Sulfur Release and Migration Characteristics of Sewage Sludge

Apr 7, 2017 - The effect of three organic calcium compound (OCC) addition on the sulfur release characteristics of sewage sludge (SS) combustion was s...
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Sulfur Release and Migration Characteristics of Sewage Sludge Combustion under the Effect of Organic Calcium Compound Addition Feng Duan,*,†,‡ Lihui Zhang,†,§ and Yaji Huang† †

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing, Jiangsu 210096, People’s Republic of China ‡ School of Energy and Environment and §School of Civil Engineering and Architecture, Anhui University of Technology, Maanshan, Anhui 243002, People’s Republic of China ABSTRACT: The effect of three organic calcium compound (OCC) addition on the sulfur release characteristics of sewage sludge (SS) combustion was studied using the KZDL-4A fast sulfur determination device. Results show that calcium magnesium acetate (CMA) has the highest desulfurization rate. Desulfurization rates of three OCCs significantly increase at a lower Ca/S molar ratio (R) stage while increase little as the R value reaches 4. Peak desulfurization rates both appear at 900 °C when the R value is 1 and 2. However, with increases of the temperature, no singular point is observed when the R value is 3. Meanwhile, the migration mechanisms of microscopic sulfur form during the combustion of SS, and its blends with CMA at different R values were investigate by an X-ray photoelectron spectroscopy (XPS) analyzer. Results showed that CMA impregnation has little effect on the SS sulfur functionalities. After combustion, mercaptan (S1), sulfide (S2), and thiophene (S3) decrease significantly, while sulfoxide (S4) and sulfate (S6) show inverse trends. Sulfone (S5) has the highest value. With an increasing R value, sulfoxide significantly decreases first and then increases little, while sulfate shows an inverse trend. The residual of SS/CMA-3 combustion has the highest value of sulfate. The first four kinds of sulfur are easier to release during the combustion, and these kinds of sulfur in SS are much easier to be captured using CMA.

1. INTRODUCTION In China, the annual production of sewage sludge (SS) was approximately 30 million tons in 2015 and the quantity increased by over 13% annually from 2007 to 2015.1,2 Because dried SS has a high content of organic matter, it may be a promising energy source. Thus, its combustion can both contribute to its volume reduction and partially substitute fossil fuels. Higher volatile matter (VM) and smaller fixed carbon (FC) yields contribute to the relatively lower calorific value of dried SS.3 Therefore, to obtain a near heat quantity in the same combustor/incinerator, a 2 or more times feeding rate of dried SS is needed in comparison to that using coal as the fuel. Although dried SS has relatively lower sulfur and nitrogen contents, the pollutant emissions from incineration are still a problem of the safe disposal of SS. Li et al. studied the pollutant release characteristics from a SS incinerator.4 Results indicated that the highest observed SO2 emission was 167.191 mg/m3 when the incineration temperature was 850 °C and oxygen was used as the agent. Inorganic calcium-based compounds (ICCs) are typically used in the stabilization and flocculation stages of wastewater treatment. Results showed that the ICC present in SS ash plays a self-desulfurizing role when ICC is added during the flocculation stage.5 especially, the nitrogen contents of some SS are much higher and can be reaching to 6 wt %.6,7 The NOx emission from the SS combustion also cannot be neglected. Recently, a study on the simultaneous decrease of SO2 and NOx emissions has attracted much attention as a result of its economics and applicability for an existing power plant. An © XXXX American Chemical Society

