Characteristics of hematite and fly ash during chemical looping combustion of sewage sludge

Chemical Engineering Journal 268 (2015) 236–244

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Characteristics of hematite and fly ash during chemical looping combustion of sewage sludge Xin Niu, Laihong Shen ⇑, Haiming Gu, Shouxi Jiang, Jun Xiao Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China

h i g h l i g h t s  CLC is an attractive technology for processing sewage sludge.  No char bypassed to air reactor during CLC of sewage sludge.  The reactivity of hematite shows a slight decrease during 10 h operation.  Phosphorus speciation in CLC fly ash is beneficial to agriculture.

a r t i c l e

i n f o

Article history: Received 28 November 2014 Received in revised form 16 January 2015 Accepted 17 January 2015 Available online 24 January 2015 Keywords: Sewage sludge Chemical looping combustion Hematite Phosphorus

a b s t r a c t Chemical looping combustion (CLC) of sewage sludge is an appropriate solution of how to manage the continuously increasing sewage sludge production and at the same time, how to rationally use renewable resource. This work attempted to investigate the combustion performance of sewage sludge in a continuous CLC unit based hematite oxygen carrier. Besides, the characteristics of hematite and fly ash during chemical looping combustion of sewage sludge were elaborated. Compared to either bituminous coal or anthracite, sewage sludge was a unique solid fuel in the term of high combustion efficiency even at 800 °C. Additionally, there were no char particles bypassed to air reactor during CLC of sewage sludge. During 10 h continuous operation, hematite shows a slightly decrease reactivity, indicating a good long-term reactivity of the oxygen carrier used. Although some ash particles deposited on the surface of hematite, no reaction between sludge ash and hematite were identified. The only phosphate identified incinerated sludge ash was Ca2P2O7. However, the form of phosphorus in fly ash based on CLC was CaH2P2O7 and CaHPO4. The presence of steam in flue gas and the reduction atmosphere can significantly accelerate the generation of CaH2P2O7 and CaHPO4. Then, possible options of utilizing sludge ash on agriculture were discussed. Ó 2015 Elsevier B.V. All rights reserved.

1. Introduction Sewage sludge is one of the major solid wastes generated from municipal wastewater treatment [1,2]. It is a complex heterogeneous mixture of micro-organisms, synthetic organic compounds, inorganic materials and moisture. Additionally, it contains heavy materials and essential nutrients (like N, P, S and Mg) [1,3]. The surge of industrialization and urbanization coupled with increasingly stringent sludge reuse regulations and increasing public pressure, is forcing both public and private sludge generators to develop environmental friendly sewage sludge treatment routes [4,5]. It is anticipated that upcoming sludge treatment should accentuate upon reuse energy and recovery phosphorus from sludge. ⇑ Corresponding author. E-mail address: [email protected] (L. Shen). http://dx.doi.org/10.1016/j.cej.2015.01.063 1385-8947/Ó 2015 Elsevier B.V. All rights reserved.

Among various options, chemical looping combustion (CLC) is one of the feasible alternatives. Sewage sludge treatment using CLC technology has been successfully tested using Ni-based oxygen carrier [6]. CLC is a novel combustion technology based on transfer oxygen from air to fuel by a metal oxide, named oxygen carrier, which is continuously circulating between air and fuel reactor. It is an innovative combustion technology with inherent separation of CO2 [7]. When solid fuel (coal, biomass or sewage sludge) is used, in situ gasification chemical looping combustion (iG-CLC) is one of the feasible options [8]. In this process, solid fuel is directly introduced to fuel reactor where the gasification of solid fuel and subsequent reactions with oxygen carrier particles will occur simultaneously [7,9]. Then, the reduced oxygen carrier circulates to air reactor where it is re-oxidized by air. The adequate oxygen carrier is a cornerstone in the successful development of a CLC system, which should have high reactivity,

