Investigation of Maize Straw Char Briquette Ash Fusion Characteristics

Feb 23, 2017 - microscopy with energy-dispersive spectrometry, Fourier transform infrared spectroscopy, and X-ray fluorescence. The high fusion tenden...
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Investigation of MS char briquettes ash fusion characteristics and the influence of phosphorous additives Qian Wang, Kuihua Han, Jie Gao, Jiamin Wang, and Chun-mei Lu Energy Fuels, Just Accepted Manuscript • DOI: 10.1021/acs.energyfuels.7b00047 • Publication Date (Web): 23 Feb 2017 Downloaded from http://pubs.acs.org on February 27, 2017

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Investigation of MS char briquettes ash fusion characteristics and the influence of phosphorous additives Qian Wang, Kuihua Han*, Jie Gao, Jiamin Wang, Chunmei Lu1 1

School of Energy and Power Engineering, Shandong University, Jinan, China *Corresponding author ([email protected])

Abstract The purpose of this article is to explore the fuel and ash characteristics of maize straw char briquettes. The bio-char briquettes were made from different pyrolysis temperatures and additives. The pyrolysis and combustion process were performed in a fix bed furnace system. Ash fusion temperatures were tested in an ash fusion point analyzer. Pyrolysis product property under different pretreatment were analyzed by a combination of XRD, SEM-EDS, FTIR and XRF. High fusion tendency of MS char was observed thorough ash fusion indexes. The ST decreased with a deduced base-to-acid and elevated pyrolysis temperature. Aromatization became obviously in a high temperature pyrolysis product. KH2PO4 and Ca(PO3)2 were detected in MS-ADP and MS-CPM, accompanied by the decreasing intensity of KCl. Phosphorous grains attached on fibers, reacted and transformed the K element into K-Ca-P compounds. As a result, ash fusion temperatures and fusion phenomena were greatly improved by adding NH4H2PO4 and Ca(H2PO4)2 in the pretreatment. Key words Bio-char briquette; pyrolysis; ash fusion temperature; additives 1. Introduction Biomass is regarded as one of the renewable energy resources to replace the traditional fossil fuel in the aspect of carbon credit [1]. Agricultural residuals and herbaceous biomass distribute intensively and can be utilized both in small domestic

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stoves and large scale power industries. However, the raw biomass may be not suitable for direct thermal conversion, due to the low heating values, low energy volumes and difficulty in storage and transportation [2]. Based on this, it is inconvenient to use the raw biomass directly in many applications without any pre-treatment and upgrading treatment [37]. Densification and pyrolysis are two available upgrading technologies to improve the properties of biomass. The physical and combustion behavior are improved through densification, contributing to larger variety of lignocellulosic materials as fuels [3]. The decomposition of cellulose and hemicellulose during pyrolysis promotes the homogenization of biomass [4]. Use of biomass char pyrolysis from 250-350°C has the benefits in lower SO2 and NOx pollutant emission factors rather than raw biomass in power plants [38]. High grade and value fuel can be obtained through a combination of these two pretreatments. Torrefied pellets can improve volumetric energy density and decreases the transportation cost especially for long distance transportation dramatically [5]. China, as a great agricultural country, the development of pyrolysis briquettes can improve the profitability of pyrolysis residual agricultural briquettes exported to other countries. On the other hand, bio-char briquettes even has the potential for replacing the natural gas and oil in power generation [6]. Typically, the production chain of torrefied biomass pellets are usually torrefaction firstly and then densification. However, fiber structure and moisture content changes in pyrolysis, bad cohesiveness and flow ability induces worse compaction characteristic of bio-char compared to parent materials, so pyrolysis biomass is not a suitable resource for densification [7]. The durability and strength of the pellets are significantly improved by the additional pyrolysis tar [8]. So, a reverse procedure of densification and pyrolysis, utilizing the pyrolysis tar as binders, should be considered

