10–12determined that N/C ratio remains stable at temperatures of 600 °C–800 °C. Xie et al.13reported that accumulated amounts of NH3 released are first maximized; then, the amounts decrease sharply. Furthermore, heating rate affects nitrogen transformation and migration with an increase in pyrolysis temperature; this increase is attributed to the secondary reaction.8, 14 To date, research on SS pyrolysis has mainly focused on the N-containing species in tar and gas products.4Tian et al.15 noted that HCN is the main NOX precursor that accounts for 80% of the total nitrogen in the pyrolysis process. Aznar et al.16 reported that NH3 yield decreases gradually with increasing temperature; furthermore, N2 yield increases distinctively during SS gasification. Zhang
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et al.17, 18 presented a basic insight into SS-N transformation and identified amine-N, heterocyclic-N, and nitrile-N compounds as three important intermediates. In addition, HCN yields are nearly half those of NH3 during the microwave pyrolysis of SS. On the basis of the aforementioned studies, knowledge regarding fuel-N distribution during SS pyrolysis remains inadequate. Moreover, the relationship between sludge source and SS-N conversion during pyrolysis has rarely been studied. The present study investigates the conversions of SS-N into primary nitrogen products(i.e., char-N, NH3-N, HCN-N, and HCN/NH3 ratios) during pyrolysis in three different SS treatment processes at temperatures of 400 °C–800 °C. This study not only determines the effects of temperature on SS-N conversion into char-N and gas-N (i.e., HCN and NH3) but also the influence of sewage treatment processes on the SS-N transformations at elevated temperatures. Understanding SS-N conversion during pyrolysis is important in minimizing the emission of nitrogen oxides (N-X). This understanding may also enhance pyrolysis process control and maximize the clean utilization of sludge as an energy source. These outcomes can contribute significantly to sustainable development.
2. Experimental 2.1. SS sample. Dewatered sludge was sampled from three municipal wastewater plants in
Shenyang, China. The source and waste water treatment process of the sludge samples are provided in Table 1 (i.e., raw sludge, Beibu sludge, Mantanghe sludge, and Xiannvhe sludge are denoted as RS, BS, MS and XS, respectively). The samples were pulverized to facilitate their passage through a 100-mesh sieve (< 150 µm), desiccated at 105 °C for 24 h, and then stored in an airtight container
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before use. An ultimate analysis was conducted in an elemental analyzer (German Vario ELII). The results of proximate and ultimate analyses on the samples are listed in Table 2. 2.2. Pyrolysis. Pyrolysis experiments were performed by placing 5 g (± 0.5 mg) of dried sludge into a fixed-bed quartz reactor (60 mm outer diameter, 600 mm length) under 20 ml/min N2 flow at a heating rate of 20 °C/min. Upon reaching the required temperatures, the injected samples were held for 20 min until no significant gas release was observed. Carrier gas was then injected into the system for 20 min to purge residual gas. In each experiment, the volatile substances generated through sludge pyrolysis passed through several dichloromethane-containing condensers that were placed in ice baths. Bio-gas was transferred from the tar trap to the bubbling solutions. The HCN and NH3 in the pyrolysis gas were collected by bubbling in NaOH (0.2mol/L) and H2SO4 (0.1 mol/L) solutions, respectively. The residual chars in the reactors were collected and stored in airtight containers until cooled to room temperature. Char weight was determined by measuring the reactor and sludge/char weights before and after an experiment. The char yields are shown in Table 3. 2.3. Product analysis methods. A Thermo ESCALAB250 spectrometer equipped with Al Kα radiation was used to determine the SS-N functional forms in the raw sludge and in the chars. The analysis was conducted five times at different surface positions to obtain spectra and representative data of sufficient quality. To compensate for sample charging, the sludge samples were referenced to the C1s peak at 284.6 eV, and the peak areas reflected the relative contents of different components for semi-quantitative analysis. Five types of SS-N, namely, N-6 (398.8 ± 0.4 eV), protein-N (P-N) (399.7 ± 0.4 eV), N-5 (400.2 ± 0.3 eV), N-Q (401.4 ± 0.3 eV), and N-X (402.9 ± 0.5 eV) were quantified.1, 7, 15, 16
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The HCN and NH3 absorbed in the solutions were quantified through spectrophotometric method (HJ484-2009 and HJ 535-2009). All of the experiments were conducted in triplicate and averaged to obtain the final results.
3. Results and discussion 3.1.Sludge char-N 3.1.1. Effect of temperature on the char-N of BS. To determine the effects of temperature on nitrogen transformation in pyrolysis solids, the nitrogen forms in BS were first determined through XPS. In each case, the individual peaks and the total simulated spectra are shown along with the actual spectra. Figure 1 clearly indicates that the N1s spectra of BS-Raw exhibit one peak, whereas the N1s spectra of its chars mainly display double peaks and three peaks, which can be explained by the fact that SS-N forms changes with increasing temperature.
