Reed Black Liquor Combustion in Fluidized Bed for Direct

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Reed Black Liquor Combustion in Fluidized Bed for Direct Causticization with Limestone as Bed Material Xiaoyu Ji, Rushan Bie,* Pei Chen, and Wenbo Gu School of Energy Science and Engineering, Harbin Institute of Technology, Harbin 150001, P.R. China ABSTRACT: For a reliable and efficient process for the recovery of energy and alkali of reed black liquor (BL), the experiments of reed BL combustion in a fluidized bed with limestone as bed material for direct causticization were conducted at temperatures from 700 to 900 °C. The products in the direct causticizing process were identified by XRD, and the recovery rate of alkali was measured. In addition, the mechanism of inhibiting bed material agglomeration was studied. The results show that a direct causticizing system successfully recovers NaOH when using limestone as bed material, and the recovery rate of NaOH is more than 87.4%. Na2O·2SiO2 produced in the combustion process is the main binder of agglomeration. The limestone has a strong effect on inhibiting bed agglomeration, as the decomposition of limestone can provide CaO for the reaction with silica forming high-melting Ca2SiO4 and 3CaO·Al2O3·3SiO2 instead of low-melting Na2O·2SiO2.

1. INTRODUCTION Black liquor, which is the waste liquid generated in the chemical pulping process of papermaking, is the major cause of serious water pollution in the paper industry.1,2 Generally, it can be divided into wood and straw pulp based on the different raw materials. The BL mainly consists of alkali lignin, hemicellulose, polysaccharides, extractives, and some inorganic compounds from the chemicals used in the pulping process.3 Thus, the BL is also a potential energy source for pulp mills. For the economics of the paper industry, the recovery of the chemicals and energy of BL is very important. Nowadays, the wood BL can be burned in a Tomlinson recovery boiler for energy generation and chemical recovery. However, it is very difficult to apply the Tomlinson recovery technology to the straw BL because it has higher contents of silica and chlorine, higher viscosity, and a lower heat value compared with wood BL.4−6 Therefore, some alternative recovery technologies that are safer, easier, and more efficient have been developed over the past 20 years.7−9 Among them, the fluidized bed alkali recovery technology is a well-established and widely used method for BL treatment. In this system, the BL is burned in a fluidized bed to recover the energy and chemicals. It has many advantages, such as better fuel flexibility, higher heat transfer rate, lower investment and emission level, and so forth. However, the problem of bed material agglomeration was found when burning the straw BL in the fluidized bed.10−13 Related studies have shown that the agglomeration is mainly attributed to the presence of alkali elements,14−18 chlorine,19 and silicon.11,20 Low-melting alkali eutectics (such as NaCl and KCl) and sodium silicates will be generated during the BL combustion.10,11,21−23 Under high temperature, these lowmelting compounds will melt and cause the “melt-induced” and “coating-induced” agglomeration of bed material.24−26 The Indian Shreyans company can remove 90% chlorine in raw materials by water leaching; then, the Na2CO3 particles can be successfully produced by burning the straw BL in the fluidized bed.27 However, low-melting sodium silicate was produced during the combustion, which causes defluidization when the combustion temperature is higher than 730 °C. The calcium© 2016 American Chemical Society

based zeolite was used as bed material to burn reed BL in our previous study, and the defluidization did not occur within 4 h below 850 °C.28 However, to recover the alkali, the bed material should be soaked in water after combustion. The sodium salts (mainly Na2CO3) are dissolved in water to form green liquor. Then, CaO is added to the green liquor, and NaOH is recovered from the causticizing reaction,29 as shown in eqs 1 and 2. CaO + H 2O = Ca(OH)2

(1)

Ca(OH)2 + Na 2CO3 = 2NaOH + CaCO3

(2)