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as a possible method to produce the bio-char pellet/briquette with a better physical property. However, ash-related problems such as ash-slagging, sintering and fouling induced by potassium and sodium elements still do not be inhibited by pyrolysis [2]. Compared with other methods to mitigate ash related problem, mixing mineral based additive is an attractive solution due to its easy operation. Adding additives in the densification process to study the conversion mechanism in the pyrolysis, which finally improve ash fusion phenomenon is an effective way. Phosphorus tends to dominate over silicon element for alkali capture, contributing to the formation of high temperature melting calcium potassium phosphate [9]. Thus, phosphorus based additives have benefit influences on ash related problems [11-12]. Li [10] have researched the NH4H2PO4 could decreased the emission of gaseous KCl. Qi [31] found that NH4H2PO4 increased the ash fusion temperatures of five kinds of biomass in different degrees. Calcium element contribute to the formation of high temperature melting calcium potassium phosphate and silicates, so combination of calcium and phosphorus as additives (such as Ca(H2PO4)2 may improve the ash melting phenomenon [12,35]. In addition, Ca(H2PO4)2 can increase carbon retention and strength bio-char stabilization [36]. Although both NH4H2PO4 and Ca(H2PO4)2 can improve the ash property during biomass combustion, little research focuses on the influence of phosphorous based additives on the pyrolysis process and product property. Although Li researched that NH4H2PO4 decreased gaseous potassium release ratios during the carbonization process [13], the influence of additives on the bio-char ash melting properties were not considered. Based on the above description, maize straw, as a typical kind of agricultural residual with low ash fusion temperature is investigated in this research. The influence of

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pyrolysis temperature on micromorphology, crystal structure, functional group, ash composition and the ash fusion temperatures were analyzed. The influence phosphorus based additives on the pyrolysis, the ash fusion property and the mechanism were explained. Finally, a fix bed combustion system was utilized to observe the morphology of combustion products. 2. Materials and methods 2.1 Samples In this study, the raw maize straws (MS) were collected from Shandong province. The potassium content was 1.8%, and was measured by inductively coupled plasma– atomic emission spectrometry (ICP–AES, Thermo Fisher Scientific, IRIS). Two phosphorous based additives, calcium phosphate monobasic (Ca(H2PO4)2, CPM in abbreviation) and ammonium dihydrogen phosphate (NH4H2PO4, ADP in abbreviation) (Kemiou Chemical Reagent Co.) were mixed mechanically with pulverized MS. A previous study by members of our research group indicates a molar ratio of PO43−/K between 1:1 and 2:1 was a satisfying addition ratio [10]. So the P/K molar ratio is 1.5 in this research. The briquettes were produced in a cylindrical shape mold, and the bio-char briquettes are acquired from directly pyrolysis of biomass briquette in a fix bed pyrolysis system under N2 atmosphere. The flow rate of N2 was 2L/min. The char preparation process and fix bed pyrolysis system is shown in Figure 1(a).The pyrolysis temperature were settled from room temperature to 250, 350, 450, 550 and 650 °C with a heating rate of 10 °C/min and a duration time of 1 hour. The acquired char briquettes are denoted as A-MS-B, where A represents pyrolysis temperature, B represents the additive of ADP or CPM. The proximate and ultimate analysis of acquired MS char briquettes are listed in Table 1, and the fuels are shown in Figure 1(b). It is obviously that the surface color of 250-MS-char is dark brown, which is

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different from the MS char briquettes acquired from higher pyrolysis temperature. With the increased pyrolysis temperature, the volumes of the briquettes present decreasing tendency. 2.2 Analysis methods The main phase analysis of the bio-char was identified XRD (X-ray powder diffractometry, D/MAX-2500PC). The Fourier transform infrared spectrometry (FTIR, VERTEX 70) was used to record the absorption peaks in the spectral region from 4000 to 400 cm-1. The MS char morphology analysis and the element distribution in the bio-char particle and were performed by using a scanning electron microscopy with energy dispersive spectrometer (SEM-EDS, ZEISS Supra 55). The main chemical composition of bio-char briquettes ashes (in the form of oxides) were accomplished with XRF (X-ray fluorescence, ZSX Primus II). The ashes used for XRF test were prepared according to GB/T 28731-2012 (China), after the samples combust at 550 °C for 2 hours. 2.3 Ash fusion test The ash fusion temperatures are described by four characteristic temperatures: initial deformation temperature (DT), softening temperature (ST), hemispherical temperature (HT) and flow temperature (FT). The test method was in accordance with GB/T 30326-2014 (China), the deformation tests of ash cone were recorded by an automatic ash fusion temperature analyzer (YX-HRD). ST was widely regarded as an evaluation index in coal analysis. However, due to the lack of sensitivity of ST, HT and FT in biomass ashes, DT was suggested to be evaluation index by researchers [14]. To ensure the accuracy of test, each sample was tested three times, the average value is calculated. Ash fusion tendencies were applied to predict through the use of several criterion