organic calcium compound (OCC) has the ability to simultaneously remove SO2 and NOx.8−10 Now studies of OCC addition are mainly focused on the characteristics of coal combustion and its pollutant reduction.11,12 Carboxylic and hydrocarbonyl radicals from OCC calcination can be relevant to NOx reduction under reducing conditions.11 However, SS has a distinct fuel characteristic compared to coal, and its inorganic part could have a detrimental effect on the reduction of NOx.13 Calcium oxide as the main residual produced from OCC calcination is effective in capturing SO2. Because SS still contains the P content, calcium oxide may react with S or P to form CaSO4 or Ca3(PO4)2. ICC and OCC could also interact with phosphor-based compounds in competition with sulfurbased compounds. OCC can also be used in the wastewater treatment to have a 2-fold benefit in the process as both a desulfurization agent and flocculant or stabilizing agent. However, this technique needs further improvements to reduce the cost of OCC at present. Additional studies evaluating the OCC addition for the pollutant reduction of SS combustion still need to be performed. Particularly, the carboxylic and hydrocarbonyl radicals released from OCC have combustible ability. This significantly affects the thermal decomposition characteristics of sludge blended with OCC14 and the pore characteristics of the residual.15 Therefore, the sulfur release and migration characteristics during the combustion of SS blended with OCCs are different from those of individual SS Received: November 8, 2016 Revised: March 14, 2017 Published: April 7, 2017 A

DOI: 10.1021/acs.energyfuels.6b02947 Energy Fuels XXXX, XXX, XXX−XXX

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Energy & Fuels Table 1. Calorific Value and Results of Proximate and Ultimate Analyses of Dried SS ultimate analysis (wt %) SS

proximate analysis (wt %)

C

H

O

N

S

V

FC

A

M

calorific value (MJ/kg)

26.85

4.08

11.34

4.15

0.89

43.17

4.23

46.35

6.25

13.09

CA, and SS/CP represent the SS blended with CMA, CA, and CP, respectively. The number in the names of blended fuels represents the R value. 2.2. Experimental Setup and Test Processing. The KZDL-4A fast sulfur determination device (made by HBTY Co., China) was used to measure the sulfur release from combustion of SS and blended fuels. Figure 1 shows the experimental system. This experimental system

combustion, and still not enough studies have been performed on this topic. In particular, the role of sulfur forms in SS during its combustion has recently become the subject of intense investigations. X-ray photoelectron spectroscopy (XPS) is one of the non-destructive techniques that can aid in identifying elements (except H) on a solid sample surface.16−19 This method has been applied to identify the functionalities of the sulfur compounds in coal,16,18,19 SS,16 and biomass.19 Folgueras et al.20 investigated the sulfur retention in ashes from combustion of three bituminous coals and a SS. Li et al.,16 Harrison et al.,21 and Merino et al.22 found that SS has sulfurcontaining functionality content similar to that of coal. Li et al. also investigated the characteristics of sulfur in individual SS, coal, and their mixture during combustion,16 and they found that these functionalities in SS showed the different trends after combustion compared to coal. However, the conversion and migration mechanisms of microscopic sulfur forms during combustion of SS blended with OCC were seldom present. Thus far, the first aim in this study is to explore the sulfur release characteristics during SS combustion under the effect of OCC addition. In this test, the effects of OCC type, Ca/S molar ratio (R) value, and reaction temperature were investigated by the KZDL-4A fast sulfur determination device. The second aim is to discuss the conversion and migration mechanisms of microscopic sulfur forms in SS and its blended fuels with OCC. In this test, the effects of the impregnation, combustion, and R value on the transformation mechanism of sulfur-containing functionalities in samples were studied by the XPS analyzer.

Figure 1. Sulfur determination analyzer system: (1) control and analytical unit, (2) fuel feeding pole, (3) quartz boat, (4) thermocouple, (5) high-temperature furnace, (6) electrolytic tank, (7) stirrer, (8) flow meter, (9) extraction pump, and (10) purification unit. consists of a high-temperature furnace, fuel feeding pole, electrolytic tank, control and analytical unit, and purification unit. The coulometric titration method was applied for KZDL-4A to accurately quantify sulfur in solution. When SO2 generated from SS combustion was pumped into the electrolytic tank, its reaction with water will destroy the potential balance between iodine and potash iodide, resulting in a dramatic shift of potential generation in the working electrode. Therefore, the moles of sulfur in solution can be calculate by the current magnitude and current duration. The detail reactions are as follows:

