Energy & Fuels 2009, 23, 683–689
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Formation Mechanism of Agglomeration Caused by Burning NSSC Black Liquor in a Fluidized Bed Incinerator Rushan Bie,* Ying Zhao, Zhigang Chen, Jie Lu, and Lidan Yang School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, China ReceiVed August 30, 2008. ReVised Manuscript ReceiVed NoVember 12, 2008
Experiments were carried out to investigate the agglomeration mechanism of neutral sulfite semi-chemical (NSSC) black liquor combusted in a bench-scale fluidized bed. The combustion experiments were focused on the influence of different operating conditions at temperatures ranging from 600 to 800 °C and at additive dosages from 0 to 30% with different additives (kaolin, calcium carbonate, and alumina). Defluidization resulting from agglomeration occurred frequently in the experiments. Samples of the bed material collected after the experiments were characterized by scanning electron microscopy, energy dispersive X-ray detection and X-ray diffractometry. There were explainable differences in the experimental phenomena and surface characteristics of the sinters depending on different operating conditions in this study, which can also be found in other studies. The mechanism has also explained that the additive is effective in solving the agglomeration problem. The experimental results indicate that the formation of the agglomeration in the fluidized bed burning NSSC black liquor depends mainly on the temperature, additive dosage, and additive species.
1. Introduction In general, neutral sulfite semi-chemical (NSSC) pulp is produced under alkaline conditions by cooking the raw materials (wood chips) using sodium sulfite and soda (or baking soda). Sodium sulfite is added as a cooking chemical in order to obtain the weight percentage, with respect to dry wood chips, of about 8-14%. Soda is added as a buffer in order to get the weight percentage to dry wood chips to be about 3-5%. After cooking, the wastewater that is removed from the pulp is called the NSSC black liquor. Black liquor, a kind of pulping wastewater generated during the chemical cooking process, is now the biggest threat to the water environment. It is a highly viscous liquor that contains about 60% organic matter (alkali lignin, hemicellulose, polysaccharides, extractives) and 40% inorganic cooking chemicals on a dry basis.1 It is a residue that is formed after the cooking of wood, straw, or other fibrous plants in the production of pulp and paper. It can be burned as a biomass fuel in a recovery boiler to recover the heat energy, and the inorganic residue, which contains mainly alkali, can be reused as pulping chemicals. Hence, it could offer potential advantages in both energy sources and environmental benefits. Alkali recovery technology is the most effective method to treat black liquor of pulp production plants. It has evoked the wide interest of many researchers in recent years in China. However, it is difficult to carry out for small pulp plants and hence causes the most serious pollution, because the traditional alkali recovery systems need a high initial investment. In recent years, fluidized bed technology in environmental pollution control has shown many great advantages: high thermal capacity, complete particle mixing, high heat and mass transfer * To whom correspondence should be addressed. Phone/fax: +86-45186413232; e-mail:
[email protected]. (1) Sricharoenchaikul, V.; Frederick, W. J.; Agrawal, P. Carbon distribution in char residue from gasification of kraft black liquor. Biomass Bioenergy 2003, 25 (2), 209–220.
