Effect of organic calcium compounds on combustion characteristics of rice husk, sewage sludge, and bituminous coal: Thermogravimetric investigation

Bioresource Technology 181 (2015) 62–71

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Bioresource Technology journal homepage: www.elsevier.com/locate/biortech

Effect of organic calcium compounds on combustion characteristics of rice husk, sewage sludge, and bituminous coal: Thermogravimetric investigation Zhang Lihui a, Duan Feng a,b, Huang Yaji a,⇑ a b

Key Laboratory of Energy Thermal Conversion and Control of Ministry of Education, School of Energy and Environment, Southeast University, Nanjing 210096, China School of Energy and Environment, Anhui University of Technology, Maanshan 243002, Anhui Province, China

h i g h l i g h t s  Cofiring behavior of three fuels blended with organic calcium compound is studied.  Effect of OCC on the rice husk combustion is not obvious due to compensation effect.  Combustion performance indexes for Shenhua coal impregnated by OCC were improved.  Negative effect is found for sludge because of its lower Ti and higher VM content.

a r t i c l e

i n f o

Article history: Received 24 November 2014 Received in revised form 8 January 2015 Accepted 9 January 2015 Available online 17 January 2015 Keywords: Organic calcium compound Combustion Biomass Sewage sludge Coal

a b s t r a c t Experiments were conducted in a thermogravimetric analyzer to assess the enhancement of combustion characteristics of different solid fuels blended with organic calcium compounds (OCCs). Rice husk, sewage sludge, and bituminous coal, and two OCC were used in this study. Effect of different mole ratios of calcium to sulfur (Ca/S ratio) on the combustion characteristics were also investigated. Results indicated that combustion performance indexes for bituminous coal impregnated by OCC were improved, however, an inverse trend was found for sewage sludge because sewage sludge has lower ignition temperature and higher volatile matter content compared to those of OCC. For rice husk, effect of added OCC on the combustion characteristics is not obvious. Different solid fuels show different combustion characteristics with increases of Ca/S ratio. The maximum combustion performance indexes appear at Ca/S ratios of 1:1, 2:1, and 3:1 for OCC blended with Shenhua coal, rice husk, and sewage sludge, respectively. Ó 2015 Elsevier Ltd. All rights reserved.

1. Introduction China is an agricultural country and has plenty of biomass resources including many agriculture residues; several different biomass fuels are presently used and introduced on the Chinese market in favor of fossil fuels (Zhou et al., 2012). However, oxygen is the main component of biomass and is of special relevance to the calorific value (Duan et al., 2013a,b). The oxygen contents in biomass are greater than those in fossil fuel, resulting in lower calorific value of biomass. Meanwhile, greater moisture and smaller fixed carbon (FC) contents have a negative impact to the calorific value of sewage sludge (Cong et al., 2013; Werther and Ogada, ⇑ Corresponding author. Tel./fax: +86 25 8379 4744. E-mail addresses: [email protected], [email protected] (Y. Huang). http://dx.doi.org/10.1016/j.biortech.2015.01.041 0960-8524/Ó 2015 Elsevier Ltd. All rights reserved.

1999). To achieve the same heating output of combustor, more biomass or sewage sludge is needed to be burned due to their lower calorific value. Besides, the sulfur and nitrogen content of these alternative fuels are generally considered to be low. However, the nitrogen contents of logging residual, rape stalk, rice straw, and wheat straw range from 0.3 to 0.8 wt.%. Obernberger et al. (2006) proposed that elevated emissions of NOx when using biomass can be expected when fuel-N concentrations are above 0.6 wt% (dry basis). This could especially occur for the combustion of straw, cereals, grasses, grains and fruit residues. Specially, the nitrogen contents of seed and some sewage sludge are much higher (Crutzen et al., 2007; He et al., 2014). The nitrogen content of rapeseed and some sewage sludge can be reaching to 4 and 6 wt.%, respectively. Therefore, the total pollutant emissions from these alternative fuels combustion cannot be neglected. Calcium

