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Effect of pyrolysis temperature on lignite char properties and slurrying ability
Fuel Processing Technology 134 (2015) 52–58
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Effect of pyrolysis temperature on lignite char properties and slurrying ability Yan Li 1, Zhi-Hua Wang ⁎,1, Zhen-Yu Huang 1, Jian-Zhong Liu 1, Jun-Hu Zhou 1, Ke-Fa Cen 1 State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 2 April 2014 Received in revised form 6 January 2015 Accepted 8 January 2015 Available online 3 February 2015 Keywords: Lignite Pyrolysis Lignite char–water slurry Slurrying ability Functional group Pore structure
a b s t r a c t This work investigates the influence of different pyrolysis temperatures in lignite char properties and slurrying ability. Baorixile lignite was pyrolyzed under nitrogen atmosphere from low to high temperatures. The pyrolyzed chars were then used in preparing lignite char–water slurry fuels. The apparent viscosities and rheological behaviors of different slurries were obtained by using a rotating viscometer. Pyrolysis can effectively improve lignite char–water slurry concentration, and the influence of such mechanism can be determined by analyzing oxygen-containing functional groups and pore structures. Result suggests that pyrolysis not only removes the moisture content but also causes an apparent increase in lignite coal rank. Pyrolysis also evidently reduces the number of oxygen-containing functional groups and the hydrophilicity of lignite char, consequently improving slurry concentration but decreasing slurry static stability. The pore structure of the lignite char changes significantly after pyrolysis. With increasing pyrolysis temperature, average pore diameter initially decreases and eventually increases, whereas specific surface area and pore volume exhibit an opposite trend. Experimental results reveal that the slurry concentration of lignite char does not increase monotonically with the increase of pyrolysis temperature and 600 °C–800 °C is the most suitable pyrolysis temperature range for lignite slurrying ability improvement. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Coal water slurry (CWS), as a fuel derived from coal, consists of 60%– 70% coal, 29%–39% water, and ~1% additives. This fuel was first developed as an oil replacement for slurry combustion during the energy crisis in the 1980s. Since then, CWS has been widely used in combustion and gasification process. With the rapid development of integrated gasification in combined cycle (IGCC) and coal gasification in the recent years, CWS fuel has been recognized as an important fuel for high-pressure gasifier with high energy efficiency. CWS feedstocks have been extended from bituminous coal to petroleum coke [1] and lignite [2] because of the short supply of high-rank coal. Given its high pore volume and high moisture content, it was extremely challenging for lignite to produce highly concentrated CWS fuel. Thus, the use of low-rank coal, which has abundant reserves accounting for approximately 53% of the global total coal reserve, has attracted the attention of researchers around the world. Various lignite upgrading technologies have been reported in the literature [3–5]. Lignite is characterized by having an enriched volatile
⁎ Corresponding author at: Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, China. E-mail address: [email protected] (Z.-H. Wang). 1 Tel.: +86 571 87,953,162; fax: +86 571 87951616.
http://dx.doi.org/10.1016/j.fuproc.2015.01.007 0378-3820/© 2015 Elsevier B.V. All rights reserved.
matter content; a high H/C ratio; a large amount of carboxyl groups, carbonyl groups, and methylene; and a positive reactive behavior. As such, lignite is highly reactive during pyrolysis. Pyrolysis has been widely investigated as a thermal treatment upgrading method that can upgrade lignite quality in terms of heat value, stability, and moisture absorption [6–8]. After being pyrolyzed, lignite can be converted into liquid, gas, and solid products. Kaji et al. [9] found that after pyrolysis at 400 °C, the water-holding capacity of lignite char decreased by more than two-thirds compared to that of raw lignite. Previous studies indicate that pyrolysis can increase specific surface area [10] and reduce oxygen content [11], which is preferable for making high-concentration lignite char–water slurry (LCWS). Fan et al. [12] investigated the additive effects on slurrying activity and pointed that lignite char–water slurrying ability is considerably higher than that of raw coal. However, most of the coal pyrolyses were conducted at a low or moderate temperature range [13,14], lacking the understanding of high temperature pyrolyzed char properties. The primary aim of this work is to investigate the effect of the pyrolysis temperature, from low to high range, on slurrying ability of lignite char by using selected additives as dispersant and stabilizer to gain insight into the global comprehensive utilization of lignite. Baorixile (BRXL), a typical lignite with total water concentration of 25.68% from Inner Mongolia, where it is one of the largest reserve areas of lignite in China, was studied in this article. Pyrolysis was performed in the absence of oxygen and at a temperature ranging from 400 °C to 1000 °C. The influence of different pyrolysis temperatures on LCWS
Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
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Fig. 1. Schematic diagram of the experimental setup.
properties, including slurry concentration, rheological behavior, and stability, was investigated. 2. Material and methods 2.1. Experimental setup As Fig. 1 shows, the experimental setup, which consists of a char reactor with power supply, a LCWS preparation process, and two analysis systems, namely, char/coal property analysis system and LCWS/CWS property analysis system. BRXL was first pyrolyzed in a nitrogen atmosphere furnace at different temperatures. Char properties, involving proximate and ultimate analyses, functional groups, and physical structures were tested accordingly. Pyrolyzed chars were made into LCWS, whose solid loading, rheological behavior, and static stability were evaluated with the use of the LCWS/CWS property analysis system. 2.2. Material In the experiment, air-dried raw coal was milled to particles of less than 75 μm to ensure the thoroughness of pyrolysis and the homogeneity of the lignite char. As the moisture content is one of the main challenges in the preparation of high-concentration LCWS, coal was oven-dried at 105 °C for 2 h; the slurrying abilities of which were then compared with that of the pyrolyzed lignite chars. Results of the proximate and ultimate analyses of raw coal and oven-dried coal are listed in Tables 1 and 2. 2.3. Methods 2.3.1. Pyrolysis process Pyrolysis experiments were conducted in a controlled atmosphere furnace (HMX1700-30). The temperature ranged from 400 °C–
1000 °C, with an interval of 100 °C at a heating rate of 10 °C/min and an isothermal treatment of 30 min. Approximately 300 g of raw coal powder was paved in a ceramic container under N2 gas environment. After a complete purging process by N2 gas, a sufficient amount of N2 was fed into the furnace until a tiny positive pressure was achieved to guarantee an inert atmosphere. 2.3.2. Lignite char–water slurry preparation The LCWS was prepared as follows. First, the required masses of char/coal powder, deionized water, and additives were calculated and weighed in predetermined ratios. A copolymer of methylene naphthalene, sulfonate styrene, and sulfonate maleate (an NDF additive) was adopted as a dispersant and a stabilizer in this experiment. The abovementioned dispersant accounted for 0.8% of the weight of the char/coal samples. The coal/char powder was gradually added into the dispersant solution under low speed agitation of 200 rpm. The solution was then stirred at 1000 rpm for 20 min to ensure that the dispersant solution with coal/char powder is mixed well. During the preparation of LCWS/CWS, the temperature was maintained at 25 °C–30 °C. 2.3.3. Physicochemical characterization of lignite char Functional group analyses of raw/dried coal and lignite chars were performed on a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer. During the test, KBr pellets were prepared by grinding 2.5 mg of specimens with 200 mg of KBr. The FTIR spectra of the specimens for the 4000 cm− 1–400 cm− 1 region were studied by curve-fitting analysis. The pore structure of raw coal and lignite chars was determined by using an automatic surface area and a porosity analyzer (TriStar 3000, MicroMeritics). The samples were kept under 200 °C in a vacuum for 3 h to remove the contaminants inside the pore structures. Then, a 0.5 g sample was retained in liquid nitrogen for analysis. The pore structure properties were assessed during nitrogen absorption and separation
Table 1 Proximate and ultimate analyses of coal and different temperature pyrolyzed chars. Specimen
Raw coal 400 °C char 500 °C char 600 °C char 700 °C char 800 °C char 900 °C char 1000 °C char
Qnet,ad (J/g)
Proximate analysis (%) Mad
Ad
VMd
FCd
8.68 1.41 1.76 1.00 1.04 0.66 0.52 0.62
10.50 12.05 13.63 14.90 15.46 16.03 16.52 16.74
37.89 25.60 15.61 7.17 3.64 2.09 1.65 1.31
51.61 62.35 70.76 77.93 80.89 81.88 81.84 81.95
22,989 27,278 29,000 29,891 29,702 28,930 28,825 28,639
Ultimate analysis (%, ad) C
H
N
St
O
60.06 70.25 75.55 80.02 80.60 82.01 82.30 82.50
3.38 3.51 2.93 1.22 2.00 0.56 0.17 0
0.59 0.91 1.03 0.80 0.91 0.59 0.60 0.46
0.11 0.19 0.18 0.19 0.20 0.18 0.18 0.17
17.59 11.85 5.16 2.02 0 0.08 0 0
Note: ar = as received. ad = air dry basis. d = dry basis. M = moisture content. A = ash content. V = volatile matter. FC = fixed carbon.