2. EXPERIMENTAL SECTION 2.1. Material. Raw SS samples from the conventional municipal treatment plant in Nanjing was used, and its moisture yield is higher than 90%. First, raw SS was dried in the sun for 5 days to remove most moisture, and dried SS was prepared by heating samples at 105 °C in an electric oven for 1.5 h. The dried SS was stored in a polyethylene bag and was sealed to avoid direct contact with environmental air. Table 1 gives the calorific value and results of proximate and ultimate analyses of dried SS. As seen in this table, the dried SS still has the moisture yield of 6.25% because the surface-adsorbed water in SS is hard to remove using normal drying methods.23 Calcium acetate (CA), calcium propionate (CP), and calcium magnesium acetate (CMA) were selected as the OCCs in this test. CA, CP, and CMA were chemically synthesized using the methods previously reported.14,24 Prior to characterization, each OCC sample and dried SS was finely ground and sieved to a powder of size less than 0.15 mm. The Ca/S mole ratio used in this experiment ranges from 1 to 4. In this study, the quantity of the OCC additive is determined according to the sulfur content in SS and R value. For the case of a nearly uniform distribution of OCC in solid fuels, OCCs and SS are dispensed into into bottles and well-dispersed in water. The suspension is stirred for 8 h at room temperature with a magnetic stir bar. Afterward, blended fuels are also prepared by heating samples at 105 °C in an electric oven for 24 h. Similar to the blended fuels, the same procedure completed with the individual fuels alone to ensure the mixing procedure did not affect the results achieved. New names of blended fuels, such as SS, SS/CMA-1, SS/CA-2, SS/CP-3, etc., are proposed to identify the different samples. Specifically, SS/CMA, SS/

anode reaction

2I− − 2e → I 2 cathode reaction

2H++2e → H 2 oxidized reaction

I 2 + H 2SO3 + H 2O → 2I− + H 2SO4 + 2H+ In this test, silicon carbide (SiC) was used as the heating element of the feedback-controlled high-temperature furnace, whose temperature can range from room temperature to 1500 °C. A total of 5 min was needed to adjust the air flow to reach steady state before beginning experiments. The fixed air flow rate of 1 L/min was controlled by the flowmeter, which was placed together with the extraction pump. Then, a high-temperature furnace was preheated to the setting reaction temperature. After that, the fuel of 50 mg was placed in a quartz boat and was pushed into the center of the quartz tube in the furnace to start the combustion. The flue gas was pumped by an extraction pump into the electrolytic tank and reacts with water to form sulfurous acid. The amount of sulfur in solution was calculated according to Faraday’s law of electrolysis. Besides, the X-ray fluorescence (XRF) spectrograph (Thermo Fisher Scientific, Waltham, MA, U.S.A.) was used to detect the ash composition from sample combustion. XPS analysis was performed by the ESCALAB250 spectrometer (Thermo Fisher Scientific, Waltham, MA, U.S.A.) equipped with a Mg B

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Energy & Fuels X-ray source (1253.6 eV). The spectra data were analyzed by the software of XPSPEAK to calculate the relative content of sulfurcontaining functionalities. Table 2 gives the working conditions in this test.

Table 2. Working Conditions parameter

unit

value

Sulfur Release Characteristic Test R value 1, 2, 3, and 4 OCC CA, CP, and CMA temperature °C 800, 850, 900, 950, and 1000 air flow rate L/min 1 total mass of sample mg 50 Sulfur-Containing Functionality Transformation Test sample SS and SS/CMA R value 1, 2, 3, and 4 temperature °C 900

2.3. Data Processing. The sulfur release fraction can be calculated according to eq 1

ψ=

SSO2 mST

× 100%

(1)

where ψ is the sulfur release fraction of the samples (%) and SSO2 represents the total SO2 release from combustion, which can be obtained by the KZDL-4A fast sulfur determination device (mg), In this test, the total sampling time is 5 min, and the time interval is 0.6 s. ST is the sulfur content in SS from proximate analysis (wt %), and m is the total weight of the SS in the blended fuels (mg). The desulfurization rate was calculated according to eq 2

η=

Se − Ss × 100% Se

(2)

where η is the desulfurization rate of the samples using OCCs (%) and Se and Ss represent the sulfur release from the combustion of individual SS and its blended fuels with OCCs, respectively (mg).