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indexes for MS char ashes. The inclinations of ash fouling degree in the previous research are listed in Table 2, and the index calculation formulas are expressed below [15-17]. Although the biomass composition is various, these indexes are wildly used as a secure decision of fusion tendency. In addition, the ash fusion relationships between fusion temperature and chemical composition can be determined. 2.4 Combustion process The fix bed furnace system is shown in Figure 1(a). The porcelain boats with char briquette samples were placed in a fix bed combustion system. The atmosphere was provided by N2 and O2 cylinders with a flow ratio of 79/21, and the total flow rate was 2L/min. The combustion temperature was 1000 °C and the samples were exposed in high temperature atmosphere for 30 minutes. After cooling down, the visual observation of heating ashes was investigated to investigate the phase transformation. 3. Results and discussion 3.1 Influence of pyrolysis temperature on ash compositions and fusion tendency The ash composition of major inorganic elements in the form of oxides acquired from XRF analysis is shown in Figure 2. Like most biomass ash, the MS char ashes still compose high content of SiO2, K2O and CaO, high concentration of these contents are contributed to the formation of low-melting temperature products, which are negative for boiler surface [18]. The chlorine content in the bio-char ash are relative high, the presence of Cl in the form of KCl or NaCl accelerate the corrosion and ash deposition rates, even the temperature is below the fusion point of pure salts [19]. Ash fusion indexes calculated according to ash contents by formulas are listed in Table 3. All the raw MS char samples are in the range of high fusion inclination area ( RB / A >0.7, Fu >40, SR ≤65, SiO2/Al2O3>2.65, AI>0.34, ST1390 1260~1390 40

SR

>72

65~72

≤65

SiO2/Al2O3

2.65

AI

0.34

base-to-acid: RB / A = RB / RA = ( Fe2O3 + CaO + MgO + Na2O + K 2 O + P2O5 ) / ( SiO2 + Al2 O3 + TiO2 ) fouling index: Fu = RB / A ( Na2O + K 2O) base-to acid: RB / A+ P = ( Fe2O3 + CaO + MgO + Na2O + K 2O + P2O5 ) / ( SiO2 + Al2O3 + TiO2 ) slag viscosity index: S R = SiO2 × 100 / ( SiO2 + Fe2O3 + CaO + MgO)

alkali index: AI = ( K 2 O + Na2 O )gA / Qar ,gr , A-ash content, %; Qar ,gr -gross calorific value as dried basis, GJ/kg

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Table 3. Ash fusion indexes of MS-char briquette samples acquired from different pyrolysis temperature 250-MS 350-MS 450-MS 550-MS 650-MS RA

21.529

22.225

22.743

23.373

24.507

RB

67.821

67.316

67.232

66.010

64.977

RB / A

3.150

3.029

2.956

2.824

2.651

Fu

139.555 131.452 126.228 118.899 106.850

SR

46.569

46.990

47.052

48.370

48.672

SiO2/Al2O3 27.517

27.858

31.277

31.417

30.668

2.605

3.743

4.600

5.164

5.111

1081

1050

1039

1030

958

AI ST

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Table 4. Location of indicator bands, corresponding the vibration and assignment to functional group Location wavenumber/cm-1

Functional group

790

C-H bending vibration

1031

Si-O vibration and P-O stretching

1440

Aromatic C=O stretching

1573

Aromatic C=C vibration

1636

Alkenes C=C stretching

1710

Carboxyl C=O stretching

2916

C-H stretching

3330

O-H stretching

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Table 5. Ash fusion temperature of MS-char briquette samples acquired from different situations DT

Pyrolysis

ST

HT

FT

temperature

MS

250

1006

1198

1274

1081

1340

1409

1127

1451

1423

1189

1493

1500

350

988

1160

1254

1050

1327

1380

1108

1478

1410

1190

1485

1450

450

975

1141

1244

1039

1355

1370

1057

1450

1404

1190

1488

1424

550

958

1137

1175

1030

1380

1378

1068

1340

1411

1198

1435

1495

650

920

1327

1300

958

1448

1380

1128

1485

1426

1201

>1500

>1500

MS-ADP MS-CPM

MS

MS-ADP MS-CPM

MS

MS-ADP MS-CPM

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MS-ADP MS-CPM

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List of Figures Figure 1. Schematic diagram of MS char briquette samples production procedure (a) fix bed furnace system (b) visual observation of acquired MS-char briquettes samples Figure 2. Ash composition of MS-char briquette samples acquired from different situations, wt% Figure 3. Relationships between DT/ST between (a) RB / A and (b) pyrolysis temperature Figure 4. FTIR spectra of MS char briquette samples acquired from different situations (a) MS; (b) MS-ADP; (c) MS-CPM Figure 5. X-ray diffraction analysis of MS char briquette samples acquired from different situations (a) MS; (b) MS-ADP; (c) MS-CPM Figure 6. SEM observation of MS char briquette samples acquired from different situations Figure 7. SEM and EDS qualitative maps of 650-MS-CPM at high phosphorus position Figure 8. The appearance of MS char briquettes ashes after combusting under 1000 °C