After causticizing, the aqueous solution of NaOH is reused to dissolve alkali in the pulping process of papermaking. The CaCO3 is removed mechanically and sent to the rotary kiln where CaCO3 is calcined to CaO using natural gas as an auxiliary fuel. It can be seen that the process is complicated, which gives high capital costs. Furthermore, the calcination of CaCO3 would cause additional energy consumption. In recent years, direct causticizing technology was investigated to improve the efficiency of the traditional alkali recovery process. Nagai30 sprayed Fe2O3 into a furnace as an automatic causticizing agent when the wood BL was burned in a fluidized bed. The Fe2O3 can react with Na2CO3 to produce Na2Fe2O4 in the temperature range 900−1000 °C. Then, the NaOH can be recovered from hydrolysis of Na2Fe2O4. However, with the high silicon content in straw BL, the Na2Fe2O4 cannot be hydrolyzed easily. In addition, it has been proven that this technology is unsuitable for commercial pulp mills because of the formation of large amounts of iron oxide dust.31 BL gasification with direct causticizing using TiO2 has also been studied by some researchers.32−36 However, because of the high cost of TiO2, it cannot be widely used in industry. In this paper, limestone was used as the bed material for directly causticizing and inhibiting bed agglomeration when Received: April 11, 2016 Revised: May 26, 2016 Published: June 14, 2016 5791

DOI: 10.1021/acs.energyfuels.6b00847 Energy Fuels 2016, 30, 5791−5798

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Energy & Fuels reed BL was burned in a fluidized bed. The limestone will decompose into CaO when the temperature is higher than 825 °C. The reactions shown in eqs 1 and 2 will occur after the bed material and reaction products are soaked in water, and NaOH can be directly recovered when the CaCO3 is recycled as a direct causticizing agent after drying. As can be seen, the NaOH can be recovered without adding CaO, and the calcination of CaCO3 in a kiln is not necessary. Therefore, the direct causticizing process using limestone can significantly simplify the recovery process and avoid additional energy consumption. Meanwhile, the CaO can react with low-melting sodium silicate to form high-melting compounds, such as Na2O·2CaO· 3SiO2 and Na2O·3CaO·6SiO2 during the BL combustion,37,38 thus inhibiting the agglomeration of bed material. The reactions are as follows: Na 2O·3SiO2 + 2CaO = Na 2O·2CaO·3SiO2

seen from Table 1 that the contents of Si and Cl in reed BL are much higher than those in wood BL,39 and its calorific value is relatively low. The limestone with a grain size of 0.1−1 mm (the harmonic mean diameter is 0.529 mm) obtained from Harbin, Northeast China, was employed as bed material. The plot of the minimum fluidization velocity (Umf) as a function of the bed temperature is shown in Figure 1. The element composition of limestone as determined by XRF is shown in Table 2. The bed material is completely new for each experiment.

(3)

Na 2O·3SiO2 + 3SiO2 + 3CaO = Na 2O·3CaO·6SiO2 (4)

The work presented in this paper is applied to reed BL because the amount of pulp produced in China made using reed has increased significantly in recent years. The objective of this work is to study the effect of limestone on direct causticizing and inhibiting agglomeration as reed BL is burned in a fluidized bed. The products in the direct alkali recovery process were identified, and the alkali recovery rate was measured. In addition, the mechanism of inhibiting agglomeration at different temperatures was investigated. This work aims to provide a more reliable and efficient process for the recovery of energy and alkali of reed BL.

Figure 1. Minimum fluidization velocity (Umf) of bed material at different temperatures.