efficiency, uniform temperature distribution, low SOx and NOx emission, etc. Hence, the fluidized bed alkali recovery technology is a potential process for dealing with the black liquor from the scientific and industrial points of view. It is important to choose the right kind of bed material for fluidized bed combustors. Early studies in India, Denmark, Australia, etc. employed amphoteric oxide (Fe2O3) as fluidized bed material, and scholars from Japan reported on the incineration reaction of black liquor with Fe2O3 within a temperature range from 900 to 1000 °C in the fluidized bed.2 However, it has a limitation for kraft or NSSC black liquor, because Fe2O3 could react with sulfide to produce Fe2S3, which hinders the recycling of Na2S. On the other hand, Fe(OH)3 coming from the hydration of incineration productions can form colloids that could negatively affect the recovery of alkali. The mechanisms of the reactions involved have been discussed extensively. In particular when using TiO2 as the bed material, the process underwent considerable development in the 1990s by Canada’s University of New Bruswick.3-7 However, the high cost made the application of TiO2 difficult to put into practice. Shreyans Company of India researched that the problems caused by amphoteric oxide can be effectively avoided by using Na2CO3 (2) Nagai, C. Development of iron oxide bubbling fluidized bed in direct causticization process. Kami Pa Gikyoshi/Jap. Tappi J. 2001, 55 (3), 101– 108. (3) Pels, J. R.; Zeng, L.; van Heiningen, A. R. P. Direct causticization of kraft black liquor with TiO2 in a fluidized bed-identification and analysis of sodium titanates. J. Pulp Pap. Sci. 1997, 23 (12), 549–554. (4) Zeng, L.; Pels, J. R.; van Heiningen, A. R. P. Direct causticization of kraft black liquor solids with TiO2 in a fluidized bed. Tappi J. 2000, 83 (12), 53. (5) Zeng, L.; van Heiningen, A. R. P. Pilot fluidized-bed testing of kraft black liquor gasification and its direct causticization with TiO2. J. Pulp Pap. Sci. 1997, 23 (11), 511–516. (6) Zeng, L.; van Heiningen, A. R. P. Sulfur distribution during air gasification of kraft black liquor solids in a fluidized bed of TiO2 particals. Pulp Pap. Can. 1996, 100 (6), 58–62. (7) Zeng, L.; van Heiningen, A. R. P. Carbon gasification of kraft black liquor solids in a presence of TiO2 in a fluidized bed. Energy Fuels 2000, 14 (1), 83–88.
10.1021/ef800725f CCC: $40.75 2009 American Chemical Society Published on Web 12/23/2008
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Bie, et al. Table 1. Ultimate Analysis of NSSC Black Liquor on a Dry Basis sample NSSC black liquor (dry basis)
C (%)
H (%)
O (%)
Na (%)
S (%)
Cl (%)
K (%)
N (%)
36.32
3.43
35.17
18.35
5.45
0.21
1.03
0.04
Table 2. Proximate Analysis of NSSC Black Liquor on a Dry Basis sample
Figure 1. XRD analysis of the kaolin.
as the bed material to deal with wheat black liquor in the fluidized bed reactor at temperatures between 650 and 700 °C.8 Another widely studied phenomenon is black liquor swellingl; Gea et al.9 presented data that showed the influence of the reactor temperature and the gas composition on the swelling during devolatilization and compared the swelling characteristics of a straw black liquor with similar published data on black liquors from kraft pulping of wood. Kari10 reported the swelling, pyrolysis, corrosion, and reactivity of the black liquor on laboratory tests. Olazaar et al.11 suggested that the loss of fluidization due to bed agglomeration was found to be the main problem in the fluidized bed reactor used. Some investigations had been undertaken to study the mechanism of the black liquor gasification.1,12-14 There is, relative to other biomass residues, little data on kraft wood black liquor and soda straw black liquor agglomeration in fluidized beds. Information about the agglomeration of the NSSC black liquor is even more limited. Whitty15 indicated that different types of black liquors could behave very differently in the same recovery boiler under the same operating conditions. Therefore, a specific study of each type of liquor is necessary to characterize their behaviors during the fluidized bed alkali recovery process. This paper describes a case study on the bed material agglomeration as NSSC black liquor combusted in a fluidized bed. The aim is to determine the effect of temperature, additive dosage, and additive species on agglomeration in the fluidized bed. Experimental analyses and measurements of bed agglomerates are conducted using several approaches including scanning electron microscopy (SEM), energy dispersive X-ray detection (EDX), and X-ray diffraction (XRD). 