Article Cite This: Energy Fuels 2018, 32, 11710−11717
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Experimental Study of Alkali Salt Adsorption in a Circulating Fluidized Bed Boiler Hang Shi, Yang Zhang, Yuxin Wu,* and Junfu Lyu
Energy Fuels 2018.32:11710-11717. Downloaded from pubs.acs.org by YORK UNIV on 12/17/18. For personal use only.
Key Laboratory for Thermal Science and Power Engineering of Ministry of Education, Department of Energy and Power Engineering, Tsinghua University, Beijing 100084, China ABSTRACT: Spraying the concentrated wastewater into a circulating fluidized bed (CFB) boiler furnace is an economical way to process the saline wastewater generated in coal gasification. However, if the wastewater is overdosed and the alkali salt mass fraction exceeds the capability of the bed material, the excess alkali components in the flue gas may raise not only boiler fouling and corrosion problems but also environmental problems. Therefore, the alkali salt adsorption capability of the bed material is a crucial parameter for the overall feasibility and security of this process. Experiments were conducted under laboratory conditions using analytical devices and a pilot-scale 3.3 MWth hot CFB test rig to understand the NaCl (determined through component analysis of the wastewater) adsorption characteristics and mechanism. The lab-scale experimental results showed that the NaCl adsorption capacity of the bed material for a blending ratio (BR, the ratio of the NaCl weight to the bed material weight) of 8% is higher than that for BRs lower than 8%. The pilot-scale experimental results showed that the Na mass fraction could reach 1.55% in the bed material and 1.24% in the fly ash. Component analysis results indicated that the NaCl adsorption process, which was highly related to the calcination temperature and BR, was boht physical and chemical in the lab-scale experiments but only chemical in the pilot-scale experiments. The NaCl adsorption capacity of the bed material was lower in the pilot-scale CFB boiler than in a muffle furnace at the same BR.
1. INTRODUCTION Coal gasification is a clean and efficient method of coal utilization.1 However, a large amount of saline wastewater is generated during the coal gasification process. There are up to 130 kinds of organic matter that are difficult to degrade in the saline wastewater. Besides, the content of sodium salts in typical saline wastewater can reach up to 4000 mg/L.5,6 To meet the wastewater discharge standards, wastewater treatment systems have to be complex and expensive. Effective and economical wastewater treatment is still a big challenge.2−4 Therefore, a low-cost method for treatment of organic-matterrich, high-alkali-content wastewater is urgently needed by the coal gasification industries. Coal gasifiers are usually built with a concomitant coal-fired boiler, and the circulating fluidized bed (CFB) boiler is a popular choice because of the fuel flexibility and low-cost emission control. A large amount of medium- to hightemperature nonmelting ash suitable for NaCl adsorption is generated during the combustion process in a CFB boiler. Spraying the wastewater into a coal-fired CFB boiler furnace is a potential method to solve the aforementioned wastewater treatment problems. First, the organic matter in the wastewater will be completely burned out and converted to harmless components in the furnace because of its medium- to hightemperature environment (800−900 °C). Second, the alkali salt matter in the wastewater can be adsorbed by the bed material, namely, ash generated from the coal combustion process. In this method, NaCl can be injected into a furnace in the form of either concentrated wastewater or crystalline salt. Regardless of the form of the NaCl injected into the furnace, the alkali salt adsorption capability of the bed material is a crucial parameter for the overall feasibility and security of this process. If the wastewater is overdosed and the alkali salt mass © 2018 American Chemical Society