3. RESULTS AND DISCUSSION 3.1. Sulfur Release Characteristics. 3.1.1. Sulfur Release Characteristics Using Different OCCs. Figure 2 shows the sulfur release characteristics at different operating conditions. Figure 2A shows the sulfur release characteristics from combustion of individual SS, SS/CMA-2, SS/CA-2, and SS/ CP-2, respectively. In this figure, the reaction temperature is set to 900 °C and the R values of three blended fuels are all fixed at 2. It can be seen in this figure that the beginning and ending times of sulfur release in four curves all appear at about 20 and 160 s, respectively. The final sulfur release fraction of individual SS combustion is 85.84%, indicating that sulfur in SS cannot decompose completely under this temperature. The final sulfur release fractions of SS/CMA-2, SS/CA-2, and SS/CP-2 are 23.82, 24.72, and 32.58%, respectively, which are much lower than that of individual SS. SO2 is the main pollutant emission during sludge combustion. The decreased final sulfur release fraction after SS blended with OCCs suggests that SO2 emission can be captured by the solid residual from OCC calcination. The desulfurization rate of SS blended with different OCCs can be calculated according to eq 2, and these values of SS/ CMA-2, SS/CA-2, and SS/CP-2 are 73.25, 71.20, and 62.04%, respectively. Results show that CMA has the highest SO2 capture ability. The reasons are as follows: First, the inner grains of the solid residual after OCC calcination contain an

Figure 2. Sulfur release characteristics at different operating conditions: (A) sulfur release curve under different organic calcium salts, (B) effect of the R value on the desulfurization rate, and (C) effect of the temperature on sulfur removal efficiency of CMA under different R values.

amount of unreacted CaO. The SO2 emission diffuses from the flue gas to the particle surface, permeates into the inner part of the adsorbent, and reacts with inner CaO on the pore surface. Therefore, the main factor that significantly affects SO2 adsorption is the specific surface area (SBET) of the residual after OCC calcination. For CMA, the total SBET of its residual calcined at a temperature of 900 °C is 46.6 m2/g. Its value is the highest among these three OCCs.15 Secondarily, in comparison to other OCCs, CMA has more abundant pore structures after calcination, which can be attributed to the fact that MgO in CMA can be used as an inert metal skeleton to support the CMA structure. Third, the decomposition of lignin and humic acid in SS during the combustion also increases the specific surface area of the fixed carbon and ash, which is helpful to adsorb SO2.1,25 However, the interaction on the SS combustion and OCC calcination will change the pore characteristics of blended fuels. The OCC type has different effects on the pore C

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Energy & Fuels Table 3. Composition Analysis of Ash after Solid Sample Combustion at Different R Values (%)

a

sample

SiO2

P2O5

Fe2O3

Al2O3

CaO

MgO

K2O

TiO

Na2O

MnO

SO3

NDa

SS SS/CMA-1 SS/CMA-2 SS/CMA-3 SS/CMA-4

39.05 38.00 34.98 32.81 30.84

15.41 15.86 17.01 16.23 15.56

15.11 13.34 12.88 13.78 14.14

13.01 12.62 12.31 11.88 11.21

8.03 10.97 13.75 16.26 19.06

3.48 3.59 3.93 3.96 4.12

2.73 2.37 2.17 1.93 1.89

1.09 1.05 1.00 0.96 0.94

1.03 1.12 0.81 0.82 0.82

0.33 0.30 0.30 0.32 0.32

0.17 0.25 0.37 0.51 0.53

0.56 0.53 0.49 0.54 0.57

ND = not detected.

