Fuel 267 (2020) 117175
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Full Length Article
Sodium transformation simulation with a 2-D CFD model during circulating ﬂuidized bed combustion
Jieqiang Jia, Leming Chenga, , Li Niea,b, Liyao Lia, Yangjun Weia a b
State Key Laboratory of Clean Energy Utilization, Institute of Thermal Power Engineering, Zhejiang University, Hangzhou 310027, PR China Dongfang Boiler Group Co. Ltd., Zigong 643001, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Sodium transformation Circulating ﬂuidized bed Numerical simulation
Investigation of the release behaviors of alkali and alkaline earth metals is necessary to understand the mechanisms of ash-related problems such as fouling, slagging, and corrosion when burning fuel with high alkali contents. Using a 2-D circulating ﬂuidized bed (CFB) combustion model combined with a sodium migration model, sodium transformation was predicted in a 30 kW CFB combustor. Simulation results from the 2-D model are in agreement with those from the 3-D model and experiments. Calculations and analysis of sodium transformation with diﬀerent parameters, including furnace temperature, excess air, and secondary air ratio, were performed. The results show that a higher furnace temperature leads to a higher number of deposited sodium and ash particles on the probe surface. Increasing the amount of excess air or decreasing the secondary air ratio may reduce the amounts of deposited sodium and ash particles.
1. Introduction Fouling, deposition, and slagging on heat transfer surfaces are severe problems caused by burning coal with high contents of alkali and alkaline earth metals. For example, the high sodium content of Zhundong (ZD) coal plays a very important role in ash-related issues when using this coal. An alternative to pulverized coal (PC) boilers [1–3], circulating ﬂuidized bed (CFB) combustion is a promising choice for obtaining clean, eﬃcient utilization of high alkali coal due to the lower furnace temperature and larger coal particle sizes suitable for this process. In addition, solid particles falling down the water wall in CFB boilers may inhibit the growth of deposits. Several studies have been performed on ash deposition and sodium transformation in the CFB system. Song et al. investigated the slagging and fouling characteristics of ZD coal during CFB gasiﬁcation [4,5], as well as the sodium transformation behavior, in a 0.4 t/d CFB furnace . Sodium in the bottom ash or ﬂy ash were collected and analyzed at diﬀerent furnace temperatures and atmospheres in a 0.25 t/d CFB system . Focusing on CFB combustion, Liu et al. studied ash deposition behavior and presented valuable results on slag morphology and chemical and mineral compositions in a 30 kW CFB system . These authors also studied the eﬀects of kaolin addition  and cocombustion on the ash deposition  and proposed eﬀective methods
to solve or mitigate ash-related problems. Generally, ash deposition and sodium migration can be aﬀected by operating parameters during CFB combustion. Qi et al. recommended that sludge ash was an appropriate bed material  and proposed that slagging and ash deposition were aﬀected greatly by wall temperature . Liu et al.  concluded that ash deposition increased with increasing bed temperature. Song et al.  reported that the sodium content of ash increased as the air equivalence ratio decreased due to the eﬀect of carbon inhibition. Zhou et al. [13,14] observed that the probe surface temperature had a signiﬁcant eﬀect on the growth, mineralogy, and microstructure of fouling deposits in a 300 kW test furnace. Speciﬁcally, the deposit thickness increased when the surface probe temperature was decreased . The average eﬀective heat conductivity of the ash deposit increased linearly with deposit growth . However, the eﬀect of atmosphere on sodium transformation is absent in literature, despite the importance of this parameter. Considering the high cost and variable conditions of ﬁeld experiments, theory and numerical simulation oﬀer an eﬀective, reliable way to study ash-related issues. In our previous work, a detailed sodium migration model was developed to study the fate of sodium in high alkali ZD coal during CFB combustion  using a 3-D comprehensive CFB combustion model (Com-CFD-CFB-model) that included hydrodynamics, coal combustion, and heat transfer . This sodium migration model combines several sub-models such as sodium species
Corresponding author. E-mail address: [email protected]
https://doi.org/10.1016/j.fuel.2020.117175 Received 26 September 2019; Received in revised form 20 January 2020; Accepted 21 January 2020 0016-2361/ © 2020 Elsevier Ltd. All rights reserved.