2. Experimental Methods 2.1. Materials. The black liquor used in this study had been provided by a Chinese paper mill where the raw material is wood (8) Song, D. L.; Kuang, S. J. Fluidized bed technology for black liquor from agricultural residues. World Pulp Paper 2001, 21 (1), 44–47. (9) Gea, G.; Muyrillo, M. B.; Arauzo, J.; Frederick, W. J. Swelling behavior of black liquor from soda pulping of wheat straw. Energy Fuels 2003, 17 (1), 46–53. (10) Kari, S. Black-liquor gasification: Results from laboratory research and rig tests. Bioresour. Technol. 1993, 46 (1∼2), 145–151. (11) Olazaar, M.; Aguado, R.; Sanchez, J. L.; Bilbao, R.; Arauzo, J. Thermal procession of straw black liquor in fluidized and spouted bed. Energy Fuels 2002, 16 (6), 1417–1424. (12) Sa′nchez, J. L.; Gonzalo, A.; Gea, G.; Bilbao, R.; Arauzo, J. Straw black liquor steam reforming in a fluidized bed reactor. Effect of temperature and bed substitution at pilot scale. Energy Fuels 2005, 19 (5), 2140–2147. (13) Whitty, K.; Backman, R.; Hupa, M. Influence of char formation conditions on pressurized black liquor gasification rates. Carbon 1998, 36 (11), 1683–1692. (14) Demirbas, A. Pyrolysis and steam gasification processes of black liquor. Energy ConVers. Manage. 2002, 43 (7), 877–884. (15) Whitty, K.; Backman, R.; Forsse´n, M.; Hupa, M.; Rainio, J.; Sorvari, V. Liquor-to-liquor differences in combustion and gasification processes: pyrolysis behaviour and char reactivity. J. Pulp Pap. Sci. 1997, 23 (3), 119–128.
NSSC black liquor (dry basis)
volatile (%)
ash (%)
fixed carbon (%)
HHV (MJ · kg-1)
50.62
24.17
25.21
14.98
Table 3. Oxide Analysis of NSSC Black Liquor Ash by XRF sample
Na2O (%)
SO3 (%)
Cl (%)
K2O (%)
NSSC black liquor ash
93.79
3.75
0.34
2.12
chips of broadleaf. The NSSC pulp is cooked with sodium sulfite and soda as the buffer. Hence, there are mainly sodium salts as the inorganic composition in the black liquor. The NSSC black liquor has a solid content of 43.78% by weight. The ultimate analysis of the NSSC black liquor solids is obtained in a CHNS elemental analyzer, an inductively coupled plasma atomic emission spectrometer, and a chloride-ion titration analyzer, which is shown in Table 1. The proximate analysis of the NSSC black liquor solids is obtained using a muffle furnace and a calorimeter, which is shown in Table 2. High ash contents and a large quantity of alkali metal (Na) are found in the black liquor, and other major elements are S and K, which are found with concentrations of more than 1%. The elemental compositions (as % of oxide) analysis of the NSSC black liquor ash as determined by XRF is shown in Table 3. The bed material consisted of 400 g of Na2CO3 (analytical purity) with particle diameters between 0.2 and 0.71 mm in the experiments, corresponding to a bed height of 300 mm, in order to cover over the thermocouple. Because the NSSC black liquor mostly converts to Na2CO3 after combustion, adopting Na2CO3 as the bed material does not introduce further matter. An additional advantage is that Na2CO3 pellets can be fluidized easily and then recovered directly or causticized to recover NaOH; the problems caused by amphoteric oxide can be effectively avoided. Additives were mixed into the black liquor equably, these are Al2O3 (analytical purity), CaCO3 (analytical purity), and kaolin (chemical purity, components are shown in Figure 1). In all experiments, particle size distributions were approximately in the same proportion as those of the bed material. 2.2. Experimental Apparatus and Procedure. The work described in this paper was performed on a bench-scale bubbling fluidized bed, shown schematically in Figure 2, that consists of four parts: reactor section, dosing section, temperature control section, and data acquisition section. The bed is made of a high-temperature-resistant nickel-chromium stainless steel tube with a height of 600 mm and an inner diameter of 40 mm. A perforated steel air distributor is located at the bottom of the bed. Below this position is the preheat zone, and above it is the combustion zone. The bed temperature is automatically controlled by the temperature controller, which allows the bed temperature to vary in the range of 273-1273 K. The combustion and flow behaviors can be observed through a window at the top of the bed. The pressure drop over the bed was monitored by a pressure gauge. The pressure versus time profile and visual observation were the indicators of defluidization. The pressure versus time profile was used to evaluate the defluidization time.16,17 (16) Chirone, R.; Miccio, F; Scala, F. Mechanism and prediction of bed agglomeration during fluidized bed combustion of a biomass fuel: Effect of the reactor scale. J. Chem. Eng. 2006, 123 (3), 71–80. (17) Scala, F.; Chirone, R. Characterization and early detection of bed agglomeration during the fluidized bed combustion of olive husk. Energy Fuels 2006, 20 (1), 120–132.