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Figure 1. Nls peaks for the raw BS and its chars (a) raw BS; (b)500°C; (c)600°C; (d)700°C; (e)800°C
Figure 2 shows the normalized relative intensities (% normalized peak intensities) of the XPS N1s spectra of BS at different temperatures. Figure 3 summarizes the loss percentages of P-N, N-6, N-5, and N-Q, and loss percentages could be calculated from equation1.
R−N The loss percentage(%) = 100 × 1 − SS − N
(1)
R-N is the residue P-N, N-6, N-5, and N-Q content in char at different temperatures, respectively; SS-N is the P-N, N-6, N-5, and N-Q content in raw SS accordingly. Figure 1, Figure 2 and Figure 3 illustrate as follows: (1)four peaks, namely, P-N, N-6, N-Q, and N-5(Figure 1a), were detected in the N1s spectra of raw BS. P-N accounted for approximately 50.45% of the total area and was the major nitrogen functionality in the SS. The percentages of the
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other functionalities were 32.61%,10.67%, and 6.27%. This finding may be explained by the fact that SS contains a significant amount of organic matter that are attributed to (dead) bacteria.15, 17–19 The P-N content in chars decreases gradually with an increase in pyrolysis temperatures. The content is 14.08% at 500 °C and only 0.8% at 800 °C, which suggests that most of the protein may have decomposed before reaching 500 °C.17 (2) Unlike in raw BS, the appearance of N-X varies significantly in chars. An increase in temperature increases bound N-X in the SS-N fraction; the content at 800 °C is almost 5.7 times that at 500 °C. During the reaction, the location of the residual nitrogen in char gradually moves to the edge of the heterocyclic structure (N-6 and/or N-5). This movement enhances N-X production. In addition, Li et al.5 determined that the presence of N-X is mainly attributed to the combination of oxygenic functional group and N-6 on char surfaces. (3) N-6 content decreases remarkably in char when temperature ranges from 500 °C to 700 °C; the decreasing trend turns slow when temperature is in the range of 700 °C–800 °C (Figure 3). Both N-Q and N-X contents increase simultaneously; this occurrence implies that N-6 may be converted into N-Q and/or N-X. Moreover, N-Q can be readily decomposed in the low-temperature range (e.g.,before 600 °C). At 500 °C, the N-5 content in char was higher than that in raw BS. This content then decreases at temperatures of 600 °C–800 °C. The N-5 increase in char at 500 °C may be attributed mainly to the conversion of N-Q and/or P-N. The downtrend of N-6 content in char was slower than that of N-5 at temperatures of 500 °C–700 °C (Figure3). As a result, N-6 may be more stable than N-5 in char. The former remains in the char, which is consistent with the findings obtained with coal.9
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Figure 2.The normalized relative intensities of XPS N1s peaks of BS at different temperatures
Figure 3.Percentages of P-N, N-6, N-5, and N-Q loss in BS at different temperatures
3.1.2. Effect of the sewage treatment process on the char-N of sludge samples. The nitrogen functionality fractions of the sludge samples are depicted in Figure 4. In particular, Figure 4(a) illustrates that P-N and N-6 accounted for approximately 80% of the total nitrogen in the three raw sludge samples, whereas N-5 and N-6 were the dominant nitrogen functionalities in coal.1, 4, 5
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Figure 4. Fractions of nitrogen functionalities in the raw sludge and its chars (a)raw sludge; (b)600°C; (c)800°C
Comparing of Figures 4(a) and 4(b) obtains the following observations: (1)At 600 °C, the catabolic rate of P-N is faster than that of N-6; thus, N-6 content is higher than that of P-N in char. N-6 then becomes the dominant nitrogen functionality in char. (2)The rate of N-6 removal from char increases in thefollowing order: MS< BS < XS. Furthermore, the N-6 removal rate is faster in XS than in BS and MS. A probable reason for these observations is the fact that XS (i.e., humus sludge) originates from the fall off of biological filter biofilms and that both BS (A/O) and MS are activated sludge processes. The distinction between humus sludge and activated sludge lies in the fact that biofilm is a layered structure whose activity improves when biofilm thickness is approximately 100–300 µm. Biofilm consists of an anaerobic layer that is composed of anaerobic and facultative