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0.25
process. The pore structure and surface area can be obtained by using the BET isothermal equation.
2.3.5. Determining viscosity and rheological behavior The rheological behavior of LCWS/CWS, which refers to the shear stress–shear rate or apparent viscosity–shear rate dependence, was obtained by using a HAAKE VT 550 rotating viscometer. A sufficient amount of LCWS/CWS was poured into the container, and the temperature was maintained at (20 ± 1)°C by using a thermostatic water bath. In accordance with the GB/T18856.4-2002 method [15], the measurement procedure was inputted into the instrument control system: shearing rate is increased from 0 to 100 s−1 at a constant speed and then held constant at 100 s−1 for 5 min. The viscosity data were recorded every 30 s during the shear rate stability period. The apparent viscosity of the slurry was the average value of the 10 recorded viscosity data. After that, the shear rate is dropped to zero at a constant speed. The slurry rheological behavior was determined by the relationship between apparent viscosity and the corresponding shear rate during its rising period. 2.3.6. Static stability measurements The static stability of LCWS/CWS is appraised by the water separation ratio, which is defined as the mass ratio of separated water to the total slurry. In this experiment, a sufficient amount of slurry was sealed in a closed container for 7 d, and water separated after that static storage. The separated water was taken out by a burette. 3. Results and discussion 3.1. Lignite char properties As shown in Table 1, with rising pyrolysis temperature, both moisture content and volatile matter yield decreased, whereas fixed carbon yield increased. This phenomenon is particularly noticeable between the raw coal and the lignite char at 400 °C whose moisture content was reduced from 8.68% to 1.41% air dry basis, indicating that pyrolysis could dehydrate lignite effectively and that the water-fixing ability of lignite would significantly decline after being pyrolyzed. The volatile content is reduced from 37.89% of raw coal to 1.31% of 1000 °C lignite char due to the decomposition of the organic matter [6]. Such volatile content is the most active part of the coal components, and it is usually released at a relatively low temperature. Therefore, a large amount of volatile reduction occurs between 400 °C and 600 °C. Elemental ratio of O/C in specimens is an important parameter in the analysis of slurry making ability. Generally, the higher the O/C ratio is, the worse the slurrying ability is. The O/C ratio in coal greatly reflects the presence of oxygen-containing functional groups, including carbonyl, hydroxyl, and carboxyl. Polar functional group content and O/C ratio decrease with an increase in coal metamorphic rank [16], causing also an increase in slurrying ability. As Fig. 2 shows, the O/C ratio diminished drastically with the rising pyrolysis temperature, which is attributed to
0.20
O/C
0.15
0.10
0.05
0.00 0
100
200
300
400
500
600
700
800
900 1000
o
Pyrolysis temperature ( C ) Fig. 2. O/C ratio in raw coal, dry coal and lignite chars.
the decomposition of oxygen-containing functional groups into gas or tar during pyrolysis. The higher the pyrolysis terminal temperature, the more extensive the decomposition of oxygen-containing functional groups and accordingly the lower the lignite char O/C ratio. The pyrolysis process leads to transformations in lignite resembling those occurring with upgradation of coal rank. 3.2. Chemical characteristics Coal is composed of different macromolecules whose branchedchain structure is quite complicated, typically including hydroxyl, carboxyl, methyl, aromatic rings and so on. Functional group characteristics of samples could directly determine the maximum internal moisture content and the hydrophilic extent of coal, which entirely impacts the slurry making ability. FTIR has been widely used to analyze the functional groups in coal and their variation with different thermal treatments [17,18]. Fig. 3 shows similar functional groups for the dry coal, the 400 °C and 1000 °C chars but with different absorption intensities in fingerprint bands. The characteristic peaks can be classified into three types relevant for this work: aliphatic absorption, aromatic absorption, and absorption of oxygen-containing groups, including: 1050 cm−1 (C_O), 1260 cm−1 (C–O–C), 1450 cm− 1–1380 cm− 1 (aliphatic bending vibration), 1600 cm− 1 (the stretching vibration of aromatic rings), 1700 cm− 1 (carboxyl absorption), 2925 cm−1–2850 cm−1 (aliphatic stretching vibration), and 3600 cm− 1–3200 cm−1 (hydroxyl absorption). Lower 0.225 0.200
C=O and -COOH
0.175
C-O
-OH
Raw coal Dry coal O
400 C char O 600 C char O 700 C char O 800 C char O 1000 C char
C-O-C
0.150
Absorbance
2.3.4. Slurry concentration measurement The actual slurry concentration was measured in accordance with the standard method of GB/T 18856.2-2002. Approximately 3 g of slurry was placed into the crucible for drying in an oven at 105 °C for 2 h. The percentage of dry solid quality divided by the initial slurry quality is the actual slurry concentration.
0.125 0.100 0.075 0.050
Table 2 Proximate and ultimate analyses of the dry coal. Specimen
Proximate analysis (%, d) M
Oven dried coal
A
V
FC
Qnet,ad (J/g)
0.025 0.000
Ultimate analysis (%, d) C
H
N
St
O
0
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Wavenumber (cm -1)
0.46 9.21 37.24 53.09 25,388 66.38 3.95 0.97 0.10 18.93 Fig. 3. FTIR spectra of raw coal, dry coal and lignite chars.
Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
1.03
55
1.01
lignite char at 400 °C and 1.031 BRXL lignite char at 1000 °C, indicating a sequential upgradation of the coal rank and aromatisation of the lignite char structure with the increase of the pyrolysis temperature. The aromaticity index increases with the extent of devolatilization increases as chars represent different stages of devolatilization which is in accordance with the results of the proximate analysis.
1.00
3.3. Pore structure
Aar/Aal
1.02
0.99
0.98
raw coal dry coal 400
500
600
700
800
900
1000
o
Pyrolysis temperature ( C ) Fig. 4. Aar/Aal concentration as a function of pyrolysis temperature.
