Behavior of Phosphorus during Co-gasification of Sewage Sludge and

Apr 4, 2012 - sewage sludge, but special attention should be paid to the high phosphorus content in sewage sludge. Phosphorus volatilization during th...
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Behavior of Phosphorus during Co-gasification of Sewage Sludge and Coal Qiang Zhang, Haifeng Liu,* Weifeng Li, Jianliang Xu, and Qinfeng Liang Key Laboratory of Coal Gasification of Ministry of Education, East China University of Science and Technology, Shanghai 200237, People’s Republic of China ABSTRACT: The gasifier represents an attractive alternative to the well-established thermal treatment system for municipal sewage sludge, but special attention should be paid to the high phosphorus content in sewage sludge. Phosphorus volatilization during the co-gasification process of municipal sewage sludge and coal mixtures was investigated using a laboratory-scale highfrequency furnace. The transformation of phosphorus-containing compounds in slag at different temperatures was also studied. The results indicate that the volatilization ratio of phosphorus monotonically increases with the increasing gasification temperature and the volatilization of phosphorus mainly takes place during the pyrolysis process. The organophosphorus compound makes up the main part of volatized phosphorus when the pyrolysis temperature is not higher than 1100 °C. Inorganic phosphorus does not volatilize obviously until 1200 °C. After gasification, most phosphorus in mixtures deposits in slag in the form of phosphorus-containing glass.

1. INTRODUCTION The generation of sewage sludge has increased during recent years, and the management of this waste produced in wastewater treatment plants is turning into a serious problem. Shortage of landfill space and risk of environmental pollution led to an urgent requirement of new treatment methods. Processed in an entrain flow gasifier with coal, sewage sludge gasification represents an attractive alternative to convert this solid waste to gaseous products with low levels of SO2 and NOx emissions, benign solid wastes, and zero or low wastewater discharges.1−3 However, the high content of phosphorus in sewage sludge, usually 10−1000 times higher than coal, may cause a drawback of this technology.4,5 On one hand, special attention should be paid to the influence of phosphorus on the gasification performance of fuels. On the other hand, we are very concerned for the fate of phosphorus in the gasification process, considering that sewage sludge is an important carrier of phosphorus. Many researchers have carried out a significant number of investigations on the behavior of phosphorus under different thermal processes. Bourgel et al.6 predicted that phosphorus evaporates up to about 50% of the initial P in the form of PO2, PO, and (P2O3)2 only in the richer P sludge, using thermodynamic calculations (FactSage). Phosphorus was found to cause deactivation of selective catalytic reduction (SCR) DeNOx by Beck and co-workers,7−10 by investigating the behavior of phosphorus in the flue gas during the combustion of high-phosphate fuels. Zhang et al.11 found that both the organically bound fraction of phosphorus and its inorganic species in sewage sludge appeared to vaporize readily during combustion with coal. Matinde et al.12 investigated the gasification of phosphorus from municipal sewage sludge ash during the carbothermic reduction process and found that phosphorus was dominantly vaporized at around 800 °C in the form of PO and PO2 and in the temperature range of 1000−1427 °C as a metallic phase phosphorus gas of P2. Previous works have © 2012 American Chemical Society

shown that there are many factors that influence the vaporization and distribution of phosphorus in products. Nevertheless, few data are available regarding to the transformation and vaporization of phosphorus during the gasification process, especially for co-gasification of sludge and coal at higher temperatures. The purpose of the present work is to characterize the release and transformation of phosphorus during the coal/sewage sludge co-gasification process. The release of phosphorus from different mixtures of coal/sewage sludge is studied in a highfrequency reactor. The volatilization ratio of phosphorus from mixtures, in both pyrolysis and gasification processes at different temperatures, is tested. To interpret the observed release behavior, phosphorus evolution in slag is characterized by X-ray diffraction (XRD), Raman spectroscopy, and scanning electron microscopy (SEM) analysis. The influences of the gasification temperature and ash composition are discussed.