Fuel 267 (2020) 117175
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C d F fh out p s se sup
A C C’ D m M Q R S V v Y
area gas concentration correction gas concentration diameter of the furnace mass molecular weight air ﬂow rates emission rate inlet areas volume ﬂow rate velocity mass fraction
cross section deposit feeding coal ﬂy ash outlet of the furnace probe solid phase secondary air superﬁcial velocity
Greek symbols Ρ γ ζ ξ
density surface tension collection eﬃciency sticking probability
the cluster renewal model .
release, homogeneous and heterogeneous reactions, vapor condensation, particles deposition (inertial impaction and thermophoresis), and shedding. The model was validated in a CFB test rig. Calculations were performed to obtain the distribution of sodium compounds in gas, ash, and deposits. The 3-D simulation provided good furnace operating predictions, but required too much computation time. If a 2-D model based on the 3D model could provide key results representing the actual process as well as the 3-D model, the simulation eﬃciency would improve and variable parameter analysis could be performed quickly. In our previous study, a 2-D simpliﬁed calculation method has been developed and its reliability has been conﬁrmed . To satisfy these considerations, a 2-D combustion and sodium migration model was developed in this work. Results from the 2-D calculations were veriﬁed with those form the 3-D calculations and experiments. The 2-D model was used to investigate the inﬂuence of operating parameters on sodium transformation, including furnace temperature, excess air, and secondary air ratio. The results can guide the selection of appropriate operating parameters that mitigate ash problems during CFB combustion.
2.2. Sodium migration model The sodium migration model  contains sub-models including sodium species release, homogeneous and heterogeneous reactions, vapor condensation, particle deposition (inertial impaction and thermophoresis), and shedding. As coal particles are fed into the furnace, sodium in the coal undergoes complicated physical and chemical reactions. First, H2O-soluble sodium is assumed to be released in the form of NaCl during the devolatilization stage and NH4Ac-soluble sodium is considered to be released in the form of Na during the char combustion stage, as shown in (R 1-1)−(R 1-3) in Table 1 . The Na release reactions are found in the literature [19,20], and the reaction coeﬃcients of the Na compounds in (R 1-1)−(R 1-3) are determined based on our previous experiments . After vaporizing, gaseous sodium participates in homogeneous and heterogeneous reactions. NaCl and Na2SO4 are considered stable gaseous species during the combustion process. Reactions (R 1-4)−(R 1-9) in Table 1 are the primarily formation and destruction reactions for NaCl and Na2SO4 . For heterogeneous reactions, absorption of alkali vapor by aluminum silicate clay is considered (R 1-10). Saturated gaseous sodium species condense on the particle surface and/or on the heating exchanging surface. The vapor condensation mass ﬂuxes depend on the local partial pressure and saturation pressure of the speciﬁc gaseous sodium species. In addition, inertial impaction and thermophoresis are involved in the deposition mechanisms. The deposition rate of particles is expressed by the total particle mass ﬂux, impact eﬃciency, and sticking probability.
2. Model description In this work, a 2-D calculation model was developed based on the 3D comprehensive combustion model (Com-CFD-CFB-model)  and combined with the sodium migration model from our previous work . The structures of the gas–solid inlets and boundary conditions of the calculated object were modiﬁed for this 2-D model. Detailed descriptions of the 3-D comprehensive combustion and sodium migration models can be found in the references [15,16]. Only the main points of these two models are described below.
Table 1 Reactions of the sodium migration model.
2.1. 3-D comprehensive combustion model The previously developed 3-D comprehensive combustion model (Com-CFD-CFB-model)  is fundamental to simulations of sodium transformation. This model incorporates gas–solid hydrodynamics, coal combustion, and heat transfer on heat exchange surfaces in the furnace. The Eulerian-Eulerian (E-E) model is combined with the energy minimization multiscale (EMMS) drag model to simulate the gas–solid hydrodynamics of this process. The gas phase is modeled with the renormalization group (RNG) k − ε turbulence model. Five sub-models are incorporated in the coal combustion simulation, including moisture evaporation, devolatilization, homogenous reactions, char combustion, and NOx/SO2 emissions. Bed to wall heat transfer is calculated based on 2
Volatile → aCH4 + bH2 + cCO2 + dCO + eH2 O + fTar + gH2 S + hNH3 + iHCN + jNaCl + kHCl
C (Nx Sy Naz ) + (1/2 + x /2 + y ) O2 → CO + xNO + ySO2 + zNa
C (Nx Sy Naz ) + (1 + x /2 + y ) O2 → CO2 + xNO + ySO2 + zNa
R R R R R R R
Na + HCl → NaCl + H Na + O2 → NaO2 NaO2 + HCl → NaCl + HO2 Na + SO2 → NaSO2 NaSO2 + NaO2 → Na2 SO4 2NaCl + SO2 + H2 O + 0.5O2 → Na2 SO4 + 2HCl Six Aly Oz + −Na (g ) → Six Aly Oz − Na
1-4 1-5 1-6 1-7 1-8 1-9 1-10
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4. Boundary conditions
Particle shedding is important due to the high density of the solid suspension in the CFB system and is dependent on the forces acting on the deposited particles: drag force, gravity, adhesion force, and contact force.