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Energy & Fuels, Vol. 23, 2009 685 Table 5. Particle Size Distributions at Different Temperatures particle size (mm)
20 °C (room temperature)
600 °C (%)
650 °C (%)
700 °C (%)
750 °C (%)
800 °C (%)
0-0.2 0.2-0.355 0.355-0.5 0.5-0.71 0.71-1 >1
0 15.3 7.6 73.6 3.5 0
0.8 8.8 17.4 70.5 2.5 0
1.6 11.2 16.1 68.2 2.9 0
3.1 13.8 11.4 65.7 4.3 1.7
6.3 14.8 7.9 49.9 3.3 17.8
8.3 9.5 4.7 45.5 4.2 27.8
Table 6. EDX Analyses of Particles Sampled at Different Temperatures Without Additive
Figure 2. Schematic diagram of experimental apparatus (1) pressure gauge; (2) temperature controller; (3) thermocouple; (4) peristaltic pump; (5) observation window; (6) silica gel desiccator; (7) flue gas analyzer; (8) heating element; (9) refractory; (10) insulation; (11) air distributor; (12) plenum box; (13) drainage pipe; (14) air heater; (15) flow meter; (16) air compressor.
Figure 3. Defluidization time at different temperatures. Table 4. Agglomeration Property at Different Bed Temperatures bed temperature (°C)
fluidization trait
600 650 700
stable stable unstable
750 800
unstable bally unstable
agglomeration intensity
burnout behavior
not almost not comparatively severe severe more severe
incompleteness incompleteness comparative completeness completeness completeness
The defluidization is marked by a sharp decrease in the pressure drop over the bed. In addition, the fluidization of the bed can be observed through the observation window. In the test, defluidization was observed through the window while the pressure drop began to decrease rapidly at the same time. As soon as defluidization occurred, the black liquor feed was stopped. The time interval between the start of black liquor feeding to defluidization occurrence of the bed is defined as the defluidization time. A small amount of flue gas is induced by a pump to a flue gas analyzer (KM9106 made in Britian) to monitor the concentrations of O2, CO2, and CO. A silica gel desiccator removes the water in the gas before it enters the analyzer. The experiments were divided into three groups, the first was to determine the influence of incineration temperature on agglomeration without additive, the second was to determine the influence of
bed temperature (°C)
Na2O (%)
Al2O3 (%)
SiO2 (%)
SO3 (%)
Cl (%)
K2O (%)
Fe2O3 (%)
600 650 700 750 800
79.68 81.67 80.54 84.06 85.25
1.55 0.86 1.51 1.05 1.56
2.70 2.60 3.14 2.70 1.21
6.44 6.37 6.57 4.83 4.85
1.87 1.67 1.50 1.18 0.40
4.90 3.89 3.82 3.63 4.00
2.86 2.94 2.92 2.55 2.73
additive dosage on agglomeration, and the third was to ascertain the influence of additive species on agglomeration. After the fluidized bed was heated to 200 °C, a small stream of air was fed into the chamber through the air distributor. When the temperature reached the preset requirement, the air was supplied by an air compressor at a feeding rate of 0.2 m3 · h-1. With the average feeding rate of 240 g · h-1, the NSSC black liquor was sprayed by means of a peristaltic pump at dilute phase region, dried as it fell into the bed, mixed with bed material, and finally burnt in the dense bed. After the black liquor combusted stably for 10 min, the concentrations of O2, CO2, and CO were continuously monitored. During the experimental run, the burnout behavior of flue gas was examined by concentrations of exhaust gases. The extent of black liquor (char particles) burnout was evaluated by loss on ignition (LOI), which was measured by igniting the pulverized, sintered sample for 3 h at 900 °C ((25 °C) in a muffle furnace. The combustion and flow behavior were observed through the observation window. When the experiment finished, the bed was cooled to room temperature and the agglomerate was collected for analysis. 2.3. Analytical Determinations. Physical and chemical changes of the agglomeration were ascertained from SEM, EDX and XRD. Many analyses were carried out for different samples, including three types of sinters formed from the test in the bed reactor at different temperatures, different amounts of additive, and different additive species, respectively. From these analyses, the microstructure of agglomerates, the mineral compounds present in agglomerates, and the elements on their surfaces can be obtained, especially the possible mineral transformation during the sinter formation.