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compounds are often used as absorbent of removing a variety of harmful emissions of flue gas, and as catalyst on coal gasification process (Abbasian et al., 1990; Clemens et al., 1998; Risnes et al., 2003; Wu et al., 2003). Particularly using organic calcium compound (OCC) as an additive is an alternative method to simultaneously remove these pollutants (Niu et al., 2011). Currently, extensive experiments in test rigs and power plants have been made to investigate the combustion behavior of OCC and coal (Nimmo et al., 2004; Patsias et al., 2005; Shemwell et al., 2000; Tayyeb Javed et al., 2009). Until now, our knowledge of using OCC as an additive for biomass or sewage sludge combustion to dual remove SO2/NOx is still deficient. Especially, the chemical composition of sewage sludge and biomass are quite different from fossil fuels, and they have difference combustion characteristics compared to fossil fuels combustion (Vassilev et al., 2010). The volatile matter (VM) content of OCC is between that of biomass/sludge and coal. The VM content of blended fuel will vary with the additional amount of OCC, which may have significant effect on the combustibility characteristics of blended fuels. Meanwhile, existence of interactions between solid fuels and OCC during combustion can lead to variations of fuel reactivity and burnout characteristics because combustion performance may not necessarily demonstrate synergistic interactions or simple additive behaviors as expected (He et al., 2014). Some investigators found that the stability of combustion was reduced when the fuel was blended with OCC, which can be attributed to the higher VM content of OCC (Patsias et al., 2005). Therefore, it is meaningful to screen OCC of good fuel quality for combustion with different solid fuels at a wide range of Ca/S ratio before designing combustion scenarios. Many studies had found that the calcined CMA has the greater SO2 removal rate at lower temperatures than conventional SO2 absorbent (Nimmo et al., 2004; Patsias et al., 2005). Meanwhile, CP has more volatile matter and combustible content compared to other OCCs (Niu et al., 2011); therefore, CP and CMA are used in this study. However, commercially available OCC is expensive due to the relatively high cost of producing acetic acid from natural gas and due to its limited markets (Yoneda et al., 2001). In this study, modified calcium magnesium acetate (MCMA) was used as an alternative OCC due to its low price since it is synthesized for lower-cost raw materials (Valor et al., 2002). In this study, rice husk (RH), sewage sludge (SS), and Shenhua bituminous coal (SHC) are selected as solid fuels; calcium propionate (CP) and MCMA are respectively added to different kinds of solid fuels according to the Ca/S ratio from 1:1 to 3:1. In the work presented in this manuscript experimental and theoretical studies were undertaken to (a) compare combustion behaviors of three individual solid fuels and their blended fuels with OCC, (b) analyze the combustion behaviors for three solid fuels with MCMA with a wide range of Ca/S ratio, and (c) assess the enhancement of the combustion characteristics of three solid fuels blended with OCC by calculate the kinetic parameters using the Coats–Redfern method.

2. Methods 2.1. Materials Two different OCC (i.e. CP and MCMA) were used to blend with three solid fuels. CP used in this study is analytical pure. MCMA was chemically synthesized from magnesium oxide (MgO), calcium hydroxide (Ca(OH)2) and acetic acid at a determined mole ratio. For preparation of MCMA, a quantity of acetic acid is determined at an excess mole ratio of 50% and it is expected that MgO and Ca(OH)2 could be mostly transformed into organic radical based compounds under the effect of acetic acid. The volatile matter content of CP and MCMA are 42.16% and 39.37%, respectively. Three different solid fuels including RH, SS and SHC are used in this study. RH used in this study was collected from South-East China, which was one of the most common agricultural residues. SS was collected from Maanshan sewage treatment plant. SHC is one of the main coal varieties for power generation in China. Ultimate and proximate analyses and calorific values of three solid fuels are given in Table 1. The higher heating value (HHV) of all the fuel samples was measured through IKA C2000 CONTROL bomb calorimeter. As shown in Table 1, the VM content of three solid fuels follows the order RH > SS > SHC. Meanwhile, VM content of SS is very close to those of OCCs. VM content of RH is higher, while that of SHC is lower than those of OCCs. Therefore, the selection of these fuels is more conducive to the thermal behavior comparison of different solid fuels under the effect of OCCs. The combustion profiles of blended fuels were compared in this study. To easily identify different blends, these blended fuels were designated as RH-CP-R1, SS-CP-R2, SHC-MCMA-R3, and so on. Specifically, RH, SS, and SHC represent rice husk, sewage sludge, and Shenhua bituminous coal, respectively. CP and MCMA represent two OCC. ‘‘R’’ represents the Ca/S ratio, and the number after ‘‘R’’ stands for different Ca/S ratios (i.e. ‘‘1’’, ‘‘2’’, and ‘‘3’’ represents 1:1, 2:1, and 3:1, respectively). Blended fuels were prepared carefully for subsequent thermal behavior analysis according to the method introduced in the published literature (Niu et al., 2009). 2.2. Thermogravimetric analysis Thermogravimetric analysis was determined using NETZSCH STA 449C Thermogravimetric Analyzer based on the widely used TGA method (García et al., 1995; He et al., 2014). The mass loss percentage (TG signal) and mass loss rate (DTG signal, differential from TG) of samples were recorded continuously under non-isothermal conditions with a temperature range of 30–900 °C at a linear heating rate of 20 °C/min. A sample mass of 5 mg was used for the thermogravimetric analysis in an air atmosphere at a gas flow rate of 20 ml/min. Compressed air is used in this study. Analysis of each sample was repeated twice and repeatability was checked. The repeatability of the tests was more than 99%. Coats–Redfern model was used in this study to calculate the parameter of reaction kinetics. Three characteristic temperatures including ignition

Table 1 Ultimate and proximate analyses and calorific values of solid fuels. Material

RH SS SHC

Ultimate analysis (wt.%, db)

Proximate analysis (wt.%, db)

Calorific (MJ/kg, db)

C

H

Oa

N

S

VM

FC

Ash

HHV

39.83 28.71 74.13

4.16 4.36 4.82

41.07 12.12 14.77

0.93 4.43 1.06

0.15 0.94 0.31

67.91 46.05 29.45

18.23 4.51 65.64

13.86 49.44 4.91

14.28 13.09 31.15

db, dry basis. a Calculated by difference on dry basis.