A veriﬁcation calculation case (case 1) of the 2-D model was carried out ﬁrst. The boundary conditions of the 2-D model were set to the same values as those in the 3-D model in our previous work , as listed in Table 2. This case was used to verify the correctness of the 2-D model.
2.3. 2-D model treatment As mentioned in the Introduction, the 2-D model was developed based on the 3-D Com-CFD-CFB-model to improve the computation eﬃciency. The principles to construct the 2-D model included: (1) The 2-D plane was from a cross section of the 3-D structure that contained a suitable number of openings. (2) The superﬁcial gas velocity in the 2-D model was equal to that of the 3-D model. (3) The velocities of gases and solids at each inlet were the same in both the 2-D and 3-D models. The aim is to replace the 3-D calculation with a 2-D calculation to obtain the key results quickly. Detailed descriptions are given for the following simulation of a 30 kW CFB test rig.
a PA and SA refer to primary air and secondary airﬂow rate, respectively. To investigate the eﬀects of furnace temperature, excess air, and secondary air ratio on the sodium transformation, nine cases were set and calculated, as listed in Table 3. Table 4 gives proximate and ultimate analyses of ZD coal. 5. Results and discussion The 2-D calculation results were compared with the results from the 3-D calculations and experiments to validate the 2-D model. Then, the eﬀects of furnace temperature, excess air, and secondary air ratio on the sodium distributions and ash deposition were computed and are discussed in this section.
3. Simulation object and 2-D simulation implementation In this work, the calculation object was a 30 kW CFB test rig , as shown in Fig. 1. The height of the cylindrical furnace was 4.2 m and its inner dimeter was 0.13 m. The gas temperature and pressure inside the furnace were measured by thermocouples (T1–T9) and pressure taps (P1–P2), respectively. In the 2-D model, the test rig was simpliﬁed to a two-dimensional plane that is a section from the 3-D structure (marked by the coarse red line in Fig. 1). All input and output openings were included in the section except the coal feeder. The model was subsequently modiﬁed to add the coal inlet port situated at the same height, as shown in Fig. 2. Six secondary air inlets (S1–S6) were located along the furnace height. The ash deposition probe, with an outer diameter of 0.03 m, was inserted into the furnace through the port at S4. In this model, the deposition probe was simpliﬁed to a circle plane. Solids exiting from the outlets returned to the furnace through a user-deﬁned function (UDF) program. To keep the penetration depth of the secondary air constant between the two models, the velocities of the secondary air in the 2-D model were set to the same value as those in the 3-D model, as shown in Eq. (1). The areas of the secondary air inlets were modiﬁed and determined by Eq. (2), where Qse,2D is the volume ﬂow rate of secondary air and calculated using Eq. (3). Gas and solids velocities at the coal feeder and solids return inlet were modiﬁed with the same method.
vse,2D = vse,3D
Sse,2D = Qse,2D × Sse,3D/ Qse,3D
Qse,2D = vsup,2D × D × (Qse,3D/ Qtotal,3D )
5.1. Validation Fig. 4 shows the gaseous temperature distributions along the furnace in experiments  and 2-D and 3-D simulations . This ﬁgure shows that the models are in agreement with the experimental results. The furnace temperature is higher at the bottom of the furnace due to the rapid burning of volatiles and char particles. As the furnace height is increased, the gaseous temperature experiences a mild decrease because of the heat loss through the furnace wall. The averaged deviation of the bed temperature between the 2-D model and 3-D model is 1.3%. Table 5 shows the ﬂue gas compositions of O2, N2O, NO, and SO2 at the outlet of the furnace. The results from the models and experimental results agree within a reasonable range. The average deviations between the 2-D model and 3-D models of O2, NO, and SO2 are 2.2%, 2.8%, and 15%, respectively. The distributions of sodium in the furnace are given in Table 6. From the calculation results, most of the sodium is retained in the ﬂy ash and exits the furnace, while less sodium remains as deposits on the probe surface. These simulation results agree well with the experimental results. The calculation time of the 2-D model is reduced by 75% when compared with that of the 3-D model.