3. Results and Discussion 3.1. The Influence of Bed Temperature. The results of the combustion tests show that the intensity of agglomeration grows and the flow of the bed material becomes worse with increasing incineration temperature. The microstructure and mineral compounds in the sinter also change to a certain degree. When the operating temperature exceeds the melting points of the eutectics, these eutectics melt to flow easily toward the surfaces of other particles by particle collisions. When the system does not undertake enough stress to detach the two particles, they remain attached to each other. As the agglomeration process continues, the agglomerate makes the bed material uneven and changes the size and density of the bed material. Table 4 shows the experimental phenomena and agglomeration property at different bed temperatures (600, 650, 700, 750, and 800 °C, respectively). Analysis of Table 4 suggests that the bed temperature is an important parameter to evaluate the black liquor combustion. As the temperature increases, the concentration of CO and LOI decrease, implying that the black
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Figure 4. SEM analyses of particles sampled at different temperatures without additive.
Figure 5. XRD analysis of particles sampled at 750 °C without additive. Table 7. Agglomeration Property with Different Additive Dosages at 750 °C additive dosage (%)
fluidization trait
agglomeration intensities
burnout behavior
0 10
unstable unstable
completeness completeness
30
stable
severe comparatively severe almost not
completeness
liquor burns out completely. As the temperature increases and the defluidization time decreases, the sinter size and the degree of agglomeration increase. The defluidization time as a function of the bed temperature is plotted in Figure 3. The result shows that the influence of the temperature on the defluidization time is significant. Fluidization of the bed material is still stable after 4 h of experiment time at 600 and 650 ° C, but the defluidization time at 800 °C is less than that at 750 or 700 °C. The result is consistent with those of C. L. Lin,18 who found that the defluidization time decreases with increasing temperature. (18) Lin, C. L.; Wey, M. Y. The effect of mineral compositions of waste and operating conditions on particle agglomeration/defluidization during incineration. Fuel 2004, 83 (17-18), 2335–2343.
Figure 6. Defluidization time with different additive dosages at 750 °C. Table 8. Particle Size Distributions with Different Additive Dosages at 750 °C particle size (mm)
0 (%)
10% (%)
30% (%)
0-0.2 0.2-0.355 0.355-0.5 0.5-0.71 0.71-1 >1
6.3 14.8 7.9 49.9 3.3 17.8
8.2 15.6 10.8 50.5 4.6 10.2
8.8 13.1 12.8 51.5 5.4 8.4
Particle size distributions of the bed material samples at different temperatures were measured and are summarized in Table 5. It can be seen that the particle size distributions at the five temperatures are comparable; after combustion, bed material of the particle size below 0-0.2 mm is higher than before combustion. From considerations of all particle size distribution data, it can be found that the bed materials of oversieve size range (>1 mm) increases with increasing temperature. It is also shown that the degree of agglomeration increases. 3.1.1. SEM-EDX Analysis. The SEM photographs with a variation of bed temperature from 600 to 800 °C are shown in Figure 4. The photographs show the surfaces of the bed materials
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Figure 7. SEM analyses of particles sampled with different additive dosages at 750 °C. Table 11. Particle Size Distributions Using Different Additives particle size (mm) 0-0.2 0.2-0.355 0.355-0.5 0.5-0.71 0.71-1 >1
Figure 8. XRD analysis of particles sampled with 30% Al2O3 added at 750 °C.