In Figure 2B, the desulfurization rates under the addition of CA and CP show trends similar to that under CMA addition. The desulfurization rate of SS/CP-1 is only 34.55%, which is much lower than those of SS/CMA-1 and SS/CA-1. In our previous study,14 for blended fuels with the same addition ratio, the order of the ignition temperature is SS/CP > SS/CMA > SS. Therefore, we assumed that the pores of SS/CP-1 are underdeveloped, resulting in a lower specific surface area and desulfurization rate. However, the starting decomposition temperature of CP is lower than that of other OCCs,29 and the ignition temperature of SS/CP decreases at higher CP addition. Results showed that the ignition temperature difference between SS/CMA-2 and SS/CP-2 is only 7.2 °C.14 Released VM during the decomposition of blended fuels results in the enrichment structure of the pore and an increasing SBET in the residual,30 which improves its SO2 capture efficiency. Therefore, the desulfurization rate of SS/CP-2 significantly increases to 62.04%, and that of SS/CP-3 increases to 72.82%. At this condition, the difference of the desulfurization rate between SS/CMA and SS/CP decreases with increases of the R value. 3.1.3. Sulfur Release Characteristics at Different Reaction Temperatures. Figure 2C shows the effect of the reaction temperature on the desulfurization rate under CMA addition. In this figure, the reaction temperature ranges from 800 to 1000 °C. Meanwhile, the final sulfur release fractions of individual SS combustion at different temperatures are also present in this figure. As seen in this figure, the final sulfur release fraction of individual SS combustion almost increases linearly with increases of the reaction temperature. SS/CMA with a higher R value has a higher desulfurization rate. As known to all, the best temperature range for capturing sulfur using the conventional calcium compounds is from 850 to 900 °C. However, the lower and higher temperature zones both become broadened when CMA was blended with SS in this figure. At a lower reaction temperature of 800 °C, the final sulfur release fraction of individual SS combustion is 78.2% and the desulfurization rates of SS/CMA-1, SS/CMA-2, and SS/ CMA-3 are 41.38, 56.05, and 75.86%, respectively. Also, for individual SS combustion under a reaction temperature of 1000 °C, the final sulfur release fraction is 96.85%, while the desulfurization rates of SS/CMA-1, SS/CMA-2, and SS/CMA3 are 40.60, 54.98, and 85.15%, respectively. For all conditions, CMA shows a strong depression on SO2 emission, which can be attributed to a porous structure formation of the solid residual, resulting from the release of carboxylic and hydrocarbonyl radicals. However, the three blended fuels show different desulfurization characteristics with increases of the reaction temperature. For SS/CMA-1 and SS/CMA-2, peak desulfurization rates at a reaction temperature of 900 °C are observed. The desulfurization rate increases first with the reaction temperature. The pore structure of the residual has not developed fully at a lower

characteristics during the blended fuel combustion. The mean SBET of the sludge blended with some OCCs becomes smaller because of the relatively small value of the residual after OCC calcination. However, the number of pores in blended fuels and the mean specific surface area still increase when sludge blended with CMA during the combustion.15 This significantly improves the adsorption capacity of the residual from blended fuel combustion. 3.1.2. Sulfur Release Characteristics at Different R Values. Figure 2B illustrates the effect of the R value on the desulfurization rate of blended fuels. In this figure, three OCCs are used to blend with sludge and the reaction temperature is set to 900 °C. It can be seen from this figure that CMA shows the highest desulfurization rate among the three OCCs for all conditions. Meanwhile, the desulfurization rate of CMA increases sharply from 60.47 to 82.46% when the R value increases from 1 to 3. However, its value slightly increases to 82.72% as the R value reaches 4, suggesting that an excessive R value has little effect on the increasing desulfurization rate. The composition analysis of ashes from sample combustion is given in Table 3. From this table, we can observe that there is an amount of CaO in the original SS and SO2 may be adsorbed to a certain extent. However, the CaO content that remained in the ash is mixed with other compositions. The collapse of the pore structure may arise as a result of the melting of glassy substances in the ash during the longer combustion time at a temperature of 900 °C, which decreases the sulfur removal efficiency significantly.15 As seen in this table, the CaO and MgO concentrations increase accordingly with the value of the R value because these two elements are the main content in CMA. The VM of CMA is higher than that of other OCCs, suggesting a more complex pore structure as a result of its VM release during the combustion. This also significantly increases the sulfur removal characteristics, which can be proven by the SO3 concentration in ash in this table. The SO3 concentration significantly increases to 0.51% when the R value is 3, indicating that more sulfur remained in the residual. Besides, the SO3 concentration increases little for SS/CMA-4. Results are also consistent with those of the desulfurization rate of SS/CMA (Figure 2B). Similar to previous studies,26−28 the calcium compound caused a decrease in SO2 emission, which is likely because of the following reactions R1 and R2. CaO + SO2 → CaSO3