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environmentally harmless and high resistance to attrition and agglomeration [7,10,11]. Additionally, low cost materials as oxygen carriers are relevant option to process a solid fuel in iG-CLC because the system will lose some oxygen carrier particles during separation of ash out of the fuel reactor [7,11]. Although Ni-based oxygen carrier shows high reactivity and good performance when combustion sewage sludge [6], nickel is more expensive than other metal oxides and it is harmful to environment. Considering this, hematite, composed of Fe2O3, is preferred for use in sewage sludge combustion in a CLC process. Mattisson et al. demonstrated that hematite showed superior rate of both reduction and oxidation which made it feasible to be employed in a CLC system [12]. Furthermore, different solid fuels were tested using iron ore as oxygen carrier in CLC process [9,13]. The results show that both iron ore and the oxide scale work well as oxygen carrier for combustion pet coke, char-coal, lignite and two kinds of bituminous coals. Moreover, both oxygen carriers increase their reactivity with time. Additionally, the continuous operation has been accomplished to evaluate the performance of iron ore as oxygen carrier for combustion solid fuels [13,14]. It can be found that hematite shows good behavior as an oxygen carrier in a CLC process for combustion coal or biomass. Ash accumulation is an important problem in the CLC process of solid fuel that requires special attention and analysis. Although ash can be separated by a cyclone because it has a much lower density than oxygen carrier, there are still some residual ash left in the system, especially using high ash content fuels [15]. The effect of ash on oxygen carrier can vary depending on the ash content, the nature of ash, the experimental conditions and the oxygen carrier used [16,17]. Most of ashes are detrimental and reduced the reactivity of oxygen carrier, except the lignite ash enriched in CaO [15,17,18]. For example, Azis et al. [17] found that an increase in gas conversion for lignite and coal gasification ash was obtained at relatively high ash loading because of the beneficial effect of Ca (in lignite ash) and Fe (in coal gasification ash) acting as a catalyst or an oxygen carrier. Bao et al. also demonstrated that the ash mainly composed of CaSO4 and Fe2O3 could act as oxygen carrier and increase the reactivity of carrier [15]. Besides, Ksepko [19] reports that sludge ash can be effectively used as a low-cost, valuable oxygen carrier in CLC process. In addition, it should be mentioned that a large amount of Ca is presented in sludge ash [20], which has been shown to be an efficient additive in oxygen carrier to promote the reactivity of carrier and catalyze gasification process [21,22]. Thus, the interaction between sludge ash and oxygen carrier cannot be ignored. Furthermore, phosphorus (P) is enriched in sludge fly ash during sewage sludge combustion in a fluidized bed reactor [23]. Additionally, P is indispensable as an essential nutrient for all living organisms and cannot be replaced [24]. Thus, recovery P from sludge ash is especially important in the light of the shrinking global phosphate rock reserves and increases in demand for P fertilizer in agriculture [25]. The reaction atmosphere had a significant effect on the crystalline structure in sludge ash. The phosphorus speciation in fly ash based on CLC is CaHPO4 and CaH2P2O7, which are not detectable based on air combustion [6]. In this work, the combustion of sewage sludge using chemical looping technology based on hematite oxygen carrier was carried out in a continuous CLC unit. The effect of fuel reactor temperature on gas distribution was analyzed. Subsequently, the CLC performances of both Shenhua bituminous coal and Huaibei anthracite were used for comparison. This work also characterized the morphologies of the fresh and used hematite after 10 h operation by X-ray diffraction (XRD), Brunauer–Emmett–Teller (BET), scanning electron microscope (SEM) and X-ray fluoroscopy (XRF) analyses. Furthermore, the characteristic of sludge ash from the outlet of fuel

reactor was analyzed and the behavior of phosphorus in fly ash was determined. 2. Experimental 2.1. Oxygen carrier and sewage sludge Hematite was supplied by Nanjing Steel Manufacturing Company. Samples were crushed and sieved to 0.1–0.3 mm. After this, the samples were calcined at 980 °C for 3 h in a muffle oven to completely oxidize and increase the mechanical strength. Table 1 shows the chemical compositions of the fresh material. De-watered sewage sludge supplied by Jurong wastewater treatment plant was used as solid fuel in the experiment. Table 2 shows the proximate and ultimate analyses of the sewage sludge together with the lower heating value, as well as Shenhua bituminous coal and Huaibei anthracite. The sample was sieved to a size range of 0.2–0.45 mm. 2.2. Experimental setup and procedure 2.2.1. 1 kWth continuous unit Experiments were conducted in a 1 kWth continuous unit made of stainless steel, as shown in Fig. 1. The facility has been described in detail in our previous work [26]. This unit is composed of two interconnected fluidized bed reactors connected by a loop-seal and a cyclone. The fuel reactor is a rectangular spout-fluid bed, with a cross section of 50  30 mm2, and a height of 1000 mm. Sewage sludge is fed by a screw feeder at the bottom of this bed with an Ar stream in order to avoid gas backflow from the reactor. Steam is used as gasifying gas. The loop-seal, with a cross section of 34  30 mm2 and a height of 370 mm, transfers reduced oxygen carrier particles to the air reactor. The fluidizing gas in loop-seal is steam, which also acts as gasifying agent. The oxidation of carrier takes place in air reactor, consisting of a fast fluidized bed with 25 mm inner diameter and 1600 mm height. Air is introduced to the bottom of air reactor to oxidize carrier. The unit is electrically heated in an oven to supply heat for start-up and compensate heat loss during operation. Temperatures in the bed of air reactor and fuel reactor are monitored as well as the pressure drops in important locations of the system. The outlet gas from air reactor and fuel reactor is sampled by gas bags for offline analysis. CO, CO2, H2, CH4 and O2 concentration in flue gas is analyzed by a NGA2000 type gas analyzer (EMERSON Company, USA) measuring 0.00–20.00 vol.% CO, 0.00–100.00 vol.% CO2, 0.00–10.00 vol.% CH4, 0.00–25.00 vol.% O2 and 0.0–50.0 vol.% H2. The concentrations of C2–C4 hydrocarbons are analyzed by a gas chromatograph (GC). In this test, the gas flow introduced to air reactor was 0.84 m3/h. The steam flows supply for loop-seal and spout-fluid bed were 0.21 kg/h and 0.15 kg/h, respectively. The Ar stream for the pneumatic medium for conveying sewage sludge particles into fuel reactor was 0.27 m3/h. The sludge flow rate was 100 g/h, corresponding to a thermal power of 278 Wth. 2.2.2. Fixed bed reactor A batch fixed bed reactor made of stainless steel was used to investigate the reactivity of both fresh hematite and the one