intensity of the infrared spectrum absorption around 1700 cm−1 indicates a decrease in the carbonyl or carbonyl groups, demonstrating the destruction and reduction of oxygen functional groups during the pyrolysis process. The absorption intensity of aliphatic hydrocarbon (2925 cm− 1–2850 cm− 1), including symmetric and anti-symmetric stretching vibrations of methyl and methylene, recedes remarkably along with the increasing temperature of the thermal treatment. The reduction of methyl and methylene indicates the breakdown of aliphatic side chains, which probably turned into methane and other releases. The stretching vibration area at 3600 cm−1–3200 cm−1 of hydroxyl declines as a function of pyrolysis temperature compared to raw coal, which is dominantly assigned to different types of hydroxyl breakdown and water reduction in samples. With the increasing pyrolysis temperature, the absorption peak becomes narrower and gradually concentrates at 3360 cm− 1, which is an illustration of the hydroxyl type reduction. Some functional groups are enhanced marginally when the pyrolysis temperature is above 700 °C, which could have originated from the blocks of pore structures due to the disproportionation of CO and incomplete decomposition of functional groups at a relatively high pyrolysis temperature [19]. High temperature pyrolyzed chars contain less hydrophilic groups and possess a strong intrinsic hydrophobic property, which principally enhances the slurrying ability. Therefore, pyrolysis treatment is beneficial to the improvement of lignite slurrying ability as confirmed by Liu et al. [20]. The ratio of aromatic/aliphatic (Aar/Aal) was proposed to characterize the aromaticity and rank of coal related to the variations in a macromolecule during the coal transformation process [21]. The higher the value, the higher the aromaticity and the coal maturation. The absorption at 1600 cm− 1–1500 cm− 1 and absorption at 2925 cm−1– 2850 cm−1 represent aromatic ring content and aliphatic hydrocarbon content, respectively. The Aar/Aal values as a function of the pyrolysis temperature of each sample are shown in Fig. 4. There is a sustaining increase in the Aar/Aal value, from 0.991 BRXL raw coal to 1.012 BRXL
Moisture will be eliminated firstly during the early stage of the pyrolysis process, and then follows the decomposition of oxygencontaining functional groups into gas and tar. During these processes, the original pore structures are destroyed at different extents based on the different pyrolysis temperatures. The pore structure variation as a function of the pyrolysis temperature is depicted in Table 3. The average pore diameter of BRXL appears to sharply decrease as the pyrolysis temperature increases, but at 1000 °C there is a tail tilt that happens. The specific surface area and pore volume, however, decrease with an increase in the pyrolysis temperature, which is in accordance with the previous research findings of Ling et al. [22]. A variety of new pores were generated owing to the decomposition of volatile matter during the pyrolysis process, which increased the specific surface area and pore volume of lignite char dramatically compared to that of raw coal. However, some parts of the surface of char were covered and the pore entrance was blocked due to the CO disproportionation reaction at a relatively high pyrolysis temperature, resulting in a decrease in the specific surface area and pore volume [19,23]. Conclusion from the FTIR analysis further proves that some parts of the pores were blocked up and further pyrolysis was prevented. This result fits well with the data on municipal solid waste reported in a previous literature [18]. The oven dry process has little influence on the pore structure properties because the pore structure properties of oven-dried coal are quite similar to that of raw coal. Generally, the pore structure could be divided into the micropore (diameter b 2 nm), mesopore (diameter from 2 nm to 30 nm), and macropore (diameter N 30 nm) [24]. The pore structure alteration along with the pyrolysis temperature is shown in Fig. 5. Due to the decomposition of oxygen-containing functional groups, the macropore, mesopore, and micropore structures, especially the mesopore, simultaneously increase along with the pyrolysis temperature. Meanwhile, the O/C ratio reduces massively and the hydrophobic property of the char increases a lot, thereby improving the LCWS solid loading. The effective volume of the micropore is important to slurry stability. The slurry with a certain micropore volume could accelerate the precipitate generation
Table 3 Pore structure properties of raw coal, dry coal and lignite chars. Specimen
Average pore diameter (nm)
Specific surface area (m2/g)
Pore volume (mm3/g)
Raw coal Dry coal 400 °C char 600 °C char 800 °C char 1000 °C char
14.5 15.9 9.85 2.31 2.29 2.62
5.53 4.75 7.31 202 216 135
20 19 18 120 120 88
Fig. 5. Pore structure alteration as a function of pyrolysis temperature.
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and go bad with slurry stability [25]. It is obvious that the micropore volumes of the lignite chars are higher than that of the raw coal and ovendried coal. In a nutshell, pyrolysis had a little negative effect on slurry stability.
3.4. Slurrying ability of LCWS/CWS The slurry concentration at a fixed apparent viscosity (FVSC), usually at 1 Pas, is one of the important parameters of CWS. In the current study, the maximum FVSC is defined as the slurry concentration at a shearing rate of 100 s−1 at 1 Pas. If the maximum FVSC is higher, the slurrying ability would be better [26]. Linear interpolation is adopted to calculate the maximum FVSC, as shown in Fig. 6. As Fig. 7 shows, the FVSC of BRXL has dramatically improved from 45.97% of raw coal to 61.84% of 700 °C treated lignite char, which is comparable to that of an Australian sub-bituminous coal water slurry with an FVSC of 63.7% [27] but a bit lower than the petroleum coke sludge with an FVSC of 69.04% [28]. Compared to raw coal, the FVSC of oven-dried lignite only rises up a little at an optimum of 49.38%, which is way lower than the standard of the slurry industrial utilization. The oven drying process could only get rid of the external moisture but scarcely affected internal moisture let alone the volatile. Therefore, the internal moisture and the abundant oxygen-containing functional groups are responsible for hindering the slurry concentration improvement. Whereas, pyrolysis could cut down the two slurrying ability adverse factors in lignite chars considerably and promote the coal rank tremendously. Internal moisture and oxygen concentration reduction through pyrolysis (as shown in Tables 1 and 2) signifies the lignite char hydrophilic abatement and LCWS free water increment. An increase in water mobility in the slurry lowers the slurry apparent viscosity and improves the slurry concentration [28]. However, the FVSC of lignite chars does not increase monotonically along with the pyrolysis temperature. The maximum FVSC of BRXL is 61.84% at 700 °C. After that, the FVSC of the lignite chars drops slightly as the pyrolysis temperature goes up. The FVSC of the BRXL lignite char at 1000 °C drops to 58.64%, which seems unsatisfactory against the requirement for slurry fuel gasification and combustion [29]. The FVSC of the lignite chars starts to decrease at high pyrolysis temperatures not only because some part of the pore structure is covered and the hydrophilic functional groups decomposed incompletely during pyrolysis at a high temperature, but also because the amount of free water entering the pore is severely constricted. In summary, pyrolysis could improve the lignite slurrying ability and become an alternative approach for lignite upgrading technology.
Fig. 6. FVSC calculated by linear interpolation.
Fig. 7. FVSC characteristics along with pyrolysis temperature.
3.5. Rheological behavior of LCWS Rheological behavior is normally an important parameter for CWS pumping. Therefore, the preliminary tests on the flow and deformation of the slurry, typically the relationship between the shear rate and shear stress, were conducted. Most CWS account for the behavior of nonNewtonian fluids and a preferable CWS should be pseudoplastic fluid [30]. The apparent viscosity of the pseudoplastic fluid decreases with an increase in the sheer rate and performs a sheer-thinning rheological behavior. The Herschel–Bulkley model fits well with rheological characteristic parameters [30,31]: τ ¼ τy þ K γ
n
where τ is the shearing stress (Pa), τy the yielding stress (Pa), K the consistency coefficient (Pa·s), γ the shear rate (s−1), and n the rheological index. The flow patterns of CWS/LCWS can be determined by the rheological index of the fluids used: n = 1, Newtonian fluid; n N 1, dilatant fluid; and n b 1, pseudoplastic fluid. The rheological behavior of BRXL CWS/LCWS seems fairly changeable as shown in Fig. 8(a–c). The suspensions of raw coal, dry coal, and lignite chars pyrolyzed at 800 °C–1000 °C demonstrate a sheer-thinning characteristic. Their apparent viscosities generally lower along with the increasing sheering rate which implies that BRXL CWS and LCWS at 800 °C– 1000 °C are considered pseudoplastic fluid. While the LCWS of chars pyrolyzed at 400 °C–700 °C display a characteristic of a sheer-thickening dilatant fluid. Dilatant phenomenon depends on slurry solid loading, particle shape, particle size distribution, and additive type, especially apt to happen in high concentration suspension and high sheering action [31]. Hence, the LCWS at 400–700 °C with a relative high slurry concentration which is shown in Fig. 7 appears to be a dilatant fluid. Usually, as for the same slurry, yielding stress τy rises along with the increasing slurry solid loading because the solid particles in slurries would be more condensed at high slurry concentration and yielding stress would strengthen when being sheared. Consistency coefficient K represents slurry viscosity variation, and the higher the K, the denser the slurry and the higher the apparent viscosity. The rheological indexes n of the dilatant and pseudoplastic fluids are contrasting. It turns out that if the slurry concentration is lower, the n value would be closer to 1.0 and the slurry resembles Newtonian fluid more, because of the higher flowing capability of water performed as a lubricant in the low solid loading slurry. The plots of BRXL LCWS rheological parameters, including τy, K, and n, at a slurry solid loading of 60.0% are shown in Fig. 9. Both yielding stress τy and consistency coefficient K depress first and then elevate along with the increasing pyrolysis temperature, implying that the moderate pyrolysis temperature is the best choice for strengthening
Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
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(a)
(b) Fig. 9. Rheological characteristic parameters of BRXL LCWS at a slurry concentration of 60.0%.
