Influence of BaCO3 on chlorine fixation, combustion characteristics and KCl conversion during biomass combustion

Fuel 208 (2017) 82–90

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Full Length Article

Influence of BaCO3 on chlorine fixation, combustion characteristics and KCl conversion during biomass combustion Qian Wang, Juan Chen, Kuihua Han ⇑, Jiamin Wang, Chunmei Lu School of Energy and Power Engineering, Shandong University, Jinan, China

h i g h l i g h t s
BaCO3 had better performance on chlorine fixation from simulation and experimental results.
Observation of BaSO4 and BaCl2 showed that BaCO3 fixed chlorine and sulfur simultaneously.
The index S increased with the addition of BaCO3 in TG/DTG analysis.
The appropriate additive ratio between biomass, NH4H2PO4 and BaCO3 could be K:P:Ba = 1:1.5:1.

a r t i c l e

i n f o

Article history: Received 22 March 2017 Received in revised form 31 May 2017 Accepted 1 July 2017

Keywords: Chlorine Barium Combustion characteristics Biomass Potassium

a b s t r a c t The influence of BaCO3 on chlorine fixation and combustion characteristics was investigated in a fixed bed system and thermal analyzer. Both simulation and experimental results show the stability of BaCl2 was much better than CaCl2 at temperature higher than 800 °C. The chlorine fixation of cotton stalk was improved 5.35 times by BaCO3. The barium chlorides in the ashes were mainly BaCl2 and Ba (ClO2)2. Obvious peaks of BaSO4 showed the ability of BaCO3 for fixing both chorine and sulfur simultaneously. With the addition of BaCO3, the Tf and S increased and the E decreased in different degree. The reactivity of elements in KCl, NH4H2PO4 and BaCO3 were deduced as: Cl-Ba < Ba-P < K-P. Excessive addition of BaCO3 may decrease the production of potassium phosphates. The reasonable additive ratio between biomass, NH4H2PO4 and BaCO3 could be K:P:Ba = 1:1.5:1. Ó 2017 Elsevier Ltd. All rights reserved.

1. Introduction Biomass accumulates the same amount of CO2 by photosynthesis during the growth as it produces during the combustion, so it is regarded as an CO2 neutral resource. Considering the global warming problems caused by CO2, biomass is a promising alternative energy resources [1]. Although biomass has been widely used, the harmful emitted gas should not be ignored. Chlorine is a minor constituent during the growth of biomass, and is also prominent in municipal solid wastes (MSW) [2]. The releases of the alkali metal and chlorine during combustion and other heating process cannot be neglected [3]. The potassium content in herbaceous biomass is higher than that in ligneous biomass. Chlorine is mainly released as HCl or KCl [12]. The potassium and chlorine contents in biomass are mainly in the form of inorganics. So the chlorine content in biomass is almost exponentially decreases with increase of volatiles ⇑ Corresponding author. E-mail address: [email protected] (K. Han). http://dx.doi.org/10.1016/j.fuel.2017.07.005 0016-2361/Ó 2017 Elsevier Ltd. All rights reserved.

in biomass [13]. The ash compositions and the form of chlorine in the fuel can be a significant contributor to corrosion, slagging and fouling [4–7]. Chlorine in the form of HCl and Cl2 can also result in low temperature corrosion of precipitators areas and other post-air heater equipment. The most severe corrosion problem in biomass-fired boilers are associated with deposition alkali chlorides on heated tube surfaces [8]. The schematic of chlorine conversion and tube oxide corrosion are shown in Fig. S1 [9–11]. Alkali chlorides (MCl, M = K and Na) are typically alkali halide salts released during the combustion. Cl2 plays a role of catalyst, a loose and non-protective adherent metal oxide layer are formed (Path⑥ in Fig. S1) [14]. The combination of chlorine and alkali metals also increases the ultrafine PM formation [15]. So, inhibiting the chlorine in the form of HCl or fly ashes is urgent to be solved. Among chlorine removal methods utilized in modern industrial applications, the Dry Sorbent Injection (DSI) process is an attractive way in recent years considering the benefit of increasing removal efficiency and easy management [16,17]. The dry powdered calcium-based sorbents (i.e. CaCO3, CaO and Ca(OH)2), or sodium-based alkali (i.e. NaCO3) are injected into the furnace.