Table 1 Compositions of the fresh hematite (wt.%). Fe2O3

SiO2

Al2O3

P2 O5

CaO

SO3

Others

83.21

7.06

5.13

0.38

0.24

0.21

3.77

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Table 2 Proximate and ultimate analyses of sewage sludge, SH bituminous coal and HB anthracite. SH bituminous coal

HB anthracite

Proximate analysis Moisture 4.89 Ash 55.86 Volatile matter 35.88 Fixed carbon 3.37

Sewage sludge

6.01 4.76 35.10 54.13

1.01 9.82 8.82 80.35

Ultimate analysis Carbon Hydrogen Nitrogen Sulfur Oxygen LHV (MJ/kg)

69.57 4.30 1.03 0.52 13.81 27.1

80.85 4.62 1.39 1.32 0.99 34.92

21.55 3.82 3.76 0.55 9.57 10.31

N 2 + O2 Fig. 2. Schematic diagram of batch fixed bed reactor.

2.3. Data evaluation

Oven

Carbon conversion efficiency (gconversion ) represents the carbon converted to gas in the sludge and is defined as the fraction of carbon introduced which is converted to gas in both fuel reactor and air reactor:

CO2 + H2O

gconversion ¼

F C;FR þ F C;AR F C;sludge

ð1Þ

where F C;FR and F C;AR is the molar flow of total carbonaceous gas leaving the fuel reactor and air reactor, respectively. F C;sludge is the molar carbon in the sludge flow introduced in the system:

Air reactor

Loopseal

Fuel reactor

Sewage sludge

Ar

Air

Steam

Fig. 1. Schematic diagram of 1 kWth continuous CLC facility.

experienced 10 h in 1 kWth unit during CLC of sewage sludge, as shown in Fig. 2. The reactor, 32 mm I.D. and 570 mm height, was heated by an external furnace, with the temperature monitored by a K-type thermocouple. Before each test, a sample of 40 g oxygen carrier was placed in the reactor and then heated to the reaction temperature 900 °C. When the desired temperature reached, the gas flow used was a mixture of N2 and CO with flow rates of 450 mL/min (STP) and 50 mL/min (STP), respectively. The flow rates of gases were controlled by mass flow controllers. The outlet gas was sampled by gas bag per 4 min for offline analysis. The reaction continued for 80 min.

F C;FR ¼

xCO2 ;FR þ xCO;FR þ xCH4 ;FR þ mxCm Hn ;FR  F N2 ;FR xN2 ;FR

ð2Þ

F C;AR ¼

xCO2 ;AR þ xCO;AR þ xCH4 ;AR  ðF Air;AR  0:79Þ xN2 ;AR

ð3Þ

where xi;FR is the i species percentage in the fuel reactor outlet. F N2 ;FR is the N2 molar flow in the fuel reactor inlet. xi;AR is the i species percentage in the air reactor outlet. F Air;AR is the air molar flow in the air reactor inlet. Carbon capture efficiency (gCC ) is defined as the ratio of the carbon converted to gas in the fuel reactor to the total carbonaceous gas flow leaving the two reactors:

gCC ¼

F C;FR F C;FR þ F C;AR

ð4Þ

The combustion efficiency in the fuel reactor (gcomb;FR ) evaluates the degree of combustion with respect to the fraction of fuel converted in the fuel reactor. It is defined as the quotient between the oxygen required to fully burn unconverted gases and the oxygen demanded for complete combustion of the sewage sludge converted in the fuel reactor:     0:5F H2 O;FR þ F CO2 ;FR þ 0:5F CO;FR out  0:5F H2 O;FR þ 0:5Osludge in gcomb;FR ¼ O2;demand;sludge