(c)
Fig. 8. Rheological behavior of CWS/LCWS of BRXL. (a) Rheological behavior of BRXL CWS at 105 °C, (b) rheological behavior of BRXL LCWS at 500 °C and (c) rheological behavior of BRXL LCWS at 900 °C.
LCWS rheological behavior. It can be concluded from Fig. 7 that BRXL LCWS pyrolyzed at 700 °C is the optimally capable slurry. Consequently 700 °C LCWS achieves the optimal flowing capability at the same slurry concentration, which originates from the lowest yielding stress and consistency coefficient of 700 °C LCWS. The rheological indexes are influenced by many factors, especially slurry concentration. As shown in Fig. 9, the rheological indexes change scarcely for different temperatures of the LCWS at a slurry concentration of around 60%. 3.6. Static stability Usually, the water separation ratio is used as an indicator of slurry stability. Normally, a lower water separation ratio suggests that the water and coal/char powder coupled well with each other in the
suspension and the slurry is also comparatively stable. Fig. 10 shows the suspension water separation ratio at different pyrolysis temperatures. The experimental condition is retained for 7 days with the stabilizer of an NDF additive. The overall water separation ratios of the samples are below 10% in Fig. 10. As for the LCWS pyrolyzed under 800 °C, the water separation ratio lessens along with the increase of the slurry concentration, implying that the higher the slurry concentration, the better the slurry stability. Solid particles and liquid in the slurry interact with each other to form a kind of structural units caused by stirring and dispersion during the slurry preparation process. These structural units are in random order in non-Newtonian fluid and build some space grid structure which could effectively stop coal/char powder reunion and causing precipitation. Compared to low-concentration slurry, there are much more solid particles in high solid loading slurry, which leads to the construction of an intensive space grid structure and stronger resistance force of particle precipitation. For such reason, high concentration slurry seems to be more stable. The stability of high-temperature (higher than 800 °C) pyrolyzed LCWS decreases with an increase in slurry concentration. The lignite char pore structure would be damaged by the high pyrolysis temperature and part of the pores would be blocked, so less mobile water could enter into the pore. In that case, hightemperature pyrolyzed LCWS reaches a saturated state at a comparatively low slurry concentration. Continually adding solid powders after LCWS reaches a saturated state which causes too high solid loading. Consequently, the mutual acting forces among the solid particles are much stronger than the forces between solid particles and water molecules, thereby resulting in easy water separation and low LCWS concentration when pyrolyzed at a high temperature. With the existence of a variety of hydrophilic groups, such as carboxyl and hydroxyl, raw coal and oven-dried coal water slurry static stability is much stronger than that of LCWS. Coal water slurry dispersant is a kind of surfactant which is composed of hydrophilic groups and hydrophobic groups. The hydrophilic groups will couple with hydrone in the suspension and the hyrophobic groups will combine with the hydrophobic substance on the surface of coal. The surface structure of the coal/char will be recombined and some pores will be unfolded in a dispersant solution during the stirring process. As a consequence, the undecomposed hydrophilic groups, such as carboxyl and hydroxyl, will perform a role in water fastening. Therefore, the hydrophilic groups in lignite chars decrease at the beginning and increase later with an increase in pyrolysis temperature, which are consistent with LCWS stability. Conclusively, the hydrophilic groups have a negative effect on the slurry concentration enhancement but they have a positive effect on slurry stability strengthening.
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Fig. 10. CWS/LCWS static stability.
4. Conclusions 1. The moisture content of the lignite char pyrolyzed at 400 °C drops to 1.41% and 0.62% at 1000 °C. The volatile content of the lignite char drops to 1.31% at 1000 °C. The O/C ratio decreases from 0.220 of raw coal to about 0 of 1000 °C lignite char, indicating that pyrolysis causes a kind of artificial maturation of coal. 2. Thermal modification reduces the quantity of oxygen-containing functional groups of the coals and chars and strengthens the hydrophobic properties of the pyrolyzed chars. The ratio of aromatic/aliphatic, as a representation of coal rank, increases persistently from 0.991 of raw coal to 1.031 of 1000 °C lignite char. 3. The average pore diameter of the lignite char sharply reduces before 800 °C and slightly increases after 800 °C along with the increase of the pyrolysis temperature. Specific area and pore volume are indirectly proportional to the average pore diameter. The oven dry process barely has an effect on the pore structures of lignite. 4. The slurrying ability of the LCWS is obviously improved by low to high temperature pyrolyses. The FVSC of the lignite char is enhanced to more than 60% compared with raw coal (45.97%). However, the FVSC of the lignite char does not monotonically increase along with the increasing pyrolysis temperature. Taking into account the slurrying ability and energy consumption during the lignite upgrading process, the most suitable pyrolysis temperature is between 600 °C and 800 °C. 5. LCWS exhibits a distinct rheological behavior in raw/dry coal water slurry, which may be possibly due to the discrepant slurry concentration at different pyrolysis temperatures. LCWS is not as stable as the raw coal and dry coal water slurry due to the reduction of hydrophilic groups. Acknowledgments This work was supported by the National Basic Research Program of China (2012CB214906) and the Program of Introducing Talents of Discipline to University (B08026). References [1] X.Y. Ma, Y.F. Duan, M. Liu, H.F. Li, Influence of sewage sludge on the rheological properties of petroleum coke–water slurry, Asia-Pacific Journal of Chemical Engineering 8 (2013) 453–460.