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Nomenclature S k(T) f(a)

DSI Dry Sorbent Injection TG/DTG Thermogravimetric/differential thermogravimetric analysis g The chlorine fixation ratio, % x1 The chlorine contents in the initial sample, % x2 The chlorine contents in the residual ash, % The masses of the initial sample, mg m1 m2 The masses of the residual ash, mg b The changing rate of chlorine fixation ratio Ti The ignition temperature, °C The burnout temperature, °C Tf Tmax The peak temperature, °C DTGmax The maximum combustion rate, %/s DTGmean The average combustion rate, %/s

a

t A E R T b n R2

CaCO3 absorbed HCl at comparatively low temperature of 400 °C, and Na2CO3 absorbed HCl at higher temperature range of 400– 500 °C [19]. No matter using Ca-based, Na-based or mixed Ca-Na sorbents, the removal of HCl were mainly studied at temperatures up to 650 °C [20–22], so the ideal reaction temperature zone of traditional sorbents is relatively low. In addition, injecting dry power additives is not suitable for small stoves. As a result, finding a better high temperature chlorine fixation additive, mixing the additive with the raw biomass before briquetting to improve the fuel property, would benefit for extensive utilization of biomass. Barium is an alkaline earth element and it occurs principally in the form of barite (barium sulfate, BaSO4) and witherite (barium carbonate, BaCO3). BaCO3 is mainly used in glass, ceramic and building material industries [23]. BaO was more inclined to release the oxygen ion for less affinity to oxygen than CaO [24]. The decomposition temperature of CaSO4 shifted to higher range due to the addition of BaCO3 [25]. The Ca-Ba sorbents reduced the HCl concentration in the temperature range of 800–900 °C, and the BaCl2 phases was confirmed by XRD [26]. However, few studies investigate the influence of BaCO3 on the chlorine fixation behavior during biomass combustion. Barium residue is the solid industrial waste and the residue contains approximately 30.92% of BaCO3 [31]. Studying the BaCO3 as an additive are also benefit for the utilization of barium residue on theoretical foundation. In addition, the combustion characteristic need to be studied to understand whether the BaCO3 could enhance combustion property or not. Several technologies have also been proposed to reduce the direct condensation of alkali vapors, including leaching, utilization of additives, co-firing and fuel sulfation [18]. Using additives is easy and convenient. In our previous study, NH4H2PO4 was an ideal potassium fixation additive, which could improve the potassium fixation ratio and melting temperatures during biomass combustion [39–41]. The possible negative product by R1 and R2 need to be noticed. So the BaCO3 could compensate for the lack chlorine fixation. The comprehensive influence and mechanism of NH4H2PO4 and BaCO3 on KCl conversion should also be considered.

2NH4 H2 PO4 þ 2KCl þ CaO ! K2 CaP2 O7 þ 2NH4 Cl þ 2H2 O

ðR1Þ

nNH4 H2 PO4 þ nKCl ! ðKPO3 Þn þ nNH4 Cl þ nH2 O

ðR2Þ

In this paper, the BaCO3 is used as a chlorine fixation additive. In order to make a comparison, two calcium based additives (CaCO3 and CaO) are also tested. The novelty of this research is as following: to find an additive (BaCO3) with better chlorine fixation ability and combustion characteristics, and to seek the interaction mechanism between NH4H2PO4 and BaCO3 on KCl conversion.

The The The The The The The The The The The The

combustion characteristic index, %2/(s°C3) function of reaction rate constant differential conversion function mass loss ratio,% reaction time, s frequency factor, A1 activation energy, kJ/mol ideal gas constant, 8.31 J/(molK) reaction temperature, °C heating rate, °C/s reaction order correlation coefficient

2. Materials and methods 2.1. Simulations on chemical equilibrium Thermodynamic calculation was conducted to the reactions products by using HSC Chemistry software (Outokumpu, Finland). The calculation was based on minimization of Gibbs free energy. The amounts of the initial reactants, the reaction pressure and the temperature range were input, the composition of all the products will be calculated [30]. Due to the restrictions in experimental conditions, quantitative measurements of HCl, BaCl2 and some other products simultaneously were hard. The thermal equilibrium calculation results are benefit to analyze the reaction mechanism. Thermodynamic equilibrium calculation was beneficial to investigate the species evolution of products. 2.2. Materials Two kinds of biomass (cotton stalk, maize straw) were collected in the suburb of Shandong province of China. The raw materials were pulverized and sized into 180 lm prior to use. The sieved samples were dried at 105 °C for at least 24 h and then sealed. Proximate, ultimate and chlorine content analyses of raw materials as well as their ash chemistry obtained using an X-ray fluorescence (XRF) analyzer are listed in Table 1. The proximate analysis was performed according to GB/T 28731-2012, and ultimate analysis was determined using an elemental analyzer (leco tru spec CHN) and sulfur analyzer (leco S144DR). The content of chlorine was determined by titration method according to GB/T 3558-2014. Individual biomass was blended with different analytical pure additives according to different molar ratio. The additives were barium carbonate (BaCO3), calcium oxide (CaO), calcium carbonate (CaCO3) and ammonium dihydrogen phosphate (NH4H2PO4). The briquetting process was proceeds in a small scale hydraulic molding machine, 5 g ± 0.1 g of the samples were put into a cylinder mold with a diameter of 25 mm, and pressed under the pressure of 15 MPa in 60 s. The acquired briquettes were in the length of 8–9 mm. 2.3. Fix bed furnace experiment system As shown in Fig. S2, the combustion experiment was conducted in a laboratory horizontal fixed bed system. The furnace was constituted by corundum tube with an inner diameter of 80 mm and a length of 1000 mm. The length of constant temperature zoon of fix bed furnace was 200 mm. The reaction temperature was measured