ð5Þ where O2demand;sludge is the molar flow of oxygen demanded for sludge completely combustion. The CO conversion ratio is defined as the percentage of the outlet CO2 concentration to the total concentrations of CO and CO2 in the off-gas at time t in a batch fixed bed reactor:

X. Niu et al. / Chemical Engineering Journal 268 (2015) 236–244

nCO2 nCO þ nCO2

239

3. Results and discussion

Table 2, the amount of volatile matter and fixed carbon was 35.88% and 3.37%, respectively. The gaseous products from volatile matter gasification in the spout-fluid bed were much more than those from fixed carbon gasification. Additionally, sludge devolatilization proceeded with a much faster reaction rate compared to char gasification. Thus, sewage sludge can be burned completely in fuel reactor and there are no carbon reached to air reactor.

3.1. Effect of fuel reactor temperature on sludge CLC process

3.2. Comparison of CLC performance between sewage sludge and coal

To evaluate the suitability of hematite oxygen carrier for combustion sewage sludge, some experimental tests were carried out under continuous operation in a CLC unit. The fuel reactor temperature was varied from 800 to 900 °C, and each temperature was maintained at steady state for at last 60 min. Fig. 3 shows the variation of gas compositions (dry basis and N2 free) at the outlet stream from fuel reactor as a function of fuel reactor temperature with hematite. It can be seen that CO2 as the main product increased tremendously from 66.41% at 800 °C to 85.35% at 900 °C. Besides, CO and CH4 were also detected as not completely oxidized products of devolatilization and gasification and their concentration decreased with temperature. Additionally, there are no H2 detected during CLC of sewage sludge. A possible explanation to this phenomenon is that H2 as a gasification product has a higher reaction rate with Fe2O3 than CO [27]. Nevertheless, the reactivity of all of CO, H2 and CH4 with Fe2O3 increased with temperature. Moreover, sewage sludge gasification in fuel reactor was highly strengthened by the rise of temperature [28]. The CO or H2 oxidation with Fe2O3 was more temperature dependent than the gasification gases generation from sewage sludge gasification, which would justify the reduction of these unburnt gases’ presence. Besides, the evolution of C2–C4 hydrocarbons in the flue gas of fuel reactor was analyzed, shown in Fig. 3. Only C2H4 was detected and its concentration decreased with temperature from 4.44% to 0.79%. However, there are no C2–C4 hydrocarbons detected in the similar operating temperature (880–900 °C) using both Shenhua bituminous coal and Huaibei anthracite as solid fuel in this unit [29]. Besides, the amounts of C2–C4 were much lower than that reference for sewage sludge gasification in a bubbling bed reactor using alumina catalyst in similar experimental conditions [30]. During sludge gasification in bubbling bed reactor, the percentage of C2H4 decreased from around 5.2% at 800 °C to 3.4% at 850 °C, which was almost 1.5 times that obtained in present test. This indicates that the presence of hematite can accelerate the C2H4 reforming reaction rate. As for air reactor, there were no presences of CO, CO2, CH4 or H2 in any of the analyses preformed. According to the results in