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temperature (Ti, °C), maximum combustion rate temperature (Tm, °C), and burnout temperature (Tb, °C) derived from TG–DTG curves, and three indexes including comprehensive combustibility index, S (%2/(°C3 min2)), ignition index, Zi (%/(min3)), and burnout index, Hj (%/(min4)) are used to evaluate the combustion reactivity of all samples. The detail calculation process are identified following the previously literature (Zhang et al., 2015).

3. Results and discussion 3.1. Solid fuels and OCCs individual combustion Fig. 1 illustrates the combustion profiles of individual solid fuels and OCCs under air atmosphere. As shown in this figure, solid fuels combustion process generally consisted of dehydration stage I, devolatilization and combustion stage II, char combustion stage III, and burnout stage. The approximate ranges of these stages have been identified in Fig. 1A. However, different solid fuels show the distinct different combustion characteristics (Fig. 1). RH and SS individual combustions have three mass loss stages, while SHC individual combustion has only two. As seen in Fig. 1A, there are

StageII

Stage I

100

only two distinct stages for individual SHC combustion because VM and FC combustion stages (stage II and III) have no distinct boundary. Main combustion of SHC was characterized by only stage III. This suggests that the decomposition of VM during SHC combustion requires a higher temperature as FC, resulting in remarkable mass loss and simultaneous oxidation of heavy VM and FC (Niu et al., 2009). Also in this figure, stage I (30–172.8 °C) was the initial water evaporation stage and two decomposition stages (stage II and III) governed the intense combustion process of RH (Ganesh et al., 1992). RH mainly composes of cellulose, hemicellulose and lignin. Therefore, the first combustion stage was extended from 172.8 to 401 °C while the second combustion stage III was from 401 to 584 °C, which was similar to main combustion process of other biomass under comparable conditions as reported by Sait et al. (2012). Two pronounced peaks were observed. Peak temperature at 314.6 °C during stage II may be related to the step wise decomposition of cellulose and lignin while peak temperature around 498.2 °C could be attributed to the residual lignin decomposition and remain char oxidation. Moreover, mass loss in stage II (48.78%) was higher than that in stage III (31.33%), suggesting more volatile content in RH.

Burnout stage

StageIII

0

90 -2

Mass loss /%

70 -4

60 50

A

-6

40

SS_TG SHC_TG RH_TG SS_DTG SHC_DTG RH_DTG

30 20 10 0

0

100

200

-8

Mass loss rate /% min

-1

80

-10

300

400

500

600

700

800

-12

900

o

0

90

-2

80

-4

70

-6

B TG_CP TG_MCMA DTG_CP DTG_MCMA

60 50

-8 -10 -12

40

-14

30 0

100

200

300

400

500

600

700

800

900

o

Temperature/ C Fig. 1. TG and DTG curves for individual combustion (A) solid fuels and (B) OCCs.

-1

100

Mass loss rate/% min

Mass loss/%

Temperature / C

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100

0

90 -2

Mass loss /%

70

A

60

Mass loss rate /% min

-1

80

-4

TG

50

SHC SHC-CP-R2 SHC-MCMA-R2

40

-6

DTG

30

-8

SHC SHC-CP-R2 SHC-MCMA-R2

20

-10

10 0

100

200

300

400

500

600

700

800

900

-12

o

Temperature / C 100

0

90 -2

-4

TG

60

RH RH-CP-R2 RH-MCMA-R2

50 40

Mass loss rate /% min

B

70

Mass loss /%

-1

80

-6

DTG RH RH-CP-R2 RH-MCMA-R2

30 20

-8

-10

10 0

100

200

300

400

500

600

700

800

900

o

Temperature / C

100

0

80

C -2

TG

70

SS SS-MCMA-R2 SS-CP-R2

60

DTG

-3

SS SS-MCMA-R2 SS-CP-R2

50

Mass loss rate/% min

Mass loss/%

-1

-1

90

-4 40

100

200

300

400

500

600

700

800

900

o

Temperature/ C Fig. 2. TG and DTG curves for the co-combustion of fuel and different organic calcium salts (A) SHC, (B) RH and (C) SS.