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Zhang, Static stability and rheological behavior of lignite char–water mixture, Fuel 104 (2013) 7–13. [13] K. Miura, K. Hashimoto, K. Mae, H. Wakiyasu, Flash pyrolysis of coal–methanol slurry, Kagaku Kogaku Ronbunshu 20 (1994) 926–933. [14] F.R. Meng, J.L. Yu, A. Tahmasebi, Y.N. Han, H. Zhao, J. Lucas, T. Wall, Characteristics of chars from low-temperature pyrolysis of lignite, Energy & Fuels 28 (2014) 275–284. [15] China National Coal Standardization Technology Committee, GB/T 18856.118856.14-2002 test methods for coal water mixtures [S], Standard Press of China, Beijing, 2002. [16] B. Tan, L.N. Qu, F.R. Wang, R.L. Hu, X.Y. Kang, Experimental study on mechanism of hydroxyl variation and the critical temperatures in the process of coal oxidation at low temperature, Disaster Advances 6 (2013) 315–321. [17] W. Zuzana, P. Oldrich, S. Ivana, M. Vladimir, Effect of bitumious coal properties on carbon dioxide and methane high pressure sorption, Fule 139 (2015) 115–124. [18] Y.T. Tang, X.Q. Ma, Z.Y. Lai, H. Lin, J. Wu, Char characteristics of municipal solid waste prepared under N-2 and CO2 atmospheres, Journal of Analytical and Applied Pyrolysis 101 (2013) 193–198. [19] N. Tsubouchi, H. Hashimoto, Y. Ohtsuka, Catalytic performance of limonite in the decomposition of ammonia in the coexistence of typical fuel gas components produced in an air-blown coal gasification process, Energy & Fuels 21 (2007) 3063–3069. [20] M.Q. Liu, J.Z. Liu, R.K. Wang, Effects of pyrolysis temperature on slurry ability of lignite semi-coke, Proceeding of the CSEE 33 (2013) 36–43. [21] J. Kister, M. Guiliano, C. Largeau, S. Derenne, E. Casadevall, Characterization of chemical-structure, degree of maturation and oil potential of torbanites (type- kerogens) by quantitative FT-IR spectroscopy, Fuel 69 (1990) 1356–1361. [22] L. Wei, Y.S. Tan, Y.Z. Han, J.T. Zhao, J.H. Wu, D.K. Zhang, Hydrogen production by methane cracking over different coal chars, Fuel 90 (2011) 3473–3479. [23] S.P. Gao, J.F. Wang, Z.Q. Wang, J.T. Zhao, Y.T. Fang, Effect of CO on the CH4 evolution during fast pyrolysis of lignite in reductive atmospheres, Journal of Analytical and Applied Pyrolysis 106 (2014) 104–111. [24] X. Li, Z. Li, Q.B. Xia, H.X. Xi, Effects of pore sizes of porous silica gels on desorption activation energy of water vapour, Applied Thermal Engineering 27 (2007) 869–876. [25] Chiwei. SCLBY. Characteristic of pore size distribution and slurry ability of coal, Journal of Fuel Chemistry and Technology 25 (1996) 434–439. [26] R.K. Wang, J.Z. Liu, Y.J. Yu, Y.X. Hu, J.H. Zhou, K.F. Cen, The slurrying properties of coal water slurries containing raw sewage sludge, Energy & Fuels 25 (2011) 747–752. [27] R. Chen, M. Wilson, Y.K. Leong, P. Bryant, H. Yang, D.K. Zhang, Preparation and rheology of biochar, lignite char and coal slurry fuels, Fuel 90 (2011) 1689–1695. [28] R.K. Wang, J.Z. Liu, F.Y. Gao, J.H. Zhou, K.F. 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Fuel Processing Technology journal homepage: www.elsevier.com/locate/fuproc
Effect of pyrolysis temperature on lignite char properties and slurrying ability Yan Li 1, Zhi-Hua Wang ⁎,1, Zhen-Yu Huang 1, Jian-Zhong Liu 1, Jun-Hu Zhou 1, Ke-Fa Cen 1 State Key Laboratory of Clean Energy Utilization, Zhejiang University, Hangzhou 310027, China
a r t i c l e
i n f o
Article history: Received 2 April 2014 Received in revised form 6 January 2015 Accepted 8 January 2015 Available online 3 February 2015 Keywords: Lignite Pyrolysis Lignite char–water slurry Slurrying ability Functional group Pore structure
a b s t r a c t This work investigates the influence of different pyrolysis temperatures in lignite char properties and slurrying ability. Baorixile lignite was pyrolyzed under nitrogen atmosphere from low to high temperatures. The pyrolyzed chars were then used in preparing lignite char–water slurry fuels. The apparent viscosities and rheological behaviors of different slurries were obtained by using a rotating viscometer. Pyrolysis can effectively improve lignite char–water slurry concentration, and the influence of such mechanism can be determined by analyzing oxygen-containing functional groups and pore structures. Result suggests that pyrolysis not only removes the moisture content but also causes an apparent increase in lignite coal rank. Pyrolysis also evidently reduces the number of oxygen-containing functional groups and the hydrophilicity of lignite char, consequently improving slurry concentration but decreasing slurry static stability. The pore structure of the lignite char changes significantly after pyrolysis. With increasing pyrolysis temperature, average pore diameter initially decreases and eventually increases, whereas specific surface area and pore volume exhibit an opposite trend. Experimental results reveal that the slurry concentration of lignite char does not increase monotonically with the increase of pyrolysis temperature and 600 °C–800 °C is the most suitable pyrolysis temperature range for lignite slurrying ability improvement. © 2015 Elsevier B.V. All rights reserved.
1. Introduction Coal water slurry (CWS), as a fuel derived from coal, consists of 60%– 70% coal, 29%–39% water, and ~1% additives. This fuel was first developed as an oil replacement for slurry combustion during the energy crisis in the 1980s. Since then, CWS has been widely used in combustion and gasification process. With the rapid development of integrated gasification in combined cycle (IGCC) and coal gasification in the recent years, CWS fuel has been recognized as an important fuel for high-pressure gasifier with high energy efficiency. CWS feedstocks have been extended from bituminous coal to petroleum coke [1] and lignite [2] because of the short supply of high-rank coal. Given its high pore volume and high moisture content, it was extremely challenging for lignite to produce highly concentrated CWS fuel. Thus, the use of low-rank coal, which has abundant reserves accounting for approximately 53% of the global total coal reserve, has attracted the attention of researchers around the world. Various lignite upgrading technologies have been reported in the literature [3–5]. Lignite is characterized by having an enriched volatile
⁎ Corresponding author at: Institute for Thermal Power Engineering, Zhejiang University, Hangzhou, China. E-mail address: [email protected] (Z.-H. Wang). 1 Tel.: +86 571 87,953,162; fax: +86 571 87951616.
http://dx.doi.org/10.1016/j.fuproc.2015.01.007 0378-3820/© 2015 Elsevier B.V. All rights reserved.
matter content; a high H/C ratio; a large amount of carboxyl groups, carbonyl groups, and methylene; and a positive reactive behavior. As such, lignite is highly reactive during pyrolysis. Pyrolysis has been widely investigated as a thermal treatment upgrading method that can upgrade lignite quality in terms of heat value, stability, and moisture absorption [6–8]. After being pyrolyzed, lignite can be converted into liquid, gas, and solid products. Kaji et al. [9] found that after pyrolysis at 400 °C, the water-holding capacity of lignite char decreased by more than two-thirds compared to that of raw lignite. Previous studies indicate that pyrolysis can increase specific surface area [10] and reduce oxygen content [11], which is preferable for making high-concentration lignite char–water slurry (LCWS). Fan et al. [12] investigated the additive effects on slurrying activity and pointed that lignite char–water slurrying ability is considerably higher than that of raw coal. However, most of the coal pyrolyses were conducted at a low or moderate temperature range [13,14], lacking the understanding of high temperature pyrolyzed char properties. The primary aim of this work is to investigate the effect of the pyrolysis temperature, from low to high range, on slurrying ability of lignite char by using selected additives as dispersant and stabilizer to gain insight into the global comprehensive utilization of lignite. Baorixile (BRXL), a typical lignite with total water concentration of 25.68% from Inner Mongolia, where it is one of the largest reserve areas of lignite in China, was studied in this article. Pyrolysis was performed in the absence of oxygen and at a temperature ranging from 400 °C to 1000 °C. The influence of different pyrolysis temperatures on LCWS
Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
53
Fig. 1. Schematic diagram of the experimental setup.