2. EXPERIMENTAL SECTION 2.1. Materials. Dewatered municipal sewage sludge used in the present study was obtained from the Longhua wastewater treatment plant (located in Shanghai, China), which uses the A2/O process for water treatment and aluminum salt for phosphorus precipitation. Proximate and ultimate analyses of the received sewage sludge and Shenfu coal from Inner Mongolia are shown in Table 1. The moisture content of received sewage sludge is about 81 wt %, and its ash, volatile matter, and nitrogen contents are fairly higher than coal. The phosphorus content of sewage sludge samples is almost 100 times higher than coal. Prior to use, all of the samples were dried and ground to less than 125 μm. Sludge powder was mixed with a varying amount of coal powder to make the total phosphorus content in the mixture be 0.5, 1.0, 1.5, 2.0, 2.5, and 2.85 wt % (labeled as M0.5, M1.0, M1.5, M2.0, and sewage sludge). Received: January 2, 2012 Revised: April 4, 2012 Published: April 4, 2012 2830

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Table 1. Main Properties of Materials ultimate analysis (wt %)a

proximate analysis (wt %) b

sample

Mar

coal sewage sludge

7.20 80.58

Adc

c

Vd

7.56 41.98

35.50 50.97

FCd

c

56.94 7.05

Cd

Hd

Nd

St,d

Pd (wt %)

Qgr,d (MJ kg−1)

71.72 32.02

4.67 2.93

1.51 5.18

0.71 1.28

0.029 2.85

28.36 16.73

Ultimate analysis is on a dried basis. bMar refers to moisture on a received basis. cAd, Vd, and FCd refer to ash, volatile matter, and fixed carbon on a dried basis. a

as CO, H2, CH4, and CO2 in pyrolysis gases were measured by gas chromatography of GC9790 (Fuli, China) in parallel experiments. Fuels and part of chars (slags) were digested according to United States Environmental Protection Agency (U.S. EPA) method 3052. Their phosphorus content were then determined according to the phosphomolybdenum blue spectrophotometric method. Phosphorus in original fuel was then divided into solid phosphorus (exist in slag) and gaseous phosphorus (exist in gaseous products) states after gasification. Results show that the recovery of phosphorus was 98− 101%. It proved that this method is reliable, and in the following experiment, only gaseous phosphorus was determined. Fuels were ashed in a muffle furnace at 815 °C according to the Chinese Standard GB/T1574-2007. Chemical compositions of ashes were determined by XRF-1800 of Shimadzu Corporation. Ash fusion temperatures of samples were determined under reducing atmosphere by a HR-A5 AFT autoanalyzer (Kaiyuan, China) according to Chinese Standard GB/T219-2008. On the basis of the “Seger Cone” method, the measurements are carried out by heating the ash cone at a rate of 15 °C/min from room temperature to 900 °C and then 5 °C/min to maximum the temperature to 1600 °C. During this process, deformation of cone with respect to the temperature is photographed. The initial deformational temperature (DT), softening temperature (ST), hemispherical temperature (HT), and fusion temperature (FT) were recorded according to the specific shapes of the ash cones.2 Raman spectroscopy and X-ray diffractogram of slags were obtained by using an inVia Reflex laser Micro-Raman spectroscopy system (Renishaw, U.K.) and a Rigaku D/max-2550VB/PC diffractometer (Rigaku, Japan), respectively. The surface morphology characteristics of the slag were studied using a S-4800 scanning electron micrograph by Hitachi HTA.

Figure 1. High-frequency furnace reaction system: (1) high-frequency current generator, (2) fuel, (3) to absorption bottles, (4) quartz sampling tube, (5) Mo crucible, (6) quartz tube reactor, (7) thermocouple and meter, and (8) compressed gas cylinders.

2.2. Experimental Procedure. The experimental device used in this study is shown in Figure 1, which is capable of operation at a temperature up to 1350 °C at atmospheric pressure. The quartz tube reactor and the molybdenum crucible within it were placed in the center of the induction coil, which connected to the high-frequency current generator. Experiments were carried out under the temperature range from 900 to 1300 °C. The quartz tube reactor was purged with argon, and then, the molybdenum crucible (20 mm inner diameter and 25 mm depth with corundum lining) was heated to the target temperature in a high-frequency alternating magnetic field. Fuel samples (300 ± 10 mg each time) were carried into the preheated crucible from the top of the reactor by argon (for pyrolysis) or carbon dioxide (for gasification). Gaseous products were carried out rapidly by the carrier gas. Chars and slags were cooled to room temperature and then stored for subsequent analyses. 2.3. Sampling and Analysis. A detachable quartz sample tube was inserted into the reactor for gas sampling, as shown in Figure 1. The temperature of the sampling point (as shown in point A of Figure 1) was kept at about 450 °C by adjusting the position of the crucible at each experiment temperature. Gaseous products was extracted and absorbed by saturated solution of HNO3−Br2. Phosphorus-containing products in gas could be oxidized to orthophosphate through the following reactions:13