fraction exceeds the capability of the bed material, the excess alkali components in the flue gas will raise boiler fouling and corrosion problems.7−11 In addition, it is important to confirm whether the bed material and fly ash after adsorption of NaCl will lead to any potential environmental risks, e.g., if the NaCl were adsorbed through physical adsorption, it may go back into the environment finally, and the proposed method would fail. Thus, it is necessary to investigate the alkali salt adsorption characteristics of the bed material under CFB boiler furnace conditions. Recently, a lot of studies12−22 have been carried out on adsorption of alkali salts (mostly NaCl and KCl) by various substances at the medium to high temperature (700−1200 °C). The adsorption characteristics of silicon additives (silica, diatomaceous earth, and clay), alumina additives (bauxite, activated bauxite, and clay), and other adsorbents12 (graphite, calcined limestone, and Utah ash) have been deeply studied. All of the additives can adsorb the alkali salts at medium to high temperature.13−15 Recent studies16−18 showed that silicon additives were more active than alumina additives, and the optimal temperature for sodium capture by the silicon additives is about 1000 °C. However, Dou et al.19 studied the adsorption of NaCl using several adsorbents at 840 °C and found that activated Al2O3, bauxite, and activated bauxite took the top three spots in terms of adsorption efficiency. Temperature can greatly affect the alkali adsorption process. Lee et al.20 and Escobar et al.15 found that bauxite adsorbed alkali through both chemical and physical adsorption at 800− Received: August 21, 2018 Revised: October 23, 2018 Published: October 29, 2018 11710
DOI: 10.1021/acs.energyfuels.8b02896 Energy Fuels 2018, 32, 11710−11717
Article
Energy & Fuels 900 °C, while Luthra and Leblanc21 reported that bauxite and activated bauxite adsorbed alkali through only physical adsorption at 800−900 °C. Some other adsorbents, e.g., diatomaceous earth, kaolin, and acid clay, adsorbed the alkali salt through chemical adsorption,15,20 while the alkali salt adsorption process on Utah ash was a combination of complex physical and chemical adsorption.12 It is generally believed that the adsorption of alkali salt vapor is related to the chemical composition of the adsorbent when the temperature is below 1200 °C, and adsorbents with higher SiO2/Al2O3 content are more prone to adsorb the alkali salt by combined chemical and physical adsorption.22 Previous studies provide good references to understand the alkali salt adsorption on different adsorbents at various temperatures. In the present study, NaCl was selected to represent the alkali salts in the wastewater through component analysis (see section 2.1). To the best of the authors’ knowledge, no studies on the absorption of NaCl by the bed material at medium to high temperature have been reported in the open literature. In addition, in all of the reported experimental adsorption studies, NaCl was first vaporized before it was carried to the adsorbent surface by the carrier gas. In such cases, the concentration of the NaCl vapor was determined by the saturation vapor pressure at the given temperature. However, when the concentrated wastewater is injected into a CFB boiler, the blending ratio (BR, the ratio of the NaCl weight to the bed material weight) rather than the NaCl vapor concentration is more practical in engineering practice. Thus, the adsorption behaviors of the bed material at different temperatures and BRs are more relevant. However, there are no related studies in the literature. In this work, experimental studies were conducted to investigate the characteristics and mechanism of NaCl adsorption on the bed material at different temperatures and BRs. Pilot-scale experiments were also conducted to study the real adsorption process in a CFB boiler.
for 2 h to remove residual carbon so as to prevent its impact on the experimental results. The loss on ignition of bed material between 800 and 950 °C was less than 0.1%, which indicates that the experimental error caused by the experimental method is less than 0.1%. The maximum alkali salt adsorption capacity of the bed material during the calcination time was determined when the weight of the calcined NaCl/bed material mixture did not decrease after recalcination for 10 min. The particle size distribution of the bed material was 30−400 μm, and d0.1, d0.5, and d0.9 were 60.2, 114.0, and 214.9 μm, respectively. The X-ray fluorescence (XRF) results for the bed material before mixing are listed in Table 1. The bed material was rich in SiO2 and Al2O3, which were considered as the main compositions that reacted with the alkali salt.