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Table 1 Proximate, ultimate, element analysis and ash compositions of biomass (wt%, on air dry basis). Sample

Cotton stalk Maize straw

Proximate analysis

a

Element analysis b

M

A

FC

V

C

H

O

N

S

Cl

K

8.22 7.84

14.74 8.63

15.16 14.37

61.88 69.16

41.84 42.33

5.41 5.78

28.22 33.07

1.30 1.06

0.27 0.31

0.34 0.65

1.42 1.80

Ash composition

Cotton stalk Maize straw

MgO

Al2O3

SiO2

P2 O5

SO3

Cl

K2O

CaO

Fe2O3

Others

3.74 0.74

7.13 2.74

8.94 0.34

30.80 58.81

4.14 2.08

4.79 3.09

2.90 5.63

17.50 20.45

15.23 3.89

3.94 0.52

0.89 1.72

FC = Fixed Carbon; V = Volatile Matters; A = Ashes; M = Moisture. O was calculated by difference.

2.4. Thermogravimetric analysis The influence of BaCO3 on combustion characteristics was performed by the thermogravimetric/differential thermogravimetric (TG/DTG) system (TGA/SDTA851e instrument, Mettler-Toledo Company). About 10 mg ± 0.1 mg of samples were put in the analyzer. The selected reaction temperature range was from 30 °C to 1000 °C with a heating rate of 20 °C/min. The reaction gas flow rate (air) was 60 ml/min and protective gas flow rate (N2) was 20 ml/ min. TG curves were obtained by continually recording the mass loss with increasing temperature. DTG curves were calculated simultaneously by differentiating TG curves. 2.5. Measurement The chlorine fixation ratio was calculated by Eq. (1):

xm g ¼ 2 2  100% x1 m1

ð1Þ

where g was the chlorine fixation ratio, %; x1 and x2 represented the chlorine contents in the initial sample and residual ash, respectively, %; m1 and m2 represented the masses of the initial sample and residual ash, respectively, mg. The measurement was repeated three times at each condition. In order to have a close look at the chlorine retention ability in the ash phase, the changing rate (b) of chlorine fixation ratio was introduced to this study and calculated by Eq. (2):

b¼

gbiomassþadditiv e  gbiomass gbiomass

ð2Þ

To obtain the chemical composition of ashes, an X-ray powder diffraction (XRD) was used to identify the main crystalline phases.

BaCO3/HCl or CaO/HCl was set as 2 kmol:4 kmol. The production equilibrium amount was calculated by the software. Fig. 1 shows the main production equilibrium amount versus temperature. The HCl represented dangerous emission, the CaO and BaCO3 represented additives. The lower amount of HCl, CaO and BaCO3, the better chlorine fixation ability. At 600 °C, almost all the HCl were transformed into CaCl2 or BaCl2.

CaO þ 2HCl ! CaCl2 þ H2 O

ðR3Þ

BaCO3 þ 2HCl ! BaCl2 þ H2 O þ CO2

ðR4Þ

In Fig. 1(a), the decomposition of CaCl2 occurred at 700 °C, the backward reaction of R3 plays an important role when the temperature is high, nearly 0.093 kmol of CaCl2 decomposed when the temperature was 1200 °C. Another important issue is the production of CaO(l). CaO particles is inevitably sintered and agglomerated of the in high temperature combustion [29]. 2.00

(a) CaCl2=0.093 kmol

Equilibrium amount/kmol

by a thermocouple connected between the reactor tube and an electric heating element. High purity N2 and O2 (N2/O2 = 79/21) with the flow rate of 2 L/min were inlet into one side of tube to maintain an air atmosphere. All combustion experiments were conducted at atmospheric pressure. Accurate weighted briquette samples were loaded into the alumina boat (120 mm  60 mm  16 mm) and delivered into the position of constant temperature zoon. The reaction temperature was 500, 600, 700, 800, 900 and 1000 °C. The holding time at the desired temperature was 30 min, the alumina boat with formed ashes would be pulled out from the reaction zone.