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Table 2 The main mass loss stages of co-combustion and combustion characteristics. Samples

Stage II

Stage III

Mass loss Stage II (%)

SHC SHC-CP-R2 SHC-MCMA-R2 RH RH-CP-R2 RH-MCMA-R2 SS SS-CP-R2 SS-MCMA-R2

440–650 400–590 410–620 173–401 173–406 174–405 175–413 186–421 181–422

401–590 406–580 405–585 413–630 421–650 422–640

87.67 86.76 87.37 48.78 48.72 51.36 23.84 22.23 22.37

Characteristic temperature

Combustion performance indexes

Stage III (%)

Ti (°C)

Tb (°C)

Zi  102 (%/min3)

Hj  102 (%/min4)

S  104 (%2/°C3 min2)

31.33 28.48 29.84 25.44 23.02 24.25

447.6 401.9 411.9 273.3 277.9 278.9 234.6 248.1 240.9

639.4 578.2 589.1 584.3 571.0 576.2 622.1 640.3 634.1

1.87 2.55 2.65 5.98 5.60 5.7 2.26 2.09 2.08

0.16 0.24 0.27 0.62 0.59 0.59 0.095 0.104 0.101

2.80 3.68 3.90 3.11 3.05 3.30 0.51 0.42 0.43

0

100 90

-2

60

A

-4

TG

-6

SHC-MCMA-R1 SHC-MCMA-R2 SHC-MCMA-R3

50 40 30

DTG

-10

SHC-MCMA-R1 SHC-MCMA-R2 SHC-MCMA-R3

20 10 0

-8

100

200

300

Mass loss rate /% min

Mass loss /%

70

-1

80

-12

400

500

600

700

800

900

o

Temperature / C 7

700 650

B 6

600 5 4

500

2

4

450 3

400

2

Ti

350

2

ZiX10

Tm

2

2

HjX10

Tb

300

4

SX10

1

250 200

ZiX10 ,HjX10 ,SX10

o

Temperature / C

550

0:1

1:1

2:1

3:1

0

Ca/S ratio Fig. 3. Co-combustion profiles of SHC with MCMA at different Ca/S ratios.

As summarized in Table 1, SS has the similar fuel characteristic with RH due to its higher VM content and smaller FC content. However, the composition of SS is much complex compared with RH. As seen in Fig. 1A, the total mass loss of SS combustion is much lower than that of RH due to the higher ash content of SS. The initial water evaporation stage I (30–175.2 °C) is mainly

caused by free water, bound water and some light VM. Two distinct decomposition stages of SS are different with RH and SHC because of its little FC content, and these two decomposition stages are mainly corresponded to the decomposition and combustion of organic components in sludge. This can be attributed to the more complex VM content in SS. Different VM content in

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SS has different strength of chemical bonds, which led to a prolonged VM combustion process. The results obtained in this study are consistent with those by Folgueras et al. (2013). The first combustion stage II was extended from 412.9 to 622.1 °C, while the second combustion stage III was from 622.1 to 660 °C. Like RH combustion, stage II of SS combustion is also the rapid stage of mass loss characterized by the maximum peak of the DTG curve and the maximum value is 3.18%/min. This is mainly the decomposition and combustion of aliphatic compounds (Werther and Ogada, 1999). Meanwhile, stage III of SS combustion also has a relatively rapid mass loss; the peak value of DTG is 2.98%/min, which is very close to the last stage. The mass loss in this stage can be attributed to the fact that protein, residual lignin and sugar compounds which are more difficult to decompose (Valor et al., 2002). The FC content of SS is only 4.51 wt.%, which is much smaller than that of RH. High VM content consumes most oxygen, and ash of higher content (49.44 wt.%) takes up considerable space of SS, and decreases the chance of oxygen diffusion to the FC surface. Therefore, FC combustion is mainly occurs after the stage of VM burning, resulting in smaller mass loss rate and higher burnout time. Besides, burn out stage of

the SS presented some slight fluctuations, which could be attributed to decomposition of inorganic salts or metal carbonates therein after 660 °C (Font et al., 2001; García et al., 1995). Fig. 1B illustrates the combustion profiles of CP, and MCMA under air atmosphere. As shown in this figure, three distinct mass loss stages including dehydration stage, VM release and combustion stage, and CaCO3 decomposition stage are observed. The main organic matters of CP, and MCMA in stage II are pentanone, and acetone, respectively (Patsias et al., 2005). CaCO3 is the residue matter for all OCCs; moreover, MgCO3 is another residue for MCMA, which can be decomposed at lower temperature. The large porosity and internal surface area are generated upon the removal of the organic matters during the decomposition reaction (Han and Sohn, 2002); this significantly improves the sulfur fixation ability of residue at high temperature of stage III. Besides, the mass loss of CP in stage II is 39.47%, which is the higher than that of MCMA, suggesting that the combustible content of CP is higher. Meanwhile, CP has lower starting decomposition temperature which can be attributed to the aliphatic chain length of OCC. The aliphatic chains of MCMA and CP have two and three carbon atom, respectively. The longer aliphatic chain length will decrease the OCC’s

100

0

A TG

60

DTG 40

-1

-2

Mass loss rate/% min

Mass loss/%

80

-4

RH-MCMA-R1 RH-MCMA-R2 RH-MCMA-R3

-6

RH-MCMA-R1 RH-MCMA-R2 RH-MCMA-R3

-8

20 -10 0

100

200

300

400

500

600

700

800

900

o

Temperature/ C 8 600

B 7

Tm

Hjx10

Tb

Sx10

5

2

4

300

4 3 2

200 1 100

0:1

1:1

2:1

3:1

Ca/S ratio Fig. 4. Co-combustion profiles of RH with MCMA at different Ca/S ratios.