P4 + 16H 2O + 10Br2 → 4H3PO4 + 20HBr

(1)

3P4 + 20HNO3 + 8H 2O → 12H3PO4 + 20NO

(2)

P4 O8 + 8H 2O + 2Br2 → 4H3PO4 + 4HBr

(3)

PH3 + 4H 2O + 4Br2 → H3PO4 + 8HBr

(4)

3. RESULTS AND DISCUSSION 3.1. Existing Forms of Phosphorus in Sewage Sludge. The SMT extraction method was used for the determination of phosphate forms in sewage sludge.14 The results indicate that total phosphorus in sewage sludge is 2.85 wt % (on a dry sewage sludge basis). Organic phosphorus, nonapatite inorganic phosphorus (NAIP, not Ca-bound phosphorus) and apatite P (AIP, Ca-bound phosphorus) account for 9.0, 73.0, and 18.0 wt % of the total phosphorus, respectively. Existing forms of phosphorus in Shenfu coal were not tested, because its total phosphorus content was just 0.029 wt %. The SMT extraction test cannot determine the exact existing form of phosphorus in sewage sludge. Phosphorus-containing compounds are divided into different forms in this method mainly according to their extraction property in different acid and alkali solutions. Still, the SMT extraction test gives valuable information for understanding the behavior of phosphorus during co-gasification of sewage sludge and coal. 3.2. Volatilization of Phosphorus during Gasification. To gain insight into the volatilize behavior of phosphorus in cogasification of sewage sludge and coal, fuels were gasified by CO2 under different temperatures and total phosphorus in gas products was detected. The volatilization ratio of phosphorus is defined as total phosphous in the gas phase (g)/total phosphorus in the mixture (g) × 100%, and the result is shown in Figure 2.

Condensed tar on the inner wall of the quartz sample tube was washed carefully using a saturated solution of HNO3−Br2. Washing solution was mixed with absorption solution and heated for complete oxidation of the phosphorus-containing compound to orthophosphate. Total phosphorus in absorption solution was tested using a 723 spectrophotometer (Shanghai Spectrum Instruments Company) according to phosphomolybdenum blue spectrophotometric method. Products such 2831

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Table 2. Ash Chemical Composition and Ash Fusion Temperature of Samples ash samples

ash chemical composition (wt %)

Figure 2. Phosphorus volatilization ratio during gasification: (◆) 900 °C, (▼) 1000 °C, (▲) 1100 °C, (●) 1200 °C, and (■) 1300 °C. ash fusion temperature (°C)

The volatilization ratio of phosphorus increases at first and then decreases with the increasing of the sewage sludge ratio in fuel and monotonically increases with the rising gasification temperature. Phosphorus volatilization in fuels can be divided into two stages by the gasification temperature. When the temperature is not higher than 1100 °C, the phosphorus volatilization ratio varies within a narrow range of 1.51−5.74%. Either M1.5 or M2.0 has the highest volatile ratio at the corresponding gasification temperature. The phosphorus volatilization ratio of pure sewage sludge is a little lower than them. The volatilization ratio increases obviously and M1.5 has the highest volatile ratio of 10.79% when the gasification temperature reaches 1200 °C. The phosphorus volatilization ratio varies in a wider range of 15.22−33.51% when gasified at 1300 °C. Gasification of fuels can be divided into two steps: fuels first pyrolyse to be chars, and then the gasification reaction takes place between chars and CO2. The phosphorus volatilization ratio of fuel during pyrolysis is detected in parallel experiments, and the result is shown in Figure 3. In comparison to Figure 2,

SiO2 Al2O3 Fe2O3 MgO CaO Na2O P2O5 SO3 DTb STc HTd FTe

coal

M1.0a

sewage sludge

21.46 9.88 9.63 1.58 29.74 2.3 0.04 22.68 1127 1136 1139 1144

26.32 21.54 6.67 1.94 16.08 1.26 12.45 10.79 1112 1153 1161 1169

29.08 26.85 6.56 2.72 11.4 0.79 15.27 5.09 1116 1132 1146 1184

a

Date of chemical composition is calculated on the basis of the mixing ratio and composition of coal and sludge. bDT = initial deformational temperature. cST = softening temperature. dHT = hemispherical temperature. eFT = fusion temperature.

fusion temperatures of coal, M1.0, and sludge ashes are 1144, 1169, and 1184 °C, respectively. Shenfu coal was gasified by CO2 at different temperatures of the range from 815 to 1300 °C. XRD spectra of its slags are shown in Figure 4. Shenfu coal slag is mainly composed of

Figure 4. XRD diffractogram of Shenfu coal slag under a CO2 atmosphere.