In order to gain an in-depth understanding of the conversion behavior of sewage sludge, the values obtained in this work were compared to those obtained using Shenhua bituminous coal and Huaibei anthracite as fuel with hematite in the same continuous unit by Song et al. [29]. The comparative study with respect to the effect of fuel type on carbon capture efficiency, carbon conversion efficiency and combustion efficiency is shown in Fig. 4. It is sufficient to note that the rise of temperature leads to the monotonically increase of carbon conversion, carbon capture efficiency and combustion efficiency using either sewage sludge or coal as solid fuel. This reflects that if the reaction temperature increases, the degree of combustion with respect to the fraction of fuel converted increases and the amount of carbon converted to gases increases. As mentioned earlier, in steam-based gasification process, the increase of temperature can increase both devolatilization and gasification rate. Additionally, the reactivity of Fe2O3 with CO or H2 increases with the temperature. The combined effects result in the increase of carbon conversion, carbon capture efficiency and combustion efficiency. However, it is sufficient to note that the highest carbon conversion value is achieved using SH bituminous coal in the temperature interval (870–920 °C), followed by that using sewage sludge and finally with HB anthracite. This means that sewage sludge has a higher amount of unconverted carbon compared to SH bituminous coal in a CLC process. Additionally, carbon capture efficiency reached almost 100% in the test with sewage sludge at all temperature tested. As previously mentioned, carbon capture efficiency is defined as the fraction of introduced carbon converted to gas in the fuel reactor. This indicates that there is no char bypassed to air reactor with hematite in CLC of sewage sludge. This value ranged from 77% to 83% and 49% to 54% for SH bituminous coal and HB anthracite in the temperature interval (880–920 °C), respectively. Furthermore, the rise of temperature from 800 to 900 °C produces an increase in combustion efficiency from 56% to 76% for sewage sludge. This value in case of sewage sludge is 1.09–1.13 and 1.63–1.94 times in similar conditions in comparison to that of SH bituminous coal and HB anthracite, respectively. It can be concluded that the degree of combustion in fuel reactor with respect to the fraction of fuel introduced for sewage sludge is higher than that obtained using either SH bituminous coal or HB anthracite. Additionally, there are no char bypassed to air reactor using sewage sludge as solid fuel. The following reasons were proposed to explain the superior CLC performance using sewage sludge as solid fuel. Firstly, the mass ratio of volatiles to fixed carbon for sewage sludge is larger than that for either SH bituminous coal or HB anthracite. According to the values shown in Table 2, the mass ratio of volatiles to fixed carbon for sewage sludge was 10.65. This value for SH bituminous coal and HB anthracite was 0.65 and 0.11, respectively. Moreover, the rate of volatiles release was relatively high and the char gasification step is the limiting step of the process [31]. Thus, if a larger proportion of carbon is released as volatiles in the fuel reactor, the combustion efficiency will be higher. Additionally, sewage sludge char is more reactive than coal char.

X¼

ð6Þ

where ni (i = CO, CO2) is the i species percentage in the fuel reactor outlet.

Gas concentration (%)

100 80

CO2 60 40 20

CH4

C2H4

CO

0 800

820 840 860 880 900 o Fuel reactor temperature ( C)

Fig. 3. Outlet gas concentrations in the flue gas of the fuel reactor during sewage sludge CLC process with hematite at different fuel reactor temperatures.

X. Niu et al. / Chemical Engineering Journal 268 (2015) 236–244

Carbon conversion efficiency (%)

100 SH bituminous coal 90 Sewage sludge

80

HB anthracite

70 60

(A) 50

800

825 850 875 900 o Fuel reactor temperature ( C)

925

Carbon capture efficiency (%)

240

100

Sewage sludge

90 80 SH bituminous coal 70 60 HB anthracite 50 800

825 850 875 900 o Fuel reactor temperature ( C)

(B) 925

Combustion efficiency (%)

80 70

Sewage sludge SH bituminous coal

60 50 HB anthracite

40

(C) 30

800

825 850 875 900 925 o Fuel reactor temperature ( C)

950

Fig. 4. (A) Carbon conversion efficiency, (B) carbon capture efficiency and (C) combustion efficiency in experiments with different solid fuels at different fuel reactor temperatures.

3.3. Characteristic of used hematite It is well known that the performance of sewage sludge CLC process is not only related to the sewage sludge properties and reaction conditions, but also largely dependent on the physicochemical properties of oxygen carrier. An important aspect of oxygen carrier is the suitability to be used in a continuous CLC unit during long periods of time. Thus, the samples of solid extracted from fuel reactor using hematite as oxygen carrier after 10 h of continuous operation at the fuel reactor temperature of 900 °C were characterized by different techniques. During 10 h operation, defluidization was never detected. Fig. 5 shows the gas concentrations at the outlet of fuel reactor during 10 h of operation. There are a slight decrease of CO2 concentration and an increase of both CO and CH4 concentration during the continuous operation. CO2 concentration is in the range of 74.5–83.5%, CO concentration 10.6–16.0%, and CH4 concentration around 5.7–9.9%. A similar investigation by Song et al. using coal as solid fuel with hematite was conducted [29]. It can be found that the hematite samples show high reactivity in the continuous operation.