0

2

Zix10

2

400

2

Ti

4

6

Zix10 ,Hjx10 ,Sx10

o

Temperature / C

500

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100

0

Mass loss/ %

-1

TG SS-MCMA-R1 SS-MCMA-R2 SS-MCMA-R3

70

-2

DTG

60

SS-MCMA-R1 SS-MCMA-R2 SS-MCMA-R3

50

-3

Mass loss rate/% min

A

80

-1

90

-4

40 0

100

200

300

400

500

600

700

800

900

o

Temperature/ C 650

2.4

600

2.1

B

4

Tb

400

1.2 2

ZiX10

350

HjX10

300

4

SX10

0.6 0.3

250 200

0.9

2

2

450

2

1.5

Tm

o

Temperature / C

1.8

Ti

500

ZiX10 ,HjX10 ,SX10

550

0:1

1:1

Ca/S ratio

2:1

3:1

0.0

Fig. 5. Co-combustion profiles of SS with MCMA at different Ca/S ratios.

thermostability, and decrease the starting decomposition temperature (Valor et al., 2002). 3.2. Effect of OCC on the solid fuels combustion Fig. 2 shows the combustion profiles of solid fuels blended with two OCCs. In this figure, the Ca/S ratio is 2:1. The solid fuels used in Fig. 2A–C are SHC, RH, and SS, respectively. As seen in Fig. 2, higher-rank solid fuel exhibited a broad stage III and greater mass loss. In Fig. 2A, there is only one distinct stage III for SHC blended with OCCs. Compared to SHC individual combustion, the maximum rate of mass loss in stage III for SHC blended fuels increased after OCCs are added. TG curves of blended fuels shift to a lower temperature, suggesting that the combustion quality of blended fuels is enhanced by the added OCCs. According to Table 2, characteristic temperatures including Ti, Tm, and Tb decreases much, while characteristic indexes such as Zi, Hi, and S increase after OCCs are added. Therefore, OCCs have the fuel characteristics which can significantly improve the quality of SHC combustion due to increases of VM in the blended fuels. Meanwhile, alkali metal and alkalineearth metals added by OCC also have catalytic effect on the FC combustion (Steciak et al., 1995). In Fig. 2B, the peak values of stage II increases slightly, while the corresponding temperatures (Tm) and width of stage II both

increase compared to RH individual combustion, resulting in increases Ti. Meanwhile, the peak values and width of stage III decrease slightly for RH-CP-R2 and RH-MCMA-R2 The slightly decrease of mass loss rates in stage III can be attributed to the lower FC contents of blended fuels compared to RH individual. Therefore, the combustion characteristic indexes including Zi, Hj, and S decrease slightly after impregnation of RH by CP and MCMA. Because the VM content of OCCs are much lower than that of RH, the VM content of blended fuels decreases after impregnation, which has negative effect on the combustion. However, calcium as a kind of catalyzer acts as the active carrier of oxygen accelerates the rate of oxygen diffusion to the surface of FC (Fahmi et al., 2007; Köpsel and Halang, 1997), decreases the burnout temperature of blended fuels. Therefore, the characteristic indexes decreases slightly because of compensation effect (Table 2). According to the combustion profiles in Fig. 2C and the combustion characteristic parameters data given in Table 2, the peak of mass loss rate and the corresponding temperature Tm in stage II change little, in contrast, the peaks of mass loss rate in stage III show an upward trend for both blended fuels. Ti is associated with the rate of release of heat from early combustion of light VM, therefore, Ti and Tm of SS individual combustion were only 234.6 and 303.4 °C, respectively, which are much lower than those of RH and SHC. After impregnation the SS with CP or MCMA, the peak values of stage II

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-8

A

-10 -12

2

ln [ G(a) / T ]

-14 -16 -18 3

2

-20

a a+(1-a)ln(1-a) 1/3 2 [1-(1-a) ] -ln(1-a) 2 [-ln(1-a)]

-22 -24 -26 -28

0.00110

0.00115

0.00120

0.00125 -1

-8

0.00130

0.00135

0.00140

-1

B

-10 -12 -14 -16 2

-18 -20 -22 -24

3

-26

2

a a+(1-a)ln(1-a) 1/3 2 [1-(1-a) ] -ln(1-a) 2 [-ln(1-a)]

-28 -30 -32 -34

[-ln(1-a)] 1/2 1-(1-a) 1/3 1-(1-a) 1/4 1-(1-a) -1 (1-a) 2/3 1-2a/3-(1-a)

-36 0.0012

0.0014

0.0016

-1

0.0018

0.0020

-1

0.0022

T /K

3.3. Effect of Ca/S ratio -8

C

-10 -12 -14 -16 2

ln [ G(a) / T ]