properties, including slurry concentration, rheological behavior, and stability, was investigated. 2. Material and methods 2.1. Experimental setup As Fig. 1 shows, the experimental setup, which consists of a char reactor with power supply, a LCWS preparation process, and two analysis systems, namely, char/coal property analysis system and LCWS/CWS property analysis system. BRXL was first pyrolyzed in a nitrogen atmosphere furnace at different temperatures. Char properties, involving proximate and ultimate analyses, functional groups, and physical structures were tested accordingly. Pyrolyzed chars were made into LCWS, whose solid loading, rheological behavior, and static stability were evaluated with the use of the LCWS/CWS property analysis system. 2.2. Material In the experiment, air-dried raw coal was milled to particles of less than 75 μm to ensure the thoroughness of pyrolysis and the homogeneity of the lignite char. As the moisture content is one of the main challenges in the preparation of high-concentration LCWS, coal was oven-dried at 105 °C for 2 h; the slurrying abilities of which were then compared with that of the pyrolyzed lignite chars. Results of the proximate and ultimate analyses of raw coal and oven-dried coal are listed in Tables 1 and 2. 2.3. Methods 2.3.1. Pyrolysis process Pyrolysis experiments were conducted in a controlled atmosphere furnace (HMX1700-30). The temperature ranged from 400 °C–
1000 °C, with an interval of 100 °C at a heating rate of 10 °C/min and an isothermal treatment of 30 min. Approximately 300 g of raw coal powder was paved in a ceramic container under N2 gas environment. After a complete purging process by N2 gas, a sufficient amount of N2 was fed into the furnace until a tiny positive pressure was achieved to guarantee an inert atmosphere. 2.3.2. Lignite char–water slurry preparation The LCWS was prepared as follows. First, the required masses of char/coal powder, deionized water, and additives were calculated and weighed in predetermined ratios. A copolymer of methylene naphthalene, sulfonate styrene, and sulfonate maleate (an NDF additive) was adopted as a dispersant and a stabilizer in this experiment. The abovementioned dispersant accounted for 0.8% of the weight of the char/coal samples. The coal/char powder was gradually added into the dispersant solution under low speed agitation of 200 rpm. The solution was then stirred at 1000 rpm for 20 min to ensure that the dispersant solution with coal/char powder is mixed well. During the preparation of LCWS/CWS, the temperature was maintained at 25 °C–30 °C. 2.3.3. Physicochemical characterization of lignite char Functional group analyses of raw/dried coal and lignite chars were performed on a Nicolet 6700 Fourier transform infrared (FTIR) spectrometer. During the test, KBr pellets were prepared by grinding 2.5 mg of specimens with 200 mg of KBr. The FTIR spectra of the specimens for the 4000 cm− 1–400 cm− 1 region were studied by curve-fitting analysis. The pore structure of raw coal and lignite chars was determined by using an automatic surface area and a porosity analyzer (TriStar 3000, MicroMeritics). The samples were kept under 200 °C in a vacuum for 3 h to remove the contaminants inside the pore structures. Then, a 0.5 g sample was retained in liquid nitrogen for analysis. The pore structure properties were assessed during nitrogen absorption and separation
Table 1 Proximate and ultimate analyses of coal and different temperature pyrolyzed chars. Specimen
Raw coal 400 °C char 500 °C char 600 °C char 700 °C char 800 °C char 900 °C char 1000 °C char
Qnet,ad (J/g)
Proximate analysis (%) Mad
Ad
VMd
FCd
8.68 1.41 1.76 1.00 1.04 0.66 0.52 0.62
10.50 12.05 13.63 14.90 15.46 16.03 16.52 16.74
37.89 25.60 15.61 7.17 3.64 2.09 1.65 1.31
51.61 62.35 70.76 77.93 80.89 81.88 81.84 81.95
22,989 27,278 29,000 29,891 29,702 28,930 28,825 28,639
Ultimate analysis (%, ad) C
H
N
St
O
60.06 70.25 75.55 80.02 80.60 82.01 82.30 82.50
3.38 3.51 2.93 1.22 2.00 0.56 0.17 0
0.59 0.91 1.03 0.80 0.91 0.59 0.60 0.46
0.11 0.19 0.18 0.19 0.20 0.18 0.18 0.17
17.59 11.85 5.16 2.02 0 0.08 0 0
Note: ar = as received. ad = air dry basis. d = dry basis. M = moisture content. A = ash content. V = volatile matter. FC = fixed carbon.
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Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
0.25
process. The pore structure and surface area can be obtained by using the BET isothermal equation.
2.3.5. Determining viscosity and rheological behavior The rheological behavior of LCWS/CWS, which refers to the shear stress–shear rate or apparent viscosity–shear rate dependence, was obtained by using a HAAKE VT 550 rotating viscometer. A sufficient amount of LCWS/CWS was poured into the container, and the temperature was maintained at (20 ± 1)°C by using a thermostatic water bath. In accordance with the GB/T18856.4-2002 method [15], the measurement procedure was inputted into the instrument control system: shearing rate is increased from 0 to 100 s−1 at a constant speed and then held constant at 100 s−1 for 5 min. The viscosity data were recorded every 30 s during the shear rate stability period. The apparent viscosity of the slurry was the average value of the 10 recorded viscosity data. After that, the shear rate is dropped to zero at a constant speed. The slurry rheological behavior was determined by the relationship between apparent viscosity and the corresponding shear rate during its rising period. 2.3.6. Static stability measurements The static stability of LCWS/CWS is appraised by the water separation ratio, which is defined as the mass ratio of separated water to the total slurry. In this experiment, a sufficient amount of slurry was sealed in a closed container for 7 d, and water separated after that static storage. The separated water was taken out by a burette. 3. Results and discussion 3.1. Lignite char properties As shown in Table 1, with rising pyrolysis temperature, both moisture content and volatile matter yield decreased, whereas fixed carbon yield increased. This phenomenon is particularly noticeable between the raw coal and the lignite char at 400 °C whose moisture content was reduced from 8.68% to 1.41% air dry basis, indicating that pyrolysis could dehydrate lignite effectively and that the water-fixing ability of lignite would significantly decline after being pyrolyzed. The volatile content is reduced from 37.89% of raw coal to 1.31% of 1000 °C lignite char due to the decomposition of the organic matter [6]. Such volatile content is the most active part of the coal components, and it is usually released at a relatively low temperature. Therefore, a large amount of volatile reduction occurs between 400 °C and 600 °C. Elemental ratio of O/C in specimens is an important parameter in the analysis of slurry making ability. Generally, the higher the O/C ratio is, the worse the slurrying ability is. The O/C ratio in coal greatly reflects the presence of oxygen-containing functional groups, including carbonyl, hydroxyl, and carboxyl. Polar functional group content and O/C ratio decrease with an increase in coal metamorphic rank [16], causing also an increase in slurrying ability. As Fig. 2 shows, the O/C ratio diminished drastically with the rising pyrolysis temperature, which is attributed to
0.20
O/C
0.15
0.10
0.05
0.00 0
100
200
300
400
500
600
700
800
900 1000
o
Pyrolysis temperature ( C ) Fig. 2. O/C ratio in raw coal, dry coal and lignite chars.
the decomposition of oxygen-containing functional groups into gas or tar during pyrolysis. The higher the pyrolysis terminal temperature, the more extensive the decomposition of oxygen-containing functional groups and accordingly the lower the lignite char O/C ratio. The pyrolysis process leads to transformations in lignite resembling those occurring with upgradation of coal rank. 3.2. Chemical characteristics Coal is composed of different macromolecules whose branchedchain structure is quite complicated, typically including hydroxyl, carboxyl, methyl, aromatic rings and so on. Functional group characteristics of samples could directly determine the maximum internal moisture content and the hydrophilic extent of coal, which entirely impacts the slurry making ability. FTIR has been widely used to analyze the functional groups in coal and their variation with different thermal treatments [17,18]. Fig. 3 shows similar functional groups for the dry coal, the 400 °C and 1000 °C chars but with different absorption intensities in fingerprint bands. The characteristic peaks can be classified into three types relevant for this work: aliphatic absorption, aromatic absorption, and absorption of oxygen-containing groups, including: 1050 cm−1 (C_O), 1260 cm−1 (C–O–C), 1450 cm− 1–1380 cm− 1 (aliphatic bending vibration), 1600 cm− 1 (the stretching vibration of aromatic rings), 1700 cm− 1 (carboxyl absorption), 2925 cm−1–2850 cm−1 (aliphatic stretching vibration), and 3600 cm− 1–3200 cm−1 (hydroxyl absorption). Lower 0.225 0.200
C=O and -COOH
0.175
C-O
-OH
Raw coal Dry coal O
400 C char O 600 C char O 700 C char O 800 C char O 1000 C char
C-O-C
0.150
Absorbance
2.3.4. Slurry concentration measurement The actual slurry concentration was measured in accordance with the standard method of GB/T 18856.2-2002. Approximately 3 g of slurry was placed into the crucible for drying in an oven at 105 °C for 2 h. The percentage of dry solid quality divided by the initial slurry quality is the actual slurry concentration.
0.125 0.100 0.075 0.050
Table 2 Proximate and ultimate analyses of the dry coal. Specimen
Proximate analysis (%, d) M
Oven dried coal
A
V
FC
Qnet,ad (J/g)
0.025 0.000
Ultimate analysis (%, d) C
H
N
St
O
0
500 1000 1500 2000 2500 3000 3500 4000 4500 5000
Wavenumber (cm -1)
0.46 9.21 37.24 53.09 25,388 66.38 3.95 0.97 0.10 18.93 Fig. 3. FTIR spectra of raw coal, dry coal and lignite chars.
Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
1.03
55
1.01
lignite char at 400 °C and 1.031 BRXL lignite char at 1000 °C, indicating a sequential upgradation of the coal rank and aromatisation of the lignite char structure with the increase of the pyrolysis temperature. The aromaticity index increases with the extent of devolatilization increases as chars represent different stages of devolatilization which is in accordance with the results of the proximate analysis.
1.00
3.3. Pore structure
Aar/Aal
1.02
0.99
0.98
raw coal dry coal 400
500
600
700
800
900
1000
o
Pyrolysis temperature ( C ) Fig. 4. Aar/Aal concentration as a function of pyrolysis temperature.
intensity of the infrared spectrum absorption around 1700 cm−1 indicates a decrease in the carbonyl or carbonyl groups, demonstrating the destruction and reduction of oxygen functional groups during the pyrolysis process. The absorption intensity of aliphatic hydrocarbon (2925 cm− 1–2850 cm− 1), including symmetric and anti-symmetric stretching vibrations of methyl and methylene, recedes remarkably along with the increasing temperature of the thermal treatment. The reduction of methyl and methylene indicates the breakdown of aliphatic side chains, which probably turned into methane and other releases. The stretching vibration area at 3600 cm−1–3200 cm−1 of hydroxyl declines as a function of pyrolysis temperature compared to raw coal, which is dominantly assigned to different types of hydroxyl breakdown and water reduction in samples. With the increasing pyrolysis temperature, the absorption peak becomes narrower and gradually concentrates at 3360 cm− 1, which is an illustration of the hydroxyl type reduction. Some functional groups are enhanced marginally when the pyrolysis temperature is above 700 °C, which could have originated from the blocks of pore structures due to the disproportionation of CO and incomplete decomposition of functional groups at a relatively high pyrolysis temperature [19]. High temperature pyrolyzed chars contain less hydrophilic groups and possess a strong intrinsic hydrophobic property, which principally enhances the slurrying ability. Therefore, pyrolysis treatment is beneficial to the improvement of lignite slurrying ability as confirmed by Liu et al. [20]. The ratio of aromatic/aliphatic (Aar/Aal) was proposed to characterize the aromaticity and rank of coal related to the variations in a macromolecule during the coal transformation process [21]. The higher the value, the higher the aromaticity and the coal maturation. The absorption at 1600 cm− 1–1500 cm− 1 and absorption at 2925 cm−1– 2850 cm−1 represent aromatic ring content and aliphatic hydrocarbon content, respectively. The Aar/Aal values as a function of the pyrolysis temperature of each sample are shown in Fig. 4. There is a sustaining increase in the Aar/Aal value, from 0.991 BRXL raw coal to 1.012 BRXL
Moisture will be eliminated firstly during the early stage of the pyrolysis process, and then follows the decomposition of oxygencontaining functional groups into gas and tar. During these processes, the original pore structures are destroyed at different extents based on the different pyrolysis temperatures. The pore structure variation as a function of the pyrolysis temperature is depicted in Table 3. The average pore diameter of BRXL appears to sharply decrease as the pyrolysis temperature increases, but at 1000 °C there is a tail tilt that happens. The specific surface area and pore volume, however, decrease with an increase in the pyrolysis temperature, which is in accordance with the previous research findings of Ling et al. [22]. A variety of new pores were generated owing to the decomposition of volatile matter during the pyrolysis process, which increased the specific surface area and pore volume of lignite char dramatically compared to that of raw coal. However, some parts of the surface of char were covered and the pore entrance was blocked due to the CO disproportionation reaction at a relatively high pyrolysis temperature, resulting in a decrease in the specific surface area and pore volume [19,23]. Conclusion from the FTIR analysis further proves that some parts of the pores were blocked up and further pyrolysis was prevented. This result fits well with the data on municipal solid waste reported in a previous literature [18]. The oven dry process has little influence on the pore structure properties because the pore structure properties of oven-dried coal are quite similar to that of raw coal. Generally, the pore structure could be divided into the micropore (diameter b 2 nm), mesopore (diameter from 2 nm to 30 nm), and macropore (diameter N 30 nm) [24]. The pore structure alteration along with the pyrolysis temperature is shown in Fig. 5. Due to the decomposition of oxygen-containing functional groups, the macropore, mesopore, and micropore structures, especially the mesopore, simultaneously increase along with the pyrolysis temperature. Meanwhile, the O/C ratio reduces massively and the hydrophobic property of the char increases a lot, thereby improving the LCWS solid loading. The effective volume of the micropore is important to slurry stability. The slurry with a certain micropore volume could accelerate the precipitate generation
Table 3 Pore structure properties of raw coal, dry coal and lignite chars. Specimen
Average pore diameter (nm)
Specific surface area (m2/g)
Pore volume (mm3/g)
Raw coal Dry coal 400 °C char 600 °C char 800 °C char 1000 °C char
14.5 15.9 9.85 2.31 2.29 2.62
5.53 4.75 7.31 202 216 135
20 19 18 120 120 88
Fig. 5. Pore structure alteration as a function of pyrolysis temperature.
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Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
and go bad with slurry stability [25]. It is obvious that the micropore volumes of the lignite chars are higher than that of the raw coal and ovendried coal. In a nutshell, pyrolysis had a little negative effect on slurry stability.
3.4. Slurrying ability of LCWS/CWS The slurry concentration at a fixed apparent viscosity (FVSC), usually at 1 Pas, is one of the important parameters of CWS. In the current study, the maximum FVSC is defined as the slurry concentration at a shearing rate of 100 s−1 at 1 Pas. If the maximum FVSC is higher, the slurrying ability would be better [26]. Linear interpolation is adopted to calculate the maximum FVSC, as shown in Fig. 6. As Fig. 7 shows, the FVSC of BRXL has dramatically improved from 45.97% of raw coal to 61.84% of 700 °C treated lignite char, which is comparable to that of an Australian sub-bituminous coal water slurry with an FVSC of 63.7% [27] but a bit lower than the petroleum coke sludge with an FVSC of 69.04% [28]. Compared to raw coal, the FVSC of oven-dried lignite only rises up a little at an optimum of 49.38%, which is way lower than the standard of the slurry industrial utilization. The oven drying process could only get rid of the external moisture but scarcely affected internal moisture let alone the volatile. Therefore, the internal moisture and the abundant oxygen-containing functional groups are responsible for hindering the slurry concentration improvement. Whereas, pyrolysis could cut down the two slurrying ability adverse factors in lignite chars considerably and promote the coal rank tremendously. Internal moisture and oxygen concentration reduction through pyrolysis (as shown in Tables 1 and 2) signifies the lignite char hydrophilic abatement and LCWS free water increment. An increase in water mobility in the slurry lowers the slurry apparent viscosity and improves the slurry concentration [28]. However, the FVSC of lignite chars does not increase monotonically along with the pyrolysis temperature. The maximum FVSC of BRXL is 61.84% at 700 °C. After that, the FVSC of the lignite chars drops slightly as the pyrolysis temperature goes up. The FVSC of the BRXL lignite char at 1000 °C drops to 58.64%, which seems unsatisfactory against the requirement for slurry fuel gasification and combustion [29]. The FVSC of the lignite chars starts to decrease at high pyrolysis temperatures not only because some part of the pore structure is covered and the hydrophilic functional groups decomposed incompletely during pyrolysis at a high temperature, but also because the amount of free water entering the pore is severely constricted. In summary, pyrolysis could improve the lignite slurrying ability and become an alternative approach for lignite upgrading technology.
Fig. 6. FVSC calculated by linear interpolation.
Fig. 7. FVSC characteristics along with pyrolysis temperature.