Figure 9. Defluidization time using different additives. Table 9. EDX Analyses of Particles Sampled with Different Additive Dosages at 750 °C additive dosage (%)
Na2O (%)
Al2O3 (%)
SiO2 (%)
SO3 (%)
Cl (%)
K2O (%)
Fe2O3 (%)
0 10 30
84.06 55.90 36.28
1.05 32.41 52.59
2.70 2.19 0.38
4.83 3.21 3.69
1.18 0.96 0.69
3.63 3.24 4.26
2.55 2.09 2.11
Table 10. Agglomeration Property Using Different Additives at 750 °C additive
fluidization trait
alumina
unstable
calcium carbonate
comparatively stable stable
kaolin
agglomeration intensities
burnout behavior
comparatively severe comparatively severe almost not
completeness completeness completeness
clearly. At low temperatures (600 and 650 °C), we can clearly see that the border is clear and maintains a certain distance between particles and that the bed material is not agglomerated. The degree of agglomeration increased with increasing incineration temperature. At 700 °C, the shape of some particles
alumina (%)
calcium carbonate (%)
kaolin (%)
8.2 15.6 10.8 50.5 4.6 10.2
8.1 7.7 41.9 20.1 12.8 9.4
2.1 3.6 51.2 21.8 16.1 5.2
becomes indistinguishable, but many interstices can also be seen on the surface of the sinter. However, unlike the porous coating of the low temperature, at higher temperatures (750 and 800 °C) the bed material coating shows a solid structure, indicating that the coating was molten. According to previous research,19-21 eutectics exist in agglomerates when the system contains alkali metals. It is suggested that the formation of the low melting point compounds might be the main route leading to the clinker formation. This reveals that the structure of particles depends on the reaction temperature. From the EDX analyses of sinters, it can be concluded that, in the temperature range from 600 to 800 °C, the elements of the high content are Na, K, S, and Cl. Other elements such as Si, Fe, etc. are impurities, which come from the experimental process, and can be ignored in the forthcoming analyses. This indicates that particles were connected via compounds with low melting points. These compounds might be sodium salt that forms eutectics with a low melting point, which flow easily to form the liquid bridge leading to the sinter formation in the study, and the potassiumrich coating on the particles sampled might contribute to the stickiness at higher temperatures. It is noteworthy to consider that, from EDX analyses of agglomerations, the content of Cl is higher under 750 °C than at 800 °C, where more HCl produced from chlorinated organic compounds will be removed at higher temperatures.22 3.1.2. XRD Analysis. The phase compositions of the five particles sampled at different temperature as determined by XRD analysis shows that the only three crystalline phases detected by XRD are Na2CO3, Na2SO4, and NaCl. The XRD results indicate that the forms of inorganic matter are almost insensitive (19) Yan, R.; Liang, D. T.; Laursen, K.; Li, Y.; Tsen, L.; Tay, J. H. Formation of bed agglomeration in a fluidized multi-waste incinerator. Fuel 2003, 82 (7), 843–851. (20) Zevenhoven-Onderwater, M.; Backman, R.; Skrifvars, B. J.; Hupa, M.; Liliendahl, T.; Rose´n, C.; Sjo¨stro¨m, K.; Engvall, K.; Hallgren, A. The ash chemistry in fluidized bed gasification of biomass fuels. Part II: Ash behaviour prediction versus bench scale agglomeration. Fuel 2001, 80 (10), 1503–1512. (21) Lin, W. G.; Dam-Johansen, K.; Frandsen, F. Agglomeration in biofuel fired fluidized bed combustors. Chem. Eng. J 2003, 96 (1-3), 171– 185. (22) Bie, R. S.; Li, S. Y.; Yang, L. D. Reaction mechanism of CaO with HCl in incineration of wastewater in fluidized bed. Chem. Eng. Sci. 2005, 60 (3), 609–616.