Fig. 3 shows the combustion profiles of SHC with MCMA at different Ca/S ratios. Fig. 3A shows the TG and DTG curves of combustion. In this figure, Ca/S ratio increases from 1:1 to 3:1. The peak value of stage III appears at Ca/S ratio of 1:1, and this value at Ca/S of 2:1 is the lowest. Meanwhile, there is another mass loss appears at burnout stage, and this peak shows a small upward trend with increases of Ca/S ratio. This may be attributed to the decomposition of calcium carbonate. Fig. 3B shows the effect of Ca/S ratio on the characteristic temperatures and combustion performance indexes. As seen in this figure, the ignition temperatures and the burnout temperatures at different Ca/S ratios decrease much compared with that of individual SHC. Decreased Ti at higher Ca/S ratio can be attributed to the fact that the devolatilization temperature of MCMA is lower than that of SHC, and VM content of blended fuels increases with Ca/S ratio. However, the decreasing rate of these temperatures did not show linear trend with increasing of Ca/S ratio. For SHC-MCMA, the peak values of stage III decreases firstly, and then increases with increasing of Ca/S ratio. Therefore, SHC-MCMA-R1 exhibits greatest Zi (2.80  102), Hj (0.35  102), and S (1.03  104) among all SHC blended fuels. Fig. 4 shows the combustion profiles of RH with MCMA at different Ca/S ratios. As seen in Fig. 4A, the three peaks of stage II at different Ca/S ratios appear at the same or similar time. RHMCMA-R2 has the maximum peak value; while the peak value of RH-MCMA-R1 is lowest. The peak of stage III of RH-MCMA-R1 appears at about 505.8 °C, which is larger than that of individual SH combustion (498.2 °C), and the peak value is also decreased from 3.93 to 3.38%/min. However, the two other conditions have the inverse trend. Specially, the peak of stage III RH-MCMA-R2

[-ln(1-a)] 1/2 1-(1-a) 1/3 1-(1-a) 1/4 1-(1-a) -1 (1-a) 2/3 1-2a/3-(1-a) T /K

ln [G(a)/T ]

decreases slightly, while the corresponding temperatures compared to SS individual combustion, therefore, Ti of SS-CP-R2 and SS-MCMA-R2 increase by 13.5 and 11.6 °C compared to SS individual combustion. Meanwhile, the negative effect of OCCs on the SS combustion is observed at stage III. The peak values of stage III decreases much, and width of stage III increase compared to SS individual combustion, resulting in high Tb. Tb of SS-CP-R2 and SS-MCMA-R2 are 640.3 and 642.1 °C, respectively. High Tb led to a prolonged combustion process, suggesting that more complex intense synergistic effect may occur during main combustion of SS and its blended fuels. The combustion characteristic indexes including Zi, and S both decrease, while Hj changes little after impregnation of SS by CP and MCMA. These can be attributed to the increase of Ti, Tm and Tb. Besides, the increase of Tb and Tm is offset by the decreases in width of stage II, resulting in little changes of Hj. Therefore, different blended fuels show different combustion characteristic after impregnation by OCCs. This phenomenon may be attributed to the different synergistic effects resulting from apparently different ignition temperatures of the solid fuels and OCCs. In Table 2, Ti range followed the order SS (234.6 °C)
-18 -20 -22 3

-24 -26

2

a a+(1-a)ln(1-a) 1/3 2 [1-(1-a) ] -ln(1-a) 2 [-ln(1-a)]

-28 -30 -32 -34 0.0012

0.0014

0.0016

[-ln(1-a)] 1/2 1-(1-a) 1/3 1-(1-a) 1/4 1-(1-a) -1 (1-a) 2/3 1-2a/3-(1-a) 0.0018

-1

-1

0.0020

0.0022

0.0024

T /K

Fig. 6. Curves indicating the solid-state mechanisms by using Coats Redfern method (A) SHC, (B) RH and (C) SS.

appears at 485.8 °C, and its value increases to 4.07%/min. Fig. 4B shows the effect of Ca/S ratio on the characteristic temperatures and combustion performance indexes. As seen in this figure, the ignition temperatures at different Ca/S ratios increase little, while burnout temperatures decrease slightly compared to SH individual combustion. Increased Ti at higher Ca/S ratio can be attributed to the fact that the devolatilization temperature of MCMA is higher than that of RH, and VM content of blended fuels decreases with