Figure 3. Phosphorus volatilization ratio during pyrolysis: (◇) 900 °C, (▽) 1000 °C, (△) 1100 °C, (○) 1200 °C, and (□) 1300 °C.

the phosphorus volatilization ratio of fuels during the pyrolysis progress under different temperatures is very close to the gasification process, which proves that the volatilization of phosphorus during fuel gasification mainly takes place during the pyrolysis process. 3.3. Evolution of Crystal Minerals in Slags. Shenfu coal, M1.0, and sewage sludge are chosen to investigate the evolution of crystal minerals in slag, and their ash chemical compositions are shown in Table 2. Elements detected in Shenfu coal ash are mainly Si, Ca, Al, Fe, and S. In sewage sludge, the Al content is quite high because aluminum salts are used to precipitate phosphorus during the wastewater treatment process. The ash

Figure 5. XRD diffractogram of sewage sludge slag under a CO2 atmosphere.

quartz (SiO2), anhydrite (CaSO4), hematite (Fe2O3), and a small amount of lime at 815 °C. Quartz, anhydrite, and hematite, decrease or disappear with the increasing temperature, accompanied by the formation of akermanite (Ca2MgSi2O7) and anorthite [(Ca, Na)(Si, Al)4O8]. Obvious 2832

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Figure 6. Diffraction intensities of main minerals in sludge slag.

amounts of gehlenite and akermanite are still found in slag at 1200 °C. Magnesium mainly exists in refractory compounds, such as akermanite and augite, which can be observed even at 1300 °C. It can be seen that mineral evolution in Shenfu coal is mainly in the form of different calciumcontaining silicates and takes place among the SiO2−CaO− Al2O3 system. No phosphorus mineral is found in Shenfu slags, because the phosphorus content in Shenfu slag is quite low. Sewage sludge was gasified by CO2 at different temperatures of the range from 815 to 1300 °C. XRD spectra of its slags are shown in Figure 5. Calcium iron phosphate [Ca9Fe(PO4)7] and aluminum phosphate (AlPO4) are found to be the main phosphorus-containing minerals at 815 °C. Different from the violent changes of minerals in Shenfu coal slags, mineral species in sewage sludge slag are relatively stable from 900 to 1300 °C. Mineral evolution in sewage sludge slags at different temperature takes place mainly between phosphates, such as AlPO4, Ca9Fe(PO4)7, and Ca4(Mg, Fe)5(PO4)6. Intensity changes of four main kinds of minerals are shown in Figure 6. Anorthite is the only calcium-containing silicate observed during this process. The diffraction intensity of quartz decreases with the increasing temperature. The diffraction intensity of calcium iron phosphate decreases with the increasing temperature and almost disappears at 1300 °C. Stanfieldite [Ca4(Mg, Fe)5(PO4)6] is first found in the slag at 900 °C, and its diffraction intensity reaches a maximum at 1000 °C. A consistent phenomenon has been found by Adam et al.15 In addition, no akermanite (Ca 2MgSi 2O7) and augite [Ca(Fe, Mg)Si 2O 6] are found because of the formation of stanfieldite by the Mg element. Figure 7 shows the XRD diffractogram of M1.0 slags obtained from 900 to 1300 °C under a CO2 atomosphere. In comparison to Figure 5, minerals in M1.0 slag are quite similar as in sewage sludge and so are their change trends against temperature. However, the diffraction intensity of calcium iron phosphate is always higher than stanfieldite in M1.0 slag samples, which is contrary to sewage sludge slags. This is because the Fe/P ratio increases after mixing and iron shows a

Figure 7. XRD diffractogram of M1.0 slag under a CO 2 atmosphere.

Table 3. Molar Ratio of Elements to Phosphorus in Different Samplesa samples

Si/P

Al/P

Ca/P

Fe/P

Mg/P

M0.5 M1.0 M1.5 M2.0 M2.5 sewage sludge

3.73 2.82 2.52 2.38 2.29 2.25

3.25 2.75 2.60 2.52 2.47 2.45

3.15 1.79 1.35 1.14 1.01 0.94

0.88 0.57 0.47 0.43 0.40 0.38

0.48 0.38 0.34 0.33 0.32 0.31

a

Calculated on the basis of the mixing ratio and composition of coal and sludge.