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Figure 10. SEM analyses of particles sampled using different additives at 750 °C.
Figure 11. XRD analysis of particles sampled using different additives at 750 °C.
to change in the range of bed temperature. The XRD analysis of the sinter at 750 °C is shown in Figure 5. It does not agree with qualitative SEM/EDX results in which Na, K, S, and Cl are present between particles. It is likely that the other eutectics of K in the connecting bridge can be considered too thin to be detected by XRD. The melting points of Na2CO3, Na2SO4, and
NaCl are 851, 882, and 801 °C, respectively. When the three alkali metal salts are mixed, they can form lower melting point eutectics NaCl-Na2CO3-Na2SO4. The melting point of eutectics is only 612 °C, in which, for example, 50% NaCl, 26% Na2SO4, and 24% Na2CO3 are mixed. Therefore, it is suggested that these melting alkali metal salt materials easily form the liquid phase and become very sticky materials that can cause very rapid defluidization of the fluidized bed, eventually leading to catastrophic sintering during the high temperature combustion. 3.2. The Influence of Additive Dosage. The results of the combustion tests of adding different additive (Al2O3) dosages at the four temperatures (650, 700, 750, and 770 °C) elucidated that the intensity of agglomeration was lessening and that the flow of the bed material became increasingly better with increasing additive dosage. The experimental phenomena and agglomeration property at different Al2O3 dosages (0, 10, and 30%, respectively) at 750 °C is shown in Table 7. It suggests that the additive dosage also plays an important role in combustion of black liquor. Just as during the operating condition of adding 10% Al2O3, agglomeration mitigation is not obvious. If 30% Al2O3 is added, the effect on the flow of bed materials is better and agglomeration is decreased. As the additive dosage increases, the degree of agglomeration decreases. The defluidization time as a function of the additive dosage is plotted in Figure 6. The result suggests that Al2O3 dosage is a significant factor in the defluidization time. The flow of the bed material can be only maintained for 55min, but it can be kept for 220 min when 30% Al2O3 is added. It is a notable find that the additive could improve the combustion of black liquor. The influence of different additive dosages for particle size distributions of bed material samples at 750 °C were measured and summarized in Table 8. It can be seen that bed materials of oversieve size range (>1 mm) decreased as Al2O3 dosage increased. It is also shown that the degree of agglomeration decreases. 3.2.1. SEM-EDX Analysis. The SEM photographs with microstructure change of sinters when adding 0, 10, and 30% additive at 750 °C are shown in Figure 7. The surface of the sinter when 10% Al2O3 was added to the black liquor is similar to that without Al2O3. Unlike the porous coating that formed when 30% Al2O3 was added to the black liquor, the two sinter coatings of adding 0 and 10% Al2O3 denote that the coatings had been molten. EDX analyses from Table 9 show that as the additive dosage increases, the content of Al increases, and the content of Na decreases. It is suggested that as the additive dosage increases, less sodium salt eutectics are generated to move to other surfaces of the agglomerates. Hence, the surface
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Table 12. EDX Analyses of Particles Sampled Using Different Additives at 750 °C additive kaolin calcium carbonate alumina
Na2O (%)
Al2O3 (%)
SiO2 (%)
SO3 (%)
Cl (%)
K2O (%)
Fe2O3 (%)
CaO (%)
46.01 50.86
8.16 0.03
15.31 2.13
4.20 3.20
1.63 2.33
1.32 2.14
1.95 1.51
21.42 37.80
55.90
32.41
2.19
3.21
0.96
3.24
2.09
area covered by the eutectics decreases, as seen in Figure 7c, where a thinner layer of eutectics is present than in Figure 7a and b. 3.2.2. XRD Analysis. The crystalline phases of the sinters that were added when 10 and 30% Al2O3 was introduced into the black liquor are Al2O3, Na2CO3, Na2SO4, and NaCl by XRD analysis. The XRD analysis of the sample after adding 30% additive at 750 °C is shown in Figure 8. Since XRD results indicate that Al2O3 does not react with sodium salts to form the new high melting crystalline solid phases (such as NaAlO2) in any of the samples at 750 °C, the results indicate that Al2O3 as a neutral medium could reduce the formation of lower melting point eutectics NaCl-Na2CO3-Na2SO4 to alleviate the degree of agglomeration. On the other hand, Al2O3 obstructs these eutectics melts to flow easily toward the surface of other particles to make the bed material even and the fluidization stable. 