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Ca/S ratio. However, calcium and magnesium in MCMA would act as an oxygen carrier to accelerate the velocity diffusion to FC surface, resulting in decreased Tb. Therefore, under such an interactive circumstance, the combustion performance indexes changes little at different Ca/S ratios. Besides, the sulfur content of RH is the lowest of three solid fuels, therefore, the additive amount of MCMA according to Ca/S ratio is less, and the combustion of MCMA cannot affect that of RH too much. Fig. 5 shows the combustion profiles of SS with MCMA at different Ca/S ratios. Fig. 5A shows the TG and DTG curves of combustion, while Fig. 5B shows the effect of Ca/S ratio on the combustion characteristic parameters. As seen in Fig. 5A, the peak values of stage I and III show the inverse trend with increases of Ca/S ratio. In stage I, the peak value of DTG curves decreases with increases of Ca/S, which can be attributed to the fact that the moisture content of blended fuel decreases at higher Ca/S ratio. Besides, the sulfur content of SS is the highest among three solid fuels, therefore, the additive amount of MCMA is also the highest according to the same Ca/S ratio. However, the peak value of DTG curves in stage II increases with Ca/S ratio. As discussed above, the ignition temperatures of SS blended fuels at lower Ca/S ratio are significantly affected by the additive of MCMA because of the higher devolatilization temperature of MCMA. Compared to individual SS combustion, Ti increases by 8.4 and 6.4 °C when the Ca/S ratios are 1:1 and 2:1, respectively (Fig. 5B). However, the width of stage II increases at Ca/S ratio of 3:1, resulting in decreases of Ti. Increasing peak value also can be observed in stage III, and this trend is becoming more obvious at higher Ca/S ratio. Specially, the peak value of DTG curve at Ca/S ratio of 3:1 increases significantly. This also result in the maximum mass loss rate appears in stage III, and not in stage II. Meanwhile, Tm significantly increases from 306.6 to 560.4 °C, and Tb decreases from 634.1 to 595.2 °C when the Ca/S ratio increases from 2:1 to 3:1. Accordingly, Zi and S decrease first, and then increase with increases of Ca/S ratio. Hj increases with the Ca/S ratio. All the maximum values of combustion performance indexes appear at Ca/S ratio of 3:1. This phenomenon may be attributed to the significant synergistic effects resulting from MCMA decomposition reactions and combustion of residual lignin, protein, and sugar compounds. At lower Ca/S ratio, the negative effect caused by higher decomposition temperature of MCMA are prevailed, resulting in decreases Ti, Tb, Zi, and S. In this study, the maximum additive amount of MCMA appears at Ca/S ratio of 3:1. The mass loss caused by MCMA decomposition in stage III becomes obvious, and this significant improves the combustion of residual lignin, protein, and sugar compounds. Therefore, combustion performance indexes become better at higher Ca/S ratio. 3.4. Kinetic analysis Two important kinetic parameters including activation energy (E) and pre-exponential factor (LnA) are used to investigate the

combustion behavior of three solid fuels, and their blended fuels with MCMA at different Ca/S ratios. According to the different combustion stages in TG–DTG curves, data derived from individual stages are applied for kinetics analysis through linear regression by selecting the most suitable reaction order. Fig. 6 shows the calculate data of solid fuels combustion using different reaction models. The solid fuels corresponding to Fig. 6A–C are SHC, RH, and SS, respectively. In this study, 11 common reaction mechanisms including diffusion model, random nucleation mode, shrinking unreacted-core mode, and so on, are used, and their model functions are also listed in Fig. 6. As shown in this figure, the second-order random nucleation mechanism is the best fitting for SHC, RH, and SS. The equations of these reaction mechanisms are:

GðaÞ ¼ ½lnð1  aÞ

2

ð1Þ

The kinetic parameters of solid fuels blended with MCMA are calculated according to this reaction mechanism, and the results are given in Table 3. As shown in this table, reliable model with correlation coefficient (R2) ranging from 0.9525 to 0.9954, which agreed with claims from combustions of solid fuels, and their blended fuels with MCMA. Moreover, kinetic parameters derived from TG–DTG curves are in good agreement in qualitative terms. Thus, these kinetics data are used to compare the combustion characteristics. In previous study, a two-stage decomposition kinetics was proposed to assess RH and SS combustions. In this study, to take into consideration the delayed combustion of heavy VM and FC in solid fuels, a modified two-stage combustion scheme was employed. And light VM release and combustion stage and heavy VM and FC combustion stage are considered as the important two stages. Therefore, E1 and LnA1 represent activation energy and pre-exponential factor in the light VM release and combustion stage, respectively; E2 and LnA2 represent activation energy and pre-exponential factor in the heavy VM and FC combustion stage, respectively (He et al., 2014). Higher-rank solid fuels present great activation energy and preexponential factor, which follows the order SHC > RH > SS. However, the activation energy values for different blended fuels show different trend compared with their individual combustion. SHC and its blended fuels at all Ca/S ratios show only one sharp peak in stage III, which may be associated with intense synergistic effects caused by overlap combustion of VM and FC. Therefore, only one decomposition stage is discussed for SHC and its blended fuels. As seen in Table 3, activation energy for blended fuels decreases by 21.79–30.08 kJmol1 after impregnation with MCMA, confirming the better reactivity of blended fuels compared with SHC individual combustion. For RH and its blended fuels combustion, RH shows lower E1 and LnA1 (180.93 kJ mol1 and 35.72 min1, respectively) in the first combustion stage and lower E2 and LnA2 (220.16 kJ mol1