Table 4. Pyrolysis Gas Yield at 1200 °C pyrolysis gas yield (mmol/g)

2833

samples

H2

CH4

CO

∑(H2 + CH4 + CO)

CO2

M0.5 M1.0 M1.5 M2.0 M2.5 sewage sludge

10.06 9.39 11.06 10.70 10.82 10.89

1.50 1.94 2.20 2.83 2.67 2.61

7.54 8.48 7.28 7.72 8.39 8.46

19.10 19.81 20.54 21.25 21.88 21.96

1.37 1.65 1.51 1.89 1.90 1.90

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Figure 8. SEM−EDS analysis of slag.

SiO2 can lead to the volatilization of phosphorus through reactions such as 5 and 8.16,17

special binding force with phosphorus.12 It is noticeable that the only crystalline mineral detected in M1.0 slag at 1300 °C is calcium iron phosphate; moreover, its diffraction intensity is also the lowest in three samples, which suggests the possibility of better fluidity of mixture slag. 3.4. Volatilization Mechanism of Phosphorus. According to SMT and XRD test results, inorganic phosphorus makes up 91% of total phosphorus mainly in the form of aluminum phosphate and calcium iron phosphate in sewage sludge. They are all nonvolatile at low temperatures. It is reasonable to deduce that released phosphorus comes from organic phosphorus when the gasification (pyrolysis) temperature is not higher than 1100 °C. The rest of organic phosphorus transforms to inorganic phosphorus. This is why all volatilization ratios are lower than 9% when the temperature is not higher than 1100 °C. The phosphorus volatilization ratio increases obviously when the gasification temperature reaches 1200 °C because of the vaporization of inorganic phosphorus. Elements in slag can be divided into two groups based on the role that they play in inorganic phosphorus volatilization. The first group includes metal elements of Ca, Fe, Mg, and Al; Si makes up the other group in the form of SiO2. Elements such as Fe, Mg, and Al can form phosphoric minerals in the form of Ca9M(PO4)6(F, Cl, OH), by partly replacing Ca of apatite [Ca5(PO4)3(F, Cl, OH)]. They are all nonvolatile at low temperatures.16 Therefore, the first group elements can improve the deposition of phosphorus into slag. Conversely,

2Ca3(PO4 )2 + 6SiO2 + 10C → 6CaSiO3 + 10CO + P4 (5)

2Ca3(PO4 )2 + 6SiO2 → 6CaSiO3 + P4O10

(6)

P4 O10 + 10C → P4 + 10CO

(7)

6Ca3(PO4 )2 + 10CH4 + 18SiO2 about 1100 ° C

⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯⎯→ 18CaSiO3 + 3P4 + 10CO + 20H 2O

(8)

Reaction 5 is the overall reaction in white phosphorus production. An “acid displacement mechanism” is considered the most likely course of this reaction: SiO2 reacts with calcium phosphate and produces P4O10, which will not take place until 2300 °C, and then P2O10 is reduced by coke to P4 at about 800 °C, as shown in reactions 6 and 7.16,17 It has been proven by practice that coke can reduce the temperature of reaction 5 to 1250−1500 °C, depending upon the ratio of SiO2/ Ca3(PO4)2 and C/Ca3(PO4)2. A gas-reducing agent, such as CH4, H2, and CO, can further decrease the reaction temperatures to 1100, 1200, and 1250 °C, respectively.16 For this reason, inorganic phosphorus begins to vaporize notably, and the first phosphorus volatilization ratio jumps when the gasification temperature increases from 1100 to 1200 °C. Considering that the quantity of reducing gases and contacting opportunity of the heterogeneous reaction are quite limited, the 2834