3.3. The Influence of Additives. The experimental phenomena and agglomeration property were studied by adding each of the following additives: alumina, calcium carbonate, and kaolin, which account for 10% of the black liquor at 750 °C. From Table 10, it is suggested that the best effect is obtained by kaolin added, followed by calcium carbonate, whereas alumina addition performs worst. The defluidization time of different additives is plotted in Figure 9. The result suggests that the flow of the bed material with adding Al2O3 into the black liquor can only be maintained for 70 min, but it can be kept for 220 min when kaolin is added. It is found that the addition of kaolin is most effective for improving the combustion of black liquor. Particle size distributions of bed material samples using different additives at 750 °C were measured and are summarized in Table 11. It can be found that bed material with kaolin additive has less particles of oversieve size range (>1 mm) than those of Al2O3 and CaCO3 additives. It is also shown that the degree of agglomeration is lightest with the addition of kaolin. 3.3.1. SEM-EDX Analysis. The SEM photographs with microstructure changes due to the addition of 10% of different additives at 750 °C are shown in Figure 10. The structure of the sinter border, which was formed when 10% kaolin was added into the black liquor, is clearer than those with calcium carbonate and alumina added. It can be seen that there is clear evidence of the effect of added kaolin, resulting in a better additive than calcium carbonate or alumina. EDX analyses from Table 12 show that the major elements of each sinter at different
additives are consistent with the composition of the black liquor and additives. 3.3.2. XRD Analysis. The crystalline phases of sinters analyzed by XRD with different additives are shown in Figure 11. For kaolin and calcium carbonate, XRD results at 750 °C reveal that the crystalline form of calcium carbonate was partly decomposed into calcium oxide. Al2O3 · 2SiO2 · 2H2O component of kaolin reacted with sodium salts to form new high melting crystalline solid phases (NaAlSi3O8). So the additives calcium carbonate and alumina with high melting points could alleviate the degree of agglomeration because of their physical effect. The additive kaolin with a high melting point could alleviate the degree of agglomeration because of the results of physical and chemical effects. 4. Conclusions Fluidized bed technology is an effective method for dealing with black liquor. The experimental results show that agglomerate behavior in the fluidized bed is very complex. The agglomeration and defluidization phenomena in a fluidized bed of black liquor are caused by the high alkali metal salts (sodium salts) content in the bed material and black liquor when the bed temperature is over 650 °C. The agglomeration and defluidization processes are very sensitive to temperature. The eutectics generated were found to cause the degree of agglomeration to increase as the operating temperature increased. SEM analyses showed that intensity of agglomeration was growing as the temperature increased. EDX and XRD analyses show that the melting of NaCl-Na2CO3-Na2SO4 compounds was clearly identified as the coating layer on the sinter surfaces because of their low melting points, which causes the formation of agglomerates and eventually defluidization during incineration. When different Al2O3 additive dosages were added to the black liquor, it was shown that it can be effective in reducing the agglomeration problem. Moreover, the fluidization trait of the bed material was still stable when 30% Al2O3 was added to the black liquor at 750 °C. When additives, such as kaolin, calcium carbonate, and alumina were added into the black liquor, it found that these added materials lowered the degree of agglomeration and that the effect of adding kaolin was the best. Hence, the fluidization of the system was extended and the generation of agglomeration was delayed. In terms of SEM, EDX, and XRD analyses, additives were believed to play an important role in reducing the formation of lower melting point eutectics. Acknowledgment. The authors gratefully acknowledge the financial support from the Natural Science Foundation of Heilongjiang Province (B0317). Thanks also go to the School of Materials Science and Engineering at Harbin Institute of Technology for their analytical assistance. EF800725F