(R1)

2CaO + 2SO2 + O2 → 2CaSO4

(R2)

However, as seen in this table, the SS also contains many mineral substances, which can also fix sulfur. Therefore, the mechanism of CMA adding on the desulfurization process needs to be further investigated with the help of XPS in this study. D

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fitting the S 2p spectrogram. The peak area of each component represents the relative content. In this figure, the peak area of S5 is the largest, while that of S4 is at a minimum, which can be ignored. It also can be seen in Figure 3B that SS/CMA-3 has sulfur functionalities similar to those of SS, indicating that impregnation of CMA has little effect on the general framework of SS sulfur functionalities. However, some differences in component evolution compared to that of SS are observed according to the statistic data in Table 4. From this table, the binding energies of six components for SS and SS/CMA-3 have little difference and these data all fall within the binding energy range of each sulfur-containing functionality published in the literature.17−19 In comparison of the corresponding relative contents of SS and SS/CMA-3, it is found that the relative contents of sulfur functionalities decrease, except for those of S2 and S6, after impregnation. As typical for S3, its value significantly decreases from 24.62 to 17.65% after impregnation. This is because part of organic sulfur in SS begins decomposition during the process of impregnation and released sulfur is captured by the organic calcium adsorbent, resulting in increases of S2 and S6 in SS/CMA-3. 3.2.2. Effect of the Combustion on the Sulfur-Containing Functionality Transformation. Figure 4 illustrates the S 2p spectrograms of the residual from sample combustion and the peak-splitting results. The samples in panels A, B, C, D, and E of Figure 4 are SS, SS/CMA-1, SS/CMA-2, SS/CMA-3, and SS/CMA-4, respectively. In these figures, the reaction temperature is 900 °C and the reaction time is 90 s. Table 5 compares the changes in sulfur functionalities of SS and SS/CMA-3. The transformation behavior of sulfur functionalities in sludge and its blended fuels can be obtained by a comparison of Figures 3A and 4A (SS) and Figures 3B and 4D (SS/CMA-3). As seen in Figure 4A, two peaks appear at a similar position in the S 2p spectrograms of the residual from SS combustion to those in Figure 3A; however, the value of the second peak is much higher than that of the first peak. In Figure 4D, only one pronounced peak is observed in the S 2p spectrograms of the residual from SS/CMA-3 combustion. In panels A and D of Figure 4, the peaks of those sulfur functionalities with a lower binding energy decrease significantly (S2) or disappear (S1 and S3). This is because alkyl sulfide with poor thermal stability is easier to release into the environment as VM; besides, it is also easier to transform into more stable functionalities under an oxidation atmosphere.31,32 As known to all, aromatic sulfur (S3) is stable in fossil fuels, such as coal, because it has gone through the long-term serious geologic process.16 However, sulfur from sulfur-containing monocyclic aromatic compounds is the main S3 in SS because the formation time and formation conditions, such as the temperature and pressure, for SS are much lower than those of coal,33,34 and this typical S3 with poor thermal stability can also be released at the temperature of 400 °C.34 Therefore, S3 also disappeared after combustion in this test. The appearance of S4 in the residual of SS combustion is in conflict with its relatively lower decomposition temperature (about 400 °C). During the SS combustion, sulfite may be formed by the process of some sulfur reacting with alkali and alkaline earth metals on the ash surface. Typically, sulfite and sulfoxide have similar binding energies, which are both around 166.0 eV. Therefore, the formation of sulfite may be contributing to the appearance of S4 after SS combustion. S5 always has the maximum value among SS, SS/CMA-3, and their residuals after combustion (Tables 4 and 5). The