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bacteria, as well as an aerobic layer composed of aerobic and facultative bacteria. Generation-time long rotifers, nematode vorticella, and other protozoa are also present in this biofilm. Activated sludge does not contain anaerobic microorganisms, and microorganism generation time is shortened. The bacteria in BS are in the endogenous respiration stage, whereas those in MS are in the slow growth stage. Thus, the anaerobic biological process significantly influences the N-6 volatiles in the sludge. The anaerobic process facilitates N-6 removal from char, and the sludge in the endogenous respiration stage is conducive to the production of such volatiles. (3)At 600 °C, the percent conversion of P-N in MS is lower than that in BS and XS. The difference between BS and XS in terms of the residual P-N content in chars is indistinctive. The probable reason for this scenario is similar to that for N-6. In addition, the sludge in the slow growth stage (MS) is not conducive to protein catabolism. (4)The rate of N-5 retention in chars is considerably lower than those of P-N and of N-6, and the potential reason is mentioned in Section 3.1.1. (5) The vestigial N-X content is also minimal in MS char; this occurrence is related to the low percent conversion of P-N and/or N-6. Figure4(c) shows that a small part of SS-N (< 10% of SS-N) was detected in the solid residue, which may suggest that the volatile release from char is almost complete at 800 °C. All five nitrogen species were retained in the chars; nonetheless, the retention rates of N-5 and N-6 are slightly higher than those of the others (i.e., N-Q, P-N, and N-X). Meanwhile, the residual contents of N-5, N-6, N-Q, and P-N in MS char are higher than those in BS and XS chars. A probable reason for this finding is the aforementioned consideration, which is related to the sewage treatment processes. 3.2 Release of N-containing gases Figure 5 indicates that temperature remarkably influences on HCN-N and NH3-N yields (% of
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SS-N) during pyrolysis. The accumulated amounts of NH3 and HCN released are roughly ordered as follows under the same temperature conditions (from 500 °C to 800 °C): MS > BS > XS. This order differs from that of char-N (i.e., P-N and/or N-6). The aerobic process facilitates the release of N-containing gases (i.e., NH3 and HCN) from sludge; the sludge in the slow growth stage is particularly conducive to the release of N-containing gases. Furthermore, findings regarding the release tendency of HCN-N and NH3-N in sludge indicate a correlation with the nitrogen content in sludge. The higher the nitrogen content in raw sludge samples is (Table 2), the greater the release of HCN-N and NH3-N.
Figure 5. Effect of temperature on the release of N-containing gases (a) NH3; (b)HCN
3.2.1. NH3-N release. As shown in Figure 5(a), the trends of NH3-N yields are similar in the three sludge samples and involve three distinctive stages that are independent of the sewage treatment processes. Conversion to NH3-N is low in the temperature range of 400 °C–500 °C, thereby suggesting that released SS volatiles are mainly present in tar. The NH3-N observed at this stage mainly originates from the primary decomposition of P-N in sludge. Partial SS volatiles are released and yield 11.98% (MS), 10.85% (BS), and 8.97% (XS) NH3 at temperatures of 500 °C–700 °C. As
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mentioned in Section 3.1.1, char-N decreases considerably at temperatures above 500 °C. Thus, the significant increase in NH3-N at temperatures of 500 °C–700 °C is mainly attributed to the thermal decomposition of ammonium-N from char and tar during pyrolysis. This inference is consistent with the findings of previous studies on SS.15–18 In addition, further increases in NH3-N yield at temperatures above 500 °C (Figure 5a) are attributed to the reactions that involve H radicals in pyrolysis solids. These reactions are similar to the reactions responsible for the formation of the majority of NH3-N during coal pyrolysis.20-21 Notably, NH3-N yields remain stable at temperatures above 700 °C. The reasons for the NH3-N yield trend at temperatures between 700 °C and 800 °C remain unclear; in fact, small quantities of N-5 and P-N are retained at 700 °C (Figure 2). Small amounts of additional NH3-N (i.e., less than 1% of the total SS-N presented in Figure 5a) can be generated from the stable char-N when temperatures range from 700 °C to 800 °C. Char-N has a more stable chemical structure and property than SS-N, which is the primary reason why NH3-N yields remain constant at temperatures above 700 °C. In addition,the observed change in NH3-N yields may also be attributed to the catalytic effect of minerals in sludge, which converts NH3 to N2 at high temperatures.12, 17–19 3.2.2. HCN-N release. As depicted in Figure 5(b), the HCN-N formation involves two distinctive stages and a pattern that differs from that of NH3-N. An increase in temperature from 400 °C to 600 °C increases bound HCN-N yields from approximately 2% to 16.06% (MS), 15.14% (BS), and 14.14% (XS), respectively. The total HCN-N yield at 600 °C is approximately equal to that of NH3-N at 700 °C (Figure 5a); the HCN-N formation rate is higher than that of NH3-N. The thermal cracking of N-6 in chars and tars is ascribed to the HCN-N formation in this temperature range,