Table 1. X-ray Fluorescence Results for the Bed Material (wt %) result
analyte
result
SiO2 Al2O3 Fe2O3 K2O
52.43% 20.54% 15.97% 2.96%
CaO TiO2 MgO Na2O
3.74% 1.47% 0.81% 0.23%
Two types of crystalline salt were concentrated from coal gasification wastewater of a coal gasification industrial park in Inner Mongolia, China. The ion chromatography (IC) test results showed that the main components of sample 1 were NaCl and Na2SO4 and that the NaCl content was higher than 95%. The main component of sample 2 was NaCl, and the NaCl content was higher than 99%. Consequently, NaCl was used as the alkali salt in both the laboratoryscale experiments and pilot-scale tests. In the laboratory-scale experiments, the process of NaCl adsorption on the bed material in CFB boilers was simulated by TGA and muffle furnace experiments. For the TGA experiments, the air flow rate was fixed at 100 mL/min, while there was no gas circulation in the muffle furnace. The NaCl vapor will escape from thermogravimetric analyzer with the air flow, resulting in a lower concentration of Na around the bed material at the same BR. As a result, the air flow will promote the evaporation of NaCl so as to influence the alkali salt adsorption. Therefore, TGA experiments were conducted to provide references for the calcination time for the muffle furnace experiments to ensure that NaCl was sufficiently adsorbed by the bed material. 2.1.1. TGA Tests. The TGA tests were used to measure the temperature−mass relation of the materials. They were performed on a TGA/DSC1/1600HT simultaneous thermal analyzer. The samples were NaCl/bed material mixtures with the desired BRs. To simulate the CFB boiler combustion conditions, the samples were calcined at 800, 850, and 950 °C for 150, 120, and 90 min, respectively. The heating rate was 30 °C/min during the heating process and −30 °C/ min during the cooling process. The air flow rate was fixed at 100 mL/min. The experimental conditions are listed in Table 2. 2.1.2. Muffle Furnace Tests. The pretreated bed material was calcined in a GW-300C muffle furnace, which can operate stably over the temperature range of 300−1000 °C. Similar to the TGA experiments, the samples in the muffle furnace tests were premixed
2. EXPERIMENTAL APPROACH In this work, the alkali salt adsorption capacity of the bed material from a CFB boiler was measured at different temperatures and BRs. At a given BR, the adsorption capacity can be identified if the mass change of the bed material during the adsorption is obtained. Thus, hierarchical experimental studies were conducted. Thermogravimetric analyzer (TGA) and muffle furnace experiments were conducted on the laboratory scale in order to acquire fundamental data on the NaCl adsorption capacity of the bed material, and pilot-scale experiments were conducted to deeply understand the real NaCl adsorption process in a CFB boiler. The results of the laboratory- and pilot-scale experiments were then compared. The detailed experimental procedures are given in sections 2.1 and 2.2. The alkali salt adsorption capacity of the adsorbent, Ca, is defined as m − m0 Ca = c × 100% m0 (1) and the BR is defined as m BR = 1 × 100% m0
analyte
Table 2. TGA Test Conditions starting air flow rate/ temperature/°C (mL/min)
(2)
where mc is the weight of the NaCl/bed material mixture after calcination, m0 is the weight of the bed material before mixing, and m1 is the weight of NaCl mixed with the bed material. 2.1. Lab-Scale Experiments. Bed material sampled from a 300 MWe CFB boiler in Longyan, China, was selected as the adsorbent. All of the bed material used in the experiments was calcined at 850 °C
heating process adiabatic process cooling process 11711
heating rate/ (°C/min)
maintain time/min
25
100
30
−
800/850/900
100
−
150/120/90
800/850/900
100
−30
−
DOI: 10.1021/acs.energyfuels.8b02896 Energy Fuels 2018, 32, 11710−11717
Article
Energy & Fuels NaCl/bed material at the desired BRs. In each experiment, the muffle furnace was first heated to the experimental temperature, and then the mixture was put into the furnace quickly. The detailed experimental conditions are shown in Table 3. X-ray diffraction (XRD) (D8/ Advance), XRF (XRF-1800), and Brunauer−Emmett−Teller (BET) (Tristar II 3020) measurements on the calcined samples were conducted.