3.5. Rheological behavior of LCWS Rheological behavior is normally an important parameter for CWS pumping. Therefore, the preliminary tests on the flow and deformation of the slurry, typically the relationship between the shear rate and shear stress, were conducted. Most CWS account for the behavior of nonNewtonian fluids and a preferable CWS should be pseudoplastic fluid [30]. The apparent viscosity of the pseudoplastic fluid decreases with an increase in the sheer rate and performs a sheer-thinning rheological behavior. The Herschel–Bulkley model fits well with rheological characteristic parameters [30,31]: τ ¼ τy þ K γ
n
where τ is the shearing stress (Pa), τy the yielding stress (Pa), K the consistency coefficient (Pa·s), γ the shear rate (s−1), and n the rheological index. The flow patterns of CWS/LCWS can be determined by the rheological index of the fluids used: n = 1, Newtonian fluid; n N 1, dilatant fluid; and n b 1, pseudoplastic fluid. The rheological behavior of BRXL CWS/LCWS seems fairly changeable as shown in Fig. 8(a–c). The suspensions of raw coal, dry coal, and lignite chars pyrolyzed at 800 °C–1000 °C demonstrate a sheer-thinning characteristic. Their apparent viscosities generally lower along with the increasing sheering rate which implies that BRXL CWS and LCWS at 800 °C– 1000 °C are considered pseudoplastic fluid. While the LCWS of chars pyrolyzed at 400 °C–700 °C display a characteristic of a sheer-thickening dilatant fluid. Dilatant phenomenon depends on slurry solid loading, particle shape, particle size distribution, and additive type, especially apt to happen in high concentration suspension and high sheering action [31]. Hence, the LCWS at 400–700 °C with a relative high slurry concentration which is shown in Fig. 7 appears to be a dilatant fluid. Usually, as for the same slurry, yielding stress τy rises along with the increasing slurry solid loading because the solid particles in slurries would be more condensed at high slurry concentration and yielding stress would strengthen when being sheared. Consistency coefficient K represents slurry viscosity variation, and the higher the K, the denser the slurry and the higher the apparent viscosity. The rheological indexes n of the dilatant and pseudoplastic fluids are contrasting. It turns out that if the slurry concentration is lower, the n value would be closer to 1.0 and the slurry resembles Newtonian fluid more, because of the higher flowing capability of water performed as a lubricant in the low solid loading slurry. The plots of BRXL LCWS rheological parameters, including τy, K, and n, at a slurry solid loading of 60.0% are shown in Fig. 9. Both yielding stress τy and consistency coefficient K depress first and then elevate along with the increasing pyrolysis temperature, implying that the moderate pyrolysis temperature is the best choice for strengthening
Y. Li et al. / Fuel Processing Technology 134 (2015) 52–58
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(a)
(b) Fig. 9. Rheological characteristic parameters of BRXL LCWS at a slurry concentration of 60.0%.
(c)
Fig. 8. Rheological behavior of CWS/LCWS of BRXL. (a) Rheological behavior of BRXL CWS at 105 °C, (b) rheological behavior of BRXL LCWS at 500 °C and (c) rheological behavior of BRXL LCWS at 900 °C.
LCWS rheological behavior. It can be concluded from Fig. 7 that BRXL LCWS pyrolyzed at 700 °C is the optimally capable slurry. Consequently 700 °C LCWS achieves the optimal flowing capability at the same slurry concentration, which originates from the lowest yielding stress and consistency coefficient of 700 °C LCWS. The rheological indexes are influenced by many factors, especially slurry concentration. As shown in Fig. 9, the rheological indexes change scarcely for different temperatures of the LCWS at a slurry concentration of around 60%. 3.6. Static stability Usually, the water separation ratio is used as an indicator of slurry stability. Normally, a lower water separation ratio suggests that the water and coal/char powder coupled well with each other in the
suspension and the slurry is also comparatively stable. Fig. 10 shows the suspension water separation ratio at different pyrolysis temperatures. The experimental condition is retained for 7 days with the stabilizer of an NDF additive. The overall water separation ratios of the samples are below 10% in Fig. 10. As for the LCWS pyrolyzed under 800 °C, the water separation ratio lessens along with the increase of the slurry concentration, implying that the higher the slurry concentration, the better the slurry stability. Solid particles and liquid in the slurry interact with each other to form a kind of structural units caused by stirring and dispersion during the slurry preparation process. These structural units are in random order in non-Newtonian fluid and build some space grid structure which could effectively stop coal/char powder reunion and causing precipitation. Compared to low-concentration slurry, there are much more solid particles in high solid loading slurry, which leads to the construction of an intensive space grid structure and stronger resistance force of particle precipitation. For such reason, high concentration slurry seems to be more stable. The stability of high-temperature (higher than 800 °C) pyrolyzed LCWS decreases with an increase in slurry concentration. The lignite char pore structure would be damaged by the high pyrolysis temperature and part of the pores would be blocked, so less mobile water could enter into the pore. In that case, hightemperature pyrolyzed LCWS reaches a saturated state at a comparatively low slurry concentration. Continually adding solid powders after LCWS reaches a saturated state which causes too high solid loading. Consequently, the mutual acting forces among the solid particles are much stronger than the forces between solid particles and water molecules, thereby resulting in easy water separation and low LCWS concentration when pyrolyzed at a high temperature. With the existence of a variety of hydrophilic groups, such as carboxyl and hydroxyl, raw coal and oven-dried coal water slurry static stability is much stronger than that of LCWS. Coal water slurry dispersant is a kind of surfactant which is composed of hydrophilic groups and hydrophobic groups. The hydrophilic groups will couple with hydrone in the suspension and the hyrophobic groups will combine with the hydrophobic substance on the surface of coal. The surface structure of the coal/char will be recombined and some pores will be unfolded in a dispersant solution during the stirring process. As a consequence, the undecomposed hydrophilic groups, such as carboxyl and hydroxyl, will perform a role in water fastening. Therefore, the hydrophilic groups in lignite chars decrease at the beginning and increase later with an increase in pyrolysis temperature, which are consistent with LCWS stability. Conclusively, the hydrophilic groups have a negative effect on the slurry concentration enhancement but they have a positive effect on slurry stability strengthening.
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Fig. 10. CWS/LCWS static stability.
4. Conclusions 1. The moisture content of the lignite char pyrolyzed at 400 °C drops to 1.41% and 0.62% at 1000 °C. The volatile content of the lignite char drops to 1.31% at 1000 °C. The O/C ratio decreases from 0.220 of raw coal to about 0 of 1000 °C lignite char, indicating that pyrolysis causes a kind of artificial maturation of coal. 2. Thermal modification reduces the quantity of oxygen-containing functional groups of the coals and chars and strengthens the hydrophobic properties of the pyrolyzed chars. The ratio of aromatic/aliphatic, as a representation of coal rank, increases persistently from 0.991 of raw coal to 1.031 of 1000 °C lignite char. 3. The average pore diameter of the lignite char sharply reduces before 800 °C and slightly increases after 800 °C along with the increase of the pyrolysis temperature. Specific area and pore volume are indirectly proportional to the average pore diameter. The oven dry process barely has an effect on the pore structures of lignite. 4. The slurrying ability of the LCWS is obviously improved by low to high temperature pyrolyses. The FVSC of the lignite char is enhanced to more than 60% compared with raw coal (45.97%). However, the FVSC of the lignite char does not monotonically increase along with the increasing pyrolysis temperature. Taking into account the slurrying ability and energy consumption during the lignite upgrading process, the most suitable pyrolysis temperature is between 600 °C and 800 °C. 5. LCWS exhibits a distinct rheological behavior in raw/dry coal water slurry, which may be possibly due to the discrepant slurry concentration at different pyrolysis temperatures. LCWS is not as stable as the raw coal and dry coal water slurry due to the reduction of hydrophilic groups. Acknowledgments This work was supported by the National Basic Research Program of China (2012CB214906) and the Program of Introducing Talents of Discipline to University (B08026). References [1] X.Y. Ma, Y.F. Duan, M. Liu, H.F. Li, Influence of sewage sludge on the rheological properties of petroleum coke–water slurry, Asia-Pacific Journal of Chemical Engineering 8 (2013) 453–460.
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