Table 2. Elemental Analysis of Limestone by XRF

2. MATERIALS AND METHODS 2.1. Materials. The reed BL used in this study was provided by a paper mill in Daqing, Northeast China. It has a solid content of 40% by weight. The ultimate and proximate analyses of reed BL and some other typical BL on dry basis are shown in Table 1.The contents of C, O, H, S, and N were obtained by an elemental analyzer (Vario micro cube, Germany) according to the ASTM-D3176 testing standard. Other elements such as K, Cl, Si, and Na were detected by X-ray fluorescence (XRF) (AXIOS-PW4400, Netherlands). The proximate analyses were obtained by drying oven, muffle furnace, and calorimeter (YX-ZR/Q 9704, China) using GB/T212-2008 standards. It can be

C H O N S Cl K Na Si other volatile fixed carbon ash LHV (MJ/kg)

RBL

wood BL50

Ultimate Wt % 33.76 35.0 4.15 3.60 36.37 33.90 0.38 0.10 0.95 5.50 1.57 0.50 1.92 2.20 17.64 19.0 1.78 1.48 0.2 Promixate Wt % 50.00 25.61 24.39 13.35

wt (%)

element

wt (%)

O Na Mg Al Si P

38.36 0.068 0.189 0.397 0.019 1.102

Ca Ti Mn Fe K S

43.30 0.025 0.064 0.245 0.118 0.013

2.2. Apparatus. The experimental apparatus for reed BL combustion is schematically shown in Figure 2, which consists of a fluidized bed reactor, temperature control system, BL feeding system, air feeding system, and data acquisition section. The fluidized bed reactor is made of a nickel−chromium stainless steel tube with an inner diameter of 80 mm and a height of 1.5 m. A perforated stainless steel plate located at the bottom of the bed acts as the gas distributor. Air is fed into the furnace by an air compressor and a flow-meter is used to regulate the fluidization velocity. The reed BL is fed into the furnace by a peristaltic pump, and the feeding rate of BL is controlled by adjusting the rotating speed of the peristaltic pump. The reactor is heated by resistance wires, which are wrapped by refractory and insulation material to prevent heat loss. The bed temperature is continuously monitored by three K-type thermocouples, and a temperature controller (TDW, China) is used to adjust the bed temperature in the range of 20−1000 °C. A flue gas analyzer (KM9106, UK) is used to monitor the concentrations of O2 and CO in flue gas. A silica gel desiccator removes the water in the flue gas before it enters into the analyzer. 2.3. Procedures. The limestone was first added into the furnace as the bed material. The weight and stationary height of the bed material was 3000 g and 500 mm, respectively. Then, the fluidized bed was heated by an electric heating system to the setting temperature (700, 750, 800, 850, or 900 °C) and the fluidizing air volume was set to 2.8 m3/h. When the bed temperature reached the set value, the BL was fed

Table 1. Ultimate and Proximate Analysis of Reed BL and Wood BL on Dry Basis fuel

element

wood BL12 32.99 4.00 35.03 0.11 5.02 0.17 0.81 21.87 0.00

22.85 13.09 5792


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Figure 3. CO emission at different temperatures. Figure 2. Schematic diagram of experimental system: (1) induced draft fan, (2) thermocouple, (3) temperature controller, (4) peristaltic pump, (5) observation window, (6) resistance wire, (7) refractories, (8) insulation material, (9) air distribution plate, (10) plenum box, (11) discharge port, (12) flow meter, (13) air compressor, (14) silicone absorber, and (15) flue gas analyzer.

different temperatures. It can be seen that the concentration of CO decreases with increasing temperature below 850 °C and then shows little change above 850 °C. This means that the reed BL burns out completely above 850 °C. The fluidization of bed material was very good at all temperatures, which was shown in the combustion experiments. Defluidization did not occur in the 4 h of combustion, and the agglomerates were not found after discharge of the bed material in the temperature range of 700−900 °C. Compared with the results of our previous studies that defluidization occurred within 25 min at 800 °C with Na2CO3 or SiO2 as bed material,10,13 the present experiments show that the limestone has a better effect on inhibiting bed agglomeration. Table 3 shows the harmonic mean particle size of bed material samples at different temperatures. It is known that the