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which may be expressed as equation 2. A clear decrease in N-6 content in char in keeping with the HCN-N yield increase in the pyrolysis process was obtained at 500-600 °C(Figure 2). Furthermore, HCN-N is also generated from the cracking of cyclic amides that serve as primary pyrolysis products.22–23 The secondary cracking of tar-N compounds into gaseous products contribute to HCN-N formation at temperatures ranging from 500 °C to 600 °C.17–19 HCN-N yield was almost constant at temperatures above 600 °C. The increases in HCN-N yields from 600 °C to 800 °C less than 1% of the total SS-N in the three sludge samples, which may indicate that HCN-N formation rates turn slow and the residual nitrogen in chars facilitates NH3 formation. Chen et al.3 and Yu et al.24 suggested that HCN-N on the char surfaces can be hydrogenated to form NH3-N. In the current study, the slow heating rate and enhanced condensation stabilize heterocyclic-N; as a result, higher temperature that induces reaction rises.25–26
3.2.3. HCN/NH3ratio. Figure 6 illustrates that HCN/NH3 ratio is sensitive to temperature; this ratio initially increases and then decreases with increasing temperature. As shown in Figure 6: (1) HCN/NH3 ratio was low and less than 1 at temperatures below approximately 440 °C. As stated previously, the thermal decomposition of unstable P-N in the sludge accounted for the part of the
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NH3 formation over this temperature range. Furthermore, the HCN yield from N-6 was low. (2) HCN/NH3 ratio increased significantly when temperature increases from 440 °C to 600 °C. As stated previously, nitrogen mainly exists as N-6 in chars when the temperature ranges from 500 °C to 600 °C; this occurrence is the main contributor to HCN-N formation. Moreover, SS-N that can be converted into NH3-N at low temperatures may also be converted into HCN-N at high temperatures. These phenomena are the potential reasons why this stage reports a high HCN/NH3 ratio. (3)HCN/NH3 ratio decreased from 600 °C to 700 °C, and then was almost constant in the range of 700 °C–800 °C. N-6 content dropped as a result of the early reaction process, which reduces the corresponding amount of HCN-N. Furthermore, N-6 in char is readily converted into N-Q26, whereas N-Q contributes to NH3-N formation at high temperatures, and may be expressed as equation 3.21 HCN/NH3 ratio is almost constantin the range of 700 °C–800 °C. In particular, those of BS and MS are close to 1. NH3-N and HCN-N formations may be initiated through competition/selectivity. (4)The HCN/NH3 ratio of XS is maximized under a constant temperature in the range of 600 °C–800 °C; this outcome may be related to the biofilm process described in Section 3.1.2.
Figure 6. Effect of temperature on HCN and NH3 ratios
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3.3 Nitrogen distributions in pyrolysis products
Figure 7.Distribution of BS products at 500 °C, 600 °C, 700 °C, and 800 °C; distribution of MS and XS products at 600 °C and 800 °C. c: char-N; g: gas-N; o: other-N
The distribution of products generated during SS pyrolysis is displayed in Figure 7. Aside from the char-N compounds, HCN-N, and NH3-N, the corresponding other-Ns are labeled as tar-N and other N-containing gases. These other N-containing gases include N2, NOX, and HNCO.3, 11 Figure 7 shows that gas-N yield (i.e., NH3 and HCN) increases and constitutes 10%–40% of SS-N with increasing temperature (500 °C–800 °C). Consequently, this pyrolysis can reduce HCN-N and NH3-N emissions by controlling the production of intermediate compounds at the given temperature range. An increase in temperature lowers the bound nitrogen concentration in the char fraction; this concentration is dependent on the pyrolysis process of minimizing the rates of volatile removal and nitrogen removal. Figures 7 and Table 3 indicate that nitrogen removal rate is higher than the volatile consumption rate. The gas-N and char-N contents of XS are minimal when temperature ranges between 600 °C and 800 °C. The volatile nitrogen in XS may be converted into tar-N, which is an occurrence that may be related to low nitrogen content, to high ash content (Table 2), and to sewage treatment process (XS is humus sludge).
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4. Conclusions The nitrogen present in the form of protein-N(P-N), pyridine-N(N-6), pyrrole-N(N-5) and quaternary-N(N-Q) in raw sludge samples, the relative contents of P-N and N-6 are larger, both account for 80% of total nitrogen. Overall, the percent conversion from char-N to gas-N increases gradually when temperature increases from 500 °C to 800 °C; NH3 was in the present investigation the main product from P-N conversion during char pyrolysis, a small amount of P-N is converted into N-5; the majority of N-6 tends to be transformed into HCN at 400 °C–600 °C. 400-800
NH3
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N-5
N-Q
HCN
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40 080 0
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N-X
N2
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Main route
Figure 8. Proposed nitrogen transformation routes during SS pyrolysis
On the basis of the experimental results, various forms of nitrogen transformation routes are summarized and showed in Figure 8:(1)At 400-440 °C, the catabolic rate of P-N is faster than that of N-6, the HCN yield from N-6 is low, and HCN/NH3 ratio