1.95

CaCl2

CaO CaCl2(g)

HCl(g) CaO(l) 1.90

HCl(g)=0.18 kmol

0.11

0.00 600

700

800

900

1000

1100

1200

Temperature/°C 2.000

(b) Equilibrium amount/kmol

a b

Na2O

BaCl2=0.006 kmol 1.995

1.990

BaCl2

BaCO3

HCl(g) BaCl(OH)(g)

BaCl2(g)

0.006

HCl=0.006 kmol

3. Results and discussion 0.000

3.1. Chlorine fixation 3.1.1. Equilibrium amounts calculation Supposing the reaction products of pure additives and HCl were BaCl2 and CaCl2 shown in R3 and R4, the initial molar ratio of

600

700

800

900

1000

1100

1200

Temperature/°C Fig. 1. The main production equilibrium amount versus temperature (a) reaction between CaO and HCl; (b) reaction between BaCO3 and HCl.

Q. Wang et al. / Fuel 208 (2017) 82–90

85

In Fig. 1(b), the decomposition of BaCl2 occurred at around 800 °C. It was deduced that the backward reaction of R4 occurred with increasing temperature. When the temperature exceeded 1100 °C, the increasing amount of BaCO3 began to flatten and even decreased, accompanied by the generation of BaClOH. According to products, R5 and R6 were deduced. These results indicate that BaCl2 is more stable than CaCl2, so BaCO3 has better performance on absorbing or capturing HCl.

BaCO3 þ HCl ! BaClOH þ CO2

ðR5Þ

BaCl2 þ H2 O ! BaClOH þ 2HCl

ðR6Þ

3.1.2. Self-chlorine fixation characteristics The chlorine fixation ratio versus temperature of biomass themselves are shown in Fig. 2. There was a relative good self-chlorine fixation ability for both maize straws and cotton stalks under 500–600 °C. The rapid decreases of g between 600 and 700 °C, from 48.8% to 20.60% for the cotton stalk and from 49.05% to 25.91% for the maize straw. It was justified that a significant fraction of chlorine element was released in the form of gaseous KCl [27], most chlorine released when the temperature was over 800 °C. It is proved that large amounts of chlorine were released between 600 and 800 °C. 3.1.3. Types of additive Fig. 3 shows the performance of CaCO3, CaO and BaCO3 on chlorine fixation ability during biomass combustion. The addition molar ratio between additives and biomass was 1.5. The g changing rates were relative low at 500–600 °C. This is due to the good self-chlorine fixation characteristics of biomass. However, the curves of bCaO and bCaCO3 presented a upside down ‘‘V” in the investigated temperature zone. At 500–800 °C, the forward reaction played a leading role in the reversible reaction R3. When temperature was above 800 °C, decomposition of CaCl2 resulted in reduction of both bCaO and bCaCO3 [28]. The performance of BaCO3 on chlorine fixation was better. Even though the trend of gBaCO3 went down continuously with increased temperature, the bBaCO3 still kept a slight increase when the temperature was over 800 °C.

increased additive molar ratio. At 1000 °C, the gBaCO3 increased greatly from 14% to 25% as the Ba/Cl molar ratio changed from 1.5 to 3.5. However, the difference of bBaCO3 between Ba/Cl = 2.5 and Ba/Cl = 3.5 was not such obvious. Excessive addition of additives may induce instable combustion, so a reasonable addition ratio of Ba/Cl should be 2.5.

3.1.4. Molar ratio of additives Chlorine fixation characteristics of cotton stalks mixed with BaCO3 with a Ba/Cl molar ratio of 1.5, 2.5 and 3.5 are shown in Fig. 4. The higher value of Ba/Cl, the better performance of chlorine fixation. When the combustion temperature was higher than 700 °C, the difference of bBaCO3 start to become obviously with

3.1.5. XRD patterns Figs. 5 and 6 represent the XRD patterns of cotton stalk ashes acquired at 800 and 1000 °C. The major components of ashes acquired at 800 °C in Fig. 5(a) were SiO2, calcium-silicon and calcium-aluminum-silicon compounds. There was a change in the crystalline structure of cotton stalks mixed with BaCO3 with

Fig. 2. The influence of temperature on self-chlorine fixation ability.