into the furnace with a steady rate of 228 g/h. Owing to the fact that the fluidized air must be large enough to ensure the fluidization and the combustion amount of BL was limited in our experiments, the excess air coefficient was 4.16 during combustion. The residence time of the bed material in the furnace is 0.5−1 h when the fluidized bed is used in industry. Therefore, in this study, the test time was set to 4 h to ensure the limestone can meet the requirements of industrial application. The fluidization status was observed through a window at the top of the bed. In the test, the defluidization is defined as any condition at which a well-fluidized bed loses fluidization, whether partial or total. The heating of reactor and feeding of reed BL were terminated as soon as the defluidization occurred. After the black liquor burned stably for 10 min, some of flue gas was pumped into the flue gas analyzer to continuously monitor the concentrations of O2 and CO. After each experiment, the interior of the reactor was visually inspected, and the bed material was collected and sieved for agglomerate separation; then, the size distribution of bed material was measured. Selected samples of agglomerates and bed material were ground to powder for XRF and X-ray diffraction (XRD) (D/max-rB, Japan) analyses to identify the composition of elements and compounds. In the alkali recovery process, the bed material was soaked into water to recover NaOH by direct causticization; then, the bed material was filtered out after fully mixing with water. The concentrations of Na+ and K+ in the soaking solution were measured by an inductively coupled plasma optical emission spectrometer (ICP-OES) (Optima 5300DV, USA), and the Cl− concentration was measured by liquid chromatography−mass spectrometry (LC-MS) (ICS3000, USA).

Table 3. Harmonic Mean Diameter of Bed Material after Combustion at Different Temperatures temperature

original particles

harmonic mean diameter (mm)

0.529

700 °C 750 °C 800 °C 850 °C 900 °C 0.537

0.541

0.498

0.474

0.477

mean size of the limestone particles tends to increase at 700− 750 °C and decrease at 800−900 °C from the initial bed to the final bed. The increase is caused by the inorganic salts generated in combustion of BL deposits on the surface of bed particles. The reasons for the decrease might be as follows: (1) decomposition of CaCO3, (2) breakage of bed particles by the collision, and (3) the inorganic salts generated in BL combustion forming particles that are much smaller than the origin bed particles. Table 4 gives the elemental composition of bed material at different temperatures as detected by XRF analysis. It shows that the contents of sodium, chlorine, and silicon in bed material added at least 0.3, 0.06, and 0.31% after combustion, respectively, which indicates that some inorganic salts containing sodium, chlorine, and silicon (such as Na2CO3, CaCl2, and Na2O·nSiO2, etc.) were produced during the combustion of reed BL. Moreover, it is observed that the sulfur content in bed material at 850 °C was much higher than that at other temperatures. This is because CaO was produced by

3. RESULTS AND DISCUSSION 3.1. Fluidized Bed Combustion Experiments. For direct alkali recovery, the combustion temperature of reed BL must be higher than the decomposition temperature of limestone. Therefore, the reed BL combustion experiments were conducted in a lab-scaled fluidized bed with limestone as the bed material at 700, 750, 800, 850, and 900 °C to verify whether the reed BL can be burned well at the requisite temperature without the occurrence of defluidization. In addition, the effect of limestone on inhibiting agglomeration can be investigated. Figure 3 shows the CO emission at 5793


