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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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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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