Table 3. Test Conditions for Muffle Furnace Experiments temperature/°C
BR/%
calcination time/min
800/825 850/875 900/925/950
0, 1, 2, 3, 5, 8 0, 1, 2, 3, 5, 8 0, 1, 2, 3, 5, 8
130 85 40
2.2. Pilot-Scale Experiments. As shown in Figure 1, the 3.3 MWth pilot-scale CFB boiler, built by the Taiyuan Boiler Group, has a complete CFB device and one heat-insulated cyclone separator. Figure 2. Temperature distribution along the furnace height.
The weight losses of NaCl and the NaCl/bed material mixture (the BRs were 8.7%/9.0%/7.4% at 800/850/900 °C) are shown in Figures 3 and 4, respectively.
Figure 1. System diagram for the 3.3 MWth pilot-scale CFB boiler. Legend: (1) coal bunker; (2) limestone bunker; (3) sand bunker; (4) water-cooled tubes; (5) furnace; (6) slagging pipe; (7) ammonia; (8) separator; (9) loop seal; (10) material-returning air fan; (11−12) high-temperature economizer; (13−14) low-temperature economizer; (15−16) air preheater; (17) valve; (18) half-dry desulfurization; (19) precipitator; (20) primary air fan; (21) secondary air fan; (22) induced-draft fan; (23) chimney. Figure 3. TG/DTG curves of NaCl at different temperatures. In the pilot-scale experiments, the tests started after the combustion parameters were stable, providing a relatively stable combustion environment for the adsorption of NaCl in the CFB boiler. Figure 2 shows the temperature distribution along the furnace height. The temperature was measured using a group of K-type thermocouples. During the tests, the dense-phase temperature was maintained at 855 ± 5 °C, and the dilute-phase temperature was 810 ± 5 °C. NaCl, at a mass flow rate of 10.8 kg/h, was continuously dosed with the feeding coal, whose mass flow rate was 500 kg/h. Once the system approached the steady state, the bed material and the fly ash were sampled. The bed material before the cyclone separator was collected through a small, self-made cyclone separator that was directly inserted into the center of the upper furnace. The bed material after the cyclone separator was collected through the loop seal. The fly ash was collected through a fly ash sampler at the desulfurizing tower. The alkali salt adsorption capacity of the bed material in the pilot-scale CFB boiler was then analyzed.
Figure 3 shows that the residual mass of NaCl is less than 1.2%. NaCl is completely evaporated when the temperature is higher than 800 °C. The influence of the residual mass of NaCl can be neglected in the experiments. It can be seen from Figure 4 that the complete adsorption process takes less than 130/85/40 min after the time for the
3. RESULTS AND DISCUSSION 3.1. NaCl Adsorption Characteristics of the Bed Material. 3.1.1. Determination of the Calcination Time. The calcination time was determined by TGA experiments.
Figure 4. TG/DTG curves of calcination experiments. 11712
DOI: 10.1021/acs.energyfuels.8b02896 Energy Fuels 2018, 32, 11710−11717
Article
Energy & Fuels heating process is deducted. The time required for the complete adsorption process decreases as the calcination temperature increases. Therefore, in the subsequent muffle furnace experiments, 130, 85, and 40 min were chosen as the calcination times for the cases with the calcination temperatures of 800, 850, and 900 °C, respectively, to ensure that NaCl can be sufficiently adsorbed by the bed material. 3.1.2. Influences of Calcination Temperature and BR on the Alkali Salt Adsorption Capacity and Adsorption Form of the Bed Material. The influences of calcination temperature and BR on alkali salt adsorption capacity of the bed material are shown in Figure 5. It can be seen that the alkali salt
Table 4. Volume-Average Particle Sizes of the Calcined NaCl/Bed Material Mixtures volume-average particle size/μm BR/%
850 °C
900 °C
950 °C
0 1 2 3 5 8
128.18 127.30 127.41 130.86 131.85 136.00
127.48 127.12 127.31 128.34 130.75 134.2
127.64 127.33 127.47 128.21 130.02 133.76
Figure 5. Influences of the calcination temperature and BR on the alkali salt adsorption capacity of the bed material in muffle furnace experiments.