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generated by the hydration of CaO. The sodium salts and other phases containing Si and Cl were not detected, which conflicts with the XRF results. Taking into account the fact that the phase content, which is less than 1%, is difficult to detect by XRD, the concentrations of other compounds are too low to be detected compared with CaCO3, CaO, and Ca(OH)2. Moreover, the high-melting matters such as Na2O·2CaO·3SiO2 and Na2O·3CaO·6SiO2, theoretically produced from the reaction between CaO and sodium silicate, are also not detected. It can be inferred that the physical effects might be the main mechanisms for limestone to inhibit the agglomeration. The CaO (melting point of 2580 °C) powder generated from the decomposition of CaCO3 coats on the bed particles and obstructs the low-melting melts to flow along the surface of bed particles to inhibit the agglomeration. 3.2. Direct Alkali Recovery Experiment. The existence of CaO and increasing content of sodium in bed material indicates that the recovery of alkali by direct causticizing technology can be realized. The reed BL burns out completely at 850 °C based on the above results, and this temperature is also the optimum temperature for desulphurization in the fluidized bed. Therefore, the temperature of 850 °C was selected as the temperature for direct alkali recovery. The combustion time was increased to 6 h to increase the sodium content fed into the furnace to prepare for the measurement of alkali recovery rate. The reed BL was burned in a fluidized bed with limestone as the bed material for 6 h at 850 °C, and the fluidization of bed material was very good. A few agglomerates with particle sizes of less than 2 mm (measured by standard screen) were found after the experiment. It can be concluded that the capability of limestone to inhibit agglomeration is weakened gradually with increasing combustion time. XRD analysis was carried out on the

Table 4. XRF Analysis of Bed Material at Different Temperaturesa

a

element (wt %)

700 °C

750 °C

800 °C

850 °C

900 °C

O Na Mg Al Si P S K Ca Cl Ti Mn Fe Sr

46.45 0.37 0.13 0.08 0.33 0.01 0.02 0.08 52.25 0.06 0.05 0.07 0.09 0.01

45.08 0.42 0.12 0.10 0.40 0.01 0.03 0.08 53.54 0.06 0.04 0.05 0.10 0.01

42.63 0.52 0.18 0.26 0.60 0.02 0.04 0.15 55.12 0.07 0.04 0.09 0.26 0.02

41.21 0.60 0.18 0.23 0.71 0.01 0.08 0.14 56.42 0.07 0.04 0.08 0.22 0.01

41.71 0.55 0.19 0.24 0.74 0.02 0.04 0.15 55.78 0.07 0.04 0.08 0.37 0.02

All data were normalized to 100%.

decomposition of limestone, and it has the best reactivity at 850 °C due to high surface area, which can offer more available active sites. As a result, much more SO2 is adsorbed by CaO.40 Figure 4 shows the crystalline compounds in bed material detected by XRD at different temperatures. The CaO is detected at all temperatures as a minor phase at the temperatures of 700−800 °C and as a major phase at 850 and 900 °C. This comes from the decomposition of CaCO3 according to the studies of Geysen41 and Xiao.42 The CaCO3 (major phase at all temperatures) and Ca(OH)2 (minor phase at 700−800 °C and major phase at 850−900 °C) are two other compounds detected in bed material, and the Ca(OH)2 is

Figure 4. XRD analysis of bed material after combustion at different temperatures. 5794


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Figure 5. XRD analysis of the agglomerate.

stirring. The XRD analysis (Figure 6) shows that the deposit is CaCO3 from the reaction as shown in eq 7. It will be reused as a direct causticizing agent after filtering.

agglomerates, and the results are shown in Figure 5. It can be seen that Na2Si2O5 (Na2O·2SiO2) was detected in the agglomerates. Dean43 pointed out that the pure Na2O·2SiO2 melts at a temperature of 874 °C, and related studies have shown that the melting point of Na2O·2SiO2 decreases under the action of SiO2. The lowest melting temperature is 789 °C, corresponding to a eutectic molar composition of 27% Na2O and 73% SiO2.22 Hence, it can be concluded that the Na2O· 2SiO2 causes the agglomeration of bed material. The Na2O· 2SiO2 melts into viscous liquid under high temperatures and then acts as the adhesive to bond the bed particles. According to Hrma,44 Osborn,45 and Gillott,46 the formation of Na2O· 2SiO2 takes place due to the reactions between sodium salts and silica as follows: Na 2CO3 + 2SiO2 = Na 2O·2SiO2 + CO2

(5)

Na 2CO3 + SiO2 = Na 2SiO3 + CO2

(6)