Fig. 3. The influence of types of additives on chlorine fixation (g) and changing rate of chlorine fixation (b): (a) cotton stalk; (b) maize straw.

Fig. 4. The influence of molar ratio on chlorine fixation (g) and changing rate of chlorine fixation (b).

Q. Wang et al. / Fuel 208 (2017) 82–90

BaCO3 þ 2KCl þ H2 O ! BaCl2 þ 2KOH þ CO2

ðR7Þ

BaCO3 þ 2KCl þ 2O2 þ H2 O ! BaðClO2 Þ2 þ 2KOH þ CO2

ðR8Þ

(a) Intensity

Ca6(SiO4)(Si2O7)(OH)2 CaAl2(Si2Al2)O10(OH)2

0 20

40

60

80

2 /°

(b)

SiO2

720

CaMg2Si2O7 CaSiO3

480

Ca6(SiO4)(Si2O7)(OH)2 CaO

240

0 20

40

60

80

2 /°

(c)

SiO2

480

Ca2MgSi2O7 CaSiO3

320

BaCO3 BaSO4 BaCl2

0

SiO2

1920

CaAl2Si2O8 Ca2SiO4

1280 640 0 40

60

80

2 /°

(b) SiO2

1920

CaAl2Si2O8 BaCO3

1280

20

40

60

80

2 /°

(d)

SiO2

390

Intensity

Ca2MgSi2O7 CaSiO3

260

BaCO3 BaSO4

130

BaCl2

0 20

40

60

80

2θ/° Fig. 6. XRD patterns of cotton stalk ashes formed at 1000 °C: (a) no additive; (b) CaO (Ca/Cl = 1.5); (c) BaCO3 (Ba/Cl = 1.5); (d) BaCO3 (Ba/Cl = 2.5).

(a) Intensity

CaSiO3

480

160

3.2.1. Combustion characteristic analysis TG/DTG profiles of combustion of cotton stalks and maize straw with and without BaCO3 are shown in Fig. S3. The molar ratio of Ba/ Cl was 1.5. Three characteristic temperatures were identified including the ignition temperature (Ti), the burnout temperature (Tf), and the peak temperature (Tmax). The maximum combustion rate (DTGmax), the average combustion rate (DTGmean), and the combustion characteristic index (S) were used to evaluate the combustion process more reasonably [32,33]. The higher the combustion rate, and the value S, the better the combustion activity of fuel. The combustion characteristic parameters calculated on the basis of thermogravimetric analysis are listed in Table 2.

Intensity

CaMg2Si2O7

240

3.2. Thermodynamic analysis

20

SiO2

720

Intensity

a Ba/Cl molar ratio of 2.5 (Fig. 5(b)). The intensity of CaAl2Si2O8 got stronger. Except the excessive unreacted component of the BaCO3, the chlorine contents were transformed into BaCl2 and Ba(ClO2)2. As shown in Fig. 6(a), the SiO2 intensities were decreased as the combustion temperature increased to 1000 °C. The silicon was more inclined to combined with alkaline-earth metals to produce alkali silicates. Fig. 6(b) shows the XRD patterns of the cotton stalk and CaO in a Ca/Cl molar ratio of 1.5. There was no big difference in composition between them. The intensities of CaCl2 peaks were so weak to detect. Fig. 8(c) and (d) are the main composition of the cotton stalk and BaCO3 in a Ba/Cl molar ratio of 1.5 and 2.5, respectively. There was no obvious detection of barium-chloride compounds when the BaCO3 addition ratio of 1.5. However, a relatively strong intensity of BaSO4 existed. This phenomenon demonstrated that Ba2+ was preferentially combined with SO24 . When BaCO3 addition ratio increased to 2.5, both peak intensities of BaSO4 and BaCl2 was stronger. The XRD patterns illustrated that chlorine in biomass retained in ashes in the form of BaCl2 or Ba(ClO2)2, and BaCO3 had the ability of fixing sulfur and chlorine simultaneously. According to element equilibrium, assuming the chlorine exist in the form of KCl in biomass, the chlorine fixation mechanism is summarized in R7 and R8.

Intensity

86

BaCl2

The influence of BaCO3 on Ti and T1max were not apparent. The addition of BaCO3 accelerated the average combustion property to some degree. The DTG1max and DTG2max of raw biomass samples increased. And the DTGmean and the S were also increased. The catalyst effect of BaCO3 on biomass combustion are mainly caused by two aspects: firstly, as an alkali salt, BaCO3 promotes the emission of volatile matters in a certain degree. It prevents the formation of stable chemical structures, which reduces the ignition temperature. Secondly, complex salts are formed by reaction between basic ions in BaCO3 and oxygenic base on the surface of char. The BaCO3 can be a carrier of oxygen when the oxygen is absorbed on the surface, promoting the transfer of oxygen to the fixed carbon, finally attributes to the fixed carbon combustion [34,35].