adsorption capacity increases nearly linearly as the BR increases over the range of 1−8%, and generally, the NaCl adsorption capacity decreases as the calcination temperature increases at the same BR. The experimental data show that the maximum NaCl adsorption capacities of the bed material were acquired when the BR is 8% in the tested BR ranges given a fixed temperature. The maximum values were 5.56%, 4.65%, 4.34%, 4.13%, 2.96%, 3.02%, and 2.74% at 800, 825, 850, 875, 900, 925, and 950 °C, respectively. Under the experimental conditions, NaCl melts at the calcination temperature, and part of the NaCl vapor is released to the gas phase while the rest of the NaCl is either physically or chemically adsorbed on the bed material. 3.2. Mechanism of NaCl Adsorption on the Bed Material. In order to reveal the mechanism of NaCl adsorption on the bed material, detailed analysis of the calcined NaCl/bed material mixture was conducted. 3.2.1. Brunauer−Emmett−Teller Test Results. Previous studies (e.g., refs 23−25) demonstrated that the destruction of the pore structure caused by the high temperature might lead to sintering of the adsorbents, resulting in a decrease of the alkali salt adsorption capacity of the adsorbents. To verify whether the Ca decrease found in Figure 5 was due to the pore structure change of the bed material, particle size and BET analyses were carried out, and the results are shown in Table 4 and Figure 6. Table 4 shows that the volume-average particle size slightly increases as the BR increases from 2% to 8% and that it increases more as the calcination temperature decreases when the BR is higher than 3%. The effect of the calcination temperature on the particle size is negligible. Figure 6 presents the Barrett−Joyner−Halenda (BJH) pore volume distributions of the samples calcined at 850, 900, and 950 °C. Generally, the pore size of the calcined NaCl/bed material mixtures was within the range of 2−20 nm. The
Figure 6. BJH specific pore volume distributions of samples calcined at (a) 850, (b) 900, and (c) 950 °C in muffle furnace experiments.
specific pore volume of the bed material hardly changed as the calcination temperature increased from 850 to 950 °C. Therefore, the Ca decrease caused by the increased calcination temperature was not dominated by the pore structure change of the bed material. 3.2.2. X-ray Diffraction Test Results. Figure 7 presents the XRD test results for the calcined samples. The main occurrence modes of Na in the bed material are NaAlSiO4 and NaCl at 800−900 °C and mainly NaAlSiO4 when the calcination temperature is higher than 900 °C. Plenty of studies (e.g., refs 25−28) have confirmed that the SiO2/Al2O3rich adsorbents chemically adsorb alkali salts mainly through the following two approaches: 2NaOH + SiO2 V Na 2SiO3 +H 2O
(3)
2NaCl(g) + H 2O(g) + Al 2O3 ·xSiO2 (s) V Na 2O·Al 2O3 ·xSiO2 (s) + 2HCl(g) 11713
(4)
DOI: 10.1021/acs.energyfuels.8b02896 Energy Fuels 2018, 32, 11710−11717
Article
Energy & Fuels
Figure 7. XRD test results of samples calcined at (a) 800, (b) 850, (c) 900, and (d) 950 °C in muffle furnace experiments. Legend: (1) SiO2; (2) NaAlSiO4; (3) Fe2O3; (4) Al2SiO5; (5) NaCl; (6) Na2O; (7) TiO2.
The Na mass fraction in the calcined NaCl/bed material mixture increases as the BR increases from 0% to 8% (Figure 8) and first increases and then decreases with increasing calcination temperature, reaching the maximum value at 850 °C (Figure 9). In comparison, the trend of the Cl mass fraction is closely related to the calcination temperature. The Cl mass fraction increases as the calcination temperature increases from 800 to 850 °C and then falls to a very low level (