temperature, and this will compromise the reagent reactivity. As the reaction temperature exceeds 900 °C, a higher reaction temperature improves the reaction rate; however, the pore structure of the residual may collapse as a result of the melting of a glassy substance in ash. Therefore, the reaction temperature of 900 °C is the critical temperature for CMA decomposition and its porous structure formation. For SS/CMA-3, the desulfurization rate is much higher than that of other conditions. Besides, its value increases steadily from 75.86 to 85.15% as the reaction temperature increases from 800 to 1000 °C, and no singular point is observed in this curve. This can be attributed to the improved pore characteristics of CMA at a higher R value. As discussed in the published literature,15 mesopores are dominant in the CMA residual. Released VM enriches the mesopore amount and increases the SBET of the residual. 3.2. Sulfur-Containing Functionality Transformation. 3.2.1. Effect of the Impregnation. Panels A and B of Figure 3

Figure 3. S 2p XPS spectra of SS and its blended fuel: (A) SS and (B) SS/CMA-3.

show the S 2p spectrograms of SS and SS/CMA-3 and the peak-splitting results, respectively. Table 4 gives the binding energy, peak area, and concentrations of sulfur-containing functionalities derived from the fitted S 2p components for SS and SS/CMA-3. It can be seen from Figure 3A that two pronounced peaks are observed in the S 2p spectrogram of SS, indicating that the sulfur functionalities in SS are more complex. Six components, including S1−S6, are obtained by E

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Energy & Fuels Table 4. Characteristics of Sulfur Functionalities in SS and SS/CMA-3 SS

SS/CMA-3

symbol

functionality

binding energy (eV)

peak area

relative content (%)

binding energy (eV)

peak area

relative content (%)

S1 S2 S3 S4 S5 S6

mercaptan sulfide thiophene sulfoxide sulfone sulfate

162.54 163.27 164.11 166.15 168.31 169.54

301.41 1050.05 1327.02 0.10 2494.79 215.88

5.59 19.48 24.62 0.00 46.29 4.01

162.65 163.25 164.19 166.16 168.25 169.42

229.25 1065.46 817.56 0.10 1988.62 530.15

4.95 23.01 17.65 0.00 42.94 11.45

Figure 4. S 2p XPS spectra of SS and its blended fuels.

value. In Table 5, the S6 value significantly increases from 4.01 to 33.89% after SS combustion. This can be attributed to the fact that the ash from SS combustion contains many alkali and alkaline earth metals, which may promote the adsorption part of SO2 to a certain extent,1 resulting in an increasing S6 content. Besides, when S6 for SS and SS/CMA-3 is compared, it can be seen that the increase in the residual of SS/CMA-3 is greater than that of SS and its value significantly increases from 11.45 to 49.85% after SS/CMA-3 combustion. This is because the CMA addition significantly increases the desulfurization effect.

Table 5. Evolution of Sulfur Functionalities in Samples (%) sample SS (raw) SS (residual) SS/CMA-3 (raw) SS/CMA-3 (residual)

S1

S2

S3

5.59

19.48 1.01 23.01

24.62

4.95

S4 22.48

17.65 2.24

S5

S6

46.29 42.62 42.94 47.90

4.01 33.89 11.45 49.85

reasons are as follows: First, S5 as the oxide sulfur functionality binding to char and ash is not easily released during combustion. Second, part of S1−S4 releases as the VM, while the rest is transformed into S5, resulting in an increasing S5 F

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Energy & Fuels 3.2.3. Sulfur-Containing Functionality Transformation at Different R Values. More information is obtained in Figure 4; the S 2p spectrograms of the residual from sample combustion in this figure show similar trends. All spectrograms only have only one peak. According to the peak-splitting results in these figures, S5 and S6 have the obvious peaks, indicating that their relative contents are much higher than those of others. Similar to those of SS and SS/CMA-3, the peaks of those sulfur functionalities with a lower binding energy in other samples decrease significantly (S2) or disappear (S1 and S3). Figure 5 illustrates the effect of the R value on the relative ratio of sulfur-containing functionalities. In this figure, the first

desulfurization test, suggesting that the value of R ranging from 2 to 3 is the optimum range for the desulfurization process during the combustion of SS blended fuels with CMA.