Ba(ClO2)2

640 0 20

40

60

80

2 /° Fig. 5. XRD patterns of cotton stalk ashes formed at 800 °C: (a) no additive; (b) BaCO3 (Ba/Cl = 2.5).

3.2.2. Kinetic parameter analysis The kinetic equation of weight loss can be formulated as the function of reaction rate constant (k(T)) described by the Arrhenius equation and differential conversion function (f(a)) [36,37].


 da E f ðaÞ ¼ kðTÞf ðaÞ ¼ A exp  RT dt

ð3Þ

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Q. Wang et al. / Fuel 208 (2017) 82–90

5 Equilibrium amount/kmol

Equilibrium amount/kmol

(b)

(a)

4 3 2 1 0

4 3

BaCl2

2

BaCl2(g)

KCl KCl(g)

K4P2O7 N4H4Cl8

1 0

500

600

700

800

900

500

1000 1100 1200

600

700

900

10

(c)

6

800

1000 1100 1200

T/°C

T/°C

5

Equilibrium amount/kmol

Equilibrium amount/kmol

HCl(g) KPO3

K3PO4

4 3 2 1

(d)

8 6 4 2 0

0 500

600

700

800

900

1000 1100 1200

500

600

700

T/°C

800

900

1000 1100 1200

T/°C

Fig. 7. The major potassium and chlorine products in the simulation results: (a) KCl: NH4H2PO4:BaCO3 = 10 kmol:10 kmol:10 kmol; (b) KCl: NH4H2PO4:BaCO3 = 10 kmol:10 kmol:15 kmol; (c) KCl: NH4H2PO4:BaCO3 = 15 kmol:10 kmol:10 kmol; (d) KCl: NH4H2PO4:BaCO3 = 10 kmol:15 kmol:10 kmol.

25

10

(a)

(b)

KCl:NH 4H2PO4:BaCO3=10:10:10 KCl:NH 4H2PO4:BaCO3=10:10:15

20

8

KCl:NH 4H2PO4:BaCO3=15:10:10

15 10

KCl(g)/%

KCl/%

KCl:NH 4H2PO4:BaCO3=10:15:10

6 4 2

5

0 0 500

600

700

800 900 T/°C

1000 1100 1200

500

80

(c)

70

40

60

35

50

HCl(g)/%

BaCl 2 /%

45

30 25 20

700

800 900 T/°C

1000 1100 1200

700

800

1000 1100 1200

(d)

40 30 20

15

10

10 5 500

600

0 600

700

800

900

T/°C

1000 1100 1200

500

600

900

T/°C

Fig. 8. The conversion rate of products with different additive ratio: (a) KCl; (b) KCl(g); (c) BaCl2; (d) HCl.

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Q. Wang et al. / Fuel 208 (2017) 82–90

Table 2 Combustion characteristics parameters of samples. Samples

Ti/°C

Tf/°C

T1max/°C

DTG1max/(%/s)

T2max/°C

DTG2max/(%/s)

DTGmean/(%/s)

S  109/(%2/(s°C3))

Cotton stalk Cotton stalk-BaCO3 Maize straw Maize straw-BaCO3

291 289 274 270

548 515 534 514

316 315 306 303

1.179 1.228 0.826 1.224

420.3 415.3 459 450.6

0.673 0.695 0.227 0.237

0.106 0.121 0.108 0.117

2.687 3.466 2.217 3.837

where a is the mass loss ratio; t is time; A is the frequency factor; E is the activation energy; R is the ideal gas constant, 8.31 J/(molK); T is the reaction temperature. In non-isothermal kinetics, the heating rate (b = dT/dt) is constant in a linear heating program, the above equation is transformed into:


 da A E f ðaÞ ¼ exp  RT dT b

ð4Þ

In Coats–Redfern method, f ðaÞ ¼ ð1  aÞn is selected to be the reaction mechanism function. n is the reaction order. Previous work assumed reaction order of biomass combustion is about 1 [38]. However, considering the diversity of biomass, the regression line corresponding to combustion patterns could be achieved by optimizing the reaction order of n from 0.33, 0.5, 0.67, 1, 1.25, 1.5 and 2 in this study. Through variable separation, integration, logarithm, and simplification [37,41], Eq. (4) leads to:

ln

ln

  lnð1  aÞ T2

¼ ln

" # 1  ð1  aÞ1n T 2 ð1  nÞ Defining ln

h

AR E  bE RT

¼ ln

AR E  bE RT

ðn ¼ 1Þ

ð5Þ

ðn–1Þ

ð6Þ

i h i aÞ1n ðn ¼ 1Þ or ln 1ð1 ðn–1Þ; x = 1T ; a =  RE T 2 ð1nÞ

lnð1aÞ T2

AR and b ¼ ln bE0 , a fitting line y = ax + b can be plotted, the slope (a)

and intercept (b) estimate values of E and A, respectively. The most satisfactory reaction order and regression line are selected and calculated through the criterion of the correlation coefficient (R2). The deduced data and fitting lines during two intense combustion stages are given in Table 3 and Fig. S4. Whether adding BaCO3 or not, it was observed that the reaction order of the first stage (0.33) was lower than the second stage (2), and activation energy of first stage was much higher than that of the second stage. The addition of BaCO3 presented a slight decrease of activation energy compared to the raw sample. BaCO3 weak the formation of stable chemical structure, therefore the activation energy of complex pyrolysis reaction is decreased. It also decreased the activation energy of C þ O2 ! CO2 . 3.3. KCl transformation by both NH4H2PO4 and BaCO3 3.3.1. Simulation of KCl, NH4H2PO4 and BaCO3 Theoretically, the NH4H2PO4 should improve potassium fixation but induce high HCl release. In this work, BaCO3 could accelerate

chlorine fixation. However, these two additives on potassium and chlorine transformation and competitive mechanism are not considered. Most chlorine in biomass was released by evaporation of KCl. So equilibrium amounts major K and Cl products between KCl, NH4H2PO4 and BaCO3 was predicted by HSC Chemistry. The initial reactant molar ratio of K/Cl:P:Ba was 10:10:10, and then increased 1.5 times of each reactant. The results are shown in Fig. 7. In Fig. 7(a), most KCl were transformed into KPO3, K3PO4 and K4P2O7, accompanied by the byproduct of N4H4Cl8. The N4H4Cl8 was not stable and then decomposed into HCl until 950 °C. KCl(g) was released after 1000 °C. The BaCl2 and KPO3 got highest at 600 °C, then decreased with the slight generation of HCl(g) and BaCl2(g). As a result, the addition of NH4H2PO4 and BaCO3 in a molar ratio of K/Cl:P:Ba = 10:10:10 cannot control the HCl(g) perfectly. when the additive amount of BaCO3 is 15 kmol, the results are shown in Fig. 7(b). The productive amount of HCl(g) was below 0.5 kmol even the temperature was 1200 °C. No more BaCO3 existed, and this was due to excessive barium were combined with KPO3 or NH4H2PO4 to produce Ba3(PO4)2, so the potassium were mainly in the form of K3PO4. When the additive amount of KCl is set to 15 kmol, the results are shown in Fig. 7(c). Except the reacted KCl, the excessive chlorine reacted with BaCO3 to produce BaCl2, and potassium were combined with NH4H2PO4 to produce K3PO4. The content of KCl decreased sharply with the increased temperature until 800 °C. The amount of HCl(g) was not increased greatly due to the production of stable BaCl2, which was stable even in the high temperature. Fig. 7(d) show the simulation results when the initial amount of NH4H2PO4 is 15 kmol. On the one hand, phosphorus combined with potassium to produce KPO3, no more other forms of potassium phosphates exist; on the other hand, excessive phosphorus was inclined to combine with Ba2+ to produce Ba3(PO4)2, so the amount of BaCl2 was relative low, and the introduction of NH+4 accelerated the HCl(g) generation. However, both KCl and KCl(g) were low, which illustrated the good ability of NH4H2PO4 on KCl transformation. 3.3.2. The comparison of KCl transformation In order to make a comparison of KCl transformation quantitatively, the conversion rates of four products (KCl, KCl(g), BaCl2 and HCl(g)) are defined by the specific value between production amount and original additive KCl amount (KCl = 10 kmol or 15 kmol). The conversion rats are shown in Fig. 8.