Na 2SiO3 + SiO2 = Na 2O·2SiO2

(7)

2NaCl + 2SiO2 + H 2O = Na 2O·2SiO2 + 2HCl

(8)

Figure 5 also shows that Ca2SiO4 and 3CaO·Al2O3·3SiO2 were detected in agglomerates. This indicates that CaO has combined with silicon according to the study of Zhu47 and Barin,48 as shown in eqs 9 and 10. 2CaO + SiO2 = Ca 2SiO4

Al 2O3 + 3CaO + 3SiO2 = 3CaO ·Al 2O3 ·3SiO2

Figure 6. XRD analysis of the deposit.

For verifying whether the NaOH can be successfully recovered, some soaking liquid was taken out and put into an iron crucible. Then, the iron crucible was heated to separate the compounds in soaking solution by crystallization. Because the NaOH can react with the CO2 to produce Na2CO3, the whole heating process was carried out in a nitrogen atmosphere. The crystals were analyzed by XRD, and the results are shown in Figure 7. It was found that NaOH was the dominant phase in crystals, which proves that the direct alkali recovery technology proposed in this paper is feasible. Moreover, the crystals also contain NaCl as a minor phase and K3Na(SO4)2 as the major phase. The concentrations of Na+, K+, and Cl− in soaking solution were measured to calculate the alkali recovery rate. The samples were diluted by a factor of 500 using pipettes and volumetric flasks before the measurement to make the concentrations of Na+, K+, and Cl− in the optimum measurement range of ICP-

(9) (10)

The SiO2 reacts with CaO to form Ca2SiO4 and 3CaO· Al2O3·3SiO2 instead of Na2O·2SiO2. As a result, the amount of low-melting Na2O·2SiO2 is significantly reduced. Moreover, the Ca2SiO4 and 3CaO·Al2O3·3SiO2 both have high melting points. They may cover the surface of the bed material, thus preventing the bonding of bed particles to each other. It is shown that in the combustion of reed BL, the limestone inhibits the agglomeration not only by simple physical effects but also by chemical effects. For the alkali to be recovered, all of the bed material was cooled to room temperature and then soaked in deionized water. Some deposits appeared in soaking solution before 5795


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Figure 7. XRD analysis of crystals.

Table 5. Experimental Conditions and the Alkali Recovery Rate ion concentrations in soaking solution (g/L) no.

mass of reed BL (g)

soaking solution volume (L)

Na+

K+

Cl−

sodium recovery rate (%)

rate of Na into NaOH (%)

1 2 3 average

1365 1371 1358

7.5 7.5 7.5

11.57 11.91 11.66

1.06 1.21 1.11

0.38 0.41 0.43

90.10 92.34 91.26 91.23

86.53 88.40 87.34 87.42

sodium to NaOH were 86.53, 88.40, and 87.34%, respectively. The derivation between measurement values and average value was less than 5%, indicating that the results were accurate enough. The average value of the measurements from three tests are used as the final data. The results show that the direct causticizing system using limestone as bed material has a high recovery efficiency. Approximately 91.2% of sodium fed into system can be recovered, and 87.4% of the sodium can be converted to NaOH. In the reuse of recovered alkali, the accumulation of chlorine and silicon is a problem that can promote bed agglomeration. In this paper, most of the silicon is converted to water-insoluble or high-melting compounds according to the above research. In addition, most of chlorine is converted to gaseous HCl as shown in eq 8. Hence, the problem of accumulating chlorine and silicon is solved. As a result, the recovered alkali solution can be directly reused in the pulping process of papermaking. A small amount of residual chlorine in the soaking solution can be removed by adding AgNO3 to produce AgCl. Then, the AgCl is filtered out from the solution. The direct causticizing technology with limestone as bed material should be wellapplied in the pulp and paper industry. The cause of sodium loss in direct causticizing experiments mainly consists of three parts: the first is the generation of water-insoluble Na2O·2SiO249 and K3Na(SO4)2, which cannot be causticized into NaOH; the second is the release of sodium in gas phase during the combustion of reed BL; and the third is the fine particles generated by breakage of bed material, which carries the sodium out of the furnace.