The BET surface area and total pore volume of fresh and used hematite were determined by a Micrometric ASAP 2020. Compared to the fresh hematite, the porous properties of the used one has a significant change after 10 h continuous operation. The BET surface area decreased from 0.8722 m2/g of fresh hematite to 0.1698 m2/g of the used one, and the total pore volume decreased from 4.269  103 to 1.622  103 cm3/g. the remarkable decrease of BET area can be ascribed to the agglomeration during long-term operation. Fig. 6 shows the SEM images of fresh and used hematite after 10 h of operation. The images A and C are taken at lower magnification (500) for fresh and used hematite particles whereas B and D are at higher magnification of 40,000 for the same. More pores were observed in the surface of the particle after operation with

100 Gas concentration (%)

The second reason is the presence of some indigenous inorganic species in sludge, such as the oxides and salts of alkaline (K) and alkaline earth (Ca) metals, as shown in Table 4. Those compounds act as catalysts which can not only enhance pyrolysis and gasification [32], but also promote the reactivity of hematite. Bao et al. found that the addition of foreign ions (K+, Na+ or Ca2+) could significantly promote the reduction rate of ilmenite [21,22]. Additionally, the consumption of CO and H2, which has an inhibiting effect on steam gasification, can increase the fuel conversion rate [33].

CO2

80 60 40

CO

20 0

0

100

200

300 400 Time (min)

CH4

500

600

Fig. 5. Gas concentrations at the outlet of fuel reactor (dry Ar free) during 10 h of operation at the fuel reactor temperature of 900 °C.

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Fig. 6. SEM images of the (A) and (B) fresh and (C) and (D) used hematite samples after 10 h of operation.

sewage sludge, which can be attributed to the high fluidization in air reactor and the accumulative effect of long-term alternating reactions of reduction and oxidation [34]. Furthermore, the surface of used particle appears to be much coarser than the surface of fresh one. In addition, Fig. 6(D) of the used hematite particle shows that no agglomeration resulting from the interaction between sludge ash and oxygen carrier is detected, which is in line with the results using SH bituminous coal or HB anthracite as fuel in the same unit [29]. Fig. 7A and B shows the EDX results of fresh and used hematite particles, respectively. It is evident that the fresh particles consist mainly of Fe, Si, Al and O. The weight percentages of Fe, Si, Al and O were 67.36%, 0.46%, 1.10% and 31.08%, respectively. In the fresh hematite, the mass ratio of Fe to O was 2.17, which was similar to the theoretical Fe and O weight percentage in Fe2O3 (Fe-70%, O-30%). However, the used particles showed the presence of Ca along with Fe, Al, Si and O. The ash analysis of sludge revealed the presence of Si, Al, Fe and Ca in ash with 34%, 13%, 18% and 8%, respectively. It is again proved that there are interactions between sewage sludge ash and hematite particles in fuel reactor. The weight percentages of Fe, Si, Al, O and Ca in used samples were 66.05%, 2.28%, 1.20%, 29.87% and

Fe

O

(A)

0.60%, respectively. The oxygen demanded percentage for SiO2, Al2O3 and CaO was 3.87%. Thus, the mass ratio of Fe to O was 2.54 in used samples. Hence, this can be proved that the reduction product is Fe3O4. XRD analyses of fresh and used hematite particles were performed to probe whether there was interaction between sludge ash and hematite. Fig. 8 shows the XRD spectra of fresh and used hematite. The major crystalline phases identified in the fresh samples were Fe2O3 and SiO2. After 10 h continuous reaction with sewage sludge in a CLC process, there are new chemical compounds formed. Fe2O3 and Fe3O4 were the main iron oxide and no peak suggests the presence of Fe or FeO phases. It seems that there are a part of Fe2O3 unconverted to Fe3O4. These observations strongly support that there are enough oxygen-carrier available to fully oxidize sewage sludge. This can be again verified that the Fe3O4 is the only reduction product. Additionally, the other crystalline phase detected was CaAl2Si2O8. The presence of this new phase suggests the deposition of sludge ash on oxygen carrier. Moreover, the chemical composition of the used particles is shown in Table 3. After 10 h operation, the used hematite has a Fe2O3 content of 69.12 wt.%. The reactivity of used hematite was tested in a batch fixed bed reactor.

Fe

O

Fe

(B)

Fe

AlSi 0 1 2 3 4 5 Full Scale 4283 cts Cursor:0.000

Fe 6

7

Ca

8

9 keV

Si Al

Ca 0 1 2 3 4 5 6 Full Scale 4283 cts Cursor:0.000

Fig. 7. EDX analysis of (A) fresh and (B) used hematite samples after 10 h of operation.