Table 3 Combustion kinetic parameters of samples. Samples

Temperature range/°C

n

E/(kJ/mol)

A (s1)

R2

Cotton stalk

267–328 419–443

0.33 2

76.867 16.219

1.35E + 05 1.11E  01

0.995 0.973

Cotton stalk-BaCO3

267–328 419–443

0.33 2

70.146 10.371

3.36E + 04 2.73E  02

0.992 0.979

Maize straw

245–316 383–477

0.33 2

78.746 2.822

2.94E + 05 1.98E  03

0.994 0.990

Maize straw-BaCO3

245–316 383–477

0.33 2

66.243 2.750

1.71E + 04 1.83E  03

0.996 0.986

Q. Wang et al. / Fuel 208 (2017) 82–90

30

Ba/Cl=3.5 24

Ba/Cl=2.5 η/ %

18

Ba/Cl=1.5 cotton stalk-NH4H2PO4

12

(P/K=1.5) 6 0 -0.5

0.0

0.5

Ba/K

1.0

1.5

2.0

89

curves of bCaO and bCaCO3 presented a distinct upside down ‘‘V” in the investigated 500–1000 °C, reached highest at around 800 °C. The performances of BaCO3 on chlorine fixation was better than CaO and CaCO3 especially at temperature range between 900 and 1000 °C. XRD patterns of the retained ashes illustrated that barium chlorides were mainly BaCl2 and Ba(ClO2)2. In addition, obvious peaks of BaSO4 showed the ability of BaCO3 on fixing both chlorine and sulfur simultaneously. Thermal analysis results show that BaCO3 increased DTGmean and S, decreased E to some degree, so the BaCO3 could be considered as a catalyst during combustion. The reactivity between elements in KCl, NH4H2PO4 and BaCO3 were deduced as: Cl-Ba < Ba-P < K-P. Over excessive addition of BaCO3 may influence the production of potassium phosphates, the appropriate additive ratio could be K:P:Ba = 1:1.5:1.

Fig. 9. Effect of Ba/K on g for cotton stalks with NH4H2PO4 combustion.

Acknowledgements When the temperature was below 800 °C, the conversion rate of KCl was high. Then the decreasing speed became flatten due to the reaction became difficult. The additive ratio of NH4H2PO4 had a great influence on chlorine contents. When the initial NH4H2PO4 amount was 15 kmol, both amount of KCl retention amount and KCl(g) conversion amount was lowest, showing a good potassium transformation ability of NH4H2PO4. However, the BaCl2 conversion ratio was lowest, the highest value was only 22.41% when the temperature was 700 °C. Moreover, chlorine content were released in the form of HCl(g), when the temperature was higher than 950 °C, more than 75% of chlorine content in KCl were transformed into HCl. In general, due to the high temperature stability of BaCl2, the BaCl2 conversion rates were mainly influenced by additive ratio rather than temperature. The higher BaCO3 was, the lower HCl(g) released. However, excessive BaCO3 were inclined to combined with phosphorus. And the reactivity between elements are deduced as: Cl-Ba < Ba-P < K-P. The excessive addition of BaCO3 could accelerate KCl(g) release ratio. When BaCO3 was 15 kmol, the incensement of KCl(g) release was only less than 1% even at 1200 °C. However, individually increase the amount of NH4H2PO4 could increase the HCl(g) greatly. 3.3.3. The experiment of chlorine fixation by NH4H2PO4 and BaCO3 From the simulation results, the control of HCl should be considered when NH4H2PO4 was used as a potassium fixed additive. The simulation assumed that potassium and chlorine in biomass were all in the form of KCl. However, the elements in biomass is complex and the K/Cl molar ratio is not 1. Fig. 9 shows the chlorine fixation ratio of cotton stalk with both NH4H2PO4 and BaCO3 on the basis of K:P:Ba = 1:1.5:0, 1:1.5:0.5, 1:1.5:1 and 1:1.5:1.5. So the molar ratio of Ba/Cl was calculated as 1.87, 3.75 and 5.62 respectively. The combustion temperature was 1000 °C. From the bottom to top, the horizontal dash lines were used to make a comparison, they were the relative g of cotton stalks only with BaCO3 in different Ba/Cl (as shown in Fig. 4). The g of cotton stalks only with NH4H2PO4 was lower than 5%, however, after the addition of BaCO3, the chlorine fixation rate increased obviously. But the g on the basis of Ba/K = 0.5 was still lower. When the addition ratio of Ba/K was 1.5, the g was 25.2%. In this research, K:P:Ba = 1:1.5:1 was relative reasonable, the g was around 22.2% when both NH4H2PO4 and BaCO3 were added. Excessive addition of BaCO3 may influence the production of potassium phosphates. And the effect of volatile combustibles is proposed to study in the future. 4. Conclusion The chlorine content in biomass was released to the gas phase in significant amounts at 600–700 °C during combustion. The

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