where vse,2D and vse,3D are the secondary air velocities in the 2-D model and 3-D model, respectively. Sse,2D, and Sse,3D are the secondary air inlet areas in the 2-D model and 3-D model, respectively. Qse,2D and Qse,3D are the secondary airﬂow rates in the 2-D model and 3-D model, respectively. vsup,2D is the superﬁcial gas velocity in the 2-D model. D is the diameter of the furnace. Qtotal,3D is the total airﬂow rate in the 3-D model. Approximately 10,000 grid cells were contained in the meshes. The mesh structure near the probe is shown in Fig. 3. A ﬁnite volume method was used in the computation of the mathematical models. All convective terms were solved by the ﬁrst-order upwind discretization scheme. The semi-implicit method for pressurelinked equation (SIMPLE) algorithm was selected for pressure–velocity coupling. The heterogeneous reaction rates, as well as the condensation and deposition ﬂuxes, were coded by UDFs and coupled with Fluent (ANYSYS, Canonsburg, PA, USA).
Fig. 1. 3-D geometry. 3
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Table 4 Proximate and ultimate analysis. Proximate analysis
Aad Vad Mad Mt Qnet,ar
6.43 w% 27.91 w% 11.17 w% 23.42 w% 19589 kJ/kg
Cad Had Nad Sad Oad Nat Ash density
62.89 w% 3.04 w% 0.55 w% 0.51 w% 15.41 w% 0.226 w% 2400 kg/m3
A, V, and M refer to ash, volatile, and moisture, respectively. ar, ad, and t refer to as received basis, air dried basis, and total content, respectively. b
Fig. 2. 2-D geometry.
Fig. 3. Mesh structure.
Table 2 Boundary conditions in the simulation. Parameter
Superﬁcial gas velocity Coal feed rate Primary air velocity Secondary air velocity SA/(PA + SA)a Primary air temperature Secondary air temperature Solid return temperature Probe surface temperature
m/s kg/s m/s m/s – K K K K
4.0 0.019 0.635 0.399 0.3 300 300 1123 700
Fig. 4. Gaseous temperature. Table 5 Gas species contents at the outlet (6% O2).
2-D calculation 3-D calculation15 Experiment15
Table 3 Parameters for the diﬀerent calculation cases.
Case Case Case Case Case Case Case Case Case
1 2 3 4 5 6 7 8 9
SA/(PA + SA)
1150 1100 1213 1150 1150 1150 1150 1150 1150
1.79 1.79 1.79 1.66 1.61 1.52 1.79 1.79 1.79
0.3 0.3 0.3 0.3 0.3 0.3 0.2 0.4 0.5
9.2 9.0 8.4
16.0 25.3 7.6
203.6 198.0 199.6
222.5 192.0 174.6
Table 6 Sodium distributions.
2-D calculation 3-D calculation15 Experiment15
Emission as gas phase (kg/m2·s)
Emission in the ash (kg/m2·s)
Retained in deposits (kg/m2·s)
5.63 × 10−5 7.77 × 10−5 –
2.73 × 10−3 2.56 × 10−3 2.88 × 10−3
3.92 × 10−7 3.13 × 10−7 3.36 × 10−7
species content in the left Y-axis exhibits the opposite trend. At the outlet of the furnace, the concentrations of both NaCl and Na2SO4 decrease as the temperature increases. As the furnace temperature increases from 1100 K to 1150 K and 1213 K (case 1, 2, 3), both the coal feed rate and total airﬂow rate increase, causing an increase in the volumetric ﬂow rate of gas (V). The O2 concentration (CO2) of diﬀerent cases also varies. These two aspects result in diﬀerent eﬀects of temperature on the dependent variables on the left and right axes in Fig. 5.