Fe 7

8

9 keV

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X. Niu et al. / Chemical Engineering Journal 268 (2015) 236–244

Intensity ( x 100 counts)

16 12

1

(A) Fresh Hematite

1

8 1

4 0 12

3

3

(B) Used Hematite

8 3

4 0 10

15

4-CaAl2Si2O8

1 1

1

1

1

1

1 2

1

3

1

1 2 3 32

2

4

4

20

2-Fe3O4

3-SiO2 1

1

1

1-Fe2O3

25

3

30

35

40

3

45

2

1

1 3

21

50

55

1 3

3

60

65

1 2 1

70

75

80

O

2-Theta( ) Fig. 8. XRD analyses of the fresh and used hematite samples after 10 h of operation.

Fig. 9 shows a detailed comparison of the CO conversion ratio with time using fresh and used hematite. The maintenance of the carrier’s reactivity can be seen for the used hematite during the transformation of Fe2O3 into Fe3O4. Additionally, It takes around 20 min for the transformation of Fe2O3 into Fe3O4 using either fresh or used hematite as oxygen carrier. When compared to fresh hematite, the CO conversion becomes progressively lower during the further reduction from Fe3O4 to FeO, corresponding to a lower oxygen carrier reactivity. According to the XRD analysis of used hematite as shown in Fig. 8, Fe3O4 is the only reduction phase in CLC of sewage sludge in a 1 kWth continuous unit. Thus, this fact justifies the slightly decrease reactivity of used hematite after long-term CLC of sewage sludge in the stage transforming of Fe2O3 into Fe3O4.

Table 3 Compositions of the used hematite (wt.%). Fe2O3

SiO2

Al2O3

P2O5

CaO

SO3

Others

69.12

12.91

14.05

0.67

1.48

0.05

1.72

CO Conversion (X)

1.0 0.9 0.8 0.7 0.6 0.5

3.4. Characteristic of sludge fly ash

Fresh hematite Used hematite

0

10

20

30 40 50 Time (min)

60

70

80

Fig. 9. Gas concentration with time for fresh and used hematite during reduction. N2: 450 mL/min (STP), CO: 50 mL/min (STP). Temperature: 900 °C.

In general, the following series of reactions for hematite reduction occurred during reaction with CO:

Fe2 O3 ! Fe3 O4 ! FeO ! Fe

ðR1Þ

3Fe2 O3 þ CO ! 2Fe3 O4 þ CO2

ðR2Þ

Fe3 O4 þ CO ! 3FeO þ CO2

ðR3Þ

FeO þ CO ! Fe þ CO2

ðR4Þ

The reaction (R2) is an irreversible reaction, which will theoretically cause complete conversion of the reducing gas CO. Nevertheless, the reaction (R3) is a reversible reaction. This reflects that incomplete conversion of CO occurs when further reducing Fe2O3 past Fe3O4.

The ash generated in the 1 kWth CLC unit is of two types, i.e. fly ash and bottom ash. Generally, the fly ash constitutes 80% of the total ash produced in a coal combustion power plant [35]. Thus, the behaviors of sludge fly ash based on CLC at 900 °C was investigated and compared to sludge incinerated ash obtained during incinerating sewage sludge in a muffle oven at 900 °C for 2 h. Table 4 shows the chemical composition of the investigated sludge ashes. For simplicity, the fly ash particles based on CLC and the sludge ash particles obtained during incineration were defined as CLC ash and AI ash, respectively. According to equilibrium calculation of elements, CLC ash contained around 45% of reduced hematite. The ash mass flow was estimated as the ratio of the amount of ash elutriated out from reactor to the reaction time and therefore the value was 1.6 g/min. As a result, the two fractions fly ash and bottom ash make 94.5 and 5.5 mass%, respectively, of the total amount of ash. As shown in Table 4, both AI ash and CLC ash are mainly composed of Fe, Si, Al, P, Ca, K, Mg and S. The Fe content in sludge ash was quite high because a part of hematite particles were elutriated out from fuel reactor. Although the contents of nutrient elements, such as P and K, in CLC ash decreased as compared to those in AI ash, their concentrations were much higher compared to many other kinds of biomass (such as crop residues and urban composted material) [36].

Table 4 Chemical composition of both sludge incineration ash and CLC fly ash (wt.%).