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promotion of reducing gases on phosphorus volatilization is also limited. When the reaction temperature increases to 1300 °C, reaction 5 happens. Fused minerals aggravate this action, because it is much higher than the ash fuse temperature (AFT) of mixtures. Therefore, phosphorus volatilization ratios at 1300 °C are much higher than at 1200 °C. As shown in Figures 5 and 7, the diffraction intensities of quartz drop obviously during the same time for participating in the reaction. Tables 3 and 4 show the element ratio to phosphorus and reducing gas yields at 1200 °C of different fuels, respectively. It can be seen from Table 3 that the ratio of Si to phosphorus decreases with the increase of the phosphorus content, which is helpful for the deposition of phosphorus. However, ratios of Al/P, Ca/P, Fe/P, and Mg/P decrease too, which will make contrary effects. Coupled with the increasing yield of reducing gases, the phosphorus volatilization ratio of mixtures increases initially and then decreases with the increase of the phosphorus content. 3.5. Structural Characterization of Amorphous Minerals in Slags. Data from various sources, including optical and electron microscopy, XRD, and geochemical data, indicate that the crystalline substance usually represents the dominant component present in ash samples at low temperatures.18 According to the result of the phosphorus volatilization ratio test at 1300 °C, no more than 33.51% of total phosphorus vaporizes during the gasification process. However, according to XRD test results (see section 3.3), crystalline phosphoric minerals either disappear or decrease to very low intensity from 815 to 1300 °C. Combination with that phosphorus is called the network former, such as silicon and germanium. It obeys Zachariasen’s rules and is easy to form glasses.17,19 It is reasonable to deduce that most phosphorus in slag under high temperatures transformed to the glass phase. Figure 8 shows SEM−energy-dispersive spectrometry (EDS) analysis results of sewage sludge slag (A) and M1.0 slag (B) at 1300 °C under a CO2 atmosphere. Because glass phases do not have special forms but crystals do, Figure 8 proves that there is phosphorus in the glass phase. However, no special relationship can be found between phosphorus and other elements, because the quantitative relationship between glass and crystalline minerals is hard to determine. Raman spectroscopy was used to determine the chemical structure of phosphorus in glass, and the results are shown in Figure 9. The structure of glass can be described using Qi terminology, where i represents the number of bridging oxygens per tetrahedral in Si or P network.20,21 The structural units of glass in Shenfu coal slag are predominantly Q3Si (1093 cm−1) and Q4Si (1150 cm−1), with only a few percentages of Q2Si (1032 cm−1) units.22 The Raman spectrum of sewage sludge at 1300 °C presents three bands that are different from Shenfu coal (Figure 9c). The weak band near 700 cm−1 is attributed to the symmetric stretching mode of P−O−P bridging oxygens.20,22−25 The band at 970 cm−1 is ascribed to the terminal P−O stretching vibrations, and the band at 1240 cm−1 is assigned to the symmetric stretch of the PO terminal oxygens.20,23−26 In addition, stretching of P−O in PO43− (orthophosphate) gives rise to a narrow sharp band near 930 cm−1 in sewage sludge and M1.0 slag, at low temperatures.25,27 When it comes to slag of M1.0, the band at 700 cm−1 is not obvious and the band at 1240 cm−1 is lower too because of less phosphorus. Phosphorus in sewage sludge slag is connected with Al, Si, or both networks via

Figure 9. Raman spectra of slag: (a) Shenfu coal, (b) M1.0, and (c) sewage sludge.

P−O−T bridges (T = Al + Si), accompanied by orthophosphate and pyrophosphate complexes. However, in M1.0 slag, phosphorus mainly exists in orthophosphate complexes as well as Q1P units.

4. CONCLUSION Phosphorus in sewage sludge is much more than in Shenfu coal, mainly in the form of inorganic phosphates. Volatilization of phosphorus and the evolution of minerals in slags have been investigated during the coal/sewage sludge co-gasification process. The phosphorus volatilization ratio increases monotonically with the increasing gasification temperature. In addition, phosphorus volatilization mainly takes place during the pyrolysis process. The organophosphorus compound makes up the main part of volatized phosphorus when the gasification temperature is not higher than 1100 °C. Inorganic phosphorus does not volatilize obviously until 1200 °C through reacting with silicon dioxide and reduced by reducing gas or carbon. No more than 33.51% of total phosphorus releases even at 1300 °C. Therefore, phosphates make up an important part of sludge and coal/sludge mixture slags. Most phosphorus in sewage sludge is deposited in slag as part of glass after gasification, and it is conducive for phosphorus control.

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AUTHOR INFORMATION

Corresponding Author

*Telephone: 86-21-64251418. Fax: 86-21-64251312. E-mail: hfl[email protected]. Notes

The authors declare no competing financial interest. 2835

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (Grant 21176079), the New Century Excellent Talents in University (NCET-08-0775), and the National Key Program of Basic Research in China (2010CB227005).

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