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Figure 1. Schematic diagram of MS char briquette samples production procedure (a) fix bed furnace system (b) visual observation of acquired MS-char briquettes samples

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50

MgO Al2O3 SiO2

40

P2O5 ash composition/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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SO3

30

Cl K2O CaO TiO2

20

MnO Fe2O3

10

others 0 250-MS

350-MS

450-MS samples

550-MS

650-MS

Figure 2. Ash composition of MS-char briquette samples acquired from different situations, wt%

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(a) 1092

pyrolysis temperature 250 °C

450 °C 550 °C

975 650 °C

2.64

ST DT Linear fit of ST, R2=0.929 Linear fit of DT, R2=0.936

1053 temperature/°C

350 °C

1014

936

(b) 1092

ST DT Linear fit of ST, R2=0.988 Linear fit of DT, R2=0.986

1053 temperature/°C

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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1014

975

936

2.76

2.88

3.00

3.12

200

RB/A

300

400 500 pyrolysis temperature/°C

600

700

Figure 3. Relationships between DT/ST between (a) RB / A and (b) pyrolysis temperature

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(a) 1.2

3330

2916

1573

1440

650-MS

1031 790

(b) 1.2

550-MS

1573

1031

550-MS-ADP

0.6 450-MS 1710 350-MS

0.6 450-MS-ADP 1710

0.4 350-MS-ADP

1636

0.2

0.2 250-MS-ADP

250-MS

0.0

0.0 4500

4000

(c) 1.2

3500

3330

3000

2916

2500 2000 -1 1500 wave number/cm

1610

1440

1000

500

4500

4000

3500

3000

2500 2000 1500 wave number/cm-1

1031

650-MS-CPM 1.0 550-MS-CPM 0.8

0.6 450-MS-CPM 1710 0.4

1440

0.8 transmittance/%

transmittance/%

2916

1.0

0.8

0.4

3330 650-MS-ADP

1.0

transmittance/%

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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350-MS-CPM

0.2 250-MS-CPM 0.0 4500

4000

3500

3000

2500 2000 1500 wave number/cm-1

1000

500

Figure 4. FTIR spectra of MS char briquette samples acquired from different situations (a) MS; (b) MS-ADP; (c) MS-CPM

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1000

500

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250-MS

1300

250-MS-ADP

(b) 2000

(a) 2600 1950

002 101

650 002

1140

350-MS-ADP

350-MS

0 KCl SiO2

760

450-MS

0

002

1140

450-MS-ADP

380 KCl SiO2

760 380 002

550-MS-ADP

550-MS

0 1140

KCl SiO2

760 380

650-MS

002

100

380 0 10

20

30

40

50 2θ/(°)

60

70

80

1000

90

KH2PO4

101

500 0

002

1140

KCl KH2PO4

760 380 0 1140

002

KCl KH2PO4

760

SiO2

380 0 1140

002

KCl KH2PO4

760

SiO2

380 0

1140

KCl SiO2

760

002

1500

650-MS-ADP

0 1140

KCl KH2PO4

002

760

0 10

SiO2

100

380 20

30

40

50 2θ/(°)

60

70

80

550-MS-CMP

450-MS-CMP

350-MS-CMP

250-MS-CMP

(c) 2000

650-MS-CMP

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002

1500 1000

KCl SiO2

101

500 0 1140

002

KCl SiO2

760

Ca(PO3)2

380 0

002

1140 760

KCl SiO2

380

Ca(PO3)2

0 1140

002

KCl SiO2

760

Ca(PO3)2

100

380

CaCl2

1400 0 1050

KCl SiO2

002

700

0 10

Ca(PO3)2

100

350 20

30

40

50 2θ/(°)

CaCl2 60

70

80

90

Figure 5. X-ray diffraction analysis of MS char briquette samples acquired from different situations (a) MS; (b) MS-ADP; (c) MS-CPM

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Figure 6. SEM observation of MS char briquette samples acquired from different situations

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Figure 7. SEM and EDS qualitative maps of 650-MS-CPM at high phosphorus position

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Figure 8. The appearance of MS char briquettes ashes after combusting under 1000 °C

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