Incineration ash CLC fly ash

Fe

Si

Al

P

Ca

K

Mg

S

Others

20.47 52.92

36.37 22.90

14.86 10.71

10.74 4.41

8.01 4.89

3.43 1.94

3.16 0

1.03 0.14

1.93 2.09

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Intensity ( x 100 counts)

30

(A) Sludge Incineration Ash 1

20

1

2

20

2-CaAl2 SiO8

3-Fe2 O3

4-Fe3 O4

5-Ca2 P2 O7

6-CaHPO4

7-CaH2 P2 O7

10 0

1-SiO2

52

1

11

12

1

1

1

1

6 1 13 1 4

1

31

3 4 1

50

55

3

1

11

1

(B) Sludge CLC ash 1 4

10 1 2

0 10

15

2

3 7

20

6 4 7

25

3 6

30

35

40

45

O

4

3

1

4

60

3

65

11

3

70

4 1

75

80

2-Theta( ) Fig. 10. XRD analyses of sludge incineration ash and CLC fly ash.

The XRD analyses of CLC ash and AI ash are performed to explore the crystalline minerals in the samples, as shown in Fig. 10. From the XRD curves of AI ash, it can be observed that the predominant minerals was SiO2 and CaAl2Si2O8. In CLC ash, Fe3O4, Fe2O3, CaAl2Si2O8 and SiO2 were the dominating crystalline compounds. The prevalent forms of phosphorus in AI ash were Ca2P2O7. During CLC of sewage sludge, the phosphorus combined hydrogen to form CaH2P2O7 and CaHPO4. The species of phosphorus pathway in the CLC unit has been proposed by a simplified reaction as follows:

Ca2 P2 O7 þ H2 O ! 2CaHPO4

ðR5Þ

The relationship of the equilibrium constant with temperature is illustrated in Fig. 11. The constant Kp is equal to 1 at 40 °C, and it decreases monotonously with temperature. This suggests that the increase of temperature impels the process of reaction (R5) towards the negative direction, leading to the decomposition of CaHPO4. As a result, CaHPO4 is generated in the outlet pipeline of fuel reactor. Additionally, the reducing atmosphere can efficiently promote the reaction. During thermal treatment of sewage sludge, organic matter and thus organic contaminants are degraded almost completely. There are two main aspects to explain the influence of fly ash on agriculture. Firstly, phosphorus is more enriched in the fly ash than in the bed ash [37]. About 75–98% of phosphorus leaving the CFB boiler retained in the fly ash [25]. This means that fly ash can be used as fertilizer or secondary raw material for fertilizer production. During CLC of sewage sludge, phosphorus is in the form of CaH2P2O7 and CaHPO4 that have a higher acid solubility than Ca2P2O7 which is the main species in incineration ash. However, the amount of ashes applied to agricultural land is restricted by their

Equilibrium constant (logKp)

2 1

heavy metal contents [38]. Secondly, sludge ash tends to be used as a ameliorant for improving soil quality because sludge ash is alkaline and contains high CaO content [35]. Considering the possible use of sludge ash on agriculture, it should be note that fly ash based on CLC is a more interesting resource than phosphorus rock or incineration ash. 4. Conclusions Sewage sludge generated during municipal waste water treatment was processed using chemical looping combustion technology, and a feasibility study was performed in a continuous CLC unit. The combustion performance of sewage sludge was elaborated. Besides, the characteristics of hematite and fly ash during chemical looping combustion of sewage sludge were investigated. The increase of fuel reactor temperature intensified gasification step and the subsequent reduction process for all solid fuel tested, leading to the increase of carbon conversion and combustion efficiency. When compared to bituminous coal and anthracite, sewage sludge shows superiority in terms of high volatile and low fixed carbon content. This results in high combustion efficiency using sewage sludge as fuel. Additionally, there are no char bypassed to air reactor during CLC of sewage sludge. Moreover, hematite shows slightly decrease reactivity during 10 h continuous operation, indicating a good long-term reactivity of the hematite used. The main form of phosphorus in incinerated sludge ash is Ca2P2O7. During CLC process, the phosphorus combines hydrogen to form CaH2P2O7 and CaHPO4. The presence of steam in the flue gas of fuel reactor can significantly accelerate the generation of CaH2P2O7 and CaHPO4. Nevertheless, the reaction mechanism needs further research. Considering the possible use of sludge ash on agriculture, fly ash based on CLC is a more interesting resource than phosphorus rock or incineration ash. Acknowledgements

0 We gratefully acknowledge the support of this research work by the National Natural Science Foundation of China (Grant Nos. 51476029, 51276037 and 51406035), China Postdoctoral Science Foundation (2014M551489).

-1 -2

Ca 2 P2 O7 +H 2 O → 2CaHPO4

-3 -4

References

-5 -6

0

150

300 450 600 Temperature (oC)

750

900

Fig. 11. Equilibrium constant of equilibrium constant of Ca2P2O7–H2O as a function of temperature.

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