Table 3 Thermal kinetic results of all samples blended with MCMA. Samples

Temperature range (°C)

E1 (kJ mol1)

LnA1 (min1)

R2

Temperature range (°C)

E2 (kJ mol1)

LnA2 (min1)

R2

SHC SHC-MCMA-R1 SHC-MCMA-R2 SHC-MCMA-R3 RH RH-MCMA-R1 RH-MCMA-R2 RH-MCMA-R3 SS SS-MCMA-R1 SS-MCMA-R2 SS-MCMA-R3

440–640 420–600 410–600 410–590 175–408 180–408 175–405 180–407 175–413 187–420 182–422 191–397

307.64 282.18 277.56 285.85 180.93 192.38 191.24 185.25 114.80 118.88 117.01 114.94

46.86 40.63 42.53 43.91 35.72 37.60 37.35 36.66 22.59 23.15 22.78 22.62

0.9806 0.9759 0.9797 0.9808 0.9937 0.9926 0.9954 0.9871 0.9856 0.9864 0.9879 0.9831

408–580 408–600 405–575 407––585 413–630 420–570 422–640 397–600

220.16 226.39 237.30 227.02 211.39 221.47 220.07 203.96

33.74 34.73 36.59 34.49 30.66 31.60 31.61 30.30

0.9740 0.9730 0.9758 0.9741 0.9739 0.9739 0.9764 0.9734

L. Zhang et al. / Bioresource Technology 181 (2015) 62–71

and 33.74 min1, respectively) in the second combustion stage. This trend is also can be observed in the process of combustion of SS and its blended fuels. E1 of all blended samples increases after adding MCMA at different Ca/S ratios, suggesting the increases of activation energy to initiate decomposition of blended fuels, and confirming better reactivity of RH and SS than blended fuels at low temperature. Results in this table do not show a linear variation of activation energy and pre-exponential factor with increases of Ca/S ratio. Take RH-MCMA for example, E1 for RH-MCMA-R1 and RHMCMA-R2 are relatively higher (192.38 and 191.24 kJ mol1, respectively), while E1 for RH-MCMA-R3 is the lowest (185.25 kJ mol1). SS blended fuels combustions at low temperature have the similar trend. SS-MCMA-R3 has the smallest value of E1 (114.94 kJ mol1), which is in relation to their Ti values. This reflects that RH-MCMA-R3 and SS-MCMA-R3 may be more reactive to ignite than other blended fuels. E2 and LnA2 for RH blended fuels both increase compared with RH. The highest E2 (237.30 kJ mol1) and LnA2 (36.59 min1) values for RH-CP-R2 suggests that higher energy is required to overcome barriers. This also proves that too higher Ca/S ratio will have negative effect on the reaction activity of RH. E2 and LnA2 of most SS blended fuels are also increase after SS added MCMA. However, SS-MCMA-R3 has the lowest E2 and LnA2, which can be attributed to the improved combustion of heavy VM under the effect of MCMA decomposition. 4. Conclusion Several comprehensible differences were found among the combustion profiles of three different solid fuels and OCC. Effect of added OCC on the rice husk combustion is not obvious. Combustion characteristics for Shenhua coal impregnated by OCC were improved. An inverse trend was found for sewage sludge impregnated by OCC. However, the combustion performance indexes increase with the Ca/S ratio. Based on the TG/DTG curves and kinetic analysis, the maximum combustion performance indexes appear at Ca/S ratios of 1:1, 2:1, and 3:1 for OCC blended with Shenhua coal, rice husk, and sewage sludge, respectively. Acknowledgements The financial support from China Postdoctoral Science Foundation (No. 2014M560382), and the National Basic Research Program of China (973 Program, No. 2013CB228106) are greatly acknowledged. Reference Abbasian, J., Rehmat, A., Leppin, D., Banerjee, D.D., 1990. Desulfurization of fuels with calcium-based sorbents. Fuel Process. Technol. 25, 1–15. Clemens, A.H., Damiano, L.F., Matheson, T.W., 1998. The effect of calcium on the rate and products of steam gasification of char from low rank coal. Fuel 77, 1017– 1020. Cong, S., Duan, F., Zhang, Y., Liu, J., Zhang, J., Zhang, L., 2013. SO2 emission from municipal sewage sludge cocombustion with bituminous coal under O2/CO2 atmosphere versus O2/N2 atmosphere. Energy Fuels 27, 7067–7071. Crutzen, P.J., Mosier, A.R., Smith, K.A., Winiwarter, W., 2007. N2O release from agrobiofuel production negates global warming reduction by replacing fossil fuels. Atmos. Chem. Phys. Discuss. 7, 11191–11205. Deng, L., Zhang, T., Che, D., 2013. Effect of water washing on fuel properties, pyrolysis and combustion characteristics, and ash fusibility of biomass. Fuel Process. Technol. 106, 712–720.

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