4. CONCLUSION The effects of three OCC addition on the sulfur release characteristics of SS combustion and the migration mechanisms of microscopic sulfur forms during the combustion of SS and its blends with CMA were investigated in this study. The main conclusions are as follows: (1) CMA has the highest desulfurization rate. Desulfurization rates of three OCCs significantly increase at a lower R value stage, while they increase little as it reaches 4. Peak desulfurization rates at a temperature of 900 °C are observed for SS/CMA-1 and SS/ CMA-2. However, no singular point is observed for SS/CMA-3 with increases of the temperature. (2) The contents of SS sulfur functionalities change little during the CMA impregnation. However, after combustion, S1, S2, and S3 decrease significantly, while S4 and S6 show inverse trends. S5 has the highest value. With an increasing R value, S4 significantly decreases first and then increases little, while S6 shows an inverse trend with that of S4. The residual of SS/CMA-3 combustion has the highest value of S6. (3) S1−S4 are easier to release during SS and its blend combustion, and these kinds of sulfur in SS are much easier to be captured using CMA. Results obtained from the XPS analysis are consistent with those from fast sulfur determination tests.

■

AUTHOR INFORMATION

Corresponding Author

*Telephone/Fax: +86-25-8379-4744. E-mail: ddffeng@126. com.

Figure 5. Effect of the R value on the relative ratio of sulfur-containing functionalities.

ORCID

Feng Duan: 0000-0003-0970-7822

three sulfur-containing functionalities are not plotted because their value are near zero. As shown in this figure, the S4 value of SS (22.48%) is much higher than those of blended fuels. This can be attributed to competitive relationship between the mineral substance of SS and CMA. The different formation paths are dependent upon the oxygen concentration around the reaction. For SS individual combustion, before alkaline and alkaline earth elements of SS react with sulfur, oxygen may partly be consumed during the fixed carbon combustion, resulting in an oxygen-lean atmosphere. Sulfites are the main formation in this condition and have the highest value among all of the samples. However, a higher CMA addition decreases the chance of the sulfur reaction with a mineral substance on the SS surface. In the majority of cases, the formation of sulfite is an intermediate stage in the process of sulfation. Sulfates are more abundant in oxygen-enriched conditions when sulfur reacts with calcium from CMA. Therefore, the S4 value decreases, while the S6 value increases when the R value increases from 1 to 3. Also in this figure, S4 and S6 values increase little as the R value increases to 4. In Figure 2B, the desulfurization rate does not increase for SS/CMA-4. That is to say, more CMA addition has little effect on the desulfurization rate for SS/CMA-4. Under this condition, more carboxylic and hydrocarbonyl radicals released from CMA addition at a higher R value improve the fixed carbon combustion and increase the Brunauer−Emmett−Teller (BET) surface area of the sample.14 This increases the chance of the mineral material reaction with SO2 and O2, resulting in a little increasing S4 value. The results are consistent with those of the XRF analysis and

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The financial support from the Nature Science Research Project of Anhui Province (Grant 1508085ME73), Colleges and Universities in Jiangsu Province Plans to Graduate Research and Innovation (KYLX15_0070), and the Foundation for Excellent Doctoral Dissertation of Southeast University is greatly acknowledged.

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NOMENCLATURE CA = calcium acetate, molecular formula of Ca(CH3COO)2 CMA = calcium magnesium acetate, molecular formula of CaMg2(CH3CO2)6 CP = calcium propionate, molecular formula of Ca(CH3CH2COO)2 FC = fixed carbon OCC = organic calcium compound ICC = inorganic calcium-based compound R = Ca/S molar ratio SS = sewage sludge SS/CMA-n = SS blended with CMA at a R value of n (n = 1, 2, 3, etc.) S1 = mercaptan S2 = sulfide S3 = thiophene S4 = sulfoxide DOI: 10.1021/acs.energyfuels.6b02947 Energy Fuels XXXX, XXX, XXX−XXX

Article

Energy & Fuels

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S5 = sulfone S6 = sulfate XPS = X-ray photoelectron spectroscopy XRF = X-ray fluorescence

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DOI: 10.1021/acs.energyfuels.6b02947 Energy Fuels XXXX, XXX, XXX−XXX