OES and LC-MS. The bed material was soaked repeatedly until Na+ was not detected in soaking solution, thus ensuring the sodium salts completely dissolved in water. The volume of deionized water was the same in each soaking. The direct causticizing experiments were conducted three times for the purpose of reducing the experimental error. Owing to the fact that there were no other compounds containing Cl and K detected by XRD except NaCl and K3Na(SO4)2, all of Cl− and K+ are assumed to be provided by these two compounds. On this basis, the total recovery rate of sodium and the rate of sodium into NaOH are calculated with eqs 11 and 12, and the results are shown in Table 5. The concentrations of Na+, K+, and Cl− in Table 5 are the results from measurement data magnified 500 times. n

ηNa =

∑i = 1 ω Nai × Vs mRBL × ω Na − RBL n

ηNaOH =

∑i = 1

(

ω Na MNa

−

1 3

× 100% ×

ωK MK

−

(11) ωCl MCl

)×V ×M

mRBL × ω Na − RBL

s

Na

× 100% (12)

in which, ηNa is the total recovery rate of sodium; ηNaOH is the rate of sodium converted to NaOH; ωNa, ωK, and ωCl are the total concentrations of Na+, K+, and Cl− in soaking solution, respectively (g/L); Vs is the volume of soaking solution (L); MNa, MK, and MCl are the molar masses of sodium, potassium, and chlorine, respectively; mRBL is mass of reed BL on dry basis (g); and ωNa−RBL is the sodium content of reed BL on dry basis (wt %). It can be seen from Table 5 that the recovery rates of sodium in the three tests were 90.10, 92.34, and 91.26%. The rate of 5796


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4. CONCLUSIONS In this paper, the effects of limestone on direct causticizing and inhibiting bed agglomeration were investigated as reed BL burned in a fluidized bed. The conclusions that can be drawn are as follows: (1) The direct causticizing technology of reed BL using limestone as bed material is feasible. NaOH was successfully recovered by direct causticization. Approximately 91.2% of sodium fed into the system can be recovered, and 87.4% of the sodium can be converted to NaOH. The generated CaCO3 can be reused as a direct causticizing agent, and the bed particles can be reused after sieving and drying. (2) The limestone has a significant effect on inhibiting bed agglomeration. At the temperature range of 700−900°C, the fluidization of bed material was very good, and the agglomerates were not found in 4 h of combustion of reed BL. However, a few agglomerates were found in bed material after 6 h of combustion at 850 °C, which indicates that the capability of limestone to inhibit agglomeration decreased gradually with increasing combustion time. (3) A combination of physical and chemical effects is the mechanism by which limestone inhibits bed agglomeration. The limestone can be calcined into CaO to react with silica to form high-melting matters such as Ca2SiO4 and 3CaO·Al2O3· 3SiO2 instead of low-melting Na2O·2SiO2. In addition, the coating formed by CaO, Ca2SiO4, and 3CaO·Al2O3·3SiO2 on the bed material leads to a deep decline of adhesion between bed particles. (4) The formation of Na2O·2SiO2 and K3Na(SO4)2 is the main reason for sodium loss. Other causes such as the release of sodium in gas phase and escaping with fine particles might also contribute to the loss of sodium. (5) The silicon is converted to water-insoluble or highmelting compounds, and most of chlorine is converted to gaseous HCl. Therefore, the problem of accumulation of chlorine and silicon in recovered alkali solution, which can promote bed agglomeration, is solved.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors gratefully acknowledge the School of Materials Science and Engineering and the School of Municipal and Environmental Engineering at Harbin Institute of Technology for their analytical assistance.

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REFERENCES

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