5.2. Eﬀect of furnace temperature The simulation results of the eﬀects of furnace temperature on sodium species contents (left Y-axis) and Na(g) emission rate (right Yaxis) are shown in Fig. 5 (case 1, 2 and 3). The Na(g) emission rate in the ﬂue gas is calculated by Eq. (5). This emission increases with increasing temperature since more sodium is released and distributed in the ﬂue gas at higher furnace temperature. However, the sodium 4
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the work of Song et al. . These authors reported that when the air equivalent ratio increased from 0.49 to 1.21, the fraction of gaseous sodium increased from 69.08% to 87.27% because of carbon inhibition . The carbon remaining in the ash exists as a carbon matrix that inhibits sodium migration from within the particle to the particle surface. Higher excess air corresponds to higher furnace temperature and lower carbon content in ash to weaken the eﬀect of carbon inhibition. In this work, as the value of excess air increases from 1.52 to 1.79, the furnace temperature also increases 10 ℃. The devolatilization (R 11) and char combustion (R 1-2, R 1-3) reactions are both enhanced since these reactions are sensitive to changes in the furnace temperature. Additionally, the amount of carbon remaining in the ﬂy ash is decreased, indicating that more Na is released during the char combustion process. Fig. 9 shows the eﬀect of excess air on the deposition rates of Na and ash. The amounts of deposited Na and ash both decrease with increasing excess air. Comparing the data in Figs. 8 and 9, opposite trends are observed for gaseous Na and deposited Na. This discrepancy indicates that at a higher excess air ratio, more sodium is retained in the ﬂue gas and less sodium accumulates on the probe surface. This result agrees well with the measurements from the 0.4 t/d CFB system in the work of Song et al. . Increasing the excess air ratio may alleviate problems caused by ash on the heat transfer surface of the furnace. Increases in the excess air ratio lead to an increase in the ﬂue gas temperature. In addition, a larger excess air ratio corresponds to a higher total gas ﬂow rate. These two aspects result in higher gas/solid velocity at the cross section of the furnace. Our previous work  illustrated that the sticking probability of the impacted particles is inversely proportional to the particle velocity, as is shown in Eq. (7). Hence, enlarging the solid velocity will decrease the sticking probability of particles. On the other hand, higher solid velocity will increase the contact force between the deposited and impacted particles and enhance the shedding process of the deposited ash particles. Therefore, increasing the excess air may reduce the amount of deposited sodium and ash particles.
Fig. 5. Eﬀect of temperature on gaseous Na.
′ CNaAc = CNaAc·
(21 − C′O2) (21 − CO2)
RNa (g ) = ρ ·V · CNaCl·MNa/ MNaCl + CNa2 SO4·2MNa/ MNa2 SO4 / Aout
where C′NaAc and CNaAc are the corrected concentrations and original concentrations of NaAc (NaAc represents NaCl and Na2SO4), respectively, ppm; C′O2 is the corrected concentration of O2 that is set to 6%; CO2 is the original concentration of O2, %; ρ is the density of the ﬂue gas, kg/m3; V is the volumetric ﬂow rate of the ﬂue gas, m3/s; Aout is the area of the furnace outlet, m2; CNaCl and CNa2SO4 are mass fractions of NaCl and Na2SO4 in the ﬂue gas, respectively, %; M represents the molecular weight, g/mol. Fig. 6 shows the eﬀect of furnace temperature on the Na deposition rate. The deposition rate of Na increases with increasing temperature. This trend agrees with the test results from Liu et al. . Considering the results in both Figs. 5 and 6, both gaseous sodium and deposited sodium rates increase with temperature. The reason for this result is that usually more coal is fed to the furnace at increased furnace temperature, so the inlet sodium content also increases. Fig. 7 shows the eﬀects of temperature on the ash deposition rate at the probe surface. The simulation results in this study are compared with those from the experiments of Liu et al. . The ash collection eﬃciency (ζ) is deﬁned in Eq. (6) to determine the ash deposition trend in Liu’s work. As shown in Fig. 7, the same trends are observed in the simulation and experimental results . This result indicates that higher temperature leads to more ash deposition. As more sodium accumulates on the probe surface at higher temperature (Fig. 6), a thicker sticky layer is formed on the probe surface, which increases the sticking probability of particles.
md A · c mF ·Yash·afh Ap
2·γ ·Ac 0.5·ms ·(vs r )2
where γ is the surface tension and vsr is the velocity of reﬂected particle that is dependent on the velocity of the impact particle. 5.4. Eﬀect of secondary air ratio From Section 5.3, the atmosphere in the furnace has a great impact on sodium migration and also inﬂuences the slagging and deposition characteristics. During combustion in the CFB, the oxidization/reduction region will change depending on the secondary air ratios. Hence,
where md is the mass of ash deposits, kg; mF is the mass of coal in the feed, kg; Yash is the mass fraction of ash in the coal, %; αfh is the ratio of ﬂy ash, %; Ac is the cross-sectional area of the gas ﬂow, m2; and Ap is the projected area of the probe, m2. 5.3. Eﬀect of excess air The eﬀects of excess air on the sodium species contents and Na(g) emission rate are given in Fig. 8 (case 1, 4, 5 and 6). This ﬁgure shows that increasing the amount of excess air will result in a higher value of RNa(g). More sodium is released during the devolatilization and char burning process when combustion is enhanced with additional injected air. Furthermore, the concentrations of gaseous sodium species increase as the excess air rises from 1.52 to 1.79, especially the gaseous NaCl content. The trend in the values of RNa(g) in Fig. 8 agrees with that from
Fig. 6. Eﬀect of temperature on Na deposition rate. 5
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bottom of furnace. As the value of SA/(SA + PA) increases, the concentration of primary air is reduced, aﬀecting combustion and lowering the temperature at the bottom of the furnace. Hence, the reaction rates of R 1-1–R 1-3 and the Na(g) emission rate are decreased. Eﬀects of secondary air ratio on Na and ash deposition rates are shown in Fig. 12. A higher secondary air ratio leads to more deposited Na and ash. As discussed for Fig. 10, a higher secondary air ratio leads to less volatilized Na, indicating that more Na is retained in the ash particle. As the value of SA/(PA + SA) increases from 0.2 to 0.5, the furnace temperature decreases, leading to a reduction of the velocity of gases and solids, which will increase the sticking probability of ﬂy ash particles and aﬀect the shedding process of the deposited ash particles. Hence, deposited Na and ash are increased. Generally, increasing the secondary air ratio could help to limit NOx emissions. However, increases in the secondary air ratio will aggravate ash-related problems. Hence, an appropriate secondary air ratio needs to be determined when designing and operating industrial CFB boilers burning high sodium coal.
Fig. 7. Eﬀect of temperature on ash deposition.
6. Conclusion In this work, a 2-D combustion model combined with a sodium migration model is described and applied to a 30 kW CFB system. The eﬀects of furnace temperature, excess air, and secondary air ratio on sodium transformation in a CFB furnace were investigated using simulations. The main conclusions are as follows. (1) A simpliﬁed 2-D combustion model combined with a sodium migration model is introduced and veriﬁed. The furnace temperature, gaseous concentrations, and sodium distributions in 2-D model agree within a reasonable range with those from the 3-D simulation and experiments. (2) The calculation time of the 2-D model is reduced by 75% when compared with that of the 3-D model. Using the 2-D treatment, the simulation eﬃciency is improved and variable parameter analysis can be performed quickly. (3) As the furnace temperature increases from 1100 to 1213, the Na(g) emission rate at the furnace outlet increases, but the concentrations of NaCl or Na2SO4 decreases. More sodium and ash deposits are collected on the heat transfer surface in the furnace, indicating that higher temperature leads to more serious ash-related problems. (4) Increasing the amount of excess air results in a higher Na(g) emission rate, whereas the amount of sodium or ash deposits decreases. Increasing the excess air ratio may help to alleviate ashrelated problems on the heating surface. (5) Decreasing the secondary air ratio from 0.5 to 0.2 increases the Na (g) emission rate and decreases the amount of sodium or ash deposits in the furnace. An appropriate secondary air ratio needs to be
Fig. 8. Eﬀect of excess air on gaseous Na.
Fig. 9. Eﬀect of excess air on the Na/ash deposition rate.
the eﬀect of secondary air ratio on sodium transformation is investigated in this section (case 1, 7, 8 and 9). Fig. 10 shows the inﬂuences of secondary air ratio on the sodium species contents and Na(g) emission rate. As the secondary air ratio increases from 0.2 to 0.5, the Na2SO4 concentration is reduced, while only a small change is observed for the NaCl content. Fig. 11 shows that the concentration of SO2 is reduced with increasing secondary air ratio. This reduced SO2 concentration helps explain the decreases in the Na2SO4(g) concentration and Na(g) emission rate RNa(g) with increasing secondary air ratio in Fig. 10. On the other hand, as shown in Fig. 4, the furnace temperature is higher at the bottom of the furnace, so the reaction rates of R 1-1–R 1-3 are higher in this area and most of the Na species are released at the
Fig. 10. Eﬀect of secondary air ratio on gaseous Na. 6
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Fig. 11. Eﬀect of secondary air ratio on SO2 concentration.
Fig. 12. Eﬀect of secondary air ratio on Na/ash deposition rate.
determined when designing and operating industrial CFB boilers burning high sodium coal. Declaration of Competing Interest The authors declare that they have no known competing ﬁnancial interests or personal relationships that could have appeared to inﬂuence the work reported in this paper. Acknowledgement This work is ﬁnancially supported by the National Key Research &